Vincent Par

Robert

Dirigée Par

PhD Rebecca Piskorowski

PhD Vivien Chevaleyre

Liset Menendez

PhD De La Prida

PhD (Rapporteur Paul Salin

PhD, PU (Président du jury Claire Legay

PhD (Examinateur Audrey Mercer

PhD, Professeur émérite Philippe Ascher

Examinateur Jean-Christophe Poncer

Hypothalamic control of hippocampal area CA2 activity

I thank Mario Penzo and Bo Li who trained me and became my friends during the year I spent in Cold Spring Harbor labs, showing me that I was capable of doing science which was critical in my decision to go for a PhD. I thank my family. In particular, I thank my mother for her never-failing support and care, and for her genuine curiosity about my topics of interest. Thank you Julie for your positive attitude. I thank my father for his attention and interest in my work. I thank my grandparents for their encouraging attitude. Thank you Françoise for helping me grow up and sparkling my scientific curiosity.

I thank my friends, who were always there for me through good and hard times, having your friendship is truly inspirational. Thank you Clément for the amazing experiences and exchanges we had together, for always pushing me forward and never letting me down. Thank you Poppy for all the happy times and music we shared, for understanding who I am and picking me up every time I fall. Thank you Joss for your unshakable positivity, genuine altruism and for always helping to keep balance in my life be it with laughter or comfort. Thank you Coralie-Anne for all the fun times and fascinating scientific discussions we shared during our thesis years, and for always looking up what is best for me. I thank Mélanie Tobin for the years of our lives that we shared during this thesis. Thank you Benoit, Arnaud, Loulou, Elsa, Lucas, Mathilde, Victor, Charles, Benjamin, Julien, Théo, Anil, Thibault, Antoine, Edith, Agathe, Pierre, Audrey, Cécile, Jean-Marie, Guillaume, Gustavo, Alex, Daphnée.

I thank Mélanie for her kindness, thoughtfulness, tolerance, joyfulness, generosity, and confidence in me, which never fail to bring me happiness and help me overcome difficulties.

I -Introduction

I.1 -Roles of the hippocampus and the hypothalamus in memory

To form memories, the brain has the remarkable ability to encode, store and retrieve information. This cognitive function is critical for survival, adaptation to a changing environment, and life in society. Memory is a complex process that encompasses several systems spanning different structures of the central nervous system. This section briefly reviews important features of memory and introduces relevant brain regions, in particular the hippocampus and hypothalamus.

From human studies, much has been learned about how the brain forms memories. Memory is divided into short-and long-term based on the time course of the retained information. These two categories are handled by different parts of the brain, as evidenced by patients with longterm memory loss who can still perform well in a working memory task that requires keeping information available during a short period of time [START_REF] Baddeley | Working memory: looking back and looking forward[END_REF]. Long-term memory is further divided in declarative and non-declarative based on the nature of the information retained. Declarative memory is the process in which items that can be explicitly expressed.

Non-declarative memory regroups unconscious forms of memory [START_REF] Cohen | Preserved learning and retention of pattern-analyzing skill in amnesia: dissociation of knowing how and knowing that[END_REF][START_REF] Squire | The medial temporal lobe memory system[END_REF]. Non-declarative memory nevertheless implies learning such that a given input will reliably trigger a corresponding output. Reflex pathways, conditioned associations, priming processes and procedural abilities fall into the non-declarative class of memory [START_REF] Squire | The medial temporal lobe memory system[END_REF][START_REF] Tulving | Priming and human memory systems[END_REF]. Declarative memory applies to the process of memory formation and recall of conscious items, either semantic if they are general facts or episodic when they involve personal information. Episodic memory is therefore defined as the memory of events concerning the self that can be explicitly recollected.

Strikingly, these different forms of long-term memory rely on specific brain structures that could be discovered in patients with lesions in the corresponding dedicated areas. Indeed, the seminal study of patient H.M. revealed a specific role for the hippocampus and related structures in episodic memory [START_REF] Squire | The Cognitive Neuroscience of Human Memory Since H[END_REF]. Following bi-lateral resection of medial temporal lobes to remove epileptic foci, H.M. developed a full anterograde and partial retrograde amnesia (Scoville and Milner, 1957). Remarkably, procedural and short-term memory were unaffected by H.M.'s lesions, and so was his retrograde long-term memory to the extent remote times before the surgery [START_REF] Sagar | Dissociations Among Processes in Remote Memory[END_REF][START_REF] Squire | The Cognitive Neuroscience of Human Memory Since H[END_REF]. Combined with clinical observations from other amnesic patients and experimental work in monkeys and rodents, these findings firmly establish a selective role for the hippocampus in episodic memory (Mishkin, 1978;Mumby, 2001;Scoville and Milner, 1957;Zola-Morgan et al., 1986;Zola-Morgan et al., 1994). Furthermore, the anterograde nature of H.M.'s amnesia revealed that the hippocampus is necessary for learning and forming new episodic memories.

Although these findings converged on a central role for the hippocampus in episodic memory, it also appeared that the hippocampus was not the sole brain structure responsible for learning and memory. Indeed, the preserved past knowledge of H.M. proved that the hippocampus is not the ultimate storage site for long-term episodic memories. Moreover, H.M.'s lesions were not restricted to the hippocampus and additional work in monkeys and rodents reported that hippocampal lesions alone leave certain aspects of memory unaffected (Mishkin, 1978;Mumby, 2001;[START_REF] Squire | The Cognitive Neuroscience of Human Memory Since H[END_REF]Zola-Morgan et al., 1994). Perhaps not too surprising given their close relations with the hippocampus, the entorhinal cortex and amygdala are also involved in episodic memory (Mishkin, 1978;[START_REF] Squire | The Cognitive Neuroscience of Human Memory Since H[END_REF]. Another brain structure implicated in memory, notably in humans, is the hypothalamus. This diencephalic area suffers extensive damage in a pathological condition called Korsakoff's syndrome [START_REF] Mair | Memory disorder in Korsakoff' s psychosis a neuropathological and neuropsychological investigation of two cases[END_REF]. Often caused by chronic alcoholism, Korsakoff's syndrome is characterized by damage to the thalamus and the hypothalamic mamillary region, resulting in memory impairments in humans and mice models of the disease (Daniel, 2005;[START_REF] Mair | Memory disorder in Korsakoff' s psychosis a neuropathological and neuropsychological investigation of two cases[END_REF]. Interestingly, the hippocampus itself is mostly spared in Korsakoff's syndrome (Daniel, 2005) suggesting that injuries to brain structures connected to the hippocampus can cause similar memory deficits as hippocampal lesions. Indeed, the mamillary region of the hypothalamus is closely associated with the hippocampus as the mamillary bodies receive hippocampal projections and the suprammillary nucleus innervates the hippocampus (Pan and McNaughton, 2004a). However, even with this, there is paucity of physiological investigations of this hypothalamohippocampal circuit.

I.2 -Brain circuits relevant to episodic memory

In order to understand how episodic memory can be formed, stored and mobilized in the central nervous system, one needs to examine the brain structures involved, how they are connected and what neurons they host. Therefore, this section describes the anatomy, global and local connectivity, and cellular composition of the hippocampal formation with emphasis on area CA2.

I.2.a -Flow of information in the canonical hippocampal loop

I.2.a.i -Hippocampal anatomy and cellular composition

The hippocampus is a bilateral archicortical structure folded under the cerebral cortex and is widely conserved in numerous species including humans and rodents. In mice, the hippocampus is a prominent structure that occupies a large volume of the brain extending on a rostro-caudal and dorso-ventral axis from a septal pole to a temporal pole in a curved shape. Besides its local circuitry, the hippocampus is connected to several brain structures. The main glutamatergic afference to the hippocampus comes from layers II and III of the entorhinal cortex (EC) [START_REF] Steward | Cells of origin of entorhinal cortical afferents to the hippocampus and fascia dentata of the rat[END_REF][START_REF] Witter | Functional organization of the extrinsic and intrinsic circuitry of the parahippocampal region[END_REF]. Another major glutamatergic input resides in the hippocampus itself, as left and right hippocampi are inter-connected [START_REF] Blackstad | Commissural connections of the hippocampal region in the rat, with special reference to their mode of termination[END_REF].

GABAergic neurons in the septum project to the hippocampus where they specifically innervate interneurons [START_REF] Freund | GABA-containing neurons in the septum control inhibitory interneurons in the hippocampus[END_REF][START_REF] Freund | Interneurons of the hippocampus[END_REF]. The septum also provides cholinergic inputs to the hippocampus [START_REF] Shute | Cholinesterase-Containing Systems of the Brain of the Rat[END_REF]. Other neuromodulatory afferences to the hippocampus include noradrenaline from the locus coeruleus [START_REF] Swanson | The central adrenergic system. An immunofluorescence study of the location of cell bodies and their efferent connections in the rat utilizing dopamine-Bhydroxylase as a marker[END_REF], serotonin from the raphe [START_REF] Freund | Serotonergic control of the hippocampus via local inhibitory interneurons[END_REF], dopamine from the ventral tegmental area and substantia nigra [START_REF] Gasbarri | Mesolimbic dopaminergic neurons innervating the hippocampal formation in the rat: a combined retrograde tracing and immunohistochemical study[END_REF]. Additional projections to the hippocampus arise from the amygdala [START_REF] Kemppainen | Projections from the posterior cortical nucleus of the amygdala to the hippocampal formation and parahippocampal region in rat[END_REF], thalamus [START_REF] Weel | Projections from the nucleus reuniens thalami to the entorhinal cortex, hippocampal field CA1, and the subiculum in the rat arise from different populations of neurons[END_REF], and hypothalamus [START_REF] Cui | Hypothalamic and other connections with dorsal CA2 area of the mouse hippocampus[END_REF][START_REF] Maglóczky | Principal cells are the postsynaptic targets of supramammillary afferents in the hippocampus of the rat[END_REF]. Efferent projections of the hippocampus are diverse as well. The major hippocampal output is the EC, thus establishing a reciprocal loop with this structure, either directly or via the subiculum [START_REF] Naber | Reciprocal connections between the entorhinal cortex and hippocampal fields CA1 and the subiculum are in register with the projections from CA1 to the subiculum[END_REF].

Reciprocal connections also consist of hippocampal projections to the septum, amygdala, and hypothalamus [START_REF] Groen | Extrinsic projections from area CA1 of the rat hippocampus: Olfactory, cortical, subcortical, and bilateral hippocampal formation projections[END_REF][START_REF] Swanson | An autoradiographic study of the organization of the efferet connections of the hippocampal formation in the rat[END_REF]. In addition, the hippocampus sends outputs to the neocortex olfactory bulb, nucleus accumbens and thalamus [START_REF] Groen | Extrinsic projections from area CA1 of the rat hippocampus: Olfactory, cortical, subcortical, and bilateral hippocampal formation projections[END_REF][START_REF] Gulyás | Stratum radiatum giant cells: a type of principal cell in the rat hippocampus[END_REF][START_REF] Jay | Selectivity of the hippocampal projection to the prelimbic area of the prefrontal cortex in the rat[END_REF][START_REF] Swanson | An autoradiographic study of the organization of the efferet connections of the hippocampal formation in the rat[END_REF].

Like most regions of the brain, the hippocampus hosts two main kinds of neurons : the majority are principal cells and the minority interneurons (INs). Classically, principal cells are excitatory as they use the neurotransmitter glutamate while interneurons release GABA and are therefore inhibitory (excepted during the development of the central nervous system [START_REF] Rivera | The K+/Cl-co-transporter KCC2 renders GABA hyperpolarizing during neuronal maturation[END_REF]). Principal cells can drive excitation within and across regions and are therefore considered as the main communication units of the brain. Interneurons are involved in local interactions but also drive inhibition between brain areas through long-range projections [START_REF] Caputi | The long and short of GABAergic neurons[END_REF]. In the hippocampal formation, neurons are segregated in different regions that have anatomical and functional specificities : the dentage gyrus (DG), and Cornu Ammoni (CA) areas CA3, CA2 and CA1 (Lorente de No, 1934). These regions are further subdivided in their radial axis corresponding to the different parts of the principal cells parallel dendritic arborization. Indeed, principal cells are organized in a laminar way such that all somas are aligned, thus forming the pyramidal layer (stratum pyramidale, SP). The dendrites of these principal cells extend in a preferential direction perpendicular to SP and are therefore aligned parallel to each other on either side of SP. The outer layer formed by basal dendrites of pyramidal neurons (PNs) is called stratum oriens (SO), and the apical dendrites fill the inner layers of the hippocampus. The juxta-somatic inner layer in area CA3 is called stratum lucidum (SL) and corresponds to axonal tracks of the dentate gyrus granule cells, the mossy fibers (MF).

Axons from area CA3 PNs, the Schaffer collaterals, run through the proximal part of the apical dendrites forming a layer called stratum radiatum (SR). Finally, the distal part of the apical dendritic arbor is contacted by afferent fibers from the entorhinal cortex (EC) in the stratum lacunosum moleculare (SLM). Drawing of Golgi stain illustrating the cytoarchitecture and connections within the different hippocampal subfields with highlighted laminar organization (adapted from Ramon y [START_REF] Redish | Independence of firing correlates of anatomically proximate hippocampal pyramidal cells[END_REF].

Within this common structural organization, principal cells from each region display specific characteristics in terms of morphology, dendritic arborization, electrophysiological properties, connectivity, and gene expression profiles (discussed here and in Section I.2.a.ii). Principal cells of the DG, the granule cells (GCs), have small ovoid cell bodies and a highly polarized dendritic tree that arborizes only on their basal side in stratum moleculare (SM) [START_REF] Claiborne | Quantitative, three-dimensional analysis of granule cell dendrites in the rat dentate gyrus[END_REF]. Their axons emerge on the apical side in the hilus and extend towards area CA3 in SL. Pyramidal neurons (PNs) of area CA3 are large cells with basal dendrites in SO and bifurcated apical ones in SL, SR and SLM. The dendritic portion in SL displays thorny excrescences (TEs) where MF from the DG make synaptic contact. Preferential axonal projections of CA3 PNs varies according to their location on the CA3a (closer to CA2)bc (closer to DG) axis. CA3a PNs give rise to extensive recurrent connections and project to proximal CA1 (closer to CA2). CA3c PNs contribute less to the recurrent system and project more towards distal CA1 (closer to the subiculum) [START_REF] Ishizuka | Organization of intrahippocampal projections originating from CA3 pyramidal cells in the rat[END_REF][START_REF] Li | The hippocampal CA3 network: An in vivo intracellular labeling study[END_REF]. CA1

PNs have smaller somas and a different apical dendritic organization than CA3. In contrast to CA3, CA1 PNs apical dendrites do not bifurcate in two apical branches but instead arborize extensively in SR where CA3 afferent axons project, and more modestly in SLM [START_REF] Bannister | Dendritic morphology of CA1 pyramidal neurones from the rat hippocampus: I. Branching patterns[END_REF]. CA2 PNs differ from both CA3 and CA1 in that they have large soma, bifurcated apical dendrites that do not display TEs and arborize very little in SR but dramatically in SLM where inputs from EC form synapses [START_REF] Chevaleyre | Strong CA2 Pyramidal Neuron Synapses Define a Powerful Disynaptic Cortico-Hippocampal Loop[END_REF][START_REF] Mercer | Characterization of Neurons in the CA2 Subfield of the Adult Rat Hippocampus[END_REF][START_REF] Piskorowski | Synaptic integration by different dendritic compartments of hippocampal CA1 and CA2 pyramidal neurons[END_REF]. Electrophysiological properties of CA2 PNs are also unique with respect to CA1 and CA3 PNs. CA2 PNs show a low membrane resistance, hyperpolarized resting membrane potential, low hyperpolarizing sag current, and no after spike hyperpolarization [START_REF] Chevaleyre | Strong CA2 Pyramidal Neuron Synapses Define a Powerful Disynaptic Cortico-Hippocampal Loop[END_REF][START_REF] Piskorowski | Age-Dependent Specific Changes in Area CA2 of the Hippocampus and Social Memory Deficit in a Mouse Model of the 22q11.2 Deletion Syndrome[END_REF][START_REF] Sun | Proximodistal Heterogeneity of Hippocampal CA3 Pyramidal Neuron Intrinsic Properties, Connectivity, and Reactivation during Memory Recall[END_REF]. Besides variations on the proximo-distal axis, PNs vary according to their somatic location on the radial axis of the hippocampus. In area CA1, deep cells closer to SO and superficial cells closer to SR differ in terms of genetic expression, afferent innervation, efferent projections, and physiological functions [START_REF] Danielson | Sublayer-Specific Coding Dynamics during Spatial Navigation and Learning in Hippocampal Area CA1[END_REF][START_REF] Lee | Parvalbumin-Positive Basket Cells Differentiate among Hippocampal Pyramidal Cells[END_REF][START_REF] Masurkar | Medial and Lateral Entorhinal Cortex Differentially Excite Deep versus Superficial CA1 Pyramidal Neurons[END_REF][START_REF] Mizuseki | Hippocampal CA1 pyramidal cells form functionally distinct sublayers[END_REF][START_REF] Valero | Determinants of different deep and superficial CA1 pyramidal cell dynamics during sharp-wave ripples[END_REF]. In particular, CA2 PNs drive stronger excitation on deep compared to superficial CA1 PNs [START_REF] Kohara | Cell type-specific genetic and optogenetic tools reveal hippocampal CA2 circuits[END_REF][START_REF] Valero | Determinants of different deep and superficial CA1 pyramidal cell dynamics during sharp-wave ripples[END_REF].

Interestingly, in vivo functional differences between deep and superficials PNs have recently been reported in area CA2 (Oliva et al., 2016a), however the cellular and circuit mechanisms underlying these observations are not yet elucidated.

Besides excitatory pyramidal neurons, the hippocampus contains inhibitory interneurons that gate the inputs and outputs of PNs, shape network oscillations and participate in synaptic plasticity. In contrast with pyramidal neurons that form a relatively homogeneous population, interneurons are extremely diverse in terms of morphology, electrophysiological properties and expression of genetic markers, reflecting their involvement in various distinct functions [START_REF] Freund | Interneurons of the hippocampus[END_REF][START_REF] Somogyi | Defined types of cortical interneurone structure space and spike timing in the hippocampus[END_REF]. INs can be classified based on morphological features that allow functional predictions such as the location of their soma, the extension of their dendritic tree, and the targets of their axonal projections. Additional genetic and electrophysiological criteria provide further refinement of often overlapping classifications.

A first functional group of INs target the dendrites of PNs and are therefore likely to control their afferent synaptic inputs. O-LM cells have their soma and dendrites located in SO and project their axon all the way to SLM [START_REF] Maccaferri | Cell surface domain specific postsynaptic currents evoked by identified GABAergic neurones in rat hippocampus in vitro[END_REF][START_REF] Mcbain | Activation of metabotropic glutamate receptors differentially affects two classes of hippocampal interneurons and potentiates excitatory synaptic transmission[END_REF], thus potentially controlling EC inputs onto distal PNs or INs dendrites [START_REF] Somogyi | Defined types of cortical interneurone structure space and spike timing in the hippocampus[END_REF]. O-LM cells express somatostatin (SOM) and fire action potentials quite regularly [START_REF] Maccaferri | Cell surface domain specific postsynaptic currents evoked by identified GABAergic neurones in rat hippocampus in vitro[END_REF]. Bi-stratified INs also target PNs and INs dendrites but in SO and SR where they potentially control afferent excitatory inputs from CA3 on proximal dendrites (Buhl et al., 1994;[START_REF] Somogyi | Defined types of cortical interneurone structure space and spike timing in the hippocampus[END_REF]. They have a soma in SP or SO, dendrites extending in SO and SR, express SOM and show little accommodation in their firing pattern [START_REF] Maccaferri | Cell surface domain specific postsynaptic currents evoked by identified GABAergic neurones in rat hippocampus in vitro[END_REF][START_REF] Pawelzik | Modulation of bistratified cell IPSPs and basket cell IPSPs by pentobarbitone sodium, diazepam and Zn2+: dual recordings in slices of adult rat hippocampus[END_REF]. SC-associated cells show a similar targeting of their axon to SR but have their soma located in SR, dendrites spanning SLM through SO, express cholecystokinin (CCK) and significantly accommodate their AP firing [START_REF] Pawelzik | Physiological and morphological diversity of immunocytochemically defined parvalbumin-and cholecystokinin-positive interneurones in CA1 of the adult rat hippocampus[END_REF][START_REF] Vida | Unitary IPSPs evoked by interneurons at the stratum radiatum-stratum lacunosum-moleculare border in the CA1 area of the rat hippocampus in vitro[END_REF]. The other broad class of INs have axons targeting the soma or axon initial segment of PNs and are therefore in position of controlling their ouput. It is especially the case for axo-axonic cells whose axon targets axon initial segment of PNs and are thus likely to affect action potential generation in these cells [START_REF] Somogyi | Defined types of cortical interneurone structure space and spike timing in the hippocampus[END_REF]. Axoaxonic cells extend their dendrites from SO through SLM, express parvalbumin (PV), and fire action potentials with either little or marked accommodation (Buhl et al., 1994a;[START_REF] Somogyi | Defined types of cortical interneurone structure space and spike timing in the hippocampus[END_REF]. The major IN type controlling PN output is the basket cell (BC) population, named after the dense peri-somatic net formed by axons from these cells (Buhl et al., 1994a).

BCs have their soma in SP or SR, their dendrites spanning all CA strata, and express either PV or CCK [START_REF] Maccaferri | Cell surface domain specific postsynaptic currents evoked by identified GABAergic neurones in rat hippocampus in vitro[END_REF][START_REF] Pawelzik | Physiological and morphological diversity of immunocytochemically defined parvalbumin-and cholecystokinin-positive interneurones in CA1 of the adult rat hippocampus[END_REF]. Remarkably, PV-expressing BCs are mostly fast-spiking whereas CCK-expressing BCs are mostly regular-spiking [START_REF] Pawelzik | Physiological and morphological diversity of immunocytochemically defined parvalbumin-and cholecystokinin-positive interneurones in CA1 of the adult rat hippocampus[END_REF].

Regardless of their type, INs can be involved in different type of wiring in hippocampal networks. The source of INs excitation can be local PNs that are subsequently inhibited by the INs : this scenario is called feedback inhibition. Feedback inhibition controls the duration and spread of excitation in PNs and can therefore pace their activity [START_REF] Bartos | Role of microcircuit structure and input integration in hippocampal interneuron recruitment and plasticity[END_REF].

Alternatively, feedforward inhibition happens when a common excitatory inputs targets both PNs and INs that can inhibit the PNs even before their AP discharge. Feedforward inhibition curtails excitation in post-synaptic PNs thus increasing the temporal precision of their AP firing [START_REF] Pouille | Enforcement of Temporal Fidelity in Pyramidal Cells by Somatic Feed-Forward Inhibition[END_REF]. In addition, INs can be engaged in mutual inhibition if they target one another, much like recurrent excitation amongst PNs but with opposite effect. Finally, certain populations of INs such as BCs are electrically coupled by gap junctions allowing for the propagation of membrane potential variations amongst these networks of INs.

Like the pyramidal neurons in area CA2, there is mounting evidence that the interneurons in this region have unique properties and potentially play significant roles in memory formation and disease. Studies performing hippocampal-wide comparison of different subclasses of interneurons based on immunological markers have revealed that CA2 comprises a peculiar composition of interneurons as compared to CA1 and CA3 [START_REF] Botcher | Distribution of interneurons in the CA2 region of the rat hippocampus[END_REF]. While most immunohistological studies overlook area CA2 when quantifying interneuron density, recent studies have performed detailed quantification of interneuron densities along with concomitant staining of CA2-specific proteins. One very thorough study performed in rat hippocampus examined the density and stratum-localization of interneurons by staining for GAD-67, parvalbumin (PV), CCK, caretinin, calbindin, reelin, somatostatin, NPY and VIP, all with CA2 borders well-defined by either PCP-4 or alpha-actinin 2 staining [START_REF] Botcher | Distribution of interneurons in the CA2 region of the rat hippocampus[END_REF]. This study and others found that area CA2 contains the highest density of PV-expressing interneurons [START_REF] Botcher | Distribution of interneurons in the CA2 region of the rat hippocampus[END_REF][START_REF] Piskorowski | Delta-Opioid Receptors Mediate Unique Plasticity onto Parvalbumin-Expressing Interneurons in Area CA2 of the Hippocampus[END_REF]. Furthermore, area CA2 contains the highest density of reelin-expressing neurons in the hippocampus [START_REF] Botcher | Distribution of interneurons in the CA2 region of the rat hippocampus[END_REF], which is very interesting given the various signaling roles of reelin in the adult hippocampus [START_REF] Kupferman | Reelin Signaling Specifies the Molecular Identity of the Pyramidal Neuron Distal Dendritic Compartment[END_REF].

Using sharp recordings, Mercer et al. performed elegant and detailed analysis of individual CA2

interneurons in hippocampal slices from rats. Their work revealed unexpected morphological and electrophysiological properties of basket cells and bistratified cells in this region (Mercer et al., 2012a;[START_REF] Mercer | SP-SR interneurones: A novel class of neurones of the CA2 region of the hippocampus[END_REF]. Area CA2 hosts two types of PV-expressing (PV+) basket cells: a minority resemble classical CA1 basket cells with narrow dendritic arborization in the septotemporal axis, no sag potential and non-adapting fast spiking firing pattern, while the majority have a broad dendritic arborization, substantial sag potential and an adapting firing pattern normally encountered in CA1 O-LM interneurons [START_REF] Mercer | Characterization of Neurons in the CA2 Subfield of the Adult Rat Hippocampus[END_REF]. Wide-arbor basket cells further differed by displaying excitatory post-synaptic potentials (EPSPs) elicited by presynaptic CA2 pyramidal neuron stimulation that were less depressing and longer in duration due to a NMDAR component (Mercer et al., 2012a). Strikingly, similar observations were made regarding bistratified cells in area CA2, which consisted of narrow and wide-arbor subpopulations, indicating that the width of dendritic arborization, rather than axonal targeting, could serve as a reliable determinant of electrophysiological properties of interneurons in area CA2 [START_REF] Mercer | Characterization of Neurons in the CA2 Subfield of the Adult Rat Hippocampus[END_REF]2012a). Finally, a novel type of interneuron was described in area CA2 with similar electrophysiological properties and morphologies as wide-arbor bistratified cells but with axons restricted to SR (SP-SR interneurons) [START_REF] Mercer | SP-SR interneurones: A novel class of neurones of the CA2 region of the hippocampus[END_REF]. How these unusual types of basket cells, bistratified cells and SP-SR interneurons participate in controlling information flow in the hippocampal network remains to be understood.

I.2.a.ii -Inter-regional connectivity and information processing

As described above, the hippocampal formation comprises different regions with specific connectivity allowing for information processing. Inputs from layers II of the medial and lateral entorhinal cortex (EC) transfer information to the DG that then targets area CA3 proximal dendrites through the mossy fibers (MF) in the stratum lucidum (SL), CA3 subsequently makes synaptic contact on the proximal dendrites of area CA1 through the Schaffer collaterals in SR and CA1 returns information to the EC, thus forming the tri-synaptic loop [START_REF] Andersen | Lamellar organization of hippocampal excitatory pathways[END_REF]. Additional routes running through the SLM allow the EC to directly impinge on the distal dendrites of pyramidal neurons in all CA regions [START_REF] Ruth | Topography between the entorhinal cortex and the dentate septotemporal axis in rats: I. Medial and intermediate entorhinal projecting cells[END_REF][START_REF] Freund | GABA-containing neurons in the septum control inhibitory interneurons in the hippocampus[END_REF][START_REF] Steward | Cells of origin of entorhinal cortical afferents to the hippocampus and fascia dentata of the rat[END_REF]. Much can be learnt about the functions of this tri-synaptic loop by considering the physiology of each region and the computation performed at each step of the circuit.

Afferent information to the hippocampus come from EC inputs that convey multimodal sensory [START_REF] Canto | What Does the Anatomical Organization of the Entorhinal Cortex Tell Us?[END_REF], metric [START_REF] Hafting | Microstructure of a spatial map in the entorhinal cortex[END_REF] and directional signals [START_REF] Taube | Head-direction cells recorded from the postsubiculum in freely moving rats. I. Description and quantitative analysis[END_REF].

Axons from the EC run through the perforant path and make glutamatergic synapses onto the dendrites of granule cells in stratum moleculare (SM) [START_REF] Deller | A novel entorhinal projection to the rat dentate gyrus: direct innervation of proximal dendrites and cell bodies of granule cells and GABAergic neurons[END_REF]. This information is processed by the dentate gyrus to allow pattern separation, i.e. the discrimination of similar inputs by sending a specific output. Indeed, dentate gyrus lesions impair spatial pattern separation in rats [START_REF] Gilbert | Dissociating hippocampal subregions: A double dissociation between dentate gyrus and CA1[END_REF]. This remarkable function of the dentate gyrus can be explained by the underlying physiology of its network. Although electrotonically compact, granule cells have a hyperpolarized resting membrane potential [START_REF] Spruston | Perforated patch-clamp analysis of the passive membrane properties of three classes of hippocampal neurons[END_REF] making it potentially difficult for EC excitatory inputs to elicit action potential firing in postsynaptic granule cells. In addition, both EC inputs and granule cells excite dentate gyrus interneurons, thus recruiting feedforward and feedback inhibition, respectively [START_REF] Freund | Interneurons of the hippocampus[END_REF][START_REF] Han | A High Degree of Spatial Selectivity in the Axonal and Dendritic Domains of Physiologically Identified Local-circuit Neurons in the Dentate Gyms of the Rat Hippocampus[END_REF][START_REF] Kneisler | Spontaneous and synaptic input from granule cells and the perforant path to dentate basket cells in the rat hippocampus[END_REF][START_REF] Sloviter | Feedforward and feedback inhibition of hippocampal principal cell activity evoked by perforant path stimulation: GABA-mediated mechanisms that regulate excitability In Vivo[END_REF]. Altogether, these characteristics confer low excitability to dentate gyrus granule cells [START_REF] Jung | Spatial selectivity of unit activity in the hippocampal granular layer[END_REF][START_REF] Penttonen | Feed-forward and feed-back activation of the dentate gyrus in vivo during dentate spikes and sharp wave bursts[END_REF]. Therefore, excitatory transmission from EC inputs is likely to result in the activation of only a sparse population of granule cells. This is believed to allow pattern separation in the dentate gyrus as similar EC inputs would result in different granule cells outputs.

Axons from the dentate gyrus granule cells form the mossy fibers (MF) that run through SL towards the very proximal part of CA3 PNs. The synapse between mossy fibers and CA3 PNs consists of a mossy presynaptic bouton and a large postsynaptic differentiation called thorny excrescence. A single granule cell axon contacts thorny excrescences of a limited number of postsynaptic CA3 PNs [START_REF] Acsády | GABAergic cells are the major postsynaptic targets of mossy fibers in the rat hippocampus[END_REF], thus keeping the pattern separated information orthogonalized. Efficacy of transmission at these synapses can be high and can reliably trigger action potential firing in CA3 PNs provided pre-synaptic granule cells fire repeatedly [START_REF] Henze | Single granule cells reliably discharge targets in the hippocampal CA3 network in vivo[END_REF]. In addition, mossy fibers also project onto CA3 interneurons [START_REF] Acsády | GABAergic cells are the major postsynaptic targets of mossy fibers in the rat hippocampus[END_REF] that in turn drive feedforward inhibition onto CA3 PNs. Much like the dentate gyrus, CA3 PNs and INs receive EC inputs in SLM [START_REF] Kiss | Entorhinal cortical innervation of parvalbumin-containing neurons (basket and chandelier cells) in the rat ammon's horn[END_REF][START_REF] Witter | Functional organization of the extrinsic and intrinsic circuitry of the parahippocampal region[END_REF]. From the information conveyed by these two upstream inputs, area CA3 performs pattern completion, i.e. builds a coherent output from partial inputs [START_REF] Rolls | A theory of hippocampal function in memory[END_REF]. Pattern completion in area CA3 has been evidenced by the reactivation of CA3 ensembles after exposure to similar contexts [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], and by impaired recall in mice lacking NMDARs in CA3 PNs [START_REF] Nakazawa | Requirement for Hippocampal CA3 NMDA Receptors in Associative Memory Recall[END_REF]. A notable feature of the CA3 network that likely underlies pattern completion is the prominent recurrent excitation between CA3 PNs [START_REF] Miles | Excitatory synaptic interactions between CA3 neurones in the guinea-pig hippocampus[END_REF].

