Ventral CA1 neurons store social memory

The ventral hippocampus mediates experience-dependent social modulation of fear in rats

Fear Conditioning by Proxy (FCbP) is a form of socially mediated fear learning, in which no-conditioned rodents acquire fear memories through social interactions with fear-conditioned rodents. This study investigates the impact of prior similar experiences on the transmission of contextual fear memories in FCbP and explores the role of the ventral hippocampus (vHPC) in the social transmission of fear. Observers were divided into two groups: those with contextual experience (D/O) and those without contextual experience- naïve (O). These rats were exposed to fear-conditioned demonstrators (D) through social interaction, and their responses to fear contexts were observed. Additionally, the effect of vHPC inactivation on fear memory transmission was examined by injecting lidocaine into the vHPC. Fear was transmitted through social interaction among experienced rats but not among naive rats. Furthermore, lidocaine injection into the vHPC inhibited the social transmission of fear memories among experienced rats. This study demonstrates that contextual fear memories can be transmitted through social interaction among experience-dependent rats but not among naive rats. That inactivation in the vHPC blocks the social transmission of contextual fear memories.

Pharmacological reduction of reverse-translated hippocampal hyperactivity in mouse: relevance for psychosis

Hippocampal hyperactivity (HH) is a potential biomarker in schizophrenia psychosis, which also appears in several other brain disorders, compromising specificity. We hypothesized that the reversal of HH in an established, reverse-translational animal preparation, coupled with a behavioral marker of psychosis may be a predictor of antipsychotic efficacy of a medication. We used a chemogenetic reverse-translational mouse preparation relevant to schizophrenia psychosis which shows HH and aberrant psychosis-relevant behaviors, specifically disrupted social recognition memory (SRM). Mice with and without HH were treated with three drugs; two known antipsychotics and one HH-reducing anticonvulsant, to assess their effects on both HH and SRM performance. All animals received one of the four treatments: vehicle (N = 15-24), haloperidol (N = 8-15), xanomeline (N = 8-13) or levetiracetam (N = 6-15) and were subsequently tested for baseline c-Fos protein expression within the hippocampal subfields (CA3 and CA1) as a measure of neuronal activity, or tested with the SRM task as a measure of social memory. All three drugs acutely reduced baseline HH compared to vehicle treatment. Subacute administration of haloperidol or xanomeline, the two drugs known to have antipsychotic activity, but not levetiracetam, normalized the SRM behavior to control levels. These results suggest that the reversal of HH alone cannot be a predictor of antipsychotic efficacy of an experimental drug and HH as a biomarker could benefit from a more sensitive readout approach.

The endocannabinoid and paracannabinoid systems in natural reward processes: possible pharmacological targets?

Natural rewards such as food, mating, and social interaction are essential for survival and species preservation, and their regulation involves a complex interplay of motivational, cognitive, and emotional processes. Over the past two decades, increasing attention has been directed toward the endocannabinoid system and its paracannabinoid counterpart as key modulators of these behaviors. This review aims to provide an integrated overview of the roles played by the endocannabinoid and paracannabinoid systems in regulating natural reward-driven behaviors, focusing on feeding, reproductive behavior, and social interaction. We highlight how the endocannabinoid system - mainly through CB1 receptor signaling - modulates central and peripheral circuits involved in energy homeostasis, reward processing, and emotional regulation. In parallel, we explore the role of paracannabinoids, such as oleoylethanolamide (OEA), palmitoylethanolamide (PEA), and stearoylethanolamide (SEA), which act primarily via non-cannabinoid receptors and contribute to the regulation of appetite, sexual motivation, and social behavior. Special attention is given to the relevance of these systems in the pathophysiology of obesity, eating disorders, sexual dysfunctions, and social impairments, as well as their potential as pharmacological targets. Overall, the evidence discussed supports a broader conceptualization of endocannabinoid and paracannabinoid signaling as pivotal regulators of natural rewards and opens new avenues for the development of targeted interventions for motivational and reward-related disorders.

Ventral hippocampus neurons encode meal-related memory

The ability to encode and retrieve meal-related information is critical to efficiently guide energy acquisition and consumption, yet the underlying neural processes remain elusive. Here we reveal that ventral hippocampus (HPCv) neuronal activity dynamically elevates between eating bouts during meal consumption and this response is predictive of performance in a foraging-related memory test for the spatial location of a previously consumed meal. Targeted recombination-mediated ablation of HPCv meal-responsive neurons impairs meal location memory without influencing food motivation or spatial memory for escape location. These HPCv meal-responsive neurons project to the lateral hypothalamic area (LHA) and are enriched in serotonin 2a receptors (5HT2aR). Either chemogenetic silencing of HPCv-to-LHA projections or intra-HPCv 5HT2aR antagonist yielded meal location memory deficits, as well as increased caloric intake driven by shorter temporal intervals between meals. Collective results identify a population of HPCv neurons in male rats that dynamically respond during eating to encode meal-related memories.

Oxytocin in periaqueductal gray plasticly regulates strain-dependent social recognition memory in mice, modeling social identity

Social identity differences are crucial for gregarious animals, impacting survival and social development. This is particularly evident in humans, where social stratification, cultural divides, and ethnic differences influence societal dynamics. Social recognition memory plays a central role in this process, maintaining social order by allowing individuals to distinguish familiar members within their group. Notably, social recognition memory exhibits differences: within a group, individuals form detailed memories of each member (individualized memory), while for out-group members, a more generalized memory of the entire group forms (categorized memory). Although this phenomenon has been explored in human studies, current research techniques and methods have limited investigations into the underlying neural mechanisms, especially their plasticity and regulatory mechanisms. This study utilizes mice to establish an experimental model for investigating differences in social recognition memory and its neural basis. We demonstrate that mice also exhibit social identity-driven memory recognition patterns. Mice form individualized memories for same-strain individuals but categorized memories for different strains, and the type of social recognition memory could be regulated by oxytocin level of ventrolateral periaqueductal gray. These findings demonstrate that oxytocin and its receptors in the ventrolateral periaqueductal gray are essential for constructing and plastically regulating intergroup social memory in mice.

Hippocampal area CA2 activity supports social investigation following an acute social stress

Neuronal activity in the hippocampus is critical for many types of memory acquisition and retrieval and influences an animal's response to stress. Moreover, the molecularly distinct principal neurons of hippocampal area CA2 are required for social recognition memory and aggression in mice. To interrogate the effects of stress on CA2-dependent behaviors, we chemogenetically manipulated neuronal activity in vivo during an acute, socially derived stressor and tested whether memory for the defeat was influenced. One day after an acute social defeat (aSD), defeated mice spent significantly less time investigating another mouse when compared to non-defeated control mice. We found that this avoidant phenotype persisted for up to one month following a single defeat encounter. When CA2 pyramidal neuron activity was inhibited with Gi-DREADD receptors during the defeat, subject mice exhibited a significantly higher amount of social avoidance one day later when compared to defeated littermates not expressing DREADDs. Moreover, CA2 inhibition during defeat caused a reduction in submissive defense behaviors in response to aggression. In vitro electrophysiology and tracing experiments revealed a circuit wherein CA2 neurons connect to caudal CA1 projection neurons that, in turn, project to corticolimbic regions including the anterior cingulate cortex. Finally, socially avoidant, defeated mice exhibited significant reductions in cFos expression in caudal hippocampal and limbic brain areas during a social investigation task 24 h after aSD. Taken together, these results indicate that CA2 neuronal activity is required to support behavioral resilience following an acute social stressor and that submissive defensive behavior during the defeat (vs. fleeing) is a predictor of future resilience to social stress. Furthermore, CA2 preferentially targets a population of caudal CA1 projection neurons that contact cortical brain regions where activity is modulated by an acute social stressor.

