Adult-Born Neurons in the Hippocampus Are Essential for Social Memory Maintenance

TAAR1 in dentate gyrus is involved in chronic stress-induced impairments in hippocampal plasticity and cognitive function

Multiple lines of evidence suggest that the trace amine-associated receptor 1 (TAAR1) holds promise as a potential target for stress-related disorders, such as treating major depressive disorder (MDD). The role of TAAR1 in the regulation of adult neurogenesis is recently supported by transcriptomic data. However, it remains unknown whether TAAR1 in dentate gyrus (DG) mediate chronic stress-induced negative effects on hippocampal plasticity and related behavior in mice. The present study consisted of a series of experiments using RNAscope, genetic approaches, behavioral tests, immunohistochemical staining, Golgi-Cox technique to unravel the effects of TAAR1 on alterations of dentate neuronal plasticity and cognitive function in the chronic social defeat stress model. The mice subjected to chronic defeat stress exhibited a noteworthy decrease in the mRNA level of TAAR1 in DG. Additionally, they exhibited compromised social memory and spatial object recognition memory, as well as impaired proliferation and maturation of adult-born dentate granule cells. Moreover, the selective knockout TAAR1 in DG mostly mimicked the cognitive function deficits and neurogenesis impairment induced by chronic stress. Importantly, the administration of the selective TAAR1 partial agonist RO5263397 during stress exposure attenuated the adverse effects of chronic stress on cognitive function, adult neurogenesis, dendritic arborization, and the synapse number of dentate neurons in DG. In summary, our findings suggest that TAAR1 plays a crucial role in mediating the detrimental effects of chronic stress on hippocampal plasticity and cognition. TAAR1 agonists exhibit therapeutic potential for individuals suffering from cognitive impairments associated with MDD.

The Mef2c Gene Dose-Dependently Controls Hippocampal Neurogenesis and the Expression of Autism-Like Behaviors

Mutations in the activity-dependent transcription factor MEF2C have been associated with several neuropsychiatric disorders. Among these, autism spectrum disorder (ASD)-related behavioral deficits are manifested. Multiple animal models that harbor mutations in Mef2c have provided compelling evidence that Mef2c is indeed an ASD gene. However, studies in mice with germline or global brain knock-out of Mef2c are limited in their ability to identify the precise neural substrates and cell types that are required for the expression of Mef2c-mediated ASD behaviors. Given the role of hippocampal neurogenesis in cognitive and social behaviors, in this study we aimed to investigate the role of Mef2c in the structure and function of newly generated dentate granule cells (DGCs) in the postnatal hippocampus and to determine whether disrupted Mef2c function is responsible for manifesting ASD behaviors. Overexpression of Mef2c (Mef2cOE ) arrested the transition of neurogenesis at progenitor stages, as indicated by sustained expression of Sox2+ in Mef2cOE DGCs. Conditional knock-out of Mef2c (Mef2ccko ) allowed neuronal commitment of Mef2ccko cells; however, Mef2ccko impaired not only dendritic arborization and spine formation but also synaptic transmission onto Mef2ccko DGCs. Moreover, the abnormal structure and function of Mef2ccko DGCs led to deficits in social interaction and social novelty recognition, which are key characteristics of ASD behaviors. Thus, our study revealed a dose-dependent requirement of Mef2c in the control of distinct steps of neurogenesis, as well as a critical cell-autonomous function of Mef2c in newborn DGCs in the expression of proper social behavior in both sexes.

Synchronized LFP rhythmicity in the social brain reflects the context of social encounters

Mammalian social behavior is highly context-sensitive. Yet, little is known about the mechanisms that modulate social behavior according to its context. Recent studies have revealed a network of mostly limbic brain regions which regulates social behavior. We hypothesize that coherent theta and gamma rhythms reflect the organization of this network into functional sub-networks in a context-dependent manner. To test this concept, we simultaneously record local field potential (LFP) from multiple social brain regions in adult male mice performing three social discrimination tasks. While LFP rhythmicity across all tasks is dominated by a global internal state, the pattern of theta coherence between the various regions reflect the behavioral task more than other variables. Moreover, Granger causality analysis implicate the ventral dentate gyrus as a main player in coordinating the context-specific rhythmic activity. Thus, our results suggest that the pattern of coordinated rhythmic activity within the network reflects the subject's social context.

