Experience-dependent plasticity in an innate social behavior is mediated by hypothalamic LTP

A Hypothalamic Perspective of Human Socioemotional Behavior

Historical evidence from stimulation and lesion studies in animals and humans demonstrated a close association between the hypothalamus and typical and atypical socioemotional behavior. A central hypothalamic contribution to regulation of socioemotional responses was also provided indirectly by studies on oxytocin and arginine vasopressin. However, a limited number of studies have so far directly investigated the contribution of the hypothalamus in human socioemotional behavior. To reconsider the functional role of the evolutionarily conserved hypothalamic region in regulating human social behavior, here I provide a synthesis of neuroimaging investigations showing that the hypothalamus is involved in multiple and diverse facets of human socioemotional behavior through widespread functional interactions with other cortical and subcortical regions. These neuroimaging findings are then integrated with recent optogenetics studies in animals demonstrating that the hypothalamus plays a more active role in eliciting socioemotional responses and is not simply a downstream effector of higher-level brain systems. Building on the aforementioned evidence, the hypothalamus is argued to substantially contribute to a continuum of human socioemotional behaviors promoting survival and preservation of the species that extends from exploratory and approaching responses facilitating social bonding to aggressive and avoidance responses aimed to protect and defend formed relationships.

The effects of different types of social interactions on the electrophysiology of neurons in the nucleus accumbens in rodents

BORLAND, J.M., The effects of different types of social interactions on the electrophysiology of neurons in the nucleus accumbens in rodents, NEUROSCI BIOBEH REV 21(1) XXX-XXX, 2024.-Sociality shapes an organisms' life. The nucleus accumbens is a critical brain region for mental health. In the following review, the effects of different types of social interactions on the physiology of neurons in the nucleus accumbens is synthesized. More specifically, the effects of sex behavior, aggression, social defeat, pair-bonding, play behavior, affiliative interactions, parental behaviors, the isolation from social interactions and maternal separation on measures of excitatory synaptic transmission, intracellular signaling and factors of transcription and translation in neurons in the nucleus accumbens in rodent models are reviewed. Similarities and differences in effects depending on the type of social interaction is then discussed. This review improves the understanding of the molecular and synaptic mechanisms of sociality.

Parenting behaviors in mice: Olfactory mechanisms and features in models of autism spectrum disorders

Rodents, along with numerous other mammals, heavily depend on olfactory cues to navigate their social interactions. Processing of olfactory sensory inputs is mediated by conserved brain circuits that ultimately trigger social behaviors, such as social interactions and parental care. Although innate, parenting is influenced by internal states, social experience, genetics, and the environment, and any significant disruption of these factors can impact the social circuits. Here, we review the molecular mechanisms and social circuits from the olfactory epithelium to central processing that initiate parental behaviors and their dysregulations that may contribute to the social impairments in mouse models of autism spectrum disorders (ASD). We discuss recent advances of the crucial role of olfaction in parental care, its consequences for social interactions, and the reciprocal influence on social interaction impairments in mouse models of ASD.

Role of neuroestrogens in the regulation of social behaviors - From social recognition to mating

In this mini-review, we summarize the brain distribution of aromatase, the enzyme catalyzing the synthesis of estrogens from androgens, and the mechanisms responsible for regulating estrogen production within the brain. Understanding this local synthesis of estrogens by neurons is pivotal as it profoundly influences various facets of social behavior. Neuroestrogen action spans from the initial processing of socially pertinent sensory cues to integrating this information with an individual's internal state, ultimately resulting in the manifestation of either pro-affiliative or - aggressive behaviors. We focus here in particular on aggressive and sexual behavior as the result of correct individual recognition of intruders and potential mates. The data summarized in this review clearly point out the crucial role of locally synthesized estrogens in facilitating rapid adaptation to the social environment in rodents and birds of both sexes. These observations not only shed light on the evolutionary significance but also indicate the potential implications of these findings in the realm of human health, suggesting a compelling avenue for further investigation.

Intrinsic Dynamics and Neural Implementation of a Hypothalamic Line Attractor Encoding an Internal Behavioral State

Line attractors are emergent population dynamics hypothesized to encode continuous variables such as head direction and internal states. In mammals, direct evidence of neural implementation of a line attractor has been hindered by the challenge of targeting perturbations to specific neurons within contributing ensembles. Estrogen receptor type 1 (Esr1)-expressing neurons in the ventrolateral subdivision of the ventromedial hypothalamus (VMHvl) show line attractor dynamics in male mice during fighting. We hypothesized that these dynamics may encode continuous variation in the intensity of an internal aggressive state. Here, we report that these neurons also show line attractor dynamics in head-fixed mice observing aggression. We exploit this finding to identify and perturb line attractor-contributing neurons using 2-photon calcium imaging and holographic optogenetic perturbations. On-manifold perturbations demonstrate that integration and persistent activity are intrinsic properties of these neurons which drive the system along the line attractor, while transient off-manifold perturbations reveal rapid relaxation back into the attractor. Furthermore, stimulation and imaging reveal selective functional connectivity among attractor-contributing neurons. Intriguingly, individual differences among mice in line attractor stability were correlated with the degree of functional connectivity among contributing neurons. Mechanistic modelling indicates that dense subnetwork connectivity and slow neurotransmission are required to explain our empirical findings. Our work bridges circuit and manifold paradigms, shedding light on the intrinsic and operational dynamics of a behaviorally relevant mammalian line attractor.

