Adult-born hippocampal neurons bidirectionally modulate entorhinal inputs into the dentate gyrus

Morris Water Maze and Contextual Fear Conditioning Tasks to Evaluate Cognitive Functions Associated With Adult Hippocampal Neurogenesis

New neurons are continuously generated and functionally integrated into the dentate gyrus (DG) network during the adult lifespan of most mammals. The hippocampus is a crucial structure for spatial learning and memory, and the addition of new neurons into the DG circuitry of rodents seems to be a key element for these processes to occur. The Morris water maze (MWM) and contextual fear conditioning (CFC) are among the most commonly used hippocampus-dependent behavioral tasks to study episodic-like learning and memory in rodents. While the functional contribution of adult hippocampal neurogenesis (AHN) through these paradigms has been widely addressed, results have generated controversial findings. In this review, we analyze and discuss possible factors in the experimental methods that could explain the inconsistent results among AHN studies; moreover, we provide specific suggestions for the design of more sensitive protocols to assess AHN-mediated learning and memory functions.

Parallel processing of sensory cue and spatial information in the dentate gyrus

During exploration, animals form an internal map of an environment by combining information about landmarks and the animal's movement, a process that depends on the hippocampus. The dentate gyrus (DG) is the first stage of the hippocampal circuit where self-motion ("where") and sensory cue information ("what") are integrated, but it remains unknown how DG neurons encode this information during cognitive map formation. Using two-photon calcium imaging in mice running on a treadmill along with online cue manipulation, we identify robust sensory cue responses in DG granule cells. Cue cell responses are stable, stimulus-specific, and accompanied by inhibition of nearby neurons. This demonstrates the existence of "cue cells" in addition to better characterized "place cells" in the DG. We hypothesize that the DG supports parallel channels of spatial and non-spatial information that contribute distinctly to downstream computations and affect roles of the DG in spatial navigation and episodic memory.

Defective Bimetallic Selenides for Selective CO2 Electroreduction to CO

CO₂ electroreduction (CO₂ RR) to CO is promising for the carbon cycle but still remains challenging. Au is regarded as the most selective catalyst for CO₂ RR, but its high cost significantly hinders its industrial application. Herein, the bimetallic CuInSe₂ is found to exhibit an Au-like catalytic feature: i) the interaction of Cu and In orbitals induces a moderate adsorption strength of CO₂ RR intermediates and favors the reaction pathway; and ii) the hydrogen evolution is energetically unfavorable on CuInSe₂ , as a surface reconstruction along with high energy change will occur after hydrogen adsorption. Furthermore, the Se vacancy is found to induce an electron redistribution, slightly tune the band structure, and optimize the CO₂ RR route of bimetallic selenide. Consequently, the Se-defective CuInSe₂ (V-CuInSe₂ ) achieves a highly selective CO production ability that is comparable to noble metals in aqueous electrolyte, and the V-CuInSe₂ cathode shows a satisfactory performance in an aqueous Zn-CO₂ cell. This work demonstrates that designing cost-effective catalysts with noble-metal-like properties is an ideal strategy for developing efficient electrocatalysts. Moreover, the class of transition bimetallic selenides has shown promising prospects as active and cost-effective electrocatalysts owing to their unique structural, electronic, and catalytic properties.

Adult hippocampal neurogenesis shapes adaptation and improves stress response: a mechanistic and integrative perspective

Adult hippocampal neurogenesis (AHN) represents a remarkable form of neuroplasticity that has increasingly been linked to the stress response in recent years. However, the hippocampus does not itself support the expression of the different dimensions of the stress response. Moreover, the main hippocampal functions are essentially preserved under AHN depletion and adult-born immature neurons (abGNs) have no extrahippocampal projections, which questions the mechanisms by which abGNs influence functions supported by brain areas far from the hippocampus. Within this framework, we propose that through its computational influences AHN is pivotal in shaping adaption to environmental demands, underlying its role in stress response. The hippocampus with its high input convergence and output divergence represents a computational hub, ideally positioned in the brain (1) to detect cues and contexts linked to past, current and predicted stressful experiences, and (2) to supervise the expression of the stress response at the cognitive, affective, behavioral, and physiological levels. AHN appears to bias hippocampal computations toward enhanced conjunctive encoding and pattern separation, promoting contextual discrimination and cognitive flexibility, reducing proactive interference and generalization of stressful experiences to safe contexts. These effects result in gating downstream brain areas with more accurate and contextualized information, enabling the different dimensions of the stress response to be more appropriately set with specific contexts. Here, we first provide an integrative perspective of the functional involvement of AHN in the hippocampus and a phenomenological overview of the stress response. We then examine the mechanistic underpinning of the role of AHN in the stress response and describe its potential implications in the different dimensions accompanying this response.

Astrocyte control of the entorhinal cortex-dentate gyrus circuit: Relevance to cognitive processing and impairment in pathology

The entorhinal cortex-dentate gyrus circuit is centrally involved in memory processing conveying to the hippocampus spatial and nonspatial context information via, respectively, medial and lateral perforant path (MPP and LPP) excitatory projections onto dentate granule cells (GCs). Here, we review work of several years from our group showing that astrocytes sense local synaptic transmission and exert in turn a presynaptic control at PP-GC synapses. Modulation of neurotransmitter release probability by astrocytes sets basal synaptic strength and dynamic range for long-term potentiation of PP-GC synapses. Intriguingly, this astrocyte control is circuit-specific, being present only at MPP-GC (not LPP-GC) synapses, which selectively express atypical presynaptic N-methyl-D-aspartate receptors (NMDAR) suitable to activation by astrocyte-released glutamate. Moreover, the astrocytic control is peculiarly dependent on the cytokine TNFα, which at constitutive levels acts as a gating factor for the astrocyte signaling. During inflammation/infection processes, increased levels of TNFα lead to uncontrolled astrocyte glutamate release, altered PP-GC circuit processing and, ultimately, impaired contextual memory performance. The TNFα-dependent pathological switch of the synaptic control from astrocytes and its deleterious consequences are observed in animal models of HIV brain infection and multiple sclerosis, conditions both known to cause cognitive disturbances in up to 50% of patients. The review also discusses open issues related to the identified astrocytic pathway: its role in contextual memory processing, potential damaging role in Alzheimer's disease, the existence of vesicular glutamate release from DG astrocytes, and the possible synaptic-like connectivity between astrocytic output sites and PP receptive sites.

Cognitive control persistently enhances hippocampal information processing

Could learning that uses cognitive control to judiciously use relevant information while ignoring distractions generally improve brain function, beyond forming explicit memories? According to a neuroplasticity hypothesis for how some cognitive behavioural therapies are effective, cognitive control training (CCT) changes neural circuit information processing^1-3. Here we investigated whether CCT persistently alters hippocampal neural circuit function. We show that mice learned and remembered a conditioned place avoidance during CCT that required ignoring irrelevant locations of shock. CCT facilitated learning new tasks in novel environments for several weeks, relative to unconditioned controls and control mice that avoided the same place during reduced distraction. CCT rapidly changes entorhinal cortex-to-dentate gyrus synaptic circuit function, resulting in an excitatory-inhibitory subcircuit change that persists for months. CCT increases inhibition that attenuates the dentate response to medial entorhinal cortical input, and through disinhibition, potentiates the response to strong inputs, pointing to overall signal-to-noise enhancement. These neurobiological findings support the neuroplasticity hypothesis that, as well as storing item-event associations, CCT persistently optimizes neural circuit information processing.

