Nan Jiang

Dario Cupolillo

Noëlle Grosjean

Séverine Deforges

Christophe Mulle

Thierry Amédée
email: thierry.amedee@u-bordeaux.fr

Plasticity of synaptic transmission and intrinsic excitability in the dentate gyrus is altered in a mouse model of Alzheimer's disease

Keywords: Alzheimer's disease, Aß plaques, Lateral perforant path, PGE 2, Population spikes, Action potential, mEPSCs Population sipke EPSP: Excitatory postsynaptic potential fEPSP: field EPSP mEPSCs: miniature excitatory postsynaptic currents EPSCs: Excitatory postsynaptic currents mGluR: Metabotropic glutamate receptors Dentate gyrus PP: Perforant pathway ACSF: Artificial cerebro-spinal fluid ISIs: Interspike intervals sexual dimorphism, hippocampus, dentate gyrus, glial cells, neuroinflammation, Alzheimer's disease, synaptic plasticity, perforant pathway Highlights Alzheimer's disease, Inflammation, PGE 2, Synaptic plasticity, Hippocampus, APP/PS1

Azheimer's disease (AD) is a neurodegenerative disease that is linked in its early stage to synaptic dysfunction and loss of synapses. Numerous clinical data obtained from patients but also experimental data obtained on mouse models of AD show that there is a sexual dimorphism evidenced by a higher amyloid plaque deposition and an early onset of memory disorders in female mice compared to male mice.

In this work, we investigated the molecular and cellular alterations of AD as well as the associated cognitive deficits in female APP/PS1 mice, a double transgenic murine model of AD. In parallel we studied the alterations of hippocampal synaptic transmission and plasticity in the stratum moleculare, a layer in the vicinity of the dentate gyrus (DG) which specifically displayed a high density of amyloid plaques. We showed the presence of numerous amyloid plaques in the DG in a larger amount in 6 month old females compared to age-matched males as well as a strong activation of astrocyte and microglia glial cells. These molecular and cellular alterations are accompanied by hippocampo-dependent memory deficits (contextual fear conditioning and novel object place recognition task) from the age of 4 months in females whereas males have no deficit until the age of 12 months. We then evidenced by LTP-induced shifts of E-S curves, which reflects an increased excitability for APP/PS1 mice.

In conclusion our results show a prominent sexual dimorphism with much earlier amyloid plaque deposition, neuroinflammatory glial activation in female vs male APP/PS1. In parallel, significant deficits in hippocampal-dependent memory are observed as well as alterations of synaptic transmission and plasticity at the PP-DG synapse, a key synapse of the integration of mnesic informations originated from the entorhinal cortex

To study at university of Bordeaux for four years with happiness and embarrassment.

During the past four years, I gain a lot in the lab. I am writing with an appreciative heart and would like to show my heartfelt gratefulness to everyone in the lab who gives help to me for the project. There are so many people to give thanks.

First and foremost, I would like to wholeheartedly thank my supervisor, Dr. Thierry AMEDEE from IINS -UMR 5297 CNRS. Dr. Thierry AMEDEE solid knowledge, strict research attitude and kindly person benefit me for the research career. His encouragements and help that has made me independent of learning in research. Carefully discussion with me for the topics, hypotheses, and up to the revision of manuscript. Every part has made a great deal of efforts.

In the meantime, I am also very thankful to the team leader Dr. Christophe Mulle, who gave me a lot guides during the four years. Discussion with him has extended my research views and encouraged me in the field of neuroscience.

Here, I would like to thank Dr. Gael Barthet from the team for his fruitful discussion of electrophysiology study, as well as the data analysis with Dr. Christophe Blanchet. Greatly thank to Jeffrey Lopez-Rojas for his invaluable help with electrophysiological recordings, the very fruitful discussions and his constant kindness.

I am also deeply appreciative to all the jury members: Dr. Valérie Fénelon, Dr. Claire Rampon and Dr. Corinne Beurrier. For their participation to my thesis committee.

In the four years, I had the precious friendship between Dr. Gael Barthet, and my colleagues Tomas Jorda, Dario Cupolillo, Ania Gonçalves and Meryl Malezieux, Pei Zhang.

They shared with me the pleasures in the lab, a precious treasure in my life. I am very grateful to Dr. Severine Deforges, Noelle Grosjean, Ruth Betterton, Ashley Kees, Eva Rodrigues and Mariela Escande for their help during the experiments.

A special thank goes to the IINS for providing me the opportunity to perform my study in a remarkable and scientific institute. Particularly, I would like to thank all the members from IINS and all the members from BIC to have the pleasure to work together for part of my project.

I would also like to thank the friends from the other lab. Dr. Lei Fan, Dr. Xiaomin Zhang, Yuxin Ma, Shuo Liu, Ye Xiao. They make my life in Bordeaux happier and beautiful.

Finally, I would also like to thank my family for their support. Thanks to my parents, Mr. Jun Jiang and Mrs. Chunling Zhang, for many years, I have provided a warm harbor for my PhD. Study. Their love gives me power and warmth, and I continue to overcome difficulties and problems and keep going forward. The completion of my doctoral thesis will be the another starting point of my career.

studied the electrical properties of DG neurons, the transmission and the plasticity of the perforant pathway -DG neurons (PP-DG synapse) in 6 month old females mouse by comparing the two genotypes APP/PS1 vs wild type (WT).

In both genotypes, DG neurons displayed two distinct populations in terms of input resistance and action potentials (APs) discharge pattern. In contrast, the resting membrane potential, the input resistance, the activation threshold and the amplitude of APs were not modified in APP/PS1 vs WT. The frequency of discharge of APs was increased in APP/PS1 without shift of E-S curve which relates EPSP-slopes to the associated AP firing probability.

Basal transmission at the PP-DG synapse was altered in the APP/PS1 mouse vs WT without alterations in the AMPA/NMDA ratio or the AMPA rectification index. The frequency of the NMDA miniature currents was increased in APP/PS1 DG neurons vs WT which suggests an increased number of silent synapses expressing NMDA receptors but devoid of AMPA receptors. The long term potentiation (LTP) of population spike amplitude was decreased by approximately 50% in APP/PS1 mice. The decrease in LTP observed in APP/PS1 was partly related to alterations in the intrinsic properties of DG neurons as

A. Alzheimer's disease

Alzheimer's disease (AD), which was first reported and described in 1906 by a clinical psychiatrist and neuroanatomist, Alois Alzheimer, is a progressive neurodegenerative disease that first display light cognitive problems such as forgetfulness. The age dependent prevalence of AD doubles every 5 years after aged 60 and increasing from a prevalence up to 40% of those aged 85 years and older [START_REF] Von Strauss | Aging and the Occurrence of Dementia: Findings From a Population-Based Cohort With a Large Sample of Nonagenarians[END_REF]. The disease is more frequent among women than men with a ratio of 1.2 to 1.5 [START_REF] Gao | The Relationships Between Age, Sex, and the Incidence of Dementia and Alzheimer Disease: A Meta-analysis[END_REF]. In 2018, about 50 million people worldwide are suffering of dementia and 60 to 70% are due to AD (World Alzheimer Report 2018). Each year there are roughly 7.7 million new reported dementia cases (WHO, 2016).

The number of worldwide cases of dementia will likely rise up to 82 million by 2030 and could possibly reach 152 million by 2050 (World Alzheimer Report 2018). The total estimated worldwide cost of dementia in 2018 is US $ 1 trillion and could rise up to US $ 2 trillion by 2030 (World Alzheimer Report 2018). Of paramount importance is the understanding of the pathogenesis and the developing of early diagnosis and effective therapy.

At present, there are no possible therapeutics available for the prevention or alleviation of the disease progression. There are continuous efforts set up by researchers worldwide to discover new therapies for AD.

a. Progression of AD

The progression rate of AD varies considerably. Usually, patients with Alzheimer's could live up to 4-8 years after diagnosis, but some of them can live as long as 20 years, depending on multiple factors. The progression of the disease depends in part on age at diagnosis but also on the general health status of the person. The early stage of AD generally lasts from 2-8 years and goes through mild cognitive impairment stage (MCI) [START_REF] Petersen | Early diagnosis of Alzheimer's disease: is MCI too late?[END_REF] before evolving to dementia (Figure 1).

A large percentage of people diagnosed with MCI have AD as an underlying cause for their cognitive impairment, although other disorders also can cause MCI. The moderate stage lasts 2-4 years and is characterized by a more serious memory deficit including long term memory and the loss of the ability to operate typical activities of daily life (for example bathing, dressing and grooming). In the late stage of AD, individuals lose the ability to respond advisably and are unable to talk with others. They will also be unable to control actions like sitting, standing and walking. In advanced disease, patients often become mute, aggressive, anxious [START_REF] David | Decreased daytime motor activity associated with apathy in Alzheimer disease: an actigraphic study[END_REF]. To study the disease, we dissertate here intricate cellular alterations during the disease developing, and how initial responses finally turn into detrimental, leading to an irreversible dysfunction of the brain.

Figure 1. The progression of AD

The figure represents a sum of the progression of AD signs from normal cognitive to MCI and dementia.

b. Pathologic features of AD

The crucial neuropathological features of AD were described by Alois Alzheimer in 1906 (Alzheimer et al., 1906). Memory deficits are the first clinical symptoms of AD [START_REF] Jacobs | Neuropsychological detection and characterization of preclinical Alzheimer's disease[END_REF] and have been studied using different techniques of neuroscience: at the macro level, there is a considerable loss of brain tissue. At the micro level, pathological hallmarks of AD include amyloid ß accumulation, tau phosphorylation, and synaptic neuronal loss.

Amyloid Hypothesis: APP Proteolytic Process

The finding of aggregates of β-amyloid (Aβ) in the extracellular space of the brain in post-mortem pieces led to the formulation of the 'Amyloid Hypothesis' (Hardy et al., 1992).

The hypothesis put forward was that the formation of Aβ plaques is the preliminary event in AD, eventually leading to cell death and dementia.

However, there are evidences that are difficult to reconcile with this hypothesis. The primary component of plaques is the Aβ peptide, a 36-to 43-amino acid peptide derived from amyloid precursor protein (APP), which is cleaved by β secretase and γ secretase [START_REF] Hamley | The amyloid beta peptide: a chemist's perspective. Role in Alzheimer's and fibrillization[END_REF] (Figure 2). Aβ peptides can be present in aggregated insoluble forms including fibrils as well as soluble oligomers [START_REF] Kayed | Common structure of soluble amyloid oligomers implies common mechanism of pathogenesis[END_REF][START_REF] Koffie | Oligomeric amyloid β associates with postsynaptic densities and correlates with excitatory synapse loss near senile plaques[END_REF]. When Aβ is in soluble oligomer, it is generally considered as the most toxic form.

Figure 2. Aβ fragments derived from APP processing

APP processes two distinct pathways: amyloidogenic and non-amyloidogenic pathway. Serial actions of APP by α-secretase and γ-secretase produces sAPPα, C83, APP intracellular domain (AICD) and P3. There is no Aβ formed through this pathway. Serial actions of APP by β-secretase (BACE1) and γ-secretase produces sAPPβ, C99, AICD and Aβ 40-42 [START_REF] Thathiah | The role of G protein-coupled receptors in the pathology of Alzheimer's disease[END_REF].

When Aβ is aggregated in the extracellular space to form Aβ plaques, these plaques are surrounded by abnormal neuronal processes known as degenerating fibers or [START_REF] Masliah | Re-evaluation of the structural organization of neuritic plaques in Alzheimer's disease[END_REF] but also by activated glial cells driving an inflammatory response (Meda et al., 1995;[START_REF] Giulian | The HHQK domain of β-amyloid provides a structural basis for the immunopathology of Alzheimer's disease[END_REF]. It is highly likely that this inflammatory response contributes to neuronal damage, although there are numerous evidences that glial cells are playing a protective role [START_REF] Mcgeer | The inflammatory response system of brain: implications for therapy of Alzheimer and other neurodegenerative diseases[END_REF][START_REF] Lucin | Immune activation in brain aging and neurodegeneration: too much or too little?[END_REF].

Hyperphosphorylation of intracellular Tau protein

Tau is a 50-to 75-kDa protein displaying six different splice forms [START_REF] Lace | A brief history of tau: the evolving view of the microtubule-associated protein tau in neurodegenerative diseases[END_REF]. It was first characterized as a microtubule-associated protein in the brain, not only synthesized and produced in neuron cell bodies but also expressed by glial cells.

Under physiological conditions the function of tau is to bind to tubulin to stabilize microtubules (Figure 3). It is proposed that neurofibrillary pathology could contribute to neuronal dysfunction and strongly correlates with the clinical progression of AD [START_REF] Ballatore | Tau-mediated neurodegeneration in Alzheimer's disease and related disorders[END_REF][START_REF] Noble | The importance of tau phosphorylation for neurodegenerative diseases[END_REF]. In AD, tau protein is hyperphosphorylated, leading to its dissociation from microtubules and has a trend to self-aggregate, forming neurofibrillary tangles (NFTs) in cell bodies (Grundke et al., 1986;[START_REF] Goedert | Cloning and sequencing of the cDNA encoding a core protein of the paired helical filament of Alzheimer disease: identification as the microtubule-associated protein tau[END_REF][START_REF] Wischik | Isolation of a fragment of tau derived from the core of the paired helical filament of Alzheimer disease[END_REF][START_REF] Wischik | Isolation of a fragment of tau derived from the core of the paired helical filament of Alzheimer disease[END_REF] which finally lead to neurodegeneration [START_REF] Avila | Tau in neurodegenerative diseases: tau phosphorylation and assembly[END_REF] (Figure 3). The abnormal phosphorylation also leads to the formation of both soluble and insoluble tau filaments [START_REF] Braak | Abnormally phosphorylated tau protein related to the formation of neurofibrillary tangles and neuropil threads in the cerebral cortex of sheep and goat[END_REF].

It is widely acknowledged that neuroinflammation plays a pivotal role in AD pathology where both Aβ plaques and NFTs co-exist with microglia and astrocytes (Heneka et al., 2010;Serrano et al., 2011). Finally a more general picture has emerged proposing that AD is the combination of Aβ and hyperphosphorylation of tau (Coello et al., 2013;Schnabel et al., 2013).

Loss of synapses and selective neuronal cell death

It is widely known that the neurons density is reduced in AD. The loss of neurons is detected by common microscopic study [START_REF] Terry | Some morphometric aspects of the brain in senile dementia of the Alzheimer type[END_REF]. As AD is developing, cognitive dysfunction becomes worse together with synapse loss and neuronal cell death. Although it is generally acknowledged that neuronal death happens during neurodegenerative diseases, the mechanism of neuronal death is often unclear. There are different mechanisms which contribute to neuronal death, such as necrosis, apoptosis, autophagic cell death and excitotoxicity [START_REF] Gorman | Dimerization of the transmembrane domain of amyloid precursor proteins and familial Alzheimer's disease mutants[END_REF].

Within the neuritic plaques, synapse loss observed in AD may occurs in still living neurons and leads to the neuronal death (Terry et al., 1991), which might be due to toxic roles of the aggregated amyloid or the soluble Aβ peptide [START_REF] Podlisny | Aggregation of secreted amyloid-protein into sodium dodecyl sulfatestable oligomers in cell culture[END_REF][START_REF] Selkoe | Toward a comprehensive theory for Alzheimer's disease. Hypothesis: Alzheimer's disease is caused by the cerebral accumulation and cytotoxicity of amyloid β-protein[END_REF] (Figure 4). Moreover, there is a regionally specific loss of neurons in AD and this specific neuronal loss in AD differs from what is found in normal aging [START_REF] West | Differences in the pattern of hippocampal neuronal loss in normal ageing and Alzheimer's disease[END_REF].

These results suggest that AD cannot be considered as a further step in the continuum of normal aging though age is certainly one of risk factors for AD.

Figure 4. Aβ and NFTs in synapses loss and neuronal death

The appearance of AD is highlighted by the Aβ formation in the brain. Along the progression of the disease, tau becomes hyperphosphorylated, a procedure thought to be determined by Aβ and increasing dendritic tau concentrations. In AD, high concentrations of tau in the dendritic compartment are involved in enhanced toxic actions of Aβ at postsynaptic terminals and therefore could be part of a toxic pathway for synapses loss and neuronal death.

Activated glial cells

Activated glial cell is a prominent feature of AD neuropathological features, both with extracellular fibrillar Aβ and intracellular neurofibrillary tangles (Selkoe et al., 1996), which are closely associated with reactive microglia and astrocytes (Figure 5) [START_REF] Sheng | Tumor necrosis factor alpha upregulates human microglial cell production of interleukin-10 in vitro[END_REF]Pozo et al., 2011). The microglia are usually inside and close to the central core of Aβ plaques, while the astrocytes generally wall-off the plaques with some of their processes in the vicinity of the amyloid central core (Bamberger et al., 2001). The kinetics to develop Aβ plaques are not known for sure, but it is plausible that such alterations develop over several years. The dialogue between microglia and astrocytes play a key role in the immune response surrounding the Aβ plaques. As a consequence, Aβ and activated glial cells could set up a harmful circle, fostering neuroinflammation and the pathology.

Hence, chronically activated microglia and astrocytes would become neurotoxic by releasing inflammatory cytokines, proteolytic enzymes, complement factors and reactive intermediaries. Regulation of glial activation can be impaired under sustained proinflammatory conditions as found in the aged brain [START_REF] Von Bernhardi | Pro-inflammatory conditions promote neuronal damage mediated by Amyloid Precursor Protein and decrease its phagocytosis and degradation by microglial cells in culture[END_REF].

c. Clinical Strategies

Clinical strategies have been focussed on four aspects of the pathology: Aβ plaques, tau tangles, neurodegeneration, and dementia. In spite of a considerable knowledge of genetics, epidemiology, neuropathological and molecular mechanisms involved in the pathology, there is still no effective cure for AD (Table 1).

β-Amyloid-Based Therapeutics

The existing drug development treatments specifically designed multiple Aβ based therapeutics. With a strong evidence supporting the amyloid hypothesis, the most likely targets would be the successful alteration of Aβ processing through β and γ-secretase. BACE1 has appeared as a likely target for the treatment of AD since it is required for the formation of Aβ. Few BACE1 inhibitors (Lanabecestat, LY-2811376 and Verubecestat) [START_REF] Stamford | Inhibitors of BACE for treating Alzheimer's disease: a fragment-based drug discovery story[END_REF][START_REF] Oehlrich | The evolution of amidine-based brain penetrant BACE1 inhibitors[END_REF][START_REF] Yan | Inhibiting BACE1 to reverse synaptic dysfunctions in Alzheimer's disease[END_REF] were tested in clinical trials, but failed to demonstrate positive effects.

Other studies have shown that non-steroidal anti-inflammatory drugs (NSAIDs) lowering Aβ 42 without obvious totally inhibition of γ-secretase activity, and it will be important to explain the potential molecular mechanism of this action to increase the relative proportion of shorter Aβ species in vitro, such as Aβ 37 and Aβ 38 , reduced prevalence of AD [START_REF] Weggen | A subset of NSAIDs lower amyloidogenic Aβ42 independently of cyclooxygenase activity[END_REF].

Removal of Aβ have been proposed and may provide an alternative method to decrease

Aβ oligomers for the treatment of AD [START_REF] Tanzi | Clearance of Alzheimer's Aβ peptide: the many roads to perdition[END_REF][START_REF] Pangalos | Disease modifying strategies for the treatment of Alzheimer's disease targeted at modulating levels of the β-amyloid peptide[END_REF]. Aβ aggregation inhibitors also have been tested with the aim to reduce Aβ load. For example, 3-Amino-1-propanesulfonic acid (3APS) which interacts with Aβ monomers reduces Aβ deposition both in transgenic mice and in humans [START_REF] Aisen | A Phase II study targeting amyloid-β with 3APS in mild-to-moderate Alzheimer disease[END_REF]Gervais et al., 2007).

Antibodies raised against Aβ (Aβ -antibodies) have also been tested and some studies have shown that they were alleviating plaque burden and cognitive deficits [START_REF] Bard | Peripherally administered antibodies against amyloid β-peptide enter the central nervous system and reduce pathology in a mouse model of Alzheimer disease[END_REF]Dodart et al., 2002;[START_REF] Lannfelt | Perspectives on future Alzheimer therapies: amyloid-β protofibrils-a new target for immunotherapy with BAN2401 in Alzheimer's disease[END_REF]. Different mechanisms could contribute to these results, such as the binding of Aβ -antibodies to Aβ forms (monomeric, oligomeric, and fibrillar) which prevent oligomerization, and increased clearance of fibrillar Aβ [START_REF] Pangalos | Disease modifying strategies for the treatment of Alzheimer's disease targeted at modulating levels of the β-amyloid peptide[END_REF]. However, clinical trials using different anti-Aβ strategies that affect the formation, aggregation and clearance of Aβ have all failed and be stopped in phase II of phase III.

Therefore, substitute hypotheses are urgently needed.

Tau-Based Therapeutics

On the contrary to strategies focused on β-amyloid, tau-related strategies have received few attentions until recently, despite the fact that a widespread tau pathology is crucial to the disease. Hence different strategies targeting the tau-dependent pathogenesis are actively studied carried, some of them being under clinical trials [START_REF] Medina | Modulation of GSK-3 as a therapeutic strategy on tau pathologies[END_REF][START_REF] Medina | New perspectives on the role of tau in Alzheimer's disease. Implications for therapy[END_REF][START_REF] Li | Tau-based therapies in neurodegeneration: opportunities and challenges[END_REF].

Attenuating hyperphosphorylation of tau through the inhibition of kinases may seem a tempting strategy [START_REF] Churcher | Tau therapeutic strategies for the treatment of Alzheimer's disease[END_REF]Mazanetz et al., 2007) but the task is complicated by the existence of numerous tau phosphorylation sites, and the ability of multiple kinases to phosphorylate individual sites [START_REF] Mi | The role of tau phosphorylation in the pathogenesis of Alzheimer's disease[END_REF]. Taking into consideration that hyperphosphorylation of tau may be achieved in vivo by multiple protein kinase activities [START_REF] Palop | Aberrant excitatory neuronal activity and compensatory remodeling of inhibitory hippocampal circuits in mouse models of Alzheimer's disease[END_REF][START_REF] Hanger | Tau phosphorylation: the therapeutic challenge for neurodegenerative disease[END_REF][START_REF] Medina | Further understanding of tau phosphorylation: implications for therapy[END_REF] few protein kinases, including GSK-3β, MARK and CDK5, have been proposed as potential therapeutic targets [START_REF] Schneider | Tau-based treatment strategies in neurodegenerative diseases[END_REF]. However, the development of protein kinase inhibitors faced toxicity and specificity, which resulted in a "dead end" during preclinical studies.

The idea that tau dispersed from microtubules result in abnormal microtubule (MT)stabilizing function has led to MT-stabilizing molecules as therapeutic target. In those MTstabilizing molecules, some anti-mitotic drugs such as paclitaxel or epothilone D have been tested in tau-overexpressing mouse models (Brunden et al., 2010).

Tau aggregation and the various tau species formed (monomers, oligomers, fibrils) along the aggregation process have also been a great of interest for potential treatment. As such, various anti-tau immunotherapy strategies to clean pathological proteins have also been tested in animal models suggesting that cleaning toxic protein species could be useful to treat tauopathies in AD [START_REF] Pedersen | Tau immunotherapy for Alzheimer's disease[END_REF].

Approaches to Inhibit Neurodegeneration

N-Methyl-D-Aspartate receptors (NMDARs) have always get the interest of researchers as potential drug targets [START_REF] Kemp | NMDA receptor pathways as drug targets[END_REF]Olivares et al., 2012;[START_REF] Zhang | Dysfunction of NMDA receptors in Alzheimer's disease[END_REF].

Accordingly, developed NMDA receptor antagonists must partially block the receptor and complete the normal functions to minimize the side effects.

The current therapy of AD includes two types: 1. the cholinesterase inhibitors (ChEIs), galantamine and rivastigmine (mild to moderate AD), donepezil (mild to severe AD); 2. noncompetitive NMDA receptor antagonist memantine (moderate to severe AD). Both ChEIs and memantine could present beneficial effects on behavior and cognition [START_REF] Patel | Combination therapy for Alzheimer's disease[END_REF][START_REF] Deardorff | A fixed-dose combination of memantine extended-release and donepezil in the treatment of moderate-to-severe Alzheimer's disease[END_REF]). In the central nervous system (CNS) disease, although glutamate induced excitotoxicity has been thought to be affected by the NMDARs antagonist (Phencyclidine, ketamine, MK801 dizocilpine and memantine), due to the high affinity and long reside time of phencyclidine, ketamine, and MK801 on the receptor, their clinical trial have been stopped [START_REF] Ellison | The N-methyl-D-aspartate antagonists phencyclidine, ketamine and dizocilpine as both behavioral and anatomical models of the dementias[END_REF]. Besides, memantine shows a low affinity and is well accepted. Therefore, it is applied to the moderate to severe AD [START_REF] Reisberg | Memantine in moderate-to-severe Alzheimer's disease[END_REF]Pierson et al., 2014;[START_REF] Song | Mechanism of NMDA receptor channel block by MK-801 and memantine[END_REF]. Furthermore, another glutamate release inhibitor (riluzole), is in phase II clinical trial for mild AD patients (Graham et al., 2017).

Increasing evidences propose that neurotoxicity could be involved in the pathogenicity of AD [START_REF] Parihar | Experimental excitotoxicity provokes oxidative damage in mice brain and attenuation by extract of Asparagus racemosus[END_REF]. Neurotoxicity may be due to the chronic inflammatory response through the production of cytokines, reactive oxygen species (ROS), excitatory amino acids, proteases and nitric oxide. The inflammatory hypothesis has been highly reported as a key component of the pathogenesis of AD. Hence, NSAIDs has been tested in numerous clinical trials [START_REF] Vlad | Protective effects of NSAIDs on the development of Alzheimer disease[END_REF][START_REF] Laino | In follow-up analysis of clinical trial, NSAIDs seem to preserve cognitive function in patients with healthy brains[END_REF].

The use of anti-oxidants for therapeutic as well as preventive treatments in AD has been questioned [START_REF] Perkins | Association of antioxidants with memory in a multiethnic elderly sample using the Third National Health and Nutrition Examination Survey[END_REF][START_REF] Masaki | Association of vitamin E and C supplement use with cognitive function and dementia in elderly men[END_REF][START_REF] Kim | The role of oxidative stress in neurodegenerative diseases[END_REF]. A study has proposed that anti-oxidant by scavenging free radicals lead to the suppression of hydrogen superoxide and thus suppressed the Aβ burden in AD (Lim et al., 2001;[START_REF] Matsubara | Melatonin increases survival and inhibits oxidative and amyloid pathology in a transgenic model of Alzheimer's disease[END_REF].

Recently, experiments also verified the supposed role of peroxisome proliferatoractivated receptor γ (PPARγ) agonists to decrease the expression of Apolipoprotein ε4 (APOε4) gene and showed neuroprotective effect during AD development in several mouse models [START_REF] Bonet-Costa | Clearing Amyloid-β through PPAR γ/ApoE Activation by Genistein is a Treatment of Experimental Alzheimer's Disease[END_REF][START_REF] Moutinho | Therapeutic potential of nuclear receptor agonists in Alzheimer's disease[END_REF]Koster et al., 2017). Several PPARγ agonists were tested for their role in AD, but owing to the large quantity health risks, including cardiovascular problems, weight gain, and fluid retention, the concept of neurodegeneration in AD attracts certain new therapeutic treatment for AD (Palop et al., 2006).

Cognitive Enhancers

Since 1970s-1980s, it was found that acetylcholine (ACh) production, ACh levels, and its receptors were extremely decreased in AD brain (Kasa et al., 1997). For many years and still today, there are no other clinical treaments available than acetylcholinesterase inhibitors (AChEIs) and memantine (non-competitive NMDA receptor antagonist). This drive other therapeutic strategies to be investigated such as the use of baicalein, a positive allosteric modulator of the GABA-A receptor, to improve cognitive dysfunctions [START_REF] Zhang | Baicalein selectively induces apoptosis in activated lymphocytes and ameliorates concanavalin a-induced hepatitis in mice[END_REF].

In addition to AChEIs, several agonists and antagonists of the serotoninergic, indirectly increase cholinergic neurotransmission are also being tested for AD. 5-hydroxytryptamine (5-HT) receptors [START_REF] Pauwels | Diverse signalling by 5-hydroxytryptamine (5-HT) receptors[END_REF] were found to be effective in animal models of AD (Butzlaff et al., 2016). 5-HT1A partial agonist S15535 enhanced learning and memory in mouse models of AD [START_REF] Millan | The role of monoamines in the actions of established and "novel" antidepressant agents: a critical review[END_REF] and 5-HT2A receptor agonists improved cognitive dysfunctions [START_REF] Zhang | The role of serotonin 5-HT2A receptors in memory and cognition[END_REF]. Last, a 5-HT6 receptor antagonist, SB271036, rescued memory impairment in a mouse model of AD by inhibiting γ-secretase activity and reducing astrocyte and microglia activation [START_REF] Yun | Serotonin 6 receptor controls Alzheimer's disease and depression[END_REF].

The application of pharmacological treatment discussed above have been demonstrated

to present a direct effect on the progressive alteration of cognition [START_REF] Neumann | Cost-effectiveness of donepezil in the treatment of mild or moderate Alzheimer's disease[END_REF][START_REF] Birks | Cholinesterase inhibitors for Alzheimer's disease[END_REF][START_REF] Tampi | Memantine: efficacy and safety in mild-to-severe Alzheimer's disease[END_REF]. Although these treatments display some beneficial aspects, numerous patients do not respond, and these compounds do not attenuate disease development [START_REF] Giacobini | Cholinesterase inhibitor therapy stabilizes symptoms of Alzheimer disease[END_REF][START_REF] Hansen | Functional outcomes of drug treatment in Alzheimer's disease[END_REF]. In this case, establishment of other therapeutics for this complex disease will be a main aim.

Aβ hypothesis

Inhibition of βsecretase

Acylguanides [START_REF] Cole | Acylguanidines as small-molecule βsecretase inhibitors[END_REF] Aminobenzthiazines [START_REF] May | Robust central reduction of amyloid-β in humans with an orally available, non-peptidic β-secretase inhibitor[END_REF] Aminoquinazolines [START_REF] Madden | Fragment-based discovery and optimization of BACE1 inhibitors[END_REF] OM99-2 [START_REF] Ghosh | Design of Potent Inhibitors for Human Brain Memapsin 2 (β-Secretase)[END_REF] Inhibition of γsecretase LY450139 [START_REF] Henley | Development of semagacestat (LY450139), a functional gamma-secretase inhibitor, for the treatment of Alzheimer's disease[END_REF] DAPT [START_REF] Morohashi | C-terminal fragment of presenilin is the molecular target of a dipeptidic γ-secretase-specific inhibitor DAPT (N-[N-(3, 5difluorophenacetyl)-l-alanyl]-S-phenylglycine t-butyl ester)[END_REF]

Prevention of Aβ aggregation

Tramiprosate [START_REF] Aisen | Alzhemed: a potential treatment for Alzheimer's disease[END_REF] Clioquinol [START_REF] Crouch | Therapeutic redistribution of metal ions to treat Alzheimer's disease[END_REF] Colostrinin [START_REF] Szaniszlo | New insights into clinical trial for Colostrinin in Alzheimer's disease[END_REF]

Increase Aβ clearance

Neprilysin [START_REF] Shirotani | Neprilysin degrades both amyloid beta peptides 1-40 and 1-42 most rapidly and efficiently among thiorphan-and phosphoramidon-sensitive endopeptidases[END_REF] IDE [START_REF] Farris | Insulin-degrading enzyme regulates the levels of insulin, amyloid beta-protein, and the beta-amyloid precursor protein intracellular domain in vivo[END_REF] ACE [START_REF] Hu | Angiotensin-converting enzyme degrades Alzheimer amyloid beta-peptide (A beta ); retards A beta aggregation, deposition, fibril formation; and inhibits cytotoxicity[END_REF] Plasmin [START_REF] Melchor | The tissue plasminogen activatorplasminogen proteolytic cascade accelerates amyloid-beta (Abeta) degradation and inhibits Abeta-induced neurodegeneration[END_REF] Anti-Aβ immunotherapy Gantenerumab (Roche) Solanezumab (Eli Lilly) Bapineuzumab (Pfizer) BAN2401 [START_REF] Logovinsky | Safety and tolerability of BAN2401-a clinical study in Alzheimer's disease with a protofibril selective Aβ antibody[END_REF][START_REF] Piton | Alzheimer's disease: advances in drug development[END_REF]

Tau hypothesis

Inhibition of tau phosphorylation GSK-3β [START_REF] Gao | GSK3: a key target for the development of novel treatments for type 2 diabetes mellitus and Alzheimer disease[END_REF] Roscovitine [START_REF] Zhang | Cyclin-dependent kinase inhibitors attenuate protein hyperphosphorylation, cytoskeletal lesion formation, and motor defects in Niemann-Pick Type C mice[END_REF] Anti-Tau immunotherapy ACI-35 vaccine (Janssen Pharmaceuticals) AADvac1 vaccine, DC8E8 antibody (Axon Neurosciences) PHF1, MC1 antibodies (Eli Lilly) MAb86 (Roche) IPN001 and IPN002 antibodies (Bristol Meyers Squibb)

Neuroinflammation

hypothesis NSAIDs CHF-5074 [START_REF] Porrini | CHF5074 (CSP-1103) induces microglia alternative activation in plaque-free Tg2576 mice and primary glial cultures exposed to betaamyloid[END_REF] Azeliragon [START_REF] Burstein | Azeliragon Phase 2b Survival Analysis Supports Beneficial Effects on Delaying Time to Cognitive Deterioration in Patients with Mild Alzheimer's Disease[END_REF] Oridonin (Wang et al., 2014) Protocatechuic [START_REF] Song | Protocatechuic acid improves cognitive deficits and attenuates amyloid deposits, inflammatory response in aged AβPP/PS1 double transgenic mice[END_REF] Acetylpuerarin [START_REF] Meng | Acetylpuerarin reduces inflammation and improves memory function in a rat model of Alzheimer's disease induced by Aβ1-42[END_REF] Indomethacin [START_REF] Sung | Modulation of nuclear factor-κB activity by indomethacin influences Aβ levels but not Aβ precursor protein metabolism in a model of Alzheimer's disease[END_REF] Mefenamic acid [START_REF] Joo | Mefenamic acid shows neuroprotective effects and improves cognitive impairment in in vitro and in vivo Alzheimer's disease models[END_REF] Ibuprofen [START_REF] Van Dam | Ibuprofen modifies cognitive disease progression in an Alzheimer's mouse model[END_REF]

B. Neuroinflammation

Over the last two decades, an increasing range of evidence has demonstrated that neuroinflammation involving the activation of glial cells played a pivotal role in the disease progression and pathology (Akiyama et al., 2000;Heneka and O'Banion, 2007;Heneka et al., 2015[START_REF] Heneka | Inflammasome signalling in brain function and neurodegenerative disease[END_REF]. There are considerable evidence that cytokines generated by activated glial cells during neuroinflammation, i.e., tumor necrosis factor alpha (TNF-α), interleukin-1β (IL-1β), and interleukin 6 (IL-6) [START_REF] Quan | Time course and localization patterns of interleukin-1β messenger RNA expression in brain and pituitary after peripheral administration of lipopolysaccharide[END_REF][START_REF] D'mello | P-Selectin-mediated monocyte-cerebral endothelium adhesive interactions link peripheral organ inflammation to sickness behaviors[END_REF][START_REF] Thomson | Peripheral inflammation is associated with remote global gene expression changes in the brain[END_REF] have profound effects on synaptic transmission and plasticity [START_REF] Pickering | Pro-inflammatory cytokines and their effects in the dentate gyrus[END_REF]Nistico et al., 2013) and may underlie cognitive and behavioral alterations [START_REF] Raison | Cytokines sing the blues: inflammation and the pathogenesis of depression[END_REF][START_REF] Galic | Cytokines and brain excitability[END_REF][START_REF] Santello | TNFα in synaptic function: switching gears[END_REF][START_REF] Lou Camara | Effects of centrally administered etanercept on behavior, microglia, and astrocytes in mice following a peripheral immune challenge[END_REF].

a. Neuroinflammation in AD

Despite the considerable corpus of data pointing out the crucial role of neuroinflammation in AD, it is still unclear if neuroinflammation is causative or correlative of the pathology (Heneka et al., 2015;[START_REF] Kinney | Inflammation as a central mechanism in Alzheimer's disease[END_REF][START_REF] Chaney | Longitudinal investigation of neuroinflammation and metabolite profiles in the APP swe× PS 1Δe9 transgenic mouse model of Alzheimer's disease[END_REF][START_REF] Lin | Neuroinflammation, Gut Microbiome, and Alzheimer's Disease[END_REF][START_REF] Cao | Peripheral immune system in aging and Alzheimer's disease[END_REF]Kumar et al., 2018). It has been suggested that Aβ deposition and NFTs activate microglia and increases level of pro-inflammatory cytokines (IL-1β, IL-6, TNF-α…) (Figure 6). These cytokines enhance APP processing via β-secretase and therefore promote Aβ production [START_REF] Blasko | Costimulatory effects of interferon-γ and interleukin-1β or tumor necrosis factor α on the synthesis of Aβ1-40 and Aβ1-42 by human astrocytes[END_REF][START_REF] Tuppo | The role of inflammation in Alzheimer's disease[END_REF]. Rodent models overexpressing mutated hAPP protein have highlighted a direct connection between the level of Aβ and increased levels of TNF-α, IL-6, IL-12, IL-1β, and IL-1α (Patel et al., 2005).

