Social housing and social isolation: Impact on stress indices and energy balance in male and female Syrian hamsters (Mesocricetus auratus)

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.

Sex-dependent regulation of social avoidance by oxytocin signaling in the ventral tegmental area

It has been hypothesized that oxytocin increases the salience of social stimuli, whether the valence is positive or negative, through its interactions with the ventral tegmental area (VTA). Indeed, oxytocin neurons project to the VTA and activate dopamine neurons that are necessary for social experiences with positive valence. Surprisingly, though, there has not been an investigation of the role of oxytocin in the VTA in mediating social experiences with negative valence (e.g., social stress). Given that there are sex differences in how oxytocin regulates the salience of positively-valenced social interactions, we hypothesized that oxytocin acting in the VTA also alters the salience of social stress in a sex-dependent manner. To test this, female and male Syrian hamsters were site-specifically infused with either saline, oxytocin (9 μM), or oxytocin receptor antagonist (90 μM) into the VTA. Subjects were then exposed to either no defeat or a single, 15 min defeat by one RA. The day following social defeat, subjects underwent a 5 min social avoidance test. There was an interaction between sex and drug treatment, such that the oxytocin antagonist increased social avoidance compared to saline treatment in socially stressed females, while oxytocin decreased social avoidance compared to saline treatment in socially stressed males. Contrary to expectations, these results suggest that oxytocin signaling generally acts to decrease social avoidance, regardless of sex. These sex differences in the efficacy of oxytocin and oxytocin receptor antagonists to alter negatively-valenced social stimuli, however, should be considered when guiding pharmacotherapies for disorders involving social deficits.

Sex differences in the impact of social status on social reward and associated mesolimbic activation

Social stress plays an important role in the etiology of many neuropsychiatric disorders and can lead to a variety of behavioral deficits such as social withdrawal. One way that social stress may contribute to psychiatric disorders is by reducing social motivation and the rewarding properties of social interactions. We investigated the impact of social stress on social reward in the context of winning versus losing agonistic encounters in Syrian hamsters (Mesocricetus auratus). First, we tested the hypothesis that social stress resulting from either stable low, or subordinate, social status or from social defeat reduces the rewarding properties of social interactions. Using an Operant Social Preference (OSP) task to measure social reward/motivation, we found that both subordinate and socially defeated males made significantly fewer entries into chambers containing novel, same-sex conspecifics compared to males who were dominant (i.e., stably won the agonistic encounters). In females, however, there were no differences in social entries between winners and losers. In a second experiment, we found more activation of the mesolimbic dopamine system (MDS) as assessed with cFos immunohistochemistry in the lateral ventral tegmental area (lVTA) and the nucleus accumbens (NAc) shell of male winners compared to losers. In females, however, there were no differences in activation in the lVTA between winners and losers. Surprisingly, however, winning females displayed significantly more activation in the NAc shell as compared to losing females, despite the lack of behavioral differences. Thus, behavioral and histological data suggest that there are sex differences in the impact of social status on social reward and associated mesolimbic activation.

Elevated estradiol during a hormone simulated pseudopregnancy decreases sleep and increases hypothalamic activation in female Syrian hamsters

Sleep disruptions are a common occurrence during the peripartum period. While physical and environmental factors associated with pregnancy and newborn care account for some sleep disruptions, there is evidence that peripartum fluctuations in estrogens may independently impact sleep. However, the impact of these large fluctuations in estrogens on peripartum sleep is unclear because it is difficult to tease apart the effects of estrogens on sleep from effects associated with the growth and development of the fetus or parental care. We therefore used a hormone-simulated pseudopregnancy (HSP) in female Syrian hamsters to test the hypothesis that pregnancy-like increases in estradiol decrease sleep in the absence of other factors. Adult female Syrian hamsters were ovariectomized and given daily hormone injections that simulate estradiol levels during early pregnancy, late pregnancy, and the postpartum period. Home cage video recordings were captured at seven timepoints and videos were analyzed for actigraphy. During "late pregnancy," total sleep time and sleep efficiency were decreased in hormone-treated animals during the white light period compared to pretest levels. Likewise, during "late pregnancy," locomotion was increased in the white light period for hormone-treated animals compared to pretest levels. These changes continued into the "postpartum period" for animals who continued to receive estradiol treatment, but not for animals who were withdrawn from estradiol. At the conclusion of the experiment, animals were euthanized and cFos expression was quantified in the ventral lateral preoptic area (VLPO) and lateral hypothalamus (LH). Animals who continued to receive high levels of estradiol during the "postpartum" period had significantly more cFos in the VLPO and LH than animals who were withdrawn from hormones or vehicle controls. Together, these data suggest that increased levels of estradiol during pregnancy are associated with sleep suppression, which may be mediated by increased activation of hypothalamic nuclei.