Serum BDNF and suicidal ideation in drug-naïve and drug-treated MDD patients: a case–control study

Background

Disturbances in structural and synaptic plasticity have been linked to depression and suicidal ideation. One of the major neurotrophic factors, the brain-derived neurotrophic factor (BDNF), is involved in the maintenance and survival of neurons and synaptic plasticity. This case–control study assesses the serum BDNF and suicidal ideation among drug-naïve and drug-treated MDD patients attending university hospitals and comparing them to healthy control. A simple random sample of 57 MDD patients and 57 age- and sex-comparable controls were enrolled. The researchers conducted a semi-structured interview to collect the demographic characteristics and disease history. Structured Clinical Interview for DSM-5 (SCID-5), Hamilton Depression Rating Scale (HDRS), and Beck Scale for Suicidal Ideation (BSS) were applied to the participants. Blood samples were collected to measure plasma BDNF level.

Results

The MDD group had lower BDNF than the control group. Within the MDD group, drug-naïve patients had significantly lower BDNF than drug-treated patients. Female patients had lower BDNF than male patients. Positive family history of MDD was associated with low BDNF. Severe and moderate cases had lower BDNF than mild cases. High BSS (≥24) was associated with low BDNF. A statistically significant positive correlation was found between BDNF and age, disease duration, duration of the current episode, and the number of previous episodes.

On the other hand, a statistically significant negative correlation was found between BDNF and age of MDD onset, HDRS, and BSS. A regression model was highly statistically significant in the prediction of HDRS. BDNF and disease duration were negatively correlated with HDRS. On the other hand, depression treatment status was not significantly associated with the HDRS prediction model.

Conclusion

Our findings extend the neurotrophic concept of depression by identifying the decreased BDNF levels as a peripheral biomarker of MDD. Our assessment of depression and suicidal ideation (SI) and their relationship to decreased BDNF levels shed light on the etiopathology of MDD and its related suicidality. They should be more studied to understand better the mechanisms by which they develop. To further explore the effect of BDNF in suicide, larger study sizes and a range of psychiatric diagnoses associated with suicide attempts are required.

Hippocampal neurogenesis promotes preference for future rewards

Adult hippocampal neurogenesis has been implicated in a number of disorders where reward processing is disrupted but whether new neurons regulate specific aspects of reward-related decision making remains unclear. Given the role of the hippocampus in future-oriented cognition, here we tested whether adult neurogenesis regulates preference for future, advantageous rewards in a delay discounting paradigm for rats. Indeed, blocking neurogenesis caused a profound aversion for delayed rewards, and biased choice behavior toward immediately available, but smaller, rewards. Consistent with a role for the ventral hippocampus in impulsive decision making and future-thinking, neurogenesis-deficient animals displayed reduced activity in the ventral hippocampus. In intact animals, delay-based decision making restructured dendrites and spines in adult-born neurons and specifically activated adult-born neurons in the ventral dentate gyrus, relative to dorsal activation in rats that chose between immediately-available rewards. Putative developmentally-born cells, located in the superficial granule cell layer, did not display task-specific activity. These findings identify a novel and specific role for neurogenesis in decisions about future rewards, thereby implicating newborn neurons in disorders where short-sighted gains are preferred at the expense of long-term health.

The Entorhinal Cortex and Adult Neurogenesis in Major Depression

Depression is characterized by impairments in adult neurogenesis. Reduced hippocampal function, which is suggestive of neurogenesis impairments, is associated with depression-related phenotypes. As adult neurogenesis operates in an activity-dependent manner, disruption of hippocampal neurogenesis in depression may be a consequence of neural circuitry impairments. In particular, the entorhinal cortex is known to have a regulatory effect on the neural circuitry related to hippocampal function and adult neurogenesis. However, a comprehensive understanding of how disruption of the neural circuitry can lead to neurogenesis impairments in depression remains unclear with respect to the regulatory role of the entorhinal cortex. This review highlights recent findings suggesting neural circuitry-regulated neurogenesis, with a focus on the potential role of the entorhinal cortex in hippocampal neurogenesis in depression-related cognitive and emotional phenotypes. Taken together, these findings may provide a better understanding of the entorhinal cortex-regulated hippocampal neurogenesis model of depression.

Boride-derived oxygen-evolution catalysts

Metal borides/borates have been considered promising as oxygen evolution reaction catalysts; however, to date, there is a dearth of evidence of long-term stability at practical current densities. Here we report a phase composition modulation approach to fabricate effective borides/borates-based catalysts. We find that metal borides in-situ formed metal borates are responsible for their high activity. This knowledge prompts us to synthesize NiFe-Boride, and to use it as a templating precursor to form an active NiFe-Borate catalyst. This boride-derived oxide catalyzes oxygen evolution with an overpotential of 167 mV at 10 mA/cm² in 1 M KOH electrolyte and requires a record-low overpotential of 460 mV to maintain water splitting performance for over 400 h at current density of 1 A/cm². We couple the catalyst with CO reduction in an alkaline membrane electrode assembly electrolyser, reporting stable C₂H₄ electrosynthesis at current density 200 mA/cm² for over 80 h.

A self-healing catalyst for electrocatalytic and photoelectrochemical oxygen evolution in highly alkaline conditions

While self-healing is considered a promising strategy to achieve long-term stability for oxygen evolution reaction (OER) catalysts, this strategy remains a challenge for OER catalysts working in highly alkaline conditions. The self-healing of the OER-active nickel iron layered double hydroxides (NiFe-LDH) has not been successful due to irreversible leaching of Fe catalytic centers. Here, we investigate the introduction of cobalt (Co) into the NiFe-LDH as a promoter for in situ Fe redeposition. An active borate-intercalated NiCoFe-LDH catalyst is synthesized using electrodeposition and shows no degradation after OER tests at 10 mA cm⁻² at pH 14 for 1000 h, demonstrating its self-healing ability under harsh OER conditions. Importantly, the presence of both ferrous ions and borate ions in the electrolyte is found to be crucial to the catalyst’s self-healing. Furthermore, the implementation of this catalyst in photoelectrochemical devices is demonstrated with an integrated silicon photoanode. The self-healing mechanism leads to a self-limiting catalyst thickness, which is ideal for integration with photoelectrodes since redeposition is not accompanied by increased parasitic light absorption.

While self-healing catalysts may survive the harsh environments used for oxygen evolution, understanding how to develop such electrocatalysts remains a challenge. Here, authors find cobalt to promote the self-healing of leached iron centers in borate-intercalated nickel-iron-cobalt oxyhydroxides.

