Abstract:
Aging is commonly associated with a decline in memory abilities, yet some individuals remain resilient to such changes. Memory processing has been shown to rely on adult neurogenesis, a form of hippocampal plasticity, but whether the integration and role of long-lived adult-born neurons (ABNs) generated during early adult life also contribute to cognitive resilience and to such inter-individual differences remain unknown. Using a pseudo-longitudinal approach in rats characterized as resilient or vulnerable to cognitive aging, we examined the survival, senescence, morphology, glutamatergic connectivity, and mitochondrial health of ABNs. To achieve this, we combined approaches based on thymidine analogues and retroviral labeling using Moloney murine leukemia viruses. While ABNs survival, entry into senescence and dendritic gross morphology did not differ between resilient and vulnerable rats, resilient animals exhibited preserved glutamatergic synaptic input and maintained mitochondrial homeostasis in the proximal dendrites of ABNs. Interestingly, bypassing this reduction in glutamatergic inputs in vulnerable rats through direct optogenetic stimulation was sufficient to rescue their memory retrieval abilities, indicating that ABNs themselves remain intrinsically functional despite reduced input. Overall, our data indicate that maintaining long-lived ABNs within the neuronal network is essential for successful cognitive aging, highlighting their potential as a therapeutic target for restoring cognitive functions in old age.

Abstract:
Hippocampal Adult-Born neurons (hABNs) play a critical role in maintaining brain homeostasis, exhibiting unique properties during their maturation. The absence of hABNs impacts surrounding neuronal networks, but the mechanisms are not fully understood. In this study, we examined how perturbations to adult hippocampal cytogenesis affect the neuronal inputs to adult-born and mature neurons in the dentate gyrus. In particular, we analyzed connectivity changes in GFAP-Tk transgenic rats with ablated neurogenesis, and in unpredictable Chronic Mild Stress (uCMS) rats with reduced neurogenesis, with a focus on sex-differences in stress-affected hABNs. Additionally, we evaluated the action of the antidepressant fluoxetine on brain connectivity. Using a virus-mediated retrograde tracing approach, we quantified synaptic inputs to mature neurons and hABNs. Our findings reveal significant impairments in both intra- and extra-hippocampal connectivity following disruptions in cytogenesis, providing new insights into hippocampal network dynamics in the context of cytogenetic impairment, depression, and therapeutic interventions.

Abstract:
Neurogenesis persists in the adult dentate gyrus (DG) of the hippocampus, playing a critical role in memory and stress adaptation. Dysregulation of this process is implicated in cognitive deficits and depressive behaviors induced by chronic stress, while classical antidepressants are known to enhance neurogenesis. The Nociceptin/Orphanin FQ (N/OFQ) system, comprising N/OFQ and its NOP receptor, modulates memory and the stress response, yet its role in adult neurogenesis remains underexplored. Here, we investigated the impact of N/OFQ signaling on neurogenesis in the mouse DG using genetic and pharmacological approaches under basal and chronic stress conditions. In constitutive NOP receptor knockout (KO) mice, adult neurogenesis was only mildly altered, with subtle changes in neuronal maturation. However, spine density in 4-week-old adult-born DG neurons increased following conditional NOP Receptor KO in the DG. The increase was specific to stubby and thin spines, while mature mushroom spine density decreased. When NOP KO was restricted to newly born neurons, no significant differences were observed in spine density suggesting that the absence of NOP receptors in mature DG neurons influences the local environment to regulate spinogenesis in adult-born neurons indirectly. Finally, chronic corticosterone exposure impaired spinogenesis in immature neurons, and this was mitigated by systemic administration of a NOP antagonist. Our findings suggest that N/OFQ signaling indirectly regulates the maturation and connectivity of adult-born neurons through modulation of local and distal inputs. This regulation may contribute to the antidepressant and pro-cognitive effects of NOP receptor antagonists.

Abstract:
The dentate gyrus (DG) of the hippocampus is a mosaic of dentate granule neurons (DGNs) accumulated throughout life. While many studies focused on the morpho-functional properties of adult-born DGNs, much less is known about DGNs generated during development, and in particular those born during embryogenesis. One of the main reasons for this gap is the lack of methods available to specifically label and manipulate embryonically-born DGNs. Here, we have assessed the relevance of the Penk(Cre) mouse line as a genetic model to target this embryonically-born population. In young animals, Penk(Cre) expression allows to tag neurons in the DG with positional, morphological and electrophysiological properties characteristic of DGNs born during the embryonic period. In addition, Penk(Cre)+ cells in the DG are distributed in both blades along the entire septo-temporal axis. This model thus offers new possibilities to explore the functions of this underexplored population of embryonically-born DGNs.

Abstract:
Hippocampal adult neurogenesis is involved in many memory processes from learning, to remembering and forgetting. However, whether or not the stimulation of adult neurogenesis is a sufficient condition to improve memory performance remains unclear. Here, we developed and validated, using ex-vivo electrophysiology, a chemogenetic approach that combines selective tagging and activation of discrete adult-born neuron populations. Then we demonstrated that, in rats, this activation can improve accuracy and strength of remote memory. These results show that stimulation of adult-born neuron activity can counteract the natural fading of memory traces that occurs with the passage of time. This opens up new avenues for treating memory problems that may arise over time.

Abstract:
Decline in episodic memory is one of the hallmarks of aging and represents one of the most important health problems facing Western societies. A key structure in episodic memory is the hippocampal formation and the dentate gyrus in particular, as the continuous production of new dentate granule neurons in this brain region was found to play a crucial role in memory and age-related decline in memory. As such, understanding the molecular processes that regulate the relationship between adult neurogenesis and aging of memory function holds great therapeutic potential. Recently, we found that Vang-Gogh like 2 (Vangl2), a core component of the Planar Cell Polarity (PCP) signaling pathway, is enriched in the dentate gyrus of adult mice. In this context, we sought to evaluate the involvement of this member of the Wnt/PCP pathway in both adult neurogenesis and memory abilities in adult and middle-aged mice. Using a heterozygous mouse model carrying a dominant-negative mutation in the Vangl2 gene, called Looptail (Vangl2(Lp)), we show that alteration in Vangl2 expression decreases the survival of adult-born granule cells and advances the onset of a decrease in cognitive flexibility. The inability of mutant mice to erase old irrelevant information to the benefit of new relevant ones highlights a key role of Vangl2 in interference-based forgetting. Taken together, our findings show that Vangl2 activity may constitute an interesting target to prevent age-related decline in hippocampal plasticity and memory.

Abstract:
Despite the central role of Rho GTPases in neuronal development, their functions in adult hippocampal neurogenesis remain poorly explored. Here, by using a retrovirus-based loss-of-function approach in vivo, we show that the atypical Rho GTPase Rnd2 is crucial for survival, positioning, somatodendritic morphogenesis, and functional maturation of adult-born dentate granule neurons. Interestingly, most of these functions are specific to granule neurons generated during adulthood since the deletion of Rnd2 in neonatally-born granule neurons only affects dendritogenesis. In addition, suppression of Rnd2 in adult-born dentate granule neurons increases anxiety-like behavior whereas its deletion in pups has no such effect, a finding supporting the adult neurogenesis hypothesis of anxiety disorders. Thus, our results are in line with the view that adult neurogenesis is not a simple continuation of earlier processes from development, and establish a causal relationship between Rnd2 expression and anxiety.

Abstract:
The dentate gyrus is one of the only brain regions that continues its development after birth in rodents. Adolescence is a very sensitive period during which cognitive competences are programmed. We investigated the role of dentate granule neurons (DGNs) born during adolescence in spatial memory and compared them with those generated earlier in life (in embryos or neonates) or during adulthood by combining functional imaging, retroviral and optogenetic tools to tag and silence DGNs. By imaging DGNs expressing Zif268, a proxy for neuronal activity, we found that neurons generated in adolescent rats (and not embryos or neonates) are transiently involved in spatial memory processing. In contrast, adult-generated DGNs are recruited at a later time point when animals are older. A causal relationship between the temporal origin of DGNs and spatial memory was confirmed by silencing DGNs in behaving animals. Our results demonstrate that the emergence of spatial memory depends on neurons born during adolescence, a function later assumed by neurons generated during adulthood.

Abstract:
Hippocampal adult neurogenesis has been associated to many cognitive, emotional, and behavioral functions and dysfunctions, and its status as a selected effect or an 'appendix of the brain' has been debated. In this review, we propose to understand hippocampal neurogenesis as the process underlying the 'Baldwin effect', a particular situation in evolution where fitness does not rely on the natural selection of genetic traits, but on 'ontogenetic adaptation' to a changing environment. This supports the view that a strong distinction between developmental and adult hippocampal neurogenesis is made. We propose that their functions are the constitution and the lifelong adaptation, respectively, of a basic repertoire of cognitive and emotional behaviors. This lifelong adaptation occurs through new forms of binding, i.e., association or dissociation of more basic elements. This distinction further suggests that a difference is made between developmental vulnerability (or resilience), stemming from dysfunctional (or highly functional) developmental hippocampal neurogenesis, and adult vulnerability (or resilience), stemming from dysfunctional (or highly functional) adult hippocampal neurogenesis. According to this hypothesis, developmental and adult vulnerability are distinct risk factors for various mental disorders in adults. This framework suggests new avenues for research on hippocampal neurogenesis and its implication in mental disorders.

Abstract:
Memory reconsolidation, the process by which memories are again stabilized after being reactivated, has strengthened the idea that memory stabilization is a highly plastic process. To date, the molecular and cellular bases of reconsolidation have been extensively investigated particularly within the hippocampus. However, the role of adult neurogenesis in memory reconsolidation is unclear. Here, we combined functional imaging, retroviral and chemogenetic approaches in rats to tag and manipulate different populations of rat adult-born neurons. We find that both mature and immature adult-born neurons are activated by remote memory retrieval. However, only specific silencing of the adult-born neurons immature during learning impairs remote memory retrieval-induced reconsolidation. Hence, our findings show that adult-born neurons immature during learning are required for the maintenance and update of remote memory reconsolidation.

Abstract:
The mechanistic target of rapamycin (mTOR) is a ubiquitously expressed kinase that acts through two complexes, mTORC1 and mTORC2, to regulate protein homeostasis, as well as long lasting forms of synaptic and behavioral plasticity. Alteration of the mTOR pathway is classically involved in neurodegenerative disorders, and it has been linked to dysregulation of cognitive functions and affective states. However, information concerning the specific involvement of the p70 S6 kinase 1 (S6K1), a downstream target of the mTORC1 pathway, in learning and memory processes and in the regulation of affective states remains scant. To fill this gap, we exposed adult male mice lacking S6K1 to a battery of behavioral tests aimed at measuring their learning and memory capabilities by evaluating reference memory and flexibility with the Morris water maze, and associative memory using the contextual fear conditioning task. We also studied their anxiety-like and depression-like behaviors by, respectively, performing elevated plus maze, open field, light-dark emergence tests, and sucrose preference and forced swim tests. We found that deleting S6K1 leads to a robust anxious phenotype concomitant with associative learning deficits; these symptoms are associated with a reduction of adult neurogenesis and neuronal atrophy in the hippocampus. Collectively, these results provide grounds for the understanding of anxiety reports after treatments with mTOR inhibitors and will be critical for developing novel compounds targeting anxiety.

Abstract:
BACKGROUND: Recent research suggests that immune dysregulation in pregnancy could be a risk factor for anxiety and depression symptoms in offspring. Whereas animal studies have demonstrated the importance of the link between perinatal cytokines and abnormal behaviors in offspring, human epidemiological studies in this area remain limited. The objectives of the study were to describe the network of cord serum cytokines at birth and test whether they are associated with subsequent anxiety and depression symptom trajectories in offspring. METHODS: We used data and biological samples from 871 mother-child pairs followed up from pregnancy to 8 years of age and participating in the French mother-child cohort EDEN (a study on the pre- and early postnatal determinants of child health and development). Cord serum cytokines were measured at birth. Children's symptoms of anxiety and depression were assessed with the emotional difficulties subscore of the Strength and Difficulties Questionnaire at ages 3, 5, and 8 years, from which trajectories of anxiety-depression symptoms were derived. RESULTS: Results showed a significant association between cord serum interleukin-7 at birth and the trajectories of children's anxiety-depression symptoms between ages 3 to 8 years (adjusted odds ratio, 0.73; 95% confidence interval, 0.57-0.93). The associations considered relevant confounders, including prenatal maternal depressive symptoms. CONCLUSIONS: Early immune changes may contribute to subsequent anxiety and depression symptoms in childhood. Beyond the understanding of mechanisms underlying the occurrence of emotional difficulties in children, our findings open avenues for future research in human and animals.

