Social context-dependent relationships between mouse dominance rank and plasma hormone levels

Sex differences in the rodent medial prefrontal cortex - What Do and Don't we know?

The prefrontal cortex, particularly its medial subregions (mPFC), mediates critical functions such as executive control, behavioral inhibition, and memory formation, with relevance for everyday functioning and psychopathology. Despite broad characterization of the mPFC in multiple model organisms, the extent to which mPFC structure and function vary according to an individual's sex is unclear - a knowledge gap that can be attributed to a historical bias for male subjects in neuroscience research. Recent efforts to consider sex as a biological variable in basic science highlight the great need to close this gap. Here we review the knowns and unknowns about how rodents categorized as male or female compare in mPFC neuroanatomy, pharmacology, as well as in aversive, appetitive, and goal- or habit-directed behaviors that recruit the mPFC. We propose that long-standing dogmatic concepts of mPFC structure and function may not remain supported when we move beyond male-only studies, and that empirical challenges to these dogmas are warranted. Additionally, we note some common pitfalls in this work. Most preclinical studies operationalize sex as a binary categorization, and while this approach has furthered the inclusion of non-male rodents it is not as such generalizable to what we know of sex as a multidimensional, dynamic variable. Exploration of sex variability may uncover both sex differences and sex similarities, but care must be taken in their interpretation. Including females in preclinical research needs to go beyond the investigation of sex differences, improving our knowledge of how this brain region and its subregions mediate behavior and health. This article is part of the Special Issue on "PFC circuit function in psychiatric disease and relevant models".

Association between social dominance hierarchy and PACAP expression in the extended amygdala, corticosterone, and behavior in C57BL/6 male mice

The natural alignment of animals into social dominance hierarchies produces adaptive, and potentially maladaptive, changes in the brain that influence health and behavior. Aggressive and submissive behaviors assumed by animals through dominance interactions engage stress-dependent neural and hormonal systems that have been shown to correspond with social rank. Here, we examined the association between social dominance hierarchy status established within cages of group-housed mice and the expression of the stress peptide PACAP in the bed nucleus of the stria terminalis (BNST) and central nucleus of the amygdala (CeA). We also examined the relationship between social dominance rank and blood corticosterone (CORT) levels, body weight, motor coordination (rotorod) and acoustic startle. Male C57BL/6 mice were ranked as either Dominant, Submissive, or Intermediate based on counts of aggressive/submissive encounters assessed at 12 weeks-old following a change in homecage conditions. PACAP expression was significantly higher in the BNST, but not the CeA, of Submissive mice compared to the other groups. CORT levels were lowest in Submissive mice and appeared to reflect a blunted response following events where dominance status is recapitulated. Together, these data reveal changes in specific neural/neuroendocrine systems that are predominant in animals of lowest social dominance rank, and implicate PACAP in brain adaptations that occur through the development of social dominance hierarchies.

A hamster model for stress-induced weight gain

This review addresses the translational relevance of animal models of stress and their effects on body weight. In humans, stress, whether chronic or acute, has often been associated with increased food intake and weight gain. In view of the current obesity epidemic, this phenomenon is especially relevant. Such observations contrast with reports with commonly used laboratory animals, especially rats and mice. In these species, it is common to find individuals gaining less weight under stress, even with potent social stressors. However, there are laboratory species that present increased appetite and weight gain under stress, such as golden hamsters. Furthermore, these animals also include metabolic and behavioral similarities with humans, including hoarding behavior which is also enhanced under stress. Consequently, we propose that our comparative perspective provides useful insights for future research on the development of obesity in humans as a consequence of chronic stress exposure.

The impact of cage dividers on mouse aggression, dominance and hormone levels

Home cage aggression in group-housed male mice is a major welfare concern and may compromise animal research. Conventional cages prevent flight or retreat from sight, increasing the risk that agonistic encounters will result in injury. Moreover, depending on social rank, mice vary in their phenotype, and these effects seem highly variable and dependent on the social context. Interventions that reduce aggression, therefore, may reduce not only injuries and stress, but also variability between cage mates. Here we housed male mice (Balb/c and SWISS, group sizes of three and five) with or without partial cage dividers for two months. Mice were inspected for wounding weekly and home cages were recorded during housing and after 6h isolation housing, to assess aggression and assign individual social ranks. Fecal boli and fur were collected to quantify steroid levels. We found no evidence that the provision of cage dividers improves the welfare of group housed male mice; The prevalence of injuries and steroid levels was similar between the two housing conditions and aggression was reduced only in Balb/c strain. However, mice housed with cage dividers developed less despotic hierarchies and had more stable social ranks. We also found a relationship between hormone levels and social rank depending on housing type. Therefore, addition of cage dividers may play a role in stabilizing social ranks and modulating the activation of hypothalamic-pituitary-adrenal (HPA) and hypothalamic-pituitary-gonadal (HPG) axes, thus reducing phenotypic variability between mice of different ranks.

Characterization of social hierarchy formation and maintenance in same-sex, group-housed male and female C57BL/6 J mice

Social hierarchies are a prevalent feature of all animal groups, and an individual's rank within the group can significantly affect their overall health, typically at the greatest expense of the lowest-ranked individuals, or omegas. These subjects have been shown to exhibit various stress-related phenotypes, such as increased hypothalamic-pituitary axis activity and increased amygdalar corticotropin-releasing factor levels compared to higher-ranked subjects. However, these findings have been primarily characterized in males and in models requiring exhibition of severe aggression. The goals of the current study, therefore, were to characterize the formation and maintenance of social hierarchies using the tube test and palatable liquid competition in same-sex groups of male and female C57BL/6 J mice. We also aimed to examine the effects of tube test-determined social rank on plasma and hypothalamic oxytocin and vasopressin levels, peptides with established roles in social behaviors and the stress response. Lastly, we assessed the effects of environmental enrichment and length of testing on the measures outlined above. Overall, we demonstrated that males and females develop social hierarchies and that these hierarchies can be determined using the tube test. While we were unable to establish a consistent connection between peptide levels and social rank, we observed transient changes in these peptides reflecting complex interactions between social rank, sex, environment, and length of testing. We also found that many male and female omegas began to exhibit passive coping behavior after repeated tube test losses, demonstrating the potential of this assay to serve as a model of chronic, mild psychosocial stress.

Bite Wounds and Dominance Structures in Male and Female African Spiny Mice (Acomys cahirinus): Implications for Animal Welfare and the Generalizability of Experimental Results

Bite wounds due to aggression in male laboratory mice (Mus musculus) are a major welfare concern, often leading to attrition, chronic activation of the innate immune system, and significant impacts on the experimental results derived from the use of these animals as models. Bite wounding within the home-cage of spiny mice (Acomys cahirinus)-a valuable research model for wound healing and menstruation-is poorly characterized. While we have anecdotally observed frequent bite wounding in Acomys, the frequency of aggression within the home-cage, the severity of the bite wounds, and the types of dominance structures remain unstudied. Here, we report that 46% of Acomys cages in our colony had at least one bite wound over the course of a year and that same-sex pairs fought in the home-cage 10% of the time during their dark/active phase. Both sexes inflicted wounds and frequently engaged in agonistic behaviors, even with stable dominance structures. We found that females inflicted less severe bite wounds in same-sex housing. Also, aged females in same-sex pairs were never observed fighting, and no bite wounds were observed in aged Acomys. These results suggest that we should consider whether bite wounding negatively impacts our experimental results since physical trauma is known to alter menstrual cycling and healing.

