Effects of pair-housing after social defeat experience on elevated plus-maze behavior in rats

Paired Housing or a Socially-Paired Context Decreases Ethanol Conditioned Place Preference in Male Rats

Alcohol abuse dramatically affects individuals’ lives nationwide. The 2020 National Survey on Drug Use and Health (NSDUH) estimated that 10.2% of Americans suffer from alcohol use disorder. Although social support has been shown to aid in general addiction prevention and rehabilitation, the benefits of social support are not entirely understood. The present study sought to compare the benefits of social interaction on the conditioned ethanol approach behavior in rats through a conditioned place preference (CPP) paradigm in which a drug is paired with one of two distinct contexts. In experiment 1A, rats were single-housed and received conditioning trials in which ethanol was paired with the less preferred context. In experiment 1B, rats underwent procedures identical to experiment 1A, but were pair-housed throughout the paradigm. In experiment 1C, rats were single-housed, but concurrently conditioned to a socially-paired context and an ethanol-paired context. By comparing the time spent between the ethanol-paired environment and the saline-paired or socially-paired environment, we extrapolated the extent of ethanol approach behavior in the pair-housed, single-housed, and concurrently conditioned rats. Our results revealed that social interaction, both in pair-housed animals or concurrently socially-conditioned animals, diminished the ethanol approach behavior, which highlights the importance of social support in addiction prevention, treatment, and recovery programs.

Sex-specific behavioral, neurobiological, and cardiovascular responses to chronic social stress in mice

Psychosocial stress promotes and links mood and cardiovascular disorders in a sex-specific manner. However, findings in animal models are equivocal, in some cases opposing human dimorphisms. We examined central nervous system (CNS), behavioral, endocrine, cardiac, and hepatic outcomes in male or female C57Bl/6 mice subjected to chronic social stress (56 days of social isolation, with intermittent social confrontation encounters twice daily throughout the final 20 days). Females exhibited distinct physiological and behavioral changes, including relative weight loss, and increases in coronary resistance, hepatic inflammation, and thigmotaxic behavior in the open field. Males evidence reductions in coronary resistance and cardiac ischemic tolerance, with increased circulating and hippocampal monoamine levels and emerging anhedonia. Shared CNS gene responses include reduced hippocampal Maoa and increased Htr1b expression, while unique responses include repression of hypothalamic Ntrk1 and upregulation of cortical Nrf2 and Htr1b in females; and repression of hippocampal Drd1 and hypothalamic Gabra1 and Oprm in males. Declining cardiac stress resistance in males was associated with repression of cardiac leptin levels and metabolic, mitochondrial biogenesis, and anti-inflammatory gene expression. These integrated data reveal distinct biological responses to social stress in males and females, and collectively evidence greater biological disruption or allostatic load in females (consistent with propensities to stress-related mood and cardiovascular disorders in humans). Distinct stress biology, and molecular to organ responses, emphasize the importance of sex-specific mechanisms and potential approaches to stress-dependent disease.

Differential impact of two paradigms of early-life adversity on behavioural responses to social defeat in young adult rats and morphology of CA3 pyramidal neurons

Early life stress (ELS) is an important factor in programing the brain for future response to stress, and resilience or vulnerability to stress-induced emotional disorders. The hippocampal formation, with essential roles in both regulating the stress circuitry and emotionality, contributes to this adaptive programing. Here, we examined the effects of early handling (EH) and maternal deprivation (MD) as mild and intense postnatal stressors, respectively, on the behavioural responses to social defeat stress in young adulthood. We also evaluated the interaction of mild and intense ELS with later social defeat (SD) stress on the morphology and dendritic spine density of Golgi-cox-stained CA3 hippocampal neurons. SD stress in adult rats, as expected, increased anxiety and depressive-like behaviours in the open field, elevated plus-maze and forced swimming test. These effects were associated with reduction of dendritic spines and soma size of CA3 neurons. Both behavioural and structural alterations were significantly ameliorated in socially defeated rats that experienced early handling (EH-SD). Basal dendrites of CA3 neurons in EH-SD rats also showed longer dendrites and more intersections with Sholl circles in the distal portion, compared to both control and SD rats. On the other hand, in socially defeated rats with maternal deprivation experience (MD-SD) the stress-induced behavioural and structural alterations were generally intensified compared to SD rats. In MD-SD rats, apical dendrites of CA3 neurons demonstrated remarkable retraction; an effect that was not detected in SD rats. The reduction of dendritic spines density on the apical dendrites of CA3 neurons was also more pronounced in MD-SD rats compared to SD rats. Dendritic arbors and spines comprise the major neuronal substrate for the circuit connectivity, and cell region-specific alterations of dendrites and spines in CA3 neurons reveal plausible mechanisms that can underlie the impact of different ELSs on risk for affective disorders in response to social stress in adulthood.

Synergistic consequences of early-life social isolation and chronic stress impact coping and neural mechanisms underlying male prairie vole susceptibility and resilience

Chronic stress can be challenging, lead to maladaptive coping strategies, and cause negative mental and physical health outcomes. Early-life adversity exposes developing young to physical or psychological experiences that risks surpassing their capacity to effectively cope, thereby impacting their lifetime physical and mental wellbeing. Sensitivity to stressful events, like social isolation, has the potential to magnify stress-coping. Chronic stress through social defeat is an established paradigm that models adverse early-life experiences and can trigger enduring alterations in behavioral and neural phenotypes. To assess the degree to which stress resilience and sensitivity stemming from early-life chronic stress impact sociability, we exposed male prairie voles to chronic social defeat stress (CSDS) during adolescence. We simultaneously exposed subjects to either social isolation (CSDS+Isol) or group housing (CSDS+Soc) during this crucial time of development. On PND41, all subjects underwent a social approach test to examine the immediate impact of isolation, CSDS, or their combined effects on sociability. Unlike the CSDS+Isol group which primarily displayed social avoidance, the CSDS+Soc group was split by individuals exhibiting susceptible or resilient stress phenotypes. Notably, the Control+Soc and CSDS+Soc animals and their cage-mates significantly gained body weight between PND31 and PND40, whereas the Control+Isol and CSDS+Isol animals did not. These results suggest that the effects of early-life stress may be mitigated by having access to social support. Vasopressin, oxytocin, and opioids and their receptors (avpr1a, oxtr, oprk1, oprm1, and oprd1) are known to modulate social and stress-coping behaviors in the lateral septum (LS). Therefore, we did an mRNA expression analysis with RT-qPCR of the avpr1a, oxtr, oprk1, oprm1, and oprd1 genes to show that isolation and CSDS, or their collective influence, can potentially differentially bias sensitivity of the LS to early-life stressors. Collectively, our study supports the impact and dimensionality of early-life adversity because the type (isolation vs. CSDS), duration (acute vs. chronic), and combination (isolation + CSDS) of stressors can dynamically alter behavioral and neural outcomes.

