Aggressive behavioral phenotypes in mice

Operant social self-administration in male CD1 mice

RATIONALE AND OBJECTIVE

We recently introduced a model of operant social reward in which female CD1 mice lever press for access to affiliative social interaction with a cagemate peer mouse of the same sex and strain. Here we determined the generality of the operant social self-administration model to male CD1 mice who, under certain conditions, will lever press to attack a subordinate male mouse.

METHODS

We trained male CD1 mice to lever press for food and social interaction with a same sex and strain cagemate peer under different fixed-ratio (FR) schedule response requirements (FR1 to FR6). We then tested their motivation to seek social interaction after 15 days of isolation in the presence of cues previously paired with social self-administration. We also determined the effect of housing conditions on operant social self-administration and seeking. Finally, we determined sex differences in operant social self-administration and seeking, and the effect of housing conditions on unconditioned affiliative and antagonistic (aggressive) social interactions in both sexes.

RESULTS

Male CD1 mice lever pressed for access to a cagemate peer under different FR response requirements and seek social interaction after 15 isolation days; these effects were independent of housing conditions. There were no sex differences in operant social self-administration and seeking. Finally, group-housed CD1 male mice did not display unconditioned aggressive behavior toward a peer male CD1 mouse.

CONCLUSIONS

Adult socially housed male CD1 mice can be used in studies on operant social reward without the potential confound of operant responding to engage in aggressive interactions.

Radioiodine Exhalation Following Oral I-131 Administration in a Mouse Model

Background: The exhalation of radioiodine following radioiodine therapy (RIT) presents a challenge in radiation protection, though the mechanisms remain incompletely understood. Previous studies have indicated that radioiodine is predominantly exhaled in an organically bound form in humans. Methods: This study investigates the chemical composition and exhaled amounts of radioiodine, as well as the impact of thyroid-targeted pharmacological interventions, using a controlled mouse model. Female Balb/c mice (25 g) were administered oral doses of radioiodine (0.1, 1, 2, 10, and 23 MBq per animal) with and without prior treatment using thyroid-blocking agents (stable iodine, perchlorate) or antithyroid drugs (carbimazole). Exhaled radioiodine was collected in metabolic cages, separating chemical forms (aerosolized iodine, elemental iodine, organically bound iodine), and quantified via scintillation counter. Results: The exhaled radioiodine activity was proportional to the administered dose (0.2-0.3%). Thyroid-blocking agents increased exhalation, shifting toward elemental iodine. Antithyroid drugs reduced exhalation but increased aerosol formation, particularly at higher I-131 doses. Organically bound iodine remained the predominant exhaled species in all groups. Conclusions: These results highlight the critical role of the thyroid in radioiodine organification. The blockade of thyroid uptake disrupted the formation of organically bound iodine, suggesting that iodine organification requires passage through the thyroid. Additionally, the results support the hypothesis that iodine metabolism outside the thyroid is less efficient, contributing to the formation of organic iodine species. Radical formation is likely a key factor in generating these volatile iodine species, with radiation-induced iodine and methyl radicals playing a role in their formation.

Neuropeptide receptor distributions in male and female Eulemur vary between female-dominant and egalitarian species

Aggression and its neurochemical modulators are typically studied in males, leaving the mechanisms of female competitive aggression or dominance largely unexplored. To better understand how competitive aggression is regulated in the primate brain, we used receptor autoradiography to compare the neural distributions of oxytocin and vasopressin receptors in male and female members of female-dominant versus egalitarian/codominant species within the Eulemur genus, wherein dominance structure is a reliable proxy of aggression in both sexes. We found that oxytocin receptor binding in the central amygdala (CeA) was predicted by dominance structure, with the members of three codominant species showing more oxytocin receptor binding in this region than their peers in four female-dominant species. Thus, both sexes in female-dominant Eulemur show a pattern consistent with the regulation of aggression in male rodents. We suggest that derived pacifism in Eulemur stems from selective suppression of ancestral female aggression over evolutionary time via a mechanism of increased oxytocin receptor binding in the CeA, rather than from augmented male aggression. This interpretation implies fitness costs to female aggression and/or benefits to its inhibition. These data establish Eulemur as a robust model for examining neural correlates of male and female competitive aggression, potentially providing novel insights into female dominance.

Visual identification of conspecifics shapes social behavior in mice

Recognizing conspecifics-others of the same species-in order to determine how to interact with them appropriately is a fundamental goal of animal sensory systems. It has undergone selective pressure in nearly all species. Mice have a large repertoire of social behaviors that are the subject of a rapidly growing field of study in neuroscience. Mouse social interactions likely incorporate all available sensory modalities, and the vast majority of studies have not attempted to isolate them. Our understanding of the role of vision in mouse social interactions remains overlooked, given the prominence of olfactory research in this area. To address this, we developed a behavioral platform that allowed us to present a subject mouse with the visual information of stimulus mice in isolation from olfactory, acoustic, and tactile cues. Our results indicate that the visual identification of the sex or individual identity of other mice influences behavior. These findings highlight the underappreciated role of vision in mouse social interactions and open new avenues to study the visual circuits underlying social behavior.

Efficacy of fluoxetine and (R,S)-ketamine in attenuating conditioned fear behaviors in male mice

Post-traumatic stress disorder (PTSD) is caused by exposure to a traumatic or stressful event. Symptoms related to this disorder include persistent re-experiencing of memories and fear of generalization. Current pharmacological treatments for PTSD are insufficient, with fewer than 30% of patients reporting symptom remission. This study aims to determine the efficacy of acute (R,S)-ketamine and chronic fluoxetine (FLX) in reducing fear memory and fear generalization. In rodents, fear conditioning (FC) is commonly used in the literature to induce behaviors related to symptoms of PTSD, and the open field test (OFT) can assess anxiety and fear generalization behaviors during the exploration of a novel environment. In this study, FC consisted of a white noise cue stimulus and 4 inescapable foot shocks. Treatments began 4 hours after FC. Fear and anxiety behaviors were recorded during re-exposure to the FC stimuli at 24 hours and 2 weeks. The OFT was conducted 1 day before the last FC re-exposure. Results support the combined use of acute ketamine and chronic FLX as a treatment for reducing behaviors indicative of fear memory during re-exposure at 2 weeks, but not behaviors indicative of anxiety and fear generalization in the OFT. FLX alone was most effective in reducing behaviors related to fear generalization. This study contributes to the existing literature on pharmacological treatment for fear and anxiety behaviors relating to fear memory and fear generalization. Continued research is necessary to replicate results, optimize treatment protocols, and investigate the molecular adaptations to trauma and treatment. SIGNIFICANCE STATEMENT: Up to 6% of people in the United States will develop PTSD within their lifetime, and less than half of those individuals will find relief from their symptoms given the current therapeutic options. This study offers preliminary support for the efficacy of ketamine and FLX in reducing PTSD-like behaviors induced by fear-conditioning in mice. Compared with current standard treatments, the results of the current study indicate the potential for a more effective therapeutic option for those with stress-related disorders, such as PTSD.

Behavioral Assays in the Study of Olfaction

Olfaction is a fundamental sense in most animal species. In mammals, the olfactory system comprises several subpopulations of sensory neurons located throughout the nasal cavity, which detect a variety of chemical stimuli, including odorants, intraspecies, and interspecies chemical communication cues. Some of these compounds are important for regulating innate or learned behaviors and endocrine changes in response to other animals in the environment. With a particular focus on laboratory rodent species, this chapter provides a comprehensive description of the most important behavioral assays used for studying the olfactory system and is meant to be a practical guide for those who investigate olfaction-mediated behaviors or who have an interest in deciphering the molecular, cellular, or neural mechanisms through which the sense of smell controls the generation of adaptive behavioral outputs.