Social recognition memory differences between mouse strains: On the effects of social isolation, adult neurogenesis, and environmental enrichment

Remembering conspecifics is paramount for the establishment and maintenance of groups. Here we asked whether the variability in social behavior caused by different breeding strategies affects social recognition memory (SRM). We tested the hypothesis that the inbred Swiss and the outbred C57BL/6 mice behave differently on SRM. Social memory in C57BL/6 mice endured at least 14 days, while in Swiss mice lasted 24 h but not ten days. We showed previously that an enriched environment enhanced the persistence of SRM in Swiss mice. Here we reproduced this result and added that it also increases the survival of adult-born neurons in the hippocampus. Next, we tested whether prolonged SRM observed in C57BL/6 mice could be changed by diminishing the trial duration or using an interference stimulus after learning. Neither short acquisition time nor interference during consolidation affected it. However, social isolation impaired SRM in C57BL/6 mice, similar to what was previously observed in Swiss mice. Our results demonstrate that SRM expression can vary according to the mouse strain, which shows the importance of considering this variable when choosing the most suitable model to answer specific questions about this memory system. We also demonstrate the suitability of both C57BL/6 and Swiss strains for exploring the impact of environmental conditions and adult neurogenesis on social memory.

Chronic intranasal oxytocin increases acoustic eavesdropping and adult neurogenesis

Social information gathering is a complex process influenced by neuroendocrine-modulated neural plasticity. Oxytocin (OXT) is a key regulator of social decision-making processes such as information gathering, as it contextually modulates social salience and can induce long-term structural plasticity, including neurogenesis. Understanding the link between OXT-induced plasticity and communicative awareness is crucial, particularly because OXT is being considered for treatment of social pathologies. We investigated the role of chronic OXT-dependent plasticity in attention to novel social information by manipulating the duration of time following cessation of intranasal treatment to allow for the functional integration of adult-born neurons resulting from OXT treatment. Following a 3-week delay, chronic intranasal OXT (IN-OXT) increased approach behavior of both female and male mice towards aggressive vocal playbacks of two unseen novel conspecifics, while no effect was observed after a 3-day delay. Immature neurons increased in the ventral hippocampus of females and males treated with chronic IN-OXT after the 3-week delay, indicating a potential association between ventral hippocampal neurogenesis and approach/acoustic eavesdropping. The less the mouse approached, the higher the level of neurogenesis. Contrary to expectations, the correlation between ventral hippocampal neurogenesis and approach behavior was not affected by IN-OXT, suggesting that other plasticity mechanisms underlie the long-term effects of chronic OXT on social approach. Furthermore, we found a negative correlation between ventral hippocampal neurogenesis and freezing behavior. Overall, our results demonstrate that chronic IN-OXT-induced long-term plasticity can influence approach to vocal information and we further reinforced the link between neurogenesis and anxiety.

An inhibitory circuit-based enhancer of DYRK1A function reverses Dyrk1a-associated impairment in social recognition

Heterozygous mutations in the dual-specificity tyrosine phosphorylation-regulated kinase 1a (Dyrk1a) gene define a syndromic form of autism spectrum disorder. The synaptic and circuit mechanisms mediating DYRK1A functions in social cognition are unclear. Here, we identify a social experience-sensitive mechanism in hippocampal mossy fiber-parvalbumin interneuron (PV IN) synapses by which DYRK1A recruits feedforward inhibition of CA3 and CA2 to promote social recognition. We employ genetic epistasis logic to identify a cytoskeletal protein, ABLIM3, as a synaptic substrate of DYRK1A. We demonstrate that Ablim3 downregulation in dentate granule cells of adult heterozygous Dyrk1a mice is sufficient to restore PV IN-mediated inhibition of CA3 and CA2 and social recognition. Acute chemogenetic activation of PV INs in CA3/CA2 of adult heterozygous Dyrk1a mice also rescued social recognition. Together, these findings illustrate how targeting DYRK1A synaptic and circuit substrates as "enhancers of DYRK1A function" harbors the potential to reverse Dyrk1a haploinsufficiency-associated circuit and cognition impairments.