Modulation of aggression by social novelty recognition memory in the hippocampal CA2 region

The dorsal CA2 subregion (dCA2) of the hippocampus exerts a critical role in social novelty recognition (SNR) memory and in the promotion of social aggression. Whether the social aggression and SNR memory functions of dCA2 are related or represent independent processes is unknown. Here we investigated the hypotheses that an animal is more likely to attack a novel compared to familiar animal and that dCA2 promotes social aggression through its ability to discriminate between novel and familiar conspecifics. To test these ideas, we conducted a multi-day resident intruder (R-I) test of aggression towards novel and familiar conspecifics. We found that mice were more likely to attack a novel compared to familiarized intruder and that silencing of dCA2 caused a more profound inhibition of aggression towards a novel than familiarized intruder. To explore whether and how dCA2 pyramidal neurons encode aggression, we recorded their activity using microendoscopic calcium imaging throughout the days of the R-I test. We found that a fraction of dCA2 neurons were selectively activated or inhibited during exploration, dominance, and attack behaviors and that these signals were enhanced during interaction with a novel compared to familiarized conspecific. Based on dCA2 population activity, a set of binary linear classifiers accurately decoded whether an animal was engaged in each of these forms of social behavior. Of particular interest, the accuracy of decoding aggression was greater with novel compared to familiarized intruders, with significant cross-day decoding using the same familiar animal on each day but not for a familiar-novel pair. Together, these findings demonstrate that dCA2 integrates information about social novelty with signals of behavioral state to promote aggression towards novel conspecifics.

Independent inhibitory control mechanisms for aggressive motivation and action

Social behaviors often consist of a motivational phase followed by action. Here we show that neurons in the ventromedial hypothalamus ventrolateral area (VMHvl) of mice encode the temporal sequence of aggressive motivation to action. The VMHvl receives local inhibitory input (VMHvl shell) and long-range input from the medial preoptic area (MPO) with functional coupling to neurons with specific temporal profiles. Encoding models reveal that during aggression, VMHvl shellvgat+ activity peaks at the start of an attack, whereas activity from the MPO-VMHvlvgat+ input peaks at specific interaction endpoints. Activation of the MPO-VMHvlvgat+ input promotes and prolongs a low motivation state, whereas activation of VMHvl shellvgat+ results in action-related deficits, acutely terminating attack. Moreover, stimulation of MPO-VMHvlvgat+ input is positively valenced and anxiolytic. Together, these data demonstrate how distinct inhibitory inputs to the hypothalamus can independently gate the motivational and action phases of aggression through a single locus of control.

Maternal Aggression Driven by the Transient Mobilisation of a Dormant Hormone-Sensitive Circuit

Aggression, a sexually dimorphic behaviour, is prevalent in males and typically absent in virgin females. Following parturition, however, the transient expression of aggression in adult female mice protects pups from predators and infanticide by male conspecifics. While maternal hormones are known to elicit nursing, their potential role in maternal aggression remains elusive. Here, we show in mice that a molecularly defined subset of ventral premammillary (PMvDAT) neurons, instrumental for intermale aggression, switch from quiescence to a hyperexcitable state during lactation. We identify that the maternal hormones prolactin and oxytocin excite these cells through actions that include T-type Ca2+ channels. Optogenetic manipulation or genetic ablation of PMvDAT neurons profoundly affects maternal aggression, while activation of these neurons impairs the expression of non-aggression-related maternal behaviours. This work identifies a monomorphic neural substrate that can incorporate hormonal cues to enable the transient expression of a dormant behavioural program in lactating females.

Role of estrogen in treatment of female depression

Depression is a neurological disorder that profoundly affects human physical and mental health, resulting in various changes in the central nervous system. Despite several prominent hypotheses, such as the monoaminergic theory, hypothalamic-pituitary-adrenal (HPA) axis theory, neuroinflammation, and neuroplasticity, the current understanding of depression's pathogenesis remains incomplete. Importantly, depression is a gender-dimorphic disorder, with women exhibiting higher incidence rates than men. Given estrogen's pivotal role in the menstrual cycle, it is reasonable to postulate that its fluctuating levels could contribute to the pathogenesis of depression. Estrogen acts by binding to a diversity of receptors, which are widely distributed in the central nervous system. An abundance of research has established that estrogen and its receptors play a crucial role in depression, spanning pathogenesis and treatment. In this comprehensive review, we provide an in-depth analysis of the fundamental role of estrogen and its receptors in depression, with a focus on neuroinflammation, neuroendocrinology, and neuroplasticity. Furthermore, we discuss potential mechanisms underlying the therapeutic effects of estrogen in the treatment of depression, which may pave the way for new antidepressant drug development and alternative treatment options.

A dedicated hypothalamic oxytocin circuit controls aversive social learning

To survive in a complex social group, one needs to know who to approach and, more importantly, who to avoid. In mice, a single defeat causes the losing mouse to stay away from the winner for weeks1. Here through a series of functional manipulation and recording experiments, we identify oxytocin neurons in the retrochiasmatic supraoptic nucleus (SOROXT) and oxytocin-receptor-expressing cells in the anterior subdivision of the ventromedial hypothalamus, ventrolateral part (aVMHvlOXTR) as a key circuit motif for defeat-induced social avoidance. Before defeat, aVMHvlOXTR cells minimally respond to aggressor cues. During defeat, aVMHvlOXTR cells are highly activated and, with the help of an exclusive oxytocin supply from the SOR, potentiate their responses to aggressor cues. After defeat, strong aggressor-induced aVMHvlOXTR cell activation drives the animal to avoid the aggressor and minimizes future defeat. Our study uncovers a neural process that supports rapid social learning caused by defeat and highlights the importance of the brain oxytocin system in social plasticity.

Anatomically distributed neural representations of instincts in the hypothalamus

Artificial activation of anatomically localized, genetically defined hypothalamic neuron populations is known to trigger distinct innate behaviors, suggesting a hypothalamic nucleus-centered organization of behavior control. To assess whether the encoding of behavior is similarly anatomically confined, we performed simultaneous neuron recordings across twenty hypothalamic regions in freely moving animals. Here we show that distinct but anatomically distributed neuron ensembles encode the social and fear behavior classes, primarily through mixed selectivity. While behavior class-encoding ensembles were spatially distributed, individual ensembles exhibited strong localization bias. Encoding models identified that behavior actions, but not motion-related variables, explained a large fraction of hypothalamic neuron activity variance. These results identify unexpected complexity in the hypothalamic encoding of instincts and provide a foundation for understanding the role of distributed neural representations in the expression of behaviors driven by hardwired circuits.