Dopamine D2 receptors in the dorsomedial prefrontal cortex modulate social hierarchy in male mice

Social hierarchy greatly influences behavior and health. Both human and animal studies have signaled the medial prefrontal cortex (mPFC) as specifically related to social hierarchy. Dopamine D1 receptors (D1Rs) and D2 receptors (D2Rs) are abundantly expressed in the mPFC, modulating its functions. However, it is unclear how DR-expressing neurons in the mPFC regulate social hierarchy. Here, using a confrontation tube test, we found that most adult C57BL/6J male mice could establish a linear social rank after 1 week of cohabitation. Lower rank individuals showed social anxiety together with decreased serum testosterone levels. D2R expression was significantly downregulated in the dorsal part of mPFC (dmPFC) in lower rank individuals, whereas D1R expression showed no significant difference among the rank groups in the whole mPFC. Virus knockdown of D2Rs in the dmPFC led to mice being particularly prone to lose the contests in the confrontation tube test. Finally, simultaneous D2R activation in the subordinates and D2R inhibition in the dominants in a pair switched their dominant-subordinate relationship. The above results indicate that D2Rs in the dmPFC play an important role in social dominance. Our findings provide novel insights into the divergent functions of prefrontal D1Rs and D2Rs in social dominance, which may contribute to ameliorating social dysfunctions along with abnormal social hierarchy.

Early life adversity shapes social subordination and cell type-specific transcriptomic patterning in the ventral hippocampus

Adverse events in early life can modulate the response to additional stressors later in life and increase the risk of developing psychiatric disorders. The underlying molecular mechanisms responsible for these effects remain unclear. Here, we uncover that early life adversity (ELA) in mice leads to social subordination. Using single-cell RNA sequencing (scRNA-seq), we identified cell type-specific changes in the transcriptional state of glutamatergic and GABAergic neurons in the ventral hippocampus of ELA mice after exposure to acute social stress in adulthood. These findings were reflected by an alteration in excitatory and inhibitory synaptic transmission induced by ELA in response to acute social stress. Finally, enhancing the inhibitory network function through transient diazepam treatment during an early developmental sensitive period reversed the ELA-induced social subordination. Collectively, this study significantly advances our understanding of the molecular, physiological, and behavioral alterations induced by ELA, uncovering a previously unknown cell type-specific vulnerability to ELA.

Topical delivery of mitogen-activated protein kinase inhibitor binimetinib prevents the development of cutaneous neurofibromas in neurofibromatosis type 1 mutant mice

Cutaneous neurofibromas (cNFs) are a hallmark of patients with the neurofibromatosis type 1 (NF1) genetic disorder. These benign nerve sheath tumors, which can amount to thousands, develop from puberty onward, often cause pain and are considered by patients to be the primary burden of the disease. Mutations of NF1, encoding a negative regulator of the RAS signaling pathway, in the Schwann cell (SCs) lineage are considered to be at the origin of cNFs. The mechanisms governing cNFs development are poorly understood, and therapeutics to reduce cNFs are missing, mainly due to the lack of appropriate animal models. To address this, we designed the Nf1-KO mouse model that develops cNFs. Using this model, we found that cNFs development is a singular event and goes through 3 successive stages: initiation, progression, and stabilization characterized by changes in the proliferative and MAPK activities of tumor SCs. We found that skin trauma accelerated the development of cNFs and further used this model to explore the efficacy of the MEK inhibitor binimetinib to cure these tumors. We showed that while topically delivered binimetinib has a selective and minor effect on mature cNFs, the same drug prevents their development over long periods.

5-HT1A receptor in the central amygdala and 5-HT2A receptor in the basolateral amygdala are involved in social hierarchy in male mice

Most social animals self-organize into dominance hierarchies that strongly influence their behavior and health. The serotonin (5-HT) system is believed to play an important role in the formation of social hierarchy. 5-HT receptors are abundantly expressed in the amygdala, which is considered as the central node for the perception and learning of social hierarchy. In this study, we assessed the functions of various 5-HT receptor subtypes related to social rank determination in different subregions of the amygdala using the confrontation tube test in mice. We revealed that most adult C57BL/6 J male mice exhibited a linear social rank after a few days of cohousing. The tube test ranks were slightly related to anxiety-like behavioral performance. After the tube test, the amygdala and 5-HT neurons in the dorsal raphe nucleus were activated in lower-rank individuals. Quantitative real-time polymerase chain reaction analysis revealed that despite the high expression of 5-HT1A receptor mRNA in the central amygdala (CeA), 5-HT2A receptor mRNA expression was downregulated in the basolateral amygdala (BLA) in higher-rank individuals. The dominant-subordinate relationship between mouse pairs could be switched via pharmacological modulation of these receptors in CeA and BLA, suggesting that these expression changes are essential for establishing social ranks. Our findings provide novel insights into the divergent functions of 5-HT receptors in the amygdala related to social hierarchy, which is closely related to our health and welfare.

Genetically Identical Mice Express Alternative Reproductive Tactics Depending on Social Conditions in the Field

In many species, establishing and maintaining a territory is critical to survival and reproduction, and an animal's ability to do so is strongly influenced by the presence and density of competitors. Here we manipulate social conditions to study the alternative reproductive tactics displayed by genetically identical, age-matched laboratory mice competing for territories under ecologically realistic social environmental conditions. We introduced adult males and females of the laboratory mouse strain (C57BL/6J) into a large, outdoor field enclosure containing defendable resource zones under one of two social conditions. We first created a low-density social environment, such that the number of available territories exceeded the number of males. After males established stable territories, we introduced a pulse of intruder males and observed the resulting defensive and invasive tactics employed. In response to this change in social environment, males with large territories invested more in patrolling but were less effective at excluding intruder males as compared to males with small territories. Intruding males failed to establish territories and displayed an alternative tactic featuring greater exploration as compared to genetically identical territorial males. Alternative tactics did not lead to equal reproductive success-males that acquired territories experienced greater survival and had greater access to females.

Wound inflammation post-orchiectomy affects the social dynamic of Nelore bulls

BACKGROUND

Confinement of cattle imposes spatial restrictions and predisposes to aversive social encounters that can lead to contusions, wounds, pain, stress, fright, and reduced productivity. Although endogenous testosterone concentrations are linked to agonistic dominance behaviors in males, it is unknown whether decreased blood testosterone concentrations after castration alter social hierarchy rank in Nelore bulls. Therefore, in this study, we investigated the impact of the surgical would inflammation post-orchiectomy on social dynamics in a group of Nelore bulls (Bos indicus). Fourteen Nelore (Bos indicus) bulls were castrated and assessed pre- and post-surgically. Parameters evaluated were agonistic (mounting, headbutting, and fighting) and affiliative (head-play) behavior, plasma testosterone concentrations, average daily weight gain (ADG), and a score for severity of post-surgical infection. Exploratory statistics included social network analysis (SNA), hierarchy rank delta (Δ), and principal component analysis (PCA). Furthermore, statistical inferences included the Wilcoxon test, multiple logistic regression models, and Spearman's correlation.

RESULTS

The social dynamic of Nelore bulls was modified after castration based on the findings of the SNA and the PCA. The moderate correlation between the postoperative inflammation level with the Δ, and the significant effect of this level in the logistic model post-castration were partially attributed to effects of pain on social relations.

CONCLUSIONS

Our findings suggest the severity of post-surgical inflammation, which has an association with pain intensity, was closely associated with changes in the social hierarchy.

The role of the prefrontal cortex in social interactions of animal models and the implications for autism spectrum disorder

Social interaction is a complex behavior which requires the individual to integrate various internal processes, such as social motivation, social recognition, salience, reward, and emotional state, as well as external cues informing the individual of others' behavior, emotional state and social rank. This complex phenotype is susceptible to disruption in humans affected by neurodevelopmental and psychiatric disorders, including autism spectrum disorder (ASD). Multiple pieces of convergent evidence collected from studies of humans and rodents suggest that the prefrontal cortex (PFC) plays a pivotal role in social interactions, serving as a hub for motivation, affiliation, empathy, and social hierarchy. Indeed, disruption of the PFC circuitry results in social behavior deficits symptomatic of ASD. Here, we review this evidence and describe various ethologically relevant social behavior tasks which could be employed with rodent models to study the role of the PFC in social interactions. We also discuss the evidence linking the PFC to pathologies associated with ASD. Finally, we address specific questions regarding mechanisms employed by the PFC circuitry that may result in atypical social interactions in rodent models, which future studies should address.