The inflammatory procedure in AD is promoted by the activation of microglia and astrocytes via pro-inflammatory molecules and associated signaling pathways, therefore resulting in synaptic impairment, neuronal loss, as well as the other inflammatory molecules produced [START_REF] Perlmutter | Microangiopathy, the vascular basement membrane and Alzheimer's disease: a review[END_REF][START_REF] Mcgeer | Microglia in degenerative neurological disease[END_REF][START_REF] Aisen | Inflammation and Alzheimer's disease: mechanisms and therapeutic strategies[END_REF]. In spite of the fact that, the role of Aβ as a potential molecule to induce the inflammation is not clear, the aggregation applies an effect through activation of caspases and transcription factors including NF-κB and AP-1, which prone to produce many inflammation mediators (IL-1β, TNF-α, and IL-6) and further β amyloid [START_REF] Ho | Mechanisms of cell signaling and inflammation in Alzheimer's disease[END_REF]. Pro-inflammatory cytokines (TNFα and IL-1β and IL-6) act directly on the neurons and cause apoptosis.

Pro-inflammatory cytokines might be injurious to synapses by themselves and thus impair neuronal function such as long term potentiation (LTP) of synaptic transmission [START_REF] Curran | The pro-inflammatory cytokine interleukin-18 impairs long-term potentiation and NMDA receptor-mediated transmission in the rat hippocampus in vitro[END_REF][START_REF] Griffin | Interleukin-1 mediates Alzheimer and Lewy body pathologies[END_REF][START_REF] Hickman | Microglial dysfunction and defective β-amyloid clearance pathways in aging Alzheimer's disease mice[END_REF][START_REF] Tachida | Interleukin--1β up--regulates TACE to enhance α--cleavage of APP in neurons: resulting decrease in Aβ production[END_REF][START_REF] Ojala | Interleukin-18 increases expression of kinases involved in tau phosphorylation in SH-SY5Y neuroblastoma cells[END_REF]Dugan et al., 2009;[START_REF] Park | Tumor necrosis factor-alpha mediated signaling in neuronal homeostasis and dysfunction[END_REF][START_REF] Chakrabarty | IFN-γ promotes complement expression and attenuates amyloid plaque deposition in amyloid β precursor protein transgenic mice[END_REF][START_REF] Zhao | The contribution of activated astrocytes to Aβ production: implications for Alzheimer's disease pathogenesis[END_REF][START_REF] Kitazawa | Blocking IL-1 signaling rescues cognition, attenuates tau pathology, and restores neuronal β-catenin pathway function in an Alzheimer's disease model[END_REF]Spooren et al., 2011;[START_REF] Sutinen | Proinflammatory interleukin-18 increases Alzheimer's disease-associated amyloid-β production in human neuron-like cells[END_REF]). In animals, in vivo [START_REF] Murray | Evidence that increased hippocampal expression of the cytokine interleukin-1β is a common trigger for age-and stress-induced impairments in long-term potentiation[END_REF][START_REF] Vereker | The inhibitory effect of interleukin-1β on long-term potentiation is coupled with increased activity of stress-activated protein kinases[END_REF] and in vitro [START_REF] Bellinger | Interleukin 1β inhibits synaptic strength and long-term potentiation in the rat CA1 hippocampus[END_REF][START_REF] Tong | Brain-derived neurotrophic factor-dependent synaptic plasticity is suppressed by interleukin-1β via p38 mitogen-activated protein kinase[END_REF] electrophysiological studies have demonstrated that IL-1β decreases LTP in the hippocampus. While IL-18 also suppresses LTP in the hippocampus, the IL-1 receptor antagonist (IL1RA) reverse the impairment of LTP by IL-18 [START_REF] Curran | The pro-inflammatory cytokine interleukin-18 impairs long-term potentiation and NMDA receptor-mediated transmission in the rat hippocampus in vitro[END_REF], thus illustrating that IL-18 affects LTP through an IL-1 dependent pathway. Cytokines can locally affect synaptic transmission between glia and neuron signaling. For example, a recent study has shown that TNFα enhances presynaptic activity via glutamate/NMDAR by measuring the frequency of miniature excitatory postsynaptic currents (mEPSCs) in mouse hippocampal slices [START_REF] Habbas | Neuroinflammatory TNFα impairs memory via astrocyte signaling[END_REF].

Neurons also seem to lead to the accumulation of neuroinflammatory products in AD, although neurons were normally thought to be passive onlookers during neuroinflammation.

Neurons generate several molecules such as IL-1, IL-6 and TNF-α (Tchelingerian et al., 1996;[START_REF] Friedman | Cytokines regulate expression of the type 1 interleukin-1 receptor in rat hippocampal neurons and glia[END_REF]. It has been demonstrated that neurons are available to work as the origin of complement proteins [START_REF] Shen | Neuronal expression of mRNAs for complement proteins of the classical pathway in Alzheimer brain[END_REF][START_REF] Strohmeyer | Detection of complement alternative pathway mRNA and proteins in the Alzheimer's disease brain[END_REF], (pentraxins, Creactive protein, amyloid P, prostanoids, NOS) [START_REF] Yasojima | Human neurons generate C-reactive protein and amyloid P: upregulation in Alzheimer's disease[END_REF][START_REF] Heneka | Role for peroxisome proliferator-activated receptor--γ in Alzheimer's disease[END_REF][START_REF] Pavlov | The cholinergic anti-inflammatory pathway[END_REF][START_REF] Simard | Bone marrowderived microglia play a critical role in restricting senile plaque formation in Alzheimer's disease[END_REF]. Proinflammatory molecules produced by neurons may actually promote further neuroinflammatory actions and result in even more serious neuronal impairment as well as the pathological development of AD.

Microglia

Microglia are the resident brain macrophages and are usually considered as "chef d'orchestre" of the brain inflammatory response during infections and disease. Microglia accounts for 5%-12% of all glial cells in the rodent brain and for 0.5%-16% in human brain [START_REF] Lawson | Heterogeneity in the distribution and morphology of microglia in the normal adult mouse brain[END_REF][START_REF] Mittelbronn | Local distribution of microglia in the normal adult human central nervous system differs by up to one order of magnitude[END_REF][START_REF] Gomez-Nicola | Microglial dynamics and role in the healthy and diseased brain: a paradigm of functional plasticity[END_REF].

In the course of neurodegenerative diseases, microglia become activated and phagocytic, proliferates and undergoes profound morphological modifications [START_REF] Ransohoff | Microglial physiology: unique stimuli, specialized responses[END_REF][START_REF] Glass | Mechanisms underlying inflammation in neurodegeneration[END_REF]. However, it is still debated if these modifications are beneficial or detrimental to the organism, especially in the light of the time course of the disease [START_REF] Sierra | Janus-faced microglia: beneficial and detrimental consequences of microglial phagocytosis[END_REF].

Microglia take also part in forming appropriate neuronal connectivity during CNS development by pruning supernumerary and improper synapses to reinforce the appropriate ones based on neuronal activity and experience [START_REF] Pierre | Neonatal microglia: the cornerstone of brain fate[END_REF]. Additionally, microglia modulate in the adult brain complicated cognitive process such as learning and memory [START_REF] Parkhurst | Microglia promote learning-dependent synapse formation through brain-derived neurotrophic factor[END_REF][START_REF] Udeochu | Microglia communication: parallels between aging and Alzheimer's disease[END_REF]. Microglia are quiescent in resting status with small cell body and branching processes. Under the condition of disease, microglia turn into actived microglia [START_REF] Stence | Dynamics of microglial activation: A confocal time--lapse analysis in hippocampal slices[END_REF]. Differing to the resting status, activated microglia are recognized by their amoeboid procedures, release of both pro-inflammatory and antiinflammatory molecules, as well as phagocytic removal of apoptotic cells [START_REF] Gehrmann | Microglia: intrinsic immuneffector cell of the brain[END_REF][START_REF] Neumann | Debris clearance by microglia: an essential link between degeneration and regeneration[END_REF]. Microglia continuously scan the brain parenchyma by sending and retracting its processes (Nimmerjahn et al., 2005;[START_REF] Davalos | ATP mediates rapid microglial response to local brain injury in vivo[END_REF][START_REF] Haynes | The P2Y12 receptor regulates microglial activation by extracellular nucleotides[END_REF][START_REF] Tremblay | The role of microglia in the healthy brain[END_REF] and play an important role in the plasticity of neuronal circuits.

During the course of AD, Aβ plaques are considered by microglial cells as a danger signal inducing a series of morphological and functional changes. These cells are termed "reactive" [START_REF] Bornemann | Aβ-induced inflammatory processes in microglia cells of APP23 transgenic mice[END_REF]. This innate immune reaction is considered to be beneficial to the organism if it settles down the danger signal. If not, microglia might topple in a chronically activated status leading to detrimental functions. Hence, reactive microglia become tightly involved in Aβ deposition via cell membrane receptors (SCARA1, CD36, CD14, α6β1 integrin, CD47, and Toll-like receptors) in brains from AD patients and AD mouse models [START_REF] Mcgeer | Reactive microglia in patients with senile dementia of the Alzheimer type are positive for the histocompatibility glycoprotein HLA-DR[END_REF][START_REF] Griffin | Brain interleukin 1 and S-100 immunoreactivity are elevated in Down syndrome and Alzheimer disease[END_REF][START_REF] Tooyama | Reactive microglia express class I and class II major histocompatibility complex antigens in Alzheimer's disease[END_REF][START_REF] Reed-Geaghan | CD14 and tolllike receptors 2 and 4 are required for fibrillar Aβ-stimulated microglial activation[END_REF][START_REF] Frenkel | Scara1 deficiency impairs clearance of soluble amyloid-β by mononuclear phagocytes and accelerates Alzheimer's-like disease progression[END_REF][START_REF] Gold | β-amyloid, microglia, and the inflammasome in Alzheimer's disease[END_REF]Heela et al., 2017). Furthermore, the expression of some proliferation and inflammation markers by reactive microglia (CSF1, IL34, TGFß) correlate with disease severity [START_REF] Olmos-Alonso | Pharmacological targeting of CSF1R inhibits microglial proliferation and prevents the progression of Alzheimer's-like pathology[END_REF], also suggesting a pathological role of microglia proliferation and activation in disease progression.

Astroglia

Astrocytes are also CNS-resident glial cells and respond to pathological stimuli through reactive gliosis aimed at neuroprotection [START_REF] Sofroniew | Astrocytes: biology and pathology[END_REF][START_REF] Burda | Reactive gliosis and the multicellular response to CNS damage and disease[END_REF].

However, morphological remodeling of astrocytes may lead to cognitive deficits via reducing the number of synapses [START_REF] Zorec | Memory formation shaped by astroglia[END_REF] and might contribute to synaptic weakness and death. Like microglia in AD, astrocytes are surrounding Aβ plaques [START_REF] Medeiros | Astrocytes: conductors of the Alzheimer disease neuroinflammatory symphony[END_REF] but interestingly, their activation precede Aβ plaque deposition in APP transgenic mice, that is at an early stage of the pathology [START_REF] Heneka | Acute treatment with the PPARγ agonist pioglitazone and ibuprofen reduces glial inflammation and Aβ1-42 levels in APPV717I transgenic mice[END_REF].

In AD, reactive astrocytes go through several complex alterations that vary between brain regions and happen at early stage of the disease, even before Aβ deposition [START_REF] Kummer | Ear2 deletion causes early memory and learning deficits in APP/PS1 mice[END_REF]. Reactive astrocytes are characterized by enhanced expression of glial fibrillary acidic protein (GFAP) and also marks of functional abnormality [START_REF] Olabarria | Age-dependent decrease in glutamine synthetase expression in the hippocampal astroglia of the triple transgenic Alzheimer's disease mouse model: mechanism for deficient glutamatergic transmission?[END_REF]. In mice models of AD, the early response is marked by astrocytes function decline, which might have profound and lasting effects on synaptic function, because astrocytes are key players of synaptic transmission [START_REF] Kang | Astrocyte-mediated potentiation of inhibitory synaptic transmission[END_REF][START_REF] Olabarria | Concomitant astroglial atrophy and astrogliosis in a triple transgenic animal model of Alzheimer's disease[END_REF][START_REF] Olabarria | Age-dependent decrease in glutamine synthetase expression in the hippocampal astroglia of the triple transgenic Alzheimer's disease mouse model: mechanism for deficient glutamatergic transmission?[END_REF]. Similar to microglia, in response to Aβ, astrocytes release pro-inflammatory cytokines, nitric oxide, and other cytotoxic molecules fueling therefore the neuroinflammatory process.

Cytokines

Microglia and astrocytes are the main source of pro-inflammatory cytokines in AD.

Cytokines take part to almost every step of the neuroinflammatory process but also Aβ deposition. Activation of microglia is designed by cytokines. Hence, in old APP/PS1 mice (15 months of age) the pathological accumulation of Aβ seems directly related to increased levels of pro-inflammatory cytokines, including TNFα, IL-6, IL-1α and IL-1ß [START_REF] Patel | Inflammatory cytokine levels correlate with amyloid load in transgenic mouse models of Alzheimer's disease[END_REF].

Interestingly, in vitro incubation of microglia with soluble Aβ 1-42 increase proinflammatory cytokines production ( [START_REF] Lue | Inflammatory repertoire of Alzheimer's disease and nondemented elderly microglia in vitro[END_REF] such as IL-1β [START_REF] Akama | β-Amyloid stimulation of inducible nitric oxide synthase in astrocytes is IL-1β-and TNF-αdependent, and involves a TNF-α receptorassociated factor-and NF kappa B-inducing kinase-dependent signaling mechanism[END_REF]. High levels of the proinflammatory cytokine IL-1β are also found in microglial cells nearby Aβ plaques in AD patients.

Intracellular caspases proteases family are central of apoptosis and inflammation. In the caspase's family, caspase 1 activation is required to subsequent cleavage of interleukin 1β

and interleukin 18 into their active forms [START_REF] Schroder | The inflammasomes[END_REF][START_REF] Van De Veerdonk | Inflammasome activation and IL-1β and IL-18 processing during infection[END_REF]. Aβ can initiate NRLP3 through lysosomal impairment in microglia [START_REF] Halle | The NALP3 inflammasome is involved in the innate immune response to amyloid-β[END_REF]. Increased active caspase 1 levels are found in AD brains and APP/PS1 mice. In addition, APP/PS1 mice lacking of NLRP3 or caspase 1 are mainly prevented from spatial memory impairment, hippocampal synaptic plasticity loss and related behavioural dysfunction associated with AD [START_REF] Heneka | NLRP3 is activated in Alzheimer's disease and contributes to pathology in APP/PS1 mice[END_REF]. Lacking of NLRP3 or caspase 1 in APP/PS1 mice appeared to change microglial from a proinflammatory type to an anti-inflammatory type [START_REF] Heneka | NLRP3 is activated in Alzheimer's disease and contributes to pathology in APP/PS1 mice[END_REF].

Even more, activation of microglia with different proinflammatory factors resulted activation of apoptotic caspase 8 and caspase 3 as well as caspase 7. Caspase 3 activation regulates NF-κB activity through PKCδ and increases the levels of neurotoxic proinflammatory factors, such as IL-1β, TNFα. Inhibition of these caspases prevented microglia activation and neurotoxicity [START_REF] Fricker | Caspase inhibitors protect neurons by enabling selective necroptosis of inflamed microglia[END_REF]. Pharmacological investigations to inhibit activated caspases have been described to reasonable play neuroprotective roles in AD mouse models [START_REF] Rohn | Caspase activation in transgenic mice with Alzheimer-like pathology: results from a pilot study utilizing the caspase inhibitor, Q-VD-OPh[END_REF][START_REF] Biscaro | Inhibition of microglial activation protects hippocampal neurogenesis and improves cognitive deficits in a transgenic mouse model for Alzheimer's disease[END_REF].

Under the head injury condition, IL-1β could promote Aβ deposition by increasing the expression of β-APP [START_REF] Goldgaber | Interleukin 1 regulates synthesis of amyloid beta-protein precursor mRNA in human endothelial cells[END_REF][START_REF] Forloni | Expression of amyloid precursor protein mRNAs in endothelial, neuronal and glial cells: modulation by interleukin-1[END_REF] through the βAPP promoter. [START_REF] Buxbaum | Cholinergic agonists and interleukin 1 regulate processing and secretion of the Alzheimer beta/A4 amyloid protein precursor[END_REF][START_REF] Mrak | Glial cytokines in Alzheimer's disease: review and pathogenic implications[END_REF]. On the contrary, proinflammatory signalling pathways can be shown in APP/PS1 mouse model where transgenic expressed IL-1β in APP/PS1 mice resulted in strong neuroinflammation but decreased Aβ plaque pathology [START_REF] Shaftel | Sustained hippocampal IL-1β overexpression mediates chronic neuroinflammation and ameliorates Alzheimer plaque pathology[END_REF][START_REF] Ghosh | Sustained interleukin-1β overexpression exacerbates tau pathology despite reduced amyloid burden in an Alzheimer's mouse model[END_REF].

IL-12 and IL-23, two pro-inflammatory cytokines, are produced by leucocytes and microglia in mouse models of AD (Berg et al., 2012), and their inhibition via the common subunit (p40) subsequently attenuate AD-like pathology and reverse cognitive deficits [START_REF] Vom Berg | Inhibition of IL-12/IL-23 signaling reduces Alzheimer's disease-like pathology and cognitive decline[END_REF][START_REF] Tan | IL12/23 p40 inhibition ameliorates Alzheimer's disease-associated neuropathology and spatial memory in SAMP8 mice[END_REF]. An increase of CSF levels of the proinflammatory cytokine TNFα together with a concomitant decrease of the anti-inflammatory cytokine TGFβ has been reported in patients with mild cognitive impairment at risk to develop AD [START_REF] Tarkowski | Intrathecal inflammation precedes development of Alzheimer's disease[END_REF] highlighting the detrimental role of neuroinflammation in the pathogenesis of AD.

Prostaglandins E2

Prostaglandin E2 (PGE 2 ) plays pivotal functions in neuroinflammation. PGE 2 is produced from arachidonic acid by the microsomal prostaglandin-E2 synthase (mPGES) and by two rate-limiting enzymes, Cox-1 and Cox-2 (Smith et al., 1991) which are expressed by hippocampal neurons (Yasojima et al., 1999), astrocytes and microglia (Nieves et al., 2012) (Figure 7). Whereas Cox-1 is constitutively expressed, Cox-2 and mPGES are strongly activated by and during neuroinflammation (Nieves et al., 2012).

PGE 2 plays an important role in the pathophysiology of AD. It is present at high concentration in the course of the disease and by fostering the production of others inflammatory molecules (TNF-α, IL 1β, ROS, nitrite) may lead to neuronal death [START_REF] Gehrmann | Microglia: intrinsic immuneffector cell of the brain[END_REF]Montine et al., 1999;[START_REF] Aloisi | Immune function of microglia[END_REF][START_REF] Combrinck | Levels of CSF prostaglandin E2, cognitive decline, and survival in Alzheimer's disease[END_REF][START_REF] Wei | Inflammatory cytokines and spontaneous preterm birth in asymptomatic women: a systematic review[END_REF]Shi et al., 2012). Study also proposes that PGE 2 promotes Aβ generation in a mouse model [START_REF] Hoshino | Prostaglandin E2 stimulates the production of amyloid-β peptides through internalization of the EP4 receptor[END_REF]. PGE 2 can bind to four different subtypes of G-protein-coupled receptors (EP1-4) which regulate adenylyl cyclase (AC) activity and/or phosphoinositol turnover and intracellular calcium mobilization (Breyer et al., 2001). Depending on which subtype of receptor it is acting, PGE 2 may have opposite effects on neuroinflammation.

Activation of EP1 receptor triggers an increase of the concentration of intracellular Ca 2+ , whereas activation of the EP2/EP4 receptors are linked to increased levels of intracellular cAMP via Gs [START_REF] Honda | Cloning and expression of a cDNA for mouse prostaglandin E receptor EP2 subtype[END_REF][START_REF] Bastien | Cloning, functional expression, and characterization of the human prostaglandin E2 receptor EP2 subtype[END_REF][START_REF] Nishigaki | Identification of prostaglandin E receptor 'EP2'cloned from mastocytoma cells as EP4 subtype[END_REF]. Conversely, EP3 receptor couples negatively to cAMP generation via Gi [START_REF] Breyer | Alternative splicing generates multiple isoforms of a rabbit prostaglandin E2 receptor[END_REF](Breyer et al., , 2001)).

Consistent with data of both favorable and toxic results of COX activity, more and more study shows both beneficial and toxic effects for EP receptors [START_REF] Andreasson | Emerging roles of PGE2 receptors in models of neurological disease[END_REF][START_REF] Johansson | Microarray analysis of the in vivo response of microglia to Aβ peptides in mice with conditional deletion of the prostaglandin EP2 receptor[END_REF].

1) EP1 receptor

EP1 receptor is expressed in the hippocampus [START_REF] Kawano | Prostaglandin E 2 EP1 receptors: downstream effectors of COX-2 neurotoxicity[END_REF]. It is coupled to Gαq, and its activation leads to an increase in intracellular Ca 2+ concentration. The role of EP1 receptor in neurodegenerative disease is still not clear but it may play a role in AD as its genetic deletion in APP/PS1 mice attenuated memory loss induced by ischemic insult and reduced the Aβ plaque load [START_REF] Zhen | PGE2 EP1 receptor exacerbated neurotoxicity in a mouse model of cerebral ischemia and Alzheimer's disease[END_REF].

2) EP2 receptor

By contrast, the role of EP2 receptor has been well assessed in neurodegenerative models. EP2 receptor is positively coupled via Gs to the generation of cAMP and is densely expressed in pyramidal neurons of hippocampus where it modulates synaptic transmission (Sang et al., 2005). The genetic deletion of EP2 in APP/PS1 mice leads to reduced brain lipid oxidation and decreased Aβ levels, possible via decreased the activity of BACE-1 [START_REF] Tamagno | Oxidative stress increases expression and activity of BACE in NT2 neurons[END_REF]Liang et al., 2005). EP2 receptor is a key mediator of the microglial response to

Aβ. The blockade of microglial EP2 receptor inhibits the ability of microglia to engulf Aβ [START_REF] Shie | Microglial EP2 as a New Target to Increase Amyloid β Phagocytosis and Decrease Amyloid β--Induced Damage to Neurons[END_REF] possibly through the blockade of the expression of CD36, a key player in Aβ phagocytosis by microglia [START_REF] Li | Prostaglandin E2 receptor subtype 2 regulation of scavenger receptor CD36 modulates microglial Aβ42 phagocytosis[END_REF]. The specific genetic deletion of microglial EP2 receptors in APP/PS1 mice induces alterations in growth factor levels, working memory and synaptic density ( [START_REF] Johansson | Microarray analysis of the in vivo response of microglia to Aβ peptides in mice with conditional deletion of the prostaglandin EP2 receptor[END_REF].

3) EP3 receptor

EP3 receptor is coupled to G proteins through a Gi/o subunit (Hatae et al., 2002) which negatively regulates AC activity. In APP/PS1 mice, PGE 2 -EP3 signaling pathway is associated with the neuroinflammatory response to Aβ and its genetic deletion reduced proinflammatory gene expression, IL-1β and TNFα production and oxidative stress (Shi et al., 2012). More recently we have shown that PGE 2 -EP3 pathway was responsible for the impairment of hippocampal presynaptic long term plasticity at the Mf/CA3 synapse in aged APP/PS1 mice and that impairment was fully reversed by the specific blockade of EP3 receptor (Maingret et al., 2017).

4) EP4 receptor

EP4 receptor is positively coupled via Gs to the generation of cAMP and expressed in neurons at low levels [START_REF] Li | Misoprostol, an anti-ulcer agent and PGE2 receptor agonist, protects against cerebral ischemia[END_REF]. EP4 receptor may provide neuroprotection in AD models and hypoxic damage [START_REF] Liang | Signaling via the prostaglandin E 2 receptor EP4 exerts neuronal and vascular protection in a mouse model of cerebral ischemia[END_REF]. In agreement to neuroprotective action, the stimulation of EP4 receptors in cultured microglia decreases the response to LPS (Minami et al., 2008;[START_REF] Shi | The prostaglandin E2 E-prostanoid 4 receptor exerts anti-inflammatory effects in brain innate immunity[END_REF] and its selective genetic deletion in microglia extends neuroinflammatory status [START_REF] Shi | The prostaglandin E2 E-prostanoid 4 receptor exerts anti-inflammatory effects in brain innate immunity[END_REF]. The stimulation of EP4 receptors in primary culture of microglia reduces levels of Aβ 42 -induced inflammatory factors (TNFα, IL-12, iNOS, Cox-2) and conditional deletion of microglial EP4 receptors in APP/PS1 mice conversely increased inflammatory gene expression, oxidative protein modification and Aβ plaques load in brain at early stages of pathology [START_REF] Woodling | Suppression of Alzheimer-associated inflammation by microglial prostaglandin-E2 EP4 receptor signaling[END_REF]. All together, these results highlight an anti-inflammatory and neuroprotective role of microglial EP4 signaling pathway in AD mice model.

b. Nonsteroidal anti-inflammatory drugs (NSAIDs)

Numerous epidemiological studies have shown that chronic intake of NSAIDs reduced the prevalence of AD (McGeer et al., 2007). Then clinical trials revealed that NSAIDs, when given to asymptomatic patients, reduced AD incidence whereas they have deletarious effects on AD pathogenesis in its later stages [START_REF] Szekely | Non-steroidal anti-inflammatory drugs and Alzheimer's disease: the epidemiological evidence[END_REF]Breitner et al., 2011;[START_REF] Wang | Anti-inflammatory drugs and risk of Alzheimer's disease: an updated systematic review and meta-analysis[END_REF][START_REF] Zhu | Can inflammation be resolved in Alzheimer's disease?[END_REF][START_REF] Kinney | Inflammation as a central mechanism in Alzheimer's disease[END_REF].

NSAIDs inhibit cyclooxygenase activity by blocking Cox-1 and Cox-2 [START_REF] Kaufmann | Cyclooxygenases and the central nervous system[END_REF][START_REF] Vlad | Protective effects of NSAIDs on the development of Alzheimer disease[END_REF]. Although NSAIDs are likely to be the safest drug option for AD, meta-analysis of different cohorts using NSAIDs on short, intermediate and long term treatment yielded no conclusive results [START_REF] Etminan | Effect of non-steroidal anti-inflammatory drugs on risk of Alzheimer's disease: systematic review and meta-analysis of observational studies[END_REF].

The chronic intake of NSAIDs by patients treated for inflammatory pathologies for more than 24 months clearly reduced the prevalence (40 to 70 %) of AD [START_REF] Stewart | Risk of Alzheimer's disease and duration of NSAID use[END_REF][START_REF] Veld | Nonsteroidal antiinflammatory drugs and the risk of Alzheimer's disease[END_REF][START_REF] Lindsay | Risk factors for Alzheimer's disease: a prospective analysis from the Canadian Study of Health and Aging[END_REF]McGeer et al., 2007) whereas most of the clinical trials looking at the effects of NSAIDs on AD's patients failed to show positive results or even yielded deletarious side effects mostly on the gastrointestinal tractus (McGeer et al., 2007).

In an effort to reduce side effects, Cox-2 specific inhibitors (nimesulide: [START_REF] Aisen | Effects of rofecoxib or naproxen vs placebo on Alzheimer disease progression: a randomized controlled trial[END_REF]rofecoxib: Aisen et al., 2003[START_REF] Thal | A randomized, double-blind, study of rofecoxib in patients with mild cognitive impairment[END_REF]celecoxib: Soininen et al., 2007;Lyketsos et al., 2007) have been tested but also failed to demonstrate beneficial effects on AD's patients.

A subset of NSAIDs known as "selective Aβ 42 -lowering agents" (SALAs) which modify γsecretase cleavage away from Aβ 42 to Aβ 40 and Aβ 38 also failed to reduce AD risk [START_REF] Szekely | No advantage of Aβ42-lowering NSAIDs for prevention of Alzheimer dementia in six pooled cohort studies[END_REF].

From all these studies it seems that effects of NSAIDs on neuroinflammation may be beneficial in the very early stages of AD but may become detrimental for later stages and even worsen the clinical symptoms (Breitner et al., 2011;[START_REF] Leoutsakos | Effects of non--steroidal anti--inflammatory drug treatments on cognitive decline vary by phase of pre--clinical Alzheimer disease: findings from the randomized controlled Alzheimer's Disease Anti--inflammatory Prevention Trial[END_REF].

C. Synaptic dysfunction

The human brain comprises about 86 billion neurons and supposedly ten times more glial cells [START_REF] Herculano-Houzel | The human brain in numbers: a linearly scaled-up primate brain[END_REF]. This unique network processes and transfers messages in the form of chemical and electrical signals (Figure 8). Transmission between neurons happens at special junctions named synapses. Synaptic development and connectivity is crucial to maintain neuronal network activity and normal brain functions. It is now widely accepted that processing but also storage of the information within the brain involves structural and functional synaptic plasticity. This plasticity is considered to be the foundation of learning and memory in the brain (for review see [START_REF] Malenka | LTP and LTD: an embarrassment of riches[END_REF] and has awarded the Nobel Prize in Physiology or Medicine in 2000 to Eric Kandel. When it comes to pathology, it is doubtless that the improper loss of synapses and/or dysfunction in neuronal transmission can be the cause of various psychiatric and neurologic diseases, such as mental retardation [START_REF] Pfeiffer | The state of synapses in fragile X syndrome[END_REF], schizophrenia [START_REF] Stephan | Synaptic plasticity and dysconnection in schizophrenia[END_REF]), Parkinson's disease [START_REF] Calabresi | A convergent model for cognitive dysfunctions in Parkinson's disease: the critical dopamine-acetylcholine synaptic balance[END_REF]), autism (Sudhof et al., 2008), AD (Selkoe et al., 2002), compulsive behavior [START_REF] Welch | Cortico-striatal synaptic defects and OCD-like behaviours in Sapap3-mutant mice[END_REF], and addiction [START_REF] Kauer | Synaptic plasticity and addiction[END_REF].

a. Intrinsic and synaptic plasticities

Intrinsic plasticity

Passive and active membrane properties are related to ionic channels expressed at the plasma membrane. Resting membrane potential (RMP), membrane resistance (R in ) and the voltage threshold and/or the minimal current intensity (also termed rheobase) needed to generate action potentials (APs) are considered as passive membrane properties. Frequency of spike activity, i.e. discharge of APs, firing pattern i.e. frequency and/or spike frequency adaptation are considered as active membrane properties. Any modification of these nonsynaptic intrinsic membrane properties will change neuronal activity and likely modify circuit properties [START_REF] Santos | Network excitability dysfunction in Alzheimer's disease: insights from in vitro and in vivo models[END_REF]. As such, Ca 2+ influx occuring during APs triggers Ca 2+ release (CICR) from intracellular stores mediated by the inositol 1,4,5-trisphosphate (IP 3 ) receptor (IP 3 R) via the activation of ryanodine receptor (RyR) channels [START_REF] Berridge | Neuronal calcium signaling[END_REF][START_REF] Berridge | The endoplasmic reticulum: a multifunctional signaling organelle[END_REF]. Interestingly, in early stages of AD, increased RyR mediated Ca 2+ signaling has been reported to affect small conductance calcium activated potassium (SK) channels through a decreased expression of FK506 binding protein that related to RyR2 activity ( [START_REF] Gant | FK506-binding protein 1b/12.6: a key to aging-related hippocampal Ca2+ dysregulation?[END_REF][START_REF] Gant | Reversal of aging-related neuronal Ca2+ dysregulation and cognitive impairment by delivery of a transgene encoding FK506-binding protein 12.6/1b to the hippocampus[END_REF]. This could result in a decline of neuronal excitability. Intracellular Ca 2+ rise also activates large conductance potassium (BK) channels, which can modulate AP firing by accelerating the membrane repolarization [START_REF] Stutzmann | Enhanced ryanodine receptor recruitment contributes to Ca2+ disruptions in young, adult, and aged Alzheimer's disease mice[END_REF][START_REF] Chakroborty | Deviant ryanodine receptor-mediated calcium release resets synaptic homeostasis in presymptomatic 3xTg-AD mice[END_REF]Chakroborty et al., 2012) and lead also to a decline of neuronal excitability.

Consequently, alteration of neuronal firing properties might induce functional deficits in the early stages of AD [START_REF] Rowan | Synaptic plasticity disruption by amyloid β protein: modulation by potential Alzheimer's disease modifying therapies[END_REF][START_REF] Trommer | ApoE isoform-specific effects on LTP: blockade by oligomeric amyloid-β1-42[END_REF][START_REF] Cuevas | Soluble Aβ 1-40 Peptide Increases Excitatory Neurotransmission and Induces Epileptiform Activity in Hippocampal Neurons[END_REF][START_REF] Palmer | Hippocampal synaptic activity, pattern separation and episodic-like memory: implications for mouse models of Alzheimer's disease pathology[END_REF].

Long term synaptic plasticity

Long term synaptic plasticity is an activity-dependent ability of a synapse to display a strengthened or weakened activity over time, on minutes to hours. The idea of synaptic plasticity and its involvement in learning was first proposed by Ramon y Cajal, who reported that the amount of neurons in the brain did not seem to change noticeably during lifetime [START_REF] Ramón Y Cajal | The Croonian lecture.-La fine structure des centres nerveux[END_REF][START_REF] Aston-Jones | Locus coeruleus neurons in monkey are selectively activated by attended cues in a vigilance task[END_REF]. The major forms of synaptic plasticity are long-term potentiation (LTP) and long-term depression (LTD), which may happen at either excitatory or inhibitory synapses, through presynaptic and/or postsynaptic mechanisms. Both mechanisms of plasticities are believed to be associated with information storage and consequently in learning and memory as well as other physiological procedures. The levels of intracellular Ca 2+ approached by the activation of NMDA receptors have an important role in characterization of long term potentiation (LTP) or long-term depression (LTD) [START_REF] Lisman | A mechanism for the Hebb and the anti-Hebb processes underlying learning and memory[END_REF][START_REF] Lisman | The CaM kinase II hypothesis for the storage of synaptic memory[END_REF]. Based on the fact that, weak activation of the NMDA receptor (left) would induce low concentration increases of Ca 2+ , leading to the activation of calcineurin (CaN) over CaMKII, leading to LTD. Conversely, strong activation of the NMDA receptor (right) would increase Ca 2+ levels enough to activate CaMKII and PKC, leading to the generation of LTP. It was suggested that the activation of calcium sensitive adenylyl cyclases by strong activation of NMDA receptor would result in the activation of cAMP dependent protein kinase (PKA).