Stable, active CO2 reduction to formate via redox-modulated stabilization of active sites

Electrochemical reduction of CO₂ (CO₂R) to formic acid upgrades waste CO₂; however, up to now, chemical and structural changes to the electrocatalyst have often led to the deterioration of performance over time. Here, we find that alloying p-block elements with differing electronegativities modulates the redox potential of active sites and stabilizes them throughout extended CO₂R operation. Active Sn-Bi/SnO₂ surfaces formed in situ on homogeneously alloyed Bi_0.1Sn crystals stabilize the CO₂R-to-formate pathway over 2400 h (100 days) of continuous operation at a current density of 100 mA cm⁻². This performance is accompanied by a Faradaic efficiency of 95% and an overpotential of ~ −0.65 V. Operating experimental studies as well as computational investigations show that the stabilized active sites offer near-optimal binding energy to the key formate intermediate *OCHO. Using a cation-exchange membrane electrode assembly device, we demonstrate the stable production of concentrated HCOO^– solution (3.4 molar, 15 wt%) over 100 h.

Stable electrochemical reduction to formate is still challenging. Here, the authors demonstrate a redox-modulation and active-site stabilization strategy for CO₂ to formate conversion over 100 days of continuous operation at 100 mA/cm² with a cathodic energy efficiency of 70%.

Evidences for Adult Hippocampal Neurogenesis in Humans

The rodent hippocampus generates new neurons throughout life. This process, named adult hippocampal neurogenesis (AHN), is a striking form of neural plasticity that occurs in the brains of numerous mammalian species. Direct evidence of adult neurogenesis in humans has remained elusive, although the occurrence of this phenomenon in the human dentate gyrus has been demonstrated in seminal studies and recent research that have applied distinct approaches to birthdate newly generated neurons and to validate markers of adult-born neurons. Our data point to the persistence of AHN until the 10th decade of human life, as well as to marked impairments in this process in patients with Alzheimer's disease. Moreover, our work demonstrates that the methods used to process and analyze postmortem human brain samples can limit the detection of various markers of AHN to the point of making them undetectable. In this Dual Perspectives article, we highlight the critical methodological aspects that should be strictly controlled in human studies and the robust evidence that supports the occurrence of AHN in humans. We also put forward reasons that may account for current discrepancies on this topic. Finally, the unresolved questions and future challenges awaiting the field are highlighted.

De novo DNA methylation controls neuronal maturation during adult hippocampal neurogenesis

Adult neurogenesis enables the life‐long addition of functional neurons to the hippocampus and is regulated by both cell‐intrinsic molecular programs and behavioral activity. De novo DNA methylation is crucial for embryonic brain development, but its role during adult hippocampal neurogenesis has remained unknown. Here, we show that de novo DNA methylation is critical for maturation and functional integration of adult‐born neurons in the mouse hippocampus. Bisulfite sequencing revealed that de novo DNA methyltransferases target neuronal enhancers and gene bodies during adult hippocampal neural stem cell differentiation, to establish neuronal methylomes and facilitate transcriptional up‐regulation of neuronal genes. Inducible deletion of both de novo DNA methyltransferases Dnmt3a and Dnmt3b in adult neural stem cells did not affect proliferation or fate specification, but specifically impaired dendritic outgrowth and synaptogenesis of newborn neurons, thereby hampering their functional maturation. Consequently, abolishing de novo DNA methylation modulated activation patterns in the hippocampal circuitry and caused specific deficits in hippocampus‐dependent learning and memory. Our results demonstrate that proper establishment of neuronal methylomes during adult neurogenesis is fundamental for hippocampal function.

Adult Neurogenesis and Antidepressant Treatment: The Surprise Finding by Ron Duman and the Field 20 Years Later

Of Duman's many influential findings, the finding that long-term treatment with antidepressant drugs produces an increase in neurogenesis in the subgranular zone of the adult hippocampus may be one of the most enduring and far-reaching. This novel discovery and his decades of continued research in the field led to a new hypothesis about the mechanism of action of antidepressants, providing a critical step in our understanding of the neurotrophic hypothesis of depression and synaptic plasticity. It is now accepted that antidepressant treatments can oppose and even reverse the effects of stress on the brain and on newly born hippocampal cells, possibly via neurotrophic factors, which Duman had continued to explore. Furthermore, ablation studies have shown preclinically that hippocampal neurogenesis may be necessary for some of the clinical effects of antidepressant drugs. Duman's laboratory continued to interrogate neurotrophins and synaptic plasticity, demonstrating that newer clinically approved antidepressant compounds also affect neurogenesis and synaptic plasticity. In this review, we summarize Duman's original findings and discuss the current state of the field of neurogenesis with respect to animal models and human studies and the implications of those findings on the field of drug discovery.

AAV ablates neurogenesis in the adult murine hippocampus

Recombinant adeno-associated virus (rAAV) has been widely used as a viral vector across mammalian biology and has been shown to be safe and effective in human gene therapy. We demonstrate that neural progenitor cells (NPCs) and immature dentate granule cells (DGCs) within the adult murine hippocampus are particularly sensitive to rAAV-induced cell death. Cell loss is dose dependent and nearly complete at experimentally relevant viral titers. rAAV-induced cell death is rapid and persistent, with loss of BrdU-labeled cells within 18 hr post-injection and no evidence of recovery of adult neurogenesis at 3 months post-injection. The remaining mature DGCs appear hyperactive 4 weeks post-injection based on immediate early gene expression, consistent with previous studies investigating the effects of attenuating adult neurogenesis. In vitro application of AAV or electroporation of AAV2 inverted terminal repeats (ITRs) is sufficient to induce cell death. Efficient transduction of the dentategyrus (DG)- without ablating adult neurogenesis- can be achieved by injection of rAAV2-retro serotyped virus into CA3. rAAV2-retro results in efficient retrograde labeling of mature DGCs and permits in vivo two-photon calcium imaging of dentate activity while leaving adult neurogenesis intact. These findings expand on recent reports implicating rAAV-linked toxicity in stem cells and other cell types and suggest that future work using rAAV as an experimental tool in the DG and as a gene therapy for diseases of the central nervous system should be carefully evaluated.

Dentate granule cells encode auditory decisions after reinforcement learning in rats

Auditory-cued goal-oriented behaviors requires the participation of cortical and subcortical brain areas, but how neural circuits associate sensory-based decisions with goal locations through learning remains poorly understood. The hippocampus is critical for spatial coding, suggesting its possible involvement in transforming sensory inputs to the goal-oriented decisions. Here, we developed an auditory discrimination task in which rats learned to navigate to goal locations based on the frequencies of auditory stimuli. Using in vivo calcium imaging in freely behaving rats over the course of learning, we found that dentate granule cells became more active, spatially tuned, and responsive to task-related variables as learning progressed. Furthermore, only after task learning, the activity of dentate granule cell ensembles represented the navigation path and predicts auditory decisions as early as when rats began to approach the goals. Finally, chemogenetic silencing of dentate gyrus suppressed task learning. Our results demonstrate that dentate granule cells gain task-relevant firing pattern through reinforcement learning and could be a potential link of sensory decisions to spatial navigation.

