Distinct Contribution of Adult-Born Hippocampal Granule Cells to Context Encoding

The relationship between adult hippocampal neurogenesis and cognitive impairment in Alzheimer's disease

Neurogenesis persists throughout adulthood in the hippocampus and contributes to specific cognitive functions. In Alzheimer's disease (AD), the hippocampus is affected by pathology and functional impairment early in the disease. Human AD patients have reduced adult hippocampal neurogenesis (AHN) levels compared to age-matched healthy controls. Similarly, rodent AD models show a decrease in AHN before the onset of the classical hallmarks of AD pathology. Conversely, enhancement of AHN can protect against AD pathology and ameliorate memory deficits in both rodents and humans. Therefore, impaired AHN may be a contributing factor of AD-associated cognitive decline, rather than an effect of it. In this review we outline the regulation and function of AHN in healthy individuals, and highlight the relationship between AHN dysfunction and cognitive impairments in AD. The existence of AHN in humans and its relevance in AD patients will also be discussed, with an outlook toward future research directions. HIGHLIGHTS: Adult hippocampal neurogenesis occurs in the brains of mammals including humans. Adult hippocampal neurogenesis is reduced in Alzheimer's disease in humans and animal models.

Targeting high-affinity nicotinic receptors protects against the functional consequences of β-amyloid in mouse hippocampus

The accumulation of β-amyloid oligomers is a hallmark of Alzheimer's disease, inducing neural and network dysfunction in the early stages of pathology. The hippocampus is affected early in the pathogenesis of AD, however the impact of soluble β-amyloid on the dentate gyrus (DG) subregion of the hippocampus and its interaction with nicotinic acetylcholine receptors (nAChRs) within this region are not known. Using a localized model of over-expression, we show that β-amyloid induces early-onset neuronal hyperactivity and hippocampal-dependent memory deficits in mice. Further, we find the DG region to be under potent and sub-type specific nicotinic control in both healthy and pathophysiological conditions, with targeted receptor inhibition leading to a mnemonic rescue against localized amyloidosis. We show that while neurogenesis and synaptic functions are not severely affected in our model, reducing β2-containing nAChR function is associated with the promotion of young adult-born neurons within the pathological network, suggesting a possible protective mechanism. Our data thus reveal the DG network level changes which occur in the early-stages of β-amyloid accumulation and highlight the downstream consequences of targeted nicotinic neuromodulation.

Adult neurogenesis improves spatial information encoding in the mouse hippocampus

Adult neurogenesis is a unique form of neuronal plasticity in which newly generated neurons are integrated into the adult dentate gyrus in a process that is modulated by environmental stimuli. Adult-born neurons can contribute to spatial memory, but it is unknown whether they alter neural representations of space in the hippocampus. Using in vivo two-photon calcium imaging, we find that male and female mice previously housed in an enriched environment, which triggers an increase in neurogenesis, have increased spatial information encoding in the dentate gyrus. Ablating adult neurogenesis blocks the effect of enrichment and lowers spatial information, as does the chemogenetic silencing of adult-born neurons. Both ablating neurogenesis and silencing adult-born neurons decreases the calcium activity of dentate gyrus neurons, resulting in a decreased amplitude of place-specific responses. These findings are in contrast with previous studies that suggested a predominantly inhibitory action for adult-born neurons. We propose that adult neurogenesis improves representations of space by increasing the gain of dentate gyrus neurons and thereby improving their ability to tune to spatial features. This mechanism may mediate the beneficial effects of environmental enrichment on spatial learning and memory.

Transformation of spatial representations along hippocampal circuits

The hippocampus is thought to provide the brain with a cognitive map of the external world by processing various types of spatial information. To understand how essential spatial variables such as direction, position, and distance are transformed along its circuits to construct this global map, we perform single-photon widefield microendoscope calcium imaging in the dentate gyrus and CA3 of mice freely navigating along a narrow corridor. We find that spatial activity maps in the dentate gyrus, but not in CA3, are correlated after aligning them to the running directions, suggesting that they represent the distance traveled along the track in egocentric coordinates. Together with population activity decoding, our data suggest that while spatial representations in the dentate gyrus and CA3 are anchored in both egocentric and allocentric coordinates, egocentric distance coding is more prevalent in the dentate gyrus than in CA3, providing insights into the assembly of the cognitive map.

Cross-species analysis uncovers the mitochondrial stress response in the hippocampus as a shared mechanism in mouse early life stress and human depression

Depression, or major depressive disorder, poses a significant burden for both individuals and society, affecting approximately 10.8% of the general population. This psychiatric disorder leads to approximately 800,000 deaths per year. A combination of genetic and environmental factors such as early life stress (ELS) increase the risk for development of depression in humans, and a clear role for the hippocampus in the pathophysiology of depression has been shown. Nevertheless, the underlying mechanisms of depression remain poorly understood, resulting in a lack of effective treatments. To better understand the core mechanisms underlying the development of depression, we used a cross-species design to investigate shared hippocampal pathophysiological mechanisms in mouse ELS and human depression. Mice were subjected to ELS by a maternal separation paradigm, followed by RNA sequencing analysis of the adult hippocampal tissue. This identified persistent transcriptional changes linked to mitochondrial stress response pathways, with oxidative phosphorylation and protein folding emerging as the main mechanisms affected by maternal separation. Remarkably, there was a significant overlap between the pathways involved in mitochondrial stress response we observed and publicly available RNAseq data from hippocampal tissue of depressive patients. This cross-species conservation of changes in gene expression of mitochondria-related genes suggests that mitochondrial stress may play a pivotal role in the development of depression. Our findings highlight the potential significance of the hippocampal mitochondrial stress response as a core mechanism underlying the development of depression. Further experimental investigations are required to expand our understanding of these mechanisms.

Multiphoton imaging of hippocampal neural circuits: techniques and biological insights into region-, cell-type-, and pathway-specific functions

Significance

The function of the hippocampus in behavior and cognition has long been studied primarily through electrophysiological recordings from freely moving rodents. However, the application of optical recording methods, particularly multiphoton fluorescence microscopy, in the last decade or two has dramatically advanced our understanding of hippocampal function. This article provides a comprehensive overview of techniques and biological findings obtained from multiphoton imaging of hippocampal neural circuits.

Aim

This review aims to summarize and discuss the recent technical advances in multiphoton imaging of hippocampal neural circuits and the accumulated biological knowledge gained through this technology.

Approach

First, we provide a brief overview of various techniques of multiphoton imaging of the hippocampus and discuss its advantages, drawbacks, and associated key innovations and practices. Then, we review a large body of findings obtained through multiphoton imaging by region (CA1 and dentate gyrus), cell type (pyramidal neurons, inhibitory interneurons, and glial cells), and cellular compartment (dendrite and axon).

Results

Multiphoton imaging of the hippocampus is primarily performed under head-fixed conditions and can reveal detailed mechanisms of circuit operation owing to its high spatial resolution and specificity. As the hippocampus lies deep below the cortex, its imaging requires elaborate methods. These include imaging cannula implantation, microendoscopy, and the use of long-wavelength light sources. Although many studies have focused on the dorsal CA1 pyramidal cells, studies of other local and inter-areal circuitry elements have also helped provide a more comprehensive picture of the information processing performed by the hippocampal circuits. Imaging of circuit function in mouse models of Alzheimer's disease and other brain disorders such as autism spectrum disorder has also contributed greatly to our understanding of their pathophysiology.

