Ventral hippocampal lesions affect anxiety but not spatial learning

Hippocampal Synaptic Plasticity: Integrating Memory and Anxiety Impairments in the Early Stages of Alzheimer's Disease

A decline in hippocampal function has long been associated with the progression of cognitive impairments in patients with Alzheimer's disease (AD). The disruption of hippocampal synaptic plasticity [primarily the reduction of long-term potentiation LTP] by excess production of soluble beta-amyloid (Aβ) has long been accepted as the mechanism by which AD pathology impairs memory, at least during the early stages of AD pathogenesis. However, the premise that hippocampal LTP underpins the formation of associative, long-term memories has been challenged. Here, we consider evidence that this canonical view of LTP needs to be refined. Similarly, the view that the hippocampus simply supports memory ignores the wealth of data showing that the hippocampus is functionally heterogeneous along its septo-temporal axis. The ventral (but not the dorsal) hippocampus plays a major role in modulating emotional reactions to conflict. Here, we suggest that hippocampal LTP is not involved in forming long-term associative memories, but instead contributes to the disambiguation of overlapping memories in situations of conflict and associative interference. This conceptualisation of hippocampal synaptic plasticity may help explain how early-stage AD pathology may impact both memory and anxiety.

Models of human hippocampal specialization: a look at the electrophysiological evidence

From an anatomical perspective, the concept that the anterior and posterior hippocampus fulfill distinct cognitive roles may seem unsurprising. When compared with the posterior hippocampus, the anterior region is proportionally larger, with visible expansion of the CA1 subfield and intimate continuity with adjacent medial temporal lobe (MTL) structures such as the uncus and amygdala. However, the functional relevance emerging from these anatomical differences remains to be established in humans. Drawing on both rodent and human data, several models of hippocampal longitudinal specialization have been proposed. For the brevity and clarity of this review, we focus on human electrophysiological evidence supporting and contravening these models with limited inclusion of noninvasive data. We then synthesize these data to propose a novel longitudinal model based on the amount of contextual information, drawing on previous conceptions described within the past decade.

Contribution of prefrontal cortex and ventral hippocampus to anxiety in young epileptic mice

Children with epilepsy are particularly vulnerable to anxiety disorders, where these disorders are frequently underdiagnosed and untreated. Despite the high prevalence of anxiety in epilepsy, the underlying neurobiological mechanisms are not fully understood. The medial prefrontal cortex (mPFC) and ventral hippocampus (vHPC) are key brain regions implicated in the genesis and modulation of anxiety, and their interactions play a crucial role in emotional processing including anxiety. We utilized a pilocarpine-induced epilepsy model in young mice (7 weeks old) to assess anxiety-like behaviors using the open field test (OFT), light/dark box, and elevated plus maze (EPM). Local field potential (LFP) recordings were conducted to examine theta power and coherence between the mPFC and vHPC. LFP recordings revealed significantly altered theta power variation in both the mPFC and vHPC during exposure to anxiogenic contexts, suggesting the involvement of these regions in anxiety in the young epileptic mice. Notably, theta-frequency synchrony between the mPFC and vHPC was not significantly altered in the young epileptic mice, indicating that altered theta power rather than inter-regional synchrony may underlie anxiety behaviors in young epileptic mice. Furthermore, we demonstrated that chemogenetic inhibition of excitatory neurons in the mPFC and vHPC reduced anxiety levels in young epileptic mice. Altogether, our findings highlight the critical contributions of mPFC and vHPC to the pathogenesis of comorbid anxiety in epilepsy. These findings underscore the potential therapeutic significance of modulating the activity in these two regions as means to alleviate anxiety in a youth epilepsy population.

Astrocytes contribute to the functional differentiation of the hippocampal longitudinal axis during reward and aversion processing in the adult male rat

This study aims to investigate whether glial cells, in particular putative astrocytes, contribute to functional distinctions between the dorsal (DH), intermediate (IH), and ventral (VH) hippocampus. To evaluate this, we performed three different behavioral tasks (i.e., Morris water maze; MWM, Passive avoidance; PA, T-maze place preference; TPP) to determine whether the DH, IH, and VH are necessary for each task. Sensitivity of behavioral tasks was confirmed using lidocaine (2 %, 1 μl) reversible inactivation. Subsequently, we examined the effects of silencing astrocytes, using fluorocitrate (FC, 1 mM/1 μl), into the DH, IH, and VH on these tasks. The effects of drugs were examined separately. We observed that injection of FC into the DH resulted in a significant impairment in MWM performance. In contrast, while FC injections into the IH or VH did not prevent platform localization during the acquisition phase, rats showed difficulty recalling the target zone during the retrieval phase. In the PA test, FC injection into the VH impaired task learning and memory. During the acquisition phase, FC injection into the DH or IH did not differ from the control in the number of shocks; however, during retrieval, there was a significant decrease in the latency before entering the dark chamber. The TPP test performance was impaired by FC injection in the IH. In sum, we show that glial cells, especially astrocytes in specific functional regions of the hippocampus, play distinct roles in processing aversive and rewarding experiences and contribute to the functional organization of the hippocampal longitudinal axis.

Hippocampal contextualization of social rewards in mice

Acquiring and exploiting memories of rewarding experiences is critical for survival. The spatial environment in which a rewarding stimulus is encountered regulates memory retrieval. The ventral hippocampus (vH) has been implicated in contextual memories involving rewarding stimuli such as food, social cues or drugs. Yet, the neuronal representations and circuits underlying contextual memories of socially rewarding stimuli are poorly understood. Here, using in vivo electrophysiological recordings, in vivo one-photon calcium imaging, and optogenetics during a social reward contextual conditioning paradigm in male mice, we show that vH neurons discriminate between contexts with neutral or acquired social reward value. The formation of context-discriminating vH neurons following learning was contingent upon the presence of unconditioned stimuli. Moreover, vH neurons showed distinct contextual representations during the retrieval of social reward compared to fear contextual memories. Finally, optogenetic inhibition of locus coeruleus (LC) projections in the vH selectively disrupted social reward contextual memory by impairing vH contextual representations. Collectively, our findings reveal that the vH integrates contextual and social reward information, with memory encoding of these representations supported by input from the LC.

Representations of stimulus meaning in the hippocampus

The ability to discriminate and categorize the meaning of environmental stimuli and respond accordingly is essential for survival. The ventral hippocampus (vHPC) controls emotional and motivated behaviors in response to environmental cues and is hypothesized to do so in part by deciphering the positive or negative quality of these cues. Yet, what features of the environment are represented in the activity patterns of vCA1 neurons, and whether the positive or negative meaning of a stimulus is present at this stage, remains unclear. Here, using 2-photon calcium imaging across six different experimental paradigms, we consistently found that vCA1 ensembles encode the identity, sensory features, and intensity of learned and innately salient stimuli, but not their overall valence. These results offer a reappraisal of vCA1 function, wherein information corresponding to individual stimulus features and their behavioral saliency predominates, while valence-related information is attached elsewhere.

Understanding OLM interneurons: Characterization, circuitry, and significance in memory and navigation

Understanding the intricate mechanisms underlying memory formation and retention relies on unraveling how the hippocampus, a structure fundamental for memory acquisition, is organized. Within the complex hippocampal network, interneurons play a crucial role in orchestrating memory processes. Among these interneurons, Oriens-Lacunosum Moleculare (OLM) cells emerge as key regulators, governing the flow of information to CA1 pyramidal cells. In this review, we explore OLM interneurons in detail, describing their mechanisms and effects on memory processing, particularly in spatial and contextual memory tasks. Our aim is to provide a detailed understanding of how OLM interneurons contribute to the dynamic landscape of memory formation and retrieval.