Contributions of associative and non-associative learning to the dynamics of defensive ethograms

Defensive behavior changes based on threat intensity, proximity, and context of exposure, and learning about danger-predicting stimuli is critical for survival. However, most Pavlovian fear conditioning paradigms focus only on freezing behavior, obscuring the contributions of associative and non-associative mechanisms to dynamic defensive responses. To thoroughly investigate defensive ethograms, we subjected male and female adult C57BL/6 J mice to a Pavlovian conditioning paradigm that paired footshock with a serial compound stimulus (SCS) consisting of distinct tone and white noise (WN) stimulus periods. To investigate how associative and non-associative mechanisms affect defensive responses, we compared this paired SCS-footshock group with four control groups that were conditioned with either pseudorandom unpaired presentations of SCS and footshock, shock only, or reversed SCS presentations with inverted tone-WN order, with paired or unpaired presentations. On day 2 of conditioning, the paired group exhibited robust freezing during the tone period with switching to explosive jumping and darting behaviors during the WN period. Comparatively, the unpaired and both reverse SCS groups expressed less tone-induced freezing and rarely showed jumping or darting during WN. Following the second day of conditioning, we observed how defensive behavior changed over two extinction sessions. During extinction, the tone-induced freezing decreased in the paired group, and mice rapidly shifted from escape jumping during WN to a combination of freezing and darting. The unpaired, unpaired reverse, and shock-only groups displayed defensive tail rattling and darting during the SCS, with minimal freezing and jumping. Interestingly, the paired reverse group did not jump to WN, and tone-evoked freezing was resistant to extinction. These findings demonstrate that non-associative factors promote some defensive responsiveness, but associative factors are required for robust cue-induced freezing and high-intensity flight expression.

The role of gene-environment interactions in social dysfunction: Focus on preclinical evidence from mouse studies

Human and animal research has demonstrated that genetic and environmental factors can strongly modulate behavioral function, including the expression of social behaviors and their dysfunctionalities. Several genes have been linked to pathologies characterized by alterations in social behaviors, e.g., aggressive/antisocial personality disorder (ASPD), or autism spectrum disorder (ASD). Environmental stimulation (e.g., physical exercise, environmental enrichment) or adversity (e.g., chronic stress, social isolation) may respectively improve or impair social interactions. While the independent contribution of genetic and environmental factors to social behaviors has been assessed in a variety of human and animal studies, the impact of their interactive effects on social functions has been less extensively investigated. Genetic mutations and environmental changes can indeed influence each other through complex mutual effects, e.g., inducing synergistic, antagonistic or interactive behavioral outcomes. This complexity is difficult to be disentangled in human populations, thus encouraging studies in animal models, especially in the mouse species which is the most suitable for genetic manipulations. Here we review the available preclinical evidence on the impact of gene-environment interactions on social behaviors and their dysfunction, focusing on studies in laboratory mice. We included findings combining naturally occurring mutations, selectively bred or transgenic mice with multiple environmental manipulations, including positive (environmental enrichment, physical exercise) and aversive (social isolation, maternal separation, and stress) experiences. The impact of these results is critically discussed in terms of their generalizability across mouse models and social tests, as well as their implications for human studies on social dysfunction.

Automated home cage monitoring of an aging colony of mice-Implications for welfare monitoring and experimentation

Introduction

Our understanding of laboratory animal behavior and the implications of husbandry activities on their wellbeing remains incomplete. This is especially relevant with an aging colony as their activity patterns may change as they mature. Home Cage Monitoring (HCM) provides valuable insights into mouse activity within the animal's own environment and can shed light on acclimatization periods and responses to husbandry activities such as cage changing. The aim of this study was to monitor and explore changes in the activity and rest disturbance (RDI) patterns of an aging colony of male and female C57/BL6 mice.

Methods

The mice were housed in the Digitally Ventilated Cage® system, for up to 18 months of age. Data was then downloaded to investigate how the activity patterns and RDI of the mice changed over time. Habituation, aging and cage change assessments were conducted using linear mixed models, while cage separation and stereotypic behavior investigations were conducted by visual inspection of the data.

Results

As expected during the study, mice were less active during the light phase compared to the dark phase. However, on arrival mice displayed heightened activity and RDI during the light phase and reduced activity and RDI during the dark phase, taking several days to adjust to baseline "acclimatized" patterns. With age, overall activity significantly decreased from 5 months until 14 months of age, after which it increased back toward baseline levels. We also observed activity spikes during our monitoring of this colony. Prolonged housing can lead to alarming stereotypic behaviors in animals. Cages of mice flagged for potential stereotypy displayed sustained activity spikes in the light and dark phases. Spikes in activity during the dark phase were much more pronounced than in the light phase. Cage changing led to an increase in the light phase activity and RDI compared to the previous day, with no observed difference in the dark phase post-cage change. This effect remained consistent as the animals aged.

Discussion

This study explores changes in the activity patterns of an aging colony of male and female C57/BL6 mice housed in the Digitally Ventilated Cage® system. We identified distinct aging phases concerning activity and RDI differences and a potential new welfare application for the DVC®, specifically for early detection of stereotypy. In conclusion, the adoption of HCM systems should be considered for long-term animal housing from both a welfare and behavioral perspective.

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 androgen receptor (AR) α knockout (KO) animals for this study since this gene 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.

Socially activated neurons in the anterior cingulate cortex are essential for social behavior in mice

Social behavior, defined as any mode of communication between conspecifics is regulated by a widespread network comprising multiple brain structures. The anterior cingulate cortex (ACC) serves as a hub region interconnected with several brain regions involved in social behavior. Because the ACC coordinates various behaviors, it is important to focus on a subpopulation of neurons that are potentially involved in social behavior to clarify the precise role of the ACC in social behavior. In this study, we aimed to analyze the roles of a social stimulus-responsive subpopulation of neurons in the ACC in social behavior in mice. We demonstrated that a subpopulation of neurons in the ACC was activated by social stimuli and that silencing the social stimulus-responsive subpopulation of neurons in the ACC significantly impaired social interaction without affecting locomotor activity or anxiety-like behavior. Our current findings highlight the importance of the social stimulus-responsive subpopulation of neurons in the ACC for social behavior and the association between ACC dysfunction and impaired social behavior, which sheds light on therapeutic interventions for psychiatric conditions.

Contributions of associative and non-associative learning to the dynamics of defensive ethograms

Defensive behavior changes based on threat intensity, proximity, and context of exposure, and learning about danger-predicting stimuli is critical for survival. However, most Pavlovian fear conditioning paradigms focus only on freezing behavior, obscuring the contributions of associative and non-associative mechanisms to dynamic defensive responses. To thoroughly investigate defensive ethograms, we subjected male and female adult C57BL/6J mice to a Pavlovian conditioning paradigm that paired footshock with a serial compound stimulus (SCS) consisting of distinct tone and white noise (WN) stimulus periods. To investigate how associative and non-associative mechanisms affect defensive responses, we compared this paired SCS-footshock group with four control groups that were conditioned with either pseudorandom unpaired presentations of SCS and footshock, shock only, or reversed SCS presentations with inverted tone--WN order, with paired or unpaired presentations. On day 2 of conditioning, the paired group exhibited robust freezing during the tone period with switching to explosive jumping and darting behaviors during the WN period. Comparatively, the unpaired and both reverse SCS groups expressed less tone-induced freezing and rarely showed jumping or darting during WN. Following the second day of conditioning, we observed how defensive behavior changed over two extinction sessions. During extinction, the tone-induced freezing decreased in the paired group and mice rapidly shifted from escape jumping during WN to a combination of freezing and darting. The unpaired, unpaired reverse, and shock-only groups displayed defensive tail rattling and darting during the SCS, with minimal freezing and jumping. Interestingly, the paired reverse group did not jump to WN, and tone-evoked freezing was resistant to extinction. These findings demonstrate that non-associative factors promote some defensive responsiveness, but associative factors are required for robust cue-induced freezing and high-intensity flight expression.