Use of a Preclinical Natural Transmission Model to Study Antiviral Effects of a Carbohydrate-Binding Module Therapy against SARS-CoV-2 in Hamsters

The emergence of severe acute respiratory syndrome coronavirus (SARS-CoV-2) and its expansion to a worldwide pandemic resulted in efforts to assess and develop interventions to reduce the disease burden. Despite the introduction of vaccine programmes against SARS-CoV-2, global incidence levels in early 2022 remained high, demonstrating a need for the development of physiologically relevant models, which are essential for the identification of alternative antiviral strategies. The hamster model of SARS-CoV-2 infection has been widely adopted due to similarities with humans in terms of host cell entry mechanism (via ACE2), and aspects of symptomology and virus shedding. We have previously described a natural transmission hamster model that better represents the natural course of infection. In the present study, we have conducted further testing of the model using the first-in-class antiviral Neumifil, which has previously shown promise against SARS-CoV-2 after a direct intranasal challenge. Neumifil is an intranasally delivered carbohydrate-binding module (CBM) which reduces the binding of viruses to their cellular receptor. By targeting the host cell, Neumifil has the potential to provide broad protection against multiple pathogens and variants. This study demonstrates that using a combination of a prophylactic and therapeutic delivery of Neumifil significantly reduces the severity of clinical signs in animals infected via a natural route of transmission and indicates a reduction of viral loads in the upper respiratory tract. Further refinements of the model are required in order to ensure the adequate transmission of the virus. However, our results provide additional data to the evidence base of Neumifil efficacy against respiratory virus infection and demonstrate that the transmission model is a potentially valuable tool for testing antiviral compounds against SARS-CoV-2.

Aggression Results in the Phosphorylation of ERK1/2 in the Nucleus Accumbens and the Dephosphorylation of mTOR in the Medial Prefrontal Cortex in Female Syrian Hamsters

Like many social behaviors, aggression can be rewarding, leading to behavioral plasticity. One outcome of reward-induced aggression is the long-term increase in the speed in which future aggression-based encounters is initiated. This form of aggression impacts dendritic structure and excitatory synaptic neurotransmission in the nucleus accumbens, a brain region well known to regulate motivated behaviors. Yet, little is known about the intracellular signaling mechanisms that drive these structural/functional changes and long-term changes in aggressive behavior. This study set out to further elucidate the intracellular signaling mechanisms regulating the plasticity in neurophysiology and behavior that underlie the rewarding consequences of aggressive interactions. Female Syrian hamsters experienced zero, two or five aggressive interactions and the phosphorylation of proteins in reward-associated regions was analyzed. We report that aggressive interactions result in a transient increase in the phosphorylation of extracellular-signal related kinase 1/2 (ERK1/2) in the nucleus accumbens. We also report that aggressive interactions result in a transient decrease in the phosphorylation of mammalian target of rapamycin (mTOR) in the medial prefrontal cortex, a major input structure to the nucleus accumbens. Thus, this study identifies ERK1/2 and mTOR as potential signaling pathways for regulating the long-term rewarding consequences of aggressive interactions. Furthermore, the recruitment profile of the ERK1/2 and the mTOR pathways are distinct in different brain regions.

Impact of spaceflight stressors on behavior and cognition: A molecular, neurochemical, and neurobiological perspective

The response of the human body to multiple spaceflight stressors is complex, but mounting evidence implicate risks to CNS functionality as significant, able to threaten metrics of mission success and longer-term behavioral and neurocognitive health. Prolonged exposure to microgravity, sleep disruption, social isolation, fluid shifts, and ionizing radiation have been shown to disrupt mechanisms of homeostasis and neurobiological well-being. The overarching goal of this review is to document the existing evidence of how the major spaceflight stressors, including radiation, microgravity, isolation/confinement, and sleep deprivation, alone or in combination alter molecular, neurochemical, neurobiological, and plasma metabolite/lipid signatures that may be linked to operationally-relevant behavioral and cognitive performance. While certain brain region-specific and/or systemic alterations titrated in part with neurobiological outcome, variations across model systems, study design, and the conspicuous absence of targeted studies implementing combinations of spaceflight stressors, confounded the identification of specific signatures having direct relevance to human activities in space. Summaries are provided for formulating new research directives and more predictive readouts of portending change in neurobiological function.