Ceftriaxone Inhibits Conditioned Fear and Compulsive-like Repetitive Marble Digging without Central Nervous System Side Effects Typical of Diazepam-A Study on DBA2/J Mice and a High-5HT Subline of Wistar-Zagreb 5HT Rats

Background: Ceftriaxone upregulates GLT1 glutamate transporter in the brain and may have anti-CFC and anti-OCD effects. Methods: Twenty WZ-5HT rats were used to investigate the effects of ceftriaxone on obsessive-compulsive (OCD)-like behaviour in the marble-burying (MB) test, freezing behaviour in contextual fear conditioning (CFC) and expression of GLT1 protein in the hippocampus or amygdala using immunoblots. Fifteen DBA/2J mice were used in the MB test. We also compared diazepam with ceftriaxone in open-field, beam-walking, and wire-hanging tests on 47 DBA/2J mice. Ceftriaxone (200 mg/kg) and saline were applied intraperitoneally, once daily for 7 (rats) or 5 (mice) consecutive days. A single dose of diazepam (1.5-3.0 mg/kg) or saline was injected 30 min before the behavioural tests. Results: Ceftriaxone significantly diminished OCD-like behaviour (↓ number of marbles buried) and freezing behaviour in CFC context session (↑ latencies, ↓ total duration, ↓ duration over four 2 min periods of the session) but increased GLT1 protein expression in the amygdala and hippocampus of rats. Diazepam induced sedation, ataxia and myorelaxation in mice. Ceftriaxone did not have these side effects. Conclusions: The results of this study confirm the anti-CFC and anti-OCD effects of ceftriaxone, which did not produce the unwanted effects typical of diazepam.

The homogenous hippocampus: How hippocampal cells process available and potential goals

We present here a view of the firing patterns of hippocampal cells that is contrary, both functionally and anatomically, to conventional wisdom. We argue that the hippocampus responds to efference copies of goals encoded elsewhere; and that it uses these to detect and resolve conflict or interference between goals in general. While goals can involve space, hippocampal cells do not encode spatial (or other special types of) memory, as such. We also argue that the transverse circuits of the hippocampus operate in an essentially homogeneous way along its length. The apparently different functions of different parts (e.g. memory retrieval versus anxiety) result from the different (situational/motivational) inputs on which those parts perform the same fundamental computational operations. On this view, the key role of the hippocampus is the iterative adjustment, via Papez-like circuits, of synaptic weights in cell assemblies elsewhere.

In vivo structural connectivity of the reward system along the hippocampal long axis

Recent work has identified a critical role for the hippocampus in reward-sensitive behaviors, including motivated memory, reinforcement learning, and decision-making. Animal histology and human functional neuroimaging have shown that brain regions involved in reward processing and motivation are more interconnected with the ventral/anterior hippocampus. However, direct evidence examining gradients of structural connectivity between reward regions and the hippocampus in humans is lacking. The present study used diffusion MRI (dMRI) and probabilistic tractography to quantify the structural connectivity of the hippocampus with key reward processing regions in vivo. Using a large sample of subjects (N = 628) from the human connectome dMRI data release, we found that connectivity profiles with the hippocampus varied widely between different regions of the reward circuit. While the dopaminergic midbrain (ventral tegmental area) showed stronger connectivity with the anterior versus posterior hippocampus, the ventromedial prefrontal cortex showed stronger connectivity with the posterior hippocampus. The limbic (ventral) striatum demonstrated a more homogeneous connectivity profile along the hippocampal long axis. This is the first study to generate a probabilistic atlas of the hippocampal structural connectivity with reward-related networks, which is essential to investigating how these circuits contribute to normative adaptive behavior and maladaptive behaviors in psychiatric illness. These findings describe nuanced structural connectivity that sets the foundation to better understand how the hippocampus influences reward-guided behavior in humans.

Activity of ventral hippocampal parvalbumin interneurons during anxiety

Anxiety plays a key role in guiding behavior in response to potential threats. Anxiety is mediated by the activation of pyramidal neurons in the ventral hippocampus (vH), whose activity is controlled by GABAergic inhibitory interneurons. However, how different vH interneurons might contribute to anxiety-related processes is unclear. Here, we investigate the role of vH parvalbumin (PV)-expressing interneurons while mice transition from safe to more anxiogenic compartments of the elevated plus maze (EPM). We find that vH PV interneurons increase their activity in anxiogenic EPM compartments concomitant with dynamic changes in inhibitory interactions between PV interneurons and pyramidal neurons. By optogenetically inhibiting PV interneurons, we induce an increase in the activity of vH pyramidal neurons and persistent anxiety. Collectively, our results suggest that vH inhibitory microcircuits may act as a trigger for enduring anxiety states.

Approach/Avoidance Behavior to Novel Objects is Correlated with the Serotonergic and Dopaminergic Systems in the Brown Rat (Rattus norvegicus)

The brown rat (Rattus norvegicus) is known to show three types of behavioral responses to novel objects. Whereas some rats are indifferent to novel objects, neophobic and neophilic rats show avoidance and approach behavior, respectively. Here, we compared the dopaminergic, serotonergic, and noradrenergic systems immunohistochemically among these rats. Trapped wild rats and laboratory rats were first individually exposed to the novel objects in their home cage. Wild rats were divided into neophobic and indifferent rats depending on their behavioral responses. Similarly, laboratory rats were divided into neophilic and indifferent rats. Consistent with the behavioral differences, in the paraventricular nucleus of the hypothalamus, Fos expression in corticotropin-releasing hormone-containing neurons was higher in the neophobic rats than in the indifferent rats. In the anterior basal amygdala, the neophobic rats showed higher Fos expression than the indifferent rats. In the posterior basal amygdala, the neophobic and neophilic rats showed lower and higher Fos expressions than the indifferent rats, respectively. When we compared the neuromodulatory systems, in the dorsal raphe, the number of serotonergic neurons and Fos expression in serotonergic neurons increased linearly from neophobic to indifferent to neophilic rats. In the ventral tegmental area, Fos expression in dopaminergic neurons was higher in the neophilic rats than in the indifferent rats. These results demonstrate that approach/avoidance behavior to novel objects is correlated with the serotonergic and dopaminergic systems in the brown rat. We propose that the serotonergic system suppresses avoidance behavior while the dopaminergic system enhances approach behavior to novel objects.

The chemokine XCL1 functions as a pregnancy hormone to program offspring innate anxiety

Elevated levels of cytokines in maternal circulation increase the offspring's risk for neuropsychiatric disease. Because of their low homeostatic levels, circulating maternal cytokines during normal pregnancies have not been considered to play a role in fetal brain development and offspring behavior. Here we report that the T/NK cell chemotactic cytokine XCL1, a local paracrine immune signal, can function as a pregnancy hormone and is required for the proper development of placenta and male offspring approach-avoidance behavior. We found that circulating XCL1 levels were at a low pregestational level throughout pregnancy except for a midgestational rise and fall. Blunted elevation in maternal plasma XCL1 in dams with a genetic 5HT1A receptor deficit or following neutralization by anti-XCL1 antibodies increased the expression of tissue damage associated factors in WT fetal placenta and led to increased innate anxiety and stress reactivity in the WT male offspring. Therefore, chemokines like XCL1 may act as pregnancy hormones to regulate placenta development and offspring emotional behavior.

Amygdala-hippocampus somatostatin interneuron beta-synchrony underlies a cross-species biomarker of emotional state

Emotional responses arise from limbic circuits including the hippocampus and amygdala. In the human brain, beta-frequency communication between these structures correlates with self-reported mood and anxiety. However, both the mechanism and significance of this biomarker as a readout vs. driver of emotional state remain unknown. Here, we show that beta-frequency communication between ventral hippocampus and basolateral amygdala also predicts anxiety-related behavior in mice, both on long timescales (∼30 min) and immediately preceding behavioral choices. Genetically encoded voltage indicators reveal that this biomarker reflects synchronization between somatostatin interneurons across both structures. Indeed, synchrony between these neurons dynamically predicts approach-avoidance decisions, and optogenetically shifting the phase of synchronization by just 25 ms is sufficient to bidirectionally modulate anxiety-related behaviors. Thus, back-translation establishes a human biomarker as a causal determinant (not just predictor) of emotional state, revealing a novel mechanism whereby interregional synchronization that is frequency, phase, and cell type specific controls emotional processing.