Cell proliferation and cell death levels in the dentate gyrus correlate with home range size among adult male meadow voles

Neurogenesis occurs throughout adulthood within the dentate gyrus, and evidence indicates that these new neurons play a critical role in both spatial and social memory. However, a vast majority of past research on adult neurogenesis has involved experiments with captive mice and rats, making the generalizability of results to natural settings questionable. We assessed the connection between adult neurogenesis and memory by measuring the home range size of wild-caught, free-ranging meadow voles (Microtus pennsylvanicus). Adult male voles (n = 18) were captured, fitted with radio collars, and released back into their natural habitat, where each vole's home range was assessed using 40 radio-telemetry fixes over the course of 5 evenings. Voles were then recaptured, and brain tissue was collected. Cellular markers of cell proliferation (pHisH3, Ki67), neurogenesis (DCX), and pyknosis were labeled on histological sections and then quantified using either fluorescent or light microscopy. Voles with larger home ranges had significantly higher pHisH3+ cell densities within the granule cell layer and subgranular zone (GCL+SGZ) of the dentate gyrus and higher Ki67+ cell densities in the dorsal GCL+SGZ. Voles with larger ranges also had significantly higher pyknotic cell densities in the entire GCL+SGZ and in the dorsal GCL+SGZ. These results support the hypothesis that cell proliferation and cell death within the hippocampus are involved with spatial memory formation. However, a marker of neurogenesis (DCX+) was not correlated with range size, suggesting that there may be selective cellular turnover in the dentate gyrus when a vole is ranging through its environment.

Optimizing memory performance and emotional states: multi-level enhancement of adult hippocampal neurogenesis

Adult hippocampal neurogenesis (AHN) plays a key role in modulating memory and emotion processing. A fundamental question remains on how to effectively modulate AHN to improve hippocampal function. Here, we review recent work on how distinct aspects of hippocampal neurogenesis, including the number, maturation state, and activity of adult-born neurons (ABNs), contribute to overall hippocampal function. We propose multi-level enhancement of hippocampal neurogenesis with the combination of increased number, elevated activity, and enhanced maturation of ABNs as a potential strategy to optimize overall hippocampal performance. In addition, integration of ABNs induces significant remodeling of the local hippocampal circuits, which may in turn modulates brain-wide network dynamics. We discuss recent progress on how integration of ABNs contributes to local hippocampal circuit and brain-wide network dynamics during behavior.

Postnatal development of hippocampal CA2 structure and function during the emergence of social recognition of peers

It is now well-established that the hippocampal CA2 region plays an important role in social recognition memory in adult mice. The CA2 is also important for the earliest social memories, including those that mice have for their mothers and littermates, which manifest themselves as a social preference for familiarity over novelty. The role of the CA2 in the development of social memory for recently encountered same-age conspecifics, that is, peers, has not been previously reported. Here, we used a direct social interaction test to characterize the emergence of novelty preference for peers during development and found that at the end of the second postnatal week, pups begin to significantly prefer novel over familiar peers. Using chemogenetic inhibition at this time, we showed that CA2 activity is necessary for the emergence of novelty preference and for the ability to distinguish never encountered from recently encountered peers. In adulthood, the CA2 region is known to integrate a large number of inputs from various sources, many of which participate in social recognition memory, but previous studies have not determined whether these afferents are present at adult levels by the end of the second postnatal week. To explore the development of CA2 inputs, we used immunolabeling and retrograde adenovirus circuit tracing and found that, by the end of the second postnatal week, the CA2 is innervated by many regions, including the dentate gyrus, supramammillary nucleus of the hypothalamus, the lateral entorhinal cortex, and the median raphe nucleus. Using retroviral labeling of postnatally generated granule cells in the dentate gyrus, we found that mossy fiber projections to the CA2 mature faster during development than those generated in adulthood. Together, our findings indicate that the CA2 is partially mature in afferent connectivity by the end of the second postnatal week, connections that likely facilitate the emergence of social recognition memory and preference for novel peers.