Considering Organismal Physiology in Laboratory Studies of Rodent Behavior

Any experiment conducted in a rodent laboratory is done so against the backdrop of each animal's physiological state at the time of the experiment. This physiological state can be the product of multiple factors, both internal (e.g., animal sex, strain, hormone cycles, or circadian rhythms) and external (e.g., housing conditions, social status, and light/dark phases). Each of these factors has the potential to influence experimental outcomes, either independently or via interactions with others, and yet there is little consistency across laboratories in terms of the weight with which they are considered in experimental design. Such discrepancies-both in practice and in reporting-likely contribute to the perception of a reproducibility crisis in the field of behavioral neuroscience. In this review, we discuss how several of these sources of variability can impact outcomes within the realm of common learning and memory paradigms. Expected final online publication date for the Annual Review of Neuroscience, Volume 45 is July 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Unravelling the Neuroinflammatory Mechanisms Underlying the Effects of Social Defeat Stress on Use of Drugs of Abuse

The immune system provides the first line of the organism's defenses, working to maintain homeostasis against external threats and respond also to internal danger signals. There is much evidence to suggest that modifications of inflammatory parameters are related to vulnerability to develop mental illnesses, such as depression, autism, schizophrenia, and substance use disorders. In addition, not only are inflammatory parameters related to these disorders, but stress also induces the activation of the immune system, as recent preclinical research demonstrates. Social stress activates the immune response in the central nervous system through a number of mechanisms; for example, by promoting microglial stimulation, modifying peripheral and brain cytokine levels, and altering the blood brain barrier, which allows monocytes to traffic into the brain. In this chapter, we will first deal with the most important short- and long-term consequences of social defeat (SD) stress on the neuroinflammatory response. SD experiences (brief episodes of social confrontations during adolescence and adulthood) induce functional modifications in the brain, which are accompanied by an increase in proinflammatory markers. Most importantly, inflammatory mechanisms play a significant role in mediating the process of adaptation in the face of adversity (resilience vs susceptibility), allowing us to understand individual differences in stress responses. Secondly, we will address the role of the immune system in the vulnerability and enhanced sensitivity to drugs of abuse after social stress. We will explore in depth the effects seen in the inflammatory system in response to social stress and how they enhance the rewarding effects of drugs such as alcohol or cocaine. To conclude, we will consider pharmacological and environmental interventions that seek to influence the inflammatory response to social stress and diminish increased drug intake, as well as the translational potential and future directions of this exciting new field of research.

Social buffering in horses is influenced by context but not by the familiarity and habituation of a companion

Social buffering occurs when the presence of one animal attenuates another’s stress response during a stressful event and/or helps the subject to recover more quickly after a stressful event. Inconsistent previous results might reflect previously unrecognised contextual influences, such as the nature of the stimulus presented or social factors. We addressed these issues in a two-part study of horses paired with familiar (16 subjects) or unfamiliar (16 subjects) companions. Each subject performed 4 tests in a counterbalanced order: novel object test (static ball)—alone or with companion; and umbrella opening test—alone or with companion. Social buffering was significantly influenced by the nature of the stimulus presented, but not by companion’s habituation status or familiarity. Importantly, the stimulus used produced differential effects on behavioural and physiological measures of buffering. A companion significantly reduced behavioural response (reactivity) in the novel object test but not in the umbrella test. However, heart rate recovered more quickly for subjects with a companion in the umbrella test but not in the novel object test. We propose that circumstances which permit greater contextual processing may facilitate demonstration of behavioural effects of social buffering, whereas buffering in response to startling events may be manifest only during post-event physiological recovery.

The effect of traumatic-like stress exposure on alterations in the temporal social behavior of a rodent population

Though the relationship between traumatic stress and social behavior, which has been explored for years, is dynamic and largely estimated between dyads, little is known about the causal effects of traumatic stress exposure on the time-dependent dynamic alterations in the social behaviors on a large-group level. We thus investigated the effect of a single prolonged stress (SPS) exposure, a classical animal model that recapitulates posttraumatic stress disorder (PTSD)-like symptoms in rodents, on the spatiotemporal, social behavior changes within a large group of cohabiting rats. One-half of thirty-two Sprague-Dawley rats were assigned to the experimental group and subjected to SPS treatment administered two weeks after baseline social behavior recording; the other half served as the controls. Each group of rats (n = 16) was housed in one of two large custom-made cylinders. We used an automatic tracking system to record the behavioral indices of social behavior of the rats before SPS exposure, on the SPS exposure day, during a 7-day-long quiescent period after SPS treatment, as well as during subsequent behavioral test days. In addition to SPS-induced PTSD-like behaviors, SPS induced a time-dependent, oscillating change in active/passive social behaviors that lasted for 3 weeks. SPS treatment decreased active social behaviors (especially affiliative behaviors) but increased passive social behaviors (e.g. huddling) immediately following stress exposure. Increased active social interactions were observed during the early phase after SPS treatment; while increased passive social behaviors were observed during the late phase after SPS treatment. These dynamic changes were repeatedly observed when the rats underwent subsequent stressful behavioral tests and challenges. SPS induced a long-term, time-dependent oscillating change in indices of the social behavior. These changes may serve as an adaptive mechanism, and their manifestations critically depended on the time course following the traumatic stress exposure.

