Social Control of Hypothalamus-Mediated Male Aggression

Dopaminergic dominance in the ventral medial hypothalamus: A pivotal regulator for methamphetamine-induced pathological aggression

Methamphetamine (METH) abuse is associated with a spectrum of behavioral consequences, among which heightened aggression presents a significant challenge. However, the causal role of METH's impact in aggression and its target circuit mechanisms remains largely unknown. We established an acute METH exposure-aggression mouse model to investigate the role of ventral tegmental area (VTA) dopaminergic neurons and ventral medial hypothalamus VMH glutamatergic neuron. Our findings revealed that METH-induced VTA dopamine excitability activates the ventromedial hypothalamus (VMH) glutamatergic neurons, contributing to pathological aggression. Notably, we uncovered a dopaminergic transmission within the VTA-VMH circuit that exclusively functioned under METH influence. This dopaminergic pathway emerged as a potential key player in enabling dopamine-related pathological aggression, with heightened dopaminergic excitability implicated in various psychiatric symptoms. Also, the modulatory function of this pathway opens new possibilities for targeted therapeutic strategies for intervention to improve treatment in METH abuse and may have broader implications for addressing pathological aggression syndromes.

An Aggressive Interaction Rapidly Increases Brain Androgens in a Male Songbird during the Non-breeding Season

Aggression is a crucial behavior that impacts access to limited resources in different environmental contexts. Androgens synthesized by the gonads promote aggression during the breeding season. However, aggression can be expressed during the non-breeding season, despite low androgen synthesis by the gonads. The brain can also synthesize steroids ("neurosteroids"), including androgens, which might promote aggression during the non-breeding season. Male song sparrows, Melospiza melodia, are territorial year-round and allow the study of seasonal changes in the steroid modulation of aggression. Here, we quantified steroids following a simulated territorial intrusion (STI) for 10 min in wild adult male song sparrows during the breeding and non-breeding seasons. Using liquid chromatography-tandem mass spectrometry (LC-MS/MS), we examined 11 steroids: pregnenolone, progesterone, corticosterone, dehydroepiandrosterone, androstenedione, testosterone, 5α-dihydrotestosterone, 17β-estradiol, 17α-estradiol, estriol, and estrone. Steroids were measured in blood and 10 microdissected brain regions that regulate social behavior. In both seasons, STI increased corticosterone in the blood and brain. In the breeding season, STI had no rapid effects on androgens or estrogens. Intriguingly, in the non-breeding season, STI increased testosterone and androstenedione in several behaviorally relevant regions, but not in the blood, where androgens remained non-detectable. Also in the non-breeding season, STI increased progesterone in the blood and specific brain regions. Overall, rapid socially modulated changes in brain steroid levels are more prominent during the non-breeding season. Brain steroid levels vary with season and social context in a region-specific manner and suggest a role for neuroandrogens in aggression during the non-breeding season.

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.

Sexually dimorphic control of aggression by androgen signaling in a cichlid

Innate social behaviors like aggression are modulated by sex steroid hormones such as androgens and estrogens. However, we know little about how the same hormone regulates similar behaviors in both sexes. We investigated the role of androgenic signaling in the regulation of aggression in Astatotilapia burtoni, a social fish in which males and females perform similar aggressive behaviors. We used ARa knockout (KO) animals for this study, which was recently shown to be required for male-typical aggression and mating. Surprisingly, ARα KO females did not show deficits in aggression. We also determined that females lacking the other AR, ARβ, showed normal levels of aggression. Blocking both ARs pharmacologically confirmed that neither AR is necessary for aggression in females. However, ARα KO males showed clear deficits in attacks. Thus, in A. burtoni there appears to be a sexual dimorphism in the role of ARα in the control of aggression.

Sex differences in aggression and its neural substrate in a cichlid fish

Aggression is ubiquitous among social species and functions to maintains social dominance hierarchies. The African cichlid fish Astatotilapia burtoni is an ideal study species for studying aggression due to their unique and flexible dominance hierarchy. However, female aggression in this species and the neural mechanisms of aggression in both sexes is not well understood. To further understand the potential sex differences in aggression in this species, we characterized aggression in male and female A. burtoni in a mirror assay. We then quantified neural activation patterns in brain regions of the social behavior network (SBN) to investigate if differences in behavior are reflected in the brain with immunohistochemistry by detecting the phosphorylated ribosome marker phospho-S6 ribosomal protein (pS6), a marker for neural activation. We found that A. burtoni perform both identical and sex-specific aggressive behaviors in response to a mirror assay. We observed sex differences in pS6 immunoreactivity in the Vv, a homolog of the lateral septum in mammals. Males but not females had higher ps6 immunoreactivity in the ATn after the aggression assay. The ATn is a homolog of the ventromedial hypothalamus in mammals, which is strongly implicated in the regulation of aggression in males. Several regions also have higher pS6 immunoreactivity in negative controls than fish exposed to a mirror, implicating a role for inhibitory neurons in suppressing aggression until a relevant stimulus is present. Male and female A. burtoni display both similar and sexually dimorphic behavioral patterns in aggression in response to a mirror assay. There are also sex differences in the corresponding neural activation patterns in the SBN. In mirror males but not females, the ATn clusters with the POA, revealing a functional connectivity of these regions that is triggered in an aggressive context in males. These findings suggest that distinct neural circuitry underlie aggressive behavior in male and female A. burtoni, serving as a foundation for future work investigating the molecular and neural underpinnings of sexually dimorphic behaviors in this species to reveal fundamental insights into understanding aggression.

