Alana Tamar Wong

Daniela Kaufer

Francois Tronche I Am

Dissecting the functional and morphological contributions of the glucocorticoid receptor gene in neural progenitor cells of the hippocampus by

Stress hormones are known as one of the strongest and most ecologically relevant mediators of adult neurogenesis. A lingering question in adult neurogenesis is whether these hormones, known as corticosteroids (cortisol in humans, corticosterone in rodents), act directly on neural progenitor cells (NPCs), or indirectly through secreted factors or changes in network activity. Additionally, the functional contributions of this impact are largely speculative. To address these unknowns, we generated a transgenic mouse model whose glucocorticoid receptors (GRs) could be inducibly inactivated specifically in NPCs. GRs are the main target that corticosteroids bind to when elevated during stress. We investigated the effect of this cell-specific GR knockout model on hippocampal survival and differentiation and found them to be similarly affected by chronic corticosterone treatment compared to controls. This implies that corticosterone-suppressed neurogenesis and its impact on morphology is indirect, and GR in other cells may be mediating the effects. Furthermore, mice with GR inactivation in newborn neurons behaved similarly to controls in all tasks observed under basal levels of corticosterone. When mice were chronically treated with corticosterone, however, controls exhibited an anxious phenotype in novelty-suppressed feeding (NSF), light/dark box, and elevated Omaze, whereas transgenic mice behaved like untreated control groups in all anxiety measures except latency to feed in NSF. Neither corticosterone nor inactivation of GR in adult-born neurons altered depressionlike behaviors in the forced swim test, nor percent freezing in contextual fear discrimination. Lastly, we found that corticosterone increased the rate of learning in 1-trial contextual fear conditioning, an effect not mediated by reducing GR signaling in the neurogenic pool. These results highlight the functional contributions of adult neurogenesis as well as how their GRs mediate anxiety-relevant behaviors irrespective of suppressed neurogenesis.

have been honored with the help and support of many people I would like to take the opportunity to gratefully acknowledge.

First and foremost, I am so grateful to my Ph.D. advisors:

humor and always so generously helping me troubleshoot with my own projects and concerns at a moments notice. They made my life easier and brighter, and I am so grateful for all of their advice, concern, and smiles.

In particular, Id like to thank Kereshmeh Taravosh-Lahn for help troubleshooting with tissue staining; Shawn Shirazi for his dependable technical support; and a very special thank you to Liz Kirby, for not only her time and advice on my data, but her critical help with statistics and lightning speed email responses; you are ridiculously kind.

The Tronche lab members and colleagues as well were incredibly helpful to this research. Not only did they constantly help me with translation, but they also introduced me to Paris, helped me with a multitude of issues as they arose, and helped troubleshoot my own experimental problems. In particular, I would like to thank Camille Baranowski for teaching and helping me perform behavior tasks as well as mouse perfusions;

Nicholas Bouveyron and Jacques Barik for helping take care of the mice, setting up my breeding pairs, and genotyping; Sebastian Fernandez and Stephanie Daumas, who helped train me on and troubleshoot behavioral paradigms; and Samah Karaki, who gave so much of her time in helping me take corticosterone samples, conduct experiments, transition data, and being such a kind friend.

I am also so grateful to the incredibly bright and thoughtful students who helped conduct and analyze this research (and research not included here): Amrita Krishnamurthy, Casey Tsai, Fanny Du, Ken Brown, Chris Wang, and Eva Freund. In particular, Eva Freund was an important asset, spending long hours helping conduct behavior experiments, weighing and caring for the mice, changing corticosterone bottles, and scoring behavior data; I was never so proud as when Id see her fall asleep after hours of counting paw entries. Furthermore, I greatly benefited from Ken Brown and Chris Wang for the enormous amount of time, commitment, and attentive focus they spent slicing brains and scoring behavior data. I am also very appreciative to Ken for mounting tissue (immensely useful) and Chris for our conversations about culture and explaining my namesake. These talented individuals dedicated an enormous amount of time and energy into my project and were a delight to work beside.

Additionally, I want to thank all my friends in Berkeley and Paris that gave me so many memories over the years that I will forever cherish and hold dear. I particularly want to thank some friends who have been a dependable support structure for me: Trang Cao, for listening to all my complaining, always boosting me up and building my confidence, giving helpful advice, and for the numerous delicious meals she fed me or left out for me especially when Id return home from lab after midnight; Lydia Wood, for giving me a home and a friendship that has been an invaluable net of support, her esteemed advice, and thoughtful insights for my research; Kevin Duncan, for supporting me throughout my dissertation writing with his friendship and backhealing powers; and Ilios Galil, for also giving me a home, not just an apartment, but a city, for all the late night rides home or to lab, and for those special moments when you'd listen to me vent about research and not talk.

Those were great.

I especially want to thank my family. Doing a PhD requires a great amount of dedication and as a result, neglect of others. I am so thankful for their tremendous moral support over the years that kept me from falling down the rabbit hole. They have been my rock, listening and empathizing over any or all my experimental pitfalls and being the first people to cheer for me when things go well. I don't know how to thank them fully, for they have been and done so much for me. At the very least, I am so appreciative to them just for always being there. In particular, I want to thank Lara Wong, for answering the phone day and night, listening to me vent, practice talks, or bounce ideas off of, and entertaining me while I do microscopy; and Marsha Wong, for definitely listening to me vent the most, finding ways to motivate me and enrich my life, being constantly updated on my welfare, and supporting me in everything I wanted to do except quit. Their love and encouragement has inspired my work and ambitions.

To all the people that kept me sane over the years and made my Ph.D. experience what is became, thank you. Last but not least, I must thank the rats and mice who, perhaps unknowingly, dedicated their lives to my goals and the pursuit of advancing science. I hope I treated you well even though you contributed a significant amount of noise and variability to my data. And most importantly, to 7-year-old Alana, who was the driving force behind all of this, pushing me to not just finish my Ph.D., but seek out knowledge and contribute to science. For you, I sacrificed a decent salary, better fashion sense, and social skills. I hope I made you happy.

To Berkeley and Paris: it was the best of times, it was the worst of times; I wouldn't trade it for the world.

LIST OF ABBREVIATIONS

Scope of thesis

The research in this thesis is an investigation of the role the glucocorticoid receptor (GR) plays in hippocampal neurogenesis and functioning. To achieve this, we have generated a novel mouse model that inactivated the GR gene in nestin-expressing cells of the adult mouse brain.

In this dissertation, I report abolishing GR expression specifically in a population of neuronal progenitor cells (type 1 and 2a; NPCs) of the hippocampus by using an inducible Cre/loxP recombinase strategy. We found that GR gene inactivation in these cells did not change the survival or differentiation rate of neurogenesis for mice under basal levels of corticosterone. Similar to controls, differentiation of NPCs was also sensitive to chronic corticosterone treatment in mice with GR gene inactivation. Functional studies further revealed that GR gene inactivation in NPCs reduced anxiety-like behaviors only when mice were under chronic treatment of stress hormones. There was no effect of GR gene inactivation on depression-like behaviors, contextual fear conditioning, or pattern separation, a hippocampal-dependent associative memory ability. This was true regardless of whether mice were treated with corticosterone or not. These results provide a novel mouse model to examine GR-mediated processes in NPCs and suggest a functional role in anxietylike behaviors mediated perhaps independently from changes in neurogenesis.

1.2

Stress and neurogenesis in the mammalian central nervous system

The stress response in the brain

The body strives to maintain homeostasis, or optimal biological functioning [START_REF] Selye | Forty years of stress research: principal remaining problems and misconceptions[END_REF][START_REF] Cannon | A Charles Richel ses amis, ses collègues, ses élèves[END_REF][START_REF] Mcewen | Stress, adaptation, and disease. Allostasis and allostatic load[END_REF] . In order to achieve this, it needs to have systems in place that respond to the ever-changing external and internal events or stimuli that it faces. External events can range from being physical (e.g. injury, heat stroke, starvation), to psychological, such as fear of threat. These psychological events, whether real or imagined, stimulate the stress response. Two of these responses are called the norepinephrine-sympathetic adrenomedullary system (NE-SAM) and the hypothalamic-pituitary-adrenocortical system (HPA). These two systems work together to increase (or redirect) energy resources, particularly critical when an organism is in survival mode (for review [START_REF] De Kloet | Brain Corticosteroid Receptor Balance in Health and Disease[END_REF][START_REF] Gunnar | Brain and behavior interface: stress and the developing brain[END_REF] ).

When the brain perceives a stressor, this triggers the stress response (Figure 1; for review [START_REF] Herman | Neuronal circuit regulation of the hypothalamo-pituitaryadrenocortical stress axis[END_REF] ). The hypothalamus activates the NE-SAM system-releasing epinephrine (adrenaline) and norepinephrine into the blood stream for fast reactions [START_REF] Sapolsky | How do glucocorticoids influence stress responses? Integrating permissive, suppressive, stimulatory, and preparative actions[END_REF] . These are the immediate physiological changes that occur, such as an increase in heart rate, breathing, and metabolism, and a decrease in digestion and growth. The hypothalamus, also activated by the HPA system, releases, via a portal system, corticotrophin-releasing hormone or factor (CRH/F) and vasopressin onto the anterior pituitary to evoke the release of adrenocorticotropin (ACTH). ACTH then reacts with the adrenal glands to secrete glucocorticoids (i.e. cortisol in humans, corticosterone in rodents; hereafter referred to as cort) into the bloodstream [START_REF] Gunnar | Brain and behavior interface: stress and the developing brain[END_REF] . These lipophilic hormones can then pass through the blood brain barrier and circulate throughout the brain, indirectly affecting DNA transcription. This cascade of events in the HPA axis can take approximately 20 minutes [START_REF] Gunnar | Brain and behavior interface: stress and the developing brain[END_REF] , however, the effect on protein synthesis can be persisting [START_REF] De Kloet | Stress and cognition: Are corticosteroids good or bad guys?[END_REF] .

Under stress conditions, circadian rhythms are overridden, and corticosterone secretion is at its peak [START_REF] Ferguson | Overexpression of mineralocorticoid and transdominant glucocorticoid receptor blocks the impairing effects of glucocorticoids on memory[END_REF][START_REF] Nichols | Do glucocorticoids contribute to brain aging?[END_REF] . The effects this asserts can depend on things such as whether the stressor is acute, chronic, predictable, or controllable [START_REF] Koolhaas | Stress revisited: A critical evaluation of the stress concept[END_REF] . The HPA axis has a negative feedback loop, such that corticosterone can inhibit both the further production of ACTH and the secretion of CRF, all with the goal of stabilizing the response [START_REF] Kendall | The significance of the adrenals for adaptation to mineral metabolism[END_REF] .

However, as many people have experienced, when this system is not balanced, it can lead to more serious conditions, anxiety-related disorders, depression, or even memory impairment [START_REF] De Kloet | Stress, genes and the mechanism of programming the brain for later life[END_REF][START_REF] De Kloet | Glucocorticoid signaling and stress-related limbic susceptibility pathway: about receptors, transcription machinery and microRNA[END_REF][START_REF] Holsboer | The corticosteroid receptor hypothesis of depression[END_REF] .

Whether under basal or stress conditions, corticosterone can indirectly affect transcription through two steroid receptors: the mineralocorticoid receptor (MR), and the glucocorticoid receptor (GR) [START_REF] De Kloet | Brain Corticosteroid Receptor Balance in Health and Disease[END_REF][START_REF] Joëls | Functional actions of corticosteroids in the hippocampus[END_REF][START_REF] Reul | Two receptor systems for corticosterone in rat brain: microdistribution and differential occupation[END_REF] . The MR has a 10-fold higher affinity for binding corticosterone, and thus, is usually fully occupied under basal conditions [START_REF] De Kloet | Brain mineralocorticoid receptor diversity: functional implications[END_REF] . The expression pattern of MRs is more dense in the hippocampus [START_REF] De Kloet | Brain Corticosteroid Receptor Balance in Health and Disease[END_REF][START_REF] Joëls | Functional actions of corticosteroids in the hippocampus[END_REF] , an area of the brain highly relevant for stress and memory [START_REF] Squire | Memory and the hippocampus: a synthesis from findings with rats, monkeys, and humans[END_REF] . When conditions are more stressful and thus corticosterone is at greater levels, it begins to occupy GRs [START_REF] Ferguson | Overexpression of mineralocorticoid and transdominant glucocorticoid receptor blocks the impairing effects of glucocorticoids on memory[END_REF][START_REF] Gunnar | Brain and behavior interface: stress and the developing brain[END_REF] . These receptors are ubiquitously expressed throughout the brain, with higher expression density in the limbic regions, particularly the hippocampus [START_REF] De Kloet | Brain Corticosteroid Receptor Balance in Health and Disease[END_REF][START_REF] Joëls | Functional actions of corticosteroids in the hippocampus[END_REF] . Both MR and GR can be found in the cytosol and upon binding corticosterone, translocate into the nucleus where they can directly promote or inhibit transcription (Figure 3) [START_REF] De Kloet | Brain Corticosteroid Receptor Balance in Health and Disease[END_REF][START_REF] Tronche | Genetic dissection of glucocorticoid receptor function in mice[END_REF] .

Under basal conditions, the HPA axis functions in a circadian rhythm on a 24-hour cycle [START_REF] Mcewen | Stress, adaptation, and disease. Allostasis and allostatic load[END_REF][START_REF] De Kloet | Brain Corticosteroid Receptor Balance in Health and Disease[END_REF] . This typically consists of lower corticosterone secretion as the animals go to sleep and greater secretion as the animals begin to wake up (Figure 2). Although this secretion is pulsatile, averages lie predictably along this rhythm [START_REF] Santarelli | Requirement of hippocampal neurogenesis for the behavioral effects of antidepressants[END_REF][START_REF] David | Neurogenesis-dependent and -independent effects of fluoxetine in an animal model of anxiety/depression[END_REF][START_REF] Sahay | Increasing adult hippocampal neurogenesis is sufficient to improve pattern separation[END_REF][START_REF] Surget | Antidepressants recruit new neurons to improve stress response regulation[END_REF][START_REF] Kitamura | Adult neurogenesis modulates the hippocampus-dependent period of associative fear memory[END_REF] .

GR is comprised of an N-terminus region, followed by a DNA-binding domain (DBD), a hinge region, a ligand-binding domain (LBD), and a C-terminus (Figure 4) [START_REF] Hollenberg | Multiple and cooperative trans-activation domains of the human glucocorticoid receptor[END_REF][START_REF] Tao | Mapping of glucocorticoid receptor DNA binding domain surfaces contributing to transrepression of NF-kappa B and induction of apoptosis[END_REF][START_REF] Tissing | Molecular determinants of glucocorticoid sensitivity and resistance in acute lymphoblastic leukemia[END_REF] . In the cytosol, GR is already bound to other chaperones inhibiting its DBD from being exposed [START_REF] Kawata | Steroid receptor signalling in the brain--lessons learned from molecular imaging[END_REF] . When corticosterone binds to the LBD, GR changes its conformational structure, releasing previously bound chaperones, and allowing it to dimerize and pass into the nucleus [START_REF] Luisi | Crystallographic analysis of the interaction of the glucocorticoid receptor with DNA[END_REF][START_REF] Reichardt | DNA binding of the glucocorticoid receptor is not essential for survival[END_REF] . Here it can directly affect transcription by binding onto DNA fragments known as glucocorticoidresponse-elements (GRE), located upstream of a gene promoter (Figure 3) [START_REF] Zilliacus | Structural determinants of DNA-binding specificity by steroid receptors[END_REF] . By binding to GREs, GR can Neurons in the hypothalamus release CRH and arginine vasopressin (AVP), which induce the secretion of ACTH from the pituitary. ACTH then triggers the adrenal cortex to produce glucocorticoids known as corticosteroids (cort).

Corticosterone can pass through the blood brain barrier and interact with two receptors, MR and GR. Activation of these receptors can trigger feedback loops that can inhibit further activity of the HPA axis and return the system to a homeostatic point. Photo credit [START_REF] Lupien | Effects of stress throughout the lifespan on the brain, behaviour and cognition[END_REF] .

promote or inhibit (negative GREs) transcription of RNA that codes for proteins. Alternatively, GR can bind to other nuclear transcription factors, sequestering their activity, and thus, indirectly promoting or inhibiting transcription. Without the actions of this steroid receptor, the animal could not survive, as we know from genetic mutations that GR gene inactivation is lethal at birth [START_REF] Tronche | Genetic dissection of glucocorticoid receptor function in mice[END_REF] .

Although the focus of this research is on GR-mediated stress, there are other stress-related molecules that may play a role in the stress response, including dopamine, serotonin, BDNF, VEGF, glutamate, and NMDA [START_REF] Saaltink | Stress, glucocorticoid receptors, and adult neurogenesis: a balance between excitation and inhibition?[END_REF][START_REF] Schoenfeld | Stress, stress hormones, and adult neurogenesis[END_REF] . Despite these other mediators of the stress response, we focused on corticosterone because of its robust effect on NPCs. While mature neurons are found to express both MR and GR, newborn neuronal cells only express GR (see Figure 5) [START_REF] Garcia | Age-dependent expression of glucocorticoid-and mineralocorticoid receptors on neural precursor cell populations in the adult murine hippocampus[END_REF] . Thus, it may not be surprising that high levels During rodents' night cycle (awake), corticosterone secretion is higher and as they enter their light cycle (sleep), levels begin to decline. Photo credit [START_REF] Rankin | Characterizing dynamic interactions between ultradian glucocorticoid rhythmicity and acute stress using the phase response curve[END_REF] .

Figure 3. The glucocorticoid receptor can regulate gene transcription. Due to their size and lipophilic nature, glucocorticoids (i.e. cort) can pass through the cell membrane. When they bind to GR, this induces a conformational change that releases GR from a complex with heat-shock proteins (hsp) allowing GR to translocate into the nucleus. Here, GR can affect transcription as a dimer bound to GREs, as well as a monomer, interfering with other factors (e.g. AP-1 and NF-kB). Photo credit [START_REF] De Kloet | Brain mineralocorticoid receptor diversity: functional implications[END_REF] .

of circulating corticosterone, such as that caused by environmental stressors, affect the development of these cells.

Neurogenesis in the hippocampus

During brain development, neural stem cells, responding to both internal and external cues, multiply (proliferate), migrate, and mature (differentiate) into neurons (a process referred to as neurogenesis) or glia (referred to as gliogenesis) (for review [START_REF] Temple | The development of neural stem cells[END_REF] ). Gliogenesis, more specifically, refers to astrocytes (astrogenesis) or oligodendrocytes (oligodendrogenesis). These mature cells are some of the main players that constitute the brain and contribute to its function. This developmental process goes through stages that become increasingly more restrictive in cell-type and self-renewal [START_REF] Gage | Mammalian neural stem cells[END_REF] . It culminates with the mature cell, neuron or glia, that is fully differentiated and no longer mitotic (Figure 5) [START_REF] Kempermann | Milestones of neuronal development in the adult hippocampus[END_REF] . Referred to broadly as neurogenesis (since most cells become neurons), this process occurs in the pre-or early post-natal stages of all vertebrate mammals [START_REF] Morsink | The dynamic pattern of glucocorticoid receptor-mediated transcriptional responses in neuronal PC12 cells[END_REF][START_REF] Heck | A distinct modulating domain in glucocorticoid receptor monomers in the repression of activity of the transcription factor AP-1[END_REF][START_REF] Beato | Control of transcription by steroid hormones[END_REF][START_REF] Zhao | Mechanisms and functional implications of adult neurogenesis[END_REF][START_REF] Aimone | Adult neurogenesis: integrating theories and separating functions[END_REF][START_REF] Cayre | The common properties of neurogenesis in the adult brain: from invertebrates to vertebrates[END_REF] .

Grandfather of neuroscience, Ramón y Cajal, established the long-held dogma that after this initial period, the brain no longer retained this regenerative capacity: "In the adult centres, the nerve paths are something fixed, ended, and immutable. Everything may die, nothing may be regenerated" 47 .

It was not until the 1960s, with more sophisticated tools and scientific methods, that it was discovered that neurogenesis takes place in the adult brain as well. Although the results were not widely accepted, Altman and colleagues (1965) first presented evidence of mammalian adult neurogenesis using autoradiography and light microscopy of general cytological stains [START_REF] Altman | Autoradiographic and histological evidence of postnatal hippocampal neurogenesis in rats[END_REF] . This result was further supported by combining autoradiography and high-resolution electron microscopy to show evidence that adult-born cells with tritiated thymidine uptake, a marker of cell division, exhibited definite neuronal morphology [START_REF] Kaplan | Neurogenesis in the adult rat: electron microscopic analysis of light radioautographs[END_REF] . Still, mammalian adult neurogenesis was not firmly established until the 1990s, when the use of more advanced techniques in cell culturing and immunohistochemistry provided evidence of adult neural progenitor cells that were multipotent in vitro [START_REF] Lois | Proliferating subventricular zone cells in the adult mammalian forebrain can differentiate into neurons and glia[END_REF][START_REF] Ray | Proliferation, differentiation, and long-term culture of primary hippocampal neurons[END_REF][START_REF] Reynolds | Generation of neurons and astrocytes from isolated cells of the adult mammalian central nervous system[END_REF] , and in vivo [START_REF] Gould | Adrenal hormones suppress cell division in the adult rat dentate gyrus[END_REF][START_REF] Eriksson | Neurogenesis in the adult human hippocampus[END_REF][START_REF] Kuhn | Neurogenesis in the dentate gyrus of the adult rat: age-related decrease of neuronal progenitor proliferation[END_REF][START_REF] Gage | Multipotent progenitor cells in the adult dentate gyrus[END_REF][START_REF] Palmer | FGF-2-responsive neuronal progenitors reside in proliferative and quiescent regions of the adult rodent brain[END_REF][START_REF] Richards | De novo generation of neuronal cells from the adult mouse brain[END_REF] . Now widely accepted, adult neurogenesis has been demonstrated across species, including bird [START_REF] Nottebohm | From bird song to neurogenesis[END_REF] , tree shrew 60 , mouse [START_REF] Van Praag | Running increases cell proliferation and neurogenesis in the adult mouse dentate gyrus[END_REF] , rats [START_REF] Kaplan | Neurogenesis in the adult rat: electron microscopic analysis of light radioautographs[END_REF][START_REF] Gould | Adrenal hormones suppress cell division in the adult rat dentate gyrus[END_REF][START_REF] Woolley | Differentiation of newly born neurons and glia in the dentate gyrus of the adult rat[END_REF] , monkeys [START_REF] Gould | Neurogenesis in the neocortex of adult primates[END_REF] , and humans [START_REF] Eriksson | Neurogenesis in the adult human hippocampus[END_REF][START_REF] Gross | Neurogenesis in the adult brain: death of a dogma[END_REF][START_REF] Curtis | Progenitor cells and adult neurogenesis in neurodegenerative diseases and injuries of the basal ganglia[END_REF] . Not only did the discovery of this phenomenon overturn a long-held central dogma of neuroscience, but it also enlightened us to a new form of adult plasticity that could contribute to learning processes and be a potential source for treating damage to, disorders, or diseases of the brain.

There are two locations where adult neurogenesis is found to occur: the subventricular zone (SVZ) of the lateral ventricles, and the subgranular zone (SGZ) of the hippocampal dentate gyrus (DG) [START_REF] Woolley | Differentiation of newly born neurons and glia in the dentate gyrus of the adult rat[END_REF][START_REF] Corotto | Neurogenesis persists in the subependymal layer of the adult mouse brain[END_REF] . Within the hippocampus, as cells proliferate, some daughter cells become NPCs which further mature into neurons (approximately 70-90%), or other glial cells (~10%) [START_REF] Schoenfeld | Stress, stress hormones, and adult neurogenesis[END_REF][START_REF] Woolley | Differentiation of newly born neurons and glia in the dentate gyrus of the adult rat[END_REF][START_REF] Steiner | Differential regulation of gliogenesis in the context of adult hippocampal neurogenesis in mice[END_REF][START_REF] Cameron | Adult neurogenesis produces a large pool of new granule cells in the dentate gyrus[END_REF][START_REF] Abrous | Adult neurogenesis: from precursors to network and physiology[END_REF] . Typically, these maturing cells migrate into the granular cell layer, differentiate into granule neurons, and integrate into the circuitry by extending the appropriate projections to their CA3 target area [START_REF] Markakis | Adult-generated neurons in the dentate gyrus send axonal projections to field CA3 and are surrounded by synaptic vesicles[END_REF] and acquiring electrophysiological properties that make them indistinguishable from the adjacent, older neurons (Figure 5) [START_REF] Van Praag | Functional neurogenesis in the adult hippocampus[END_REF] . Across species, the rate and degree of neurogenesis is varied. For example, rats have ~9000 newborn cells surviving after 1 week and ~70% of these cells are already neurons by 2 weeks; whereas mice have ~3000 newborn cells surviving after 1 week (although they have similar levels of newborn cells per area to rats at this time) and still, less than 50% have developed into mature neurons by the 4 week time point [START_REF] Snyder | Adult-born hippocampal neurons are more numerous, faster maturing, and more involved in behavior in rats than in mice[END_REF] . Nonetheless, in a matter of weeks depending on the species, adult NPCs can become functionally responsive neurons [START_REF] Snyder | Adult-born hippocampal neurons are more numerous, faster maturing, and more involved in behavior in rats than in mice[END_REF][START_REF] Liu | Generation of functional inhibitory neurons in the adult rat hippocampus[END_REF] . Although NPCs are restricted to two discrete brain regions and comprise less than 10% of the total DG neuronal population [START_REF] Cameron | Adult neurogenesis produces a large pool of new granule cells in the dentate gyrus[END_REF] , their long-term, regular self-renewal has remained a conserved mechanism across species. Understandably, this begs the question of what significant role they play in brain function.