Emulation as a behavioral strategy underlying spatial observational learning in rats

Humans and several non-human species have shown the ability to learn by observing an experimented conspecific. A basic form of this learning is spatial observational learning (sOL), where a naïve animal improves their accuracy in a spatial task after witnessing a demonstrator solve the same task. This ability has gained neurophysiological support with the discovery of hippocampal CA1 social place cells, which encoded the position of others, and the role of CA2 and ventral CA1 neurons in processing and storing social memory. However, the direct role of the hippocampus in accomplishing sOL and the behavioral changes adopted by the observer animal have not been fully understood. Observational learning can be explained by behavioral processes such as imitation, emulation, or local enhancement. We used a modified version of the oasis maze to unravel the strategy unfolded by naïve observer rats during sOL. Our results suggest that emulation is the primary strategy implemented by observers by switching from a free-foraging approach to goal-directed behavior. Furthermore, the pharmacological inactivation of the hippocampus during the observation period impeded sOL, revealing the necessity of this structure for engaging in this behavioral change. Our results propose that the hippocampus is necessary for the internal representation of the demonstrator in the space and their movement towards a particular area and for the animal comprehension of the behavioral purpose of others during observational learning.

Capturing dynamic neuronal responses to dominant and subordinate social hierarchy members with catFISH

Dominance hierarchies are key to social organization in group-living species, requiring individuals to recognize their own and others' ranks. This is particularly complex for mid-ranking animals, who navigate interactions with higher- and lower-ranking individuals. Using in situ hybridization, we examined how mid-ranked mice's brains respond to dominant and subordinate stimuli by labeling activity-induced immediate early genes and neuronal markers. We show that distinct neuronal populations in the amygdala and hippocampus respond differentially across social contexts. In the basolateral amygdala and dorsal endopiriform, glutamatergic Slc17a7+ neurons, particularly dopamine-receptive Slc17a7+Drd1+ neurons, show elevated IEG expression in response to social stimuli, with a higher response to dominant over subordinate animals. Similar response patterns are observed among GABAergic Slc32a1+Oxtr+ neurons in the medial amygdala. We also identified distinct neural ensembles selectively active in response to dominant and subordinate animals by examining cell reactivation over repeated stimulus presentations. We find a higher degree of reactivation among Slc17a7+Oxtr+ ensembles in the endopiriform when the same individual was presented twice in succession. A similar pattern was observed among Oxtr+ neurons in the dentate gyrus hilus, while the inverse was observed among Slc17a7+Avrp1b+Oxtr+ neurons in distal CA2CA3, suggesting distinct encoding or recollection mechanisms across hippocampal subregions. We also highlight methodological advances showing that IEG responses are shaped by stimulus duration and the identity of the IEG and timepoint at which expression is measured. This work lays the foundation for further precise, cell type-resolved investigation into how the brain processes social information.

Altered activity within the social behavior neural network in adolescent rats following prenatal alcohol exposure and/or early-life adversity

BACKGROUND

Social behavior relies on the dynamic, complex, and coordinated activity of a highly conserved "social behavior neural network," which includes the olfactory bulb (OB), piriform cortex (PCX), lateral septum, medial prefrontal cortex (mPFC), amygdala, paraventricular nucleus of the hypothalamus (PVN), and ventral hippocampus. Prenatal alcohol exposure (PAE) is known to disrupt social behavior development, leading to lifelong social functioning impairments. Individuals with PAE are at heightened risk of experiencing early-life adversity (ELA), which independently affects social behavior development; however, little is known about the combined effects of PAE and ELA on social behavior.

METHODS

We previously demonstrated that PAE and ELA impact social recognition memory throughout adolescence; here, we combine animal models of PAE and ELA to gain insight into both independent and interactive effects of these insults on social behavior network neural activity in both early and late adolescent male and female rats. We measured neural activity (c-fos mRNA expression) across the network following social recognition memory testing.

RESULTS

Our findings indicate that both PAE and ELA are associated with altered neural activity in regions supporting social recognition memory, notably the OB, PCX, mPFC, amygdala, and PVN. The direction of these effects and specific regions impacted vary by sex and age at assessment. Importantly, different brain areas exhibit distinct sensitivities to each type of insult, with PAE generally resulting in hypoactivity of the amygdala and ELA altering mPFC activity.

CONCLUSIONS

These data contribute to a more complete social neurobehavioral profile, accounting for both PAE and ELA, to support earlier diagnoses and interventions.

The ventral CA2 region of the hippocampus and its differential contributions to social memory and social aggression

The dorsal and ventral regions of the CA1 field of the hippocampus play distinct roles in the encoding of cognitive vs. emotional behaviors, respectively. Whether this distinction applies to other hippocampal fields and other behaviors is unclear. Here, we focus on the hippocampal CA2 field and compare the properties and behavioral roles of its dorsal (dCA2) and ventral (vCA2) regions. Although dCA2 is known to be required for social memory and to promote social aggression, the role of vCA2 is unknown. We report that a defined CA2 region extends to the extreme ventral pole of the hippocampus, with certain distinctions to dCA2. Unlike dCA2, chemogenetic silencing of vCA2 pyramidal neurons did not impair social memory. Similar to dCA2, vCA2 was required to promote social aggression. Thus, consistent with the CA1 region, CA2 may be differentially tuned to support cognitive compared with emotional processes along its dorsal to ventral axis.

Anterior cingulate cortex parvalbumin and somatostatin interneurons shape social behavior in male mice

The anterior cingulate cortex (ACC) is essential for social behavior, and its dysfunction is implicated in social interaction deficits in autism. Pyramidal neuron activity in the ACC is modulated by parvalbumin (PV) and somatostatin (SST) interneurons, though their specific roles in social interactions remain unclear. Here, we demonstrate that PV and SST interneurons differentially contribute to the regulation of social interactions. In a Shank3-deficient autistic model, the expression of Kcnh7, a risk gene for autism, is reduced in both PV and SST interneurons. Knocking out Kcnh7 in either interneuron subtype leads to social interaction deficits. Furthermore, projections from the lateral posterior thalamic nucleus (mediorostral part, LPMR) to PV interneurons and from the ventral hippocampus (vHPC) to SST interneurons differentially modulate social interactions. These findings provide new insights into the distinct roles of PV and SST interneurons in social processes and their contributions to autism-related pathophysiology.