Impact of social dominance hierarchy on PACAP expression in the extended amygdala, corticosterone, and behavior in C57BL/6 male mice

The natural alignment of animals into social dominance hierarchies produces adaptive, and potentially maladaptive, changes in the brain that influence health and behavior. Aggressive and submissive behaviors assumed by animals through dominance interactions engage stress-dependent neural and hormonal systems that have been shown to correspond with social rank. Here, we examined the impact of social dominance hierarchies established within cages of group-housed laboratory mice on expression of the stress peptide pituitary adenylate cyclase-activating polypeptide (PACAP) in areas of the extended amygdala comprising the bed nucleus of the stria terminalis (BNST) and central nucleus of the amygdala (CeA). We also quantified the impact of dominance rank on corticosterone (CORT), body weight, and behavior including rotorod and acoustic startle response. Weight-matched male C57BL/6 mice, group-housed (4/cage) starting at 3 weeks of age, were ranked as either most-dominant (Dominant), least-dominant (Submissive) or in-between rank (Intermediate) based on counts of aggressive and submissive encounters assessed at 12 weeks-old following a change in homecage conditions. We found that PACAP expression was significantly higher in the BNST, but not the CeA, of Submissive mice compared to the other two groups. CORT levels were lowest in Submissive mice and appeared to reflect a blunted response following social dominance interactions. Body weight, motor coordination, and acoustic startle were not significantly different between the groups. Together, these data reveal changes in specific neural/neuroendocrine systems that are predominant in animals of lowest social dominance rank, and implicate PACAP in brain adaptations that occur through the development of social dominance hierarchies.

Female dominance hierarchies influence responses to psychosocial stressors

Social species form dominance hierarchies to ensure survival and promote reproductive success. Traditionally studied in males, rodent hierarchies are considered despotic, and dominant social rank results from a history of winning agonistic encounters. By contrast, female hierarchies are thought to be less despotic, and rank is conferred by intrinsic traits. Both social buffering and elevated social status confer resilience to depression, anxiety, and other consequences of chronic stress. Here, we investigate whether female social hierarchies and individual traits related to social rank likewise influence stress resilience. We observe the formation of dyadic female hierarchies under varying conditions of ambient light and circadian phase and subject mice to two forms of chronic psychosocial stress: social isolation or social instability. We find that stable female hierarchies emerge rapidly in dyads. Individual behavioral and endocrinological traits are characteristic of rank, some of which are circadian phase dependent. Further, female social rank is predicted by behavior and stress status prior to social introduction. Other behavioral characteristics suggest that rank is motivation-based, indicating that female rank identity serves an evolutionarily relevant purpose. Rank is associated with alterations in behavior in response to social instability stress and prolonged social isolation, but the different forms of stress produce disparate rank responses in endocrine status. Histological examination of c-Fos protein expression identified brain regions that respond to social novelty or social reunion following chronic isolation in a rank-specific manner. Collectively, female rank is linked to neurobiology, and hierarchies exert context-specific influence upon stress outcomes.

The role of androgens and estrogens in social interactions and social cognition

Gonadal hormones are becoming increasingly recognized for their effects on cognition. Estrogens, in particular, have received attention for their effects on learning and memory that rely upon the functioning of various brain regions. However, the impacts of androgens on cognition are relatively under investigated. Testosterone, as well as estrogens, have been shown to play a role in the modulation of different aspects of social cognition. This review explores the impact of testosterone and other androgens on various facets of social cognition including social recognition, social learning, social approach/avoidance, and aggression. We highlight the relevance of considering not only the actions of the most commonly studied steroids (i.e., testosterone, 17β-estradiol, and dihydrotestosterone), but also that of their metabolites and precursors, which interact with a plethora of different receptors and signalling molecules, ultimately modulating behaviour. We point out that it is also essential to investigate the effects of androgens, their precursors and metabolites in females, as prior studies have mostly focused on males. Overall, a comprehensive analysis of the impact of steroids such as androgens on behaviour is fundamental for a full understanding of the neural mechanisms underlying social cognition, including that of humans.

Production of multimodal signals to assert social dominance in white-lipped peccary (Tayassu pecari)

In this study we aimed to examine whether the 'redundancy' (a backup function to ensure the signal transmission) or 'multiple messages' (sensory communication system in combination) hypothesis would explain the function of multimodal communication of white-lipped peccaries (Tayassu pecari-WLPs). We also aimed to assess the individual factors (the social rank and sex of the sender) influencing the production of, and responses to unimodal and multimodal signals. We determined the social rank of 21 WLPs living in two captive groups and quantified the production of unimodal and multimodal signals when displaying threatening and submissive behaviors. WLPs most often produce multimodal signals independent of a previous unimodal signal failure, which suggests that they were adding more information, such as the sender's size, rather than merely increasing efficacy by engaging a different receiver's sensory channel. There was no effect of the sender's sex in the production of, and responses to, multimodal signals. However, the higher the sender's social rank, the greater the production of multimodal signals when WLPs were displaying threatening behaviors; whereas the lower the sender's social rank, the greater the production of multimodal signals when displaying submission behaviors. Multimodal signals elicited more non-aggressive responses than did the unimodal signals when displaying a threat. Moreover, the higher the sender's social rank, the greater the occurrence of non-aggressive responses to multimodal signals when displaying a threat; whereas the opposite occurred when displaying submission. Our findings support the 'multiple messages' hypothesis to explain the function of multimodal signaling during agonistic interactions in WLPs. Additionally, both the production of, and responses to, multimodal signals are related to the sender's social rank. These results allow us to suggest that the production of multimodal signals may have a key role in mitigating conflict and thus promoting group cohesion among white-lipped peccaries.

Winter madness: Melatonin as a neuroendocrine regulator of seasonal aggression

Individuals of virtually all vertebrate species are exposed to annual fluctuations in the deterioration and renewal of their environments. As such, organisms have evolved to restrict energetically expensive processes and activities to a specific time of the year. Thus, the precise timing of physiology and behavior is critical for individual reproductive success and subsequent fitness. Although the majority of research on seasonality has focused on seasonal reproduction, pronounced fluctuations in other non-reproductive social behaviors, including agonistic behaviors (e.g., aggression), also occur. To date, most studies that have investigated the neuroendocrine mechanisms underlying seasonal aggression have focused on the role of photoperiod (i.e., day length); prior findings have demonstrated that some seasonally breeding species housed in short "winter-like" photoperiods display increased aggression compared with those housed in long "summer-like" photoperiods, despite inhibited reproduction and low gonadal steroid levels. While fewer studies have examined how the hormonal correlates of environmental cues regulate seasonal aggression, our previous work suggests that the pineal hormone melatonin acts to increase non-breeding aggression in Siberian hamsters (Phodopus sungorus) by altering steroid hormone secretion. This review addresses the physiological and cellular mechanisms underlying seasonal plasticity in aggressive and non-aggressive social behaviors, including a key role for melatonin in facilitating a "neuroendocrine switch" to alternative physiological mechanisms of aggression across the annual cycle. Collectively, these studies highlight novel and important mechanisms by which melatonin regulates aggressive behavior in vertebrates and provide a more comprehensive understanding of the neuroendocrine bases of seasonal social behaviors broadly.