Androgen receptor deficiency is associated with reduced aromatase expression in the ventromedial hypothalamus of male cichlids

Social behaviors are regulated by sex steroid hormones, such as androgens and estrogens. However, the specific molecular and neural processes modulated by steroid hormones to generate social behaviors remain to be elucidated. We investigated whether some actions of androgen signaling in the control of social behavior may occur through the regulation of estradiol synthesis in the highly social cichlid fish, Astatotilapia burtoni. Specifically, we examined the expression of cyp19a1, a brain-specific aromatase, in the brains of male A. burtoni lacking a functional ARα gene (ar1), which was recently found to be necessary for aggression in this species. We found that cyp19a1 expression is higher in wild-type males compared to ar1 mutant males in the anterior tuberal nucleus (ATn), the putative fish homolog of the mammalian ventromedial hypothalamus, a brain region that is critical for aggression across taxa. Using in situ hybridization chain reaction, we determined that cyp19a1+ cells coexpress ar1 throughout the brain, including in the ATn. We speculate that ARα may modulate cyp19a1 expression in the ATn to govern aggression in A. burtoni. These studies provide novel insights into the hormonal mechanisms of social behavior in teleosts and lay a foundation for future functional studies.

Identifying behavioral links to neural dynamics of multifiber photometry recordings in a mouse social behavior network

Distributed hypothalamic-midbrain neural circuits orchestrate complex behavioral responses during social interactions. How population-averaged neural activity measured by multi-fiber photometry (MFP) for calcium fluorescence signals correlates with social behaviors is a fundamental question. We propose a state-space analysis framework to characterize mouse MFP data based on dynamic latent variable models, which include continuous-state linear dynamical system (LDS) and discrete-state hidden semi-Markov model (HSMM). We validate these models on extensive MFP recordings during aggressive and mating behaviors in male-male and male-female interactions, respectively. Our results show that these models are capable of capturing both temporal behavioral structure and associated neural states. Overall, these analysis approaches provide an unbiased strategy to examine neural dynamics underlying social behaviors and reveals mechanistic insights into the relevant networks.

The genetic architecture of the human hypothalamus and its involvement in neuropsychiatric behaviours and disorders

Despite its crucial role in the regulation of vital metabolic and neurological functions, the genetic architecture of the hypothalamus remains unknown. Here we conducted multivariate genome-wide association studies (GWAS) using hypothalamic imaging data from 32,956 individuals to uncover the genetic underpinnings of the hypothalamus and its involvement in neuropsychiatric traits. There were 23 significant loci associated with the whole hypothalamus and its subunits, with functional enrichment for genes involved in intracellular trafficking systems and metabolic processes of steroid-related compounds. The hypothalamus exhibited substantial genetic associations with limbic system structures and neuropsychiatric traits including chronotype, risky behaviour, cognition, satiety and sympathetic-parasympathetic activity. The strongest signal in the primary GWAS, the ADAMTS8 locus, was replicated in three independent datasets (N = 1,685-4,321) and was strengthened after meta-analysis. Exome-wide association analyses added evidence to the association for ADAMTS8, and Mendelian randomization showed lower ADAMTS8 expression with larger hypothalamic volumes. The current study advances our understanding of complex structure-function relationships of the hypothalamus and provides insights into the molecular mechanisms that underlie hypothalamic formation.

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.

Transcriptionally defined amygdala subpopulations play distinct roles in innate social behaviors

Social behaviors are innate and supported by dedicated neural circuits, but the molecular identities of these circuits and how they are established developmentally and shaped by experience remain unclear. Here we show that medial amygdala (MeA) cells originating from two embryonically parcellated developmental lineages have distinct response patterns and functions in social behavior in male mice. MeA cells expressing the transcription factor Foxp2 (MeAFoxp2) are specialized for processing male conspecific cues and are essential for adult inter-male aggression. By contrast, MeA cells derived from the Dbx1 lineage (MeADbx1) respond broadly to social cues, respond strongly during ejaculation and are not essential for male aggression. Furthermore, MeAFoxp2 and MeADbx1 cells show differential anatomical and functional connectivity. Altogether, our results suggest a developmentally hardwired aggression circuit at the MeA level and a lineage-based circuit organization by which a cell's embryonic transcription factor profile determines its social information representation and behavioral relevance during adulthood.