Sphingosine-1-phosphate receptor 3 in the medial prefrontal cortex promotes stress resilience by reducing inflammatory processes

Stress can promote the development of psychiatric disorders, though some individuals are more vulnerable to stress compared to others who are more resilient. Here we show that the sphingosine-1-phosphate receptor 3 (S1PR3) in the medial prefrontal cortex (mPFC) of rats regulates resilience to chronic social defeat stress. S1PR3 expression is elevated in the mPFC of resilient compared to vulnerable and control rats. Virally-mediated over-expression of S1PR3 in the mPFC produces a resilient phenotype whereas its knock-down produces a vulnerable phenotype, characterized by increased anxiety- and depressive-like behaviors, and these effects are mediated by TNFα. Furthermore, we show that S1PR3 mRNA in blood is reduced in veterans with PTSD compared to combat-exposed control subjects and its expression negatively correlates with symptom severity. Together, these data identify S1PR3 as a regulator of stress resilience and reveal sphingolipid receptors as important substrates of relevance to stress-related psychiatric disorders.

Social Housing Conditions Modulate the Long-Lasting Increase in Cocaine Reward Induced by Intermittent Social Defeat

Social defeat is considered the most representative animal model for studying the consequences of social stress. Intermittent social defeat (ISD) has proved to enhance the response to cocaine hedonic properties. In the present research, we evaluated if different social housing conditions, as housing with a familiar conspecific or with a female, exert a protective effect modulating the negative consequences of ISD as the increased sensitivity to cocaine and the induction of anxiety-like behavior. To achieve this objective, non-stressed or ISD OF1 male mice were divided into five different experimental groups according to their social environment: standard housing (four adult males per cage); male adolescent or adult in pairs (two males per cage); and adult males housed with a female for a short or long period (3 days vs. the whole duration of the study). Anxiety-like behavior was evaluated 19 days after the last episode of ISD using an elevated plus maze (EPM), and 24 h later the animals underwent a conditioned place preference paradigm (CPP) induced by a sub-threshold dose of cocaine (1 mg/kg). Following CPP, biological samples were taken to measure striatal levels of interleukin 6 (IL-6) and plasmatic levels of oxytocin (OT). Our results confirmed that ISD animals housed in standard condition displayed an anxious phenotype, developed CPP and had increased levels of IL-6 in the striatum. However, animals housed with a female or with a familiar male since adolescence did not develop CPP and were protected against the anxiogenic and neuroinflammatory potential of ISD stress. In the group of animals paired with a female throughout the experimental procedure, an increase in OT levels may have underlain this buffering effect, while the protective effect of being housed with a familiar male mouse seems to be related with a better resolution of the stress response. The present results expand our knowledge of the neurobiology of vulnerability to drug addiction and highlight the benefit of social support for recovery from the adverse effects of social stress.

Social defeat stress before pregnancy induces depressive-like behaviours and cognitive deficits in adult male offspring: correlation with neurobiological changes

BACKGROUND

Epidemiological surveys and studies with animal models have established a relationship between maternal stress and affective disorders in their offspring. However, whether maternal depression before pregnancy influences behaviour and related neurobiological mechanisms in the offspring has not been studied.

RESULTS

A social defeat stress (SDS) maternal rat model was established using the resident-intruder paradigm with female specific pathogen-free Wistar rats and evaluated with behavioural tests. SDS maternal rats showed a significant reduction in sucrose preference and locomotor and exploratory activities after 4 weeks of stress. In the third week of the experiment, a reduction in body weight gain was observed in SDS animals. Sucrose preference, open field, the elevated-plus maze, light-dark box, object recognition, the Morris water maze, and forced swimming tests were performed using the 2-month-old male offspring of the female SDS rats. Offspring subjected to pre-gestational SDS displayed enhanced anxiety-like behaviours, reduced exploratory behaviours, reduced sucrose preference, and atypical despair behaviours. With regard to cognition, the offspring showed significant impairments in the retention phase of the object recognition test, but no effect was observed in the acquisition phase. These animals also showed impairments in recognition memory, as the discrimination index in the Morris water maze test in this group was significantly lower for both 1 h and 24 h memory retention compared to controls. Corticosterone, adrenocorticotropic hormone, and monoamine neurotransmitters levels were determined using enzyme immunoassays or radioimmunoassays in plasma, hypothalamus, left hippocampus, and left prefrontal cortex samples from the offspring of the SDS rats. These markers of hypothalamic-pituitary-adrenal axis responsiveness and the monoaminergic system were significantly altered in pre-gestationally stressed offspring. Brain-derived neurotrophic factor (BDNF), cyclic adenosine monophosphate response element binding protein (CREB), phosphorylated CREB (pCREB), and serotonin transporter (SERT) protein levels were evaluated using western blotting with right hippocampus and right prefrontal cortex samples. Expression levels of BDNF, pCREB, and SERT in the offspring were also altered in the hippocampus and in the prefrontal cortex; however, there was no effect on CREB.

CONCLUSION

We conclude that SDS before pregnancy might induce depressive-like behaviours, cognitive deficits, and neurobiological alterations in the offspring.

Common and differential transcriptional responses to different models of traumatic stress exposure in rats

The effect of six different traumatic stress protocols on the transcriptome of the rat adrenal gland was examined using RNA sequencing. These protocols included chronic variable stress, chronic shock, social defeat and social isolation. The response of the transcriptome to stress suggested that there are genes that respond in a universal or stress modality-independent manner, as well as genes that respond in a stress modality-specific manner. Using a small number of the genes selected from the modality-independent set of stress-sensitive genes, a sensitive and robust measure of chronic stress exposure was developed. This stress-sensitive gene expression (SSGE) index could detect chronic traumatic stress exposure in a wide range of different stress models in a manner that was relatively independent of the modality of stress exposure and that paralleled the intensity of stress exposure in a dose-dependent manner. This measure could reliably distinguish control and stressed individuals in the case of animals exposed to the most intense stress protocols. The response of a subset of the modality-specific genes could also distinguish some types of stress exposure, based solely on changes in the pattern of gene expression. The results suggest that it is possible to develop diagnostic measures of traumatic stress exposure based solely on changes in the level of expression of a relatively small number of genes.