Flexibility of neural circuits regulating mating behaviors in mice and flies

Mating is essential for the reproduction of animal species. As mating behaviors are high-risk and energy-consuming processes, it is critical for animals to make adaptive mating decisions. This includes not only finding a suitable mate, but also adapting mating behaviors to the animal's needs and environmental conditions. Internal needs include physical states (e.g., hunger) and emotional states (e.g., fear), while external conditions include both social cues (e.g., the existence of predators or rivals) and non-social factors (e.g., food availability). With recent advances in behavioral neuroscience, we are now beginning to understand the neural basis of mating behaviors, particularly in genetic model organisms such as mice and flies. However, how internal and external factors are integrated by the nervous system to enable adaptive mating-related decision-making in a state- and context-dependent manner is less well understood. In this article, we review recent knowledge regarding the neural basis of flexible mating behaviors from studies of flies and mice. By contrasting the knowledge derived from these two evolutionarily distant model organisms, we discuss potential conserved and divergent neural mechanisms involved in the control of flexible mating behaviors in invertebrate and vertebrate brains.

Vasopressin mediates nonapeptide and glucocorticoid signaling and social dynamics in juvenile dominance hierarchies of a highly social cichlid fish

Early-life social experience can strongly affect adult behavior, yet the behavioral mechanisms underlying developmental trajectories are poorly understood. Here, we use the highly social cichlid, Burton's Mouthbrooder (Astatotilapia burtoni) to investigate juvenile social status and behavior, as well as the underlying neuroendocrine mechanisms. We placed juveniles in pairs or triads and found that they readily establish social status hierarchies, with some group structural variation depending on group size, as well as the relative body size of the group members. Next, we used intracerebroventricular injections to test the hypothesis that arginine vasopressin (AVP) regulates juvenile social behavior and status, similar to adult A. burtoni. While we found no direct behavioral effects of experimentally increasing (via vasotocin) or decreasing (via antagonist Manning Compound) AVP signaling, social interactions directed at the treated individual were significantly altered. This group-level effect of central AVP manipulation was also reflected in a significant shift in whole brain expression of genes involved in nonapeptide signaling (AVP, oxytocin, and oxytocin receptor) and the neuroendocrine stress axis (corticotropin-releasing factor (CRF), glucocorticoid receptors (GR) 1a and 1b). Further, social status was associated with the expression of genes involved in glucocorticoid signaling (GR1a, GR1b, GR2, mineralocorticoid receptor), social interactions with the dominant fish, and nonapeptide signaling activity (AVP, AVP receptor V1aR2, OTR). Together, our results considerably expand our understanding of the context-specific emergence of social dominance hierarchies in juveniles and demonstrate a role for nonapeptide and stress axis signaling in the regulation of social status and social group dynamics.

Distinct immune and transcriptomic profiles in dominant versus subordinate males in mouse social hierarchies

Social status is a critical factor determining health outcomes in human and nonhuman social species. In social hierarchies with reproductive skew, individuals compete to monopolize resources and increase mating opportunities. This can come at a significant energetic cost leading to trade-offs between different physiological systems. In particular, changes in energetic investment in the immune system can have significant short and long-term effects on fitness and health. We have previously found that dominant alpha male mice living in social hierarchies have increased metabolic demands related to territorial defense. In this study, we tested the hypothesis that high-ranking male mice favor adaptive immunity, while subordinate mice show higher investment in innate immunity. We housed 12 groups of 10 outbred CD-1 male mice in a social housing system. All formed linear social hierarchies and subordinate mice had higher concentrations of plasma corticosterone (CORT) than alpha males. This difference was heightened in highly despotic hierarchies. Using flow cytometry, we found that dominant status was associated with a significant shift in immunophenotypes towards favoring adaptive versus innate immunity. Using Tag-Seq to profile hepatic and splenic transcriptomes of alpha and subordinate males, we identified genes that regulate metabolic and immune defense pathways that are associated with status and/or CORT concentration. In the liver, dominant animals showed a relatively higher expression of specific genes involved in major urinary production and catabolic processes, whereas subordinate animals showed relatively higher expression of genes promoting biosynthetic processes, wound healing, and proinflammatory responses. In spleen, subordinate mice showed relatively higher expression of genes facilitating oxidative phosphorylation and DNA repair and CORT was negatively associated with genes involved in lymphocyte proliferation and activation. Together, our findings suggest that dominant and subordinate animals adaptively shift immune profiles and peripheral gene expression to match their contextual needs.

Social status mediated variation in hypothalamic transcriptional profiles of male mice

Animals of different social status exhibit variation in aggression, territorial and reproductive behavior as well as activity patterns, feeding, drinking and status signaling. This behavioral and physiological plasticity is coordinated by underlying changes in brain gene transcription. Using Tag-based RNA sequencing (Tag-seq), we explore RNA transcriptomes from the medial preoptic area (mPOA) and ventral hypothalamus (vHYP) of male mice of different social ranks in a dominance hierarchy and detect candidate genes and cellular pathways that underlie status-related plasticity. Within the mPOA, oxytocin (Oxt) and vasopressin (Avp) are more highly expressed in subdominant mice compared to other ranks, while nitric oxide synthase (Nos1) has lower expression in subdominant mice. Within the vHYP, we find that both orexigenic and anorexigenic genes involved in feeding behavior, including agouti-related peptide (Agrp), neuropeptide-Y (Npy), galanin (Gal), proopiomelanocortin (Pomc), and Cocaine- and Amphetamine-Regulated Transcript Protein prepropeptide (Cartpt), are less expressed in dominant animals compared to more subordinate ranks. We suggest that this may represent a reshaping of feeding circuits in dominant compared to subdominant and subordinate animals. Furthermore, we determine several genes that are positively and negatively associated with the level of despotism (aggression) in dominant males. Ultimately, we identify hypothalamic genes controlling feeding and social behaviors that are differentially transcribed across animals of varying social status. These changes in brain transcriptomics likely support phenotypic variation that enable animals to adapt to their current social status.

What's wrong with my experiment?: The impact of hidden variables on neuropsychopharmacology research

The field of neuropsychopharmacology relies on behavioral assays to quantify behavioral processes related to mental illness and substance use disorders. Although these assays have been highly informative, sometimes laboratories have unpublished datasets from experiments that "didn't work". Often this is because expected outcomes were not observed in positive or negative control groups. While this can be due to experimenter error, an important alternative is that under-appreciated environmental factors can have a major impact on results. "Hidden variables" such as circadian cycles, husbandry, and social environments are often omitted in methods sections, even though there is a strong body of literature documenting their impact on physiological and behavioral outcomes. Applying this knowledge in a more critical manner could provide behavioral neuroscientists with tools to develop better testing methods, improve the external validity of behavioral techniques, and make better comparisons of experimental data across institutions. Here we review the potential impact of "hidden variables" that are commonly overlooked such as light-dark cycles, transport stress, cage ventilation, and social housing structure. While some of these conditions may not be under direct control of investigators, it does not diminish the potential impact of these variables on experimental results. We provide recommendations to investigators on which variables to report in publications and how to address "hidden variables" that impact their experimental results.

Rapid-Onset Antidepressant-Like Effect of Nelumbinis semen in Social Hierarchy Stress Model of Depression

Depression is a disease with increasing prevalence worldwide, and it is necessary to develop a therapeutic agent with better efficacy than existing antidepressant drugs. Antidepressants that act on the glutamatergic nervous system, such as ketamine, have a rapid-onset antidepressant effect and are effective against treatment-resistant depression. However, because of the addictive potential of ketamine, alternative substances without psychological side effects are recommended. In particular, many natural compounds have been tested for their antidepressant effects. The antidepressant effects of Nelumbinis semen (NS) have been tested in many studies, along with the various actions of NS on the glutamatergic system. Thus, it was expected that NS might have a rapid-onset antidepressant effect. To test the antidepressant potential, despair and anhedonic behaviors were measured after administering NS to mice exposed to social hierarchy stress (SHS), and biochemical changes in the prefrontal cortex and hippocampus were analyzed. NS reduced despair-like responses in the forced swim test and tail suspension test. Mice exposed to SHS showed depression-like responses such as increased despair, reduced hedonia, and an anxiety-like response in the novelty suppressed feeding test. NS, but not fluoxetine, improved those depression-like behaviors after acute treatment, and NBQX, an AMPA receptor blocker, inhibited the antidepressant-like effects of NS. The antidepressant-like effect of NS was related to enhanced phosphorylation of mTOR in the prefrontal cortex and dephosphorylation of GluR1 S845 in the hippocampus. Since NS has shown antidepressant-like potential in a preclinical model, it may be considered as a candidate for the development of antidepressants in the future.