Use of magnetic resonance structural imaging to identify disease progression in patients with mild cognitive impairment: A voxel-based morphometry and surface-based morphometry study

Voxel-based morphometry (VBM) and surface-based morphometry (SBM) based on magnetic resonance structural imaging were used to identify disease progression in mild cognitive impairment (MCI) patients. A retrospective analysis was conducted on 154 MCI patients from the Alzheimer's Disease Neuroimaging Initiative (ADNI) database, with 62 patients classified into the progressive MCI (pMCI) group and 92 patients into the stable MCI (sMCI) group. VBM and SBM were employed to identify structural differences between sMCI and pMCI patients, and differential features were extracted for model construction. The logistic regression method was used to establish relevant index models, and the DeLong test was used to compare the diagnostic performance of the different models. Additionally, 51 patients from the National Alzheimer's Coordinating Center (NACC) database were used as an external validation set to further validate the clinical efficacy of the model. Significant structural differences between pMCI and sMCI patients were revealed through VBM and SBM analyses. Volume reductions were observed in the frontal and temporal lobes, and cortical thinning occurred in the left inferior and superior parietal cortices. Reduced gyrification was observed in the bilateral insular gyrus. The structural joint model, which combines volume and cortical indices, demonstrated higher diagnostic accuracy compared to the joint scale index model that combines the Mini-Mental State Examination (MMSE) and Montreal Cognitive Assessment (MOCA) indices. The findings indicate that combined VBM and SBM analysis offers a sensitive and noninvasive approach to detect structural biomarkers of MCI progression, providing a practical tool for early risk stratification and personalized clinical management.

Diminished Social Memory and Hippocampal Correlates of Social Interactions in Chronic Social Defeat Stress Susceptibility

Background

Susceptibility to chronic stress has been associated with depression, a mood disorder that highly implicates the hippocampus. Hippocampal contribution to stress susceptibility has been supported by findings in mice following chronic social defeat stress (CSDS). However, little is known about the role of hippocampal activity in determining the development of stress susceptibility.

Methods

We used the UCLA Miniscope to longitudinally measure the activity of dorsal CA1 hippocampal neurons during CSDS. In addition to examining the representation of social information by these neurons, we compared social memory in mice that were either susceptible or resilient to CSDS.

Results

We observed more stable dorsal CA1 correlates of social interaction and social memory in CSDS-resilient mice. Such changes were absent in CSDS-susceptible mice and accompanied by greater social memory impairments.

Conclusions

CSDS susceptibility may be supported by hippocampal social cognitive processes, as reflected in diminished hippocampal representations of social information and greater impairment in social memory in suspectible compared with resilient mice.

Neural connectivity of oxytocin receptor-expressing neurons in the nucleus accumbens and their role in social attachment

Oxytocin receptor (OXTR) activity in the nucleus accumbens (NAc) is critical for pair bonding in prairie voles. Oxtr knockdown or pharmacological blockade in this region prevents mating-induced partner preferences, while overexpression facilitates bonding. However, no prior work has selectively interrogated or manipulated Oxtr-expressing neurons during dynamic bonding behaviors. We have developed an Oxtr-P2A-Cre prairie vole line that enables direct access to specific Oxtr neural populations. We utilized Oxtr-P2A-Cre prairie voles to express inhibitory DREADDs selectively in OXTR-expressing NAc neurons. Inhibiting NAc OXTR cells during initial cohabitation did not affect subsequent partner preference formation; however, inhibition during partner preference testing increased partner-directed huddling behavior, revealing a complex role for these neurons in social interactions. Using a viral tracing approach, we found that NAc OXTR-expressing neurons receive prominent inputs from the medial prefrontal cortex, hippocampus, thalamus, and hypothalamus, while projecting strongly to the ventral pallidum, ventral tegmental area, and lateral hypothalamus. Our cell-type-specific manipulation reveals how oxytocin receptor signaling in the NAc may modulate emotional state and facilitate the complex social behaviors underlying monogamous pair bonding. This Cre-recombinase approach demonstrates the utility of cell-type-specific targeting for elucidating oxytocin neural circuit mechanisms regulating emotional and social behavior in prairie voles.

Serotonin and neurotensin inputs in the vCA1 dictate opposing social valence

The ability to evaluate valence of a social agent based on social experience is essential for an animal's survival in its social group1. Although hippocampal circuits have been implicated in distinguishing novel and familiar conspecifics2-7, it remains unclear how social valence is constructed on the basis of social history and what mechanisms underlie the heightened valence versatility in dynamic relationships. Here we demonstrate that the ventral (v)CA1 integrates serotonin (5-HT) inputs from the dorsal raphe and neurotensin inputs from the paraventricular nucleus of the thalamus (PVT) to determine positive or negative valence of conspecific representations. Specifically, during an appetitive social interaction 5-HT is released into the vCA1 and disinhibits pyramidal neurons through 5-HT1B receptors, whereas neurotensin is released during an aversive social interaction and potentiates vCA1 neurons directly through NTR1s. Optogenetic silencing of dorsal raphe 5-HT and PVT neurotensin inputs into the vCA1 impairs positive and negative social valence, respectively, and excitation flexibly switches valence assignment. These results show how aversive and rewarding social experiences are linked to conspecific identity through converging dorsal raphe 5-HT and PVT neurotensin signals in the vCA1 that instruct opposing valence, and represent a synaptic switch for flexible social valence computation.

Brain circuits that regulate social behavior

Social interactions are essential for the survival of individuals and the reproduction of populations. Social stressors, such as social defeat and isolation, can lead to emotional disorders and cognitive impairments. Furthermore, dysfunctional social behaviors are hallmark symptoms of various neuropsychiatric disorders, including autism spectrum disorder (ASD) and post-traumatic stress disorder (PTSD). Consequently, understanding the neural circuit mechanisms underlying social behaviors has become a major focus in neuroscience. Social behaviors, which encompass a wide range of expressions and phases, are regulated by complex neural networks. In this review, we summarize recent progress in identifying the circuits involved in different types of social behaviors, including general social investigation, social preference, mating, aggression, parenting, prosocial behaviors, and dominance behaviors. We also outline the circuit mechanisms associated with social deficits in neuropsychiatric disorders, such as ASD, schizophrenia, and PTSD. Given the pivotal role of rodents in social behavior research, our review primarily focuses on neural circuits in these animals. Finally, we propose future research directions, including the development of specific behavioral paradigms, the identification of circuits involved in motor output, the integration of activity, transcriptome, and connectome data, the multifunctional roles of neurons with multiple targets, and the interactions among multiple brain regions.

A molecularly defined mPFC-BLA circuit specifically regulates social novelty preference

Social novelty preference is an important aspect of social interaction for evaluating new threats and opportunities for survival, but the underlying neuronal mechanism remains unclear. Here, we identify a molecularly defined medial prefrontal cortex (mPFC) excitatory neuron subtype, located in layer 5 expressing Il1rapl2, which is highly associated with social deficit disorders in genome-wide association studies and might be responsible for regulating social novelty preference. Using an Il1rapl2-Cre mouse line, we show that chemogenetic activation of the mPFC Il1rapl2-expressing neurons impairs social novelty preference but with little effect on sociability. In addition, fiber photometry recording indicates that this neuron subtype is inhibited when mice interact with novel but not with familiar mice. Furthermore, viral tracing and terminal manipulation reveal that basolateral amygdala (BLA)-projecting Il1rapl2+ neurons mediate the social novelty preference. Thus, our study uncovers a molecularly defined mPFC-BLA circuit that specifically regulates social novelty preference, highlighting that specific neuron subtypes and circuits could modulate distinct aspects of social behaviors.