Abstract:
Rnd proteins constitute a subfamily of Rho GTPases represented in mammals by Rnd1, Rnd2 and Rnd3. Despite their GTPase structure, their specific feature is the inability to hydrolyse GTP-bound nucleotide. This aspect makes them atypical among Rho GTPases. Rnds are regulated for their expression at the transcriptional or post-transcriptional levels and they are activated through post-translational modifications and interactions with other proteins. Rnd proteins are mainly involved in the regulation of the actin cytoskeleton and cell proliferation. Whereas Rnd3 is ubiquitously expressed, Rnd1 and 2 are tissue-specific. Increasing data has described their important role during development and diseases. Herein, we describe their involvement in physiological and pathological conditions with a focus on the neuronal and vascular systems, and summarize their implications in tumorigenesis.

Abstract:
The calcium/calmodulin-dependent protein kinase II (CaMKII) is a ubiquitous and central player in Ca(2+) signaling that is best known for its functions in the brain. In particular, the ? isoform of CaMKII has been the subject of intense research and it has been established as a central regulator of neuronal plasticity. In contrast, little attention has been paid to CaMKII?, the other predominant brain isoform that interacts directly with the actin cytoskeleton, and the functions of CaMKII? in this organ remain largely unexplored. However, recently, the perturbation of CaMKII? expression has been associated with multiple neuropsychiatric and neurodevelopmental diseases, highlighting CAMK2B as a gene of interest. Herein, after highlighting the main structural and expression differences between the ? and ? isoforms, we will review the specific functions of CaMKII?, as described so far, in neuronal development and plasticity, as well as its potential implication in brain diseases.

Abstract:
During aging, some individuals are resilient to the decline of cognitive functions whereas others are vulnerable. These inter-individual differences in memory abilities have been associated with differences in the rate of hippocampal neurogenesis measured in elderlies. Whether the maintenance of the functionality of neurons generated throughout adult life is linked to resilience to cognitive aging remains completely unexplored. Using the immediate early gene Zif268, we analyzed the activation of dentate granule neurons born in adult (3-month-old), middle-aged (12-month-old), or senescent (18-month-old) rats (n = 96) in response to learning when animals reached 21 months of age. The activation of neurons born during the developmental period was also examined. We show that adult-born neurons can survive up to 19 months and that neurons generated 4, 10, or 19 months before learning, but not developmentally born neurons, are activated in senescent rats with good learning abilities. In contrast, aged rats with bad learning abilities do not exhibit activity-dependent regulation of newborn cells, whatever their birthdate. In conclusion, we propose that resilience to cognitive aging is associated with responsiveness of neurons born during adult life. These data add to our current knowledge by showing that the aging of memory abilities stems not only from the number but also from the responsiveness of adult-born neurons.

Abstract:
Neuronal activity initiates transcriptional programs that shape long-term changes in plasticity. Although neuron subtypes differ in their plasticity response, most activity-dependent transcription factors (TFs) are broadly expressed across neuron subtypes and brain regions. Thus, how region- and neuronal subtype-specific plasticity are established on the transcriptional level remains poorly understood. We report that in young adult (i.e., 6-8

Abstract:
Mild-traumatic brain injury (mTBI) represents ~80% of all emergency room visits and increases the probability of developing long-term cognitive disorders in children. To date, molecular and cellular mechanisms underlying post-mTBI cognitive dysfunction are unknown. Astrogliosis has been shown to significantly alter astrocytes' properties following brain injury, potentially leading to significant brain dysfunction. However, such alterations have never been investigated in the context of juvenile mTBI (jmTBI). A closed-head injury model was used to study jmTBI on postnatal-day 17 mice. Astrogliosis was evaluated using glial fibrillary acidic protein (GFAP), vimentin, and nestin immunolabeling in somatosensory cortex (SSC), dentate gyrus (DG), amygdala (AMY), and infralimbic area (ILA) of prefrontal cortex in both hemispheres from 1 to 30 days postinjury (dpi). In vivo T2-weighted-imaging (T2WI) and diffusion tensor imaging (DTI) were performed at 7 and 30 dpi to examine tissue level structural alterations. Increased GFAP-labeling was observed up to 30 dpi in the ipsilateral SSC, the initial site of the impact. However, vimentin and nestin expression was not perturbed by jmTBI. The morphology of GFAP positive cells was significantly altered in the SSC, DG, AMY, and ILA up to 7 dpi that some correlated with magnetic resonance imaging changes. T2WI and DTI values were significantly altered at 30 dpi within these brain regions most prominently in regions distant from the impact site. Our data show that jmTBI triggers changes in astrocytic phenotype with a distinct spatiotemporal pattern. We speculate that the presence and time course of astrogliosis may contribute to pathophysiological processes and long-term structural alterations following jmTBI.

Abstract:
Adult hippocampal neurogenesis plays a critical role in a wide spectrum of hippocampus-dependent functions. Brain pathologies that involve the hippocampus like epilepsy, stroke, and traumatic brain injury, are commonly associated with cognitive impairments and mood disorders. These insults can affect neural stem cells and the subsequent neurogenic cascade in the hippocampus, resulting in the induction of aberrant neurogenesis, which is thought to compromise hippocampal network function, thereby hampering hippocampus-dependent behavior. We here summarize recent preclinical literature on hippocampal insult-induced changes in neurogenesis and based on that, we propose that normalizing aberrant neurogenesis post-insult may help to prevent or rescue behavioral deficits which could help develop novel therapeutic strategies.

Abstract:
Perturbation of CaMKIIbeta expression has been associated with multiple neuropsychiatric diseases, highlighting CaMKIIbeta as a gene of interest. Yet, in contrast to CaMKIIalpha, the specific functions of CaMKIIbeta in the brain remain poorly explored. Here, we reveal a novel function for this CaMKII isoform in vivo during neuronal development. By using in utero electroporation, we show that CaMKIIbeta is an important regulator of radial migration of projection neurons during cerebral cortex development. Knockdown of CaMKIIbeta causes accelerated migration of nascent pyramidal neurons, whereas overexpression of CaMKIIbeta inhibits migration, demonstrating that precise regulation of CaMKIIbeta expression is required for correct neuronal migration. More precisely, CaMKIIbeta controls the multipolar-bipolar transition in the intermediate zone and locomotion in the cortical plate through its actin-binding and -bundling activities. In addition, our data indicate that a fine-tuned balance between CaMKIIbeta and cofilin activities is necessary to ensure proper migration of cortical neurons. Thus, our findings define a novel isoform-specific function for CaMKIIbeta, demonstrating that CaMKIIbeta has a major biological function in the developing brain.

Abstract:
In nonhuman mammals and in particular in rodents, most granule neurons of the dentate gyrus (DG) are generated during development and yet little is known about their properties compared with adult-born neurons. Although it is generally admitted that these populations are morphologically indistinguishable once mature, a detailed analysis of developmentally born neurons is lacking. Here, we used in vivo electroporation to label dentate granule cells (DGCs) generated in mouse embryos (E14.5) or in neonates (P0) and followed their morphological development up to 6 months after birth. By comparison with mature retrovirus-labeled DGCs born at weaning (P21) or young adult (P84) stages, we provide the evidence that perinatally born neurons, especially embryonically born cells, are morphologically distinct from later-born neurons and are thus easily distinguishable. In addition, our data indicate that semilunar and hilar GCs, 2 populations in ectopic location, are generated during the embryonic and the neonatal periods, respectively. Thus, our findings provide new insights into the development of the different populations of GCs in the DG and open new questions regarding their function in the brain.

Abstract:
Renewed discussion about whether or not adult neurogenesis exists in the human hippocampus, and the nature and strength of the supporting evidence, has been reignited by two prominently published reports with opposite conclusions. Here, we summarize the state of the field and argue that there is currently no reason to abandon the idea that adult-generated neurons make important functional contributions to neural plasticity and cognition across the human lifespan.

Abstract:
Ciliary neurotrophic factor (CNTF) potently decreases food intake and body weight in diet-induced obese mice by acting through neuronal circuits and pathways located in the arcuate nucleus (ARC) of the hypothalamus. CNTF also exerts pro-inflammatory actions within the brain. Here we tested whether CNTF modifies energy balance by inducing inflammatory responses in the ARC and whether these effects depend upon the mechanistic target of rapamycin complex 1 (mTORC1) pathway, which regulates both energy metabolism and inflammation. To this purpose, chow- and high fat diet (HFD)- fed mice lacking the S6 kinase 1 (S6K1(-/-)), a downstream target of mTORC1, and their wild-type (WT) littermates received 12 days continuous intracerebroventricular (icv) infusion of the CNTF analogue axokine (CNTFAx15). Behavioral, metabolic and molecular effects were evaluated. Central chronic administration of CNTFAx15 decreased body weight and feed efficiency in WT mice only, when fed HFD, but not chow. These metabolic effects correlated with increased number of iba-1 positive microglia specifically in the ARC and were accompanied by significant increases of IL-1beta and TNF-alpha mRNA expression in the hypothalamus. Hypothalamic iNOS and SOCS3 mRNA, molecular markers of pro-inflammatory response, were also increased by CNTFAx15. All these changes were absent in S6K1(-/-) mice. This study reveals that CNTFAx15 requires a functional S6K1 to modulate energy balance and hypothalamic inflammation in a diet-dependent fashion. Further investigations should determine whether S6K1 is a suitable target for the treatment of pathologies characterized by a high neuroinflammatory state.

Abstract:
Progenitors of cortical glutamatergic neurons (Glu progenitors) are usually thought to switch fate before birth to produce astrocytes. We used fate-mapping approaches to show that a large fraction of Glu progenitors persist in the postnatal forebrain after closure of the cortical neurogenesis period. Postnatal Glu progenitors do not accumulate during embryonal development but are produced by embryonal radial glial cells that persist after birth in the dorsal subventricular zone and continue to give rise to cortical neurons, although with low efficiency. Single-cell RNA sequencing reveals a dysregulation of transcriptional programs, which parallels changes in m(6)A methylation and correlates with the gradual decline in cortical neurogenesis observed in vivo. Rescuing experiments show that postnatal progenitors are partially permissive to genetic and pharmacological manipulations. Our study provides an in-depth characterization of postnatal Glu progenitors and identifies potential therapeutic targets for promoting brain repair.

Abstract:
The hippocampus is the main locus for adult dentate gyrus (DG) neurogenesis. A number of studies have shown that aberrant DG neurogenesis correlates with many neuropsychiatric disorders, including drug addiction. Although clear causal relationships have been established between DG neurogenesis and memory dysfunction or mood-related disorders, evidence of the causal role of DG neurogenesis in drug-seeking behaviors has not been established. Here we assessed the role of new DG neurons in cocaine self-administration using an inducible transgenic approach that selectively depletes adult DG neurogenesis. Our results show that transgenic mice with decreased adult DG neurogenesis exhibit increased motivation to self-administer cocaine and a higher seeking response to cocaine-related cues. These results identify adult hippocampal neurogenesis as a key factor in vulnerability to cocaine addiction.

Abstract:
The amyloid precursor protein (APP) harbors physiological roles at synapses and is central to Alzheimer's disease (AD) pathogenesis. Evidence suggests that APP intracellular domain (AICD) could regulate synapse function, but the underlying molecular mechanisms remain unknown. We addressed AICD actions at synapses, per se, combining in-vivo AICD expression, ex-vivo AICD delivery or APP knock-down by in utero electroporation of shRNAs with whole-cell electrophysiology. We report a critical physiological role of AICD in controlling GluN2B-containing NMDA receptors (NMDARs) at immature excitatory synapses, via a transcription-dependent mechanism. We further show that AICD increase in mature neurons, as reported in AD, alters synaptic NMDAR composition to an immature-like GluN2B-rich profile. This disrupts synaptic signal integration, via over-activation of SK channels, and synapse plasticity, phenotypes rescued by GluN2B antagonism. We provide a new physiological role for AICD, which becomes pathological upon AICD increase in mature neurons. Thus, AICD could contribute to AD synaptic failure.