Septotemporal Variation of Information Processing in the Hippocampus of Fmr1 KO Rat

INTRODUCTION

Fragile X messenger ribonucleoprotein (FMRP) is a protein involved in many neuronal processes in the nervous system including the modulation of synaptic transmission. The loss of FMRP produces the fragile X syndrome (FXS), a neurodevelopmental disorder affecting synaptic and neuronal function and producing cognitive impairments. However, the effects of FXS on short-term processing of synaptic inputs and neuronal outputs in the hippocampus have not yet been sufficiently clarified. Furthermore, it is not known whether dorsal and ventral hippocampi are affected similarly or not in FXS.

METHOD

We used an Fmr1 knockout (KO) rat model of FXS and recordings of evoked field potentials from the CA1 field of transverse slices from both the dorsal and the ventral hippocampi of adult rats.

RESULTS

Following application of a frequency stimulation protocol consisting of a ten-pulse train and recordings of fEPSP, we found that the dorsal but not ventral KO hippocampus shows altered short-term synaptic plasticity. Furthermore, applying the frequency stimulation protocol and recordings of population spikes, both segments of the KO hippocampus display altered short-term neuronal dynamics.

CONCLUSIONS

These data suggest that short-term processing of synaptic inputs is affected in the dorsal, not ventral, FXS hippocampus, while short-term processing of neuronal output is affected in both segments of the FXS hippocampus in a similar way. These FXS-associated changes may have significant impact on the functions of the dorsal and ventral hippocampi in individuals with FXS.

Nicotine use disorder and Neuregulin 3: Opportunities for precision medicine

Nicotine use disorder remains a major public health emergency despite years of trumpeting the consequences of smoking. This is likely due to the complex interplay of genetics and nicotine exposure across the lifespan of these individuals. Genetics influence all aspects of life, including complex disorders such as nicotine use disorder. This review first highlights the critical neurocircuitry underlying nicotine dependence and withdrawal, and then describes the cellular signaling mechanisms involved. Finally, current genetic, genomic, and transcriptomic evidence for new drug development of smoking cessation aids is discussed, with a focus on the Neuregulin 3 Signaling Pathway.

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.

Urolithin A Inhibits Anterior Basolateral Amygdala to Ventral Hippocampal CA1 Circuit to Ameliorate Amyloid-β-Impaired Social Ability

Background

Anxiety and social withdrawal are highly prevalent among patients with Alzheimer's disease (AD). However, the neural circuit mechanisms underlying these symptoms remain elusive, and there is a need for effective prevention strategies.

Objective

This study aims to elucidate the neural circuitry mechanisms underlying social anxiety in AD.

Methods

We utilized 5xFAD mice and conducted a series of experiments including optogenetic manipulation, Tandem Mass Tag-labeled proteome analysis, behavioral assessments, and immunofluorescence staining.

Results

In 5xFAD mice, we observed significant amyloid-β (Aβ) accumulation in the anterior part of basolateral amygdala (aBLA). Behaviorally, 6-month-old 5xFAD mice displayed excessive social avoidance during social interaction. Concurrently, the pathway from aBLA to ventral hippocampal CA1 (vCA1) was significantly activated and exhibited a disorganized firing patterns during social interaction. By optogenetically inhibiting the aBLA-vCA1 pathway, we effectively improved the social ability of 5xFAD mice. In the presence of Aβ accumulation, we identified distinct changes in the protein network within the aBLA. Following one month of administration of Urolithin A (UA), we observed significant restoration of the abnormal protein network within the aBLA. UA treatment also attenuated the disorganized firings of the aBLA-vCA1 pathway, leading to an improvement in social ability.

Conclusions

The aBLA-vCA1 circuit is a vulnerable pathway in response to Aβ accumulation during the progression of AD and plays a crucial role in Aβ-induced social anxiety. Targeting the aBLA-vCA1 circuit and UA administration are both effective strategies for improving the Aβ-impaired social ability.

Polygenic risk for depression and anterior and posterior hippocampal volume in children and adolescents

BACKGROUND

Depression has frequently been associated with smaller hippocampal volume. The hippocampus varies in function along its anterior-posterior axis, with the anterior hippocampus more strongly associated with stress and emotion processing. The goals of this study were to examine the associations among parental history of anxiety/depression, polygenic risk scores for depression (PGS-DEP), and anterior and posterior hippocampal volumes in children and adolescents. To examine specificity to PGS-DEP, we examined associations of educational attainment polygenic scores (PGS-EA) with anterior and posterior hippocampal volume.

METHODS

Participants were 350 3- to 21-year-olds (46 % female). PGS-DEP and PGS-EA were computed based on recent, large-scale genome-wide association studies. High-resolution, T1-weighted magnetic resonance imaging (MRI) data were acquired, and a semi-automated approach was used to segment the hippocampus into anterior and posterior subregions.

RESULTS

Children and adolescents with higher polygenic risk for depression were more likely to have a parent with a history of anxiety/depression. Higher polygenic risk for depression was significantly associated with smaller anterior but not posterior hippocampal volume. PGS-EA was not associated with anterior or posterior hippocampal volumes.

LIMITATIONS

Participants in these analyses were all of European ancestry.

CONCLUSIONS

Polygenic risk for depression may lead to smaller anterior but not posterior hippocampal volume in children and adolescents, and there may be specificity of these effects to PGS-DEP rather than PGS-EA. These findings may inform the earlier identification of those in need of support and the design of more effective, personalized treatment strategies.

DECLARATIONS OF INTEREST

none.

DECLARATIONS OF INTEREST

None.

Ketogenic diet changes microglial morphology and the hippocampal lipidomic profile differently in stress susceptible versus resistant male mice upon repeated social defeat

Psychological stress confers an increased risk for several diseases including psychiatric conditions. The susceptibility to psychological stress is modulated by various factors, many of them being modifiable lifestyle choices. The ketogenic diet (KD) has emerged as a dietary regime that offers positive outcomes on mood and health status. Psychological stress and elevated inflammation are common features of neuropsychiatric disorders such as certain types of major depressive disorder. KD has been attributed anti-inflammatory properties that could underlie its beneficial consequences on the brain and behavior. Microglia are the main drivers of inflammation in the central nervous system. They are known to respond to both dietary changes and psychological stress, notably by modifying their production of cytokines and relationships among the brain parenchyma. To assess the interactions between KD and the stress response, including effects on microglia, we examined adult male mice on control diet (CD) versus KD that underwent 10 days of repeated social defeat (RSD) or remained non-stressed (controls; CTRLs). Through a social interaction test, stressed mice were classified as susceptible (SUS) or resistant (RES) to RSD. The mouse population fed a KD tended to have a higher proportion of individuals classified as RES following RSD. Microglial morphology and ultrastructure were then analyzed in the ventral hippocampus CA1, a brain region known to present structural alterations as a response to psychological stress. Distinct changes in microglial soma and arborization linked to the KD, SUS and RES phenotypes were revealed. Ultrastructural analysis by electron microscopy showed a clear reduction of cellular stress markers in microglia from KD fed animals. Furthermore, ultrastructural analysis showed that microglial contacts with synaptic elements were reduced in the SUS compared to the RES and CTRL groups. Hippocampal lipidomic analyses lastly identified a distinct lipid profile in SUS animals compared to CTRLs. These key differences, combined with the distinct microglial responses to diet and stress, indicate that unique metabolic changes may underlie the stress susceptibility phenotypes. Altogether, our results reveal novel mechanisms by which a KD might improve the resistance to psychological stress.