Social subordination is associated with better cognitive performance and higher theta coherence of the mPFC-vHPC circuit in male rats

Social dominance hierarchy is considered an influential factor on cognitive performance. The spatial working memory (SWM) is inversely related to dominance status after the formation of social hierarchy. However, their neural underpinings are poorly understood. The medial prefrontal cortex (mPFC) and ventral hippocampus (vHPC) play pivotal roles in social hierarchy and SWM. To investigate the associations between social hierarchy and SWM and their neural circuit (mPFC-vHPC), we used twenty one natal male Wistar rats after weaning (3 rats per cage, 7 cages in total). In the 9th postnatal week, the tube test was started to determine the relative social rank in each cage (dominant, middle-ranked, subordinate). One month after living in the hierarchy, we implanted electrodes in mPFC and vHPC. One week following recovery, the SWM test was performed using T-maze with two difficulty levels (30s and 5min delays between trials) while recording the local field potentials. The percentage of correct responses showed no significant difference among three different social groups. However, subordinates demonstrated significantly lower latency in reaching the goal arm, while middle-ranked rats exhibited the longest latency in 30s delay. Electrophysiological data revealed significantly higher theta correlation and coherence of the mPFC-vHPC circuit in subordinates. Although theta rhythm synchronization was reduced in all social ranks by increasing task difficulty, the subordinates maintained better task performance and less reduction of theta coherence. These findings underscore the association between social hierarchy and working memory performance within the mPFC-vHPC circuit, highlighting the influence of social rank on implicated circuit.

Layer-specific input to medial prefrontal cortex is linked to stress susceptibility

Stress response is essential for adapting to an ever-changing environment. However, the mechanisms that render some individuals susceptible to stress are poorly understood. While chronic stress is known to induce dendritic atrophy and spine loss in medial prefrontal cortex (mPFC), its impact on synapses made by long-range projections terminating on the mPFC remains unknown. Here, we labeled synapses on male mouse mPFC dendrites formed by ventral hippocampus (VH), basolateral amygdala (BLA) and ventral tegmental area (VTA) long-range afferents using different-colored eGRASP constructs. We obtained multispectral 3D-images of the mPFC covering all cortical laminae, and automatically segmented the dendrites and synapses. In layer II/III, the relative abundances and spatial organizations of VH-mPFC and BLA-mPFC synapses changed similarly in stress resilient (SR) and stress susceptible (SS) mice when compared to stress naïve (SN) mice. In layers Vb and VI, on the other hand, the percentage of BLA-mPFC synapses increased and that of VH-mPFC decreased only in SS mice. Moreover, the distances of VH synapses to their corresponding closest BLA synapses decreased and the distances of BLA synapses to their corresponding closest VH synapses increased in the SS group. Consistently, the percentage of single dendritic segments receiving input from multiple brain regions increased in the SS group, suggesting that long-range synaptic inputs to deep layers of mPFC were disorganized in SS mice. Our findings demonstrate afferent- and lamina-specific differential reorganization of synapses between different stress phenotypes, suggesting specific roles for different long-range projections in mediating the stress response.

Integrating the COM-B model into behavioral neuroscience: A framework for understanding animal behavior

In light of the intricate nature of animal behavior regulation, a theoretical model is proposed, grounded in the COM-B (Capability, Opportunity, Motivation - Behavior) framework, which has gained considerable traction in the domain of human behavioral intervention. When extending the COM-B model to behavioral neuroscience, we first discuss the utility of the model in animal research, particularly its capacity to integrate environmental and social factors, and enhance cross-species comparisons, including animal-to-human translations and evolutionary considerations. The subsequent discussion then summarizes the advantages of utilizing the COM-B model in neuroscience are summarized, including the facilitation of a systems-level understanding of behavior and the establishment of a link between neural mechanisms and specific behavioral components. The experimental design for the application of the COM-B model in neuroscience is proposed to elucidate the brain regulatory processes that govern behavior. Finally, three specific examples are provided to illustrate the theoretical considerations, namely feeding and social behavior, and the role of neuromodulators in the control of behavior.

Essential Role of the Anterior Piriform Cortex in Mediating Social Novelty Output via a Top-Down Circuit

Social novelty is indispensable for a wide range of social behaviors. The medial prefrontal cortex (mPFC), along with other social information hubs, composes the foundational circuitry of social novelty. However, the precise circuit mechanisms that govern social novelty processing remain elusive. The piriform cortex, as the largest olfactory cortex, receives extensive innervation from top-down centers that dictate social behavior. Here, it is shown that the anterior piriform cortex (APC) exhibited an increase in gamma event incidence during social engagement in male mice. In vivo electrophysiology and fiber photometry reveal that APC pyramidal neurons respond more intensely to novel mice than familiar ones. Intriguingly, silencing APC neurons selectively impairs social novelty processing, yet leaves the basic olfactory discrimination capabilities intact. Moreover, the APC inherits social cues from the mPFC and sends feedback projections to the olfactory bulb (OB) to modulate social novelty. These findings unveil the APC's role as extending well beyond olfaction, encompassing a specialized function in social novelty recognition in male mice.

Compensatory Regulation of Excitation/Inhibition Balance in the Ventral Hippocampus: Insights from Fragile X Syndrome

The excitation/inhibition (E/I) balance is a critical feature of neural circuits, which is crucial for maintaining optimal brain function by ensuring network stability and preventing neural hyperexcitability. The hippocampus exhibits the particularly interesting characteristics of having different functions and E/I profiles between its dorsal and ventral segments. Furthermore, the hippocampus is particularly vulnerable to epilepsy and implicated in Fragile X Syndrome (FXS), disorders associated with heightened E/I balance and possible deficits in GABA-mediated inhibition. In epilepsy, the ventral hippocampus shows heightened susceptibility to seizures, while in FXS, recent evidence suggests differential alterations in excitability and inhibition between dorsal and ventral regions. This article explores the mechanisms underlying E/I balance regulation, focusing on the hippocampus in epilepsy and FXS, and emphasizing the possible mechanisms that may confer homeostatic flexibility to the ventral hippocampus in maintaining E/I balance. Notably, the ventral hippocampus in adult FXS models shows enhanced GABAergic inhibition, resistance to epileptiform activity, and physiological network pattern (sharp wave-ripples, SWRs), potentially representing a homeostatic adaptation. In contrast, the dorsal hippocampus in these FXS models is more vulnerable to aberrant discharges and displays altered SWRs. These findings highlight the complex, region-specific nature of E/I balance disruptions in neurological disorders and suggest that the ventral hippocampus may possess unique compensatory mechanisms. Specifically, it is proposed that the ventral hippocampus, the brain region most prone to hyperexcitability, may have unique adaptive capabilities at the cellular and network levels that maintain the E/I balance within a normal range to prevent the transition to hyperexcitability and preserve normal function. Investigating the mechanisms underlying these compensatory responses in the ventral hippocampus and their developmental trajectories may offer novel insights into strategies for mitigating E/I imbalances in epilepsy, FXS, and potentially other neuropsychiatric and neurodevelopmental disorders.