Abstract:
Recent hypotheses propose that one prerequisite to obtain a rapid antidepressant (AD) effect would reside in processes of synaptic reinforcement occurring within the dentate gyrus (DG) of the hippocampus independently from neurogenesis. However, to date no relationship has been established between an increased DG synaptic plasticity, and rapid AD-like action. To the best of our knowledge, this study shows for the first time that inducing a long-term potentiation (LTP) within the DG by stimulating the perforant pathway (PP) is sufficient to induce such effects. Thus, Sprague-Dawley rats having undergone a successful LTP displayed a significant reduction of immobility when passed acutely 3 days thereafter in the forced swimming test (FST). Further, in a longitudinal paradigm using the pseudo-depressed Wistar-Kyoto rat strain, LTP elicited a decrease of FST immobility after only 2 days, whereas the AD desipramine was not effective before 16 days. In both models, the influence of LTP was transient, as it was no more observed after 8-9 days. No effects were observed on the locomotor activity or on anxiety-related behavior. Theta-burst stimulation of a brain region anatomically adjacent to the PP remained ineffective in the FST. Immunoreactivity of DG cells for phosphorylated histone H3 and doublecortin were not modified three days after LTP, indicating a lack of effect on both cell proliferation and neurogenesis. Finally, depleting brain serotonin contents reduced the success rate of LTP but did not affect its subsequent AD-like effects. These results confirm the 'plastic DG' theory of rapid AD efficacy. Beyond, they point out stimulations of the entorhinal cortex, from which the PP originates, as putative new approaches in AD research.Molecular Psychiatry advance online publication, 9 May 2017; doi:10.1038/mp.2017.94.

Abstract:
Cell proliferation and neuroinflammation in the adult hypothalamus may contribute to the pathogenesis of obesity. Here we tested whether the intertwining of these two processes has a role in the metabolic changes caused by three weeks of saturated high-fat diet (HFD) consumption.As compared to chow, HFD-fed mice rapidly increased body weight and fat mass, and specifically showed increased microglia number in the arcuate nucleus (ARC) of the hypothalamus. Microglia expansion required the adequate presence of fats and carbohydrates in the diet, since feeding mice a very high-fat, very low-carbohydrate diet did not affect cell proliferation. Blocking HFD-induced cell proliferation by central delivery of the antimitotic drug arabinofuranosyl cytidine (AraC) blunted food intake, body weight gain and adiposity. AraC treatment completely prevented the increase in the number of activated microglia in the ARC, the expression of the pro-inflammatory cytokine TNFalpha in microglia and the recruitment of the NF-kappaB pathway, while restoring hypothalamic leptin sensitivity. Central blockade of cell proliferation also normalized circulating levels of the cytokines leptin and IL-1beta and decreased peritoneal pro-inflammatory CD86-IR macrophages number.These findings suggest that inhibition of diet-dependent microglia expansion hinders body weight gain while preventing central and peripheral inflammatory responses due to caloric overload.

Abstract:
Maternal stress is associated with an altered mother-infant relationship that endangers offspring development, leading to emotional/behavioral problems. However, little research has investigated the stress-induced alterations of the maternal brain that could underlie such a disruption of mother-infant bonding. Olfactory cues play an extensive role in the coordination of mother-infant interactions, suggesting that motherhood may be associated to enhanced olfactory performances, and that this effect may be abolished by maternal stress. To test this hypothesis, we analyzed the impact of motherhood under normal conditions or after gestational stress on olfactory functions in C57BL/6 J mice. We report that gestational stress alters maternal behavior and prevents both mothers' ability to discriminate pup odors and motherhood-induced enhancement in odor memory. We investigated adult bulbar neurogenesis as a potential mechanism of the enhanced olfactory function in mothers and found that motherhood was associated with an increased complexity of the dendritic tree of newborn neurons. This motherhood-evoked remodeling was totally prevented by gestational stress. Altogether, our results may thus provide insight into the neural changes that could contribute to altered maternal behavior in stressed mothers.

Abstract:
Brain electroporation is a rapid and powerful approach to study neuronal development. In particular, this technique has become a method of choice for studying the process of radial migration of projection neurons in the embryonic cerebral cortex. This method has considerably helped to describe in detail the different steps of radial migration and to characterize the molecular mechanisms controlling this process. Delineating the complexities of neuronal migration is critical to our understanding not only of normal cerebral cortex formation but also of neurodevelopmental disorders resulting from neuronal migration defects. Here, we describe in detail the protocols to perform in utero or ex vivo electroporation of progenitor cells in the ventricular zone of the cerebral cortex with the aim of studying the process of radial migration of projection neurons during embryonic development. (c) 2016 by John Wiley & Sons, Inc.

Abstract:
The human newborn is highly dependent on parental care for its survival but also for the healthy development of its brain. A large body of literature demonstrates the impact of early life adversity, even during the prenatal period, on the adult's health. The susceptibility to neuropsychiatric diseases is often potentiated by early stress. If there is an agreement that a critical developmental period exists, the mechanisms underlying the long term effects of early life adversity are still poorly understood. Recent studies in animals highlight the involvement of epigenetic processes in the transmission of such vulnerabilities, notably via modifications in germ cells, which can be transmitted in the next generations.

Abstract:
LAMP5 is member of the LAMP family of membrane proteins. In contrast to the canonical members of this protein family, LAMP1 and LAMP2, which show widespread expression in many tissues, LAMP 5 is brain specific in mice. In C. elegans, the LAMP5 ortholog UNC-46 has been suggested to act a trafficking chaperone, essential for the correct targeting of the nematode vesicular GABA-transporter UNC-47. We show here that in the mouse brain LAMP5 is expressed in subpopulations of GABAergic forebrain neurons in the striato-nigral system and the olfactory bulb. The protein was present at synaptic terminals, overlapping with the mammalian vesicular GABA-transporter VGAT. In LAMP5-deficient mice localization of the transporter was unaffected arguing against a conserved role in VGAT trafficking. Electrophysiological analyses in mutants showed alterations in short term synaptic plasticity suggesting that LAMP5 is involved in controlling the dynamics of evoked GABAergic transmission. At the behavioral level, LAMP5 mutant mice showed decreased anxiety and deficits in olfactory discrimination. Altogether, this work implicates LAMP5 function in GABAergic neurotransmission in defined neuronal subpopulations.

Abstract:
Laboratory rodents provided chronic unlimited access to running wheels display increased neurogenesis in the hippocampal dentate gyrus. In addition, recent studies indicate that such an access to wheels stimulates dendritic arborization in newly formed neurons. However, (i) the presence of the running wheel in the housing environment might also bear intrinsic influences on the number and shape of new neurons and (ii) the dendritic arborization of new neurons might be insensitive to moderate daily running activity (i.e. several hours). In keeping with these uncertainties, we have examined neurogenesis and dendritic arborization in newly formed granular cells in adult C57Bl/6N male mice housed for 3 weeks under standard conditions, with a locked wheel, with a running wheel set free 3 h/day, or with a running wheel set permanently free. The results indicate that the presence of a blocked wheel in the home cage increased cell proliferation, but not the number of new neurons while running increased in a duration-dependent manner the number of newborn neurons, as assessed by DCX labeling. Morphological analyses of the dendritic tree of newborn neurons, as identified by BrdU-DCX co-staining, revealed that although the presence of the wheel stimulated their dendritic architecture, the amplitude of this effect was lower than that elicited by running activity, and was found to be running duration-dependent. This article is protected by copyright. All rights reserved.

Abstract:
New dentate granule cells (GCs) are generated in the hippocampus throughout life. These adult-born neurons are required for spatial learning in the Morris water maze (MWM). In rats, spatial learning shapes the network by regulating their number and dendritic development. Here, we explored whether such modulatory effects exist in mice. New GCs were tagged using thymidine analogs or a GFP-expressing retrovirus. Animals were exposed to a reference memory protocol for 10-14 days (spaced training) at different times after newborn cells labeling. Cell proliferation, cell survival, cell death, neuronal phenotype, and dendritic and spine development were examined using immunohistochemistry. Surprisingly, spatial learning did not modify any of the parameters under scrutiny including cell number and dendritic morphology. These results suggest that although new GCs are required in mice for spatial learning in the MWM, they are, at least for the developmental intervals analyzed here, refractory to behavioral stimuli generated in the course of learning in the MWM. (c) 2015 Wiley Periodicals, Inc.

Abstract:
During the last decade, the questions on the functionality of adult neurogenesis have changed their emphasis from if to how the adult-born neurons participate in a variety of memory processes. The emerging answers are complex because we are overwhelmed by a variety of behavioral tasks that apparently require new neurons to be performed optimally. With few exceptions, the hippocampal memory system seems to use the newly generated neurons for multiple roles. Adult neurogenesis has given the dentate gyrus new capabilities not previously thought possible within the scope of traditional synaptic plasticity. Looking at these new developments from the perspective of past discoveries, the science of adult neurogenesis has emerged from its initial phase of being, first, a surprising oddity and, later, exciting possibility, to the present state of being an integral part of mainstream neuroscience. The answers to many remaining questions regarding adult neurogenesis will come along only with our growing understanding of the functionality of the brain as a whole. This, in turn, will require integration of multiple levels of organization from molecules and cells to circuits and systems, ultimately resulting in comprehension of behavioral outcomes.

Abstract:
Adult neurogenesis occurs in the dentate gyrus of the hippocampus, which is a key structure in learning and memory. Adult-generated granule cells have been shown to play a role in spatial memory processes such as acquisition or retrieval, in particular during an immature stage when they exhibit a period of increased plasticity. Here, we demonstrate that immature and mature neurons born in the dentate gyrus of adult rats are similarly activated in spatial memory processes. By imaging the activation of these two different neuron generations in the same rat and by using the immediate early gene Zif268, we show that these neurons are involved in both spatial memory acquisition and retrieval. These results demonstrate that adult-generated granule cells are involved in memory beyond their immaturity stage. This article is protected by copyright. All rights reserved.

Abstract:
The mammalian cerebral cortex contains a high variety of neuronal subtypes that acquire precise spatial locations and form long or short-range connections to establish functional neuronal circuits. During embryonic development, cortical projection neurons are generated in the areas lining the lateral ventricles and they subsequently undergo radial migration to reach the position of their final maturation within the cortical plate. The control of the neuroblast migratory behavior and the coordination of the migration process with other neurogenic events such as cell cycle exit, differentiation and final maturation are crucial to normal brain development. Among the key regulators of cortical neuron migration, the small GTP binding proteins of the Rho family and the atypical Rnd members play important roles in integrating intracellular signaling pathways into changes in cytoskeletal dynamics and motility behavior. Here we review the role of Rnd proteins during cortical neuronal migration and we discuss both the upstream mechanisms that regulate Rnd protein activity and the downstream molecular pathways that mediate Rnd effects on cell cytoskeleton.

Abstract:
Interactions between genes and environment are a critical feature of development and both contribute to shape individuality. They are at the core of vulnerability resiliency for mental illnesses. During the early postnatal period, several brain structures involved in cognitive and emotional processing, such as the hippocampus, still develop and it is likely that interferences with this neuronal development, which is genetically determined, might lead to long-lasting structural and functional consequences and increase the risk of developing psychopathology. One particular target is adult neurogenesis, which is involved in the regulation of cognitive and emotional processes. Insights into the dynamic interplay between genes and environmental factors in setting up individual rates of neurogenesis have come from laboratory studies exploring experience-dependent changes in adult neurogenesis as a function of individual's genetic makeup. These studies have implications for our understanding of the mechanisms regulating adult neurogenesis, which could constitute a link between environmental challenges and psychopathology.

Abstract:
The serotonin2C receptor (5-HT2CR) is known to control dopamine (DA) neuron function by modulating DA neuronal firing and DA exocytosis at terminals. Recent studies assessing the influence of 5-HT2CRs on cocaine-induced neurochemical and behavioral responses have shown that 5-HT2CRs can also modulate mesoaccumbens DA pathway activity at post-synaptic level, by controlling DA transmission in the nucleus accumbens (NAc), independently of DA release itself. A similar mechanism has been proposed to occur at the level of the nigrostriatal DA system. Here, using in vivo microdialysis in freely moving rats and molecular approaches, we assessed this hypothesis by studying the influence of the 5-HT2CR agonist Ro 60-0175 on cocaine-induced responses in the striatum. The intraperitoneal (i.p.) administration of 1 mg/kg Ro 60-0175 had no effect on the increase in striatal DA outflow induced by cocaine (15 mg/kg, i.p.). Conversely, Ro 60-0175 inhibited cocaine-induced Fos immunoreactivity and phosphorylation of the DA and c-AMP regulated phosphoprotein of Mr 32 kDa (DARPP-32) at threonine 75 residue in the striatum. Finally, the suppressant effect of Ro 60-0175 on cocaine-induced DARPP-32 phosphorylation was reversed by the selective 5-HT2CR antagonist SB 242084 (0.5 mg/kg, i.p.). In keeping with the key role of DARPP-32 in DA neurotransmission, our results demonstrate that 5-HT2CRs are capable of modulating nigrostriatal DA pathway activity at post-synaptic level, by specifically controlling DA signaling in the striatum.