Roles of AMPA receptors in social behaviors

As a crucial player in excitatory synaptic transmission, AMPA receptors (AMPARs) contribute to the formation, regulation, and expression of social behaviors. AMPAR modifications have been associated with naturalistic social behaviors, such as aggression, sociability, and social memory, but are also noted in brain diseases featuring impaired social behavior. Understanding the role of AMPARs in social behaviors is timely to reveal therapeutic targets for treating social impairment in disorders, such as autism spectrum disorder and schizophrenia. In this review, we will discuss the contribution of the molecular composition, function, and plasticity of AMPARs to social behaviors. The impact of targeting AMPARs in treating brain disorders will also be discussed.

Social isolation and aggression training lead to escalated aggression and hypothalamus-pituitary-gonad axis hyperfunction in mice

Although the participation of sex hormones and sex hormone-responsive neurons in aggressive behavior has been extensively studied, the role of other systems within the hypothalamus-pituitary-gonadal (HPG) axis remains elusive. Here we assessed how the gonadotropin-releasing hormone (GnRH) and kisspeptin systems are impacted by escalated aggression in male mice. We used a combination of social isolation and aggression training (IST) to exacerbate mice's aggressive behavior. Next, low-aggressive (group-housed, GH) and highly aggressive (IST) mice were compared regarding neuronal activity in the target populations and hormonal levels, using immunohistochemistry and ELISA, respectively. Finally, we used pharmacological and viral approaches to manipulate neuropeptide signaling and expression, subsequently evaluating its effects on behavior. IST mice exhibited enhanced aggressive behavior compared to GH controls, which was accompanied by elevated neuronal activity in GnRH neurons and arcuate nucleus kisspeptin neurons. Remarkably, IST mice presented an increased number of kisspeptin neurons in the anteroventral periventricular nucleus (AVPV). In addition, IST mice exhibited elevated levels of luteinizing hormone (LH) in serum. Accordingly, activation and blockade of GnRH receptors (GnRHR) exacerbated and reduced aggression, respectively. Surprisingly, kisspeptin had intricate effects on aggression, i.e., viral ablation of AVPV-kisspeptin neurons impaired the training-induced rise in aggressive behavior whereas kisspeptin itself strongly reduced aggression in IST mice. Our results indicate that IST enhances aggressive behavior in male mice by exacerbating HPG-axis activity. Particularly, increased GnRH neuron activity and GnRHR signaling were found to underlie aggression whereas the relationship with kisspeptin remains puzzling.

Social threat alters the behavioral structure of social motivation and reshapes functional brain connectivity

Traumatic social experiences redefine socially motivated behaviors to enhance safety and survival. Although many brain regions have been implicated in signaling a social threat, the mechanisms by which global neural networks regulate such motivated behaviors remain unclear. To address this issue, we first combined traditional and modern behavioral tracking techniques in mice to assess both approach and avoidance, as well as sub-second behavioral changes, during a social threat learning task. We were able to identify previously undescribed body and tail movements during social threat learning and recognition that demonstrate unique alterations into the behavioral structure of social motivation. We then utilized inter-regional correlation analysis of brain activity after a mouse recognizes a social threat to explore functional communication amongst brain regions implicated in social motivation. Broad brain activity changes were observed within the nucleus accumbens, the paraventricular thalamus, the ventromedial hypothalamus, and the nucleus of reuniens. Inter-regional correlation analysis revealed a reshaping of the functional connectivity across the brain when mice recognize a social threat. Altogether, these findings suggest that reshaping of functional brain connectivity may be necessary to alter the behavioral structure of social motivation when a social threat is encountered.

Influence of sex hormones on the aggressive behavior during peck order establishment and stabilization in meat and egg type chickens

In the poultry industry, broiler and layer strains are genetically selected for different purposes (e.g., high meat-yield and high egg-production). Genetic selection for productivity can have unintended consequences on the behavioral repertoire of the birds, including aggression. Alongside the increasing societal concern regarding the welfare of animal in agriculture, the number of countries that are advocating the prohibition of using battery cages for laying hens has resulted in the transition and adoption of cage-free or free-range systems. Thus, both broiler and layer chickens are housed in large flocks rather than housed individually in cages. Housing birds in groups increases the opportunity for birds to engage in social behaviors, including aggression, that are used to establish social status. Aggressive interactions are associated with the risk of injury and the potential for a subordinate animal to have unmet needs (e.g., access to feed). The aim of this study was to characterize the relationships among aggressive behavior, neurobiology, and hormones during peck order establishment and social hierarchy stabilization of 2 divergently selected strains (meat- and egg-type chicken). Meat-type strains performed more male on male (P < 0.001), male on female (P < 0.0001), and female on female (P < 0.0001) non-reciprocal aggression behavior (NRA) than egg-type strains. Greater serum testosterone and estradiol concentrations in the weeks after the peck order establishment were observed in meat-type birds compared those in egg-type birds for both males and females (all P < 0.05). Greater (P < 0.05) cellular densities of androgen receptors, but not estrogen receptors, were observed in the hypothalamus of meat-type birds compared to egg-type birds. These findings suggest that greater sex hormone concentrations in the meat-type birds may be a consequence of genetic selection for rapid growth resulting in more sex hormones-induced aggressive behavior.

Improved Memory and Lower Stress Levels in Male Mice Co-Housed with Ovariectomized Female Mice

Aggressiveness, expressed by fighting, is a frequent problem in group-housed laboratory male mice and results in increased stress, injury, and death. One way to prevent fighting is by pairing the male mice with ovariectomized female mice to provide a compatible companion. However, the effect of these housing conditions remains unclear. Therefore, we aimed to evaluate behavior and stress levels in two different housing conditions, pair-housed with an ovariectomized female and group-housed with other males. Behavioral tests were performed to assess stress and anxiety-like behavior. Moreover, the corticosterone levels in plasma were measured by ELISA. Based on home cage behavior assessment, pair-housed male mice showed no signs of fighting, not even after isolation and regrouping. Our results also showed that the pair-housed males had a better memory and demonstrated less anxiety-like behavior. Subsequently, the pair-housed male mice had a larger reduction in corticosterone levels compared to group-housed males. Overall, pair-housing reduced anxiety-like behavior and stress levels in male mice compared to standard group-housing.

Effects of social housing conditions on ethanol-induced behavioral sensitization in Swiss mice

RATIONALE

In previous animal model studies, it was shown that drug sensitization is dependent upon physical environmental conditions. However, the effects of social housing conditions on drug sensitization is much less known.

OBJECTIVE

The aim of the present study was to investigate the effects of social conditions, through the size of housing groups, on ethanol stimulant effects and ethanol-induced behavioral sensitization in mice.

MATERIALS AND METHODS

Male and female Swiss mice were housed in groups of different sizes (isolated mice, two mice per cage, four mice per cage and eight mice per cage) during a six-week period. A standard paradigm of ethanol-induced locomotor sensitization was then started with one daily injection of 2.5 g/kg ethanol for 8 consecutive days.