Rac1 in parvalbumin neurons of the medial prefrontal cortex governs rapid forgetting of social memory

Social memory can undergo rapid forgetting at first according to the Ebbinghaus forgetting curve, for which the underlying mechanism remains entirely unknown. Here, we reported that rapid forgetting of social memory did not occur as indicated by social preference on stranger 2 (S2) over stranger 1 (S1) mouse, tested shortly after social interaction with S1. However, rapid forgetting of both social and object memories occurred as indicated by no social or object preference, respectively, when the constitutive active (CA) variant of Rac1 was knocked-in parvalbumin (PV) but not somatostatin (SST) neurons of the brain. Furthermore, rapid forgetting of only social memory occurred if this CA variant was knocked-in PV but not SST neurons of the medial prefrontal cortex (mPFC). By contrast, rapid forgetting of social memory was prevented by the dominant negative (DN) variant of Rac1 knocked-in PV neurons of the mPFC. Moreover, fiber photometry revealed that PV but not SST neurons of the mPFC generated dual calcium peaks to delineate each social interaction event. Thus, PV-specific Rac1 activity of the mPFC is both necessary and sufficient for controlling social behavior via rapid forgetting of social memory, providing a novel understanding of social behaviors under health and disease conditions.

Hippocampus carbonic anhydrase 1 via ERK pathway may be involved in depressive like behaviors

Major depressive disorder (MDD) is a destructive mental disease, yet the mechanism is still not clear. Carbonic anhydrase, an efficient catalyst for CO2 conversion to carbonate and protons, could affect many functions, such as memory formation recognition. Lately, we illustrated that carbonic anhydrase 1 (CAR1) knockout (CAR1-/-) mice could lead to depressive-like behaviors, but the underlying molecular mechanism is unknown. Herein, we attempted to explore whether CAR1 knockout could result in transcriptional changes thus involve in depressive like behaviors. The present study revealed that compared with WT mice, the CAR1 Knockout (CAR1-/-) mice led to depressive-like behavior. According to the microarray profiling, a couple of disturbed signaling pathways are found in CAR1-/- mice. Proteins like GluR1 and GABA Aα1 were validated compared to control groups by western blotting, while NMDAR 2A was increased in RNA level compared to the control group. More interestingly, the proteins might be related to the ERK signal pathway, MAPK, and RSK decreased in protein level compared to the control group. Moreover, this decline could be restored when CAR1 was overexpressed in the ventral hippocampus of CAR1-/- mice and depressive-like phenotypes were also rescued. Our dataset suggests that CAR1 might influence depressive-like behavior through the ERK signal pathway, which may provide novel insights and evidences to MDD study.

Cutting-edge methodologies for tagging and tracing active neuronal coding in the brain

Decoding the neural substrates that underlie learning and behavior is a fundamental goal in neuroscience. Identifying "key players" at the molecular, cellular, and circuit levels has become possible with recent advancements in molecular technologies offering high spatiotemporal resolution. Immediate-early genes are effective markers of neural activity and plasticity, allowing for the identification of active cells involved in memory-based behavior. A calcium-dependent labeling system coupled with light or biochemical proximity labeling allows characterization of active cell ensembles and circuitry across broader brain regions within short time windows, particularly during transient behaviors. The integration of these systems expands the ability to address diverse research questions across behavioral paradigms. This review examines current molecular systems for activity-dependent labeling, highlighting their applications in identifying specific cell ensembles and circuits relevant to various scientific questions and further discuss their significance, along with future directions for the development of innovative methodologies.

Variations of Aberrant Volume, Activity, and Network Connectivity of Hippocampus in Adolescent Male Rats Exposed to Juvenile Stress

BACKGROUND

Childhood is a crucial period for brain development, and short-term juvenile stress has demonstrated long-lasting effects on cognitive and cellular functions in the hippocampus. However, the influence of such stress on the brain's overall network remains unclear.

METHODS

In this study, we employed functional magnetic resonance imaging (fMRI) to explore the effects of transient wild stress on juvenile male rats. Pregnant rats were purchased and housed in a specific pathogen-free (SPF) environment, with pups separated by sex on postnatal day 21 (PD21). From PD27 to PD29, male rats were subjected to transient wild stress, which included forced swimming, elevated platform exposure, and restraint stress. Following stress exposure, all animals were carefully maintained and scanned at 42 days of age (PD42) using fMRI. Structural analysis was performed using voxel-based morphometry (VBM) to assess changes in gray matter volume, while functional activity was evaluated through regional homogeneity (ReHo) and voxel-wise functional connectivity.

RESULTS

The results showed significant reductions in gray matter volume in several brain regions in the stress group, including the periaqueductal gray (PAG), entorhinal cortex (Ent), and dentate gyrus (DG). In terms of functional activity, cortical regions, particularly the primary somatosensory areas, exhibited decreased activity, whereas increased activity was observed in the PAG, DG, and medulla. Furthermore, functional connectivity analysis revealed a significant reduction in connectivity between the DG and entorhinal cortex, while the DG-PAG connectivity was significantly enhanced.

CONCLUSIONS

These findings suggest that juvenile stress leads to profound alterations in both brain structure and function, potentially disrupting emotional regulation and memory processing by affecting the development and connectivity of key brain regions.

Environmental exposure to common pesticide induces synaptic deficit and social memory impairment driven by neurodevelopmental vulnerability of hippocampal parvalbumin interneurons

Environmental exposure to pesticides at levels deemed safe by regulatory agencies has been linked to increased risk for neurodevelopmental disorders. Yet, the mechanisms linking exposure to these disorders remain unclear. Here, we show that maternal exposure to the pesticide deltamethrin (DM) at the no observed adverse effect level (NOAEL) disrupts long-term potentiation (LTP) in the hippocampus of adult male offspring three months after exposure, a phenotype absent in female offspring. Clonazepam, a GABAa receptor agonist, rescued this deficit, indicating impaired hippocampal GABAergic signaling. Recordings from CA1 pyramidal neurons, complemented by MALDI mass spectrometry imaging, showed an imbalance in excitatory/inhibitory tone. Using a combination of parvalbumin (PV)-Cre transgenic mice and hippocampal injection of designer receptors exclusively activated by designer drugs (DREADDs), we show that developmental DM exposure reduces hippocampal PV interneuron intrinsic firing. DREADD activation rescued both PV interneuron firing and LTP deficits. Complementary behavioral experiments revealed a deficit in social memory, a behavior relevant to autism spectrum disorder (ASD) symptomatology, which was restored by DREADD activation. Overall, these results establish a novel mechanistic link between maternal exposure to DM at the NOAEL and known cellular, circuital, and behavioral vulnerabilities, indicating it is a potential driver in the exposome of ASD.

Engrams across diseases: Different pathologies - unifying mechanisms?

Memories are our reservoir of knowledge and thus, are crucial for guiding decisions and defining our self. The physical correlate of a memory in the brain is termed an engram and since decades helps researchers to elucidate the intricate nature of our imprinted experiences and knowledge. Given the importance that memories have for our lives, their impairment can present a tremendous burden. In this review we aim to discuss engram malfunctioning across diseases, covering dementia-associated pathologies, epilepsy, chronic pain and psychiatric disorders. Current neuroscientific tools allow to witness the emergence and fate of engram cells and enable their manipulation. We further suggest that specific mechanisms of mnemonic malfunction can be derived from engram cell readouts. While depicting the way diseases act on the mnemonic component - specifically, on the cellular engram - we emphasize a differentiation between forms of amnesia and hypermnesia. Finally, we highlight commonalities and distinctions of engram impairments on the cellular level across diseases independent of their pathogenic origins and discuss prospective therapeutic measures.