Abstract:
In keeping with its ability to control the mesoaccumbens dopamine (DA) pathway, the serotonin2C receptor (5-HT2C R) plays a key role in mediating the behavioral and neurochemical effects of drugs of abuse. Studies assessing the influence of 5-HT2C R agonists on cocaine-induced responses have suggested that 5-HT2C Rs can modulate mesoaccumbens DA pathway activity independently of accumbal DA release, thereby controlling DA transmission in the nucleus accumbens (NAc). In the present study, we assessed this hypothesis by studying the influence of the 5-HT2C R agonist Ro 60-0175 on cocaine-induced behavioral, neurochemical and molecular responses. The i.p. administration of 1 mg/kg Ro 60-0175 inhibited hyperlocomotion induced by cocaine (15 mg/kg, i.p.), had no effect on cocaine-induced DA outflow in the shell, and increased it in the core subregion of the NAc. Furthermore, Ro 60-0175 inhibited the late-onset locomotion induced by the subcutaneous administration of the DA-D2 R agonist quinpirole (0.5 mg/kg), as well as cocaine-induced increase in c-Fos immunoreactivity in NAc subregions. Finally, Ro 60-0175 inhibited cocaine-induced phosphorylation of the DA and c-AMP regulated phosphoprotein of Mr 32 kDa (DARPP-32) at threonine residues in the NAc core, this effect being reversed by the selective 5-HT2C R antagonist SB 242084 (0.5 mg/kg, i.p.). Altogether, these findings demonstrate that 5-HT2C Rs are capable of modulating mesoaccumbens DA pathway activity at post-synaptic level by specifically controlling DA signaling in the NAc core subregion. In keeping with the tight relationship between locomotor activity and NAc DA function, this interaction could participate in the inhibitory control of cocaine-induced locomotor activity.

Abstract:
New neurons are continuously produced in the adult dentate gyrus of the hippocampus, a key structure in learning and memory. It has been shown that adult neurogenesis is crucial for normal memory processing. However, it is not known whether neurons born during the developmental period and during adulthood support the same functions. Here, we demonstrate that neurons born in neonates (first postnatal week) are activated in different memory processes when they are mature compared to neurons born in adults. By imaging the activation of these two different neuron generations in the same rat and using the IEG Zif268 and Fos, we show that these neurons are involved in discriminating dissimilar contexts and spatial problem solving, respectively. These findings demonstrate that the ontogenetic stage during which neurons are generated is crucial for their function within the memory network.

Abstract:
A transcriptional programme initiated by the proneural factors Neurog2 and Ascl1 controls successive steps of neurogenesis in the embryonic cerebral cortex. Previous work has shown that proneural factors also confer a migratory behaviour to cortical neurons by inducing the expression of the small GTP-binding proteins such as Rnd2 and Rnd3. However, the directionality of radial migration suggests that migrating neurons also respond to extracellular signal-regulated pathways. Here we show that the Plexin B2 receptor interacts physically and functionally with Rnd3 and stimulates RhoA activity in migrating cortical neurons. Plexin B2 competes with p190RhoGAP for binding to Rnd3, thus blocking the Rnd3-mediated inhibition of RhoA and also recruits RhoGEFs to directly stimulate RhoA activity. Thus, an interaction between the cell-extrinsic Plexin signalling pathway and the cell-intrinsic Ascl1-Rnd3 pathway determines the level of RhoA activity appropriate for cortical neuron migration.

Abstract:
The cerebral cortex is the site of higher human cognitive and motor functions. Histologically, it is organized into six horizontal layers, each containing unique populations of molecularly and functionally distinct excitatory projection neurons and inhibitory interneurons. The stereotyped cellular distribution of cortical neurons is crucial for the formation of functional neural circuits and it is predominantly established during embryonic development. Cortical neuron development is a multiphasic process characterized by sequential steps of neural progenitor proliferation, cell cycle exit, neuroblast migration and neuronal differentiation. This series of events requires an extensive and dynamic remodeling of the cell cytoskeleton at each step of the process. As major regulators of the cytoskeleton, the family of small Rho GTPases has been shown to play essential functions in cerebral cortex development. Here we review in vivo findings that support the contribution of Rho GTPases to cortical projection neuron development and we address their involvement in the etiology of cerebral cortex malformations.

Abstract:
In utero electroporation is a rapid and powerful technique to study the development of many brain regions. This approach presents several advantages over other methods to study specific steps of brain development in vivo, from proliferation to synaptic integration. Here, we describe in detail the individual steps necessary to carry out the technique. We also highlight the variations that can be implemented to target different cerebral structures and to study specific steps of development.

Abstract:
The mechanisms governing the expansion of neuron number in specific brain regions are still poorly understood. Enlarged neuron numbers in different species are often anticipated by increased numbers of progenitors dividing in the subventricular zone. Here we present live imaging analysis of radial glial cells and their progeny in the ventral telencephalon, the region with the largest subventricular zone in the murine brain during neurogenesis. We observe lineage amplification by a new type of progenitor, including bipolar radial glial cells dividing at subapical positions and generating further proliferating progeny. The frequency of this new type of progenitor is increased not only in larger clones of the mouse lateral ganglionic eminence but also in cerebral cortices of gyrated species, and upon inducing gyrification in the murine cerebral cortex. This implies key roles of this new type of radial glia in ontogeny and phylogeny.

Abstract:
The functional relevance of septo-hippocampal cholinergic (SHC) degeneration to the degradation of hippocampus-dependent declarative memory (DM) in aging and Alzheimer's disease (AD) remains ill-defined. Specifically, selective SHC lesions often fail to induce overt memory impairments in animal models. In spite of apparent normal performance, however, neuronal activity within relevant brain structures might be altered by SHC disruption. We hypothesized that partial SHC degeneration may contribute to functional alterations within memory circuits occurring in aging before DM decline. In young adult mice, we studied the effects of behaviorally ineffective (saporin-induced) SHC lesions - similar in extent to that seen in aged animals - on activity patterns and functional connectivity between three main neural memory systems: the septo-hippocampal system, the striatum and the amygdala that sustain declarative, procedural and emotional memory, respectively. Animals were trained in a radial maze procedure dissociating the human equivalents of relational/DM and non-R/DM expressions in animals. Test-induced Fos activation pattern revealed that the partial SHC lesion significantly altered the brain's functional activities and connectivity (co-activation pattern) despite the absence of overt behavioral deficit. Specifically, hippocampal CA3 hyperactivity and abnormal septo-hippocampo-amygdalar inter-connectivity resemble those observed in aging and prodromal AD. Hence, SHC neurons critically coordinate hippocampal function in concert with extra-hippocampal structures in accordance with specific mnemonic demand. Although partial SHC degeneration is not sufficient to impact DM performance by itself, the connectivity change might predispose the emergence of subsequent DM loss when, due to additional age-related insults, the brain can no longer compensate the holistic imbalance caused by cholinergic loss.

Abstract:
The generation of neurons by neural stem cells is a highly choreographed process that requires extensive and dynamic remodelling of the cytoskeleton at each step of the process. The atypical RhoGTPase Rnd3 is expressed by progenitors in the embryonic brain but its role in early steps of neurogenesis has not been addressed. Here we show that silencing Rnd3 in the embryonic cerebral cortex interferes with the interkinetic nuclear migration of radial glial stem cells, disrupts their apical attachment and modifies the orientation of their cleavage plane. These defects are rescued by co-expression of a constitutively active form of cofilin, demonstrating that Rnd3-mediated disassembly of actin filaments coordinates the cellular behaviour of radial glial. Rnd3 also limits the divisions of basal progenitors via a distinct mechanism involving the suppression of cyclin D1 translation. Interestingly, although Rnd3 expression is controlled transcriptionally by Ascl1, this proneural factor is itself required in radial glial progenitors only for proper orientation of cell divisions.

Abstract:
The functional relevance of septo-hippocampal cholinergic (SHC) degeneration to the degradation of hippocampus-dependent declarative memory (DM) in aging and Alzheimer's disease (AD) remains ill-defined. Specifically, selective SHC lesions often fail to induce overt memory impairments in animal models. In spite of apparent normal performance, however, neuronal activity within relevant brain structures might be altered by SHC disruption. We hypothesized that partial SHC degeneration may contribute to functional alterations within memory circuits occurring in aging before DM decline. In young adult mice, we studied the effects of behaviorally ineffective (saporin-induced) SHC lesions - similar in extent to that seen in aged animals - on activity patterns and functional connectivity between three main neural memory systems: the septo-hippocampal system, the striatum and the amygdala that sustain declarative, procedural and emotional memory, respectively. Animals were trained in a radial maze procedure dissociating the human equivalents of relational/DM and non-R/DM expressions in animals. Test-induced Fos activation pattern revealed that the partial SHC lesion significantly altered the brain's functional activities and connectivity (co-activation pattern) despite the absence of overt behavioral deficit. Specifically, hippocampal CA3 hyperactivity and abnormal septo-hippocampo-amygdalar inter-connectivity resemble those observed in aging and prodromal AD. Hence, SHC neurons critically coordinate hippocampal function in concert with extra-hippocampal structures in accordance with specific mnemonic demand. Although partial SHC degeneration is not sufficient to impact DM performance by itself, the connectivity change might predispose the emergence of subsequent DM loss when, due to additional age-related insults, the brain can no longer compensate the holistic imbalance caused by cholinergic loss.

Abstract:
The dentate gyrus (DG) and the olfactory bulb (OB) are two regions of the adult brain in which new neurons are integrated daily in the existing networks. It is clearly established that these newborn neurons are implicated in specific functions sustained by these regions and that different factors can influence neurogenesis in both structures. Among these, life events, particularly occurring during early life, were shown to profoundly affect adult hippocampal neurogenesis and its associated functions like spatial learning, but data regarding their impact on adult bulbar neurogenesis are lacking. We hypothesized that prenatal stress could interfere with the development of the olfactory system, which takes place during the prenatal period, leading to alterations in adult bulbar neurogenesis and in olfactory capacities. To test this hypothesis we exposed pregnant C57Bl/6J mice to gestational restraint stress and evaluated behavioral and anatomic consequences in adult male offspring. We report that prenatal stress has no impact on adult bulbar neurogenesis, and does not alter olfactory functions in adult male mice. However, it decreases cell proliferation and neurogenesis in the DG of the hippocampus, thus confirming previous reports on rats. Altogether our data support a selective and cross-species long-term impact of prenatal stress on neurogenesis.

Abstract:
BACKGROUND: Adult hippocampal neurogenesis, which is involved in the physiopathology of hippocampal functions, is genetically determined and influenced by early life events. However, studies on the interaction of these determining forces are lacking. This prompted us to investigate whether adult hippocampal neurogenesis can be modulated by maternal care and whether this influence depends upon the genetic background of the individual. METHODS: We used a model of fostering that allows singling out the influence of the genetic make-up of the pups on the outcome of maternal behavior. Mice from two different inbred strains (C57BL/6J and DBA/2J) known to differ in their baseline neurogenesis as well as in their sensitivity to the influence of environmental experiences were raised by nonrelated mothers from the AKR/Ola (AKR) and C3H/He (C3H) strains exhibiting low- and high-pup-oriented behavior, respectively. Neurogenesis was then assessed in the dentate gyrus of the adult adopted C57BL/6J and DBA/2J mice. RESULTS: We show that both the number and the morphological features of newborn granule cells in the dentate gyrus are determined by the maternal environment to which mice were exposed as pups and that this sensitivity to maternal environment is observed only in genetically vulnerable subjects. CONCLUSIONS: Altogether, our data indicate interplay between early environment and the genetic envelop of an individual in determining adult hippocampal neurogenesis. Our experimental approach could thus contribute to the identification of factors determining the neurogenic potential of the adult hippocampus.

Abstract:
A crucial event in the birth of a neuron is the detachment of its apical process from the neuroepithelium. In this issue of Neuron, Rousso et al. (2012) show that repression of N-cadherin by Foxp transcription factors disrupts apical adherens junctions and triggers neurogenesis.