These recurrent excitatory connections between CA3 PNs, together with sparse but strong input from the dentate gyrus [START_REF] Geiger | Dynamic Control of Presynaptic Ca2+ Inflow by Fast-Inactivating K+ Channels in Hippocampal Mossy Fiber Boutons[END_REF][START_REF] Henze | Single granule cells reliably discharge targets in the hippocampal CA3 network in vivo[END_REF] and the recruitment of interneurons that drive feedback inhibition [START_REF] Miles | Synaptic excitation of inhibitory cells by single CA3 hippocampal pyramidal cells of the guinea-pig in vitro[END_REF], are thought to allow the reactivation of "meaningful" CA3 PNs ensembles from sparse dentate gyrus inputs. It should be noted that pattern completion is highly specific as small changes in context can lead to orthogonalization of spatial representation in area CA3 [START_REF] Leutgeb | Distinct Ensemble Codes in Hippocampal Areas CA3 and CA1[END_REF].

CA3 PNs in turn project onto CA1 PNs proximal dendrites via the Schaffer collaterals in SR.

The CA3 -CA1 PN synapses have been shown to bring about modest levels of excitation [START_REF] Larkman | Quantal analysis of excitatory synaptic action and depression in hippocampal slices[END_REF][START_REF] Sayer | The time course and amplitude of EPSPs evoked at synapses between pairs of CA3/CA1 neurons in the hippocampal slice[END_REF]. Afferent CA3 axons also recruit feedforward inhibition onto CA1 PNs by exciting local CA1 interneurons [START_REF] Lacaille | Postsynaptic potentials mediated by excitatory and inhibitory amino acids in interneurons of stratum pyramidale of the CA1 region of rat hippocampal slices in vitro[END_REF]. Entorhinal inputs provide another source of excitation [START_REF] Yeckel | Feedforward excitation of the hippocampus by afferents from the entorhinal cortex: redefinition of the role of the trisynaptic pathway[END_REF] to CA1 PNs and to interneurons that can contribute in feedforward inhibition [START_REF] Kiss | Entorhinal cortical innervation of parvalbumin-containing neurons (basket and chandelier cells) in the rat ammon's horn[END_REF]. However, excitatory synapses formed in SLM by EC axons are inefficient in driving CA1 PNs action potential firing because of EPSPs generated in SLM are dampened by filtering on their way from the distal dendrites to the soma [START_REF] Andreasen | Factors determining the efficacy of distal excitatory synapses in rat hippocampal CA1 pyramidal neurones[END_REF][START_REF] Golding | Factors mediating powerful voltage attenuation along CA1 pyramidal neuron dendrites[END_REF][START_REF] Magee | Dendritic hyperpolarization-activated currents modify the integrative properties of hippocampal CA1 pyramidal neurons[END_REF]. Therefore, EC inputs require summation with other sources of excitation to drive activity in postsynaptic CA1 PNs. This puts area CA1 in a good position to act as an integrator of current experience through EC inputs and past experience "kept in memory" relayed by the DG -CA3 -CA1 path.

Furthermore, excitatory recurrent connections are rare in area CA1 [START_REF] Deuchars | CA1 pyramid-pyramid connections in rat hippocampus in vitro: Dual intracellular recordings with biocytin filling[END_REF] while feedback inhibition is prominent (Ali and Thomson, 1998;Ali et al., 1998;Buhl et al., 1994a). Altogether, these properties of the CA1 network are thought to allow weighted representations of past and present experience to be formed and sent to outside of the hippocampus. Efferent projections from CA1 PNs target the entorhinal cortex, directly or via the subiculum, hence closing the loop [START_REF] Naber | Reciprocal connections between the entorhinal cortex and hippocampal fields CA1 and the subiculum are in register with the projections from CA1 to the subiculum[END_REF]. The accumulated knowledge of the functions of the tri-synaptic loop led to the idea that it supports memory formation in the hippocampus through pattern separation in the DG, pattern completion in area CA3, and integration in area CA1. However, in vivo testing of this view by disruption of the transmission between CA3 and CA1 led to unexpected results. Indeed, major aspects of hippocampal-dependent spatial memory were unaffected by lesions or genetic block of the CA3 -CA1 transmission [START_REF] Brun | Place Cells and Place Recognition Maintained by Direct Entorhinal-Hippocampal Circuitry[END_REF][START_REF] Nakashiba | Transgenic Inhibition of Synaptic Transmission Reveals Role of CA3 Output in Hippocampal Learning[END_REF]. The mono-synaptic EC -CA1 loop was proposed to rescue mnemonic functions of the hippocampus in the absence of the tri-synaptic loop. However, this interpretation is in contradiction with the inability of EC inputs to efficiently drive CA1 PNs. Shortly thereafter, the discovery of a di-synaptic loop bypassing the DG and area CA3 but rather involving area CA2 provided an alternative hypothesis for the rescue of hippocampal memory formation in the absence of transmission between CA3 and CA1 [START_REF] Chevaleyre | Strong CA2 Pyramidal Neuron Synapses Define a Powerful Disynaptic Cortico-Hippocampal Loop[END_REF].

I.2.b -Integration of area CA2 inputs and outputs in the hippocampal circuit

I.2.b.i -Intra-hippocampal connectivity of area CA2

To understand and support the role of area CA2 in information transfer through the hippocampus, one musts characterize its connections with other hippocampal areas. The intrahippocampal inputs to area CA2 have been studied using tracing, optogenetics and electrophysiology in vivo and ex vivo. Area CA2 receives direct input in SLM from the EC [START_REF] Bartesaghi | Parallel activation of field CA2 and dentate gyrus by synaptically elicited perforant path volleys[END_REF][START_REF] Chevaleyre | Strong CA2 Pyramidal Neuron Synapses Define a Powerful Disynaptic Cortico-Hippocampal Loop[END_REF][START_REF] Cui | Hypothalamic and other connections with dorsal CA2 area of the mouse hippocampus[END_REF][START_REF] Hitti | The hippocampal CA2 region is essential for social memory[END_REF][START_REF] Kohara | Cell type-specific genetic and optogenetic tools reveal hippocampal CA2 circuits[END_REF]. In vivo recordings from anesthetized guinea pigs reported that area CA2 is the first CA region to be active in response to EC input stimulation [START_REF] Bartesaghi | Parallel activation of field CA2 and dentate gyrus by synaptically elicited perforant path volleys[END_REF], indicating a strong synaptic connection. This synaptic input was further elucidated with acute mouse brain slice recordings, revealing that mild electrical stimulation of these inputs is sufficient to make CA2 pyramidal neurons fire action potentials (APs) [START_REF] Chevaleyre | Strong CA2 Pyramidal Neuron Synapses Define a Powerful Disynaptic Cortico-Hippocampal Loop[END_REF]. This strong synaptic connection in area CA2 contrasts with distal CA1 cortical inputs, which has both high levels of feedforward inhibition as well as a large Ih current that prevents strong excitation of CA1 PNs [START_REF] Chevaleyre | Strong CA2 Pyramidal Neuron Synapses Define a Powerful Disynaptic Cortico-Hippocampal Loop[END_REF][START_REF] Nolan | A Behavioral Role for Dendritic Integration HCN1 Channels Constrain Spatial Memory and Plasticity at Inputs to Distal Dendrites of CA1 Pyramidal Neurons[END_REF]). Thus, it is possible that cortical inputs directly excite CA2 PNs, which then project to CA1 PNs, forming a di-synaptic hippocampal circuit. The EC -CA2 connection was further examined using conventional retrograde tracing from area CA2 in mice. Cui et al. reported afferent projections from primarily layer II EC cells, with a few layer III cells [START_REF] Cui | Hypothalamic and other connections with dorsal CA2 area of the mouse hippocampus[END_REF]. Further, the generation of two different transgenic mouse lines expressing Cre recombinase specifically in CA2 pyramidal neurons allowed Hitti and Siegelbaum with the Amigo2-Cre line and Kohara et al. with the MAP3K15-Cre line to examine this input in a highly controlled manner. Both studies confirmed a strong layer II EC with sparcely labeled layer III cells following retrograde rabies tracing [START_REF] Hitti | The hippocampal CA2 region is essential for social memory[END_REF][START_REF] Kohara | Cell type-specific genetic and optogenetic tools reveal hippocampal CA2 circuits[END_REF]. DG granule cell mossy fibers form functional synapses with CA2 pyramidal neurons, as shown by optogenetic stimulation [START_REF] Kohara | Cell type-specific genetic and optogenetic tools reveal hippocampal CA2 circuits[END_REF]. Mossy fiber inputs onto CA2 PNs have been shown to differ from the well-studied DG -CA3 synapse. As originally observed by Lorenté de No, CA2 pyramidal neurons lack the thorny excressences that are stereotypical for the mossy fiber -CA3 synapse (No 1934) [START_REF] Kohara | Cell type-specific genetic and optogenetic tools reveal hippocampal CA2 circuits[END_REF]. Furthermore, the DG -CA2 synapse has relatively much lower levels of excitatory transmission as compared to CA3 [START_REF] Sun | Proximodistal Heterogeneity of Hippocampal CA3 Pyramidal Neuron Intrinsic Properties, Connectivity, and Reactivation during Memory Recall[END_REF].

This input is capable of driving feedforward inhibition by recruitment of CA2 interneurons [START_REF] Kohara | Cell type-specific genetic and optogenetic tools reveal hippocampal CA2 circuits[END_REF]. Interestingly, it has been proposed from immunohistological studies that newborn DG granule cells project their axons to area CA2 [START_REF] Llorens-Martín | Novel connection between newborn granule neurons and the hippocampal CA2 field[END_REF]. This synapse merits further examination, as the DG was unlabeled in retrograde studies performed in the rat [START_REF] Cui | Hypothalamic and other connections with dorsal CA2 area of the mouse hippocampus[END_REF], indicating a potential species-specific difference. Furthermore, in the Amigo2-cre line, this connection was not identified with retrograde tracing [START_REF] Hitti | The hippocampal CA2 region is essential for social memory[END_REF] but was with experiments performed in the MAP3K15-Cre line [START_REF] Kohara | Cell type-specific genetic and optogenetic tools reveal hippocampal CA2 circuits[END_REF]. These differences may be due to technical differences between vectors and transgenic lines.

Conventional and rabies-based retrograde labeling in mice showed that CA2 pyramidal neurons receive inputs from both CA3 PNs as well as CA2 PNs [START_REF] Cui | Hypothalamic and other connections with dorsal CA2 area of the mouse hippocampus[END_REF][START_REF] Hitti | The hippocampal CA2 region is essential for social memory[END_REF]. Interestingly, stimulation of CA3 Schaffer collaterals in SR on acute slices from mice revealed that this input is dominated by a strong feedforward inhibitory drive in area CA2, unlike in CA1 [START_REF] Chevaleyre | Strong CA2 Pyramidal Neuron Synapses Define a Powerful Disynaptic Cortico-Hippocampal Loop[END_REF]. This strong feedforward inhibitory transmission between CA3 and CA2 strongly contrasts with the strong direct excitation and limited feedforward inhibition observed following distal EC input stimulation [START_REF] Chevaleyre | Strong CA2 Pyramidal Neuron Synapses Define a Powerful Disynaptic Cortico-Hippocampal Loop[END_REF]. In the context of information transfer, this large feedforward inhibition recruited by area CA3 onto CA2 indicates that area CA2 may not be recruited by CA3 neurons and may be bypassed in the tri-synaptic circuit. This alternative pathway for information flow had been previously proposed based on field recordings and voltage-sensitive dye imaging studies of entire hippocampal slices [START_REF] Sekino | Delayed Signal Propagation via CA2 in Rat Hippocampal Slices Revealed by Optical Recording[END_REF]. This study found that, in the majority of instances, no activity was detected in area CA2 following stimulation of the hilus. However, in some instances, a delayed activation of CA2 was detected that led to a larger and delayed signal propagation of area CA1. This early and insightful work may be revealing a potential role of area CA2 in hippocampal network function. Remarkably, the PV+ population of interneurons is the substrate of a long-term depression of the CA3 to CA2 feedforward inhibition [START_REF] Piskorowski | Delta-Opioid Receptors Mediate Unique Plasticity onto Parvalbumin-Expressing Interneurons in Area CA2 of the Hippocampus[END_REF] that subsequently allows CA2 to be recruited by CA3 inputs [START_REF] Nasrallah | Inhibitory Plasticity Permits the Recruitment of CA2 Pyramidal Neurons by CA3[END_REF]. Hence, the inhibitory gating of area CA2 activity is also unique and shows dysfunctions in schizophrenia due to a loss of PV+ interneurons [START_REF] Piskorowski | Age-Dependent Specific Changes in Area CA2 of the Hippocampus and Social Memory Deficit in a Mouse Model of the 22q11.2 Deletion Syndrome[END_REF].

The intra-hippocampal outputs of area CA2 have been investigated with tracing experiments, ex vivo electrophysiology and genetically restricted optogenetics. Interestingly, by selectively infecting EC neurons with rabies viruses in mice and conventional retrograde tracing in both mice and rats, Rowland et al. showed that area CA2 sends a return projection to EC layer II neurons [START_REF] Rowland | Transgenically Targeted Rabies Virus Demonstrates a Major Monosynaptic Projection from Hippocampal Area CA2 to Medial Entorhinal Layer II Neurons[END_REF]. Although this observation stands alone from other studies using either conventional or genetically targeted anterograde viral tracing from area CA2 [START_REF] Cui | Hypothalamic and other connections with dorsal CA2 area of the mouse hippocampus[END_REF][START_REF] Hitti | The hippocampal CA2 region is essential for social memory[END_REF], the retrograde method used by Rowland et al. is not restricted to a specific pre-synaptic cell type and is potentially more efficient in labeling. This intriguing finding merits replication and further investigation. Using unilateral viral anterograde labeling of CA2 neurons in mice, Cui et al. reported ipsi-and contra-lateral projections to areas CA1, CA2 and CA3 [START_REF] Cui | Hypothalamic and other connections with dorsal CA2 area of the mouse hippocampus[END_REF]. Paired recordings from CA2 and CA1 pyramidal neurons on acute hippocampal slices from mice directly proved an excitatory monosynaptic connection from CA2 to CA1 that appeared several-fold stronger than the CA3 to CA1 connection [START_REF] Chevaleyre | Strong CA2 Pyramidal Neuron Synapses Define a Powerful Disynaptic Cortico-Hippocampal Loop[END_REF]. Taking advantage of the Amigo2-Cre transgenic mouse line, Hitti et al. also reported axons of CA2 pyramidal neurons projecting to every CA area [START_REF] Hitti | The hippocampal CA2 region is essential for social memory[END_REF]. The CA2 to CA1 inputs were further examined with optogenetics using the MAP3K15-Cre mouse line, revealing that CA2 PNs have different excitatory drive along the radial CA1 axis, providing a much stronger excitation to deep CA1 pyramidal neurons [START_REF] Kohara | Cell type-specific genetic and optogenetic tools reveal hippocampal CA2 circuits[END_REF]. This is quite interesting, as the deep and superficial CA1 pyramidal neurons project to different cortical and limbic regions [START_REF] Lee | Parvalbumin-Positive Basket Cells Differentiate among Hippocampal Pyramidal Cells[END_REF]. Using a different CA2-specific mouse Cre line driven by the calcium voltage-gated channel auxiliary subunit gamma 5 (CACNG5) promoter region, Boehringer et al. examined CA2 pyramidal neuron output to areas CA1 and CA3 in acute slices. This study showed that, while area CA2 recruits direct excitation and feedforward inhibition in both areas CA1 and CA3, the inhibitory drive of CA2 predominates in CA3 [START_REF] Boehringer | Chronic Loss of CA2 Transmission Leads to Hippocampal Hyperexcitability[END_REF]. In summary, by projecting to every CA subfield, CA2 pyramidal neurons are poised to act on the entire hippocampal network: there is a reciprocal control of E/I balance between areas CA2 and CA3, whereas CA2 acts very strongly to preferentially excite deep CA1 pyramidal neurons, thereby influencing hippocampal output.

I.2.b.ii -Area CA2 long-range afferences and efferences

In addition to forming the hub of an intrinsic hippocampal network, area CA2 is connected with several extrahippocampal structures. Retrograde tracing from area CA2 and anterograde labeling of vasopressinergic neurons in the paraventricular nucleus of the hypothalamus (PVN) revealed that area CA2 is targeted by inputs containing vasopressin [START_REF] Cui | Hypothalamic and other connections with dorsal CA2 area of the mouse hippocampus[END_REF]. The PVN is strongly activated in stressful and social situations and results in the release of oxytocin and vasopressin throughout the brain (reviewed by [START_REF] Iovino | Synaptic inputs of neural afferent pathways to vasopressin-and Oxytocin-secreting neurons of Supraoptic and Paraventricular hypothalamic nuclei[END_REF], thus area CA2 is likely to be directly modulated during these circumstances. Additional long-range inputs to area CA2 have been described from the medial septum, diagonal band of Broca and median raphé that were labeled after retrograde marker injections in area CA2 of mice [START_REF] Cui | Hypothalamic and other connections with dorsal CA2 area of the mouse hippocampus[END_REF].

Projections from these 3 regions were confirmed with rabies-based retrograde tracing from CA2 pyramidal neurons in the Amigo2-Cre and MAP3K15-Cre mouse lines [START_REF] Hitti | The hippocampal CA2 region is essential for social memory[END_REF][START_REF] Kohara | Cell type-specific genetic and optogenetic tools reveal hippocampal CA2 circuits[END_REF].

Another hypothalamic projection to area CA2 arises from neurons in the supramammillary nucleus (SuM) [START_REF] Haglund | The projection of the supramammillary nucleus to the hippocampal formation: An immunohistochemical and anterograde transport study with the lectin PHA-L in the rat[END_REF][START_REF] Vertes | PHA-L analysis of projections from the supramammillary nucleus in the rat[END_REF]. Albeit a relatively small area, the SuM has a significant role in the central nervous system through its connections with numerous brain structures. It receives inputs from other hypothalamic nuclei and also from the raphe, the habenula, the central gray, the septum and cortical regions [START_REF] Pan | The supramammillary area: its organization, functions and relationship to the hippocampus[END_REF]. The SuM outputs are diverse as well and include reciprocal connections several hypothalamic areas, the raphe, the central gray, the septum and the cingulate and infralimbic cortex [START_REF] Pan | The supramammillary area: its organization, functions and relationship to the hippocampus[END_REF][START_REF] Vertes | PHA-L analysis of projections from the supramammillary nucleus in the rat[END_REF]. In addition, the SuM sends projections to the locus coeruleus, the thalamus, the amygdala, the EC and the hippocampus [START_REF] Pan | The supramammillary area: its organization, functions and relationship to the hippocampus[END_REF][START_REF] Vertes | PHA-L analysis of projections from the supramammillary nucleus in the rat[END_REF]. Neurons in the SuM are chemically diverse and have been reported to express calretinin, CCK, VGluT2, VGAT, dopamine, VIP, substance P and nitric oxide synthase (NOS) [START_REF] Pan | The supramammillary area: its organization, functions and relationship to the hippocampus[END_REF]). In the hippocampus, afferent axons originate from calretinin-, VGluT2-, VGAT-, NOS-, VIP-, CCK-and substance P-expressing neuronal populations in the SuM [START_REF] Berger | Human and monkey fetal brain development of the supramammillary-hippocampal projections: A system involved in the regulation of theta activity[END_REF]Borhegyi and Leranth, 1997;[START_REF] Haglund | The projection of the supramammillary nucleus to the hippocampal formation: An immunohistochemical and anterograde transport study with the lectin PHA-L in the rat[END_REF][START_REF] Kiss | The supramammillo-hippocampal and supramammillo-septal glutamatergic/aspartatergic projections in the rat: a combined [3H]daspartate autoradiographic and immunohistochemical study[END_REF][START_REF] Pedersen | Supramammillary glutamate neurons are a key node of the arousal system[END_REF]. Interestingly, a subpopulation of SuM neurons has been shown to project both to the hippocampus and to the septum [START_REF] Borhegyi | The supramammillary nucleus innervates cholinergic and GABAergic neurons in the medial septum-diagonal band of Broca complex[END_REF][START_REF] Vertes | Collateral projections from the supramammillary nucleus to the medial septum and hippocampus[END_REF]. In contrast, inputs to area CA2 and the DG are likely provided by different populations of SuM neurons (Borhegyi and Leranth, 1997;[START_REF] Soussi | Heterogeneity of the supramammillary-hippocampal pathways: evidence for a unique GABAergic neurotransmitter phenotype and regional differences[END_REF]. Indeed, SuM afferent axons to the DG co-express glutamatergic (VGluT2) and GABAergic markers (VGAT or GAD65) [START_REF] Boulland | Vesicular Glutamate and GABA Transporters Sort to Distinct Sets of Vesicles in a Population of Presynaptic Terminals[END_REF][START_REF] Soussi | Heterogeneity of the supramammillary-hippocampal pathways: evidence for a unique GABAergic neurotransmitter phenotype and regional differences[END_REF]. These axons co-release glutamate and GABA onto DG granule cells [START_REF] Pedersen | Supramammillary glutamate neurons are a key node of the arousal system[END_REF] and potentially interneurons [START_REF] Leranth | Morphological evidence that hypothalamic substance Pcontaining afferents are capable of filtering the signal flow in the monkey hippocampal formation[END_REF][START_REF] Nitsch | GABAergic neurons in the rat dentate gyrus are innervated by subcortical calretinin-containing afferents[END_REF]. Transmission from the SuM to the DG has been shown to facilitate population spikes evoked by performant path stimulation of EC inputs, potentially by increasing activity in granule cells and decreasing it in interneurons [START_REF] Mizumori | A comparison of supramammillary and medial septal influences on hippocampal field potentials and single-unit activity[END_REF].

In contrast, area CA2 is targeted by VGluT2-expressing SuM axons only that presumably form synapses exclusively on pyramidal neurons in the rat [START_REF] Kiss | The supramammillo-hippocampal and supramammillo-septal glutamatergic/aspartatergic projections in the rat: a combined [3H]daspartate autoradiographic and immunohistochemical study[END_REF][START_REF] Maglóczky | Principal cells are the postsynaptic targets of supramammillary afferents in the hippocampus of the rat[END_REF][START_REF] Soussi | Heterogeneity of the supramammillary-hippocampal pathways: evidence for a unique GABAergic neurotransmitter phenotype and regional differences[END_REF]. These CA2-projecting SuM axons also express calretinin and substance P [START_REF] Berger | Human and monkey fetal brain development of the supramammillary-hippocampal projections: A system involved in the regulation of theta activity[END_REF]Borhegyi and Leranth, 1997;[START_REF] Nitsch | Calretinin immunoreactivity in the monkey hippocampal formation-II. Intrinsic gabaergic and hypothalamic non-gabaergic systems: An experimental tracing and co-existence study[END_REF]. This projection has also been reported in mice [START_REF] Cui | Hypothalamic and other connections with dorsal CA2 area of the mouse hippocampus[END_REF][START_REF] Hitti | The hippocampal CA2 region is essential for social memory[END_REF][START_REF] Kohara | Cell type-specific genetic and optogenetic tools reveal hippocampal CA2 circuits[END_REF] and primates [START_REF] Berger | Human and monkey fetal brain development of the supramammillary-hippocampal projections: A system involved in the regulation of theta activity[END_REF], where it is present in prenatal hippocampus and proposed to play a role in hippocampal rhythms necessary for proper development. Until now, functional investigation of the glutamatergic SuM -CA2 transmission has never been carried out, and neither has the neuromodulation of hippocampal networks by substance P released from SuM.

Neurons in area CA2 also send projections outside the hippocampal circuit to a number of brain regions, sometimes establishing reciprocal connections. Tracing studies in rodents indicate that this is the case for the SuM, the medial septum and the diagonal band of Broca [START_REF] Cui | Hypothalamic and other connections with dorsal CA2 area of the mouse hippocampus[END_REF]. The lateral septum has also been reported as a long-range output of area CA2 by anterograde tracings in mice [START_REF] Cui | Hypothalamic and other connections with dorsal CA2 area of the mouse hippocampus[END_REF]. These extrahippocampal projections were not observed when using the Amigo2-Cre specific of CA2 pyramidal neurons mouse line [START_REF] Hitti | The hippocampal CA2 region is essential for social memory[END_REF] and are thus potentially due to a different population of neurons in area CA2, either long-range projecting interneurons or a subpopulation of pyramidal neurons not labeled by this Cre line.

The tendency of long-range reciprocal connections between area CA2 and other cerebral structures raises the possibility of mutual influences between these regions and the hippocampal network via area CA2. However, functional characterizations of these bi-directional connections are still missing and should provide a more comprehensive understanding of the interplay between area CA2 and the rest of the brain.

In summary, the cellular composition, neuronal physiology, intrinsic and extrinsic connectivity patterns of area CA2 make it uniquely positioned in the hippocampal network. A general feature of area CA2 is its low overall excitability conferred by a high density of INs and hyperpolarized non-resistive PNs. As opposed to CA1, the net input received by CA2 PNs from CA3 is dominated by feedforward inhibition. Interestingly, this CA3 -CA2 feedforward inhibition is highly plastic whereas the CA3 -CA2 excitatory synapse is not. Also contrasting with CA1, distal EC inputs efficiently provide excitation to CA2 PNs thus potentially bypassing the trisynaptic loop. Area CA2 strongly drives excitation in area CA1 and provides inhibition back to area CA3, thereby possibly isolating the di-synaptic loop from the tri-synaptic one.

Interestingly, area CA2 receives and sends long range inputs and outputs that may favor or prevent activity of CA2 PNs thus influencing the rest of the hippocampal network by switching between CA3-and CA2-dominated loops. Illustrations in C, D and E are inspired by [START_REF] Chevaleyre | Strong CA2 Pyramidal Neuron Synapses Define a Powerful Disynaptic Cortico-Hippocampal Loop[END_REF].

I.3 -Physiological features of hippocampal-dependent memory formation

While being conceptually useful, the information transfer theory derived from neuronal connectivity within hippocampal networks provides only a "static" view of the system that does not reflect its operational states. To alleviate this limitation, this section details physiological activity patterns in the hippocampus and related functions with highlights on the contributions of area CA2.

I.3.a -Network activity patterns underlying learning and memory

I.3.a.i -Theta and gamma oscillations

PNs of the hippocampus are organized laminarly with their somas aligned and their dendrites parallel, thus forming electrical dipoles. In such a network, synchronous changes in membrane potential from populations of neurons generate an electrical field potential that can be recorded extra-cellularly either locally (local field potential, LFP) or superficially (electroencephalogram, EEG). This allows the examination of activity in neuronal networks in vivo and led to the discovery of several patterns occurring during different brain states : theta, beta, gamma, sharp waves / ripples (SWR), large and small irregular amplitude (LIA and SIA) [START_REF] Vanderwolf | Hippocampal electrical activity and voluntary movement in the rat[END_REF]. Theta and gamma rhythms are present during active wake and REM sleep while SWR occur during quiet wake and slow wave sleep (SWS) when LIA is predominant [START_REF] Buzsáki | Hippocampal sharp wave-ripple: A cognitive biomarker for episodic memory and planning[END_REF].

Theta waves correspond to rhythmic oscillations ranging from 6 to 12 Hz (in rodents) encountered in the hippocampus and several related brain structures, either as proper field potential oscillations or as rhythmic AP discharges in this frequency band. These include a number cortical areas, the amygdala, septum, hypothalamus, thalamus, VTA and raphe [START_REF] Buzsáki | Theta Oscillations in the Hippocampus[END_REF][START_REF] Vertes | Brainstem-diencephalo-septohippocampal systems controlling the theta rhythm of the hippocampus[END_REF]. In the hippocampus, theta oscillations are prominent in all layers and subregions although their amplitude is largest in CA1 [START_REF] Buzsáki | Theta Oscillations in the Hippocampus[END_REF]. Of note, hippocampal networks are current generators of theta oscillations but not their sole pacemaker, as isolation of the hippocampus from the septum abolishes the hippocampal theta [START_REF] Vertes | Brainstem-diencephalo-septohippocampal systems controlling the theta rhythm of the hippocampus[END_REF]. Indeed, rhythmic activity in the theta range exists in brain structures afferent to the hippocampus : namely the septum, EC and SuM (Alonso and García-Austt, 1987a;1987b;[START_REF] Kocsis | Characterization of neurons of the supramammillary nucleus and mammillary body that discharge rhythmically with the hippocampal theta rhythm in the rat[END_REF][START_REF] Stewart | Firing relations of lateral septal neurons to the hippocampal theta rhythm in urethane anesthetized rats[END_REF]. EC inputs impinging on distal dendrites bring about rhythmic excitation generating current sinks in SLM [START_REF] Buzsáki | Theta Oscillations in the Hippocampus[END_REF]. In addition, the septum is critical for hippocampal theta waves as it provides both permissive cholinergic and pacemaking GABAergic inputs [START_REF] Buzsáki | Theta Oscillations in the Hippocampus[END_REF][START_REF] Pan | The supramammillary area: its organization, functions and relationship to the hippocampus[END_REF][START_REF] Vertes | Brainstem-diencephalo-septohippocampal systems controlling the theta rhythm of the hippocampus[END_REF]. Remarkably, hippocampal neurons possess a repertoire of conductances allowing intrinsic resonance in the theta and gamma frequency range [START_REF] Wang | Neurophysiological and Computational Principles of Cortical Rhythms in Cognition[END_REF]. Therefore, although septal cholinergic afferents to the hippocampus are diffuse and do not contribute to the post-synaptic potentials underlying theta, acetylcholine released by these fibers causes modifications in these conductances leading to a global depolarization of hippocampal neurons [START_REF] Madison | Voltage clamp analysis of cholinergic action in the hippocampus[END_REF]. This effect of elevated cholinergic tone setting hippocampal networks in a theta prone state led to the definition of an atropine-sensitive theta, named after the cholinergic antagonist atropine that abolishes it as opposed to an atropine-resistant theta [START_REF] Kramis | Two types of hippocampal rhythmical slow activity in both the rabbit and the rat: Relations to behavior and effects of atropine, diethyl ether, urethane, and pentobarbital[END_REF]. Further contribution of the septum to hippocampal theta is brought about by rhythmic inhibition of local INs by hippocampus-projecting GABAergic septal INs [START_REF] Freund | GABA-containing neurons in the septum control inhibitory interneurons in the hippocampus[END_REF][START_REF] Hangya | GABAergic Neurons of the Medial Septum Lead the Hippocampal Network during Theta Activity[END_REF][START_REF] Tóth | Disinhibition of rat hippocampal pyramidal cells by GABAergic afferents from the septum[END_REF]. Peri-somatic targeting hippocampal INs entrained by periodic septal inhibition in turn trigger rhythmic inhibitory post-synaptic potentials in PNs, thus generating a current source in SP [START_REF] Buzsáki | Theta Oscillations in the Hippocampus[END_REF]. Finally, rhythmic activation of the intra-hippocampal CA3 -CA1 SC contribute to another current sink in CA1 SR [START_REF] Buzsáki | Theta Oscillations in the Hippocampus[END_REF]. Altogether, specific sets of conductances and rhythmic excitatory and inhibitory synaptic inputs on different dendritic compartments trigger oscillations of the membrane potential of hippocampal neurons at theta frequencies. This sets windows of excitability that differ depending on the neuronal population, thus leading to action potential firing by PNs and INs at specific preferential phases of the theta cycle [START_REF] Fox | Hippocampal theta rhythm and the firing of neurons in walking and urethane anesthetized rats[END_REF][START_REF] Klausberger | Brain-stateand cell-type-specific firing of hippocampal interneurons in vivo[END_REF][START_REF] Ikemoto | The Supramammillary Nucleus Mediates Primary Reinforcement via GABAA Receptors[END_REF]. Indeed, average firing rates of PNs are highest around the negative peak of the CA1 SP theta and are preceded by INs from the pyramidal layer whose maximal discharges occurs 60° earlier (Csicsvari et al., 1999). It should be noted that these are only indicative values as preferred theta phases vary across cell types and depends on behavior (as discussed later in Section I.3.b.i -). Still, PNs of areas CA3, CA2 and CA1 of the hippocampus seem to form a relatively homogeneous population with regards to theta oscillations : recent in vivo studies that examined PN properties in different CA regions reported either no differences [START_REF] Kay | A hippocampal network for spatial coding during immobility and sleep[END_REF][START_REF] Mankin | Hippocampal CA2 Activity Patterns Change over Time to a Larger Extent than between Spatial Contexts[END_REF], or slightly stronger theta modulation and earlier preferred theta phase of CA3 PNs compared to CA2 and CA1 PNs (Oliva et al., 2016a).