Classical LTP is dependent the activation of post-synaptic NMDA receptors and the insertion of new AMPA receptors, whereas classical LTD would be the result of the activation of NMDA receptors, AMPA receptors as well as kainate receptors and metabotropic glutamate receptors (mGluRs) ( Collingridge et al., 2009;[START_REF] Granger | Expression mechanisms underlying long-term potentiation: a postsynaptic view, 10 years on[END_REF] (Figure 9).

Changes in both LTP and LTD have been considered to be involved in AD [START_REF] Wang | Soluble oligomers of β amyloid (1-42) inhibit long-term potentiation but not long-term depression in rat dentate gyrus[END_REF][START_REF] Shankar | Amyloid-β protein dimers isolated directly from Alzheimer's brains impair synaptic plasticity and memory[END_REF][START_REF] Li | Soluble oligomers of amyloid β protein facilitate hippocampal long-term depression by disrupting neuronal glutamate uptake[END_REF].

1) Long-term potentiation (LTP)

LTP is a long lasting enhancement in the strength of synaptic transmission involving either presynaptic or postsynaptic mechanisms or both, was first reported in rabbit hippocampus (Bliss and Gardner-Medwin, 1973).

LTP has been involved in a number of physiological events such as the building of new dendritic spines, increments in perforated postsynaptic densities (collecting more presynaptic input), and triggers an enlargement of spine heads [START_REF] Van Harreveld | Swelling of dendritic spines in the fascia dentata after stimulation of the perforant fibers as a mechanism of post-tetanic potentiation[END_REF][START_REF] Maletic-Savatic | Rapid dendritic morphogenesis in CA1 hippocampal dendrites induced by synaptic activity[END_REF]Nagërl et al., 2004;[START_REF] Bosch | Structural plasticity of dendritic spines[END_REF]. Hence, LTP alterations in the hippocampus in AD mice models have been extensively studied. LTP comprises two phases: early and late. The early phase depends upon protein kinase activation leading to the phosphorylation of synaptic AMPA receptors, enhanced activity, insertion of new AMPA receptors into the postsynaptic density leading to the enhancement of synaptic transmission.

In the late phase, raised calcium levels at the postsynaptic side and constant activation of kinases (PKC, PKMzeta, and CamKIIa) trigger the activation of transcription factors, such as CREB. In vitro, the exogenous application of Aß triggers an impairment of LTP [START_REF] Chen | Impairment of hippocampal long--term potentiation by Alzheimer amyloid β--peptides[END_REF][START_REF] Li | Soluble Aβ oligomers inhibit long-term potentiation through a mechanism involving excessive activation of extrasynaptic NR2B-containing NMDA receptors[END_REF]Jo et al., 2011;[START_REF] Varga | Amyloid-β 1-42 Disrupts Synaptic Plasticity by Altering Glutamate Recycling at the Synapse[END_REF] but genetic deletion or treatment with blocking antibody showed that endogenous Aß is necessary for hippocampal LTP in vivo [START_REF] Puzzo | Endogenous amyloid--β is necessary for hippocampal synaptic plasticity and memory[END_REF]. This study together with other reports [START_REF] Puzzo | Picomolar amyloid-β positively modulates synaptic plasticity and memory in hippocampus[END_REF][START_REF] Garcia-Osta | Amyloid beta mediates memory formation[END_REF], suggest that endogenous low levels of Aß do have a physiological function in synaptic plasticity.

LTP is also impaired in tau mutant mice [START_REF] Fa | Extracellular tau oligomers produce an immediate impairment of LTP and memory[END_REF][START_REF] Gelman | Differences in synaptic dysfunction between rTg4510 and APP/PS1 mouse models of Alzheimer's disease[END_REF]. These alterations were observed in old mice, likely the result of high levels of accumulated tau [START_REF] Chong | Synaptic dysfunction in hippocampus of transgenic mouse models of Alzheimer's disease: A multi-electrode array study[END_REF][START_REF] Sydow | Tau-induced defects in synaptic plasticity, learning, and memory are reversible in transgenic mice after switching off the toxic Tau mutant[END_REF]. Moreover, similar age-dependent results on LTP were also reported in mutant mice overexpressing normal human tau [START_REF] Polydoro | Age-dependent impairment of cognitive and synaptic function in the htau mouse model of tau pathology[END_REF].

Whereas tau acts on LTP by itself or downstream of Aß or even in concert with Aß is still debated (Jones et al., 2014).

2) Long-term depression (LTD)

LTD is a long lasting decline of synaptic strength. It may occur by several ways, which not surprisingly have opposite effects to those observed in LTP, including internalization of AMPA receptors (Dudek and Bear, 1992;[START_REF] Massey | Long-term depression: multiple forms and implications for brain function[END_REF][START_REF] Collingridge | Long-term depression in the CNS[END_REF], and calcineurin activation [START_REF] Mulkey | An essential role for protein phosphatases in hippocampal long-term depression[END_REF][START_REF] Li | Caspase-3 activation via mitochondria is required for long-term depression and AMPA receptor internalization[END_REF].

LTD is believed to be important for the clearing of old memories and in circumstances requiring behavioral adaptability [START_REF] Collingridge | Long-term depression in the CNS[END_REF]. This "forgetting" feature of LTD could be disrupted during AD causing memory problems (Jones et al., 2014). As for LTP, the molecular mechanisms of LTD which could be targeted by AD are poorly unknown.

Interestingly, Aβ induces AMPA and NMDA receptors internalization through the same calcineurin-mediated pathways involved in LTD [START_REF] Wang | β-amyloid-mediated inhibition of NMDA receptor-dependent long-term potentiation induction involves activation of microglia and stimulation of inducible nitric oxide synthase and superoxide[END_REF][START_REF] Snyder | Regulation of NMDA receptor trafficking by amyloid-β[END_REF]Hsieh et al., 2006;[START_REF] Koffie | Alzheimer's disease: synapses gone cold[END_REF].

Knockout mice for tau at the CA1 region of hippocampus show impairment of LTD [START_REF] Kimura | Microtubule-associated protein tau is essential for long-term depression in the hippocampus[END_REF] suggesting an important role of tau in synaptic transmission. Besides, mutation of tau was found to be connected with glutamate signaling as well as excitotoxicity in vivo, further proposing a role for tau in modulation of synaptic transmission [START_REF] Hunsberger | P301L tau expression affects glutamate release and clearance in the hippocampal trisynaptic pathway[END_REF].

Short term synaptic plasticity

Short term plasticity (STP) takes place on milliseconds to few seconds scale [START_REF] Zucker | Short-term synaptic plasticity[END_REF][START_REF] Abbott | Synaptic computation[END_REF]. At some synapses, a decrease of neurotransmitter release contributes to short term depression by decreasing the release probability or reducing the readily releasable pool of vesicles [START_REF] Eccles | Nature of the" endplate potential" in curarized muscle[END_REF][START_REF] Feng | Studies on the neuromuscular junction. XXVI. The changes of the end-plate potential during and after prolonged stimulation[END_REF] (Figure 10). At other synapses, increases of neurotransmitter release probability [START_REF] Katz | The role of calcium in neuromuscular facilitation[END_REF] contribute to short term depression by saturating of local calcium buffer [START_REF] Felmy | Probing the intracellular calcium sensitivity of transmitter release during synaptic facilitation[END_REF][START_REF] Blatow | Ca2+ buffer saturation underlies paired pulse facilitation in calbindin-D28k-containing terminals[END_REF] or increasing of calcium concentration at the presynaptic terminal [START_REF] Kamiya | Residual Ca2+ and short-term synaptic plasticity[END_REF][START_REF] Atluri | Determinants of the time course of facilitation at the granule cell to Purkinje cell synapse[END_REF][START_REF] Sippy | Acute changes in shortterm plasticity at synapses with elevated levels of neuronal calcium sensor-1[END_REF].

STP plasticity such as post-tetanic potentiation (PTP) and paired pulse facilitation (PPF) play essential roles in long-term memory formation and hence, STP impairments cause long lasting memory formation deficits ( [START_REF] Silva | Impaired learning in mice with abnormal short-lived plasticity[END_REF]. Consistent long-term memory activity relies on Ca 2+ mediated STP to convert short term to long term memory and set up synapse specificity [START_REF] Frey | Synaptic tagging and long-term potentiation[END_REF][START_REF] Redondo | Making memories last: the synaptic tagging and capture hypothesis[END_REF].

STP is not a trivial synaptic mechanism but seems instead to be a property to keep the balance between excitation and inhibition. Depending on the type of STP triggered at a particular synapse, it will shape the transferred information which suggests that the same presynaptic site may carry different information to distinct postsynaptic cells [START_REF] Markram | Potential for multiple mechanisms, phenomena and algorithms for synaptic plasticity at single synapses[END_REF]. In this case, short-term facilitation and depression carry information in both low and high frequencies range [START_REF] Fuhrmann | Coding of temporal information by activity-dependent synapses[END_REF]. These studies highlight the crucial roles of STP in information processing and strongly suggest the physiological relevance of STP in different cognitive tasks as for example, decision making or working memory [START_REF] Shyu | Short-term synaptic plasticity in the nociceptive thalamic-anterior cingulate pathway[END_REF][START_REF] Deng | Abnormal presynaptic short-term plasticity and information processing in a mouse model of fragile X syndrome[END_REF]. The characteristics of postsynaptic receptors could be also involved in short term plasticity. When exposed to neurotransmitter, the desensitization of postsynaptic receptors could lead to a decrease synaptic activity upon repeated activation [START_REF] Trussell | Desensitization of AMPA receptors upon multiquantal neurotransmitter release[END_REF][START_REF] Jones | The impact of receptor desensitization on fast synaptic transmission[END_REF]Chen et al., 2002;[START_REF] Sun | Single and multiple vesicle fusion induce different rates of endocytosis at a central synapse[END_REF][START_REF] Xu-Friedman | Ultrastructural contributions to desensitization at cerebellar mossy fiber to granule cell synapses[END_REF][START_REF] Wadiche | Multivesicular release at climbing fiber-Purkinje cell synapses[END_REF][START_REF] Foster | Interaction of postsynaptic receptor saturation with presynaptic mechanisms produces a reliable synapse[END_REF][START_REF] Harrison | Receptor occupancy limits synaptic depression at climbing fiber synapses[END_REF].

In early AD stage, a number of synaptic pathophysiological mechanisms induced by ryanodine receptors (RyR) of excessive Ca 2+ have been reported, such as an enhanced frequency of spontaneous synaptic potentials, decreased PPF ratio as well as increase of synaptic depression [START_REF] Chakroborty | Deviant ryanodine receptor-mediated calcium release resets synaptic homeostasis in presymptomatic 3xTg-AD mice[END_REF](Chakroborty et al., , 2012;;[START_REF] Briggs | beta amyloid peptide plaques fail to alter evoked neuronal calcium signals in APP/PS1 Alzheimer's disease mice[END_REF]. Published studies have demonstrated that sizeable vesicle depletion happens at high release probability synapses, which has been illustrated in several studies of AD mice [START_REF] Goussakov | NMDA-mediated Ca2+ influx drives aberrant ryanodine receptor activation in dendrites of young Alzheimer's disease mice[END_REF][START_REF] Scimemi | The number and organization of Ca2+ channels in the active zone shapes neurotransmitter release from Schaffer collateral synapses[END_REF]. For example, at excitatory CA3/CA1 synapses of mouse hippocampus, several functional Ca 2+ channels may be located in the active zone at variable sites. This location results in a far and wide distribution of release probability through the vesicles at the active zone, and the depletion of the vesicles adjacent to Ca 2+ channels can contribute to the Ca 2+ dependent STP at these synapses.

Activity-dependent modulation of population spike (PS) plasticity

Population spike (PS) which can be recorded with extracellular electrodes is a composite response built by the extracellular summation of action currents during synchronous APs firing. With appropriate external stimulation, it is possible to trigger a potentiation of the PS which gives rise to an increase in the PS amplitude.

Noteworthy, at the LPP-DG synapse, the PS amplitude is comparatively larger than the amplitude of the excitatory postsynaptic potential (EPSP) [START_REF] Bliss | Long--lasting potentiation of synaptic transmission in the dentate area of the anaesthetized rabbit following stimulation of the perforant path[END_REF][START_REF] Andersen | Possible mechanisms for long--lasting potentiation of synaptic transmission in hippocampal slices from guinea--pigs[END_REF]Yanagihashi et al., 1992). In theory, an increase of the PS amplitude goes along with an increase of the field EPSP (fEPSP). Nonetheless, this close relation between PS and EPSP may varies while LTP is induced by tetanic stimulation. The potentiation of PS is relatively greater than the EPSP. This suggests that the potentiation mechanism underlying spikes is to some extent independent from the synaptic potentiation [START_REF] Bliss | Long--lasting potentiation of synaptic transmission in the dentate area of the anaesthetized rabbit following stimulation of the perforant path[END_REF][START_REF] Mozzachiodi | More than synaptic plasticity: role of nonsynaptic plasticity in learning and memory[END_REF]. Therefore, the increase in PS amplitude is not sensus stricto the exact equivalent of an increase in EPSP which is synaptic in essence. In other words, the mechanisms of an increase of PS amplitude may rely both on synaptic and non-synaptic mechanisms whereas an increase in the EPSP reflects synaptic mechanisms.

The "non-synaptic" part of PS potentiation has been observed in different brain regions with different LTP protocols [START_REF] Abraham | Long--term potentiation involves enhanced synaptic excitation relative to synaptic inhibition in guinea--pig hippocampus[END_REF][START_REF] Chavez-Noriega | The EPSP-spike (ES) component of long-term potentiation in the rat hippocampal slice is modulated by GABAergic but not cholinergic mechanisms[END_REF]. It shows an enhancement of the intrinsic excitability in the postsynaptic neurons via modulation of voltage Na + channels but also other channels (Ca 2+ channels, K + channels) [START_REF] Taube | Mechanisms of long-term potentiation: EPSP/spike dissociation, intradendritic recordings, and glutamate sensitivity[END_REF][START_REF] Wathey | Computer simulations of EPSP-spike (ES) potentiation in hippocampal CA1 pyramidal cells[END_REF][START_REF] Bernard | Expression of EPSP/spike potentiation following low frequency and tetanic stimulation in the CA1 area of the rat hippocampus[END_REF][START_REF] Savonenko | Episodic-like memory deficits in the APPswe/PS1dE9 mouse model of Alzheimer's disease: Relationships to β-amyloid deposition and neurotransmitter abnormalities[END_REF][START_REF] Fink | Short trains of theta frequency stimulation enhance CA1 pyramidal neuron excitability in the absence of synaptic potentiation[END_REF].

In some cases, alterations in intrinsic excitability without obvious changes in synaptic strengh, i.e. LTP or LTD, have been reported. In the CA1 hippocampus, high frequency stimulation potentiates EPSPs, but this potentiation in neither a prerequisite for, nor a correlation of a potentiation of PS amplitude [START_REF] Taube | Mechanisms of long-term potentiation: EPSP/spike dissociation, intradendritic recordings, and glutamate sensitivity[END_REF]. Short trains

(2-3 s) of Schaffer collateral fiber stimulation delivered at 5 Hz induce increase in the excitability of CA1 neurons in the absence of synaptic potentiation [START_REF] Fink | Short trains of theta frequency stimulation enhance CA1 pyramidal neuron excitability in the absence of synaptic potentiation[END_REF].

Furthermore, both EPSP and PS plasticities were found related to the activity of NMDARs [START_REF] Jester | Associative EPSP----spike potentiation induced by pairing orthodromic and antidromic stimulation in rat hippocampal slices[END_REF][START_REF] Lu | In CA1 pyramidal neurons of the hippocampus protein kinase C regulates calcium-dependent inactivation of NMDA receptors[END_REF]. Remarkably, non-synaptic changes can promote synaptic modulations [START_REF] Byrne | Cellular analysis of associative learning[END_REF] such as depolarization of the resting membrane potential, could promote the induction of synaptic plasticity which is necessary for memory storage [START_REF] Crow | Molecular pathology of schizophrenia: more than one disease process?[END_REF][START_REF] Mozzachiodi | Changes in neuronal excitability serve as a mechanism of long-term memory for operant conditioning[END_REF]. Generally, these results show that neuronal activity can be modified respectively in different sections within the same cell [START_REF] Losonczy | Compartmentalized dendritic plasticity and input feature storage in neurons[END_REF] and reveal therefore that modulation of neuronal activity are more intricate than previously thought.

Miniature excitatory postsynaptic currents (mEPSCs)

Miniature excitatory postsynaptic current (mEPSC) is due to the spontaneous release of a single vesicle into the synapse in the absence of the occurence of APs as they are recorded in the presence of TTX. Synaptic plasticity mechanisms modifying neuronal activity can be studied at the level of mEPSCs [START_REF] Bekkers | Origin of variability in quantal size in cultured hippocampal neurons and hippocampal slices[END_REF][START_REF] Turrigiano | Activity-dependent scaling of quantal amplitude in neocortical neurons[END_REF][START_REF] O'brien | Activity-dependent modulation of synaptic AMPA receptor accumulation[END_REF][START_REF] Watt | Activity coregulates quantal AMPA and NMDA currents at neocortical synapses[END_REF] which the frequency and the amplitude are related to pre and postsynaptic loci of action respectively [START_REF] Prange | Correlation of miniature synaptic activity and evoked release probability in cultures of cortical neurons[END_REF][START_REF] Ling | Protein kinase Mζ enhances excitatory synaptic transmission by increasing the number of active postsynaptic AMPA receptors[END_REF].

The increase in mEPSCs frequency depends on calcium influx but not on the activation of voltage-dependent Ca 2+ channels (Gray et al., 1996;[START_REF] Capogna | Calcium-independent actions of alpha-latrotoxin on spontaneous and evoked synaptic transmission in the hippocampus[END_REF][START_REF] Sharma | Modulation of presynaptic store calcium induces release of glutamate and postsynaptic firing[END_REF]. mEPSCs frequency changes are often associated to changes in the transmitter release probability [START_REF] Scholz | Inhibition of quantal transmitter release in the absence of calcium influx by a G protein-linked adenosine receptor at hippocampal synapses[END_REF][START_REF] Debanne | Paired--pulse facilitation and depression at unitary synapses in rat hippocampus: quantal fluctuation affects subsequent release[END_REF][START_REF] Prange | Correlation of miniature synaptic activity and evoked release probability in cultures of cortical neurons[END_REF][START_REF] Sharma | Modulation of presynaptic store calcium induces release of glutamate and postsynaptic firing[END_REF], but mEPSCs frequency can change without having an effect on the transmitter release [START_REF] Eliot | Modulation of spontaneous transmitter release during depression and posttetanic potentiation of Aplysia sensory-motor neuron synapses isolated in culture[END_REF][START_REF] Yoshihara | Selective effects of neuronal-synaptobrevin mutations on transmitter release evoked by sustained versus transient Ca2+ increases and by cAMP[END_REF][START_REF] Béïque | PSD--95 regulates synaptic transmission and plasticity in rat cerebral cortex[END_REF]. Hence, the increase of docked vesicles number can lead to an increase of mEPSC frequency [START_REF] Tyler | BDNF enhances quantal neurotransmitter release and increases the number of docked vesicles at the active zones of hippocampal excitatory synapses[END_REF][START_REF] Murthy | Inactivity produces increases in neurotransmitter release and synapse size[END_REF][START_REF] Lauri | Activity blockade increases the number of functional synapses in the hippocampus of newborn rats[END_REF] but also the increase of the number of functional release sites, i.e. via the unmasking of "silent synapses" into functional ones [START_REF] Isaac | Evidence for silent synapses: implications for the expression of LTP[END_REF][START_REF] Liao | Activation of postsynaptically silent synapses during pairing-induced LTP in CA1 region of hippocampal slice[END_REF][START_REF] Durand | Long-term potentiation and functional synapse induction in developing hippocampus[END_REF][START_REF] Liao | Activation of silent synapses by rapid activity-dependent synaptic recruitment of AMPA receptors[END_REF].

Changes in mEPSCs amplitude are considered to reflect postsynaptic mechanisms.

Unless if the quantal size of a vesicle is modified, the modification of mEPSCs amplitude is believed to reflect a change in the receptor number [START_REF] Turrigiano | Activity-dependent scaling of quantal amplitude in neocortical neurons[END_REF][START_REF] Watt | Activity coregulates quantal AMPA and NMDA currents at neocortical synapses[END_REF][START_REF] Ling | Protein kinase Mζ enhances excitatory synaptic transmission by increasing the number of active postsynaptic AMPA receptors[END_REF]. But it is also possible that increases or decreases in the glutamate content of synaptic vesicles lead to the change in mEPSC amplitude. [START_REF] Watt | Activity coregulates quantal AMPA and NMDA currents at neocortical synapses[END_REF][START_REF] Liu | Variability of neurotransmitter concentration and nonsaturation of postsynaptic AMPA receptors at synapses in hippocampal cultures and slices[END_REF][START_REF] Mcallister | Nonsaturation of AMPA and NMDA receptors at hippocampal synapses[END_REF]. Although there is no direct study supporting alterations in glutamate content in vesicles, there is direct study for alterations of sensitivity in postsynaptic receptors [START_REF] Turrigiano | Activity-dependent scaling of quantal amplitude in neocortical neurons[END_REF][START_REF] Lissin | Activity differentially regulates the surface expression of synaptic AMPA and NMDA glutamate receptors[END_REF][START_REF] O'brien | Activity-dependent modulation of synaptic AMPA receptor accumulation[END_REF].

Numerous data report that soluble Aβ increases neuronal activity [START_REF] Bausch | Plasticity of both excitatory and inhibitory synapses is associated with seizures induced by removal of chronic blockade of activity in cultured hippocampus[END_REF][START_REF] Busche | Critical role of soluble amyloid-β for early hippocampal hyperactivity in a mouse model of Alzheimer's disease[END_REF]. Aβ 1-40 triggers an increase in the frequency of mEPSCs together with firing activity [START_REF] Cuevas | Soluble Aβ 1-40 Peptide Increases Excitatory Neurotransmission and Induces Epileptiform Activity in Hippocampal Neurons[END_REF]. Pharmacologically study to identify synaptic activity indicated that the activity of NMDA mediated mEPSCs was obviously increased, implying enhanced excitation and synaptic plasticity [START_REF] Berberich | The role of NMDAR subtypes and charge transfer during hippocampal LTP induction[END_REF][START_REF] Hoe | The effects of amyloid precursor protein on postsynaptic composition and activity[END_REF].

b. Synaptic dysfunction in AD 1. Synaptic activity and Aβ homeostasis

AD, at least in its early stage is considered to involve synaptic dysfunction before synaptic and neuronal loss (Selkoe et al., 2002;Jacobsen et al., 2006;Sheng et al., 2012). In good agreement, spatial and contextual learning and memory deficits reported in a mouse model of AD appear prior to Aβ plaques and tau pathology [START_REF] Billings | Intraneuronal Aβ causes the onset of early Alzheimer's disease-related cognitive deficits in transgenic mice[END_REF]. Therefore, it is no surprise that synapses appear as a major therapeutical target to at least alleviate the burden of the disease and possibly maintain cognitive and functional abilities. Furthermore, the brain location of Aβ plaques and tau tangles, the two main molecular hallmarks of AD, allows to link morphologic alterations to cognitive impairments such as learning and memory with synaptic dysfunction.

It is well known that Aβ peptide, lead to morpholo-functional alterations and numerous harmful effects on neurons (Strooper et al., 2016). Aβ oligomers directly promote neuronal apoptosis via cell-surface receptors (TRAIL, TNF-related apoptosis-inducing ligand death death receptors 4 and 5) [START_REF] Fossati | TRAIL death receptors DR4 and DR5 mediate cerebral microvascular endothelial cell apoptosis induced by oligomeric Alzheimer's Aβ[END_REF]. Furthermore, the long-term buildup of toxic Aβ species triggers DNA damage, injury of cellular organelles and dysregulation of intracellular calcium level -all of them leading to cell death [START_REF] Kayed | Molecular mechanisms of amyloid oligomers toxicity[END_REF]. Although

Aβ under certain conditions can bring rapid impairment of synaptic plasticity [START_REF] Cullen | Block of LTP in rat hippocampus in vivo by β-amyloid precursor protein fragments[END_REF]Lambert et al., 1998;[START_REF] Shankar | Amyloid-β protein dimers isolated directly from Alzheimer's brains impair synaptic plasticity and memory[END_REF] as well as changes of dendritic spine morphology and nerve terminals of dendritic spines (Rönicke et al., 2011) within hours, it takes years for Aβ accumulation to have noticeable and deleterious effects.

There are numerous data supporting synaptic pathological effects of Aβ [START_REF] Hsia | Plaque-independent disruption of neural circuits in Alzheimer's disease mouse models[END_REF][START_REF] Mucke | High-level neuronal expression of Aβ1-42 in wild-type human amyloid protein precursor transgenic mice: synaptotoxicity without plaque formation[END_REF]Walsh et al., 2002;[START_REF] Kamenetz | APP processing and synaptic function[END_REF][START_REF] Lacor | Aβ oligomer-induced aberrations in synapse composition, shape, and density provide a molecular basis for loss of connectivity in Alzheimer's disease[END_REF].

Depending on its extracellular concentration, Aβ may have opposite effects: low concentrations (100-200 pM) facilitate excitatory activity and may are involved in memory formation [START_REF] Shankar | Natural oligomers of the Alzheimer amyloid-beta protein induce reversible synapse loss by modulating an NMDA-type glutamate receptor-dependent signaling pathway[END_REF][START_REF] Puzzo | Picomolar amyloid-β positively modulates synaptic plasticity and memory in hippocampus[END_REF][START_REF] Puzzo | Endogenous amyloid--β is necessary for hippocampal synaptic plasticity and memory[END_REF][START_REF] Garcia-Osta | Amyloid beta mediates memory formation[END_REF] while higher levels (>40 nM) suppress it (Yankner et al., 1990). As Aβ depresses synapses and impairs synaptic plasticity (Hsieh et al., 2006;[START_REF] Shankar | Amyloid-β protein dimers isolated directly from Alzheimer's brains impair synaptic plasticity and memory[END_REF], the activity-dependent Aβ accumulation [START_REF] Cirrito | Synaptic activity regulates interstitial fluid amyloid-β levels in vivo[END_REF] could be considered as a feedback mechanism protecting against synaptic hyperactivation and excitotoxicity [START_REF] Kamenetz | APP processing and synaptic function[END_REF].

In early stages of AD, in addition to the increased expression of RyR, its function is regulated by PS1 and/or PS2 [START_REF] Shilling | Suppression of InsP3 receptor-mediated Ca2+ signaling alleviates mutant presenilinlinked familial Alzheimer's disease pathogenesis[END_REF][START_REF] Del Prete | Ryanodine receptors: physiological function and deregulation in Alzheimer disease[END_REF]. PS is known to interact with proteins of the neurotransmitter release machinery and its genetic deletion impairs synaptic plasticity [START_REF] Barthet | Presenilin-mediated cleavage of APP regulates synaptotagmin-7 and presynaptic plasticity[END_REF]. At postsynaptic level, intracellular Ca 2+ rise is enlarged by RyR-mediated CICR in dendritic spines which is required for the initiation of long term synaptic plasticity [START_REF] Rossi | Benefits from dietary polyphenols for brain aging and Alzheimer's disease[END_REF][START_REF] Goussakov | NMDA-mediated Ca2+ influx drives aberrant ryanodine receptor activation in dendrites of young Alzheimer's disease mice[END_REF]Briggs et al., 2016).

In summary, further investigations on physiological functions of Aβ will be most wanted to decipher new targets at which possible AD therapeutics could be performed.

1) Aβ-induced presynaptic dysfunction

Vesicles which are released within the synapse are present at presynaptic terminals and their release is controled by a constellation of proteins [START_REF] Garner | Molecular mechanisms of presynaptic differentiation[END_REF]. In AD, Aβ accumulates at presynaptic terminals (Ye et al., 2017) and not surprisingly may impair presynaptic mechanisms. In addition, Aβ oligomers co-localize with voltage-gated calcium channels to stimulate presynaptic calcium influx increasing therefore the release probability of neurotransmitter, leading to an unusually high level of glutamate in the extracellular space (Moreira et al., 2011). Aβ oligomers can also damage axonal transport [START_REF] Decker | Amyloid-β peptide oligomers disrupt axonal transport through an NMDA receptor-dependent mechanism that is mediated by glycogen synthase kinase 3β in primary cultured hippocampal neurons[END_REF].

The mechanisms by which Aβ disturbs axonal transport of calcium influx through NMDA receptors activation, resulting in cytosolic calcium upregulation as well as the activation of calcineurin. GSK-3β dependent activation of calcineurin setup a new link between toxic Aβ and AD pathology [START_REF] Pigino | Disruption of fast axonal transport is a pathogenic mechanism for intraneuronal amyloid beta[END_REF][START_REF] Decker | Amyloid-β peptide oligomers disrupt axonal transport through an NMDA receptor-dependent mechanism that is mediated by glycogen synthase kinase 3β in primary cultured hippocampal neurons[END_REF][START_REF] Ramser | Amyloid-β oligomers induce tau-independent disruption of BDNF axonal transport via calcineurin activation in cultured hippocampal neurons[END_REF][START_REF] Gan | Dendritic and axonal mechanisms of Ca2+ elevation impair BDNF transport in Aβ oligomer-treated hippocampal neurons[END_REF]. A recent study has shown that the axonal binding of Aβ oligomers was leading to neuronal degeneration [START_REF] Baleriola | Axonally synthesized ATF4 transmits a neurodegenerative signal across brain regions[END_REF].

Synaptic vesicles are the key players that pass on synaptic signals and Aβ could affect

the key procedures of vesicle cycling, including trafficking, release, and recycling [START_REF] Ovsepian | Synaptic vesicle cycle and amyloid β: biting the hand that feeds[END_REF]. Additionally, Aβ may damage vesicle recycling by lowering the recycling pool and increasing the resting pool (Park et al., 2013).

2) Aβ-induced postsynaptic dysfunction

Aβ oligomers interact with the postsynaptic side to alter the structure, the composition, and the function of the synapse [START_REF] Koffie | Oligomeric amyloid β associates with postsynaptic densities and correlates with excitatory synapse loss near senile plaques[END_REF]. The activity of Aβ oligomers at postsynaptic sites can be mediated via their interaction with membrane molecules, eventually leading to synaptotoxicity.

NMDA and AMPA receptors are the main ionotropic glutamate receptors expressed by postsynaptic excitatory synapses (Scheefhals & MacGillavry, 2018). The activation of NMDA receptor promotes calcium influx and triggers intracellular signaling pathways such as calcineurin-mediated Ca 2+ signaling and Ca 2+ /calmodulin-dependent protein kinase II (CaMKII) (Scheefhals & MacGillavry, 2018). Consequently, evoking dysregulation of intracellular signaling cascades may induce the disruption of neuronal transport in primary neurons (Rui et al., 2006;[START_REF] Decker | Amyloid-β peptide oligomers disrupt axonal transport through an NMDA receptor-dependent mechanism that is mediated by glycogen synthase kinase 3β in primary cultured hippocampal neurons[END_REF]. Similarly, the removal of AMPA receptors from excitatory synapses can be induced by increased Aβ levels, which result in dendritic spines loss and synaptic depression [START_REF] Chang | AMPA receptor downscaling at the onset of Alzheimer's disease pathology in double knockin mice[END_REF]Hsieh et al., 2006;[START_REF] Miñano-Molina | Soluble oligomers of amyloid-β peptide disrupt membrane trafficking of α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor contributing to early synapse dysfunction[END_REF][START_REF] Guntupalli | Amyloid-β-induced dysregulation of AMPA receptor trafficking[END_REF].

Metabotropic glutamate receptors 5 (mGluR5) play a key role in modulating transmission at glutamatergic synapses and NMDA receptor-dependent synaptic plasticity (Harney et al., 2006) (Figure 11). In addition, mGluR5 seem essential in Aβ-induced synaptotoxicity and could be a possible therapeutical target for AD [START_REF] Kumar | Therapeutic potential of mGluR5 targeting in Alzheimer's disease[END_REF]. Aβ induces the clustering and the overstabilization of mGluR5 at the postsynaptic side and thus may lead to early synaptic failure [START_REF] Renner | Deleterious effects of amyloid β oligomers acting as an extracellular scaffold for mGluR5[END_REF].

Synaptic activity and tau homeostasis

Tau is present at synapses in both physiological and pathological conditions (Pooler et al., 2014). Normal tau is detected at both pre and postsynaptic terminales in control humains brains but a hyperphosphorylated form of tau (pS396/pS404) is found in AD synaptosomes, a key location to mediate proteotoxicity which disrupts synapses in AD (Tai et al., 2012) (Figure 11).

Figure 11. Aβ and Tau pathology in AD

Aβ may be transported to the synapse by apoE4 and binds to postsynaptic receptors, leads to an increase in calcium levels, activation of CaN, caspase-3 and downstream synaptic receptors internalization. Tau has also been implicated in synapse dysfunction induced by Aβ, and phosphorylated forms of tau (pTau) are transported "through" synaptic activity. However, which forms of pTau are transferred and how they are transported remains unknown.

Numerous publications have demonstrated that when administrated to extracellular cultured neuronal cells, tau oligomers promoted tau neuronal uptake and enhanced intracellular calcium levels [START_REF] Gomez-Ramos | Extracellular tau promotes intracellular calcium increase through M1 and M3 muscarinic receptors in neuronal cells[END_REF]. In mice, in vivo application of tau oligomers induces mitochondrial and synaptic dysfunction [START_REF] Berger | Accumulation of pathological tau species and memory loss in a conditional model of tauopathy[END_REF][START_REF] Lasagna-Reeves | Tau oligomers impair memory and induce synaptic and mitochondrial dysfunction in wild-type mice[END_REF].

Strikingly, synaptic activity induced in mouse cultured cortical neurons promoted tau phosphorylation, translocation of endogenous tau from the dendritic shaft to spines whereas the addition of Aβ oligomers induced mislocalization of tau into the spines at rest i.e. without synaptic activation, and blocked the activity-dependent enrichment of tau within spines (Frandemiche et al., 2014).

The abnormal synaptic localization of tau and the disruption of the activity-dependent enrichment of tau within spines by Aβ oligomers could play a role in the progression of the pathology [START_REF] Hoover | Tau mislocalization to dendritic spines mediates synaptic dysfunction independently of neurodegeneration[END_REF][START_REF] Roberson | Amyloid-β/Fyn-induced synaptic, network, and cognitive impairments depend on tau levels in multiple mouse models of Alzheimer's disease[END_REF]Tai et al., 2012). Furthermore, synaptic activity and Aβ likely promote tau-phosphorylation on different sites (Frandemiche et al., 2014) leading to different forms of phosphorylated tau (toxic vs non-toxic) which may have different impact on synaptic transmission and plasticity.