Adult-born neurons promote cognitive flexibility by improving memory precision and indexing

Adult neurogenesis in the hippocampal dentate gyrus (DG) is an extraordinary form of plasticity fundamental for cognitive flexibility. Recent evidence showed that newborn neurons differentially modulate input to the infra- and supra-pyramidal blades of the DG during the processing of spatial and contextual information, respectively. However, how this differential regulation by neurogenesis is translated into different aspects contributing cognitive flexibility is unclear. Here, we increased adult-born neurons by a genetic expansion of neural stem cells and studied their influence during navigational learning. We found that increased neurogenesis improved both memory precision and flexibility. Interestingly, each of these gains was associated with distinct subregional patterns of activity and better separation of memory representations in the DG-CA3 network. Our results highlight the role of adult-born neurons in promoting memory precision and indexing and suggests their anatomical allocation within specific DG-CA3 compartments, together contributing to cognitive flexibility.

Adult Hippocampal Neurogenesis and Affective Disorders: New Neurons for Psychic Well-Being

A paradigm shift in neuroscience was the discovery that new neurons are constantly produced in the adult mammalian brain of several species, including Homo sapiens. These new-born cells are formed in some main neurogenic niches, including the subventricular zone (SVZ) at the margin of the lateral ventricle and subgranular zone (SGZ) in the hippocampal dentate gyrus (DG). In the DG, neuroblasts derive from SGZ progenitors and migrate to the hippocampal granular layer becoming adult granule cells, which are integrated into functional adult circuits. It has been confirmed that adult hippocampal neurogenesis (AHN) is a long-lasting phenomenon in the human brain. The functions of hippocampal new-born cells are not fully established. Experimental studies suggest that they have unique electrophysiological properties, including hyperexcitability, which enable them to regulate adult granule cells. Their specific function depends on the anatomical hippocampal location along the hippocampal dorsal-ventral axis. Dorsal hippocampus plays a more defined role on spatial learning and contextual information, while the ventral hippocampus is more related to emotional behavior, stress resilience and social interaction. Several reports suggest a role for AHN in pattern separation, cognitive flexibility, forgetting and reversal learning. It has been proposed that deficits in AHN might impair normal DG function, including pattern separation and cognitive flexibility, which could play a role on the etiology of affective disorders, such as depression, anxiety and post-traumatic stress disorder (PTSD). In this paper, we review recent scientific evidence suggesting that impairment of AHN may underlie the pathophysiology of affective disorders even in humans and that neurogenesis-inspired therapies may be a promising approach to reduce symptoms of affective disorders in humans.

MRI-guided histology of TDP-43 knock-in mice implicates parvalbumin interneuron loss, impaired neurogenesis and aberrant neurodevelopment in amyotrophic lateral sclerosis-frontotemporal dementia

Amyotrophic lateral sclerosis and frontotemporal dementia are overlapping diseases in which MRI reveals brain structural changes in advance of symptom onset. Recapitulating these changes in preclinical models would help to improve our understanding of the molecular causes underlying regionally selective brain atrophy in early disease. We therefore investigated the translational potential of the TDP-43Q331K knock-in mouse model of amyotrophic lateral sclerosis-frontotemporal dementia using MRI. We performed in vivo MRI of TDP-43Q331K knock-in mice. Regions of significant volume change were chosen for post-mortem brain tissue analyses. Ex vivo computed tomography was performed to investigate skull shape. Parvalbumin neuron density was quantified in post-mortem amyotrophic lateral sclerosis frontal cortex. Adult mutants demonstrated parenchymal volume reductions affecting the frontal lobe and entorhinal cortex in a manner reminiscent of amyotrophic lateral sclerosis-frontotemporal dementia. Subcortical, cerebellar and brain stem regions were also affected in line with observations in pre-symptomatic carriers of mutations in C9orf72, the commonest genetic cause of both amyotrophic lateral sclerosis and frontotemporal dementia. Volume loss was also observed in the dentate gyrus of the hippocampus, along with ventricular enlargement. Immunohistochemistry revealed reduced parvalbumin interneurons as a potential cellular correlate of MRI changes in mutant mice. By contrast, microglia was in a disease activated state even in the absence of brain volume loss. A reduction in immature neurons was found in the dentate gyrus, indicative of impaired adult neurogenesis, while a paucity of parvalbumin interneurons in P14 mutant mice suggests that TDP-43Q331K disrupts neurodevelopment. Computerized tomography imaging showed altered skull morphology in mutants, further suggesting a role for TDP-43Q331K in development. Finally, analysis of human post-mortem brains confirmed a paucity of parvalbumin interneurons in the prefrontal cortex in sporadic amyotrophic lateral sclerosis and amyotrophic lateral sclerosis linked to C9orf72 mutations. Regional brain MRI changes seen in human amyotrophic lateral sclerosis-frontotemporal dementia are recapitulated in TDP-43Q331K knock-in mice. By marrying in vivo imaging with targeted histology, we can unravel cellular and molecular processes underlying selective brain vulnerability in human disease. As well as helping to understand the earliest causes of disease, our MRI and histological markers will be valuable in assessing the efficacy of putative therapeutics in TDP-43Q331K knock-in mice.

Nuclear isoform of FGF13 regulates post-natal neurogenesis in the hippocampus through an epigenomic mechanism

The hippocampus is one of two niches in the mammalian brain with persistent neurogenesis into adulthood. The neurogenic capacity of hippocampal neural stem cells (NSCs) declines with age, but the molecular mechanisms of this process remain unknown. In this study, we find that fibroblast growth factor 13 (FGF13) is essential for the post-natal neurogenesis in mouse hippocampus, and FGF13 deficiency impairs learning and memory. In particular, we find that FGF13A, the nuclear isoform of FGF13, is involved in the maintenance of NSCs and the suppression of neuronal differentiation during post-natal hippocampal development. Furthermore, we find that FGF13A interacts with ARID1B, a unit of Brahma-associated factor chromatin remodeling complex, and suppresses the expression of neuron differentiation-associated genes through chromatin modification. Our results suggest that FGF13A is an important regulator for maintaining the self-renewal and neurogenic capacity of NSCs in post-natal hippocampus, revealing an epigenomic regulatory function of FGFs in neurogenesis.

Increasing Adult Hippocampal Neurogenesis Promotes Resilience in a Mouse Model of Depression

Many studies evaluated the functional role of adult hippocampal neurogenesis (AHN) and its key role in cognitive functions and mood regulation. The effects of promoting AHN on the recovery of stress-induced symptoms have been well studied, but its involvement in stress resilience remains elusive. We used a mouse model enabling us to foster AHN before the exposure to unpredictable chronic mild stress (UCMS) to evaluate the potential protective effects of AHN on stress, assessing the depressive-like phenotype and executive functions. For this purpose, an inducible transgenic mouse model was used to delete the pro-apoptotic gene Bax from neural progenitors four weeks before UCMS, whereby increasing the survival of adult-generated neurons. Our results showed that UCMS elicited a depressive-like phenotype, highlighted by a deteriorated coat state, a higher immobility duration in the tail suspension test (TST), and a delayed reversal learning in a water maze procedure. Promoting AHN before UCMS was sufficient to prevent the development of stressed-induced behavioral changes in the TST and the water maze, reflecting an effect of AHN on stress resilience. Taken together, our data suggest that increasing AHN promotes stress resilience on some depressive-like symptoms but also in cognitive symptoms, which are often observed in MD.