Abstract:
Impairment of working memory is one of the most important deleterious effects of marijuana intoxication in humans, but its underlying mechanisms are presently unknown. Here, we demonstrate that the impairment of spatial working memory (SWM) and in vivo long-term depression (LTD) of synaptic strength at hippocampal CA3-CA1 synapses, induced by an acute exposure of exogenous cannabinoids, is fully abolished in conditional mutant mice lacking type-1 cannabinoid receptors (CB(1)R) in brain astroglial cells but is conserved in mice lacking CB(1)R in glutamatergic or GABAergic neurons. Blockade of neuronal glutamate N-methyl-D-aspartate receptors (NMDAR) and of synaptic trafficking of glutamate alpha-amino-3-hydroxy-5-methyl-isoxazole propionic acid receptors (AMPAR) also abolishes cannabinoid effects on SWM and LTD induction and expression. We conclude that the impairment of working memory by marijuana and cannabinoids is due to the activation of astroglial CB(1)R and is associated with astroglia-dependent hippocampal LTD in vivo.

Abstract:
Adult neurogenesis occurs in the dentate gyrus of the hippocampus, which is a key structure in learning and memory. It is believed that adult-born neurons exert their unique role in information processing due to their high plasticity during immature stage that renders them malleable in response to environmental demands. Here, we demonstrate that, in rats, there is no critical time window for experience-induced dendritic plasticity of adult-born neurons as spatial learning in the water maze sculpts the dendritic arbor of adult-born neurons even when they are several months of age. By ablating neurogenesis within a specific period of time, we found that learning was disrupted when the delay between ablation and learning was extended to several months. Together, these results show that mature adult-born neurons are still plastic when they are functionally integrated into dentate network. Our results suggest a new perspective with regard to the role of neo-neurons by highlighting that even mature ones can provide an additional source of plasticity to the brain to process memory information.

Abstract:
Until recently, it was believed that the introduction of new neurons in neuronal networks was incompatible with memory function. Since the rediscovery of adult hippocampal neurogenesis, behavioral data demonstrate that adult neurogenesis is required for memory processing. We examine neurocomputational studies to identify which basic mechanisms involved in memory might be mediated by adult neurogenesis. Mainly, adult neurogenesis might be involved in the reduction of catastrophic interference and in a time-related pattern separation function. Artificial neuronal networks suggest that the selective recruitment of new-born or old neurons is not stochastic, but depends on environmental requirements. This leads us to propose the novel concept of 'soft-supervision'. Soft-supervision would be a biologically plausible process, by which the environment is able to influence activation and learning rules of neurons differentially.

Abstract:
New neurons are continuously produced in the adult dentate gyrus of the hippocampus. It has been shown that one of the functions of adult neurogenesis is to support spatial pattern separation, a process that transforms similar memories into nonoverlapping representations. This prompted us to investigate whether adult-born neurons are required for discriminating two contexts, i.e., for identifying a familiar environment and detect any changes introduced in it. We show that depleting adult-born neurons impairs the animal's ability to disambiguate two contexts after extensive training. These data suggest that the continuous production of new dentate neurons plays a crucial role in extracting and separating efficiently contextual representation in order to discriminate features within events.

Abstract:
Hyperforin is one of the main bioactive compounds that underlie the antidepressant actions of the medicinal plant Hypericum perforatum (St. John's wort). However, the effects of a chronic hyperforin treatment on brain cells remains to be fully addressed. The following study was undertaken to further advance our understanding of the biological effects of this plant extract on neurons. Special attention was given to its impact on the brain-derived neurotrophic factor (BDNF) receptor TrkB and on adult hippocampal neurogenesis since they appear central to the mechanisms of action of antidepressants. The consequences of a chronic hyperforin treatment were investigated on cortical neurons in culture and on the brain of adult mice treated for 4 wk with a daily injection (i.p.) of hyperforin (4 mg/kg). Its effects on the expression of the cyclic adenosine monophosphate response element-binding protein (CREB), phospho-CREB (p-CREB), TrkB and phospho-TrkB (p-TrkB) were analysed by Western blot experiments and its impact on adult hippocampal neurogenesis was also investigated. Hyperforin stimulated the expression of TRPC6 channels and TrkB via SKF-96365-sensitive channels controlling a downstream signalling cascade involving Ca2+, protein kinase A, CREB and p-CREB. In vivo, hyperforin augmented the expression of TrkB in the cortex but not in the hippocampus where hippocampal neurogenesis remained unchanged. In conclusion, this plant extract acts on the cortical BDNF/TrkB pathway leaving adult hippocampal neurogenesis unaffected. This study provides new insights on the neuronal responses controlled by hyperforin. We propose that the cortex is an important brain structure targeted by hyperforin.

Abstract:
In utero electroporation (IUE) has become a powerful technique to study the development of different regions of the embryonic nervous system (1-5). To date this tool has been widely used to study the regulation of cellular proliferation, differentiation and neuronal migration especially in the developing cerebral cortex (6-8). Here we detail our protocol to electroporate in utero the cerebral cortex and the hippocampus and provide evidence that this approach can be used to study dendrites and spines in these two cerebral regions. Visualization and manipulation of neurons in primary cultures have contributed to a better understanding of the processes involved in dendrite, spine and synapse development. However neurons growing in vitro are not exposed to all the physiological cues that can affect dendrite and/or spine formation and maintenance during normal development. Our knowledge of dendrite and spine structures in vivo in wild-type or mutant mice comes mostly from observations using the Golgi-Cox method( 9). However, Golgi staining is considered to be unpredictable. Indeed, groups of nerve cells and fiber tracts are labeled randomly, with particular areas often appearing completely stained while adjacent areas are devoid of staining. Recent studies have shown that IUE of fluorescent constructs represents an attractive alternative method to study dendrites, spines as well as synapses in mutant / wild-type mice (10-11) (Figure 1A). Moreover in comparison to the generation of mouse knockouts, IUE represents a rapid approach to perform gain and loss of function studies in specific population of cells during a specific time window. In addition, IUE has been successfully used with inducible gene expression or inducible RNAi approaches to refine the temporal control over the expression of a gene or shRNA (12). These advantages of IUE have thus opened new dimensions to study the effect of gene expression/suppression on dendrites and spines not only in specific cerebral structures (Figure 1B) but also at a specific time point of development (Figure 1C). Finally, IUE provides a useful tool to identify functional interactions between genes involved in dendrite, spine and/or synapse development. Indeed, in contrast to other gene transfer methods such as virus, it is straightforward to combine multiple RNAi or transgenes in the same population of cells. In summary, IUE is a powerful method that has already contributed to the characterization of molecular mechanisms underlying brain function and disease and it should also be useful in the study of dendrites and spines.

Abstract:
Vascular-specific growth factor angiopoietin-2 (Ang2) is mainly involved during vascular network setup. Recently, Ang2 was suggested to play a role in adult neurogenesis, affecting migration and differentiation of adult neuroblasts in vitro. However, to date, no data have reported an effect of Ang2 on neurogenesis during embryonic development. As we detected Ang2 expression in the developing cerebral cortex at embryonic day E14.5 and E16.5, we used in utero electroporation to knock down Ang2 expression in neuronal progenitors located in the cortical ventricular zone (VZ) to examine the role of Ang2 in cortical embryonic neurogenesis. Using this strategy, we showed that radial migration from the VZ toward the cortical plate of Ang2-knocked down neurons is altered as well as their morphology. In parallel, we observed a perturbation of intermediate progenitor population and the surrounding vasculature. Taken together, our results show for the first time that, in addition to its role during brain vasculature setup, Ang2 is also involved in embryonic cortical neurogenesis and especially in the radial migration of projection neurons.

Abstract:
Proneural genes such as Ascl1 are known to promote cell cycle exit and neuronal differentiation when expressed in neural progenitor cells. The mechanisms by which proneural genes activate neurogenesis--and, in particular, the genes that they regulate--however, are mostly unknown. We performed a genome-wide characterization of the transcriptional targets of Ascl1 in the embryonic brain and in neural stem cell cultures by location analysis and expression profiling of embryos overexpressing or mutant for Ascl1. The wide range of molecular and cellular functions represented among these targets suggests that Ascl1 directly controls the specification of neural progenitors as well as the later steps of neuronal differentiation and neurite outgrowth. Surprisingly, Ascl1 also regulates the expression of a large number of genes involved in cell cycle progression, including canonical cell cycle regulators and oncogenic transcription factors. Mutational analysis in the embryonic brain and manipulation of Ascl1 activity in neural stem cell cultures revealed that Ascl1 is indeed required for normal proliferation of neural progenitors. This study identified a novel and unexpected activity of the proneural gene Ascl1, and revealed a direct molecular link between the phase of expansion of neural progenitors and the subsequent phases of cell cycle exit and neuronal differentiation.

Abstract:
Adult neurogenesis is a process by which the brain produces new neurons once development has ceased. Adult hippocampal neurogenesis has been linked to the relational processing of spatial information, a role attributed to the contribution of newborn neurons to long-term potentiation (LTP). However, whether newborn neurons also influence long-term depression (LTD), and how synaptic transmission and plasticity are affected as they incorporate their network, remain to be determined. To address these issues, we took advantage of a genetic model in which a majority of adult-born neurons can be selectively ablated in the dentate gyrus (DG) and, most importantly, in which neurogenesis can be restored on demand. Using electrophysiological recordings, we show that selective reduction of adult-born neurons impairs synaptic transmission at medial perforant pathway synapses onto DG granule cells. Furthermore, LTP and LTD are largely compromised at these synapses, probably as a result of an increased induction threshold. Whereas the deficits in synaptic transmission and plasticity are completely rescued by restoring neurogenesis, these synapses regain their ability to express LTP much faster than their ability to express LTD. These results demonstrate that both LTP and LTD are influenced by adult neurogenesis. They also indicate that as newborn neurons integrate their network, the ability to express bidirectional synaptic plasticity is largely improved at these synapses. These findings establish that adult neurogenesis is an important process for synaptic transmission and bidirectional plasticity in the DG, accounting for its role in efficiently integrating novel incoming information and in forming new memories.

Abstract:
Little is known of the intracellular machinery that controls the motility of newborn neurons. We have previously shown that the proneural protein Neurog2 promotes the migration of nascent cortical neurons by inducing the expression of the atypical Rho GTPase Rnd2. Here, we show that another proneural factor, Ascl1, promotes neuronal migration in the cortex through direct regulation of a second Rnd family member, Rnd3. Both Rnd2 and Rnd3 promote neuronal migration by inhibiting RhoA signaling, but they control distinct steps of the migratory process, multipolar to bipolar transition in the intermediate zone and locomotion in the cortical plate, respectively. Interestingly, these divergent functions directly result from the distinct subcellular distributions of the two Rnd proteins. Because Rnd proteins also regulate progenitor divisions and neurite outgrowth, we propose that proneural factors, through spatiotemporal regulation of Rnd proteins, integrate the process of neuronal migration with other events in the neurogenic program.

Abstract:
Understanding the cellular mechanisms underlying learning and memory is a major challenge in neurobiology. Structural and functional changes occurring in the hippocampus such as synaptic remodeling and long-term potentiation are key signatures of long-term memory processes. The discovery of a de novo hippocampal production of neurons in the adult brain has been a breakthrough in the field of plasticity and memory, introducing a new actor that could sustain memory processes. Here we will review our current knowledge on the role of these adult new neurons in memory. In particular we will provide evidence showing that they are required for learning and memory and that an alteration in their production rate or maturation leads to memory impairments. Through a thorough survey of the literature, we will also acknowledge that there are many controversies regarding the specific role played by newborn neurons. The emerging picture is that they are involved in the establishment of spatiotemporal relationships among multiple environmental cues for the flexible use of the acquired information. Indeed, newborn neurons have been found to be required for separating events based on their spatial and temporal characteristics, a process that preserves the uniqueness of a memory representation. Thus, adult-born neurons are required for allocentric space representation, for long-term memory retention and for flexible inferential memory expression. Finally, we will conclude by highlighting directions for future research, emphasizing that the exact participation of newborn neurons in memory processes will not be approached without considering the hippocampal network in general.

Abstract:
In the mammalian brain, the dentate gyrus and the olfactory bulb are regions where new neurons are continuously added. While the functional consequences of continuous hippocampal neurogenesis have been extensively studied, the role of olfactory adult-born neurons remains elusive. In particular, the involvement of these newborn neurons in odor processing is still a matter of debate. We demonstrate a critical impact of both the age of new neurons and the memory processes involved (learning vs recall) in the recruitment of newborn cells. Thus, odor stimulation preferentially recruited immature neurons over more mature ones (2 weeks old vs 5 and 9 weeks old), whereas associative learning based on odor discrimination preferentially recruited mature neurons (5-9 weeks old). Furthermore, while mature neurons were activated by this associative learning, they were not activated by long-term memory recall, indicating that the contribution of newborn neurons in olfactory functions depends also on the memory process involved. Our data thus show that newborn neurons are indeed involved in odor processing and that their recruitment is age- and memory process-dependent.