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.

Acute administration of fluoxetine increases social avoidance and risk assessment behaviors in a sex- and social stress-dependent manner in Syrian hamsters (Mesocricetus auratus)

Most studies investigating the effects of acute administration of selective serotonin reuptake inhibitors (SSRI) on responses to social stress have been conducted with males. This is despite the fact that SSRIs remain the primary pharmacotherapy for social stress-related disorders for both sexes and that the prevalence of these disorders is twofold higher in women than in men. To determine whether acute treatment with the SSRI, fluoxetine, alters behavioral responses to social defeat stress in a sex- or social stress-dependent manner, male and female Syrian hamsters were subjected to one of three social defeat conditions: no defeat (placed into an empty resident aggressor (RA) cage), a single defeat by one RA for 15 min, or three consecutive defeats using different RAs for 5 min each. The day following social defeat, subjects were infused with either vehicle or fluoxetine (20 mg/kg, I.P.) 2 h prior to a 5 min social avoidance test. Overall, we found that fluoxetine increased social vigilance regardless of sex or defeat condition. We also found that fluoxetine affected social avoidance in a sex by stress intensity interaction, such that fluoxetine increased avoidance in no defeat males and in males defeated once but significantly increased avoidance in females only after three defeats. These data suggest that treatment with an SSRI could initially exacerbate the effects of social stress in both sexes. These data also emphasize the importance of including sex as a biological variable when investigating the efficacy of pharmacotherapy for stress-related disorders.

Mean Girls: Social Stress Models for Female Rodents

Social stressors are known to have strong negative impacts on mental health. There is a long history of preclinical social defeat stress studies in rodents focusing on males that has produced important insights into the neural mechanisms that modulate depression- and anxiety-related behavior. Despite these impressive results, a historical weakness of rodent social stress models has been an under-representation of studies in females. This is problematic because rates of depression and anxiety are higher in women versus men. Recently there has been a surge of interest in adapting social stress methods for female rodents. Here we review new rodent models that have investigated numerous facets of social stress in females. The different models have different strengths and weaknesses, with some model systems having stronger ethological validity with other models having better access to molecular tools to manipulate neural circuits. Continued use and refinement of these complementary models will be critical for addressing gaps in understanding the function of neural circuits modulating depression- and anxiety-related behavior in females.

Science, sensitivity and the sociozoological scale: Constituting and complicating the human-animal boundary at the 1875 Royal Commission on Vivisection and beyond

Arnold Arluke and Clinton Sanders (1996) have argued that human societies index both humans and animals as belonging to particular rungs of the social hierarchy. They term this multispecies ranking the "sociozoological scale". This paper will investigate how claims at the 1875 Royal Commission on Vivisection about the sensitivity of particular species and breeds not only reflected assumptions about human social hierarchy but also blurred the boundaries between the human and the animal in the process. It will further be shown how these claims were informed by 18th and 19th century humanitarianism, classism, scientific racism and evolutionary theory, and how these influences combined in claims-making about the relative capacity of particular animals to sense pain and ethical duties towards them that followed from this sensitivity. Particular attention will be given to the opposing efforts of commissioners Thomas Henry Huxley and Richard Holt Hutton to demarcate human and animal sensitivity and exempt companion animals from vivisection respectively. The paper concludes by considering the sociozoological orders constituted by the 1876 Cruelty to Animals Act, particularly through its focus on calculating pain, and the legacy and limitations of this constitution.

From human wellbeing to animal welfare

What does it mean to be "well" and how might such a state be cultivated? When we speak of wellbeing, it is of ourselves and fellow humans. When it comes to nonhuman animals, consideration turns to welfare. My aim herein is to suggest that theoretical approaches to human wellbeing might be beneficially applied to consideration of animal welfare, and in so doing, introduce new lines of inquiry and practice. I will review current approaches to human wellbeing, adopting a triarchic structure that delineates hedonic wellbeing, eudaimonic wellbeing, and social wellbeing. For each, I present a conceptual definition and a review of how researchers have endeavored to measure the construct. Drawing these three domains of research together, I highlight how these traditionally anthropocentric lines of inquiry might be extended to the question of animal welfare - namely by considering hedonic welfare, eudaimonic welfare, and social welfare as potentially distinguishable and complementary components of the broader construct of animal welfare.