An inhibitory circuit-based enhancer of Dyrk1a function reverses Dyrk1a -associated impairment in social recognition

Heterozygous mutations in the Dual specificity tyrosine-phosphorylation-regulated kinase 1a Dyrk1a gene define a syndromic form of Autism Spectrum Disorder. The synaptic and circuit mechanisms mediating Dyrk1a functions in social cognition are unclear. Here, we identify a social experience-sensitive mechanism in hippocampal mossy fiber-parvalbumin interneuron (PV IN) synapses by which Dyrk1a recruits feedforward inhibition of CA3 and CA2 to promote social recognition. We employ genetic epistasis logic to identify a cytoskeletal protein, Ablim3, as a synaptic substrate of Dyrk1a. We demonstrate that Ablim3 downregulation in dentate granule cells of adult hemizygous Dyrk1a mice is sufficient to restore PV IN mediated inhibition of CA3 and CA2 and social recognition. Acute chemogenetic activation of PV INs in CA3/CA2 of adult hemizygous Dyrk1a mice also rescued social recognition. Together, these findings illustrate how targeting Dyrk1a synaptic and circuit substrates as "enhancers of Dyrk1a function" harbors potential to reverse Dyrk1a haploinsufficiency-associated circuit and cognition impairments.

Highlights

Dyrk1a in mossy fibers recruits PV IN mediated feed-forward inhibition of CA3 and CA2Dyrk1a-Ablim3 signaling in mossy fiber-PV IN synapses promotes inhibition of CA3 and CA2 Downregulating Ablim3 restores PV IN excitability, CA3/CA2 inhibition and social recognition in Dyrk1a+/- mice Chemogenetic activation of PV INs in CA3/CA2 rescues social recognition in Dyrk1a+/- mice.

Neuronal ensemble dynamics in social memory

A large body of evidence suggests that cognitive functions rely on the coordination of ensembles of neurons across brain circuits. One example is social memory, the ability to recognize and remember other conspecifics. A broad range of brain regions have been implicated in social behaviors and memory processes. At the single-cell level, neurons from different brain areas have responded to specific social features. The coordination of these ensembles both within a region and across structures is required to support social memory and decision-making. The synchronous activation of these neuronal ensembles could allow for the integration of different aspects of a social episode into a unified representation of experience. In this review, recent results on the circuit basis and physiological mechanisms of social memory are discussed, from a systems neuroscience perspective. An integrative framework of the neuronal ensemble dynamics supporting this fundamental cognitive ability is proposed.

In the pursuit of new social neurons. Neurogenesis and social behavior in mice: A systematic review

Social behaviors have become more relevant to our understanding of the human nervous system because relationships with our peers may require and modulate adult neurogenesis. Here, we review the pieces of evidence we have to date for the divergence of social behaviors in mice by modulation of adult neurogenesis or if social behaviors and the social environment can drive a change in neurogenic processes. Social recognition and memory are deeply affected by antimitotic drugs and irradiation, while NSC transgenic mice may run with lower levels of social discrimination. Interestingly, social living conditions can create a big impact on neurogenesis. Social isolation and social defeat reduce the number of new neurons, while social dominance and enrichment of the social environment increase their number. These new “social neurons” trigger functional modifications with amazing transgenerational effects. All of these suggest that we are facing two bidirectional intertwined variables, and the great challenge now is to understand the cellular and genetic mechanisms that allow this relationship to be used therapeutically.