Social deficits in BTBR T+ Itpr3tf/J mice vary with ecological validity of the test

Translational value of mouse models of neuropsychiatric disorders depends heavily on the accuracy with which they replicate symptoms observed in the human population. In mouse models of autism spectrum disorder (ASD) these include, among others, social affiliation, and communication deficits as well as impairments in understanding and perception of others. Most studies addressing these issues in the BTBR T+ Itpr3tf/J mouse, an idiopathic model of ASD, were based on short dyadic interactions of often non-familiar partners placed in a novel environment. In such stressful and variable conditions, the reproducibility of the phenotype was low. Here, we compared physical conditions and the degree of habituation of mice at the time of testing in the three chambered social affiliation task, as well as parameters used to measure social deficits and found that both the level of stress and human bias profoundly affect the results of the test. To minimize these effects, we tested social preference and network dynamics in mice group-housed in the Eco-HAB system. This automated recording allowed for long-lasting monitoring of differences in social repertoire (including interest in social stimuli) in BTBR T+ Itpr3tf/J and normosocial c57BL/6J mice. With these observations we further validate the BTBR T+ Itpr3tf/J mouse as a model for ASD, but at the same time emphasize the need for more ecological testing of social behavior within all constructs of the Systems for Social Processes domain (as defined by the Research Domain Criteria framework).

Female-biased effects of aging on a chimeric hemagglutinin stalk-based universal influenza virus vaccine in mice

To determine if biological sex and age intersect to affect universal influenza vaccine-induced immunity, adult and aged male and female C57BL/6 mice were sequentially immunized with a chimeric-hemagglutinin (cHA) stalk-based H1 vaccine. Adult mice developed greater quantity and quality of H1-stalk antibodies, that were more cross-reactive with other group 1, but not group 2, influenza viruses, than aged mice. The vaccine did not induce neutralizing or hemagglutination inhibition antibodies, but rather antibody-dependent cellular cytotoxicity, which was greater in adult than aged mice. Vaccinated adult mice were better protected than aged mice after challenge with 2009 H1N1 virus, experiencing less morbidity and having lower pulmonary virus titers. The age-associated decline in immunity and protection was consistently greater among females than males, with the reduction in immunity and protection for aged as compared with adult females often being the sole comparison driving the overall age-associated significant differences. The age-associated reduction in stalk-based immunity in females was not, however, associated with changes in estradiol. To determine if the better antibodies in adults could be utilized to protect aged mice, serum was passively transferred from vaccinated adult mice into naïve sex-matched aged mice. Even with transferred serum from young adult mice, aged females still suffered greater morbidity than aged males. These data suggest there are sex-dependent effects of aging on cHA-based universal influenza virus vaccine-induced immunity that cannot be reversed through transfer of serum from young animals. The lack of consideration of sex-specific effects of aging on immunity could hinder efforts toward universal vaccines.

Manifestations of domination: Assessments of social dominance in rodents

Social hierarchies are ubiquitous features of virtually all animal groups. The varying social ranks of members within these groups have profound effects on both physical and emotional health, with lower-ranked individuals typically being the most adversely affected by their respective ranks. Thus, reliable measures of social dominance in preclinical rodent models are necessary to better understand the effects of an individual's social rank on other behaviors and physiological processes. In this review, we outline the primary methodologies used to assess social dominance in various rodent species: those that are based on analyses of agonistic behaviors, and those that are based on resource competition. In synthesizing this review, we conclude that assays based on resource competition may be better suited to characterize social dominance in a wider variety of rodent species and strains, and in both males and females. Lastly, albeit expectedly, we demonstrate that similarly to many other areas of preclinical research, studies incorporating female subjects are lacking in comparison to those using males. These findings emphasize the need for an increased number of studies assessing social dominance in females to form a more comprehensive understanding of this behavioral phenomenon.

Hot-headed peckers: thermographic changes during aggression among juvenile pheasants (Phasianus colchicus)

In group-living vertebrates, dominance status often covaries with physiological measurements (e.g. glucocorticoid levels), but it is unclear how dominance is linked to dynamic changes in physiological state over a shorter, behavioural timescale. In this observational study, we recorded spontaneous aggression among captive juvenile pheasants (Phasianus colchicus) alongside infrared thermographic measurements of their external temperature, a non-invasive technique previously used to examine stress responses in non-social contexts, where peripheral blood is redirected towards the body core. We found low but highly significant repeatability in maximum head temperature, suggesting individually consistent thermal profiles, and some indication of lower head temperatures in more active behavioural states (e.g. walking compared to resting). These individual differences were partly associated with sex, females being cooler on average than males, but unrelated to body size. During pairwise aggressive encounters, we observed a non-monotonic temperature change, with head temperature dropping rapidly immediately prior to an attack and increasing rapidly afterwards, before returning to baseline levels. This nonlinear pattern was similar for birds in aggressor and recipient roles, but aggressors were slightly hotter on average. Our findings show that aggressive interactions induce rapid temperature changes in dominants and subordinates alike, and highlight infrared thermography as a promising tool for investigating the physiological basis of pecking orders in galliforms. This article is part of the theme issue 'The centennial of the pecking order: current state and future prospects for the study of dominance hierarchies'.

DomArchive: a century of published dominance data

Dominance behaviours have been collected for many groups of animals since 1922 and serve as a foundation for research on social behaviour and social structure. Despite a wealth of data from the last century of research on dominance hierarchies, these data are only rarely used for comparative insight. Here, we aim to facilitate comparative studies of the structure and function of dominance hierarchies by compiling published dominance interaction datasets from the last 100 years of work. This compiled archive includes 436 datasets from 190 studies of 367 unique groups (mean group size 13.8, s.d. = 13.4) of 135 different species, totalling over 243 000 interactions. These data are presented in an R package alongside relevant metadata and a tool for subsetting the archive based on biological or methodological criteria. In this paper, we explain how to use the archive, discuss potential limitations of the data, and reflect on best practices in publishing dominance data based on our experience in assembling this dataset. This archive will serve as an important resource for future comparative studies and will promote the development of general unifying theories of dominance in behavioural ecology that can be grounded in testing with empirical data. This article is part of the theme issue 'The centennial of the pecking order: current state and future prospects for the study of dominance hierarchies'.

Behavioural and physiological plasticity in social hierarchies

Individuals occupying dominant and subordinate positions in social hierarchies exhibit divergent behaviours, physiology and neural functioning. Dominant animals express higher levels of dominance behaviours such as aggression, territorial defence and mate-guarding. Dominants also signal their status via auditory, visual or chemical cues. Moreover, dominant animals typically increase reproductive behaviours and show enhanced spatial and social cognition as well as elevated arousal. These biobehavioural changes increase energetic demands that are met via shifting both energy intake and metabolism and are supported by coordinated changes in physiological systems including the hypothalamic-pituitary-adrenal and hypothalamic-pituitary-gonadal axes as well as altered gene expression and sensitivity of neural circuits that regulate these behaviours. Conversely, subordinate animals inhibit dominance and often reproductive behaviours and exhibit physiological changes adapted to socially stressful contexts. Phenotypic changes in both dominant and subordinate individuals may be beneficial in the short-term but lead to long-term challenges to health. Further, rapid changes in social ranks occur as dominant animals socially ascend or descend and are associated with dynamic modulations in the brain and periphery. In this paper, we provide a broad overview of how behavioural and phenotypic changes associated with social dominance and subordination are expressed in neural and physiological plasticity. This article is part of the theme issue 'The centennial of the pecking order: current state and future prospects for the study of dominance hierarchies'.