Methods and Challenges in Investigating Sex-Specific Consequences of Social Stressors in Adolescence in Rats: Is It the Stress or the Social or the Stage of Development?

Adolescence is a time of social learning and social restructuring that is accompanied by changes in both the hypothalamic-pituitary-gonadal axis and the hypothalamic-pituitary-adrenal (HPA) axis. The activation of these axes by puberty and stressors, respectively, shapes adolescent development. Models of social stress in rats are used to understand the consequences of perturbations of the social environment for ongoing brain development. This paper reviews the challenges in investigating the sex-specific consequences of social stressors, sex differences in the models of social stress used in rats and the sex-specific effects on behaviour and provides an overview of sex differences in HPA responding to stressors, the variability in pubertal development and in strains of rats that require consideration in conducting such research, and directions for future research.

Estrogen Withdrawal Increases Postpartum Anxiety via Oxytocin Plasticity in the Paraventricular Hypothalamus and Dorsal Raphe Nucleus

BACKGROUND

Estrogen increases dramatically during pregnancy but quickly drops below prepregnancy levels at birth and remains suppressed during the postpartum period. Clinical and rodent work suggests that this postpartum drop in estrogen results in an estrogen withdrawal state that is related to changes in affect, mood, and behavior. How estrogen withdrawal affects oxytocin (OT) neurocircuitry has not been examined.

METHODS

We used a hormone-simulated pseudopregnancy followed by estrogen withdrawal in Syrian hamsters, a first for this species. Ovariectomized females were given daily injections to approximate hormone levels during gestation and then withdrawn from estrogen to simulate postpartum estrogen withdrawal. These hamsters were tested for behavioral assays of anxiety and anhedonia during estrogen withdrawal. Neuroplasticity in OT-producing neurons in the paraventricular nucleus of the hypothalamus and its efferent targets was measured.

RESULTS

Estrogen-withdrawn females had increased anxiety-like behaviors in the elevated plus maze and open field tests but did not differ from control females in sucrose preference. Furthermore, estrogen-withdrawn females had more OT-immunoreactive cells and OT messenger RNA in the paraventricular nucleus of the hypothalamus and an increase in OT receptor density in the dorsal raphe nucleus. Finally, blocking OT receptors in the dorsal raphe nucleus during estrogen withdrawal prevented the high-anxiety behavioral phenotype in estrogen-withdrawn females.

CONCLUSIONS

Estrogen withdrawal induces OT neuroplasticity in the paraventricular nucleus of the hypothalamus and dorsal raphe nucleus to increase anxiety-like behavior during the postpartum period. More broadly, these experiments suggest Syrian hamsters as a novel organism in which to model the effects of postpartum estrogen withdrawal on the brain and anxiety-like behavior.

Ingestion of probiotic (Lactobacillus helveticus and Bifidobacterium longum) alters intestinal microbial structure and behavioral expression following social defeat stress

Social stress exacerbates anxious and depressive behaviors in humans. Similarly, anxiety- and depressive-like behaviors are triggered by social stress in a variety of non-human animals. Here, we tested whether oral administration of the putative anxiolytic probiotic strains Lactobacillus helveticus R0052 and Bifidobacterium longum R0175 reduces the striking increase in anxiety-like behavior and changes in gut microbiota observed following social defeat stress in Syrian hamsters. We administered the probiotic at two different doses for 21 days, and 16S rRNA gene amplicon sequencing revealed a shift in microbial structure following probiotic administration at both doses, independently of stress. Probiotic administration at either dose increased anti-inflammatory cytokines IL-4, IL-5, and IL-10 compared to placebo. Surprisingly, probiotic administration at the low dose, equivalent to the one used in humans, significantly increased social avoidance and decreased social interaction. This behavioral change was associated with a reduction in microbial richness in this group. Together, these results demonstrate that probiotic administration alters gut microbial composition and may promote an anti-inflammatory profile but that these changes may not promote reductions in behavioral responses to social stress.