The impact of physical exercise on hippocampus, in physiological condition and ageing-related decline: current evidence from animal and human studies

Physical exercise (PE), notoriously, promotes a state of general well-being, throughout the entire human lifespan. Moreover, maintaining an adequate and regular PE habit results to be a powerful preventive factor towards many diseases and may also help in managing existing pathological conditions. PE induces structural and functional changes in various districts of the body, determining biological and psychological benefits. Additionally, in elderly, PE might represent a remarkable tool reducing cognitive impairments related to the normal aging processes and it has also been found to have an impact in neurodegenerative diseases such as Alzheimer's disease. The present review aims to provide an overview about PE effects on hippocampus, since it is one of the brain regions most susceptible to aging and, therefore, involved in diseases characterized by cognitive impairment.

Gamma rhythm communication between entorhinal cortex and dentate gyrus neuronal assemblies

Gamma oscillations are thought to coordinate the spike timing of functionally specialized neuronal ensembles across brain regions. To test this hypothesis, we optogenetically perturbed gamma spike timing in the rat medial (MEC) and lateral (LEC) entorhinal cortices and found impairments in spatial and object learning tasks, respectively. MEC and LEC were synchronized with the hippocampal dentate gyrus through high- and low-gamma-frequency rhythms, respectively, and engaged either granule cells or mossy cells and CA3 pyramidal cells in a task-dependent manner. Gamma perturbation disrupted the learning-induced assembly organization of target neurons. Our findings imply that pathway-specific gamma oscillations route task-relevant information between distinct neuronal subpopulations in the entorhinal-hippocampal circuit. We hypothesize that interregional gamma-time-scale spike coordination is a mechanism of neuronal communication.

Adult-born granule cell mossy fibers preferentially target parvalbumin-positive interneurons surrounded by perineuronal nets

Adult-born granule cells (abGCs) integrate into the hippocampus and form connections with dentate gyrus parvalbumin-positive (PV+) interneurons, a circuit important for modulating plasticity. Many of these interneurons are surrounded by perineuronal nets (PNNs), extracellular matrix structures known to participate in plasticity. We compared abGC projections to PV+ interneurons with negative-to-low intensity PNNs to those with high intensity PNNs using retroviral and 3R-Tau labeling in adult mice, and found that abGC mossy fibers and boutons are more frequently located near PV+ interneurons with high intensity PNNs. These results suggest that axons of new neurons preferentially stabilize near target cells with intense PNNs. Next, we asked whether the number of abGCs influences PNN formation around PV+ interneurons, and found that near complete ablation of abGCs produced a decrease in the intensity and number of PV+ neurons with PNNs, suggesting that new neuron innervation may enhance PNN formation. Experience-driven changes in adult neurogenesis did not produce consistent effects, perhaps due to widespread effects on plasticity. Our study identifies abGC projections to PV+ interneurons with PNNs, with more presumed abGC mossy fiber boutons found near the cell body of PV+ interneurons with strong PNNs.

Formation and integration of new neurons in the adult hippocampus

Neural stem cells (NSCs) generate new neurons throughout life in the mammalian brain. Adult-born neurons shape brain function, and endogenous NSCs could potentially be harnessed for brain repair. In this Review, focused on hippocampal neurogenesis in rodents, we highlight recent advances in the field based on novel technologies (including single-cell RNA sequencing, intravital imaging and functional observation of newborn cells in behaving mice) and characterize the distinct developmental steps from stem cell activation to the integration of newborn neurons into pre-existing circuits. Further, we review current knowledge of how levels of neurogenesis are regulated, discuss findings regarding survival and maturation of adult-born cells and describe how newborn neurons affect brain function. The evidence arguing for (and against) lifelong neurogenesis in the human hippocampus is briefly summarized. Finally, we provide an outlook of what is needed to improve our understanding of the mechanisms and functional consequences of adult neurogenesis and how the field may move towards more translational relevance in the context of acute and chronic neural injury and stem cell-based brain repair.

Functional differentiation in the transverse plane of the hippocampus: An update on activity segregation within the DG and CA3 subfields

Decades of neuroscience research in rodents have established an essential role of the hippocampus in the processing of episodic memories. Based on accumulating evidence of functional segregation in the hippocampus along the longitudinal axis, this role has been primarily ascribed to the dorsal hippocampus. More recent findings, however, demonstrate that functional segregation also occurs along transverse axis of the hippocampus, within the hippocampal subfields CA1, CA2, CA3, and the dentate gyrus (DG). Because the functional heterogeneity within CA1 has been addressed in several recent articles, here we discuss behavioral findings and putative mechanisms supporting generation of asymmetrical activity patterns along the transverse axis of DG and CA3. While transverse subnetworks appear to discretely contribute to the processing of spatial, non-spatial, temporal, and social components of episodic memories, integration of these components also occurs, especially in the CA3 subfield and possibly downstream, in the cortical targets of the hippocampus.

Ion-channel regulation of response decorrelation in a heterogeneous multi-scale model of the dentate gyrus

Heterogeneities in biological neural circuits manifest in afferent connectivity as well as in local-circuit components such as neuronal excitability, neural structure and local synaptic strengths. The expression of adult neurogenesis in the dentate gyrus (DG) amplifies local-circuit heterogeneities and guides heterogeneities in afferent connectivity. How do neurons and their networks endowed with these distinct forms of heterogeneities respond to perturbations to individual ion channels, which are known to change under several physiological and pathophysiological conditions? We sequentially traversed the ion channels-neurons-network scales and assessed the impact of eliminating individual ion channels on conductance-based neuronal and network models endowed with disparate local-circuit and afferent heterogeneities. We found that many ion channels differentially contributed to specific neuronal or network measurements, and the elimination of any given ion channel altered several functional measurements. We then quantified the impact of ion-channel elimination on response decorrelation, a well-established metric to assess the ability of neurons in a network to convey complementary information, in DG networks endowed with different forms of heterogeneities. Notably, we found that networks constructed with structurally immature neurons exhibited functional robustness, manifesting as minimal changes in response decorrelation in the face of ion-channel elimination. Importantly, the average change in output correlation was dependent on the eliminated ion channel but invariant to input correlation. Our analyses suggest that neurogenesis-driven structural heterogeneities could assist the DG network in providing functional resilience to molecular perturbations.

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Perturbations at one scale result in a cascading impact on physiology across scales.

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Heterogeneous multi-scale models used to assess the impact of ion-channel deletion.

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Mapping of structural components to functional outcomes is many-to-many.

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Differential & variable impact of ion channel deletion on response decorrelation.

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Neurogenesis-induced structural heterogeneity confers resilience to perturbations.