Neuropeptide Signaling is Required to Implement a Line Attractor Encoding a Persistent Internal Behavioral State

Internal states drive survival behaviors, but their neural implementation is not well understood. Recently we identified a line attractor in the ventromedial hypothalamus (VMH) that represents an internal state of aggressiveness. Line attractors can be implemented by recurrent connectivity and/or neuromodulatory signaling, but evidence for the latter is scant. Here we show that neuropeptidergic signaling is necessary for line attractor dynamics in this system, using a novel approach that integrates cell type-specific, anatomically restricted CRISPR/Cas9-based gene editing with microendoscopic calcium imaging. Co-disruption of receptors for oxytocin and vasopressin in adult VMH Esr1 + neurons that control aggression suppressed attack, reduced persistent neural activity and eliminated line attractor dynamics, while only modestly impacting neural activity and sex- or behavior-tuning. These data identify a requisite role for neuropeptidergic signaling in implementing a behaviorally relevant line attractor. Our approach should facilitate mechanistic studies in neuroscience that bridge different levels of biological function and abstraction.

Hypothalamic control of innate social behaviors

Sexual, parental, and aggressive behaviors are central to the reproductive success of individuals and species survival and thus are supported by hardwired neural circuits. The reproductive behavior control column (RBCC), which comprises the medial preoptic nucleus (MPN), the ventrolateral part of the ventromedial hypothalamus (VMHvl), and the ventral premammillary nucleus (PMv), is essential for all social behaviors. The RBCC integrates diverse hormonal and metabolic cues and adjusts an animal's physical activity, hence the chance of social encounters. The RBCC further engages the mesolimbic dopamine system to maintain social interest and reinforces cues and actions that are time-locked with social behaviors. We propose that the RBCC and brainstem form a dual-control system for generating moment-to-moment social actions. This Review summarizes recent progress regarding the identities of RBCC cells and their pathways that drive different aspects of social behaviors.

Neural dynamics in the limbic system during male social behaviors

Sexual and aggressive behaviors are vital for species survival and individual reproductive success. Although many limbic regions have been found relevant to these behaviors, how social cues are represented across regions and how the network activity generates each behavior remains elusive. To answer these questions, we utilize multi-fiber photometry (MFP) to simultaneously record Ca2+ signals of estrogen receptor alpha (Esr1)-expressing cells from 13 limbic regions in male mice during mating and fighting. We find that conspecific sensory information and social action signals are widely distributed in the limbic system and can be decoded from the network activity. Cross-region correlation analysis reveals striking increases in the network functional connectivity during the social action initiation phase, whereas late copulation is accompanied by a "dissociated" network state. Based on the response patterns, we propose a mating-biased network (MBN) and an aggression-biased network (ABN) for mediating male sexual and aggressive behaviors, respectively.

Sex differences in dominance relationships in Syrian hamsters

Dominance relationships are identified by changes in agonistic behavior toward specific individuals. While there are considerable individual and species differences in dominance relationships, sex differences are poorly understood in rodent models because aggression among female rodents is rare. The aim of this study was to characterize sex differences in the formation and maintenance of dominance relationships in same-sex pairs of male and female Syrian hamsters. We pooled data from multiple projects in our lab to evaluate dominance interactions in 68 male dyads and 88 female dyads. In each project, animals were matched with a partner similar in age, sex, and estrous cycle and we exposed animals to daily social encounters for two weeks in a resident-intruder format. We found that female hamsters were quicker to attack and attacked at higher rates compared to males regardless of dominance status. In addition, resident female hamsters were quicker to attack and attacked at higher rates than intruder females, but aggression in males did not depend on residency status. Female subordinates were quicker to submit and fled at higher rates from their dominant counterparts compared to male subordinates. Intruder subordinate females were quicker to submit and fled at higher rates than resident subordinate females. Females were also more resistant than males to becoming subordinate in that they fought back more consistently and were more likely to reverse their dominance status. These findings indicate that dominance relationships are less stable in females compared to males and that residency status has a larger impact on agonistic behavior in females than males. Overall, differences in how males and females display territorial aggression can lead to sex differences in the establishment and maintenance of dominance relationships.

Nitric oxide synthase contributes to the maintenance of LTP in the oxytocin-mRFP1 neuron of the rat hypothalamus

Oxytocin (OXT) is a neuropeptide hormone that plays a critical role in nociception. Long-term potentiation (LTP) is a major form of synaptic plasticity in the central nervous system. Recently, LTP has been reported in the hypothalamus; however, data on LTP in hypothalamic OXT-ergic neurons are unclear. Furthermore, the signaling pathways for hypothalamic OXT-ergic neuronal LTP and its physiological significance remain unknown. Herein, we aimed to investigate the induction of hypothalamic OXT-ergic neuronal LTP and its synaptic mechanism using OXT-monomeric red fluorescent protein 1 transgenic rats to visualize and record from OXT-ergic neurons. The hypothalamic paraventricular nucleus (PVN) OXT-ergic neuronal LTP induced by the pairing protocol was dependent on N-methyl-D-aspartate receptor (NMDAR). Furthermore, nitric oxide synthase (NOS) is required to maintain the LTP regardless of the NMDARs. In addition, hypothalamic OXT-ergic neuronal LTP was not induced in the adjuvant arthritis rat model but increased excitatory postsynaptic currents were detected. LTP in hypothalamic OXT-ergic neurons in the PVN in the presence of NOS may be involved in neuronal changes during OXT synthesis in chronic inflammation.

Hypothalamus, Neuropeptides and Socioemotional Behavior

A large body of evidence from old stimulation and lesion studies on the hypothalamus in animals and humans demonstrates that this subcortical area significantly affects socioemotional behavior [...].

A neural circuit for male sexual behavior and reward

Male sexual behavior is innate and rewarding. Despite its centrality to reproduction, a molecularly specified neural circuit governing innate male sexual behavior and reward remains to be characterized. We have discovered a developmentally wired neural circuit necessary and sufficient for male mating. This circuit connects chemosensory input to BNSTprTac1 neurons, which innervate POATacr1 neurons that project to centers regulating motor output and reward. Epistasis studies demonstrate that BNSTprTac1 neurons are upstream of POATacr1 neurons, and BNSTprTac1-released substance P following mate recognition potentiates activation of POATacr1 neurons through Tacr1 to initiate mating. Experimental activation of POATacr1 neurons triggers mating, even in sexually satiated males, and it is rewarding, eliciting dopamine release and self-stimulation of these cells. Together, we have uncovered a neural circuit that governs the key aspects of innate male sexual behavior: motor displays, drive, and reward.