Besides the septum and EC, the SuM is of particular interest for the generation and control of theta oscillations in the hippocampus by extra-hippocampal sources. It has been described as the first nucleus where neurons display phasic rather than tonic firing in the so-called "ascending theta synchronizing path" that also comprises the reticular nucleus pontis oralis (RPO), posterior hypothalamus (PH), septum and hippocampus [START_REF] Kirk | Frequency Modulation of Hippocampal Theta by the Supramammillary Nucleus, and Other Hypothalamo-Hippocampal Interactions: Mechanisms and Functional Implications[END_REF]. Indeed, neurons in the SuM fire in bursts at theta frequency coherent with hippocampal theta in anaesthetized rats [START_REF] Kirk | Evidence for differential control of posterior hypothalamic, supramammillary, and medial mammillary theta-related cellular discharge by ascending and descending pathways[END_REF][START_REF] Kocsis | Characterization of neurons of the supramammillary nucleus and mammillary body that discharge rhythmically with the hippocampal theta rhythm in the rat[END_REF]. The SuM is in close relation with both the hippocampus by direct projections and the septum through reciprocal connections [START_REF] Borhegyi | Dual projection from the medial septum to the supramammillary nucleus in the rat[END_REF][START_REF] Leranth | A population of supramammillary area calretinin neurons terminating on medial septal area cholinergic and lateral septal area calbindin-containing cells are aspartate/glutamatergic[END_REF][START_REF] Vertes | PHA-L analysis of projections from the supramammillary nucleus in the rat[END_REF][START_REF] Vertes | Collateral projections from the supramammillary nucleus to the medial septum and hippocampus[END_REF]. Of note, theta activity in the SuM is not solely relayed by its interactions with the septum as it remains upon inactivation of the septum [START_REF] Kirk | Supramammillary Neural Discharge Patterns and Hippocampal EEG[END_REF][START_REF] Kirk | Evidence for differential control of posterior hypothalamic, supramammillary, and medial mammillary theta-related cellular discharge by ascending and descending pathways[END_REF]. Under anesthesia, the SuM influence over hippocampal theta is massive as evidenced by a reduction in frequency and amplitude of theta oscillations in the hippocampus upon disruption of SuM activity by pharmacological agents or lesions [START_REF] Kirk | Mapping the differential effects of procaine on frequency and amplitude of reticularly elicited hippocampal rhythmical slow activity[END_REF][START_REF] Mcnaughton | Contribution of synapses in the medial supramammillary nucleus to the frequency of hippocampal theta rhythm in freely moving rats[END_REF][START_REF] Thinschmidt | The supramammillary nucleus: Is it necessary for the mediation of hippocampal theta rhythm?[END_REF]. Although less pronounced, this effect remains in freely behaving animals [START_REF] Mcnaughton | Contribution of synapses in the medial supramammillary nucleus to the frequency of hippocampal theta rhythm in freely moving rats[END_REF][START_REF] Pan | The medial supramammillary nucleus, spatial learning and the frequency of hippocampal theta activity[END_REF]2002). Altogether, these findings point out the SuM as a critical actor in the regulation of hippocampal theta oscillations, either through its direct projections to the hippocampus or via the septum.

Often concurring with theta, gamma waves are higher-frequency oscillations (30-100 Hz) also occurring throughout hippocampal layers and subregions, as well as in other brain areas including the cortex [START_REF] Buzsáki | Mechanisms of Gamma Oscillations[END_REF][START_REF] Colgin | Rhythms of the hippocampal network[END_REF]. Detailed analysis of gamma oscillations reveal that they can be subdivided based on their frequency, origin, amplitudephase coupling relation to theta, and laminar organization [START_REF] Belluscio | Cross-Frequency Phase-Phase Coupling between Theta and Gamma Oscillations in the Hippocampus[END_REF][START_REF] Bragin | Gamma (40-100 Hz) oscillation in the hippocampus of the behaving rat[END_REF][START_REF] Colgin | Frequency of gamma oscillations routes flow of information in the hippocampus[END_REF][START_REF] Schomburg | Theta Phase Segregation of Input-Specific Gamma Patterns in Entorhinal-Hippocampal Networks[END_REF]. First, fast-gamma (also referred to as epsilon) spans frequencies from 90 to 150 Hz, is largest at the theta trough, and is confined to CA1 SP [START_REF] Belluscio | Cross-Frequency Phase-Phase Coupling between Theta and Gamma Oscillations in the Hippocampus[END_REF]. Next, mid-gamma occupies the 50 to 90 Hz band of frequencies, shows maximal amplitude at the peak of theta, and is localized in SLM and DG consistent with its entorhinal origin [START_REF] Belluscio | Cross-Frequency Phase-Phase Coupling between Theta and Gamma Oscillations in the Hippocampus[END_REF][START_REF] Bragin | Gamma (40-100 Hz) oscillation in the hippocampus of the behaving rat[END_REF][START_REF] Chrobak | Gamma oscillations in the entorhinal cortex of the freely behaving rat[END_REF][START_REF] Colgin | Frequency of gamma oscillations routes flow of information in the hippocampus[END_REF][START_REF] Csicsvari | Mechanisms of Gamma Oscillations in the Hippocampus of the Behaving Rat[END_REF][START_REF] Schomburg | Theta Phase Segregation of Input-Specific Gamma Patterns in Entorhinal-Hippocampal Networks[END_REF]. Finally, slow-gamma ranges from 30 to 50 Hz, is highest during the descending phase of theta, originates in area CA3 and therefore predominates in CA1 SR [START_REF] Belluscio | Cross-Frequency Phase-Phase Coupling between Theta and Gamma Oscillations in the Hippocampus[END_REF][START_REF] Bragin | Gamma (40-100 Hz) oscillation in the hippocampus of the behaving rat[END_REF][START_REF] Chrobak | Gamma oscillations in the entorhinal cortex of the freely behaving rat[END_REF][START_REF] Colgin | Frequency of gamma oscillations routes flow of information in the hippocampus[END_REF][START_REF] Csicsvari | Mechanisms of Gamma Oscillations in the Hippocampus of the Behaving Rat[END_REF][START_REF] Schomburg | Theta Phase Segregation of Input-Specific Gamma Patterns in Entorhinal-Hippocampal Networks[END_REF].

Much like theta oscillations, cellular and circuit mechanisms underlying gamma oscillations have been examined using various approaches. Modelling studies based on physiological data indicate that gamma oscillations can emerge in neuronal networks consisting either of interneurons only engaged in mutual inhibition and driven by a common excitatory source (I-I or ING model), or of pyramidal cells and interneurons through recurrent excitation and feedback inhibition (E-I or PING model) [START_REF] Buzsáki | Mechanisms of Gamma Oscillations[END_REF][START_REF] Wang | Neurophysiological and Computational Principles of Cortical Rhythms in Cognition[END_REF]. In support of the latter, in vivo examination of action potential firing relative to the gamma cycle revealed that PNs discharge before INs in the recurrent CA3 network, but also drive CA1 INs consistent with feedback and feedforward inhibitory circuits respectively [START_REF] Csicsvari | Mechanisms of Gamma Oscillations in the Hippocampus of the Behaving Rat[END_REF]. Given the intrinsic nature of slow gamma in hippocampal networks, several studies took advantage of the possibility to induce gamma oscillations in acute slices to further elucidate the mechanism involved. Indeed, elevation of the cholinergic tone by bath application of carbachol (CCh) induces gamma oscillations ex vivo that resemble the slow-gamma seen in vivo [START_REF] Fisahn | Cholinergic induction of network oscillations at 40 Hz in the hippocampus in vitro[END_REF][START_REF] Mann | Perisomatic Feedback Inhibition Underlies Cholinergically Induced Fast Network Oscillations in the Rat Hippocampus In Vitro[END_REF]. These approaches notably allowed to validate the central role of fastspiking PV-expressing basket cells in gamma oscillations, as they generate an active current source in SP through rhythmic perisomatic inhibition [START_REF] Gulyás | Parvalbumin-Containing Fast-Spiking Basket Cells Generate the Field Potential Oscillations Induced by Cholinergic Receptor Activation in the Hippocampus[END_REF][START_REF] Mann | Perisomatic Feedback Inhibition Underlies Cholinergically Induced Fast Network Oscillations in the Rat Hippocampus In Vitro[END_REF].

In addition, they confirmed that intra-hippocampal gamma oscillations build up through recurrent excitation and feedback inhibition in area CA3 and are subsequently transferred to area CA1 by feedforward inhibition [START_REF] Mann | Perisomatic Feedback Inhibition Underlies Cholinergically Induced Fast Network Oscillations in the Rat Hippocampus In Vitro[END_REF][START_REF] Oren | Synaptic Currents in Anatomically Identified CA3 Neurons during Hippocampal Gamma Oscillations In Vitro[END_REF][START_REF] Zemankovics | Feedforward Inhibition Underlies the Propagation of Cholinergically Induced Gamma Oscillations from Hippocampal CA3 to CA1[END_REF]. Area CA2 has yet to be incorporated in this scheme although its intra-and extra-hippocampal connections indicate that it is involved in gamma oscillations. Indeed, area CA2 displays recurrent excitation [START_REF] Cui | Hypothalamic and other connections with dorsal CA2 area of the mouse hippocampus[END_REF][START_REF] Hitti | The hippocampal CA2 region is essential for social memory[END_REF] and feedback inhibition (Mercer et al., 2012a;[START_REF] Mercer | SP-SR interneurones: A novel class of neurones of the CA2 region of the hippocampus[END_REF] much like CA3 and is therefore endowed with the necessary features for generating gamma oscillations. Furthermore, its strong excitatory drive from EC [START_REF] Chevaleyre | Strong CA2 Pyramidal Neuron Synapses Define a Powerful Disynaptic Cortico-Hippocampal Loop[END_REF], reciprocal inhibitory influences with CA3 [START_REF] Boehringer | Chronic Loss of CA2 Transmission Leads to Hippocampal Hyperexcitability[END_REF], and potent excitatory output onto CA1 [START_REF] Chevaleyre | Strong CA2 Pyramidal Neuron Synapses Define a Powerful Disynaptic Cortico-Hippocampal Loop[END_REF][START_REF] Kohara | Cell type-specific genetic and optogenetic tools reveal hippocampal CA2 circuits[END_REF], places area CA2 in an ideal position to participate in mid-, slow-, and fast-gamma respectively. In addition, one can predict a role for area CA2 in theta phase coupling of gamma amplitude as it receives theta-locked inputs from the SuM [START_REF] Kirk | Evidence for differential control of posterior hypothalamic, supramammillary, and medial mammillary theta-related cellular discharge by ascending and descending pathways[END_REF][START_REF] Kocsis | Characterization of neurons of the supramammillary nucleus and mammillary body that discharge rhythmically with the hippocampal theta rhythm in the rat[END_REF]. Finally, in vivo and ex vivo work describe the initiation site of slow-gamma waves in area CA3a, i.e. in the vicinity of CA2 which may have been overlooked in these studies [START_REF] Csicsvari | Mechanisms of Gamma Oscillations in the Hippocampus of the Behaving Rat[END_REF][START_REF] Mann | Perisomatic Feedback Inhibition Underlies Cholinergically Induced Fast Network Oscillations in the Rat Hippocampus In Vitro[END_REF].

I.3.a.ii -Sharp wave / ripples

In contrast with prolonged theta oscillations that dominate the hippocampal EEG during active wake and REM sleep, brief and fast oscillatory events called sharp wave / ripples (SWRs) emerge in states of quiet wakefulness and SWS [START_REF] Buzsáki | Hippocampal sharp wave-ripple: A cognitive biomarker for episodic memory and planning[END_REF]. SWRs consist of a short (~100 ms) dendritic negative deflection of the local field potential (LFP) in CA1 SR (the sharp wave) superimposed with a fast (140-200 Hz) oscillation in the pyramidal layer (the ripple) [START_REF] Buzsáki | Hippocampal sharp wave-ripple: A cognitive biomarker for episodic memory and planning[END_REF]. Contrary to theta and mid-gamma that are brought about in hippocampal networks by extrinsic inputs, SWRs are intrinsically generated as they remain after lesion of afferent structures to the hippocampus (Suzuki and Smith, 1988). Indeed, SWRs originate in areas CA3 and CA2 whose recurrent excitatory connections allow a buildup of excitation culminating in burst firing of PNs [START_REF] Buzsáki | Hippocampal sharp waves: Their origin and significance[END_REF]Csicsvari et al., 1999b;[START_REF] Mizuseki | Activity dynamics and behavioral correlates of CA3 and CA1 hippocampal pyramidal neurons[END_REF]Oliva et al., 2016;[START_REF] Schlingloff | Mechanisms of Sharp Wave Initiation and Ripple Generation[END_REF]. Initiation of SWRs might involve a post-inhibitory rebound of PN excitation as a trigger and additional disinhibition by silencing CCK-expressing INs through cannabinoids released by CA3 PNs as they start firing APs [START_REF] Hájos | Input-Output Features of Anatomically Identified CA3 Neurons during Hippocampal Sharp Wave/Ripple Oscillation In Vitro[END_REF][START_REF] Lasztóczi | Terminal Field and Firing Selectivity of Cholecystokinin-Expressing Interneurons in the Hippocampal CA3 Area[END_REF][START_REF] Papatheodoropoulos | Patterned activation of hippocampal network (∼10 Hz) during in vitro sharp wave-ripples[END_REF]. CA3 PNs then synchronously excite CA1 cells through the SC thus generating a large sink in CA1 SR, the sharp wave [START_REF] Buzsáki | Hippocampal sharp waves: Their origin and significance[END_REF]Csicsvari et al., 1999b). In response to CA3 strong excitatory inputs, CA1 PNs rhythmically fire APs seen as the repeated negative peaks in CA1 SP signing the ripple [START_REF] Schlingloff | Mechanisms of Sharp Wave Initiation and Ripple Generation[END_REF][START_REF] Schomburg | The Spiking Component of Oscillatory Extracellular Potentials in the Rat Hippocampus[END_REF]. In addition, phasic excitation from CA3 PNs recruits PV-expressing BCs that in turn provide perisomatic inhibition to CA1 (Csicsvari et al., 1999a;[START_REF] Hájos | Input-Output Features of Anatomically Identified CA3 Neurons during Hippocampal Sharp Wave/Ripple Oscillation In Vitro[END_REF]. Both feedforward from CA3 and feedback inhibition from CA1 produce synchronized IPSPs in CA1 SP that contribute to the upward peaks on the ripple (Csicsvari et al., 1999a;[START_REF] Schomburg | The Spiking Component of Oscillatory Extracellular Potentials in the Rat Hippocampus[END_REF]. Although triggered by a build-up of excitation in the CA3 recurrent network, SWRs require adequate GABAergic transmission that paces action potential firing during this fast oscillation [START_REF] Schlingloff | Mechanisms of Sharp Wave Initiation and Ripple Generation[END_REF][START_REF] Stark | Pyramidal Cell-Interneuron Interactions Underlie Hippocampal Ripple Oscillations[END_REF]. This phasic inhibition is likely provided by PV-expressing BCs as they fire APs at ripple frequencies phase-locked to the oscillation through mutual inhibition (Csicsvari et al., 1999a;[START_REF] Hájos | Input-Output Features of Anatomically Identified CA3 Neurons during Hippocampal Sharp Wave/Ripple Oscillation In Vitro[END_REF][START_REF] Klausberger | Brain-stateand cell-type-specific firing of hippocampal interneurons in vivo[END_REF][START_REF] Schlingloff | Mechanisms of Sharp Wave Initiation and Ripple Generation[END_REF][START_REF] Stark | Pyramidal Cell-Interneuron Interactions Underlie Hippocampal Ripple Oscillations[END_REF]. Finally, SWRs are likely terminated by hyperpolarization of PNs through activation of GABAB receptors following high interneuronal activity (English et al., 2014). When considered at the scale of hippocampal circuits, SWRs have been shown to travel from CA3a / CA2 towards CA3c and then reach CA1 where superficial PNs are more entrained than deep PNs [START_REF] Csicsvari | Ensemble Patterns of Hippocampal CA3-CA1 Neurons during Sharp Wave-Associated Population Events[END_REF]Oliva et al., 2016a;[START_REF] Stark | Pyramidal Cell-Interneuron Interactions Underlie Hippocampal Ripple Oscillations[END_REF][START_REF] Valero | Determinants of different deep and superficial CA1 pyramidal cell dynamics during sharp-wave ripples[END_REF]. Discharging CA1 PNs subsequently transfer information outside the hippocampus to cortical areas [START_REF] Buzsáki | Hippocampal sharp wave-ripple: A cognitive biomarker for episodic memory and planning[END_REF].

While PNs typically increase their action potential firing during SWRs, there are intriguing findings from in vivo studies that describe non-stereotypic firing patterns of CA2 pyramidal neurons during SWRs, indicating that this region is contributing to hippocampal network dynamics in an unusual way. Kay et al. performed in vivo recordings of SWRs and hippocampal unit activity in all CA regions in rats during sleep, rest and while performing a spatial memory task [START_REF] Kay | A hippocampal network for spatial coding during immobility and sleep[END_REF]. They reported two populations of putative pyramidal cells in area CA2 that had opposite modulations of their firing rates during SWRs. CA2 P-units ("positively modulated") showed increased firing rates during SWRs, much like cells in other hippocampal subfields. Conversely, CA2 N-units ("non-positively modulated") fired less APs during SWRs.

Unlike CA2-P, CA3 and CA1 units that classically showed increased activity during locomotion and SWRs, CA2 N-units displayed increased firing rates during non-SWR immobility [START_REF] Kay | A hippocampal network for spatial coding during immobility and sleep[END_REF]. This unusual firing behavior in area CA2 was supported by experiments using high density silicon probes inserted parallel to the transverse axis of the hippocampus that simultaneously recorded potentials in every layer of each CA region during awake immobility and non-REM sleep (Oliva et al., 2016a). In this study, the authors show that a population of pyramidal neurons in CA2 ramp up their activity prior to SWR initiation after which they become silent, while another population fires more APs during SWRs (Oliva et al., 2016a). Furthermore, Valero et al. used sharp electrodes to record CA2 pyramidal firing in vivo and reported a reduction in firing during SWRs [START_REF] Valero | Determinants of different deep and superficial CA1 pyramidal cell dynamics during sharp-wave ripples[END_REF]. Lastly, in vivo whole-cell recordings in area CA2 showed that the membrane potential of PNs undergoes a hyperpolarization during SWR [START_REF] Matsumoto | 3-Hz subthreshold oscillations of CA2 neurons in vivo[END_REF]. Interestingly, pyramidal neurons in area CA2 have also been proposed to participate in the generation of SWR, as deep cells ramp up their activity prior to SWR initiation and superficial cells fire earliest during SWR events compared to CA3 and CA1 (Oliva et al., 2016a). Moreover, SWRs could also be generated in area CA2 and directly transferred to CA1 (Oliva et al., 2016a).

I.3.b -Mnemonic functions of the hippocampus and hypothalamus

I.3.b.i -Spatial coding and memory

A major aspect of episodic memory supported by the hippocampus is the formation and storage of internal representations of the environment. This spatial coding is achieved by place cells, i.e. pyramidal neurons that increase their action potential firing when the animal visits a discrete location called place field [START_REF] Christian | Behavioral functions and hippocampal cell types: evidence for two nonoverlapping populations in the rat[END_REF][START_REF] O'keefe | Place units in the hippocampus of the freely moving rat[END_REF][START_REF] O'keefe | The hippocampus as a spatial map. Preliminary evidence from unit activity in the freely-moving rat[END_REF]. Location-specific activity of place cells is likely to be the result of the processing by hippocampal networks of directional, metric and multimodal sensory information provided to the hippocampus by upstream structures. Indeed, EC hosts grid cells that fire action potentials repeatedly at discrete location of an animal's displacement through the environment, thus providing a metric of distance travelled [START_REF] Hafting | Microstructure of a spatial map in the entorhinal cortex[END_REF]. In addition, head direction cells that discharge action potentials preferentially when the animal's head faces certain angles were found in areas related to the hippocampus [START_REF] Taube | Head-direction cells recorded from the postsubiculum in freely moving rats. I. Description and quantitative analysis[END_REF]. Remarkably, place cells are highly specific of a particular location, such that each place displays usually only one fairly small place field in a given environment [START_REF] Muller | Spatial firing patterns of hippocampal complex-spike cells in a fixed environment[END_REF][START_REF] O'keefe | Place units in the hippocampus of the freely moving rat[END_REF]. In addition, changing environments causes modifications of place cell features : place fields can change location, disappear or appear from new place cells [START_REF] Bostock | Experience-dependent modifications of hippocampal place cell firing[END_REF][START_REF] Muller | Spatial firing patterns of hippocampal complex-spike cells in a fixed environment[END_REF]. Indeed, only a small fraction of hippocampal PNs are actively recruited as place cell in a particular environment [START_REF] Thompson | Place cells and silent cells in the hippocampus of freely-behaving rats[END_REF]. It follows that different combinations of place cells can provide specific internal representations of different environments [START_REF] Wilson | Dynamics of the hippocampal ensemble code for space[END_REF]. This leads to the question of how a particular set of PNs is selected to represent a given environment. Contrary to sensory systems, there is no known topological organization of place cells in the hippocampus, as nearby PNs do not tend to code for locations close in space [START_REF] Redish | Independence of firing correlates of anatomically proximate hippocampal pyramidal cells[END_REF]. Instead, spatial representations are first learned and then kept in memory : place cells emerge as an animal begins to explore an environment and subsequently form a stable representation of the environment [START_REF] Frank | Hippocampal Plasticity across Multiple Days of Exposure to Novel Environments[END_REF][START_REF] Lever | Long-term plasticity in hippocampal place-cell representation of environmental geometry[END_REF][START_REF] Wilson | Dynamics of the hippocampal ensemble code for space[END_REF]. Another mnemonic property of hippocampal spatial coding is evidenced by the remapping of spatial representations upon environmental variations to different degrees [START_REF] Bostock | Experience-dependent modifications of hippocampal place cell firing[END_REF][START_REF] Leutgeb | Independent Codes for Spatial and Episodic Memory in Hippocampal Neuronal Ensembles[END_REF][START_REF] Muller | Spatial firing patterns of hippocampal complex-spike cells in a fixed environment[END_REF].

Interestingly, two types of remapping reminiscent of pattern separation and completion have been described : global remapping in which place fields change locations and rate remapping when place fields remain at their location but the corresponding place cells vary their action potential firing [START_REF] Leutgeb | Independent Codes for Spatial and Episodic Memory in Hippocampal Neuronal Ensembles[END_REF].

Of note, both theta oscillation and SWR play important roles in spatial coding. As an animal crosses a place field, the action potential firing of the corresponding place cell gradually shifts its preferred theta phase from late to early in the cycle, a phenomenon called phase precession [START_REF] O'keefe | Phase relationship between hippocampal place units and the EEG theta rhythm[END_REF]. As place fields are successively visited, the AP discharge of corresponding place cells is distributed at different phases of theta cycles (Skaggs et al., 1996).

Therefore, place cells fire in an organized manner reflecting the animal's path during theta epochs, thus forming theta sequences. Strikingly, sleep enhances the correlation of place cells with nearby place fields (Skaggs and McNaughton, 1996;[START_REF] Wilson | Reactivation of hippocampal ensemble memories during sleep[END_REF].

This observation suggests that a plasticity process occurs during sleep and stabilizes the information encoded in theta sequences during wake. SWRs are an ideal candidate as their disruption during sleep impairs memory consolidation [START_REF] Girardeau | Selective suppression of hippocampal ripples impairs spatial memory[END_REF]. Furthermore, theta sequences re-emerge during SWRs as compressed replays [START_REF] Lee | Memory of Sequential Experience in the Hippocampus during Slow Wave Sleep[END_REF].

Altogether, these findings critically link theta oscillations and SWR to spatial coding, learning and memory. However, the precise role of the different hippocampal subregions, especially that of area CA2, in these functions is not fully understood.

Place cells have been discovered and extensively studied in areas CA1 and CA3 but only recently characterized in area CA2. Surprisingly, several studies have reported striking differences in spatial coding in area CA2 as compared to CA1 and CA3. Place cells in area CA2 are more abundant, display larger and more numerous place fields per place cell and fire more action potentials on average without increased peak "in-field" firing leading to a lower spatial information content than in area CA1 and CA3, as evidenced by in vivo recordings in behaving rats [START_REF] Alexander | Social and novel contexts modify hippocampal CA2 representations of space[END_REF][START_REF] Lee | Neural Population Evidence of Functional Heterogeneity along the CA3 Transverse Axis: Pattern Completion versus Pattern Separation[END_REF][START_REF] Lu | Topography of Place Maps along the CA3-to-CA2 Axis of the Hippocampus[END_REF][START_REF] Mankin | Hippocampal CA2 Activity Patterns Change over Time to a Larger Extent than between Spatial Contexts[END_REF]Oliva et al., 2016b). Other differences include lower theta phase precession [START_REF] Fernández-Ruiz | Entorhinal-CA3 Dual-Input Control of Spike Timing in the Hippocampus by Theta-Gamma Coupling[END_REF][START_REF] Mankin | Hippocampal CA2 Activity Patterns Change over Time to a Larger Extent than between Spatial Contexts[END_REF]Oliva et al., 2016b). Together with this low-precision spatial coding, space representation in area CA2 is less specific than in neighboring areas: CA2 place cells fail to remap on changing of enclosure [START_REF] Lu | Topography of Place Maps along the CA3-to-CA2 Axis of the Hippocampus[END_REF][START_REF] Mankin | Hippocampal CA2 Activity Patterns Change over Time to a Larger Extent than between Spatial Contexts[END_REF], or on displacement of local or global cues [START_REF] Lee | Neural Population Evidence of Functional Heterogeneity along the CA3 Transverse Axis: Pattern Completion versus Pattern Separation[END_REF].

These observations raise the question of what role, if any, area CA2 plays in spatial coding in the hippocampal network. By recording place cell activity in every hippocampal CA subfield as rats successively explored different familiar environments during sessions spanning up to 2 days, Mankin et al. were able to examine CA2 place cell remapping [START_REF] Mankin | Hippocampal CA2 Activity Patterns Change over Time to a Larger Extent than between Spatial Contexts[END_REF].

Strikingly, they showed that patterns of activity in area CA2 varied as the same environment was presented repeatedly for several hours. Contextual changes, however, were found to induce little differences in CA2 remapping. Thus, the authors hypothesized that remapping in area CA2 provides an internal representation of elapsed time, due to a combination of rate remapping and place field gain/loss or drift [START_REF] Mankin | Hippocampal CA2 Activity Patterns Change over Time to a Larger Extent than between Spatial Contexts[END_REF]. Similar observations were made in different experimental approaches in two studies [START_REF] Lee | Neural Population Evidence of Functional Heterogeneity along the CA3 Transverse Axis: Pattern Completion versus Pattern Separation[END_REF][START_REF] Lu | Topography of Place Maps along the CA3-to-CA2 Axis of the Hippocampus[END_REF]. Using simultaneous recordings along the CA3 to CA1 axis, Lu et al. compared remapping upon exposure to rooms with different wall colors or to different rooms. They reported lower global and rate remapping induced by context in area CA2 than CA3 and preferential remapping in area CA2 with times increasing from 10 min to up to 31 h [START_REF] Lu | Topography of Place Maps along the CA3-to-CA2 Axis of the Hippocampus[END_REF]. Lee et al. examined the coherence of place cell activity in CA2 and different CA3 subfields as rats were exposed to an environment where local and global cues were rotated between sessions. This paradigm enabled them to show that place cells in area CA2 were more prone to follow local cues and rotate rather than to remap. Further, they characterized CA2 place cell ensembles as less stable than CA3 in coding the same environment as time passed across 4 days [START_REF] Lee | Neural Population Evidence of Functional Heterogeneity along the CA3 Transverse Axis: Pattern Completion versus Pattern Separation[END_REF]. In summary, these studies report that area CA2 shows low discrimination between different familiar environments [START_REF] Mankin | Hippocampal CA2 Activity Patterns Change over Time to a Larger Extent than between Spatial Contexts[END_REF], undergoes poor partial and global remapping when challenged with varying environments [START_REF] Lu | Topography of Place Maps along the CA3-to-CA2 Axis of the Hippocampus[END_REF] and that area CA2 displays more pattern completion dictated by local cues than remapping [START_REF] Lee | Neural Population Evidence of Functional Heterogeneity along the CA3 Transverse Axis: Pattern Completion versus Pattern Separation[END_REF].

Alexander et al. challenged rats with familiar or novel social or object stimulations presented in subsequent sessions of exploration of a familiar arena. Recordings of CA1 and CA2 place cells in this paradigm revealed remapping in area CA2 induced by novel or familiar social exposure and by novel but not familiar object presentation, suggesting that spatial representations in this region can be specific depending on which cues are presented [START_REF] Alexander | Social and novel contexts modify hippocampal CA2 representations of space[END_REF].

Interestingly, using immediate-early gene expression to label active cells, Wintzer et al. showed that different cell ensembles are activated in area CA2 upon exposure to a familiar versus novel environment, similar to other CA regions. However, when only small contextual changes were introduced, only area CA2 displayed different active cell ensembles, hence functioning as a conflict detector within tens of minutes of exposure [START_REF] Wintzer | The Hippocampal CA2 Ensemble Is Sensitive to Contextual Change[END_REF]. On a similar time scale, spatial representation of a familiar environment is disrupted in area CA2 by interleaved exposure to a known but different context, although discrimination between contexts is itself low [START_REF] Mankin | Hippocampal CA2 Activity Patterns Change over Time to a Larger Extent than between Spatial Contexts[END_REF].

The apparent discrepancy between both studies can be reconciled, as the introduction of a subtle change to a familiar environment [START_REF] Wintzer | The Hippocampal CA2 Ensemble Is Sensitive to Contextual Change[END_REF] and exposure to a different but familiar environment [START_REF] Mankin | Hippocampal CA2 Activity Patterns Change over Time to a Larger Extent than between Spatial Contexts[END_REF] are essentially discrete but salient variations of external surroundings conflicting with internal representations detected by area CA2 that then remaps within tens of minutes. Overall, from these experiments, spatial coding in area CA2 seems broad and unspecific, potentially allowing binding of temporal elements with space but can be sharpened by social, novel or conflicting stimulations. Whether these seemingly unrelated functions are really different aspects of a specific type of episodic memory remains unclear.

Given that blocking area CA2 synaptic output only disrupts social memory, future work will clarify the relationship of these spatial functions by encoding social aspects of episodic memory.

All the above experiments examine the role of area CA2 in spatial encoding during the traditional paradigm of place cell activity, that is, when the animal is actively moving through a receptive field. However, when animals are stationary, spatial information is still maintained in the absence of place cell firing. A recent breakthrough concerning the hippocampal mechanisms allowing this spatial encoding during immobility has shown that cells in area CA2 play a central and unexpected role [START_REF] Kay | A hippocampal network for spatial coding during immobility and sleep[END_REF].