Regulation of adult-born and mature neurons in stress response and antidepressant action in the dentate gyrus of the hippocampus

The dentate gyrus (DG) of the hippocampus has been implicated in the regulation of stress responses, and in the pathophysiology and treatment of depression. This review discusses the cellular changes caused by chronic stress and the cellular role of the DG in stress-induced behavioral changes and its antidepressant-like effects. Regarding adult-born neurogenic processes in the DG, chronic stress, such as repeated social defeat, suppresses cell proliferation during and immediately after stress; however, this effect is transient. The subsequent differentiation and survival processes are differentially regulated depending on the timing and sensitivity of stress. The activation of young adult-born neurons during stress contributes to stress resilience, while the transient increase in the survival of adult-born neurons after the cessation of stress seems to promote stress susceptibility. In mature granule neurons, the predominant cells in the DG, synaptic plasticity is suppressed by chronic stress. However, a group of mature granule neurons is activated by chronic stress. Chronic antidepressant treatment can transform mature granule neurons to a phenotype resembling that of immature neurons, characterized as "dematuration". Adult-born neurons suppress the activation of mature granule neurons during stress, indicating that local neural interactions within the DG are important for the stress response. Elucidating the stress-associated context- and timing-dependent cellular changes and functions in the DG will provide insights into stress-related psychiatric diseases.

Atypical perineuronal nets in the CA2 region interfere with social memory in a mouse model of social dysfunction

Social memory dysfunction is an especially devastating symptom of many neuropsychiatric disorders, which makes understanding the cellular and molecular processes that contribute to such abnormalities important. Evidence suggests that the hippocampus, particularly the CA2 region, plays an important role in social memory. We sought to identify potential mechanisms of social memory dysfunction in the hippocampus by investigating features of neurons, glia, and the extracellular matrix (ECM) of BTBR mice, an inbred mouse strain with deficient social memory. The CA2 is known to receive inputs from dentate gyrus adult-born granule cells (abGCs), neurons known to participate in social memory, so we examined this cell population and found fewer abGCs, as well as fewer axons from abGCs in the CA2 of BTBR mice compared to controls. We also found that BTBR mice had fewer pyramidal cell dendritic spines, in addition to fewer microglia and astrocytes, in the CA2 compared to controls. Along with diminished neuronal and glial elements, we found atypical perineuronal nets (PNNs), specialized ECM structures that regulate plasticity, in the CA2 of BTBR mice. By diminishing PNNs in the CA2 of BTBR mice to control levels, we observed a partial restoration of social memory. Our findings suggest that the CA2 region of BTBR mice exhibits multiple cellular and extracellular abnormalities and identify atypical PNNs as one mechanism producing social memory dysfunction, although the contribution of reduced abGC afferents, pyramidal cell dendritic spine, and glial cell numbers remains unexplored.

Inhibition of Hippocampal Neurogenesis Starting in Adolescence Increases Anxiodepressive Behaviors Amid Stress

Stressors during the adolescent period can affect development of the brain and have long-lasting impacts on behavior. Specifically, adolescent stress impairs hippocampal neurogenesis and can increase risk for anxiety, depression, and a dysregulated stress response in adulthood. In order to model the functional effects of reduced hippocampal neurogenesis during adolescence, a transgenic neurogenesis ablation rat model was used to suppress neurogenesis during the adolescent period and test anxiodepressive behaviors and stress physiology during adulthood. Wildtype and transgenic (TK) rats were given valganciclovir during the first two weeks of adolescence (4-6 weeks old) to knock down neurogenesis in TK rats. Starting in young adulthood (13 weeks old), blood was sampled for corticosterone at several time points following acute restraint stress to measure negative feedback of the stress response, and rats were tested on a battery of anxiodepressive tests at baseline and following acute restraint stress. Although TK rats had large reductions in both cell proliferation during adolescence, as measured by bromodeoxyuridine (BrdU), and ongoing neurogenesis in adulthood (by doublecortin), resulting in decreased volume of the dentate gyrus, negative feedback of the stress response following acute restraint was similar across all rats. Despite similar stress responses, TK rats showed higher anxiety-like behavior at baseline. In addition, only TK rats had increased depressive-like behavior when tested after acute stress. Together, these results suggest that long-term neurogenesis ablation starting in adolescence produces hippocampal atrophy and increases behavioral caution and despair amid stressful environments.