A Review of the Effects of Some Extrinsic Factors on Mice Used in Research

Animals have been used in research for over 2,000 y. From very crude experiments conducted by ancient scholars, animal research, as a science, was refined over hundreds of years to what we know it as today. However, the housing conditions of animals used for research did not improve significantly until less than 100 years ago when guidelines for housing research animals were first published. In addition, it was not until relatively recently that some extrinsic factors were recognized as a research variable, even when animals were housed under recommended guidelines. For example, temperature, humidity, light, noise, vibration, diet, water, caging, bedding, etc., can all potentially affect research using mice, contributing the inability of others to reproduce published findings. Consequently, these external factors should be carefully considered in the design, planning, and execution of animal experiments. In addition, as recommended by others, the housing and husbandry conditions of the animals should be described in detail in publications resulting from animal research to improve study reproducibility. Here, we briefly review some common, and less common, external factors that affect research in one of the most popular animal models, the mouse.

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.

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.

Breaking Through the Bottleneck: Krogh's Principle in Behavioral Neuroendocrinology and the Potential of Gene Editing

In 1929, August Krogh wrote that for every question in biology, there is a species or collection of species in which pursuing such questions is the most appropriate for achieving the deepest insights. Referred to as "Krogh's Principle," these words are a guiding force for many biologists. In practice, Krogh's principle might guide a biologist interested in studying bi-parental care to choose not to use lab mice, in which the female does most of the parenting, but instead study species in which bi-parental care is present and clearly observable, such as in certain poison dart frogs. This approach to pursuing biological questions has been fruitful, with more in-depth insights achievable with new technologies. However, up until recently, an important limitation of Krogh's principle for biologists interested in the functions of certain genes, was certain techniques were only available for a few traditional model organisms such as lab mice, fruit flies (Drosophila melanogaster), zebrafish (Danio rerio) and C. elegans (Caenorhabditis elegans), in which testing the functions of molecular systems on biological processes can be achieved using genetic knockout (KO) and transgenic technology. These methods are typically more precise than other approaches (e.g., pharmacology) commonly used in nontraditional model organisms to address similar questions. Therefore, some of the most in-depth insights into our understanding of the molecular control of these mechanisms have come from a small number of genetically tractable species. Recent advances in gene editing technology such as CRISPR (Clustered Regularly Interspersed Short Palindromic Repeats)/Cas9 gene editing as a laboratory tool has changed the insights achievable for biologists applying Krogh's principle. In this review, we will provide a brief summary on how some researchers of nontraditional model organisms have been able to achieve different levels of experimental precision with limited genetic tractability in their non-traditional model organism in the field of behavioral neuroendocrinology, a field in which understanding tissue and brain-region specific actions of molecules of interest has been a major goal. Then, we will highlight the exciting potential of Krogh's principle using discoveries made in a popular model species of social behavior, the African cichlid fish Astatotilapia burtoni. Specifically, we will focus on insights gained from studies of the control of social status by sex steroid hormones (androgens and estrogens) in A. burtoni that originated during field observations during the 1970s, and have recently culminated in novel insights from CRISPR/Cas9 gene editing in laboratory studies. Our review highlighting discoveries in A. burtoni may function as a roadmap for others using Krogh's principle aiming to incorporate gene editing into their research program. Gene editing is thus a powerful complimentary laboratory tool researchers can use to yield novel insights into understanding the molecular mechanisms of physiology and behavior in non-traditional model organisms.

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.

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.

Non-angry aggressive arousal and angriffsberietschaft: A narrative review of the phenomenology and physiology of proactive/offensive aggression motivation and escalation in people and other animals

Human aggression typologies largely correspond with those for other animals. While there may be no non-human equivalent of angry reactive aggression, we propose that human proactive aggression is similar to offense in other animals' dominance contests for territory or social status. Like predation/hunting, but unlike defense, offense and proactive aggression are positively reinforcing, involving dopamine release in accumbens. The drive these motivational states provide must suffice to overcome fear associated with initiating risky fights. We term the neural activity motivating proactive aggression "non-angry aggressive arousal", but use "angriffsberietschaft" for offense motivation in other animals to acknowledge possible differences. Temporal variation in angriffsberietschaft partitions fights into bouts; engendering reduced anti-predator vigilance, redirected aggression and motivational over-ride. Increased aggressive arousal drives threat-to-attack transitions, as in verbal-to-physical escalation and beyond that, into hyper-aggression. Proactive aggression and offense involve related neural activity states. Cingulate, insular and prefrontal cortices energize/modulate aggression through a subcortical core containing subnuclei for each aggression type. These proposals will deepen understanding of aggression across taxa, guiding prevention/intervention for human violence.

Hypothalamic neurons that mirror aggression

Social interactions require awareness and understanding of the behavior of others. Mirror neurons, cells representing an action by self and others, have been proposed to be integral to the cognitive substrates that enable such awareness and understanding. Mirror neurons of the primate neocortex represent skilled motor tasks, but it is unclear if they are critical for the actions they embody, enable social behaviors, or exist in non-cortical regions. We demonstrate that the activity of individual VMHvlPR neurons in the mouse hypothalamus represents aggression performed by self and others. We used a genetically encoded mirror-TRAP strategy to functionally interrogate these aggression-mirroring neurons. We find that their activity is essential for fighting and that forced activation of these cells triggers aggressive displays by mice, even toward their mirror image. Together, we have discovered a mirroring center in an evolutionarily ancient region that provides a subcortical cognitive substrate essential for a social behavior.