Surprisingly, area CA2 N-units fire more action potentials during non-SWR epochs of immobility and were found to code for spatial location during immobility and sleep [START_REF] Kay | A hippocampal network for spatial coding during immobility and sleep[END_REF]. These cells fired in a very location-specific manner that inversely correlated with locomotion. This was accompanied by a hippocampal-wide pattern of activity distinct from SWR called N-wave [START_REF] Kay | A hippocampal network for spatial coding during immobility and sleep[END_REF]. Thus, area CA2 hosts a peculiar subpopulation of pyramidal neurons that entrain the hippocampal network allowing spatial coding during immobility and sleep, which potentially allows subsets of cells in CA1 to code for nesting position (Jarosiewicz et al., 2002).

Hence, in accordance with its unique and extensive intra-and extra-hippocampal connectivity, area CA2 appears to be critically involved in triggering hippocampal wide patterns of activity relevant for spatial coding during immobility and sleep (N-wave) as well as memory consolidation (SWR). This raises the issue of understanding how a region displaying poor spatial coding during exploration could then orchestrate hippocampal events tied to spatial memory during rest.

Information on this matter has be gained by manipulating area CA2 activity by using tetanus toxin light-chain expression to block CA2 pyramidal neuron output chronically in mice [START_REF] Boehringer | Chronic Loss of CA2 Transmission Leads to Hippocampal Hyperexcitability[END_REF]. It was this study that revealed a loss of feedforward inhibition from CA2 to CA3 that leads to hyperexcitability in area CA3 [START_REF] Boehringer | Chronic Loss of CA2 Transmission Leads to Hippocampal Hyperexcitability[END_REF]. This transformed normal CA1 and CA3 place field firing into spatially-triggered network hyperexcitability events with increased theta power preceded by a ramping up of CA1 and CA3 pyramidal neuron firing. In addition, in mice with synaptic transmission blocked in CA2 pyramidal neurons, SWRs were partially replaced by epileptiform-like discharges.

Interestingly, these mice showed impairments of contextual habituation. When acutely inhibiting CA2 pyramidal neurons with inhibitory DREADDs, Boehringer et al. observed an abnormal remapping of CA1 and CA3 pyramidal neurons that tended to cluster their firing into hotspots. Altogether, these results indicate a critical role of CA2-mediated inhibition of CA3 that potentially parses hippocampal network excitability and allows for physiological levels of enhanced excitability inside a place field. Hence, although area CA2 is not encoding spatial information during exploration, this region is involved in shaping place field activity in CA1 and CA3.

What makes area CA2 special in regards to spatial coding ? Besides its unique PN properties, interneuronal composition, and relations to other hippocampal subregions, area CA2 is directly influenced by the SuM which is involved in spatial learning and memory. Given the importance of theta oscillations in this process, it is only natural to suggest a mnemonic role for SuM as it controls the hippocampal theta. Indeed, several studies have reported a joint effect on theta and memory after interfering with SuM activity [START_REF] Pan | The supramammillary area: its organization, functions and relationship to the hippocampus[END_REF]. Silencing of SuM with a GABAergic agonist leads to a decreased hippocampal theta together with delayed spatial learning [START_REF] Pan | The medial supramammillary nucleus, spatial learning and the frequency of hippocampal theta activity[END_REF]. Impairments of spatial memory [START_REF] Aranda | Temporary inactivation of the supramammillary area impairs spatial working memory and spatial reference memory retrieval[END_REF][START_REF] Shahidi | Effect of reversible inactivation of the supramammillary nucleus on spatial learning and memory in rats[END_REF], working memory [START_REF] Aranda | Supramammillary and adjacent nuclei lesions impair spatial working memory and induce anxiolitic-like behavior[END_REF][START_REF] Shahidi | Effect of reversible inactivation of the supramammillary nucleus on spatial learning and memory in rats[END_REF], and contextual fear memory [START_REF] Pan | The role of the medial supramammillary nucleus in the control of hippocampal theta activity and behaviour in rats[END_REF] have also been described upon SuM lesion or pharmacological block of action potential firing in SuM neurons. In addition, disruption of serotoninergic neuromodulation in SuM causes deficits in spatial learning accompanied by alterations of hippocampal theta [START_REF] Gutiérrez-Guzmán | Serotonin depletion of supramammillary/posterior hypothalamus nuclei produces place learning deficiencies and alters the concomitant hippocampal theta activity in rats[END_REF][START_REF] Hernández-Pérez | Supramammillary serotonin reduction alters place learning and concomitant hippocampal, septal, and supramammillar theta activity in a Morris water maze[END_REF].

Therefore, regardless of some discrepancies between studies likely due to variable approaches (eg [START_REF] Pan | The medial supramammillary nucleus, spatial learning and the frequency of hippocampal theta activity[END_REF]vs 2002), the SuM is clearly involved in spatial learning and memory although the underlying mechanisms are yet to be determined.

I.3.b.ii -Emotional, novel and social aspects of memory

The hippocampus is well known for its role in the formation of episodic memory, a person's unique memory of specific events that encloses the questions "who ?", "what ?", "where ?" and "when ?" (Squire et al., 2007). The spatial aspect of this memory has been well studied, in particular with the discovery of place cells (O' Keefe and Dostrovsky, 1971a). The other aspects of episodic memory have been much less studied.

There is compelling evidence that area CA2 plays a central role in social aspects of memory formation. Wersinger et al. first presented evidence for this role of area CA2 in this form of memory. Using repetitive exposure of a male mouse to a female mouse (10-min exposures every 5 min, repeated ten times), they showed that wild-type males are able to discriminate between novel and familiar stimulus mice more quickly than mice lacking the vasopressin V1b receptor (Wersinger et al., 2002). They also showed using a more challenging paradigm (30min interval between trials) that V1b receptor KO mice did not appear to have reduced interaction with a familiar female mouse. A few years later, the V1b receptor was shown to be expressed only in discreet regions of the brain, with a very high expression in area CA2 (Young et al., 2006). These data suggest that vasopressin release in area CA2 acting on V1b receptors plays a role in social recognition memory.

The importance of CA2 in social memory was further confirmed by two lesion studies. The first study used transgenic mice (Amigo2-cre) to express the tetanus toxin specifically in CA2 pyramidal neurons in order to prevent transmission between these cells and their targets (Hitti and Siegelbaum, 2014a). The second study performed an excitotoxic lesion of dorsal CA2 by injecting NMDA in area CA2 in rats [START_REF] Stevenson | Lesions to the CA2 region of the hippocampus impair social memory in mice[END_REF]. Both studies reported a strong impairment in social memory formation, gauged by the lack of decrease in interaction time between the experimental subject and a familiar conspecific. Thus, both studies concluded that area CA2 is necessary for the formation of social recognition memory. In agreement with this idea, optogenetic activation of vasopressinergic fibers in area CA2 has been shown to strongly increase the duration of social memory in mice [START_REF] Smith | Targeted activation of the hippocampal CA2 area strongly enhances social memory[END_REF]. Altogether, these data show that area CA2 is necessary for social memory formation and that modulation of activity in CA2 with PVN-mediated vasopressin release results in an increase in social memory duration.

While the PVN vasopressinergic innervation of area CA2 seems critical for its social functions, little is known about the role of the SuM to CA2 connection. However, area CA2 is involved in conflict detection [START_REF] Wintzer | The Hippocampal CA2 Ensemble Is Sensitive to Contextual Change[END_REF] and the SuM has been shown to be highly activated during novelty [START_REF] Ito | Three-dimensional distribution of Fos-positive neurons in the supramammillary nucleus of the rat exposed to novel environment[END_REF]. Therefore, SuM inputs to area CA2 might participate to the detection of salient novel information. In line with this hypothesis, the SuM is involved rewardrelated tasks [START_REF] Ikemoto | The Supramammillary Nucleus Mediates Primary Reinforcement via GABAA Receptors[END_REF][START_REF] Ikemoto | Rewarding Effects of AMPA Administration into the Supramammillary or Posterior Hypothalamic Nuclei But Not the Ventral Tegmental Area[END_REF], thus potentially conveying emotional signals to area CA2. Although direct evidence is missing, the variations of CA2 activity seen upon exposure to novel objects or animal (Alexander et al., 2016a) might be brought about by the inputs it receives from the SuM.

I.4 -Goals of the thesis

In this section, we have presented the anatomy, connectivity, physiology and functions of the hippocampus, a cerebral structure critically involved in learning and memory. The description of the specificities of each hippocampal region has highlighted their particular roles in information processing. Even so, this introduction also revealed that several aspects of hippocampal circuitry and physiology are still not well understood. In particular, the contributions of area CA2 to hippocampal network activity remain underexplored. As suggested by its role in social memory, spatial coding and SWR generation, area CA2 is clearly involved in important hippocampal functions. In addition, hippocampal rhythms are influenced by extra-hippocampal inputs amongst which the SuM directly connects area CA2. Therefore, understanding the patterns of activity underlying hippocampal-dependent learning and memory requires area CA2 to be taken into consideration.

Because the SuM participates to the control of hippocampal theta oscillations and projects directly to area CA2, the main goal of this thesis was to characterize the functional aspects of this connection. First, given theta states are associated with oscillatory activity of hippocampal neurons receiving cholinergic inputs from the septum, we wished to determine the properties of cellular and network activity in area CA2 under conditions of elevated cholinergic tone. This question was addressed in a first study using carbachol application to induce gamma-like oscillations in acute hippocampal slices from mice. Then, we aimed to decipher the local circuit engaged by the long-range SuM input in area CA2 and its influence over CA2 pyramidal neurons activity. We focused on this point in a second study using optogenetics to stimulate SuM afferent fibers to area CA2 and probe the consequences of their activation ex vivo.

II -Material and methods

II.1 -Animals

All procedures involving animals were performed in accordance with institutional regulations.

Animal sample sizes were estimated using power tests with standard deviations and ANOVA values from pilot experiments. A 15 % failure rate was assumed to account for stereotaxic injection errors and slice preparation complications. No alterations of neurotransmission were observed in the transgenic lines used in this study.

Use of the Tg(Slc17ab-icre)10Ki mouse line (VGluT2-Cre) we used the Tg(Slc17ab-icre)10Ki mouse line that was previously generated [START_REF] Borgius | A transgenic mouse line for molecular genetic analysis of excitatory glutamatergic neurons[END_REF] to express the Cre recombinase under the control slc17a6 gene coding for the vesicular glutamate transporter isoform 2 (VGluT2).

Use of the Pvalbtm1(cre)Arbr/J mouse line (PV-Cre) we used the Pvalbtm1(cre)Arbr/J mouse line that was previously generated [START_REF] Hippenmeyer | A developmental switch in the response of DRG neurons to ETS transcription factor signaling[END_REF] to express the Cre recombinase under the control Pvalbm gene coding for parvalbumin (PV).

Use of the Cacng5-Cre mouse line we used the Cacng5-Cre mouse line that was previously generated [START_REF] Boehringer | Chronic Loss of CA2 Transmission Leads to Hippocampal Hyperexcitability[END_REF] to express the Cre recombinase under the control cacng5 gene coding for the voltage-dependent calcium channel gamma subunit 5 specifically expressed in CA2 PNs.

Generation of the csf2rb2-Cre mouse line by the McHugh laboratory

The bacterial artificial chromosome (BAC) RP23-233L7 (BACPAC Resources Center) containing Mus musculus colony stimulating factor 2 receptor, beta 2, low-affinity (granulocyte-macrophage) (csf2rb2) was modified using the Quick & Easy BAC Modification Kit (Gene Bridges GmbH) to insert a Cre-FRT-Amp-FRT cassette at the start site of csf2rb2 translation. The ampicillin (Amp) marker was deleted utilizing Flp-706 recombination and the Cre-modified BAC's were confirmed by PCR analysis. The confirmed Cre-modified RP23-233L7 BAC was subsequently purified using the QIAGEN Large Construct Kit (QIAGEN). To prepare the modified BAC for microinjection, 50 µg of Cre-modified RP23-233L7 BAC was linearized using AscI (New England Biolabs (NEB)) and then buffer exchanged using the Sepharose CL-4B matrix (Pharmacia/Pfizer) into an injection buffer composed of 10 mM Tris-HCl, pH 7.4, 0.1 mM EDTA, and 100 mM NaCl. The purified BAC was injected into C57BL/6 fertilized pronuclei and fertilized blastocysts were implanted into pseudopregnant females.

From the microinjections, three csf2rb2 B6 founders (lines 252, 267, 271) were generated. The 252 csf2rb2-Cre line was determined to express Cre selectively in the supramammillary region using immunohistochemistry with the anti-Cre antibody (AB24607, AbCam). The 252 line was maintained in a pure C57BL/6 background for all the experiments herein described.

II.2 -Stereotaxic viral injections

Surgical procedure Animals were anaesthetized with ketamine (100 mg/kg) and xylazine (7 mg/kg). 500 nL of virus was injected into the brain of 4 week-old male wild type C57BL6, Tg(Slc17ab-icre)10Ki (VGluT2-Cre) mice, csf2rb2-cre, Pvalbtm1(cre)Arbr/J (PV-Cre) or Cacng5-Cre mice at 100 nL/min and the injection cannula was left at the injection site for 10 minutes following infusion.

SuM was targeted by unilateral midline injections. Bilateral injections were performed in dorsal CA2.

Viral titers

The

Injections sites coordinates

The loci of the injection sites were as follows: anterior-posterior relative to bregma: -2.8 mm for SuM, -1.6 mm for CA2; medial-lateral relative to midline: 0 mm for SuM, 1.9 mm for CA2;

dorsal-ventral relative to surface of the brain: 4.75 mm for SuM, 1.4 mm for CA2.

II.3 -Acute hippocampal slices preparation

Transverse hippocampal slices were prepared at least 3 weeks after viral. In the case of mice and conditions. Longer trains of 2 minutes at 10 Hz with a light power of 25 mW/mm² were used for assessing the effect of SuM inputs on CA2 PN bursting. These trains were triggered manually when the cell had reached a critical pre-burst VM inferred from online examination of previous bursts as a point of no return in the depolarization leading to burst discharge.

Variability in light onset was minimized and light-on bursts were interleaved with light-off bursts to provide internal control. For the experiments performed in CA1, we used 50 ms long light pulses of 25 mW/mm² delivered at 10 Hz for 1 second repeated every 10 seconds.

II.6 -Immunochemistry and cell identification

Midbrains containing the injection site were examined post-hoc to ensure that infection was restricted to the SuM.

II.7 -Data analysis and statistics

Electrophysiological recordings were analyzed using IGORpro (Wavemetrics) and Clampfit (Molecular devices) software.

Whole-cell recordings

For the clustering of CA2 PN action potentials (APs) in bursts, APs in whole cell recordings were detected automatically by threshold with IGOR TaroTools followed by visual inspection.

Kinetics of membrane potential (VM) variations in the vicinity of bursts were measured by linear fits of the VM trace as it changed from ACSF-VM to CCh-VM or from inter-burst-VM plus 5 mV to burst-VM minus 5 mV. AP phases were defined as the phase of the oscillation in area CA2

SP at the time of the AP. Whole-cell recordings during which CA2 PNs did not fire APs or went into depolarization block were excluded from analysis.

For accurate measurements of the kinetics and latencies of SuM light-evoked post-synaptic responses, the following detection process was used. For each cell, average traces were used to create a template waveform that was then fitted to individual traces and measurements were performed on the fitted traces. When only amplitudes of responses were needed, standard average peak detection was used.

When assessing the effect of SuM photostimulation on CA2 PN bursting, comparison of lighton versus light-off bursts in CA2 PNs was performed by aligning all bursts to the pre-burst VM defined as the mean VM at light onset for each cell.

LFP recordings

For the analysis of CCh-induced oscillations, LFPs were digitally highpass filtered at 1 Hz and lowpass filtered at 1 kHz post hoc using IGOR DSP filters prior to Fourier analysis. Timefrequency analysis was performed on sequential 60 seconds long bins of signal with fast Fourier transform applied to time windows of 1.5 seconds with 50 % overlap multiplied by a Hanning window. Gamma power was defined as the area under the power spectral density between 20 and 40 Hz. LFP recordings that failed to display gamma power above baseline (taken before CCh application) or that were contaminated by noise were excluded from analysis. Wavelet analysis was performed on time windows up to 60 seconds of raw signal decimated to 1 kHz, by computing the complex Morlet wavelet transform of the LFP between 15 and 45 Hz with Fourier scaling. Phases were extracted from the frequency bearing the maximal magnitude at each time point. Cycles ran from -π to π with a phase of -π defined at the trough of oscillations.

Peak-to-peak cycle averages were constructed by averaging raw LFP traces between peaks detected as zero crossings of the derivatives of the corresponding bandpass filtered traces between 20 and 40 Hz with a custom designed digital finite impulse response filter of -60 dB attenuation.

For the detection of extracellular AP firing, LFPs were digitally highpass filtered at 1 kHz post hoc to isolate spikes using IGOR DSP filter. APs were then detected automatically by threshold with IGOR TaroTools followed by visual inspection. Circular statistics were used to evaluate the strength of the AP phase-coupling. For each burst of APs, a mean vector (r) was constructed by summing unitary vectors (R) with angle defined by the phase of each AP in the burst and normalizing to the number of APs (n) in the burst: |r| = |R| / n. This yielded a mean vector (r) for each burst with an angle representing the preferred phase of APs in the burst and a radius inversely proportional to the dispersion of individual AP phases around a cycle. Significance of the phase coupling was assessed for each burst by computing the Rayleigh probability of the corresponding mean vector. The circular (SD) was taken as SD = √ -2 ln |r|.

Statistics

II.8 -Experimental strategy

Investigation of single-neuron and LFP activity during carbachol-induced oscillations

In this first set of experiments, transverse acute hippocampal slices were prepared from 6-10 week-old mice. After a baseline recording period of 1-5 minutes, 10 µM CCh was continuously applied to these slices while the following experiments were performed. This concentration of CCh was chosen to be consistent with several previous studies working on similar preparations [START_REF] Fisahn | Cholinergic induction of network oscillations at 40 Hz in the hippocampus in vitro[END_REF][START_REF] Gulyás | Parvalbumin-Containing Fast-Spiking Basket Cells Generate the Field Potential Oscillations Induced by Cholinergic Receptor Activation in the Hippocampus[END_REF][START_REF] Mann | Perisomatic Feedback Inhibition Underlies Cholinergically Induced Fast Network Oscillations in the Rat Hippocampus In Vitro[END_REF][START_REF] Oren | Synaptic Currents in Anatomically Identified CA3 Neurons during Hippocampal Gamma Oscillations In Vitro[END_REF][START_REF] Zemankovics | Feedforward Inhibition Underlies the Propagation of Cholinergically Induced Gamma Oscillations from Hippocampal CA3 to CA1[END_REF]. Theta and gamma rhythms in hippocampal networks are critical for learning and memory [START_REF] Buzsáki | Memory, navigation and theta rhythm in the hippocampalentorhinal system[END_REF][START_REF] Colgin | Rhythms of the hippocampal network[END_REF]. Cellular mechanisms underlying gamma oscillations in area CA3 and CA1 are well documented [START_REF] Csicsvari | Mechanisms of Gamma Oscillations in the Hippocampus of the Behaving Rat[END_REF][START_REF] Fisahn | Cholinergic induction of network oscillations at 40 Hz in the hippocampus in vitro[END_REF][START_REF] Mann | Perisomatic Feedback Inhibition Underlies Cholinergically Induced Fast Network Oscillations in the Rat Hippocampus In Vitro[END_REF][START_REF] Oren | Synaptic Currents in Anatomically Identified CA3 Neurons during Hippocampal Gamma Oscillations In Vitro[END_REF][START_REF] Zemankovics | Feedforward Inhibition Underlies the Propagation of Cholinergically Induced Gamma Oscillations from Hippocampal CA3 to CA1[END_REF], however, as of now, this activity in area CA2 has not been examined. Recent in vivo recordings have revealed that neuronal activity in area CA2 is critical for several aspects of hippocampal physiology including initiation of sharp waves / ripples (SWR) (Oliva et al., 2016)and are capable of encoding spatial information during immobility [START_REF] Kay | A hippocampal network for spatial coding during immobility and sleep[END_REF]. These important roles indicate that a better understanding of area CA2 network oscillations is required to better understand hippocampal function.

Thus, we set out to examine the cellular and network mechanisms governing rhythmic activity in area CA2. Area CA3 has been shown to intrinsically generate slow gamma activity in acute slice preparations [START_REF] Fisahn | Cholinergic induction of network oscillations at 40 Hz in the hippocampus in vitro[END_REF][START_REF] Mann | Perisomatic Feedback Inhibition Underlies Cholinergically Induced Fast Network Oscillations in the Rat Hippocampus In Vitro[END_REF][START_REF] Oren | Synaptic Currents in Anatomically Identified CA3 Neurons during Hippocampal Gamma Oscillations In Vitro[END_REF]) and shares similarities with area CA2 in terms of network recurrence, inputs and outputs [START_REF] Boehringer | Chronic Loss of CA2 Transmission Leads to Hippocampal Hyperexcitability[END_REF][START_REF] Chevaleyre | Strong CA2 Pyramidal Neuron Synapses Define a Powerful Disynaptic Cortico-Hippocampal Loop[END_REF][START_REF] Hitti | The hippocampal CA2 region is essential for social memory[END_REF][START_REF] Kohara | Cell type-specific genetic and optogenetic tools reveal hippocampal CA2 circuits[END_REF][START_REF] Sun | Proximodistal Heterogeneity of Hippocampal CA3 Pyramidal Neuron Intrinsic Properties, Connectivity, and Reactivation during Memory Recall[END_REF]. We focused our study on pharmacologically-induced ex vivo gamma-like activity in area CA2. A wealth of studies focused on CA3 and CA1 have provided evidence that gamma oscillations in the low range, i.e. 30 to 50 Hz, are generated in area CA3 and propagate to area CA1 through feedforward inhibition [START_REF] Csicsvari | Mechanisms of Gamma Oscillations in the Hippocampus of the Behaving Rat[END_REF][START_REF] Zemankovics | Feedforward Inhibition Underlies the Propagation of Cholinergically Induced Gamma Oscillations from Hippocampal CA3 to CA1[END_REF]. Both CA2 and CA3 PNs reciprocally recruit strong feed-foward inhibition over one another [START_REF] Boehringer | Chronic Loss of CA2 Transmission Leads to Hippocampal Hyperexcitability[END_REF][START_REF] Chevaleyre | Strong CA2 Pyramidal Neuron Synapses Define a Powerful Disynaptic Cortico-Hippocampal Loop[END_REF], and CA2 PNs also project onto CA1 PNs [START_REF] Chevaleyre | Strong CA2 Pyramidal Neuron Synapses Define a Powerful Disynaptic Cortico-Hippocampal Loop[END_REF][START_REF] Kohara | Cell type-specific genetic and optogenetic tools reveal hippocampal CA2 circuits[END_REF]. Therefore, description of cellular and network activity in area CA2 will help the understanding of gamma oscillations by incorporating area CA2 in the hippocampal circuit and taking into account its mutual influences with CA3 and its contribution to driving activity in CA1. To this end, we recorded simultaneous local field potentials (LFPs) from area CA2 and CA3 and obtained whole-cell patch-clamp recordings from CA2 PNs while superfusing acute hippocampal slices with the cholinergic agonist carbachol (CCh). This approach allowed us to compare network activity in area CA2 and CA3, to explore the activity of CA2 PNs in this condition and relate it to the ongoing oscillation.

We observed that application of the cholinergic agonist carbachol reliably induced gamma-like oscillations in areas CA3 and CA2 as recorded with local field potentials Comparison of such oscillations between both regions revealed that the power of the signal was lower in area CA2.

Patch-clamp recordings of CA2 pyramidal neurons in these conditions revealed their ability to rhythmically fire bursts of action potentials. Excitatory and inhibitory synaptic inputs contributed to the shaping of these action potentials bursts in CA2 PNs. However, synaptic transmission was not necessary as burst firing persisted when blocked. CA2 PN activity was influenced by the network activity as evidenced by phase-coupling of action potential firing to the ongoing field oscillation during bursts. Therefore, our findings reveal the ability of the CA2 neuronal network to undergo gamma-like activity similar to area CA3 and highlight the pattern of action potential firing in CA2 PNs in the presence of increased cholinergic tone.

III.1.b -Results

Carbachol induces gamma-like activity in area CA2

Both in vivo and ex vivo studies have provided evidence that area CA3 can intrinsically generate oscillations in the low gamma range [START_REF] Csicsvari | Mechanisms of Gamma Oscillations in the Hippocampus of the Behaving Rat[END_REF][START_REF] Fisahn | Cholinergic induction of network oscillations at 40 Hz in the hippocampus in vitro[END_REF][START_REF] Mann | Perisomatic Feedback Inhibition Underlies Cholinergically Induced Fast Network Oscillations in the Rat Hippocampus In Vitro[END_REF][START_REF] Oren | Synaptic Currents in Anatomically Identified CA3 Neurons during Hippocampal Gamma Oscillations In Vitro[END_REF][START_REF] Zemankovics | Feedforward Inhibition Underlies the Propagation of Cholinergically Induced Gamma Oscillations from Hippocampal CA3 to CA1[END_REF]. Given the newly discovered roles of area CA2 network activity in fundamental aspects of hippocampal physiology such as SWR generation (Oliva et al., 2016) and spatial coding [START_REF] Kay | A hippocampal network for spatial coding during immobility and sleep[END_REF], and because it shares properties in terms of inputs, outputs and recurrent connections with area CA3 [START_REF] Chevaleyre | Strong CA2 Pyramidal Neuron Synapses Define a Powerful Disynaptic Cortico-Hippocampal Loop[END_REF][START_REF] Cui | Hypothalamic and other connections with dorsal CA2 area of the mouse hippocampus[END_REF][START_REF] Hitti | The hippocampal CA2 region is essential for social memory[END_REF][START_REF] Kohara | Cell type-specific genetic and optogenetic tools reveal hippocampal CA2 circuits[END_REF][START_REF] Sun | Proximodistal Heterogeneity of Hippocampal CA3 Pyramidal Neuron Intrinsic Properties, Connectivity, and Reactivation during Memory Recall[END_REF], we sought to determine how neuronal networks in area CA2 behave when challenged with an increase in cholinergic tone. To this end, we simultaneously recorded local field Further analysis was performed using wavelet transforms with Morlet basis on subsets of data spanning 60 seconds during which gamma power was maximal, to extract the magnitude of frequency components in the signal from 15 to 45 Hz [START_REF] Oren | Synaptic Currents in Anatomically Identified CA3 Neurons during Hippocampal Gamma Oscillations In Vitro[END_REF]. This method revealed that the magnitude and dominant frequency of oscillations were fluctuating over time in the

CA2 pyramidal cells undergo slow rhythmic bursting activity during CCh-induced oscillations

Gamma oscillations in area CA3 result from the synchronized activity of pyramidal cells that build up excitation through their excitatory recurrent conncetions and then recruit feedback inhibition mediated by peri-somatic targeting interneurons which repolarize them, hence completing a cycle [START_REF] Csicsvari | Mechanisms of Gamma Oscillations in the Hippocampus of the Behaving Rat[END_REF][START_REF] Fisahn | Cholinergic induction of network oscillations at 40 Hz in the hippocampus in vitro[END_REF][START_REF] Mann | Perisomatic Feedback Inhibition Underlies Cholinergically Induced Fast Network Oscillations in the Rat Hippocampus In Vitro[END_REF][START_REF] Oren | Synaptic Currents in Anatomically Identified CA3 Neurons during Hippocampal Gamma Oscillations In Vitro[END_REF][START_REF] Zemankovics | Feedforward Inhibition Underlies the Propagation of Cholinergically Induced Gamma Oscillations from Hippocampal CA3 to CA1[END_REF]. Given CCh application triggers gamma-like oscillations in area CA2 that were somewhat different from gamma-like oscillations in CA3, and because CA2 pyramidal neurons display unique properties [START_REF] Chevaleyre | Strong CA2 Pyramidal Neuron Synapses Define a Powerful Disynaptic Cortico-Hippocampal Loop[END_REF]Srinivas et al., 2017;[START_REF] Pagani | Role of the vasopressin 1b receptor in rodent aggressive behavior and synaptic plasticity in hippocampal area CA2[END_REF][START_REF] Piskorowski | Age-Dependent Specific Changes in Area CA2 of the Hippocampus and Social Memory Deficit in a Mouse Model of the 22q11.2 Deletion Syndrome[END_REF][START_REF] Sun | Dendritic Na+ spikes enable cortical input to drive action potential output from hippocampal CA2 pyramidal neurons[END_REF][START_REF] Talley | Cns distribution of members of the twopore-domain (KCNK) potassium channel family[END_REF] This CCh-induced depolarization was slow (6.5 ± 0.8 mV/min) and variable as CA2 PNs took several minutes (6.4 ± 0.7 min) to reach the CCh-VM (Table III Wavelet analysis of LFP signal relative to CA2 PNs bursting revealed no changes in gamma magnitude or peak frequency related to bursts (wavelet magnitudes were 0.25 ± 0.03 during bursts and 0.24 ± 0.3 between bursts, n = 29; Wilcoxon signed-rank test, p = 0.29; peak frequencies were 27 ± 0.7 Hz during bursts and 26 ± 0.7 Hz between bursts, n = 29; Wilcoxon signed-rank test, p = 0.62). This indicates that, although synaptic inputs impact both network oscillations and CA2 PNs bursting, bursts of action potentials in CA2 PNs do not reflect epochs of increased synchronous network activity. However, it is well documented that different neuronal populations preferentially fire action potentials at specific phases of the gamma oscillation [START_REF] Csicsvari | Mechanisms of Gamma Oscillations in the Hippocampus of the Behaving Rat[END_REF][START_REF] Mann | Perisomatic Feedback Inhibition Underlies Cholinergically Induced Fast Network Oscillations in the Rat Hippocampus In Vitro[END_REF][START_REF] Oren | Synaptic Currents in Anatomically Identified CA3 Neurons during Hippocampal Gamma Oscillations In Vitro[END_REF][START_REF] Zemankovics | Feedforward Inhibition Underlies the Propagation of Cholinergically Induced Gamma Oscillations from Hippocampal CA3 to CA1[END_REF], a phenomenon that relates neuronal activity and synaptic inputs to the activity of the networks they are embedded in. Classically, CA3 PNs fire shortly after the trough of slow gamma oscillations recorded from SP ex vivo [START_REF] Oren | Synaptic Currents in Anatomically Identified CA3 Neurons during Hippocampal Gamma Oscillations In Vitro[END_REF][START_REF] Zemankovics | Feedforward Inhibition Underlies the Propagation of Cholinergically Induced Gamma Oscillations from Hippocampal CA3 to CA1[END_REF] while CA3 interneurons prefer later phases [START_REF] Oren | Synaptic Currents in Anatomically Identified CA3 Neurons during Hippocampal Gamma Oscillations In Vitro[END_REF][START_REF] Zemankovics | Feedforward Inhibition Underlies the Propagation of Cholinergically Induced Gamma Oscillations from Hippocampal CA3 to CA1[END_REF]. In order to check if these relations of phase-coupling firing of neurons with the oscillation were present in CA2 PNs as well, we extracted the timing of action potential firing relative to the CCh-induced oscillation during bursts (Figure III.1.8B). We found that the more than half of CA2 PNs had their firing modulated by the field oscillation during bursts (55.2 %, n = 16 / 29).

Examination of individual bursts from each CA2 PN revealed that action potential firing showed significant phase coupling only in a fraction of bursts (18.5 ± 3.4 %, n = 16) which was

III.2 -Hypothalamic control of hippocampal area CA2 activity by direct excitation and feedforward inhibition III.2.a -Introduction

The hippocampus is critical for memory formation and spatial navigation [START_REF] Buzsáki | Memory, navigation and theta rhythm in the hippocampalentorhinal system[END_REF][START_REF] Eichenbaum | Can We Reconcile the Declarative Memory and Spatial Navigation Views on Hippocampal Function?[END_REF]), yet basic questions persist regarding the circuitry and cellular components allowing these processes. In particular, it has recently been discovered that area CA2 of the hippocampus forms the basis of a hippocampal-wide network that encodes location during immobility and sleep [START_REF] Kay | A hippocampal network for spatial coding during immobility and sleep[END_REF]. Furthermore, lesions in area CA2 result in reduced social memory [START_REF] Hitti | The hippocampal CA2 region is essential for social memory[END_REF][START_REF] Stevenson | Lesions to the CA2 region of the hippocampus impair social memory in mice[END_REF].