Abstract:
Scribble (Scrib) is a key regulator of apicobasal polarity, presynaptic architecture, and short-term synaptic plasticity in Drosophila. In mammals, its homolog Scrib1 has been implicated in cancer, neural tube closure, and planar cell polarity (PCP), but its specific role in the developing and adult nervous system is unclear. Here, we used the circletail mutant, a mouse model for PCP defects, to show that Scrib1 is located in spines where it influences actin cytoskeleton and spine morphing. In the hippocampus of these mutants, we observed an increased synapse pruning associated with an increased number of enlarged spines and postsynaptic density, and a decreased number of perforated synapses. This phenotype was associated with a mislocalization of the signaling pathway downstream of Scrib1, leading to an overall activation of Rac1 and defects in actin dynamic reorganization. Finally, Scrib1-deficient mice exhibit enhanced learning and memory abilities and impaired social behavior, two features relevant to autistic spectrum disorders. Our data identify Scrib1 as a crucial regulator of brain development and spine morphology, and suggest that Scrib1(crc/+) mice might be a model for studying synaptic dysfunction and human psychiatric disorders.

Abstract:
Chronic voluntary wheel-running activity has been reported to hypersensitise central CB1 receptors in mice. On the other hand, pharmacological findings suggest that the CB1 receptor could be involved in wheel-running behaviour and in running-induced neurogenesis in the hippocampus. We analysed wheel-running behaviour for 6 weeks and measured its consequences on hippocampal neurogenesis in CB1 knockout (CB1(-/-)) animals, compared to wild-type (CB1(+/+)) littermates. Because wheel running has been shown to affect locomotor reactivity in novel environments, memory for aversive events and depression-like behaviours, we also assessed these behaviours in control and running CB1(+/+) and CB1(-/-) mice. When compared with running CB1(+/+) mice, the distance covered weekly by CB1(-/-) mice was decreased by 30-40%, an observation accounted for by decreased time spent and maximal velocity on the wheels. Analyses of running distances with respect to the light/dark cycle revealed that mutant covered less distance throughout both the inactive and the active phases of that cycle. Locomotion in an activity cage, exploration in an open field, and immobility time in the forced swim test proved insensitive to chronic wheel running in either genotype. Wheel running, per se, did not influence the expression and extinction of cued fear memory but counteracted in a time-dependent manner the deficiency of extinction measured in CB1(-/-) mice. Hippocampal neurogenesis, assessed by doublecortin labelling of immature neurons in the dentate gyrus, was lowered by 40% in control CB1(-/-) mice, compared to control CB1(+/+) mice. Although CB1(-/-) mice ran less than their wild-type littermates, the 6-week running protocol increased neurogenesis to similar extents (37-39%) in both genotypes. This study suggests that mouse CB1 receptors control wheel running but not its neurogenic consequences in the hippocampus.

Abstract:
Neurogenesis in the hippocampus is characterized by the birth of thousand of cells that generate neurons throughout life. The fate of these adult newborn neurons depends on life experiences. In particular, spatial learning promotes the survival and death of new neurons. Whether learning influences the development of the dendritic tree of the surviving neurons (a key parameter for synaptic integration and signal processing) is unknown. Here we show that learning accelerates the maturation of their dendritic trees and their integration into the hippocampal network. We demonstrate that these learning effects on dendritic arbors are homeostatically regulated, persist for several months, and are specific to neurons born during adulthood. Finally, we show that this dendritic shaping depends on the cognitive demand and relies on the activation of NMDA receptors. In the search for the structural changes underlying long-term memory, these findings lead to the conclusion that shaping neo-networks is important in forming spatial memories.

Abstract:
The dogma according to which 'once the development of the central nervous system ended, generation of neurons was impossible' has been challenged by the discovery that new neurons are created in specific regions of the adult mammalian brain. This discovery has been one of the most controversial of modern neuroscience. One of these regions is the dentate gyrus of the hippocampal formation, a key structure in memory. Here we will review our current knowledge on the role of adult hippocampal neurogenesis in memory and in the pathophysiology of memory. In particular we will review evidence showing that adult-born neurons are required for learning and memory and that an alteration of their production rate leads to memory impairments. We also discuss how neurogenesis is finely shaped by learning for the purpose of mnemonic information processing.

Abstract:
Adult hippocampal neurogenesis is a unique example of structural plasticity, the functional role of which has been a matter of intense debate. New transgenic models have recently shown that neurogenesis participates in hippocampus-mediated learning. Here, we show that transgenic animals, in which adult hippocampal neurogenesis has been specifically impaired, exhibit a striking increase in anxiety-related behaviors. Our results indicate that neurogenesis plays an important role in the regulation of affective states and could be the target of new treatments for anxiety disorders.

Abstract:
Stressors occurring during pregnancy can alter the developmental trajectory of offspring and lead to, among other deleterious effects, cognitive deficits and hyperactivity of the hypothalamo-pituitary-adrenal axis. A recent feature of the prenatal stress (PS) model is its reported influence on structural plasticity in hippocampal formation, which sustains both cognitive functions and stress responsiveness. Indeed, we and others have previously reported that males exposed to stress in utero are characterized by a decrease in hippocampal cell proliferation, and consequently neurogenesis, from adolescence to senescence. Recent studies in females submitted to PS have reported conflicting results, ranging from no effect to a decrease in cell proliferation. We hypothesized that changes in cell proliferation in PS female rats are age dependent. To address this issue, we examined the impact of PS on hippocampal cell proliferation in juvenile, young, middle-aged and old females. As hypothesized, we found an age-dependent effect of PS in female rats as cell proliferation was significantly decreased only when animals reached senescence, a time when adrenal gland weight also increased. These data suggest that the deleterious effects of PS on hippocampal cell proliferation in females are either specific to senescence or masked during adulthood by protective factors.

Abstract:
BACKGROUND: In mammals, new neurons are added to the olfactory bulb (OB) throughout life. Most of these new neurons, granule and periglomerular cells originate from the subventricular zone (SVZ) lining the lateral ventricles and migrate via the rostral migratory stream toward the OB. Thousands of new neurons appear each day, but the function of this ongoing neurogenesis remains unclear. METHODOLOGY/PRINCIPAL FINDINGS: In this study, we irradiated adult mice to impair constitutive OB neurogenesis, and explored the functional impacts of this irradiation on the sense of smell. We found that focal irradiation of the SVZ greatly decreased the rate of production of new OB neurons, leaving other brain areas intact. This effect persisted for up to seven months after exposure to 15 Gray. Despite this robust impairment, the thresholds for detecting pure odorant molecules and short-term olfactory memory were not affected by irradiation. Similarly, the ability to distinguish between odorant molecules and the odorant-guided social behavior of irradiated mice were not affected by the decrease in the number of new neurons. Only long-term olfactory memory was found to be sensitive to SVZ irradiation. CONCLUSION/SIGNIFICANCE: These findings suggest that the continuous production of adult-generated neurons is involved in consolidating or restituting long-lasting olfactory traces.

Abstract:
This study demonstrates that a prolyl hydroxylase inhibitor, FG-0041, is able, in combination with the ROCK inhibitor, Y-27632, to initiate differentiation of mesenchymal stem cells (MSCs) into neuron-like cells. FG-0041/Y-27632 co-treatment provokes morphological changes into neuron-like cells, increases neuronal marker expression and provokes modifications of cell cycle-related gene expression consistent with a cell cycle arrest of MSC, three events showing the engagement of MSC towards the neuronal lineage. Moreover, as we observed in our previous studies with cobalt chloride and desferroxamine, the activation of HIF-1 by this prolyl hydroxylase inhibitor is potentiated by Y-27632 which could explain at least in part the effect of this co-treatment on MSC neuronal differentiation. In addition, we show that this co-treatment enhances neurite outgrowth and tyrosine hydroxylase expression in PC12 cells. Altogether, these results evidence that concomitant inhibition of prolyl hydroxylases and ROCK represents a relevant protocol to initiate neuronal differentiation.

Abstract:
Many studies showed beneficial effects of either erythropoietin (EPO) or mesenchymal stem cells (MSCs) treatment in cerebral ischemia. In addition to a neuroprotective role, not only EPO but also MSC favors neurogenesis and functional recovery. In an attempt to further improve postischemic tissue repair, we investigated the effect of a systemic administration of MSC, in the presence or not of EPO, on neurogenesis and functional recovery in a transient focal cerebral ischemia model in the adult rat. Twenty-four hours after ischemia, the rats were divided into four groups, namely vehicle, MSC, EPO, and MSC+EPO, and received a single intravenous injection of MSC (2 x 10(6) cells) and/or a repeated intraperitoneal administration of EPO (1,000 UI/kg) for 3 days. The lesion volume, the MSC outcome, neurogenesis, and functional recovery were assessed 51 days after ischemia. The results showed that cellular proliferation and neurogenesis were increased along the lateral ventricle wall in the MSC+EPO group, whereas no significant effect was observed in groups receiving MSC or EPO alone. This effect was accompanied by an improvement of mnesic performances. Mesenchymal stem cells expressing neuronal or glial markers were detected in the ischemic hemisphere. These results suggest that EPO could act in a synergistic way with MSC to potentiate the postischemic neurogenesis.

Abstract:
Adult hippocampal neurogenesis is influenced by a variety of stimuli, including exercise, but the mechanisms by which running affects neurogenesis are not yet fully understood. Because beta-endorphin, which is released in response to exercise, increases cell proliferation in vitro, we hypothesized that it could exert a similar effect in vivo and mediate the stimulatory effects of running on neurogenesis. We thus analyzed the effects of voluntary wheel-running on adult neurogenesis (proliferation, differentiation, survival/death) in wild-type and beta-endorphin-deficient mice. In wild-type mice, exercise promoted cell proliferation evaluated by sacrificing animals 24 h after the last 5-bromo-2'-deoxyuridine (BrdU) pulse and by using endogenous cell cycle markers (Ki67 and pH(3)). This was accompanied by an increased survival of 4-wk-old BrdU-labeled cells, leading to a net increase of neurogenesis. Beta-endorphin deficiency had no effect in sedentary mice, but it completely blocked the running-induced increase in cell proliferation; this blockade was accompanied by an increased survival of 4-wk-old cells and a decreased cell death. Altogether, adult neurogenesis was increased in response to exercise in knockout mice. We conclude that beta-endorphin released during running is a key factor for exercise-induced cell proliferation and that a homeostatic balance may regulate the final number of new neurons.

Abstract:
The importance of maternal care in shaping an individual's phenotype in health and disease is becoming more and more apparent in both human and animal studies. However, in mouse studies using inbred strains or knockout mice to analyze the genetic influences on the development of normal and aberrant behavioral phenotypes, maternal behavior is very poorly characterized and often ignored. This study provides an extensive analysis of spontaneous maternal behavior of inbred mice in three conditions: (1) comparing two commonly used strains, (2) analyzing the impact of adopting pups from the same strain (intrastrain cross-fostering) and (3) analyzing the impact of adopting pups from a different strain (interstrain cross-fostering). For each condition, maternal behavior was analyzed continuously over 23-h periods on postnatal days 2, 4, 6 and 9. We report that (1) the maternal behavior of C57BL/6J and DBA/2J dams toward their biological offspring is highly similar, (2) intrastrain cross-fostering has minimal impact on maternal behavior of C57BL/6J and DBA/2J dams, (3) interstrain cross-fostering does not modify the strain differences in maternal care observed between AKR and C3H/He mothers and (4) the pup strain does influence the amount of maternal behavior shown by both mothers in interstrain cross-fostering. These latter findings demonstrate that both mother strain and pup strain are key determinants of maternal behavior.

Abstract:
A dysfunction of retinoid hippocampal signaling pathway has been involved in the appearance of affective and cognitive disorders. However, the underlying neurobiological mechanisms remain unknown. Hippocampal granule neurons are generated throughout life and are involved in emotion and memory. Here, we investigated the effects of vitamin A deficiency (VAD) on neurogenesis and memory and the ability of retinoic acid (RA) treatment to prevent VAD-induced impairments. Adult retinoid-deficient rats were generated by a vitamin A-free diet from weaning in order to allow a normal development. The effects of VAD and/or RA administration were examined on hippocampal neurogenesis, retinoid target genes such as neurotrophin receptors and spatial reference memory measured in the water maze. Long-term VAD decreased neurogenesis and led to memory deficits. More importantly, these effects were reversed by 4 weeks of RA treatment. These beneficial effects may be in part related to an up-regulation of retinoid-mediated molecular events, such as the expression of the neurotrophin receptor TrkA. We have demonstrated for the first time that the effect of vitamin A deficient diet on the level of hippoccampal neurogenesis is reversible and that RA treatment is important for the maintenance of the hippocampal plasticity and function.