1) Tau-induced presynaptic dysfunction

In the course of AD, hyperphosphorylation of tau leads to its dissociation from microtubules, followed by diminished cytoskeletal solidity and impaired axonal transport [START_REF] Wagner | Cellular phosphorylation of tau by GSK-3 beta influences tau binding to microtubules and microtubule organisation[END_REF][START_REF] Patrick | Conversion of p35 to p25 deregulates Cdk5 activity and promotes neurodegeneration[END_REF]. Additionally, extracellular tau oligomers can be taken up by neurons, accumulated within axons and dysregulate axonal transport [START_REF] Wu | Small misfolded Tau species are internalized via bulk endocytosis and anterogradely and retrogradely transported in neurons[END_REF][START_REF] Swanson | Extracellular tau oligomers induce invasion of endogenous tau into the somatodendritic compartment and axonal transport dysfunction[END_REF]. pTau binds to synaptic vesicles via its N-terminal domain and synaptogyrin-3 (a transmembrane vesicle protein) and interferes with presynaptic synaptic vesicles (SV) mobilization and release rate as well as down regulation of neurotransmission. [START_REF] Zhou | Tau association with synaptic vesicles causes presynaptic dysfunction[END_REF][START_REF] Mcinnes | Synaptogyrin-3 mediates presynaptic dysfunction induced by tau[END_REF]. Accumulation of presynaptic pTau triggers a transient increase of spontaneous neurotransmitter release mediated by intracellular calcium release. This is followed by SV exocytosis block, leading to consistent synaptic transmission failure owing to the consequent toxicity activities [START_REF] Moreno | Blocking effects of human tau on squid giant synapse transmission and its prevention by T-817 MA[END_REF][START_REF] Moreno | Tau pathology-mediated presynaptic dysfunction[END_REF].

In an inducible mouse model expressing pro-aggregant tau, a severe depletion of synaptic vesicles pool was observed together with an impaired LTD [START_REF] Decker | Pro-aggregant Tau impairs mossy fiber plasticity due to structural changes and Ca++ dysregulation[END_REF] and the accute addition of recombinant human tau oligomers produced the impairment of LTP (Fà et al., 2016).

2) Tau-induced postsynaptic dysfunction

The accumulation of tau into dendritic spines leads to the decreased expression of both NMDA and AMPA receptors, induced impaired excitatory synaptic transmission and memory deficits [START_REF] Hoover | Tau mislocalization to dendritic spines mediates synaptic dysfunction independently of neurodegeneration[END_REF][START_REF] Xia | Pseudophosphorylation of Tau at distinct epitopes or the presence of the P301L mutation targets the microtubule-associated protein Tau to dendritic spines[END_REF][START_REF] Kim | Caspase-cleaved tau exhibits rapid memory impairment associated with tau oligomers in a transgenic mouse model[END_REF][START_REF] Yin | Tau accumulation induces synaptic impairment and memory deficit by calcineurinmediated inactivation of nuclear CaMKIV/CREB signaling[END_REF][START_REF] Zhao | Caspase-2 cleavage of tau reversibly impairs memory[END_REF]. Lack of tau in tau -/-mutant mice disrupts the postsynaptic Src kinase Fyn localization resulting in the reduction of NMDA receptor phosphorylation and in fine protection against Aβ-induced excitotoxicity (Ittner et al., 2010;[START_REF] Roberson | Amyloid-β/Fyn-induced synaptic, network, and cognitive impairments depend on tau levels in multiple mouse models of Alzheimer's disease[END_REF]. In contrast to the Aβ toxicity mediated by phosphorylated tau protein, at least in the early stage of the disease, sitespecific phosphorylation of tau mediated by p38 MAPKinase inhibits Aβ toxicity [START_REF] Ittner | Site-specific phosphorylation of tau inhibits amyloid-β toxicity in Alzheimer's mice[END_REF].

c. Synaptic dysfunction at hippocampus

Following entorhinal cortex, hippocampus is one of the earliest affected brain region in AD (Arriagada et al., 1992). AD pathological lesions and neuronal loss are observed in multiple regions including hippocampus as well as trans-entorhinal and entorhinal cortices [START_REF] Hyman | Alzheimer's disease: cell-specific pathology isolates the hippocampal formation[END_REF](Hyman et al., , 1990;;[START_REF] Geddes | Plasticity of hippocampal circuitry in Alzheimer's disease[END_REF]Isla et al., 1996;[START_REF] Kordower | Loss and atrophy of layer II entorhinal cortex neurons in elderly people with mild cognitive impairment[END_REF]. The hippocampus is part of the limbic system, and plays important roles in the consolidation of information from short-term memory to long-term memory, and in spatial memory that enables navigation in rodents. As such, it is generally acknowledged that memory deficits in AD involve synaptic dysfunction and synaptic plasticity impairments in the hippocampus (Selkoe, 2002;[START_REF] Morrison | The ageing cortical synapse: hallmarks and implications for cognitive decline[END_REF].

The hippocampus (Figure 12) has a general shape of a seahorse and therefore was named after by the Venetian anatomist Julius Caesar Aranzi (1587). It comprises the dentate gyrus (DG) formed by a condensed layer of granule cells bodies making a v-shape and a ram's horn (Cornu Ammonis) comprising CA4 (in humans), CA3, CA2 and CA1. The CA fields present densely packed pyramidal neurons [START_REF] Hyman | Neuron numbers in Alzheimer's disease: cellspecific pathology[END_REF]Cajal, 1901;[START_REF] De Nó | Studies on the structure of the cerebral cortex. II. Continuation of the study of the ammonic system[END_REF]. The main input to the hippocampus is the entorhinal cortex (EC) through perforant pathway (PP).

This pathway generates mostly from layers II and III of EC. Cells from layer II project to DG and CA3, while layer III neurons project to CA1 and the subiculum. Axons from granule cells of DG which project to CA3 pyramidal neurons are named mossy fibers. CA3 pyramidal neurons mainly project to CA1 via Schaffer collaterals [START_REF] Amaral | The three-dimensional organization of the hippocampal formation: a review of anatomical data[END_REF]. These two routes, EC → CA1 and EC → DG → CA3 → CA1, drive CA1 activity and CA1 pyramidal neurons projects back into deep layer of EC (Figure 12).

Perforant path (PP)/Dentate Gyrus (DG) synapses

The major excitatory input to DG is the perforant path (PP) made of glutamatergic projections, which form functional connections during the 3 rd week of development (Esposito et al., 2005;[START_REF] Ge | GABA regulates synaptic integration of newly generated neurons in the adult brain[END_REF][START_REF] Zhao | Activation of group II metabotropic glutamate receptors attenuates both stress and cueinduced ethanol-seeking and modulates c-fos expression in the hippocampus and amygdala[END_REF][START_REF] Toni | Synapse formation on neurons born in the adult hippocampus[END_REF]. In the 4 th week, the strength of excitatory afferents enhances greatly whereas the membrane input resistance is still high, an intrinsic property of immature neurons (Mongiat et al., 2009). According to this, immature granule cells (GCs) are much easier to fire AP than mature neurons [START_REF] Ge | A critical period for enhanced synaptic plasticity in newly generated neurons of the adult brain[END_REF] and therefore intrinsic properties of GCs may highly affect the memory encoding procedure [START_REF] Rolls | A computational theory of hippocampal function, and empirical tests of the theory[END_REF].

The perforant path consists of two subfields: the medial perforant path (MPP) and the lateral perforant path (LPP) (McNaughton, 1980). The MPP initiates from medial EC and projects to the middle third of the molecular cell layer, while the LPP initiates from the lateral EC and projects to the outmost third of the molecular cell layer. The excitatory input from both MPP and LPP onto GCs have different properties of short-term and long-term plasticity (McNaughton et al., 1980;[START_REF] Bramham | Activation of AP5--sensitive NMDA receptors is not required to induce LTP of synaptic transmission in the lateral perforant path[END_REF]Colino et al., 1993;[START_REF] Patten | The benefits of exercise on structural and functional plasticity in the rodent hippocampus of different disease models[END_REF]. The MPP processes spatio-temporal informations involving NMDAR-dependent plasticity whereas the LPP processes novel object information through opiate receptor dependent plasticity [START_REF] Hargreaves | Major dissociation between medial and lateral entorhinal input to dorsal hippocampus[END_REF]Hunsaker et al., 2007Hunsaker et al., , 2013) (Figure 13).

Both MPP and LPP take glutamate as main neurotransmitter [START_REF] White | Glutamate as transmitter of hippocampal perforant path[END_REF][START_REF] Storm-Mathisen | Glutamate in hippocampal pathways[END_REF], but present different pharmacological and physiological features such as L-2-amino-4-phosphonobutyric acid (L-AP4) acting through the inhibition of voltage-dependent calcium channels (VDCCs) and/or potassium currents, leading to the suppression of glutamate release [START_REF] Forsythe | Presynaptic glutamate receptors depress excitatory monosynaptic transmission between mouse hippocampal neurones[END_REF]. et al., 1990). Short term synaptic plasticity events such as PPF and PTP also vary between LPP and MPP (McNaughton, 1980).

When it comes to AD, both pathological tau and amyloid collaboratively increases cytotoxicity leading to PP degeneration. Aβ oligomer were demonstrated to be transported from cell bodies in the entorhinal cortex to the PP, and partly from the posterior parietal cortex (Morales et al., 2012;Bero et al., 2012).

Most interestingly, tau pathology seems confined initially to PP [START_REF] García-Sierra | The extent of neurofibrillary pathology in perforant pathway neurons is the key determinant of dementia in the very old[END_REF][START_REF] De Calignon | Propagation of tau pathology in a model of early Alzheimer's disease[END_REF][START_REF] Gao | GSK3: a key target for the development of novel treatments for type 2 diabetes mellitus and Alzheimer disease[END_REF][START_REF] Polydoro | Soluble pathological tau in the entorhinal cortex leads to presynaptic deficits in an early Alzheimer's disease model[END_REF]. These data strongly support the concept of a ''spread'' of the pathology following the perforant pathway. Furthermore, PP projection to DG is specifically vulnerable in terms of synaptic plasticity in AD murine models [START_REF] Palop | Aberrant excitatory neuronal activity and compensatory remodeling of inhibitory hippocampal circuits in mouse models of Alzheimer's disease[END_REF].

Figure13. Synaptic circuits of PP/DG in hippocampus

The schematic shows the arrangement of afferent inputs to the molecular layer of the DG. Inputs from lateral perforant path (LPP), medial perforant path (MPP) connect the outer, middle molecular layer, respectively. Hippocampal granule cells bodies (yellow) located at the dentate granule cell body layer (DG).

Mossy fibre (Mf) /CA3 synapses

Since the first classified of CA3 region in the hippocampus [START_REF] De Nó | Studies on the structure of the cerebral cortex. II. Continuation of the study of the ammonic system[END_REF], there has been more and more interest in the different CA3 neuron characteristics. Intrinsic functional connectivity in CA3 is a network of highly structured connection [START_REF] Willshaw | Non-holographic associative memory[END_REF][START_REF] Marr | Simple memory: a theory for archicortex[END_REF][START_REF] Hopfield | Neural networks and physical systems with emergent collective computational abilities[END_REF][START_REF] Mcnaughton | Hippocampal synaptic enhancement and information storage within a distributed memory system[END_REF]Treves et al., 1994;[START_REF] Bennett | Dynamics of the CA3 pyramidial neuron autoassociative memory network in the hippocampus[END_REF]Lisman et al., 1999;[START_REF] Rolls | A quantitative theory of the functions of the hippocampal CA3 network in memory[END_REF]. CA3 circuits display extensive excitatory interconnections between CA3 pyramidal cells (PCs) through associational and commissural fibres (for review see [START_REF] Rebola | Operation and plasticity of hippocampal CA3 circuits: implications for memory encoding[END_REF] but also connections to inhibitory neurons [START_REF] De Nó | Studies on the structure of the cerebral cortex. II. Continuation of the study of the ammonic system[END_REF]. This region is as likely as involved in forming spatial representations and episodic memories of initial acquisition stages [START_REF] Scoville | Loss of recent memory after bilateral hippocampal lesions[END_REF][START_REF] Nadel | The Hippocampus as a Cognitive Map[END_REF][START_REF] Kesner | A computational theory of hippocampal function, and tests of the theory: new developments[END_REF].

CA3 region receives inputs from the entorhinal cortex either directly via the PP or from DG through mossy fibres (Mfs) [START_REF] Amaral | The three-dimensional organization of the hippocampal formation: a review of anatomical data[END_REF]. Mf form giant boutons [START_REF]Miniature excitatory postsynaptic currents (mEPSCs)[END_REF](6)(7)(8) µm in diameter) that have an average of 20 release sites per bouton. The Mf/CA3 pathway acts as a filter that transforms complex coded signals to specific hippocampal information, required for memory encoding. This procedure really depends on the firing pattern of DG granule cells (GCs) and leads to more positive stimulus performance in the CA3 region (Leutgeb et al., 2007).

Pattern separation and pattern completion are often considered as associated with DG and CA3 region, respectively, with the DG granule cell and Mf setting the pathway to CA3

(Figure 14). However, CA3 pyramidal cells also send 'back' to DG [START_REF] Scharfman | The CA3 "backprojection" to the dentate gyrus[END_REF].

The projection from CA3 to DG might have an effect on the circuits in the DG and CA3 [START_REF] Penttonen | Feed--forward and feed--back activation of the dentate gyrus in vivo during dentate spikes and sharp wave bursts[END_REF][START_REF] Lisman | Recall of memory sequences by interaction of the dentate and CA3: a revised model of the phase precession[END_REF], and this pathway might contribute to the information process. A study in the computational model showed that this backprojection has an important role in the ability of the computational model to present pattern separation and completion [START_REF] Myers | Pattern separation in the dentate gyrus: a role for the CA3 backprojection[END_REF]. In the hippocampus, only CA3 contains recurrent pathway: more than 95% of input to CA3 region is from CA3 recurrent connections. These associated fibers might promote the pattern completion [START_REF] Guzowski | Ensemble dynamics of hippocampal regions CA3 and CA1[END_REF].

Importantly, Mf-CA3 synapses present a wide range of short term plasticity (frequency facilitation, paired-pulse facilitation, post-tetanic potentiation) but also presynaptic LTP and LTD that are depending on NMDAR activation (Nicoll and Schmitz, 2005;[START_REF] Rebola | Operation and plasticity of hippocampal CA3 circuits: implications for memory encoding[END_REF]. Furthermore, NMDARs contribute to LTP characterized by a selective enhancement of NMDA receptor-mediated transmission which requires coactivation of NMDA and mGlu5 receptors and a postsynaptic calcium rise at Mf-CA3 synapses [START_REF] Kwon | Long-term potentiation selectively expressed by NMDA receptors at hippocampal mossy fiber synapses[END_REF].

Up to now, most of the studies in AD mouse models have been done at the archetypical CA3-CA1 synapse which is characterized by a canonical postsynaptic LTP. On the opposite, few studies have investigated Mf-CA3 synapses alterations in AD (Witton et al., 2010;Marchetti and Marie, 2011;Wilke et al., 2014;Silva et al., 2016;Maingret et al., 2017).

In APP/PS1 mice at 6 months of age, postsynaptic LTP of associative/commissural (A/C) inputs to CA3 pyramidal cells was totally impaired (Silva et al., 2016) whereas the presynaptic LTP at the Mf-CA3 synapse was unaffected (Maingret et al., 2017). Interestingly, APP/PS1 mice at 6 months of age displayed altered fear conditionning (Kilgore et al., 2010).

Aβ-overproducing Tg2576 mouse display impairement of short term frequency facilitation and absence of NMDAR independent LTP at the Mf pathway (Witton et al., 2010).

On the contrary, tau oligomers lead to presynaptic and postsynaptic morphological changes (toxic function) at the Mf linked to the CA3 region in mutant human tau mice [START_REF] Decker | Pro-aggregant Tau impairs mossy fiber plasticity due to structural changes and Ca++ dysregulation[END_REF].

Figure 14. Synaptic circuit of Mf/CA3 in hippocampus

Mossy fibre (Mf) projection contains axons of the DG granule cells in the hippocampus.

Schaffer collaterals (SC)/CA1 synapses

CA3 are important for the fast combination different information and pattern completion, while CA1 may be helpful in identifying a new event or a new context (Lisman et al., 2001;Kumaran et al., 2007). In the hippocampus, CA1 receives projections from CA3

through Schaffer collaterals (SC) [START_REF] Li | The hippocampal CA3 network: an in vivo intracellular labeling study[END_REF] (Figure 15). Different layers of entorhinal cortex (EC) transport vary kinds of information to the CA3 and CA1 regions [START_REF] Sargolini | Conjunctive representation of position, direction, and velocity in entorhinal cortex[END_REF]. This has lead to detailed physiological studies of these regions in the hippocampus.

Figure 15. Synaptic circuit of CA3/CA1 in hippocampus

Sites of termination of axons of CA1 and CA3 pyramidal cells Regional differences in firing patterns corresponding to different environmental factors have been found [START_REF] Lee | Comparison of population coherence of place cells in hippocampal subfields CA1 and CA3[END_REF][START_REF] Leutgeb | Distinct ensemble codes in hippocampal areas CA3 and CA1[END_REF][START_REF] Leutgeb | Independent codes for spatial and episodic memory in hippocampal neuronal ensembles[END_REF][START_REF] Vazdarjanova | Differences in hippocampal neuronal population responses to modifications of an environmental context: evidence for distinct, yet complementary, functions of CA3 and CA1 ensembles[END_REF]Dragoi and Buzsaki, 2006;[START_REF] Lee | The relationship between the field-shifting phenomenon and representational coherence of place cells in CA1 and CA3 in a cue-altered environment[END_REF]Leutgeb and Leutgeb, 2007;[START_REF] Alvernhe | Different CA1 and CA3 representations of novel routes in a shortcut situation[END_REF]. When testing rat in a spatial environment, both CA1 and CA3 neurons are selectively firing when the animal moves to particular places in the environment.

These 'place cells' are thought to set as parts of a cognitive map in the environment [START_REF] O'keefe | Hippocampal place units in the freely moving rat: why they fire where they fire[END_REF] or representations of places where significant events happen [START_REF] Eichenbaum | The hippocampus, memory, and place cells: is it spatial memory or a memory space?[END_REF][START_REF] Eichenbaum | Hippocampus: cognitive processes and neural representations that underlie declarative memory[END_REF]. Thus, investigations of age-related alterations in the firing patterns of place cells maybe informative of disruptions of memory procedure in aged hippocampus. Spatial information processed by place cells is changed in aged rats in several studies (Rosenzweig et al., 2003). Particularly, many studies have indicated that CA3 and CA1 place cells of aged rats could not form new spatial or task information quickly [START_REF] Tanila | Brain aging: impaired coding of novel environmental cues[END_REF][START_REF] Oler | Age--related deficits in the ability to encode contextual change: A place cell analysis[END_REF][START_REF] Wilson | Place cell rigidity correlates with impaired spatial learning in aged rats[END_REF][START_REF] Wilson | Cognitive aging and the hippocampus: how old rats represent new environments[END_REF][START_REF] Wilson | Age-associated alterations of hippocampal place cells are subregion specific[END_REF].

In the hippocampus, synaptic plasticity dysfunction of AD has been demonstrated by the application of Aβ peptides and strongly inhibits LTP induction at Schaffer collateral/CA1 synapses [START_REF] Chen | Impairment of hippocampal long--term potentiation by Alzheimer amyloid β--peptides[END_REF]. This result is also complementary to an early in vivo study

showing impaired maintenance of CA1 -LTP in rats injected i.c.v. with Aβ 1-40 or Aβ 1-42 [START_REF] Cullen | Block of LTP in rat hippocampus in vivo by β-amyloid precursor protein fragments[END_REF]. Another study illustrates that basic synaptic transmission was disrupted at SC/CA1 synapse in htau mouse with an age dependent mode [START_REF] Polydoro | Age-dependent impairment of cognitive and synaptic function in the htau mouse model of tau pathology[END_REF]

II. Study objectives

Alzheimer's disease (AD) is a progressive neurodegenerative disease characterized by early cognitive deficits linked to synaptic dysfunction (Selkoe, 2002;Jacobsen et al., 2006;Sheng et al., 2012). Numerous evidences show that neuroinflammation plays a key role in the pathogenesis of AD (Haruhiko et al., 2000;[START_REF] Rojo | Neuroinflammation: implications for the pathogenesis and molecular diagnosis of Alzheimer's disease[END_REF] and most importantly, neuroinflammatory molecules like prostanoids and cytokines are well known to modulate synaptic transmission and plasticity (Sang et al., 2006;[START_REF] Yang | Cyclooxygenase-2 in synaptic signaling[END_REF][START_REF] Riazi | Microglia-dependent alteration of glutamatergic synaptic transmission and plasticity in the hippocampus during peripheral inflammation[END_REF].

Multiple epidemiological studies have revealed that chronic intake of nonsteroidal antiinflammatory drugs (NSAIDs) reduced considerably the prevalence of AD (McGeer and McGeer, 2007) and clinical trials have shown that when given to asymptomatic patients, NSAIDs reduced AD incidence (Breitner et al., 2011). Among the proinflammatory molecules generated during AD, prostaglandin E2 (PGE 2 ) plays a pivotal role [START_REF] Yang | Cyclooxygenase-2 in synaptic signaling[END_REF]. PGE 2 is produced from arachidonic acid by two rate limiting enzymes, Cox-1 and

Cox-2 and binds with four main subtypes of receptors, that is EP1, EP2, EP3, and EP4 (Breyer et al., 2001). In a previous study we demonstrated that the presynaptic long-term potentiation (LTP) at hippocampal Mf-CA3 synapse was impaired in APP/PS1 males due to the engagement of a PGE 2 -EP3 signaling pathway (Maingret et al., 2017). Mf-CA3 synapse is characterized by prominent presynaptic short term plasticity such as paired-pulse facilitation (PPF), frequency facilitation (FF), and presynaptic long-term plasticity (LTP). LTP has been described to be involved in the novel memory fast encoding (Kesner et al., 2007), a procedure particularly affected in AD.

Among different major risk factors for developing the disease revealed by epidemiological and genetic studies (for review see Hickman et al., 2016), surprisingly sex, which is one of the major risk, has received relatively little attention so far. However, there is a prominent sexual dimorphism in the human pathology, with a much higher prevalence (two to three times) of AD-related cognitive decline and greater AD neuropathology (Buckwalter et al., 1993;Letenneur et al., 1994;Henderson and Buckwalter, 1994;Brayne et al., 1995;Fratiglioni et al., 1997;Andersen et al., 1999;Ruitenberg et al., 2001;Corder et al., 2004;Barnes et al., 2005;Sinforiani et al., 2010) in women compared to men. Remarquably, this sexual dimorphism is also found in mouse models of AD with females displaying precocious and stronger memory deficits (King et al., 1999;Clinton et al., 2007;Pistell et al., 2008;Carroll et al., 2010) as well as higher levels of Aβ accumulation in the brain (APP23:

Sturchler-Pierrat and Staufenbiel, 2000; 3xTg-AD: Carroll et al., 2010), than males. In APP/PS1 mice, Aβ deposition could be detected at 6 months, leading to a massive load of Aβ plaques in the hippocampus and cortex by 9 months (Jankowsky et al., 2004) further increased at 12 months [START_REF] Garcia-Alloza | Characterization of amyloid deposition in the APPswe/PS1dE9 mouse model of Alzheimer disease[END_REF]. Higher levels of insoluble Aβ 42 and Aβ plaque load are observed in females compared to age-matched males (Wang et al., 2003;Taniuchi et al., 2007;Gallagher et al., 2013) but also earlier deficits in several hippocampaldependent spatial memory tasks (Taniuchi et al., 2007;Gallagher et al., 2013;Richetin et al., 2017). Amyloid-β peptides (Aβ) are one of the most important hallmark in AD pathogenesis (Robin Roychaudhuri et al., 2009). Neuroinflammation in AD is reported to be associated with Aβ, likely leading to microglia and astrocytes activation [START_REF] Frozza | Lipid-core nanocapsules improve the effects of resveratrol against Aβ-induced neuroinflammation[END_REF]. Strikingly in vitro and in vivo studies have reported that neuronal electrical activity was enhanced in the vicinity of Aβ plaques [START_REF] Busche | Critical role of soluble amyloid-β for early hippocampal hyperactivity in a mouse model of Alzheimer's disease[END_REF]Busche & Konnerth, 2016). Female transgenic mice displayed both earlier memory deficits and more severe Aβ plaques deposition compared to age-matched males (Richetin et al., 2017;[START_REF] Yang | Sex differences in neuropathology and cognitive behavior in APP/PS1/tau triple-transgenic mouse model of Alzheimer's disease[END_REF]. Therefore, we first investigated presynaptic LTP at Mf-CA3 synapses in APP/PS1 female mice at 4 and 6 months, an age for which behavioural impairments are already present. Unexpectedly, we did not observe any difference between WT and APP/PS1 female mice. Then we chose to study synaptic transmission and plasticity in a region which is highly enriched with Aβ plaques, the stratum moleculare in the vicinity of the dentate gyrus. This region receives axonal projections from entorhinal cortex via the perforant pathway (PP) which convey spatial informations (Deng et al., 2010) and make synapses onto DG granule cells. PP projections carry to DG granule cells cortical inputs responsible for pattern separation and spatial information and memory encoding [START_REF] Hyman | Perforant pathway changes and the memory impairment of Alzheimer's disease[END_REF]Bennet et al., 2016). The PP is divided into two parts: the lateral perforant pathway (LPP) and the medial perforant pathway (MPP). MPP and LPP display different properties in term of synaptic plasticity (Petersen et al., 2013) allowing to distinguish between LPP and MPP. In this study, we investigated whether abundant Aβ deposition in female APP/PS1 mice shows alerations of behaviours, neuron intrinsic properties and basal transmissions, as well as LTP at LPP/DG area.

Introduction

Alzheimer's disease (AD) is a progressive and devastating neurodegenerative disease which is responsible for more than 70 % of dementia in the elderly population worldwide (in "2018

Alzheimer's disease fact and figures", 2018). AD is characterized by histopathological hallmarks, i.e. the extracellular deposit of Aβ plaques mostly in the cortex and the hippocampus and the intraneuronal accumulation of fibrillary tangles of hyperphosphorylated Tau (Querfurth and LaFerla, 2010). These molecular events are associated with profound structural changes in the brain such as the shrinkage of the cortex, the enlargement of ventricules and the severe atrophy of the hippocampus.

Despite tremendous efforts both targeted at basic and clinical studies over the last three decades, there is still neither a treatment or a cure for AD nor efficient treatments to alleviate the symptoms. Nevertheless, at least in its early stage, AD is thought to affect synaptic circuits through alterations of synaptic transmission and plasticity, especially in the entorhinal cortex-hippocampal circuitry (for review see Selkoe, 2002;Spires-Jones and Hyman, 2014).

Impairment of synaptic transmission and plasticity has been extensively studied over the last 20 years using transgenic mouse model of AD (for review see Marchetti and Marie, 2011).

These studies have revealed that soluble Aβ and numerous inflammatory molecules produced by the activation of glial cells contributed to the pathophysiology of the disease and its progression (Akiyama et al., 2000;Heneka and O'Banion, 2007;Heneka et al., 2015).

Epidemiological and genetic studies point to major risk factors for developing the disease (age, susceptibility genes, diabetes of type 2, obesity) (for review see Hickman et al., 2016) but surprisingly sex, which is one of the major risk, has received relatively little attention so far.

In amyloidogenic mouse models of AD, female mice display precocious and stronger memory deficits (Carroll et al., 2010;Clinton et al., 2007;King et al., 1999;Pistell et al., 2008) as well as higher levels of Aβ accumulation in the brain (APP23: Sturchler-Pierrat and Staufenbiel, 2000; 3xTg-AD: Carroll et al., 2010), than males. In APP/PS1 female mice, higher levels of insoluble Aβ 42 and Aβ plaque load are observed compared to age-matched males (Wang et al., 2003;Taniuchi et al., 2007;Gallagher et al., 2013). Paralleling these events, age-related deficits in several hippocampal-dependent spatial memory tasks have been reported in APP/PS1 male mice (Jankowsky et al., 2004;[START_REF] Puoliväli | Hippocampal Ab42 levels correlate with spatial memory deficit in APP and PS1 double transgenic mice[END_REF][START_REF] Savonenko | Episodic-like memory deficits in the APPswe/PS1dE9 mouse model of Alzheimer's disease: Relationships to β-amyloid deposition and neurotransmitter abnormalities[END_REF] but occuring earlier in females (8-9 months), an age for which males are spared (Gallagher et al., 2013;Taniuchi et al., 2007;Richetin et al., 2017).

This sexual dimorphism is of high interest as it is also found in the human pathology, with a much higher prevalence (two to three times) of AD-related cognitive decline and greater AD neuropathology (Buckwalter et al., 1993;Letenneur et al., 1994;Henderson and Buckwalter, 1994;Brayne et al., 1995;Fratiglioni et al., 1997;Andersen et al., 1999;Ruitenberg et al. 2001;Corder et al., 2004;Barnes et al., 2005;Sinforiani et al., 2010) in women compared to men.

In the present work we have compared the hippocampal progression of Aβ plaque deposition in male and female APP/PS1 mice at 3, 6 and 9 months of age in relation to the neuroinflammatory status. We focussed on the dentate gyrus (DG), a key region of the excitatory trisynaptic hippocampal pathway which receives axonal projections from the entorhinal cortex and conveys non-spatial as well as spatial informations (Deng et al., 2010).

We then studied hippocampus-dependent spatial memory using two different one-trial behavioral paradigms, i.e., contextual fear conditioning (CFC) and novel object place recognition task (NORT) in both gender at different age. We then studied if and how the intrinsic properties of DG granule cells and the properties of synaptic transmission and plasticity at the perforant pathway -DG circuit were altered in APP/PS1 female mice at 6 months of age.

The main findings of our study are plural: APP/PS1 females display earlier Aβ plaques load than males in the stratum moleculare of the DG that are paralleled by neuroinflammatory activation of glial cells. These hippocampal molecular and cellular alterations are accompanied by strong memory impairments in female but not male mice even in old animals (12 months of age). We show that APP/PS1 female mice at 6 months of age displayed alterations in intrinsic excitability in DG cells, as well as synaptic alterations at the PP-DG circuit.

Material and Methods

Ethical approval

Animal anesthesia and euthanasia were carried out in accordance with the Animal Protection of ethical standards and the French legislation concerning animal experimentation and were approved by the University of Bordeaux / CNRS Animal care and Use Committee (authorization #2573). All efforts to improve the animal welfare and to minimize animals suffering were made.

Animals

The animals used in this study were males and females APPswe695/PS1ΔE9 termed APP/PS1 obtained from Jackson Laboratory (Bar Harbor, ME, USA) and their wild-type (WT) littermates (C57BL6/J). Briefly, the APP/PS1 mice express a chimeric mouse/human amyloid precursor protein APPswe (mouse APP695 harboring a human Aβ domain and mutations K595N and M596L linked to a Swedish familial AD) and a human presenilin 1 mutated in familial AD (PS1ΔE9; deletion of exon 9). These bigenic mice were created by co-injection of both transgenes allowing for a co-segregation of the transgenes as a single locus (Jankowsky et al., 2004). Mice were generated in our animal facility from double-transgenic APP/PS1 males mated with C57BL/6J females. Transgenic mice (APP/PS1) and age-matched non-transgenic littermates (WT) were allowed free access to food and water and maintained in a 12 hours' dark-light cycle. Mice were genotyped and systematically regenotyped after each experiments (Transcriptomics Platform, Neurocentre Magendie, Bordeaux, France). For all experiments, females have been used without consideration to their estrous cycle.

Acute hippocampal slices

In accordance with Animal Protection Association of ethical standards and the French

Electrophysiological studies on acute hippocampal slices

For recording, slices were transferred into a recording chamber where they were submerged and continuously perfused with an oxygenated (95% 0 2 and 5% CO 2 ) extracellular solution at EPSP amplitude/slope ratio histograms before and after HFS were generated as follows: at the start of the experiment, for each cell the amplitude of sub-threshold EPSPs (i.e. not triggering APs) was measured in current-clamp mode (the membrane potential was set to around -70 mV), and plotted vs the EPSP slope. Then a linear regression was fitted yielding a slope value.

The cell was rejected if the goodness of fit, S y.x , which quantifies the scatter of the data points around the best fit line was < 1.5. After this, the cell was switched to voltage-clamp mode and held at -70 mV. A baseline was recorded for 5 min and then the HFS protocol (3 bursts of 100 stimuli at 100 Hz with a 10 s interburst interval) was applied followed by a period of 20 min.

Then the cell was returned back to current-clamp mode (set around -70 mV) and a new EPSP amplitude/slope ratio histogram was generated.

Immunohistological staining and analysis

WT and APP/PS1 male and female mice at 3, 6 and 9 months of age were anesthetized with intraperitoneal administration of pentobarbital (50 mg/kg body weight) and were fixed by transcardial perfusion with buffered saline and 50 ml of 4% paraformaldehyde (PFA). Brains

Enzyme immunoassay determination of PGE 2

The concentration of PGE 2 was quantified both in WT and APP/PS1 at

Statistical analysis

In order to minimize biases as much as possible, experimenters were kept blind to the

Results

Aβ plaques and neuroinflammation develops faster in APP/PS1 females than males

Males and females WT and APP/PS1 mice at 3, 6 and 9 months of age were compared in terms of number of Aβ plaques and glial activation (Fig. 1). While there are only very few and small Aβ plaques in APP/PS1 male at 9 months of age, there is a heavy load of Aβ plaques in APP/PS1 female at the same age (Fig. 1A). Strikingly, Aβ plaques are not uniformely distributed throughout the hippocampus but are clearly concentrated in the stratum moleculare (delineated by the white dotted-line) of the DG. At a higher magnification (lower panel), Aβ plaques appear either associated with a dense core commonly referred as "compact" dense-core plaques or diffuse without a dense core known as "cotton wool plaque" [START_REF] Le | Cotton Wool Plaques in Non-Familial Late-Onset Alzheimer Disease[END_REF]. Compact plaques are formed by a dense amyloid core (anti-Aβ 1-16 , white signal) surrounded by a spider web of microglia (Iba-1, green signal) walled off by activated astrocytes (GFAP, red signal). Cotton wool plaques are larger structures devoid of a dense amyloid core but nevetheless surrounded by activated microglia and astrocytes. There is a progressive increase in the number of Aβ plaques in APP/PS1 males and females (Fig. 1B) To further evaluate the neuroinflammatory status at different age, gender and genotype, double immunostaining of microglia (Iba-1) and astrocytes (GFAP) were performed. As shown in Fig. 1C, in WT females at 6 months of age, astrocytes displayed elongated bodies with long and thin processes whether in APP/PS females at the same age, astrocytes displayed rounded cell bodies with shortened and enlarged processes, a morphological hallmark of activated astrocytes [START_REF] Wilhelmsson | Redefining the concept of reactive astrocytes as cells that remain within their unique domains upon reaction to injury[END_REF] for review see [START_REF] Schiweck | Important Shapeshifter: Mechanisms Allowing Astrocytes to Respond to the Changing Nervous System During Development, Injury and Disease[END_REF].

starting
Microglia also displayed a typical phenotype of reactive cells (Nimmerjahn et al., 2005) shifting from a quiescent morphology to an activated status with enlargement of cell bodies and shorter and thicker processes (Fig. 1D).

These data provide clear evidence that deposits of Aβ plaques and glial activation occur faster and stronger in APP/PS1 females vs males and are in good agreement with previous studies reporting higher level of Aβ plaques load (Taniuchi et al., 2007;Gallagher et al., 2013;[START_REF] Sierksma | Behavioral and neurobiological effects of prenatal stress exposure in male and female APPswe/PS1dE9 mice[END_REF] and abundant hypertrophic astrocytes in the DG (Richetin et al., 2017) in APP/PS1 females compared to age-matched males.