A critical period of neuronal activity results in aberrant neurogenesis rewiring hippocampal circuitry in a mouse model of epilepsy

In the mammalian hippocampus, adult-born granule cells (abGCs) contribute to the function of the dentate gyrus (DG). Disruption of the DG circuitry causes spontaneous recurrent seizures (SRS), which can lead to epilepsy. Although abGCs contribute to local inhibitory feedback circuitry, whether they are involved in epileptogenesis remains elusive. Here, we identify a critical window of activity associated with the aberrant maturation of abGCs characterized by abnormal dendrite morphology, ectopic migration, and SRS. Importantly, in a mouse model of temporal lobe epilepsy, silencing aberrant abGCs during this critical period reduces abnormal dendrite morphology, cell migration, and SRS. Using mono-synaptic tracers, we show silencing aberrant abGCs decreases recurrent CA3 back-projections and restores proper cortical connections to the hippocampus. Furthermore, we show that GABA-mediated amplification of intracellular calcium regulates the early critical period of activity. Our results demonstrate that aberrant neurogenesis rewires hippocampal circuitry aggravating epilepsy in mice.

Neural Circuitry-Neurogenesis Coupling Model of Depression

Depression is characterized by the disruption of both neural circuitry and neurogenesis. Defects in hippocampal activity and volume, indicative of reduced neurogenesis, are associated with depression-related behaviors in both humans and animals. Neurogenesis in adulthood is considered an activity-dependent process; therefore, hippocampal neurogenesis defects in depression can be a result of defective neural circuitry activity. However, the mechanistic understanding of how defective neural circuitry can induce neurogenesis defects in depression remains unclear. This review highlights the current findings supporting the neural circuitry-regulated neurogenesis, especially focusing on hippocampal neurogenesis regulated by the entorhinal cortex, with regard to memory, pattern separation, and mood. Taken together, these findings may pave the way for future progress in neural circuitry-neurogenesis coupling studies of depression.

The unique plasticity of hippocampal adult-born neurons: Contributing to a heterogeneous dentate

The dentate gyrus (DG) of the hippocampus is evolutionarily conserved as one of the few sites of adult neurogenesis in mammals. Although there is clear evidence that neurogenesis is necessary for healthy hippocampal function, whether adult-born neurons are simply integrated into existing hippocampal networks to serve a similar purpose to that of developmentally born neurons or whether they represent a discrete cell population with unique functions remains less clear. In this review, we consider evidence for discrete cellular, synaptic, and structural features of adult-born DG neurons, suggesting that neurogenesis contributes to the formation of a heterogeneous DG. We therefore propose that hippocampal neurogenesis creates a specialized neuronal subpopulation that may play a key role in hippocampal functions like episodic memory. We note critical gaps in this extensive body of work, including a general failure to include female animals in relevant research and a need for more precise consideration of intrahippocampal neuroanatomy.

Adult neurogenesis augmentation attenuates anhedonia and HPA axis dysregulation in a mouse model of chronic stress and depression

Major depressive disorder is a common debilitating mental health problem that represents one of the leading causes of disability. Up to date, the therapeutic targets and approaches are still limited. Adult hippocampal neurogenesis (AHN) has been proposed as a critical contributor to the pathophysiology and treatment of depression, altering the hippocampal control over stress response at network, neuroendocrine and behavioral levels. These findings together have suggested that manipulating AHN may be a promising therapeutic strategy for depression. To investigate this question, we assessed whether increasing adult neurogenesis would be sufficient to produce antidepressant-like effects at behavioral and neuroendocrine levels in a mouse model of depression; the unpredictable chronic mild stress (UCMS). For this purpose, we used a bi-transgenic mouse line (iBax) in which AHN increase was induced by deletion of the pro-apoptotic gene Bax from the neural progenitors following the tamoxifen-dependent action of CreERT2 recombinases. UCMS induced a syndrome that is reminiscent of depression-like states, including anhedonia (cookie test), physical changes (coat deterioration, reduced weight gain), anxiety-like behaviors (higher latency in the novelty-supressed feeding -NSF- test), passive stress-coping behaviors (immobility in the forced swim test -FST-) and a blunted hypothalamo-pituitary-adrenal (HPA) axis reactivity to acute stress in addition to AHN decrease. Tamoxifen injection reversed the AHN decrease as well as partly counteracted UCMS effects on the cookie test and HPA axis but not for the coat state, weight gain, NSF test and FST. Taken together, our results suggest that a strategy directing at increasing AHN may be able to alleviate some depression-related behavioral and neuroendocrine dimensions of UCMS, such as anhedonia and HPA axis reactivity deficits, but may be hardly sufficient to produce a complete recovery.

Functional and Structural Correlates of Impaired Enrichment-Mediated Adult Hippocampal Neurogenesis in a Mouse Model of Prenatal Alcohol Exposure

Background:

Fetal alcohol spectrum disorders (FASDs) are associated with a wide range of cognitive deficiencies.

Objective:

We previously found that gestational exposure to moderate levels of alcohol in mice throughout the 1st-2nd human trimester-equivalents for brain development results in profound impairment of the hippocampal neurogenic response to enriched environment (EE) in adulthood, without altering baseline neurogenesis rate under standard housing (SH). However, the functional and structural consequences of impaired EE-mediated neurogenesis in the context of prenatal alcohol exposure (PAE) have not been determined.

Results:

Here, we demonstrate that PAE-EE mice display impaired performance on a neurogenesis-dependent pattern discrimination task, broadened behavioral activation of the dentate gyrus, as assessed by expression of the immediate early gene, c-Fos, and impaired dendritic branching of adult-generated dentate granule cells (aDGCs).

Conclusions:

These studies further underscore the impact of moderate gestational alcohol exposure on adult hippocampal plasticity and support adult hippocampal neurogenesis as a potential therapeutic target to remediate certain neurological outcomes in FASD.

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

Throughout adulthood, the dentate gyrus continues to produce new granule cells, which integrate into the hippocampal circuitry. New neurons have been linked to several known functions of the hippocampus, including learning and memory, anxiety and stress regulation, and social behavior. We explored whether transgenic reduction of adult-born neurons in mice would impair social memory and the formation of social dominance hierarchies. We used a conditional transgenic mouse strain [thymidine kinase (TK) mice] that selectively reduces adult neurogenesis by treatment with the antiviral drug valganciclovir (VGCV). TK mice treated with VGCV were unable to recognize conspecifics as familiar 24 h after initial exposure. We then explored whether reducing new neurons completely impaired their ability to acquire or retrieve a social memory and found that TK mice treated with VGCV were able to perform at control levels when the time between exposure (acquisition) and reexposure (retrieval) was brief. We next explored whether adult-born neurons are involved in dominance hierarchy formation by analyzing their home cage behavior as well as their performance in the tube test, a social hierarchy test, and did not find any consistent alterations in behavior between control and TK mice treated with VGCV. These data suggest that adult neurogenesis is essential for social memory maintenance, but not for acquisition nor retrieval over a short time frame, with no effect on social dominance hierarchy. Future work is needed to explore whether the influence of new neurons on social memory is mediated through connections with the CA2, an area involved in social recognition.