Measuring neurogenesis

There are different methods of measuring neurogenesis. Four of these strategies involve labeling newborn cells through the use of exogenous markers, endogenous markers, retro-viral markers, or genetic techniques. Exogenous labeling of proliferative cells involves injecting a solution of either tritiated thymidine, or a thymidine analog called 5'-bromo-2'-deoxyuridane (BrdU), which incorporates itself during DNA replication by replacing the thymidine nucleoside [START_REF] Duque | Different effects of bromodeoxyuridine and [3H]thymidine incorporation into DNA on cell proliferation, position, and fate[END_REF][START_REF] Taupin | BrdU immunohistochemistry for studying adult neurogenesis: paradigms, pitfalls, limitations, and validation[END_REF] . Thus, when BrdU is present, any replicating cell in S-phase will take -4), which proliferate more rapidly, although with limited self-renewal, and lineage is more restricted. Note that type-2b cells were not found to express GR. The next stage marks the early postmitotic period, where cells differentiate and migrate into the GCL. Finally, the last two postmitotic stages are characterized by NeuN expression and spine formation. This progresses until the cell is functionally integrated, extending its axons to the CA3 region, and is now a mature granule cell with large and complex synapses. Adapted from [START_REF] Saaltink | Stress, glucocorticoid receptors, and adult neurogenesis: a balance between excitation and inhibition?[END_REF][START_REF] Garcia | Age-dependent expression of glucocorticoid-and mineralocorticoid receptors on neural precursor cell populations in the adult murine hippocampus[END_REF][START_REF] Kempermann | Milestones of neuronal development in the adult hippocampus[END_REF][START_REF] Zhao | Mechanisms and functional implications of adult neurogenesis[END_REF][START_REF] Hooijdonk | Glucocorticoid receptor knockdown and adult hippocampal neurogenesis[END_REF] .

up this analog into its DNA structure and by using immunofluorescence, anti-BrdU antibody can identify them throughout their lifecycle. By thoughtfully planning the paradigm and temporal order of events, one can examine proliferation, survival, and differentiation of the cells. For example, differentiation can be qualitatively and quantitatively measured with anti-BrdU co-labeled with other cell markers if the timing between BrdU administration and tissue fixation provided enough time for the newborn cell to mature. These other cell markers can identify distinct or overlapping stages of development, such as nestin for progenitor cells, doublecortin (DCX) for immature neurons, or NeuN for mature neurons (Figure 5). By co-labeling with antibodies for distinct cell types, one can for track cell fate.

Another method of measuring neurogenesis involves immunofluorescence of endogenous markers of cell division such as Ki67 or proliferating cell nuclear antigen (PCNA) [START_REF] Taupin | BrdU immunohistochemistry for studying adult neurogenesis: paradigms, pitfalls, limitations, and validation[END_REF][START_REF] Cooper-Kuhn | Is it all DNA repair? Methodological considerations for detecting neurogenesis in the adult brain[END_REF] . Ki67 is a protein that is naturally expressed during the S-phase of the cell division cycle [START_REF] Dayer | Short-term and long-term survival of new neurons in the rat dentate gyrus[END_REF][START_REF] Kee | The utility of Ki-67 and BrdU as proliferative markers of adult neurogenesis[END_REF] . PCNA is a protein that also naturally exists during cell division [START_REF] Cameron | Distinct populations of cells in the adult dentate gyrus undergo mitosis or apoptosis in response to adrenalectomy[END_REF] . By immuno-labeling with antibodies against themselves, one can quantitatively and qualitatively measure proliferation. A drawback of this method, however, is that the cell is only identified while replicating, making survival or differentiation measures impossible. Still, it is a less-invasive and simpler way of examining proliferation at the time of death.

Labeling of adult neurogenesis is also possible using particular retroviruses. These retroviruses are a type of virus that only enters the cell when it is dividing. Thus, this method only targets proliferating cells. If the retrovirus is bound to a fluorescent protein, similar to the BrdU method, when the virus is present, any replicating cell will be infected and using immunofluorescence, it can be identified throughout its lifecycle [START_REF] Zhao | Mechanisms and functional implications of adult neurogenesis[END_REF][START_REF] Van Praag | Functional neurogenesis in the adult hippocampus[END_REF] .

The last method mentioned includes genetic techniques such as Cre/lox recombination. Cre is a recombinase protein that can be inserted into the promoter region of any gene such that when the gene is being expressed, Cre will be active concomitantly [START_REF] Reichardt | Molecular genetic analysis of glucocorticoid signaling using the Cre/loxP system[END_REF] . When active, Cre locates and excises any DNA contained between (floxed by) head-to-tail loxP fragments. Then Cre recombines the genetic coding (minus the floxed DNA segment) and from that point onward, the cell's genetic code is permanently altered, including its progeny [START_REF] Carlén | Genetic visualization of neurogenesis[END_REF] . Both Cre and loxP are genetic manipulations not occurring naturally in mammals. One way this technique can be used to label progenitor cells could be by using a mouse that has a gene with Cre in the promoter region of a proliferative cell marker (i.e. nestin) and a gene with loxP floxing a STOP codon preceding a yellow fluorescent protein (YFP). The Cre will remove the STOP codon, causing YFP to be visible only in nestin-expressing cells. This method, although more intricate, would label all nestin-expressing cells from birth. A more clever alternative technique is through the use of CreERT2, a form of Cre that is inducible at any selected time point [START_REF] Imayoshi | Temporal regulation of Cre recombinase activity in neural stem cells[END_REF][START_REF] Lagace | Dynamic contribution of nestin-expressing stem cells to adult neurogenesis[END_REF][START_REF] Chen | Inducible site-specific recombination in neural stem/progenitor cells[END_REF] . This type of Cre is controlled by a mutated estrogen receptor (ERT2) that is only active when induced exogenously by an estrogen analog such as tamoxifen [START_REF] Chen | Inducible site-specific recombination in neural stem/progenitor cells[END_REF][START_REF] Schwenk | Temporally and spatially regulated somatic mutagenesis in mice[END_REF] . Unlike Cre, which becomes active as soon as nestin (for example) is expressed, CreERT2 becomes active when both tamoxifen is bound to its ERT2 and nestin is expressed. Thus, using this inducible CreERT2 genetic technique, proliferative cells can be labeled and tracked for proliferative, survival, and differentiation analyses [START_REF] Lagace | Dynamic contribution of nestin-expressing stem cells to adult neurogenesis[END_REF][START_REF] Kellendonk | Inducible site-specific recombination in the brain[END_REF][START_REF] Bonaguidi | In vivo clonal analysis reveals self-renewing and multipotent adult neural stem cell characteristics[END_REF][START_REF] Imayoshi | Genetic methods to identify and manipulate newly born neurons in the adult brain[END_REF] .

Stress-induced regulation of neurogenesis

Both the stress response and neurogenesis appear to be well-conserved mechanisms that provide an organism with the ability to cope and perhaps more importantly, adapt to future occurrences [START_REF] Hooijdonk | Glucocorticoid receptor knockdown and adult hippocampal neurogenesis[END_REF][START_REF] Champagne | Fundamental aspects of the impact of glucocorticoids on the (immature) brain[END_REF][START_REF] Petrik | The neurogenesis hypothesis of affective and anxiety disorders: are we mistaking the scaffolding for the building?[END_REF] . It is not so surprising then that the hippocampus is the brain structure where the stress-sensitive limbic system and one of two neurogenic niches converge. The hippocampus is critically involved in both learning and memory, two plastic processes that necessitate the continuous need to receive, reorganize, relay, and readjust to new information [START_REF] Squire | Memory and the hippocampus: a synthesis from findings with rats, monkeys, and humans[END_REF][START_REF] Chauveau | The hippocampus and prefrontal cortex are differentially involved in serial memory retrieval in non-stress and stress conditions[END_REF][START_REF] Mcclelland | Why there are complementary learning systems in the hippocampus and neocortex: insights from the successes and failures of connectionist models of learning and memory[END_REF][START_REF] Rosenbaum | The case of K.C.: contributions of a memory-impaired person to memory theory[END_REF] . The regenerating NPCs of the hippocampus are well situated to react to the environment as they are found predominantly in the direct vicinity of the vasculature [START_REF] Palmer | Vascular niche for adult hippocampal neurogenesis[END_REF] . This would allow for stress hormones circulating in the blood to quickly communicate with these regenerating cells. Thus, one of the first discovered and widely studied mediators of neurogenesis was stress [START_REF] Gould | Adrenal steroids regulate postnatal development of the rat dentate gyrus: I. Effects of glucocorticoids on cell death[END_REF] .

Neurogenesis is not only regulated by stress, but also other stimuli that are associated with elevated corticosterone, such as voluntary exercise, learning tasks, and enriched environments [START_REF] Brown | Enriched environment and physical activity stimulate hippocampal but not olfactory bulb neurogenesis[END_REF][START_REF] Kronenberg | Physical exercise prevents age-related decline in precursor cell activity in the mouse dentate gyrus[END_REF][START_REF] Leuner | Learning enhances the survival of new neurons beyond the time when the hippocampus is required for memory[END_REF][100][101] . The effects of stress on neurogenesis can be during the proliferation, survival, or differentiation of the cell. The responses can vary depending on the type of stressor (i.e. restraint, predator smell, tail suspension) and the duration of its application (i.e. acute, chronic) 102 . Each stage of neurogenesis appears to be a plastic progression with the potential to be influenced by stress and corticosterone.

Stress and glucocorticoids affect cell proliferation

The first stage in neurogenesis is when the cell undergoes mitosis and multiplies itself. This proliferative stage can be sensitive to environmental stressors depending on the duration of the stress. The effect of acute and chronic stress has been studied in a variety of animals, including mouse, rat, tree shrew, and marmoset monkey, and is generally found to be inhibitory.

Acute

Many studies have found that acute stress causes decreases in proliferation [START_REF] Sapolsky | How do glucocorticoids influence stress responses? Integrating permissive, suppressive, stimulatory, and preparative actions[END_REF]103 . Acute stress usually involves one instance or one day of a stressful paradigm. In one study, rats were exposed to odors from a fox, a natural predator, or nonthreatening stimuli (mint or orange). Only acute exposure to fox odor was found to decrease the number of newborn cells 104 . Additionally, rats underwent adrenalectomies to remove glucocorticoids from their system, and then were given low levels of corticosterone for maintaining normal functioning. When these rats were exposed to the odors now, there was no effect on proliferation, implying that the stress effect suppressing proliferation was driven by raised corticosterone levels 104 . Other researchers confirmed these results, finding predator odor caused inhibition of proliferation 105,106 . Decreased proliferation was also seen in acute exposure to a social defeat paradigm 107 , but not always significantly 108 . A psychosocial stress, also referred to as resident-intruder, was found to inhibit proliferation of NPCs [START_REF] Gould | Neurogenesis in the dentate gyrus of the adult tree shrew is regulated by psychosocial stress and NMDA receptor activation[END_REF]109 , as well as an acute duration of unpredictable stressors 110 , such as forced cold swim and cold immobilization. Similarly, after a day of inescapable shock, proliferation again was shown to be suppressed 111 . During early life, rat pups exposed to an adult male odor showed suppressed proliferation 112 . While all these studies refer to suppressed neurogenesis in the SGZ of the hippocampus, neurogenesis in the SVZ has also been examined but found to show no effect of acute stress on cell production 111,113 . This suggests that the effect of stress on proliferation is specific to the hippocampus and not due to the uptake or labeling of BrdU. The duration of the decreased proliferation may vary if experienced during the early postnatal stage or adulthood. In one study that looked at rat pups, they found that early life stress from maternal separation caused reduced proliferation rates that lasted into adulthood 113 However, other groups found that acute stress effects on proliferation normalized within 24 hours, but these were performed on adult mice 107,114 .

Although many studies show acute stress induces inhibition of newborn neurons, there are several studies that do not confirm those results. There was no change found in proliferation in two studies of acute restraint on rats 115,116 . Likewise, acute stress from both psychosocial stress or a predator odor were not always found to suppress proliferation 117,118 , although these studies were criticized for administering BrdU shortly before the stressor, and thus were not a direct measure of proliferation change caused by the stressor [START_REF] Schoenfeld | Stress, stress hormones, and adult neurogenesis[END_REF] .

Recently, Kaufer and colleagues (2013) found that acute immobilization stress increased proliferation 119 . In this study, researchers were sensitive to pre-handling procedures that prepared both experimental and control rodents for handling the day of the stress, which may have made a significant difference as well. Increased proliferation from acute stress is also found in female rodents 110,120 and attributed to estradiol effects that are neurogenesis enhancing. In one of these studies, when female rats were exposed to predator odor, there was no change seen in proliferation, but after an ovariectomy and exogenous estradiol was added, there was an increase in cell birth 120 . Overall, it seems as though acute stress can regulate proliferation, although these effects are sensitive to the type of stressor experienced, the protocol of the paradigm, the age of the animal, the species being studied, and the time course in which proliferation was examined.

Chronic

While acute stress effects on proliferation appear to be variable, chronic stress appears to be a strong inhibitor of proliferation. Chronic stress refers to any stressor experienced repeatedly over a course of a few days to several weeks. In a chronic unpredictable stress paradigm, that can include various stressors such as forced cold swim, cold immobilization, isolation, vibration, shaking, overcrowding, wet bedding, restraint, odors, altered light schedules, or strobe lighting, rodents were found to have significant reductions in the birth of new neurons 110,[121][122][123][124] Chronic restraint for 6 hours per day for 2-3 weeks also induced inhibition of cell proliferation 115,116 . Likewise, reduced proliferation was found after chronic psychosocial stress [125][126][127] as well as chronic shock exposure 128 , and repeated social defeat 108 . During early life, rat pups that had prolonged maternal deprivation were found to have significantly reduced basal proliferation rates as adults 113 .

These effects appear to last longer than those from acute stress. Chronic stress experienced during early life exhibits inhibited proliferation into adulthood 113 while chronic stress experienced as an adult appears to recover from inhibited cell birth after 3 weeks 114,123 .

Whether acute or chronic, the question remains whether decreases in proliferation are due to a slowing or a pausing of the cell cycle, or if the cells are exiting the cell cycle or dying. This question was investigated and researchers found that when dexamethasone, a GR-specific synthetic glucocorticoid, was applied in vitro, it concomitantly reduced proliferation and increased p21 protein 129 . P21 is an enzyme involved in cell-cycle arrest by inhibiting progression from the G1 to the S phase in the cell cycle 130 . Similarly, increased p21 expression was found in NPCs 131 as well as HT-22 cells exposed to dexamethasone 132 . In one study in vivo, while stress was found to downregulate newborn cell births, this correlated with an upregulation in p27Kip1, another enzyme of cell cycle arrest 130 . Although the pathway in which stress or glucocorticoids enacts its effects on cell proliferation is still unknown, particularly whether its effect is directly or indirectly through NPCs, these results suggest that the stress and GC-induced inhibition of proliferation is mediated by a slowing of the G1-S phase in a cell cycle, and not just due to cell cycle exit.

Stress and glucocorticoids affect cell survival

Although thousands of newborn neurons are born each day, approximately 50% of these cells die within 3 weeks [START_REF] Cameron | Adult neurogenesis produces a large pool of new granule cells in the dentate gyrus[END_REF][START_REF] Dayer | Short-term and long-term survival of new neurons in the rat dentate gyrus[END_REF] . This stage is referred to as survival, and this pruning process is also sensitive to environmental input 133,134 .

Acute

Results from measurements of cell apoptosis after an acute stressor are conflicting. Cell survival was measured at different time points post-BrdU uptake from rats exposed to an acute psychosocial stressor 118 .

These time points assessed the immediate, short-term, and long-term survival of newborn neurons. The study found no change in survival rates immediately after BrdU administration, but did find a decrease in both the short-term and long-term survival rates 118 . Similarly, rats exposed to acute unpredictable stress had increased apoptosis in the hilus, SGZ, and granule cell layer (GCL) of the hippocampus 110 . The opposite effect, however, was seen in rats exposed to acute predator odor 120 . In this study, the acute stressor suppressed cell death in the DG, and thus survival rates were higher than controls 120 . In addition to decreased and increased survival rates from acute stressors, some studies find no change at all 107,121 , although these were performed with mice, suggesting that stress effects are more deleterious on particular species. The variability in the intensity, duration, and protocol of the stressor, along with differences in BrdU time course and measurement methods can perhaps account for the conflicting results found from stress on cell death.

Although most experiments are done with male rodents to control for the neurogenic enhancing effects of estrogen in females 135 , one study also tested cell survival rates in females exposed to the same predator odor as males were and found no change 120 . This emphasizes sex differences in stress-induced effects on neurogenesis, particularly since ovariectomized female rodents had vast cell death in the hippocampus that was ameliorated by exogenous estradiol hormone 120 . This could imply that any suppressive effect on survival induced by stress may be counteracted by the enhancing effect of estrogen.

Chronic

The effects of chronic stress on cell survival appear to be clearer. Cell death was increased in the DG of adult rats after experiencing chronic stress paradigms such as psychosocial stress 125 , restraint stress 115 , and unpredictable variable stress 110,136,137 . One study that measured cell death determined that the effects were restricted to the GCL, and not seen in the hilus 136 . Interestingly, when a GR antagonist, mifeprisone, was administered for 4 days after the chronic unpredictable variable stress paradigm, cell survival increased 137 .

This suggests that the suppressive effects on cell survival induced by chronic stress are mediated through the GR. These effects may be similar as well if stress is experienced in early life. Gould and colleagues examined rats that experienced prolonged maternal deprivation as pups and found that these pups also had lower survival rates for newborn neurons in adulthood 113 . This effect persisted for 1 week, however, dissipating after 3 weeks 113 . Studies on cell survival after chronic stress all seem to concur that stress, perhaps mediated through the GR, increase apoptosis of newborn NPCs.

Stress and glucocorticoids affect cell differentiation

In the rodent hippocampus, after NPCs proliferate and survive the pruning process, they begin to take on morphological, molecular, and functional characteristics of more mature cell types [START_REF] Zhao | Mechanisms and functional implications of adult neurogenesis[END_REF] . This process is referred to as differentiation and typically, 70-90% of these surviving cells become granule neurons [START_REF] Cameron | Adult neurogenesis produces a large pool of new granule cells in the dentate gyrus[END_REF][START_REF] Abrous | Adult neurogenesis: from precursors to network and physiology[END_REF] .

Although most of the research on stress effects on neurogenesis point to the proliferation stage as the mediator of neurogenic changes, differentiation appears to also be sensitive to environmental input.

Acute

Unlike the proliferation stage, there are not many studies showing an effect of acute stress on cell fate. Adult rats subjected to acute restraint were found to have no significant differences in cell differentiation 115 . Similarly, rats exposed to an acute psychosocial stressor had no changes in percentage of BrdU-positive cells that co-labeled with mature neuronal markers 118 . Since there was an overall significant reduction in the number of neurons, but not in the percentage of newborn cells that differentiated into neurons, this would indicate that changes in neurogenesis from acute stressors probably derive from the initial changes seen on proliferation.

Chronic

The effects of chronic stress on cell differentiation appear to vary from decreased neuronal fate or no change in cell fate. Both adult rats and 3-week-old mouse pups subjected to chronic restraint showed reduced neuronal differentiation 115,138 . Mice that experienced chronic social isolation displayed suppressed neuronal differentiation as well in both the GCL and SGZ, but not the hilus 139 . Likewise, exposure to a chronic shock paradigm reduced neural cell fate in rats 128 , although not always 140 . Chronic exposure to a psychosocial stressor showed suppressed neuronal differentiation in tree shrews 127 , but no effect in rats 125 . The effects on neuronal differentiation from chronic unpredictable stressors or chronic mild stressors, which can include cage tilting, wet bedding, predator sounds, empty cages with water on the bottom, reversal of the light/dark cycle, sporadic light changes, restraint, forced cold swim, water deprivation, pairing with a stressed littermate, or cage switching, were found to be nonsignificant in rats 114,136 , but significantly reduced in mice 141,142 . One study that looked at 7 weeks of chronic mild stress on mice, however, did not see this reduction as both stressed and control groups had 73% of BrdU positive cells co-labeling with NeuN 143 . One study examined a similar paradigm on three strains of mice and found a significant suppression of neuronal fate in both the males and females of all strains 142 . None of these studies found any shift in astrocytic cell fate; however, a recent study found that chronic restraint stress not only decreased neuronal differentiation in rats, but increased oligodendrogenesis 144 . In this study, not only stress induced this shift in cell fate from neurons to oligodendrocytes, but also lineage tracing of NPCs in vivo showed that after administering corticosterone, oligodendrocytic fate was increased, implying that this stress effect is a cort-mediated mechanism 144 . Overall, it appears that while acute stress does not change the fate of newborn cells, chronic stress can often reduce or alter the differentiation of newborn cells through elevated corticosterone.

Other environmental factors regulating neurogenesis

As evident from stress effects, all stages of neurogenesis appear sensitive to environmental factors, although the degree and direction of the effect is sensitive to the duration and the context of the stressor.

Interestingly, while many of the studies suggest the underlying mechanism involved is cort-mediated, it is clearly more complicated than an indirect relationship between corticosterone levels and neurogenesis production. Like stress, corticosterone levels are also increased after other environmental stimuli such as learning, environmental enrichment, and voluntary exercise [START_REF] Brown | Enriched environment and physical activity stimulate hippocampal but not olfactory bulb neurogenesis[END_REF][START_REF] Kronenberg | Physical exercise prevents age-related decline in precursor cell activity in the mouse dentate gyrus[END_REF][START_REF] Leuner | Learning enhances the survival of new neurons beyond the time when the hippocampus is required for memory[END_REF][100][101] . However unlike stress, these cort-associated stimuli have a more enhancing effect on neurogenesis [START_REF] Brown | Enriched environment and physical activity stimulate hippocampal but not olfactory bulb neurogenesis[END_REF][START_REF] Kronenberg | Physical exercise prevents age-related decline in precursor cell activity in the mouse dentate gyrus[END_REF][START_REF] Leuner | Learning enhances the survival of new neurons beyond the time when the hippocampus is required for memory[END_REF]101,[145][146][147][148] . Thus, it is suggested that the psychological outlook or context, such as predictability or controllability, of an environmental stimuli impacts cell development.

Learning

Training on hippocampal-dependent learning tasks can alter the number of newborn neurons in the DG, however, the effect can be enhancing or suppressing of cell survival depending on the protocol 149 . In studies on rats, the learning task known as eyeblink trace conditioning, a classical conditioning of the eyeblink response using a trace protocol, was found to increase proliferation and survival specifically in the DG and not in the SVZ [START_REF] Leuner | Learning enhances the survival of new neurons beyond the time when the hippocampus is required for memory[END_REF]150 . There were no changes in differentiation, but interestingly, although corticosterone levels were greater in rats being trained versus untrained, this was not the case among the rats learning the paired associations versus unpaired associations [START_REF] Leuner | Learning enhances the survival of new neurons beyond the time when the hippocampus is required for memory[END_REF] . This may imply that corticosterone is not mediating the learning effect on neurogenesis. During spatial navigation learning in the Morris water maze, rats showed enhanced survival as well 134,150,151 , although this increase was only evident in the rostral part of the external blade of the GCL 134 . This enhancement, however, was only apparent for the short-term survival of newborn neurons, since survival rates actually decreased after a week 151,152 . It is thought that the age of the newborn neurons at the time of training plays a role in whether they survive or die 152 . The duration of learning appears to impact cell survival since more trials in the spatial navigation task, as well as more days of training in a social transmission of food preference task both decrease survival of NPCs 149,152,153 . Whereas one day of training in social transmission of food preference showed increased cell survival, two days of training resulted in reduced survival of newborn neurons 153 . In this study, corticosterone levels were also found to be unchanged between the learners and the controls 153 . Whether it is the age of the newborn cells that matters, or learning induces an initial increase in survival and then greater cell death ensues with time, more research is needed to elucidate in what ways learning regulates the survival of newborn NPCs 149 .