Hippocampal-dependent memory is impaired in APP/PS1 females but not in males

Hippocampal-dependent memory was tested in APP/PS1 males and females of 3 to 12 months of age and their age-matched WT littermates using two different one-trial behavioral paradigms: contextual fear conditioning (CFC) and object location task (OLT).

Contextual fear conditioning is a hippocampal-dependent task which results in a robust long lasting memory for a rodent [START_REF] Phillips | Differential contribution of amygdala and hippocampus to cued and contextual fear conditioning[END_REF]. Our protocol (Fig. 2A) was designed to yield approximately 40-50% freezing in the retrieval test (Fig. 2B). All together, these data clearly show that memory deficits displayed by APP/PS1 mice and tested by CFC and NORT appear much earlier in females than males. These memory deficits parallel the prominent increase in neuroinflammatory markers, i.e. Aβ plaques load, astrocytic and microglial activation ahead of few months in females compared to age-matched males.

Basal synaptic transmission at the lateral perforant path -DG synapse is altered in APP/PS1 female mice

Because the Aβ plaques appeared earlier and were more abundant in the stratum moleculare of the DG in female mice, we set out to characterize the basic properties of the lateral perforant path (LPP) input which make synaptic contact to granule cells of the DG in this layer. The DG receives information from layer 2 of the entorhinal cortex via a bundle of axons, the perforant path which is subdivided into the LPP that is assumed to convey novel contextual information whereas spatial information appears to be conveyed by fibres of the medial perforant path (MPP) (Hunsaker et al., 2007).

The stimulating electrode was placed in the outer one-third molecular layer of the DG suprapyramidal blade and the recording electrode was located either in the DG granule cell layer for patch-clamp recordings as shown in Fig. 4A, or near the crest of the suprapyramidal blade for field recordings. In this way, we achieved reliable and selective activation of LPP projections to the DG. To activate selectively MPP, the electrode was carefully positioned in the middle molecular layer, that is at a distance < 100 µm from the DG granule cell layer.

Increasing stimulus intensity (0 -200 pA; 0.1 ms at 0.1 Hz) of the LPP triggered population spike the amplitude of which was plotted against stimulus intensity (Fig. 4B). Input-output relationships for stimulus intensity vs population spike amplitude were significantly different between genotypes, APP/PS1 female mice at 6 months of age always displaying a smaller population spike amplitude compared to the aged-matched WT female (Fig. 4B). LPP was also discriminated from MPP using a paired-pulse protocol (McNaughton, 1980;[START_REF] Wang | Membrane properties of identified lateral and medial perforant pathway projection neurons[END_REF]Petersen et al., 2013) which results in the facilitation of the second pulse when the LPP is stimulated but either no facilitation or even a depression of the second pulse when the MPP is stimulated (Fig. 4C). Stimuli were delivered at a low intensity, that is at 25-30% maximum pop spike amplitude to minimize as much as possible current spreading to the adjacent pathway but also to avoid paired-pulse facilitation of the MPP which is reported for higher stimulus intensity (Petersen et al., 2013). The two pulses were separated by 50 to 100 ms inter-pulse interval. Both genotypes exhibited a facilitation for the LPP but not for the MPP (Fig. 4C).

The difference in the input-output relationships between the two genotypes suggest a decreased excitability of DG cells in APP/PS1 female mice compared to WT. We next examined the relative amplitude of synaptic currents mediated by AMPA and NMDA receptors in DG granule cells of APP/PS1 and WT female mice at 6 months of age. AMPA currents were recorded using the whole-cell configuration of the patch clamp technique in voltage-clamp mode at a holding potential of -70 mV, a potential at which NMDA currents were virtually absent due to the Mg 2+ voltage-dependent block [START_REF] Kupper | Internal Mg 2+ block of recombinant NMDA channels mutated within the selectivity filter and expressed in Xenopus oocytes[END_REF]. NMDA currents were isolated at a holding potential of +40 mV in the presence of 50 µM NBQX to block AMPA and kainate currents. We found no difference in AMPA/NMDA ratio between WT (2.82 ± 0.45, n=10) and APP/PS1 mice (2.35 ± 0.39, n=9; p=0.452, Fig. 4D).

Furthermore, the AMPA rectification index, a proxy to investigate for possible change of AMPA receptor function did not significantly differ between both genotypes (WT:0.51 ± 0.06, n=10; APP/PS1:0.49 ± 0.08, n=9; p=0.794, Fig. 4E) suggesting no major alteration in AMPA receptor function in APP/PS1 female mice.

We then recorded spontaneous AMPA and NMDA miniature excitatory postsynaptic currents (mEPSCs) from DG cells to investigate if they were altered in APP/PS1 compared to WT mice. AMPA mEPSCs (mAMPA) were recorded at a holding membrane potential of -70 mV in the presence of 0.5 µM TTX to block action potentials and 10 µM bicuculline to block mIPSCs due to GABA-A receptors. For NMDA mEPSCs (mNMDA), recordings were made in nominal Mg 2+ free solution with 50 µM D-serine (co-agonist of the NMDA receptor, [START_REF] Mothet | D-serine is an endogenous ligand for the glycine site of the N-methyl-Daspartate receptor[END_REF] and in the presence of 10 µM NBQX to block AMPA receptors. Synaptic activity was recorded for 15 minutes and all events occuring during this period were counted for both amplitude and frequency analysis.

The average mean frequency of mAMPA currents was similar between both genotype (WT:

14.00 ± 2.50 events/min, n=16; APP/PS1: 18.50 ± 2.91 events/min, n=15, p=0.249). In contrast, the frequency of NMDA currents was significantly larger in APP/PS1 mice (17.12 ± 1.29 events/min, n=14) when compared to WT (4.60 ± 0.43 events/min, n=13; *** p<0.001) (Fig. 5A).

The average mean amplitude of mAMPA (WT: 8.19 ± 0.27 pA, n=15; APP/PS1: 9.22 ± 0.51 pA, n=15, p=0.088) and of mNMDA (WT: 15.50 ± 0.94 pA, n=15; APP/PS1: 17.55 ± 1.09 pA, n=14, p=0.166) currents were similar between both genotype (Fig. 5A).

No significant differences were observed for the mean 10-90% rise time and the time constant (Tau) of the decay between genotypes for both mAMPA and mNMDA currents. The mean rise time of mAMPA and mNMDA currents was 0.65 ± 0.07 msec (n=15) and 0.77 ± 0.08 msec (n=15) respectively for WT mice and 0.93 ± 0.13 msec (n=14) and 0.69 ± 0.05 msec (n=14) respectively for APP/PS1 mice (Fig. 5B). The decay of the currents was monoexponential with a Tau of 6.22 ± 0.35 msec (n=15) for WT mAMPA and 4.92 ± 0.21 msec (n=14) for APP/PS1 mAMPA (Fig. 5B). For mNMDA, Tau was 15.27 ± 2.01 msec (n=15) for WT and 19.06 ± 1.68 msec (n=14) for APP/PS1 (Fig. 5B).

The cumulative probability analysis of mAMPA amplitudes showed a significant difference between WT and APP/PS1 mice (KS test, *** p>0.0001; 2556 events of 16 cells for WT and 3884 events of 15 cells for APP/PS1 mice) (Fig. 5C) whereas the amplitude distribution histogram was not statistically different between genotypes (paired t test, p=0.126) (Fig. 5C).

The cumulative probability analysis of mNMDA amplitudes also showed a significant difference between WT and APP/PS1 mice (KS test, *** p>0.0001; 861 events of 15 cells for WT and 3461 events of 14 cells for APP/PS1 mice) but also the amplitude distribution histogram (paired t test, * p<0.05) (Fig. 5C).

In summary these results show a decreased basal transmission and an increased frequency of mNMDA currents for APP/PS1 vs WT mice.

The firing activity of type 1 DG granule cells is increased in APP/PS1 mice

Adult DG granule cells have been reported to be composed of a continuum of newly born and mature neurons reflecting their dynamic integration in the DG throughout the entire life of the organism [START_REF] Aimone | Resolving new memories: a critical look at the dentate gyrus, adult neurogenesis, and pattern separation[END_REF][START_REF] Nakashiba | Young dentate granule cells mediate pattern separation, whereas old granule cells facilitate pattern completion[END_REF]. These two populations of DG neurons display different intrinsic properties, such as input resistance (Rin) and firing patterns, which have been used as specific hallmarks of the degree of maturation and integration in the circuitry. This prompted us to ask whether intrinsic properties of DG cells were differentially affected by Aβ pathology in APP/PS1 mice model. Whole-cell patch clamp recordings were carried out in DG cells from 6 months-old WT and APP/PS1 mice. Figure 6A shows representative electrotonus and action potentials (APs) in a DG granule cell from a WT mouse in response to short (200 µs) depolarizing current step injections (10 µA increments) in current-clamp mode at a membrane potential close to -70 mV. We did not observe significant differences between WT and APP/PS1 mice for input resistance (R in ), total amplitude of AP, amplitude of the overshoot, time to peak of the AP, time to reach the half-maximal amplitude (t 50 ), threshold potential (Table 1). The resting membrane potential of DG granule cells was on average slightly more hyperpolarized in APP/PS1 mice compared to WT mice (-72.7 ± 0.9, n=23; -75.4 ± 0.7, n=15 respectively, * p<0.05, Table 1).

The E-S curve (relates EPSP-slopes to the associated AP firing probability) was generated by applying increasing stimuli to span the all range, i.e. from a sub-threshold EPSP evoking no action potential to an EPSP evoking an action potential with 100% probability. There were no significant differences between genotypes in the E-S curves with a EC 50 value of 5.96±0.38 (n=15) for WT and 5.85±0.39 (n=15) for APP/PS1; p=0.849 (Fig. 6A).

In order to discriminate the putative two types of DG cells, we then applied long current steps (250 ms) of 10 pA increments from -20 pA up to 200 pA to evoke trains of APs. As shown in Fig. 6B, DG granule cells could be divided into two distinct categories, termed type 1 and type 2 according to [START_REF] Nenov | Impaired firing properties of dentate granule neurons in an Alzheimer's disease animal model are rescued by PPARγ agonism[END_REF]. Type 1 DG cells exhibited spikes starting from a moderate intensity, increasing in frequency following steps increments (Fig. 6B). At current steps of high intensity (>120 pA), type 1 DG cells lacked sustained firing, APs became progressively smaller in amplitude and ceased before the end of the current step (Fig. 6B). In contrast, type 2 DG cells fired throughout all the current step without failure and exhibited firing frequencies linearly proportional to the intensity of the current step, i.e. the maximum number of APs was always observed for the highest current intensity (Fig. 6B; Fig. 6C).

Whatever the intensity of the current steps, WT DG cells significantly fired fewer spikes that APP/PS1 DG cells for both type 1 and type 2 (Fig. 6C; *** p<0.0001, two-way ANOVA).

WT type 1 DG cells had a high R in (368±20 MΩ, n=14) whereas WT type 2 DG cells had a significant lower R in (279±19 MΩ, n=16, ** p=0.0033) (Table 2). In APP/PS1 2).

To gain further information on ionic mechanisms that may underlie different spiking patterns between type 1 and type 2 DG cells according to genotype, we studied the first three interspike intervals (Fig. 6D). 6D).

We also examined the last three interspike intervals in type 2 DG cells, as they were spiking throughout all the current step without failure, to search for possible differences, with respect to the genotype, in spike frequency adaptation, i.e. a reduction of the firing rate to a stimulus of constant intensity. Whatever the interval, i.e. the third, the second or the first, they were no differences between both genotype (third spike interval: WT: 31.6±1.7 ms, n=15 and APP/PS1: 28.9±1.1 ms, n=14, p=0.206; second spike interval: WT: 30.3±1.5 ms, n=17 and APP/PS1: 31.6±1.4 ms, n=14, p=0.551; first spike interval: WT: 34.5±1.4 ms, n=17 and APP/PS1: 33.3±1.8 ms, n=14, p=0.623) (Fig. 6E).

In summary these results show a selective increase in firing activity for type 1 DG granule cells (supposedly less mature; Mongiat et al., 2009[START_REF] Spampanato | Properties of doublecortin expressing neurons in the adult mouse dentate gyrus[END_REF][START_REF] Van Praag | Functional neurogenesis in the adult hippocampus[END_REF][START_REF] Nenov | Impaired firing properties of dentate granule neurons in an Alzheimer's disease animal model are rescued by PPARγ agonism[END_REF] We also examined if the probability of discharge of the postsynaptic DG granule cells in response to excitatory synaptic stimulation was differentially affected in WT vs APP/PS1 mice. HFS induced a large shift to the left of the EPSP-to-Spike (E-S) potentiation curves (Fig. 7B) in both genotypes although with a larger amplitude for APP/PS1 mice (EC 50 WT: 7E).

Discussion

Our findings show prominent sex-differences in the development of Aβ plaques load and glial activation in a transgenic mouse model of AD. The molecular and cellular alterations observed in the DG of APP/PS1 female mice paralleled functional deficits revealed by hippocampal-dependent memory tasks. Most interestingly, these alterations are concomittant with modifications of the plasticity of synaptic transmission and intrinsic excitability in the perforant path to DG granule cells circuitry, a key region of the integration of mnesic informations originated from the entorhinal cortex which display a high density of Aβ plaques.

Sex-differences in Aβ load and glial activation in the DG region

Our results clearly show that Aβ load starts much earlier in APP/PS1 females as compared to age-matched APP/PS1 males. Whatever the age, Aβ plaques burden was always significantly higher in females vs males.

According to Jankowsky et al (2004), APP/PS1 mice display a high ratio Aβ 42 / Aβ 40 which accelerates Aβ aggregation into Aβ plaques starting around 6 months of age. APP/PS1 females at 8-9 months of age display roughly twice as many Aβ plaques in the hippocampus and two to three times higher levels of Aβ 42 (Gallagher et al., 2013;Taniuchi et al., 2007) than age-matched males. This sex-difference prolongs during aging and is still present in APP/PS1 up to 17 months of age (Wang et al., 2003).

The physiological basis underlying this sexual dimorphism is still elusive but few potential mechanisms have been proposed to support it such as the increased expression of BACE-1 in APP/PS1 females leading to an enhanced production of Aβ (Gallagher et al., 2013), a modified proliferation of hippocampal progenitors (Tanuichi et al., 2007) yielding less DG adult-born neurons with a reduced life expectancy in APP/PS1 females (Richetin et al., 2017) but also an increased activation of glial cells (microglia, Tanuichi et al., 2007;astrocytes, Richetin et al., 2017). Obviously, the putative influence of sex hormones has also been investigated. In vitro estrogen has been shown to be neuroprotective against Aβ through its ability to decrease lipid peroxidation [START_REF] Behl | 17-β estradiol protects neurons from oxidative stress-induced cell death in vitro[END_REF][START_REF] Gridley | Low concentrations of estradiol reduce β-amyloid (25-35)-induced toxicity, lipid peroxidation and glucose utilization in human SK-N-SH neuroblastoma cells[END_REF]. Estrogen supplementation has been reported to delay the onset in AD and to reduce its prevalence in woman (Henderson et al., 2000) but further trials suggested that estrogen supplementation might have no effect on the course of the disease in elderly woman [START_REF] Fillit | The role of hormone replacement therapy in the prevention of Alzheimer disease[END_REF][START_REF] Cholerton | Estrogen and Alzheimer's disease: the story so far[END_REF].

There is considerable evidence that neuroinflammation is a major component of AD (Akiyama et al., 2000;Heneka and O'Banion, 2007;Heneka et al., 2015) contributing to its progression and its pathogenicity. Furthermore, and relevant to sex-differences, it has been suggested that the differential vulnerability to AD between man and woman could be mediated by a sex-specific systemic inflammatory response in early AD [START_REF] Varma | Alpha-2 macroglobulin in Alzheimer's disease: a marker of neuronal injury through the RCAN1 pathway[END_REF].

The cellular players of neuroinflammation i.e. microglia, the resident immune-competent brain cells, and astrocytes release a plethora of inflammatory molecules such as proinflammatory cytokines, prostanoids, nitric oxide and other cytotoxic molecules when activated. The agenda of glial activation in the course of AD is still a matter of debate, but it is doubtless that Aβ is a potent neuroinflammatory molecule which activates glial cells and thereby exacerbates the neuroinflammatory response (Heneka et al., 2015). In the present work, hallmarks of activated glial cells were evidenced in APP/PS1 females and appeared much earlier than in age-matched APP/PS1 males. In APP/PS1 females, abundant hypertrophic astrocytes (see [START_REF] Schiweck | Important Shapeshifter: Mechanisms Allowing Astrocytes to Respond to the Changing Nervous System During Development, Injury and Disease[END_REF] and ameboïd microglia were densely expressed in the stratum moleculare of the DG, a region affected early by the spatiotemporal progression of the disease [START_REF] Hurtado | Ab accelerates the spatiotemporal progression of tau pathology and augments tau amyloidosis in an Alzheimer mouse model[END_REF]. Activated glial cells were distributed in the stratum moleculare and densely packed around Aβ aggregates to form Aβ plaques.

Sex-differences in hippocampal-dependent memory tasks

Our results show that APP/PS1 females exhibit much earlier spatial memory impairments than age-matched males. In order to relate hippocampal Aβ load and neuroinflammatory glial activation, we chose to compare mnemonic performances of APP/PS1 females and males in two behavioral tasks (contextual fear conditioning and object location task) testing hippocampal-dependent memory. APP/PS1 females were impaired as early as 4 months of age, i.e. an age for which there are only very few Aβ plaques, whereas age-matched males were spared. Remarquably, APP/PS1 males displayed unaltered CFC behavior until at least 12 months of age, despite a noticeable hippocampal Aβ plaques load (Maingret et al., 2017).

As noticed by Richetin et al (2017) and strenghened by our study, it suggests that hippocampal-dependent memory deficits were not directly linked to Aβ plaques but more likely to the presence of Aβ peptides. Interestingly, it has also been suggested that impaired synaptic integration of disabled adult-born neurons related to the neuroinflammatory activation of astrocytes in APP/PS1 females could be involved in those hippocampaldependent memory deficits (Richetin et al., 2017).

Electrophysiological properties of type 1 and type 2 DG granule cells in WT and APP/PS1 mice

Our results show, supported by previous studies, that granule cells in the DG can be segregated into two distinct populations i.e. type 1 and type 2, with different intrinsic properties, based on R in and firing patterns [START_REF] Schmidt-Hieber | Enhanced synaptic plasticity in newly generated granule cells of the adult hippocampus[END_REF]Mongiat et al., 2009;[START_REF] Nenov | Impaired firing properties of dentate granule neurons in an Alzheimer's disease animal model are rescued by PPARγ agonism[END_REF]. Type 1 DG cells are considered to be young neurons whereas type 2 cells correspond to more mature cells [START_REF] Van Praag | Functional neurogenesis in the adult hippocampus[END_REF]Mongiat et al., 2009;[START_REF] Spampanato | Properties of doublecortin expressing neurons in the adult mouse dentate gyrus[END_REF]. Remarquably, the proportion of type 1 and type 2 was not affected by the genotype in contrast to what was reported by [START_REF] Nenov | Impaired firing properties of dentate granule neurons in an Alzheimer's disease animal model are rescued by PPARγ agonism[END_REF]. Type 1 DG cells had a high R in and did not fire APs throughout the entire duration of the current step whether type 2 DG cells has a low R in but fired APs without failure during the 250 ms current step. Notably, we observed no differences in R in due to genotype whereas [START_REF] Nenov | Impaired firing properties of dentate granule neurons in an Alzheimer's disease animal model are rescued by PPARγ agonism[END_REF] reported a significant difference of R in for type 2 DG cells between WT and single transgenic Tg2576 mice.

We have no firm explanation for these discrepancies but they might be related to differences between Nenov's data and ours. First, in our study, we were using only female mice whether male and female were used indifferently in Nenov's study. Second the transgenic mouse model was different (single transgenic Tg2576 for [START_REF] Nenov | Impaired firing properties of dentate granule neurons in an Alzheimer's disease animal model are rescued by PPARγ agonism[END_REF]; double transgenic APP/PS1 in our work) and last the age was also different (9 months old mice for [START_REF] Nenov | Impaired firing properties of dentate granule neurons in an Alzheimer's disease animal model are rescued by PPARγ agonism[END_REF]; 6 months old mice in our work). Interestingly, [START_REF] Nenov | Impaired firing properties of dentate granule neurons in an Alzheimer's disease animal model are rescued by PPARγ agonism[END_REF] suggested that differences observed in WT vs Tg2576 DG cells could be due to a decrease in proliferation, survival, maturation or integration of newborn DG neurons within DG. If true, the younger age of our mice (6 months vs 9 months in Nenov's study) could blunt differences due to these mechanisms. Worthy of note, the estrous cycle is known to alter hippocampal spinogenesis and spine densities [START_REF] Gould | Gonadal steroids regulate dendritic spine density in hippocampal pyramidal cells in adulthood[END_REF][START_REF] Hara | Estrogen effects on cognitive and synaptic health over the lifecourse[END_REF] and adult-born DG cells in APP/PS1 female mice at older age than in the present study (i.e. 7-9 months of age) display severely impaired dendritic spine density contributing to reduced survival rate (Richetin et al., 2015).

Differences in the plasticity of intrinsic excitability in DG granule cells between WT and APP/PS1 female mice

Whatever the intensity of the current steps, APP/PS1 DG cells fired significantly more spikes than WT DG cells for both type 1 and type 2 hence revealing an increased excitability of DG cells in APP/PS1 female mice. An increased excitability, although not statistically significant, was also reported for Tg2576 type 2 DG cells [START_REF] Nenov | Impaired firing properties of dentate granule neurons in an Alzheimer's disease animal model are rescued by PPARγ agonism[END_REF]. To caracterize further the firing pattern, we examined both the instantaneous firing frequency by measuring the interspike interval (ISI) during the first three intervals and the spike frequency adaptation by measuring ISI for the last three intervals for type 1 and type 2 DG cells. Type 1 APP/PS1 DG cells showed a significant reduction in ISI during the first three intervals compared to WT but not for type 2. No differences in spike frequency adaptation was observed for type 2 DG cells between genotypes.

The distinct firing patterns of type 1 and type 2 DG cells imply a diversity of underlying voltage-dependent ion channels and their different contribution may be related to the observed changes in firing frequency in APP/PS1 DG cells. Hence, a decreased expression of somatic Na + channels produced an increase of the instantaneous spiking frequency in PS/APP double transgenic mice (Brown et al., 2011). Related synaptic mechanims may also impact neuronal excitability such as a relative decrease in GABAergic tonus reported in cortical neurons from APP23/PS45 double transgenic mice [START_REF] Busche | Clusters of hyperactive neurons near amyloid plaques in a mouse model of Alzheimer's disease[END_REF] but unlikely to be involved in our experimental conditions always in the presence of bicuculline.

Synaptic plasticity and intrinsic excitability in DG granule cells of WT vs APP/PS1 females

The difference in the input-output relationships between WT and APP/PS1 at the LPP-DG synapse lead us to investigate if the synaptic content in AMPA and NMDA receptors were affected by the genotype. Both the AMPA/NMDA ratio and the AMPA rectification index were similar between genotypes suggesting neither major change in NMDA receptor activity nor in AMPA receptor function. Strikingly, the frequency of NMDA minis but not AMPA minis was increased in APP/PS1 DG granule cells. The most reasonable explanation for this increased frequency of NMDA minis would be an increased number of silent synapses expressing NMDA receptors in APP/PS1 DG granule cells but devoid of functional AMPA receptors. Remarquably, an increase in hippocampal silent synapses in transgenic mouse models of AD, by removal and/or internalization of AMPA receptors, has been previously reported (Hsieh et al., 2006;Price et al., 2014, reviewed in Chater and[START_REF] Chater | The role of AMPA receptors in postsynaptic mechanisms of synaptic plasticity[END_REF], although in our conditions we found no decrease in AMPA receptor mediated synaptic transmission.

Our data show a strong impairment of HFS-induced potentiation of the population spike amplitude in APP/PS1 female mice, when compared to aged-matched WT female mice. Thus it is important to unravel the intermingled relationships between synaptic plasticity and plasticity of intrinsic excitability of DG granule cells to decipher how they might impact each other. The LTP protocol induced a prominent modulation of the E-S curve linking EPSPs slope to the probability of firing an action potential. HFS elicited significant shifts to the left for both genotype with a larger shift for APP/PS1 vs WT female mice.

Taken together, these results indicated that synaptic potentialisation elicited changes in intrinsic excitability of DG granule cells with a larger increase for APP/PS1 vs WT. Strikingly, the increase in CA1 neuronal activity induced by Aβ oligomers impaired by half the potentialisation of population spike amplitude [START_REF] Lei | Soluble Aβ oligomers impair hippocampal LTP by disrupting glutamatergic/GABAergic balance[END_REF].

Interestingly, we did not observe any significant changes in the threshold for action potentials and only a fairly minor change (less than 3 mV) in the resting membrane potentials that could account for excitability changes. However, we observed a significant difference in the EPSP amplitude/slope ratio before and after HFS for APP/PS1 but not WT DG cells. As EPSP slope was measured during the first two milliseconds which reflects mainly the AMPA component of synaptic transmission, these data suggested that changes in dendritic processing of synaptic input in APP/PS1 DG cells could account for excitability changes. Interestingly, changes in intrinsic excitability of DG cells have been recently attributed to the modulation of dendritic A-type potassium channels [START_REF] Lopez-Rojas | Plasticity of intrinsic excitability in mature granule cells of the dentate gyrus[END_REF] and dendritic hyperexcitability induced by Kv4.2 deficiencies in the hippocampal CA1 region was reported in a single transgenic (hAPP) mouse model of AD [START_REF] Hall | Tau-dependent Kv4.2 depletion and dendritic hyperexcitability in a mouse model of Alzheimer's disease[END_REF].

Prostanoïds and LTP impairment in APP/PS1 females

In a previous work, we reported that PGE 2 -EP3 signaling pathway was impairing hippocampal presynaptic LTP at the mossy fiber -CA3 synapse in aged APP/PS1 males (Maingret et al., 2017). In the present work, neither PGE 2 -EP2/EP4 nor PGE 2 -EP3 pathways were involved in the impairment of PSA-LTP at LPP-DG synapses. Furthermore, in contrast with our previous study (Maingret et al., 2017), hippocampal levels of PGE 2 were not increased with time in APP/PS1 vs WT females. Interestingly, deficiency of estrogen augments PGE 2 levels in ovariectomized female rats (Wang et al., 2014) which suggests that female sex hormones exert a tonic brake on PGE 2 levels and might explain why PGE 2 levels

were not altered in APP/PS1 vs WT females.

In conclusion, neuroinflammatory hallmarks of AD are more precocious in double transgenic female mouse model of AD when compared to age-matched males. These histopathological alterations are concomittant of strong and specific impairment of hippocampal-dependent memory task together with non-synaptic and synaptic modifications at the hippocampal PP-DG circuitry. Hence, our results are clear evidence that 1 -results originated from studies using males and females together must be taken with extreme caution as males and females clearly behave differently, 2 -depending on the hippocampal region and on the gender, mechanisms responsible for LTP impairment are different and 3 -more studies aiming at deciphering mechanisms on mouse models of AD in females are most wanted.

B. APP/PS1 female mice do not present impaired presynaptic LTP at Mf-CA3 synapses

In a previous study we reported that presynaptic LTP at hippocampal MF/CA3 synapses was impaired in APP/PS1 males (Maingret et al., 2017). We then investigated if the presynaptic LTP at Mf-CA3 synapses was also impaired in APP/PS1 female mice by recording fEPSPs. fEPSPs were evoked by minimal stimulation intensity at low frequency (0.1 Hz). The fEPSP slope is taken as a proxy to measure the excitatory drive to CA3 neurons.

Basal fEPSP slope were measured as 99.99 ±3.75 % for WT female mice and 99.99 ± 3.75 % for APP/PS1 female mice at 4 months and were not significantly different. Long term synaptic potentiation (LTP) was triggered by high frequency tetanic stimulation (HFS, 3 x 100 Hz, every 10 s). The relative change in the fEPSP slope was 168.8 ± 13.59 % (n=8) of baseline during 30-40 min after HFS for WT and 154.3 ± 17.4 % (n=8, p > 0.05; Fig. 2A) for APP/PS1. The total blockade of fEPSPs by mGluR agonist LCCG-1 confirmed the recording of Mf-CA3 synaptic transmission (Fig. 2A).

Then we questioned whether LTP was altered in female mice at 6 months, an age for which both molecular and cellular hallmarks of AD were present, together with behavioral deficits. LTP was 163.8 ± 6.3 % (n=8) for WT and 148.78 ± 5.52 % (n=8, p > 0.05) for APP/PS1 (Fig. 2B).

Therefore, Mf-CA3 synaptic plasticity was not impaired in APP/PS1 female mice at 4 and 6 months of age despite prominent AD's alterations.

C. Impaired LTP of population spikes in APP/PS1 mice at LPP/DG synapses

Then we investigated if synaptic potentiation was affected in APP/PS1 female mice at an other synapse. We chose to investigate the lateral perforant pathway -DG synapse because of its location in the stratum moleculare of the DG, a region displaying a massive load of Aβ plaques at 6 months of age. To do so, we measured the population spike amplitude (PSA), which reflects APs discharge, between the upper positive deflection and the lower negative deflection (Fig. 3A). The basal stimulation intensity was chosen to evoke 25-30% of maximal PSA. This allowed us to compare the response of neurons in DG to the same excitatory drive before and after HFS (Fig. 3A). The relative potentiation of PSA was 186.6 ± 14.33 % (n=17)

for WT and significantly impaired in APP/PS1 (146.5 ± 8.52 %, n=16, * p < 0.05; Fig. 3B).

Interestingly, LTP in WT male mice was significantly lower than in age-matched WT female mice (male: 149.8 ± 9.39 %, n=15; female: 186.6 ± 14.33 %, n=17, * p < 0.05). LTP was purely NMDA-dependent as completely blocked by AP-V (Fig. 3B).

D. PGE 2 and LTP at LPP-DG synapses a. PGE 2

PGE 2 is a key neuroinflammatory molecule and of particular interest in AD studies (McGeer and McGeer, 2007). Having shown that PGE 2 impaired synaptic plasticity at the Mf-CA3 synapse (Maingret et al., 2017), we investigated the effects of PGE 2 at LPP-DG synapse during the process of the neuroinflammation in APP/PS1 female mice.

Bath application of PGE 2 (10 µM) had no effect on synaptic basal transmission in WT female mice at LPP-DG synapses (baseline: 99.9 ± 1.77%, n=8; PGE 2 : 99.58 ± 1.04%, n=8, p > 0.05; Fig. 4). These data indicated that exogenous application of PGE 2 did not affect basal transmission (Fig. 4 B).

E. APs number in APP/PS1 vs WT female mice

At the maximal current intensity (200 pA) and for a 250 ms step, we studied the number of spikes during the first and the last 50 ms of the step (Fig. 6A). During the first 50 ms, type I DG cells (WT: 2.92 ± 0.21, n=13; APP/PS1: 3.70 ± 0.20, n=17; * p < 0.05) but not type II DG cells (WT: 2.94 ± 0.24, n=17; APP/PS1: 3.50 ± 0.20, n=14) showed a significant increase of spikes number in APP/PS1 vs WT mice (Fig. 6B). For the last 50 ms, both type I and type II DG cells did not show significant spikes number between WT and APP/PS1 mice (type I WT: 0.53 ± 0.14, n=13; type I APP/PS1: 1 ± 0.24, n=17; type II WT: 1.23 ± 0.13, n=17; type II APP/PS1: 1.57 ± 0.17, n=14) (Fig. 6C). However, WT type II DG cells significantly fired more spikes than type I DG cells (Type I: 0.53 ± 0.14, n=13; Type II: 1.23 ± 0.13, n=17; ** p < 0.05) (Fig. 6C).

IV. Discussion -Conclusion

Gender Differences in LTP of population spike amplitude (PSA)

The physiological basis underlying gender differences is still elusive, but it is known for a long time that estrogen modulates neuronal cell activity in the hippocampus [START_REF] Teyler | Gonadal steroids: effects on excitability of hippocampal pyramidal cells[END_REF][START_REF] Foy | 17-α-estradiol and 17-β-estradiol in hippocampus[END_REF][START_REF] Wong | Long-term and short-term electrophysiological effects of estrogen on the synaptic properties of hippocampal CA1 neurons[END_REF]. As such, the significant larger increase in LTP in WT female mice vs male mice could be related to estrogen levels.

Interestingly, in female rats, a large LTP was observed during proestrus when estrogen was expressed at a high level [START_REF] Warren | LTP varies across the estrous cycle: enhanced synaptic plasticity in proestrus rats[END_REF]. Furthermore, HFS did not induce LTP in males as large as in females when estrogen levels were low. In ovariectomized rats, administration of 17-β-estradiol promoted induction of LTP, showing that estrogen could indeed modulates LTP [START_REF] Montoya | Estrogen facilitates induction of long term potentiation in the hippocampus of awake rats[END_REF]. Our results showed that WT female mice displayed significantly larger LTP when compared to males. Because all experiments have been done without consideration of the estrous cycle, we cannot conclude on a possible effect of estrogen but such effect is plausible and may deserve further investigation.

Aβ impaired LTP at LPP-DG synapse

In APP/PS1 female mice, DG region displayed a massive load of Aβ plaques when compared to other hippocampal regions. Entorhinal cortex is the main input to hippocampus via DG for new memory encoding, and mostly prone to dysfunction in early AD. Evidences suggest that Aβ may affect neuronal activity [START_REF] Cirrito | Synaptic activity regulates interstitial fluid amyloid-β levels in vivo[END_REF] and affect LTP [START_REF] Wang | β-amyloid-mediated inhibition of NMDA receptor-dependent long-term potentiation induction involves activation of microglia and stimulation of inducible nitric oxide synthase and superoxide[END_REF][START_REF] Gong | Ubiquitin hydrolase Uch-L1 rescues β-amyloid-induced decreases in synaptic function and contextual memory[END_REF][START_REF] Ahmed | Curcuminoids enhance memory in an amyloidinfused rat model of Alzheimer's disease[END_REF]. Our results showed the partial impairment of LTP in APP/PS1 female mice of 6 months of age at LPP-DG synapses. Many factors could contribute to this impairment, a decrease in the probability of transmitter release, a decrease in the reliability or the strength of individual synapses, a decrease in the number of functional synapses or receptors.

PGE 2

PGE 2 is of particular interest because its level is increased in the CSF of AD patients (Montine et al., 1999;[START_REF] Combrinck | Levels of CSF prostaglandin E2, cognitive decline, and survival in Alzheimer's disease[END_REF]. PGE 2 can bind to four subtypes of receptors (EP1-4) which are all expressed in the hippocampus and its effects may vary accordingly.

PGE 2 is produced mostly from COX-2 signaling pathway [START_REF] Brock | Arachidonic acid is preferentially metabolized by cyclooxygenase-2 to prostacyclin and prostaglandin E2[END_REF][START_REF] Vidensky | Neuronal overexpression of COX-2 results in dominant production of PGE 2 and altered fever response[END_REF]Chen and Bazan, 2005) and it is an important modulator of hippocampal synaptic transmission and plasticity [START_REF] Bazan | COX-2 as a multifunctional neuronal modulator[END_REF]Chen et al., 2002;Chen and Bazan, 2005).

Here we show that PGE 2 via the activation of EP3 receptors inhibits a presynaptic form of LTP at Mf-CA3 synapse whereas it had no effects on LTP at LPP-DG synapse whatever the subtype of EP receptor. Furthermore, to our surprise, hippocampal levels of PGE 2 were not increased with time in APP/PS1 females vs WT females.