Steroid hormones in hair and fresh wounds reveal sex specific costs of reproductive engagement and reproductive success in wild house mice (Mus musculus domesticus)

Not only males but also females compete over reproduction. In a population of free-living house mice (Mus musculus domesticus), we analyzed how (metabolic) costs of aggressive interactions (reflected in fresh wounds and long-term corticosterone concentrations in hair) are predicted by individual reproductive physiology and reproductive success in males and females. Over eight years, we studied wounds and reproduction of more than 2800 adults under naturally varying environmental conditions and analyzed steroid hormones from more than 1000 hair samples. Hair corticosterone were higher and wounds more frequent in males than females. In males, wound occurrence increased with increasing breeding activity in the population, without affecting hair corticosterone levels. Unexpectedly, individual male reproductive success did not predict wounds, while hair corticosterone increased with increasing levels of hair testosterone and reproductive success. High corticosterone in hair of males might therefore reflect metabolic costs of fighting over reproduction. In females, hair corticosterone was generally lower than in males and high levels did not impede pregnancy. Reproductive investment (reflected in hair progesterone) was dissociated from reproductive success. Occasional wounds in females indicated individuals without recent reproductive success and revealed reproductive competition, presumably driven by instability in the social environment. In both sexes, corticosterone increased with age, but there was no evidence that received overt aggression, as indicated by wounds or elevated corticosterone, suppressed reproductive physiology. Our results diverge from laboratory findings and emphasize the need to also study animals in their natural environment in order to understand the complexity of their behavioral physiology.

Exploring Refinement Strategies for Single Housing of Male C57BL/6JRj Mice: Effect of Cage Divider on Stress-Related Behavior and Hypothalamic-Pituitary-Adrenal-Axis Activity

Introduction: Single housing of laboratory mice is a common practice to meet experimental needs, or to avoid intermale aggression. However, single housing is considered to negatively affect animal welfare and may compromise the scientific validity of experiments. The aim of this study was to investigate whether the use of a cage with a cage divider, which avoids physical contact between mice while maintaining sensory contact, may be a potential refinement strategy for experiments in which group housing of mice is not possible.

Methods: Eight-week-old male C57BL/6JRj mice were single housed, pair housed or pair housed with a cage divider for four (experiment 1) or ten (experiment 2) weeks, after which we performed an open field test, Y-maze spontaneous alternation test, elevated plus maze test, an auditory fear conditioning task, and assessed responsiveness of the hypothalamic-pituitary-adrenal (HPA) axis.

Results: Housing conditions did not affect body weight, exploratory activity, anxiety, working memory, fear memory processing or markers for HPA-axis functioning in either experiment 1 or experiment 2. There was an increased distance traveled in mice housed with a cage divider compared to pair housed mice after 4 weeks, and after 10 weeks mice housed with a cage divider made significantly more arm entries in the Y-maze spontaneous alternation test.

Conclusion: Taken together, our study did not provide evidence for robust differences in exploratory activity, anxiety, working memory and fear memory processing in male C57BL/6JRj mice that were single housed, pair housed or pair housed with a cage divider.

Individual history of winning and hierarchy landscape influence stress susceptibility in mice

Social hierarchy formation is strongly evolutionarily conserved. Across species, rank within social hierarchy has large effects on health and behavior. To investigate the relationship between social rank and stress susceptibility, we exposed ranked male and female mice to social and non-social stressors and manipulated social hierarchy position. We found that rank predicts same sex social stress outcomes: dominance in males and females confers resilience while subordination confers susceptibility. Pre-existing rank does not predict non-social stress outcomes in females and weakly does so in males, but rank emerging under stress conditions reveals social interaction deficits in male and female subordinates. Both history of winning and rank of cage mates affect stress susceptibility in males: rising to the top rank through high mobility confers resilience and mice that lose dominance lose stress resilience, although gaining dominance over a subordinate animal does not confer resilience. Overall, we have demonstrated a relationship between social status and stress susceptibility, particularly when taking into account individual history of winning and the overall hierarchy landscape in male and female mice.

Comparing axon regeneration in male and female mice after peripheral nerve injury

Axons in the adult mammalian central nervous system fail to regenerate after injury. By contrast, spontaneous axon regeneration occurs in the peripheral nervous system (PNS) due to a supportive PNS environment and an increase in the intrinsic growth potential induced by injury via cooperative activation of multifaceted biological pathways. This study compared axon regeneration and injury responses in C57BL/6 male and female mice after sciatic nerve crush (SNC) injury. The extent of axon regeneration in vivo was indistinguishable in male and female mice when observed at 3 days after SNC injury, and primary dorsal root ganglion (DRG) neurons from injured, male and female mice extended axons to a similar length. Moreover, the induction of selected regeneration-associated genes (RAGs), such as Atf3, Sprr1a, Gap43, Sox11, Jun, Gadd45a, and Smad1 were comparable in male and female DRGs when assessed by quantitative real-time reverse transcription polymerase chain reaction. Furthermore, the RNA-seq analysis of male and female DRGs revealed that differentially expressed genes (DEGs) in SNC groups compared to sham-operated groups included many common genes associated with neurite outgrowth. However, we also found that a large number of genes in the DEGs were sex dependent, implicating the involvement of distinct gene regulatory network in the two sexes following peripheral nerve injury. In conclusion, we found that male and female mice mounted a comparable axon regeneration response and many RAGs were commonly induced in response to SNC. However, given that many DEGs were sex-dependently expressed, future studies are needed to investigate whether they contribute to peripheral axon regeneration, and if so, to what extent.

N6 -Methyladenosine Modification in Chronic Stress Response Due to Social Hierarchy Positioning of Mice

Appropriately responding to stressful events is essential for maintaining health and well-being of any organism. Concerning social stress, the response is not always as straightforward as reacting to physical stressors, e.g., extreme heat, and thus has to be balanced subtly. Particularly, regulatory mechanisms contributing to gaining resilience in the face of mild social stress are not fully deciphered yet. We employed an intrinsic social hierarchy stress paradigm in mice of both sexes to identify critical factors for potential coping strategies. While global transcriptomic changes could not be observed in male mice, several genes previously reported to be involved in synaptic plasticity, learning, and anxiety-like behavior were differentially regulated in female mice. Moreover, changes in N⁶-methyladenosine (m⁶A)-modification of mRNA occurred associated with corticosterone level in both sexes with, e.g., increased global amount in submissive female mice. In accordance with this, METTL14 and WTAP, subunits of the methyltransferase complex, showed elevated levels in submissive female mice. N⁶-adenosyl-methylation is the most prominent type of mRNA methylation and plays a crucial role in processes such as metabolism, but also response to physical stress. Our findings underpin its essential role by also providing a link to social stress evoked by hierarchy building within same-sex groups. As recently, search for small molecule modifiers for the respective class of RNA modifying enzymes has started, this might even lead to new therapeutic approaches against stress disorders.

Dynamic neurogenomic responses to social interactions and dominance outcomes in female paper wasps

Social interactions have large effects on individual physiology and fitness. In the immediate sense, social stimuli are often highly salient and engaging. Over longer time scales, competitive interactions often lead to distinct social ranks and differences in physiology and behavior. Understanding how initial responses lead to longer-term effects of social interactions requires examining the changes in responses over time. Here we examined the effects of social interactions on transcriptomic signatures at two times, at the end of a 45-minute interaction and 4 hours later, in female Polistes fuscatus paper wasp foundresses. Female P. fuscatus have variable facial patterns that are used for visual individual recognition, so we separately examined the transcriptional dynamics in the optic lobe and the non-visual brain. Results demonstrate much stronger transcriptional responses to social interactions in the non-visual brain compared to the optic lobe. Differentially regulated genes in response to social interactions are enriched for memory-related transcripts. Comparisons between winners and losers of the encounters revealed similar overall transcriptional profiles at the end of an interaction, which significantly diverged over the course of 4 hours, with losers showing changes in expression levels of genes associated with aggression and reproduction in paper wasps. On nests, subordinate foundresses are less aggressive, do more foraging and lay fewer eggs compared to dominant foundresses and we find losers shift expression of many genes in the non-visual brain, including vitellogenin, related to aggression, worker behavior, and reproduction within hours of losing an encounter. These results highlight the early neurogenomic changes that likely contribute to behavioral and physiological effects of social status changes in a social insect.