Repeated social defeat stress impairs attentional set shifting irrespective of social avoidance and increases female preference associated with heightened anxiety

Repeated social defeat stress (R-SDS) induces multiple behavioral changes in mice. However, the relationships between these behavioral changes were not fully understood. In the first experiment, to examine how the social avoidance is related to R-SDS-impaired behavioral flexibility, 10-week-old male C57BL/6N mice received R-SDS followed by the social interaction test and the attentional set shifting task. R-SDS impaired attentional set shifting irrespective of the development of social avoidance. In the second experiment, to examine whether R-SDS affects sexual preference and how this behavioral change is related to the social avoidance and R-SDS-heightened anxiety, another group of 10-week-old male C57BL/6N mice were subjected to R-SDS followed by the social interaction test, the female encounter test and the elevated plus maze test. The anxiety was heightened in the defeated mice without social avoidance, but not in those which showed social avoidance. Furthermore, female preference was increased specifically in the defeated mice which showed heightened anxiety, but was not related to the level of social avoidance. Together, these results showed that attentional set shifting is more sensitive to R-SDS than social interaction, and that female preference is affected by R-SDS in association with heightened anxiety rather than the social avoidance.

A Social Safety Net: Developing a Model of Social-Support Figures as Prepared Safety Stimuli

Although the presence of social-support figures (e.g., close friends and family members) is known to increase feelings of safety, reduce threat responses, and improve health, the route by which these effects occur is not well understood. One explanation is that social-support figures are members of a powerful category of safety signals—prepared safety stimuli. Here, we review research demonstrating that social-support figures act as prepared safety stimuli and explore the impact that these unique safety stimuli have on fear-learning processes. According to recent work, the presence of social-support figures both reduces fear acquisition and enhances fear extinction, ultimately decreasing perceptions of threat. These findings shed light on the route by which social support buffers against threat and illustrate the unique properties of prepared safety stimuli and how they might be used to improve mental and physical health outcomes.

An Improved Model of Physical and Emotional Social Defeat: Different Effects on Social Behavior and Body Weight of Adolescent Mice by Interaction With Social Support

Social stress is a prevalent etiological environmental factor that can affect health, especially during adolescence. Either experiencing or witnessing a traumatic event during adolescence can increase the risk of psychiatric disorders, such as PTSD. The present study attempted to establish an improved social stress model to better distinguish the effects of physical and emotional social stress on the behavior and physiology of adolescent mice. In addition, we investigated how social support affected these stress-induced changes in social behavior. On PND 28, male littermates were exposed to either physical stress (PS) or emotional stress (ES), afterwards, half of them were paired-housed and the others were singly housed. The PS exposed mice were directly confronted with a violent aggressor using the social defeat stress (SDS) paradigm for 15 min/trial (with the total of 10 trials randomly administered over a week), while the ES exposed mice were placed in a neighboring compartment to witness the PS procedure. Our results indicate that both stressors induced an effective stress response in adolescent mice, but PS and ES had differential influence in the context of relevant social anxiety/fear and social interaction with peers. Additionally, social support following stress exposure exerted beneficial effects on the social anxiety/fear in ES exposed mice, but not on PS exposed mice, suggesting that the type of stressor may affect the intervention efficacy of social support. These findings provide extensive evidence that physical and emotional stressors induce different effects. Moreover, ES exposed mice, rather than PS exposed mice, seemed to benefit from social support. In summary, the study suggests that this paradigm will be helpful in investigating the effects of psychological intervention for the treatment of stress-related psychiatric disorders.

Adversity in early adolescence promotes an enduring anxious phenotype and increases serotonergic innervation of the infralimbic medial prefrontal cortex

Stress during early development produces lasting effects on psychopathological outcomes. We analysed the impact of prior intermittent, physical stress (IPS) during early adolescence (PD 22-33) on anxiety-like behaviour of female rats in adulthood. After behavioural testing, we used immunohistochemistry for the 5-HT transporter (SERT) to evaluate 5-HT innervation profiles in the medial prefrontal cortex (mPFC) and ventral hippocampus (VH). Administration of IPS (i.e., water immersion, elevated platform, foot shock) in early adolescence increased rats' anxiety-like behaviour in the elevated plus-maze but had no effects in the shock-probe burying test. In the social interaction test, IPS decreased social interaction, and this effect was driven by selective decreases in the frequency of playfighting with no evident changes in contact and investigative behaviours. Selective stress-induced increases in the density of SERT-ir positive fibres were found in the infralimbic (IL) subregion of the mPFC but not in the cingulate or prelimbic (PL) subregions. IPS in early adolescence did not affect 5-HT innervation profiles in any sub-fields of the VH. Our findings confirm and extend on earlier evidence that stress during early adolescence promotes the emergence of an anxious phenotype and provide novel evidence that these effects are associated with increased 5-HT innervation of the IL mPFC.

Unpacking the buffering effect of social support figures: Social support attenuates fear acquisition

Social support is associated with positive health outcomes, and research has demonstrated that the presence, or even just a reminder, of a social-support figure can reduce psychological and physiological responses to threats. However, the mechanisms underlying this effect are unclear, and no previous work has examined the impact of social support on basic fear learning processes, which have implications for threat responding. This study examined whether social support inhibits the formation of fear associations. After conducting a fear-conditioning procedure in which social-support stimuli were paired with conditional stimuli during fear acquisition, we found that the threat of shock was not associated with conditional stimuli paired with images of social-support figures, but was associated with stimuli paired with images of strangers. These findings indicate that social support prevents the formation of fear associations, reducing the amount of learned fears people acquire as they navigate the world, consequently reducing threat-related stress.