Immediate-early gene expression studies have shown that this area may detect conflict between present and previous experience [START_REF] Wintzer | The Hippocampal CA2 Ensemble Is Sensitive to Contextual Change[END_REF]. In contrast to areas CA1 and CA3, the place cell activity [START_REF] Alexander | Social and novel contexts modify hippocampal CA2 representations of space[END_REF][START_REF] Lee | Neural Population Evidence of Functional Heterogeneity along the CA3 Transverse Axis: Pattern Completion versus Pattern Separation[END_REF][START_REF] Lu | Topography of Place Maps along the CA3-to-CA2 Axis of the Hippocampus[END_REF][START_REF] Mankin | Hippocampal CA2 Activity Patterns Change over Time to a Larger Extent than between Spatial Contexts[END_REF], hippocampal connectivity of area CA2 [START_REF] Chevaleyre | Strong CA2 Pyramidal Neuron Synapses Define a Powerful Disynaptic Cortico-Hippocampal Loop[END_REF][START_REF] Kohara | Cell type-specific genetic and optogenetic tools reveal hippocampal CA2 circuits[END_REF] and synaptic plasticity [START_REF] Piskorowski | Delta-Opioid Receptors Mediate Unique Plasticity onto Parvalbumin-Expressing Interneurons in Area CA2 of the Hippocampus[END_REF][START_REF] Zhao | Synaptic Plasticity (and the Lack Thereof) in Hippocampal CA2 Neurons[END_REF] in this region have only very recently begun being examined ex vivo and in vivo.

The hypothalamic supramammillary nucleus (SuM) has been shown to project to the dentate gyrus (DG) and hippocampal area CA2 in several species, including rodents and humans [START_REF] Berger | Human and monkey fetal brain development of the supramammillary-hippocampal projections: A system involved in the regulation of theta activity[END_REF][START_REF] Haglund | The projection of the supramammillary nucleus to the hippocampal formation: An immunohistochemical and anterograde transport study with the lectin PHA-L in the rat[END_REF][START_REF] Wyss | Evidence for an input to the molecular layer and the stratum granulosum of the dentate gyrus from the supramammillary region of the hypothalamus[END_REF]. There is evidence that the SuM is involved in learning processes, as spatial memory is impaired following silencing of the SuM with pharmacological methods [START_REF] Aranda | Temporary inactivation of the supramammillary area impairs spatial working memory and spatial reference memory retrieval[END_REF][START_REF] Shahidi | Effect of reversible inactivation of the supramammillary nucleus on spatial learning and memory in rats[END_REF] or lesions [START_REF] Aranda | Supramammillary and adjacent nuclei lesions impair spatial working memory and induce anxiolitic-like behavior[END_REF]. Furthermore, analysis of cFos expression revealed that this nucleus is highly activated by exposure to a novel environment [START_REF] Ito | Three-dimensional distribution of Fos-positive neurons in the supramammillary nucleus of the rat exposed to novel environment[END_REF]. Under urethane anesthesia, SuM neurons fire bursts of action potentials (AP) phase-locked with the hippocampal theta rhythm [START_REF] Kocsis | Characterization of neurons of the supramammillary nucleus and mammillary body that discharge rhythmically with the hippocampal theta rhythm in the rat[END_REF]. The SuM plays a prominent role in the modulation of the hippocampal theta-frequency [START_REF] Pan | The medial supramammillary nucleus, spatial learning and the frequency of hippocampal theta activity[END_REF]2002), possibly by its direct projection to the hippocampus, or by modulation of the medial septum [START_REF] Borhegyi | Dual projection from the medial septum to the supramammillary nucleus in the rat[END_REF][START_REF] Vertes | PHA-L analysis of projections from the supramammillary nucleus in the rat[END_REF]. Serotonin depletion of the SuM leads to deficiencies in spatial learning in the morris water maze, and results in altered hippocampal theta activity [START_REF] Gutiérrez-Guzmán | Serotonin depletion of supramammillary/posterior hypothalamus nuclei produces place learning deficiencies and alters the concomitant hippocampal theta activity in rats[END_REF][START_REF] Hernández-Pérez | Supramammillary serotonin reduction alters place learning and concomitant hippocampal, septal, and supramammillar theta activity in a Morris water maze[END_REF].

Even with the anatomical and in vivo data, the properties and consequences of SuM activation on area CA2 activity remain unexplored. In this study, we use a combination of histological, ex vivo electrophysiological and optogenetic approaches, to specifically examine the effects of SuM input stimulation on neuronal activity in hippocampal area CA2. We used a viral retrograde approach to identify the SuM cells that project to area CA2 in mice. Then, we used viral injections of Cre-dependent constructs in the SuM of VGluT2-Cre and Csf2rb2-Cre transgenic mice to selectively express channelrhodopsin-2. We performed whole-cell patchclamp recordings in hippocampal slices from these mice while photostimulating SuM axons to examine their targets and the consequences of their activation in area CA2. We found that SuM axons target both pyramidal neurons (PNs) and interneurons (INs) to which they provide glutamatergic excitation. The SuM excitatory drive was significantly stronger on basket celltype INs than any other neuronal population. Consequently, we observed substantial feedforward inhibition onto PNs recruited by SuM stimulation. In addition, the excitatory/inhibitory drive from SuM was more inhibitory in deep compared to superficial PN subpopulations, regardless of their CA2 or CA3 morphology. Finally, we found that this inhibition was capable of modulating the action potential jittering of CA2 and CA3a PNs, and thus is likely capable of modulating excitability in this area of the hippocampus.

III.2.b -Results

The supramammillary projection to the hippocampus has been described in retrograde and anterograde studies in the rat, guinea pig and monkey [START_REF] Borhegyi | The supramammillary nucleus innervates cholinergic and GABAergic neurons in the medial septum-diagonal band of Broca complex[END_REF][START_REF] Haglund | The projection of the supramammillary nucleus to the hippocampal formation: An immunohistochemical and anterograde transport study with the lectin PHA-L in the rat[END_REF][START_REF] Kiss | The supramammillo-hippocampal and supramammillo-septal glutamatergic/aspartatergic projections in the rat: a combined [3H]daspartate autoradiographic and immunohistochemical study[END_REF][START_REF] Soussi | Heterogeneity of the supramammillary-hippocampal pathways: evidence for a unique GABAergic neurotransmitter phenotype and regional differences[END_REF]. In order to determine which cells in the mouse SuM promoter, the Tg(Slc17ab-icre)10Ki line [START_REF] Borgius | A transgenic mouse line for molecular genetic analysis of excitatory glutamatergic neurons[END_REF]. In parallel, we used a novel mouse line, the Csf2rb2-Cre line that selectively expresses Cre recombinase in the SuM. We found that with both strategies we could reproducibly restrict expression of ChR2-EYFP in the SuM, and avoid infecting nearby hypothalamic regions that also may project to the and primates [START_REF] Berger | Human and monkey fetal brain development of the supramammillary-hippocampal projections: A system involved in the regulation of theta activity[END_REF]Borhegyi and Leranth, 1997b;1997a;[START_REF] Halasy | Distribution and origin of vesicular glutamate transporter 2-immunoreactive fibers in the rat hippocampus[END_REF][START_REF] Kiss | The supramammillo-hippocampal and supramammillo-septal glutamatergic/aspartatergic projections in the rat: a combined [3H]daspartate autoradiographic and immunohistochemical study[END_REF][START_REF] Soussi | Heterogeneity of the supramammillary-hippocampal pathways: evidence for a unique GABAergic neurotransmitter phenotype and regional differences[END_REF]. In the CA regions, the SuM fiber projection area was clearly restricted to area CA2, as defined by expression of the CA2-specific markers PCP4 and

SuM axons provide excitatory glutamatergic input to pyramidal neurons in area CA2

In order to better understand the cellular targets and consequences of SuM input activity in area CA2, we applied the above experimental strategy to express ChR2-EYFP in SuM axonal fibers and activated projecting axons with pulses of 440 nm light in acute hippocampal slices while Dentate gyrus granule cells receive mono-synaptic glutamatergic as well as mono-and disynaptic GABAergic inputs from the SuM Besides the release of glutamate by SuM axons, histological and physiological studies have described co-release of GABA by SuM inputs onto granule cells (GCs) in the dentate gyrus [START_REF] Pedersen | Supramammillary glutamate neurons are a key node of the arousal system[END_REF][START_REF] Soussi | Heterogeneity of the supramammillary-hippocampal pathways: evidence for a unique GABAergic neurotransmitter phenotype and regional differences[END_REF]. Therefore, we wished to verify that our novel EPSCs could be compared between different cell populations. However, not every photostimulation gave rise to an EPSC leading to an average success rate that seemed highest in BCs, although this difference did not reach statistical significance (Table III.2.6). In addition, BCs appeared to receive more excitation from SuM inputs than other cells types, as EPSCs were larger in BCs than in PNs (Table III.2.6). EPSCs recorded in BCs also had faster kinetics than in PNs (Table III.

Parvalbumin-expressing basket cells mediate the feedforward inhibition recruited by SuM

In the hippocampus, BCs express either cholecystokinin (CCK) or parvalbumin (PV) [START_REF] Klausberger | Neuronal Diversity and Temporal Dynamics: The Unity of Hippocampal Circuit Operations[END_REF]. We found that in response to a 1 second depolarizing pulse, most BCs that received strong SuM excitatory input displayed very fast AP firing with little accommodation in the AP firing frequency (Table III [START_REF] Pawelzik | Physiological and morphological diversity of immunocytochemically defined parvalbumin-and cholecystokinin-positive interneurones in CA1 of the adult rat hippocampus[END_REF]. In contrast, CCK-expressing BCs show a lower firing frequency and more accommodation during the train [START_REF] Pawelzik | Physiological and morphological diversity of immunocytochemically defined parvalbumin-and cholecystokinin-positive interneurones in CA1 of the adult rat hippocampus[END_REF]. This result suggests that BCs connected by the SuM may be expressing PV. To directly confirm this hypothesis, we performed post-hoc immunostaining of recorded interneurons that received strong excitation from SuM input. Because of the dialysis inherent to the whole-cell recording conditions, we encountered staining difficulties for multiple cells. However, PVimmunoreactivity could unequivocally be detected in either the soma or dendrites of 7 connected BCs (Figure III.2.9D). Therefore, this data demonstrates that at least a fraction of the recorded BCs connected by the SuM are expressing PV. We have previously shown that the SuM controls the timing of CA2 PN action potential firing by recruiting feedforward inhibition mediated by PV-expressing (PV+) basket cells (BCs). This population of interneurons (INs) is critically involved in virtually all hippocampal rhythms including theta, gamma, and SWR [START_REF] Fuchs | Recruitment of Parvalbumin-Positive Interneurons Determines Hippocampal Function and Associated Behavior[END_REF][START_REF] Gulyás | Parvalbumin-Containing Fast-Spiking Basket Cells Generate the Field Potential Oscillations Induced by Cholinergic Receptor Activation in the Hippocampus[END_REF][START_REF] Hájos | Input-Output Features of Anatomically Identified CA3 Neurons during Hippocampal Sharp Wave/Ripple Oscillation In Vitro[END_REF][START_REF] Klausberger | Complementary Roles of Cholecystokinin-and Parvalbumin-Expressing GABAergic Neurons in Hippocampal Network Oscillations[END_REF][START_REF] Korotkova | NMDA Receptor Ablation on Parvalbumin-Positive Interneurons Impairs Hippocampal Synchrony, Spatial Representations, and Working Memory[END_REF][START_REF] Royer | Control of timing, rate and bursts of hippocampal place cells by dendritic and somatic inhibition[END_REF]. This prompted us to examine the effect of SuM stimulation on area CA2 activity and the consequences on CA1 PN firing in a physiologically active hippocampal network. Slow gamma oscillations are generated intrinsically in the hippocampus, and this oscillation can be induced in acute hippocampal slices by elevating the cholinergic tone [START_REF] Fisahn | Cholinergic induction of network oscillations at 40 Hz in the hippocampus in vitro[END_REF][START_REF] Mann | Perisomatic Feedback Inhibition Underlies Cholinergically Induced Fast Network Oscillations in the Rat Hippocampus In Vitro[END_REF][START_REF] Oren | Synaptic Currents in Anatomically Identified CA3 Neurons during Hippocampal Gamma Oscillations In Vitro[END_REF][START_REF] Zemankovics | Feedforward Inhibition Underlies the Propagation of Cholinergically Induced Gamma Oscillations from Hippocampal CA3 to CA1[END_REF]. We have previously observed that application of the cholinergic agonist carbachol (CCh) induces gamma-like oscillations in area CA2 and triggers burst firing

III.3.b -Results

Carbachol influences the strength and dynamics of SuM transmission onto area CA2

We have previously shown that SuM inputs provide direct excitation and feedforward inhibition to area CA2 PNs resulting in an overall inhibitory drive. This feedforward inhibition is mediated by PV+ BCs that fire action potentials in response to SuM axon photostimulation. In order to study the functional consequences of activating SuM inputs to area CA2 under conditions in which neurons are active, we applied CCh to induce network activity in acute hippocampal slices from injected VGluT2-Cre and Csf2rb2-Cre animals expressing ChR2 in SuM axons.

Activation of cholinergic receptors has profound and diverse effects on several hippocampal targets including the depolarization of PNs and the majority of INs, as well as the modulation of glutamate and GABA release [START_REF] Alger | Muscarinic cholinergic receptors modulate inhibitory synaptic rhythms in hippocampus and neocortex[END_REF][START_REF] Cobb | Cholinergic modulation of hippocampal cells and circuits[END_REF][START_REF] Hasselmo | The role of acetylcholine in learning and memory[END_REF].

Because of this, we first assessed whether mono-synaptic excitation and di-synaptic inhibition To achieve this, the dynamics of VM depolarization were monitored for several bursts to determine a "pre-burst" VM level that reliably preceded burst firing ( Subsequent "light-on" bursts were interleaved with "light-off" ones serving as internal controls.

Both light-on and light-off bursts were analyzed by aligning to the mean pre-burst VM corresponding to the onset of the SuM photostimulation for each cell (Figure III.3.2C). This approach revealed that light-on bursts were significantly delayed with regards to the pre-burst

IV -Discussion and future directions

The work performed during the course of this thesis explored several aspects of area CA2 physiology and connections with intra-and extra-hippocampal structures in order to better understand the role of area CA2 in hippocampal circuits and its implications in learning and memory. Because area CA2 only recently began to be investigated by in vivo experiments and had never been studied in the context of ongoing network activity ex vivo, our first goal was to examine the characteristics of CCh-induced gamma-like oscillations in area CA2. Next, as tracing studies reported the innervation of area CA2 by SuM inputs relevant to theta oscillations, our project was to decipher the micro-circuit engaged by the long-range connection from SuM to area CA2. Finally, given CA2 PNs strongly project to area CA1 that constitutes the main hippocampal output, we probed the control of area CA2 activity by SuM inputs and the consequences on activity downstream in area CA1. In this section, we discuss our main experimental findings, place them in the larger context of hippocampal physiology, and propose future directions to complement these studies.

IV.1 -Roles of area CA2 in hippocampal networks and rhythms

In this first series of experiments, we describe the cellular and circuit events underlying network oscillations in area CA2 and provide evidence of how they are related, in comparison with area CA3. Using a pharmacological model of gamma oscillations in acute hippocampal slices, we were able to compare network activity in area CA2 and CA3, as well as elucidate the corresponding dynamics of neuronal activity. We found that the CA2 neuronal network undergoes oscillatory activity in the gamma range in the presence of the cholinergic agonist CCh, similar to what has been reported in area CA3. In these conditions, we observed a peculiar pattern of activity in individual CA2 PNs consisting of bursts of action potentials. Blocking synaptic inputs revealed that CA2 PNs are intrinsically prone to bursting. However, the properties of these bursts were influenced by the activity of the surrounding network as AP timing is likely modulated by the ongoing oscillation. Altogether, these observations highlight important specific characteristics of area CA2 neuronal circuits to bear in mind when considering global hippocampal rhythms.

IV.1.a -Similarities and differences of network activity in area CA2 and

CA3

Like CA3, area CA2 possess fundamental features required for rhythmogenesis in a neuronal network : recurrent excitation [START_REF] Cui | Hypothalamic and other connections with dorsal CA2 area of the mouse hippocampus[END_REF][START_REF] Hitti | The hippocampal CA2 region is essential for social memory[END_REF] and feedback inhibition (Mercer et al., 2012a;[START_REF] Mercer | SP-SR interneurones: A novel class of neurones of the CA2 region of the hippocampus[END_REF]. The dynamics of cellular activity in relation to the network oscillations have been clarified in area CA3 by a wealth of studies demonstrating that different neuronal populations fire action potentials in an organized manner during the gamma cycle [START_REF] Csicsvari | Mechanisms of Gamma Oscillations in the Hippocampus of the Behaving Rat[END_REF][START_REF] Mann | Perisomatic Feedback Inhibition Underlies Cholinergically Induced Fast Network Oscillations in the Rat Hippocampus In Vitro[END_REF][START_REF] Oren | Synaptic Currents in Anatomically Identified CA3 Neurons during Hippocampal Gamma Oscillations In Vitro[END_REF][START_REF] Zemankovics | Feedforward Inhibition Underlies the Propagation of Cholinergically Induced Gamma Oscillations from Hippocampal CA3 to CA1[END_REF], linking synaptic currents generating the oscillation with spiking output of neurons in the network. While the basic mechanisms are likely the same in area CA2, a number of peculiarities in this region suggest that it may behave differently than CA3. First, area CA2 hosts the highest density of interneurons, PV+ in particular, in the hippocampus [START_REF] Botcher | Distribution of interneurons in the CA2 region of the rat hippocampus[END_REF][START_REF] Piskorowski | Delta-Opioid Receptors Mediate Unique Plasticity onto Parvalbumin-Expressing Interneurons in Area CA2 of the Hippocampus[END_REF]. It is well established that PV+ interneurons are central to gamma oscillations [START_REF] Fuchs | Recruitment of Parvalbumin-Positive Interneurons Determines Hippocampal Function and Associated Behavior[END_REF][START_REF] Gulyás | Parvalbumin-Containing Fast-Spiking Basket Cells Generate the Field Potential Oscillations Induced by Cholinergic Receptor Activation in the Hippocampus[END_REF][START_REF] Korotkova | NMDA Receptor Ablation on Parvalbumin-Positive Interneurons Impairs Hippocampal Synchrony, Spatial Representations, and Working Memory[END_REF]. Thus, the level of inhibition brought about by these interneurons might be higher in CA2 than CA3 PNs.

Second, CA2 PNs have a particularly low membrane resistance [START_REF] Chevaleyre | Strong CA2 Pyramidal Neuron Synapses Define a Powerful Disynaptic Cortico-Hippocampal Loop[END_REF][START_REF] Piskorowski | Age-Dependent Specific Changes in Area CA2 of the Hippocampus and Social Memory Deficit in a Mouse Model of the 22q11.2 Deletion Syndrome[END_REF][START_REF] Sun | Proximodistal Heterogeneity of Hippocampal CA3 Pyramidal Neuron Intrinsic Properties, Connectivity, and Reactivation during Memory Recall[END_REF], causing synaptic inputs received by a CA2 PN to have a lower impact on VM than in a CA3 or CA1 PN. Third, CA2 PNs fire action potentials at lower frequencies and with higher rhythmicity than CA3 and CA1 PNs [START_REF] Chevaleyre | Strong CA2 Pyramidal Neuron Synapses Define a Powerful Disynaptic Cortico-Hippocampal Loop[END_REF][START_REF] Sun | Proximodistal Heterogeneity of Hippocampal CA3 Pyramidal Neuron Intrinsic Properties, Connectivity, and Reactivation during Memory Recall[END_REF]. Altogether, these observations predict that CA2 PNs will be harder to bring to firing threshold than CA3 PNs but would stay active longer once they start to spike. The sparsity of long bursts of action potentials reported here matches these predictions.

When considering rhythms in the whole hippocampal network, a striking feature is that the same oscillation is present in different subregions in which signals lead or lag relative to one another [START_REF] Csicsvari | Mechanisms of Gamma Oscillations in the Hippocampus of the Behaving Rat[END_REF][START_REF] Patel | Traveling Theta Waves along the Entire Septotemporal Axis of the Hippocampus[END_REF]. Obviously, the type of wiring that connects each subregion will determine how the oscillatory wave will travel. Studying the sequential activation of neurons and the relative timing of the oscillation in different subregions allows to identify where the signal originates and how it propagates through the hippocampus.

Importantly, slow gamma oscillations have been described to be generated in area CA3 and propagate to CA1 through feedforward inhibition [START_REF] Csicsvari | Mechanisms of Gamma Oscillations in the Hippocampus of the Behaving Rat[END_REF][START_REF] Zemankovics | Feedforward Inhibition Underlies the Propagation of Cholinergically Induced Gamma Oscillations from Hippocampal CA3 to CA1[END_REF]. This raises the question of the place of area CA2 in this circuit, as area CA2 and CA3 mainly inhibit each other through feedforward inhibition while CA2 PNs strongly excite CA1 represent hyper-synchronous events. On the contrary, blocking inhibitory transmission alone resulted in large epileptiform discharges recorded in the LFP which were never observed in control conditions. Hence, the pattern of activity that we describe in CA2 PNs does not originate from bouts of hyper-excitability but rather reflects physiological dynamics of CA2 PNs action potential firing that is shaped by synaptic inputs.

Finally, an interesting question is the consequence that bursts of action potentials in CA2 PNs will have on their post-synaptic targets in area CA3 and CA1. CA2 PNs strongly excite CA1

PNs [START_REF] Chevaleyre | Strong CA2 Pyramidal Neuron Synapses Define a Powerful Disynaptic Cortico-Hippocampal Loop[END_REF][START_REF] Kohara | Cell type-specific genetic and optogenetic tools reveal hippocampal CA2 circuits[END_REF] and drive both excitation and feedforward inhibition onto CA3 PNs [START_REF] Boehringer | Chronic Loss of CA2 Transmission Leads to Hippocampal Hyperexcitability[END_REF]. Therefore, bursts of action potentials in CA2 PNs will elicit trains of EPSPs and IPSPs around theta frequency for several seconds in CA1 and CA3 PNs, the consequences of which merit further investigation.

IV.1.c -Integration of area CA2 in theta and gamma hippocampal rhythms

Although we describe several aspects of area CA2 activity in gamma oscillations, our study could not fully examine the role of area CA2 in oscillations that are absent in acute hippocampal slices. However, predictions can be made based on the connectivity patterns of area CA2 with brain structures involved in theta and gamma oscillations, and from evidence provided by in vivo experiments. Indeed, area CA2 is connected by the septum, the EC, area CA3, the DG, and the hypothalamus [START_REF] Cui | Hypothalamic and other connections with dorsal CA2 area of the mouse hippocampus[END_REF][START_REF] Hitti | The hippocampal CA2 region is essential for social memory[END_REF][START_REF] Kohara | Cell type-specific genetic and optogenetic tools reveal hippocampal CA2 circuits[END_REF][START_REF] Pan | The supramammillary area: its organization, functions and relationship to the hippocampus[END_REF]. Much like other hippocampal subregions, area CA2 likely receives thetalocked inhibition from the septum and excitatory inputs at theta and gamma frequencies from the EC. In contrast with other CA areas, CA2 is contacted by the SuM, providing an additional rhythmic input during theta oscillations. Area CA2 further differs from CA1 by receiving strong excitatory distal inputs from EC and a potent inhibitory drive from CA3. Somewhat surprisingly still, in vivo studies do not report major differences of theta modulation or preferred phase of PNs from CA2 and other CA regions [START_REF] Kay | A hippocampal network for spatial coding during immobility and sleep[END_REF][START_REF] Mankin | Hippocampal CA2 Activity Patterns Change over Time to a Larger Extent than between Spatial Contexts[END_REF]Oliva et al., 2016).

Conversely, CA2 PNs show lower amounts of theta phase precession than CA1 PNs, arguably because of different relative timing of EC and CA3 inputs between those regions [START_REF] Fernández-Ruiz | Entorhinal-CA3 Dual-Input Control of Spike Timing in the Hippocampus by Theta-Gamma Coupling[END_REF]. Therefore, the uniquely strong excitatory drive from EC and the additional theta-locked input from SuM likely dynamically influence the behavior of CA2 PNs during theta and gamma oscillations. Further, the importance of area CA2 in hippocampal rhythms has been directly tested by blocking synaptic transmission from CA2 PNs [START_REF] Boehringer | Chronic Loss of CA2 Transmission Leads to Hippocampal Hyperexcitability[END_REF].

This manipulation leads to hippocampal hyperexcitability, a global increase in theta and gamma power, and a shift in CA3 PNs preferred theta phase [START_REF] Boehringer | Chronic Loss of CA2 Transmission Leads to Hippocampal Hyperexcitability[END_REF], thus revealing a clear contribution of area CA2 to hippocampal rhythms.

IV.2 -Relevance of SuM inputs to hippocampal area CA2 physiology

In the second study, we provide direct evidence for a functional connection between the hypothalamus and the hippocampus. Using stereotaxic injection of viral vectors in combination with transgenic mouse lines to express channelrhodopsin in a projection-specific manner, we have been able to selectively stimulate SuM axons in area CA2 of the hippocampus, allowing for the direct examination of synaptic transmission. This approach yielded novel functional physiological information about the SuM post-synaptic targets and overall consequences of activation. We found that, consistent with previous anatomical studies, SuM inputs into area CA2 are entirely glutamatergic. However, in contrast to previous reports, we found that SuM Glutamatergic innervation of area CA2 by the SuM has been previously described by tracing studies in rats [START_REF] Kiss | The supramammillo-hippocampal and supramammillo-septal glutamatergic/aspartatergic projections in the rat: a combined [3H]daspartate autoradiographic and immunohistochemical study[END_REF][START_REF] Soussi | Heterogeneity of the supramammillary-hippocampal pathways: evidence for a unique GABAergic neurotransmitter phenotype and regional differences[END_REF] and reported to form synapses exclusively onto PNs [START_REF] Maglóczky | Principal cells are the postsynaptic targets of supramammillary afferents in the hippocampus of the rat[END_REF] Aside from regional distinctions such as CA2 versus CA3, hippocampal PNs have been described to differ according to the location of their soma along the radial axis of the pyramidal layer. Numerous physiological differences between deep and superficial PNs have been described in area CA1 [START_REF] Danielson | Sublayer-Specific Coding Dynamics during Spatial Navigation and Learning in Hippocampal Area CA1[END_REF][START_REF] Mizuseki | Hippocampal CA1 pyramidal cells form functionally distinct sublayers[END_REF], and a recent in vivo study revealed that area CA2 also hosts deep and superficial PNs with functional differences (Oliva et al., 2016a)

IV.2.b -SuM input and space coding in area CA2

Unlike in areas CA3 and CA1, place cells in area CA2 have been shown to be less precise and unstable [START_REF] Kay | A hippocampal network for spatial coding during immobility and sleep[END_REF]Mankin et al., 2015a;Oliva et al., 2016a). However, a recent study by Kay et al. revealed that area CA2 forms the basis of a hippocampal network that encodes location during immobility and sleep [START_REF] Kay | A hippocampal network for spatial coding during immobility and sleep[END_REF]. More precisely, the activity of certain CA2 PNs (N units) was found to negatively correlate with sharp wave ripples (SWR).

Consistently, CA2 PNs firing has been shown to decrease during SWR [START_REF] Valero | Determinants of different deep and superficial CA1 pyramidal cell dynamics during sharp-wave ripples[END_REF], and the deep subpopulation of CA2 PNs ramp up their activity before suppression of firing upon SWR occurrence (Oliva et al., 2016b). Furthermore, the activity of N units negatively correlated with locomotive speed, and hippocampal theta power. The firing of these cells was found to show PV immuno-reactivity, are sensitive to MOR and DOR activation, and their selective silencing reduces SuM driven feed-foward inhibition of area CA2 PNs. BCs are central actors of hippocampal theta and gamma oscillations (Csicsvari et al., 2003a;Mann et al., 2005a;Oren et al., 2006a;[START_REF] Zemankovics | Feedforward Inhibition Underlies the Propagation of Cholinergically Induced Gamma Oscillations from Hippocampal CA3 to CA1[END_REF]. Specifically, PV+ fast-spiking INs participate to both theta and gamma oscillations, and are involved in hippocampal learning (Fuchs et al., 2007a;Gulyás et al., 2010a;Korotkova et al., 2010a). It is believed that during oscillatory activity in the hippocampus, specific interneuron subtypes act to control timing windows to synchronize pyramidal cell discharge. We found that SuM stimulation delays action potential firing and strongly reduces action potential jitter. Thus, the inhibitory post-synaptic targets of SuM axons combine all the characteristics for efficiently affecting hippocampal oscillations.

In addition to its role in theta oscillations, the SuM has been implicated in the regulation of brain states by controlling arousal versus sleep [START_REF] Pedersen | Supramammillary glutamate neurons are a key node of the arousal system[END_REF]. Indeed, DG-projecting SuM neurons are labelled by c-fos after REM sleep [START_REF] Billwiller | Differential origin of the activation of dorsal and ventral dentate gyrus granule cells during paradoxical (REM) sleep in the rat[END_REF][START_REF] Renouard | The supramammillary nucleus and the claustrum activate the cortex during REM sleep[END_REF]. Chemogenetic activation or inhibition of VGluT2+ SuM neurons respectively increases and decreases wake as well as EEG power in the theta and gamma bands [START_REF] Pedersen | Supramammillary glutamate neurons are a key node of the arousal system[END_REF].

However, the experimental strategy employed in this study does not allow for discrimination between the relative contributions of SuM inputs to CA2 and to DG in driving arousal.

IV.2.d -Gating of area CA2 activity by PV+ INs and significance for pathologies

The density of PV+ INs in area CA2 is strikingly higher than in neighboring areas CA3 and CA1 (Botcher et al., 2014a;Piskorowski and Chevaleyre, 2013a). This population of INs has been shown to play a powerful role in controlling the activation of CA2 PNs by CA3 inputs [START_REF] Nasrallah | Inhibitory Plasticity Permits the Recruitment of CA2 Pyramidal Neurons by CA3[END_REF]Piskorowski and Chevaleyre, 2013b). We show in this study that longrange inputs from the SuM can strongly recruit PV+ BCs, which in turn inhibit PNs in this area.

Hence, both intra-hippocampal inputs from CA3 and long-range inputs from the SuM converge onto PV+ INs to control CA2 PN excitability and output.

Postmortem studies have reported losses of PV+ INs in area CA2 in several pathological contexts including bipolar disorder [START_REF] Benes | A reduction of nonpyramidal cells in sector CA2 of schizophrenics and manic depressives[END_REF]), Alzheimer's disease [START_REF] Brady | Parvalbumin-immunoreactive neurons in the hippocampal formation of Alzheimer's diseased brain[END_REF], and schizophrenia [START_REF] Benes | A reduction of nonpyramidal cells in sector CA2 of schizophrenics and manic depressives[END_REF]et al., 2004) (for a review see [START_REF] Chevaleyre | Hippocampal area CA2: An overlooked but promising therapeutic target[END_REF]. Consistent with these reports, in a mouse model of the 22q11.2 deletion syndrome, we found a loss of PV staining and deficit of inhibitory transmission in area CA2 that were accompanied by impairments in social memory [START_REF] Piskorowski | Age-Dependent Specific Changes in Area CA2 of the Hippocampus and Social Memory Deficit in a Mouse Model of the 22q11.2 Deletion Syndrome[END_REF]. We postulate that the PV+ INs altered during pathological conditions may be the same population of PV+ BCs recruited by long-range SuM inputs. Thus, the loss of function of PV+ INs in area CA2 could disrupt proper long-range connection between the hippocampus as the hypothalamus and possibly contribute to some of the cognitive impairments observed in schizophrenia animal models. Further, pharmacological mouse models of schizophrenia have reported increased c-fos immunoreactivity in the SuM as well as memory impairments [START_REF] Castañé | PCP-based mice models of schizophrenia: differential behavioral, neurochemical and cellular effects of acute and subchronic treatments[END_REF]. Although several alterations in these models of schizophrenia could lead to deficits of hippocampal-dependent behavior, abnormalities of the SuM projection onto area CA2 appear as a potential mechanism that warrants further investigation.