Whole Transcriptome Analysis of Hypothalamus in Mice during Short-Term Starvation

Molecular profiling of the hypothalamus in response to metabolic shifts is a critical cue to better understand the principle of the central control of whole-body energy metabolism. The transcriptional responses of the rodent hypothalamus to short-term calorie restriction have been documented. However, studies on the identification of hypothalamic secretory factors that potentially contribute to the control of appetite are lacking. In this study, we analyzed the differential expression of hypothalamic genes and compared the selected secretory factors from the fasted mice with those of fed control mice using bulk RNA-sequencing. We verified seven secretory genes that were significantly altered in the hypothalamus of fasted mice. In addition, we determined the response of secretory genes in cultured hypothalamic cells to treatment with ghrelin and leptin. The current study provides further insights into the neuronal response to food restriction at the molecular level and may be useful for understanding the hypothalamic control of appetite.

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.

Illicit drug use and violence

This chapter reviews evidence on the relationship between illicit drug use and violence, specifically cannabis, stimulant drugs, and opioids. It summarizes findings of systematic reviews of evidence on cannabis, stimulant drugs, and opioids. It also examines evidence from epidemiological studies of drug use among violent offenders and of violence among persons who use drugs, intervention studies, animal studies, human laboratory studies, and human neuroimaging studies. More studies have examined cannabis because of its higher prevalence of use. There is an association between cannabis use and violence, suggestive evidence of a dose-response relationship between the frequency of cannabis use and violence, and a stronger association in persons with psychoses. There is similar emerging evidence on stimulant use and violence, but evidence on opioids is very limited. There is limited and mixed evidence from intervention studies that reducing drug use reduces violence. Animal and human studies provide potential biological explanations for these associations. The association between cannabis use and violence is most consistent but limited by study heterogeneity and lack of control for potential confounders. It is unclear whether these associations are causal or reflect reverse causation or the effects of confounding.

Genetic dissection of steroid-hormone modulated social behavior: Novel paralogous genes are a boon for discovery

Research across species has led to important discoveries on the functions of steroid hormones in the regulation of behavior. However, like in many fields, advancements in transgenic and mutagenic technology allowed mice to become the premier genetic model for conducting many experiments to understand how steroids control social behavior. Since there has been a general lack of parallel methodological developments in other species, many of the findings cannot be generalized. This is especially the case for teleost fish, in which a whole-genome duplication produced novel paralogs for key steroid hormone signaling genes. In this review, we summarize technical advancements over the history of the field of neuroendocrinology that have led to important insights in our understanding of the control of social behavior by steroids. We demonstrate that early mouse genetic models to understand these mechanisms suffered from several issues that were remedied by more precise transgenic technological advancements. We then highlight the importance of CRISPR/Cas9 gene editing tools that will in time bridge the gap between mice and non-traditional model species for understanding principles of steroid hormone action in the modulation of social behavior. We specifically highlight the role of teleost fish in bridging this gap because they are 1) highly genetically tractable and 2) provide a novel advantage in achieving precise genetic control. The field of neuroendocrinology is entering a new "gene editing revolution" that will lead to novel discoveries about the roles of steroid hormones in the regulation and evolutionary trajectories of social behavior.

High-Frequency Local Field Potential Oscillations May Modulate Aggressive Behaviors in Mice

Aggressive behavior is one of congenital social behaviors in many species, which could be promoted by social neglect or isolation in the early stages of life. Many brain regions including the medial prefrontal cortex (mPFC), medial amygdala (MeA) and ventromedial hypothalamus (VMH) are demonstrated to relate to aggressive behavior; however, the dynamic patterns of neural activities during the occurrence of this behavior remain unclear. In this study, 21-day-old male CD-1 mice were reared in social isolation conditions and cohousing conditions for two weeks. Aggressive behaviors of each subject were estimated by the resident–intruder test. Simultaneously, the local field potentials of mPFC, MeA and VMH were recorded for exploring differences in the relative power spectra of different oscillations when aggressive behaviors occurred. The results showed that the following: (1) Compared with the cohousing mice, the socially isolated mice exhibited more aggression. (2) Regardless of “time condition” (pre-, during- and post- attack), the relative power spectra of beta band in the cohousing mice were significantly greater than those in the socially isolated mice, and inversely, the relative power spectra of gamma band in the cohousing mice were significantly smaller than those in the socially isolated mice. (3) The bilateral mPFC exhibited significantly smaller beta power spectra but greater gamma power spectra compared with other brain areas regardless of rearing patterns. (4) For the right VMH of the socially isolated mice, the relative power spectra of the gamma band during attacks were significantly greater than those before attack. These results suggest that aggressive behaviors in mice could be shaped by rearing patterns and that high-frequency oscillations (beta and gamma bands) may engage in mediating aggressive behaviors in mice.

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.