Estrogen-responsive neural circuits governing male and female mating behavior in mice

Decades of knockout analyses have highlighted the crucial involvement of estrogen receptors and downstream genes in controlling mating behaviors. More recently, advancements in neural circuit research have unveiled a distributed subcortical network comprising estrogen-receptor or estrogen-synthesis-enzyme-expressing cells that transforms sensory inputs into sex-specific mating actions. This review provides an overview of the latest discoveries on estrogen-responsive neurons in various brain regions and the associated neural circuits that govern different aspects of male and female mating actions in mice. By contextualizing these findings within previous knockout studies of estrogen receptors, we emphasize the emerging field of "circuit genetics", where identifying mating behavior-related neural circuits may allow for a more precise evaluation of gene functions within these circuits. Such investigations will enable a deeper understanding of how hormone fluctuation, acting through estrogen receptors and downstream genes, influences the connectivity and activity of neural circuits, ultimately impacting the manifestation of innate mating actions.

Circuits underlying social function and dysfunction

Substantial advances have been made toward understanding the genetic and environmental risk factors for autism, a neurodevelopmental disorder with social impairment as a core feature. In combination with optogenetic and chemogenetic tools to manipulate neural circuits in vivo, it is now possible to use model systems to test how specific neural circuits underlie social function and dysfunction. Here, we review the literature that has identified circuits associated with social interest (sociability), social reward, social memory, dominance, and aggression, and we outline a preliminary roadmap of the neural circuits driving these social behaviors. We highlight the neural circuitry underlying each behavioral domain, as well as develop an interactive map of how these circuits overlap across domains. We find that some of the circuits underlying social behavior are general and are involved in the control of multiple behavioral aspects, whereas other circuits appear to be specialized for specific aspects of social behavior. Our overlapping circuit map therefore helps to delineate the circuits involved in the various domains of social behavior and to identify gaps in knowledge.

Social experience and pheromone receptor activity reprogram gene expression in sensory neurons

Social experience and pheromone signaling in olfactory neurons affect neuronal responses and male courtship behaviors in Drosophila. We previously showed that social experience and pheromone signaling modulate chromatin around behavioral switch gene fruitless, which encodes a transcription factor necessary and sufficient for male sexual behaviors. Fruitless drives social experience-dependent modulation of courtship behaviors and physiological sensory neuron responses to pheromone; however, the molecular mechanisms underlying this modulation of neural responses remain less clear. To identify the molecular mechanisms driving social experience-dependent changes in neuronal responses, we performed RNA-seq from antennal samples of mutants in pheromone receptors and fruitless, as well as grouped or isolated wild-type males. Genes affecting neuronal physiology and function, such as neurotransmitter receptors, ion channels, ion and membrane transporters, and odorant binding proteins are differentially regulated by social context and pheromone signaling. While we found that loss of pheromone detection only has small effects on differential promoter and exon usage within fruitless gene, many of the differentially regulated genes have Fruitless-binding sites or are bound by Fruitless in the nervous system. Recent studies showed that social experience and juvenile hormone signaling co-regulate fruitless chromatin to modify pheromone responses in olfactory neurons. Interestingly, genes involved in juvenile hormone metabolism are also misregulated in different social contexts and mutant backgrounds. Our results suggest that modulation of neuronal activity and behaviors in response to social experience and pheromone signaling likely arise due to large-scale changes in transcriptional programs for neuronal function downstream of behavioral switch gene function.

A hypothalamic pathway that suppresses aggression toward superior opponents

Aggression is costly and requires tight regulation. Here we identify the projection from estrogen receptor alpha-expressing cells in the caudal part of the medial preoptic area (cMPOAEsr1) to the ventrolateral part of the ventromedial hypothalamus (VMHvl) as an essential pathway for modulating aggression in male mice. cMPOAEsr1 cells increase activity mainly during male-male interaction, which differs from the female-biased response pattern of rostral MPOAEsr1 (rMPOAEsr1) cells. Notably, cMPOAEsr1 cell responses to male opponents correlated with the opponents' fighting capability, which mice could estimate based on physical traits or learn through physical combats. Inactivating the cMPOAEsr1-VMHvl pathway increased aggression, whereas activating the pathway suppressed natural intermale aggression. Thus, cMPOAEsr1 is a key population for encoding opponents' fighting capability-information that could be used to prevent animals from engaging in disadvantageous conflicts with superior opponents by suppressing the activity of VMHvl cells essential for attack behaviors.

To see is to experience: Aggression neurons light up when witnessing a fight

Fighting is an intense experience not only for the executors but also for the observers. In the current issue of Cell, Yang et al. identified hypothalamic aggression mirror neurons, activated during both physical fighting and witnessing a fight, possibly representing a neural mechanism for understanding social experiences in other minds.

An approximate line attractor in the hypothalamus encodes an aggressive state

The hypothalamus regulates innate social behaviors, including mating and aggression. These behaviors can be evoked by optogenetic stimulation of specific neuronal subpopulations within MPOA and VMHvl, respectively. Here, we perform dynamical systems modeling of population neuronal activity in these nuclei during social behaviors. In VMHvl, unsupervised analysis identified a dominant dimension of neural activity with a large time constant (>50 s), generating an approximate line attractor in neural state space. Progression of the neural trajectory along this attractor was correlated with an escalation of agonistic behavior, suggesting that it may encode a scalable state of aggressiveness. Consistent with this, individual differences in the magnitude of the integration dimension time constant were strongly correlated with differences in aggressiveness. In contrast, approximate line attractors were not observed in MPOA during mating; instead, neurons with fast dynamics were tuned to specific actions. Thus, different hypothalamic nuclei employ distinct neural population codes to represent similar social behaviors.