Exercise

Voluntary exercise seems to be a strong inducer of neurogenesis. Although running can be seen as stressful and both stress and exercise increase corticosterone levels 154 , it is important to distinguish involuntary exercise (i.e. forced swim), which has suppressive effects on neurogenesis, from voluntary exercise (i.e. wheel running), which has enhancing effects on neurogenesis 147 . Rodents given access to a running wheel, running approximately 48km per day 146 were compared to rodents with an immobilized running wheel in their cage to control for environment. In each study of exercise-induced changes on neurogenesis, an increase in the number of newborn neurons was significantly greater in the rodents allowed to exercise [START_REF] Van Praag | Running increases cell proliferation and neurogenesis in the adult mouse dentate gyrus[END_REF][START_REF] Brown | Enriched environment and physical activity stimulate hippocampal but not olfactory bulb neurogenesis[END_REF][START_REF] Kronenberg | Physical exercise prevents age-related decline in precursor cell activity in the mouse dentate gyrus[END_REF]145,147 . This number was often greater than double compared to controls 145 . This increase in proliferation went down over time, however, was still greater than age-matched controls, suggesting exercise contributes to successful aging [START_REF] Kronenberg | Physical exercise prevents age-related decline in precursor cell activity in the mouse dentate gyrus[END_REF] . Additionally, the survival of these NPCs was promoted when examined 4 weeks after BrdU injections [START_REF] Van Praag | Running increases cell proliferation and neurogenesis in the adult mouse dentate gyrus[END_REF]146 . Voluntary exercise also increased cell fate toward a neuronal type in most studies [START_REF] Van Praag | Running increases cell proliferation and neurogenesis in the adult mouse dentate gyrus[END_REF][START_REF] Brown | Enriched environment and physical activity stimulate hippocampal but not olfactory bulb neurogenesis[END_REF]145 , but not all 146 . This increased neurogenesis was specific to the hippocampus and not found in the SVZ or olfactory bulb [START_REF] Brown | Enriched environment and physical activity stimulate hippocampal but not olfactory bulb neurogenesis[END_REF] and appears to affect both sexes [START_REF] Van Praag | Running increases cell proliferation and neurogenesis in the adult mouse dentate gyrus[END_REF][START_REF] Brown | Enriched environment and physical activity stimulate hippocampal but not olfactory bulb neurogenesis[END_REF][START_REF] Kronenberg | Physical exercise prevents age-related decline in precursor cell activity in the mouse dentate gyrus[END_REF]145,146 . Thus, it appears that exercise is an environmental stimulus that can promote each developmental stage of neurogenesis.

Environmental enrichment

Neurogenesis can also be modulated by an enriched environment (EE). An EE entails more nesting material, tunnels, toys, a running wheel, and extra food treats, such as cheese, crackers, apples, and popcorn.

When exposed to this environment, while most studies find no changes in proliferation of hippocampal NPCs [START_REF] Brown | Enriched environment and physical activity stimulate hippocampal but not olfactory bulb neurogenesis[END_REF]101,147,148,155 , there was an enhanced survival effect for these newborn neurons for both mice 101,147 and rats 148,156 . Interestingly, there does not appear to be sex differences, as many of these studies showed similar results for both male and female rodents. Although some studies found increased neuronal cell fate due to this stimulating environment [START_REF] Brown | Enriched environment and physical activity stimulate hippocampal but not olfactory bulb neurogenesis[END_REF]147,155 , it was not seen in others 101,148 . This effect was specific to the SGZ of the DG as it was not found in the SVZ or migratory NPCs in the olfactory bulb [START_REF] Brown | Enriched environment and physical activity stimulate hippocampal but not olfactory bulb neurogenesis[END_REF] . Corticosterone levels were found to be increased in both mice and rats exposed to EEs 157,158 , suggesting the effect could be cort-mediated. It is suggested that this enhancement of neurogenesis is either due to the novelty of the environment, since mice continuously living in an EE compared to mice removed from an EE have reduced survival rates 159 , or it is simply from increased motor activity (and not learning), since all cages have running wheels and exercise alone showed similar results 147 .

The role of glucocorticoids in hippocampal neuroplasticity

Environmental stimuli, such as stress, learning, exercise, and enriched environments, are all associated with elevations in corticosterone levels, yet can have opposing effects on neurogenesis. Therefore the mechanisms driving these changes appear to be more complex. It is important thus to distinguish potential mediators, such as corticosterone, and assess how these factors affect neurogenesis in isolation.

Glucocorticoids modulate neurogenesis

Glucocorticoids directly modulate neurogenesis

Although stress is found to reduce neurogenesis in the DG, the underlying mechanisms are far from understood. Substantial evidence suggests that corticosterone plays an important role 103 . Whereas the hippocampus is both richly endowed with adrenal steroid receptors and shows structural and functional changes due to corticosterone manipulations 160 , it is now clear that corticosterone alone mediates neurogenesis. Experiments that removed all endogenous corticosterone by adrenalectomy (ADX) found that proliferation of newborn neurons was increased in both young [START_REF] Gould | Adrenal hormones suppress cell division in the adult rat dentate gyrus[END_REF]161,162 and aged rats 163 . Although ADX generally induced more apoptosis throughout the hippocampus, NPCs had enhanced survival without circulating corticosterone after 4 weeks [START_REF] Gould | Adrenal hormones suppress cell division in the adult rat dentate gyrus[END_REF]164 . Additionally, a lack of corticosterone also suppressed neuronal differentiation [START_REF] Gould | Adrenal hormones suppress cell division in the adult rat dentate gyrus[END_REF]161,163,165 . Another study blocked HPA-axis activity by blocking receptors for corticotrophinreleasing factor (CRF) and vasopressin (V1b), two upstream regulators of corticosterone secretion 143 .

Researchers found that the decrease in the birth and differentiation of newborn cells induced by chronic stress was reversed by this method of inhibiting corticosterone secretion 143 . Adding a low dose of corticosterone post-ADX helps maintain diurnal rhythms, while controlling for any stress-induced surge in corticosterone levels. This would just allow for MR occupation, but suppressed GR activity since there is no rise in the corticosterone levels. In this circumstance, the increase in proliferation from a stressor was prevented [START_REF] Gould | Adrenal hormones suppress cell division in the adult rat dentate gyrus[END_REF]104 although one study reported no change with differentiation 161 . Whereas removing or reducing corticosterone availability promoted neurogenesis, increasing corticosterone availability inhibited neurogenesis 131,161,162,164,165 .

Both acute and chronic treatment with corticosterone reduced proliferation in vivo 119,131,161,162,164,165 and in vitro 119,131 , and was found reduced specifically in the dorsal region and not ventral region of the hippocampus 119 . This reduction of newborn NPCs was ~30% compared to NPC production in controls 162 , but only seemed to occur from corticosterone exposure within the first 18 days of cell birth 164 . Survival rates of NPCs were also decreased by exogenous increases in cort 164 , as well as differentiation to a neural cell fate 131,165 . Thus, it appears clear that corticosterone alone can regulate all stages of neurogenesis.

GR modulates neurogenesis

Studies suggest that the mechanism by which corticosterone mediates neurogenesis is through the GC receptor. As described earlier, corticosterone binds to two receptors, MR and GR, at different affinities.

Whereas MR is fully occupied at basal levels of corticosterone secretion during the diurnal rhythm, GR activity is induced when corticosterone levels are raised, such as that seen during stressful encounters [START_REF] Gunnar | Brain and behavior interface: stress and the developing brain[END_REF][START_REF] De Kloet | Brain mineralocorticoid receptor diversity: functional implications[END_REF]166 .

Additionally, GR is expressed in NPCs and throughout their maturation, whereas MR is only expressed in the mature stage [START_REF] Garcia | Age-dependent expression of glucocorticoid-and mineralocorticoid receptors on neural precursor cell populations in the adult murine hippocampus[END_REF] . Thus, GR appears to be a direct mediator of cort-induced suppression of neurogenesis. When adrenalectomized rats given low doses of corticosterone to maintain basal levels were treated with an MR antagonist, proliferation was enhanced (such as that seen in adrenalectomized-only rats) 167 . When the same rats were treated with a GR antagonist, there were no effects, most likely because GR is not occupied at this level of corticosterone, but when rats were given additional corticosterone, blocking GR reversed the cortinduced suppression of neurogenesis 167 . Similarly, other studies found that pharmalogical blockade of GR with antagonist, mifeprisone, blocked the effects on proliferation, survival, and differentiation from both cortinduced 168 and chronic stress-induced suppression 137,169 . Not only does blocking GR stop suppression of neurogenesis from corticosterone or stress, but also stimulating GR further drives suppression of neurogenesis 129,131,167,170,171 . Several reports have confirmed that treating NPCs in vitro with a GR-specific agonist, namely dexamethasone (Dex), causes reduced proliferation 129,131,167,170 . Suppression of neuronal differentiation is also seen from Dex-treated NPCs 171 , but not always 129 . Overall, it appears that the GR is a potential mediator for the effects of corticosterone on NPCs.

Other cort-driven mediators of neurogenesis

Cort-induced suppression of neurogenesis may directly act through GR, but there are also several other mediators through which corticosterone may indirectly cause this effect. These could be growth and hormone factors, cell cycle inhibitors, or excitatory amino acids like those involved in glutamate signaling.

Although the mechanism is unclear, corticosterone can inhibit expression of growth factors as well as their receptors [172][173][174] . One of the growth factors suppressed by chronic corticosterone treatment is BDNF [175][176][177] .

Interestingly, while corticosterone levels are increased during both stress and exercise, it has opposing effects for these stimuli; BDNF, however, while suppressed during stress, is actually increased during exercise 177 .

Furthermore, changes in BDNF are also correlated with changes in neurogenesis [178][179][180] . Similarly, insulin growth factor (IGF) was found to promote neurogenesis 181 . This growth factor can also be regulated by glucocorticoids 172,182,183 . Other growth and hormone factors that have been shown to be responsive to corticosterone include fibroblast growth factor 2 (FGF2) 119,184,185 , epidermal growth factor 186,187 , and CRF 143 .

Additionally, all of these growth and hormone factors have also been shown to alter the ability of NPCs to proliferate and differentiate (see review [START_REF] Abrous | Adult neurogenesis: from precursors to network and physiology[END_REF] ). Thus, stress and cort-specific effects on hippocampal plasticity may indeed be indirectly mediated by these factors.

The effect of corticosterone on neurogenesis can be derived from its slowing or pausing of the cell cycle during NPC proliferation 102 . Although corticosterone treatment is associated with changes in cell cycle inhibitors, such as cyclin D1, cyclin D2, CdK4, CdK6, p21Cip1, and p27Kip1 [START_REF] Abrous | Adult neurogenesis: from precursors to network and physiology[END_REF]188,189 , it is not clear the pathway in which it is able to mediate them. Despite this, cell cycle inhibitors, while controlling proliferation and development, can thus control neurogenesis as well.

Levels of corticosterone availability also regulate glutamate signaling [START_REF] Saaltink | Stress, glucocorticoid receptors, and adult neurogenesis: a balance between excitation and inhibition?[END_REF]190,191 . Whereas cort-free adrenalectomized rodents have decreased glutamate in the hippocampus 192 , high levels of corticosterone can induce excess glutamate secretion 193 . This excitatory amino acid has been a long-recognized modulator of neurogenesis as well since both stress-and cort-induced suppression of neurogenesis can be saved by blocking glutamate receptors, namely NMDA receptors 162,[194][195][196] . It appears thus that corticosterone may exert its influence on neurogenesis by indirectly acting through glutamate and the NMDA receptor.

Glucocorticoids modulate anxiety and cognitive performance

Although there is a wealth of literature describing how various stress-induced hormonal, morphological, and synaptic changes can affect behavior and cognition, for purposes of this review, I will focus on corticosteroids (for review [START_REF] Gunnar | Brain and behavior interface: stress and the developing brain[END_REF]197 ). Research from both animal and human studies have demonstrated acute and chronic corticosterone influences on anxiety as well as cognitive performance, particularly on memory tasks [START_REF] De Kloet | Stress and cognition: Are corticosteroids good or bad guys?[END_REF][198][199][200][201][202][203] . Furthermore, whereas my research focused on the hippocampal structure, I will describe these cort-associated changes in hippocampal-dependent functions.

Memory

One hippocampal-dependent process that is most impacted by corticosterone is memory. Not only are there many different forms of memory, but also each form can be further broken down into stages 204 . Two forms of memory most affected by adrenal hormones are declarative memory, which describes the storage of facts and events, and associative memory, which describes the forming of relationships between facts and events 204 . All of these measures of memory performance, it should be noted, are sensitive to many factors such as species and strain, time of day tested, task protocol, what is measured in the task, concentration of corticosterone, administration route of corticosterone, and duration of corticosterone treatment. Declarative memory can be subdivided into different phases of acquisition, consolidation, and retrieval 205 . The cortinduced impact on declarative memory is most often measured using spatial maze tasks, and the cort-induced impact on associate memory can be measured by contextual conditioning, trace conditioning, and pattern separation.

Although the literature on declarative memory can appear confusing, it has become apparent that the strongest effect of corticosterone is on the consolidation and retrieval phases of memory 204 . More specifically, increased corticosterone availability, particularly during or after emotionally arousing experiences, leads to enhanced consolidation of memories 206 . In contrast, it also suppresses retrieval of this information if elevated shortly before retention testing 204,205 . When endogenous corticosterone is removed by ADX, studies have found that the consolidation of memory for emotionally arousing events was impaired [207][208][209] . This effect was attenuated if GR was activated with agonist Dex after training 209 . Furthermore, acute corticosterone treatment given post-training improved the consolidation of emotionally arousing memories [209][210][211][212] . Chronically high doses of corticosterone, however, can have less of an effect or even impair consolidation in a dose-dependent inverted-U fashion 203 , although due to the chronic nature of the dosing, it is difficult to tease apart which stage of memory it is acting upon. Overall, these effects are believed to be GR-dependent since blocking GR with intra-cerebroventricular infusions of GR antagonists before or immediately after training impaired spatial memory in a water maze 208,213 and mice with partial GR gene inactivation also show poor memory consolidation 213 . These results clearly show corticosterone, through GR activation, is able to manipulate the consolidation stage of memory.

While consolidation of spatial memories is mostly improved with corticosterone, retrieval of the memory is impaired 166,214 . Memory retention was tested in rats 24 hours after being trained in a spatial navigation task called Morris Water Maze 215 . Rats that received corticosterone treatment shortly before testing displayed impaired retrieval 215 . Humans also show this effect when given stress doses of cortisone one hour before a retrieval test of recently learned verbal material 216 . Similarly, high levels of circulating corticosterone as well as infusions of GR agonist also worsen memory recall of previously learned information 205 . These studies demonstrate that corticosterone impairs memory retrieval, which depends, at least in part, on GR activation.

Another aspect of hippocampal-dependent memory that is affected by corticosterone is associative memory. When examined through trace conditioning, a hippocampal-dependent eyeblink association task, it was shown that acute administration of corticosterone enhanced trace conditioning 30 minutes post-injection, but not 24 hours later 217 . Furthermore, adrenalectomized rats did not demonstrate a stress-induced enhancement, even when low dose corticosterone was added to maintain basal levels 217 . This implies that not only do high levels of corticosterone help improve associative memory, but that it may act through the GR.

Indeed, GR-knockdown mice showed impaired memory association in a contextual fear-conditioning task.

Compared with controls, mice with reduced GR levels in newborn DG cells had reduced context-induced freezing, a measurement of associative memory ability 218 . This can also be compared to rats under chronic corticosterone treatment, which had enhanced freezing to the context 219 . Overall, these studies show that both acutely and chronically increased corticosterone levels help consolidate memories in an associative memory task and that GR plays a role in this enhancement.

Just as important it is to be able to associate events and facts together, it is also important to be able to distinguish between similar events occurring close in time or that contain similar features. This control of associative memory is termed pattern separation. Aging, which is associated with increased corticosterone levels, adversely affects pattern separation in both rodents and humans [220][221][222][223][224][225] . Although still unknown, given evidence of corticosterone enhancing the duration and association of memories, but aging-induced increased corticosterone levels impairing pattern separation, it will be interesting to discover whether corticosterone plays a role in this ability to separate distinct memory patterns.

Anxiety

The association of a fear (real or imagined) with a neutral stimulus can lead to a maladaptive emotional state referred to as anxiety. High levels of corticosterone, like those induced by repeated chronic stress, can play a role in this fearful behavior 226 . These behaviors, induced by maladaptive associations, are There is a vast amount of literature suggesting that corticosterone may play a role in depression (for review [START_REF] De Kloet | Stress and cognition: Are corticosteroids good or bad guys?[END_REF] ). This complex pathology is often associated with increased immobility (and decreased mobility) in the Porsolt forced swim test, a behavioral assay used for screening antidepressant compounds 241,242 . Rodents are presented to an inescapable swim task and assessed for depression-like behaviors such as increased time spent immobile, decreased time spent actively climbing, and decreased latency to become immobile 243 .

Whereas depression and antidepressants are both associated with opposing levels of corticosterone secretion [244][245][246] , results from studies of cort-induced depression-like states are unclear. Rats with no endogenous corticosterone due to ADX spend less time immobile, and acute administration of GR-agonist Dex increases immobility time to that of control levels 247 . This GR-induced increase was not seen in rats given acute corticosterone injections 243 , suggesting acute corticosterone treatment and acute GR activity does not cause greater immobility compared to controls. Yet another study found that corticosterone acutely administered to mice through their drinking water 24 hours prior to forced swim actually reduced immobility, implying corticosterone may even have an antidepressant-like effect 248 . When corticosterone levels are kept chronically elevated, mice showed no differences in the forced swim task [START_REF] David | Neurogenesis-dependent and -independent effects of fluoxetine in an animal model of anxiety/depression[END_REF] , however rats show reduced mobility and increased immobility in a dose-dependent manner 243 . Both brain-specific GR mutant mice and GR antisense mice showed reduced depression-like behavior 239,240 . In contrast, partial GR mutant mice exhibited increased helplessness, a feature of depression, after stress exposure, and overexpression of GR evoked reduced helplessness after stress exposure 249 . While corticosterone, through GR activation, may be involved in depression-like behaviors, assessment of this psychopathology is clearly sensitive to species, corticosterone concentration, treatment duration, method of administration, and protocol parameters.

The role of neurogenesis in hippocampal function

Evidence has been accumulating that describes a role for adult neurogenesis in hippocampal

functioning, yet the exact function is unclear. The contribution of neurogenesis to hippocampal behavior is complicated whereby neurogenic niches along the dorsal-ventral hippocampal axis have differential connectivity, and new neurons at different stages in their maturation will display distinct properties [START_REF] Aimone | Adult neurogenesis: integrating theories and separating functions[END_REF]250,251 .

Regardless, collective research suggests adult neurogenesis is involved in cognitive processes such as learning, memory, and emotional regulation [START_REF] Zhao | Mechanisms and functional implications of adult neurogenesis[END_REF][START_REF] Aimone | Adult neurogenesis: integrating theories and separating functions[END_REF]252 .

Neurogenesis modulates cognitive performance

Albeit controversial, roles for adult neurogenesis in learning and memory have been demonstrated in studies examining trace eyeblink conditioning, trace fear conditioning, contextual fear conditioning, spatial memory, place and object recognition memory, and pattern separation (for review [START_REF] Zhao | Mechanisms and functional implications of adult neurogenesis[END_REF]252 ). Several methods have been employed to ablate or decrease neurogenesis by low-dose irradiation, systemic antimitotic drug treatment, and genetically engineered mice that target neural progenitor cells [START_REF] Zhao | Mechanisms and functional implications of adult neurogenesis[END_REF] . Rats treated with an antimitotic drug to eliminate proliferating cells showed impaired conditioning in the trace eyeblink and trace fear conditioning, but not contextual fear conditioning 253 . Likewise, reduced neurogenesis in conditional tlxknockout mice had no effect on contextual fear conditioning 254 , however other studies have found impaired learning on contextual fear conditioning when examined in irradiated rodents and conditionally-induced glial fibrillary acidic protein (GFAP) knockout mice 255,256 . Effects on spatial memory are also elusive 149 . Irradiation on both newly weaned mice and inducible tlx-knockout adult mice caused poor spatial memory in the Morris water maze, but not in the Barnes maze 254,257 . Conversely, irradiated adult mice had impaired spatial memory in the Barnes maze and not the Morris water maze 258 . Moreover, many other studies did not find differences in spatial learning at all when neurogenesis was reduced or ablated 253,255,[259][260][261] . Deficits in place and object recognition memory were found after irradiation 256,259 , however these effects may be sensitive to the time interval between learning and testing as well as contextual setting since shorter intervals in a simpler context did not produce object recognition impairment in rats that were irradiated nor treated with antimitotic drugs 259,262 . As for pattern separation abilities, increased neurogenesis improved contextual fear discrimination 263 and correlated with better spatial pattern separation 264 . Correspondingly, ablated and reduced neurogenesis impaired spatial pattern separation [265][266][267] . Pattern separation, however, is broadly defined and the role of neurogenesis may be better discerned if this term is conceptualized in terms of memory resolution 268 . Overall, more research is necessary to specifically identify cognitive processes regulated by neurogenesis since current research results vary most likely due to differences in the species, experimental designs, and the assumptions and techniques used to target the contributions of newborn neurons 252 .

NG modulates anxiety and depression

Aside from changes in cognitive abilities, neurogenesis may contribute to emotional behaviors as well 269 . It has been suggested that neurogenesis is involved in depression since blocking the production of newborn neurons also blocked the behavioral response to antidepressants [START_REF] Santarelli | Requirement of hippocampal neurogenesis for the behavioral effects of antidepressants[END_REF][START_REF] David | Neurogenesis-dependent and -independent effects of fluoxetine in an animal model of anxiety/depression[END_REF][START_REF] Surget | Antidepressants recruit new neurons to improve stress response regulation[END_REF]270 . Additionally, some studies have supported the hypothesis that neurogenesis has potential relevance to anxiety disorders 271 . The literature remains controversial though as to whether neurogenesis directly contributes to anxiety or the etiology of depression 269,270 . It is important to note that anxiety and affective disorders (i.e. depression) are diagnostically different and current methods to examine these states look at symptoms and not etiology [START_REF] Petrik | The neurogenesis hypothesis of affective and anxiety disorders: are we mistaking the scaffolding for the building?[END_REF] .

The effects of reduced or ablated neurogenesis on anxiety-and depression-like behaviors appear to depend on whether the animals are naïve or stressed prior to examination [START_REF] Petrik | The neurogenesis hypothesis of affective and anxiety disorders: are we mistaking the scaffolding for the building?[END_REF] . Most of the current research agrees that ablating neurogenesis in naïve or nonstressed rodents does not impact depression-like behaviors [START_REF] Petrik | The neurogenesis hypothesis of affective and anxiety disorders: are we mistaking the scaffolding for the building?[END_REF] . When examining whether a lack of neurogenesis exhibits an anxious phenotype, Abrous and colleagues found that naïve transgenic mice with pro-apoptotic protein over-expressed in newborn neuronal cells showed reduced time spent and number of entries in the open arms of the elevated plus maze, as well as increased latency to emerge and time spent in the cylinder of the light/dark emergence test 272 . These anxiety-related behaviors appear to be due to an increase in the fear of potential threats and not just a decrease in novelty exploration since depleted neurogenesis increased predator avoidance but not novelty exploration 272 . Interestingly, these neurogenesis-depleted mice did not exhibit depressive-like behaviors in the forced swim test. Another study found stressed GFAP knockout mice, which had inhibited neurogenesis, showed increased latency to feed in the novelty-suppressed feeding task; however, these mice did exhibit depressive-like behaviors in the forced swim task and sucrose preference, a test of anhedonia 273 . In contrast, many studies altogether did not find that ablated neurogenesis created an anxious or depressive phenotype 10- 14 . Overall, a meta-analysis of all the current research found that most studies do not show that reduced neurogenesis yields depression-like behavior when the animal is naïve [START_REF] Petrik | The neurogenesis hypothesis of affective and anxiety disorders: are we mistaking the scaffolding for the building?[END_REF]274,275 , although it may yield affective behavior if the animal is stressed [START_REF] Petrik | The neurogenesis hypothesis of affective and anxiety disorders: are we mistaking the scaffolding for the building?[END_REF] . Interestingly, upregulated neurogenesis using a genetic approach had no effect on novelty-suppressed feeding, forced swim, nor open field exploration in both low and high stress conditions [START_REF] Sahay | Increasing adult hippocampal neurogenesis is sufficient to improve pattern separation[END_REF] . It is still also unclear whether intact neurogenesis contributes to antidepressant efficacy. Whereas the underlying mechanisms that cause depression most likely rely on various aspects of brain functioning, like changes in neurotrophic factors or synaptic plasticity, as well as other brain regions, such as the frontal lobes or amygdala, it would be surprising if adult hippocampal neurogenesis influenced them all [START_REF] Petrik | The neurogenesis hypothesis of affective and anxiety disorders: are we mistaking the scaffolding for the building?[END_REF] . Thus, although there may be a link between adult hippocampal neurogenesis and mood, it is not clear how this association is mechanistically connected 276 .

Functional implications of stress-induced changes to neurogenesis

Whether stress hormones impact adult neurogenesis in a functionally significant way is largely unanswered. As discussed, corticosterone is elevated during experiences such as stress, learning, and exercise, all of which can affect memory and emotional states. These hormones can directly alter transcription through the MR and GR, only the latter of which is expressed in newborn cells of the hippocampus. The significance of this expression is unknown, but the activity of GR in these cells can potentially regulate the proliferation, survival, and differentiation stages of neuronal development. Like stress, these changes in neurogenesis are associated with changes in hippocampal-dependent memory processes as well as anxiety and depression, although more research is needed to determine the nature of its effect. Interestingly, these functional changes seem to arise when the animal is under duress and not in a naïve state. Both stress and neurogenesis are well-conserved mechanisms designed to help the organism learn and adjust to future environmental stimuli. Thus, in expanding our understanding of the functional role of neurogenesis, it would be important to know if the changes incurred during neurogenesis due to stress reactivity lead to changes in behavior or cognition. Equally important, whether cort-activation of GRs in newborn neurons modifies anxiety, depression, or hippocampal-dependent memory processes remains a lingering question and a compelling new avenue of investigation.