Pathway specific interventions reveal the multiple roles of ventral hippocampus projections in cognitive functions

Since the 1950s study of Scoville and Milner on the case H.M., the hippocampus has attracted neuroscientists' attention. The hippocampus has been traditionally divided into dorsal and ventral parts, each of which projects to different brain structures and mediates various functions. Despite a predominant interest in its dorsal part in animal models, especially regarding episodic-like and spatial cognition, recent data highlight the role of the ventral hippocampus (vHPC), as the main hippocampal output, in cognitive processes. Here, we review recent studies conducted in rodents that have used advanced in vivo functional techniques to specifically monitor and manipulate vHPC efferent pathways and delineate the roles of these specific projections in learning and memory processes. Results highlight that vHPC projections to basal amygdala are implicated in emotional memory, to nucleus accumbens in social memory and instrumental actions and to prefrontal cortex in all the above as well as in object-based memory. Some of these hippocampal projections also modulate feeding and anxiety-like behaviours providing further evidence that the "one pathway-one function" view is outdated and future directions are proposed to better understand the role of hippocampal pathways and shed further light on its connectivity and function.

Ventral CA1 Population Codes for Anxiety

The ventral hippocampus is a critical node in the distributed brain network that controls anxiety. Using miniature microscopy and calcium imaging, we recorded ventral CA1 (vCA1) neurons in freely moving mice as they explored variants of classic behavioral assays for anxiety. Unsupervised behavioral segmentation revealed clusters of behavioral motifs that corresponded to exploratory and vigilance-like states. We discovered multiple vCA1 population codes that represented the anxiogenic features of the environment, such as bright light and openness, as well as the moment-to-moment anxiety state of the animals. These population codes possessed distinct generalization properties: neural representations of anxiogenic features were different for open field and elevated plus/zero maze tasks, while neural representations of moment-to-moment anxiety state were similar across both experimental contexts. Our results suggest that anxiety is not tied to the aversive compartments of these mazes but is rather defined by a behavioral state and its corresponding population code that generalizes across environments.

Ketogenic diet alters microglial morphology and changes the hippocampal lipidomic profile distinctively in stress susceptible versus resistant male mice upon repeated social defeat

Psychological stress confers an increased risk for several diseases including psychiatric conditions. The susceptibility to psychological stress is modulated by various factors, many of them being modifiable lifestyle choices. The ketogenic diet (KD) has emerged as a dietary regime that offers positive outcomes on mood and health status. Psychological stress and elevated inflammation are common features of neuropsychiatric disorders such as certain types of major depressive disorder. KD has been attributed anti-inflammatory properties that could underlie its beneficial consequences on the brain and behavior. Microglia are the main drivers of inflammation in the central nervous system. They are known to respond to both dietary changes and psychological stress, notably by modifying their production of cytokines and relationships among the brain parenchyma. To assess the interactions between KD and the stress response, including effects on microglia, we examined adult male mice on control diet (CD) versus KD that underwent 10 days of repeated social defeat (RSD) or remained non-stressed (controls; CTRLs). Through a social interaction test, stressed mice were classified as susceptible (SUS) or resistant (RES) to RSD. The mouse population fed a KD tended to have a higher proportion of individuals classified as RES following RSD. Microglial morphology and ultrastructure were then analyzed in the ventral hippocampus CA1, a brain region known to present structural alterations as a response to psychological stress. Distinct changes in microglial soma and arborization linked to the KD, SUS and RES phenotypes were revealed. Ultrastructural analysis by electron microscopy showed a clear reduction of cellular stress markers in microglia from KD fed animals. Furthermore, ultrastructural analysis showed that microglial contacts with synaptic elements were reduced in the SUS compared to the RES and CTRL groups. Hippocampal lipidomic analyses lastly identified a distinct lipid profile in SUS animals compared to CTRLs. These key differences, combined with the distinct microglial responses to diet and stress, indicate that unique metabolic changes may underlie the stress susceptibility phenotypes. Altogether, our results reveal novel mechanisms by which a KD might improve the resistance to psychological stress.

The past and present of therapeutic strategy for Alzheimer's diseases: potential for stem cell therapy

Alzheimer's disease (AD), a progressive neurodegenerative disease characterized by cognitive dysfunction and neuropsychiatric symptoms, is the most prevalent form of dementia among the elderly. Amyloid aggregation, tau hyperphosphorylation, and neural cell loss are the main pathological features. Various hypotheses have been proposed to explain the development of AD. Some therapeutic agents have shown clinical benefits in patients with AD; however, many of these agents have failed. The degree of neural cell loss is associated with the severity of AD. Adult neurogenesis, which governs cognitive and emotional behaviors, occurs in the hippocampus, and some research groups have reported that neural cell transplantation into the hippocampus improves cognitive dysfunction in AD model mice. Based on these clinical findings, stem cell therapy for patients with AD has recently attracted attention. This review provides past and present therapeutic strategies for the management and treatment of AD.

Reduced Vrk2 expression is associated with higher risk of depression in humans and mediates depressive-like behaviors in mice

BACKGROUND

Genome-wide association studies (GWAS) have reported single-nucleotide polymorphisms (SNPs) in the VRK serine/threonine kinase 2 gene (VRK2) showing genome-wide significant associations with major depression, but the regulation effect of the risk SNPs on VRK2 as well as their roles in the illness are yet to be elucidated.

METHODS

Based on the summary statistics of major depression GWAS, we conducted population genetic analyses, epigenome bioinformatics analyses, dual luciferase reporter assays, and expression quantitative trait loci (eQTL) analyses to identify the functional SNPs regulating VRK2; we also carried out behavioral assessments, dendritic spine morphological analyses, and phosphorylated 4D-label-free quantitative proteomics analyses in mice with Vrk2 repression.

RESULTS

We identified a SNP rs2678907 located in the 5' upstream of VRK2 gene exhibiting large spatial overlap with enhancer regulatory marks in human neural cells and brain tissues. Using luciferase reporter gene assays and eQTL analyses, the depression risk allele of rs2678907 decreased enhancer activities and predicted lower VRK2 mRNA expression, which is consistent with the observations of reduced VRK2 level in the patients with major depression compared with controls. Notably, Vrk2-/- mice exhibited depressive-like behaviors compared to Vrk2+/+ mice and specifically repressing Vrk2 in the ventral hippocampus using adeno-associated virus (AAV) lead to consistent and even stronger depressive-like behaviors in mice. Compared with Vrk2+/+ mice, the density of mushroom and thin spines in the ventral hippocampus was significantly altered in Vrk2-/- mice, which is in line with the phosphoproteomic analyses showing dysregulated synapse-associated proteins and pathways in Vrk2-/- mice.

CONCLUSIONS

Vrk2 deficiency mice showed behavioral abnormalities that mimic human depressive phenotypes, which may serve as a useful murine model for studying the pathophysiology of depression.

Execution of new trajectories toward a stable goal without a functional hippocampus

The hippocampus is a critical component of a mammalian spatial navigation system, with the firing sequences of hippocampal place cells during sleep or immobility constituting a "replay" of an animal's past trajectories. A novel spatial navigation task recently revealed that such "replay" sequences of place fields can also prospectively map onto imminent new paths to a goal that occupies a stable location during each session. It was hypothesized that such "prospective replay" sequences may play a causal role in goal-directed navigation. In the present study, we query this putative causal role in finding only minimal effects of muscimol-induced inactivation of the dorsal and intermediate hippocampus on the same spatial navigation task. The concentration of muscimol used demonstrably inhibited hippocampal cell firing in vivo and caused a severe deficit in a hippocampal-dependent "episodic-like" spatial memory task in a watermaze. These findings call into question whether "prospective replay" of an imminent and direct path is actually necessary for its execution in certain navigational tasks.