Gonadal steroid hormone receptors in the medial amygdala contribute to experience-dependent changes in stress vulnerability

Social experience can generate neural plasticity that changes how individuals respond to stress. Winning aggressive encounters alters how animals respond to future challenges and leads to increased plasma testosterone concentrations and androgen receptor (AR) expression in the social behavior neural network. In this project, our aim was to identify neuroendocrine mechanisms that account for changes in stress-related behavior following the establishment of dominance relationships over a two-week period. We used a Syrian hamster model in which acute social defeat stress increases anxiety-like responses in a conditioned defeat test in males and in a social avoidance test in females. First, we administered flutamide, an AR antagonist, via intraperitoneal injections daily during the establishment of dominance relationships in male hamsters. We found that pharmacological blockade of AR prevented a reduction in conditioned defeat in dominant males and blocked an upregulation of AR in the posterior dorsal medial amygdala (MePD) and posterior ventral medial amygdala (MePV), but not in the ventral lateral septum. Next, we administered flutamide into the posterior aspects of the medial amygdala (MeP) prior to acute social defeat stress or prior to conditioned defeat testing in males. We found that pharmacological blockade of AR in the MeP prior to social defeat, but not prior to testing, increased the conditioned defeat response in dominant males and did not alter behavior in subordinates. Finally, we developed a procedure to establish dominance relationships in female hamsters and investigated status-dependent changes in plasma steroid hormone concentrations, estrogen receptor alpha (ERα) immunoreactivity, and defeat-induced social avoidance. We found that dominant female hamsters showed reduced social avoidance regardless of social defeat exposure as well as increased ERα expression in the MePD, but no status-dependent changes in the concentration of plasma steroid hormones. Overall, these findings suggest that achieving and maintaining stable social dominance leads to sex-specific neural plasticity in the MeP that underlies status-dependent changes in stress vulnerability.

Social isolation uncovers a circuit underlying context-dependent territory-covering micturition

The release of urine, or micturition, serves a fundamental physiological function and, in many species, is critical for social communication. In mice, the pattern of urine release is modulated by external and internal factors and transmitted to the spinal cord via the pontine micturition center (PMC). Here, we exploited a behavioral paradigm in which mice, depending on strain, social experience, and sensory context, either vigorously cover an arena with small urine spots or deposit urine in a few isolated large spots. We refer to these micturition modes as, respectively, high and low territory-covering micturition (TCM) and find that the presence of a urine stimulus robustly induces high TCM in socially isolated mice. Comparison of the brain networks activated by social isolation and by urine stimuli to those upstream of the PMC identified the lateral hypothalamic area as a potential modulator of micturition modes. Indeed, chemogenetic manipulations of the lateral hypothalamus can switch micturition behavior between high and low TCM, overriding the influence of social experience and sensory context. Our results suggest that both inhibitory and excitatory signals arising from a network upstream of the PMC are integrated to determine context- and social-experience-dependent micturition patterns.

Sex differences in fear memory consolidation via Tac2 signaling in mice

Memory formation is key for brain functioning. Uncovering the memory mechanisms is helping us to better understand neural processes in health and disease. Moreover, more specific treatments for fear-related disorders such as posttraumatic stress disorder and phobias may help to decrease their negative impact on mental health. In this line, the Tachykinin 2 (Tac2) pathway in the central amygdala (CeA) has been shown to be sufficient and necessary for the modulation of fear memory consolidation. CeA-Tac2 antagonism and its pharmacogenetic temporal inhibition impair fear memory in male mice. Surprisingly, we demonstrate here the opposite effect of Tac2 blockade on enhancing fear memory consolidation in females. Furthermore, we show that CeA-testosterone in males, CeA-estradiol in females and Akt/GSK3β/β-Catenin signaling both mediate the opposite-sex differential Tac2 pathway regulation of fear memory.

Group housing and social dominance hierarchy affect circadian activity patterns in mice

In this study, we investigated the effect of social environment on circadian patterns in activity by group housing either six male or six female mice together in a cage, under regular light-dark cycles. Based on the interactions among the animals, the social dominance rank of individual mice was quantitatively established by calculating Elo ratings. Our results indicated that, during our experiment, the social dominance hierarchy was rapidly established, stable yet complex, often showing more than one dominant mouse and several subordinate mice. Moreover, we found that especially dominant male mice, but not female mice, displayed a significantly higher fraction of their activity during daytime. This resulted in reduced rhythm amplitude in dominant males. After division into separate cages, male mice showed an enhancement of their 24 h rhythm, due to lower daytime activity. Recordings of several physiological parameters showed no evidence for reduced health as a potential consequence of reduced rhythm amplitude. For female mice, transfer to individual housing did not affect their daily activity pattern. We conclude that 24 h rhythms under light-dark cycles are influenced by the social environment in males but not in females, and lead to a decrement in behavioural rhythm amplitude that is larger in dominant mice.

A Systematic Review and Meta-Analysis of the Relationship Between Social Dominance Status and Common Behavioral Phenotypes in Male Laboratory Mice

Background: Social dominance status (e.g., dominant or subordinate) is often associated with individual differences in behavior and physiology but is largely neglected in experimental designs and statistical analysis plans in biomedical animal research. In fact, the extent to which social dominance status affects common experimental outcomes is virtually unknown. Given the pervasive use of laboratory mice and culminating evidence of issues with reproducibility, understanding the role of social dominance status on common behavioral measures used in research may be of paramount importance.

Methods: To determine whether social dominance status—one facet of the social environment—contributes in a systematic way to standard measures of behavior in biomedical science, we conducted a systematic review of the existing literature searching the databases of PubMed, Embase, and Web of Science. Experiments were divided into several domains of behavior: exploration, anxiety, learned helplessness, cognition, social, and sensory behavior. Meta-analyses between experiments were conducted for the open field, elevated plus-maze, and Porsolt forced swim test.

Results: Of the 696 publications identified, a total of 55 experiments from 20 published studies met our pre-specified criteria. Study characteristics and reported results were highly heterogeneous across studies. A systematic review and meta-analyses, where possible, with these studies revealed little evidence for systematic phenotypic differences between dominant and subordinate male mice.

Conclusion: This finding contradicts the notion that social dominance status impacts behavior in significant ways, although the lack of an observed relationship may be attributable to study heterogeneity concerning strain, group-size, age, housing and husbandry conditions, and dominance assessment method. Therefore, further research considering these secondary sources of variation may be necessary to determine if social dominance generally impacts treatment effects in substantive ways.

Neural and Hormonal Control of Sexual Behavior

Gonadal hormones contribute to the sexual differentiation of brain and behavior throughout the lifespan, from initial neural patterning to "activation" of adult circuits. Sexual behavior is an ideal system in which to investigate the mechanisms underlying hormonal activation of neural circuits. Sexual behavior is a hormonally regulated, innate social behavior found across species. Although both sexes seek out and engage in sexual behavior, the specific actions involved in mating are sexually dimorphic. Thus, the neural circuits mediating sexual motivation and behavior in males and females are overlapping yet distinct. Furthermore, sexual behavior is strongly dependent on circulating gonadal hormones in both sexes. There has been significant recent progress on elucidating how gonadal hormones modulate physiological properties within sexual behavior circuits with consequences for behavior. Therefore, in this mini-review we review the neural circuits of male and female sexual motivation and behavior, from initial sensory detection of pheromones to the extended amygdala and on to medial hypothalamic nuclei and reward systems. We also discuss how gonadal hormones impact the physiology and functioning of each node within these circuits. By better understanding the myriad of ways in which gonadal hormones impact sexual behavior circuits, we can gain a richer and more complete appreciation for the neural substrates of complex behavior.