CHAPTER 2

REGULATION OF ADULT HIPPOCAMPAL NEUROGENESIS BY GLUCOCORTICOID RECEPTORS IN NEWBORN NEURONS 2.1 Abstract

Although it is well known that glucocorticoids (GCs), corticosterone in rodents or cortisol in humans (hereafter referred to as cort), can potently regulate neural stem/progenitor cells (NPCs), the underlying molecular mechanisms that mediate this effect remain unknown. When elevated, as occurring during stress, corticosterone binds to the GC receptor (GR), which can directly bind to and affect cellular DNA. We sought to determine whether chronic treatment of corticosterone suppresses adult neurogenesis by directly acting upon GRs in NPCs or indirectly through other mediators. To investigate this, we generated a novel mouse model lacking GR gene function only in NPCs. These mice then received chronic treatment of corticosterone through their drinking water and were examined for changes in NPC survival and neuronal differentiation. Although all groups examined had comparable numbers of NPCs surviving after 4 weeks, chronic corticosterone treatment ultimately suppressed neurogenesis by reducing neuronal differentiation. This effect was not blocked by GR gene inactivation in NPCs (GR NPCKO ). These results demonstrate that chronic corticosterone treatment can regulate the differentiation of NPCs into mature neurons, and that this effect may be mediated indirectly through other signaling pathways of corticosterone.

Introduction

In the adult brain, a small population of NPCs continually multiplies and matures into functioning neurons that integrate into the cytoarchitecture and contribute to the functioning of the hippocampus [START_REF] Schoenfeld | Stress, stress hormones, and adult neurogenesis[END_REF]276 .

These nestin-expressing stem cells mostly become neurons by progressing through stages of proliferation, survival, and differentiation, collectively known as neurogenesis [START_REF] Kempermann | Milestones of neuronal development in the adult hippocampus[END_REF][START_REF] Lagace | Dynamic contribution of nestin-expressing stem cells to adult neurogenesis[END_REF] . The hippocampus is a structure critical for memory and learning; correspondingly, neurogenesis appears to play an important role in the consolidation, retrieval, and association of memories [START_REF] Zhao | Mechanisms and functional implications of adult neurogenesis[END_REF][START_REF] Aimone | Adult neurogenesis: integrating theories and separating functions[END_REF]252 .

One of the most studied and pervasive regulators of this phenomenon is stress, or more specifically, stress hormones known as glucocorticoids [START_REF] Schoenfeld | Stress, stress hormones, and adult neurogenesis[END_REF]102 . During the stress response, glucocorticoids known as corticosteroids (cort) are elevated and can inhibit neurogenesis 277 . Interestingly, during learning and exercise, elevated corticosterone is also associated with promoting neurogenesis [START_REF] Van Praag | Running increases cell proliferation and neurogenesis in the adult mouse dentate gyrus[END_REF]149,152,278 . Corticosterone is both an ecologically and medicinally important hormone that can be both damaging and beneficial to brain function.

Despite this, the underlying mechanisms by which corticosterone can manipulate neurogenesis in the hippocampus remain unclear.

One of the ways in which corticosterone can influence neurogenesis is by altering genetic expression through the mineralocorticoid-(MR) and glucocorticoid receptors (GR). When MR or GR is bound to corticosterone, it can pass into the cell nucleus and directly affect DNA transcription [START_REF] Sapolsky | How do glucocorticoids influence stress responses? Integrating permissive, suppressive, stimulatory, and preparative actions[END_REF]279 . The expression of these nuclear receptors is highly enriched in the hippocampus 280,281 , and play a role in emotional behavior, learning, memory, and mood disorders [START_REF] De Kloet | Brain Corticosteroid Receptor Balance in Health and Disease[END_REF][START_REF] Saaltink | Stress, glucocorticoid receptors, and adult neurogenesis: a balance between excitation and inhibition?[END_REF]203,239,282,283 . Interestingly, only GR is expressed in the proliferative newborn cells [START_REF] Garcia | Age-dependent expression of glucocorticoid-and mineralocorticoid receptors on neural precursor cell populations in the adult murine hippocampus[END_REF] , and both activation and blockage of their GR in vitro has been demonstrated to either suppress or enhance neurogenesis, respectively 129,131 . Thus, evidence suggests that the GR plays an important role in regulating adult neurogenesis and is a potential mechanism for cort-induced suppression.

Other studies have attempted to elucidate the role of GR in hippocampal neuroplasticity, but have not yet isolated their role in the proliferative cells. Although GR-specific agonists and antagonists have demonstrated an effect on neurogenesis in vivo 137,[167][168][169] , this could be indirectly mediated by the GRs in mature neurons or astroglia. Additionally, a recent study knocked down GR with lentiviral-delivered short hairpin RNA into the hippocampal dentate gyrus (DG) to examine the role of GR in neurogenesis 218 . They found GR knockdown increased neuronal differentiation, however this effect was examined 1 week after cell division, whereas mature neuronal differentiation usually takes at least 4 weeks in the mouse brain [START_REF] Snyder | Adult-born hippocampal neurons are more numerous, faster maturing, and more involved in behavior in rats than in mice[END_REF] . If this measure is accurate, it is not clear if GR knockdown increased the speed of differentiation or the percentage of cells differentiating. Also, this effect was examined under basal corticosterone levels, which are usually too low to even occupy GR 284 . Furthermore, although the lentiviral-mediated knockdown targeted the neurogenic niche, it can infect any neighboring cells, including differentiated neurons and astroglia. While it appears GR can regulate neurogenesis, it remains unclear whether cort-induced suppression of neurogenesis, as experienced during stress, is directly mediated through the GR in NPCs.

To investigate this hypothesis, we generated a mouse with adult NPCs that lacked GRs. By pairing mice that express tamoxifen-inducible Cre recombinase under the control of the nestin promoter [START_REF] Lagace | Dynamic contribution of nestin-expressing stem cells to adult neurogenesis[END_REF] and mice that contained Cre sensitive GR alleles (GR loxP/loxP ) 239 , we created a mouse model that can be induced to lose GR gene function specifically in nestin-expressing cells and their progeny. Nestin is an intermediate filament protein used to identify immature type1 and type 2a NPCs 285,286 , which are the only NPC types found to express GR [START_REF] Garcia | Age-dependent expression of glucocorticoid-and mineralocorticoid receptors on neural precursor cell populations in the adult murine hippocampus[END_REF] . The synthetic CreERT2 recombinase was generated by fusing the Cre open reading frame with a mutated estrogen-receptor ligand-binding domain that binds tamoxifen, but not estrogens. Adult mice were induced with tamoxifen, activating Cre activity leading to GR gene ablation. Using this novel mouse model, we examined how chronic treatment of corticosterone affects neurogenesis and whether this effect is mediated by the GR in NPCs.

Results

Generation of inducible GR gene inactivation in adult Nestin+ cells

In order to examine how the GR in adult hippocampal NPCs contributes to neurogenesis, we used a transgenic approach allowing for temporal control of genetic recombination restricted to NPCs (Figure 1). First, mice expressing a tamoxifen-inducible form of Cre recombinase (Cre ERT2 ) under nestin (marker for NPCs) transcriptional control [START_REF] Lagace | Dynamic contribution of nestin-expressing stem cells to adult neurogenesis[END_REF] were bred to a R26R-YFP reporter line to generate mice with a surrogate marker for recombination (Nestin-CreERT2/R26R-YFP). Next, we mated mice carrying GR loxP/loxP alleles 239 with the Nestin-CreERT2/R26R-YFP line for at least two generations to produce a GR (loxP/loxP) ;R26YFP +/-;Tg(Nestin-

Verification of the inducible GR gene inactivation in adult Nestin+ cells

We used two approaches to verify the efficacy of recombinase activity in vivo. Using

Chronic cort-treatment does not affect NPC survival

To examine adult neurogenesis in this cell-specific GR (NestinCreERT2) model, the experimental design was as follows: GR (NestinCreERT2) mice were treated with tamoxifen or vehicle (controls) at 5-6 weeks of age and then allowed at least two weeks for recovery (Fig 2a

Cort-suppressed differentiation of NPCs may be indirectly mediated

Finally, we sought to determine whether chronic corticosterone treatment reduced neurogenesis by reducing neuronal differentiation, and if this effect was directly mediated through the GR in NPCs. Mice from each group were administered BrdU 4 weeks prior to perfusion. This allowed for cell lineage tracking from newborn precursor to mature neuron [START_REF] Snyder | Adult-born hippocampal neurons are more numerous, faster maturing, and more involved in behavior in rats than in mice[END_REF] . We measured the number of hippocampal cells that were BrdU positive (proliferating cell marker), and of those, which were co-labeled as NeuN positive (mature neuronal marker) in the DG and hilus (Fig 3). When comparing the percent of BrdU positive cells co-labeling for NeuN, a two way ANOVA of genotype x corticosterone revealed a significant effect of corticosterone (p=0.003, F (1,15) =12. 8), but no effect of genotype nor interaction (Fig 3a). This suggests that chronic cort-treatment reduced neurogenesis overall in both control and GR NPCKO groups. Further post-hoc analysis revealed a significant effect of corticosterone between controls (p=0.0087) as well as between GR NPCKO s (p<0.05;

Neuman-Keuls), but no significant differences between noncort-treated groups or between cort-treated groups that chronic corticosterone treatment reduces neuronal differentiation in vivo, and it is not reversed or attenuated by GR gene inactivation in NPCs.

Discussion

We have generated mice using an inducible Cre-lox system to conditionally lose GR function in an adult population of NPCs. We were able to show that chronic corticosterone treatment in vivo did not affect the survival rate of NPCs in the mouse hippocampus, however it did inhibit neuronal differentiation. This effect was not blocked by loss of GR in NPCs. In the present study, we show that GR gene function in NPCs is not necessary for cort-induced suppression of neurogenesis, implying that this effect of corticosterone may be mediated indirectly.

This is the first mouse model of GR function that specifically targets adult NPCs and did not require surgery. Previous studies have investigated the function of the GR in vivo using pharmalogical agents and found they played a significant role in the survival 169 , and proliferation of NPCs 167,168 . These studies, however, looked at the general role of GR in the brain and were not specifically targeting any cell type making it difficult to assess whether the effect of corticosterone on neurogenesis is mediated by cell-autonomous GRs.

Comparatively, Tronche and colleagues created a transgenic mouse with GR gene inactivation in NPCs, but since it was not inducible, all neural cells lost GR function from birth 239 , making it impossible to study the role of GR in adult neurogenesis. A recently published study used RNA-interference to knockdown GR within the adult neurogenic niche, however these mice had intrahippocampal injections of lentivirus, which is invasive and not specific to proliferating cells 218 . Here we report that we were able to isolate the direct role of GR function in a discrete population of proliferative cells.

In our study, chronic treatment of corticosterone does not appear to affect survival. Although this supports other results [START_REF] David | Neurogenesis-dependent and -independent effects of fluoxetine in an animal model of anxiety/depression[END_REF]110,287 , our results are unclear without more information on the effects on proliferation.

Many studies have previously demonstrated that chronic stress, as well as chronic corticosterone treatment, significantly suppresses the proliferative nature of NPCs [START_REF] David | Neurogenesis-dependent and -independent effects of fluoxetine in an animal model of anxiety/depression[END_REF]108,110,115,116,119,[121][122][123][124][125][126][127][128]131,161,162,164,165 . Thus, if in fact corticosterone treatment also inhibited proliferation in our study, since there was no difference in the number of NPCs at 4 weeks post-BrdU injection, this would mean that noncort-treated mice had greater pruning among NPCs. In other words, cort-treated controls had less cell death. If proliferation was not affected in the GR NPCKO s, this would imply that GR NPCKO s have greater cell death, regardless of corticosterone treatment. It is worth noting that proliferation was affected in the transgenic mice with GR gene inactivation in all brain cells (F. Tronche, unpublished data), however as mentioned, whether this is also the case for NPC-specific GR gene inactivation is currently unknown. Moreover, if corticosterone had a similar effect on proliferation of GR NPCKO s compared to controls, this would imply that GR gene inactivation in NPCs has no effect on NPC survival. Given these points, in addition to a larger sample size, it would be important to know the effect of GR NPCKO s on proliferation in order to conclude that GR in NPCs does not affect survival rates.

Although many studies report chronic corticosterone treatment reduces neurogenesis, this effect appears to be due to reduced proliferation 119,131,161,162,164,165,288 . It is important to examine each stage of NPC development to understand how glucocorticoids regulate the rate of neurogenesis. This information will allow for greater elucidation of cort-mediated pathways that affect hippocampal cytoarchitecture. In line with other studies [START_REF] David | Neurogenesis-dependent and -independent effects of fluoxetine in an animal model of anxiety/depression[END_REF]165 , we find that chronic corticosterone treatment can also inhibit the differentiation of NPCs into mature neurons. This cort-induced suppression appears to be mediated independently of GR activation in NPCs, however, a larger sample size would make these results more convincing.

If in fact GR ablation in NPCs does not block cort-induced suppression of neuronal differentiation, there could be several explanations. First, it could be due to experimental design. It is possible that in our mouse model, GR was not sufficiently knocked out of enough NPCs. Whereas tamoxifen-induced Cre recombinase is not 100% efficient and some NPCs (i.e. type 2b) do not express nestin and so their GR gene remains functional, thus, it remains possible that not enough of the population was affected to block corticosterone influence. Furthermore, it could be that our chronic cort-treatment was too extended that other cort-induced pathways compensated for the lack of GR. This latter explanation also suggests another interpretation of the results, however-that neurogenesis can be regulated indirectly.

Although NPCs can express GR, GR is not expressed ubiquitously in NPCs. Whereas GR was not found to be expressed at all in one earlier study 289 , a more recent finding showed that only 13% of newborn BrdU+ cells expressed GR [START_REF] Garcia | Age-dependent expression of glucocorticoid-and mineralocorticoid receptors on neural precursor cell populations in the adult murine hippocampus[END_REF] . According to this study, GR was only expressed in approximately half of NPCs at each developmental stage, with the exception of type 2b cells (0% express GR). It was not until cells reached a post-mitotic stage that they all started expressing the GR [START_REF] Garcia | Age-dependent expression of glucocorticoid-and mineralocorticoid receptors on neural precursor cell populations in the adult murine hippocampus[END_REF] . Although this study was done on female mice, there is no known reason to assume that male mice would have a greater percentage of NPCs expressing GR. Thus, if corticosterone does indeed suppress neurogenesis by directly acting through GRs in NPCs, it can only be affecting at most 50% of the NPC pool. This lack of GR in earlier stages of neurogenesis may imply two contrasting hypotheses: either GR expression during early stages is not as functionally active as it is during the post-mitotic stage and thus, corticosterone indirectly influences NPC behavior; or GR expression is always functionally active and it is such that corticosterone directly influences the fraction of GR-expressing NPCs, which is sufficient enough to cause overall suppressed differentiation. Since our results show that when mice have a significant inactivation of GR genes in NPCs, even further reducing the sub-population of GRexpressing NPCs, they still show similarly reduced differentiation, this implies that the former hypothesis is more accurate. That being, corticosterone might be indirectly influencing NPC behavior.

One of the cort-mediated pathways indirectly influencing NPC behavior could still be through GR activation, but through GR in mature neurons or astroglia. Since much of the literature has shown that GR plays a role in cort-induced suppression of neurogenesis, it may be that the GR in NPCs alone is insufficient to drive this effect. Many studies have demonstrated GR expression in mature neurons [START_REF] Garcia | Age-dependent expression of glucocorticoid-and mineralocorticoid receptors on neural precursor cell populations in the adult murine hippocampus[END_REF]290 , astrocytes 174,185,291- and neurogenesis are implicated in psychopathologies and mood disorders [START_REF] Gunnar | Brain and behavior interface: stress and the developing brain[END_REF][START_REF] De Kloet | Stress and cognition: Are corticosteroids good or bad guys?[END_REF]197,276,304 . Additionally, they are shown to play a role in regulating memory processes [START_REF] Zhao | Mechanisms and functional implications of adult neurogenesis[END_REF][START_REF] Aimone | Adult neurogenesis: integrating theories and separating functions[END_REF]204,252 . Our novel mouse model of GR gene inactivation specific to NPCs in the adult brain allows for a better understanding of the molecular mechanisms that are mediated by corticosterone to regulate adult neurogenesis. This is relevant both to endogenously elevated corticosterone as induced by stress, learning, and exercise, as well as exogenously administered for medicinal purposes. Furthermore, our transgenic mice will allow for a better understanding of the role that GR plays in newborn neurons and potentially contribute to the development of new neuropharmalogical therapies.

Materials and Methods

Nestin-CreERT2 and GRloxP/loxP transgenic mice. To selectively inactivate the GR gene in neural progenitor cells, we generated GR loxP/loxP ; R26R-YFP; Tg(NesCreERT2) mice (thereafter denominated NesCreERT2) mice were used as controls. Vehicle-and tamoxifen-induced GR loxP/loxP mice were also tested alongside controls to verify no effects of Cre or tamoxifen.

60 mice (n=10 per group) were used in experiment. For analysis of gene inactivation efficiency, 10 mice per group (-tam/-corticosterone, -tam/+corticosterone, +tam/-corticosterone, +tam/+cort) were used, however, 5

could not be analyzed as a result of 1 death and 4 exclusions due to inefficient recombination in NPCs.

We restricted our analysis to male mice. Animals were bred and raised under a 12h light/dark cycle;

temperature was 22±2˚C and humidity 60±5%. Food and water were supplied ad libitum. Experiments were performed in accordance with French (Ministere de l'Agriculture et de la Forêt, 87-848) and European Economic Community (EEC, 86-6091) guidelines for the care of laboratory animals.

Genotypes were determined by PCR analysis of tail DNA samples as described previously [START_REF] Lagace | Dynamic contribution of nestin-expressing stem cells to adult neurogenesis[END_REF]239 . Adult mice were genotyped using the following primers: Tamoxifen induction. Tamoxifen (Sigma T5648) was light-protected and dissolved in 10% ethanol and suspended in 90% sunflower seed oil [START_REF] Lagace | Dynamic contribution of nestin-expressing stem cells to adult neurogenesis[END_REF] . Mice received daily injections at 180mg/kg, i.p. for 5 days when they were 5-6 weeks old. Control mice received vehicle (sunflower oil). Mice receiving tamoxifen were only grouphoused with other tamoxifen-receiving mice; likewise, only vehicle (oil)-receiving mice had vehicle-receiving littermates. This guarded against possible cross-contamination of tamoxifen.

CreERT2: CreERT2 (
Corticosterone treatment. Corticosterone (Sigma C2505) was prepared as described previously [START_REF] David | Neurogenesis-dependent and -independent effects of fluoxetine in an animal model of anxiety/depression[END_REF] . It was dissolved at 35 µg/mL in a water solution of 0.45% (wt/vol) hydroxypropyl-beta-cyclodextrine (Sigma 332593) by sonication for two hours. Corticosterone treatment was given to mice in opaque water bottles to protect it from light, changed twice a week, and available ad libitum.

BrdU labeling. To trace cell lineage, 31-32 days prior to perfusion, mice were administered with bromodeoxyuridane (150mg/kg, i.p. dissolved in saline; Sigma B5002) twice daily for 3 days.

Immunohistochemistry. Mice anesthetized with pentobarbital euthanasia solution were perfused transcardially with ice cold 0.1M phosphate buffered saline (PBS) followed by 4% paraformaldehyde (PFA).

Brains were removed, fixed overnight in PFA at 4˚C, and transferred to 0.1M PBS. Serial sections were cut coronally at 35 µm using a vibrotome (Vibratome 3000 Plus; Ted Pella, Inc). Staining consisted of 3 x 5min washes in 1xPBS, blocking and permeabilizing in a 1xPBS solution of 5% normal donkey serum (Jackson 017-000-121) and 0.3% tritonX for 30-60 minutes, followed by overnight incubation at 4˚C with primaries.

Tissue was then washed 3x5min in PBS, treated with secondary antibody for two hours at room temperature, washed again, fixed in 4% PFA for 15minutes, then washed again. Tissue preparation for BrdU co-labeling then proceeded with treatment of 0.9% saline for 5 minutes, followed by an acid wash in 2N HCL at 37˙C for 30 minutes. Tissue was then washed 3x5minutes, blocked, and incubated with BrdU antibody overnight. The following primary antibodies were used on free-floating sections: rat monoclonal anti-BrdU (Abcam 6326;

Microscopy and quantification.

Confocal laser scanning microscopy (Zeiss) was used for counting fluorescent-labeled cells. It was performed using a 40X-oil objective on a 1 in 10 series of sections through the entire DG. In all cases, 8 hippocampi per animal of each experimental group were randomly selected within each series and analyzed. For survival and differentiation analysis, 32 mice (n=4-6 per group) were given BrdU 4 weeks prior to perfusion; of these, only 27 were analyzed due to 1 death, 1 damaged brain tissue, and 3 exclusions due to inefficient recombination in NPCs: -tam/-corticosterone, n=6; -tam/+corticosterone, n=5; +tam/-corticosterone, n=4; +tam/+corticosterone, n=4; Gr loxP/loxP -tam/+corticosterone, n=4; Gr loxP/loxP +tam/+corticosterone, n=4. BrdU+ cells were counted in 8 hippocampi of 2 series and multiplied by 10 to represent total numbers throughout DG and hilus. Area of DG and hilus were measured using Stereo Investigator software.

Statistical analysis.

Results are expressed as means±s.e.m. Statistical analysis was performed using Student t-tests when comparing effect of corticosterone between controls, and two-way ANOVA for comparing all groups. Analyses were followed by Dunnett's or Newman-Keuls post-hoc tests for pairwise comparisons, as appropriate. * indicates p<0.05.

Figures and Legends

Introduction

The conservation of adult neurogenesis across invertebrate (for review [START_REF] Cayre | The common properties of neurogenesis in the adult brain: from invertebrates to vertebrates[END_REF] ) and vertebrate (for review [START_REF] Abrous | Adult neurogenesis: from precursors to network and physiology[END_REF] ) species suggests a fundamental biological significance that is at the forefront of exploration. This plasticity appears to be restricted to two regions of the mammalian brain: the subventricular zone of the lateral ventricles, and the subgranular zone in the hippocampus 305 , further suggesting that the distinct functions of these structures allow them to benefit from the continual production of new neural cells. Within the mouse hippocampus, the process of a newborn progenitor cell developing into a functional granule neuron takes approximately 4 weeks [306][307][308] . Although this population of adult-born granule cells (abGCs) makes up only around 10% of the dentate gyrus (DG) 309 , due to their hyper-excitability, greater integration capacity, and extensive connections, it is believed to make a disproportionate functional contribution 310 . It is well known that the hippocampus is critical for memory and learning [311][312][313] , and significantly involved in mood-related behaviors 314 . Accordingly, abGCs are implicated in memory and learning processes such as contextual conditioning and pattern separation 218,221,268,315 , as well as anxiety-like behaviors, and depression 252,272,276,316- 318 . Although adult neurogenesis contributes to these aspects of hippocampal-dependent functions, the degree to which is controversial.

Further indication of the functional relevance of hippocampal neurogenesis is their responsiveness to the external environment, particularly stress. This implies hippocampal neurogenesis may affect behaviors associated with stress and adaptation. An elevation in circulating glucocorticoids, as triggered during stress, has been demonstrated to impact the behavior of newborn neurons 103,279 . During stress, the elevated glucocorticoid known as corticosterone in rodents, and cortisol in humans (hereafter called cort), impacts hippocampal plasticity not only through neurogenesis, but also synaptic integrity, cell migration, apoptosis, long term potentiation (LTP), and CA3 dendritic remodeling 279,[319][320][321] . To what degree any of these effects are functionally significant, however, is yet to be determined.

What is known, nonetheless, is that corticosterone affects a variety of hippocampal functions, such as anxiety-and depression-like behaviors, as well as memory processing 197,200,243,279,322 . This can be seen in Cushing's syndrome, a state of hypercortisolism, in which patients commonly suffer from memory dysfunction as well as mood disorders like depression and anxiety 197 . Considering that both the corticosterone response during stress as well as adult neurogenesis are conserved mechanisms, it would seem apparent that there would be a functional advantage of abGCs being responsive to corticosterone signals. Corticosterone actions are mediated by two receptors: the glucocorticoid receptor (GR) and the mineralocorticoid receptor (MR).

Whereas MR binds to corticosterone with an affinity 10-fold higher than GR, GR does not become occupied (activated) until corticosterone levels are high, i.e. at the circadian peak and during stress [START_REF] Reul | Two receptor systems for corticosterone in rat brain: microdistribution and differential occupation[END_REF]280,323,324 .

Furthermore, while abGCs express both MR and GR, only GR is expressed earlier on during the proliferative stages [START_REF] Garcia | Age-dependent expression of glucocorticoid-and mineralocorticoid receptors on neural precursor cell populations in the adult murine hippocampus[END_REF] . This implicates GR as an important target in newborn neurons during high levels of corticosterone.

In the brain, GR is thought to regulate emotional behavior and cognitive functions [START_REF] De Kloet | Brain Corticosteroid Receptor Balance in Health and Disease[END_REF]283 . While it was shown that general GR gene inactivation is lethal [START_REF] Tronche | Genetic dissection of glucocorticoid receptor function in mice[END_REF] , and brain-wide GR gene inactivation reduces anxiety-like behaviors 239 , it is unknown if GR gene inactivation specifically in NPCs is behaviorally relevant as well.