Vocal recognition of partners by female prairie voles

Recognizing conspecifics is vital for differentiating mates, offspring, and social threats. Individual recognition is often reliant upon chemical or visual cues but can also be facilitated by vocal signatures in some species. In common laboratory rodents, playback studies have uncovered communicative functions of vocalizations, but scant behavioral evidence exists for individual vocal recognition. Here, we find that the socially monogamous prairie vole (Microtus ochrogaster) emits behavior-dependent vocalizations that can communicate individual identity. Vocalizations of individual males change after bonding with a female; however, acoustic variation across individuals is greater than within-individual variation. Critically, females behaviorally discriminate their partner's vocalizations from a stranger's, even if emitted to another stimulus female. These results establish the acoustic and behavioral foundation for individual vocal recognition in prairie voles, where neurobiological tools enable future studies revealing its causal neural mechanisms.

Memory-related neurophysiological mechanisms in the hippocampus underlying stress susceptibility

Stress-induced psychiatric symptoms, such as increased anxiety, decreased sociality, and depression, differ considerably across individuals. The cognitive model of depression proposes that biased negative memory is a crucial determinant in the development of mental stress-induced disorders. Accumulating evidence from both clinical and animal studies has demonstrated that such biased memory processing could be triggered by the hippocampus, a region well known to be involved in declarative memories. This review mainly describes how memory-related neurophysiological mechanisms in the hippocampus and their interactions with other related brain regions are involved in the regulation of stress susceptibility and discusses potential interventions to prevent and treat stress-related psychiatric symptoms. Further neurophysiological insights based on memory mechanisms are expected to devise personalized prevention and therapy to confer stress resilience.

Social odors drive hippocampal CA2 place cell responses to social stimuli

Hippocampal region CA2 is essential for social memory processing. Interaction with social stimuli induces changes in CA2 place cell firing during active exploration and sharp wave-ripples during rest following a social interaction. However, it is unknown whether these changes in firing patterns are caused by integration of multimodal social stimuli or by a specific sensory modality associated with a social interaction. Rodents rely heavily on chemosensory cues in the form of olfactory signals for social recognition processes. To determine the extent to which social olfactory signals contribute to CA2 place cell responses to social stimuli, we recorded CA2 place cells in rats freely exploring environments containing stimuli that included or lacked olfactory content. We found that CA2 place cell firing patterns significantly changed only when social odors were prominent. Also, place cells that increased their firing in the presence of social odors alone preferentially increased their firing during subsequent sharp wave-ripples. Our results suggest that social olfactory cues are essential for changing CA2 place cell firing patterns during and after social interactions. These results support prior work suggesting CA2 performs social functions and shed light on processes underlying CA2 responses to social stimuli.

The Long-Term Effects of Chronic Unpredictable Mild Stress Experienced During Adolescence Could Vary Depending on Biological Sex

Sex differences in the neurobiology of responses to chronic stress have been widely discussed but remain poorly understood. We found that chronic unpredictable mild stress (CUMS) experienced during adolescence induced different behavioral patterns in adult males and females. Immunohistochemical analysis of the CA1 field of the dorsal and ventral hippocampus revealed no quantitative or morphological changes in astrocytes in the long term after CUMS. Real-time PCR analysis showed no increase in the expression level of SigmaR1 after CUMS relative to individual housekeeping genes. Analysis of mouse cerebral cortex homogenates showed that IL-1β levels only decreased after CUMS in males. However, the SigmaR1 levels were significantly higher in the CUMS groups than in the control groups in both sexes. It can be concluded that biological sex and age influence the response to CUMS, although not in all cases. Further studies are needed to understand the effects of chronic stress on males and females. This is important because men and women have different risks for stress and mental disorders.

Hippocampal coding of identity, sex, hierarchy, and affiliation in a social group of wild fruit bats

Social animals live in groups and interact volitionally in complex ways. However, little is known about neural responses under such natural conditions. Here, we investigated hippocampal CA1 neurons in a mixed-sex group of five to 10 freely behaving wild Egyptian fruit bats that lived continuously in a laboratory-based cave and formed a stable social network. In-flight, most hippocampal place cells were socially modulated and represented the identity and sex of conspecifics. Upon social interactions, neurons represented specific interaction types. During active observation, neurons encoded the bat's own position and head direction, together with the position, direction, and identity of multiple conspecifics. Identity-coding neurons encoded the same bat across contexts. The strength of identity coding was modulated by sex, hierarchy, and social affiliation. Thus, hippocampal neurons form a multidimensional sociospatial representation of the natural world.

Heterogeneity of anti-Caspr2 antibodies: specificity and pathogenicity

Maternal anti-Caspr2 (Contactin-associated protein-like 2) antibodies have been associated with increased risk for autism spectrum disorder (ASD). Previous studies have shown that in utero exposure to anti-Caspr2 antibodies results in a phenotype with ASD-like features in male mice. Here we ask whether four newly generated antibodies against Caspr2 are pathogenic to the developing fetal brain and whether they function through similar means. Our results show that the novel anti-Caspr2 antibodies recognize different epitopes of Caspr2. In utero exposure to these antibodies elicits differential ASD-like phenotypes in male offspring, tested in the social interaction, open field, and light-dark tasks. These results demonstrate variability in the antigenic specificity and pathogenicity of anti-Caspr2 antibodies which may have clinical implications.

Endopiriform neurons projecting to ventral CA1 are a critical node for recognition memory

The claustrum complex is viewed as fundamental for higher-order cognition; however, the circuit organization and function of its neuroanatomical subregions are not well understood. We demonstrated that some of the key roles of the CLA complex can be attributed to the connectivity and function of a small group of neurons in its ventral subregion, the endopiriform (EN). We identified a subpopulation of EN neurons by their projection to the ventral CA1 (ENvCA1-proj. neurons), embedded in recurrent circuits with other EN neurons and the piriform cortex. Although the ENvCA1-proj. neuron activity was biased toward novelty across stimulus categories, their chemogenetic inhibition selectively disrupted the memory-guided but not innate responses of mice to novelty. Based on our functional connectivity analysis, we suggest that ENvCA1-proj. neurons serve as an essential node for recognition memory through recurrent circuits mediating sustained attention to novelty, and through feed-forward inhibition of distal vCA1 neurons shifting memory-guided behavior from familiarity to novelty.

Central amygdala NPBWR1 neurons facilitate social novelty seeking and new social interactions

The formation of new social interactions is vital for social animals, but the underlying neural mechanisms remain poorly understood. We identified CeANpbwr1 neurons, a population in central amygdala expressing neuropeptide B/W receptor-1 (NPBWR1), that play a critical role in these interactions. CeANpbwr1 neurons were activated during encounters with unfamiliar, but not with familiar, mice. Manipulations of CeANpbwr1 neurons showed that their excitation is essential for maintaining physical interactions with novel conspecifics. Activation of CeANpbwr1 neurons alleviated social deficits induced by chronic social defeat stress, suggesting therapeutic potential. Conversely, overexpression of human NPBWR1 in CeANpbwr1 neurons reduced activity of these neurons and impaired social interactions with unfamiliar mice. This effect was absent in a polymorphic variant of the human NPBWR1 gene (404A>T). These findings highlight how CeANpbwr1 neurons promote social novelty seeking and reveal a complex interplay between NPBWR1 genetic variations and social behavior.