Intra-hippocampal cis-P tau microinjection induces long-term changes in behavior and synaptic plasticity in mice

BACKGROUND

Alzheimer's disease is accompanied by an abnormal high accumulation of cis-P tau. However, the long-term changes in behavior following tau accumulation remains under debate. The present study investigated the long-term effects of tauopathy on learning and memory, synaptic plasticity, and hippocampal cell numbers.

RESULTS

Cis-P tau was microinjected into the dorsal hippocampus to generate Alzheimer's like-disease model in C57BL/6 mice. Cis-P tau injected animals showed a significant impairment in learning and memory in Y-maze and Barnes maze tests. In another group of animals, the generation of long-term potentiation (LTP) was evaluated in hippocampal slices 7 months after cis-P tau injection. LTP induction was disrupted only in the dorsal but not ventral hippocampal slices. The basal synaptic transmission was also reduced in dorsal hippocampal slices. In addition, hippocampal sampling was done, and the number of cells was assessed by Nissl staining. Obtained results indicated that the number of survived cells was significantly reduced in the dorsal and ventral hippocampus of cis P-tau injected animals compared to the animals in control group. However, the decrement of cell number was higher in the dorsal compared to the ventral hippocampus.

CONCLUSIONS

In conclusion, intra-hippocampal cis-P tau injection produced learning and memory impairment at 7 months after its injection. This impairment might result from LTP disruption and a significant decrease in the number of neurons in the dorsal hippocampus.

Corticosterone injection into the dorsal and ventral hippocampus impairs fear memory reconsolidation in a time-dependent manner in rats

Reconsolidation is an active process induced following the reactivation of previously consolidated memories. Recent studies suggest brain corticosteroid receptors may participate in the modulation of fear memory reconsolidation. Glucocorticoid receptors (GRs), with 10-fold lower affinity than mineralocorticoid receptors (MRs), are mainly occupied during the peak of the circadian rhythm, and after stress, so they probably have a more critical role than MRs in memory phases during stressful situations. This study investigated the role of dorsal and ventral hippocampal (DH and VH) GRs and MRs on fear memory reconsolidation in rats. Male Wistar rats with surgically implanted bilaterally cannulae at the DH and VH were trained and tested in an inhibitory avoidance task. The animals received bilateral microinjections of vehicle (0.3 µl/side), corticosterone (3 ng/0.3 µl/side), the GRs antagonist RU38486 (3 ng/0.3 µl/side), or the MRs antagonist spironolactone (3 ng/0.3 µl/side) immediately after memory reactivation. Moreover, drugs were injected into VH 90 minutes after memory reactivation. Memory tests were performed 2, 9, 11, and 13 days after memory reactivation. Results indicated that injection of corticosterone into the DH but not VH immediately after memory reactivation significantly impaired fear memory reconsolidation. Moreover, corticosterone injection into VH 90 minutes after memory reactivation impaired fear memory reconsolidation. RU38486 reversed these effects but not spironolactone. These findings indicate that corticosterone injection into the DH and VH via GRs activation impairs the reconsolidation of fear memory in a time-dependent manner.

Activation of ventral hippocampal CB1 receptors inhibits ketamine-induced anxiogenic-like behavior: Alteration of BDNF/c-Fos levels in the mouse hippocampus

Considering the increasing usage of ketamine as a recreational drug with hallucinogenic properties and also scarce studies about receptor systems responsible for its effects, in the present study we aimed to investigate whether the activation of the ventral hippocampal (VH) CB1 cannabinoid receptors affects the anxiety-like behaviors induced by ketamine. Also, the levels of BDNF and c-Fos proteins in the mouse hippocampus were measured following the treatments. For this purpose, male NMRI mice were cannulated bilaterally in the VH with a stereotaxic apparatus. Anxiety properties and protein changes were measured using elevated plus-maze (EPM) and western blotting respectively. The results revealed that intraperitoneal (i.p.) administration of ketamine (5-20 mg/kg) significantly decreased the percentage of open arm time (%OAT) and open arm entry (%OAE) in the EPM with no alteration in the locomotor activity suggesting an anxiogenic-like behavior to ketamine. Furthermore, ketamine administration (10 mg/kg, i.p.) increased BDNF and c-Fos levels in the hippocampus. Interestingly, activation of the VH CB1 receptors by ACPA (0.5-4 ng/mouse) inhibited the anxiogenic-like behaviors produced by ketamine, whereas the microinjection of the same doses of ACPA into VH by itself had no effect on the EPM parameters. Hippocampal levels of BDNF and c-Fos decreased after treatment with combined ketamine with ACPA. These results suggest the therapeutic potency of cannabinoid receptor agonists for ketamine-induced anxiogenic-related responses. This effect might be at least partially mediated by the alteration of BDNF and c-Fos signaling in the hippocampus.

Environmental conditions of recognition memory testing induce neurovascular changes in the hippocampus in a sex-specific manner in mice

Experiences are linked to emotions impacting memory consolidation and associated brain neuronal circuits. Posttraumatic stress disorder is an example of strong negative emotions affecting memory processes by flashbacks of past traumas. Stress-related memory deficits are also observed in major depressive disorder (MDD). We recently highlighted that sex-specific blood-brain barrier (BBB) alterations underlie stress responses in mice and human depression. However, little is known about the relationship between emotional valence, memory encoding and BBB gene expression. Here, we investigated the effects of novel object recognition (NOR) test, an experience considered of neutral emotional valence, on BBB properties in dorsal vs ventral hippocampus (HIPP) in the context of various environmental conditions (arena size, handling, age). The HIPP is a brain area central for learning and memory processes with the dorsal and ventral subregions being associated with working memory vs reference memory retrieval, respectively. Expression of genes related to BBB integrity are altered in line with learning and memory processes in a region- and sex-specific manner. We observed correlations between poor learning, anxiety, stress-induced corticosterone release and changes in BBB-associated gene expression. Comparison of BBB transcriptomes between sexes also revealed profound differences at baseline in both ventral and dorsal HIPP. Finally, we identified circulating vascular biomarkers, such as sE-selectin and matrix metallopeptidase 9 (MMP-9), altered following NOR exposure supporting that recognition memory formation has an impact on the neurovasculature. Although deemed as a neutral valence test, NOR experimental conditions can shift it toward a negative valence, impacting performance and highlighting the need to minimize anxiety when performing this commonly used test in mice.

Extracellular zinc regulates contextual fear memory formation in male rats through MMP-BDNF-TrkB pathway in dorsal hippocampus and basolateral amygdala

Large amount of zinc (100 µM even up to 300 µM) is released from the nerve terminals in response to high frequency neuronal stimulation in certain brain regions including hippocampus and amygdala. However, its precise pharmacological effect is poorly understood. Here, we investigated the role of extracellular zinc (endogenous zinc) and exogenous zinc in memory formation using contextual fear conditioning (CFC) model. Male Sprague Dawley rats were trained for fear conditioning followed by in vivo microdialysis for collection of microdialysate samples from CA1 and CA3 regions of hippocampus and basolateral amygdala (BLA). Extracellular zinc chelator CaEDTA, BDNF scavenger TrkB-Fc, exogenous 7,8-DHF and matrix metalloproteinases (MMP) inhibitor were infused into the CA1 and CA3 regions of hippocampus and BLA after CFC. Different doses of exogenous zinc hydroaspartate were administered intraperitoneally immediately after CFC. We found that CFC increased the level of extracellular zinc in the hippocampus and BLA. Infusing the CaEDTA, TrkB-Fc and MMP inhibitor into the CA1 and CA3 regions of hippocampus and BLA disrupted the fear memory formation. Furthermore, administration of TrKB agonist 7,8-DHF reversed the inhibitory effect of CaEDTA on fear memory formation, suggesting that extracellular zinc may regulate fear memory formation via the BDNF-TrKB pathway. We also found that high dose of exogenous zinc hydroaspartate supplementation increased extracellular zinc levels in brain and enhanced fear memory formation. Altogether, these findings indicate that extracellular zinc may participate in formation of contextual fear memory through MMP-BDNF-TrkB pathway in the hippocampus and BLA.