Whereas adult neurogenesis appears to contribute to functions similarly affected by high levels of corticosterone, the question remains whether abGCs mediate any of these cort-induced effects on behavior and cognition. To address this question, we irreversibly inactivated GR gene signaling specifically in adult NPCs and their progeny, thereby reducing the capacity of the abGC population to respond to stress signals,

i.e. elevated corticosterone. In this novel GR mutant mouse model specific to adult neurogenesis (GR NPCKO ),

we investigated the functional relevance of GR-signaling in abGCs. Mice received 4 weeks of chronic corticosterone treatment through their drinking water to induce an anxious state (as described [START_REF] David | Neurogenesis-dependent and -independent effects of fluoxetine in an animal model of anxiety/depression[END_REF] ), and were then tested on a battery of behavioral tests under continued treatment. These behavioral tests included commonly used protocols to measure behaviors associated with anxiety and depression, as well as cognitive abilities such as contextual fear conditioning and pattern separation. This study explores not only how corticosterone impacts behavior, but also functional contributions of adult neurogenesis, or in other words, how the response of abGCs to chronic corticosterone treatment affects mood and memory.

Results

Generation of mice with reduced GR signaling in adult born granule cells.

To investigate the role played by GR-signaling in abGCs, we generated a transgenic nestin-CreERT2 mouse line with inducible GR gene inactivation (see chapter 2). CreERT2 is a chimeric Cre recombinase enzyme obtained by fusing the Cre open reading frame and a mutated ligand-binding domain (LBD) of the estrogen receptor. In the presence of tamoxifen, a synthetic ligand of the estrogen receptor, the CreERT2 protein is released from chaperone proteins in the cytoplasm and translocates to the nucleus where is can excise DNA segments flanked by loxP sites. In the GR (NesCreERT2) model, CreERT2 was only present in cells expressing nestin, an intermediate filament protein found mostly in stem cells and used to identify type1 and type 2a NPCs [START_REF] Lagace | Dynamic contribution of nestin-expressing stem cells to adult neurogenesis[END_REF]325 . These mice were homozygous for GR alleles containing exon 3 flanked by loxP. Deletion of this segment excises the DNA binding domain and abolishes the expression of the LBD in the GR gene, resulting in a nonfunctional GR 239 specifically in NPCs.

Mice were induced with tamoxifen at 5-6 weeks old (young adulthood) and given at least 2 weeks for recovery (Fig 1a). This technique was successful in knocking out GR in approximately half of the proliferative cells in the hippocampus that express GR (see Fig1g from chapter 2) thereby reducing the capacity of the neurogenic pool to respond to elevated corticosterone levels. We then chronically treated half of the induced GR NPCKO mice and half of the non-induced control mice with low-dose corticosterone and looked at several aspects of hippocampal-dependent behavior and cognition. First, we examined whether there was an effect on locomotion, or their physical activity in a novel environment. Cort-treatment has reduced effect on anxiety-like behaviors in GR NPCKO s.

Next, to examine whether GR in abGCs plays an important role in mediating anxiety-like behavior in a stressful environment, we used three behavioral paradigms that test the natural avoidance behavior of mice:

novelty-suppressed feeding, light/dark box, and elevated Omaze (Fig 2). In novelty-suppressed feeding (NSF),

we measured how long it took before food-deprived mice approached as well as ate food from the aversive (i.e. lit and exposed) center platform (Fig 2a). For latency to approach, two-way ANOVA revealed a significant effect of genotype (p=0.019, F (1,30) =6. 18), but only a near significant interaction effect (genotype x corticosterone, p=0.062). Further post-hoc analysis, however, showed a significant difference between the untreated and cort-treated controls (p<0.05, Dunnett), but not the GR NPCKO s. Cort-treated controls were also significantly different from both GR NPCKO groups (p<0.05, Neuman-Keuls). Thus, under chronic cort-treatment, we found that the controls on corticosterone took significantly longer to approach the food, while the behavior of the GR NPCKO s were not affected by corticosterone (Fig 2a). The latency to eat revealed only an effect of corttreatment (p=0.002, F (1, 31) =0.596), which post-hoc analysis revealed was largely driven by a significant difference between cort-treated GR NPCKO mice and both untreated mice groups (p>0.05, Neuman-Keuls).

Furthermore, there was not a significant effect between all groups in an analysis of percent of mice not feeding (p=0.09, Mantel-Cox log rank).

In light/dark box, we measured the latency to exit the dark compartment (and enter the lit compartment), and the frequency of entries (4 paws) and time spent in the lit compartment (Fig 2b). Two-way ANOVA revealed a significant effect of genotype in both the latency to enter and time spent in the lit compartment (p=0.013, F (1,3) =7.06 and p=0.020, F (1,31) =5.97, respectively), while further post-hoc analysis showed cort-treated controls took significantly longer to exit compared to untreated controls (p<0.05, Dunnett's), as well as untreated and cort-treated GR NPCKO s (p<0.05, Neuman-Keuls). Interestingly, cort-treated GR NPCKO s behaved similarly to untreated GR NPCKO s. This interaction effect was most apparent in the number of 4 paw exits, which proved significant in two-way ANOVA (p=0.037, F (1,31) . This again suggests that while anxiety-like behavior in control mice is affected by cort-treatment, the behavior of GR NPCKO s is unaffected. There were no significant differences found among all groups in the latency to exit in this task.

GR NPCKO mice do not show depression-like phenotype.

Elevated levels of corticosterone are often associated with depressive-like symptoms 197,200,243,279,322 , and NPCs have been shown to be involved in the behavioral effects of antidepressants [START_REF] Santarelli | Requirement of hippocampal neurogenesis for the behavioral effects of antidepressants[END_REF][START_REF] David | Neurogenesis-dependent and -independent effects of fluoxetine in an animal model of anxiety/depression[END_REF]318 , thus, we sought to investigate whether the GR in abGCs plays a role in depressive-like behaviors. To examine this, we used the mouse Porsolt forced swim test, which is a well-established clinical model of stress-coping ability 197,241 . In this task, the behavioral response to stress is measured in the time spent floating (immobility), active (climbing), and the latency to float (Fig 3a-c). Although there appears to be a trend of chronic cort-treatment affecting both controls and GR NPCKO s in both the latency to float and time spent immobile, two-way ANOVA revealed no significant differences across all groups (Fig 3a -b). Similarly, mice did not differ significantly in the time spent climbing (Fig 3c). Further post-hoc analyses confirmed no significant differences among groups, suggesting that neither chronic cort-treatment nor GR signaling in NPCs alone drives a depression-like phenotype.

GR NPCKO s are successful at 1-trial contextual fear conditioning.

Contextual fear conditioning is a hippocampal-dependent function that studies have demonstrated can be impaired by corticosterone administration 237,326 , as well as ablated neurogenesis 255,327,328 . To determine whether impairment may be due to corticosterone effects (via GR) specifically on abGCs, we next examined contextual fear conditioning in our cort-treated controls and GR NPCKO mice. On day 0, mice were allowed to explore a novel environment for 3 minutes before they received a 2sec-0.75mV foot shock (Fig 4a). During this time before foot shock, there was minimal freezing (0-5% of total time). The next day, all mice demonstrated a significant increase in freezing compared to day 0 (three-way ANOVA, p<0.0001, F (1,33) =100.18) verifying that all mice were able to learn and recall an association between contextual cues and an electrical shock (Fig 4b-c). Interestingly, when only comparing treatment within control groups, conditioning was not affected by corticosterone per se (p=0.56), but was affected by the interaction of corticosterone over time (two-way ANOVA, p=0.0457, F (1,16) =4.697), indicating a change in the rate of learning (Fig 4b). This effect is more clearly evident by comparing the differences in %freezing in untreated versus cort-treated controls (p=0.03) (Fig 4d). Our GR NPCKO mice did not significantly block this effect. According to three-way ANOVA, there was no effect of genotype (p=0.86), and only a near significant interaction effect of genotype x corticosterone x day (p=0.06). Two-way ANOVA did not find a significant interaction for the difference in Although all mice appeared able to learn the contextual fear association, and discriminate contexts within group, this ability was not significantly different between groups (Fig 6). The ability to discriminate these environments was measured per day with the following ratio: the difference in percent time spent freezing in chamber A (foot shock) versus chamber B (neutral) divided by the sum of percent time spent freezing in both chamber A and B. Repeated measures three-way ANOVA revealed a significant effect of day (p=0.0013, F (11,33) =2. 87), but no effect of genotype (p=0.218), corticosterone (p=0.965), nor an interaction effect (p=0.096) when comparing discrimination ratios across groups (Fig 6a-b). Comparing just the effect of corticosterone on control mice revealed a significant interaction between day and corticosterone treatment (p=0.0002, F (11,17) =3.52, repeated measures two-way ANOVA), which appears to be driven by the last 2 days since compared between groups per day, while scores in figure 5 (d-g) are compared within group across time.

Overall, these results suggest all groups had comparable abilities in discriminating similar contexts.

Discussion

To distinguish GR signaling pathways within abGCs that mediate behavior from those throughout the nervous system, we generated an inducible GR mutant mouse line in which nestin-driven, Cre-mediated inactivation of the GR gene takes place selectively in NPCs. By waiting until mice were adults to induce gene inactivation, we were able to examine how adult neurogenesis is behaviorally relevant to hippocampal functioning. We found that the functional significance of GR gene inactivation in abGCs was not apparent in our tests unless mice are experiencing chronic corticosterone exposure. In all tests administered, GR NPCKO mice exhibited normal behavior under basal conditions and were indistinguishable from their control littermates. It was not until mice were experiencing chronic stress-like conditions that differences in behavioral phenotypes emerged. Whereas chronic corticosterone treatment induced an anxious phenotype in control mice, it had a reduced effect on anxiety-like behaviors in GR NPCKO s. All mice behaved similarly in locomotor and open field tests, which measure physical activity in a novel environment, indicating that changes in anxiety-like behaviors were not due to differences in their motivation or ability to explore. These results highlight the role of adult neurogenesis as being an environmentally responsive adaptive mechanism.

The selective disruption of GR signaling pathways in abGCs significantly reduced fearful behavior when mice were under chronic corticosterone exposure. This suggests that abGCs can attenuate anxiety in novel environments via suppressed GR activity or even that perhaps anxiety in novel environments can be induced thru GR signaling in abGCs. The anxiety-reduced phenotype of cort-treated GR NPCKO the behavior of cort-treated GR NPCKO mice was indistinguishable from untreated mice. Lastly, we saw similar avoidance behavior in NSF. Cort-treated controls took significantly longer to approach the brightly lit, center platform containing food, whereas GR NPCKO blocked this cort-induced behavior. Although GR is expressed in many hippocampal cells, these results suggest that the cort-activated GR signaling specifically in abGCs can induce anxiety-like behavior, or at least, can attenuate anxiety-like behavior when the signaling is deficient.

It is worth noting that this effect was not robust in all anxiety-relevant measures. For example, it was not seen in the latency to eat, which could be due to mice not being hungry enough (although all mice ate immediately post-test) or this measurement of the NSF test could be more related to depression-like symptoms [START_REF] Petrik | The neurogenesis hypothesis of affective and anxiety disorders: are we mistaking the scaffolding for the building?[END_REF]272,273 rather than anxiety. Indeed, several studies have used this test as a measure of antidepressant efficacy [START_REF] Santarelli | Requirement of hippocampal neurogenesis for the behavioral effects of antidepressants[END_REF]329 . Interestingly, we also did not see this effect in the open field test. Although often used as a measure of physical activity and motivation, we did not see reduced time spent in the center of the open field, whereas we do see that effect in other anxiety-relevant tests. This may be due to experimental procedures, such as the size of the arena used, the length of the experiment, and what area we decided constitutes as being in the center. These changes may elucidate behavioral differences. Additionally, although there is an apparent trend of GR NPCKO blocking the effect of corticosterone on anxiety-like behavior, there was only a statistically significant interaction effect (genotype x cort) for the number of 4 paw entries in the light/dark box, and near significant interaction effect for the number of 4 paw entries and time spent in the open arms for the elevated Omaze. An increased sample size may help clarify the strength of the effect; otherwise changes in the testing procedures or corticosterone treatment might reveal stronger differences. Be that as it may, it could be that GR signaling in abGCs contributes finer aspects to these behaviors, and are not solely responsible for anxiety in its entirety. The contribution of adult neurogenesis to these behavioral nuances will need to be disentangled over time.

Future studies on GR NPCKO blocking anxiety-relevant behaviors will be necessary to demonstrate whether this effect is truly regulated by a cell-specific signaling pathway. It is possible that it is not abGCspecific inactivation of the GR gene attenuating cort-induced responses. First, it could be just overall reduced GR signaling in the DG that drives our effect, irrespective of cell type. Unfortunately, the ideal control group addressing this concern would be GR gene inactivation in ~10% of matured granule cells, a difficult manipulation 276 . Secondly, there could be other signaling pathways, such as altered MR activity or a decrease in NMDA receptors, that also result in the same effect of blocking cort-induced anxiety-like behaviors. Thus, GR gene inactivation may not be necessary to induce or attenuate these fearful behaviors. Thirdly, it could be any disruption in hippocampal signaling can mediate anxiety-related behaviors. Lastly, there could indeed be a strong functional effect of GR gene inactivation in NPCs, however for the effect to be clearly evident, there needs to be 100% GR gene inactivation in all hippocampal stem/progenitor cell types. More thorough and advanced techniques will help address these issues for the future. In any case, our results demonstrate that reduced GR signaling in abGCs is sufficient for suppressing cort-induced anxiety-like behaviors.

Other studies have supported the idea that either GR or neurogenesis plays a role in anxiety-related behaviors. It is well documented that chronic elevation of corticosterone levels results in an anxious phenotype 226 . Exogenous administration of corticosterone has shown to increase latency to feed in NSF Soon after discovering elevated corticosterone can inhibit proliferation of newborn neurons in the hippocampus, Gould and colleagues proposed that since high levels of corticosterone are often found in patients with affective disorders, perhaps the changes in hippocampal neurogenesis were regulating mood [START_REF] Gould | Adrenal hormones suppress cell division in the adult rat dentate gyrus[END_REF] .

The research that addressed this proposal has been controversial. Many experiments manipulating neurogenesis saw no effect on anxiety-like behaviors [START_REF] Santarelli | Requirement of hippocampal neurogenesis for the behavioral effects of antidepressants[END_REF][START_REF] David | Neurogenesis-dependent and -independent effects of fluoxetine in an animal model of anxiety/depression[END_REF][START_REF] Sahay | Increasing adult hippocampal neurogenesis is sufficient to improve pattern separation[END_REF][START_REF] Surget | Antidepressants recruit new neurons to improve stress response regulation[END_REF][START_REF] Kitamura | Adult neurogenesis modulates the hippocampus-dependent period of associative fear memory[END_REF] , although this could be due to techniques used to target neurogenesis [START_REF] Schoenfeld | Stress, stress hormones, and adult neurogenesis[END_REF] . Other studies, however, found that altered neurogenesis did result in an anxious phenotype 272,330 . In one study, mice with a profound decrease in NPCs showed decreased number of entries and time spent in the open arms for the elevated Omaze, as well as increased latency to emerge and reduced time spent in the lit portion of the light/dark box 272 .

Since elevated corticosterone can both suppress neurogenesis and promote an anxious phenotype (via GR), and some studies found that suppressed neurogenesis results in an anxious phenotype, we questioned whether the corticosterone effect on neurogenesis was driving this anxious phenotype. By inactivating the GR gene in nestin expressing cells, and hence abGCs, we did not find a change in anxiety-like responses. It was only after mice were chronically exposed to excess corticosterone did they begin to exhibit an anxiolytic effect of their mutation. This is the first study we know of that has examined mood regulation acting through GR in abGCs. Overall, our findings advance our understanding of not only the functional contributions of abGCs, but also the mechanisms underlying anxiety-relevant responses.

Aside from anxiety, we also investigated how GR signaling in abGCs affects depression-like symptoms. While it appears that elevated corticosterone and adult neurogenesis are relevant to depression, their direct relationship with depression is not clear. Half of people suffering from depression were found to have hypercortisolism 331 and normalizing their corticosterone levels not only led to symptom relief, but also the extent of which predicted relapse 279 . Likewise, half of Cushing patients have depression and correcting their hypercortisolism reduced their psychopathology 332 . Co-treating with antidepressants and anti-glucocorticoids improves both the time it takes to treat and its efficacy in psychotic depression 333 , and corticosterone synthesis inhibitor, metyrapone, is found to be effective in treating this affective disorder as well 334 . Oddly though, dexamethasone (dex), a GR agonist, and RU38486, a GR antagonist, were both found to separately alleviate symptoms of depression 335,336 . Dex, however, was not only found to poorly penetrate the brain, but also suppress endogenous corticosterone, thus, it often acts as an antiglucocorticoid in the brain [337][338][339] . The forced swim test has demonstrated both reliable and predictive validity on the efficacy of all antidepressants effective in humans for the last several decades [START_REF] Santarelli | Requirement of hippocampal neurogenesis for the behavioral effects of antidepressants[END_REF]227,242,340,341 . This test correlates increased immobility, and decreased activity and latency to immobility with a depressive phenotype 227,340 . It was then discovered that x-ray ablation of hippocampal neurogenesis prevented antidepressant efficacy on this test [START_REF] Santarelli | Requirement of hippocampal neurogenesis for the behavioral effects of antidepressants[END_REF]287,329 and ever since, it has been proposed that antidepressants function through the abGCs [START_REF] Santarelli | Requirement of hippocampal neurogenesis for the behavioral effects of antidepressants[END_REF]126,342 . Additionally, it was thought that since the hippocampus is often found reduced in depression, it may be due to impaired neurogenesis 279 . Although evidence appears to link neurogenesis and depression together, most studies do not find reduced neurogenesis causes depressive behavior [START_REF] Petrik | The neurogenesis hypothesis of affective and anxiety disorders: are we mistaking the scaffolding for the building?[END_REF]253,260,274,275 , and state that they are not well correlated 255,343 .

To our knowledge, we are the first to examine this relationship between glucocorticoids and neurogenesis and their effect on depression-like behavior. Although glucocorticoids are believed to play a causative role in the etiology of depression [START_REF] De Kloet | Stress, genes and the mechanism of programming the brain for later life[END_REF]344 , the amount and duration of corticosterone administration are key determinants for the expression of a depressive phenotype 197 . Studies have found increased immobility in the FST after treating mice orally with 25µg/mL of corticosterone for 2 weeks 345 and through their drinking water with 20µg/mL for 2 months, although this latter experiment was in 3-week-old mice 346 . We chose to chronically administer low dose corticosterone (35µg/mL) through drinking water (to naturally and more ecologically mimic the circadian and pulsatile nature of corticosterone release and the stress response) over 6-7 weeks prior to and throughout testing. Although there was a trend of increased immobility and decreased latency to float, neither measurement was significantly different from nontreated controls. Either corticosterone was at too low of a dose for not enough time or exogenously administered corticosterone is not a reliably effective method to induce depression-like behavior in the FST. It could also be due to the mouse line we used, whereas Balb/c mice are typically more prone to exhibiting depressive-like responses 347 . Hen and colleagues found similar results to ours after treating similar mouse species with corticosterone and not finding an increase in depressive-like behaviors in the FST [START_REF] David | Neurogenesis-dependent and -independent effects of fluoxetine in an animal model of anxiety/depression[END_REF] , however only total mobility was reported, and not latency to immobility or immobility alone. They did find this treatment protocol of corticosterone increased the latency to feed in NSF, another test used to demonstrate affective impairment 272 , although we did not find the same effect in our study. Interestingly, even though mice treated with corticosterone did not exhibit a depressive phenotype compared to untreated mice, mice treated with corticosterone and fluoxetine, an antidepressant, showed significantly greater mobility compared to untreated and cort-treated mice [START_REF] David | Neurogenesis-dependent and -independent effects of fluoxetine in an animal model of anxiety/depression[END_REF] . This underlies that actual antidepressant efficacy is independent from the etiology underlying depression, or in other words, fluoxetine may be masking a depressive phenotype only by inflating opposing behaviors.

Moreover, neither GR NPCKO nor GR NPCKO under cort-treatment had any significant effect on depression-like behaviors. These results may be different if mice were initially exhibiting a depressive-like state, or if we had examined how they respond to antidepressants. It was proposed that the functional importance of abGCs may only be revealed when the animal is in a depressive-like state 348 . In any case, our results appear to support the majority of research (76% of current studies) demonstrating that manipulations of abGCs have no effect on mood-related tests in naïve rodents [START_REF] Petrik | The neurogenesis hypothesis of affective and anxiety disorders: are we mistaking the scaffolding for the building?[END_REF] . It is worth noting that even though our GR NPCKO s on corticosterone demonstrate anxiolytic, but not antidepressant-like behavior, depression and anxiety-related behaviors may indeed be differentially associated with the hippocampus 349 as they are distinct diagnostic disorders [START_REF] Petrik | The neurogenesis hypothesis of affective and anxiety disorders: are we mistaking the scaffolding for the building?[END_REF] .

Next, we examined hippocampal-dependent cognitive functions that may be affected by GR signaling in abGCs. The acquisition of contextual information associated with an unconditioned stimulus has been regarded as a hippocampal-dependent ability, involving the DG and amygdala 350,351 . This ability can be measured in a task known as contextual fear conditioning, in which a rodent is placed in a novel environment to explore and receives a foot shock soon thereafter in this environment. Typically 24 hours later, animals are replaced in this environment to gauge their memory of the previous event. Evidence of this association is exhibited as freezing behavior, an adaptive species-specific defense mechanism in anticipation of an aversive event 352 . Both corticosterone and GR activation are involved in fear conditioning 214 such that stress-or cortmediated activation of GR promotes memory consolidation, particularly due to it being an emotionally arousing experience 214,353,354 . Several studies have verified that chronic corticosterone administration in both rats and mice enhance contextual fear conditioning 219,237,326,355 and removal of corticosterone through adrenalectomy impairs this behavior 214,356,357 . Furthermore, it has also been demonstrated extensively that GR signaling affects freezing behavior in this context 354,[357][358][359][360][361] and blocking GR with an antagonist before conditioning impairs freezing behavior 362 . At the same time, evidence has suggested that the cells mediating this function are granule cells in the dorsal DG, such that activation or inhibition of these cells impairs contextual encoding 317 . Some studies have narrowed this cell type to the abGCs because arrest of NPCs through either X-irradiation or genetic ablation techniques has significantly impaired contextual fear conditioning 255,256,309,328,[363][364][365][366] . However, other studies have not found NPCs as mediators in contextual fear conditioning [START_REF] Sahay | Increasing adult hippocampal neurogenesis is sufficient to improve pattern separation[END_REF]253,254,266,367,368 . For example, neither arrest of neurogenesis through x-irradiation [START_REF] Sahay | Increasing adult hippocampal neurogenesis is sufficient to improve pattern separation[END_REF] or genetic ablation 253,262,266 , nor enhanced neurogenesis [START_REF] Sahay | Increasing adult hippocampal neurogenesis is sufficient to improve pattern separation[END_REF] affected freezing behaviors in rodents compared to controls.

This conflicting data may be explained though by discrepancies in the experimental design and NPC targeting technique 367 . For example, only high-dose irradiation, and not a low dose, impaired fear conditioning in one experiment 369 ; another experiment demonstrated only single-trial, and not multiple trial conditioning, is affected by neurogenesis arrest 327,328 ; furthermore, this reduced conditioning effect is only seen after at least 4-6 weeks have passed post-arrest 327 . Thus, it appears that the effect of corticosterone, via GR, in either the dorsal DG or amygdala can regulate the association of contextual and aversive information, and it is possible that abGCs are necessary for this function as well.

Thus, we decided to investigate whether GR signaling in abGCs that were at least 6 weeks old were involved in contextual fear conditioning. We found that none of our mice had impaired fear conditioning through our manipulations. Furthermore, in our control mice, corticosterone alone enhanced the rate of learning as seen by significantly increased freezing behaviors over time. This effect was not seen between our untreated and cort-treated GR NPCKO mice, but there was not a significant interaction for this effect compared with controls. Also, although both GR NPCKO groups exhibited less freezing behavior than controls, it was not a significant effect. Our results verify other studies showing that corticosterone enhances associative memory in the contextual fear conditioning paradigm, although our experimental design cannot separate whether it is enhanced acquisition or retrieval that drives the increased freezing behavior. Furthermore, we demonstrate that reduced GR signaling in abGCs is not sufficient enough to impair this type of learning and memory, and although it appears there is a reduced effect of corticosterone treatment on GR NPCKO s compared to controls, this did not prove to be significant. Our data suggests that corticosterone promotes contextual fear conditioning, but not through GR signaling in the abGCs.

Our results are in conflict with another study that investigated the role of GR signaling in NPCs 218 . This latter study used lentiviral constructs to deliver short hair-RNA to knockdown GR in the DG and found impaired freezing behavior 218 . Although infected cells appear to be mostly NPCs, lentivirus is not specific for NPCs. Thus, this design may have affected a wider population of cells than ours. Importantly, this study also used a different mouse line than ours, specifically they used Balb/c mice, which are known to be more sensitive to stress, have increased anxiety-like behaviors, and enhanced contextual fear conditioning compared to other strains 142,[370][371][372] . This appears to be an important distinction; one study found ablated neurogenesis affected novel object preference, another hippocampal-dependent memory task, only in 129/SvEv mice and not C57B1/6J mice 327 . Additionally, Fitzsimons et al (2013) used 0.4mA for a foot shock 218 , whereas our shock was 0.75mA. It may be that our aversive stimulus was too intense to see the subtle contributions of GR signaling in abGCs to this behavior. Overall, there seems to be many differences between our mice, experimental designs, and perhaps even scoring strategies that result in decreased freezing behaviors for their GR mutant mice but not in our mice.