Conclusions

Multiphoton imaging has revealed much regarding region-, cell-type-, and pathway-specific mechanisms in hippocampal function and dysfunction in health and disease. Future technological advances will allow further illustration of the operating principle of the hippocampal circuits via the large-scale, high-resolution, multimodal, and minimally invasive imaging.

Mapping the current trends and hotspots of adult hippocampal neurogenesis from 2004-2023: a bibliometric analysis

Objective

We utilized bibliometric and data visualization techniques to discern the primary research domains and emerging frontiers in the field of adult hippocampal neurogenesis (AHN).

Methods

We systematically searched the Web of Science database for AHN-related articles published between 2004 and 2023. The retrieved articles were filtered based on publication types (articles and reviews) and language (English). We employed CiteSpace, VOSviewer, and the online bibliometric platform (bibliometric.com) to visualize and analyze the collected data.

Results

In total, 1,590 AHN-related publications were discovered, exhibiting a steady increase in yearly publications over time. The United States emerged as the leading contributor in AHN research in terms of both publication quantity and national influence. Among all research institutions in the field of AHN, the University of California System exhibited the highest impact. Kempermann, Gerd was the most active author. The publications of the top three active authors primarily focused on the functions of AHN, and reversing hippocampal damage and cognitive impairment by improving AHN. An analysis of reference co-citation clustering revealed 8 distinct research clusters, and the notable ones included "adult hippocampal neurogenesis," "neurogenesis," "hippocampus," "dentate gyrus," "neural stem cell," and "depression." Additionally, a burst keyword detection indicated that 'anxiety' is a current research hotspot in the field of AHN.

Conclusion

This in-depth bibliographic assessment of AHN offers a deeper insight into the present research hotspots in the field. The association between AHN and cognitive diseases, such as Alzheimer's disease (AD) and anxiety, has emerged as a prominent research hotspot.

Modelling adult neurogenesis in the aging rodent hippocampus: a midlife crisis

Contrary to humans, adult hippocampal neurogenesis in rodents is not controversial. And in the last three decades, multiple studies in rodents have deemed adult neurogenesis essential for most hippocampal functions. The functional relevance of new neurons relies on their distinct physiological properties during their maturation before they become indistinguishable from mature granule cells. Most functional studies have used very young animals with robust neurogenesis. However, this trait declines dramatically with age, questioning its functional relevance in aging animals, a caveat that has been mentioned repeatedly, but rarely analyzed quantitatively. In this meta-analysis, we use data from published studies to determine the critical functional window of new neurons and to model their numbers across age in both mice and rats. Our model shows that new neurons with distinct functional profile represent about 3% of the total granule cells in young adult 3-month-old rodents, and their number decline following a power function to reach less than 1% in middle aged animals and less than 0.5% in old mice and rats. These low ratios pose an important logical and computational caveat to the proposed essential role of new neurons in the dentate gyrus, particularly in middle aged and old animals, a factor that needs to be adequately addressed when defining the relevance of adult neurogenesis in hippocampal function.

Exploring memory-related network via dorsal hippocampus suppression

Memory is a complex brain process that requires coordinated activities in a large-scale brain network. However, the relationship between coordinated brain network activities and memory-related behavior is not well understood. In this study, we investigated this issue by suppressing the activity in the dorsal hippocampus (dHP) using chemogenetics and measuring the corresponding changes in brain-wide resting-state functional connectivity (RSFC) and memory behavior in awake rats. We identified an extended brain network contributing to the performance in a spatial-memory related task. Our results were cross-validated using two different chemogenetic actuators, clozapine (CLZ) and clozapine-N-oxide (CNO). This study provides a brain network interpretation of memory performance, indicating that memory is associated with coordinated brain-wide neural activities.

Significance Statement

Successful memory processes require coordinated activity in a large-scale brain network, extending beyond a few key, well-known brain regions like the hippocampus. However, the specific brain regions involved and how they orchestrate their activity that is pertinent to memory processing remain unclear. Our study, using a chemogenetics-rsfMRI- behavior approach in awake rats, elucidates a comprehensive framework of the extended memory-associated network. This knowledge offers a broader interpretation of memory processes, enhancing our understanding of the neural mechanisms behind memory function, particularly from a network perspective.

Memory circuits in dementia: The engram, hippocampal neurogenesis and Alzheimer's disease

Here, we provide an in-depth consideration of our current understanding of engrams, spanning from molecular to network levels, and hippocampal neurogenesis, in health and Alzheimer's disease (AD). This review highlights novel findings in these emerging research fields and future research directions for novel therapeutic avenues for memory failure in dementia. Engrams, memory in AD, and hippocampal neurogenesis have each been extensively studied. The integration of these topics, however, has been relatively less deliberated, and is the focus of this review. We primarily focus on the dentate gyrus (DG) of the hippocampus, which is a key area of episodic memory formation. Episodic memory is significantly impaired in AD, and is also the site of adult hippocampal neurogenesis. Advancements in technology, especially opto- and chemogenetics, have made sophisticated manipulations of engram cells possible. Furthermore, innovative methods have emerged for monitoring neurons, even specific neuronal populations, in vivo while animals engage in tasks, such as calcium imaging. In vivo calcium imaging contributes to a more comprehensive understanding of engram cells. Critically, studies of the engram in the DG using these technologies have shown the important contribution of hippocampal neurogenesis for memory in both health and AD. Together, the discussion of these topics provides a holistic perspective that motivates questions for future research.

Maladaptation of dentate gyrus mossy cells mediates contextual discrimination deficit after traumatic stress

Fear overgeneralization is a maladaptive response to traumatic stress that is associated with the inability to discriminate between threat and safety contexts, a hallmark feature of post-traumatic stress disorder (PTSD). However, the neural mechanisms underlying this deficit remain unclear. Here, we show that traumatic stress exposure impairs contextual discrimination between threat and safety contexts in the learned helplessness (LH) model. Mossy cells (MCs) in the dorsal hippocampus are suppressed in response to traumatic stress. Bidirectional manipulation of MC activity in the LH model reveals that MC inhibition is causally linked to impaired contextual discrimination. Mechanistically, MC inhibition increases the number of active granule cells in a given context, significantly overlapping context-specific ensembles. Our study demonstrates that maladaptive inhibition of MCs after traumatic stress is a substantial mechanism underlying fear overgeneralization with contextual discrimination deficit, suggesting a potential therapeutic target for cognitive symptoms of PTSD.