Aβ and intrinsic excitability

Series of studies show that Aβ accumulation and abnormal excitatory neuronal activity are associated [START_REF] Buckner | Molecular, structural, and functional characterization of Alzheimer's disease: evidence for a relationship between default activity, amyloid, and memory[END_REF][START_REF] Mintun | 11C]PIB in a nondemented population: potential antecedent marker of Alzheimer disease[END_REF][START_REF] Seeley | Neurodegenerative diseases target large-scale human brain networks[END_REF][START_REF] Sperling | Amyloid deposition is associated with impaired default network function in older persons without dementia[END_REF] but the molecular mechanisms are still illdefined.

It has been demonstrated that over activation of NMDA receptor has a crucial effect on increased excitability, which is thought to play a key role in AD. More specifically, Aβ oligomers has been illustrated to enhance NMDA receptor mediated currents in rat DG [START_REF] Wu | beta-Amyloid selectively augments NMDA receptor-mediated synaptic transmission in rat hippocampus[END_REF] and to enhance NMDA receptor mediated hippocampal neuronal firing in vivo (Molnár et al., 2004). The interaction of Aβ oligomers with NMDA receptor promotes a Ca 2+ influx [START_REF] De Felice | Abeta oligomers induce neuronal oxidative stress through an N-methyl-D-aspartate receptor-dependent mechanism that is blocked by the Alzheimer drug memantine[END_REF] which induces an increase of intracellular calcium stocks and in turn the activation of ryanodine receptor (RyR) mediated Ca 2+ induced Ca 2+ release (CICR) from endoplasmic reticulum stores (Lima et al., 2011). Ultimately, this lead to persistant neuronal depolarization and the activation of synaptotoxic calcium associated pathways [START_REF] Ferreira | The Abeta oligomer hypothesis for synapse failure and memory loss in Alzheimer's disease[END_REF].

In conclusion, the amyloid hypothesis of AD postulates that accumulation of Aβ in the brain is the primary influence driving AD pathogenesis [START_REF] Selkoe | Toward a comprehensive theory for Alzheimer's disease. Hypothesis: Alzheimer's disease is caused by the cerebral accumulation and cytotoxicity of amyloid β-protein[END_REF][START_REF] Cirrito | Synaptic activity regulates interstitial fluid amyloid-β levels in vivo[END_REF][START_REF] Hu | Extracellular Forms of Aβ and Tau from iPSC Models of Alzheimer's Disease Disrupt Synaptic Plasticity[END_REF]. The formation of Aβ is the result of cellular process via successive enzymatic cuts of APP by β-secretase and γ-secretase (Querfurth et al., 2010). In the present work we have highlighted the presence of Aβ plaques in the hippocampus in both APP/PS1 male and female mice and demonstrated that the deposition was occuring earlier and heavier in female mice than in age-matched male mice.

Numerous publications have reported that Aβ was inducing synaptic dysfunction through changes of neuronal intrinsic excitability in AD brain [START_REF] Zhang | The other side of the engram: experience-driven changes in neuronal intrinsic excitability[END_REF].

Hence, application of soluble Aβ to cultured neurons [START_REF] Cuevas | Soluble Aβ 1-40 Peptide Increases Excitatory Neurotransmission and Induces Epileptiform Activity in Hippocampal Neurons[END_REF], brain slices [START_REF] Minkeviciene | Amyloid beta-induced neuronal hyperexcitability triggers progressive epilepsy[END_REF][START_REF] Varga | Abeta(1-42) enhances neuronal excitability in the CA1 via NR2B subunit-containing NMDA receptors[END_REF][START_REF] Ren | Persistent sodium currents contribute to Abeta1-42-induced hyperexcitation of hippocampal CA1 pyramidal neurons[END_REF] or in vivo (Orbán et al., 2010;[START_REF] Busche | Critical role of soluble amyloid-β for early hippocampal hyperactivity in a mouse model of Alzheimer's disease[END_REF], induced neuronal hyperexcitability in the hippocampus. These data are consistent with some AD mouse models for which increased Aβ levels are involved in altered neuronal activity, spontaneous activity and hyperexcitability activity [START_REF] Palop | Aberrant excitatory neuronal activity and compensatory remodeling of inhibitory hippocampal circuits in mouse models of Alzheimer's disease[END_REF]Palop and Mucke, 2010;Brown et al., 2011;[START_REF] Kerrigan | Characterization of altered intrinsic excitability in hippocampal CA1 pyramidal cells of the Abeta-overproducing PDAPP mouse[END_REF]. Alterations of synaptic activity is considered as an early event leading to cognitive deficits appearing in the course of AD [START_REF] Mesulam | Neuroplasticity failure in Alzheimer's disease: bridging the gap between plaques and tangles[END_REF][START_REF] Srivareerat | Chronic psychosocial stress exacerbates impairment of cognition and long-term potentiation in beta-amyloid rat model of Alzheimer's disease[END_REF][START_REF] Srivareerat | Chronic nicotine restores normal Abeta levels and prevents short-term memory and E-LTP impairment in Abeta rat model of Alzheimer's disease[END_REF][START_REF] Alkadhi | Intensification of long-term memory deficit by chronic stress and prevention by nicotine in a rat model of Alzheimer's disease[END_REF][START_REF] Alkadhi | Chronic psychosocial stress exacerbates impairment of synaptic plasticity in beta-amyloid rat model of Alzheimer's disease: prevention by nicotine[END_REF].

In the hippocampus, DG granule cells are susceptible to AD pathology (A.T. [START_REF] Dao | Comparison of the Effect of Exercise on Late-Phase LTP of the Dentate Gyrus and CA1 of Alzheimer's Disease Model[END_REF]. Following these findings, our data showed that in the DG area, the depostion of Aβ plaques paralleled a decreased excitability of APP/PS1 female DG granule cells detected

shown by the input/output (I/O) relationships. Hippocampal LTP is generally accepted to be an important molecular mechanism related to learning and memory processes [START_REF] Abraham | Properties and mechanisms of LTP maintenance[END_REF][START_REF] Malenka | LTP and LTD: an embarrassment of riches[END_REF]. It has been proposed that depression of basal synaptic transmission and LTP reported in mouse models of AD could be related to the activation of presynaptic Ca 2+ activated K + channels, which modifies in turn excitability and transmitter release [START_REF] Ye | Activation of large-conductance Ca(2+)-activated K(+) channels depresses basal synaptic transmission in the hippocampal CA1 area in APP (swe/ind) TgCRND8 mice[END_REF].

The present study showed an increased intrinsic excitability and LTP impairment at LPP/DG synapse in a mouse model of AD. Although increased excitability activity and its cellular and functional consequences are typically reported in mouse models of AD, the mechanisms of Aβ-induced neuronal hyperexcitability remain to be clarified. Neuronal brain activity is also tightly controlled via a balance between excitation and inhibition neurotransmission and an imbalance may lead to hyperexcitability. NMDARs play an essential role in the hyperexcitability, which is also considered to be a potential target of AD.

Abnormal firing activity can seriously weaken or impair LTP in vitro [START_REF] Schubert | Kindling-induced changes in plasticity of the rat amygdala and hippocampus[END_REF][START_REF] Salmani | Epileptogenic insult alters endogenous adenosine control on long-term changes in synaptic strength by theta pattern stimulation in hippocampus area CA1[END_REF]. Interestingly, LTP and increased excitability activity may crossover. Therefore, attenuating excessive neuronal excitability activity could reverse HFS induced LTP impairment [START_REF] Albensi | Electrical stimulation protocols for hippocampal synaptic plasticity and neuronal hyper-excitability: are they effective or relevant?[END_REF]. According to these reports, administration of levetiracetam prevented aberrant network activity in hAPP mice and restored learning and memory function [START_REF] Sanchez | Levetiracetam suppresses neuronal network dysfunction and reverses synaptic and cognitive deficits in an Alzheimer's disease model[END_REF] and had similar actions in patients with mild cognitive impairments (MCI) that have significant brain Aβ deposition [START_REF] Bakker | Reduction of hippocampal hyperactivity improves cognition in amnestic mild cognitive impairment[END_REF].

Collectively, our data show that Aβ-induced excessive excitatory activity in the hippocampus may have a crucial role in the development of early synaptic functional deficits in AD.

V. Perspectives

Although gender differences have been reported for many years, our understanding of the potential bases underlying these differences is relatively poor Joseph et al., 1978;Williams et al., 1990;Roof et al., 1993;Gresack and Frick, 2003;Clinton et al., 2007;Richetin et al., 2017). Clearly, more research is needed to investigate the etiology of sex differences in memory and synaptic functions.

Our studies may help to clarify that Aβ may affect hippocampal-dependent memory task and neuronal activity in a sex dimorphic manner. These findings could have significant benefits for a differential preventive treatment of the human pathology (Claxton et al., 2013;Zissimopoulos et al., 2017;Ferretti et al., 2018). The complex network of gender differences should be taken into account when designing models for disease prevention and diagnosis as well.

To further address Aβ effects on neuronal activity, it would be necessary to evaluate it taking into consideration distances between recorded neuron and Aβ plaques [START_REF] Busche | Clusters of hyperactive neurons near amyloid plaques in a mouse model of Alzheimer's disease[END_REF]Ovsepian et al., 2016). In this respect, Ab form is an important issue (oligomers vs plaques) as soluble Aβ oligomers are described to impact more severely synaptic and cognitive dysfunction than plaques (Selkoe et al., 2008;[START_REF] Shankar | Amyloid-β protein dimers isolated directly from Alzheimer's brains impair synaptic plasticity and memory[END_REF]Mucke and Selkoe, 2012). Therefore, it will be relevant to assay Aβ40/Aβ42 levels in hippocampal homogenates from APP/PS1 females vs males and to compare them to the number of Ab plaques.

Although the putative effects of Aβ on LTD have not been studied in the present work, it is known that Ab may facilitate LTD [START_REF] Snyder | Regulation of NMDA receptor trafficking by amyloid-β[END_REF][START_REF] Li | Soluble oligomers of amyloid β protein facilitate hippocampal long-term depression by disrupting neuronal glutamate uptake[END_REF]Chen et al., 2013) by involving mGluR, p38MAPK and caspase-3 activities. Strikingly, same Ab -activated intracellular signaling pathways are involved in the blockade of hippocampal LTP [START_REF] Wang | β-amyloid-mediated inhibition of NMDA receptor-dependent long-term potentiation induction involves activation of microglia and stimulation of inducible nitric oxide synthase and superoxide[END_REF] which suggests that a crossover pathway could reinforce Ab induced LTP impairments (reviewed in Mucke and Selkoe, 2012).

Whereas we have evaluated how the intrinsic properties of type 1 and type 2 DG cells were affected in APP/PS1 female mice, we have not investigated if the impaired LTP was different depending on the subtype of DG cells. This is an important issue as type 1 and type 2 are signatures of the degree of maturation and circuitry integration of DG cells which can be differentially impacted by the pathology.

So far, almost all clinical trials have failed and the number of AD patients is continuously growing. Therefore, there is an urgent need for new studies. Deciphering relationships between Aβ-associated aberrant excitatory neuronal activity and cognitive deficits in AD could present valuable insights into the mechanisms of early stages of AD to lead to new therapeutic methods.

Indeed, disruptions of the balance between excitatory and inhibitory activity are likely involved in the early processes of cognitive deficits in AD [START_REF] Palop | Aberrant excitatory neuronal activity and compensatory remodeling of inhibitory hippocampal circuits in mouse models of Alzheimer's disease[END_REF]Leonard and McNamara, 2007). Here, we focused only on excitatory inputs onto granule cells in the DG but it would be necessary to study inhibitory activity that was also reported to be associated with impairments of both short and long term plasticity in DG [START_REF] Palop | Aberrant excitatory neuronal activity and compensatory remodeling of inhibitory hippocampal circuits in mouse models of Alzheimer's disease[END_REF]. As such, prominent increases of neuronal activity in hippocampal networks may possibly coexist with altered glutamatergic transmission in our mouse model of AD and the complex relationships between excitatory and inhibitory activity need to be further clarified.

Introduction

Alzheimer's disease (AD) is a progressive and devastating neurodegenerative disease characterized by deficits in learning and memory processes. There is considerable evidence that neuroinflammation involving the activation of glial cells contributes to the disease progression and pathology (Akiyama et al., 2000;Heneka and O'Banion, 2007). As such, epidemiological studies have revealed that chronic intake of nonsteroidal anti-inflammatory drugs (NSAIDs) reduced the prevalence of AD (McGeer and McGeer, 2007). Furthermore, clinical trials have revealed that NSAIDs, when given to asymptomatic patients, reduce AD incidence, whereas they have adverse effect on AD pathogenesis in its later stages (Breitner et al., 2011).

When activated in neuroinflammatory conditions, glia release a plethora of neuroinflammatory molecules such as inflammatory cytokines, chemokines, and prostanoids [START_REF] Eikelenboom | Inflammatory mechanisms in Alzheimer's disease[END_REF]. The prostanoid prostaglandin E 2 (PGE 2 ) plays pivotal functions in inflammation [START_REF] Bos | Prostanoids and prostanoid receptors in signal transduction[END_REF]. PGE 2 is produced from arachidonic acid by the microsomal prostaglandin-E2 synthase (mPGES) and by 2 ratelimiting enzymes, Cox-1 and Cox-2 (Smith et al.,1991), which are expressed by hippocampal neurons (Yasojima et al., 1999), astrocytes and microglia (Font-Nieves et al., 2012). Although Cox-1 is constitutively expressed, Cox-2 and mPGES are strongly activated by and during neuroinflammation (Font-Nieves et al., 2012). PGE 2 can bind to 4 different subtypes of G-proteinecoupled receptors (EP1-4), which regulate adenylyl cyclase (AC) activity and/or phosphoinositol turnover and intracellular calcium mobilization (Breyer et al., 2001). PGE 2 plays an important role in the pathophysiology of AD. It is a primary target of NSAIDs, and its level is elevated together with the expression of Cox-2 in the brain of AD patients [START_REF] Kitamura | Increased expression of cyclooxygenases and peroxisome proliferator-activated receptorgamma in Alzheimer's disease brains[END_REF]Montine et al.,1999;Yasojima et al.,1999). Both in vitro and in vivo, PGE 2 stimulates amyloid beta (Aß) production by microglial cells, astrocytes, and neurons [START_REF] Hoshino | Involvement of prostaglandin E2 in production of amyloid-beta peptides both in vitro and in vivo[END_REF]. Selective inhibition of Cox-2 by NS398 acutely prevents the Aß-induced impairment of longterm potentiation (LTP) of a-amino-3-hydroxy-5methyl-4isoxazolepropionic acid receptorsemediated synaptic transmission at CA3-CA1 synapses [START_REF] Kotilinek | Cyclooxygenase-2 inhibition improves amyloid-beta-mediated suppression of memory and synaptic plasticity[END_REF]. Exogenous addition of PGE 2 did not impair by itself LTP inwild-type mice but preventedthe ability of NS398 to restore LTP impairment induced by synthetic Aß suggesting complex interactions between PGE 2 , Aß, and postsynaptic forms of LTP [START_REF] Kotilinek | Cyclooxygenase-2 inhibition improves amyloid-beta-mediated suppression of memory and synaptic plasticity[END_REF]. AD, at least in its early stage, is thought to involve synaptic dysfunction and loss [START_REF] Jacobsen | [END_REF]Selkoe, 2002;Sheng et al., 2012). Connections between the dentate gyrus (DG) and the hippocampal CA3 region through mossy fiber (Mf) synapses have been proposed to participate in the rapid encoding of novel memory (Kesner, 2007), a mnemonic process particularly affected in AD conditions. Mf synapses onto CA3 pyramidal cells display a wide dynamic range of presynaptic plasticity, including prominent shortterm plasticity and a form of LTP that is independent of N-methylDaspartate (NMDA) receptors [START_REF] Henze | Single granule cells reliably discharge targets in the hippocampal CA3 network in vivo[END_REF][START_REF] Salin | Distinct short-term plasticity at two excitatory synapses in the hippocampus[END_REF]. Alterations in Mf synaptic function in the context of AD have only been studied in senescent (24-to 25-month old) Tg2576 mice that is long after a massive load of amyloid plaques (Witton et al., 2010).

Here we thus studied Mf-CA3 synaptic function in the APPswe/ PS1ΔE9 (APP/PS1) mouse model of AD in relation with PGE 2 . APP/ PS1 mice are characterized by an over-production of Aß protein leading to a noticeable load of amyloid plaques around 12 months of age (Jankowsky et al., 2004) and display synaptic dysfunction [START_REF] Volianskis | Episodic memory deficits are not related to altered glutamatergic synaptic transmission and plasticity in the CA1 hippocampus of the APPswe/PS1dE9-deleted transgenic mice model of ß-amyloidosis[END_REF] and learning deficits [START_REF] Lagadec | Early temporal short-term memory deficits in double transgenic APP/PS1 mice[END_REF][START_REF] Reiserer | Impaired spatial learning in the APPSwe + PSEN1DeltaE9 bigenic mouse model of Alzheimer's disease[END_REF]. We report that acute application of PGE 2 impairs presynaptic Mf-LTP in young mice through activation of EP3 receptors. In APP/PS1 mice, presynaptic Mf-LTP was noticeably impaired at 12 months of age in parallel with a building up of endogeneous levels of PGE 2 in the hippocampus. Mf-LTP could be fully rescued by blockade of EP3 receptors. Considering the diverse physiological roles of PGE 2 , directly targeting the EP3 receptor may prove to be a more specific therapeutical strategy in AD than the global inhibition of prostaglandins by NSAIDs.

Materials and methods

Ethical approval

Animal anesthesia and euthanasia procedures were carried out in accordance with the Animal Protection Association of ethical standards and the French legislation concerning animal experimentation and were approved by the University of Bordeaux/CNRS Animal Care and Use Committee (#55).

Animals

The animals used in this study were male APP/PS1 mice obtained from Jackson Laboratory (Bar Harbor, ME, USA) and their wild-type (WT) littermates (C57BL6/J). The APP/PS1 mice express a chimeric mouse/human amyloid precursor protein APPswe (mouse APP695 harboring a human Aß domain and mutations K595N and M596L linked to Swedish familial AD) and a human presenilin 1 mutated in familial AD (PS1ΔE9; deletion of exon 9). These bigenic mice were created by co-injection of both trangenes allowing for a co-segregation of the transgenes as a single locus (Jankowsky et al., 2004). Mice were allowed free access to food and water and maintained in a 12 hours darkelight cycle. Mice were genotyped and systematically regenotyped after each experiment using transcript-specific primers (Table S1).

Acute hippocampal slices

C57BL6/J mice (P19-P21) were sacrificed by cervical dislocation. The brain was quickly removed from the skull and chilled in ice-cold low-calcium artificial CSF containing the following (in mM): 87 NaCl, 2.5 KCl, 1.25 NaH 2 PO 4 , 0.5 CaCl 2 , 7 MgCl 2 , 10 glucose, 25 NaHCO 3 and 75 sucrose with pH 7.4 adjusted by saturating with carbogen (95% O 2 and 5% CO 2 ), and an osmolarity of 300-310 mOsm. Isolated brains were glued onto the stage of a vibratome (VT 1200S, Leica Microsystems, Nussloch, Germany), and parasagittal hippocampal slices (320 µm) were cut. Slices were then transferred in an extracellular solution containing the following (in mM): 129 NaCl, 2.5 KCl, 1.25 NaH 2 PO 4 , 2 CaCl 2 , 1 MgCl 2 , 11 glucose, 25 NaHCO 3 equilibrated with 95% O 2 and 5% CO 2 (310-320 mOsm) for 30 minutes at 33 °C and thereafter maintained at room temperature until required.

Because of the well-documented difficulty in preparing healthy acute brain slices from mature adult rodents and in accordance with Animal Protection Association of ethical standards and the French legislation, APP/PS1 and WT mice at 6, 9, and 12 months were deeply anesthetized with a mix of ketamine (80 mg/kg; intraperitoneal) and xylazine (16 mg/kg; intraperitoneal) and then intracardially perfused for 2-3 minutes with a protective solution at 4°C containing the following (in mM): (310)(311)(312)(313)(314)(315)(316)(317)(318)(319)(320). When the solution coming out of the heart was free of blood, the mouse was sacrificed by decapitation, and the brain was quickly removed from the skull and stored in the same solution. Parasagittal brain slices (320 µm) were cut in the protective solution for which sodium chloride was replaced by sucrose, and then incubated at 33 C for 30 minutes in a resting solution containing the following (in mM): 103 NaCl, 2 KCl, 1.25 NaH 2 PO 4 , 0.27 CaCl 2 , 8 MgCl 2 , 22 glucose, 26 NaHCO 3 , 0.4 ascorbic acid equilibrated with 95% O 2 , and 5% CO 2 (300-310 mOsm). Slices were thereafter maintained at room temperature in the same solution until required.

Electrophysiological studies on acute hippocampal slices

For recording, slices were transferred into a recording chamber where they were submerged and continuously perfused with an oxygenated (95% O 2 and 5% CO 2 ) extracellular solution at 32 °C-34 °C. Whole-cell patch clamp recordings (3)(4) electrodes, 70 mV holding potential) were made from hippocampal CA3 pyramidal neurons visualized by infrared videomicroscopy. Patch clamp electrodes were pulled out from borosilicate glass (GF 150 F-10) and filled with an internal solution containing the following (in mM): 140 CsCH 3 SO 3 , 2 MgCl 2 , 4 NaCl, 5 phospho-creatine, 2 Na 2 ATP, 0.2 EGTA, 10 HEPES, and 0.33 GTP (pH 7.3) adjusted with CsOH. Bicuculline (10 µM) was added to the bath to inhibit g-aminobutyric acid-A receptors. Voltage-clamp recordings were performed on CA3 pyramidal neurons identified with a differential interference contrast microscope (Eclipse FN-1, Nikon, Champigny sur Marne, France) equipped with an infrared camera (VX 44, Till Photonics, Gräfelfing, Germany) using an Axopatch-200B amplifier (Axon Instruments, Sunnyvale, CA, USA). Signals were filtered at 2 kHz and digitized at 5 kHz via a DigiData 1322A interface (Axon Instruments). Series resistance (10-20 MΩ) was monitored during the recording using a 10 mV hyperpolarizing voltage step of 50 ms length occurring at the beginning of each recording. Neurons were rejected if more than a 20% change in series resistance occurred during the experiment. Neurons with a holding current exceeding 300 pA at a holding potential of -70 mV were also rejected. Data were collected and analyzed using pClamp software 9.2 (Axon Instruments).

Mf-CA3 excitatory postsynaptic currents (EPSCs) were evoked by minimal intensity stimulation [START_REF] Marchal | Postnatal maturation of mossy fibre excitatory transmission in mouse CA3 pyramidal cells: a potential role for kainate receptors[END_REF]. A glass microelectrode (~1 µm tip diameter) was placed in the hilus of the DG to stimulate Mfs. The baseline stimulation frequency for all experiments was 0.1 Hz. Mf synaptic currents were identified according to the following criteria: robust low-frequency facilitation (ratio 1/0.1 Hz), low release probability at 0.1 Hz, rapid rise times of individual EPSC ((~1 ms), and EPSCs decays free of secondary peaks that may indicate the presence of polysynaptic contamination. The group II mGluR agonist (2S,1'S,2'S)-2-(Carboxycyclopropyl) glycine (LCCG)-1 (10 µM) was applied routinely to confirm that Mfs were actually stimulated. Extracellular recordings were made from the stratum lucidum using glass electrodes (1-2 MΩ) filled with extracellular solution. For subsequent experiments on basal transmission and synaptic plasticities, the stimulus strength was set to a level that produced 50% of the maximal responses.

2.5. Tissue processing and laser capture microdissection C57/Bl6J mice (Jackson Laboratory, Bar Harbor, ME, USA) at P21 were anesthetized with intraperitoneal administration of pentobarbital (50 mg/kg body weight), and the brain was removed and rapidly frozen in heptane and stored at -80 °C until sectioning. For slicing, the frozen brain was mounted onto a chuck with Tissue-Tek O.C.T compound (Sakura Finetek France, Villeneuve-d'Ascq, France) and placed in a Leica CM3050S cryostat (Leica Microsystems, Wetzlar, Germany) for 20 minutes to equilibrate with the chamber temperature of 20 °C. Then, 60-µm-thick sections of the hippocampus region were cut and placed on polyethylene naphthalate (PEN) Membrane Frame slides (Carl Zeiss, Munich, Germany) under RNAase-free conditions. Then, the slides were immediately placed on dry ice and stained on the same day using cresyl violet. Briefly, slides were transferred from dry ice into ice-cold 95% ethanol for 40 seconds and incubated in 75% ethanol for 30 seconds and in 50% ethanol for 30 seconds. Specimens were briefly stained in 1% cresyl violet solution. Tissue sections were dehydrated through 50% ethanol (30 seconds), 75% ethanol (30 seconds), 95% ethanol (30 seconds), followed by two 40 seconds incubations in anhydrous 100% ethanol. Slides were dried for 5 minutes at room temperature. Immediately after dehydratation, laser capture microdissection (LCM) was performed using a P.A.L.M. MicroBeam microdissection system Version 4.8 equipped with a P.A.L.M. RoboSoftware (P.A.L.M. Microlaser Technologies AG, Bernried, Germany). Microdissection was performed at 5X magnification. Samples were collected in adhesives caps (P.A.L.M. Microlaser Technologies AG, Bernried, Germany).

2.6. RNA isolation, reverse transcription, and quantitative PCR To limit RNA degradation during isolation, samples were collected for up to 30 minutes per slide, after which the LCM caps were placed on a sterile microcentrifuge tube containing 350 µL of lysis buffer. The samples were stored at -80 °C until extraction was done. Total RNA was extracted from microdissected tissues, i.e., CA3 and DG, using the RNeasy micro Kit (Qiagen, Hilden, Germany) according to the manufacturer's protocol and eluted with 14 µL of RNAse-free water. The concentration of RNA was determined using Nanodrop 1000 and the integrity of RNA was determined using RNA 6000 Pico Kit and Bioanalyzer (Agilent Technologies, Santa Clara, CA, USA). The RNA quality was assessed according to the RNA integrity number. A RNA integrity number threshold value of 7 was fixed, and samples below this value were discarded. 0.2 µg of total RNA was reverse transcribed to cDNA using RevertAid Premium Reverse Transcriptase (Fermentas) and primed with oligo-dT primers (Fermentas) and random primers (Fermentas). Quantitative polymerase chain reaction (q-PCR) was performed using a LightCycler 480 Real-Time PCR System (Roche, Meylan, France). q-PCR reactions were done in duplicate for each sample, using transcript-specific primers (Table S2), cDNA (4 ng), and LightCycler 480 SYBR Green I Master (Roche) in a final volume of 10µL. The PCR data were exported and analyzed with Gene Expression Analysis Software Environment developed at the Neurocentre Magendie. For the determination of the reference gene, the geNorm method was used [START_REF] Bustin | The MIQE guidelines: minimum information for publication of quantitative realtime PCR experiments[END_REF]. Relative expression analysis was corrected for PCR efficiency and normalized against the houskeeping gene succinate dehydrogenase complex subunit A (Sdha). The relative level of expression was calculated using the comparative (2 -ΔΔCT ) method [START_REF] Livak | Analysis of relative gene expression data using real-time quantitative PCR and the 2(-Delta Delta C(T))[END_REF].

Immunohistological staining and analysis

WT and APP/PS1 mice at 6, 9, and 12 months of age were anesthetized with intraperitoneal administration of pentobarbital (50 mg/kg body weight) and were fixed by transcardial perfusion with buffered saline and 50 mL of 4% paraformaldehyde (PFA). Brains were removed, postfixed in 4% PFA overnight, and cut into 40 um-thick frontal sections on a vibratome. Sections were collected into PBS 0.1 M and stored at 4 °C until the staining protocol. After a thorough wash in TBST (tris buffer sodium: 0.0384-M trisma base 0.1263-M NaCl, pH: 7.4 þ 0.1% Tween 20), free-floating sections were incubated with TBST-10% Normal Goat Serume0.3% Triton X-100, for 1 hour at room temperature. Then sections were incubated with primary antibody in TBST-3.5% Normal Donkey Serum overnight at 4 °C (chicken antieglial fibrillary acidic protein (GFAP): 1/1000 USBiologicals; rabbit antieionized calcium-binding adapter molecule (Iba)-1: 1/5000 Wako; mouse anti-Aß 1-16 : 1/150 Millipore). After washes in TBST, sections were incubated with secondary antibodies for 1 hour (Goat anti-chicken Alexa 647, anti-mouse Alexa 555, and anti-rabbit Alexa 488: 1/500, Invitrogen). Slices were finally washed and mounted in mounting medium (Fluoromount + DAPI, SouthernBiotech). Slices were imaged using the up-right widefield microscope Leica DM5000 (Leica Microsystems, Nanterre, France) using objectives HC PL Fluotar 20X NA 0.5. Fluorescence excitation was performed by a LED SOLA Light (Lumencor, Beaverton, USA). Images were obtained by the resolutive cameras CoolSnap HQ2 (Photometrics, Tucson, USA) and a cooled QICAM (QImaging, Surrey, Canada). A galvanometric stage (Leica Microsystems) allowed the z stack reconstructions. The mosaics were made using a motorized stage Scan (Märzhäuser, Wetzlar, Germany). This system was controlled by MetaMorph software (Molecular Devices, Sunnyvale, USA). Analyses were performed with Image J software (National Institutes of Health, New York, USA) on the maximum intensity fluorescence projections of the 3D mosaics. The distribution of GFAP positive cells (number of astrocytes per mm 2 ), GFAP labelling, and Iba-1 positive cells (number of microglia per mm 2 ) were represented as box plot for which the top of each box indicates the 75th quartile (Q3), the bottom indicates the 25th quartile (Q1), and the bar inside the box is the median (Q2). The whiskers extend out to the most extreme data point that is at most 1.5 times the interquartile range above the third quartile (Q3 + 1.5 x [Q3-Q1]) or below the first quartile (Q3 -1.5 x [Q3 -Q1]).

Enzyme immunoassay determination of PGE 2

The concentration of PGE 2 was quantified both in WT and APP/PS1 at 6, 9,12, and 15 months of age. After anesthesia induction by intraperitoneal administration of pentobarbital (50 mg/kg body weight), the hippocampus was rapidly dissected and immediately frozen into liquid nitrogen. The concentration of PGE 2 present in the hippocampus was quantified with a specific PGE 2 competitive EIA kit (ACETM EIA kit, Cayman Chemical, Ann Arbor, MI, USA) following the procedures detailed in the instructions. The detection limit of this EIA kit was 15 pg/mL (80% B/Bo). Absorbance was measured at 415 nm by a microplate reader model 680 (Bio-Rad, Marnes-la-Coquette, France).

Statistical analysis

To eliminate bias, experimenters were kept blind to the genotype of the mice until all analysis was completed. Values are presented as mean standard error of the mean of n experiments. Unless stated otherwise, a Student t test for 2 groups and a 1-way analysis of variance with Bonferonni's test for more than 2 groups were used for statistical comparison. The p-values given are 2-tailed and were considered significant if p < 0.05. Statistical analysis was performed with Prism 5.0 (GraphPad Software, La Jolla, CA, USA).

Drugs

LCCG-1, bicuculline, D-2-amino-5 phosphonopentanoate (D-AP5), and CGP55845 were obtained from Tocris biosciences. Sulprostone, PGE 2 , and forskolin were obtained from SigmaeAldrich. ONO compounds were a generous gift from ONO Pharmaceuticals Co Ltd (Japan).

Results

PGE 2 impairs presynaptic long-term potentiation at MfeCA3 synapses

We tested the effects of PGE 2 on Mf-EPSCs recorded from CA3 pyramidal cells of P19-P21 wildtype mice using the whole-cell voltage clamp mode of the patch clamp technique (Fig. 1A). Mf-EPSCs evoked by minimal intensity stimulation at low frequency (0.1 Hz) were not affected by bath application of 1-10 µM PGE 2 (Fig. 1B). Mf-CA3 synapses display prominent forms of presynaptic short-term plasticity (Nicoll and Schmitz, 2005), including paired-pulse facilitation (PPF: 2 stimulations separated by 10-200 ms), and frequency facilitation (FF: switching from 0.1 Hz to >1 Hz tonic stimulation). PGE 2 (10 µM) neither affected the pronounced PPF (Fig. 1C) nor FF (Fig. 1D). Last, we investigated possible effects of PGE 2 on post-tetanic potentiation, another form of presynaptic short-term plasticity triggered by a single train of 100 stimuli at 100 Hz and characterized by a robust increase in Mf-EPSCs amplitude lasting several minutes. As shown in Fig. 1E, PGE 2 (10 µM) did not affect post-tetanic potentiation.

Altogether, exogenous application of PGE 2 affected neither basal synaptic transmission nor different forms of presynaptic short-term plasticity at Mf-CA3 synapses in young (P19-P21) mice. We then investigated whether PGE 2 affected LTP of Mf-EPSCs (Mf-LTP), a presynaptic form of plasticity which is independent of NMDA receptors (Nicoll and Schmitz, 2005). In the control condition, a highfrequency stimulus (HFS: 3 trains of 100 stimuli at 100 Hz) triggered presynaptic Mf-LTP as measured by the mean amplitude of Mf-EPSCs recorded 30-40 minutes after HFS (Fig. 1F). Bath application of 10-µM PGE 2 starting 10 minutes before HFS fully prevented Mf-LTP (Fig. 1F). In contrast, when applied after the induction of Mf-LTP (PGE 2 AI), PGE 2 had no effect (Fig. 2A, right panel). These results indicate that PGE 2 impairs the induction of Mf-LTP but not its maintenance.

PGE 2 impairs

Mf-LTP by acting on EP3 receptors PGE 2 can bind to 4 subtypes of Gproteinecoupled receptors, namely EP1, EP2, EP3, and EP4 (Breyer et al., 2001). Markedly, the expression of messenger RNAs (mRNAs) coding for EP2 and EP3 receptors has been reported in mouse and rat hippocampus [START_REF] Zhu | Heterogeneous expression and regulation of hippocampal prostaglandin E2 receptors[END_REF]. Thus, we investigated which subtype of PGE 2 receptor was involved in the impairment of Mf-LTP. Sulprostone (10 µM), a potent EP3 receptor agonist [START_REF] Coleman | New evidence with selective agonists and antagonists for the subclassification of PGE 2 -sensitive (EP) receptors[END_REF] fully prevented Mf-LTP, exactly mimicking the effect of PGE 2 (Fig. 2A). In contrast, ONO-AE1-259-01 (1 µM), a specific EP2 receptor agonist [START_REF] Mori | The prostanoid EP(2) receptor agonist ONO-AE1-259-01 protects against glutamateinduced neurotoxicity in rat retina[END_REF] had no effect on Mf-LTP (142 ± 26, n = 5; p = 0.903; Fig. 2A). ONO-AE3-240, a specific EP3 receptor antagonist fully prevented the impairment of LTP induced by PGE 2 (Fig. 2A, left panel). Altogether these results are strong evidence that the EP3 receptor is solely responsible for the impairment of LTP. The lack of effect of the EP2 agonist rules out the involvement of the EP4 receptor since it is coupled to the same intracellular signaling pathway as EP2. Finally, because it is coupled to phosphatidylinositol-calcium second messenger system, the EP1 receptor does not regulate AC activity and therefore is unlikely to be involved in presynaptic plasticity at Mf-CA3 synapses. Therefore, we focused on EP3 receptors in the rest of the study.