IV.2.e -Differential modulation of DG and area CA2 by SuM inputs and consequences on hippocampus-dependent memory formation

Area CA2 is not the sole hippocampal target of SuM inputs as the DG also receives innervation from the SuM [START_REF] Haglund | The projection of the supramammillary nucleus to the hippocampal formation: An immunohistochemical and anterograde transport study with the lectin PHA-L in the rat[END_REF][START_REF] Vertes | PHA-L analysis of projections from the supramammillary nucleus in the rat[END_REF]. Interestingly, afferents to area CA2 and the DG likely originate from distinct neurons in the SuM and might therefore serve different functions [START_REF] Kiss | The supramammillo-hippocampal and supramammillo-septal glutamatergic/aspartatergic projections in the rat: a combined [3H]daspartate autoradiographic and immunohistochemical study[END_REF]Soussi et al., 2010a). While previous studies and our own work have established that VGluT2-expressing SuM inputs target area CA2 where they control CA2

PN excitability through feedforward inhibition, the physiology of SuM transmission to the DG is most likely different. Indeed, SuM axons have been shown to co-release glutamate and GABA on DG granule cells [START_REF] Boulland | Vesicular Glutamate and GABA Transporters Sort to Distinct Sets of Vesicles in a Population of Presynaptic Terminals[END_REF][START_REF] Pedersen | Supramammillary glutamate neurons are a key node of the arousal system[END_REF]Soussi et al., 2010b; and our own observations). In addition, we demonstrated that SuM inputs also recruit feedforward inhibition onto DG granule cells. These observations raise the question of the net effect of SuM driven excitation and inhibition have on GCs. Although the combination of direct and feedforward inhibition tends to suggest an overall inhibitory effect of SuM transmission onto GCs, in vivo results describing an excitatory effect of SuM stimulation argue for a predominance of SuM-driven excitation in DG GCs [START_REF] Mizumori | A comparison of supramammillary and medial septal influences on hippocampal field potentials and single-unit activity[END_REF]. In contrast with CA2 PNs, DG GCs are highly resistive and will therefore respond readily to synaptic inputs [START_REF] Spruston | Perforated patch-clamp analysis of the passive membrane properties of three classes of hippocampal neurons[END_REF]. In addition, overall effects of SuM transmission on DG GCs will depend on the distribution of direct excitatory and inhibitory synapses on the dendritic tree of GCs as well as the location targeted by INs mediating the feedforward inhibition. Hence, the influence of SuM inputs onto DG GC activity is difficult to predict and might vary during different brain states. Current-clamp recordings of DG GCs coupled with SuM stimulation are required to resolve this issue.

Because the SuM, area CA2 and the DG have separately been described to participate in specific hippocampal memory functions, a particularly interesting challenge is to determine how these regions interact or act independently on each function. For instance, SuM activity is involved in working, spatial and contextual fear memory as well as novelty-and reward-laden behaviors [START_REF] Aranda | Supramammillary and adjacent nuclei lesions impair spatial working memory and induce anxiolitic-like behavior[END_REF][START_REF] Gutiérrez-Guzmán | Serotonin depletion of supramammillary/posterior hypothalamus nuclei produces place learning deficiencies and alters the concomitant hippocampal theta activity in rats[END_REF][START_REF] Hernández-Pérez | Supramammillary serotonin reduction alters place learning and concomitant hippocampal, septal, and supramammillar theta activity in a Morris water maze[END_REF][START_REF] Ikemoto | The Supramammillary Nucleus Mediates Primary Reinforcement via GABAA Receptors[END_REF][START_REF] Ikemoto | Rewarding Effects of AMPA Administration into the Supramammillary or Posterior Hypothalamic Nuclei But Not the Ventral Tegmental Area[END_REF][START_REF] Ito | Three-dimensional distribution of Fos-positive neurons in the supramammillary nucleus of the rat exposed to novel environment[END_REF][START_REF] Pan | The medial supramammillary nucleus, spatial learning and the frequency of hippocampal theta activity[END_REF]2002;2004b;Shahidi et al., 2004a;2004b). Therefore, one may ask which aspects of hippocampal functions carried out by CA2 such as social memory and novelty detection (Hitti and Siegelbaum, 2014b;[START_REF] Wintzer | The Hippocampal CA2 Ensemble Is Sensitive to Contextual Change[END_REF] are due to SuM modulation of CA2 output. Or when considering the DG, how does SuM participates in pattern separation and working memory [START_REF] Gilbert | Dissociating hippocampal subregions: A double dissociation between dentate gyrus and CA1[END_REF][START_REF] Xavier | Dentate gyrus-selective colchicine lesion and disruption of performance in spatial tasks: difficulties in "place strategy" because of a lack of flexibility in the use of environmental cues?[END_REF].

In collaboration with our investigation of hippocampal circuits engaged by SuM inputs, the that can elicit to action potential firing (Chevaleyre and Siegelbaum, 2010b;[START_REF] Sun | Dendritic Na+ spikes enable cortical input to drive action potential output from hippocampal CA2 pyramidal neurons[END_REF].

During theta oscillations, both SuM and EC neurons rhythmically fire at theta frequency [START_REF] Buzsáki | Theta Oscillations in the Hippocampus[END_REF][START_REF] Kirk | Evidence for differential control of posterior hypothalamic, supramammillary, and medial mammillary theta-related cellular discharge by ascending and descending pathways[END_REF][START_REF] Kocsis | Characterization of neurons of the supramammillary nucleus and mammillary body that discharge rhythmically with the hippocampal theta rhythm in the rat[END_REF]. Therefore, depending on their phase relation relative to the theta cycle, these excitatory inputs could summate in CA2 PNs and influence the timing of their action potential discharge. Second, SuM-driven excitation of area CA2 PNs likely prevails in conditions of decreased activity from PV+ BCs. As discussed above, it might be the case in pathologies such as schizophrenia (Piskorowski et al., 2016b).

Besides pathological conditions, PV+ INs in area CA2 can be depressed by a delta opioid receptor-dependent long-term depression of inhibition (iTLD) that can be induced by 10 Hz stimulation of CA3 inputs [START_REF] Nasrallah | Inhibitory Plasticity Permits the Recruitment of CA2 Pyramidal Neurons by CA3[END_REF]Piskorowski and Chevaleyre, 2013b).

Interestingly, preliminary results from our laboratory indicate this iLTD naturally occurs in mice exposed to an enriched environment. In these conditions, the SuM-driven feedforward inhibition mediated by PV+ INs would be reduced, thus allowing a stronger impact of excitatory transmission from SuM inputs onto CA2 PNs. How this modification of E/I drive from SuM affects CA2 PNs remains to be determined. Of note, environmental enrichment has been described to increase theta-nested gamma oscillations in the hippocampus [START_REF] Shinohara | Experience enhances gamma oscillations and interhemispheric asymmetry in the hippocampus[END_REF]. Given its rhythmic activity with theta, the SuM might contribute to this effect through modified influence over area CA2.

IV.3 -Control of hippocampal activity and output by SuM inputs

In the third study, we report the physiological impact of SuM inputs to area CA2 and the consequences on downstream CA1 in conditions of high cholinergic tone. Using geneticallyrestricted optogenetics to selectively stimulate SuM axons on acute hippocampal slices superfused with the cholinergic agonist CCh, we investigated the responses of CA2 and CA1

PNs to the activation of SuM inputs during ongoing network activity. This strategy enabled us to decipher the SuM-CA2-CA1 circuit and to quantify the events occurring at each step. First, we demonstrated that SuM direct excitatory and feedforward inhibitory drives are dampened in amplitude but prolonged in time by CCh. Next, we showed that recruitment of inhibition by SuM inputs exerts a temporal control on the bursting of action potentials by CA2 PNs. Finally, we found that the CA2 output shaped by SuM inputs triggers a strong and prolonged reduction of CA1 activity. Therefore, our data provide novel functional information regarding the physiology of hippocampal circuits and highlight a powerful control of hippocampal output by long-range SuM inputs.

IV.3.a -Cholinergic modulation of SuM inputs

Quantification of the SuM light-evoked EPSC and IPSC amplitudes and PPRs in CA2 PNs revealed that CCh decreases glutamate release from SuM axons. While this explanation is straight forward for mono-synaptic excitation, additional factors need to be accounted for when considering the feedforward inhibition recruited by SuM input. First, the dynamics of the excitatory synapses formed by SuM axons onto PNs and INs might differ, although no evidence suggests that it is the case. Second, CCh causes a mixed effect on inhibitory transmission as it depolarizes most interneurons but decreases GABA release ( [START_REF] Alger | Muscarinic cholinergic receptors modulate inhibitory synaptic rhythms in hippocampus and neocortex[END_REF][START_REF] Cobb | Cholinergic modulation of hippocampal cells and circuits[END_REF]. Therefore, one can predict that weaker excitation from SuM would be necessary Concerning the pre-synaptic inhibition of SuM axons by CCh, the possibility of pre-synaptic expression of cholinergic receptors by SuM fibers afferent to the hippocampus cannot be ruled out, although tracing studies never reported this. Indeed, cholinergic pre-synaptic inhibition of excitatory transmission has been described in other hippocampal circuits [START_REF] Hasselmo | The role of acetylcholine in learning and memory[END_REF].

Alternatively, depression of glutamate release from SuM axons could be due to other neuromodulators released by hippocampal neurons when challenged with CCh. In any case, the reduction of SuM light-evoked EPSC and IPSC amplitudes in CCh comes with a decreased short-term depression of these inputs. This is particularly interesting in the context of rhythmic activity of SuM neurons during theta oscillations when the cholinergic tone is high. Under these conditions, the reduced depression of excitatory and inhibitory transmission from SuM would allow its influence over hippocampal post-synaptic targets to be sustained in time. Albeit initially less powerful, the SuM drive would have a stronger impact over time when acetylcholine concentation is high. This likely allows SuM inputs to reliably recruit feedforward inhibition over time, causing significant delays of CA2 PN bursting (Figure 2). At a more global scale, low short-term depression of SuM transmission probably participates in synchronizing theta oscillations through reciprocal connections with other structures displaying rhythmic activity, such as the septum (Pan and McNaughton, 2004a).

IV.3.b -Synchronization of CA2 activity by SuM inhibitory drive

In an effort to assess the consequences of SuM inputs onto area CA2 activity in a physiological way, we report that the timing of bursts of action potential discharged by CA2 PNs can be controlled by the SuM inhibitory drive. However, the experimental strategy employed here comes with a number of caveats. First, the onset of SuM inputs photostimulation is clearly biased towards times preceding bursts in the recorded CA2 PN. This choice was made for the following reasons : light-stimulation of SuM axons recruits feedforward inhibition, therefore its impact would have been limited if triggered earlier in the inter-burst interval when CA2 PNs are hyperpolarized. On the contrary, the amount of feedforward inhibition might not have been [START_REF] Ogier | Action of tachykinins in the rat hippocampus: modulation of inhibitory synaptic transmission[END_REF][START_REF] Ogier | Action of tachykinins in the hippocampus: Facilitation of inhibitory drive to GABAergic interneurons[END_REF]. The CA3 to CA1 excitatory drive is reduced by substance P, potentially because of increased recruitment of inhibition [START_REF] Kouznetsova | Modulation by substance P of synaptic transmission in the mouse hippocampal slice[END_REF]. Interestingly, substance P causes a hyperpolarization of CA1 PNs but potentializes the depolarizing effect of subsequent application of CCh [START_REF] Kouznetsova | Facilitation of cholinergic transmission by substance P methyl ester in the mouse hippocampal slice preparation[END_REF]. Although it is difficult to directly relate this study to our own, the hyperpolarization of CA1 PNs seen upon bath application of substance P make it a likely candidate neuropeptide to explain our results. Indeed, preliminary experiments performed in area CA2 indicate that sustained photostimulation of SuM fibers can trigger release of substance P. Using 2 minutes-long trains of light pulses to stimulate SuM inputs, we observed a decreased amplitude of the CA3 to CA2 feedforward inhibition and obtained a similar effect by bath application of substance P. Therefore, these preliminary experiments give precedent to further investigate the role of substance P in the SuM-driven silencing of CA1 activity. Future experiments will be performed with the same stimulation protocol of SuM inputs while blocking the neurokinin 1 receptor of substance P.

Alternatively, potent and repeated excitation of CA2 and CA1 INs, either directly by SuM or by CA2 PNs through post-inhibitory rebound of excitation, might release a variety of neuromodulators intrinsic to the hippocampus that could cause the suppression of CA1 activity.

Neuromodulatory signaling such as CCK, VIP, endocannabinoids, mGluRs, could be implicated in the SuM effect on area CA1, and all of which potentially interfere with cholinergic pathways. Replication of our protocol in the presence of pharmacological blockers will help identify the signaling molecules responsible. In addition, chemogenetic inhibition of specific populations of INs, or of CA2 PNs, will allow to decipher their respective contributions in this phenomenon. Finally, characterizing the INs responsible for the CA2 to CA1 feedforward inhibition will help to understand the influence of SuM on CA1 through CA2 PNs.

Regardless of its origin and mechanism, the silencing of CA1 by SuM activity has important functional potential implications. During hippocampal theta oscillations, CA1 is influenced by CA3, entorhinal and septal inputs that all fire rhythmically (Buzsáki, 2002a). At the same time, SuM neurons fire bursts of action potentials at theta frequency (Buzsáki, 2002a;Kirk et al., 1996a;Kocsis and Vertes, 1994a). Therefore, through its influence over area CA2, the SuM likely participates in the generation of theta oscillations in area CA1 by phasically hyperpolarizing CA1 PNs. In addition, CA2-projecting SuM neurons send collaterals to the septum [START_REF] Borhegyi | The supramammillary nucleus innervates cholinergic and GABAergic neurons in the medial septum-diagonal band of Broca complex[END_REF][START_REF] Vertes | Collateral projections from the supramammillary nucleus to the medial septum and hippocampus[END_REF]; our own observations), and area CA2 sends outputs to the septum as well [START_REF] Cui | Hypothalamic and other connections with dorsal CA2 area of the mouse hippocampus[END_REF]. Furthermore, the septum targets both area CA2 and the SuM [START_REF] Cui | Hypothalamic and other connections with dorsal CA2 area of the mouse hippocampus[END_REF]Hitti and Siegelbaum, 2014a;[START_REF] Kohara | Cell type-specific genetic and optogenetic tools reveal hippocampal CA2 circuits[END_REF];

  Figure I.2.1. Hippocampal anatomy in a transverse slice. ....................................................................

Figure

  Figure I.2.2. Flow of information in the tri-synaptic loop of the hippocampus. ......................................

Figure

  Figure I.2.3. Integrated view of area CA2 in the intra-and extra-hippocampal circuits. .......................

Figure

  Figure I.3.1. Activity of area CA2 from in vivo experiments. .................................................................

Figure

  Figure II.8.1. Experimental workflow of the study on CCh-induced network activity.............................

Figure

  Figure II.8.2. Experimental workflow of the study on SuM input to area CA2. ......................................

Figure

  Figure II.8.3. Experimental workflow of the study on the consequences of SuM activation on area CA2 and CA1 activity. ...........................................................................................................................

Figure

  Figure III.1.1. Carbachol induces gamma-like oscillations in hippocampal area CA2 and CA3. ..........

Figure

  Figure III.1.2. Temporal relations of CCh-induced oscillations from area CA2 and CA3. .....................

Figure

  Figure III.1.3. Effects of chemogenetic silencing of CA2 PNs on CCh-induced oscillations. ................

Figure

  Figure III.1.4. CA2 PNs fire bursts of APs following CCh application. ..................................................

Figure

  Figure III.1.5. Characteristics of charbachol-induced CA2 PN spontaneous activity. ...........................

Figure

  Figure III.1.6. Effect of CCh on spontaneous excitatory and inhibitory events in CA2 PNs. .................

Figure

  Figure III.1.7. Excitatory and inhibitory transmission shape bursts of action potentials in CA2 pyramidal cells. ..............................................................................................................................................

Figure

  Figure III.1.8. Bursts of action potentials in CA2 pyramidal cells are modulated by the ongoing field oscillation. .....................................................................................................................................

Figure

  Figure III.2.1. Selective labeling of SuM neurons that project to hippocampal area CA2. ....................

Figure

  Figure III.2.2. CA2-projecting SuM neurons express VGluT2. ..............................................................

Figure

  Figure III.2.3. SuM input provides excitatory glutamatergic transmission to diverse population of PNs in area CA2. ......................................................................................................................................

Figure

  Figure III.2.4. Reconstruction of pre-synaptic SuM fibers and post-synaptic PNs in area CA2. ...........

Figure

  Figure III.2.5. SuM light-evoked mono-synaptic excitation as well as mono-and di-synaptic inhibition onto DG GCs. ...............................................................................................................................

Figure

  Figure III.2.6. SuM axon stimulation recruits feedforward inhibition. ..................................................

Figure

  Figure III.2.7. Area CA2 PNs receive a net inhibitory drive from SuM that controls summation and AP firing properties. ..........................................................................................................................

Figure

  Figure III.2.8. SuM inputs strongly excite basket cell interneurons in area CA2. ................................

Figure

  Figure III.2.9. SuM inputs provide excitation to Parvalbumin-expressing BCs. ..................................

Figure

  Figure III.2.10. Parvalbumin-expressing BCs mediate the feedforward inhibition recruited by photostimulation of SuM fibers. ..................................................................................................

Figure

  Figure III.3.1. Reduction of SuM excitatory and inhibitory transmission by carbachol. .......................

Figure

  Figure III.3.2. SuM control of CA2 PN bursting activity. ......................................................................

Figure

  Figure III.3.3. Immediate consequences of SuM activation on CA1 activity. ......................................

Figure

  Figure III.3.4. Delayed consequences of SuM activation on CA1 activity. ..........................................

Figure

  Figure IV.3.1. Integrated view of brain state-dependent SuM and area CA2 activity. ........................

  4. Characteristics of SuM light-evoked responses in deep and superficial pyramidal neurons in areas CA2 and CA3a. ...............................................................................................................

Figure

  Figure I.2.1. Hippocampal anatomy in a transverse slice.

Figure

  Figure I.2.2. Flow of information in the tri-synaptic loop of the hippocampus.

Figure

  Figure I.2.3. Integrated view of area CA2 in the intra-and extra-hippocampal circuits.

Figure

  Figure I.3.1. Activity of area CA2 from in vivo experiments.

  Finally, we asked what consequences SuM inputs had on area CA2 during carbachol-induced network activity and what the resulting output of area CA2 onto area CA1 was. These investigations were carried out in collaboration with in vivo approaches from the McHugh laboratory complementing our third study performed with optogenetics on acute hippocampal slices.

  injected with AAV.Synapsin.DIO.hM4D(Gi)-mcherry, slices were prepared 6 weeks after viral injection. Animals were deeply anaesthetized with ketamine (100 mg/kg) and xylazine (7 mg/kg), and perfused transcardially with a N-methyl-D-glucamin-based (NMDG) cutting solution containing the following (in mM): NMDG 93, KCl 2.5, NaH2PO4 1.25, NaHCO3 30, HEPES 20, glucose 25, thiourea 2, Na-ascorbate 5, Na-pyruvate 3, CaCl2 0.5, MgCl2 10. Brains were then rapidly removed, hippocampi were dissected out and placed upright into an agar mold and cut into 400 µm thick transverse slices (Leica VT1200S) in the same cutting solution at 4 °C. Slices were transferred to an immersed-type chamber and maintained in artificial cerebrospinal fluid (ACSF) containing the following (in mM) : NaCl 125, KCl 2.5, NaH2PO4 1.25, NaHCO3 26, glucose 10, Na-pyruvate 2, CaCl2 2, MgCl2 1. Slices were incubated at 32 °C for approximately 20 minutes then maintained at room temperature for at least 45 minutes prior to transfert to a recording chamber allowing dual perfusion of the slice with ACSF at 5 mL/min at 30 °C.II.4 -Electrophysiological recordingsWhole-cell or cell-attached patch-clamp of recordings of PNs or INs and local field potential (LFP) recordings were obtained from the CA2, CA1 and CA3 regions. Whole-cell patch-clamp recordings were performed with potassium-or cesium-based intracellular solutions containing the following (in mM): K-or Cs-methyl sulfonate 135, KCl 5, EGTA-KOH 0.1, HEPES 10, NaCl 2, MgATP 5, Na2GTP 0.4, Na2-phosphocreatine 10 and biocytin (4 mg/mL). Series resistance were < 20 MΩ and were not compensated in voltage-clamp, bridge balance was applied in current-clamp. An experimentally determined liquid junction potential of approximately 2 mV was not corrected for.Cell-attached recordings were performed using patch pipettes (2-5 MΩ) filled with ACSF for monitoring bursts of APs. Alternatively, regular K + -or Cs + -based intracellular solution was used for cell-attached recordings when assessing AP firing in response to SuM photostimulation because these experiments required subsequent whole-cell recordings. LFPs were obtained from area CA2, CA1 and CA3 strata pyramidale (SP) using patch pipettes (2-5 MΩ) filled with 1 M NaCl. Data were obtained using a Multiclamp 700B amplifier, sampled at 10 kHz and digitized using a Digidata. The pClamp10 software was used for data acquisition. Pharmacological agents were added to ACSF at the following concentrations (in μM): 10 CCh to activate cholinergic receptors, 10 NBQX and 50 D-APV to block AMPA and NMDA receptors, 1 SR95531 and 2 CGP55845A to block GABAA and GABAB receptors, 1 DAMGO to activate µ-opioid receptors (MOR), 0.5 DPDPE to activate δ-opioid receptors (DOR), 10 clozapine N-oxide (CNO) to activate hM4D(Gi) DREADDs, 100 4-Aminopyridine (4-AP) to block Kv1 potassium channels and 0.2 TTX to prevent sodic action potential (AP) generation.II.5 -Optogenetic stimulationChR2 was excited by 440 nm light delivered by a LED attached to the epifluorescence port of the microscope. Light stimulations trains consisted of 2-10 pulses, 0.5-2 ms long, delivered at 10 Hz, repeated every 10-20 seconds for at least 20 sweeps. Power-response curves of SuM post-synaptic responses in CA2 PNs were carried out with light intensities ranging from 0 to 45 mW/mm². We routinely used a light intensity of 25 mW/mm² which was experimentally determined as the lowest irradiance allowing TTX-sensitive maximal responses in all cell types

  Histological proceduresPost-hoc reconstruction of neuronal morphology and SuM axonal projections were performed on slices and midbrain tissue following overnight incubation in 4 % paraformaldehyde in phosphate buffered saline (PBS). Midbrain sections were re-sliced sagitally to 100 µm thick sections. Slices were permeabilized with 0.2 % triton in PBS and blocked overnight with 3 % goat serum in PBS with 0.2 % triton. Primary antibody (life technologies) incubation was carried out in 3 % goat serum in PBS overnight at 4 °C. Channelrhodopsin-2 was detected by chicken primary antibody to GFP (Life technologies) (1:10,000 dilution) and a alexa488conjugated goat-anti chick secondary. Other primary antibodies used were mouse anti-RGS14 (Neuromab) (1:300 dilution), rabbit anti-PCP4 (Sigma) (1:600 dilution), guinea pig anti-VGluT2 antibody (Milipore) (1:10,000 dilution), rabbit anti-parvalbumin antibody (Swant) (dilution 1:2000). Alexa-546-conjugated streptavidin (life technologies), secondary antibodies and far-red neurotrace (life technologies) incubations were carried out in block solution for 4 hours at room temperature. Images were collected with a Zeiss 710 laser-scanning confocal microscope.Classification of neuronal populationsReconstructed neurons were classified as either PNs or INs based on the extension and localization of their dendrites and axons. CA1, CA2 and CA3 PNs were identified based on their somatic localization, dendritic arborization and presence of thorny excrescences (TE).Among INs with somas located in the pyramidal layer (stratum pyramidale, SP), discrimination between BCs and non-BCs was achieved based on the restriction of their axons to SP or not, respectively. When available, firing patterns upon injection of depolarizing current step injection, action potential (AP) half-width, amount of repolarizing sag current upon hyperpolarization from -70 mV to -100 mV by current step injection, membrane resistance (RM) and capacitance (CM) were additionally used for cell identification. CA2 and CA3a PNs displayed similar firing patterns, AP width, sag current, RM and CM. In contrast, INs had faster firing rates, shorter AP width, higher RM and lower CM than PNs. BCs further differed from non-BCs by the presence of a larger sag current. All recorded neurons that could not be unequivocally identified as PNs or INs were excluded from analysis.

  LFP recordings were obtained from area CA2 and CA3b SP to evaluate the characteristics of CCh-induced oscillations in both regions and their relation. A subset of these experiments involved bilateral injections of AAVs carrying a Cre-dependent inhibitory DREADD construct targeted in area CA2 of Cacng5-Cre mice that express the Cre recombinase specifically in CA2 PNs. These injections were performed on 4 week-old mice, and slices were prepared 6 weeks after that. 10 µM CNO was applied 10 minutes after CCh to these slices to selectively silence CA2 PNs expressing the hM4D(Gi)-mcherry receptor and examine the consequences on LFP signals from area CA2 and CA3b. The pattern of AP firing of CA2 PNs upon CCh application was first assessed by cell-attached recordings from putative CA2 PNs and then by whole-cell patch-clamp recordings of CA2 PNs with post hoc confirmation of neuronal identity. When monitoring AP firing induced by CCh in whole-cell configuration, we used current-clamp mode without current injection. In a few instances, transient application of CCh followed by VM depolarization through DC current injection after CCh wash-out were used to evaluate the role of CCh in the initial depolarization and subsequent bursts of APs in CA2 PNs. In a subset of experiments, slices were cut with a scalpel blade between area CA2 and CA3a in cutting solution to isolate area CA2 from upstream inputs. Voltage-clamp recordings of CA2 PNs allowed to quantify the effect of CCh on membrane resistance and on spontaneous excitatory and inhibitory post-synaptic currents. The role of synaptic inputs in shaping bursts of APs in CA2 PNs was assessed by application of 10 µM NBQX + 50 µM APV and / or 1 µM SR95531 + 2 µM CGP5584A, 1-2 minutes prior to CCh wash-in. Simultaneous recordings of LFP in CA2 SP and whole-cell patch-clamp of CA2 PN were performed to probe the temporal relation of CA2 PN AP firing to the field oscillation.

Figure

  Figure II.8.1. Experimental workflow of the study on CCh-induced network activity. Diagrams illustrating the methods used in Section III.1.b

Figure II.8. 2 .

 2 Figure II.8.2. Experimental workflow of the study on SuM input to area CA2. Diagrams illustrating the methods used in Section III.2.b

Figure

  Figure II.8.3. Experimental workflow of the study on the consequences of SuM activation on area CA2 and CA1 activity.

  potentials (LFPs) in area CA2 and CA3 strata pyramidale (SP) on acute hippocampal slices and applied the cholinergic agonist carbachol (CCh)(Figure III.1.1A). We assessed the presence and strength of oscillations over time by performing sequential fast Fourier transforms and measuring the average gamma power per minute (defined as the area under the power spectral density curve between 20 and 40 Hz). Upon bath application of 10 µM CCh, oscillatory activity could be detected from both CA2 and CA3 SP with peaks of power spectral density in the gamma range(Figure III.1.1B). These oscillations occurred after 1 minute of CCh application and had a significant gamma power for 15 minutes after CCh application. This time window was used for analysis (Figure III.1.1B; n = 34; Kruskal-Wallis test, p = 0.007 for CA2, p = 0.003 for CA3; Wilcoxon signed-rank test between before and after CCh, p < 0.001 for CA2, p < 0.001 for CA3). We observed that network oscillations contained a significantly greater gamma power in area CA3 than CA2 (Figure III.1.1C; gamma powers were 388 ± 64.6 µV² in CA2 and 623 ± 112 µV² in CA3, n = 34; Wilcoxon signed-rank test, p = 0.005) with no difference in peak frequencies (Figure III.1.1C; peak frequencies were 23 ± 1.0 Hz in CA2 and 24 ± 1.3 Hz in CA3, n = 34; paired Student T test, p = 0.53).

Figure

  Figure III.1.1. Carbachol induces gamma-like oscillations in hippocampal area CA2 and CA3.

  Figure III.1.2C) while being similar overall(Figure III.1.1C). Therefore, we extracted phases of oscillations at frequencies bearing the maximal magnitude at each time point[START_REF] Oren | Synaptic Currents in Anatomically Identified CA3 Neurons during Hippocampal Gamma Oscillations In Vitro[END_REF] in CA2 and CA3 to examine the temporal relation between signals from both regions.However, cross-correlation analysis did not reveal a significant lag between field potential oscillations in CA2 and CA3 (CA3 lag versus CA2 = 0.62 ± 0.46 ms, n = 34; one-sample T test, p = 0.15) nor their respective phases (CA3 lag versus CA2 = 0.97 ± 1.23 ms, n = 34; one-sample T test, p = 0.44).

Figure

  Figure III.1.2. Temporal relations of CCh-induced oscillations from area CA2 and CA3. A. Diagram illustrating the simultaneous LFP recordings from area CA2 and CA3 SP in acute hippocampal slices with application of 10 µM CCh. B. Normalized magnitude components of the wavelet transforms of raw LFPs recorded in SP from area CA2 (top) and CA3 (bottom), warm colors indicate high magnitude. C. Power spectral density functions of LFPs recorded in SP from area CA2 (red line) and CA3 (black line) showing peaks in the gamma range.

Figure

  Figure III.1.3. Effects of chemogenetic silencing of CA2 PNs on CCh-induced oscillations. A. Diagram illustrating the experimental strategy used for chemogenetic silencing of CA2 PNs. B. Top, time course of the mean gamma power of LFPs recorded in area CA2 (red) and CA3 (black) in 10 µM CCh upon 10 µM CNO application (n = 16; error bars represent SEM). Bottom, example spectrograms of signals recorded in area CA2 and CA3 during this time course, warm colors indicate high power. C. Comparison of the power of CCh-induced oscillations recorded from area CA2 (red) and CA3 (black) before and after 10 µM CNO application (individual cells shown as grey lines, population average shown as horizontal line, error bars represent SEM, n = 16; Wilcoxon signed-rank test, p < 0.001 for CA2, p = 0.159 for CA3).

  , we asked how PNs in area CA2 behave during CCh-induced oscillations(Figure III.1.4A). First, we assessed the firing of action potentials of cells in area CA2 by performing cell-attached recordings with ACSF-filled pipettes to leave the intra-cellular content unperturbed. Prior to CCh application, most cells did not fire action potentials (n = 14 of 15 cells). Although several cells remained silent, the majority showed action potential firing upon CCh wash-in (n = 9 of 15 cells). Interestingly, spiking activity from these cells was often structured in bursts (n = 8 of 9 cells) rather than tonic(Figure III.1.4B). This bursting pattern could either be sustained throughout CCh application (n = 5 cells) or transition to a tonic pattern (n = 3 cells). As burst firing seemed to be the most common type of activity displayed by putative PNs, we wished to confirm the identity of bursting cells and characterize them further. Albeit non-invasive, cellattached recordings provide limited information on the cell type, electrophysiological properties and synaptic responses of the recorded neuron. Therefore, we turned to whole-cell patch-clamp recordings of CA2 PNs together with LFP recordings from area CA2 SP to access both the supra-and sub-threshold dynamics of CA2 PN VM and to relate them to the network activity. Using current-clamp mode without current injection, we could study the evolution of CA2 PNs membrane potential (VM) during the course of CCh application (Table III.1.1, Figure III.1.4C and Figure III.1.5A-L). Upon 10 µM CCh wash-in, CA2 PNs increased their membrane potential from resting at -75 ± 1.2 mV (referred to as ACSF-VM) to a depolarized level of -64 ± 1.0 mV (referred to as CCh-VM) (Table III.1.1, Figure III.1.4C and Figure III.1.5C; n = 25).