RESULTS

The results show that social housing conditions affect the acute stimulant effects of ethanol. The highest stimulant effects were observed in socially isolated mice and then gradually decreased as the size of the group increased. Although the rate of ethanol sensitization did not differ between groups, the ultimate sensitized levels of ethanol-induced stimulant effects were significantly reduced in mice housed in groups of eight.

CONCLUSIONS

These results are consistent with the idea that higher levels of acute and sensitized ethanol stimulant effects are observed in mice housed in stressful housing conditions, such as social isolation.

Systemic histone deacetylase inhibition ameliorates the aberrant responses to acute stress in socially isolated male mice

Adverse experiences in early life can induce maladaptive responses to acute stress in later life. Chronic social isolation during adolescence is an early life adversity that can precipitate stress-related psychiatric disorders. We found that male mice after 8 weeks of adolescent social isolation (SI) have markedly increased aggression after being exposed to 2 h of restraint stress (RS), which was accompanied by a significant increase of AMPA receptor- and NMDA receptor-mediated synaptic transmission in prefrontal cortex (PFC) pyramidal neurons of SIRS males. Compared to group-housed counterparts, SIRS males exhibited a significantly decreased level of histone H3 acetylation in PFC. Systemic administration of class I histone deacetylase inhibitors, romidepsin or MS-275, ameliorated the aggressive behaviour, as well as general social interaction deficits, of SIRS males. Electrophysiological recordings also found normalization of PFC glutamatergic currents by romidepsin treatment of SIRS male mice. These results revealed an epigenetic mechanism and intervention avenue for aggression induced by chronic social isolation. KEY POINTS: Adolescent chronic social isolation can precipitate stress-related psychiatric disorders. A significant increase of glutamatergic transmission is found in the prefrontal cortex (PFC) of socially isolated male mice exposed to an acute stress (SIRS). Treatment with class I histone deacetylase (HDAC) inhibitors ameliorates the aggressive behaviour and social interaction deficits of SIRS males, and normalizes glutamatergic currents in PFC neurons. It provides an epigenetic mechanism and intervention avenue for aberrant stress responses induced by chronic social isolation.

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.

Automated behavioral analysis reveals that mice employ a bait-and-switch escape mechanism to de-escalate social conflict

Intraspecies aggression has profound ecological and evolutionary consequences, as recipients can suffer injuries, decreases in fitness, and become outcasts from social groups. Although animals implement diverse strategies to avoid hostile confrontations, the extent to which social influences affect escape tactics is unclear. Here, we used computational and machine-learning approaches to analyze complex behavioral interactions as mixed-sex groups of mice, Mus musculus, freely interacted. Mice displayed a rich repertoire of behaviors marked by changes in behavioral state, aggressive encounters, and mixed-sex interactions. A prominent behavioral sequence consistently occurred after aggressive encounters, where males in submissive states quickly approached and transiently interacted with females immediately before the aggressor engaged with the same female. The behavioral sequences were also associated with substantially fewer physical altercations. Furthermore, the male's behavioral state and the interacting partners could be predicted by distinct features of the behavioral sequence, such as kinematics and the latency to and duration of male-female interactions. More broadly, our work revealed an ethologically relevant escape strategy influenced by the presence of females that may serve as a mechanism for de-escalating social conflict and preventing consequential reductions in fitness.

Reduction of aggressive behaviour following hypothalamic deep brain stimulation: Involvement of 5-HT1A and testosterone

BACKGROUND

Aggressive behaviour (AB) may occur in patients with different neuropsychiatric disorders. Although most patients respond to conventional treatments, a small percentage continue to experience AB despite optimized pharmacological management and are considered to be treatment-refractory. For these patients, hypothalamic deep brain stimulation (pHyp-DBS) has been investigated. The hypothalamus is a key structure in the neurocircuitry of AB. An imbalance between serotonin (5-HT) and steroid hormones seems to exacerbate AB.

OBJECTIVES

To test whether pHyp-DBS reduces aggressive behaviour in mice through mechanisms involving testosterone and 5-HT.

METHODS

Male mice were housed with females for two weeks. These resident animals tend to become territorial and aggressive towards intruder mice placed in their cages. Residents had electrodes implanted in the pHyp. DBS was administered for 5 h/day for 8 consecutive days prior to daily encounters with the intruder. After testing, blood and brains were recovered for measuring testosterone and 5-HT receptor density, respectively. In a second experiment, residents received WAY-100635 (5-HT_1A antagonist) or saline injections prior to pHyp-DBS. After the first 4 encounters, the injection allocation was crossed, and animals received the alternative treatment during the next 4 days.

RESULTS

DBS-treated mice showed reduced AB that was correlated with testosterone levels and an increase in 5-HT1_A receptor density in the orbitofrontal cortex and amygdala. Pre-treatment with WAY-100635 blocked the anti-aggressive effect of pHyp-DBS.

CONCLUSIONS

This study shows that pHyp-DBS reduces AB in mice via changes in testosterone and 5-HT1_A mechanisms.

Contextual modifiers of healthspan, lifespan, and epigenome in mice under chronic social stress

Sustained life stress and low socioeconomic status are among the major causes of aging-related diseases and decreased life expectancy. Experimental rodent models can help to identify the underlying mechanisms, yet very few studies address the long-term consequences of social stress on aging. We conducted a randomized study involving more than 300 male mice of commonly used laboratory strains (C57BL/6J, CD1, and Sv129Ev) chosen for the spontaneous aggression gradient and stress-vulnerability. Mice were exposed to a lifelong chronic psychosocial stress protocol to model social gradients in aging and disease vulnerability. Low social rank, inferred based on a discretized aggression index, was found to negatively impact lifespan in our study population. However, social rank interacted with genetic background in that low-ranking C57BL/6J, high-ranking Sv129Ev, and middle-ranking CD1 mice had lower survival, respectively, implying a cost of maintaining a given social rank that varies across strains. Machine learning linear discriminant analysis identified baseline fat-free mass as the most important predictor of mouse genetic background and social rank in the present dataset. Finally, strain and social rank differences were significantly associated with epigenetic changes, most significantly in Sv129Ev mice and in high-ranking compared to lower ranking subjects. Overall, we identified genetic background and social rank as critical contextual modifiers of aging and lifespan in an ethologically relevant rodent model of social stress, thereby providing a preclinical experimental paradigm to study the impact of social determinants of health disparities and accelerated aging.

Anesthetic-loaded nanodroplets with focused ultrasound reduces agitation in Alzheimer's mice

OBJECTIVE

Alzheimer's disease (AD) is often associated with neuropsychiatric symptoms, including agitation and aggressive behavior. These symptoms increase with disease severity, ranging from 10% in mild cognitive impairment to 50% in patients with moderate-to-severe AD, pose a great risk for self-injury and injury to caregivers, result in high rates of institutionalization and great suffering for patients and families. Current pharmacological therapies have limited efficacy and a high potential for severe side effects. Thus, there is a growing need to develop novel therapeutics tailored to safely and effectively reduce agitation and aggressive behavior in AD. Here, we investigate for the first time the use of focused ultrasound combined with anesthetic-loaded nanodroplets (nanoFUS) targeting the amygdala (key structure in the neurocircuitry of agitation) as a novel minimally invasive tool to modulate local neural activity and reduce agitation and aggressive behavior in the TgCRND8 AD transgenic mice.

METHODS

Male and female animals were tested in the resident-intruder (i.e., aggressive behavior) and open-field tests (i.e., motor agitation) for baseline measures, followed by treatment with active- or sham-nanoFUS. Behavioral testing was then repeated after treatment.

RESULTS

Active-nanoFUS neuromodulation reduced aggressive behavior and agitation in male mice, as compared to sham-treated controls. Treatment with active-nanoFUS increased the time male mice spent in social-non-aggressive behaviors.