Sex-dependent effects of social status on the regulation of arginine-vasopressin (AVP) V1a, oxytocin (OT), and serotonin (5-HT) 1A receptor binding and aggression in Syrian hamsters (Mesocricetus auratus)

Dominance status in hamsters is driven by interactions between arginine-vasopressin V1a, oxytocin (OT), and serotonin 1A (5-HT1A) receptors. Activation of V1a and OT receptors in the anterior hypothalamus (AH) increases aggression in males, while decreasing aggression in females. In contrast, activation of 5-HT1A receptors in the AH decreases aggression in males and increases aggression in females. The mechanism underlying these differences is not known. The purpose of this study was to determine if dominance status and sex interact to regulate V1a, OT, and 5-HT1A receptor binding. Same-sex hamsters (N = 47) were paired 12 times across six days in five min sessions. Brains from paired and unpaired (non-social control) hamsters were collected immediately after the last interaction and processed for receptor binding using autoradiography. Differences in V1a, OT, and 5-HT1A receptor binding densities were observed in several brain regions as a function of social status and sex. For example, in the AH, there was an interaction between sex and social status, such that V1a binding in subordinate males was lower than in subordinate females and V1a receptor density in dominant males was higher than in dominant females. There was also an interaction in 5-HT1A receptor binding, such that social pairing increased 5-HT1A binding in the AH of males but decreased 5-HT1A binding in females compared with unpaired controls. These results indicate that dominance status and sex play important roles in shaping the binding profiles of key receptor subtypes across the neural circuitry that regulates social behavior.

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

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

Silence, Solitude, and Serotonin: Neural Mechanisms Linking Hearing Loss and Social Isolation

For social animals that communicate acoustically, hearing loss and social isolation are factors that independently influence social behavior. In human subjects, hearing loss may also contribute to objective and subjective measures of social isolation. Although the behavioral relationship between hearing loss and social isolation is evident, there is little understanding of their interdependence at the level of neural systems. Separate lines of research have shown that social isolation and hearing loss independently target the serotonergic system in the rodent brain. These two factors affect both presynaptic and postsynaptic measures of serotonergic anatomy and function, highlighting the sensitivity of serotonergic pathways to both types of insult. The effects of deficits in both acoustic and social inputs are seen not only within the auditory system, but also in other brain regions, suggesting relatively extensive effects of these deficits on serotonergic regulatory systems. Serotonin plays a much-studied role in depression and anxiety, and may also influence several aspects of auditory cognition, including auditory attention and understanding speech in challenging listening conditions. These commonalities suggest that serotonergic pathways are worthy of further exploration as potential intervening mechanisms between the related conditions of hearing loss and social isolation, and the affective and cognitive dysfunctions that follow.

Social experience and sex-dependent regulation of aggression in the lateral septum by extrasynaptic δGABAA receptors

RATIONALE

Understanding the neurobiological mechanisms mediating dominance and competitive aggression is essential to understanding the development and treatment of various psychiatric disorders. Previous research suggests that these mechanisms are both sexually differentiated and influenced substantially by social experience. In numerous species, GABA_A receptors in the lateral septum have been shown to play a significant role in aggression in males. However, very little is known about the role of these GABA_A receptors in female aggression, the role of social experience on GABA_A receptor-mediated aggression, or the roles of different GABA_A subtypes in regulating aggression.

OBJECTIVES

Thus, in the following set of experiments, we determined the role of social experience in modulating GABA_A receptor-induced aggression in both male and female Syrian hamsters, with a particular focus on the GABA_A receptor subtype mediating these effects.

RESULTS

Activation of GABA_A receptors in the dorsal lateral septum increased aggression in both males and females. Social housing, however, significantly decreased the ability of GABA_A receptor activation to induce aggression in males but not females. No significant differences were observed in the effects of GABA_A receptor activation in dominant versus subordinate group-housed hamsters. Finally, examination of potential GABA_A receptor subtype specificity revealed that social housing decreased the ratio of δ extrasynaptic to γ₂ synaptic subunit GABA_A receptor mRNA expression in the anterior dorsal lateral septum, while activation of δ extrasynaptic, but not γ₂ synaptic, GABA_A receptors in the dorsal lateral septum increased aggression.

CONCLUSIONS

These data suggest that social experience can have profound effects on the neuronal mechanisms mediating aggression, especially in males, and that δ extrasynaptic GABA_A receptors may be an important therapeutic target in disorders characterized by high levels of aggression.