A temperature-regulated circuit for feeding behavior

Both rodents and primates have evolved to orchestrate food intake to maintain thermal homeostasis in coping with ambient temperature challenges. However, the mechanisms underlying temperature-coordinated feeding behavior are rarely reported. Here we find that a non-canonical feeding center, the anteroventral and periventricular portions of medial preoptic area (apMPOA) respond to altered dietary states in mice. Two neighboring but distinct neuronal populations in apMPOA mediate feeding behavior by receiving anatomical inputs from external and dorsal subnuclei of lateral parabrachial nucleus. While both populations are glutamatergic, the arcuate nucleus-projecting neurons in apMPOA can sense low temperature and promote food intake. The other type, the paraventricular hypothalamic nucleus (PVH)-projecting neurons in apMPOA are primarily sensitive to high temperature and suppress food intake. Caspase ablation or chemogenetic inhibition of the apMPOA→PVH pathway can eliminate the temperature dependence of feeding. Further projection-specific RNA sequencing and fluorescence in situ hybridization identify that the two neuronal populations are molecularly marked by galanin receptor and apelin receptor. These findings reveal unrecognized cell populations and circuits of apMPOA that orchestrates feeding behavior against thermal challenges.

Anatomical and molecular features of the amygdalohippocampal transition area and its role in social and emotional behavior processes

The amygdalohippocampal transition area (AHi) has emerged as a critical nucleus of sociosexual behaviors such as mating, parenting, and aggression. The AHi has been overlooked in rodent and human amygdala studies until recently. The AHi is hypothesized to play a role in metabolic and cognitive functions as well as social behaviors based on its connectivity and molecular composition. The AHi is small nucleus rich in neuropeptide and hormone receptors and is contiguous with the ventral subiculum of the hippocampus-hence its designation as a "transition area". Literature focused on the AHi can be difficult to interpret because of changing nomenclature and conflation with neighboring nuclei. Here we summarize what is currently known about AHi structure and development, connections throughout the brain, molecular composition, and functional significance. We aim to delineate current knowledge regarding the AHi, identify potential functions with supporting evidence, and ultimately make clear the importance of the AHi in sociosexual function.

Inhibition of the medial amygdala disrupts escalated aggression in lactating female mice after repeated exposure to male intruders

Virgin female laboratory mice readily express pup care when co-housed with dams and pups. However, pup-sensitized virgins fail to express intruder-directed aggression on a single session of testing. To study whether repeated testing would affect the onset and dynamics of maternal or intruder-directed aggression, we tested dams and their accompanying virgins from postpartum day 4 to 6. Repeated testing led to escalated aggression towards male intruders in dams, but virgins never developed aggression. In dams, inhibition of the medial amygdala using DREADD (designer receptors exclusively activated by designer drugs) vectors carrying the hM4Di receptor blocked the expected increase in maternal aggression on the second testing day. Our data support that the onset of maternal aggression is linked to physiological changes occurring during motherhood, and that medial amygdala, a key centre integrating vomeronasal, olfactory and hormonal information, enables the expression of escalated aggression induced by repeated testing. Future studies selectively targeting specific neuronal populations of the medial amygdala are needed to allow a deeper understanding of the control of experience-dependent aggression increase, a phenomenon leading to the high aggression levels found in violent behaviours.

The onset of maternal aggression in mice is dependent on physiological changes that occur during pregnancy and lactation, and the medial amygdala is key in the expression of escalated aggression induced by repeated testing.

Sensory-thresholded switch of neural firing states in a computational model of the ventromedial hypothalamus

The mouse ventromedial hypothalamus (VMH) is both necessary and sufficient for defensive responses to predator and social threats. Defensive behaviors typically involve cautious approach toward potentially threatening stimuli aimed at obtaining information about the risk involved, followed by sudden avoidance and flight behavior to escape harm. In vivo neural recording studies in mice have identified two major populations of VMH neurons that either increase their firing activity as the animal approaches the threat (called Assessment+ cells) or increase their activity as the animal flees the threat (called Flight+ cells). Interestingly, Assessment+ and Flight+ cells abruptly decrease and increase their firing activity, respectively, at the decision point for flight, creating an escape-related “switch” in functional state. This suggests that the activity of the two cell types in VMH is coordinated and could result from local circuit interactions. Here, we used computational modeling to test if a local inhibitory feedback circuit could give rise to key features of the neural activity seen in VMH during the approach-to-flight transition. Starting from a simple dual-population inhibitory feedback circuit receiving repeated trains of monotonically increasing sensory input to mimic approach to threat, we tested the requirement for balanced sensory input, balanced feedback, short-term synaptic plasticity, rebound excitation, and inhibitory feedback exclusivity to reproduce an abrupt, sensory-thresholded reciprocal firing change that resembles Assessment+ and Flight+ cell activity seen in vivo. Our work demonstrates that a relatively simple local circuit architecture is sufficient for the emergence of firing patterns similar to those seen in vivo and suggests that a reiterative process of experimental and computational work may be a fruitful avenue for better understanding the functional organization of mammalian instinctive behaviors at the circuit level.

Transformations of neural representations in a social behaviour network

Mating and aggression are innate social behaviours that are controlled by subcortical circuits in the extended amygdala and hypothalamus^1-4. The bed nucleus of the stria terminalis (BNSTpr) is a node that receives input encoding sex-specific olfactory cues from the medial amygdala^5,6, and which in turn projects to hypothalamic nuclei that control mating^7-9 (medial preoptic area (MPOA)) and aggression^9-14 (ventromedial hypothalamus, ventrolateral subdivision (VMHvl)), respectively¹⁵. Previous studies have demonstrated that male aromatase-positive BNSTpr neurons are required for mounting and attack, and may identify conspecific sex according to their overall level of activity¹⁶. However, neural representations in BNSTpr, their function and their transformations in the hypothalamus have not been characterized. Here we performed calcium imaging^17,18 of male BNSTpr^Esr1 neurons during social behaviours. We identify distinct populations of female- versus male-tuned neurons in BNSTpr, with the former outnumbering the latter by around two to one, similar to the medial amygdala and MPOA but opposite to VMHvl, in which male-tuned neurons predominate^6,9,19. Chemogenetic silencing of BNSTpr^Esr1 neurons while imaging MPOA^Esr1 or VMHvl^Esr1 neurons in behaving animals showed, unexpectedly, that the male-dominant sex-tuning bias in VMHvl was inverted to female-dominant whereas a switch from sniff- to mount-selective neurons during mating was attenuated in MPOA. Our data also indicate that BNSTpr^Esr1 neurons are not essential for conspecific sex identification. Rather, they control the transition from appetitive to consummatory phases of male social behaviours by shaping sex- and behaviour-specific neural representations in the hypothalamus.