Lastly, another cognitive ability that we tested was contextual fear discrimination. This test is often used as a measure of the ability for pattern separation, a skill highly associated with hippocampal neurogenesis 268,373 . Pattern separation is described as a neurophysiological computation that disassociates similar patterns of neural activity such that similar memories (representations) can be distinguished from one another 315 . Although this ability is difficult to measure because of current limitations in our understanding of how this information is neurally represented 374 , it is thought to be exhibited in the contextual fear discrimination task 315,373,375 . This task assesses a rodent's ability to disambiguate two highly similar contexts from one another by measuring its fear responses in each context. This was initially discovered to be a hippocampaldependent ability because electrolytic lesions of the hippocampus resulted in higher freezing behaviors in the nonshock context, and thus, less discrimination between the two contexts 376 . Further investigation showed that reduced neurogenesis impaired discrimination of similar contexts 265,266,377 . Ablated neurogenesis from Xirradiation resulted in reduced discrimination of similar spatial locations in radial arm maze 265 and overexpression of the apoptotic Bax protein in NPCs reduced the difference in discriminative freezing responses in contextual fear discrimination 266 . Correspondingly, enhanced neurogenesis by knocking out the Bax protein in NPCs improved contextual fear discrimination [START_REF] Sahay | Increasing adult hippocampal neurogenesis is sufficient to improve pattern separation[END_REF] . Whereas corticosterone enhances fear conditioning, we examined whether corticosterone had a similar effect on fear discrimination. Additionally, we investigated whether this ability was mediated by GR signaling in abGCs. Comparing discrimination ability across groups showed that all mouse groups performed similarly in the contextual fear discrimination paradigm. This was not due to impaired association learning, as all groups had increased freezing levels in both contexts over the course of the experiment. This is also not due to impaired discrimination as individual analysis of each group revealed an overall significant difference in freezing levels per context. Although this analysis showed specific days in which groups began consistently demonstrating a significant difference in freezing between contexts, this does not suggest particular groups were discriminating significantly sooner when compared to other groups. It only demonstrates each group was successful at discriminating contexts at some point during the course of the experiment. Examining individual days across groups highlights this result. Here we found that even though there may be a significant difference in freezing levels between contexts for one group on one day, does not mean that this difference is significantly different from the difference in freezing levels per context for another group. Thus, although all mice learned the contextual fear association and could discriminate the contexts at some point during the course of the experiment, their ability to discriminate was not significantly different from each other. This result is similar to previous research studying the effect of chronic immobilization stress on mice with Nr1 knockout in their NMDA receptors (NMDAR) in their CA3 neurons compared to controls 378 . In this study, both transgenic mice and controls were equally capable of discriminating a novel context after chronic stress 378 . Similarly, chronic corticosterone did not appear to differentially impact our GR NPCKO s compared to controls. This further strengthens evidence that GR signaling in abGCs is not prominently involved in contextual fear discrimination.

Whereas neurogenesis has been shown to be necessary for normal pattern separation 265,379 , the lack of effect of GR NPCKO s on contextual fear discrimination could indicate other mechanisms are more critical for this type of memory processing. In particular, NMDA receptors seem to be important for pattern separation 373,376,380 . One study that knocked out the Nr1 subunit in NMDAR in the granule cells of the DG found that while mutant mice had normal fear conditioning responses, they had increased freezing behavior in the nonshock context, and thus reduced discrimination 373 . Also, knock out of the Nr2B subunit of the NMDARs specifically in abGCs resulted in impaired fear discrimination 380 . These NMDARs regulate the glutamatergic and calcium signaling in abGCs, which can lead to long-term changes in synaptic strength and connectivity 380,381 . Thus, although the NR2B knockout mouse model did not affect cell survival, it did reduce dendritic complexity. This may affect the DG/CA3 circuit that facilitates pattern separation 380 .

It is possible that our experimental design did not capture the role GR signaling plays in abGCs as well. One aspect that may make a difference is the timing between context presentations. A recent study on pattern separation found that odor discrimination was impaired by lesions in the ventral DG, but only if the two similar odors were presented 60 seconds apart, since there was no effect if they were presented within 15 seconds from one another 382 . This implies that the DG is important for discriminating similar patterns within a particular time frame. It could be the case that if we altered the length of time between context exposures, it would reveal deficits or improvements in pattern separation for our GR NPCKO mice. Other possible design changes in our experiment that could exhibit behavioral differences include reducing the intensity of the foot shock, and increasing the dissimilarity between the contexts, since mice appeared to be freezing too often in the nonshock context. These changes could help elucidate whether GR signaling in abGCs is truly involved in contextual fear discrimination, or whether other mechanisms, such NMDAR signaling, are a more relevant pathway to continue investigating.

Adult neurogenesis represents a form of neural plasticity that has been conserved across species and may not only influence human brain functioning, but also be manipulated for purposes of repair 100 . Over the last decade, much research has been dedicated to understanding the functional importance of adult neurogenesis. While evidence has suggested roles in mood regulation and memory processing [START_REF] Zhao | Mechanisms and functional implications of adult neurogenesis[END_REF][START_REF] Aimone | Adult neurogenesis: integrating theories and separating functions[END_REF]252,269,276 , results have been inconsistent perhaps due to varied experimental designs that examine these complex cognitive abilities too broadly. Whereas stress, via corticosterone signaling, impacts the behaviors of newborn neurons, and corticosterone, via GR signaling in the brain, regulates similar emotional and cognitive processes as proposed relevant to adult neurogenesis [START_REF] De Kloet | Brain Corticosteroid Receptor Balance in Health and Disease[END_REF]283 , we investigated how chronic corticosterone exposure and GR signaling specific to abGCs affects anxiety-related behaviors, depression-like phenotypes, contextual fear conditioning, and pattern separation in a contextual fear discrimination paradigm. Our results

showed that our chronic corticosterone treatment increased anxiety-like behaviors, did not induce a depressive phenotype, promoted contextual fear conditioning, and did not significantly influence contextual fear discrimination. Reduced GR signaling in abGCs had no impact on any of the behavioral and cognitive functions measured. Interestingly, the functional contributions of our GR NPCKO mice were not revealed until mice were subjected to chronic corticosterone exposure. While no changes were seen in depression-like behaviors, fear conditioning, or contextual discrimination, reducing GR signaling in abGCs significantly attenuated cort-induced anxiety-like behaviors to novel environments.

These behaviors are symptomatic to anxiety disorders, the most common types of mental illness in the US 383 . While the underlying psychopathology remains elusive, anxiety disorders remain a pervasive problem in society, affecting 25% of the population 384 and costing the US government upwards of $45 billion per year 385 . It is clear that reducing the outcome of this disorder would be a major benefit to society.

Additionally, a substantial number of people receive repeated corticosterone injections and glucocorticoid treatments without fully understanding the neurological impact. Our results not only reveal functional contributions of cell-specific GR signaling, but also highlight the relevance of abGCs in responding to environmental changes. Expanding our knowledge of these mechanisms and their functional relevance, both what they do and do not regulate, is critical for better understanding brain functioning, as well as developing more effective therapeutic strategies and novel neuropharmalogical targets.

Materials and Methods

Nestin-CreER T2 and GR loxP/loxP transgenic mice. A mouse model with inducible gene inactivation was used to remove GRs from NPCs and their progeny in the adult hippocampus (mouse line is described in more detail in CHAPTER 2). Briefly, mice homozygous for the Gr loxP allele 239 were mated with the bitransgenic line expressing the CreERT2 recombinase gene under control of the nestin promoter [START_REF] Lagace | Dynamic contribution of nestin-expressing stem cells to adult neurogenesis[END_REF] and a R26R-YFP reporter strain (Jackson Laboratories). This mating required two generations to obtain mutant animals (GR loxP/loxP ;Tg(NesCre ERT2 );Tg(R26R-YFP)), thereafter designated as GR (NesCreERT2) , and a further one to amplify the colony. Activation of CreERT2 is induced by tamoxifen; this leads to Cre recombinase excising the floxed GR gene selectively in nestin-expressing cells, thus creating a temporally controlled nestin-specific loss of GR function and active YFP expression. Vehicle-induced GR (NesCreERT2) mice were used as controls. Vehicleand tamoxifen-induced GR loxP/loxP mice were also tested alongside controls to verify no effects of Cre or tamoxifen. All lines were maintained on a C57BL/6 background and only male mice were used. Ten mice from each group were used in experiment; four mice were excluded from analyses, however, due to post-mortem genetic verification. Animals were bred and raised under a 12h light/dark cycle; temperature was 22±˚C and humidity 60±5%. Food and water were supplied ad libitum. Experiments were performed in accordance with French (Ministere de l'Agriculture et de la Forêt, 87-848) and European Economic Community (EEC, 86-6091) guidelines for the care of laboratory animals.

Tamoxifen induction. Tamoxifen (Sigma T5648) was light protected and dissolved in 10% ethanol and suspended in 90% sunflower seed oil [START_REF] Lagace | Dynamic contribution of nestin-expressing stem cells to adult neurogenesis[END_REF] . Mice received daily injections at 180mg/kg, i.p. for 5 days when they were 5-6 weeks old. Control mice received vehicle (sunflower oil). Mice receiving tamoxifen were only group housed with other tamoxifen-receiving mice; likewise, only vehicle (oil)-receiving mice had vehicle-receiving littermates. This guarded against possible cross-contamination of tamoxifen.

Corticosterone treatment. Corticosterone (Sigma C2505) was prepared as described previously [START_REF] David | Neurogenesis-dependent and -independent effects of fluoxetine in an animal model of anxiety/depression[END_REF] . It was dissolved at 35 µg/mL in a water solution of 0.45% (wt/vol) hydroxypropyl-beta-cyclodextrine (Sigma 332593) by sonication for two hours. Corticosterone treatment was given to mice in opaque water bottles to protect it from light, changed twice a week, and available ad libitum. Behavior testing started after 4 weeks of corticosterone treatment and treatment continued throughout all behavioral testing.

General behavioral apparatus and procedures

Locomotor. Mice were introduced into circular chambers (4.5 cm width, 17 cm external diameter) crossed by four infrared captors (1.5cm above the base) placed at every 90˚ (Imetronic, Bordeaux, France). Locomotor activity was measured by number of quarter turns every 5 minutes for 5 hours. Quarter turns were counted when mice interrupted two successive beams, thus having traveled a quarter of the circular corridor.

Open field. Motor activity was quantified over 9 minutes in a 100x100x30cm2 white Plexiglas field with the center brightly illuminated (~980 lux) compared to the corners (~100 lux). The computer recorded the session and defined grid lines that divided the box into 9 equal squares, with the center square consisting of four lines 33.3 cm from the wall. The mice were acclimated to the room at least 15 minutes prior to testing and the open field was cleaned between mice.

Light/dark test. The light/dark test was conduced in a chamber measuring 45x20x25cm. Less than half of the chamber is an enclosed opaque dark box (black PVC) with a small opening to allow passage (5cm x 5cm) into the lit portion (white PVC). The lit portion was approximately 50 lux, while the dark portion was approximately 2 lux. The mice were acclimated to the room at least 15 minutes prior to testing and chambers were cleaned between mice. Mice were placed in the dark chamber and testing began once the lid to the dark chamber was closed. Mice were given 9 minutes to freely explore both chambers in a quiet environment.

Elevated Omaze. The testing apparatus consisted of an elevated (80 cm) opaque plastic platform in the shape of an "O" (70cm diameter). The ring was divided into 4 alternating open (100 lux) and closed (60 lux) arms.

Closed arms contained 8 cm high walls. Mice were placed facing within the closed arm and allowed to freely explore the platform for 6 minutes while being recorded. A mouse was considered to exit or enter into arms when all 4 paws were introduced. Platform was cleaned between mice and any animals that fell off the platform during testing were excluded from analyses.

Novelty-suppressed feeding. Mice were food-deprived for 16 hours prior to testing and their weights were recorded pre-and post-deprivation. The testing apparatus consisted of a 45x45x30cm box covered with approximately 2cm of fresh bedding. Two food pellets were secured to a white paper platform in the center of the arena. A light was shone above the center onto the food as well, such that the box center was 800 lux and the corners were 100 lux. Mice were placed in the corner of the box and recorded for 30 minutes. Once a mouse ate, defined as sitting on its haunches and biting the pellet with its forepaws, or reached the 30 minutes, the center light was turned off and they were given 6g of food in the box corner and 5 minutes to continue feeding. After this, mice were reweighed and returned to their homecage. Boxes were cleaned in between mice.

Porsolt forced swim. As described previously 241 , mice were placed into clear cylinders (30cm tall, 9.5cm diameter, filled with 15cm of 22˚C-24˚C water) and videotaped for the entire session. Immobility, defined as floating with minimal limb movement to keep head afloat, climbing behavior, and latency to float were measured for 6 minutes; only the last 4 minutes were analyzed.

Contextual fear discrimination. Mice were exposed to two contextually similar chambers, but only one of which was always paired with a two second shock (0.75mA). In this task, they were evaluated for their ability to discriminate between the two environments, as measured by their freezing behavior. Both chambers were essentially equal in appearance, including a grid floor, but with a few differences: chamber A was cleaned between mice (grid and waste tray) with 75% ethanol, and had closed doors with a houselight and fan, metal walls, and a lemon scent in the bedding under the grid floor, and the mice were transported there in an opaque enclosed bucket with a layer of bedding; chamber B was cleaned with non-ethanol cleaner, and had open doors (natural light), no running fan, blue plastic walls, and an orange scent in the bedding under the grid floor, and mice were transported there in a clear open bucket with no bedding. The experiment proceeded as follows: on day 0, mice were placed in chamber A. They were given 185s to explore, then received the shock, and were removed 15 seconds later and returned to their homecage. On day 1, mice were re-exposed to chamber A as in day 0. Two hours later, they were placed in chamber B. In chamber B, they were given 185s

to explore and then returned to their homecage. For days 2-13, mice were exposed to both chambers in a random manner with a 2 hour delay between chambers. There was no training (rest) for either chamber on day 10. Mice were recorded in both chambers and freezing, defined as complete stillness except for necessary breathing movements, was measured every 5 seconds for 3 minutes. Corticosterone treatment preceding novelty-suppressed feeding significantly increased the latency to approach food for control mice, but not for GRNPC KO ones. However, cort-treatment similarly affected latency to eat for control mice compared to GRNPC KO mice. (B) Cort-treated control mice exhibited significantly increased latency to enter the aversive lit compartment compared to GRNPC KO mice. Similarly, cort-treated control mice exhibited a trend of decreased time spent in the lit compartment, but no effect of corticosterone was seen on GRNPC KO mice. Likewise, the number of 4 paws entries showed a significant interaction effect between genotype and corticosterone treatment. There were no differences between GR NPCKO mice with or without cort-treatment for all parameters measured in the light-dark box. Chamber A always administered a foot shock, while chamber B was always neutral. Freezing was measured depicted for all groups on individual days. There were no significant differences exhibited between levels of contextual freezing for any group on day 4 (C). However, by day 9, cort-treated controls were showing a significant contextual difference in freezing behavior (D). By day 12 (E), this difference persisted in cort-treated controls, and was now found in nontreated and cort-treated GRNPC KO mice. This difference remained significant for these groups by day 13 (F) and appeared in nontreated controls as well. Results are expressed as mean ± s.e.m. *, p<0.05; §, p<0.01; #, p<0.001.

Figures and Legends

Introduction to general discussion

Stress is one of the strongest and most ecologically relevant mediators of adult neurogenesis. It has been shown that this occurs through stress-induced elevations in corticosteroids, which act through GR to reduce proliferation and differentiation of NPCs (see CHAPTER 1). How GR signaling regulates this process and whether it has any impact on behavior or cognition has long been speculated but is still unknown. While many studies have shown that disrupted GR signaling impacts mood and memory functions, the functional impact of newborn neurons on brain function is still a hot controversial topic. This is due to different methods of ablating neurogenesis that only occasionally elicit behavioral deficits [START_REF] Schoenfeld | Stress, stress hormones, and adult neurogenesis[END_REF][START_REF] Petrik | The neurogenesis hypothesis of affective and anxiety disorders: are we mistaking the scaffolding for the building?[END_REF] . To carefully manipulate the role of GR signaling within newborn neurons and investigate whether it can mediate behavior would necessitate newer animal models. Previous research on GR function using various pharmacological and genetic manipulations in animal models has been conducted. However, since GR influences a pleiotropy of functions that vary among hippocampal cell types 386 , it is important to isolate the role GR signaling in NPCs to understand how these regenerative cells contribute to behavioral responses under stress.

To investigate whether the effects of stress on neurogenesis are responsible for some of the stressinduced emotional and cognitive changes, we examined GR signaling within NPCs. A goal of my dissertation was to isolate the effects of GR specifically in newborn neurons and characterize their contributions to cell development and behavior. We generated an inducible Cre-mediated inactivation of GR gene function specifically in nestin-expressing cells and their progeny. After this genotype was induced in adulthood, we measured changes in neurogenesis as discussed in CHAPTER 2. Our results suggested that the effect of corticosterone directly on NPCs was not the driving force to suppress differentiation in vivo and this reduced neurogenesis must be indirectly mediated through GR in other cell types. We then characterized how this cellspecific GR gene inactivation model responded behaviorally under either basal levels of corticosterone or chronic corticosterone treatment. These characterizations were made in CHAPTER 3 using non-induced GR NPCKO mice as a control. We found that control mice had increased anxiety-like behaviors under chronic corticosterone treatment, however, cort-treated GR NPCKO s behaved similarly to untreated controls.

In this chapter, I will discuss the contributions of these findings to our understanding of GR function and adult neurogenesis in a broader context. I will compare our GR NPCKO model to other GR animal models, assessing its advantages and disadvantages. I will then propose how my results fit in with our current understanding of GR function and highlight the adaptive role of newborn neurons to novel encounters. Lastly, implications and future directions of this research will be discussed.

Cre-mediated GR gene inactivation in NPCs

A comparison of our GR NPCKO mouse model with other models

Previous research has helped elucidate GR functioning through assorted pharmacological and genetic models. Various GR agonist and antagonists can be injected or consumed to manipulate GR function, however, these methods are transient, partial, nonspecific, and are typically incapable of passing the blood brain barrier 214,387 . Whereas more controlled manipulation of GR is not possible (or unethical) in humans, advanced genetic techniques have generated mouse models that control expression and function of GR.

While some of these mouse models have demonstrated GR affects on anxiety-and depressive-like behaviors, spatial or contextual memory, HPA regulation, and basal corticosterone levels, others have not (see Table 4.1). This may be due to the technique employed, the expression levels, the tissue affected, and any side effects of the manipulation that make it difficult to deduce GR influence. Our GR NPCKO model attempts to correct for these concerns by being cell-type specific (NPCs), inducible, centrally noninvasive, and having complete GR gene inactivation in affected cells.

Since GR has a pleiotropy of effects that are tissue-specific [START_REF] De Kloet | Stress, genes and the mechanism of programming the brain for later life[END_REF]386,388,389 , we focused on adult newborn neurons and how GR signaling in this regenerative pool of cells influences function. Some of the first mouse models of GR function reduced expression, however, this reduction occurred in many different types of tissue 240,249,[390][391][392] . Indiscriminate targeting introduces peripheral changes in immune function and metabolism that conflict correlations between GR and behavior. Thus, even though all of these studies found impaired HPA regulation 240,249,[390][391][392] , unsurprisingly, the behavior results are conflicting. A more recent study knocked down GR expression in the neurogenic niche and purported direct effects of GR signaling regulating neurogenesis 218 . Although this study had a more tightly controlled manipulation of GR expression, knock down was achieved by short hairpin RNA interference delivered by lentiviral injections. Lentivirus can enter any cell type, thus while it may have infected some newborn neurons in the neurogenic niche it was injected into, the possibility of infecting different cell types at different stages of maturity could not be controlled for with this technique. A more substantial ablation of GR function was achieved by the Cre/lox recombination method 239,[393][394][395] . In these studies, GR function was ablated throughout the brain 239 , or throughout the limbic system [393][394][395] . This GR inactivation model appeared to have reduced anxiety-like behaviors, however, due to increased general locomotor activity and hyperadrenalism in the paraventricular nucleus, attributing suppressed anxiety to solely GR is confounded by these added variables. To our knowledge, ours is the first study that specifically addressed GR function in a specific cell type, NPCs.

Our GR NPCKO model was also advantageous compared to past and current models by being inducible.

Although prenatal deletion of GR is lethal 239,396 , previous studies have examined partial GR deficiency from birth through adulthood [START_REF] Reichardt | DNA binding of the glucocorticoid receptor is not essential for survival[END_REF]239,249,393,394,[397][398][399][400] . The results of these studies, however, are difficult to distinguish from developmental side effects and compensatory mechanisms that may arise over time. By temporally controlling GR gene inactivation, we could bypass these confounds as well as isolate the role of GR in adult neurogenesis.

There can always be inflammatory and other damaging side effects that result from injections into brain tissue. By using CreERT2 recombination, our mouse model can be induced by intraperitoneal injections of tamoxifen. Other methods that manipulated GR function used invasive procedures that require anesthesia as well as surgical incisions into brain tissue 218,401,402 . Although these methods allow for temporal control of GR function and use carefully constructed controls when examining differences, the possibility of anesthesia, inflammation, cell death, or infections alone or in combination with GR changes can confound behavioral and physiological results.

Another advantage of our mouse model is that the GR genes in cells targeted by Cre recombination are consequently inactivated, a mutation stably passed onto their progeny. Other methods to study GR function, such as antisense RNA inhibition, short hairpin RNA interference, and use of GR heterozygous mice (Gr-/+), can only reduce GR expression within cells 218,240,249,390,391 . One study attempted to ablate GR function by deleting exon 2 in the GR alleles 392 , however this deletion still expressed a GR gene that was at least partially functional [START_REF] Hooijdonk | Glucocorticoid receptor knockdown and adult hippocampal neurogenesis[END_REF] . Our GR loxP alleles target exon 3, which when excised, causes a frame shift in the transcriptional readout, ultimately resulting in a nonfunctional GR. Additionally, other studies have examined overexpression of GR 249,398,403 or impaired the morphology of GR [START_REF] Reichardt | DNA binding of the glucocorticoid receptor is not essential for survival[END_REF][399][400][401][402] to examine function. Importantly, these have allowed for comparison of functions, although not all of these studies were designed to examine GR function 401,402 . Furthermore, impaired dimerization, although not a complete inactivation of GR gene function, allowed for GR-induced actions to be examined more mechanistically. In any case, none of the aforementioned GR mouse models can precisely examine the (cell-autonomous) contributions of GR activity in adult newborn neurons to cytoarchitecture, or behavior and cognition. By being cell-type specific (NPCs), inducible, centrally noninvasive, and having complete loss of GR function in affected cells, our GR NPCKO model can address these questions.

Assessment of our GR NPCKO mouse model

Despite the advantages our GR NPCKO mouse model offers us in studying adult neurogenesis, there are several disadvantages to our method as well. First, nestin is a protein expressed in most NPC cell types (type 1, 2a, and 2b), however, not all types, that being type 3 [START_REF] Garcia | Age-dependent expression of glucocorticoid-and mineralocorticoid receptors on neural precursor cell populations in the adult murine hippocampus[END_REF][START_REF] Kempermann | Milestones of neuronal development in the adult hippocampus[END_REF]107,363 . This NPC type is slightly more restricted in its developmental potential compared to type 1, and though it is more proliferative, these abilities are limited [START_REF] Kempermann | Milestones of neuronal development in the adult hippocampus[END_REF] . Thus, not every hippocampal NPC was affected. Additionally, our nestin-CreERT2 mouse targeted nestin-expressing cells in the subventricular zone as well. By examining hippocampal-dependent functions, we attempted to separate the role of GR in each area, however, it is always possible GR NPCKO in the subventricular zone adds to these effects. Moreover, Cre recombination is not 100% efficient and does require an injection of tamoxifen. Although controls injected with tamoxifen behaved similarly to noninjected controls, an ideal model would not require this slight stressor. Overall, GR gene inactivation in our mice was sufficiently reduced by 50%. Though this was not a complete GR inactivation in NPCs, due to non-nestin-expressing NPCs, and inefficient Cre recombination and tamoxifen induction, we believe this can also be a benefit to our model, as it more accurately resembles naturally occurring dysfunction with the GR.

Another disadvantage to our experimental model is not all controls groups were possible for comparison. In particular, GR gene inactivation specifically in ~10% of other (matured prior to adulthood) granule cells would be an appropriate control. Any differences between our GR NPCKO s and this one would suggest that the results were truly due to GR function specifically in NPCs and not just a reduction in overall GR signaling. An ideal GR NPCKO model may also include more temporal control over GR function, such as those achieved through optogenetics. In these mice, behavior testing could be compared prior to or after GR loss, such that one could examine cognitive skills due to GR NPCKO and then reexamine these skills once GR function was reinstated. Undoubtedly, proper controls for re-exposure to similar paradigms would need to be implemented. It is important also to acknowledge the results from mouse models may differ significantly from those performed on rats. It would be an interesting comparison to examine GR function, particularly in NPCs, within the rat species since much stress research is outlined in the rat, and neurogenesis is more proliferative and contributes more abGCs to the DG in the rat as well [START_REF] Snyder | Adult-born hippocampal neurons are more numerous, faster maturing, and more involved in behavior in rats than in mice[END_REF] . Unfortunately, genetic manipulations of GR (or anything) in a rat model are typically not successful or currently available. In any case, these additional controls and techniques are advantages our model does not provide.