Dysfunction of AMPA receptor GluA3 is associated with aggressive behavior in human

Inappropriate aggression in humans hurts the society, families and individuals. The genetic basis for aggressive behavior, however, remains largely elusive. In this study, we identified two rare missense variants in X-linked GRIA3 from male patients who showed syndromes featuring aggressive outbursts. Both G630R and E787G mutations in AMPA receptor GluA3 completely lost their ion channel functions. Furthermore, a guanine-repeat single nucleotide polymorphism (SNP, rs3216834) located in the first intron of human GRIA3 gene was found to regulate GluA3 expression with longer guanine repeats (rs3216834-10G/-11G) suppressing transcription compared to the shorter ones (-7G/-8G/-9G). Importantly, the distribution of rs3216834-10G/-11G was elevated in a male violent criminal sample from Chinese Han population. Using GluA3 knockout mice, we showed that the excitatory neurotransmission and neuronal activity in the medial prefrontal cortex (mPFC) was impaired. Expressing GluA3 back into the mPFC alleviated the aggressive behavior of GluA3 knockout mice, suggesting that the defects in mPFC explained, at least partially, the neural mechanisms underlying the aggressive behavior. Therefore, our study provides compelling evidence that dysfunction of AMPA receptor GluA3 promotes aggressive behavior.

Deletion of the P/Q-Type Calcium Channel from Serotonergic Neurons Drives Male Aggression in Mice

Aggressive behavior is one of the most conserved social interactions in nature and serves as a crucial evolutionary trait. Serotonin (5-HT) plays a key role in the regulation of our emotions, such as anxiety and aggression, but which molecules and mechanisms in the serotonergic system are involved in violent behavior are still unknown. In this study, we show that deletion of the P/Q-type calcium channel selectively from serotonergic neurons in the dorsal raphe nuclei (DRN) augments aggressive behavior in male mice, while anxiety is not affected. These mice demonstrated increased induction of the immediate early gene c-fos and in vivo serotonergic firing activity in the DRN. The ventrolateral part of the ventromedial hypothalamus is also a prominent region of the brain mediating aggression. We confirmed a monosynaptic projection from the DRN to the ventrolateral part of the ventromedial hypothalamus, and silencing these projections with an inhibitory designer receptor exclusively activated by a designer drug effectively reduced aggressive behavior. Overall, our findings show that deletion of the P/Q-type calcium channel from DRN neurons is sufficient to induce male aggression in mice and regulating its activity may serve as a therapeutic approach to treat violent behavior.SIGNIFICANCE STATEMENT In this study, we show that P/Q-type calcium channel is mediating aggression in serotonergic neurons from the dorsal raphe nucleus via monosynaptic projections to the ventrolateral part of the ventromedial hypothalamus. More importantly, silencing these projections reduced aggressive behavior in mice and may serve as a therapeutic approach for treating aggression in humans.

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.

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.

Hypothalamic syndrome

Hypothalamic syndrome (HS) is a rare disorder caused by disease-related and/or treatment-related injury to the hypothalamus, most commonly associated with rare, non-cancerous parasellar masses, such as craniopharyngiomas, germ cell tumours, gliomas, cysts of Rathke's pouch and Langerhans cell histiocytosis, as well as with genetic neurodevelopmental syndromes, such as Prader-Willi syndrome and septo-optic dysplasia. HS is characterized by intractable weight gain associated with severe morbid obesity, multiple endocrine abnormalities and memory impairment, attention deficit and reduced impulse control as well as increased risk of cardiovascular and metabolic disorders. Currently, there is no cure for this condition but treatments for general obesity are often used in patients with HS, including surgery, medication and counselling. However, these are mostly ineffective and no medications that are specifically approved for the treatment of HS are available. Specific challenges in HS are because the syndrome represents an adverse effect of different diseases, and that diagnostic criteria, aetiology, pathogenesis and management of HS are not completely defined.

From Reductionism Toward Integration: Understanding How Social Behavior Emerges From Integrated Circuits

Complex social behaviors are emergent properties of the brain's interconnected and overlapping neural networks. Questions aimed at understanding how brain circuits produce specific and appropriate behaviors have changed over the past half century, shifting from studies of gross anatomical and behavioral associations, to manipulating and monitoring precisely targeted cell types. This technical progression has enabled increasingly deep insights into the regulation of perception and behavior with remarkable precision. The capacity of reductionist approaches to identify the function of isolated circuits is undeniable but many behaviors require rapid integration of diverse inputs. This review examines progress toward understanding integrative social circuits and focuses on specific nodes of the social behavior network including the medial amygdala, ventromedial hypothalamus (VMH) and medial preoptic area of the hypothalamus (MPOA) as examples of broad integration between multiple interwoven brain circuits. Our understanding of mechanisms for producing social behavior has deepened in conjunction with advances in technologies for visualizing and manipulating specific neurons and, here, we consider emerging strategies to address brain circuit function in the context of integrative anatomy.