Dentate gyrus is needed for memory retrieval

The hippocampus is crucial for acquiring and retrieving episodic and contextual memories. In previous studies, the inactivation of dentate gyrus (DG) neurons by chemogenetic- and optogenetic-mediated hyperpolarization led to opposing conclusions about DG's role in memory retrieval. One study used Designer Receptors Exclusively Activated by Designer Drugs (DREADD)-mediated clozapine N-oxide (CNO)-induced hyperpolarization and reported that the previously formed memory was erased, thus concluding that denate gyrus is needed for memory maintenance. The other study used optogenetic with halorhodopsin induced hyperpolarization and reported and dentate gyrus is needed for memory retrieval. We hypothesized that this apparent discrepancy could be due to the length of hyperpolarization in previous studies; minutes by optogenetics and several hours by DREADD/CNO. Since hyperpolarization interferes with anterograde and retrograde neuronal signaling, it is possible that the memory engram in the dentate gyrus and the entorhinal to hippocampus trisynaptic circuit was erased by long-term, but not with short-term hyperpolarization. We developed and applied an advanced chemogenetic technology to selectively silence synaptic output by blocking neurotransmitter release without hyperpolarizing DG neurons to explore this apparent discrepancy. We performed in vivo electrophysiology during trace eyeblink in a rabbit model of associative learning. Our work shows that the DG output is required for memory retrieval. Based on previous and recent findings, we propose that the actively functional anterograde and retrograde neuronal signaling is necessary to preserve synaptic memory engrams along the entorhinal cortex to the hippocampal trisynaptic circuit.

behaviorMate: An Intranet of Things Approach for Adaptable Control of Behavioral and Navigation-Based Experiments

Investigators conducting behavioral experiments often need precise control over the timing of the delivery of stimuli to subjects and to collect the precise times of the subsequent behavioral responses. Furthermore, investigators want fine-tuned control over how various multi-modal cues are presented. behaviorMate takes an "Intranet of Things" approach, using a networked system of hardware and software components for achieving these goals. The system outputs a file with integrated timestamp-event pairs that investigators can then format and process using their own analysis pipelines. We present an overview of the electronic components and GUI application that make up behaviorMate as well as mechanical designs for compatible experimental rigs to provide the reader with the ability to set up their own system. A wide variety of paradigms are supported, including goal-oriented learning, random foraging, and context switching. We demonstrate behaviorMate's utility and reliability with a range of use cases from several published studies and benchmark tests. Finally, we present experimental validation demonstrating different modalities of hippocampal place field studies. Both treadmill with burlap belt and virtual reality with running wheel paradigms were performed to confirm the efficacy and flexibility of the approach. Previous solutions rely on proprietary systems that may have large upfront costs or present frameworks that require customized software to be developed. behaviorMate uses open-source software and a flexible configuration system to mitigate both concerns. behaviorMate has a proven record for head-fixed imaging experiments and could be easily adopted for task control in a variety of experimental situations.

Rbm8a regulates neurogenesis and reduces Alzheimer's disease-associated pathology in the dentate gyrus of 5×FAD mice

Alzheimer's disease is a prevalent and debilitating neurodegenerative condition that profoundly affects a patient's daily functioning with progressive cognitive decline, which can be partly attributed to impaired hippocampal neurogenesis. Neurogenesis in the hippocampal dentate gyrus is likely to persist throughout life but declines with aging, especially in Alzheimer's disease. Recent evidence indicated that RNA-binding protein 8A (Rbm8a) promotes the proliferation of neural progenitor cells, with lower expression levels observed in Alzheimer's disease patients compared with healthy people. This study investigated the hypothesis that Rbm8a overexpression may enhance neurogenesis by promoting the proliferation of neural progenitor cells to improve memory impairment in Alzheimer's disease. Therefore, Rbm8a overexpression was induced in the dentate gyrus of 5×FAD mice to validate this hypothesis. Elevated Rbm8a levels in the dentate gyrus triggered neurogenesis and abated pathological phenotypes (such as plaque formation, gliosis reaction, and dystrophic neurites), leading to ameliorated memory performance in 5×FAD mice. RNA sequencing data further substantiated these findings, showing the enrichment of differentially expressed genes involved in biological processes including neurogenesis, cell proliferation, and amyloid protein formation. In conclusion, overexpressing Rbm8a in the dentate gyrus of 5×FAD mouse brains improved cognitive function by ameliorating amyloid-beta-associated pathological phenotypes and enhancing neurogenesis.

Efficient encoding of aversive location by CA3 long-range projections

Memorizing locations that are harmful or dangerous is a key capability of all organisms and requires an integration of affective and spatial information. In mammals, the dorsal hippocampus mainly processes spatial information, while the intermediate to ventral hippocampal divisions receive affective information via the amygdala. However, how spatial and aversive information is integrated is currently unknown. To address this question, we recorded the activity of hippocampal long-range CA3 axons at single-axon resolution in mice forming an aversive spatial memory. We show that intermediate CA3 to dorsal CA3 (i-dCA3) projections rapidly overrepresent areas preceding the location of an aversive stimulus due to a spatially selective addition of new place-coding axons followed by spatially non-specific stabilization. This sequence significantly improves the encoding of location by the i-dCA3 axon population. These results suggest that i-dCA3 axons transmit a precise, denoised, and stable signal indicating imminent danger to the dorsal hippocampus.

Event Integration and Temporal Differentiation: How Hierarchical Knowledge Emerges in Hippocampal Subfields through Learning

Everyday life is composed of events organized by changes in contexts, with each event containing an unfolding sequence of occurrences. A major challenge facing our memory systems is how to integrate sequential occurrences within events while also maintaining their details and avoiding over-integration across different contexts. We asked if and how distinct hippocampal subfields come to hierarchically and, in parallel, represent both event context and subevent occurrences with learning. Female and male human participants viewed sequential events defined as sequences of objects superimposed on shared color frames while undergoing high-resolution fMRI. Importantly, these events were repeated to induce learning. Event segmentation, as indexed by increased reaction times at event boundaries, was observed in all repetitions. Temporal memory decisions were quicker for items from the same event compared to across different events, indicating that events shaped memory. With learning, hippocampal CA3 multivoxel activation patterns clustered to reflect the event context, with more clustering correlated with behavioral facilitation during event transitions. In contrast, in the dentate gyrus (DG), temporally proximal items that belonged to the same event became associated with more differentiated neural patterns. A computational model explained these results by dynamic inhibition in the DG. Additional similarity measures support the notion that CA3 clustered representations reflect shared voxel populations, while DG's distinct item representations reflect different voxel populations. These findings suggest an interplay between temporal differentiation in the DG and attractor dynamics in CA3. They advance our understanding of how knowledge is structured through integration and separation across time and context.

Effects of electroconvulsive shock on the function, circuitry, and transcriptome of dentate gyrus granule neurons

Therapeutic use of electroconvulsive shock (ECS) is 75% effective for the remission of treatment-resistant depression. Like other more common forms of antidepressant treatment such as fluoxetine, ECS has been shown to increase neurogenesis in the hippocampal dentate gyrus of rodent models. Yet the question of how ECS-induced neurogenesis supports improvement of depressive symptoms remains unknown. Here, we show that ECS-induced neurogenesis is necessary to improve depressive-like behavior of mice exposed to chronic corticosterone (Cort). We then use slice electrophysiology to show that optogenetic stimulation of adult-born neurons produces a greater hyperpolarization in mature granule neurons after ECS vs Sham treatment. We identify that this hyperpolarization requires the activation of metabotropic glutamate receptor 2 (mGluR2). Consistent with this finding, we observe reduced expression of the immediate early gene cFos in the granule cell layer of ECS vs Sham subjects. We then show that mGluR2 knockdown specifically in ventral granule neurons blunts the antidepressant-like behavioral effects of ECS. Using single nucleus RNA sequencing, we reveal major transcriptomic shifts in granule neurons after treatment with ECS+Cort or fluoxetine+Cort vs Cort alone. We identify a population of immature cells which has greater representation in both ECS+Cort and fluoxetine+Cort treated samples vs Cort alone. We also find global differences in ECS-vs fluoxetine-induced transcriptomic shifts. Together, these findings highlight a critical role for immature granule cells and mGluR2 signaling in the antidepressant action of ECS.