INTERPRETATION

Our results show that neuromodulation with active-nanoFUS may be a potential therapeutic tool for the treatment of neuropsychiatric symptoms, with special focus on agitation and aggressive behaviors. Further studies are necessary to establish cellular, molecular and long-term behavioral changes following treatment with nanoFUS.

An operant social self-administration and choice model in mice

Little is known about how social factors contribute to neurobiology or neuropsychiatric disorders. The use of mice allows one to probe the neurobiological bases of social interaction, offering the genetic diversity and versatility to identify cell types and neural circuits of social behavior. However, mice typically show lower social motivation compared with rats, leading to the question of whether mice should be used to model complex social behaviors displayed by humans. Studies on mouse social behavior often rely on measures such as time spent in contact with a social partner or preference for a social-paired context, but fail to assess volitional (subject-controlled) rewarding social interaction. Here, we describe a volitional social self-administration and choice model that is an extension of our previous work on rats. Using mice, we systematically compared female adolescent and adult C57BL/6 mice and outbred CD1 mice, showing that operant social self-administration, social seeking during periods of isolation and choice of social interaction over palatable food is significantly stronger in female CD1 mice than in female C57BL/6J mice, independently of age. We describe the requirements for building the social self-administration and choice apparatus and we provide guidance for studying the role of operant social reward in mice. We also discuss its use to study brain mechanisms of operant social reward, potentially extending its application to mouse models of neuropsychiatric disorders. The training commonly requires ~4 weeks for stable social self-administration and 3-4 additional weeks for tests, including social seeking and choice.

On the effect of social cue valence in contextual memory persistence

Social cues are valuable sensorial stimuli to the acquisition and retrieval of contextual memories. Here, we asked whether the valence of social cues would impact the formation of contextual memories. Adult male C57/BL6 mice were exposed to either conditioned place preference (CPP) or avoidance (CPA). As positive stimuli we used social interaction with a female (IF), while interaction with a male CD1 mice (IM) was used as negative stimulus. Contextual memory was tested 24 h and 7 days after conditioning. Aggressive behavior of CD1, as well as interaction with the female were quantified along the conditioning sessions. IM, but not IF, was salient enough to induce contextual memory estimated by the difference between the time in the conditioned context during test and habituation. Next, we chose two odors with innate behavioral responses and opposite valence to narrow down the sociability to one of its sensorial sources of information - the olfaction. We used urine from females in proestrus (U) and 2,4,5-trimethyl thiazoline (TMT), a predator odor. TMT decreased and U increased the time in the conditioned context during the test performed 24 h and 7 days after conditioning. Taken together, our results suggest that contextual memories conditioned to social encounters are difficult to stablish in mice, specially the one with positive valence. On the other hand, using odors with ecological relevance is a promising strategy to study long-term contextual memories with opposite valences. Ultimately, the behavioral protocol proposed here offers the advantage of studying contextual memories with opposite valences using unconditioned stimulus from the same sensorial category such as olfaction.

Ecological validity of social defeat stressors in mouse models of vulnerability and resilience

Laboratory mouse models offer opportunities to bridge the gap between basic neuroscience and applied stress research. Here we consider the ecological validity of social defeat stressors in mouse models of emotional vulnerability and resilience. Reports identified in PubMed from 1980 to 2020 are reviewed for the ecological validity of social defeat stressors, sex of subjects, and whether results are discussed in terms of vulnerability alone, resilience alone, or both vulnerability and resilience. Most of the 318 reviewed reports (95%) focus on males, and many reports (71%) discuss vulnerability and resilience. Limited ecological validity is associated with increased vulnerability and decreased resilience. Elements of limited ecological validity include frequent and repeated exposure to defeat stressors without opportunities to avoid or escape from unfamiliar conspecifics that are pre-screened and selected for aggressive behavior. These elements ensure defeat and may be required to induce vulnerability, but they are not representative of naturalistic conditions. Research aimed at establishing causality is needed to determine whether ecologically valid stressors build resilience in both sexes of mice.

Activation of glucagon-like peptide-1 receptors reduces the acquisition of aggression-like behaviors in male mice

Aggression is a complex social behavior, which is provoked in the defense of limited resources including food and mates. Recent advances show that the gut-brain hormone ghrelin modulates aggressive behaviors. As the gut-brain hormone glucagon-like peptide-1 (GLP-1) reduces food intake and sexual behaviors its potential role in aggressive behaviors is likely. Therefore, we investigated a tentative link between GLP-1 and aggressive behaviors by combining preclinical and human genetic-association studies. The influence of acute or repeated injections of a GLP-1 receptor (GLP-1R) agonist, exendin-4 (Ex4), on aggressive behaviors was assessed in male mice exposed to the resident-intruder paradigm. Besides, possible mechanisms participating in the ability of Ex4 to reduce aggressive behaviors were evaluated. Associations of polymorphisms in GLP-1R genes and overt aggression in males of the CATSS cohort were assessed. In male mice, repeated, but not acute, Ex4 treatment dose-dependently reduced aggressive behaviors. Neurochemical and western blot studies further revealed that putative serotonergic and noradrenergic signaling in nucleus accumbens, specifically the shell compartment, may participate in the interaction between Ex4 and aggression. As high-fat diet (HFD) impairs the responsiveness to GLP-1 on various behaviors the possibility that HFD blunts the ability of Ex4 to reduce aggressive behaviors was explored. Indeed, the levels of aggression was similar in vehicle and Ex4 treated mice consuming HFD. In humans, there were no associations between polymorphisms of the GLP-1R genes and overt aggression. Overall, GLP-1 signaling suppresses acquisition of aggressive behaviors via central neurotransmission and additional studies exploring this link are warranted.

Excessive alcohol consumption after exposure to two types of chronic social stress: intermittent episodes vs. continuous exposure in C57BL/6J mice with a history of drinking

RATIONALE

The attraction to alcohol can be greatly increased when it is consumed in a social context. While pro-social interactions can potentiate voluntary alcohol drinking under some conditions, aversive social experience (i.e., social stress) can similarly intensify alcohol consumption.

OBJECTIVE

We sought to determine how exposure to different types of chronic social stress (i.e., intermittent episodes of social defeat or continuous social stress) influences alcohol consumption and the reinforcing effects of alcohol in mice with a history of drinking.

METHODS

Separate cohorts of male C57BL/6J mice were exposed to either 10 days of continuous or intermittent social defeat stress. In experiment 1, mice were assigned to 20% w/v alcohol consumption in a two-bottle choice protocol both prior to and after exposure to social defeat stress. In a second experiment, mice engaged in an operant response sequence to gain access to alcohol wherein completion of a fixed interval (FI; 5 min) schedule was reinforced with continuous access to alcohol (fixed ratio; FR1) for up to 1.8 g/kg. Alcohol-reinforced responding and subsequent alcohol consumption were assessed daily for 4 weeks prior to the 10-day social stress exposure and for 6-week post-stress. Machine learning was implemented to standardize the analysis of defeat behaviors exhibited by the intruder mouse during confrontation with an attacking resident.

RESULTS

In mice with a prior history of alcohol drinking, intermittent episodes of social defeat stress produced a significant increase in 20% EtOH consumption in preference over concurrently available water. This increased intake persisted for at least 6 weeks after the final social stress experience. Intermittently stressed mice also accelerated their anticipatory responding during the fixed interval component of the operant response chain that was reinforced by alcohol. Neither unstressed controls nor mice exposed to continuous social stress exhibited significant increases in alcohol consumption and alcohol reinforcement.