What Is Adult Hippocampal Neurogenesis Good for?

Adult hippocampal neurogenesis is a unique and exceptional process in the mammalian brain that in a lifelong and activity-dependent way generates new excitatory principal neurons. A comprehensive view on their function in greater contexts has now emerged, revealing to which extent the hippocampus (and hence brain and mind) depend on these neurons. Due to a postmitotic period of heightened synaptic plasticity they bias incoming excitation to the dentate gyrus to non-overlapping subnetworks, resulting in pattern separation and the avoidance of catastrophic interference. Temporally, this promotes the flexible integration of novel information into familiar contexts and contributes to episodic memory, which in humans would be critical for autobiographic memory. Together these local effects represent a unique strategy to solve the plasticity-stability dilemma that all learning neuronal networks are facing. Neurogenesis-dependent plasticity also improves memory consolidation. This relates to the surprising involvement of adult neurogenesis in forgetting, which is also hypothesized to be critically relevant for negative plasticity, for example in post-traumatic stress disorder. In addition, adult-born neurons also directly mediate stress-resilience and take part in affective behaviors. Finally, the activity- and experience-dependent plasticity that is contributed by adult neurogenesis is associated with an individualization of the hippocampal circuitry. While a solid and largely consensual understanding of how new neurons contribute to hippocampal function has been reached, an overarching unifying theory that embeds neurogenesis-dependent functionality and effects on connectomics is still missing. More sophisticated multi-electrode electrophysiology, advanced ethologically relevant behavioral tests, and next-generation computational modeling will let us take the next steps.

Intracerebroventricular injection of sclerostin reduced social hierarchy and impaired neuronal dendritic complexity in mice

An increasing number of studies have demonstrated extensive functional links between bone and the brain. As a novel endocrine organ, bone has received increasing attention for its upregulatory functions in the brain. Sclerostin, a novel bone-derived endocrine molecule, secreted by osteocytes, can inhibit the bone morphogenetic protein (BMP) and wingless/integrated (Wnt) signaling pathways to regulate bone formation, but its effects on the central nervous system and neurosocial behaviors are unknown. This study investigated the effects of intracerebroventricular sclerostin injection on social-emotional behaviors in adult mice. The results showed that acute elevation of sclerostin levels in the brain could induce anxiety-like behaviors and reduce the social hierarchy of mice while reducing the dendritic complexity of pyramidal neurons in the mouse hippocampus. These data suggested that sclerostin may regulate social-emotional behaviors, providing new evidence for the existence of a bone-brain axis, new insights into the regulation of social behaviors by bone-derived endocrine molecules, and a new direction for the study of individual emotional behavior regulation.

Inhibition of adult neurogenesis reduces avoidance behavior in male, but not female, mice subjected to early life adversity

Early life adversity (ELA) increases the risk of developing neuropsychiatric illnesses such as anxiety disorders. However, the mechanisms connecting these negative early life experiences to illness later in life remain unclear. In rodents, plasticity mechanisms, specifically adult neurogenesis in the ventral hippocampus, have been shown to be altered by ELA and important for buffering against detrimental stress-induced outcomes. The current study sought to explore whether adult neurogenesis contributes to ELA-induced changes in avoidance behavior. Using the GFAP-TK transgenic model, which allows for the inhibition of adult neurogenesis, and CD1 littermate controls, we subjected mice to an ELA paradigm of maternal separation and early weaning (MSEW) or control rearing. We found that mice with intact adult neurogenesis showed no behavioral changes in response to MSEW. After reducing adult neurogenesis, however, male mice previously subjected to MSEW had an unexpected decrease in avoidance behavior. This finding was not observed in female mice, suggesting that a sex difference exists in the role of adult-born neurons in buffering against ELA-induced changes in behavior. Taken together with the existing literature on ELA and avoidance behavior, this work suggests that strain differences exist in susceptibility to ELA and that adult-born neurons may play a role in regulating adaptive behavior.