Adolescent intermittent ethanol exposure enhances adult stress effects in male rats

Binge patterns of alcohol use, prevalent among adolescents, are associated with a higher probability of developing alcohol use disorders (AUD) and other psychiatric disorders, like anxiety and depression. Additionally, adverse life events strongly predict AUD and other psychiatric disorders. As such, the combined fields of stress and AUD have been well established, and animal models indicate that both binge-like alcohol exposure and stress exposure elevate anxiety-like behaviors. However, few have investigated the interaction of adolescent intermittent ethanol (AIE) and adult stressors. We hypothesized that AIE would increase vulnerability to restraint-induced stress (RS), manifested as increased anxiety-like behavior. After AIE exposure, in adulthood, animals were tested on forced swim (FST) and saccharin preference (SP) and then exposed to either RS (90 min/5 days) or home-cage control. Twenty-four hours after the last RS session, animals began testing on the elevated plus maze (EPM), and were re-tested on FST and SP. A separate group of animals were sacrificed in adulthood after AIE and RS, and brains were harvested for immunoblot analysis of dorsal and ventral hippocampus. Consistent with previous reports, AIE had no significant effect on closed arm time in the EPM (anxiety-like behavior). However, in male rats the interaction of AIE and adult RS increased time spent in the closed arms. No effect was observed among female animals. AIE and RS-specific alterations were found in glial and synaptic markers (GLT-1, FMRP and PSD-95) in male animals. These findings indicate AIE has sex-specific effects on both SP and the interaction of AIE and adult RS, which induces a propensity toward anxiety-like behavior in males. Also, AIE produces persistent hippocampal deficits that may interact with adult RS to cause increased anxiety-like behaviors. Understanding the mechanisms behind this AIE-induced increase in stress vulnerability may provide insight into treatment and prevention strategies for alcohol use disorders.

Hippocampus-based behavioral, structural, and molecular dynamics across the estrous cycle

The activity of neurons in the rodent hippocampus contributes to diverse behaviors, with the activity of ventral hippocampal neurons affecting behaviors related to anxiety and emotion regulation, and the activity of dorsal hippocampal neurons affecting performance in learning- and memory-related tasks. Hippocampal cells also express receptors for ovarian hormones, estrogen and progesterone, and are therefore affected by physiological fluctuations of those hormones that occur over the rodent estrous cycle. In this review, we discuss the effects of cycling ovarian hormones on hippocampal physiology. Starting with behavior, we explore the role of the estrous cycle in regulating hippocampus-dependent behaviors. We go on to detail the cellular mechanisms through which cycling estrogen and progesterone, through changes in the structural and functional properties of hippocampal neurons, may be eliciting these changes in behavior. Then, providing a basis for these cellular changes, we outline the epigenetic, chromatin regulatory mechanisms through which ovarian hormones, by binding to their receptors, can affect the regulation of behavior- and synaptic plasticity-related genes in hippocampal neurons. We also highlight an unconventional role that chromatin dynamics may have in regulating neuronal function across the estrous cycle, including in sex hormone-driven X chromosome plasticity and hormonally-induced epigenetic priming. Finally, we discuss directions for future studies and the translational value of the rodent estrous cycle for understanding the effects of the human menstrual cycle on hippocampal physiology and brain disease risk.

Adolescent Δ-9-tetrahydrocannabinol exposure induces differential acute and long-term neuronal and molecular disturbances in dorsal vs. ventral hippocampal subregions

Chronic exposure to Δ-9-tetrahydrocannabinol (THC) during adolescence is associated with long-lasting cognitive impairments and enhanced susceptibility to anxiety and mood disorders. Previous evidence has revealed functional and anatomical dissociations between the posterior vs. anterior portions of the hippocampal formation, which are classified as the dorsal and ventral regions in rodents, respectively. Notably, the dorsal hippocampus is critical for cognitive and contextual processing, whereas the ventral region is critical for affective and emotional processing. While adolescent THC exposure can induce significant morphological disturbances and glutamatergic signaling abnormalities in the hippocampus, it is not currently understood how the dorsal vs. ventral hippocampal regions are affected by THC during neurodevelopment. In the present study, we used an integrative combination of behavioral, molecular, and neural assays in a neurodevelopmental rodent model of adolescent THC exposure. We report that adolescent THC exposure induces long-lasting memory deficits and anxiety like-behaviors concomitant with a wide range of differential molecular and neuronal abnormalities in dorsal vs. ventral hippocampal regions. In addition, using matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI-IMS), we show for the first time that adolescent THC exposure induces significant and enduring dysregulation of GABA and glutamate levels in dorsal vs. ventral hippocampus. Finally, adolescent THC exposure induced dissociable dysregulations of hippocampal glutamatergic signaling, characterized by differential glutamatergic receptor expression markers, profound alterations in pyramidal neuronal activity and associated oscillatory patterns in dorsal vs. ventral hippocampal subregions.

Short photoperiod modulates behavior, cognition and hippocampal neurogenesis in male Japanese quail

The mechanisms underlying the photoperiodic control of reproduction in mammals and birds have been recently clarified. In contrast, the potential impact of photoperiod on more complex, integrative processes, such as cognitive behaviors, remains poorly characterized. Here, we investigated the impact of contrasted long and short photoperiods (LP, 16 h light/day and SP, 8 h light/day, respectively) on learning, spatial orientation abilities, and emotional reactivity in male Japanese quail. In addition, we quantified cell proliferation and young cell maturation/migration within the hippocampus, a brain region involved in spatial orientation. Our study reveals that, in male quail, SP increases emotional responses and spatial orientation abilities, compared to LP. Behaviorally, SP birds were found to be more fearful than LP birds, exhibiting more freezing in the open field and taking longer to exit the dark compartment in the emergence test. Furthermore, SP birds were significantly less aggressive than LP birds in a mirror test. Cognitively, SP birds were slower to habituate and learn a spatial orientation task compared to LP birds. However, during a recall test, SP birds performed better than LP birds. From a neuroanatomical standpoint, SP birds had a significantly lower density of young neurons, and also tended to have a lower density of mature neurons within the hippocampus, compared to LP birds. In conclusion, our data reveal that, beyond breeding control, photoperiod also exerts a profound influence on behavior, cognition, and brain plasticity, which comprise the seasonal program of this species.

Behavioral and Proteomic Studies Reveal Methylglyoxal Activate Pathways Associated with Alzheimer's Disease

Diabetes is one of the major risk factors for Alzheimer's disease (AD) development. The role of elevated levels of glucose, methylglyoxal (MGO), and advanced glycation end products (AGEs) in the pathogenesis of AD is not well understood. In this pursuit, we studied the role of methylglyoxal in the pathogenesis of AD in rat models. The elevated plus-maze (EPM) behavioral study indicated that MGO induces anxiety. Treatment of telmisartan (RAGE expression inhibitor) and aminoguanidine (MGO quencher) attenuated MGO induced anxiety. Further, hippocampal proteomics demonstrated that MGO treated rats differentially regulate proteins involved in calcium homeostasis, mitochondrial functioning, and apoptosis, which may affect neurotransmission and neuronal plasticity. The hippocampal tau phosphorylation level was increased in MGO treated rats, which was reduced in the presence of aminoguanidine and telmisartan. The plasma fructosamine level was increased upon MGO treatment. Hippocampal histochemistry showed vascular degeneration and neuronal loss upon MGO treatment. This study provides mechanistic insight into the role of MGO in the diabetes-associated development of AD.