DISCUSSION

Episodic social defeat stress promotes the seeking and consumption of alcohol, extending earlier work to alcohol-experienced mice. We hypothesize that intermittent access to alcohol and intermittent episodes of social stress are additive and share common sensitizing neural mechanisms that engender excessive alcohol consumption.

To fight or not to fight: activation of the mPFC during decision to engage in aggressive behavior after ethanol consumption in a novel murine model

RATIONALE

Alcohol consumption is a common antecedent of aggressive behavior. The effects of alcohol on the decision to engage in aggression in preference over pro-social interaction are hypothesized to arise from augmented function within the medial prefrontal cortex (mPFC).

OBJECTIVE

In a newly developed procedure, we studied social decision-making in male C57BL/6 J mice based on preferentially seeking access to either sociosexual interactions with a female partner or the opportunity to attack an intruder male. While deciding to engage in aggressive vs. sociosexual behavior, corresponding neural activation was assessed via c-Fos immunoreactivity in cortical, amygdaloid and tegmental regions of interest. A further objective was to investigate how self-administered alcohol impacted social choice.

METHODS

During repeated confrontations with an intruder male in their home cage, experimental mice engaged in species-specific sequence of pursuit, threat, and attack behavior within < 2 min. Mice were then conditioned to respond at one of two separate illuminated operanda in an experimental chamber (octagon) attached to their home cage; completion of 10 responses (fixed ratio 10; FR10) was reinforced by access to either a female or a male intruder which were presented in the resident's home cage. Brains were harvested following choice between the concurrently available aggressive and sociosexual options and processed for c-Fos immunoreactivity across 10 brain regions. In two separate groups, mice were trained to rapidly self-administer ethanol prior to a social choice trial in order to examine the effects of alcohol on social choice, sociosexual, aggressive acts and postures, and concurrent c-Fos activity in the mPFC and limbic regions.

RESULTS AND DISCUSSION

Eight out of 65 mice consistently chose to engage in aggressive behavior in preference to sociosexual contact with a female when each outcome was concurrently available. Self-administered alcohol (experiment 1: 1.2 ± 0.02 g/kg; experiment 2: 0, 1.0, 1.5, and 1.8 g/kg) increased responding for the aggressive option in mice that previously opted predominantly for access to sociosexual interactions with the female. When choosing the aggressive, but not the sociosexual option, the prelimbic area of the mPFC revealed increased c-Fos activity, guiding future detailed inquiry into the neural mechanisms for aggressive choice.

Dynamic influences on the neural encoding of social valence

Social signals can serve as potent emotional triggers with powerful impacts on processes from cognition to valence processing. How are social signals dynamically and flexibly associated with positive or negative valence? How do our past social experiences and present social standing shape our motivation to seek or avoid social contact? We discuss a model in which social attributes, social history, social memory, social rank and social isolation can flexibly influence valence assignment to social stimuli, termed here as 'social valence'. We emphasize how the brain encodes each of these four factors and highlight the neural circuits and mechanisms that play a part in the perception of social attributes, social memory and social rank, as well as how these factors affect valence systems associated with social stimuli. We highlight the impact of social isolation, dissecting the neural and behavioural mechanisms that mediate the effects of acute versus prolonged periods of social isolation. Importantly, we discuss conceptual models that may account for the potential shift in valence of social stimuli from positive to negative as the period of isolation extends in time. Collectively, this Review identifies factors that control the formation and attribution of social valence - integrating diverse areas of research and emphasizing their unique contributions to the categorization of social stimuli as positive or negative.

Vocalization during agonistic encounter in Mongolian gerbils: Impact of sexual experience

Behaviors and vocalizations associated with aggression are essential for animals to survive, reproduce, and organize social hierarchy. Mongolian gerbils (Meriones unguiculatus) are highly aggressive and frequently emit calls. We took advantage of these features to study the relationship between vocalizations and aggressive behaviors in virgin and sexually experienced male and female Mongolian gerbils through the same-sex resident-intruder test. Both sexes of resident gerbils exhibited aggressive responses toward intruders. Multiparous females exhibited the most aggressive responses among the four groups. We also confirmed two groups of vocalizations during the encounters: high-frequency (>24.6 kHz) and low-frequency (<24.6 kHz). At the timing of high-frequency vocalizations observed during the tests, the vast majority (96.2%) of the behavioral interactions were non-agonistic. While, at the timing of low-frequency vocalizations observed during the tests, around half (45%) of the behavioral interactions were agonistic. Low-frequency vocalizations were observed mainly during encounters in which multiparous females were involved. These results suggest that high- and low-frequency vocalizations relate to non-agonistic and agonistic interactions, respectively. In addition to affecting aggressive behavior, sexual experience also affects vocalization during encounters. These findings provide new insights into the modulatory effects of sex and sexual experience on vocalizations during agonistic encounters.

Sex Differences in Social Cognition

In this review we explore the sex differences underlying various types of social cognition. Particular focus will be placed on the behaviors of social recognition, social learning, and aggression. Known similarities and differences between sexes in the expressions of these behaviors and the known brain regions where these behaviors are mediated are discussed. The role that the sex hormones (estrogens and androgens) have as well as possible interactions with other neurochemicals, such as oxytocin, vasopressin, and serotonin is reviewed as well. Finally, implications about these findings on the mediation of social cognition are mediated and the sex differences related to humans are considered.

Coordination of social behaviors by the bed nucleus of the stria terminalis

The bed nucleus of the stria terminalis (BNST) is a sexually dimorphic, neuropeptide-rich node of the extended amygdala that has been implicated in responses to stress, drugs of abuse, and natural rewards. Its function is dysregulated in neuropsychiatric disorders that are characterized by stress- or drug-induced alterations in mood, arousal, motivation, and social behavior. However, compared to the BNST's role in mood, arousal, and motivation, its role in social behavior has remained relatively understudied. Moreover, the precise cell types and circuits underlying the BNST's role in social behavior have only recently begun to be explored using modern neuroscience techniques. Here, we systematically review the existing literature investigating the neurobiological substrates within the BNST that contribute to the coordination of various sex-dependent and sex-independent social behavioral repertoires, focusing largely on pharmacological and circuit-based behavioral studies in rodents. We suggest that the BNST coordinates social behavior by promoting appropriate assessment of social contexts to select relevant behavioral outputs and that disruption of socially relevant BNST systems by stress and drugs of abuse may be an important factor in the development of social dysfunction in neuropsychiatric disorders.

Linking vgll3 genotype and aggressive behaviour in juvenile Atlantic salmon (Salmo salar)

We tested the possibility that vgll3, a gene linked with maturation age in Atlantic salmon (Salmo salar), may be associated with behaviour by measuring aggressiveness and feeding activity in 380 juveniles with different vgll3 genotypes. Contrary to our prediction, individuals with the genotype associated with later maturation (vgll3*LL) were significantly more aggressive than individuals with the genotype associated with earlier maturation (vgll3*EE). Individuals with higher aggression were also significantly lighter in colour and had higher feeding activity. Although higher aggression was associated with higher feeding activity, there was no association between feeding activity and vgll3 genotype. Increased aggression of vgll3*LL individuals was independent of their sex and size, and genotypes did not differ in their condition factor. These results imply that aggressive behaviour may have an energetic cost impairing growth and condition, especially when food cannot be monopolized. This may have implications for individual fitness and aquaculture practices.