A Baldwin interpretation of adult hippocampal neurogenesis: from functional relevance to physiopathology

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.

Adult Neurogenesis: A Story Ranging from Controversial New Neurogenic Areas and Human Adult Neurogenesis to Molecular Regulation

The generation of new neurons in the adult brain is a currently accepted phenomenon. Over the past few decades, the subventricular zone and the hippocampal dentate gyrus have been described as the two main neurogenic niches. Neurogenic niches generate new neurons through an asymmetric division process involving several developmental steps. This process occurs throughout life in several species, including humans. These new neurons possess unique properties that contribute to the local circuitry. Despite several efforts, no other neurogenic zones have been observed in many years; the lack of observation is probably due to technical issues. However, in recent years, more brain niches have been described, once again breaking the current paradigms. Currently, a debate in the scientific community about new neurogenic areas of the brain, namely, human adult neurogenesis, is ongoing. Thus, several open questions regarding new neurogenic niches, as well as this phenomenon in adult humans, their functional relevance, and their mechanisms, remain to be answered. In this review, we discuss the literature and provide a compressive overview of the known neurogenic zones, traditional zones, and newly described zones. Additionally, we will review the regulatory roles of some molecular mechanisms, such as miRNAs, neurotrophic factors, and neurotrophins. We also join the debate on human adult neurogenesis, and we will identify similarities and differences in the literature and summarize the knowledge regarding these interesting topics.

Generation of an Lpar1-EGFP Fusion Knock-in Transgenic Mouse Line

Lysophosphatidic acid (LPA) is a lysophospholipid that acts as an extracellular signal through the activation of cognate G protein-coupled receptors (GPCRs). There are six known LPA receptors (LPA_1-6). The first such receptor, LPA₁, was identified in the embryonic brain and has been studied extensively for gene expression throughout the body, including through studies of receptor-null mice. However, identifying receptor protein expression in situ and in vivo within living cells and tissues has been difficult because of biologically low receptor expression and variable antibody specificity. To visualize native LPA₁ receptor expression in situ, we generated a knock-in mouse produced by homologous recombination in murine embryonic stem (ES) cells to replace a wildtype Lpar1 allele with a mutant allele created by in-frame fusion of EGFP to the 4th exon of Lpar1 (Lpar1-EGFP knock-in allele). Homozygous knock-in mice appeared normal and the expected mendelian ratios of knock-in allele transmission were present in females and males. Histological assessments of the fetal and adult central nervous system (CNS) demonstrated expression patterns that were consistent with prior in situ hybridization studies. This new mouse line will be useful for studies of LPA₁ in the developing and adult CNS, as well as other tissues, and for receptor assessments in living tissues and disease models.

On the novel mechanisms for social memory and the emerging role of neurogenesis

Social memory (SM) is a key element in social cognition and it encompasses the neural representation of conspecifics, an essential information to guide behavior in a social context. Here we evaluate classical and cutting-edge studies on neurobiology of SM, using as a guiding principle behavioral tasks performed in adult rodents. Our review highlights the relevance of the hippocampus, especially the CA2 region, as a neural substrate for SM and suggest that neural ensembles in the olfactory bulb may also encode SM traces. Compared to other hippocampus-dependent memories, much remains to be done to describe the neurobiological foundations of SM. Nonetheless, we argue that special attention should be paid to neurogenesis. Finally, we pinpoint the remaining open question on whether the hippocampal adult neurogenesis acts through pattern separation to permit the discrimination of highly similar stimuli during behavior.

The temporal origin of dentate granule neurons dictates their role in spatial memory

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.