Social Odors: Alarm Pheromones and Social Buffering

In this chapter, I describe 2 types of olfactory communication in rats, which appear to arouse anxiety and relief, respectively. In alarm pheromonal communication, rats release 4-methylpentanal and hexanal from their perianal region when they are stressed. These molecules activate the anxiety circuit, including the bed nucleus of the stria terminalis, when 4-methylpentanal and hexanal are simultaneously detected by the vomeronasal system and the main olfactory system, respectively. Consequently, recipient rats show a variety of anxiety responses, depending on the threatening stimuli. In appeasing olfactory communication, non-stressed rats release an appeasing olfactory signal, which is detected by the main olfactory system of other rats. When detected, this olfactory signal suppresses activation of the basolateral complex of the amygdala and, as a result, ameliorates stress responses elicited by an auditory conditioned stimulus during social buffering phenomenon. Because social buffering appears to be based on affinity and attachment to accompanying animals, the appeasing olfactory signal may arouse relief in rats. A definition of social buffering is also proposed as we still have no set definition for the term social buffering yet.

Social stress in adolescents induces depression and brain-region-specific modulation of the transcription factor MAX

MAX is a conserved constitutive small phosphoprotein from a network of transcription factors that are extensively studied in tumorigenesis and whose functions affect cell proliferation, differentiation and death. Inspired by its higher expression during development and in regions involved in emotional behaviors, we hypothesized its involvement in cerebral changes caused by early-life stress. We studied the effects of repeated social stress during adolescence on behaviors and on MAX and its putative partner MYC. Thirty-day-old C57BL/6 male mice underwent brief daily social defeat stress from an adult aggressor for 21 days. Following social stress episodes and housing in social groups after each defeat, adolescent mice exhibit depressive-like, but not anxiety-like behaviors and show higher MAX nuclear immunoreactivity in hippocampal (HC) but not prefrontal cortical (PFC) neurons. Conversely, MAX immunoreactivity is lower in the striatum (ST) of defeated adolescents. The positive correlation between MAX and MYC levels in the PFC revealed disruptions in both the HC and ST. The changes in MAX protein levels are not due to differential gene expression or protein degradation in those regions, suggesting that posttranscriptional modifications occurred. These findings indicate that repeated, brief social defeat in adolescent male mice, combined with group housing, is a useful protocol to study a subtype of depression that is dissociated from generalized (non-social) anxiety. To our knowledge, this is the first report of an association between dysregulation of the MAX-MYC network in the brain and a behavior, suggesting a novel approach for exploiting the neuroplasticity associated with depression.

Adolescent Social Defeat Induced Alterations in Social Behavior and Cognitive Flexibility in Adult Mice: Effects of Developmental Stage and Social Condition

Negative social experiences during adolescence increase the risk of psychiatric disorders in adulthood. Using "resident-intruder" stress, the present study aimed to investigate the effects of adolescent social defeat on emotional and cognitive symptoms associated with psychiatric disorders during adulthood and the effects of the developmental stage and social condition on this process. In Experiment 1, animals were exposed to social defeat or manipulation for 10 days during early adolescence (EA, postnatal days [PND] 28-37), late adolescence (LA, PND 38-47), and adulthood (ADULT, PND 70-79) and then singly housed until the behavioral tests. Behaviors, including social avoidance of the defeat context and cortically mediated cognitive flexibility in an attentional set-shifting task (AST), were assessed during the week following stress or after 6 weeks during adulthood. We determined that social defeat induced significant and continuous social avoidance across age groups at both time points. The mice that experienced social defeat during adulthood exhibited short-term impairments in reversal learning (RL) on the AST that dissipated after 6 weeks. In contrast, social defeat during EA but not LA induced a delayed deficit in extra-dimensional set-shifting (EDS) in adulthood but not during adolescence. In Experiment 2, we further examined the effects of social condition (isolation or social housing after stress) on the alterations induced by social defeat during EA in adult mice. The adult mice that had experienced stress during EA exhibited social avoidance similar to the avoidance identified in Experiment 1 regardless of the isolation or social housing after the stress. However, social housing after the stress ameliorated the cognitive flexibility deficits induced by early adolescent social defeat in the adult mice, and the social condition had no effect on cognitive function. These findings suggest that the effects of social defeat on emotion and cognitive function are differentially affected by the developmental stage and social condition. EA may comprise a particularly sensitive developmental period in which social defeat may produce a delayed impairment in cognitive flexibility during adulthood, and the social condition following stress appears to play an important intermediary role in the development of these cognitive deficits.

Middle-range exploratory activity in adult rats suggests higher resilience to chronic social defeat

OBJECTIVE

Stressful life events play an important role in the aetiology of human mood disorders and are frequently modelled by chronic social defeat (SD) in rodents. Exploratory phenotype in rats is a stable trait that is likely related to inter-individual differences in reactivity to stress. The aim of the study was to confirm that low levels of exploratory activity (LE) are, in rodents, a risk factor for passive stress coping, and to clarify the role of medium (ME) and high (HE) exploratory disposition in the sensitivity to SD.

METHODS

We examined the effect of SD on male Wistar rats with LE, ME, and HE activity levels as measured in the exploration box. After SD, the rats were evaluated in social preference, elevated zero maze, and open-field tests. Brain tissue levels of monoamines were measured by high-performance liquid chromatography.

RESULTS

Rats submitted to SD exhibited lower weight gain, higher sucrose consumption, showed larger stress-induced hyperthermia, lower levels of homovanillic acid in the frontal cortex, and higher levels of noradrenaline in the amygdala and hippocampus. Open-field, elevated zero maze, and social preference tests revealed the interaction between stress and phenotype, as only LE-rats were further inhibited by SD. ME-rats exhibited the least reactivity to stress in terms of changes in body weight, stress-induced hyperthermia, and sucrose intake.