Exploring the dynamics of adult Axin2 cell lineage integration into dentate gyrus granule neurons

The Wnt pathway plays critical roles in neurogenesis. The expression of Axin2 is induced by Wnt/β-catenin signaling, making this gene a reliable indicator of canonical Wnt activity. We employed pulse-chase genetic lineage tracing with the Axin2-CreERT2 allele to follow the fate of Axin2+ lineage in the adult hippocampal formation. We found Axin2 expressed in astrocytes, neurons and endothelial cells, as well as in the choroid plexus epithelia. Simultaneously with the induction of Axin2 fate mapping by tamoxifen, we marked the dividing cells with 5-ethynyl-2'-deoxyuridine (EdU). Tamoxifen induction led to a significant increase in labeled dentate gyrus granule cells three months later. However, none of these neurons showed any EdU signal. Conversely, six months after the pulse-chase labeling with tamoxifen/EdU, we identified granule neurons that were positive for both EdU and tdTomato lineage tracer in each animal. Our data indicates that Axin2 is expressed at multiple stages of adult granule neuron differentiation. Furthermore, these findings suggest that the integration process of adult-born neurons from specific cell lineages may require more time than previously thought.

Robust and consistent measures of pattern separation based on information theory and demonstrated in the dentate gyrus

Pattern separation is a valuable computational function performed by neuronal circuits, such as the dentate gyrus, where dissimilarity between inputs is increased, reducing noise and increasing the storage capacity of downstream networks. Pattern separation is studied from both in vivo experimental and computational perspectives and, a number of different measures (such as orthogonalisation, decorrelation, or spike train distance) have been applied to quantify the process of pattern separation. However, these are known to give conclusions that can differ qualitatively depending on the choice of measure and the parameters used to calculate it. We here demonstrate that arbitrarily increasing sparsity, a noticeable feature of dentate granule cell firing and one that is believed to be key to pattern separation, typically leads to improved classical measures for pattern separation even, inappropriately, up to the point where almost all information about the inputs is lost. Standard measures therefore both cannot differentiate between pattern separation and pattern destruction, and give results that may depend on arbitrary parameter choices. We propose that techniques from information theory, in particular mutual information, transfer entropy, and redundancy, should be applied to penalise the potential for lost information (often due to increased sparsity) that is neglected by existing measures. We compare five commonly-used measures of pattern separation with three novel techniques based on information theory, showing that the latter can be applied in a principled way and provide a robust and reliable measure for comparing the pattern separation performance of different neurons and networks. We demonstrate our new measures on detailed compartmental models of individual dentate granule cells and a dentate microcircuit, and show how structural changes associated with epilepsy affect pattern separation performance. We also demonstrate how our measures of pattern separation can predict pattern completion accuracy. Overall, our measures solve a widely acknowledged problem in assessing the pattern separation of neural circuits such as the dentate gyrus, as well as the cerebellum and mushroom body. Finally we provide a publicly available toolbox allowing for easy analysis of pattern separation in spike train ensembles.

Subfield-specific interneuron circuits govern the hippocampal response to novelty in male mice

The hippocampus is the brain's center for episodic memories. Its subregions, the dentate gyrus and CA1-3, are differentially involved in memory encoding and recall. Hippocampal principal cells represent episodic features like movement, space, and context, but less is known about GABAergic interneurons. Here, we performed two-photon calcium imaging of parvalbumin- and somatostatin-expressing interneurons in the dentate gyrus and CA1-3 of male mice exploring virtual environments. Parvalbumin-interneurons increased activity with running-speed and reduced it in novel environments. Somatostatin-interneurons in CA1-3 behaved similar to parvalbumin-expressing cells, but their dentate gyrus counterparts increased activity during rest and in novel environments. Congruently, chemogenetic silencing of dentate parvalbumin-interneurons had prominent effects in familiar contexts, while silencing somatostatin-expressing cells increased similarity of granule cell representations between novel and familiar environments. Our data indicate unique roles for parvalbumin- and somatostatin-positive interneurons in the dentate gyrus that are distinct from those in CA1-3 and may support routing of novel information.

A bird's eye view of the hippocampus beyond space: Behavioral, neuroanatomical, and neuroendocrine perspectives

Although the hippocampus is one of the most-studied brain regions in mammals, research on the avian hippocampus has been more limited in scope. It is generally agreed that the hippocampus is an ancient feature of the amniote brain, and therefore homologous between the two lineages. Because birds and mammals are evolutionarily not very closely related, any shared anatomy is likely to be crucial for shared functions of their hippocampi. These functions, in turn, are likely to be essential if they have been conserved for over 300 million years. Therefore, research on the avian hippocampus can help us understand how this brain region evolved and how it has changed over evolutionary time. Further, there is a strong research foundation in birds on hippocampal-supported behaviors such as spatial navigation, food caching, and brood parasitism that scientists can build upon to better understand how hippocampal anatomy, network circuitry, endocrinology, and physiology can help control these behaviors. In this review, we summarize our current understanding of the avian hippocampus in spatial cognition as well as in regulating anxiety, approach-avoidance behavior, and stress responses. Although there are still some questions about the exact number of subdivisions in the avian hippocampus and how that might vary in different avian families, there is intriguing evidence that the avian hippocampus might have complementary functional profiles along the rostral-caudal axis similar to the dorsal-ventral axis of the rodent hippocampus, where the rostral/dorsal hippocampus is more involved in cognitive processes like spatial learning and the caudal/ventral hippocampus regulates emotional states, anxiety, and the stress response. Future research should focus on elucidating the cellular and molecular mechanisms - including endocrinological - in the avian hippocampus that underlie behaviors such as spatial navigation, spatial memory, and anxiety-related behaviors, and in so doing, resolve outstanding questions about avian hippocampal function and organization.

Adult Neurogenesis, Context Encoding, and Pattern Separation: A Pathway for Treating Overgeneralization

In mammals, the subgranular zone of the dentate gyrus is one of two brain regions (with the subventricular zone of the olfactory bulb) that continues to generate new neurons throughout adulthood, a phenomenon known as adult hippocampal neurogenesis (AHN) (Eriksson et al., Nat Med 4:1313-1317, 1998; García-Verdugo et al., J Neurobiol 36:234-248, 1998). The integration of these new neurons into the dentate gyrus (DG) has implications for memory encoding, with unique firing and wiring properties of immature neurons that affect how the hippocampal network encodes and stores attributes of memory. In this chapter, we will describe the process of AHN and properties of adult-born cells as they integrate into the hippocampal circuit and mature. Then, we will discuss some methodological considerations before we review evidence for the role of AHN in two major processes supporting memory that are performed by the DG. First, we will discuss encoding of contextual information for episodic memories and how this is facilitated by AHN. Second, will discuss pattern separation, a major role of the DG that reduces interference for the formation of new memories. Finally, we will review clinical and translational considerations, suggesting that stimulation of AHN may help decrease overgeneralization-a common endophenotype of mood, anxiety, trauma-related, and age-related disorders.

Adult-born granule cells facilitate remapping of spatial and non-spatial representations in the dentate gyrus

Adult-born granule cells (abGCs) have been implicated in memory discrimination through a neural computation known as pattern separation. Here, using in vivo Ca2+ imaging, we examined how chronic ablation or acute chemogenetic silencing of abGCs affects the activity of mature granule cells (mGCs). In both cases, we observed altered remapping of mGCs. Rather than broadly modulating the activity of all mGCs, abGCs promote the remapping of place cells' firing fields while increasing rate remapping of mGCs that represent sensory cues. In turn, these remapping deficits are associated with behavioral impairments in animals' ability to correctly identify new goal locations. Thus, abGCs facilitate pattern separation through the formation of non-overlapping representations for identical sensory cues encountered in different locations. In the absence of abGCs, the dentate gyrus shifts to a state that is dominated by cue information, a situation that is consistent with the overgeneralization often observed in anxiety or age-related disorders.