Role of peroxisome proliferator-activated receptors α and γ in mediating the beneficial effects of β-caryophyllene in a rat model of fragile X syndrome

β-Caryophyllene (BCP) is a naturally occurring sesquiterpene found in numerous plant species, including Cannabis sativa. BCP has shown a high safety profile and a wide range of biological functions, including beneficial effects in neurodegenerative and inflammatory diseases. Here, we used behavioral, pharmacological, and in-silico docking analyses to investigate the effects and mechanism of action of BCP in Fragile X Syndrome (FXS), the most common inherited cause of Autism Spectrum Disorder (ASD) and intellectual disability. To this aim, we used the recently validated Fmr1-Δexon 8 rat model of FXS, that is also a genetic rat model of ASD. Acute and repeated oral administration of BCP rescued the cognitive deficits displayed by Fmr1-Δexon 8 rats, without inducing tolerance after repeated administration. These beneficial effects were mediated by activation of hippocampal peroxisome proliferator-activated receptors (PPARs) α and γ, and were mimicked by the PPARα agonist Fenofibrate and the PPARγ agonist Pioglitazone. Conversely, CB2 cannabinoid receptors were not involved. Docking analyses further confirmed the ability of BCP to bind rat PPARs. Together, our findings demonstrate that hippocampal PPARs α and γ play a role in the cognitive deficits observed in a rat model of FXS, and provide first preclinical evidence about the efficacy and mechanism of action of BCP in neurodevelopmental disorders.

Unraveling sex differences in maternal and paternal care impacts on social behaviors and neurobiological responses to early-life adversity

Early-life stress (ELS) affects the development of prosocial behaviors and social-cognitive function, often leading to structural brain changes and increased psychosocial disorders. Recent studies suggest that mother- and father-child relationships independently influence social development in a sex-specific manner, but the effects of impaired father-child relationships are often overlooked. This review examines preclinical rodent studies to explore how parental neglect impacts neuroplasticity and social behaviors in offspring. We highlight that disruptions in maternal interactions may affect male pups more in uniparental rodents, while impaired paternal interactions in biparental rodents tend to impact female pups more. Due to limited research, the separate effects of maternal and paternal neglect on brain development and social behaviors in biparental species remain unclear. Addressing these gaps could clarify the sex-specific mechanisms underlying social and neurobiological deficits following parental neglect.

Ventral hippocampus to nucleus accumbens shell circuit regulates approach decisions during motivational conflict

Successful resolution of approach-avoidance conflict (AAC) is fundamentally important for survival, and its dysregulation is a hallmark of many neuropsychiatric disorders, and yet the underlying neural circuit mechanisms are not well elucidated. Converging human and animal research has implicated the anterior/ventral hippocampus (vHPC) as a key node in arbitrating AAC in a region-specific manner. In this study, we sought to target the vHPC CA1 projection pathway to the nucleus accumbens (NAc) to delineate its contribution to AAC decision-making, particularly in the arbitration of learned reward and punishment signals, as well as innate signals. To this end, we used pathway-specific chemogenetics in male and female Long Evans rats to inhibit the NAc shell projecting vHPC CA1 neurons while rats underwent a test in which cues of positive and negative valence were presented concurrently to elicit AAC. Additional behavioral assays of social preference and memory, reward and punishment cue processing, anxiety, and novelty processing were administered to further interrogate the conditions under which the vCA1-NAc shell pathway is recruited. Chemogenetic inhibition of the vCA1-NAc shell circuit resulted in animals exhibiting increased decision-making time and avoidance bias specifically in the face of motivational conflict, as the same behavioral phenotype was absent in separate conditioned cue preference and avoidance tests. vCA1-NAc shell inhibition also led to a reduction in seeking social interaction with a novel rat but did not alter anxiety-like behaviors. The vCA1-NAc shell circuit is therefore critically engaged in biasing decisions to approach in the face of social novelty and approach-avoidance conflict. Dysregulation of this circuit could lead to the precipitation of addictive behaviors in substance abuse, or maladaptive avoidance in situations of approach-avoidance conflict.

Capturing Dynamic Neuronal Responses to Dominant and Subordinate Social Hierarchy Members with catFISH

Dominance hierarchies are key to social organization in group-living species, requiring individuals to recognize their own and others' ranks. This is particularly complex for intermediate-ranking animals, who navigate interactions with higher- and lower-ranking individuals. Using in situ hybridization, we examined how the brains of intermediate-ranked mice in hierarchies respond to dominant and subordinate stimuli by labeling activity-induced immediate early genes and neuronal markers. We show that distinct neuronal populations in the amygdala and hippocampus respond differentially across social contexts. In the basolateral amygdala, glutamatergic Slc17a7+ neurons, particularly dopamine-receptive Slc17a7+Drd1+ neurons, show elevated IEG expression in response to social stimuli, with a higher response to dominant over subordinate animals. Similar patterns are observed among Slc17a7+Oxtr+ neurons in the dorsal endopiriform nucleus and GABAergic Slc32a+ neurons in the medial amygdala. We also identified distinct neural ensembles selectively active in response to dominant and subordinate hierarchy members. We find a higher degree of reactivation among Slc17a7+Oxtr+ ensembles in the dorsal endopiriform nucleus in animals repeatedly presented with the same hierarchy member, as opposed to those presented with a dominant and subordinate member. We observe a similar pattern among Oxtr+ neurons in the dentate gyrus hilus, while the inverse is observed among Slc17a7+ Avrp1b+Oxtr+ neurons in the distal CA2CA3 region. Collectively, our findings reveal how social context is associated with activity changes in social, olfactory, and memory systems in the brain at the neuronal cell type level. This work lays the foundation for further precise cell-type investigation into how the brain processes social information.

Abnormal c-Fos expression in TetTag mice containing fos-EGFP

Molecular and genetic techniques now allow selective tagging and manipulation of the population of neurons, often referred to as "engram cells," that were active during a specific experience. One common approach to labeling these cells is to use the fos-tTA transgenic mouse (TetTag). In addition to tagging cells active during learning, it is common to examine the reactivation of these cells using immediate early gene (IEG) expression as an index of neural activity. There are currently multiple TetTag lines available. The original line, cryopreserved at MMRRC, contains only the fos-tTA transgene, while Jackson Labs provides a version of the mouse that expresses both the fos-tTA and fos-shEGFP genes. In the current experiments, we examined IEG expression in these two mouse lines. Unexpectedly, we found that Jackson fos-tTA/fos-shEGFP mice express increased levels of c-Fos in the hippocampus compared to wild type animals when examined with immunohistochemistry (IHC). The expression of other IEGs, such as Arc and Egr-1, was not elevated in these mice, suggesting that the overexpression of c-Fos is not the result of increased excitability or broad changes in gene expression. qPCR revealed that Jackson fos-tTA/fos-shEGFP mice express mRNA corresponding to a c-Fos-Exon1-GFP fusion molecule, which may bind to C-Fos antibodies during IHC and inflate apparent c-Fos expression. Jackson fos-tTA/fos-shEGFP mice did not differ from their wild-type counterparts in fear expression or memory, indicating no behavioral effect of the presence of a c-Fos-GFP fusion protein. These results identify a major limitation inherent in the use of Jackson fos-tTA/fos-shEGFP mice.