Mechanisms Underlying Memory Consolidation by Adult-Born Neurons During Sleep

The mammalian hippocampus generates new neurons that incorporate into existing neuronal networks throughout the lifespan, which bestows a unique form of cellular plasticity to the memory system. Recently, we found that hippocampal adult-born neurons (ABNs) that were active during learning reactivate during subsequent rapid eye movement (REM) sleep and provided causal evidence that ABN activity during REM sleep is necessary for memory consolidation. Here, we describe the potential underlying mechanisms by highlighting distinct characteristics of ABNs including decoupled firing from local oscillations and ability to undergo profound synaptic remodeling in response to experience. We further discuss whether ABNs constitute the conventional definition of engram cells by focusing on their active and passive roles in the memory system. This synthesis of evidence helps advance our thinking on the unique mechanisms by which ABNs contribute to memory consolidation.

Behavioral response to fluoxetine in both female and male mice is modulated by dentate gyrus granule cell activity

Depression is a complex psychiatric disorder that is a major burden on society, with only ~33% of depressed patients attaining remission upon initial monotherapy with a selective serotonin reuptake inhibitor (SSRI). In preclinical studies using rodents, chronic stress paradigms, such as chronic corticosterone and social instability stress, are used to induce avoidance behaviors associated with negative affective states. Chronic fluoxetine (FLX; an SSRI) treatment reverses these chronic stress-induced behavioral changes in some, but not all mice, permitting stratification of mice into behavioral responders and non-responders to FLX. We previously reported that 5-HT1A receptors, which are Gi-coupled inhibitory receptors, on mature granule cells (GCs) in the dentate gyrus (DG) are necessary and sufficient for the behavioral, neurogenic, and neuroendocrine response to chronic SSRI treatment. Since inhibition of mature DG GCs through cell autonomous Gi-coupled receptors is critical for mounting an antidepressant response, we assessed the relationship between behavioral response to FLX and DG GC activation in FLX responders, non-responders, and stress controls in both male and female mice. Intriguingly, using disparate stress paradigms, we found that male and female behavioral FLX responders show decreased DG GC activation (as measured by cFos immunostaining) relative to non-responders and stress controls. We then show in both sexes that chronic inhibition of ventral DG GCs (through usage of Gi-DREADDs) results in a decrease in maladaptive avoidance behaviors, while ventral DG GCs stimulation with Gq-DREADDs increases maladaptive behaviors. Finally, we were able to bidirectionally control the behavioral response to FLX through modulation of DG GCs. Chronic inhibition of ventral DG GCs with Gi-DREADDs converted FLX non-responders into responders, while activation of ventral DG GCs with Gq-DREADDs converted FLX responders into non-responders. This study illustrates ventral DG GC activity is a major modulator of the behavioral response to FLX in both male and female mice.

Parallel processing streams in the hippocampus

The hippocampus performs two complementary processes, pattern separation and pattern completion, to minimize interference and maximize the storage capacity of memories. Classic computational models have suggested that the dentate gyrus (DG) supports pattern separation and the putative attractor circuitry in CA3 supports pattern completion. However, recent evidence of functional heterogeneity along the CA3 transverse axis of the hippocampus suggests that the DG and proximal CA3 work as a functional unit for pattern separation, while distal CA3 forms an autoassociative network for pattern completion. We propose that the outputs of these functional circuits, combined with direct projections from entorhinal cortex to CA1, form interconnected, parallel processing circuits to support accurate memory storage and retrieval.

Transcriptomic analysis of the signature of neurogenesis in human hippocampus suggests restricted progenitor cell progression post-childhood

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NESTIN, SOX1, and SOX4 decreased progressively from prenatal to adult age.

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KI67 and TBR2 reached zero expression level at adolescence.

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NEUROD1, DCX, PSA NCAM remained unchanged post-childhood.

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VEGF and FGF2 did not change significantly from prenatal to adult age.

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BAX and TP53 decreased progressively from prenatal to adult age.

Purpose

Immunohistological investigations have given rise to divergent perspectives about adult hippocampal neurogenesis in humans. Therefore, this study aimed to examine whether a comprehensive transcriptomic analysis of signature markers of neurogenesis, supplemented with markers of gliogenesis, vasculogenesis, cell proliferation, and apoptosis, may help discern essential aspects of adult hippocampal neurogenesis in humans.

Materials and Methods

RNA expression data for salient marker genes of neurogenesis, gliogenesis, vasculogenesis, and apoptosis in post-mortem human hippocampal tissue [from prenatal (n = 15), child (n = 5), adolescent (n = 4), and adult (n = 6) brains] were downloaded from the Allen Human Brain Atlas database (http://www.brainspan.org/rnaseq/search/index.html). Gene expression data was categorized, median values were computed, and age group-specific differential expression was subjected to statistical analysis (significance level, α = 0.01).

Results

With the exception of the genes encoding GFAP, BLBP, SOX2, and PSA-NCAM (unchanged), and the post-mitotic late maturation markers CALB1, CALB2, MAP2, and NEUN as well as the pan-neuronal marker PROX1 which were persistently expressed throughout, expression of all other genes associated with neurogenesis was steeply and progressively downregulated between perinatal life and adulthood. Interestingly, expression of the classical proliferation marker KI67 and a progenitor cell marker TBR2 were found to have reached baseline expression levels (zero expression score) at adolescence while the expression of immature neuronal, post-mitotic early and late maturation markers remained at a constant level after childhood. In contrast, markers of gliogenesis (other than PDGFRA and Vimentin) were significantly upregulated between prenatal life and childhood. Expression of the vasculogenesis markers VEGFA and FGF2 did not differ across any of the age groups studied, whereas the expression of apoptotic markers was progressively decreased after prenatal life.

Conclusions

Our findings indicate that the progression of neurogenesis from progenitor cells is highly restricted in the human brain from childhood onwards. An alternative possibility that limited neurogenesis may be continued in adolescents and adults from a developmentally arrested pool of immature neurons needs to be examined further through experimental studies.

Metabolic tuning of inhibition regulates hippocampal neurogenesis in the adult brain

Hippocampus-engaged behaviors stimulate neurogenesis in the adult dentate gyrus by largely unknown means. To explore the underlying mechanisms, we used tetrode recording to analyze neuronal activity in the dentate gyrus of freely moving adult mice during hippocampus-engaged contextual exploration. We found that exploration induced an overall sustained increase in inhibitory neuron activity that was concomitant with decreased excitatory neuron activity. A mathematical model based on energy homeostasis in the dentate gyrus showed that enhanced inhibition and decreased excitation resulted in a similar increase in neurogenesis to that observed experimentally. To mechanistically investigate this sustained inhibitory regulation, we performed metabolomic and lipidomic profiling of the hippocampus during exploration. We found sustainably increased signaling of sphingosine-1-phosphate, a bioactive metabolite, during exploration. Furthermore, we found that sphingosine-1-phosphate signaling through its receptor 2 increased interneuron activity and thus mediated exploration-induced neurogenesis. Taken together, our findings point to a behavior-metabolism circuit pathway through which experience regulates adult hippocampal neurogenesis.