Direct cortical inputs to hippocampal area CA1 transmit complementary signals for goal-directed navigation

The subregions of the entorhinal cortex (EC) are conventionally thought to compute dichotomous representations for spatial processing, with the medial EC (MEC) providing a global spatial map and the lateral EC (LEC) encoding specific sensory details of experience. Yet, little is known about the specific types of information EC transmits downstream to the hippocampus. Here, we exploit in vivo sub-cellular imaging to record from EC axons in CA1 while mice perform navigational tasks in virtual reality (VR). We uncover distinct yet overlapping representations of task, location, and context in both MEC and LEC axons. MEC transmitted highly location- and context-specific codes; LEC inputs were biased by ongoing navigational goals. However, during tasks with reliable reward locations, the animals' position could be accurately decoded from either subregion. Our results revise the prevailing dogma about EC information processing, revealing novel ways spatial and non-spatial information is routed and combined upstream of the hippocampus.

Dynamics of Adult Axin2 Cell Lineage Integration in Granule Neurons of the Dentate Gyrus

The Wnt pathway plays critical roles in neurogenesis. The expression of Axin2 is induced by Wnt/β-catenin signaling, making this gene a sensitive indicator of canonical Wnt activity. We employed pulse-chase genetic lineage tracing with the Axin2-CreERT2 allele to follow the fate of Axin2 -positive cells in the adult hippocampal formation. We found Axin2 expressed in astrocytes, neurons and endothelial cells, as well as in the choroid plexus epithelia. Simultaneously with tamoxifen induction of Axin2 fate mapping, the dividing cells were marked with 5-ethynyl-2'-deoxyuridine (EdU). Tamoxifen induction resulted in significant increase of dentate gyrus granule cells three months later; however, none of these neurons contained EdU signal. Conversely, six months after the tamoxifen/EdU pulse-chase labeling, EdU-positive granule neurons were identified in each animal. Our data imply that Axin2 is expressed at several different stages of adult granule neuron differentiation and suggest that the process of integration of the adult-born neurons from certain cell lineages may take longer than previously thought.

Sharpening the blades of the dentate gyrus: how adult-born neurons differentially modulate diverse aspects of hippocampal learning and memory

For decades, the mammalian hippocampus has been the focus of cellular, anatomical, behavioral, and computational studies aimed at understanding the fundamental mechanisms underlying cognition. Long recognized as the brain's seat for learning and memory, a wealth of knowledge has been accumulated on how the hippocampus processes sensory input, builds complex associations between objects, events, and space, and stores this information in the form of memories to be retrieved later in life. However, despite major efforts, our understanding of hippocampal cognitive function remains fragmentary, and models trying to explain it are continually revisited. Here, we review the literature across all above-mentioned domains and offer a new perspective by bringing attention to the most distinctive, and generally neglected, feature of the mammalian hippocampal formation, namely, the structural separability of the two blades of the dentate gyrus into "supra-pyramidal" and "infra-pyramidal". Next, we discuss recent reports supporting differential effects of adult neurogenesis in the regulation of mature granule cell activity in these two blades. We propose a model for how differences in connectivity and adult neurogenesis in the two blades can potentially provide a substrate for subtly different cognitive functions.

The Effects of Stress on Hippocampal Neurogenesis and Behavior in the Absence of Lipocalin-2

Lipocalin-2 (LCN2) is an acute phase protein able to bind iron when complexed with bacterial siderophores. The recent identification of a mammalian siderophore also suggested a physiological role for LCN2 in the regulation of iron levels and redox state. In the central nervous system, the deletion of LCN2 induces deficits in neural stem cells proliferation and commitment, with an impact on the hippocampal-dependent contextual fear discriminative task. Additionally, stress is a well-known regulator of cell genesis and is known to decrease adult hippocampal cell proliferation and neurogenesis. Although voluntary running, another well-known regulator of neurogenesis, is sufficient to rescue the defective hippocampal neurogenesis and behavior in LCN2-null mice by promoting stem cells' cell cycle progression and maturation, the relevance of LCN2-regulated hippocampal neurogenesis in response to stress has never been explored. Here, we show a lack of response by LCN2-null mice to the effects of chronic stress exposure at the cellular and behavioral levels. Together, these findings implicate LCN2 as a relevant mediator of neuronal plasticity and brain function in the adult mammalian brain.

Unique potential of immature adult-born neurons for the remodeling of CA3 spatial maps

Mammalian hippocampal circuits undergo extensive remodeling through adult neurogenesis. While this process has been widely studied, the specific contribution of adult-born granule cells (aGCs) to spatial operations in the hippocampus remains unknown. Here, we show that optogenetic activation of 4-week-old (young) aGCs in free-foraging mice produces a non-reversible reconfiguration of spatial maps in proximal CA3 while rarely evoking neural activity. Stimulation of the same neuronal cohort on subsequent days recruits CA3 neurons with increased efficacy but fails to induce further remapping. In contrast, stimulation of 8-week-old (mature) aGCs can reliably activate CA3 cells but produces no alterations in spatial maps. Our results reveal a unique role of young aGCs in remodeling CA3 representations, a potential that can be depleted and is lost with maturation. This ability could contribute to generate orthogonalized downstream codes supporting pattern separation.

Delayed formation of neural representations of space in aged mice

Aging is associated with cognitive deficits, with spatial memory being very susceptible to decline. The hippocampal dentate gyrus (DG) is important for processing spatial information in the brain and is particularly vulnerable to aging, yet its sparse activity has led to difficulties in assessing changes in this area. Using in vivo two-photon calcium imaging, we compared DG neuronal activity and representations of space in young and aged mice walking on an unfamiliar treadmill. We found that calcium activity was significantly higher and less tuned to location in aged mice, resulting in decreased spatial information encoded in the DG. However, with repeated exposure to the same treadmill, both spatial tuning and information levels in aged mice became similar to young mice, while activity remained elevated. Our results show that spatial representations of novel environments are impaired in the aged hippocampus and gradually improve with increased familiarity. Moreover, while the aged DG is hyperexcitable, this does not disrupt neural representations of familiar environments.

Daily Consumption of High-Polyphenol Olive Oil Enhances Hippocampal Neurogenesis in Old Female Rats

OBJECTIVE

The aim of this study is to evaluate the effect of daily consumption of high-polyphenol (HP) olive oil on neurogenesis by investigating neuronal cell proliferation and maturation in the hippocampus of old rats, and to evaluate the relationship between neurogenesis, spatial memory, and anxiety-like behavior.

METHODS

A total of 34 female, 20-22-month-old Sprague Dawley rats were divided into three groups: control group, low-polyphenol (LP) group, and high-polyphenol (HP) group. The animals were fed distilled water, LP olive oil and HP-extra virgin olive oil, respectively for 6 weeks using an oral gavage. At 43 days, animals were tested using the Morris Water Maze to evaluate spatial memory, and the Open-field test to evaluate anxiety-like behavior. Neural cell proliferation in the dentate gyrus (DG) was determined by BrdU labeling and Nestin protein expression. Neuronal maturation was determined by NeuN labeling. Synaptic density in the hippocampus and prefrontal cortex was examined by measuring Synaptophysin (SYN) levels. Hippocampal Calbindin levels were measured to assess cellular calcium metabolism.