CONCLUSION

Both low and high novelty-related activity, especially the former, are associated with elevated sensitivity to social stress. This study shows that both tails of a behavioural dimension can produce stress-related vulnerability.

Escalation of cocaine self-administration in adulthood after social defeat of adolescent rats: role of social experience and adaptive coping behavior

BACKGROUND

The link between adolescent social stress and substance abuse is modeled in social defeat of adolescent male rats, at an age when social experiences are essential for neurobehavioral maturation.

OBJECTIVE

We investigated the role of social experience and social defeat stress during adolescence on social behavior and cocaine self-administration (CocSelfAd) in early adulthood.

METHODS

We manipulated social experience by housing male rats in pairs (PH) or singly (SH) on postnatal day (P) 21. In addition, rats were subjected to social defeat from P35-44. Social behavior was measured during the first and last social defeat in PH and SH adolescents and PH adults. After assessing the behavioral response to novelty and cocaine (P57-61), intrajugular catheters were implanted and CocSelfAd was analyzed.

RESULTS

Residents were less aggressive toward PH adolescent intruders compared to PH adult intruders. Adults were submissive and defensive when attacked, whereas PH adolescents froze. In the course of repeated defeats, adolescent PH rats increased freezing, while SH rats decreased freezing. Longer attack-induced freezing after repeated defeats predicted escalated CocSelfAd in adulthood. PH controls acquired CocSelfAd more slowly than PH defeated and SH rats. Defeated PH rats increased CocSelfAd during progressive ratio schedules of reinforcement and during a 24-h continuous access binge compared to PH controls and SH defeated rats.

CONCLUSIONS

Social defeat in adolescence of PH rats caused persistent increases in adult CocSelfAd. Adolescent PH rats coped with attacks adaptively by increasing freezing behavior after repeated social defeats, a measure that predicted CocSelfAd in adulthood.

Stress, social behavior, and resilience: insights from rodents

The neurobiology of stress and the neurobiology of social behavior are deeply intertwined. The social environment interacts with stress on almost every front: social interactions can be potent stressors; they can buffer the response to an external stressor; and social behavior often changes in response to stressful life experience. This review explores mechanistic and behavioral links between stress, anxiety, resilience, and social behavior in rodents, with particular attention to different social contexts. We consider variation between several different rodent species and make connections to research on humans and non-human primates.

An updated animal model capturing both the cognitive and emotional features of post-traumatic stress disorder (PTSD)

The new-released Diagnostic and Statistical Manual of Mental Disorders (DSM-5) defines post-traumatic stress disorder (PTSD) as a "trauma and stressor-related disorder". PTSD pathogenesis relies on paradoxical changes of emotional memory processing induced by the trauma exposure and associated with emotional dysfunction. Several animal models of PTSD have been validated and are currently used. Each one mimics a particular subset of the disorder with particular emphasis, mainly driven by the past classification of PTSD in the DSM-4, on the emotional features. In view of the recent update in the DSM-5, our aim was to develop, by using well-validated paradigms, a modified model of PTSD able to mimic at the same time both the cognitive and emotional features of the disease. We exposed male rats to either a piece of worn cat collar or to a series of inescapable footshocks paired with a PTSD risk factor, i.e., social isolation. Animals were subsequently re-exposed to the conditioned contexts at different time intervals in order to test memory retention for the stressors. In addition, footshock-exposed rats were tested in the elevated-plus-maze and social interaction tests. We found that rats exposed to a cat collar exhibited an acute fear response that did not lead to enduring memory retention. Conversely, footshock-exposed rats expressed a successful retention of the stressful experience at 1, 7, 14, 21 and 56 post-exposure days. Footshock-exposed rats displayed an anxious behavioral profile in the social interaction test and a significantly reduced locomotor activity in the elevated-plus-maze test. These dysfunctions were not observed when animals were socially housed, thus highlighting a social buffering effect in the development of the pathology. Our results underline the good validity of a footshock-based paradigm paired with social isolation as a PTSD animal model, able to mimic at the same time both some of the enduring cognitive and emotional facets of the pathology.

The effects of social housing on extinction of fear conditioning in rapid eye movement sleep-deprived rats

Both human and animal research indicate that rapid eye movement sleep (REM) plays an important role in the processing of emotional information. REM is altered after fear conditioning in rats, but this alteration can be mitigated by exposure to a naïve conspecific. In addition, both the housing condition (isolated vs paired) and the experiences of rats' cagemates can influence the response to aversive events. Based on this prior work, the present study sought to determine the effects of social housing on the previously demonstrated impairment in the extinction of conditioned fear responses produced by REM deprivation. Rats were assigned to one of three housing conditions: housed with a naïve rat, housed with another fear-conditioned rat, or housed alone. The results demonstrated that rats housed with either a naïve or a fear-conditioned conspecific exhibited an impairment in the acquisition of extinction as a consequence of REM deprivation, as observed in previous studies. However, rats in the isolated condition demonstrated a trend toward an impairment only after continued extinction training. These results indicate that the effects of social housing on REM deprivation-induced impairments in learning and memory are subtle, but may explain some conflicting findings in the literature.

Physical interaction is not necessary for the induction of housing-type social buffering of conditioned hyperthermia in male rats

In social animals, housing with conspecific animals after a stressful event attenuates the subsequent adverse outcomes due to the event, and this has been called housing-type social buffering. We have previously found that housing-type social buffering attenuates the enhancement of hyperthermia and Fos expression in the paraventricular nucleus of the hypothalamus that occurs in response to an aversive conditioned stimulus in male rats. Here, we analyzed the role of physical interactions during social housing in the induction of housing-type social buffering. When a fear-conditioned subject was alone after the conditioning and then exposed to the conditioned stimulus, it showed behavioral, autonomic, and neural stress responses. However, social housing, during which physical interactions were prevented by wire mesh, attenuated these autonomic and neural stress responses, as has been seen in previous studies. These results suggested that physical interaction was not necessary for the induction of housing-type social buffering. With this social cohabitation model, we then found that social cohabitation increased Fos expression in the posterior complex of the anterior olfactory nucleus of the fear-conditioned subject. Social cohabitation also increased Fos expression in 11 brain regions, including the prefrontal cortex, the nucleus accumbens, the bed nucleus of the stria terminalis, and the medial, lateral, basal, and cortical amygdala. These results provide information about the neural mechanisms that induce housing-type social buffering.