Lastly, generation of our GR NPCKO mice for our experimental design resulted in some potential confounds. First, whereas litters had varying numbers of CreERT2-expressing male mice, it was difficult to control for the ultimate number of littermates per cage for our mice. Due to this, and any unexpected death, our mice varied in having zero to four littermates within their cage. Particularly in cages receiving chronic corticosterone treatment, stressed littermates can add additional stress to the environmental situation confounding cort-specific effects on behavior. Additionally, due to varying litter sizes, it was difficult to generate enough mice per group within a close age range. Each experimental group controlled for age differences, however, within group, this could mask or reduce effects, and any subsequent mouse deaths or removals due to inefficient recombination in the brain, may upset the balance of age discrepancies between groups.

GR signaling in the neurogenic niche

Results When manipulations were made to adult neurogenesis, that being reduced GR signaling within abGCs, there was no observable phenotype for baseline behaviors. This is not so surprising since GR is mostly occupied, and thus activated, when circulating corticosterone levels are elevated [START_REF] De Kloet | Brain Corticosteroid Receptor Balance in Health and Disease[END_REF][START_REF] Reul | Two receptor systems for corticosterone in rat brain: microdistribution and differential occupation[END_REF] . However, whereas in the absence of stress, corticosterone can be elevated naturally due to its circadian and highly pulsatile fluctuations 404 , and the simple act of handling mice or exposing them to a new environment can raise corticosterone levels 405,406 , it would not be curious to have a behavioral phenotype prior to our chronic corticosterone treatment. We did not find this in our manipulated abGCs; similarly, other studies manipulating neurogenesis do not find a phenotype for baseline behaviors (for review [START_REF] Petrik | The neurogenesis hypothesis of affective and anxiety disorders: are we mistaking the scaffolding for the building?[END_REF] ). It often appears that a behavioral phenotype is exhibited when the rodent is presented with novel environmental stimuli, communicated to the brain by elevated corticosterone levels 107,273,407 . This suggests that the functional contributions of abGCs are for adaptive purposes, adjusting emotional responses and memory associations to prepare for future scenarios. This role of abGCs would promote survival as well as reproductive success in a dynamic environment.

A recent article supports this hypothesis and elaborates that adult neurogenesis functions to regulate the stress response and assign stress salience to sensory context 408 . This idea initially arose from evidence that the dentate gyrus (DG) helps mediate the stress response and novelty detection in both rodents and humans 373,[409][410][411] . Lesions to the DG caused a reduced corticosterone response to a novel environment that did not habituate with repeated trials 412 . This appeared to be specific to novelty since the corticosterone response was normal for other stressors like laparotomy and ether exposure 412 . Further investigation revealed that novelty detection might specifically involve abGCs 265,266 . Similarly, these newborn neurons have a bidirectional relationship with stress. First, stress impacts neurogenesis, most likely in an indirect manner according to our results. Although the functional importance of abGCs is hotly contested, the sensitivity of adult neurogenesis to chronic elevations in corticosterone levels is consistently reproduced. Secondly, neurogenesis impacts the HPA axis 408 . Reduced neurogenesis resulted in enhanced corticosterone secretion, thus, dysregulating negative feedback 413 , as well as impaired recovery from a social stressor 407 . Furthermore, due to their connections to the limbic circuitry and vasculature, abGCs are well positioned to sense stress and adjust the response accordingly 408 . GR signaling within abGCs is specifically implicated in mediating stress recovery and the development of chronic stress-related conditions due to its role in regulating the gradual influx of calcium currents 414 . Overall, this evidence suggests that abGCs may be involved in assigning stress salience to novelty during learning. Thus, if this construction is dependent on the stress state of the animal, these cells might only become specifically engaged under stressful conditions 408 .

The highly plastic nature of the hippocampus and its high concentration of GRs suggest that one of its functions are to be sensitive to stressors and adapt affect and memory processing accordingly. It is proposed this may be mediated by GR signaling within abGCs, producing a behavioral phenotype conditional to stress. Thus, we suggest that this functional role is taken into consideration for future evaluations of adult neurogenesis.

How is corticosterone impacting abGC function if not through suppressed neurogenesis?

When stress is chronic or overwhelming, this may impair neurogenesis and produce maladaptive responses, such as mood dysregulation. While the link between chronic stress (via elevated stress hormones)

and anxiety-like behavior has long been recognized, the underlying mechanisms remain largely hypothetical.

Neurogenesis was first proposed to be involved when Gould and colleagues reported that both stress and corticosterone suppressed neurogenesis [START_REF] Gould | Adrenal hormones suppress cell division in the adult rat dentate gyrus[END_REF][START_REF] Gould | Adrenal steroids regulate postnatal development of the rat dentate gyrus: I. Effects of glucocorticoids on cell death[END_REF] , a finding that correlated with postmortem tissue showing reduced hippocampal volume in depressed patients [415][416][417] and patients with PTSD 418,419 . It was then followed by a study showing that chronic antidepressant treatment increased neurogenesis 420 . This established the idea that neurogenesis could be responsible for the development or treatment of affective disorders, however at this time, evidence was only correlative. To test this, Hen and colleagues compared antidepressant efficacy in mice with ablated neurogenesis to mice with intact neurogenesis [START_REF] Santarelli | Requirement of hippocampal neurogenesis for the behavioral effects of antidepressants[END_REF] . When it was shown that antidepressants

were not effective in neurogenesis-deficient mice, the "neurogenesis hypothesis of affective disorders" was established [START_REF] Petrik | The neurogenesis hypothesis of affective and anxiety disorders: are we mistaking the scaffolding for the building?[END_REF] . Since then, many reviews have discussed the cumulative evidence that is highly conflicting and conclude that while intact neurogenesis may aid in antidepressant efficacy in a stressed animal model, reducing neurogenesis does not induce anxiety-or depressive-like behaviors [START_REF] Petrik | The neurogenesis hypothesis of affective and anxiety disorders: are we mistaking the scaffolding for the building?[END_REF]271,348,[421][422][423] . This latter association was only shown to be correlative, and not causal. Indeed, the research presented in this thesis addresses this controversy. We show a model in which neurogenesis was reduced by chronic corticosterone treatment, yet remained resilient to cort-induced anxiety-like behaviors. Similarly, other studies corroborate our results, demonstrating neurogenesis ablated by x-ray irradiation led to increased resiliency in a social avoidance task with mice 107 , and mice with reduced neurogenesis from selectively deleting cell cycle checkpoint kinase, ATR, also exhibited reduced anxiety-like behaviors in three different tasks 424 . In any event, chronic corticosterone treatment still induced anxiety-like behaviors in our control mice, and our results

suggest that newborn neurons are involved. However, these behaviors were not mediated by reduced numbers of abGCs; instead, it appears they can be mediated by GR signaling within abGCs.

Our experiments demonstrate that two mouse groups with cort-suppressed neurogenesis behaved differently under chronic corticosterone treatment. Adult neurogenesis refers to newborn cells that range in developmental stage (i.e. NPCs, immature neurons, and abGCs). Thus, at each stage, these cells have varying degrees of impact on the hippocampus. Cort-suppressed neurogenesis occurs during the NPC or immature neuronal stage, a phase in which cells are not quite functionally integrated. Therefore, a reasonable hypothesis would be that these two mouse groups with cort-suppressed neurogenesis behaved differently under chronic corticosterone treatment due to GR signaling within the abGCs.

While this hypothesis requires further support, GR signaling within abGCs would be largely capable of regulating protein production, such as NMDA 162,295,296 ,AMPA 191,425 , and 5-HT 1A receptor levels 426,427 , morphology, such as dendritic arborization 218,428,429 and hippocampal positioning 218 , and synaptic potentiation, such as LTP/LDP [430][431][432] and Ca2+ signaling [433][434][435] . Any of these changes could potentially contribute to the complex set of symptoms or etiology underlying mood and affect disorders. Despite limited numbers, it is important to note that abGCs are well positioned in the hippocampus to mediate emotional responses 408 , and due to their hyperactivity, well equipped to make a disproportionately larger impact than mature granule neurons nearby 310,436 .

It is important as well to recognize that our results, while they suggest GR activation in abGCs induces anxiety-like behaviors, they only actually show that inactivating or reducing GR signaling in abGCs can attenuate cort-induced anxiety-like behaviors. This means that it is possible that GR is not the pathway responsible for fearful responses, but that somehow blocking its activity will eliminate this corticosterone effect on behavior. In other words, GR activation in abGCs does not drive this behavior, but somehow it indirectly supports the cells or actions that are significantly more responsible. One potential explanation could be if reducing GR signaling disrupts the MR/GR balance, and aberrant MR signaling enhances resiliency [START_REF] Saaltink | Stress, glucocorticoid receptors, and adult neurogenesis: a balance between excitation and inhibition?[END_REF] .

Additional evidence to suspect MR involvement comes from studies demonstrating that raised corticosterone levels could rapidly enhance glutamate release and excitatory transmission in hippocampus through pre-and post-synaptic non-genomic MR 437,438 . Another possibility could be GR activity in glial cells or even other limbic structures perpetrate these maladaptive responses and GR activity in newborn neurons functions to maintain supporting factors. A third potential explanation could be that GR deficiency in abGCs impairs overall ability to detect novelty, thus, the animal would not register being in an unfamiliar environment, suppressing fearful responses. This idea was proposed by Dranovsky & Leonardo (2012), suggesting that GR activity in abGCs functions to assign a stress value to a contextual cue (novel paradigm) the way the amygdala assigns an emotional value to a sensory cue; the latter increases the strength of the memory trace 439 , while the former may increase the anxiety-like response 408 . Nonetheless, it is important to realize our behavioral results could just be caused by an overall reduction in GR signaling in the DG, and may not be specific to abGCs. At this point, we can only speculate.

My results challenge the current neurogenesis hypothesis of affective and anxiety disorders; namely, that decreased neurogenesis results in depression or anxiety. We show our experimental group had decreased neurogenesis induced by corticosterone, however they did not exhibit a depressive or anxious phenotype. This suggests that specific GR-regulated mechanisms in abGCs are capable of suppressing anxiety-relevant behaviors. How this is accomplished warrants further investigation.

GR signaling in abGCs: implications for stress-related brain disorders

The research presented in this thesis addresses the cell-autonomous role of GR in adult neurogenesis, the functional and cellular impact of chronic corticosterone exposure, and the functional significance of not only newborn neurons, but also GR activity within newborn neurons. Human neuroimaging studies have implicated hippocampal dysfunction in mood and anxiety disorders 416,[440][441][442][443] , and these human psychopathologies are also associated with a dysregulated HPA axis, impaired GR signaling, and aberrant corticosterone levels as well [START_REF] Mcewen | Stress, adaptation, and disease. Allostasis and allostatic load[END_REF][START_REF] De Kloet | Brain Corticosteroid Receptor Balance in Health and Disease[END_REF][START_REF] Holsboer | The corticosteroid receptor hypothesis of depression[END_REF]226,283,[444][445][446] , suggesting the relevancy of this research is not restricted to animals. Additionally, patients with psychotic mood dysregulation demonstrated significantly improved mood when treated with GR antagonists 447,448 . Altogether, data presented here has implications for stress-related brain disorders, including generalized anxiety, PTSD, social anxiety, suicidality, panic disorders, and obsessive-compulsiveness, and demonstrate a model of stress resilience in spite of suppressed neurogenesis.

First, our results suggest stress-and cort-effects on neurogenesis are indirect and challenge the hypothesis that reduced neurogenesis can cause anxiety or depression. Indeed, this causal link was not found in many studies [START_REF] Santarelli | Requirement of hippocampal neurogenesis for the behavioral effects of antidepressants[END_REF][START_REF] David | Neurogenesis-dependent and -independent effects of fluoxetine in an animal model of anxiety/depression[END_REF][START_REF] Surget | Antidepressants recruit new neurons to improve stress response regulation[END_REF]253,255,260,272,273,275,287,288,407,[449][450][451][452][453] , antidepressant efficacy did not correlate with increased neurogenesis [START_REF] Petrik | The neurogenesis hypothesis of affective and anxiety disorders: are we mistaking the scaffolding for the building?[END_REF]451 , and increasing neurogenesis did not produce any anxiolytic or anti-depressant -like behavioral effects [START_REF] Sahay | Increasing adult hippocampal neurogenesis is sufficient to improve pattern separation[END_REF] . Our results suggest redirecting attention from reduced proliferation or differentiation to GR-regulated proteins or pathways in abGCs that may mediate or provide relief from anxiety-like responses.

Whereas too many people suffering from anxiety-related disorders do not receive relief from pharmacological treatment or relief is delayed, subdued, or even temporary 454,455 , understanding the etiology or underlying mechanisms of symptom relief will allow for the development of more targeted pharmaceutical therapies, as well as better prevention and intervention strategies. Though we found no causal link between a depressive phenotype and our chronic corticosterone treatment or an effect of reduced GR signaling in abGCs, it is also important that these results are taken into consideration for understanding the etiology of depressive symptoms (perhaps not GR regulated) or targets for anti-depressants. In general, studying the relationship between abGCs and stress regulation could be clinically relevant to determining antidepressant efficacy [START_REF] Petrik | The neurogenesis hypothesis of affective and anxiety disorders: are we mistaking the scaffolding for the building?[END_REF] and clarifying levels of resilience to stress 456 . Our results not only provide a model for directly studying the effects of stress (and cort) on adult hippocampal plasticity, but also advance the understanding of the underlying mechanisms of mood and affective disorders.

In addition to its relevancy to stress-related disorders, our results have implications for diseases marked by hypercortisolism, such as Cushing's disease, as well as patients receiving exogenous corticosterone treatment for various conditions, such as rheumatoid arthritis, sports injuries, and other inflammatory diseases. We show that chronic exposure to elevated corticosterone can suppress neurogenesis, induce anxiety-like behaviors, and enhance fear conditioning. Specifically reducing GR activity in newborn neurons appears to block this effect on anxiety-like responses. While this may be relevant for understanding symptoms and medication side effects, this information may also lead to the development of more targeted treatment options or therapies with reduced side effects. Overall, understanding cell-specific GR signaling and functional contributions of adult neurogenesis will better inform us of pathways involved in brain plasticity, and help develop better therapeutic strategies when these pathways are dysfunctional.

Future directions

The GR NPCKO model discussed in this dissertation highlights the stress-adaptive role of both newborn neurons and GR signaling, addressing long-lingering questions in the field regarding functional relevance. This in turn incites further queries into underlying pathways and other aspects of behavior that may be affected by additional experimental adjustments. Some of the underlying mechanisms that could be studied include the GR-independent pathways that corticosterone induces suppression of neurogenesis and whether the functional consequences of GR-deficiency on behavior is mediated differentially along the dorsal-ventral axis of the hippocampus. Hen and colleagues have demonstrated with irradiation 251 and optogenetic controlling 317 that newborn neurons in the dorsal DG appear to play a role in contextual memory processing, while neurogenesis in the ventral DG plays a role in emotional processing. This is thought due to the dorsal region projecting to associational cortical regions, while the ventral region projects to frontal cortex, amygdala, and hypothalamus [457][458][459][460] . It would be an interesting future experiment to either impair or temporally-control GR activity separately in dorsal versus ventral abGCs and characterize the behavior differences that may be attributed to each region.

Other immediate future directions include re-examining depressive-like symptoms and memory processes that may be affected by additional experimental adjustments. First, I would suggest adjusting the cort-dosing and/or treatment length to an amount that does produce depressive-like behaviors in the FST in an ecologically valid manner. I would then use this protocol to re-examine the influence of GR-signaling in abGCs. Additionally, I would also incorporate treatment with antidepressants and since these should reduce immobility in the FST, investigate whether our GR NPCKO model blocks these effects. This may help elucidate the underlying mechanisms in which antidepressants attenuate depressive symptoms. In our experimental design, mice in the FST were able to see one another during testing. I would suggest testing mice in a bordered/closed apparatus such that the behavior or presence of other mice did not influence their own behavior. Also, it would be interesting to examine these behaviors in a different mouse strain, such as Balb/c mice. These mice have higher emotional and stress reactivity 370,372 and apparently do not have the GR polymorphisms found in many mice of the C57 family 399 . Having at least one allele of this GR polymorphism correlated with a lower stress response but increased anxiety-like behaviors 399 . Any or all of these changes may reveal a role of abGCs in depressive-like symptoms via GR signaling.

In re-examining memory processes, I would better optimize the contextual fear discrimination protocol so that it was clear my control mice were successfully and consecutively discriminating for at least 3 days. This may involve changing the amplitude of the shock, the time between exposures to each context, or even the degree of dissimilarity between contexts. Much of the testing done in this experiment was performed at the end of the day. Thus, it would be interesting to see if behavior is different if testing is conducted in the morning, which is likely so due to the proposed dependence of adult neurogenesis on circadian cues 461,462 and fluctuating hormone levels 463 . Moreover, if possible, I would recommend testing all of the behaviors during the mice's dark cycle, the time in which mice are actually awake and active, although this may make it difficult to compare behavior results across the literature. It is worth noting that most behavior testing in the literature occurs during the mice's light cycle, when they prefer to sleep, thus, it would not be unsubstantiated to criticize that behavioral differences found, ours included, are confounded by sleep deprivation or disturbances.

Aside from changes to the experimental design, it would be worth testing GR NPCKO mice in other tests of pattern separation or spatial memory, and examining fear extinction as well. Mice did not receive contextual fear conditioning on day 10 of our contextual fear discrimination protocol. Interestingly, on day 11, discrimination ability appears to decrease for control mice (freezing was similar in both contexts), whereas it did not for GR NPCKO mice (freezing remained different in each context). Although there was not a significant genotype effect when comparing across all groups, there is a potential difference in fear-associated memory extinction that warrants further investigation. Whereas both GR and adult neurogenesis are implicated in memory processing, future research would benefit from thoroughly examining their roles and how they interact in these cognitive realms. Dissecting the functional and morphological contributions of the glucocorticoid receptor gene in neural progenitor cells of the hippocampus.

Abstract : Stress hormones are known as one of the strongest and most ecologically relevant mediators of adult neurogenesis. A lingering question in adult neurogenesis is whether these hormones, known as glucocorticoids (CG), act directly on neural progenitor cells (NPCs), or indirectly through secreted factors or changes in network activity. To address these unknowns, we generated a transgenic mouse model whose GC receptors (GRs) could be inducibly inactivated specifically in NPCs. We investigated the effect of this cellspecific GR knockout model on hippocampal survival and differentiation and found them to be similarly affected by chronic GC treatment compared to controls. This implies that GC-suppressed neurogenesis and its impact on morphology is indirect, and GR in other cells may be mediating the effects. Furthermore, mice with GR inactivation in newborn neurons behaved similarly to controls in all tasks observed under basal levels of GC. When mice were chronically treated with GC, however, controls exhibited an anxious phenotype in novelty-suppressed feeding (NSF), light/dark box, and elevated Omaze, whereas transgenic mice behaved like untreated control groups in all anxiety measures except latency to feed in NSF. Neither GC nor inactivation of GR in adult-born neurons altered depression-like behaviors in the forced swim test, nor percent freezing in contextual fear discrimination. Lastly, we found that GC increased the rate of learning in 1-trial contextual fear conditioning, an effect not mediated by reducing GR signaling in the neurogenic pool. These results highlight the functional contributions of adult neurogenesis as well as how their GRs mediate anxietyrelevant behaviors irrespective of suppressed neurogenesis.

Key words : neurogenesis, hippocampus, glucocorticoid receptor, stress response, behavior, conditional mutagenesis

Figure 1 .

 1 Figure 1. The HPA axis. When the brain perceives a threat, a coordinated cascade of events occurs in response.Neurons in the hypothalamus release CRH and arginine vasopressin (AVP), which induce the secretion of ACTH from the pituitary. ACTH then triggers the adrenal cortex to produce glucocorticoids known as corticosteroids (cort).Corticosterone can pass through the blood brain barrier and interact with two receptors, MR and GR. Activation of these receptors can trigger feedback loops that can inhibit further activity of the HPA axis and return the system to a homeostatic point. Photo credit9 .

Figure 2 .

 2 Figure 2. Corticosterone secretion throughout day in freely moving rat. Corticosterone plasma is released in a circadian pattern that consists of ultradian (pulsatile) oscillations. During rodents' night cycle (awake), corticosterone secretion is higher and as they enter their light cycle (sleep), levels begin to decline. Photo credit 15 .

Figure 4 .

 4 Figure 4. Structural organization of glucocorticoid receptor. Schematic 1 dimensional (1D) amino acid sequence of nuclear receptors, such as GR. Sequence C represents the DBD (where receptor interacts with DNA) and sequence E represents the LBD (where hormone binds), both shown below as 3 dimensional (3D) structures.

Figure 5 .

 5 Figure 5. Proposed development of newborn neurons in the dentate gyrus of the hippocampus. Neuronal development in the adult hippocampus can be readily identified in stages on the basis of morphology, proliferative ability, and expression of markers such as nestin, GFAP, DCX, calbindin and NeuN. Development begins from the putative stem cell (type-1 cell; stage 1) that has radial glia and astrocytic properties and potentially unlimited proliferative ability, although it is more quiescent. The next stages of neural development include transiently amplifying progenitor cells (type-2a, type-2b and type-3 cells; stages 2-4), which proliferate more rapidly, although with limited self-renewal, and lineage is more restricted. Note that type-2b cells were not found to express GR. The next stage marks the early postmitotic period, where cells differentiate and migrate into the GCL. Finally, the last two postmitotic stages are characterized by NeuN expression and spine formation. This progresses until the cell is functionally integrated, extending its axons to the CA3 region, and is now a mature granule cell with large and complex synapses. Adapted from35,37,40,44,74 .

  often measured and quantified using the following tasks: open field, light/dark box, elevated Omaze or plus maze, and novelty-suppressed feeding. Each of these tasks compares rodents' natural inclination to avoid open spaces, bright lights, and elevated platforms with their curiosity to explore novel spaces[227][228][229][230] . So while a secure, calm rodent will spend more time exploring in the center of a novel arena, an anxious rodent will hug the borders of the arena and avoid lit or exposed spaces. In studies of acute corticosterone treatment, researchers found increased exploration of open arms in the elevated Omaze 231 , but only increased risk assessment behavior, and not conventional anxiety measures, in the elevated plus maze and open field tests232 . Even acute inhibition of corticosterone by metyrapone, a corticosterone synthesis blocker, only affected the risk assessment behaviors associated with a passive coping response to anxiety 232 . When rodents were under chronic corticosterone treatment, they had increased latency to feed in noveltysuppressed feeding[START_REF] David | Neurogenesis-dependent and -independent effects of fluoxetine in an animal model of anxiety/depression[END_REF] , and they spent less time in the open arms of the elevated plus maze[233][234][235][236] , the center of the open field[START_REF] David | Neurogenesis-dependent and -independent effects of fluoxetine in an animal model of anxiety/depression[END_REF]237 , and the lit compartment of the light/dark box 238 (for review 197 ). Depending on the dose, duration, and task, it appears corticosterone can regulate various aspects of anxiety-relevant behaviors.These effects on anxiety may also be attributed to GR activity. Tronche and colleagues developed mice lacking GR in brain cells and found inhibited anxiety in the light dark box and elevated Omaze. In the light dark box task, mice showed decreased latency to exit into the lit compartment, and spent more time exploring in this compartment 239 . In the elevated Omaze, the number of entries and time spent in the open arms was increased in the GR mutants as well239 . Similarly, a separate study using mice with genetically reduced GR function also found the number of entries and time spent in the open arms of the elevated Omaze were increased, demonstrating reduced anxiety-like behavior once again 240 . These studies indicate a role for GR in regulating anxiety behaviors.

  CreERT2) (thereafter called GR(NestinCreERT2) ) mouse line with a YFP reporter(Fig 1a). DNA from mouse tails was carefully extracted and analyzed by PCR to verify genotype(Fig 1b). Both GR loxP and R26R-YFP alleles are sensitive to Cre recombinase, which catalyzes site-specific recombination between the loxP sites. Thus, upon tamoxifen administration, GR was selectively inactivated and YFP expression was selectively induced only in cells expressing nestin and all their progeny thereafter (Fig1a). Mutant mice (i.e. tamoxifen-treated GR NestinCreERT2 mice) are denominated GR NPCKO . Untreated GR NestinCreERT2 mice were used as control animals.

  immunohistochemistry, we quantified and analyzed YFP expression and co-labeling of GR and BrdU in the hippocampus of GR(NestinCreERT2) mice after administrating vehicle (control) or tamoxifen. Tamoxifen binds to the mutated estrogen receptor (ERT2) of Cre in cells expressing Nestin, allowing the Cre recombinase to excise the STOP codon in the YFP reporter and the exon 3 segment in GR loxP alleles. Thus, mice receiving vehicle (-tam) maintained an inactive YFP reporter (Fig 1c), while mice receiving tamoxifen (+tam) expressed YFP in nestin-expressing cells and their progeny (Fig 1d). Similarly, adult -tam mice maintained active GR expression in the proliferating (BrdU+) hippocampal cell population (Fig 1e), while in +tam mice, the GR gene was inactivated in the majority of this population (Fig 1f). Quantitative analysis revealed a significant difference between -tam and +tam mice for the percent of BrdU+ cells expressing GR (two-way ANOVA corticosterone x genotype, effect of genotype: p<0.0001, F (1.31) =72.59, cort: p=0.25, interaction: p=0.73) (Fig 1g).