Linking Social Cognition, Parvalbumin Interneurons, and Oxytocin in Alzheimer's Disease: An Update

Finding a cure for Alzheimer's disease (AD) has been notoriously challenging for many decades. Therefore, the current focus is mainly on prevention, timely intervention, and slowing the progression in the earliest stages. A better understanding of underlying mechanisms at the beginning of the disease could aid in early diagnosis and intervention, including alleviating symptoms or slowing down the disease progression. Changes in social cognition and progressive parvalbumin (PV) interneuron dysfunction are among the earliest observable effects of AD. Various AD rodent models mimic these early alterations, but only a narrow field of study has considered their mutual relationship. In this review, we discuss current knowledge about PV interneuron dysfunction in AD and emphasize their importance in social cognition and memory. Next, we propose oxytocin (OT) as a potent modulator of PV interneurons and as a promising treatment for managing some of the early symptoms. We further discuss the supporting evidence on its beneficial effects on AD-related pathology. Clinical trials have employed the use of OT in various neuropsychiatric diseases with promising results, but little is known about its prospective impacts on AD. On the other hand, the modulatory effects of OT in specific structures and local circuits need to be clarified in future studies. This review highlights the connection between PV interneurons and social cognition impairment in the early stages of AD and considers OT as a promising therapeutic agent for addressing these early deficits.

A circuit from the ventral subiculum to anterior hypothalamic nucleus GABAergic neurons essential for anxiety-like behavioral avoidance

Behavioral observations suggest a connection between anxiety and predator defense, but the underlying neural mechanisms remain unclear. Here we examine the role of the anterior hypothalamic nucleus (AHN), a node in the predator defense network, in anxiety-like behaviors. By in vivo recordings in male mice, we find that activity of AHN GABAergic (AHN^Vgat+) neurons shows individually stable increases when animals approach unfamiliar objects in an open field (OF) or when they explore the open-arm of an elevated plus-maze (EPM). Moreover, object-evoked AHN activity overlap with predator cue responses and correlate with the object and open-arm avoidance. Crucially, exploration-triggered optogenetic inhibition of AHN^Vgat+ neurons reduces object and open-arm avoidance. Furthermore, retrograde viral tracing identifies the ventral subiculum (vSub) of the hippocampal formation as a significant input to AHN^Vgat+ neurons in driving avoidance behaviors in anxiogenic situations. Thus, convergent activation of AHN^Vgat+ neurons serves as a shared mechanism between anxiety and predator defense to promote behavioral avoidance.

Anxiety-related activity of ventral hippocampal interneurons

Anxiety is an aversive mood reflecting the anticipation of potential threats. The ventral hippocampus (vH) is a key brain region involved in the genesis of anxiety responses. Recent studies have shown that anxiety is mediated by the activation of vH pyramidal neurons targeting various limbic structures. Throughout the cortex, the activity of pyramidal neurons is controlled by GABA-releasing inhibitory interneurons and the GABAergic system represents an important target of anxiolytic drugs. However, how the activity of vH inhibitory interneurons is related to different anxiety behaviours has not been investigated so far. Here, we integrated in vivo electrophysiology with behavioural phenotyping of distinct anxiety exploration behaviours in rats. We showed that pyramidal neurons and interneurons of the vH are selectively active when animals explore specific compartments of the elevated-plus-maze (EPM), an anxiety task for rodents. Moreover, rats with prior goal-related experience exhibited low-anxiety exploratory behaviour and showed a larger trajectory-related activity of vH interneurons during EPM exploration compared to high anxiety rats. Finally, in low anxiety rats, trajectory-related vH interneurons exhibited opposite activity to pyramidal neurons specifically in the open arms (i.e. more anxiogenic) of the EPM. Our results suggest that vH inhibitory micro-circuits could act as critical elements underlying different anxiety states.

Spread of pathological human Tau from neurons to oligodendrocytes and loss of high-firing pyramidal neurons in aging mice

Intracellular aggregation of hyperphosphorylated Tau (pTau) in the brain is associated with cognitive and motor impairments, and ultimately neurodegeneration. We investigate how human pTau affects cells and network activity in the hippocampal formation of the THY-Tau22 tauopathy model mice in vivo. We find that pTau preferentially accumulates in deep-layer pyramidal neurons, leading to neurodegeneration, and we establish that pTau spreads to oligodendrocytes. During goal-directed virtual navigation in aged transgenic mice, we detect fewer high-firing prosubicular pyramidal cells, but the firing population retains its coupling to theta oscillations. Analysis of network oscillations and firing patterns of pyramidal and GABAergic neurons recorded in head-fixed and freely moving mice suggests preserved neuronal coordination. In spatial memory tests, transgenic mice have reduced short-term familiarity, but spatial working and reference memory are surprisingly normal. We hypothesize that unimpaired subcortical network mechanisms maintain cortical neuronal coordination, counteracting the widespread pTau aggregation, loss of high-firing cells, and neurodegeneration.

Aversive Contexts Reduce Activity in the Ventral Subiculum- BNST Pathway

Many anxiety disorders can be characterized by abnormalities in detecting and learning about threats, and the inability to reduce fear responses in non-threatening environments. PTSD may be the most representative of context processing pathology, as intrusive memories are experienced in "safe" contexts. The ventral subiculum (vSUB), the main output of the ventral hippocampus, encodes environmental cues and is critical for context processing. The bed nucleus of the stria terminalis (BNST) contributes to anxiety-like behaviors as well as context fear conditioning. Given the important roles of the BNST and the vSUB in these anxiety and fear-related behaviors, and the anatomical connections between the two brain regions, the major aims of this study were to characterize the anatomy and function of the vSUB-BNST pathway. First, using the retrograde tracer cholera toxin, we mapped the topographical arrangement of the vSUB-BNST pathway. Dual retrograde tracing experiments revealed neurons projecting to the BNST and those projecting to the basolateral amygdala are distinct populations. Second, we assessed whether activity in this pathway, as indexed by FOS immunohistochemistry, was modulated by context fear conditioning. Our data reveal less activation of the vSUB-BNST pathway in both males and females in aversive contexts and the greatest activation when animals explored a neutral familiar context. In addition, the vSUB of females contained fewer GABAergic neurons compared to males. These findings suggest that the vSUB-BNST pathway is involved in eliciting appropriate responses to contexts.

Sex hormone fluctuation and increased female risk for depression and anxiety disorders: From clinical evidence to molecular mechanisms

Women are at twice the risk for anxiety and depression disorders as men are, although the underlying biological factors and mechanisms are largely unknown. In this review, we address this sex disparity at both the etiological and mechanistic level. We dissect the role of fluctuating sex hormones as a critical biological factor contributing to the increased depression and anxiety risk in women. We provide parallel evidence in humans and rodents that brain structure and function vary with naturally-cycling ovarian hormones. This female-unique brain plasticity and associated vulnerability are primarily driven by estrogen level changes. For the first time, we provide a sex hormone-driven molecular mechanism, namely chromatin organizational changes, that regulates neuronal gene expression and brain plasticity but may also prime the (epi)genome for psychopathology. Finally, we map out future directions including experimental and clinical studies that will facilitate novel sex- and gender-informed approaches to treat depression and anxiety disorders.