Figure

  Figure III.1.4. CA2 PNs fire bursts of APs following CCh application. A. Diagram illustrating the cell-attached or whole-cell recording configuration of CA2 PN in acute hippocampal slices with application of 10 µM CCh. B. Sample trace of AP bursts recorded in cell-

Figure

  Figure III.1.5. Characteristics of charbachol-induced CA2 PN spontaneous activity.

Figure

  Figure III.1.6. Effect of CCh on spontaneous excitatory and inhibitory events in CA2 PNs. A. Diagram illustrating the whole-cell recording configuration of CA2 PN in acute hippocampal slices with application of 10 µM CCh. B. Sample traces of sEPSCs recorded in a CA2 PN before (top) and

Figure

  Figure III.1.7. Excitatory and inhibitory transmission shape bursts of action potentials in CA2 pyramidal cells. A. Diagram illustrating the simultaneous whole-cell recording configuration of CA2 PNs and LFP recordings from area CA2 SP in acute hippocampal slices with application of 10 µM CCh. B. Sample trace of AP bursts recorded whole-cell from a CA2 PN following 10 µM CCh application with excitatory and inhibitory synaptic transmission blocked by application of 10 µM NBQX, 50 µM APV, 1 µM SR95531 & 2 µM CGP5584A (top) and expanded view of the demarcated window (dashed gray box, bottom). C. Sample trace of bursts recorded whole-cell from a CA2 PN upon 10 µM CCh application with excitatory transmission blocked (top; 10 µM NBQX & 50 µM APV) and expanded view of the demarcated window (dashed gray box, bottom). D. Sample trace of bursts recorded whole-cell from a CA2 PN upon 10 µM CCh application with inhibitory synaptic transmission blocked (top; 1 µM SR95531 & 2 µM CGP5584A) and expanded view of the demarcated window (dashed gray box) with the corresponding LFP recording showing epileptiform-like discharges (bottom).

  selected for further analysis(Figure III.1.8C). During these bursts, CA2 PNs showed a significant phase preference of -2.6 ± 0.2 radians (n = 16; Rayleigh test, p < 0.001) hence near the trough of the gamma-like oscillation which defined -π/π phase(Figure III.1.8B and Figure III.1.8D). Thus, this result establishes that the pattern of action potential firing within a burst is dictated by the ongoing network activity.

Figure

  Figure III.1.8. Bursts of action potentials in CA2 pyramidal cells are modulated by the ongoing field oscillation. A. Diagram illustrating the simultaneous whole-cell recording configuration of CA2 PNs and LFP recordings from area CA2 SP in acute hippocampal slices with application of 10 µM CCh. B. Sample trace of a burst recorded in a CA2 PN (top) and the corresponding normalized magnitude components

  project to hippocampal area CA2, we injected a retrograde canine adenovirus type 2 (CAV-2) into area CA2 of the hippocampus to permit the expression of Cre-recombinase (Cre) in hippocampal-projecting SuM neurons, and an adeno-associated virus (AAV) was injected into the SuM to allow the expression of EGFP under the control of Cre (Figure III.2.1A). In 5 animals the injection of retrograde CAV-2 was sufficiently targeted to area CA2, as indicated by the presence of EGFP-expressing SuM axonal fibers primarily in this hippocampal area(Figure III.2.1B). We stained for calretinin to define the boundaries of the SuM nucleus, and put our mouse anatomical data in context with the anatomical studies from rat[START_REF] Pan | The supramammillary area: its organization, functions and relationship to the hippocampus[END_REF]. Consistent with what has been described, CA2-projecting cells in the SuM are located in the medial regions. These cells were located bilaterally, ventral to the fiber bundles that traverse the SuM (Figure III.2.1C). Furthermore, we found that the majority of these cells were positive for calretinin(Figure III.2.1D).In order to functionally investigate the SuM projection to area CA2, we used an anterograde strategy in two separate transgenic mouse lines (Figure III.2.1E). It has been shown that the source of vesicular glutamate transporter 2 (VGluT2)-immunopositive boutons in area CA2 originate from the SuM[START_REF] Halasy | Distribution and origin of vesicular glutamate transporter 2-immunoreactive fibers in the rat hippocampus[END_REF], as shown in Figure III.2.2. To further assess where these VGluT2-expressing SuM cells project into the hippocampus, we injected an AAV to express channelrhodopsin(H143R)-YFP (ChR2-EYFP) under the control of Cre recombinase into the SuM of a transgenic mouse line with Cre expression controlled by the VGluT2

  hippocampus (Figure III.2.1F). In a subset of experiments involving PV-Cre animals, we used targeted injections of an AAV to express hSyn.hChR2(H134R).EYFP in all SuM neurons regardless of their Cre expression. For all experiments, injection sites were examined post hoc to ensure correct targeting of the SuM. With both the VGluT2-Cre and Csf2rb2-Cre lines of transgenic mice, we observed identical patterns of SuM fiber localization in the hippocampus. EYFP-containing SuM axons were found throughout the supragranular layer of the DG and in area CA2 (Figure III.2.1G) where they clustered around the pyramidal layer (stratum pyramidale, SP) and spread in stratum radiatum (SR) consistent with anterograde studies in rats

  RGS14, and did not spread to neighboring areas CA3 and CA1 (Figure III.2.1B and Figure III.2.1G). In order to maximize the precision of our experiments, we frequently only achieved partial infection of the SuM nucleus, as indicated by the sparseness of CHR2-EYFP-containing fibers in comparison to the number of VGluT2-stained boutons in this region (Figure III.2.2B and Figure III.2.2C).

Figure

  Figure III.2.2. CA2-projecting SuM neurons express VGluT2.

  performing whole-cell patch-clamp recordings of PNs across the hippocampal CA regions (Figure III.2.3A). Following all recordings, we performed post-hoc anatomical reconstructions of recorded cells and axonal fibers (Figure III.2.4), as well as immunohistochemical stainings for CA2-area markers. We observed that photostimulation of SuM axons elicited excitatory post-synaptic responses in 63 % of PNs (n = 166 of 263 cells) located in area CA2 (Figure III.2.3B). PNs recorded in CA1 (3 %, n = 1 of 36 cells) and CA3b (0 %, n = 0 of 11) further away from the site of SuM fibers innvervation showed no excitatory responses, revealing the specificity of SuM inputs to area CA2. PNs in this region shared similar overall dendritic morphologies but differed along two criteria. First, in stratum lucidum some PNs clearly had thorny excrescences (TE) while others had very smooth apical dendrites (Figure III.2.3C and Figure III.2.3D). Based on the presence of TEs, we classified cells as CA2 or CA3 PNs (unequivocal distinction was possible in 148 of 263 cases). Examination of electrophysiological properties of these cells revealed a higher membrane resistance in CA3 PNs than CA2 PNs, but membrane capacitance and resting membrane potential were similar (

Figure

  Figure III.2.3. SuM input provides excitatory glutamatergic transmission to diverse population of PNs in area CA2.

Figure

  Figure III.2.4. Reconstruction of pre-synaptic SuM fibers and post-synaptic PNs in area CA2.

Csf2rb2-

  Cre line recapitulated the physiological observations made by Pedersen et al. in a VGluT2-Cre line. To this end, we performed voltage-clamp recordings of GCs at -70 mV like previously, and then under conditions optimal for recording inhibitory post-synaptic currents (IPSCs), i.e., with cesium in the pipette solution and the membrane potential clamped at +10 mV (Figure III.2.5A). Photostimulation of SuM axons reliably elicited EPSCs and IPSCs recorded in GCs at -70 mV and +10 mV, respectively (Figure III.2.5B and Figure III.2.5C). Application of NBQX and D-APV abolished EPSCs (Figure III.2.5B and Figure III.2.5D; amplitudes 13 ± 3.3 pA were in control and 1.2 ± 0.4 pA in NBQX & D-APV and SR95531 & CGP55845A hence a 83 ± 6.1 % block by NBQX & D-APV and SR95531 & CGP55845A, n = 14; Wilcoxon signed-rank test, p < 0.001) and significantly reduced IPSCs amplitude (Figure III.2.5C and Figure III.2.5D; amplitudes were 74 ± 24 pA in control and 29 ± 8.6 pA in NBQX & D-APV hence a 31 ± 13 % block by NBQX & D-APV, n = 17; Wilcoxon signed-rank test, p = 0.009). Further application of the GABAA and GABAB receptor antagonists SR95531 (1 µM) and CGP55845A (2 µM) completely blocked IPSCs (Figure III.2.5C and Figure III.2.5D; amplitudes were 29 ± 8.6 pA in NBQX & D-APV and 1.3 ± 0.2 pA in NBQX & D-APV NBQX & D-APV and SR95531 & CGP55845A hence a 91 ± 2.0 % block by SR95531 & CGP55845A, n = 17; Wilcoxon signed-rank test, p < 0.001). Altogether, these data replicate the results of Pedersen et al.[START_REF] Pedersen | Supramammillary glutamate neurons are a key node of the arousal system[END_REF] showing direct release of both glutamate and GABA at the SuM to GC synapses, thus validating our Csf2rb2-Cre line. Furthermore, the reduction of light-evoked IPSCs upon blockade of excitatory transmission alone demonstrates an additional feedforward inhibitory component of the SuM drive onto GCs in the dentate gyrus.

Figure

  Figure III.2.5. SuM light-evoked mono-synaptic excitation as well as mono-and di-synaptic inhibition onto DG GCs.

  excitatory transmission by applying NBQX and D-APV which completely abolished all lightevoked IPSCs (Figure III.2.6D; amplitudes were 167 ± 40.3 pA in control and 11.6 ± 2.0 pA in NBQX & D-APV hence a 93 ± 1.9 % block by NBQX & D-APV, n = 7; paired-T test, p = 0.007). We further verified the mono-and di-synaptic natures of EPSCs and IPSCs respectively by applying 0.2 µM TTX and 100 µM 4-AP to allow depolarization of pre-synaptic terminals while blocking action-potential driven vesicular release. Under these conditions, EPSCs remained but IPSCs were abolished even with photostimulation increased to 5 ms at 45 mW/mm² (Figure III.2.6E; amplitudes were 20.2 ± 6.1 pA for EPSCs and 110 ± 50 pA for IPSCs in control and 9.3 ± 7.7 pA for EPSCs and 4.8 ± 0.7 pA for IPSCs in TTX & 4-AP hence a 61 ± 24 % block of EPSCs and 88 ± 4.2 % block of IPSCs by TTX & 4-AP, n = 7; Wilcoxon differentially influences PNs according to their radial somatic location rather than their CA2 or CA3 morphology.

Figure

  Figure III.2.6. SuM axon stimulation recruits feedforward inhibition.

Figure

  Figure III.2.7. Area CA2 PNs receive a net inhibitory drive from SuM that controls summation and AP firing properties. A. Diagram illustrating the whole-cell recording configuration of PNs in area CA2 and SuM fiber stimulation in acute hippocampal slices. B. Top, sample traces of three 10 Hz SuM light-evoked PSPs before and after blocking inhibitory transmission (control shown in black, SR95531 & CGP55845A shown in grey). Bottom, comparison of light-evoked PSP amplitudes recorded in PNs before and after application of 1 µM SR95531 & 2 µM CGP55845A (individual cells shown as grey lines, population average shown as horizontal line, error bars represent SEM, n = 14; Wilcoxon signed-rank tests, p = 0.004 for the first PSP, p = 0.013 for the second PSP, p < 0.001 for the third PSP). C. Example traces of a CA2 PN action potential firing in response to current injection in the absence (black traces) or presence of 10 Hz photostimulation of SuM inputs (red traces, during the experiment photostimulations were interleaved with controls, but are grouped here for demonstration purposes), and with further application of 1 µM SR95531& 2 µM CGP55845A (purple traces). D. Action potential onset is increased with 10 Hz SuM input photostimulation. Left, sample traces (control in black, photostimulation in red). Right, comparison of photostimulation-induced delay of AP firing in area CA2 PNs in control and with application of 1 µM SR95531 & 2 µM CGP55845A in a subset of experiments (control shown in red, n = 12, paired-T test, p = 0.016; 1 µM SR95531 & 2 µM CGP55845A shown in purple, n = 6; Wilcoxon signed-rank test, p = 0.44; individual cells shown with dots, averages shown with boxplots). E. AP jitter in CA2 PNs is reduced by activation of SuM inputs. Comparison of the standard deviation of AP timing with or without 10 Hz photostimulation in control (top, n = 12; Wilcoxon signed-rank test, p < 0.001 for the first AP, p = 0.0078 for the second AP, p = 0.0039 for the third AP; individual cells shown with grey lines, population average shown as horizontal line, error bars represent SEM) and in 1 µM SR95531 & 2 µM CGP55845A (bottom, n = 6; Wilcoxon signed-rank tests, p = 0.22 for the first AP, p = 0.16 for the second AP, p = 0.09 for the third AP).

  photostimulation of SuM axons was able to evoke AP firing (Figure III.2.8G) in multiple

Figure

  Figure III.2.8. SuM inputs strongly excite basket cell interneurons in area CA2.

Figure

  Figure III.2.9. SuM inputs provide excitation to Parvalbumin-expressing BCs.

Figure

  Figure III.2.10. Parvalbumin-expressing BCs mediate the feedforward inhibition recruited by photostimulation of SuM fibers.

  of action potentials in CA2 PNs. These bursts of action potentials in CA2 PNs are likely to have a dramatic effect on area CA1 and the resulting hippocampal output. Therefore, we asked how neuronal activity in area CA2 is modulated by the SuM during CCh-induced oscillations and what the resulting consequences are on area CA1 PN activity.To address this, we used optogenetics to photostimulate SuM projections to area CA2 and performed whole-cell patch-clamp recordings of CA2 and CA1 PNs in acute hippocampal slices following application of 10 µM CCh. Cre-dependent channelrhodopsin-2 (ChR2) was specifically expressed in SuM neurons by injection of adeno-associated viruses (AAV) in VGluT2 and Csf2rb2-Cre mouse lines. This allowed us to quantify the strength of excitatory and inhibitory drives of SuM inputs onto CA2 PNs and their effect on burst firing in CCh. We observed a reduction of SuM light-evoked excitatory (EPSC) and inhibitory post-synaptic currents (IPSC) amplitudes and paired-pulse depression in CA2 PNs upon CCh application. The net effect of SuM transmission over CA2 PNs remained inhibitory in CCh and resulted in delayed burst firing from these cells. When examining the consequences of SuM activity on CA2 output transmission in area CA1 during CCh-induced network activity, we observed a drastic reduction of action potential firing in CA1 stratum pyramidale (SP) that lasted several seconds following light-stimulation of SuM inputs. Whole-cell recordings of CA1 PNs revealed that this reduction of field activity was paralleled by a hyperpolarization of CA1 PN membrane potential (VM) that prevented action potential firing in a seconds-long time window after SuM axon photostimulation. The time course of spontaneous EPSC and IPSC frequencies in CA1PNs during this time window showed a transient increase during the light-on period followed by a lasting decrease upon termination of SuM input stimulation. Altogether, our data demonstrate that the SuM inhibitory drive received by CA2 PNs controls the timing of their burst firing of action potentials that results in a massive and lasting silencing of CA1 PNs during CCh-induced network activity. These results indicate that the hippocampal output from area CA1 is controlled by SuM input through the modulation of area CA2 activity.

from

  SuM inputs onto CA2 PNs was affected by CCh. Whole-cell patch-clamp recordings with Cs + -based internal solution were obtained from CA2 PNs and light-evoked excitatory or inhibitory post-synaptic currents from SuM inputs were monitored before and after application of CCh (Figure III.3.1A). Given both resting membrane potential and neurotransmitter release of hippocampal neurons involved in the microcircuit engaged by SuM in area CA2 were affected by CCh, we measured how the amplitudes of EPSCs and IPSCs changed with increasing light intensity. Strikingly, application of 10 µM CCh reduced the amplitude of lightevoked EPSCs throughout the power-response curve (Figure III.3.1B; n = 14; two-way ANOVA with repeated measures, p < 0.001). This effect was solely due to changes in excitatory transmission from SuM as it persisted in experiments performed with GABAA and GABAB receptors blocked by 1 µM SR95531 and 2 µM CGP55845A (Figure III.3.1C; n = 7; two-way ANOVA with repeated measures, p < 0.001). Examination of the short-term synaptic depression of light-evoked EPSCs before and after CCh wash-in revealed an increase of the paired-pulse ratio (PPR) indicating that CCh decreases the release of glutamate by pre-synaptic SuM axons (Figure III.3.1B and Figure III.

Figure

  Figure III.3.1. Reduction of SuM excitatory and inhibitory transmission by carbachol. A. Diagram illustrating the whole-cell recording configuration of PNs in area CA2 and SuM fiber stimulation in acute hippocampal slices. B, C, D. Effect of 10 µM CCh on SuM light-evoked PSCs recorded in CA2 PNs under different conditions : voltage clamp at -70mV in control (B, control shown in black, CCh shown in orange), voltage clamp at -70mV with inhibitory transmission blocked (C, SR95531 & CGP55845A shown in grey, SR95531 & CGP55845A + CCh shown in orange), and voltage clamp at +10mV (D, control shown in red, CCh shown in orange). Left, sample traces. Middle, power-response curves (B, n = 14, two-way ANOVA with repeated measures, p < 0.001; C, n = 7; two-way ANOVA with repeated measures, p < 0.001; D, n = 17; two-way ANOVA with repeated measures, p < 0.001; error bars represent SEM). Right, comparison of PPRs (B, n = 14; paired-T test, p < 0.001; C, n = 7; paired-T test, p < 0.001; D, n = 17; paired-T test, p = 0.001; individual cells shown as grey lines, population average shown as horizontal line, error bars represent SEM).

  photostimulation trains were initiated when the pre-burst VM was reached(Figure III.3.2B).

  photostimulation onset(Figure III.3.2B and Figure III.3.2C). Unfortunately, the role of inhibition recruited by SuM input in delaying bursts could not be directly tested as blocking inhibitory transmission during CCh application transforms bursts into epileptiform-like events (see FigureIII.1.7D). Even so, fast synaptic transmission from SuM axons likely accounts for the burst delay as the effect of SuM activation was abolished by blocking excitatory

Figure

  Figure III.3.3. Immediate consequences of SuM activation on CA1 activity. A. Example raster plot (top) and corresponding peri-stimulus time histogram (bottom) of CA1 multi-unit activity recorded in vivo during photostimulation of SuM input over area CA2. B. Example raster plot

Figure

  Figure III.3.4. Delayed consequences of SuM activation on CA1 activity. A. Diagram illustrating the LFP and whole-cell recording configuration of PNs in area CA1 and SuM fiber stimulation in acute hippocampal slices with application of 10 µM CCh. B. Schematic illustration of the

  inputs form synapses onto both PNs and INs in area CA2. This excitation of INs was sufficient to reliably drive feedforward inhibition onto PNs, and was capable of influencing the jitter and timing of PN action potential firing. At the population level, deep PNs received a stronger inhibitory influence from SuM inputs than superficial PNs. The excitatory drive evoked by light-stimulation of SuM inputs was significantly larger for BC INs, which we demonstrate are predominantly PV+ BCs. Indeed, chemogenetic silencing of these cells as well as reduction of their GABA release by activation of mu or delta opioids receptors decrease SuM-driven feedforward inhibition of CA2 PNs. IV.2.a -SuM inputs to area CA2 form a microcircuit where PV+ basket cells strongly inhibit deep pyramidal neurons in area CA2 and CA3

  . More precisely, Oliva et al. describe a ramping up of deep CA2 PNs activity followed by a suppression of firing prior to sharp wave ripples (SWR) onset, and a peak of superficial CA2 PNs firing preceding the SWR recorded in CA1. When clustering PNs according to their deep or superficial somatic location and quantifying the relative strength of excitation and inhibition in these cells, we observed that SuM inputs exert a stronger overall inhibitory effect on deep PNs than on superficial PNs. Thus, the SuM can differentially influence these two subpopulations of CA2 PNs, which are functionally different. Namely, the preferential inhibitory over excitatory drive of the SuM on deep PNs could participate in promoting superficial PNs activity over deep PNs.

  McHugh laboratory has been examining the behavioral effects of activating or silencing SuM projections to either area CA2 or the DG in a spatially-restricted way. Using retrograde virus injections in SuM-Cre animals to selectively express excitatory or inhibitory DREADDs or opsins in either CA2-projecting or DG-projection SuM neurons, they were able to manipulate these inputs during awake behavior. Strikingly, activation or inhibition of SuM inputs to area CA2 during a social task bi-directionally affected social memory. Activation of SuM inputs increased exploration of a familiar mouse whereas silencing these inputs prevented the enhanced exploration of a novel animal. This indicates that SuM inputs play a role in social novelty detection or proper encoding of social information. On the other hand, silencing DGprojecting neurons in a delayed alternation task impaired working memory and object recognition. Therefore, in agreement with distinct physiological features of the SuM to CA2 versus DG projections, these two circuits are specifically involved in different behaviors and hippocampal-dependent learning processes.IV.2.f -Relevance of the SuM excitatory drive to CA2 PNsOur findings clearly prove an overall inhibitory drive of SuM inputs on area CA2 PNs. This point leads to the question of the relevance of the SuM excitatory transmission received by the majority of CA2 and CA3a PNs. Several observations can help in understanding the role of SuM-driven excitation of CA2 PNs. First, CA2 PNs are strongly excited by distal EC inputs

  to bring INs to threshold but that subsequent inhibition of CA2 PNs would be less pronounced in CCh. In addition, the depolarization of INs brought about by CCh might cause INs other than PV+ BCs to be recruited by SuM input and thus participate in the feedforward inhibition of CA2 PNs. Although we did not fully address this point, we occasionally observed an incomplete block of IPSCs by further application of DAMGO after CCh indicating that other INs beside PV+ BCs might be recruited by SuM inputs in these conditions. Patch-clamp recordings of identified IN subtypes and chemogenetic silencing of specific populations of INs are necessary to make firmer conclusions on this matter.

  sufficient to terminate an ongoing burst if light-stimulation had been delivered later, especially because CA2 PNs can intrinsically burst in the absence of synaptic inputs. In addition, PV+ BCs mediating the SuM-driven feedforward inhibition are likely recruited in feedback loops when CA2 PNs fire action potentials during bursts. Although the interaction of the SuM and CA2 excitatory drives onto PV+ BCs would have been extremely interesting to study notably in terms of AP phase coupling to the oscillation, the low number of bursts and their limited duration would have made these measurements noisy. Therefore, triggering SuM activity during the rising phase of bursts seemed the best option as it also yields information about the generation of bursts through recurrent excitatory connections of CA2 PNs captured by the field spiking activity. Indeed, in parallel with the delay of CA2 PN bursting, we observed a reduction in action potential firing recorded extracellularly in CA2 SP. Because they represent the majority of neurons in the pyramidal layer, CA2 PNs are likely to be the source of action potentials recorded with a field electrode in CA2 SP. Therefore, the inhibitory effect of SuM stimulation applies to the population of CA2 PNs as a whole. Although we did not see an increased field spiking activity prior to bursts recorded whole-cell in the light-off condition, the reduction of firing rate in CA2 SP might contribute to the delay of burst firing in the light-on condition by dampening recurrent excitation between CA2 PNs. Surprisingly, we did not observe an increased firing in CA2 SP after the light-induced delay of bursts from CA2 PNs recorded whole cell. This argues for a gradual relief of SuM-driven inhibition allowing CA2 PNs to fire bursts of APs rather than a post-inhibitory rebound of excitation that would cause synchronous burst discharge of CA2 PNs. In agreement with this, CA2 PNs normally fire sparse bursts of action potentials and do not discharge together, as opposed to hyper-synchrony observed with inhibitory transmission blocked (see FigureIII.1.7D). Therefore, it is likely that the prolonged trains of SuM photostimulation suppress firing in area CA2 by differentially delaying CA2 PNs activity depending on their level of depolarization at the onset of the light.As inhibition wanes off later during light trains, CA2 PNs gradually return to burst firing.Hence, SuM activity can efficiently control area CA2 output by pacing burst firing of CA2 PNs through feedforward inhibition.IV.3.c -Transient silencing of CA1 activity by the SuM-CA2-CA1 circuitA major finding of our study is the silencing of CA1 activity resulting from photostimulation of SuM inputs onto area CA2. Indeed, activation of SuM inputs over area CA2 caused a marked and long-lasting reduction of field spiking activity, hyperpolarization, and transient increase followed by lasting decrease of spontaneous EPSCs and IPSC frequencies in area CA1. While revealing a potentially crucial role of SuM in controlling the output of the hippocampus with surprising implications for hippocampal network activity, this result proves difficult to explain given the current information available on the SuM-CA2-CA1 circuit. Solid evidences describe an inhibitory drive of SuM to area CA2 and an excitatory output of CA2 PNs onto CA1. In addition, SuM and CA2 driven feedforward inhibition onto CA1 PNs need to be considered as plausible. Finally, neuromodulatory effects of long-range SuM fibers and local hippocampal neurons engaged in this circuit may be at play. First, it appears clearly that the overall effect of SuM inputs stimulation is inhibitory in area CA1. When considering fast synaptic transmission, this might be explained by decreased excitatory drive from CA2 PNs inhibited by SuM inputs, propagation to area CA1 of the feedforward inhibition recruited by SuM inputs in area CA2, and additional feedforward inhibition from area CA2 to CA1 once CA2 PNs recover from SuMdriven inhibition. The slow aspect of CA1 silencing following SuM activation suggests that it might be mediated by a GABAB-mediated effect. However, preliminary experiments with inhibitory transmission blocked revealed that the hyperpolarization of CA1 PNs remained in these conditions. Therefore, although fast synaptic transmission among the circuit described above likely plays a part, it cannot fully explain the silencing of CA1 activity caused by SuM input activation. The slow time course of this effect and its global spread to spontaneous EPSCs and IPSCs argues for a neuromodulatory mechanism. Of note, SuM fibers afferent to the hippocampus contain substance P that could well be released during strong photostimulation of SuM inputs. In area CA1, bath application of substance P excites INs leading to increased spontaneous inhibition of both PNs and other INs

Figure

  Figure IV.3.1. Integrated view of brain state-dependent SuM and area CA2 activity. Diagrams illustrating the intrinsic and extrinsic connections of the different hippocampal areas and their relative field and firing activity during theta / gamma-and SWRs-dominated brain states.

Table III .

 III 1.1. Characteristics of charbachol-induced CA2 PN spontaneous activity. ............................ Table III.2.1. Electrophysiological properties of pyramidal neurons in SuM innervated area ............... Table III.2.2. Properties of deep and superficial pyramidal neurons in areas CA2 and CA3a .............. Table III.2.3. Characteristics of SuM light-evoked transmission onto PNs in area CA2. ......................

	Table III.2.

Table III .

 III 2.5. Electrophysiological properties of interneurons in SuM-innervated area. ......................Table III.2.6. Characteristics of SuM light-evoked transmission onto interneurons & pyramidal cells.106 Table III.3.1. Consequences of SuM photostimulation over area CA2 on CA1 PNs. .........................

  FigureIII.1.4C and Figure III.1.5G; burst-VM = -41 ± 1.0 mV) interleaved by repolarization inbetween bursts to an inter-burst-VM close to the CCh-VM(Table III.1.1, Figure III.1.4C and

	Figure III.1.5H; inter-burst-VM = -62 ± 0.8 mV). During inter-bursts intervals, CA2 PNs VM
	slowly increased from repolarized levels after a burst to depolarized levels preceding a burst
	(Figure III.1.4C). However, most of the VM variation occurred in the vicinity of bursts with a
	fast transition from inter-burst-VM to burst-VM prior to bursting (Table III.1.1, Figure III.1.4C
	and Figure III.1.5I; burst rise slope = 3.5 ± 0.5 mV/s) and from burst-VM to inter-burst-VM after
	bursting (Table III.1.1, Figure III.1.4C and Figure III.1.5J; burst decay slope = -4.6 ± 0.8 mV/s).
	These transitions demarcated bursts that lasted 3.8 ± 0.5 seconds (Table III.1.1, Figure III.1.4C
	and Figure III.1.5L) with CA2 PNs firing action potentials at frequencies in the theta range at
	14 ± 2.6 Hz (Table III.1.1, Figure III.1.4C and Figure III.1.5K). Thus, CA2 PNs undergo slow
	rhythmic bursting when challenged with increased cholinergic tone, a behavior previously
	.1.1, Figure III.1.4C, Figure
	III.1.5D and Figure III.1.5E), possibly due to differences in depth of the recording. Voltage-
	clamp measurements of CA2 PNs membrane properties before and after CCh application
	revealed that this depolarization was accompanied by a 56 ± 6.6 MΩ increase in membrane
	resistance (RM) from 40 ± 2.6 MΩ to 95 ± 7.0 MΩ (Table III.1.1, Figure III.1.5B). This indicates
	that CCh depolarizes CA2 PNs by, at least in part, closing potassium-conducting channels. The
	initial slow depolarization was followed by regular bursts of action potentials in CA2 PNs that
	could last throughout CCh application. To maintain consistency with our LFP analysis, we

analyzed bursts during the first 15 minutes following CCh wash-in. These bursts occurred at a rather low rate

(Table III.1.1, Figure III.1.4C and Figure III.1.5F

; inter-burst interval = 100 ± 14.7 s) as CA2 PNs underwent further depolarization of their VM during bursts (Table

III

.1.1, described in CA3 PNs in similar conditions

[START_REF] Cobb | Activation of nicotinic acetylcholine receptors patterns network activity in the rodent hippocampus[END_REF]

. Importantly, CCh is necessary for CA2 PNs bursting as mimicking the CCh-induced depolarization by injecting current in the absence of CCh led to tonic firing but failed to elicit bursts

(Figure III.1.4D)

. Conversely, CA2

PNs are endowed with a repertoire of conductances sufficient for CCh to induce bursting without the need of synaptic inputs as blocking all synaptic transmission and isolating area CA2 from CA3 by cuts between these regions did not abolish AP bursts in CA2 PNs

(Figure III.1.4E)

.

Table III .1.1. Characteristics of charbachol-induced CA2 PN spontaneous activity.