Investigation of Behavior and Plasma Levels of Corticosterone in Restrictive- and Ad Libitum-Fed Diet-Induced Obese Mice

Diet-induced obesity (DIO) mice models are commonly used to investigate obesity-related health problems. Until now, only sparse data exist on the influence of DIO on behavior and stress hormones in mice. The present study investigates high-fat DIO with two different feeding regimes on behavioral parameters in mice. Various behavioral tests (open field, elevated plus maze, social interaction, hotplate) were performed with female BALB/c and male C57BL/6 mice after a feeding period of twelve weeks (restrictive vs. ad libitum and normal-fat diet vs. high-fat diet) to investigate levels of anxiety and aggression. BALB/c mice were DIO-resistant and therefore the prerequisite for the behavior analyses was not attained. C57BL/6 mice fed a high-fat diet had a significantly higher body weight and fat mass compared to C57BL/6 mice fed a control diet. Interestingly, the DIO C57BL/6 mice showed no changes in their aggression- or anxiety-related behavior but showed a significant change in the anxiety index. This was probably due to a lower activity level, as other ethological parameters did not show an altered anxiety-related behavior. In the ad libitum-fed DIO group, the highest corticosterone level was detected. Changes due to the feeding regime (restrictive vs. ad libitum) were not observed. These results provide a possible hint to a bias in the investigation of DIO-related health problems in laboratory animal experiments, which may be influenced by the lower activity level.

Inheritance of social dominance is associated with global sperm DNA methylation in inbred male mice

Dominance relationships between males and their associated traits are usually heritable and have implications for sexual selection in animals. In particular, social dominance and its related male pheromones are heritable in inbred mice; thus, we wondered whether epigenetic changes due to altered levels of DNA methylation determine inheritance. Here, we used C57BL/6 male mice to establish a social dominance–subordination relationship through chronic dyadic encounters, and this relationship and pheromone covariation occurred in their offspring, indicative of heritability. Through transcriptome sequencing and whole-genome DNA methylation profiling of the sperm of both generations, we found that differential methylation of many genes was induced by social dominance–subordination in sires and could be passed on to the offspring. These methylated genes were mainly related to growth and development processes, neurodevelopment and cellular transportation. The expression of the genes with similar functions in WGBS was also differentiated by social dominance–subordination, as revealed by RNA-seq. In particular, the gene Dennd1a, which regulates neural signalling, was differentially methylated and expressed in the sperm and medial prefrontal cortex (mPFC) in paired males before and after dominance–subordination establishment, suggesting the potential epigenetic control and inheritance of social dominance-related aggression. We suggest that social dominance might be passed on to male offspring through sperm DNA methylation and that the differences could potentially affect male competition in offspring by affecting the development of the nervous system.

Acute and Delayed Effects of Stress Eliciting Post-Traumatic Stress-Like Disorder Differentially Alters Fecal Microbiota Composition in a Male Mouse Model

The association between the shift in fecal resident microbiome and social conflicts with long-term consequences on psychological plasticity, such as the development of post-traumatic stress disorder (PTSD), is yet to be comprehended. We developed an aggressor-exposed (Agg-E) social stress (SS) mouse model to mimic warzone-like conflicts, where random life-threatening interactions took place between naïve intruder mice and aggressive resident mice. Gradually these Agg-E mice developed distinct characteristics simulating PTSD-like aspects, whereas the control mice not exposed to Agg-E SS demonstrated distinct phenotypes. To further investigate the role of Agg-E SS on the resident microbiome, 16S rRNA gene sequencing was assayed using fecal samples collected at pre-, during, and post-SS time points. A time agonist shift in the fecal microbial composition of Agg-E mice in contrast to its controls suggested a persistent impact of Agg-E SS on resident microbiota. At the taxonomic level, Agg-E SS caused a significant shift in the time-resolved ratios of Firmicutes and Bacteroidetes abundance. Furthermore, Agg-E SS caused diverging shifts in the relative abundances of Verrucomicrobia and Actinobacteria. An in silico estimation of genomic potential identified a potentially perturbed cluster of bioenergetic networks, which became increasingly enriched with time since the termination of Agg-E SS. Supported by a growing number of studies, our results indicated the roles of the microbiome in a wide range of phenotypes that could mimic the comorbidities of PTSD, which would be directly influenced by energy deficiency. Together, the present work suggested the fecal microbiome as a potential tool to manage long-term effects of social conflicts, including the management of PTSD.

Behavioral Effect of Terahertz Waves in C57BL/6 Mice

Terahertz is a new radiation source with many unique advantages. In recent years, its application has rapidly expanded to various fields, but there are few studies on the individual effects of terahertz. In this study, we investigated the behavioral effects of terahertz radiation on C57BL/6 mice, and we conducted an open field test, an elevated plus maze test, a light-dark box test, a three-chamber social test, and a forced swim test to explore the effects of terahertz radiation on mice from a behavioral perspective. The results show that terahertz wave may increase anti-anxiety, anti-depression, and social interaction in mice.

Strain differences in post-castration sexual and aggressive behavior in male mice

The degree to which male sexual behavior and territorial aggression are regulated by gonadal steroid hormones depends strongly on species and experience. While castration abolishes male sexual behavior in most laboratory rodents, approximately one third of B6D2F1 mice retain the full repertoire of male sexual behaviors long term ("maters"). It is not yet known whether maters retain other behaviors that typically rely on gonadal steroids to a greater extent than non-maters. In this study, we tested aggressive behavior in B6D2F1 males and males of each parental strain (C57BL/6J and DBA/2J) in the resident intruder paradigm before and after castration, as well as male sexual behavior after castration. Before castration, B6D2F1 residents displayed more attacks compared to DBA/2J males (p < 0.05). There was no difference in attack frequency between B6D2F1 and C57BL/6J males nor between DBA/2J and C57BL/6J males (p > 0.2). A greater proportion of hybrid males demonstrated intromissions and the ejaculatory reflex compared to males of either parental strain (p < 0.01). After castration, B6D2F1 residents attacked more than C57BL/6J males, but not DBA/2J males (p < 0.05; p > 0.2). There was no difference in post-castration attack frequency between maters and non-maters (p > 0.7). Finally, residents that attacked during all 3 pre-castration resident intruder tests displayed more attacks post-castration than animals that attacked during 1 pre-castration test (p < 0.05). These data suggest that strain and experience influence the expression of aggressive behavior after castration and warrant future study in experience-induced transient increases in extragonadal testosterone.

Neuroendocrine regulation of female aggression

Classically the neurobiology of aggression has been studied exclusively in males. Thus, females have been considered mildly aggressive except during lactation. Interestingly, recent studies in rodents and humans have revealed that non-lactating females can show exacerbated and pathological aggression similarly to males. This review provides an overview of recent findings on the neuroendocrine mechanisms regulating aggressive behavior in females. In particular, the focus will be on novel rodent models of exaggerated aggression established in non-lactating females. Among the neuromodulatory systems influencing female aggression, special attention has been given to sex-steroids and sex-steroid-sensitive neuronal populations (i.e., the core nuclei of the neural pathway of aggression) as well as to the neuropeptides oxytocin and vasopressin which are major players in the regulation of social behaviors.

Targeting PSD95/nNOS by ZL006 alleviates social isolation-induced heightened attack behavior in mice

RATIONALE

Deregulated attack behaviors have devastating social consequences; however, satisfactory clinical management for the behavior is still an unmet need so far. Social isolation (SI) has been common during the COVID-19 pandemic and may have detrimental effects on mental health, including eliciting heightened attack behavior.

OBJECTIVES

This study aims to explore whether injection of ZL006 can alleviate SI-induced escalation of attack behavior in mice.

METHODS

Pharmacological tools, biochemical methods, and behavioral tests were used to explore the potential therapeutic effects of ZL006 targeting postsynaptic density 95 (PSD95)/neuronal nitric oxide synthase (nNOS) pathway on escalation of attack behavior induced by SI in mice.