Mechanisms of action of EP3 receptors at Mf-CA3 synapses

EP3 receptors are coupled to G proteins through a Gi/o subunit (Hatae et al., 2002) which negatively regulates AC activity. Presynaptic 3',5'cyclic adenosine monophosphate (cAMP) is crucial for the regulation of presynaptic mechanisms at Mf-CA3 synapses; presynaptic receptors coupled to Gs subunits, such as the 5-hydroxytryptamine 4 receptor (5-HT4) [START_REF] Kobayashi | Chronic fluoxetine bidirectionally modulates potentiating effects of serotonin on the hippocampal mossy fiber synaptic transmission[END_REF] or dopamine receptor 1 (D1) [START_REF] Kobayashi | Chronic fluoxetine selectively upregulates dopamine D 1 -like receptors in the hippocampus[END_REF] potentiate Mf-CA3 synaptic transmission. To test the hypothesis that PGE 2 negatively modulates AC activity through the activation of EP3 receptors, we investigated the effect of sulprostone on a moderate increase of AC activity induced by forskolin (2.5 µM). Forskolin progressively increased Mf-EPSC amplitude (Fig. 2B) and sulprostone (10 µM) markedly reduced this potentiation (Fig. 2C).

Both the reduction of forskolin-induced potentiation of synaptic transmission and the action on presynaptic Mf-LTP by PGE 2 are consistent with a presynaptic locus of action for PGE 2 . Unfortunately, there is currently no suitable antibody to ascertain a presynaptic localization of the EP3 receptor in Mf terminals by immunocytochemistry. Commercially available antibodies for EP3 receptors lead to a strong background staining in brain sections in both EP3 +/+ and EP3 -/-mice (data not shown), which precluded any relevant conclusion (see also Shi et al., 2012). We thus investigated the expression of mRNA coding for EP3 receptors (EP3 mRNA) using LCM of hippocampal neurons from the CA3 region and from the granule cell layer of the DG in brain slices followed by quantitative real-time reverse transcriptased quantitative polymerase chain reaction (RT-qPCR). Slices were first stained with cresyl violet to visualize better CA3 and DG regions (Fig. 3A). Gene expression level of Ptger3, which codes for the EP3 receptor, was normalized to the housekeeping gene succinate dehydrogenase subunit A (Sdha) and mRNA expression was presented relative to that in the CA3 region of P19-P21 mice. RTqPCR indicated the expression of EP3 mRNA by DG granule cells (mean threshold cycle [Ct]: 31.54 ± 0.15, n = 3) further supporting the localization of the EP3 receptor in Mfs. EP3 mRNA was also expressed, although at a much lower level, in the pyramidal cell layer of CA3 (mean Ct: 32.74 ± 0.55, n = 3), and the relative expression of EP3 mRNA was 2 fold in DG as compared to CA3 (2.1 fold 0.3, n = 3; p < 0.05) (Fig. 3B). Altogether, these results strongly suggest that PGE 2 acting on presynaptic EP3 receptors controls the modulation of Mf-CA3 synaptic transmission by cAMP.

Neuroinflammatory status of wild type versus APP/PS1 mice

It is well established that at the clinical diagnosis stage, AD brains display extracellular deposits of Aß plaques accompanied by a prominent inflammatory reaction characterized by periplaque microgliosis and astrogliosis (Akiyama et al., 2000). We characterized the neuroinflammatory status of the hippocampus in WT and APP/PS1 mice at different ages (6, 9, and 12 months of age). Aß plaques were stained by an antibody raised against Aß 1-16 . Until 9 months of age, Aß plaques were barely detectable in the hippocampus of APP/PS1 mice. At 12 months of age, Aß plaques were virtually absent in WT mice (Fig. 4A), whereas they were abundant in APP/PS1 mice (per hippocampus, WT: 0.3 ± 0.5 plaques; APP/PS1: 21 ± 5 plaques, n = 4, p < 0.001; Fig. 4B). Strikingly, Aß plaques appeared to be nonuniformly distributed throughout the hippocampus but rather concentrated in the stratum lacunosum moleculare and the molecular layer of the and McGeer, 2007) and in transgenic mouse models of AD [START_REF] Kotilinek | Cyclooxygenase-2 inhibition improves amyloid-beta-mediated suppression of memory and synaptic plasticity[END_REF]Liang et al., 2005;Shi et al., 2012). We then quantified PGE 2 within the hippocampus during aging for both genotypes. No significant difference in PGE 2 levels was observed between WT and APP/PS1 mice (Fig. 4H) at ages up to 9 months (6 months: WT: 10.0 ± 1.4 pg/mg, n = 8; APP/ PS1: 10.1± 0.84 pg/mg, n = 9; p = 0.957; 9 months: WT: 9.54 ± 0.98 pg/mg, n = 14; APP/PS1: 10.71 ± 1.51 pg/mg, n = 12; p = 0.511). In contrast, PGE 2 levels were increased in APP/PS1 mice compared to WT mice at 12 and 15 months of age (12 months: WT: 10.91± 0.9 pg/mg, n = 18; APP/PS1: 14.67 ± 1.36 pg/mg, n = 17; * p < 0.05; 15 months: WT: 10.87 ± 2.38 pg/mg, n = 6; and APP/PS1: 17.07 ± 1.42 pg/mg, n = 13; Fig. 4H). Interestingly, the expression of hippocampal EP3 mRNA was reported to increase for the same APP/PS1 mouse model (Shi et al., 2012). These results show that APP/PS1 mice displayed, together with the load of Aß plaques and a marked activation of astrocytes and microglia, increased hippocampal levels of PGE 2 .

3.5. Presynaptic Mf-LTP is impaired in 12-monthold APP/PS1 mice Since PGE 2 alters synaptic plasticity at Mf-CA3 synapses in WT mice, we investigated what could be the impact of increased levels of PGE 2 caused by neuroinflammation in APP/PS1 mice. Because of the very low rate of success in obtaining patch-clamp recordings from 6, 9, and 12-monthold mice, we recorded fEPSP in the remainder of the study. We did not observe any differences between WT and APP/PS1 mice in inputeoutput relationships for fiber volley amplitude versus fEPSP slope at 6, 9 (data not shown), and 12 months of age (Fig. 5A). Similarly, neither PPF nor FF was differentially affected between the 2 genotypes (Table 1). No difference in the extent of Mf-LTP was observed between APP/PS1 mice and their WT littermates at 9 months of age (Fig. 5B). In contrast, presynaptic Mf-LTP was impaired in APP/PS1 mice at 12 months of age (Fig. 5C).

3.6. Role of EP3 receptors in Mf-LTP deficits in APP/PS1 mice Strikingly, the alteration of Mf-LTP was concomitant with the increased levels of PGE 2 in APP/PS1 mice (see Fig. 4H). We thus tested whether the building up of ambient PGE 2 produced within the course of the pathology in APP/PS1 mice was involved in the impairment of presynaptic Mf-LTP observed at 12 months of age. On the one hand, blocking EP3 receptors with ONO-AE3-240 (1 µM) added 30 minutes before the HFS burst rescued Mf-LTP in APP/PS1 mice to a similar extent as Mf-LTP in WT mice but had no effect per se on Mf-LTP in WT mice (Fig. 5E). On the other hand, activating EP3 receptors with sulprostone (1 µM) did not significantly impair further Mf-LTP in APP/PS1 mice (Fig. 5C). Last, presynaptic Mf-LTP was strongly impaired by PGE 2 (10 µM) an effect faithfully mimicked by 1µM sulprostone (Fig. 5D), demonstrating the functionality of the PGE 2 -EP3 pathway in WT littermates.

Taken together, these results provide strong evidence that PGE 2 acting via EP3 receptors is responsible for the impairment of Mf-LTP in 12month-old APP/PS1 mice, an age where the building up of endogenous PGE 2 due to neuroinflammation, presumably reaches a critical level to impact EP3 receptors.

Discussion

Our findings demonstrate that PGE 2 , acting through the EP3 receptor subtype, inhibits presynaptic long-term plasticity at Mf-CA3 synapses. We further provide evidence that in the APP/PS1 mouse model of AD, the PGE 2 -EP3 receptor signaling pathway is responsible for the impairment of presynaptic LTP at Mf-CA3 synapses by showing that the blockade of EP3 receptors fully rescues the LTP at 12-month-old, an age characterized by a neuroinflammatory reaction and chronically elevated levels of the neuroinflammatory molecule PGE 2 . These results highlight EP3 receptors as a potential therapeutical target to alleviate synaptic dysfunction, and possibly also cognitive deficits associated with AD. (Chen and Bazan, 2005;Sang et al., 2005;[START_REF] Yang | Altered hippocampal long-term synaptic plasticity in mice deficient in the PGE2 EP2 receptor[END_REF]. Depending on the concentration, PGE 2 can either be beneficial or detrimental to hippocampal transmission and/or plasticity. Endogenous basal levels of PGE 2 regulate membrane excitability, synaptic transmission, and plasticity in hippocampal CA1 pyramidal neurons (Chen and Bazan, 2005;Chen et al., 2002) and are necessary for memory acquisition [START_REF] Rall | Intrahippocampal infusion of a cyclooxygenase-2 inhibitor attenuates memory acquisition in rats[END_REF], consolidation [START_REF] Teather | Post-training cyclooxygenase-2 (COX-2) inhibition impairs memory consolidation[END_REF], and retention [START_REF] Cowley | COX-2, but not COX-1, activity is necessary for the induction of perforant path long-term potentiation and spatial learning in vivo[END_REF][START_REF] Sharifzadeh | A time course analysis of cyclooxygenase-2 suggests a role in spatial memory retrieval in rats[END_REF][START_REF] Shaw | Deficits in spatial learning and synaptic plasticity induced by the rapid and competitive broad-spectrum cyclooxygenase inhibitor ibuprofen are reversed by increasing endogenous brain-derived neurotrophic factor[END_REF]. In the course of neuroinflammation, Cox-2 and microsomal PGE 2 synthase expression levels are increased in different mouse models of AD [START_REF] Akitake | Microsomal prostaglandin E synthase-1 is induced in Alzheimer's disease and its deletion mitigates Alzheimer's diseaselike pathology in a mouse model[END_REF][START_REF] Ferretti | Intracellular Aß-oligomers and early inflammation in a model of Alzheimer's disease[END_REF] in parallel with impairments of synaptic plasticity and cognitive deficits. These data are coherent with a key role of PGE 2 , which is further supported by the induction of cognitive deficits in transgenic mice overexpressing Cox-2 in hippocampal neurons [START_REF] Andreasson | Age-dependent cognitive deficits and neuronal apoptosis in cyclooxygenase-2 transgenic mice[END_REF].

As a part of a complex system, PGE 2 can bind to 4 different subtypes of receptors (EP1-4). Depending on the subtype, the activation of EP receptors leads to changes in the production of cAMP and/or in phosphoinositol turnover and intracellular calcium mobilization (Breyer et al., 2001). Our data show that ONO-AE1-259-01, a specific EP2 receptor agonist [START_REF] Mori | The prostanoid EP(2) receptor agonist ONO-AE1-259-01 protects against glutamateinduced neurotoxicity in rat retina[END_REF], had no effect on presynaptic Mf LTP, therefore ruling out any significant involvement of this receptor. Furthermore, as the EP4 receptor is coupled to the same intracellular signaling pathway as the EP2 receptor, namely the G s -AC-cAMP pathway leading to an increase of cAMP production, our data also rule out a significant involvement of the EP4 receptor in PGE 2 -induced impairment of Mf LTP.

Here we show that activation of EP3 receptors by either exogenous PGE 2 in WT mice or endogenous PGE 2 in APP/PS1 mice inhibits a presynaptic form of LTP at Mf-CA3 synapses. EP3 receptors couple with the G i -AC-cAMP pathway leading to inhibition of cAMP generation (Breyer et al., 2001;[START_REF] Irie | Mouse prostaglandin E receptor EP3 subtype mediates calcium signals via Gi in cDNA-transfected Chinese hamster ovary cells[END_REF]. Two isoforms of AC are expressed by hippocampal neurons; AC1 and AC8 (Wang et al., 2003;[START_REF] Xia | Distribution of mRNA for the calmodulin-sensitive adenylate cyclase in rat brain: expression in areas associated with learning and memory[END_REF]. AC8 activity is exclusively dependent on intracellular Ca 2+ [START_REF] Nielsen | Differential regulation of type I and type VIII Ca 2+ -stimulated adenylyl cyclases by Gi-coupled receptors in vivo[END_REF], whereas AC1, which is highly expressed in DG cells [START_REF] Xia | Distribution of mRNA for the calmodulin-sensitive adenylate cyclase in rat brain: expression in areas associated with learning and memory[END_REF], is both regulated by G proteins and intracellular Ca 2+ [START_REF] Willoughby | Organization and Ca 2+ regulation of adenylyl cyclases in cAMP microdomains[END_REF]. Therefore, it is likely that only AC1 may be linked to the PGE 2 -EP3esignaling pathway. Our results are consistent with other studies showing that simple deletion of AC1 [START_REF] Villacres | Type I adenylyl cyclase mutant mice have impaired mossy fiber long-term potentiation[END_REF] or double deletion of AC1/AC8 (Wang et al., 2003) do not affect basal properties at Mf-CA3 synapses. However, we observed a normalization of Mf-EPSCs amplitude following application of a moderate concentration of forskolin, an activator of AC which is known to enhance Mf-CA3 synaptic transmission. Hence, the activation of PGE 2 -EP3 signaling pathway does not directly modulate synaptic basal transmission; however, it may interact with pathways linked to AC, when previously activated. In keeping with this, we found that PGE 2 impaired presynaptic Mf-LTP, an effect mimicked by sulprostone (EP3 agonist) and blocked by ONO-AE3-240 (EP3 antagonist), whereas ONO-AE1-259-01 (EP2 agonist) had no effect. These data provide strong evidence for an effect of PGE 2 mediated by EP3 receptors, but not EP2/EP4 receptors, and indicate that PGE 2 acts by downregulating AC activity triggered during the HFS bursts. Because of the impact of EP3 receptor activation on presynaptic mechanisms, we propose that EP3 receptors are localized in presynaptic terminals. Indeed, many lines of evidence coherently point at a presynaptic localization of EP3 receptors at Mf-CA3 synapses. First, PGE 2 modulates the release of neurotransmitters by Mf through a EP3 receptor G i -AC-cAMP pathway. Second, the PGE 2 -EP3 receptor signaling pathway specifically impairs the presynaptic form of LTP at Mf-CA3 synapses. Third, DG cells express mRNA coding for EP3 receptor. Last, the PGE 2 -EP3 receptor signaling pathway regulates the hippocampal expression of presynaptic proteins such as synaptophysin, synapsin 1, SNAP-25, or VAMP-2 but not the expression of proteins belonging to the postsynaptic density (Shi et al., 2012). However, due to the lack of a suitable antibody against EP3 receptors (this study; Shi et al., 2012), we were not able to ascertain its cellular localization. All the data are consistent with EP3 receptors being expressed at Mfs, but they cannot rule out a putative glial expression of EP3 receptors [START_REF] Slawik | Microglial expression of prostaglandin EP3 receptor in excitotoxic lesions in the rat striatum[END_REF]. However, glial process has a restricted access to Mf-CA3 synaptic cleft in physiological conditions [START_REF] Rollenhagen | Structural determinants of transmission at large hippocampal mossy fiber synapses[END_REF], which seems to preclude a major role for glial EP3 receptor in the modulation of Mf synaptic plasticity. 4.2. Elevation of PGE 2 during AD-linked neuroinflammatory processes is responsible for impairment of synaptic plasticity at Mf-CA3 synapses Most relevant to our study, PGE 2 -EP3 signaling seems to be a key component in the progression of the pathology in the APP/PS1 mouse model of Alzheimer's disease (Shi et al., 2012). The present finding that PGE 2 -EP3 negatively controls presynaptic Mf-LTP in WT mice prompted us to investigate if this plasticity was altered in APP/PS1 mice during the course of the disease and if the PGE 2 -EP3 signaling pathway was involved. So far, most of the studies focussed on synaptic failure in mouse models of AD have not addressed whether and how progressive neuroinflammatory processes are linked with synaptic dysfunction. Here, we showed that presynaptic Mf-LTP was impaired in APP/PS1 mice starting at 12 months of age, whereas presynaptic short-term plasticity was not. Strikingly, this impairment paralleled the increase in hippocampal levels of PGE 2 in APP/PS1 mice that we could detect starting at 12 months of age. Furthermore, EP3 receptor blockade by ONO-AE3-240 fully rescued impaired Mf-LTP in 12-monthold APP/PS1 mice clearly supporting a key role of the PGE 2 -EP3 signaling pathway in the development of synaptic dysfunction at the Mf-CA3 synapse in neuroinflammatory processes linked to AD. Worthy to note, ONO-AE3-240 had no effect on Mf-LTP in 12-month-old WT mice suggesting that the amount of PGE 2 produced during normal aging was not sufficient to activate tonically EP3 receptors and therefore impair plasticity. Interestingly, a time point of 8 months in the APP/ PS1 mouse was proposed for the expression onset of inflammatory genes IL-1ß, iNOS, and TNFα, which is driven by an increased expression of hippocampal EP3 mRNA (Shi et al., 2012). In our study, the time course of impairment of presynaptic Mf-LTP correlates well with the progressive development of neuroinflammation characterized by a mild activation of microglia and astrocytes and elevated levels of PGE 2 . Mf-CA3 synapses seem therefore relatively spared by the pathology in terms of presynaptic plasticity, at least up to 12 months of age. In contrast, postsynaptic long-term potentiation at associative/commissural CA3 synapses is completely abolished starting at 6 months of age in APP/PS1 mice (Viana da Silva et al., 2016). These data suggest that early learning deficits involving short-term working memory [START_REF] Lagadec | Early temporal short-term memory deficits in double transgenic APP/PS1 mice[END_REF] or long-term spatial memory [START_REF] Reiserer | Impaired spatial learning in the APPSwe + PSEN1DeltaE9 bigenic mouse model of Alzheimer's disease[END_REF], which occur at 6-7 months of age may not involve presynaptic Mf-CA3 plasticity but rather may be related to alterations of postsynaptic plasticity at CA3-CA1 synapses [START_REF] Volianskis | Episodic memory deficits are not related to altered glutamatergic synaptic transmission and plasticity in the CA1 hippocampus of the APPswe/PS1dE9-deleted transgenic mice model of ß-amyloidosis[END_REF] and/or CA3-CA3 synapses (Viana da Silva et al., 2016). 4.3. Pathophysiological relevance of PGE 2 -EP3 signaling at Mf-CA3 synapses in the contex of AD Potential alterations of Mf-CA3 synaptic function and plasticity in the context of AD have only received limited attention, despite the key role of DG-CA3 circuits in the encoding of new spatial information and complex mnemonic processes (Kesner, 2007). Our study demonstrates for the first time that presynaptic Mf-LTP is specifically altered through a PGE 2 -EP3 signaling pathway in APP/ PS1 mice at 12 months of age and most importantly can be entirely rescued by the pharmacological blockade of the EP3 receptor. Therefore, in addition to being instrumental in neuroinflammation processes (Shi et al., 2012), PGE 2 -EP3 signaling is also linked to synaptic alterations. Further work is necessary to elucidate how the engagement of the PGE 2 -EP3 signaling pathway during neuroinflammatory processes contributes to ADrelated cognitive deficits. Interestingly, contextual fear conditioning is altered in APP/PS1 mice (Kilgore et al., 2010), and PGE 2 has been shown to be necessary and sufficient to induce CFC when injected in the dorsal hippocampus [START_REF] Hein | Prostaglandins are necessary and sufficient to induce contextual fear learning impairments after interleukin-1 beta injections into the dorsal hippocampus[END_REF].

In conclusion, our data demonstrate that PGE 2 signaling via EP3 receptors impairs a presynaptic form of LTP at Mf-CA3 synapses, hence identifying a potential mechanism to explain deficits in 1-trial memory in AD (Kilgore et al., 2010). This pathway, implicated in neuroinflammatory processes in the course of AD (Shi et al., 2012), may identify a presynaptic form of LTP as a new target and leads to the promising idea of a therapeutical targeting of EP3 receptors instead of an upstream global inhibition of the production of PGE 2 , which may have deleterious effects.

Fig. 5. PGE 2 -EP3 signaling pathway impairs Mf presynaptic plasticity in APP/PS1 mice aged 12 months. (A) Inputeoutput relationship between the amplitude of fiber volley and the slope of fEPSP for gradually increasing stimulation intensities in wild type animals (black circles) and APP/PS1 animals (white circles) at 12 months of age. Both genotypes (WT and APP/PS1) displayed similar inputeoutput relationships forfiber presynaptic LTP (Mf-LTP) in WT and APP/PS1 mice at 9 months of age. Mf-LTP was induced by 3 bursts of 100 stimuli at 100 Hzfiber volley amplitude versus fEPSP slope (WT: 0.21 mV/ms ± 0.08 mV/ms, n = 8; APP/PS1: 0.28 mV/ms ± 0.12 mV/ms, n = 6; p = 0.624). (B) Average time course of mossy with a 10-second interburst interval (HFS). There was no difference between the 2 genotypes for normalized EPSCs measured between 40 and 50 minutes (WT: 152 ± 14%; APP/PS1: 160 ± 11%, p = 0.67, n = 5 for both conditions). (C) Average time course of Mf-LTP in WT and APP/PS1 mice aged 12 months. Note the impairment of Mf-LTP in APP/PS1 mice (WT: 173 ± 12%, n = 15; APP/PS1: 126 ± 13%, n = 11, p < 0.001). Bar graph represents average of normalized slopes from fEPSPs. Sulprostone had no further significant effect on LTP reduction in APP/PS1 mice (APP/PS1 + sulprostone: 92 ± 26%, n = 4, p < 0.001). (D) Bar graph represents average of normalized slopes from fEPSPs in WT mice at 12 months of age either in the presence of PGE 2 or sulprostone. Note that sulprostone or PGE 2 induced similar decrease of LTP (WT: 173 ± 12%, n = 15; PGE2: 114 ± 5%, n = 9; sulprostone: 125 ± 12%, n = 7; p < 0.001). (E) Average time course of Mf-LTP illustrates the rescue of Mf-LTP in presence of ONO-AE3-240 (1 mM) in APP/PS1 mice at 12 months of age. Bar graph illustrates the average of fEPSPs slopes (WT: 173 ± 12%, n = 15; WT + ONO-AE3-240: 180 ± 4%, n = 7; APP/PS1: 126 ± 13%, n = 11; APP/PS1 + ONO-AE3-240: 163 ± 17%, n = 10; p < 0.001). In these series of experiments (not shown due to the time scale), the complete inhibition of EPSCs by group II mGluR agonist LCCG-1 (10 mM) confirmed that they were Mf-EPSCs. ***p < 0.001. Abbreviations: EP3, prostaglandin E2 receptor 3; fEPSP, field excitatory postsynaptic potential; HFS, high-frequency stimulus; Mf, mossy fiber; PGE 2 , prostaglandin E2. La diminution de PLT observée chez les souris femelles APP/PS1 pourrait être due à une diminution de la probabilité de libération du neurotransmetteur, à une diminution de la fiabilité ou du poids des synapses individuelles, à une diminution du nombre de synapses ou de récepteurs fonctionnels. De façon remarquable les oligomères Aβ activent une voie de signalisation dépendante des récepteurs NMDA qui conduit à une diminution de la PLT hippocampale par internalisation des récepteurs synaptiques AMPA et NMDA [START_REF] Kamenetz | APP processing and synaptic function[END_REF]Kessels et al., 2013;Snyder et al., 2005 ). Nous avons donc mesuré le rapport AMPA/NMDA pour évaluer cette possibilité, mais n'avons observé aucun changement de ce rapport entre les souris femelles sauvages et APP/PS1 à 6 mois. Nous avons également étudié les courants synaptiques excitateurs miniatures des cellules granulaires. Il est généralement décrit que l'amplitude des courants miniatures est reliée à des évènements postsynaptiques quand leur fréquence est attribuée à des mécanismes présynaptiques [START_REF] Turrigiano | Activity-dependent scaling of quantal amplitude in neocortical neurons[END_REF]Kim and Tsien, 2008;Ardiles et al., 2012) bien qu'une diminution de la fréquence puisse également être liée à une diminution du nombre de synapses fonctionnelles. Les résultats montrent une augmentation de la fréquence des courants miniatures médiés par les récepteurs NMDA dans les cellules granulaires du gyrus dentelé chez les souris femelles APP/PS1 par rapport aux souris femelles sauvages. Cette augmentation de la fréquence des courants miniatures NMDA pourrait être due à un nombre de synapses silencieuses (c'est à dire dépourvues de recepteurs AMPA) supérieur chez les souris femelles APP/PS1 par rapport aux souris femelles sauvages. En revanche, l'amplitude des courants miniatures AMPA ou NMDA n'est pas significativement modifié par le génotype. En conclusion, l'ensemble de ce travail montre qu'il existe des différences importantes liées au sexe avec un dépôt plus fort et précoce de plaques amyloïdes, une activation gliale neuro--inflammatoire plus importante associée à des déficits précoces de la mémoire dépendante de l'hippocampe chez les souris femelles APP/PS1 vs souris mâles APP/PS1. En parallèle de ces altérations moléculaires, cellulaires et comportementales, nous observons des déficits de la transmission et de la plasticité synaptique à la synapse voie perforante --cellules granulaires du gyrus dentelé (voie d'entrée des informations en provenance du cortex entorhinal) ainsi qu'une modification des propriétés intrinséques des cellules granulaires.

Figure 3 .

 3 Figure 3. NFTs accumulation in AD

Figure 5 .

 5 Figure 5. Activated glia cells with Aβ and NFTs

Figure 6 .

 6 Figure 6. Neuroinflammation in AD

Figure 7 .

 7 Figure 7. Prostaglandin synthesis

Figure 8 .

 8 Figure 8. Synaptic transmission

Figure 9 .

 9 Figure 9. Signaling in LTD and LTP

Figure 10 .

 10 Figure 10. Short term synaptic plasticity

Figure 12 .

 12 Figure 12. Synaptic circuits in hippocampus

  L-AP4 contributes to a significant larger depressive effect on the excitatory postsynaptic potential (EPSP) evoked by LPP than by MPP stimulation (Koerner and Cotman, 1981), while baclofen has the opposite effect (Lanthom and Cotman, 1981). Carbachol acting through postsynaptic sites, more likely suppresses MPP EPSP (Kahle and Cotman, 1989), and antagonists of NMDA receptor (D-(-)-2-amino-5phosphonovaleric acid, D-APV and 3-[( + )-2-carboxypiperazin-4-yl]-propyl-l-phosphonic acid, CPP) have been shown to decrease the late part of the MPP EPSP whereas with the LPP EPSP is unchanged (Dahl

  32-34°C. Whole-cell patch clamp recordings(3)(4) were made from dentate gyrus (DG) granule cells visualized by infrared video-microscopy.For field recordings, electrodes of about 2 MΩ were used.Patch clamp electrodes were pulled out from borosilicate glass (GF 150 F-10) and filled with an internal solution containing the following (in mM): 140 CsCH 3 SO 3 , 2 MgCl 2 , 4 NaCl, 5 phospho-creatine, 2 Na 2 ATP, 0.2 EGTA, 10 HEPES, and 0.33 GTP adjusted with CsOH (300 mOsm, pH 7.3). In some experiments a K + based internal solution containing (in mM) was used: 115 KC 6 H 11 O 7 , 10 KCl 2 , 0.2 EGTA, 0.02, CaCl 2 10 HEPES, 15 phospho-creatine, 4 MgATP and 0.3 NaGTP adjusted with KOH (300 mOsm, pH 7.3). Bicuculline (10 µM) was added to the bath to inhibit γ-Aminobutyric acid-A (GABA-A) receptors. Voltage-clamp recordings were performed on DG granule cells identified with a differential interference contrast microscope (Eclipse FN-1, Nikon, Champigny sur Marne, France) equipped with an infrared camera (VX 44, Till Photonics, Gräfelfing, Germany) using an Axopatch-200B amplifier (Axon Instruments, Sunnyvale, CA, USA). Signals were filtered at 2 kHz and digitized at 5 kHz via a DigiData 1322A interface (Axon instruments). Series resistance (10-20 MΩ) was monitored during the recording using a -10 mV hyperpolarizing voltage step of 50 ms length occurring at the beginning of each recording.To monitor the access resistance during the recording time, a hyperpolarizing voltage step (-5 mV, 10 ms) was applied at the beginning of each sweep and DG granule cells were rejected if more than a 20% change occurred during the experiment. DG granule cells with a holding current exceeding 250 pA at a holding potential of -70 mV were also rejected. Data were collected and analysed using pClamp software 9.2 (Axon Instruments). For extracellular recordings, because of the possibility of crosstalk between mossy cell fibers (MCFs) and the more proximal medial perforant path (MPP) we focused our study on lateral perforant path (LPP) transmission. To stimulate selectively the LPP, a stimulating glass electrode (1 -2 µm tip diameter) filled with extracellular recording solution was placed in the outer molecular layer of the DG (>100 µm away from the granule cell layer) as close to the hippocampal fissure as possible. In addition to the location of the stimulating electrode, we characterized the LPP on its ability to exhibit paired-pulse facilitation (Fig.4C) while the MPP does not (Fig.4C). Signals were recorded using an Axopatch-200B amplifier (Axon Instruments, Sunnyvale, CA, USA), filtered at 20 kHz and digitally sampled at 50 kHz. All experiments were performed in the presence of bicuculline (10 µM) to block GABA-A receptors. Inputoutput curves were collected by varying stimulation strength from 0 to 200 µA in 50 µA intervals. For subsequent experiments on basal transmission and synaptic plasticities, the stimulus strength was set to a level that produced about 30% of the maximal responses. The LTP-induction protocol consisted of 3 bursts of 100 stimuli at 100 Hz with a 10 s interburst interval (HFS). E-S curve, which relates EPSP slopes to the associated action potential probability, was generated by measuring slopes during the first 2 ms (reflecting mainly the fast component of AMPA-receptor synaptic transmission) and sorted in 0.5mV/ms bins to the associated action potential probability. The AP firing probability was calculated for each bin and the EC 50 value (the value of the EPSP-slope that elicits an AP with 50 % of probability) was calculated in each cell.

  were removed, postfixed in 4% PFA overnight and cut into 40 µm-thick frontal sections on a vibratome. Sections were collected into PBS 0.1M and stored at + 4°C until the staining protocol. After a thorough wash in TBST (Tris Buffer Sodium: 0.0384 M Trisma Base -0.1263 M NaCl pH: 7.4 + 0.1 % Tween 20), free-floating sections were incubated with TBST -10% Normal Goat Serum -0.3% Triton X-100, for 1 hour at room temperature. Then sections were incubated with primary antibody in TBST -3.5% Normal Donkey Serum overnight at 4°C (chicken anti -glial fibrillary acidic protein (GFAP): 1/1000 USBiologicals; rabbit anti -ionized calcium-binding adapter molecule Iba-1: 1/5000 Wako. Aβ plaques were stained either using Methoxy-X04 (a derivative of Congo red, 50 µM, Tocris) or mouse anti-Aß 1-16 : 1/150 (Millipore).After washes in TBST, sections were incubated with secondary antibodies for 1 hour (Goat anti-chicken Alexa 647, anti-mouse Alexa 555 and anti-rabbit Alexa 488: 1/500, Invitrogen).Slices were finally washed and mounted in mounting medium (Fluoromount + DAPI, SouthernBiotech). Slices were imaged using the up-right widefield microscope Leica DM5000 (Leica Microsystems, Nanterre, France) using objectives HC PL Fluotar 20X NA 0.5. Fluorescence excitation was performed by a LED SOLA Light (Lumencor, Beaverton, USA). Images were obtained by the resolutive cameras CoolSnap HQ2 (Photometrics, Tucson, USA) and a cooled QICAM (QImaging, Surrey, Canada). A galvanometric stage (Leica Microsystems) allowed the z stack reconstructions. The mosaics were made using a motorized stage Scan (Märzhäuser, Wetzlar, Germany). This system was controlled by MetaMorph software (Molecular Devices, Sunnyvale, USA). Analyses were performed with Image J software (National Institutes of Health, New York, USA) on the maximum intensity fluorescence projections of the 3D mosaics.Behavioral studies: Contextual fear conditioning (CFC)Mice were housed individually. Prior to the test, mice were handled 2 minutes per day during 5 days. At day 0, single-trial contextual fear conditioning was performed by placing mice into a conditioning chamber (context A) where they were allowed to explore for 150 secondes after which they received a mild electric footshock (2s, 0.7 mA) and were removed 28 secondes after the shock. The conditioning chamber was housed in a sound attenuated box with the four interior walls wearing a checkerboard pattern. The conditioning cage (17 cm wide, 17 cm deep and 25 cm high) was made of clear plexiglas and the floor consisted of a shock grid. Between each animal the cage was cleaned with 70% ethanol solution. At day 1, mice were reexposed to the conditioning context A then to a new context (context B) during 180 secondes for each. For the neutral context B, the chamber was housed in an identical box but cues made of round and stars pattern on a white background were placed around a rounded instead of a squared plexiglas cage. The floor grid was covered by plexiglas and bedding. The light was dimmed compared to context A and the cage was cleaned with 1% acetic acid between animals. Contextual fear memory retention was studied by measuring the freezing response in context A and context B. Automatic freezing was detected with the help of a software (Any-Maze, Ugo Basile, Gemonio, Italy).Behavioral studies: Object Location Task (OLT)Prior to the test, mice were handled 2 minutes per day during 5 days and habituated to the openfield (45 cm wide, 45 cm deep and 40 cm high) the day before the experiment during 20 minutes, in the absence of objects. The total covered distance was measured by video-tracking (Noldus-Ethovision, Nantes, France). The day of testing, mice were first placed for 10 minutes (habituation phase) in the arena where two identical plastic bricks were located 15 cm away from the walls in adjacent quadrants(A and C or B and D). Brick exploration was defined as exploration with the nose directing toward it within 2 cm. Absence of preference with one of the two bricks was checked during the habituation phase. After a delay of 30 minutes, mice were reexposed to the arena in which one of the two bricks was moved (for exemple from quadrant B to quadrant A or from quadrant D to quadrant C) in order to create a new spatial combination and mice were allowed to explore for again 10 minutes. The time spent exploring the bricks in the novel and familiar locations was measured during the first 5 minutes as mice have tendancy to loose novelty detection behavior after few minutes(Cès et al., 2018). The discrimination ratio was determined by the percentage of the time spent exploring the displaced brick over the time spent exploring both bricks.

  genotype of the mice until analysis was completed. Values are presented as mean ± SEM of n experiments. Statistical analyses were performed with Prism 7.0 (GraphPad Software, La Jolla, CA, USA). Unless stated otherwise, a Student's t-test for 2 groups and a one-way ANOVA with Bonferonni's test for more than two groups were used for statistical comparison. Data distributions were analysed using the Kolmogorov-Smirnov test; for miniature events the distribution curve was calculated for each cell and all cells averaged per condition to create a single curve. The p -values given are two-tailed and were considered significant if p<0.05.DrugsBicuculline, D-2-amino-5-phosphonopentanoate (D-AP5), LCCG-1, CGP55845 and Methoxy-X04 were obtained from Tocris. TTX, PGE 2 , D-serine and PF-04418948 were obtained from Sigma-Aldrich. NBQX was obtained from HelloBio. ONO compounds were a generous gift from ONO Pharmaceuticals Co Ltd (Japan).

  in APP/PS1 mice and no change of intrinsic excitability in type 2 DG cells. The firing properties of DG granule cells are differentially affected by synaptic potentiation in WT vs APP/PS1 miceIt has been recently reported that the intrinsic excitability of DG granule cells was highly plastic and modulated by synaptic potentiation(Lopez-Rojas et al., 2015). Interestingly, the intrinsic firing properties of DG granule cells were affected in a single transgenic mouse model of AD[START_REF] Nenov | Impaired firing properties of dentate granule neurons in an Alzheimer's disease animal model are rescued by PPARγ agonism[END_REF]. At the LPP-DG synapse, long-term potentiation (LTP) of excitatory transmission is induced by high-frequency stimulation (HFS). As shown in Fig. 7A, HFS (3 bursts of 100 stimuli at 100 Hz with a 10-second interburst interval) triggered a robust and long-lasting potentiation (LTP) of the population spike amplitude (PSA) in WT female mice (187±14%, n=17) which was completely blocked by 50 µM AP-V (n=9). This LTP was reduced by half in APP/PS1 female mice (146±8%, n=16, * p<0.05) (Fig 7. A, D). The levels reached for LTP in agematched WT male mice (150±9%, n=15, * p<0.05) were similar to those observed in APP/PS1 female mice (Fig 7. A, D).