A functional cellular framework for sex and estrous cycle-dependent gene expression and behavior

Sex hormones exert a profound influence on gendered behaviors. How individual sex hormone-responsive neuronal populations regulate diverse sex-typical behaviors is unclear. We performed orthogonal, genetically targeted sequencing of four estrogen receptor 1-expressing (Esr1+) populations and identified 1,415 genes expressed differentially between sexes or estrous states. Unique subsets of these genes were distributed across all 137 transcriptomically defined Esr1+ cell types, including estrous stage-specific ones, that comprise the four populations. We used differentially expressed genes labeling single Esr1+ cell types as entry points to functionally characterize two such cell types, BNSTprTac1/Esr1 and VMHvlCckar/Esr1. We observed that these two cell types, but not the other Esr1+ cell types in these populations, are essential for sex recognition in males and mating in females, respectively. Furthermore, VMHvlCckar/Esr1 cell type projections are distinct from those of other VMHvlEsr1 cell types. Together, projection and functional specialization of dimorphic cell types enables sex hormone-responsive populations to regulate diverse social behaviors.

Mechanically evoked defensive attack is controlled by GABAergic neurons in the anterior hypothalamic nucleus

Innate defensive behaviors triggered by environmental threats are important for animal survival. Among these behaviors, defensive attack toward threatening stimuli (for example, predators) is often the last line of defense. How the brain regulates defensive attack remains poorly understood. Here we show that noxious mechanical force in an inescapable context is a key stimulus for triggering defensive attack in laboratory mice. Mechanically evoked defensive attacks were abrogated by photoinhibition of vGAT⁺ neurons in the anterior hypothalamic nucleus (AHN). The vGAT⁺ AHN neurons encoded the intensity of mechanical force and were innervated by brain areas relevant to pain and attack. Activation of these neurons triggered biting attacks toward a predator while suppressing ongoing behaviors. The projection from vGAT⁺ AHN neurons to the periaqueductal gray might be one AHN pathway participating in mechanically evoked defensive attack. Together, these data reveal that vGAT⁺ AHN neurons encode noxious mechanical stimuli and regulate defensive attack in mice.

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.

Divergent outputs of the ventral lateral geniculate nucleus mediate visually evoked defensive behaviors

Rapid alternations between exploration and defensive reactions require ongoing risk assessment. How visual cues and internal states flexibly modulate the selection of behaviors remains incompletely understood. Here, we show that the ventral lateral geniculate nucleus (vLGN)-a major retinorecipient structure-is a critical node in the network controlling defensive behaviors to visual threats. We find that vLGNGABA neuron activity scales with the intensity of environmental illumination and is modulated by behavioral state. Chemogenetic activation of vLGNGABA neurons reduces freezing, whereas inactivation dramatically extends the duration of freezing to visual threats. Perturbations of vLGN activity disrupt exploration in brightly illuminated environments. We describe both a vLGN→nucleus reuniens (Re) circuit and a vLGN→superior colliculus (SC) circuit, which exert opposite influences on defensive responses. These findings reveal roles for genetic- and projection-defined vLGN subpopulations in modulating the expression of behavioral threat responses according to internal state.

Measuring Behavior in the Home Cage: Study Design, Applications, Challenges, and Perspectives

The reproducibility crisis (or replication crisis) in biomedical research is a particularly existential and under-addressed issue in the field of behavioral neuroscience, where, in spite of efforts to standardize testing and assay protocols, several known and unknown sources of confounding environmental factors add to variance. Human interference is a major contributor to variability both within and across laboratories, as well as novelty-induced anxiety. Attempts to reduce human interference and to measure more "natural" behaviors in subjects has led to the development of automated home-cage monitoring systems. These systems enable prolonged and longitudinal recordings, and provide large continuous measures of spontaneous behavior that can be analyzed across multiple time scales. In this review, a diverse team of neuroscientists and product developers share their experiences using such an automated monitoring system that combines Noldus PhenoTyper® home-cages and the video-based tracking software, EthoVision® XT, to extract digital biomarkers of motor, emotional, social and cognitive behavior. After presenting our working definition of a "home-cage", we compare home-cage testing with more conventional out-of-cage tests (e.g., the open field) and outline the various advantages of the former, including opportunities for within-subject analyses and assessments of circadian and ultradian activity. Next, we address technical issues pertaining to the acquisition of behavioral data, such as the fine-tuning of the tracking software and the potential for integration with biotelemetry and optogenetics. Finally, we provide guidance on which behavioral measures to emphasize, how to filter, segment, and analyze behavior, and how to use analysis scripts. We summarize how the PhenoTyper has applications to study neuropharmacology as well as animal models of neurodegenerative and neuropsychiatric illness. Looking forward, we examine current challenges and the impact of new developments. Examples include the automated recognition of specific behaviors, unambiguous tracking of individuals in a social context, the development of more animal-centered measures of behavior and ways of dealing with large datasets. Together, we advocate that by embracing standardized home-cage monitoring platforms like the PhenoTyper, we are poised to directly assess issues pertaining to reproducibility, and more importantly, measure features of rodent behavior under more ethologically relevant scenarios.