Hippocampus-Anterior Hypothalamic Circuit Modulates Stress-Induced Endocrine and Behavioral Response

Hippocampal input to the hypothalamus is known to be critically involved in mediating the negative feedback inhibition of stress response. However, the underlying neural circuitry has not been fully elucidated. Using a combination of rabies tracing, pathway-specific optogenetic inhibition, and cell-type specific synaptic silencing, the present study examined the role of hippocampal input to the hypothalamus in modulating neuroendocrine and behavioral responses to stress in mice. Transsynaptic rabies tracing revealed that the ventral hippocampus (vHPC) is monosynaptically connected to inhibitory cells in the anterior hypothalamic nucleus (AHN-GABA cells). Optogenetic inhibition of the vHPC→AHN pathway during a restraint stress resulted in a prolonged and exaggerated release of corticosterone, accompanied by an increase in stress-induced anxiety behaviors. Consistently, tetanus toxin-mediated synaptic inhibition in AHN-GABA cells produced a remarkably similar effect on the corticosterone release profile, corroborating the role of HPC→AHN pathway in mediating the hippocampal control of stress responses. Lastly, we found that chronic inhibition of AHN-GABA cells leads to cognitive impairments in both object and social recognition memory. Together, our data present a novel hypothalamic circuit for the modulation of adaptive stress responses, the dysfunction of which has been implicated in various affective disorders.

Alleviation of cognitive deficits via upregulation of chondroitin sulfate biosynthesis by lignan sesamin in a mouse model of neuroinflammation

Lignans are plant-derived compounds that act as partial estrogen agonists. Chondroitin sulfate proteoglycans (CSPGs) represent one of the major components of the extracellular matrix (ECM). Here we aimed to understand the role of sesamin (SES), a major lignan compound, in the biosynthesis and degradation of CSPGs in the mouse hippocampus because CSPGs play a key role in the regulation of cognitive functions through the promotion of adult neurogenesis. The expression of the pro-inflammatory cytokine interleukin-1β was decreased by SES administration in the hippocampus of lipopolysaccharide (LPS)-treated mice, a model of neuroinflammation-induced cognitive deficits. The expression of genes related to biosynthesis and degradation of CSPGs in the hippocampus of LPS-treated mice was both increased and decreased by SES administration. Further, the diffuse ECM labeling of CSPGs by Wisteria floribunda agglutinin (WFA) in the hippocampus of LPS-treated mice was increased by SES administration. The densities of neural stem cells, late transit-amplifying cells, and newborn-granule cells in the hippocampus of LPS-treated mice were also increased by SES administration. Moreover, SES-induced alterations in gene expression, WFA labeling, and adult neurogenesis in LPS-treated mice were more evident in the dorsal hippocampus (center of cognition) than in the ventral hippocampus (center of emotion). Neither LPS nor SES administration affected locomotor activity, anxiety-like behavior, and depression-related behavior. However, impairments in contextual memory and sensorimotor gating in LPS-treated mice were recovered by SES administration. Our results show that SES can promote adult hippocampal neurogenesis through the upregulation of CSPGs, which may alleviate cognitive deficits induced by neuroinflammation.

Parallel ventral hippocampus-lateral septum pathways differentially regulate approach-avoidance conflict

The ability to resolve an approach-avoidance conflict is critical to adaptive behavior. The ventral CA3 (vCA3) and CA1 (vCA1) subfields of the ventral hippocampus (vHPC) have been shown to facilitate avoidance and approach behavior, respectively, in the face of motivational conflict, but the neural circuits by which this subfield-specific regulation is implemented is unknown. We demonstrate that two distinct pathways from these subfields to lateral septum (LS) contribute to this divergent control. In Long-Evans rats, chemogenetic inhibition of the vCA3- LS caudodorsal (cd) pathway potentiated approach towards a learned conflict-eliciting stimulus, while inhibition of the vCA1-LS rostroventral (rv) pathway potentiated approach non-specifically. Additionally, vCA3-LScd inhibited animals were less hesitant to explore food during environmental uncertainty, while the vCA1- LSrv inhibited animals took longer to initiate food exploration. These findings suggest that the vHPC influences multiple behavioral systems via differential projections to the LS, which in turn send inhibitory projections to motivational centres of the brain.

Dual medial prefrontal cortex and hippocampus projecting neurons in the paraventricular nucleus of the thalamus

The paraventricular nucleus (PVT) of the midline thalamus is a critical higher-order cortico-thalamo-cortical integration site that plays a critical role in various behaviors including reward seeking, cue saliency, and emotional memory. Anatomical studies have shown that PVT projects to both medial prefrontal cortex (mPFC) and hippocampus (HC). However, dual mPFC-HC projecting neurons which could serve a role in synchronizing mPFC and HC activity during PVT-dependent behaviors, have not been explored. Here we used a dual retrograde adenoassociated virus (AAV) tracing approach to characterize the location and proportion of different projection populations that send collaterals to mPFC and/or ventral hippocampus (vHC) in rats. Additionally, we examined the distribution of calcium binding proteins calretinin (CR) and calbindin (CB) with respect to these projection populations in PVT. We found that PVT contains separate populations of cells that project to mPFC, vHC, and those that innervate both regions. Interestingly, dual mPFC-HC projecting cells expressed neither CR nor CB. Topographically, CB+ and CR+ containing cells clustered around dual projecting neurons in PVT. These results are consistent with the features of dual mPFC-vHC projecting cells in the nucleus reuniens (RE) and suggestive of a functional mPFC-PVT-vHC system that may support mPFC-vHC interactions in PVT-dependent behaviors.

Complement Dependent Synaptic Reorganisation During Critical Periods of Brain Development and Risk for Psychiatric Disorder

We now know that the immune system plays a major role in the complex processes underlying brain development throughout the lifespan, carrying out a number of important homeostatic functions under physiological conditions in the absence of pathological inflammation or infection. In particular, complement-mediated synaptic pruning during critical periods of early life may play a key role in shaping brain development and subsequent risk for psychopathology, including neurodevelopmental disorders such as schizophrenia and autism spectrum disorders. However, these disorders vary greatly in their onset, disease course, and prevalence amongst sexes suggesting complex interactions between the immune system, sex and the unique developmental trajectories of circuitries underlying different brain functions which are yet to be fully understood. Perturbations of homeostatic neuroimmune interactions during different critical periods in which regional circuits mature may have a plethora of long-term consequences for psychiatric phenotypes, but at present there is a gap in our understanding of how these mechanisms may impact on the structural and functional changes occurring in the brain at different developmental stages. In this article we will consider the latest developments in the field of complement mediated synaptic pruning where our understanding is beginning to move beyond the visual system where this process was first described, to brain areas and developmental periods of potential relevance to psychiatric disorders.

Genomic modules and intramodular network concordance in susceptible and resilient male mice across models of stress

The multifactorial etiology of stress-related disorders necessitates a constant interrogation of the molecular convergences in preclinical models of stress that use disparate paradigms as stressors spanning from environmental challenges to genetic predisposition to hormonal signaling. Using RNA-sequencing, we investigated the genomic signatures in the ventral hippocampus common to mouse models of stress. Chronic oral corticosterone (CORT) induced increased anxiety- and depression-like behavior in wild-type male mice and male mice heterozygous for the gene coding for brain-derived neurotrophic factor Val66Met, a variant associated with genetic susceptibility to stress. In a separate set of male mice, chronic social defeat stress (CSDS) led to a susceptible or a resilient population, whose proportion was dependent on housing conditions, namely standard housing or enriched environment. Rank-rank-hypergeometric overlap (RRHO), a threshold-free approach that ranks genes by their p value and effect size direction, was used to identify genes from a continuous gradient of significancy that were concordant across groups. In mice treated with CORT and in standard-housed susceptible mice, differentially expressed genes (DEGs) were concordant for gene networks involved in neurotransmission, cytoskeleton function, and vascularization. Weighted gene co-expression analysis generated 54 gene hub modules and revealed two modules in which both CORT and CSDS-induced enrichment in DEGs, whose function was concordant with the RRHO predictions, and correlated with behavioral resilience or susceptibility. These data showed transcriptional concordance across models in which the stress coping depends upon hormonal, environmental, or genetic factors revealing common genomic drivers that embody the multifaceted nature of stress-related disorders.