Abstract:
The dentate gyrus of the hippocampus is one of the few regions of the mammalian brain where new neurons are generated throughout adulthood. This adult neurogenesis has been proposed as a novel mechanism that mediates spatial memory. However, data showing a causal relationship between neurogenesis and spatial memory are controversial. Here, we developed an inducible transgenic strategy allowing specific ablation of adult-born hippocampal neurons. This resulted in an impairment of spatial relational memory, which supports a capacity for flexible, inferential memory expression. In contrast, less complex forms of spatial knowledge were unaltered. These findings demonstrate that adult-born neurons are necessary for complex forms of hippocampus-mediated learning.

Abstract:
BACKGROUND: Accumulating epidemiological evidence points to the role of genetic background as a modulator of the capacity of adverse early experiences to give rise to mental illness. However, direct evidence of such gene-environment interaction in the context of substance abuse is scarce. In the present study we investigated whether the impact of early life experiences on cocaine intake in adulthood depends on genetic background. In addition, we studied other behavioral dimensions associated with drug abuse, i.e. anxiety- and depression-related behaviors. METHODOLOGY/PRINCIPAL FINDINGS: For this purpose, we manipulated the maternal environment of two inbred mouse strains, the C57BL/6J and DBA/2J by fostering them with non-related mothers, i.e. the C3H/HeN and AKR strains. These mother strains show respectively high and low pup-oriented behavior. As adults, C57BL/6J and DBA/2J were tested either for cocaine intravenous self-administration or in the elevated plus-maze and forced swim test (FST). We found that the impact of maternal environment on cocaine use and a depression-related behavior depends upon genotype, as cocaine self-administration and behavior in the FST were influenced by maternal environment in DBA/2J, but not in C57BL/6J mice. Anxiety was not influenced by maternal environment in either strain. CONCLUSIONS/SIGNIFICANCE: Our experimental approach could contribute to the identification of the psychobiological factors determining the susceptibility or the resilience of certain individuals to develop psychopathologies.

Abstract:
The role of adult hippocampal neurogenesis in spatial learning remains a matter of debate. Here, we show that spatial learning modifies neurogenesis by inducing a cascade of events that resembles the selective stabilization process characterizing development. Learning promotes survival of relatively mature neurons, apoptosis of more immature cells, and finally, proliferation of neural precursors. These are three interrelated events mediating learning. Thus, blocking apoptosis impairs memory and inhibits learning-induced cell survival and cell proliferation. In conclusion, during learning, similar to the selective stabilization process, neuronal networks are sculpted by a tightly regulated selection and suppression of different populations of newly born neurons.

Abstract:
This study demonstrates that the Rho-kinase (ROCK) inhibitor, Y-27632, potentiates not only the effect of cobalt chloride (CoCl(2)) but also that of deferoxamine, another HIF-1 inducer, on mesenchymal stem cell (MSC) neuronal differentiation. HIF-1 is essential for CoCl(2)+/-Y-27632-induced MSC neuronal differentiation, since agents inhibiting HIF-1 abolish the changes of morphology and cell cycle arrest-related gene or protein expressions (p21, cyclin D1) and the increase of neuronal marker expressions (Tuj1, NSE). Y-27632 potentiates the CoCl(2)-induced decrease of cyclin D1 and nestin expressions, the increase of HIF-1 activation and EPO expression, and decreases pVHL expression. Interestingly, CoCl(2) decreases RhoA expression, an effect potentiated by Y-27632, revealing crosstalk between HIF-1 and RhoA/ROCK pathways. Moreover, we demonstrate a synergistic effect of CoCl(2) and Y-27632 on neurosphere differentiation into neurons and PC12 neurite outgrowth underlining that a co-treatment targeting both HIF-1 and ROCK pathways might be relevant to differentiate stem cells into neurons.

Abstract:
Aging is accompanied by an alteration of spatial memory, which has been related to an alteration in hippocampal plasticity. Within the dentate gyrus, new neurons are generated throughout the entire life of an individual. This neurogenesis seems to play a role in hippocampal-mediated learning and learning-induced changes in neurogenesis have been proposed to be involved in memory. However, in aged rats, little is known on the influence of learning on the early development of the adult-born neurons and on the possible involvement of learning-induced changes in neurogenesis in age-related memory deficits. To address this issue, we took advantage of the existence of spontaneous individual differences for performances observed in aged subjects in the water maze. In this task, learning can be divided into two phases, an early phase during which performances quickly improve, and a late phase during which asymptotic levels of performances are reached. We show that the influence of spatial learning on the survival of the newly born cells depends on their birth date and the memory abilities of the aged rats. In aged rats with preserved spatial memory, learning increases the survival of cells generated before learning whereas it decreases survival of cells produced during the early phase of learning. These results highlight the importance of learning-induced changes in adult-born cell survival in memory. Furthermore, they provide new insights on the possible neural mechanisms of aging of cognitive functions and show that an alteration to the steps leading to neurogenesis may be involved in the determination of individual memory abilities.

Abstract:
There is much interest to understand the mechanisms leading to the establishment, maintenance, and extinction of fear memories. The amygdala has been critically involved in the processing of fear memories and a number of molecular changes have been implicated in this brain region in relation to fear learning. Although neural cell adhesion molecules (NCAMs) have been hypothesized to play a role, information available about their contribution to fear memories is scarce. We investigate here whether polysialylated NCAM (PSA-NCAM) contributes to auditory fear conditioning in the amygdala. First, PSA-NCAM expression was evaluated in different amygdala nuclei after auditory fear conditioning at two different shock intensities. Results showed that PSA-NCAM expression was increased 24 h post-training only in animals subjected to the highest shock intensity (1mA). Second, PSA-NCAM was cleaved in the basolateral amygdaloid complex through micro-infusions of the enzyme endoneuraminidase N, and the consequences of such treatment were investigated on the acquisition, consolidation, remote memory expression, and extinction of conditioned fear memories. Intra-amygdaloid cleavage of PSA-NCAM did not affect acquisition, consolidation or expression of remote fear memories. However, intra-amygdaloid PSA-NCAM cleavage enhanced fear extinction processes. These results suggest that upregulation of PSA-NCAM is a correlate of fear conditioning that is not necessary for the establishment of fear memory in the amygdala, but participates in mechanisms precluding fear extinction. These findings point out PSA-NCAM as a potential target for the treatment of psychopathologies that involve impairment in fear extinction.

Abstract:
The role of the hippocampus in pavlovian fear conditioning is controversial. Although lesion and pharmacological inactivation studies have suggested a key role for the dorsal hippocampus in contextual fear conditioning, the involvement of the ventral part is still uncertain. Likewise, the debate is open with regard to the putative implication of each hippocampal subdivision in fear conditioning to a discrete conditioned stimulus. We explored the potential existence of dissociations occurring in the dorsal versus ventral hippocampus at the cellular level while dealing with either contextual or cued fear conditioning and focused in a molecular 'signature' linked to structural plasticity, the polysialylated form of the neural cell adhesion molecule (PSA-NCAM). We found an upregulation of PSA-NCAM expression in the dorsal (but not ventral) dentate gyrus at 24 h after contextual (but not tone) fear conditioning. Specific removal of PSA through microinfusion of the enzyme endoneuraminidase-N in the dorsal (but not ventral) hippocampus reduced freezing responses to the conditioned context. Therefore, we present evidence for a specific role of PSA-NCAM in the dorsal hippocampus in the plasticity processes occurring during consolidation of the context representation after 'standard' contextual fear conditioning. Interestingly, we also found that exposing animals just to the context induced an activation of PSA-NCAM in both dorsal and ventral dentate gyrus. Altogether, these findings highlighting the distinctive occurrence of these neuroplastic processes in the dorsal hippocampus during the standard contextual fear-conditioning task enlighten the ongoing debate about the involvement of these hippocampal subdivisions in pavlovian fear conditioning.

Abstract:
Excitotoxic lesion of the striatum provides a useful model for evaluating the excitotoxic processes involved in neurological disorders, in particular stroke diseases. The behavioural outcome after such injury is however poorly described. We have therefore investigated the potential behavioural deficits induced by a NMDA-induced excitotoxic unilateral lesion of the lateral part of the striatum, by comparison with a PBS striatal injection (sham procedure), and non-operated mice behaviour. Three groups of male adult Swiss mice were constituted: unilateral NMDA (20 nmol striatal NMDA injection), sham (striatal PBS injection), and control (healthy non-operated mice). From 14 to 29 days post-surgery, sensorimotor and mnesic tests were performed in all groups. After euthanasia, immunohistochemical stainings (NeuN and GFAP) were performed in order to assess the size of the lesion. Straight runway and passive avoidance performances revealed mild deficits related to the excitotoxic NMDA-induced lesion as compared to the sham procedure. Moreover, accelerated rotarod and Morris water maze acquisition performances also revealed deficits related to the surgery, i.e. observed in sham-operated as compared to control mice. NeuN staining revealed no striatal lesion in the sham and non-operated groups in contrast to the NMDA-injected group in which the volume of infarcted striatum was 2.4+/-0.3mm3. GFAP staining revealed a glial reaction in the lesioned striatum of NMDA animals and at the PBS injection site in sham animals. These results suggest that NMDA-induced excitotoxic lesion induces subtle long-term behavioural deficits in mice. Moreover, this study shows the importance of the sham group to investigate the behavioural deficits after excitotoxic lesion models in mice.

Abstract:
STUDY OBJECTIVES: Numerous clinical studies and sleep surveys have shown pronounced sex differences in the occurrence of insomnia and other sleep pathologies. It has been suggested that sex differences in sleep, while subtle under baseline conditions, may increase in magnitude under biological or environmental challenges. However, controlled and experimental studies on sleep under challenged conditions rarely include female subjects. In this context, we examined sex differences in sleep in the mouse, not only under baseline conditions, but also after sleep deprivation and restraint stress. DESIGN: Adult male and female C57BL/6J mice were implanted with electrodes to record sleep-wake architecture and sleep electroencephalogram under baseline conditions and after 6 hours of sleep deprivation or 1 hour of restraint stress at the beginning of the daily light phase. RESULTS: Although baseline sleep patterns slightly differed between the sexes, the homeostatic recovery response to sleep deprivation was similar. In contrast, the changes in sleep after restraint stress were markedly different between male and female mice, with males displaying a stronger initial suppression of sleep and a stronger rebound of rapid-eye-movement sleep later in the recovery phase. CONCLUSIONS: In mice, the fundamental homeostatic properties of sleep regulation may not differ between the sexes, but the way sleep is affected and disrupted by environmental influences may be sex dependent. The latter may reflect sex differences in stress sensitivity.

Abstract:
Bone-marrow-derived mesenchymal stem cells (MSCs) constitute an interesting cellular source to promote brain regeneration after neurodegenerative diseases. Recently, several studies suggested that oxygen-dependent gene expression is of crucial importance in governing the essential steps of neurogenesis such as cell proliferation, survival and differentiation. In this context, we analysed the effect of the HIF-1 (hypoxia inducible factor-1) activation-mimicking agent CoCl(2) on MSCs. CoCl(2) treatment increased the expression of the anti-proliferative gene BTG2/PC3 and decreased cyclin D1 expression. Expression of HIF-1alpha and its target genes EPO, VEGF and p21 was also upregulated. These changes were followed by inhibition of cell proliferation and morphological changes resulting in neuron-like cells, which had increased neuronal marker expression and responded to neurotransmitters. Echinomycin, a molecule inhibiting HIF-1 DNA-binding activity, blocked the CoCl(2) effect on MSCs. Additionally, by using Y-27632, we demonstrated that Rho kinase (ROCK) inhibition potentiated CoCl(2)-induced MSC differentiation in particular into dopaminergic neuron-like cells as attested by its effect on tyrosine hydroxylase expression. Altogether, these results support the ability of MSCs to differentiate into neuron-like cells in response to CoCl(2), an effect that might act, in part, through HIF-1 activation and cell-cycle arrest, and which is potentiated by inhibition of ROCK.