Copulatory and agonistic behavior in Syrian hamsters following social defeat

Syrian hamsters are highly aggressive animals that reliably defend their home territory. After social defeat, however, hamsters no longer defend their home cage but instead display submissive and defensive behavior toward an intruder, a response that we have termed conditioned defeat. Plasma testosterone is significantly reduced in Syrian hamsters following repeated defeat suggesting that social defeat might also impair copulatory behavior. The present study aimed to determine whether copulatory behavior in male Syrian hamsters is suppressed following repeated social defeats and additionally whether exposure to a hormone-primed stimulus female after social defeat reduces the behavioral response to defeat. Hamsters were paired with an aggressive opponent for one or nine defeats using a resident-intruder model, while controls were placed into the empty cage of a resident aggressor. On the day after the last treatment, half of the hamsters were paired with a receptive female for 10 min. There were no significant differences in the copulatory behavior of defeated versus non-defeated hamsters, and the opportunity to copulate had no effect on subsequent conditioned defeat testing, as defeated animals displayed significantly more submissive behavior than did non-defeated animals. The current data suggest that conditioned defeat is not necessarily a maladaptive response to social stress, at least in terms of reproductive behavior, but may instead represent a viable behavioral strategy adopted by losing animals following social defeat. Further, these data indicate that conditioned defeat is relatively persistent and stable, as the opportunity to copulate does not reduce the subsequent display of submissive behavior.

Adolescent social stress does not necessarily lead to a compromised adaptive capacity during adulthood: a study on the consequences of social stress in rats

Childhood bullying or social stress in adolescent humans is generally considered to increase the risk of developing behavioral disorders like depression in adulthood. Juveniles are hypothesized to be particularly sensitive to stressors in their environment due to the relatively late maturation of brain areas that are targeted by stress such as the prefrontal cortex and hippocampus. In our study male adolescent rats were subjected to repeated social defeat on postnatal day (PND) 28, 31 and 34 (experiment 1) or to daily social defeats between PND 35 and 42 (experiment 2). Adolescent rats in experiment 1 were socially housed in pairs with a male of similar age during and after the social defeat. In experiment 2 adolescents were housed either alone or with an age-mate for 7 days (PND 35-42) next to either a highly aggressive or a non-aggressive adult male neighbor with whom a repeated physical interaction was allowed. In experiment 1 the adolescent defeats affected subsequent play behavior with the cage mate. Socially stressed rats more frequently initiated play behavior but also adopted more frequently submissive postures during the play fights. As adults, they seemed to cope behaviorally and physiologically better with a similar exposure to a residential aggressive male rat than unstressed controls. In experiment 2 acute effects of adolescent social stress were studied on neuroplasticity markers like hippocampal cell proliferation and neurogenesis as well as hippocampal brain-derived neurotrophic factor (BDNF) levels. The 2nd experiment also studied long-term effects of the adolescent stress in the response to an adult social defeat. A few acute but minor changes in brain plasticity markers and behavior were observed but these were transient and no behavioral or physiological effects persisted into adulthood. The results from both experiments support the theory developed in the so-called "match-mismatch hypothesis" which claims that the final consequence of childhood adversity depends on how well the early life environment matches the challenges in later life. Socially stressed adolescents are rather resilient to the lasting behavioral and physiological effects of the stress exposure if they are socially housed afterward and have the ability to recover.

Socially isolated mice exhibit a blunted homeostatic sleep response to acute sleep deprivation compared to socially paired mice

Sleep is an important physiological process underlying maintenance of physical, mental and emotional health. Consequently, sleep deprivation (SD) is associated with adverse consequences and increases the risk for anxiety, immune, and cognitive disorders. SD is characterized by increased energy expenditure responses and sleep rebound upon recovery that are regulated by homeostatic processes, which in turn are influenced by stress. Since all previous studies on SD were conducted in a setting of social isolation, the impact of the social contextual setting is unknown. Therefore, we used a relatively stress-free SD paradigm in mice to assess the impact of social isolation on sleep, wakefulness and delta electroencephalogram (EEG) power during non-rapid eye movement (NREM) sleep. Paired or isolated C57BL/6J adult chronically-implanted male mice were exposed to SD for 6h and telemetric polygraphic recordings were conducted, including 18 h recovery. Recovery from SD in the paired group showed a significant decrease in wake and significant increase in NREM sleep and rapid eye movement (REM), and a similar, albeit less robust response occurred in the isolated mice. Delta power during NREM sleep was increased in both groups immediately following SD, but paired mice exhibited significantly higher delta power throughout the dark period. The increase in body temperature and gross motor activity observed during the SD procedure was decreased during the dark period. In both open field and elevated plus maze tests, socially isolated mice showed significantly higher anxiety than paired mice. The homeostatic processes altered by SD are differentially affected in paired and isolated mice, suggesting that the social context of isolation stress may adversely affect the quantity and quality of sleep in mice.