Sex- and context-dependent effects of acute isolation on vocal and non-vocal social behaviors in mice

Humans are extraordinarily social, and social isolation has profound effects on our behavior, ranging from increased social motivation following short periods of social isolation to increased anti-social behaviors following long-term social isolation. Mice are frequently used as a model to understand how social isolation impacts the brain and behavior. While the effects of chronic social isolation on mouse social behavior have been well studied, much less is known about how acute isolation impacts mouse social behavior and whether these effects vary according to the sex of the mouse and the behavioral context of the social encounter. To address these questions, we characterized the effects of acute (3-day) social isolation on the vocal and non-vocal social behaviors of male and female mice during same-sex and opposite-sex social interactions. Our experiments uncovered pronounced effects of acute isolation on social interactions between female mice, while revealing more subtle effects on the social behaviors of male mice during same-sex and opposite-sex interactions. Our findings advance the study of same-sex interactions between female mice as an attractive paradigm to investigate neural mechanisms through which acute isolation enhances social motivation and promotes social behavior.

Neural basis for estrous cycle-dependent control of female behaviors

Females display changes in distinct behaviors along the estrous cycle. Levels of circulating ovarian sex steroid hormones peak around ovulation, which occur around estrus phase of the cycle. This increase of sex hormones is thought to be important for changes in behaviors, however, neural circuit mechanisms of periodic behavioral changes in females are not understood well. Different lines of research indicate sex hormonal effects on several forms of neuronal plasticity. This review provides an overview of behavioral and plastic changes that occur in an estrous cycle-dependent manner and explores the current research linking these changes to understand neural circuit mechanisms that control female behaviors.

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 social network: Neural control of sex differences in reproductive behaviors, motivation, and response to social isolation

Social animal species present a vast repertoire of social interactions when encountering conspecifics. Reproduction-related behaviors, such as mating, parental care, and aggression, are some of the most rewarding types of social interactions and are also the most sexually dimorphic ones. This review focuses on rodent species and summarizes recent advances in neuroscience research that link sexually dimorphic reproductive behaviors to sexual dimorphism in their underlying neuronal circuits. Specifically, we present a few possible mechanisms governing sexually-dimorphic behaviors, by hypothalamic and reward-related brain regions. Sex differences in the neural response to social isolation in adulthood are also discussed, as well as future directions for comparative studies with naturally solitary species.

Home-based training program during the SARS-CoV- 2 quarantine: training load, motivation, and wellbeing in professional elite female basketball players

BACKGROUND

due to the quarantine caused by the Coronavirus disease 2019 (COVID-19), professional team-sports adapted their training routine to maintain the athletes' performance and minimize the negative emotional effects. The purpose of the present study was to describe a home-based training program and extra-activities applied during the quarantine in elite professional female basketball players, and to compare the internal training load (ITL), session rating of perceived exertion (s-RPE), motivation, and wellbeing responses before and during the quarantine.

METHODS

nine female elite players (25.7 ± 7.0 years; 180.0 ± 8.0 cm; 79.0 ± 11.0 kg) belonging to a professional basketball team performed a home-based training program during the quarantine. ITL, s-RPE, training motivation, and wellbeing were examined before (phase 1), at the beginning (phase 2), and during (phase 3) of the quarantine.

RESULTS

ITL was higher (main effect) in phase 1 (449.5 ± 62.8 a.u.) compared to phase 2 (265.5 ± 39.0 a.u.) and phase 3 (423.1 ± 18.7 a.u.); s-RPE was higher in phase 1 (4.50 ± 0.63 a.u.) compared to phase 2 (3.8 ± 0.6 a.u.), but not to phase 3 (4.37 ± 0.79 a.u.). Motivation was higher in phase 1 (3.6 ± 0.3 a.u.) compared to phase 2 (2.3 ± 0.6 a.u.) and phase 3 (2.4 ± 0.5 a.u.).

CONCLUSIONS

in conclusion, the results of the study demonstrated that extra-activities involving online athletes' socialization were implemented to rouse the motivation, but the quarantine period induced a reduction in ITL and influenced the player's training motivation.

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.

Melatonin-dependent changes in neurosteroids are associated with increased aggression in a seasonally breeding rodent

Aggression is a complex social behaviour that allows individuals to compete for access to limited resources (eg, mates, food and territories). Excessive or inappropriate aggression, however, has become problematic in modern societies, and current treatments are largely ineffective. Although previous work in mammals suggests that aggressive behaviour varies seasonally, seasonality is largely overlooked when developing clinical treatments for inappropriate aggression. Here, we investigated how the hormone melatonin regulates seasonal changes in neurosteroid levels and aggressive behaviour in Siberian hamsters, a rodent model of seasonal aggression. Specifically, we housed males in long-day (LD) or short-day (SD) photoperiods, administered timed s.c. melatonin injections (which mimic a SD-like signal) or control injections, and measured aggression using a resident-intruder paradigm after 9 weeks of treatment. Moreover, we quantified five steroid hormones in circulation and in brain regions associated with aggressive behaviour (lateral septum, anterior hypothalamus, medial amygdala and periaqueductal gray) using liquid chromatography-tandem mass spectrometry. SD hamsters and LD hamsters administered timed melatonin injections (LD-M) displayed increased aggression and exhibited region-specific decreases in neural dehydroepiandrosterone, testosterone and oestradiol, but showed no changes in progesterone or cortisol. Male hamsters also showed distinct associations between neurosteroids and aggressive behaviour, in which neural progesterone and dehydroepiandrosterone were positively correlated with aggression in all treatment groups, whereas neural testosterone, oestradiol and cortisol were negatively correlated with aggression only in LD-M and SD hamsters. Collectively, these results provide insight into a novel neuroendocrine mechanism of mammalian aggression, in which melatonin reduces neurosteroid levels and elevates aggressive behaviour.