 III

		Control	Excitatory and	Excitatory	Inhibitory	Statistics
			inhibitory	transmission	transmission
			transmission	blocked	blocked
			blocked
			(10 µM NBQX,	(10 µM NBQX,	(1 µM SR95531,
			50 µM APV,	50 µM APV)	2 µM CGP5584A)
			1 µM SR95531,
			2 µM CGP5584A)
		(n = 25)	(n = 6)	(n = 7)	(n = 6)
	ACSF-VM (mV)	-75 ± 1.2	-70 ± 4.1	-72 ± 3.1	-80 ± 1.9	1-way ANOVA test p = 0.054
	CCh-VM (mV)	-64 ± 1.0	-57 ± 2.9	-62 ± 2.1	-64 ± 1.5	1-way ANOVA test p = 0.036
			p = 0.025 vs CT			Tukey post hoc
	ΔACSF-CCh VM (mV)	11 ± 1.0	13 ± 1.4	9.5 ± 1.5	16 ± 1.9	1-way ANOVA test p = 0.065
	depolarization slope (mV/min) 6.5 ± 0.8	8.6 ± 2.0	9.2 ± 3.4	9.4 ± 2.3	Kruskal-Wallis test p = 0.52
	burst onset (min)	6.4 ± 0.7	3.5 ± 0.6	4.6 ± 1.8	4.7 ± 1.2	1-way ANOVA test p = 0.21
	inter-burst interval (s)	100 ± 14.7	36 ± 5.6	36 ± 8.3	113 ± 41.9	Kruskal-Wallis test p = 0.005
			p < 0.05 vs CT	p < 0.05 vs CT		Dunn-Holland-Wolfe post hoc
	burst-VM (mV)	-41 ± 1.0	-42 ± 2.2	-38 ± 2.1	-39 ± 1.9	1-way ANOVA test p = 0.45
	inter-burst-VM (mV)	-62 ± 0.8	-56 ± 1.1	-56 ± 1.4	-63 ± 1.9	1-way ANOVA test p < 0.001
			p = 0.006 vs CT	p = 0.001 vs CT		Tukey post hoc
			p = 0.033 vs SR	p = 0.013 vs SR
	Δburst-IBI VM (mV)	22 ± 1.3	14 ± 2.2	18 ± 2.6	24 ± 2.7	1-way ANOVA test p = 0.047
	burst rise slope (mV/s)	3.5 ± 0.5	3.6 ± 0.6	5.9 ± 1.1	59 ± 26	Kruskal-Wallis test p < 0.001
					p < 0.05 vs CT	Dunn-Holland-Wolfe post hoc
	burst decay slope (mV/s)	-4.6 ± 0.8	-4.8 ± 0.6	-4.3 ± 0.7	-13.7 ± 4.0	Kruskal-Wallis test p = 0.087
	burst duration (s)	3.8 ± 0.5	2.4 ± 0.6	2.6 ± 0.7	1.1 ± 0.3	Kruskal-Wallis test p = 0.037
					p < 0.05 vs CT	Dunn-Holland-Wolfe post hoc
	bursting frequency (Hz)	14 ± 2.6	6.3 ± 1.5	9.3 ± 3.0	23 ± 5.2	Kruskal-Wallis test p = 0.014
					p < 0.05 vs CT	Dunn-Holland-Wolfe post hoc
	ACSF-RM (MΩ)	40 ± 2.6
	CCh-RM (MΩ)	95 ± 7.0
	ΔACSF-CCh RM (MΩ)	56 ± 6.6

Table III .2.4. Characteristics of SuM light-evoked responses in deep and superficial pyramidal neurons in areas CA2 and CA3a.

 III

				EPSC
	cell type	connectivity (%)	amplitude (pA) rise time (ms) decay time (ms) latency (ms)	success rate
	CA2 PN deep	45 (n = 14 of 31)	19 ± 4.3	3.2 ± 0.3	16 ± 1.8	3.0 ± 0.6	0.42 ± 0.05*
	CA2 PN superficial 57 (n = 31 of 54)	17 ± 2.9	2.8 ± 0.2	14 ± 1.0	2.1 ± 0.2	0.48 ± 0.05
	CA3 PN deep	55 (n = 11 of 20)	13 ± 1.9	3.1 ± 0.2	13 ± 1.3	2.7 ± 0.5	0.45 ± 0.07#
	CA3 PN superficial 44 (n = 10 of 23)	36 ± 12	3.0 ± 0.3	15 ± 1.5	2.4 ± 0.3	0.73 ± 0.07*#
	Statistics	χ² test p = 0.815	Kruskal-Wallis test p = 0.196	1-way ANOVA test p = 0.614	1-way ANOVA test p = 0.514	Kruskal-Wallis test p = 0.174	1-way ANOVA test p = 0.016 Tukey post hoc
							p = 0.016*
							p = 0.031#
				IPSC
	cell type	connectivity (%)	amplitude (pA) rise time (ms) decay time (ms) latency (ms)	success rate
	CA2 PN deep	35 (n = 6 of 17)	229 ± 98.1	3.5 ± 0.8	24 ± 1.9	7.3 ± 1.4	0.44 ± 0.12
	CA2 PN superficial 32 (n = 11 of 34)	180 ± 50.8	3.9 ± 0.5	25 ± 1.9	5.9 ± 1.0	0.56 ± 0.07
	CA3 PN deep	58 (n = 7 of 12)	157 ± 20.9	3.9 ± 0.6	25 ± 2.0	6.6 ± 0.9	0.60 ± 0.08
	CA3 PN superficial 60 (n = 9 of 15)	135 ± 39.4	5.1 ± 0.5	24 ± 1.0	8.1 ± 1.3	0.49 ± 0.07
	Statistics	χ² test p = 0.418	Kruskal-Wallis test p = 0.609	1-way ANOVA test p = 0.257	1-way ANOVA test p = 0.914	1-way ANOVA test p = 0.537	1-way ANOVA test p = 0.791

  Table III.2.5). Light-evoked EPSCs and EPSPs were readily observed in the vast majority of BCs (Figure III.2.8B and Figure III.2.8D, Table III.2.6) and reached large amplitudes in some instances. An additional 26 INs with soma in SP classified as non-BCs because their axon did not target SP (Figure III.2.8C). In our recordings, these cells fired in bursts and showed little sag during hyperpolarizing current injection steps (Table III.2.5). We consistently observed no or very minor light-evoked excitatory transmission onto non-BCs (Figure III.2.8C, Table III.2.6). Furthermore, we recorded from 17 INs that had soma in stratum oriens (SO) and 9 in stratum radiatum (SR). Like non-BCs, these INs did not receive strong excitation from SuM fibers (Figure III.2.8B, Table III.2.6). This data is consistent with the conclusion that SuM input preferentially forms excitatory synapses onto basket cells in area CA2 and CA3a. To fully assess the strength of SuM inputs onto the different cell types, we examined the following parameters for each population: the connectivity, success rate, amplitude, potency, kinetics, and latencies of EPSCs as well as the resulting depolarization of the membrane potential. First, SuM inputs preferentially innervated BCs as evidenced by a higher connectivity of EPSCs in BCs than in PNs or other INs (Table III.2.6). Importantly, excitatory responses had short latencies with limited jitter (Figure III.2.6B, Table III.2.6) indicating that the connection was monosynaptic in all cell types. When voltage-clamping cells at -70 mV, light-evoked

Table III .2.5. Electrophysiological properties of interneurons in SuM-innervated area. Table III.2.6. Characteristics of SuM light-evoked transmission onto interneurons & pyramidal cells.

 III

			VM (mV)	RM (MOhm)	CM (pF)	firing adaptation index	sag (mV)
	Basket cell (n = 16)	-57.3 ± 1.38	144 ± 28.1	64.0 ± 8.70	0.74 ± 0.05	9.4 ± 1.0
	non-Basket Cell (n = 12) -55.6 ± 1.84	224 ± 46.8	52.0 ± 5.90	0.57 ± 0.06	5.9 ± 1.4
	interneuron SO (n = 6) -57.0 ± 3.16	201 ± 21.0	44.7 ± 5.31	0.61 ± 0.11	7.6 ± 1.9
	interneuron SR (n = 8) -60.1 ± 2.89	282 ± 49.8	39.6 ± 3.18	0.65 ± 0.09	8.1 ± 2.1
	Statistics	1-way ANOVA test	1-way ANOVA test	Kruskal-Wallis test	1-way ANOVA test	1-way ANOVA test
			p = 0.527		p = 0.100	p = 0.354	p = 0.238	p = 0.292
						EPSC
	cell type	connectivity (%)	amplitude (pA)	rise time (ms)	decay time (ms) latency (ms) success rate
	Pyramidal Cell	63 (n = 166 of 263)		19 ± 1.6*	3.4 ± 0.1*	15 ± 0.5*	2.9 ± 0.1	0.46 ± 0.02
	Basket Cell	82 (n = 18 of 22)		43 ± 8.7*	1.7 ± 0.3*	8.4 ± 1.3*	3.1 ± 0.4	0.59 ± 0.07
	non-Basket Cell	39 (n = 10 of 26)
	interneuron SO	12 (n = 2 of 17)			16 ± 2.8	2.6 ± 0.5	12 ± 1.4	3.4 ± 0.7	0.36 ± 0.06
	interneuron SR	11 (n = 1 of 9)
	Statistics	χ² test p = 0.006*	Kruskal-Wallis test p = 0.016 p < 0.05* post hoc Dunn-Holland-Wolfe	1-way ANOVA test p < 0.001* Tukey post hoc p < 0.001	1-way ANOVA test p < 0.001* Tukey post hoc p < 0.001	1-way ANOVA test p = 0.580	1-way ANOVA test p = 0.066

  Altogether, these results show that activation of cholinergic receptors in acute hippocampal slices decreases glutamate release from SuM inputs. This was directly demonstrated at the excitatory SuM-CA2 PN synapse and can be inferred to apply to excitation of INs by SuM as well. Consequently, CCh application reduces SuM-driven feedforward inhibition of CA2 PNs by reducing the recruitment of INs or decreasing GABA release from these INs, or both. Therefore, because short-term depression of EPSCs and IPSCs is decreased by CCh, SuM-driven excitation and inhibition in area CA2 is likely to develop more gradually over time during a train in conditions of elevated cholinergic tone.

	0.59 ± 0.06 in SR95531 & CGP55845A and 0.87 ± 0.05 in SR95531 & CGP55845A and CCh
	hence a 51 ± 11 % increase by CCh, n = 7; paired-T test, p < 0.001). Consistent with this,
	amplitudes of light-evoked IPSCs recorded in CA2 PNs were also reduced by CCh at all light
	intensities (Figure III.3.1D; n = 17; two-way ANOVA with repeated measures, p < 0.001).
	Similarly, inhibitory responses to SuM photostimulation showed less short-term depression
	after CCh application as evidenced by an increased PPR of IPSC (Figure III.3.1D; PPRs were
	0.73 ± 0.07 in control and 1.05 ± 0.05 in CCh hence a 67 ± 18 % increase by CCh, n = 17;
	paired-T test, p = 0.001).
	3.1C; PPRs were 0.65 ± 0.06 in control and 1.07 ±
	0.06 in CCh hence a 84 ± 19 % increase by CCh, n = 14; paired-T test, p < 0.001; PPRs were

Table III

 III T tests, p = 0.024 and p = 0.010). The frequency of spontaneous IPSCs underwent the same variations as that of sEPSCs, and the transient increase in sIPSC frequency was statistically significant when compared to interleaved light-off epochs(Table III.3.1, Figure III.3.4F; sIPSC

	frequency was increased by 7.6 ± 3.7 % during and decreased by 20.5 ± 4.3 % after compared
	to before photostimulation, n =12; paired-T tests, p = 0.093 and p = 0.001; sIPSC frequency
	was increased by 11.1 ± 5.5 % during and decreased by 17.6 ± 6.0 % after photostimulation
	compared to interleaved light-off epochs, n =12; paired-T tests, p = 0.032 and p = 0.008). No
	significant change in sEPSC or sIPSC amplitudes were detected (Table III.3.1). This result
	demonstrates a seconds-long global silencing of excitatory and inhibitory neurons afferent to
	CA1 PNs caused by activation of SuM inputs to area CA2. Importantly, all parameters
	measured here were stable in the light-off interleaved epochs (Table III.3.1, Figure III.3.4C-F).
	Interestingly, preliminary experiments performed with GABAA and GABAB receptors blocked
	indicate that the hyperpolarization of CA1 PNs after SuM activation remains in the absence of
	inhibitory transmission (Figure III.3.4D). This observation, if confirmed, argues for a
	neuromodulatory mechanism underlying CA1 PN hyperpolarization and reduced activity with
	SuM stimulation. In support of this hypothesis, voltage-clamp recordings of CA1 PNs at -70
	mV show a hyperpolarizing current upon SuM fiber activation that parallels the time course of
	hyperpolarization, reduced field activity, and decreased frequency of spontaneous events
	(Figure III.3.4E). Therefore, our data reveal that area CA1 is subjected to a drastic, prolonged
	and global silencing that results from modifications of area CA2 output by SuM activity. We
	postulate that the underlying mechanism of this silencing is likely to involve a change in
	conductances brought about by a yet to be identified.
	.3.1, Figure III.3.4E; sEPSC frequency was increased by 26.5 ± 7.9 % during and

decreased by 24.4 ± 2.7 % after compared to before photostimulation, n = 12; paired-T tests, p = 0.013 and p < 0.001; sEPSC frequency was increased by 36.6 ± 14.2 % during and decreased by 22.2 ± 8.3 % after photostimulation compared to interleaved light-off epochs, n = 12; paired-

  . Our experimental strategy allowed for the direct examination of the post-synaptic targets of SuM glutamatergic axons. Our results confirm that PNs in area CA2 indeed receive excitatory synapses from SuM axons. However, in contrast to what had been proposed in previous studies, we observed that SuM inputs target not only PNs but also INs in area CA2. Importantly, we identified a specific subpopulation of INs as PV+ BCs which were the cell type most potently excited by SuM. These BCs fired action potentials upon SuM input photostimulation, leading to a substantial feedforward inhibition of neighboring PNs. Consistent with the perisomatic targeting of BCs axons, recruitment of BCs by SuM resulted in the control of PNs excitability. This finding demonstrates that SuM activity can pace action potential firing in PNs through recruitment of PV+ BCs. Furthermore, while excitatory transmission from SuM was restricted to neurons in area CA2, the feedforward inhibition extended in CA3b several hundreds of micrometers away from the termination zone of the SuM fibers. This is in accordance with the pattern of projection of BCs axons that extended toward CA3b. Thus, the SuM inhibitory control of PNs excitability appears more global over both area CA2 and CA3. Of note, SuM direct excitation and feedforward inhibition were equally distributed between CA2 and CA3 PNs.

Acknowledgements

It is with sincere enthusiasm that I thank all the persons involved in the completion of this thesis. I thank Rebecca Piskorowski and Vivien Chevaleyre for taking me on as a PhD student, teaching me many technical and intellectual aspects of research, supervising my work, and guiding me through these years. Thank you for your availability, your encouragements, your insightful advices and for setting such a pleasant atmosphere in the lab. I am proud to have learnt science from you. During these years, I have not only received an invaluable training from you but also found exceptional mentors who never failed to help me move forward through good and hard times with thoughtfulness and friendship. Rebecca, thank you for your patience, your support, your trust, and your enthusiasm in passing on your expertise. I can only hope to become a researcher like you in the future. I thank the members of my thesis committee, Claire Legay, Liset Menendez de la Prida, Paul Salin, Audrey Mercer, Philippe Ascher, Jean-Christophe Poncer and Karim Benchenane. It is an honor to have you evaluate my work. I thank Manuel Mameli for being part of my progress committee.

I thank all the present and past members of the lab for useful discussions, help with experiments, and all the fun times we had. In particular, I thank Ludivine Therreau and Kaoutsar Nasrallah.

Thank you Ludivine for your invaluable help with surgeries, genotyping, and histology. Thank you Kaoutsar for helping me when I started and for your thoughtfulness and positive attitude that never failed to bring serenity in the lab, it was amazing working with you. I thank the Ecole Normale Superieure de Cachan, the French Ministère de la Recherche, and the Fondation pour la Recherche Médicale (FDT20170437387) for funding my work.

I thank the members of the McHugh lab for providing the Csf2rb2-Cre mouse line, the AAV.Synapsin. DIO.hM4D(Gi).mCherry virus, and in vivo experiments.

G. Top, hCHR2(H134R)-EYFP -expressing SuM fibers (green) and nissl staining (blue) in the hippocampus. Bottom left, higher magnification image of area CA2 with hCHR2(H134R)-EYFPexpressing SuM fibers (green) and nissl staining (blue); bottom right, hCHR2(H134R)-EYFP -expressing SuM fibers (green) and RGS14 staining (magenta) to label area CA2.

following the application of 10 µM NBQX and 50 µM D-APV (Figure III.2.3F; amplitudes were 16 ± 4.8 pA in control and 1.8 ± 0.3 pA in NBQX & D-APV hence a 89 ± 3.5 % block by NBQX & D-APV, n = 6; Wilcoxon signed-rank test, p = 0.03). These data confirm that SuM inputs provide long-range glutamatergic excitation to CA2 and CA3 PNs in area CA2.

III.3 -Influence of SuM inputs on hippocampal area CA2 drive onto CA1 III.3.a -Introduction

Crucial aspects of episodic learning and memory are supported by the hippocampus. Afferent information from the entorhinal cortex (EC) is thought to be sequentially processed by hippocampal circuits in the classical tri-synaptic loop enabling memory formation. In this view, the dentate gyrus (DG) receives EC inputs and projects to area CA3, CA3 pyramidal neurons (PNs) then send outputs to area CA1 that in turn returns information to the EC. Surprisingly, interrupting this loop by blocking the CA3 to CA1 connection does not result in a dramatic compromise of hippocampal-dependent learning and memory [START_REF] Brun | Place Cells and Place Recognition Maintained by Direct Entorhinal-Hippocampal Circuitry[END_REF][START_REF] Nakashiba | Transgenic Inhibition of Synaptic Transmission Reveals Role of CA3 Output in Hippocampal Learning[END_REF]. This observation argues for another route of information in the hippocampus that could rescue its functions in the absence of transmission from area CA3 to CA1. A good candidate for this role is the long-overlooked area CA2 as it receives strong input from the EC and potently drives area CA1, thus potentially bypassing CA3 [START_REF] Chevaleyre | Strong CA2 Pyramidal Neuron Synapses Define a Powerful Disynaptic Cortico-Hippocampal Loop[END_REF][START_REF] Kohara | Cell type-specific genetic and optogenetic tools reveal hippocampal CA2 circuits[END_REF]. However, little is known about how area CA2 influences CA1 and what the resulting hippocampal output is. Therefore, physiological investigations of the consequences of area CA2 neuronal activity on CA1 are required to better understand information flow in hippocampal circuits.

Of note, specific patterns of activity are present in hippocampal networks during the encoding, consolidation and retrieval of spatial information. Theta and gamma oscillations as well as sharp wave ripples (SWR) are critically involved in these mnemonic processes [START_REF] Fuchs | Recruitment of Parvalbumin-Positive Interneurons Determines Hippocampal Function and Associated Behavior[END_REF][START_REF] Girardeau | Selective suppression of hippocampal ripples impairs spatial memory[END_REF][START_REF] Korotkova | NMDA Receptor Ablation on Parvalbumin-Positive Interneurons Impairs Hippocampal Synchrony, Spatial Representations, and Working Memory[END_REF]. Although the roles of area CA3 and CA1 in rhythmic activity of hippocampal networks have been extensively studied, the contributions of area CA2 have rarely been addressed until lately. Remarkably, recent studies have unveiled a critical role of area CA2 in the generation of SWR and in spatial coding [START_REF] Boehringer | Chronic Loss of CA2 Transmission Leads to Hippocampal Hyperexcitability[END_REF][START_REF] Kay | A hippocampal network for spatial coding during immobility and sleep[END_REF]Oliva et al., 2016). On the other hand, the functions of area CA2 during brain states of theta and gamma oscillations are still unclear. Strikingly, area CA2 receives direct input from the hypothalamic supramammillary nucleus (SuM) [START_REF] Haglund | The projection of the supramammillary nucleus to the hippocampal formation: An immunohistochemical and anterograde transport study with the lectin PHA-L in the rat[END_REF][START_REF] Kiss | The supramammillo-hippocampal and supramammillo-septal glutamatergic/aspartatergic projections in the rat: a combined [3H]daspartate autoradiographic and immunohistochemical study[END_REF][START_REF] Maglóczky | Principal cells are the postsynaptic targets of supramammillary afferents in the hippocampus of the rat[END_REF][START_REF] Soussi | Heterogeneity of the supramammillary-hippocampal pathways: evidence for a unique GABAergic neurotransmitter phenotype and regional differences[END_REF][START_REF] Vertes | PHA-L analysis of projections from the supramammillary nucleus in the rat[END_REF]) that shows theta-locked activity [START_REF] Kirk | Evidence for differential control of posterior hypothalamic, supramammillary, and medial mammillary theta-related cellular discharge by ascending and descending pathways[END_REF][START_REF] Kocsis | Characterization of neurons of the supramammillary nucleus and mammillary body that discharge rhythmically with the hippocampal theta rhythm in the rat[END_REF] and is involved in the control of hippocampal theta oscillations [START_REF] Kirk | Mapping the differential effects of procaine on frequency and amplitude of reticularly elicited hippocampal rhythmical slow activity[END_REF][START_REF] Mcnaughton | Contribution of synapses in the medial supramammillary nucleus to the frequency of hippocampal theta rhythm in freely moving rats[END_REF][START_REF] Thinschmidt | The supramammillary nucleus: Is it necessary for the mediation of hippocampal theta rhythm?[END_REF]. Hence, area CA2 neuronal activity is likely to be modulated by SuM input during theta states and subsequently affect the output of the hippocampus via CA1. pre-light-off 10.5 ± 1.4 N/A N/A 3.9 ± 0.5 14.3 ± 0.6 12.1 ± 1.0 27.5 ± 2.5 pulse-light-off 10.5 ± 1.3 -0.01 ± 0.05 -0.02 ± 0.05 3.8 ± 0.4 14.2 ± 0.7 12.0 ± 1.1 27.2 ± 2.6 post-light-off 10.5 ± 1.4 0.01 ± 0.04 -0.01 ± 0.05 4.1 ± 0.4 14.1 ± 0.5 12.0 ± 1.1 28.2 ± 2.9

pre-light-on 11.8 ± 1.6 N/A N/A 3.9 ± 0.3 13.9 ± 0.7 12.6 ± 1.2 27.6 ± 2.3 pulse-light-on 10.6 ± 1.4 -0.20 ± 0.05 -0.07 ± 0.05 5.0 ± 0.6 14.0 ± 0.6 13.5 ± 1.4 32.0 ± 5.1 post-light-on 8.8 ± 1.2 -0.51 ± 0.12 -0.13 ± 0.06 2.9 ± 0.2 13.7 ± 0.7 10.1 ± 1.1 27.7 ± 2.9 PNs. Our data did not reveal a consistent lead or lag between oscillations from area CA2 and CA3. Whether this reflects that oscillators in area CA2 and CA3 are independent is unclear as we cannot exclude the possibility that axonal projections between regions were severed in our preparation. Future work with simultaneous recordings of gamma oscillations in area CA2 and CA3 in vivo is needed to resolve this issue.

IV.1.b -Bursts of action potentials in CA2 PNs

A major aspect of the dynamics of action potential firing in CA2 PNs unveiled by our study is that they come in bursts when CCh is present, similar to CA3 PNs [START_REF] Cobb | Activation of nicotinic acetylcholine receptors patterns network activity in the rodent hippocampus[END_REF].

Furthermore, these bursts occur repeatedly with synaptic transmission blocked. This demonstrates that CA2 PNs possess a combination of conductances allowing intrinsic bursting when challenged with CCh. Although we did not try to identify these conductances, predictions can be made from previous studies in other CA areas and prior information regarding CA2 PNs.

Intuitively, the observation that CA2 PNs bursting occurs during membrane potential depolarization and an increase in membrane resistance suggests that CCh induces closure of potassium leak channels in these cells. Indeed, signaling through muscarinic cholinergic receptors (mAchR) decreases potassium conductances IM, IKsAHP, and leak in cortical principal cells [START_REF] Alger | Muscarinic cholinergic receptors modulate inhibitory synaptic rhythms in hippocampus and neocortex[END_REF][START_REF] Cobb | Cholinergic modulation of hippocampal cells and circuits[END_REF]. In addition, Ih, INaP and Ication-Ca2+ are increased by mAchR activation [START_REF] Alger | Muscarinic cholinergic receptors modulate inhibitory synaptic rhythms in hippocampus and neocortex[END_REF][START_REF] Cobb | Cholinergic modulation of hippocampal cells and circuits[END_REF][START_REF] Fisahn | Muscarinic Induction of Hippocampal Gamma Oscillations Requires Coupling of the M1 Receptor to Two Mixed Cation Currents[END_REF][START_REF] Yamada-Hanff | Persistent Sodium Current Drives Conditional Pacemaking in CA1 Pyramidal Neurons under Muscarinic Stimulation[END_REF]. Whether these observations hold for CA2 PNs remains to be determined. Interestingly, CA2 PNs have been described to express high mRNA levels of the potassium leak TREK channel whose increased conductance accounts for excitability defects of CA2 PNs in the Df16 +/-mouse model of schizophrenia [START_REF] Piskorowski | Age-Dependent Specific Changes in Area CA2 of the Hippocampus and Social Memory Deficit in a Mouse Model of the 22q11.2 Deletion Syndrome[END_REF]. It is possible that in this mouse model, the hyperpolarized VM disrupts CA2 PN bursting, contributing to the deficits in social memory [START_REF] Piskorowski | Age-Dependent Specific Changes in Area CA2 of the Hippocampus and Social Memory Deficit in a Mouse Model of the 22q11.2 Deletion Syndrome[END_REF] and cortico-hippocampal synchrony [START_REF] Sigurdsson | Impaired hippocampalprefrontal synchrony in a genetic mouse model of schizophrenia[END_REF] observed in these mice.

While bursts of action potentials in CA2 PNs in the presence of CCh occurred with excitatory and inhibitory transmission blocked, the properties of these intrinsic bursts were different from those seen in an intact network. The observation that bursts occur more frequently in the absence of synaptic input suggests that excitation and inhibition normally structure the network excitability and paces bursts in individual CA2 PNs. This raises the issue of the potential synchrony of bursts amongst PNs in area CA2. Examination of the LFP signal during bursts did not reveal an increased gamma power during these epochs, thus arguing that bursts do not encode location during immobility and in the absence of theta oscillation. We propose that our results provide crucial mechanistic insight about CA2 N units activity. It has been shown that neuronal activity in the SuM is locked to hippocampal theta oscillations [START_REF] Kirk | Supramammillarycell firing and hippocampal rhythmical slow activity[END_REF] which is prominent during locomotion [START_REF] Buzsáki | Theta Oscillations in the Hippocampus[END_REF]. We demonstrate that long-range SuM axons, which selectively innervate this area, provide a feedforward inhibition via PV+ basket cells in area CA2. Thus, by exciting BCs, the SuM can rhythmically inhibit CA2 and CA3a PNs during theta states associated with locomotion. Conversely, decreased SuM activity during immobility would allow CA2 and CA3a PNs to fire action potentials by relieving them from inhibition. Therefore, SuM activity likely allows for a state-dependent modulation of deep versus superficial CA2 PNs.

IV.2.c -SuM input and hippocampal oscillations

Oscillatory activity of hippocampal networks in the theta and gamma range support key functions of the hippocampus and are critical for learning and memory [START_REF] Alexander | Social and novel contexts modify hippocampal CA2 representations of space[END_REF][START_REF] Vertes | Brainstem-diencephalo-septohippocampal systems controlling the theta rhythm of the hippocampus[END_REF]. Although numerous studies have yielded valuable insights into the generation, mechanisms and functions of these oscillations, the contribution of extrahippocampal structures and the consequences of their activity on the hippocampus are still not entirely understood. Indeed, the SuM has been established as a key nucleus involved in setting the frequency of the hippocampal theta rhythm [START_REF] Cui | Hypothalamic and other connections with dorsal CA2 area of the mouse hippocampus[END_REF][START_REF] Hitti | The hippocampal CA2 region is essential for social memory[END_REF][START_REF] Kohara | Cell type-specific genetic and optogenetic tools reveal hippocampal CA2 circuits[END_REF]Pan and McNaughton, 2004a) but how this control is achieved remains unclear. Activity in SuM is theta-locked and has been shown to strongly influence hippocampal theta frequency in anaesthetized rodents [START_REF] Ariffin | Nicotinic receptor mechanism in supramammillary nucleus mediates physiological regulation of neural activity in dorsal hippocampal field CA1 of anaesthetized rat[END_REF][START_REF] Jiang | Microinjection of carbachol in the supramammillary region suppresses CA1 pyramidal cell synaptic excitability[END_REF][START_REF] Kirk | Mapping the differential effects of procaine on frequency and amplitude of reticularly elicited hippocampal rhythmical slow activity[END_REF] and partially in awake animals [START_REF] Pan | The medial supramammillary nucleus, spatial learning and the frequency of hippocampal theta activity[END_REF]2002;[START_REF] Ruan | Minimal driving of hippocampal theta by the supramammillary nucleus during water maze learning[END_REF][START_REF] Shahidi | Effect of reversible inactivation of the supramammillary nucleus on spatial learning and memory in rats[END_REF][START_REF] Woodnorth | Multiple hypothalamic sites control the frequency of hippocampal theta rhythm[END_REF]. In addition, hippocampal theta activity during spatial learning in a Morris water maze is altered upon serotonin depletion in SuM [START_REF] Gutiérrez-Guzmán | Serotonin depletion of supramammillary/posterior hypothalamus nuclei produces place learning deficiencies and alters the concomitant hippocampal theta activity in rats[END_REF][START_REF] Hernández-Pérez | Supramammillary serotonin reduction alters place learning and concomitant hippocampal, septal, and supramammillar theta activity in a Morris water maze[END_REF]. SuM influence over hippocampal theta rhythm could be achieved indirectly by SuM outputs to the septum [START_REF] Jiang | Microinjection of carbachol in the supramammillary region suppresses CA1 pyramidal cell synaptic excitability[END_REF][START_REF] Vertes | Brainstem-diencephalo-septohippocampal systems controlling the theta rhythm of the hippocampus[END_REF], but also through its direct outputs to the DG and hippocampal area CA2 [START_REF] Vertes | PHA-L analysis of projections from the supramammillary nucleus in the rat[END_REF].

The micro circuitry we reveal in this work provides insight into how the SuM activity can affect hippocampal oscillatory activity. The population of INs potently excited by SuM transmission have many features that allow us to classify them as PV+ BCs. They have soma located in the somatic layer with densely packed perisomatic-targeted axons. These cells are fast spiking, Pan and McNaughton, 2004a), thus potentially establishing bi-directional connections.

Therefore, the SuM, the septum and area CA2 might form a long-distance loop that could be critical for the synchronization of theta oscillations. These perspectives provide exciting hypothesis to be tested in the future, notably by examining the reciprocal connection between area CA2 and the septum.

VI -Appendices

Abstract

The hippocampus is a brain structure critically involved in learning and memory. These functions depend on the coordinated activity of hippocampal neurons during network oscillations. Different rhythmic patterns of activity exist in the hippocampus such as oscillations in the theta and gamma range as well as sharp wave ripples, however their underlying mechanisms are not fully understood. Indeed, several brain structures connected to the hippocampus participate in the generation of these oscillations, but their respective contributions remain elusive. Notably, the hypothalamic supramammillary nucleus (SuM) is strongly involved in theta oscillations and projects to the long-overlooked hippocampal area CA2. Even so, the physiology of this hypothalamo-hippocampal long-range input has never been investigated. Interestingly, recent in vivo studies have revealed a role of area CA2 in the generation of sharp wave ripples and spatial coding, suggesting specific contributions of area CA2 to hippocampal network function. Therefore, information regarding the mechanism governing network activity in area CA2, how it is influenced by SuM inputs and the consequences on hippocampal output is required to better understand hippocampal-dependent learning and memory.

To address these questions, we combined histology, pharmacology, ex vivo electrophysiology, optogenetics and chemogenetics on acute hippocampal slices from genetically-engineered mouse lines. This first allowed us to characterize the cellular and circuit mechanisms of gammalike oscillations induced by the cholinergic agonist carbachol in area CA2. In this regime, we found that CA2 pyramidal neurons fire bursts of action potentials that show phase-coupling to the oscillation. Next, we proved that SuM inputs differentially drive mono-synaptic excitation and di-synaptic inhibition onto deep and superficial CA2 pyramidal neurons. In addition, we demonstrated that parvalbumin-expressing basket cells are strongly recruited by SuM inputs and control the timing and precision of CA2 pyramidal neurons action potential firing via feedforward inhibition. Moreover, we showed that the SuM inhibitory drive onto CA2 pyramidal neurons exerts a temporal control on their bursting of action potential in the presence of carbachol. Finally, we observed that activation of SuM inputs to area CA2 result in a prolonged reduction of activity in area CA1 under conditions of elevated cholinergic tone. Altogether, our results highlight a critical role of the SuM in controlling area CA2 activity and thereby influences the hippocampal output. To conclude, we postulate that the SuM to area CA2 connection underlies key aspects of hippocampal rhythmogenesis and associated functions.

Keywords : hippocampus, hypothalamus, area CA2, SuM, carbachol, oscillations, burst firing, parvalbumin-expressing basket cell, feedforward inhibition.