Interspecific variation in cooperative burrowing behavior by Peromyscus mice

Animals often adjust their behavior according to social context, but the capacity for such behavioral flexibility can vary among species. Here, we test for interspecific variation in behavioral flexibility by comparing burrowing behavior across three species of deer mice (genus Peromyscus) with divergent social systems, ranging from promiscuous (Peromyscus leucopus and Peromyscus maniculatus) to monogamous (Peromyscus polionotus). First, we compared the burrows built by individual mice to those built by pairs of mice in all three species. Although burrow length did not differ in P. leucopus or P. maniculatus, we found that P. polionotus pairs cooperatively constructed burrows that were nearly twice as long as those built by individuals and that opposite‐sex pairs dug longer burrows than same‐sex pairs. Second, to directly observe cooperative digging behavior in P. polionotus, we designed a burrowing assay in which we could video‐record active digging in narrow, transparent enclosures. Using this novel assay, we found, unexpectedly, that neither males nor females spent more time digging with an opposite‐sex partner. Rather, we demonstrate that opposite‐sex pairs are more socially cohesive and thus more efficient digging partners than same‐sex pairs. Together, our study demonstrates how social context can modulate innate behavior and offers insight into how differences in behavioral flexibility may evolve among closely related species.

Hippocampal Proteomic Analysis in Male Mice Following Aggressive Behavior Induced by Long-Term Administration of Perampanel

Antiepileptic drugs have been shown to be associated with inducing or exacerbating adverse psychotropic reaction, including aggressive behavior. Perampanel, the first pharmacological compound approved by the FDA in 2012, is an effective antiepileptic drug for intractable epilepsy but induces severe aggression. So far, the underlying molecular mechanisms of aggression induced by perampanel remain incompletely understood. In the present study, a model of aggressive behavior based on the clinical use of perampanel was established and resident-intruder test and open field test were performed. Changes in hippocampal protein profiles were detected by tandem mass tag (TMT) proteomics. The behavioral results indicated that long-term use of perampanel increased the aggressive behavior of C57BL/6J mice. Proteomic analysis revealed that 93 proteins were significantly altered in the hippocampus of the perampanel-treated group (corrected p < 0.05), which were divided into multiple functional groups, mainly related to synaptic function, synaptogenesis, postsynaptic density protein, neurite outgrowth, AMPA-type glutamate receptor immobilization, and others. Bioinformatic analysis showed that differentially expressed proteins were involved in synaptic plasticity and the Ras signaling pathway. Furthermore, validation results by western blot demonstrated that glutamate receptor 1 (GluA1) and phosphorylation of mitogen-activated protein kinase (ERK1/2) were notably up-regulated, and synaptophysin (Syn) and postsynaptic density 95 (PSD95) were down-regulated in perampanel-treated mice. Therefore, our results provide valuable insight into the molecular mechanisms of aggressive behavior induced by perampanel, as well as potential options for safety treatment of perampanel in the future.

A single vomeronasal receptor promotes intermale aggression through dedicated hypothalamic neurons

The pheromonal information received by the vomeronasal system plays a crucial role in regulating social behaviors such as aggression in mice. Despite accumulating knowledge of the brain regions involved in aggression, the specific vomeronasal receptors and the exact neural circuits responsible for pheromone-mediated aggression remain unknown. Here, we identified one murine vomeronasal receptor, Vmn2r53, that is activated by urine from males of various strains and is responsible for evoking intermale aggression. We prepared a purified pheromonal fraction and Vmn2r53 knockout mice and applied genetic tools for neuronal activity recording, manipulation, and circuit tracing to decipher the neural mechanisms underlying Vmn2r53-mediated aggression. We found that Vmn2r53-mediated aggression is regulated by specific neuronal populations in the ventral premammillary nucleus and the ventromedial hypothalamic nucleus. Together, our results shed light on the hypothalamic regulation of male aggression mediated by a single vomeronasal receptor.

Parenting as a model for behavioural switches

Adaptability to ethologically relevant cues is fundamental for social interactions. As such, reproductive success relies on the ability of an animal to transition between parental and nonparental states. Though driven by genetically pre-programmed circuits, these instinctive repertoires are reshaped by internal state and experience, making parenting a robust model for the study of behavioural flexibility. As a functional wiring diagram for parenting emerges in mice, we are well placed to identify neural substrates and posit associated mechanisms underlying caregiving transitions. In this review, we discuss the importance of comprehensively characterising behaviour, highlight the role of shared circuit elements for behavioural malleability and explore plastic mechanisms that might guide switches between parental and nonparental repertoires.

Plasticity of neural connections underlying oxytocin-mediated parental behaviors of male mice

The adult brain can flexibly adapt behaviors to specific life-stage demands. For example, while sexually naive male mice are aggressive to the conspecific young, they start to provide caregiving to infants around the time when their own young are expected. How such behavioral plasticity is implemented at the level of neural connections remains poorly understood. Here, using viral-genetic approaches, we establish hypothalamic oxytocin neurons as the key regulators of the parental caregiving behaviors of male mice. We then use rabies-virus-mediated unbiased screening to identify excitatory neural connections originating from the lateral hypothalamus to the oxytocin neurons to be drastically strengthened when male mice become fathers. These connections are functionally relevant, as their activation suppresses pup-directed aggression in virgin males. These results demonstrate the life-stage associated, long-distance, and cell-type-specific plasticity of neural connections in the hypothalamus, the brain region that is classically assumed to be hard-wired.