Stearidonic-Enriched Soybean Oil Modulates Obesity, Glucose Metabolism, and Fatty Acid Profiles Independently of Akkermansia muciniphila

SCOPE

Previous studies have suggested that diets rich in omega-3 and low in omega-6 long-chain polyunsaturated fatty acids (PUFAs) can limit the development of metabolic syndrome (MetS). Transgenic soybeans yielding oils enriched for omega-3 PUFAs represent a new and readily-available option for incorporating omega-3 PUFAs into diets to provide health benefits.

METHODS AND RESULTS

Transgenic soybean oils, enriched for either stearidonic acid (SDA) or eicosapentaenoic acid (EPA), are incorporated into diets to test their effects on limiting the development of MetS in a mouse model of diet-induced obesity. Supplementation with SDA- but not EPA-enriched oils improved features of MetS compared to feeding a control wild-type oil. Because previous studies have linked the gut microorganism Akkermansia muciniphila to the metabolic effects of feeding omega-3 PUFAs, the causal contribution of A. muciniphila to mediating the metabolic benefits provided by SDA-enriched diets is investigated. Although A. muciniphila is not required for SDA-induced metabolic improvements, this microorganism does modulate levels of saturated and mono-unsaturated fatty acids in host adipose tissues.

CONCLUSION

Together, these findings support the utilization of SDA-enriched diets to modulate weight gain, glucose metabolism, and fatty acid profiles of liver and adipose tissue.

Lack of AR in LepRb Cells Disrupts Ambulatory Activity and Neuroendocrine Axes in a Sex-Specific Manner in Mice

Disorders of androgen imbalance, such as hyperandrogenism in females or hypoandrogenism in males, increase risk of visceral adiposity, type 2 diabetes, and infertility. Androgens act upon androgen receptors (AR) which are expressed in many tissues. In the brain, AR are abundant in hypothalamic nuclei involved in regulation of reproduction and energy homeostasis, yet the role of androgens acting via AR in specific neuronal populations has not been fully elucidated. Leptin receptor (LepRb)-expressing neurons coexpress AR predominantly in hypothalamic arcuate and ventral premammillary nuclei (ARH and PMv, respectively), with low colocalization in other LepRb neuronal populations, and very low colocalization in the pituitary gland and gonads. Deletion of AR from LepRb-expressing cells (LepRbΔAR) has no effect on body weight, energy expenditure, and glucose homeostasis in male and female mice. However, LepRbΔAR female mice show increased body length later in life, whereas male LepRbΔAR mice show an increase in spontaneous ambulatory activity. LepRbΔAR mice display typical pubertal timing, estrous cycles, and fertility, but increased testosterone levels in males. Removal of sex steroid negative feedback action induced an exaggerated rise in luteinizing hormone in LepRbΔAR males and follicle-stimulating hormone in LepRbΔAR females. Our findings show that AR can directly affect a subset of ARH and PMv neurons in a sex-specific manner and demonstrate specific androgenic actions in the neuroendocrine hypothalamus.

Social enrichment by separated pair housing of male C57BL/6JRj mice

Laboratory male mice are often housed individually due to aggressive behavior or experimental requirements, though social isolation can cause welfare issues. As a strategy to refine housing of male mice, we introduce the separated pair housing system. A perforated transparent wall divides the cage into two compartments and allows olfactory, acoustic, and visual communication between the two mice but prevents fighting and injuries. Long-term effects of separated pair housing on well-being and distress of adult male C57BL/6JRj mice were investigated and compared with both single- and group-housed mice. Behavioral analysis after eight weeks in three different housing systems revealed no differences in burrowing performance, social interaction, anxiety, and stress hormone concentrations. However, pair-housed mice built more complex nests compared to single-housed mice and the nest position suggested that pair-housed mice preferred the close proximity to their cage mates. Moreover, pair-housed mice showed less locomotor activity compared to group- and single-housed mice. Body weight was higher in group-housed mice. All in all, no unambiguous long-term beneficial effects of pair housing on the well-being were found. However, the findings emphasized that effects of the housing systems on behavioral, physical, and biochemical parameters must be considered in the design of animal experimental studies.

Uptake of high-density lipoprotein by scavenger receptor class B type 1 is associated with prostate cancer proliferation and tumor progression in mice

High-density lipoprotein (HDL) metabolism is facilitated in part by scavenger receptor class B, type 1 (SR-B1) that mediates HDL uptake into cells. Higher levels of HDL have been associated with protection in other diseases, however, its role in prostate cancer is not definitive. SR-B1 is up-regulated in prostate cancer tissue, suggesting a possible role of this receptor in tumor progression. Here, we report that knockout (KO) of SR-B1 in both human and mouse prostate cancer cell lines through CRISPR/Cas9-mediated genome editing reduces HDL uptake into the prostate cancer cells and reduces their proliferation in response to HDL. In vivo studies using syngeneic SR-B1 WT (SR-B1+/+) and SR-B1 KO (SR-B1-/-) prostate cancer cells in WT and apolipoprotein-AI KO (apoA1-KO) C57BL/6J mice revealed that WT hosts, containing higher levels of total and HDL-cholesterol, grew larger tumors than apoA1-KO hosts with lower levels of total and HDL-cholesterol. Furthermore, SR-B1-/- prostate cancer cells formed smaller tumors in WT hosts than SR-B1+/+ cells in the same host model. Increased tumor volume was overall associated with reduced survival. We conclude that knocking out SR-B1 in prostate cancer tumors reduces HDL-associated increases in prostate cancer cell proliferation and disease progression.

Sperm sex ratio adjustment in a mammal: perceived male competition leads to elevated proportions of female-producing sperm

Mammal sex allocation research has focused almost exclusively on maternal traits, but it is now apparent that fathers can also influence offspring sex ratios. Parents that produce female offspring under conditions of intense male-male competition can benefit with greater assurance of maximized grand-parentage. Adaptive adjustment in the sperm sex ratio, for example with an increase in the production of X-chromosome bearing sperm (CBS), is one potential paternal mechanism for achieving female-biased sex ratios. Here, we tested this mechanistic hypothesis by varying the risk of male-male competition that male house mice perceived during development, and quantifying sperm sex ratios at sexual maturity. Our analyses revealed that males exposed to a competitive 'risk' produced lower proportions of Y-CBS compared to males that matured under 'no risk' of competition. We also explored whether testosterone production was linked to sperm sex ratio variation, but found no evidence to support this. We discuss our findings in relation to the adaptive value of sperm sex ratio adjustments and the role of steroid hormones in socially induced sex allocation.

Stress in groups: Lessons from non-traditional rodent species and housing models

A major feature of life in groups is that individuals experience social stressors of varying intensity and type. Social stress can have profound effects on health, social behavior, and ongoing relationships. Relationships can also buffer the experience of exogenous stressors. Social stress has most commonly been investigated in dyadic contexts in mice and rats that produce intense stress. Here we review findings from studies of diverse rodents and non-traditional group housing paradigms, focusing on laboratory studies of mice and rats housed in visible burrow systems, prairie and meadow voles, and mole-rats. We argue that the use of methods informed by the natural ecology of rodent species provides novel insights into the relationship between social stress, behavior and physiology. In particular, we describe how this ethologically inspired approach reveals how individuals vary in their experience of and response to social stress, and how ecological and social contexts impact the effects of stress. Social stress induces adaptive changes, as well as long-term disruptive effects on behavior and physiology.