RESULTS

Daily consumption of HP olive oil significantly improved cell proliferation and neuronal maturation in the DG of old rats. HP-olive oil significantly increased SYN levels in the prefrontal cortex, and nestin and calbindin levels in the hippocampus (p < 0.05). LP olive oil diet has shown no effect on any parameter (p > 0.05). We also did not find any statistically significant difference between the groups in terms of spatial memory and anxiety-like behavior (p > 0.05).

CONCLUSION

Our study is first to show that daily consumption of HP-olive oil enhances hippocampal neurogenesis in old rats, which has been confirmed by proliferation and maturation biomarkers. In addition, increased SYN and calbindin levels showed that the generated cells were also functionally developed in the HP group. We suggest that daily consumption of HP olive oil may have beneficial effects on brain aging by triggering neurogenesis.

Lactate Improves Long-term Cognitive Impairment Induced By Repeated Neonatal Sevoflurane Exposures Through SIRT1-mediated Regulation of Adult Hippocampal Neurogenesis and Synaptic Plasticity in Male Mice

Repeated neonatal exposures to sevoflurane induce long-term cognitive impairment that has been reported to have sex-dependent differences. Exercise promotes learning and memory by releasing lactate from the muscle. The study tested the hypothesis that lactate may improve long-term cognitive impairment induced by repeated neonatal exposures to sevoflurane through SIRT1-mediated regulation of adult hippocampal neurogenesis and synaptic plasticity. C57BL/6 mice of both genders were exposed to 3% sevoflurane for 2 h daily from postnatal day 6 (P6) to P8. In the intervention experiments, mice received lactate at 1 g/kg intraperitoneally once daily from P21 to P41. Behavioral tests including open field (OF), object location (OL), novel object recognition (NOR), and fear conditioning (FC) tests were performed to assess cognitive function. The number of 5-Bromo-2'- deoxyuridine positive (BrdU+) cells and BrdU+/DCX+ (doublecortin) co-labeled cells, expressions of brain-derived neurotrophic factor (BDNF), activity-regulated cytoskeletal-associated protein (Arc), early growth response 1 (Egr-1), SIRT1, PGC-1α and FNDC5, and long-term potentiation (LTP) were evaluated in the hippocampus. Repeated exposures to sevoflurane induced deficits in OL, NOR and contextual FC tests in male but not female mice. Similarly, adult hippocampal neurogenesis, synaptic plasticity-related proteins and hippocampal LTP were impaired after repeated exposures to sevoflurane in male but not female mice, which could rescue by lactate treatment. Our study suggests that repeated neonatal exposures to sevoflurane inhibit adult hippocampal neurogenesis and induce defects of synaptic plasticity in male but not female mice, which may contribute to long-term cognitive impairment. Lactate treatment rescues these abnormalities through activation of SIRT1.

Adult-born granule cells improve stimulus encoding and discrimination in the dentate gyrus

Heterogeneity plays an important role in diversifying neural responses to support brain function. Adult neurogenesis provides the dentate gyrus with a heterogeneous population of granule cells (GCs) that were born and developed their properties at different times. Immature GCs have distinct intrinsic and synaptic properties than mature GCs and are needed for correct encoding and discrimination in spatial tasks. How immature GCs enhance the encoding of information to support these functions is not well understood. Here, we record the responses to fluctuating current injections of GCs of different ages in mouse hippocampal slices to study how they encode stimuli. Immature GCs produce unreliable responses compared to mature GCs, exhibiting imprecise spike timings across repeated stimulation. We use a statistical model to describe the stimulus-response transformation performed by GCs of different ages. We fit this model to the data and obtain parameters that capture GCs' encoding properties. Parameter values from this fit reflect the maturational differences of the population and indicate that immature GCs perform a differential encoding of stimuli. To study how this age heterogeneity influences encoding by a population, we perform stimulus decoding using populations that contain GCs of different ages. We find that, despite their individual unreliability, immature GCs enhance the fidelity of the signal encoded by the population and improve the discrimination of similar time-dependent stimuli. Thus, the observed heterogeneity confers the population with enhanced encoding capabilities.

Assessments of dentate gyrus function: discoveries and debates

There has been considerable speculation regarding the function of the dentate gyrus (DG) - a subregion of the mammalian hippocampus - in learning and memory. In this Perspective article, we compare leading theories of DG function. We note that these theories all critically rely on the generation of distinct patterns of activity in the region to signal differences between experiences and to reduce interference between memories. However, these theories are divided by the roles they attribute to the DG during learning and recall and by the contributions they ascribe to specific inputs or cell types within the DG. These differences influence the information that the DG is thought to impart to downstream structures. We work towards a holistic view of the role of DG in learning and memory by first developing three critical questions to foster a dialogue between the leading theories. We then evaluate the extent to which previous studies address our questions, highlight remaining areas of conflict, and suggest future experiments to bridge these theories.

Targeting aberrant dendritic integration to treat cognitive comorbidities of epilepsy

Memory deficits are a debilitating symptom of epilepsy, but little is known about mechanisms underlying cognitive deficits. Here, we describe a Na+ channel-dependent mechanism underlying altered hippocampal dendritic integration, degraded place coding and deficits in spatial memory. Two-photon glutamate uncaging experiments revealed a marked increase in the fraction of hippocampal first-order CA1 pyramidal cell dendrites capable of generating dendritic spikes in the kainate model of chronic epilepsy. Moreover, in epileptic mice dendritic spikes were generated with lower input synchrony, and with a lower threshold. The Nav1.3/1.1 selective Na+ channel blocker ICA-121431 reversed dendritic hyperexcitability in epileptic mice, while the Nav1.2/1.6 preferring anticonvulsant S-Lic did not. We used in vivo two-photon imaging to determine if aberrant dendritic excitability is associated with altered place-related firing of CA1 neurons. We show that ICA-121431 improves degraded hippocampal spatial representations in epileptic mice. Finally, behavioural experiments show that reversing aberrant dendritic excitability with ICA-121431 reverses hippocampal memory deficits. Thus, a dendritic channelopathy may underlie cognitive deficits in epilepsy and targeting it pharmacologically may constitute a new avenue to enhance cognition.

Anatomical organization of temporally correlated neural calcium activity in the hippocampal CA1 region

Hippocampal CA1 neuronal ensembles generate sequential patterns of firing activity that contribute to episodic memory formation and spatial cognition. Here we used in vivo calcium imaging to record neural ensemble activities in mouse hippocampal CA1 and identified CA1 excitatory neuron sub-populations whose members are active across the same second-long period of time. We identified groups of hippocampal neurons sharing temporally correlated neural calcium activity during behavioral exploration and found that they also organized as clusters in anatomical space. Such clusters vary in membership and activity dynamics with respect to movement in different environments, but also appear during immobility in the dark suggesting an internal dynamic. The strong covariance between dynamics and anatomical location within the CA1 sub-region reveals a previously unrecognized form of topographic representation in hippocampus that may guide generation of hippocampal sequences across time and therefore organize the content of episodic memory.