Neural mechanisms of persistent aggression

While aggression is often conceptualized as a highly stereotyped, innate behavior, individuals within a species exhibit a surprising amount of variability in the frequency, intensity, and targets of their aggression. While differences in genetics are a source of some of this variation across individuals (estimates place the heritability of behavior at around 25-30%), a critical driver of variability is previous life experience. A wide variety of social experiences, including sexual, parental, and housing experiences can facilitate "persistent" aggressive states, suggesting that these experiences engage a common set of synaptic and molecular mechanisms that act on dedicated neural circuits for aggression. It has long been known that sex steroid hormones are powerful modulators of behavior, and also, that levels of these hormones are themselves modulated by experience. Several recent studies have started to unravel how experience-dependent hormonal changes during adulthood can create a cascade of molecular, synaptic, and circuit changes that enable behavioral persistence through circuit level remodeling. Here, we propose that sex steroid hormones facilitate persistent aggressive states by changing the relationship between neural activity and an aggression "threshold".

Deviant circadian rhythmicity, corticosterone variability and trait testosterone levels in aggressive mice

Although aggression has been linked to disturbances of circadian rhythm, insight into the neural substrate of this association is currently lacking. The suprachiasmatic nucleus (SCN) of the hypothalamus, the master circadian clock, is regulated by clock genes and known to influence the secretion of cortisosterone and testosterone, important hormones implicated in aggression. Here, we investigated deviations in the regulation of the locomotor circadian rhythm and hormonal levels in a mouse model of abnormal aggression. We tested aggressive BALB/cJ and control BALB/cByJ mice in the resident–intruder paradigm and compared them on their locomotor circadian rhythm during a 12 h light/12 h dark cycle and constant darkness. State (serum) corticosterone and trait (hair) corticosterone and testosterone levels were determined, and immunohistochemistry was performed to assess the expression of important clock proteins, PER1 and PER2, in the core and shell of the SCN at the start of their active phase. Compared with BALB/cByJ mice, aggressive BALB/cJ mice displayed: (1) a shorter free‐running period in constant darkness; (2) reduced state corticosterone variability between circadian peak and trough but no differences in corticosterone trait levels; (3) lower testosterone trait levels; (4) higher PER1 expression in the SCN shell with no changes in PER2 in either SCN subregion during the early dark phase. Together, these results suggest that aggressive BALB/cJ mice have disturbances in different components encompassing the circadian and hormonal cycle, emphasizing their value for future investigation of the causal relationship between SCN function, circadian clocks and aggression.

Anger management: Mechanisms of glutamate receptor-mediated synaptic plasticity underlying animal aggression

Excessive and recurring violent aggression is a serious concern for society and a symptom of many psychiatric diseases. Substance abuse, attack experience, and social and traumatic stress increase vulnerability to developing this type of aggression. Glutamate receptors are an intriguing target for long-term treatment. This review will assess the importance of glutamate receptors and glutamatergic pathways in aggression, focusing on the role of glutamate receptor-mediated synaptic plasticity in experience-dependent long-lasting aggression. By synthesizing what is known about glutamatergic systems in aggression, it is hoped more effective treatments can be developed.

Neural circuit mechanisms that govern inter-male attack in mice

Individuals of many species fight with conspecifics to gain access to or defend critical resources essential for survival and reproduction. Such intraspecific fighting is evolutionarily selected for in a species-, sex-, and environment-dependent manner when the value of resources secured exceeds the cost of fighting. One such example is males fighting for chances to mate with females. Recent advances in new tools open up ways to dissect the detailed neural circuit mechanisms that govern intraspecific, particularly inter-male, aggression in the model organism Mus musculus (house mouse). By targeting and functional manipulating genetically defined populations of neurons and their projections, these studies reveal a core neural circuit that controls the display of reactive male-male attacks in mice, from sensory detection to decision making and action selection. Here, we summarize these critical results. We then describe various modulatory inputs that route into the core circuit to afford state-dependent and top-down modulation of inter-male attacks. While reviewing these exciting developments, we note that how the inter-male attack circuit converges or diverges with neural circuits that mediate other forms of social interactions remain not fully understood. Finally, we emphasize the importance of combining circuit, pharmacological, and genetic analysis when studying the neural control of aggression in the future.

Periaqueductal gray neurons encode the sequential motor program in hunting behavior of mice

Sequential encoding of motor programs is essential for behavior generation. However, whether it is critical for instinctive behavior is still largely unknown. Mouse hunting behavior typically contains a sequential motor program, including the prey search, chase, attack, and consumption. Here, we reveal that the neuronal activity in the lateral periaqueductal gray (LPAG) follows a sequential pattern and is time-locked to different hunting actions. Optrode recordings and photoinhibition demonstrate that LPAG^Vgat neurons are required for the prey detection, chase and attack, while LPAG^Vglut2 neurons are selectively required for the attack. Ablation of inputs that could trigger hunting, including the central amygdala, the lateral hypothalamus, and the zona incerta, interrupts the activity sequence pattern and substantially impairs hunting actions. Therefore, our findings reveal that periaqueductal gray neuronal ensembles encode the sequential hunting motor program, which might provide a framework for decoding complex instinctive behaviors.

Hunting behavior typically contains a sequential motor program, including search, chase, attack, and consumption. Here, the authors show that periaqueductal gray neuronal ensembles encode the sequential hunting motor program, which might provide a framework for decoding complex instinctive behaviors.

Mechanisms, development, and comparative perspectives on experience-dependent plasticity in social behavior

Revealing the mechanisms underlying experience-dependent plasticity is a hallmark of behavioral neuroscience. While the study of social behavior has focused primarily on the neuroendocrine and neural control of social behaviors, the plasticity of these innate behaviors has received relatively less attention. Here, we review studies on mating-dependent changes to social behavior and neural circuitry across mammals, birds, and reptiles. We provide an overview of species similarities and differences in the effects of mating experiences on motivational and performative aspects of sexual behaviors, on sensory processing and preferences, and on the experience-dependent consolidation of sexual behavior. We also discuss recent insights into the neural mechanisms of and developmental influences on mating-dependent changes and outline promising approaches to investigate evolutionary parallels and divergences in experience-dependent plasticity.