Abstract:
BACKGROUND: Prenatal stress constitutes a developmental risk factor for later psychopathology. The behavioral disorders are sustained by neurobiological alterations including long-term reduction of hippocampal neurogenesis; its deregulation has been involved in cognitive impairments, mood disorders and addiction. A major goal is to define periods in development and strategies for intervening to prevent the effects of early stressful events. We investigated the ability of a postnatal infantile stimulation to prevent prenatal stress-induced alteration in hippocampal neurogenesis. METHODS: The influence of postnatal handling on prenatal stress-induced changes in hippocampal neurogenesis was examined in 4 and 26 month-old male rats. Three distinct phases of the neurogenesis were studied: proliferation, survival and neuronal differentiation. RESULTS: Prenatal stress reduced hippocampal cell proliferation all throughout life. Furthermore, the survival rate of newborn cells, the number of immature neurons and the number of differentiated new neurons were reduced in young and old prenatally-stressed rats. All those deleterious effects were counteracted by neonatal handling. CONCLUSIONS: These data show that finer aspects of brain shaping can be rewired by environmental influences occurring at sensitive phase of development. They also suggest that infantile stimulation may reverse the appearance of behavioral disorders induced by early life stress.

Abstract:
Ageing is accompanied by an alteration of spatial memory, a decline in hippocampal neurogenesis and a dysregulation of the hypothalamic-pituitary axis (HPA) leading to elevated levels of circulating corticosterone. However, the role of the HPA axis in age-related decline in cognitive functions and in neurogenesis decline remains unclear. We found that suppression of glucocorticoids secretion from midlife to the rest of the animals' life increases neurogenesis in old animals and prevents the emergence of age-related memory disorders. Reciprocally, aged rats with a chronic upregulation of the HPA axis exhibit not only spatial memory impairments but also very low levels of hippocampal cell proliferation and survival. Altogether, these results indicate that the extent of lifetime exposure to glucocorticoids determines the extent of age-related decline in hippocampal neurogenesis and consequently age-related cognitive dysfunctions.

Abstract:
The aim of the present study is to better understand oxygen-sensitive adaptative pathways underlying the hypoxic preconditioning-induced protection of the brain against ischemia. Using oligonucleotide microarrays, we examined the brain genomic response of adult mice following hypoxia preconditioning (8% O2 for 1 or 6 h of hypoxia with reoxygenation 12, 18, 24 h or 72 h) and ischemia (6 h), preceeded (tolerant state) or not, by preconditioning. Real-time PCR was used to validate the results. Most gene expression increases occurred during hypoxia, including those of HIF-1-dependent genes (RTP801, AM, VEGF, p21, GLUT-1), early response genes (IER3) and transcriptional factors (ATF3, C/EBPdelta). A second wave of changes occurred 24 h after reoxygenation (S100A5, TH, Calretinin, PBX3). A third one occurred during ischemia itself, revealing that hypoxic preconditioning modifies the brain genomic response to ischemia. In addition, we show that some identical genes are overexpressed by hypoxia in both neonatal and adult brains (VEGF, EPO, GLUT-1, AM, MTs, C/EBPdelta).

Abstract:
Hypoxia and ischemia in the brain often result in brain dysfunctions and neuronal death during both the neonatal and adult periods. Though the mechanisms contributing to brain injury secondary to hypoxia-ischemia are more clearly defined, there are still no pharmacological treatments available to reduce cell death in the ischemic brain. This review highlights the beneficial effects of hypoxia-inducible factors, such as the transcriptional factor hypoxia-inducible factor-1 and its target genes, as both cytoprotective and regenerative factors, and focuses in particular on one of the most well-known: erythropoietin. Altogether, the data presented in this review suggest that further insights into the role of hypoxia-inducible factors would help develop promising strategies to improve the outcome of hypoxia/ischemia-related brain pathologies.

Abstract:
During adulthood, new neurons are continuously added to the mammalian dentate gyrus (DG). An increasing number of studies have correlated changes in rates of dentate neurogenesis with memory abilities. One study based on subchronic treatment with the toxin methylazoxymethanol acetate (MAM) has provided causal evidence that neurogenesis is involved in hippocampal-dependent trace conditioning. In contrast, spatial learning is not impaired following MAM treatment. We hypothesized that this was due to the small residual number of new cells produced following MAM treatment. In the present experiment, we attempted to achieve a higher level of reduction of adult-generated cells following MAM treatment in young and aged rats. We found only a partial reduction of adult-generated cells in the DG. More importantly, independently of the age of the animals, MAM treatment at a dose necessary to reduce neurogenesis altered the overall health of the animals. In conclusion, the behavioural results obtained following subchronic treatment with high doses of MAM in adulthood must be interpreted with extreme caution.

Abstract:
The discovery that the adult mammalian brain creates new neurons from pools of stemlike cells was a breakthrough in neuroscience. Interestingly, this particular new form of structural brain plasticity seems specific to discrete brain regions, and most investigations concern the subventricular zone (SVZ) and the dentate gyrus (DG) of the hippocampal formation (HF). Overall, two main lines of research have emerged over the last two decades: the first aims to understand the fundamental biological properties of neural stemlike cells (and their progeny) and the integration of the newly born neurons into preexisting networks, while the second focuses on understanding its relevance in brain functioning, which has been more extensively approached in the DG. Here, we propose an overview of the current knowledge on adult neurogenesis and its functional relevance for the adult brain. We first present an analysis of the methodological issues that have hampered progress in this field and describe the main neurogenic sites with their specificities. We will see that despite considerable progress, the levels of anatomic and functional integration of the newly born neurons within the host circuitry have yet to be elucidated. Then the intracellular mechanisms controlling neuronal fate are presented briefly, along with the extrinsic factors that regulate adult neurogenesis. We will see that a growing list of epigenetic factors that display a specificity of action depending on the neurogenic site under consideration has been identified. Finally, we review the progress accomplished in implicating neurogenesis in hippocampal functioning under physiological conditions and in the development of hippocampal-related pathologies such as epilepsy, mood disorders, and addiction. This constitutes a necessary step in promoting the development of therapeutic strategies.

Abstract:
PURPOSE: Expression of erythropoietin (Epo) and its receptor (Epo-R) has been shown in various normal and neoplastic nonhematopoietic tissues. This study, in non-small cell lung carcinoma, was designed to investigate the previously unreported expression of Epo and Epo-R as well as hypoxia-inducible factor-1alpha (HIF-1alpha), which is known to control Epo expression. EXPERIMENTAL DESIGN: Samples from lung squamous cell carcinomas (n = 17) and adenocarcinomas (n = 12) were obtained from patients undergoing curative surgery. mRNA transcripts of Epo, Epo-R, soluble Epo-R (sEpo-R), HIF-1alpha, and factor inhibiting HIF-1 (FIH-1) were evaluated by reverse transcription-PCR, whereas localization of Epo, Epo-R, and HIF-1alpha was assessed by immunohistochemistry. RESULTS: Epo, Epo-R, sEpo-R, HIF-1alpha, and FIH-1 transcripts were detected by reverse transcription-PCR in all samples tested, but with heterogeneous levels of expression for Epo, Epo-R, and sEpo-R. Coordinated levels of mRNA were observed for HIF-1alpha and FIH-1.Epo was detected in carcinomatous cells by immunohistochemistry in 50% of samples and Epo-R was detected in 96% of samples. Co-expression of Epo and Epo-R was observed on contiguous sections from 50% of tumors. HIF-1alpha was immunolocalized in 80% of non-small cell lung carcinomas. CONCLUSION: Epo-R was expressed in almost all samples and Epo was expressed in one half of samples on immunohistochemistry and in 100% of samples by mRNA detection, suggesting a potential paracrine and/or autocrine role of endogenous Epo in non-small cell lung carcinoma. The detection of stabilized HIF-1alpha suggests a possible role in Epo expression. Moreover, in the light of these results, the potential interactions between therapeutic recombinant Epo and the putative neoplastic Epo/Epo-R signaling pathways must be considered.

Abstract:
Age-dependent cognitive impairments have been correlated with functional and structural modifications in the hippocampal formation. In particular, the brain endogenous steroid pregnenolone-sulfate (Preg-S) is a cognitive enhancer whose hippocampal levels have been linked physiologically to cognitive performance in senescent animals. However, the mechanism of its actions remains unknown. Because neurogenesis is sensitive to hormonal influences, we examined the effect of Preg-S on neurogenesis, a novel form of plasticity, in young and old rats. We demonstrate that in vivo infusion of Preg-S stimulates neurogenesis and the expression of the polysialylated forms of NCAM, PSA-NCAM, in the dentate gyrus of 3- and 20-month-old rats. These influences on hippocampal plasticity are mediated by the modulation of the gamma-aminobutyric acid receptor complex A (GABA(A)) receptors present on hippocampal neuroblasts. In vitro, Preg-S stimulates the division of adult-derived spheres suggesting a direct influence on progenitors. These data provide evidence that neurosteroids represent one of the local secreted signals controlling hippocampal neurogenesis. Thus, therapies which stimulate neurosteroidogenesis could preserve hippocampal plasticity and prevent the appearance of age-related cognitive disturbances.

Abstract:
STUDY OBJECTIVES: Gamma-hydroxybutyrate (GHB) is an endogenous neuromodulator that appears to have wide-ranging effects on vigilance and behavior. In the present study, we examined the effects of GHB on sleep-wake behavior and EEG in mice. In addition, we measured effects of GHB on body temperature and arousal or stress hormones. DESIGN: Adult male BALB/c mice were implanted with electroencephalographic and electromyographic electrodes to record vigilance states and an intraperitoneal transmitter to record body temperature. After recovery from surgery and habituation to the recording procedure, the mice were intraperitoneally injected with saline or GHB (50, 150 or 250 mg/kg) half an hour after light onset. Blood samples to measure effects of GHB on corticosterone and prolactin levels were collected in a separate group of mice. SETTING: N/A PATIENTS: N/A INTERVENTIONS: N/A RESULTS: At the lowest dose, GHB had no conspicuous effects on behavioral vigilance and electroencephalogram, nor on body temperature and endocrine measures. At the 2 higher doses, GHB induced a short period of electroencephalographic hypersynchrony in parallel to complete behavioral inactivity, an unnatural flat body posture, and nonresponsiveness to stimulation. After the highest dose of GHB, this state of reduced vigilance was associated with a decrease in body temperature, while prolactin and corticosterone levels were strongly increased. CONCLUSIONS: The results do not indicate a clear sleep-promoting effect of GHB in mice, but, at higher doses, it caused electroencephalographic hypersynchronization together with a coma-like state.

Abstract:
Life events occurring during the perinatal period have strong long-term effects. In rats, prenatal stress, postnatal maternal separations, or adoptions at different periods are known to affect behavior and reactivity to stress in offspring. To determine the role of maternal factors on differential outcome adoptions, the authors investigated interactions between pups and the adopting mothers by assessing both pups' ultrasound emissions and maternal behavior. Early and late adoptions increased mother care at the moment of adoption and during mother-infant reunion after a separation procedure. However, although early adoption induced a decrease in pups' ultrasound emissions in response to a stressful separation, later adoptions enhanced it. Results suggest a sensitive period during which fostering may change pups' and dams' behavior.

Abstract:
Environmental challenges profoundly modify phenotypes and disrupt inherent developmental programs both at functional and structural levels. As an example, we have studied the impact of these environmental influences on adult neurogenesis in the dentate gyrus. Neurogenesis results from an inherent program, participates to hippocampal network organization and, as a consequence, to the various functional abilities depending on this region, including memories. In preclinical studies of aging we have shown that phenotypes vulnerable to the development of spatial memory disorders are characterized by lower hippocampal neurogenesis. We have hypothesized that these interindividual variations in functional expression of neurogenesis in senescent subjects could be predicted early in life. Indeed, a behavioral response (novelty-induced locomotor reactivity) and a biological trait (hypothalamo-pituitary-adrenal axis activity), which are predictive of cognitive impairments later in life, are related to neurogenesis in young adult rats. This suggests that subjects starting off with an impaired neurogenesis, here rats that are high reactive to stress, are predisposed for the development of age-related cognitive disorders. We have further shown that these inter-individual differences result from early deleterious life events. Indeed, prenatal stress orients neurogenesis in pathological ways for the entire life, and precipitates age-related cognitive impairments. Altogether these data suggest first that hippocampal neurogenesis plays a pivotal role in environmentally-induced vulnerability to the development of pathological aging, and second that environmental challenges and life events orient structural developments, leading to different phenotypes.