Global remapping in granule cells and mossy cells of the mouse dentate gyrus

Hippocampal place cells exhibit spatially modulated firing, or place fields, which can remap to encode changes in the environment or other variables. Unique among hippocampal subregions, the dentate gyrus (DG) has two excitatory populations of place cells, granule cells and mossy cells, which are among the least and most active spatially modulated cells in the hippocampus, respectively. Previous studies of remapping in the DG have drawn different conclusions about whether granule cells exhibit global remapping and contribute to the encoding of context specificity. By recording granule cells and mossy cells as mice foraged in different environments, we found that by most measures, both granule cells and mossy cells remapped robustly but through different mechanisms that are consistent with firing properties of each cell type. Our results resolve the ambiguity surrounding remapping in the DG and suggest that most spatially modulated granule cells contribute to orthogonal representations of distinct spatial contexts.

Sub-cellular population imaging tools reveal stable apical dendrites in hippocampal area CA3

Anatomically segregated apical and basal dendrites of pyramidal neurons receive functionally distinct inputs, but it is unknown if this results in compartment-level functional diversity during behavior. Here we imaged calcium signals from apical dendrites, soma, and basal dendrites of pyramidal neurons in area CA3 of mouse hippocampus during head-fixed navigation. To examine dendritic population activity, we developed computational tools to identify dendritic regions of interest and extract accurate fluorescence traces. We identified robust spatial tuning in apical and basal dendrites, similar to soma, though basal dendrites had reduced activity rates and place field widths. Across days, apical dendrites were more stable than soma or basal dendrites, resulting in better decoding of the animal's position. These population-level dendritic differences may reflect functionally distinct input streams leading to different dendritic computations in CA3. These tools will facilitate future studies of signal transformations between cellular compartments and their relation to behavior.

Activation of hypothalamic-enhanced adult-born neurons restores cognitive and affective function in Alzheimer's disease

Patients with Alzheimer's disease (AD) exhibit progressive memory loss, depression, and anxiety, accompanied by impaired adult hippocampal neurogenesis (AHN). Whether AHN can be enhanced in impaired AD brain to restore cognitive and affective function remains elusive. Here, we report that patterned optogenetic stimulation of the hypothalamic supramammillary nucleus (SuM) enhances AHN in two distinct AD mouse models, 5×FAD and 3×Tg-AD. Strikingly, the chemogenetic activation of SuM-enhanced adult-born neurons (ABNs) rescues memory and emotion deficits in these AD mice. By contrast, SuM stimulation alone or activation of ABNs without SuM modification fails to restore behavioral deficits. Furthermore, quantitative phosphoproteomics analyses reveal activation of the canonical pathways related to synaptic plasticity and microglia phagocytosis of plaques following acute chemogenetic activation of SuM-enhanced (vs. control) ABNs. Our study establishes the activity-dependent contribution of SuM-enhanced ABNs in modulating AD-related deficits and informs signaling mechanisms mediated by the activation of SuM-enhanced ABNs.

Neuronal activity-related transcription is blunted in immature compared to mature dentate granule cells

Immature dentate granule cells (DGCs) generated in the hippocampus during adulthood are believed to play a unique role in dentate gyrus (DG) function. Although immature DGCs have hyperexcitable membrane properties in vitro, the consequences of this hyperexcitability in vivo remain unclear. In particular, the relationship between experiences that activate the DG, such as exploration of a novel environment (NE), and downstream molecular processes that modify DG circuitry in response to cellular activation is unknown in this cell population. We first performed quantification of immediate early gene (IEG) proteins in immature (5-week-old) and mature (13-week-old) DGCs from mice exposed to a NE. Paradoxically, we observed lower IEG protein expression in hyperexcitable immature DGCs. We then isolated nuclei from active and inactive immature DGCs and performed single-nuclei RNA-Sequencing. Compared to mature nuclei collected from the same animal, immature DGC nuclei showed less activity-induced transcriptional change, even though they were classified as active based on expression of ARC protein. These results demonstrate that the coupling of spatial exploration, cellular activation, and transcriptional change differs between immature and mature DGCs, with blunted activity-induced changes in immature cells.

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.

Delayed formation of neural representations of space in aged mice

Aging is associated with cognitive deficits, with spatial memory being very susceptible to decline. The hippocampal dentate gyrus (DG) is important for processing spatial information in the brain and is particularly vulnerable to aging, yet its sparse activity has led to difficulties in assessing changes in this area. Using in vivo two-photon calcium imaging, we compared DG neuronal activity and representations of space in young and aged mice walking on an unfamiliar treadmill. We found that calcium activity was significantly higher and less tuned to location in aged mice, resulting in decreased spatial information encoded in the DG. However, with repeated exposure to the same treadmill, both spatial tuning and information levels in aged mice became similar to young mice, while activity remained elevated. Our results show that spatial representations of novel environments are impaired in the aged hippocampus and gradually improve with increased familiarity. Moreover, while the aged DG is hyperexcitable, this does not disrupt neural representations of familiar environments.

Insight into the role of adult hippocampal neurogenesis in aging and Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia and seriously affects the quality of life of the elderly. Neurodegeneration is closely related to hippocampal dysfunction in AD patients. The hippocampus is key to creating new memories and is also one of the first areas of the brain to deteriorate with age. Mammalian neurogenesis occurs mainly in the hippocampus. Recent studies have confirmed that neurogenesis in the hippocampus is sustainable but decreases with age, which seriously affects the learning and memory function of AD patients. At present, our understanding of neurogenesis is still relatively shallow, especially pertaining to the influence and role of neurogenesis during aging and cognitive deficits in AD patients. Interestingly, many recent studies have described the characteristics of neurogenesis in animal models. This article reviews the progress of neurogenesis research in the context of aging and AD to provide new insights into neurogenesis.

Adult-born neurons inhibit developmentally-born neurons during spatial learning

Ongoing neurogenesis in the dentate gyrus (DG) subregion of the hippocampus results in a heterogenous population of neurons. Immature adult-born neurons (ABNs) have physiological and anatomical properties that may give them a unique role in learning. For example, compared to older granule neurons, they have greater somatic excitability, which could facilitate their recruitment into memory traces. However, recruitment is also likely to depend on interactions with other DG neurons through processes such as lateral inhibition. Immature ABNs target inhibitory interneurons and, compared to older neurons, they receive less GABAergic inhibition. Thus, they may induce lateral inhibition of mature DG neurons while being less susceptible to inhibition themselves. To test this we used a chemogenetic approach to silence immature ABNs as rats learned a spatial water maze task, and measured activity (Fos expression) in ABNs and developmentally-born neurons (DBNs). A retrovirus expressing the inhibitory DREADD receptor, hM4Di, was injected into the dorsal DG of male rats at 6w to infect neurons born in adulthood. Animals were also injected with BrdU to label DBNs or ABNs. DBNs were significantly more active than immature 4-week-old ABNs. Silencing 4-week-old ABNs did not alter learning but it increased activity in DBNs. However, silencing ABNs did not affect activation in other ABNs within the DG. Silencing ABNs also did not alter Fos expression in parvalbumin- and somatostatin-expressing interneurons. Collectively, these results suggest that ABNs may directly inhibit DBN activity during hippocampal-dependent learning, which may be relevant for maintaining sparse hippocampal representations of experienced events.