Neural circuit mechanisms of sex and fighting in male mice

Surviving in the animal kingdom hinges on the ability to fight competitors and to mate with partners. Dedicated neural circuits in the brain allow animals to mate and attack without any prior experience. Classical lesioning and stimulation studies demonstrated that medial hypothalamic and limbic areas are crucial for male sexual and aggressive behaviors. Moreover, recent functional manipulation tools have uncovered neural circuits critical for mating and aggression, and optical and electrophysiological recordings have revealed how socially relevant information (e.g. sex-specific sensory signals, action commands for specific behaviors, mating- and aggression-specific motivational states) is encoded in these circuits. A better understanding of the neural mechanisms of innate social behaviors will provide critical insights to how complex behavioral outputs are coordinated at the circuit level. In this paper, I review these recent studies and discuss the potential circuit logic of male sexual and aggressive behaviors in mice.

The Structural and Electrophysiological Properties of Progesterone Receptor-Expressing Neurons Vary along the Anterior-Posterior Axis of the Ventromedial Hypothalamus and Undergo Local Changes across the Reproductive Cycle

Sex hormone levels continuously fluctuate across the reproductive cycle, changing the activity of neuronal circuits to coordinate female behavior and reproductive capacity. The ventrolateral division of the ventromedial hypothalamus (VMHvl) contains neurons expressing receptors for sex hormones and its function is intimately linked to female sexual receptivity. However, recent findings suggest that the VMHvl is functionally heterogeneous. Here, we used whole recordings and intracellular labeling to characterize the electrophysiological and morphologic properties of individual VMHvl neurons in naturally cycling females and report the existence of multiple electrophysiological phenotypes within the VMHvl. We found that the properties of progesterone receptor expressing (PR+) neurons, but not PR– neurons, depended systematically on the neuron’s location along the anterior-posterior (AP) axis of the VMHvl and the phase within the reproductive cycle. Prominent among this, the resting membrane potential of anterior PR+ neurons decreased during the receptive phase, while the excitability of medial PR+ neurons increased during the non-receptive phase. During the receptive phase of the cycle, posterior PR+ neurons simultaneously showed an increase in dendritic complexity and a decrease in spine density. These findings reveal an extensive diversity of local rules driving structural and physiological changes in response to fluctuating levels of sex hormones, supporting the anatomic and functional subdivision of the VMHvl and its possible role in the orchestration of different aspects of female socio-sexual behavior.

Organization of neural circuits underlying social behavior: A consideration of the medial amygdala

The medial amygdala (MeA) is critical for the expression of a broad range of social behaviors, and is also connected to many other brain regions that mediate those same behaviors. Here, we summarize recent advances toward elucidating mechanisms that enable the MeA to regulate a diversity of social behaviors, and also consider what role the MeA plays within the broader network of regions that orchestrate social sensorimotor transformations. We outline the molecular, anatomical, and electrophysiological features of the MeA that segregate distinct social behaviors, propose experimental strategies to disambiguate sensory representations from behavioral function in the context of a social interaction, and consider to what extent MeA function may overlap with other regions mediating similar behaviors.

Neural circuits of social behaviors: Innate yet flexible

Social behaviors, such as mating, fighting, and parenting, are fundamental for survival of any vertebrate species. All members of a species express social behaviors in a stereotypical and species-specific way without training because of developmentally hardwired neural circuits dedicated to these behaviors. Despite being innate, social behaviors are flexible. The readiness to interact with a social target or engage in specific social acts can vary widely based on reproductive state, social experience, and many other internal and external factors. Such high flexibility gives vertebrates the ability to release the relevant behavior at the right moment and toward the right target. This maximizes reproductive success while minimizing the cost and risk associated with behavioral expression. Decades of research have revealed the basic neural circuits underlying each innate social behavior. The neural mechanisms that support behavioral plasticity have also started to emerge. Here we provide an overview of these social behaviors and their underlying neural circuits and then discuss in detail recent findings regarding the neural processes that support the flexibility of innate social behaviors.

Effect of Aggressive Experience in Female Syrian Hamsters on Glutamate Receptor Expression in the Nucleus Accumbens

Our social relationships determine our health and well-being. In rodent models, there is now strong support for the rewarding properties of aggressive or assertive behaviors to be critical for the expression and development of adaptive social relationships, buffering from stress and protecting from the development of psychiatric disorders such as depression. However, due to the false belief that aggression is not a part of the normal repertoire of social behaviors displayed by females, almost nothing is known about the neural mechanisms mediating the rewarding properties of aggression in half the population. In the following study, using Syrian hamsters as a well-validated and translational model of female aggression, we investigated the effects of aggressive experience on the expression of markers of postsynaptic structure (PSD-95, Caskin I) and excitatory synaptic transmission (GluA1, GluA2, GluA4, NR2A, NR2B, mGluR1a, and mGluR5) in the nucleus accumbens (NAc), caudate putamen and prefrontal cortex. Aggressive experience resulted in an increase in PSD-95, GluA1 and the dimer form of mGluR5 specifically in the NAc 24 h following aggressive experience. There was also an increase in the dimer form of mGluR1a 1 week following aggressive experience. Aggressive experience also resulted in an increase in the strength of the association between these postsynaptic proteins and glutamate receptors, supporting a common mechanism of action. In addition, 1 week following aggressive experience there was a positive correlation between the monomer of mGluR5 and multiple AMPAR and NMDAR subunits. In conclusion, we provide evidence that aggressive experience in females results in an increase in the expression of postsynaptic density, AMPARs and group I metabotropic glutamate receptors, and an increase in the strength of the association between postsynaptic proteins and glutamate receptors. This suggests that aggressive experience may result in an increase in excitatory synaptic transmission in the NAc, potentially encoding the rewarding and behavioral effects of aggressive interactions.