Interconnected neural circuits mediating social reward

Across species, social behaviors are shaped and maintained through positive reinforcement of affiliative social interactions. As with nonsocial rewards, the reinforcing properties of social interactions have been shown to involve interplay between various brain regions and the mesolimbic reward system. In this review, we summarize findings from rodent research on the neural circuits that encode and mediate different components of social reward-seeking behavior. We explore methods to parse and study social reward-related behaviors using available behavioral paradigms. We also compare the neural mechanisms that support social versus nonsocial reward-seeking. Finally, we discuss how internal state and neuromodulatory systems affect reward-seeking behavior and the neural circuits that underlie social reward.

Extrahippocampal Contributions to Social Memory: The Role of Septal Nuclei

Social memory, the ability to recognize and remember individuals within a social group, is crucial for social interactions and relationships. Deficits in social memory have been linked to several neuropsychiatric and neurodegenerative disorders. The hippocampus, especially the circuit that links dorsal CA2 and ventral CA1 neurons, is considered a neural substrate for social memory formation. Recent studies have provided compelling evidence of extrahippocampal contributions to social memory. The septal nuclei, including the medial and lateral septum, make up a basal forebrain region that shares bidirectional neuronal connections with the hippocampus and has recently been identified as critical for social memory. The focus of our review is the neural circuit mechanisms that underlie social memory, with a special emphasis on the septum. We also discuss the social memory dysfunction associated with neuropsychiatric and neurodegenerative disorders.

Social and nonsocial environmental loss have differential effects on ventral hippocampus-dependent behavior and inhibitory synaptic markers in adult male mice

In humans, psychological loss, whether social or nonsocial, can lead to clinical depression, anxiety disorders, and social memory impairments. Researchers have modeled combined social and nonsocial loss in rodents by transitioning them from social, enriched environments (EE) to individual housing, affecting behaviors related to avoidance, stress coping, and cognitive function. However, it remains unclear if these effects are driven by social or nonsocial loss. We examined the effects of nonsocial loss by housing adult male mice in EE before moving them to standard cages, where they were pair-housed, and compared this to mice experiencing complete social loss. Continuous EE reduced social investigation time while leaving social memory intact, also decreasing avoidance behavior. Nonsocial loss restored social investigation and avoidance behavior to control levels, while social loss impaired social memory and increased avoidance. In rodents, social memory and avoidance require ventral hippocampus (vHIP) neuronal oscillations, which involve parvalbumin-positive (PV+) inhibitory interneurons. We found decreased vHIP PV intensity in the social loss group, with no differences in the nonsocial loss group. Most PV+ cells are surrounded by perineuronal nets (PNNs) concentrating GABAA receptors in their lattice-like holes. Social loss decreased GABAA-δ expression, a subunit associated with extrasynaptic receptors, across PNN+ soma and in PNN holes, while nonsocial loss reduced gephyrin in these regions. These findings suggest social and nonsocial losses differentially affect vHIP function and behavior, with social loss having a more pronounced impact through mechanisms involving PV+ interneurons, PNN structure, and neurotransmitter receptor expression.

Hypothalamic regulation of hippocampal CA1 interneurons by the supramammillary nucleus

The hypothalamic supramammillary nucleus (SuM) projects heavily to the hippocampus to regulate hippocampal activity and plasticity. Although the projections from the SuM to the dentate gyrus (DG) and CA2 have been extensively studied, whether the SuM projects to CA1, the main hippocampal output region, is unclear. Here, we report a glutamatergic pathway from the SuM that selectively excites CA1 interneurons in the border between the stratum radiatum (SR) and the stratum lacunosum-moleculare (SLM). We find that the SuM projects selectively to a narrow band in the CA1 SR/SLM and monosynaptically excites SR/SLM interneurons, including vasoactive intestinal peptide-expressing (VIP+) and neuron-derived neurotrophic factor-expressing (NDNF+) cells, but completely avoids making monosynaptic contacts with CA1 pyramidal neurons (PNs) or parvalbumin-expressing (PV+) or somatostatin-expressing (SOM+) cells. Moreover, SuM activation drives spikes in most SR/SLM interneurons to suppress CA1 PN excitability. Taken together, our findings reveal that the SuM can directly regulate hippocampal output region CA1, bypassing CA2, CA3, and the DG.

Dysregulation of neuropilin-2 expression in inhibitory neurons impairs hippocampal circuit development and enhances risk for autism-related behaviors and seizures

Dysregulation of development, migration, and function of interneurons, collectively termed interneuronopathies, have been proposed as a shared mechanism for autism spectrum disorders (ASDs) and childhood epilepsy. Neuropilin-2 (Nrp2), a candidate ASD gene, is a critical regulator of interneuron migration from the median ganglionic eminence (MGE) to the pallium, including the hippocampus. While clinical studies have identified Nrp2 polymorphisms in patients with ASD, whether selective dysregulation of Nrp2-dependent interneuron migration contributes to pathogenesis of ASD and enhances the risk for seizures has not been evaluated. We tested the hypothesis that the lack of Nrp2 in MGE-derived interneuron precursors disrupts the excitation/inhibition balance in hippocampal circuits, thus predisposing the network to seizures and behavioral patterns associated with ASD. Embryonic deletion of Nrp2 during the developmental period for migration of MGE derived interneuron precursors (iCKO) significantly reduced parvalbumin, neuropeptide Y, and somatostatin positive neurons in the hippocampal CA1. Consequently, when compared to controls, the frequency of inhibitory synaptic currents in CA1 pyramidal cells was reduced while frequency of excitatory synaptic currents was increased in iCKO mice. Although passive and active membrane properties of CA1 pyramidal cells were unchanged, iCKO mice showed enhanced susceptibility to chemically evoked seizures. Moreover, iCKO mice exhibited selective behavioral deficits in both preference for social novelty and goal-directed learning, which are consistent with ASD-like phenotype. Together, our findings show that disruption of developmental Nrp2 regulation of interneuron circuit establishment, produces ASD-like behaviors and enhanced risk for epilepsy. These results support the developmental interneuronopathy hypothesis of ASD epilepsy comorbidity.

Prefrontal cortical circuits in social behaviors: an overview

Social behaviors are fundamental and intricate functions in both humans and animals, governed by the interplay of social cognition and emotions. A noteworthy feature of several neuropsychiatric disorders, including autism spectrum disorder (ASD) and schizophrenia (SCZ), is a pronounced deficit in social functioning. Despite a burgeoning body of research on social behaviors, the precise neural circuit mechanisms underpinning these phenomena remain to be elucidated. In this paper, we review the pivotal role of the prefrontal cortex (PFC) in modulating social behaviors, as well as its functional alteration in social disorders in ASD or SCZ. We posit that PFC dysfunction may represent a critical hub in the pathogenesis of psychiatric disorders characterized by shared social deficits. Furthermore, we delve into the intricate connectivity of the medial PFC (mPFC) with other cortical areas and subcortical brain regions in rodents, which exerts a profound influence on social behaviors. Notably, a substantial body of evidence underscores the role of N-methyl-D-aspartate receptors (NMDARs) and the proper functioning of parvalbumin-positive interneurons within the mPFC for social regulation. Our overarching goal is to furnish a comprehensive understanding of these intricate circuits and thereby contribute to the enhancement of both research endeavors and clinical practices concerning social behavior deficits.

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.