Blockade of D-serine signaling and adult hippocampal neurogenesis attenuates remote contextual fear memory following multiple memory retrievals in male mice

The retrieval of fear memories induces two opposing processes, reconsolidation, and extinction. The memory reconsolidation is an active process that involves gene expression and updates an existing memory. It is hypothesized that blockade of reconsolidation by manipulating the neurobiological factors, which are mechanistically involved in the process, could weaken or disrupt the original fear memory. The N-methyl-D-aspartate (NMDA) receptor and hippocampal neurogenesis play crucial roles in hippocampus-dependent memory processes, including reconsolidation. Using contextual fear conditioning paradigm with multiple retrievals, we attempted to weaken the original contextual fear memory by repeatedly disrupting retrieval-induced reconsolidation via downregulation of NMDA receptor signaling and inhibition of neurogenesis. In the first experiment, prior to fear conditioning, NMDA receptor signaling was downregulated by the genetic reduction of its co-agonist, D-serine, and the neurogenesis was dampened by focal X-ray irradiation on the hippocampus. We found that simultaneous D-serine reduction and neurogenesis dampening resulted in a progressive decrease in freezing following each retrieval, leading to an attenuation of remote contextual fear memory on day 28. In the second experiment using the same behavioral protocols, after conditioning, pharmacological approaches were conducted to simultaneously block D-serine signaling and neurogenesis, resulting in a similar suppressive effect on the remote fear memory. The present findings provide insights for understanding the role of D-serine-mediated NMDA receptor signaling and neurogenesis in memory retrieval and the maintenance of remote fear memory, and improving the efficacy of exposure-based therapy for the treatment of post-traumatic stress disorder (PTSD).

Seizure-induced hilar ectopic granule cells in the adult dentate gyrus

Epilepsy is a chronic neurological disorder characterized by hypersynchronous spontaneous recurrent seizures, and affects approximately 50 million people worldwide. Cumulative evidence has revealed that epileptogenic insult temporarily increases neurogenesis in the hippocampus; however, a fraction of the newly generated neurons are integrated abnormally into the existing neural circuits. The abnormal neurogenesis, including ectopic localization of newborn neurons in the hilus, formation of abnormal basal dendrites, and disorganization of the apical dendrites, rewires hippocampal neural networks and leads to the development of spontaneous seizures. The central roles of hilar ectopic granule cells in regulating hippocampal excitability have been suggested. In this review, we introduce recent findings about the migration of newborn granule cells to the dentate hilus after seizures and the roles of seizure-induced ectopic granule cells in the epileptic brain. In addition, we delineate possible intrinsic and extrinsic mechanisms underlying this abnormality. Finally, we suggest that the regulation of seizure-induced ectopic cells can be a promising target for epilepsy therapy and provide perspectives on future research directions.

Adult-born neurons add flexibility to hippocampal memories

Although most neurons are generated embryonically, neurogenesis is maintained at low rates in specific brain areas throughout adulthood, including the dentate gyrus of the mammalian hippocampus. Episodic-like memories encoded in the hippocampus require the dentate gyrus to decorrelate similar experiences by generating distinct neuronal representations from overlapping inputs (pattern separation). Adult-born neurons integrating into the dentate gyrus circuit compete with resident mature cells for neuronal inputs and outputs, and recruit inhibitory circuits to limit hippocampal activity. They display transient hyperexcitability and hyperplasticity during maturation, making them more likely to be recruited by any given experience. Behavioral evidence suggests that adult-born neurons support pattern separation in the rodent dentate gyrus during encoding, and they have been proposed to provide a temporal stamp to memories encoded in close succession. The constant addition of neurons gradually degrades old connections, promoting generalization and ultimately forgetting of remote memories in the hippocampus. This makes space for new memories, preventing saturation and interference. Overall, a small population of adult-born neurons appears to make a unique contribution to hippocampal information encoding and removal. Although several inconsistencies regarding the functional relevance of neurogenesis remain, in this review we argue that immature neurons confer a unique form of transience on the dentate gyrus that complements synaptic plasticity to help animals flexibly adapt to changing environments.

Astrocytes Mediate Cholinergic Regulation of Adult Hippocampal Neurogenesis and Memory Through M1 Muscarinic Receptor

BACKGROUND

In the neurogenic niches of the adult hippocampus, new functional neurons are continuously generated throughout life, and generation of these neurons has been implicated in learning and memory. Astrocytes, as components of the neurogenic niches, are critical in the regulation of adult hippocampal neurogenesis (AHN). However, little is known about how astrocytes receive and respond to extrinsic cues to regulate AHN.

METHODS

By using a transgenic strategy to conditionally delete astrocytic CRHM1 in mice and AAV (adeno-associated virus)-mediated overexpression of astrocytic CHRM1 specifically in the hippocampal dentate gyrus, we systematically investigated the role of astrocytic CHRM1 in the regulation of AHN and the underlying mechanisms using the combined approaches of immunohistochemistry, retrovirus labeling, electrophysiology, primary astrocyte cultures, immunoblotting, and behavioral assays.

RESULTS

We report that genetic ablation of CHRM1 in astrocytes led to defects in neural stem cell survival, neuronal differentiation, and maturation and integration of newborn neurons in the dentate gyrus. Astrocytic CHRM1-mediated modulation of AHN was mediated by BDNF (brain-derived neurotrophic factor) signaling. Furthermore, CHRM1 ablation in astrocytes impaired contextual fear memory. These impairments in both AHN and memory were rescued by overexpression of astrocytic CHRM1 in the dentate gyrus.

CONCLUSIONS

Our findings reveal a critical role for astrocytes in mediating cholinergic regulation of AHN and memory through CHRM1.

Adult-born dentate granule cells promote hippocampal population sparsity

The dentate gyrus (DG) gates neocortical information flow to the hippocampus. Intriguingly, the DG also produces adult-born dentate granule cells (abDGCs) throughout the lifespan, but their contribution to downstream firing dynamics remains unclear. Here, we show that abDGCs promote sparser hippocampal population spiking during mnemonic processing of novel stimuli. By combining triple-(DG-CA3-CA1) ensemble recordings and optogenetic interventions in behaving mice, we show that abDGCs constitute a subset of high-firing-rate neurons with enhanced activity responses to novelty and strong modulation by theta oscillations. Selectively activating abDGCs in their 4-7-week post-birth period increases sparsity of hippocampal population patterns, whereas suppressing abDGCs reduces this sparsity, increases principal cell firing rates and impairs novel object recognition with reduced dimensionality of the network firing structure, without affecting single-neuron spatial representations. We propose that adult-born granule cells transiently support sparser hippocampal population activity structure for higher-dimensional responses relevant to effective mnemonic information processing.

Hemisphere-specific spatial representation by hippocampal granule cells

The dentate gyrus (DG) output plays a key role in the emergence of spatial and contextual map representation within the hippocampus during learning. Differences in neuronal network activity have been observed between left and right CA1-3 areas, implying lateralization in spatial coding properties. Whether bilateral differences of DG granule cell (GC) assemblies encoding spatial and contextual information exist remains largely unexplored. Here, we employed two-photon calcium imaging of the left or the right DG to record the activity of GC populations over five consecutive days in head-fixed mice navigating through familiar and novel virtual environments. Imaging revealed similar mean GC activity on both sides. However, spatial tuning, context-selectivity and run-to-run place field reliability was markedly higher for DG place cells in the left than the right hemisphere. Moreover, the proportion of GCs reconfiguring their place fields between contexts was greater in the left DG. Thus, our data suggest that contextual information is differentially processed by GC populations depending on the hemisphere, with higher context discrimination in the left but a bias towards generalization in the right DG.