Sex-dependent effects of social isolation on the regulation of arginine-vasopressin (AVP) V1a, oxytocin (OT) and serotonin (5HT) 1a receptor binding and aggression

It is widely held that social isolation produces higher rates of mortality and morbidity and has deleterious effects on an individual's sociality. Relatedly, it is widely observed that socially isolated adult rodents display significantly higher levels of aggression when placed in a social situation than do their conspecifics living in social groups. In the following study, we investigated the effects of social isolation on several neurochemical signals that play key roles in the regulation of social behavior in adults. More specifically, we examined the effects of social isolation on vasopressin (AVP) V1a, oxytocin (OT) and serotonin (5-HT)1a receptor binding within the neural circuit controlling social behavior. Male and female Syrian hamsters were housed individually or with two other hamsters for four weeks and were then tested with a same-sex nonaggressive intruder in a neutral arena for 5 min. Social isolation significantly increased aggression in both males and females and altered receptor binding in several brain regions in a sex-dependent manner. For example, V1a receptor binding was greater in socially isolated males in the anterior hypothalamus than it was in any other group. Taken together, these data provide substantial new support for the proposition that the social environment can have a significant impact on the structural and neurochemical mechanisms regulating social behavior and that the amount and type of social interactions can produce differential effects on the circuit regulating social behavior in a sex-dependent manner.

Bundling the haystack to find the needle: Challenges and opportunities in modeling risk and resilience following early life stress

Various forms of early life adversity (ELA) have been linked with increased risk for negative health outcomes, including neuropsychiatric disorders. Understanding how the complex interplay between types, timing, duration, and severity of ELA, together with individual differences in genetic, socio-cultural, and physiological differences can mediate risk and resilience has proven difficult in population based studies. Use of animal models provides a powerful toolset to isolate key variables underlying risk for altered neural and behavioral maturational trajectories. However, a lack of clarity regarding the unique features of differing forms of adversity, lab differences in the implementation and reporting of methods, and the ability compare across labs and types of ELA has led to some confusion. Here, we highlight the diversity of approaches available, current challenges, and a possible ways forward to increase clarity and drive more meaningful and fruitful implementation and comparison of these approaches.

Sex-dependent regulation of social reward by oxytocin receptors in the ventral tegmental area

Social reward is critical for social relationships, and yet we know little about the characteristics of social interactions that are rewarding or the neural mechanisms underlying that reward. Here, we investigate the sex-dependent role of oxytocin receptors within the ventral tegmental area (VTA) in mediating the magnitude and valence of social reward. Operant and classical conditioning tests were used to measure social reward associated with same-sex social interactions. The effects of oxytocin, selective oxytocin receptor agonists, antagonists, and vehicle injected into the VTA on social reward was determined in male and female Syrian hamsters. The colocalization of FOS and oxytocin in sites that project to the VTA following social interaction was also determined. Females find same-sex social interactions more rewarding than males and activation of oxytocin receptors in the VTA is critical for social reward in females, as well as males. These studies provide support for the hypothesis that there is an inverted U relationship between the duration of social interaction and social reward, mediated by oxytocin; and that in females the dose-response relationship is initiated at lower doses compared with males. Same-sex social interaction is more rewarding in females than in males, and an inverted U relationship mediated by oxytocin may have a critical role in assigning positive and negative valence to social stimuli. Understanding these sex differences in social reward processing may be essential for understanding the sex differences in the prevalence of many psychiatric disorders and the development of gender-specific treatments of neuropsychiatric disorders.

Brain-derived neurotrophic factor signaling mitigates the impact of acute social stress

Brain-derived neurotrophic factor (BDNF) is known to promote fear learning as well as avoidant behavioral responses to chronic social defeat stress, but, conversely, this peptide can also have antidepressant effects and can reduce depressant-like symptoms such as social avoidance. The purpose of this study was to use a variety of approaches to determine whether BDNF acting on tropomyosin receptor kinase B (TrkB) promotes or prevents avoidant phenotypes in hamsters and mice that have experienced acute social defeat stress. We utilized systemic and brain region-dependent manipulation of BDNF signaling before or immediately following social defeat stress in Syrian hamsters, TrkB^F616A knock-in mice, and C57Bl/6J mice and measured the subsequent behavioral response to a novel opponent. Systemic TrkB receptor agonists reduced, and TrkB receptor antagonists enhanced, behavioral responses to social defeat in hamsters and mice. In the neural circuit that we have shown mediates defeat-induced behavioral responses, BDNF in the basolateral amygdala, but not the nucleus accumbens, also reduced social avoidant phenotypes. Conversely, knockdown in the basolateral amygdala of TrkB signaling in TrkB^F616A mice enhanced defeat-induced social avoidance. These data demonstrate that systemic administration of BDNF-TrkB drugs at the time of social defeat alters the behavioral response to the defeat stressor. These drugs appear to act, at least in part, in the basolateral amygdala and not the nucleus accumbens. These findings were generalizable to two rodent species with very different social structures and, within mice, to a variety of strains providing converging evidence that BDNF-TrkB signaling reduces anxiety- and depression-like symptoms following short-term social stress.