Enhancing adult neurogenesis promotes contextual fear memory discrimination and activation of hippocampal-dorsolateral septal circuits

Hippocampal circuitry is continuously modified by integration of adult-born dentate granule cells (DGCs). Prior work has shown that enhancing adult hippocampal neurogenesis decreases interference or overlap or conflict between ensembles of similar contexts and promotes discrimination of a shock-associated context from a similar, neutral context. However, the impact of enhanced integration of adult-born neurons on hippocampal network activity or downstream circuits such as the dorsolateral septum that mediate defensive behavioral responses is poorly understood. Here, we first replicated our finding that genetic expansion of the population of adult-born dentate granule cells (8 weeks and younger) promotes contextual fear discrimination. We found that enhanced contextual fear discrimination is associated with greater c-Fos expression in discrete hippocampal subfields along the proximo-distal and dorsoventral axis. Examination of the dorsolateral septum revealed an increase in activation of somatostatin expressing neurons consistent with recent characterization of these cells as calibrators of defensive behavior. Together, these findings begin to shed light on how genetically enhancing adult hippocampal neurogenesis affects activity of hippocampal-dorsolateral septal circuits.

[Effect of advanced maternal age on development of hippocampal neural stem cells in offspring rats]

OBJECTIVE

To study the effect of advanced maternal age (AMA) on the development of hippocampal neural stem cells in offspring rats.

METHODS

Ten 3-month-old and ten 12-month-old female Sprague-Dawley rats were housed individually with 3-month-old male rats (1:1, n=20), whose offspring rats were assigned to a control group and an AMA group. A total of 40 rats were randomly selected from each group. Immunofluorescence assay and Western blot were used to localize and determine the levels of protein expression of Nestin and doublecortin (DCX) on day 7 as well as neuronal nuclear antigen (NeuN) and glial fibrillary acidic protein (GFAP) on day 28 (n=8 for each marker). Immunofluorescence assay was also used to localize the hippocampal expression of polysialylated isoforms of neural cell adhesion molecule (PSA-NCAM) on day 14 (n=8 for each marker).

RESULTS

According to the Western blot results, the AMA group had significantly lower protein expression of DCX than the control group (P<0.05), while there were no significant differences in the protein expression of Nestin, NeuN, and GFAP between the two groups (P>0.05). According to the results of immunofluorescence assay, the AMA group had significantly lower protein expression of Nestin, DCX, and PSA-NCAM in the hippocampal dentate gyrus (DG) region than the control group (P<0.05), while there were no significant differences in the above indices in the hippocampal CA1 and CA3 regions between the two groups (P>0.05). The AMA group had significantly higher expression of NeuN in the hippocampal CA1 region than the control group (P<0.01), while there were no significant differences in the expression of NeuN in the hippocampal DG and CA3 regions between the two groups (P>0.05). The AMA group had significantly lower expression of GFAP in the hippocampal CA1, CA3, and DG regions than the control group (P<0.05).

CONCLUSIONS

AMA may cause inhibition of proliferation, survival, and migration of hippocampal neural stem cells. AMA may also affect their differentiation into neurons and astrocytes, which will eventually lead to developmental disorders of hippocampal neural stem cells in offspring rats.

Neurophotonics Approaches for the Study of Pattern Separation

Successful memory involves not only remembering over time but also keeping memories distinct. Computational models suggest that pattern separation appears as a highly efficient process to discriminate between overlapping memories. Furthermore, lesion studies have shown that the dentate gyrus (DG) participates in pattern separation. However, these manipulations did not allow identifying the neuronal mechanism underlying pattern separation. The development of different neurophotonics techniques, together with other genetic tools, has been useful for the study of the microcircuit involved in this process. It has been shown that less-overlapped information would generate distinct neuronal representations within the granule cells (GCs). However, because glutamatergic or GABAergic cells in the DG are not functionally or structurally homogeneous, identifying the specific role of the different subpopulations remains elusive. Then, understanding pattern separation requires the ability to manipulate a temporal and spatially specific subset of cells in the DG and ideally to analyze DG cells activity in individuals performing a pattern separation dependent behavioral task. Thus, neurophotonics and calcium imaging techniques in conjunction with activity-dependent promoters and high-resolution microscopy appear as important tools for this endeavor. In this work, we review how different neurophotonics techniques have been implemented in the elucidation of a neuronal network that supports pattern separation alone or in combination with traditional techniques. We discuss the limitation of these techniques and how other neurophotonic techniques could be used to complement the advances presented up to this date.

Sparse Activity of Hippocampal Adult-Born Neurons during REM Sleep Is Necessary for Memory Consolidation

The occurrence of dreaming during rapid eye movement (REM) sleep prompts interest in the role of REM sleep in hippocampal-dependent episodic memory. Within the mammalian hippocampus, the dentate gyrus (DG) has the unique characteristic of exhibiting neurogenesis persisting into adulthood. Despite their small numbers and sparse activity, adult-born neurons (ABNs) in the DG play critical roles in memory; however, their memory function during sleep is unknown. Here, we investigate whether young ABN activity contributes to memory consolidation during sleep using Ca²⁺ imaging in freely moving mice. We found that contextual fear learning recruits a population of young ABNs that are reactivated during subsequent REM sleep against a backdrop of overall reduced ABN activity. Optogenetic silencing of this sparse ABN activity during REM sleep alters the structural remodeling of spines on ABN dendrites and impairs memory consolidation. These findings provide a causal link between ABN activity during REM sleep and memory consolidation.

The Dentate Gyrus Classifies Cortical Representations of Learned Stimuli

Animals must discern important stimuli and place them onto their cognitive map of their environment. The neocortex conveys general representations of sensory events to the hippocampus, and the hippocampus is thought to classify and sharpen the distinctions between these events. We recorded populations of dentate gyrus granule cells (DG GCs) and lateral entorhinal cortex (LEC) neurons across days to understand how sensory representations are modified by experience. We found representations of odors in DG GCs that required synaptic input from the LEC. Odor classification accuracy in DG GCs correlated with future behavioral discrimination. In associative learning, DG GCs, more so than LEC neurons, changed their responses to odor stimuli, increasing the distance in neural representations between stimuli, responding more to the conditioned and less to the unconditioned odorant. Thus, with learning, DG GCs amplify the decodability of cortical representations of important stimuli, which may facilitate information storage to guide behavior.

Communication, Cross Talk, and Signal Integration in the Adult Hippocampal Neurogenic Niche

Radial glia-like neural stem cells (RGLs) in the dentate gyrus subregion of the hippocampus give rise to dentate granule cells (DGCs) and astrocytes throughout life, a process referred to as adult hippocampal neurogenesis. Adult hippocampal neurogenesis is sensitive to experiences, suggesting that it may represent an adaptive mechanism by which hippocampal circuitry is modified in response to environmental demands. Experiential information is conveyed to RGLs, progenitors, and adult-born DGCs via the neurogenic niche that is composed of diverse cell types, extracellular matrix, and afferents. Understanding how the niche performs its functions may guide strategies to maintain its health span and provide a permissive milieu for neurogenesis. Here, we first discuss representative contributions of niche cell types to regulation of neural stem cell (NSC) homeostasis and maturation of adult-born DGCs. We then consider mechanisms by which the activity of multiple niche cell types may be coordinated to communicate signals to NSCs. Finally, we speculate how NSCs integrate niche-derived signals to govern their regulation.