Processing of intraspecific chemical signals in the rodent brain

In the rodent brain, the central processing of ecologically relevant chemical stimuli involves many different areas located at various levels within the neuraxis: the main and accessory olfactory bulbs, some nuclei in the amygdala, the hypothalamus, and brainstem. These areas allow the integration of the chemosensory stimuli with other sensory information and the selection of the appropriate neurohormonal and behavioral response. This review is a brief introduction to the processing of intraspecific chemosensory stimuli beyond the secondary projection, focusing on the activity of the relevant amygdala and hypothalamic nuclei, namely the medial amygdala and ventromedial hypothalamus. These areas are involved in the appropriate interpretation of chemosensory information and drive the selection of the proper response, which may be behavioral or hormonal and may affect the neural activity of other areas in the telencephalon and brainstem.Recent data support the notion that the processing of intraspecific chemical signals is not unique to one chemosensory system and some molecules may activate both the main and the accessory olfactory system. Moreover, both these systems have mixed projections and cooperate for the correct identification of the stimuli and selection of relevant responses.

Distinct hypothalamic control of same- and opposite-sex mounting behaviour in mice

Animal behaviours that are superficially similar can express different intents in different contexts, but how this flexibility is achieved at the level of neural circuits is not understood. For example, males of many species can exhibit mounting behaviour towards same- or opposite-sex conspecifics1, but it is unclear whether the intent and neural encoding of these behaviours are similar or different. Here we show that female- and male-directed mounting in male laboratory mice are distinguishable by the presence or absence of ultrasonic vocalizations (USVs)2-4, respectively. These and additional behavioural data suggest that most male-directed mounting is aggressive, although in rare cases it can be sexual. We investigated whether USV+ and USV- mounting use the same or distinct hypothalamic neural substrates. Micro-endoscopic imaging of neurons positive for oestrogen receptor 1 (ESR1) in either the medial preoptic area (MPOA) or the ventromedial hypothalamus, ventrolateral subdivision (VMHvl) revealed distinct patterns of neuronal activity during USV+ and USV- mounting, and the type of mounting could be decoded from population activity in either region. Intersectional optogenetic stimulation of MPOA neurons that express ESR1 and vesicular GABA transporter (VGAT) (MPOAESR1∩VGAT neurons) robustly promoted USV+ mounting, and converted male-directed attack to mounting with USVs. By contrast, stimulation of VMHvl neurons that express ESR1 (VMHvlESR1 neurons) promoted USV- mounting, and inhibited the USVs evoked by female urine. Terminal stimulation experiments suggest that these complementary inhibitory effects are mediated by reciprocal projections between the MPOA and VMHvl. Together, these data identify a hypothalamic subpopulation that is genetically enriched for neurons that causally induce a male reproductive behavioural state, and indicate that reproductive and aggressive states are represented by distinct population codes distributed between MPOAESR1 and VMHvlESR1 neurons, respectively. Thus, similar behaviours that express different internal states are encoded by distinct hypothalamic neuronal populations.

Profiling of systemic and brain steroids in male songbirds: Seasonal changes in neurosteroids

Steroids are secreted by the gonads and adrenal glands into the blood to modulate neurophysiology and behaviour. In addition, the brain can metabolise circulating steroids and synthesise steroids de novo. Songbirds show high levels of neurosteroid synthesis. In the present study, we developed and validated a liquid chromatography-tandem mass spectrometry (LC-MS/MS) assay for the measurement of 10 steroids in whole blood, plasma and microdissected brain tissue (1-2 mg) of song sparrows. Our assay is highly accurate, precise, specific and sensitive. Moreover, the liquid-liquid extraction is fast, simple and effective. We quantified steroids in the blood and brain of wild male song sparrows in both breeding and non-breeding seasons. As expected, systemic androgen levels were higher in the breeding season than in the non-breeding season. Brain androgens were detectable only in the breeding season; androstenedione and 5α-dihydrotestosterone levels were up to 20-fold higher in specific brain regions than in blood. Oestrogens were not detectable in blood in both seasons. Oestrone and 17β-oestradiol were detectable in brain in the breeding season only (up to 1.4 ng g^-1 combined). Progesterone levels in several regions were higher in the non-breeding season than the breeding season, despite the lack of seasonal changes in systemic progesterone. Corticosterone levels in the blood were higher in the breeding season than in the non-breeding season but showed few seasonal differences in the brain. In general, the steroid levels presented here are lower than those in previous reports using immunoassays, because of the higher specificity of mass spectrometry. We conclude that (i) brain steroid levels can differ greatly from circulating steroid levels and (ii) brain steroid levels show region-specific seasonal patterns that are not a simple reflection of circulating steroid levels. This approach using ultrasensitive LC-MS/MS is broadly applicable to other species and allows steroid profiling in microdissected brain regions.