RESULTS

ZL006 mitigated SI-induced escalated attack behaviors and elevated nitric oxide (NO) level in the cortex of the SI mice. The beneficial effects of ZL006 lasted for at least 72 h after a single injection of ZL006. Potentiation of NO levels by L-arginine blocked the effects of ZL006. Moreover, a sub-effective dose of 7-NI in combination with a sub-effective dose of ZL006 decreased both SI-induced escalated attack behaviors and NO levels in mice subjected to SI.

CONCLUSIONS

Our study highlights the importance of the PSD95/nNOS pathway in mediating SI-induced escalation of attack behavior. ZL006 may be a promising therapeutic strategy for treating aggressive behaviors.

Establishment of the Mouse Model of Social Avoidance Induced by Female-Directed Female Aggression

Background

Most preclinical research on the effects of stress has been done on male subjects, even though women are more prone than men to experience stress-related problems. Chronic social defeat stress (CSDS) is a rodent model of psychosocial stress. However, this model has been challenged in female mouse studies since neither male nor female resident mice attack intruder females. A female-to-female CSDS model is needed to investigate the physiological and behavioral aspects.

Methods

The intruders were either male or female C57BL/6J mice, whereas the residents were male or ovariectomized (OVX) female CD-1 mice. The CD-1 aggressor mice had direct physical contact with the C57BL/6J mice for 10 min before initiating sensory contact with them for 24 h. Jump escape and freezing were evaluated during the social defeat of days 1 and 12. Experimental C57BL/6J mice underwent a social interaction test after suffering social defeat for 12 days.

Results

We found that the number of attack bites and attack latency had a significant negative correlation during the selection of aggressors. In the single-housed OVX mice, 34% of mice met the criterion of the selection of aggressors. However, single-housed OVX mice did not show sustained aggressive behavior (eg, attack bites) through the 12-day CSDS. As a result, we did not find susceptible mice during the social interaction test. In contrast, during the selection of aggressors, 42% of OVX mice housed with partners satisfied the criterion and displayed consistently aggressive behavior. CSDS produced susceptible (50%) and resilient (50%) phenotypes during the social interaction test. Notably, male and OVX female CD-1 mice housed with partners had similar amounts of attack bites and attack rates over the 12-day CSDS. Finally, we found that chronically socially defeated male and female mice displayed different coping behaviors (eg, active vs passive) with social defeat.

Conclusions

Our study demonstrates that OVX CD-1 mice housed with mates exhibited territorial aggression toward female intruders, producing susceptibility and resilience to social avoidance. Additionally, socially defeated male and female mice displayed different behavioral susceptibility to social defeat.

Multidimensional nature of dominant behavior: Insights from behavioral neuroscience

Social interactions for many species of animals are critical for survival, wellbeing, and reproduction. Optimal navigation of a social system increases chances for survival and reproduction, therefore there is strong incentive to fit into social structures. Social animals rely heavily on dominant-submissive behaviors in establishment of stable social hierarchies. There is a link between extreme manifestation of dominance/submissiveness and behavioral deviations. To understand neural substrates affiliated with a specific hierarchical rank, there is a real need for reliable animal behavioral models. Different paradigms have been consolidated over time to study the neurobiology of social rank behavior in a standardized manner using rodent models to unravel the neural pathways and substrates involved in normal and abnormal intraspecific social interactions. This review summarizes and discusses the commonly used behavioral tests and new directions for the assessment of dominance in rodents. We discuss the hierarchy inheritable nature and other critical issues regarding hierarchical rank manifestation which may help in designing social-rank-related studies that serve as promising pre-clinical tools in behavioral psychiatry.

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.

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.

The neuroscience of social feelings: mechanisms of adaptive social functioning

Social feelings have conceptual and empirical connections with affect and emotion. In this review, we discuss how they relate to cognition, emotion, behavior and well-being. We examine the functional neuroanatomy and neurobiology of social feelings and their role in adaptive social functioning. Existing neuroscience literature is reviewed to identify concepts, methods and challenges that might be addressed by social feelings research. Specific topic areas highlight the influence and modulation of social feelings on interpersonal affiliation, parent-child attachments, moral sentiments, interpersonal stressors, and emotional communication. Brain regions involved in social feelings were confirmed by meta-analysis using the Neurosynth platform for large-scale, automated synthesis of functional magnetic resonance imaging data. Words that relate specifically to social feelings were identfied as potential research variables. Topical inquiries into social media behaviors, loneliness, trauma, and social sensitivity, especially with recent physical distancing for guarding public and personal health, underscored the increasing importance of social feelings for affective and second person neuroscience research with implications for brain development, physical and mental health, and lifelong adaptive functioning.

HIV-1 Tat and morphine decrease murine inter-male social interactions and associated oxytocin levels in the prefrontal cortex, amygdala, and hypothalamic paraventricular nucleus

Many persons infected with HIV-1 (PWH) and opioid-dependent individuals experience deficits in sociability that interfere with daily living. Sociability is regulated by the prefrontal cortico-hippocampal-amygdalar circuit. Within this circuit HIV-1 trans-activator of transcription (HIV-1 Tat) and opioids can increase dendritic pathology and alter neuronal firing. Changes in sociability are also associated with dysregulation of hypothalamic neuropeptides such as oxytocin or corticotropin releasing factor (CRF) in the prefrontal cortico-hippocampal-amygdalar circuit. Accordingly, we hypothesized that the interaction of HIV-1 Tat and morphine would impair inter-male social interactions and disrupt oxytocin and CRF within the PFC and associated circuitry. Male mice were exposed to HIV-1 Tat for 8 weeks and administered saline or escalating doses of morphine twice daily (s.c.) during the last 2 weeks of HIV-1 Tat exposure. Tat attenuated aggressive interactions with an unknown intruder, whereas morphine decreased both non-aggressive and aggressive social interactions in the resident-intruder test. However, there was no effect of Tat or morphine on non-reciprocal interactions in the social interaction and novelty tests. Tat, but not morphine, decreased oxytocin levels in the PFC and amygdala, whereas both Tat and morphine decreased the percentage of oxytocin-immunoreactive neurons in the hypothalamic paraventricular nucleus (PVN). In Tat(+) or morphine-exposed mice, regional levels of CRF and oxytocin correlated with alterations in behavior in the social interaction and novelty tests. Overall, decreased expression of oxytocin in the prefrontal cortico-hippocampal-amygdalar circuit is associated with morphine- and HIV-Tat-induced deficits in social behavior.

Evolution of stress responses refine mechanisms of social rank

Social rank functions to facilitate coping responses to socially stressful situations and conditions. The evolution of social status appears to be inseparably connected to the evolution of stress. Stress, aggression, reward, and decision-making neurocircuitries overlap and interact to produce status-linked relationships, which are common among both male and female populations. Behavioral consequences stemming from social status and rank relationships are molded by aggressive interactions, which are inherently stressful. It seems likely that the balance of regulatory elements in pro- and anti-stress neurocircuitries results in rapid but brief stress responses that are advantageous to social dominance. These systems further produce, in coordination with reward and aggression circuitries, rapid adaptive responding during opportunities that arise to acquire food, mates, perch sites, territorial space, shelter and other resources. Rapid acquisition of resources and aggressive postures produces dominant individuals, who temporarily have distinct fitness advantages. For these reasons also, change in social status can occur rapidly. Social subordination results in slower and more chronic neural and endocrine reactions, a suite of unique defensive behaviors, and an increased propensity for anxious and depressive behavior and affect. These two behavioral phenotypes are but distinct ends of a spectrum, however, they may give us insights into the troubling mechanisms underlying the myriad of stress-related disorders to which they appear to be evolutionarily linked.