Figure 1 :

 1 Figure 1: APP/PS1 mice display hippocampal Aβ plaques and glial activation earlier in female than male. A. Representative micrographs of a hippocampus in WT and APP/PS1 male vs female at 9 months of age. Slices were double stained for nuclei with 4',6-diamidino-2-phenylindole (DAPI, blue signal) and for Aβ plaques with 2,5-Bis(2-(4-hydroxyphenyl)vinyl) anisole (methoxy-X04, red signal). Note the heavy load of Aβ plaques in the stratum moleculare (delineated by the white dotted-line located above the stratum granulare) of the DG of female mice. Bar scale = 250 µm. Lower panel, representative micrograph of Aß plaques (anti-Aß 1-16 antibody, white signal) surrounded by activated astrocytes (GFAP, red signal) and microglia (Iba-1, green signal). Bar scale = 10 µm. B. Bar graphs represents the number of Aβ plaques in the DG stratum moleculare (as shown in A) at different ages for male and female mice. Note the significant higher deposit of Aβ plaques for APP/PS1 female vs male mice at all age. (* p<0.05; ** p<0.01; *** p<0.001). C. Magnification of a representative DG stratum moleculare region of WT and APP/PS1 female mice. Note the morphological changes in GFAP positive cells (astrocytes) in APP/PS1 (red star and green triangle) vs WT female mice (blue triangle). D. Magnification of a representative DG stratum moleculare region of WT and APP/PS1 female mice. Note the morphological changes in Iba-1 positive cells (microglia) in APP/PS1 (red star and green triangle) vs WT female mice (blue triangle).

Figure 2 :Figure 3 :Figure 4 :

 234 Figure 2: APP/PS1 female mice develop contextual memory deficits earlier than male. A. At day 0, WT and APP/PS1 male and female mice (4, 6, 12 months) were placed in a training chamber (context A) where they received an electric footshock (2s, 0.75 mA). At day 1, mice were reexposed to the conditioning context A then to an entirely new, therefore neutral, context (context B). Freezing percentage was calculated for each animal. B. At 4 months of age, when compared to WT mice, APP/PS1 female mice show impairment of contextual fear conditioning when reexposed to context A whereas APP/PS1 male mice does not. C. At 6 months of age, APP/PS1 male mice does not still show impaired contextual fear conditioning. D. At 12 months of age, APP/PS1 male mice still do not display impaired contextual fear conditioning. (* p<0.05; ** p<0.01; *** p<0.001)

Figure 5 :

 5 Figure 5: AMPA-receptor and NMDA-receptor mediated mEPSCs in DG cells. A. Superimposed representative traces of AMPA and NMDA mEPSCs (AMPA traces are the average of 2556 and 3884 mEPSCs events for WT and APP/PS1 respectively; NMDA traces are the average of 861 and 3461 mEPSCs events for WT and APP/PS1 respectively). DG cells were held at a holding potential of -70 mV in the presence of bicuculline (10 µM), NBQX (10

Figure 6 :

 6 Figure 6: Characterization of firing properties of DG granule cells. A. Representative action potentials (APs) elicited in response to 0.2 ms depolarizing current steps with 10 µA increments in a DG granule cell from WT mouse. For each genotype (WT, black circles; APP/PS1, red circles) EPSP-slope values were calculated for the first two milliseconds and plotted versus the probability to fire action potentials. No shift in the E-S curves was observed between both genotype (WT: EC 50 =5.97, Hill slope=0.37; APP/PS1: EC 50 =5.98, Hill slope=0.39). B. Representative trains of APs elicited by long depolarizing current steps (250 ms) of increasing intensity in a WT DG cell. Whatever the genotype, type 1 and type 2 cells

Figure 7 :

 7 Figure 7: Population spike amplitude potentiation in DG granule cells. A. Left panel shows representative traces of population spike amplitude (PSA) before and after HFS. Potentiation

Figure 1 :

 1 Figure 1: Contextual fear conditioning in 18 months old males A. Experimental design for context A and context B. B. Similar freezing behavior of WT (black) and APP/PS1 (blue) mice in two highly similar contexts (context A and context B). C. CFC box. D. Contextual discrimination of WT (black) and APP/PS1 (blue) mice in very different contexts (context A and context C). *** p < 0.0001

Figure 2 :

 2 Figure 2: Presynaptic LTP at MF-CA3 synapses

Figure 3 :

 3 Figure 3: Population spike amplitude potentiation at LPP/DG synapses

Figure 4 :

 4 Figure 4: PGE2 has no effect on basal transmission

Figure 5 :

 5 Figure 5: EP4 receptor has no effect at LPP-DG synapse

Figure 6 :

 6 Figure 6: Increased spikes number in APP-PS1 mice

  Field excitatory postsynaptic potentials (fEPSPs) were evoked in response to electrical stimuli applied via another glass electrode (1-2 MΩ) filled with extracellular solution placed in the hilus of the DG. Signals were low-pass filtered at 20 kHz and digitally sampled at 50 kHz. All experiments were performed in the presence of D-AP5 (50 µM) to block NMDA receptor activity, bicuculline (10 µM), and (2S)-3-(([1S]-1-[3,4-dichlorophenyl]ethyl) amino-2 hydroxypropyl)(phenylmethyl)phosphinic acid hydrochloride (CGP) (3 µM) to block, respectively, g-aminobutyric acid-A and -B receptors. Inputeoutput curves were collected by varying stimulation strength from 40 µA to 120 µA in 40 µA intervals.

Fig. 1 .

 1 Fig. 1. PGE 2 impairs presynaptic long-term potentiation at Mf-CA3 synapses. (A) Diagram shows the positions of the stimulating electrode (located in the mossy fiber tract) and of the recording electrode (CA3 neuron, whole-cell configuration of the patch clamp technique). (B) Representative time course of EPSCs recorded in CA3 pyramidal neurons (P19-P21) in response to low frequency (0.1 Hz) stimulation of mossy fiber in control conditions (black circles) and in the presence of PGE 2 (10 µM; white circles). Note that EPSCs were evoked by minimal intensity stimulation (100 ms, 5-15 mA). EPSCs amplitude was normalized as percentage of average baseline EPSCs amplitude. Each dot corresponds to average of 5 consecutive EPSCs recorded for 8 cells. PGE 2 (10 mM) was added after 10 minutes of baseline, and EPSCs were recorded for the following 30 minutes. The complete inhibition of EPSCs by group II mGluR agonist LCCG-1 (10 µM) confirmed that they were Mf-EPSCs. Bar graph represents average amplitude of EPSCs recorded between 10 and 20 minutes (control: 69.6 ± 5.6 pA, n = 8; PGE 2 : 64.1 10.1 pA, n = 8; p = 0.28). (C) Average of 30 traces (left) illustrating paired-pulse facilitation (PPF) at 40 ms of Mf-EPSCs in control condition or in presence of PGE 2 (10 mM). Bar graph represents average paired pulse ratio at 40 ms interstimulus interval (control: 348 ± 23%, n = 12; PGE 2 : 400 ± 28%, n =12, p = 0.17). (D) Average of 50 traces (left) illustrating frequency facilitation (FF) of Mf-EPSCs when increasing the simulation frequency from 0.1 Hz to 1 Hz in control condition or in presence of PGE 2 (10 µM). Bar graphs of mean FF from 0.1 Hz to 1 Hz (control: 554 ± 34%, n = 12; PGE 2 : 613 ± 76%, n = 12, p = 0.096) (E). Representative time course of post-tetanic potentiation induced by simulation at 100 Hz during 1second. Bar graph represents average of normalized EPSCs during the 5 first minutes following the tetanic stimulation (control: 346 ± 48 %, n = 8; PGE 2 : 322 ± 55 %, n = 7, p = 0.742). (F) Time course of the effect of PGE 2 (10 µM) on mossy fiber presynaptic LTP. Mf-LTP was induced by 3 bursts of 100 stimuli at 100 Hz with a 10-second interburst interval (HFS). PGE 2 was bath applied 10 minutes before HFS. Note the complete prevention of LTP induced by PGE 2 (10 µM). Each dot corresponds to average of 6 consecutive EPSCs recorded for n cells. Data are presented as mean standard error of the mean. Abbreviations: DG, dentate gyrus; EPSCs, excitatory postsynaptic currents; LTP, long-term potentiation; Mf, mossy fiber; PGE 2 , prostaglandin E 2 ; PP, perforant pathway; SC, shaffer collaterals.

Fig. 2 .

 2 Fig. 2. PGE 2 impairs Mf-LTP by acting on EP3 receptors. (A) Left panel: ONO-AE3-240 (1 µM) (light gray symbols) rescued impairment by PGE 2 of presynaptic Mf-LTP. Right panel: Bar graph represents average of normalized EPSCs between 30 and 40 minutes after the HFS protocol in different conditions (PGE 2 AI stands for PGE 2 after induction) (control: 137 ± 18%, n = 10; PGE 2 : 83 ± 7%, n= 11; **p < 0.01; PGE2 AI: 136 ± 12%, n = 5, *p < 0.05; sulprostone: 86 ± 11%, n = 7, *p < 0.05; ONO-AE1-259-01: 142 ± 26%, n = 5; ONO-AE3-240: 159 ± 18%, n = 5; **p < 0.01 vs. sulprostone and ***p < 0.001 vs. PGE 2 ; ONO-AE3-240 + PGE 2 : 134 ± 6%, n = 5). Note that when applied after the induction of LTP, PGE 2 had no effect. Note that sulprostone (EP3 receptor agonist) fully mimicked PGE 2 induced blockade of LTP, and ONO-AE3-240 fully prevented the blockade of LTP induced by PGE 2 . ONO-AE1-259-01 (1 µM; EP2 receptor agonist) had no effect on LTP. (B) Time course of Mf-EPSCs potentiation induced by bath application of forskolin (2.5 µM) in control condition and in presence of sulprostone (10 µM). Mf-EPSCs were averaged every minute and normalized to the mean amplitude of the 10 minutes preceding bath application of forskolin. Representative currents (averages of 60 EPSCs) recorded between 20 and 30 minutes after forskolin application illustrating the effect of sulprostone on forskolin-induced potentiation of Mf-EPSCs. (C) Bar graph represents the effect of sulprostone on forskolin-induced potentiation of Mf-EPSCs (control: 198 ± 18%, n = 9; sulprostone: 128 ± 13%, n = 6, *p < 0.05). The complete inhibition of EPSCs by group II mGluR agonist LCCG-1 (10 µM) in A, B, and C confirmed that they were Mf-EPSCs. Data are presented as mean standard error of the mean. Abbreviations: EPSCs, excitatory postsynaptic currents; HFS, high-frequency stimulus; PGE 2 , prostaglandin E2.

Fig. 3 .

 3 Fig. 3. EP3 receptor expression in dentate gyrus (DG) and CA3 pyramidal layer. (A) Laser capture microdissection (LCM) of DG and CA3 neurons. Hippocampii were stained with cresyl violet. Upper panel: intact hippocampus before LCM. Left lower panel: LCM captured CA3 neuronal layer. Right lower panel: LCM captured DG. Bar = 300 mm. (B) Quantitative real-time polymerase chain reaction of hippocampal EP3 receptor mRNA derived from wild-type mice (P21) in DG and CA3 pyramidal layer. Bar graph illustrates the increase of expression in DG compared to CA3 pyramidal layer (2.1 fold 0.3, n = 3, per group; * p < 0.05).

Fig. 4 .

 4 Fig. 4. Neuroinflammatory status in APP/PS1 mice. (A) Representative micrograph of a hippocampus in a WT mouse (12 months of age) double stained for nuclei with 40,6-diamidino-2-phenylindole (DAPI) and for Aß plaques (anti-Aß 1-16 antibody). No Ab plaques were detected. Bar scale = 250 mm. (B) Representative micrograph of a hippocampus in a APP/PS1 mouse (12 months of age). Note the presence of Ab plaques (white signal) mainly distributed in the stratum lacunosum moleculare and the molecular layer (ML) of the DG. Bar scale = 250 mm. Right panel: close up of an Ab plaque (white signal) surrounded by glial cells. Note the large cell bodies with short and thick processes of astrocytes (GFAP, red signal) in the vicinity of the plaque. Microglia (Iba-1,

4. 1 .

 1 PGE2 inhibits Mf-LTP by acting on EP3 receptors likely localized presynaptically PGE 2 -and Cox-2-signaling pathways modulate hippocampal synaptic transmission and/or plasticity

Table 1 Different mechanism of therapeutic strategies in the treatment of Alzheimer's disease Hypothesis Mechnism Strategies

 1

  . Only aged htau mice displayed an alteration in PPR that is symptom of a decreased glutamate release probability. Aged htau mice display cognitive impairment in hippocampal related behavioral tasks, and changes in basic synaptic function. Additionally, HFS failed to induce LTP in aged htau mice. Together these results suggest that tau pathology may underlie agedependent learning impairment via disruption of synaptic function.

  legislation, APP/PS1 and WT mice at 6 months were deeply anaesthetized with a mix of ketamine (80 mg/kg; i.p) and xylazine (16 mg/kg; i.p) and then intracardially perfused for 2-3 minutes with a protective solution at 4°C containing the following (in mM): 2.5 KCl, 1.25 NaH 2 PO 4 , 0.5 CaCl 2 , 7 MgCl 2 , 20 glucose, 26 NaHCO 3 , 76 sucrose, 3 pyruvic acid, 5 Na ascorbate, equilibrated with 95% 0 2 and 5% CO 2 (300-310 mOsm). When the solution coming out of the heart was free of blood, the mouse was sacrificed by decapitation and the brain was quickly removed from the skull and stored in the same solution. The isolated brain was glued onto the stage of a vibratome (VT 1200S, Leica Microsystems, Nussloch, Germany) and parasagittal hippocampal slices (320 µm) were cut in the protective solution for which sodium chloride was replaced by sucrose, and then incubated at 33°C for 10-20 min in a resting solution containing the following (in mM): 125 NaCl, 2.5 KCl, 1.25 NaH 2 PO 4 , 2 CaCl 2 , 1 MgCl 2 , 20 glucose, 26 NaHCO 3 , 5 Na ascorbate equilibrated with 95% 0 2 and 5% CO 2 (300-310 mOsm). Slices were thereafter maintained at room temperature in the same solution until required. Unless stated otherwise, all experiments have been done on 6 months old female mice.

  3, 6 and 9 months of age. After anesthesia induction by intraperitoneal administration of pentobarbital (50 mg/kg body weight), the hippocampus was rapidly dissected and immediately frozen into liquid nitrogen. The concentration of PGE 2 present in the hippocampus was quantified with a specific PGE 2 competitive EIA kit (ACETM EIA kit, Cayman Chemical, Ann Arbor, MI, USA) following the procedures detailed in the instructions. The detection limit of this EIA kit was 15 pg ⁄ mL (80% B ⁄ Bo). Absorbance was measured at 415 nm by a microplate reader model 680 (Bio-Rad, Marnes-la-Coquette, France).

  at 3 months of age with significantly more Aβ plaques in females(3 months: male:

	0.04±0.03, n=44; female: 1.89±0.66, n=29, ** p<0.01; 6 months: male: 4.87±0.79, n=47;
	female: 8.48±0.77, n=56, ** p<0.01; 9 months: male: 10.14±1.42, n=37; female: 21.53±2.91,
	n=29, *** p<0.001).

  In context A, APP/PS1 males at 4 months of age showed levels of freezing nearly identical to age-matched WT littermates indicating no memory deficits (WT context A : 36.78±10.6 %, n=8; APP/PS1 context Such amounts of exploratory activity were good evidence for a lack of significant anxiety-related behavior in our experimental conditions.Next, APP/PS1 males and females of 3, 4 and 6 months of age and their age-matched WT littermates were subjected to OLT (Fig.3A). Following the 30 minutes' retention period, WT female mice spent more time exploring the novel location of the object than the familiar one at all age, so that the discrimination ratio was significantly different from 50% (3 months:In contrast and as early as 3 months of age, APP/PS1 female did not explore the novel location of the object significantly more than the fixed one(3 months: 44.17±4.66, n=15, p=0.232; 4 months: 51.6±4.91%, n=10, p=0.752; 6 months: 47.76±2.32%, n=13, p=0.354; Fig 3B) indicating therefore that they were impaired in this task at a fairly young age. Our data provide clear evidence that whatever the age, APP/PS1 mice never explored more than WT mice the novel location of the object.

	65.41±3.99%, n=14, **p<0.01; 4 months: 66.34±3.76%, n=16, ***p<0.001; 6 months:
	66.25±2.9%, n=15, **p<0.01; Fig 3B).
	At 6 months of age, APP/PS1 males still displayed no memory deficit (WT context A :
	46.59±4.94%, n=12; APP/PS1 context A : 37.99±6.36%, n=10, p=0.291) whereas APP/PS1
	females showed memory deficit as expected (WT context A : 50.23±5.09%, n=15; APP/PS1 context
	A : 30.77±6.78%, n=10, *p<0.05) (Fig. 2C). The lack of behavioral deficit in this task was
	even observed in 12 months old male mice (WT context A : 49.48±7.3 %, n=7; APP/PS1 context A :
	50.47±7.47%, n=11, p=0.929) (Fig. 2D).
	Importantly, mice for both genotype and gender at any age displayed low levels of freezing
	(data not shown) during the 150 s period before the footshock at the time of training (females
	4 months: WT:0.97±0.24%, n=14; APP/PS1:0.97±0.48%, n=9, p=0.997; males 4 months:
	WT:7.22±3.59%, n=9; APP/PS1:5.25±1.32, n=8, p=0.678; females 6 months:

A : 37.76±8.34%, n=8, p=0.942) (Fig.

2B

). In contrast, a prononced memory deficit was observed in APP/PS1 females at the same age (WT context A : 37.43±7.02 %, n=14; APP/PS1 context A : 15.2±4.79%, n=9, * p<0.05) (Fig.

2B

). When placed in a neutral context B, WT and APP/PS1 males displayed very similar low levels of freezing (WT context B : 11.56±3.18 %, n=8; APP/PS1 context B : 13.93±3.51%, n=8, p=0.624) and WT females significantly freezed longer that APP/PS1 (female WT context B : 11.57±3.06 %, n=14; female APP/PS1 context B : 3.19±0.97%, n=9, * p<0.05).

WT:5.62±1.44%, n=15; APP/PS1:3.01±0.84%, n=10, p=0.186; males 6 months: WT:1.70±0.46 %, n=12; APP/PS1:0.48±0.21%, n=10, *p<0.05; males 12 months: WT:4.16±1.92%, n=7; APP/PS1:5.73±1.14%, n=11, p=0.462).

signaling pathway is not involved in the impairment of LTP at the LPP-DG synapse

	APP/PS1 after HFS : 6.08±0.31, n=11, *** p<0.001) but not for WT mice (WT: 4.90±0.30, n=15;
	WT after HFS : 5.70±0.33, n=12, p=0.08) (Fig. 7C).
	PGE 2 -In a previous work, we reported that PGE 2 acting on EP3 receptors impaired Mf-CA3 LTP in
	WT male mice and that the involvement of an inflammatory PGE 2 -EP3 signaling pathway
	was responsible for the impairment of the LTP in aged APP/PS1 male mice (Maingret et al.,
	2017). In the present work, the LTP of PSA is significantly impaired by half in APP/PS1
	female (WT: 187±14%, n=17; APP/PS1: 146±8%, n=16, * p<0.05) (Fig 7A,7D). This
	prompted us to investigate the effects of exogenous PGE 2 and/or specific agonists and
	antagonists of different EP subtypes receptors on the LTP of PSA both in WT and in
	APP/PS1 female mice.
	(WT+ONO-AE1-259-01: 160±12%, n=17; WT+PGE 2 +PF-04418948: 169±16%, n=16;
	WT+PGE 2 +ONO-AE3-240: 154±8%, n=16) (Fig. 7D). These results ruled out the
	involvement of either PGE 2 -EP2 or PGE 2 -EP3 signaling pathway acting of LTP at the LPP-
	DG synapse in WT female mice.
	Then we searched if the impairment of LTP observed in APP/PS1 was possibly related to
	PGE 2 . Neither PF-04418948 (10 µM) nor ONO-AE3-240 (1 µM) rescued LTP
	5.96±0.38, n=15, EC 50 WT after HFS : 4.51±0.41, n=11, * p<0.05; EC 50 APP/PS1: 5.85±0.39, (APP/PS1+PF-04418948: 140±8%, n=17, p=0.619; APP/PS1+ ONO-AE3-240: 147±11%,
	n=15, EC 50 APP/PS1 after HFS : 3.92±0.27, n=10, ** p<0.01). These results show that HFS n=17, p=0.191) (Fig. 7D) which ruled out any significant involvement of inflammatory PGE 2 -
	induces long lasting changes in intrinsic plasticity of DG granule cells. The shift to the left EP2 or PGE 2 -EP3 signaling pathway in APP/PS1 female mice acting on LTP. These
	indicates an increase of excitability in DG cells induced by LTP, i.e. to a smaller EPSP slope somewhat unexpected results with respect to the precocious neuroinflammatory status of
	corresponds a higher AP firing probability (Fig. 7B). APP/PS1 female mice led us to quantified PGE 2 within the hippocampus at different ages for
	both genotypes. No significant difference in PGE 2 levels was observed between WT and
	APP/PS1 female mice at any age (3 months: WT: 17.9±2.06 pg/mg, n=13; APP/PS1:

Then we looked to the relation between the EPSP amplitude and the EPSP slope for subthreshold EPSPs (i.e. not triggering APs) before and after HFS. We found a significant increase in the EPSP amplitude/slope ratio for APP/PS1 mice (APP/PS1: 4.55±0.18, n=15; When adding 10 µM PGE 2 , we did not observe any significant impairment of LTP in WT female mice (WT: 187±14%, n=17; WT+PGE 2 : 161±15%, n=17, p=0.2). Neither a specific EP2 receptor agonist (ONO-AE1-259-01, 1µM) nor a EP2 receptor antagonist (PF-04418948, 10 µM) or a EP3 receptor antagonist (ONO-AE3-240, 1 µM) significantly altered LTP 13.07±1.31 pg/mg, n=12; p=0.073; 6 months: WT: 18.98±4.31 pg/mg, n=10; APP/PS1: 19.31±4.02 pg/mg, n=10; p=0.705; 9 months: WT: 12.44±1.41 pg/mg, n=12; APP/PS1: 12.57±1.36 pg/mg, n=11; p=0.951) (Fig.

  5 µM) and recorded for 15 minutes. NMDA mEPSCs were recorded in nominal free Mg 2+ aCSF, 4 mM Ca 2+ and D serine (50 µM). The average frequency of NMDA mEPSCs was significantly increased for APP/PS1 mice vs WT mice (WT: 4.60±0.43,

	n=13; APP/PS1: 17.12±1.29, n=14, *** p<0.001), whereas the amplitude was not (WT:
	15.46±0.94, n=15; APP/PS1: 17.55±1.09, n=14, p=0.166). Neither the frequency (WT:
	14.00±2.50, n=16; APP/PS1: 18.50±2.91, n=15, p=0.249) nor the amplitude (WT: 8.19±0.27,
	n=15; APP/PS1: 9.22±0.51, n=15, p=0.088) of AMPA mEPSCs were different. B. Kinetic
	analysis of AMPA and NMDA mEPSCs. Normalized AMPA and NMDA mEPSCs were
	superimposed for both genotypes. The rise time (10-90%) and decay time (Tau) were similar
	for AMPA mEPSCs between both genotypes (Rise time AMPA: WT 0.81±0.04, n=15;
	APP/PS1 0.88±0.04, n=15, p=0.238; Tau AMPA: WT 6.67±0.34, n=16; APP/PS1 6.00±0.25,
	n=15, p=0.130). Rise time and Tau were also similar for NMDA mEPSCs between both
	genotypes (Rise time: WT 1.22±0.12, n=13; APP/PS1 1.04±0.07, n=14, p=0.237; Tau: WT
	27.86±2.39, n=13; APP/PS1 24.94±1.34, n=14, p=0.287).

C. Cumulative probability analysis of AMPA and NMDA mEPSCs amplitude. The cumulative fraction for AMPA and NMDA mEPSCs amplitudes were significantly different between genotypes (AMPA, KS test, *** p>0.0001; 2556 events of 16 cells for WT [black curve] and 3884 events of 15 cells for APP/PS1 [red curve] mice; NMDA, KS test, *** p>0.0001; 861 events of 15 cells for WT [black curve] and 3461 events of 14 cells for APP/PS1 mice [red curve]). The distribution histogram (insets) was not statistically different between genotypes for AMPA mEPSCs amplitude (paired t test, p=0.126) and different between genotypes for NMDA mEPSCs amplitude (paired t test, * p<0.05).

Table 1 .

 1 * p= 0.0014) but larger for APP/S1 vs WT. A significant EC 50 shift in the E-S relation was induced by HFS for both Comparison between action potentials properties in DG granule cells from WT and APP/PS1 female mice. Data are mean ± SEM. Values between brackets are n values. p values were obtained with Student's t-test (amplitude: p=0.070; overshoot: p=0.064; peak time: p=0.885; RMP: * p<0.05; AP threshold: p=0.764; t 50 : p=0.848; R in : p=0.552). RMP: resting membrane potential. t 50 : time to get half amplitude of AP. R in : input resistance

	WT (WT: 5.96±0.38, n=15, WT after HFS : 4.51±0.41, n=11, * p<0.05) and APP/PS1 (APP/PS1:
	5.85±0.39, n=15, APP/PS1 after HFS : 3.92±0.27, n=10, ** p<0.01) although with a larger
	amplitude for the APP/PS1 genotype. C. EPSP amplitude/slope ratio before and after synaptic
	potentiation induced by high frequency stimulation (HFS). The amplitude/slope ratio was not
	different between WT and APP/PS1 mice before (WT: 4.90±0.30, n=15; APP/PS1: 4.55±0.18,
	n=15, p=0.332) or after synaptic potentiation induced by HFS (WT after HFS : 5.72±0.33, n=12;
	APP/PS1 after HFS : 6.08±0.31, n=11, p=0.406) whereas it was significantly increased by HFS in
	APP/PS1 mice (APP/PS1: 4.55±0.18, n=15; APP/PS1 after HFS : 6.08±0.31, n=11, *** p<0.001)
	but not for WT (WT: 4.90±0.30, n=15; WT after HFS : 5.72±0.33, n=12, p=0.332). D. Bar graph
	represents average of normalized PSA between 50 and 60 minutes after HFS protocol in
	different conditions (WT: 187±14%, n=17; APP/PS1: 146±8%, n=16; WTmale: 150±9, n=15;
	WT+PGE 2 (10µM): 161±15%, n=17; WT+EP2 agonist (ONO-AE1-259-01, 1µM): 160±12%,
	n=17; WT+PGE 2 (10 µM) +EP2 antagonist (PF, 10 µM): 169±16%, n=16; APP/PS1+EP2
	antagonist (PF, 10 µM): 140±8%, n=17. * p<0.05, t test with Welch's correction. E. Bar
	graph represents levels of PGE 2 in whole hippocampus in WT and APP/PS1 female mice at
	different ages. Note the lack of significant differences of levels of PGE 2 whatever the
	genotype or the age (3 months: WT: 17.90±2.06 pg/mg, n=13; APP/PS1:13.07±1.31 pg/mg,
	n=12, p=0.073, 6 months: WT: 18.98±4.31 pg/mg, n=10; APP/PS1: 19.31±4.02 pg/mg, n=10,
	p=0.705, 9 months: WT: 12.44±1.40 pg/mg, n=12; APP/PS1: 12.57±1.36 pg/mg, n=11,
	p=0.951).

Table 2 .

 2 Comparison of action potentials properties of type 1 and type 2 DG granule cells

	WT

between WT and APP/PS1 female mice. Data are mean ± SEM. Values between brackets are n values. p values were obtained with Student's t-test. ADP: after depolarisation. AHP: after hyperpolarisation. I thr : AP current threshold (minimal current intensity inducing an AP)

A.

Contextual Fear Conditioning in old male mice (18 months)

  As demonstrated by our results, APP/PS1 males were spared at least up to 12 months of age. We decided to investigate if males were still spared at even older ages, i.e. 18 months, and tested the ability of WT and APP/PS1 male mice at 18 months of age to discriminate between two highly similar contexts, context A vs context B, or between two very different contexts, context A vs context C. When testing the two highly similar contexts (A vs B), WT and APP/PS1 did not show differences in freezing percentages between context A vs context

	B (WT: F context A : day 1, 65.64 ± 5.89%, n=28; day 2, 70.41 ± 6.38%, n=27; day 3, 71.21 ±
	4.91%, n=27; day 4, 81.24 ± 3.37%, n=27; day 5, 79.58 ± 3.69%, n=27; day 6, 72.53 ± 4.40%,
	n=27; day 7, 77.51 ± 3.62%, n=27; F context B : day 1, 50.66 ± 5.73%, n=28; day 2, 77.92 ±
	4.55%, n=27; day 3, 78.33 ± 3.47%, n=27; day 4, 75.55 ± 3.79%, n=27; day 5, 75.17 ± 4.6%,
	n=27; day 6, 71.59 ± 4.15%, n=27; day 7, 70.64 ± 5.08%, n=27; APP/PS1: F context A : day 1,
	57.78 ± 6.19%, n=28; day 2, 80.25 ± 3.97%, n=26; day 3, 87.00 ± 2.15%, n=25; day 4, 86.41
	± 2.11%, n=26; day 5, 85.95 ± 2.62%, n=26; day 6, 83.55 ± 2.68%, n=26; day 7, 84.17 ±
	3.52%, n=27; F context B : day 1, 52.98 ± 6.11%, n=28; day 2, 81.95 ± 4.26%, n=27; day 3, 89.85
	± 3.45%, n=24; day 4, 86.12 ± 3.53%, n=25; day 5, 89.60 ± 2.31%, n=25; day 6, 87.86 ±
	3.34%, n=26; day 7, 82.54 ± 4.42%, n=27) (Fig. 1A, 1B). When testing the two very different
	contexts (A vs C), WT and APP/PS1 did discriminate equally well (WT: F context A : 68.77 ±
	3.52%, n=13; F context C : 4.10 ± 1.10%, n=11; *** p<0.0001; APP/PS1: F context A : 72.91 ±
	4.84%, n=14; F context C : 25.91 ± 6.02%, n=14, *** p<0.0001) (Fig. 1C, 1D). These results
	indicated a lack of context fear generalization in old male mice.

  2 KCl, 1.25 NaH 2 PO 4 , 0.4 CaCl 2 , 8 MgCl 2 , 22 glucose, 26 NaHCO 3 , 200 sucrose, 2.8 pyruvic acid, 0.4 ascorbic acid, equilibrated with 95% O 2 and 5% CO 2

Table 1

 1 Short-term plasticity in WT and APP/PS1 mice at 9 and 12 months of age For PPF, average of 30 traces at 40 ms interstimulus interval. For FF, average of 50 traces when increasing the stimulation frequency from 0.1 Hz to 1 Hz. Data are mean ± standard error of the mean, n = 6-8 mice per group; Student t test. Key: FF, frequency facilitation; PPF, paired-pulse facilitation; WT, wild-type mice.l'excitabilité intrinsèque. Cependant, l'application d'une stimulation haute fréquence qui déclenche une PLT provoque un déplacement de la courbe E--S qui traduit une augmentation supérieure d'excitabilité pour les souris femelles APP/PS1 vs sauvages. Ces résultats indiquent que la potentialisation synaptique induit un changement d'excitabilité intrinséque des cellules granulaires du gyrus dentelé dépendant du génotype. Nous avons donc ensuite analysé la possibilité que l'augmentation d'excitabilité présentée par les cellules granulaires du gyrus dentelé puisse résulter de modifications de l'intégration dendritique. Une façon d'étudier l'intégration dendritique consiste à établir le rapport amplitude/pente du PEPS au cours des deux premières millisecondes, c'est--à--dire lorsqu'il reflète principalement la composante rapide de la transmission synaptique des récepteurs AMPA[START_REF] Lopez-Rojas | Plasticity of intrinsic excitability in mature granule cells of the dentate gyrus[END_REF]. Le rapport amplitude/pente PEPS est similaire pour les deux génotypes mais nous avons observé une différence significative dans le rapport amplitude/pente PEPS avant et après PLT pour les souris femelles APP/PS1 mais pas pour les souris femelles suavages. Ces résultats suggérent que des modifications du traitement dendritique des entrées synaptiques dans les cellules granulaires du gyrus dentelé chez les souris femelles APP/PS1 pourraient soutendrent des modifications de l'excitabilité.

	Short-term	WT (9 mo)	APP/PS1 (9 mo)	WT (12 mo)	APP/PS1 (12 mo)
	plasticity
	PPF	381 ± 43%	369 ± 59%, p = 0.869	383 ± 75 %	282 ± 37%, p = 0.296
	FF	673 ± 100%	484 ± 61% , p = 0.148	532 ± 88 %	418 ± 123%, p = 0.432

Acknowledgements Acknowledgments

We greatly thank Jeffrey Lopez-Rojas for his invaluable help with electrophysiological recordings, the very fruitful discussions and his constant kindness. We thank Gaël Barthet for his support and fruitful comments. We thank Benoit Silvestre de Ferron for the cartoon shown in Figure 4. The microscopy was performed in the Bordeaux Imaging Center (BIC), a service unit of the CNRS-Inserm and Bordeaux University, member of the national infrastructure France BioImaging. We thank Patrice Mascalchi from the BIC for his technical support. We greatly thank Takayuki Maruyama (ONO Pharmaceuticals Co Ltd, Japan) for his generous gift of selective agonists and antagonists of EP receptors.

Grants

Nan Jiang was supported by a joint program between the University of Bordeaux and the China Scholarship Council (CSC) and awarded of a fellowship from the CSC.

Disclosures

No conflict of interest, financial or otherwise, are declared by the author(s).

Acknowledgements

The authors thank Sílvia Viana da Silva for help with electrophysiological recordings and fruitful discussions and Noelle Grosjean for genotyping APPswe/PS1ΔE9 mice. The authors thank Ashley L. Kees for careful reading of the manuscript and Benoit Silvestre de Ferron for the cartoon shown in Fig. 1A. The authors also thank Marlène Maitre (Microdissection Laser plateform, Neurocentre Magendie, Inserm U862), Thierry Lesté-Lasserre (Genotyping plateform, Neurocentre Magendie, Inserm U862) for their technical support. The microscopy was performed in the Bordeaux Imaging Center (BIC), a service unit of the CNRS-Inserm and Bordeaux University, member of the national infrastructure France BioImaging. The authors thank Fabrice Cordelières and Patrice Mascalchi from the BIC for their technical support. The authors thank Takayuki Maruyama (ONO Pharmaceuticals Co Ltd, Japan) for his generous gift of selective agonists and antagonists of EP1-4 receptors, Shuh Narumiya, Tomoyuki Furuyashiki, and Toshiro Maruyama (Kyoto University, Kyoto, Japan), for the generous gift of EP3 -/-mouse brains. Vincent Maingret was supported by a studentship from the "Conseil Régional d'Aquitaine" and from the "Fondation pour la Recherche Médicale". The authors declare no competing financial interests.

Author contributions

Disclosure statement

The authors have no conflicts of interest to disclose.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.neurobiolaging. 2016.10.012.