(

  Fig 3a). Again, examining the number of co-labeled NeuN+ cells per area by two-way ANOVA showed a significant effect of corticosterone (p=0.018, F (1.15) =7.072) (Fig 3b). Neither tam treatment nor CreERT2 genotype alone affected differentiation (data not shown). All groups had a comparable number of BrdU positive cells (Fig 2b) and no significant differences in areas measured (data not shown). These results show

  GR NesCreERT2 ), by mating ad-hoc animals, all on a C57BL/6 genetic background. The GR loxP allele contains loxP sites flanking exon 3 of the GR gene, the first zinc finger of the GR DNA-binding domain. The strategic placement of loxP around this specific part of the GR gene not only removes exon 3, but also causes a shift in the open reading frame of the gene. Potential mRNAs generated from the mutated allele would fail in translating any functioning GR protein 239 . The NesCreERT2 transgene expresses the CreERT2 recombinase gene under control of the Nestin promoter 84 . The R26R-YFP Cre-reporter allele (Jackson Laboratories) harbors a transcriptional STOP cassette preventing, in the absence of active Cre recombinase, the expression of the YFP open reading frame on the Rosa26 locus. Upon tamoxifen induction in GR (NesCreERT2) mice, Cre recombinase efficiently promotes the recombination between two head-to-tail oriented loxP sites, which leads to excision of intervening DNA, selectively in nestin-expressing cells, thus creating a nestin-specific loss of GR function and active YFP expression. Vehicle-induced GR (

  TK139) 5' (ATT-TGC-CTG-CAT-TAC-CGG-TC) 3CAT-GCT-GCT-AGG-CAA-ATG-ATC-TTA-AC) 3' GR15R (GR loxP ): 5' (CTT-CCA-CTG-CTC-TTT-TAA-AGA-AGA-C) 3' GR30R (null): 5' (GAA-TGA-GAA-TGG-CCA-TGT-ACT-AC) 3' PCR reactions were carried out at 58ºC for both CreERT2 and YFP and 60ºC for GR loxP , in a buffer containing 3 mM MgCl2. An aliquot of the PCR reaction was analyzed on an agarose gel. The CreERT2 allele gave a 350bp band; the YFP allele gave a 310bp band (560bp for wt); and the GR loxP allele gave a 450bp band (400bp for wt).

1 :

 1 500), rabbit polyclonal anti-GR (Santa Cruz 1004; 1:500), goat anti-gfp, FITC-conjugated (Rockland 600-102-215; 1:500), and rabbit polycolonal anti-NeuN (Millipore ABN78; 1:500). Secondary antibodies used were as follows: biotinylated donkey anti-rat immunoglobulin G (Jackson 712-065-153, 1:500), donkey anti-rabbit Alexa647 (Jackson 711-605-152, 1:500), and donkey anti-goat AF488 IgG(Jackson 705-545-147, 1:200).Visualization of BrdU was performed with StrepAlexa488 for 1 hour at room temperature. Tissue was treated with Dapi (Invitrogen D1306, 1:20000) when appropriate. Sections were mounted onto slides and coverslipped with DABCO.

Fig 1 .

 1 Fig 1. Generation and verification of adult mice deficient for GR in newborn neurons. (A) Strategy for inducible gene inactivation of the GR gene in adult neural progenitor cells. Transgenic mouse linesNestinCre ERT2 ;R26R-YFP, and GR loxP/loxP , were crossed for at least two generations to generate GR loxP/loxP ;NestinCre ERT2 ;R26R-YFP mouse line. That we denominated GRNPC CONT . These GRNPC CONT mice were heterozygous for the NestinCre ERT2 transgene, the R26R-YFP reporter gene that harbors a transcriptional stop cassette flanked by loxP sites, and homozygous for the GR loxP allele in which GR exon 3 was flanked by loxP sites. Upon tamoxifen administration, GRNPC CONT mice are transformed into GRNPCKO mice. The CreERT2 recombinase is activated and leads to the recombination of GRloxP allele and of R26R-YFP reporter gene. Nestin-positive cells would have an active YFP reporter and lack a functional GR. (B) Illustration of genotyping of the three different transgenes used. (C-D) Representative coronal sections immunostained with an anti-GFP antibody (marker of YFP reporter) and DAPI (nuclei stain). In absence of tamoxifen treatment (GRNPC CONT mice) there is no recombined cells in the hippocampus and therefore no YFP signal (C). After tamoxifen treatment (GRNPC KO mice) recombination can be visualized by YFP expression (D). (E-F) Double labeling for anti-GR (green) and anti-BrdU (red) in the hippocampal DG shows co-labeling in GRNPC CONT mice (E), but not in GRNPC KO mice (F). Brain sections were imaged at 40x by confocal microscopy. (G) Tamoxifen treatment led to significant gene inactivation of GR in newborn neurons (*, p<0.0001, n=8-10 per group). Results are expressed as mean±s.e.m. Scale bars represent 10µm in E-F.

Figure 2 Fig 2 .

 22 Figure 2

Figure 3 Fig 3 .

 33 Figure 3

  After 4 weeks of corticosterone treatment (see experimental design, Fig 1a), mice were placed in a dark novel environment where their movements were measured by crossing an invisible grid. Both untreated and cort-treated control and GR NPCKO mice behaved similarly to each other across 5 hours of their light cycle and overall (Fig 1b). Likewise, all groups exhibited comparable levels of physical activity in the open field test (Fig 1c). We measured the percent of time spent in the center (Fig 1c), corners, and sides (data not shown) of the arena, as well as the distance traveled, and velocity of movement. In all measures, GR NPCKO mice performed similarly to controls regardless of corttreatment.

  =4.77)(Fig 2b).In elevated Omaze, we measured the latency to exit the wall-sheltered segment (and enter the aversive, open, and more lit segment), and the frequency of entries and time spent in the lit, open segment(Fig 2c). Two-way ANOVA revealed a significant effect of genotype in the frequency of 4 paw entries (p=0.047, F(1,29) =4.29), as well as an effect of corticosterone in both the frequency of 4 paw entries (p=0.007, F(1,29) =8.468) and time spent in the lit, open segment (p=0.0004, F(1,29) =16.3). Follow-up post-hoc analysis showed that these effects were driven by the cort-treated control group, which spent significantly less time in the lit, open segment (p<0.05, Dunnett and Neuman-Keuls), and had less 4 paw entries (p<0.05, Dunnett and Neuman-Keuls) compared to all other groups. Interestingly, in both measurements, untreated and cort-treated GR NPCKO s performed similarly(Fig 2c)

  %freezing for genotype x corticosterone (p=0.11)(Fig 4d), although follow-up post-hoc analyses showed a difference in cort-treatment among controls (p<0.05, Dunnett), but no effect of corticosterone on GRNPCKO s (Fig 4d).Performance in contextual fear discrimination is unaffected by GR NPCKO s.Lastly, to examine if GR signaling in abGCs plays a role in cognitive function, we tested mice in a contextual fear discrimination paradigm (Fig5a). Whereas adult neurogenesis is both necessary and sufficient in improving contextual fear discrimination266,315 , and corticosterone is importantly involved in modulating fear226 , we sought to examine whether GR mediates this effect in the abGCs. On day 0, mice were conditioned to associate a particular context with a foot shock (Fig5a). Over the next 9 days, freezing behavior was measured in the context consistently associated with a foot shock, which reinforced this association daily, and a similar context never associated with a foot shock. All mouse groups showed significantly greater freezing over time in the shock-associated context (A), indicating successful learning of this association (p<0.0001, F (5.160) =58.62, repeated measures two-way ANOVA, data batched to account for missing values) with no differences between groups (Fig 5b). However, this association extended to the unconditioned context (B) since percent freezing values increased over time as well (p<0.0001, F (5.160) =15.05, repeated measures twoway ANOVA, data batched to account for missing values), suggesting low pattern discrimination (Fig 5c). Since mice appeared to reach a learning plateau after 9 days of training, we added a rest day (no training) as a challenge, and then resumed three more days of training. Although percent freezing in context B increased over time, each experimental group of mice was able to discriminate the contexts overall, as indicated by repeated measures two-way ANOVA (Fig 5d-g). Interestingly, only our GR NPCKO s on corticosterone treatment had a significant interaction effect of time and context when analyzed within group (p=0.039, F (11,156) =1.93), although this is not significant when analyzed across groups (Fig 5g). Further within group analysis using Bonferroni post-hoc tests showed a significant freezing difference between contexts on day 13 within untreated controls (Fig 5d), day 9, 12, and 13 within cort-treated controls (Fig 5e), day 4, 8, 12, and 13 within untreated GR NPCKO s (Fig 5f), and day 12 and 13 within cort-treated GR NPCKO s (Fig 5g).

analysis of days 1 -

 1 9 showed no effect (Fig 6a). Three-way ANOVAs comparing the effects of context, genotype, and corticosterone across individual days revealed an overall difference in context for days 2-13 (p<0.05), however again, no effect between groups. Thus, although Bonferroni post-hoc analysis showed no individual group differences in discrimination between all groups on day 4 (Fig 6c), cort-treated controls with significant difference in discrimination by day 9 (Fig 6d), cort-treated controls and all GR NPCKO s groups with significant differences in discrimination by day 12 (Fig 6e), and all groups with significant differences in discrimination by day 13 (Fig 6f), these individual group differences are not more significant than the differences in contextual freezing for other groups. It should also be noted that these scores (Fig 6c-f) are

  mice was measured in three different behavioral paradigms based on the natural avoidance behavior of mice: light/dark box, elevated Omaze, and NSF. In the light/dark box, cort-treated controls took significantly longer to exit the dark compartment, spent significantly less time in the lit compartment, and had significantly less number of 4 paw entries into the lit portion compared to untreated controls. These are all typical fearful behaviors found in an anxious phenotype (i.e. avoiding bright, open spaces). Interestingly, the behavior of cort-treated GR NPCKO mice was indistinguishable from untreated mice. This anxiolytic effect was confirmed in the elevated Omaze test. Again, mice are subjected to a novel environment in which although they have a natural tendency to explore, they prefer avoiding bright, open, and now elevated spaces. Similarly, cort-treated controls took longer to exit the enclosed arms (albeit not significantly), spent significantly less time in the open arms, and had significantly less number of 4 paw entries into the open arms compared to untreated controls. Once again,

Fig 1 .

 1 Fig 1. Normal levels of activity in cort-treated control and GRNPC KO mice. (A) Timeline for experimental protocol. (B) There were comparable levels of general locomotor and exploratory activity in a novel environment among GRNPC KO and control mice, as measured by the actimeter. (C) There was no difference among groups on all examined measures of the open field protocol.

Figure 2 Fig 2 .

 22 Figure 2

  Fig 2. Effect of corticosterone treatment on anxiety-related behaviors in GR NPCKO and control mice. (A)Corticosterone treatment preceding novelty-suppressed feeding significantly increased the latency to approach food for control mice, but not for GRNPC KO ones. However, cort-treatment similarly affected latency to eat for control mice compared to GRNPC KO mice. (B) Cort-treated control mice exhibited significantly increased latency to enter the aversive lit compartment compared to GRNPC KO mice. Similarly, cort-treated control mice exhibited a trend of decreased time spent in the lit compartment, but no effect of corticosterone was seen on GRNPC KO mice. Likewise, the number of 4 paws entries showed a significant interaction effect between genotype and corticosterone treatment. There were no differences between GR NPCKO mice with or without cort-treatment for all parameters measured in the light-dark box. (C) Cort-treatment also affected time in open arms and 4 paw entries in the elevated Omaze for control mice, but not GR NPCKO mice. There were no significant differences in latency to enter open arms. *, p<0.05 using Dunnett and Neuman-Keuls' post-hoc analyses, as appropriate. Results are expressed as mean±s.e.m (n=7-10 per group).

Figure 3 Fig 3 :

 33 Figure 3

Figure 4 Fig 4 :

 44 Figure 4

Figure 5 Fig 5 :

 55 Figure 5

Figure 6 Fig 6 :

 66 Figure 6

Scope of thesis 1.2 Stress and neurogenesis in the mammalian central nervous system

	CHAPTER 1 OUTLINE
	abGC ACTH ADX ANOVA 1.1 1.2.1 The stress response in the brain adult-born granule cell adrenocorticotropin adrenalectomy analysis of variance
	AVP BDNF BrdU INTRODUCTION arginine vasopressin brain derived neurotrophic factor bromo-deoxyuridane 1.2.2 Neurogenesis in the hippocampus 1.2.3 Measuring neurogenesis
	CamKII Cort	Ca 2+ /calmodulin dependent protein kinase corticosteroids (cortisol in humans, corticosterone for rodents) 1.2.4 Stress-induced regulation of neurogenesis
	CreERT2	Cre recombinase estrogen receptor T2 1.2.4.1 Stress and glucocorticoids regulate cell proliferation
	CRH/F DBD	corticotrophin releasing hormone/factor DNA binding domain 1.2.4.2 Stress and glucocorticoids regulate cell survival
	DCX Dex	doublecortin dexamethasone 1.2.4.3 Stress and glucocorticoids regulate cell differentiation
	DG	dentate gyrus 1.2.4.4 Other environmental factors regulate neurogenesis
	EE	enriched environment
	ERT2	estrogen receptor T2
	FGF2 FST 1.3	fibroblast growth factor 2 forced swim test
	GC	glucocorticoid
	GCL	granule cell layer
	GFAP	glial fibroblast-associated protein
	GFP	green fluorescent protein
	GR	glucocorticoid receptor
	GRE	glucocorticoid response element
	GR NPCKO	glucocorticoid receptor gene inactivation in neural progenitor cells
	HPA	hypothalamo-pituitary-adrenal
	IGF	insulin growth factor
	KD	knockdown
	KO	knockout
	LBD	ligand binding domain
	LTD	long term depression
	LTP	long term potentiation
	MR	mineralocorticoid receptor
	NE-SAM	norepinephrine-sympathetic adrenomedullary
	NeuN	Neuron-specific nuclear marker
	NG	neurogenesis
	NMDA	N-methyl-D-aspartate
	NPC	neural progenitor cell
	NSF	novelty-suppressed feeding
	OE	overexpression
	PBS	phosphate buffered solution
	PCNA	proliferating cell nuclear antigen
	PFA	paraformaldehyde
	PTSD	post traumatic stress disorder
	SEM	standard error of the mean
	SGZ	subgranular zone (of the dentate gyrus)
	SVZ	subventricular zone
	TLX	Tailless gene
	Vb1	vasopressin
	VEGF	vascular endothelial growth factor
	Wt	wild-type
	YFP	yellow fluorescent protein
	5HT1A	serotonin

The role of glucocorticoids in hippocampal neuroplasticity

1.3.1 Glucocorticoids modulate neurogenesis 1.3.2 Glucocorticoids modulate anxiety and cognitive performance

  ). Afterward, half of each of the +tam and -tam cohorts

were chronically treated with corticosteroids through their drinking water for the next 20-26 weeks (age and treatment time were counterbalanced across all groups). Four weeks prior to perfusion, 6 mice within each group were given BrdU for analysis of cell survival and fate (Fig

2a

). Control mice had similar levels of cell survival regardless of corticosterone treatment (Fig

2b

). This effect was not altered by GR ablation (two-way ANOVA, effect of cort: p=0.64; tam: p=0.96). Area measured for cell counts did not affect results as BrdU+ cells per m2 was not significantly different either (data not shown). Gr loxP/loxP mice were used as controls to verify NPC survival was not affected by CreERT2 genotype or tamoxifen alone

(3-way ANOVA, p>0.05; Fig 2b)

.

  [START_REF] David | Neurogenesis-dependent and -independent effects of fluoxetine in an animal model of anxiety/depression[END_REF] , decreased time in center of open field[START_REF] David | Neurogenesis-dependent and -independent effects of fluoxetine in an animal model of anxiety/depression[END_REF]237 , decreased time in open arms of elevated plus maze[233][234][235][236] , and decreased time spent in lit compartment of light/dark box197,238 . Whereas elevated corticosterone levels

activate GR signaling, further evidence that this anxious phenotype is via GR was demonstrated in mice with brain-specific GR gene inactivation which displayed increased latency to exit the dark compartment and decreased time spent in the lit compartment of the light/dark box 239 . Additionally, these neural GR gene inactivated mice had reduced number of entries and more time spent in the open arms of the elevated Omaze 239 .

1 Newborn neurons mediate fearful behaviors only when exposed to chronic corticosterone treatment, not to basal levels of cort

  from our research indicate that GR gene inactivation in NPCs did not alter neurogenesis nor produce an observable phenotype different from controls. It was not until the introduction of chronic corticosterone exposure that we saw differences. Cort-suppressed differentiation of NPCs still occurred, similarly to controls, yet our GR NPCKO model blocked cort-induced anxiety-like behaviors. This suggests that corticosterone (and perhaps stress) can indirectly regulate cell maturation through GR signaling in the surrounding niche and directly regulate mood-related behaviors through GR signaling in abGCs. Furthermore,

	it suggests that corticosterone (and stress) effects on neurogenesis (i.e. proliferation, survival, and
	differentiation) are not contributing to changes in anxiety-related behaviors. This brings up two additional
	queries: does adult neurogenesis become functionally relevant only when environmental stimuli (such as
	stressors) are introduced? And what is the GR pathway promoting anxiety-relevant behaviors or able to
	attenuate them when GR is blocked?
	4.3.

Dissection des rôles fonctionnels et morphologiques du récepteur des glucocorticoïdes dans les précurseurs neuronaux de l'hippocampe.

  Wong Alana -Thèse de doctorat --2014 La libération d'hormones glucocorticoïdes (GC), en réponse au stress, est un mécanisme important du contrôle de la neurogenèse chez l'adulte. Une question non résolue est de savoir si ces hormones agissent directement sur les précurseurs neuronaux (NPCs) ou indirectement, en agissant sur d'autres types cellulaires, modifiant la libération de facteurs de croissance ou l'activité de réseaux neuronaux. Afin de répondre à cette question, nous avons développé un modèle murin dans lequel le gène du récepteur des GC (GR) est invalidé, de façon inductible, dans les précurseurs neuraux adultes. Nous avons montré qu'en présence ou en absence du GR, un traitement chronique avec des GC affecte de façon similaire la différentiation et la survie des neurones nés chez l'adulte. L'effet connu des GC sur la suppression de la neurogenèse adulte n'est donc pas du à une action directe de ces hormones sur les NPCs. L'absence du GR n'affecte pas non plus le comportement des souris mutantes lorsque les GC circulent à un niveau de base. En revanche, un traitement chronique avec des GC induit chez les animaux contrôles un phénotype anxieux (observé dans les tests de novelty-suppressed feeding, light/dark box, and elevated O-maze) alors que les animaux mutés sont préservé de ce changement comportemental. De façon similaire, un traitement chronique avec des GC facilite l'apprentissage des souris contrôles lors d'un test d'apprentissage par la peur. L'invalidation du gène GR dans les NPC bloque cet effet. L'apprentissage des souris. Ces résultats précisent le rôle du GR dans le contrôle de la neurogenèse dans l'hippocampe adulte et dans la modulation des comportements de type anxieux.

Résumé :

Mots clés : Neurogenèse, Hippocampe, Récepteur des glucocorticoïdes, Réponse au stress, Comportement, Mutagenèse conditionnelle.

, and oligodendrocytes as well291,294 . Interestingly, it has been shown that GR activation in astrocytes can induce secretion of different factors that can mediate neurogenesis, such as basic fibroblast growth factor (FGF2)119 and cell cycle inhibitors293 . Additionally, stress effects on neurogenesis were found concomitantly with increases in FGF2 mRNA in the dorsal hippocampus 119 . They examined this in vitro by treating NPCs with conditioned media from cort-treated astrocytes. Not only did this affect neurogenesis, but also treating NPCs with similar levels of FGF2 alone caused a change in proliferation. Furthermore, neutralizing the FGF2 blocked the effect 119 . These results suggest that corticosterone may regulate NPC behavior indirectly through activating GR in astrocytes.Similarly, neurogenesis can be regulated when GR activation in either astrocytes or neurons dysregulates cell signaling by inhibiting both glutamate uptake and N-methyl-D-aspartate (NMDA) receptors162,295,296 . It had been shown that GR activation reduced glucose transport into both neurons and astrocytes 297 . This effect resulted in a cascade of increased levels of damaging glutamate in the synapse, which overactivated NMDA receptors, and thus, increased free cytosolic calcium295 . Not only does increased cytosolic calcium (Ca2+) signaling damage the postsynaptic neuron295 , but it has also been demonstrated to regulate all stages of neurogenesis (for review 298 ). Furthermore, blocking NMDA receptor activity prevented GR-induced suppression of neurogenesis162,296 . Although more research is needed to elucidate these pathways in vivo, it remains another possible explanation for how corticosterone can indirectly influence neurogenesis.Another way that corticosterone can influence neurogenesis indirectly is through serotonin (5HT1A) receptors. GR antagonists were shown to block the effect of corticosterone treatment on the upregulation of serotonin transporter protein levels in the hippocampus 299 , demonstrating that cort-induced GR activity promotes serotonin transporter production. Similarly, other studies have demonstrated corticosterone can regulate 5HT1A receptors in the hippocampus300,301 . Both activation and inhibition of 5HT1A receptors and transporters can cause changes in proliferation and differentiation of NPCs302,303 . These studies suggest that GR-mediated increases in 5HT1A transporter and receptor protein may regulate neurogenesis. Regardless of whether this effect is mediated directly through altered levels of 5HT1A receptors on NPCs, GR activity induced in mature neurons or astroglia may be the initial mechanism that indirectly affects NPC development.Overall, we found that chronic corticosterone treatment did not disrupt survival rates of NPCs in the adult murine hippocampus, however it did reduce neuronal differentiation. Knocking out GR in the NPCs did not attenuate this effect of cort-suppressed differentiation. Since GR is implicated in regulating neurogenesis, our results suggest that cort-suppressed neurogenesis is mediated indirectly through GR in other cells of the stem cell niche. Furthermore, it would be interesting to know if cort-suppressed proliferation is also unaffected by GR NPCKO , further suggesting that the effects of corticosterone are indirectly mediated. Both corticosteroids

Acknowledgements

Science is a collaborative process and during the years dedicated to the pursuit of this expertise, I

CHAPTER 3

FUNCTIONAL CONTRIBUTIONS OF THE GLUCOCORTICOID RECEPTOR IN ADULT NEUROGENESIS

Abstract

Corticosteroids (cort) are one of the strongest and most relevant environmental signals mediating adult neurogenesis. While the impact of these stress hormones on newborn hippocampal neurons has been demonstrated in terms of cellular changes, the functional relevance of these changes is unclear. To characterize these functional changes, we impaired cort-signaling in newborn neurons by irreversibly knocking out the glucocorticoid receptor (GR) selectively in adult neural progenitor cells (NPCs). Then, by using tests that measure anxiety-and depression-like behavior, as well as cognitive skills, such as contextual fear conditioning and pattern discrimination, we sought to determine whether changes in adult neurogenesis under chronic stress-like conditions affected hippocampal functioning. We found that chronic corticosterone treatment can induce an anxious phenotype, which is prevented by reducing GR signaling in adult-born neurons. This suggests that anxiety-like behaviors may be induced thru newborn neurons via GRs.

Additionally, we found that neither chronic corticosterone treatment nor GR gene inactivation in NPCs (GR NPCKO ) influenced depression-like behavior or performance in contextual fear discrimination. Although unaffected by GR NPCKO , chronic corticosterone treatment did, however, enhance learning in contextual conditioning. Overall, these findings show that adult neurogenesis can be functionally relevant to anxious behavior under chronic corticosterone treatment. This suggests that adult-born neurons, through GR signaling, can alter the behavioral response to stressful stimuli, an adaptive effect that can be evolutionary advantageous or dysfunctional under chronic stress.

All behavior experiments were performed during the light phase of the light/dark cycle. Mice were handled for at least 5 minutes 3 days prior to all behavior testing.

Statistical analysis.

Results are expressed as means±s.e.m. Statistical analysis was performed using two-way analysis of variance (ANOVA) for all tests except contextual fear conditioning and discrimination, which used a repeated measures three-way ANOVA. Analyses were followed by Dunnett's, Newman-Keuls, or Bonferroni post-hoc tests for pairwise comparisons, as appropriate. * indicates p<0.05.

for the first 180s in both chambers each day. Measurements over time of the percent time freezing in chamber A (B) and chamber B (C) indicated that all mice were able to learn to associate fear with a contextual setting, though not differently. This was verified by repeated measures two-way ANOVA of each group, which also showed that overall percent freezing in chamber A was significantly greater than in chamber B for nontreated controls (effect of context, p<0.0001, F (1, 185) =31.23; effect of time: p<0.0001, F (11, 185) =5.91) (D), cort-treated controls (effect of context: p<0.0001, F (1, 204) =28.30; effect of time: p<0.0001, F (11, 204) =3.76) (E), nontreated GRNPC KO mice (effect of context, p<0.0001, F (1, 183) =70.33; effect of time: p<0.0001, F (11, 183) =7.00) (F), and cort-treated GRNPC KO mice (effect of context, p<0.0001, F (1, 156) =44.57; effect of time: p<0.0001, F (11, 156) =8.66) (G). Further analysis revealed specific days these contexts significantly differed within group. For nontreated controls, this was only on day 13 (D). For cort-treated controls, day 9, 12, and 13 showed significant differences (E). For nontreated GRNPC KO