Transcriptomic Analysis Reveals Sex-Dependent Expression Patterns in the Basolateral Amygdala of Dominant and Subordinate Animals After Acute Social Conflict

The basolateral amygdala (BLA) is a critical nucleus mediating behavioral responses after exposure to acute social conflict. Male and female Syrian hamsters both readily establish a stable dominant-subordinate relationship among same-sex conspecifics, and the goal of the current study was to determine potential underlying genetic mechanisms in the BLA facilitating the establishment of social hierarchy. We sequenced the BLA transcriptomes of dominant, subordinate, and socially neutral males and females, and using de novo assembly techniques and gene network analyses, we compared these transcriptomes across social status within each sex. Our results revealed 499 transcripts that were differentially expressed in the BLA across both males and females and 138 distinct gene networks. Surprisingly, we found that there was virtually no overlap in the transcript changes or in gene network patterns in males and females of the same social status. These results suggest that, although males and females reliably engage in similar social behaviors to establish social dominance, the molecular mechanisms in the BLA by which these statuses are obtained and maintained are distinct.

Neurobiology and consequences of social isolation stress in animal model-A comprehensive review

The brain is a vital organ, susceptible to alterations under genetic influences and environmental experiences. Social isolation (SI) acts as a stressor which results in alterations in reactivity to stress, social behavior, function of neurochemical and neuroendocrine system, physiological, anatomical and behavioral changes in both animal and humans. During early stages of life, acute or chronic SIS has been proposed to show signs and symptoms of psychiatric and neurological disorders such as anxiety, depression, schizophrenia, epilepsy and memory loss. Exposure to social isolation stress induces a variety of endocrinological changes including the activation of hypothalamic-pituitary-adrenal (HPA) axis, culminating in the release of glucocorticoids (GCs), release of catecholamines, activation of the sympatho-adrenomedullary system, release of Oxytocin and vasopressin. In several regions of the central nervous system (CNS), SIS alters the level of neurotransmitter such as dopamine, serotonin, gamma aminobutyric acid (GABA), glutamate, nitrergic system and adrenaline as well as leads to alteration in receptor sensitivity of N-methyl-D-aspartate (NMDA) and opioid system. A change in the function of oxidative and nitrosative stress (O&NS) mediated mitochondrial dysfunction, inflammatory factors, neurotrophins and neurotrophicfactors (NTFs), early growth response transcription factor genes (Egr) and C-Fos expression are also involved as a pathophysiological consequences of SIS which induce neurological and psychiatric disorders.

Novel approaches to alcohol rehabilitation: Modification of stress-responsive brain regions through environmental enrichment

Relapse remains the most prominent hurdle to successful rehabilitation from alcoholism. The neural mechanisms underlying relapse are complex, but our understanding of the brain regions involved, the anatomical circuitry and the modulation of specific nuclei in the context of stress and cue-induced relapse have improved significantly in recent years. In particular, stress is now recognised as a significant trigger for relapse, adding to the well-established impact of chronic stress to escalate alcohol consumption. It is therefore unsurprising that the stress-responsive regions of the brain have also been implicated in alcohol relapse, such as the nucleus accumbens, amygdala and the hypothalamus. Environmental enrichment is a robust experimental paradigm which provides a non-pharmacological tool to alter stress response and, separately, alcohol-seeking behaviour and symptoms of withdrawal. In this review, we examine and consolidate the preclinical evidence that alcohol seeking behaviour and stress-induced relapse are modulated by environmental enrichment, and these are primarily mediated by modification of neural activity within the key nodes of the addiction circuitry. Finally, we discuss the limited clinical evidence that stress-reducing approaches such as mindfulness could potentially serve as adjunctive therapy in the treatment of alcoholism. This article is part of the Special Issue entitled "Neurobiology of Environmental Enrichment".