Behavioral effects of chronic adolescent stress are sustained and sexually dimorphic

Evidence suggests that women are more susceptible to stress-related disorders than men. Animal studies demonstrate a similar female sensitivity to stress and have been used to examine the underlying neurobiology of sex-specific effects of stress. Although our understanding of the sex-specific effects of chronic adolescent stress has grown in recent years, few studies have reported the effects of adolescent stress on depressive-like behavior. The purpose of this study was to determine if a chronic mixed modality stressor (consisting of isolation, restraint, and social defeat) during adolescence (PND 37-49) resulted in differential and sustained changes in depressive-like behavior in male and female Wistar rats. Female rats exposed to chronic adolescent stress displayed decreased sucrose consumption, hyperactivity in the elevated plus maze, decreased activity in the forced swim test, and a blunted corticosterone response to an acute forced swim stress compared to controls during both adolescence (PND 48-57) and adulthood (PND 96-104). Male rats exposed to chronic adolescent stress did not manifest significant behavioral changes at either the end of adolescence or in adulthood. These data support the proposition that adolescence may be a stress sensitive period for females and exposure to stress during adolescence results in behavioral effects that persist in females. Studies investigating the sex-specific effects of chronic adolescent stress may lead to a better understanding of the sexually dimorphic incidence of depressive and anxiety disorders in humans and ultimately improve prevention and treatment strategies.

The effects of enriching laboratory cages using various physical structures on multiple measures of welfare in singly-housed rats

The single housing of laboratory rats may be recommended in some situations such as hypothesis-driven or test-specific studies, during electroencephalogram recording of phases of sleep and after surgical procedures. However, as single housing of laboratory rats has been shown to be stressful, modification of the housing environment is needed to improve the welfare of these animals. This experiment was carried out to investigate the long-term effects of environmental enrichment on some behavioural, physiological, pathological and psychological measures of welfare. With two batches of animals, 24 rats were housed singly in either enriched cages (EC) (n = 12 cages) or unenriched cages (UC) (n = 12 cages). Behaviour was sampled every week and so was body weight and weight gain over a six-week observation period. Behaviours of the rats in the elevated plus-maze were recorded on the seventh week, whereas organ weights were recorded postmortem. The results revealed that long-term single housing of rats in super-enriched cages increased levels of indicators of good welfare including sleep, exploration, movement and feeding behaviour, body weights, weight gains and the relative weights of the thymus gland and spleen, and decreased levels of indicators of poor welfare such as stationary behaviour and the relative weight of adrenal glands. Thus, enrichment of conventional cages of newly weaned singly-housed laboratory rats with multiple physical structures appeared to improve their ability to control the environment and to promote their species-specific behaviour; changes that can ultimately result in good welfare.

Sharing stressful experiences attenuates anxiety-related cognitive and sleep impairments

Anxiety is a growing public health concern that has been shown to impair both sleep and learning, and these associations have been extensively studied in recent years. In the rodent model, oftentimes various foot-shock paradigms are employed to induce stress, and subsequent sleep recordings and/or learning task results are analyzed. Previous studies have focused primarily on an individual animal's response to stress following individual stressor exposure, thereby emulating only an isolated condition. The goal of this study was to investigate the effects of socialization on stress response, and the resultant effects on sleep architecture and aversive learning. A pair-housing/pair-exposure paradigm was utilized, and the effects of unavoidable foot-shock-induced stress on sleep architecture and aversive learning were examined. The results of the present study indicate a large, positive impact of cohabitation and shared stressful experience, as rats failed to develop sleep disturbances or learning deficits. While these results indicate the benefits and importance of companionship, the underlying mechanism of this phenomenon is yet to be elucidated.

Early adolescence as a critical window during which social stress distinctly alters behavior and brain norepinephrine activity

Many neural programs that shape behavior become established during adolescence. Adverse events at this age can have enduring consequences for both adolescent and adult mental health. Here we show that repeated social stress at different stages of adolescent development differentially affects rat behavior and neuronal activity. Early-adolescent (PND 28, EA), mid-adolescent (PND 42, MA), and adult (PND 63) rats were subjected to resident-intruder social stress (7 days) and behavior was examined 24-72 h later. In EA rats selectively, resident-intruder stress increased proactive responses in the defensive burying and forced swim tests. In adult rats, resident-intruder stress decreased burying behavior regardless of whether the animal was stressed as an adult or during early adolescence. As the locus coeruleus (LC)-norepinephrine system has been implicated in proactive defense behaviors, LC neuronal activity was quantified in separate cohorts. Stressed EA rats had elevated spontaneous LC discharge rates and diminished responses to sensory stimuli compared with controls. Microinjection of a CRF antagonist into the LC selectively inhibited neurons of stressed EA rats, suggesting that EA social stress induces tonic CRF release onto LC neurons, shifting the mode of discharge to an activated state that promotes active defensive behaviors. In all adult groups, resident-intruder stress resulted in an increased phasic response to sensory stimuli with no change in spontaneous rates. MA was a transition period during which social stress did not affect behavior or LC activity. The results suggest that social stress interacts with the brain norepinephrine system to regulate defensive strategies in an age-dependent manner.

Individual differences in novelty-seeking behavior in rats as a model for psychosocial stress-related mood disorders

Most neuropsychiatric disorders, including stress-related mood disorders, are complex multi-parametric syndromes. Diagnoses are therefore hard to establish and current therapeutic strategies suffer from significant variability in effectiveness, making the understanding of inter-individual variations crucial to unveiling effective new treatments. In rats, such individual differences are observed during exposure to a novel environment, where individuals will exhibit either high or low locomotor activity and can thus be separated into high (HR) and low (LR) responders, respectively. In rodents, a long-lasting, psychosocial, stress-induced depressive state can be triggered by exposure to a social defeat procedure. We therefore analyzed the respective vulnerabilities of HR and LR animals to long-lasting, social defeat-induced behavioral alterations relevant to mood disorders. Two weeks after four daily consecutive social defeat exposures, HR animals exhibit higher anxiety levels, reduced body weight gain, sucrose preference, and a marked social avoidance. LR animals, however, remain unaffected. Moreover, while repeated social defeat exposure induces long-lasting contextual fear memory in both HR and LR animals, only HR individuals exhibit marked freezing behavior four weeks after a single social defeat. Combined, these findings highlight the critical involvement of inter-individual variations in novelty-seeking behavior in the vulnerability to stress-related mood disorders, and uncover a promising model for posttraumatic stress disorder.

Is full physical contact necessary for buffering effects of pair housing on social stress in rats?

Our previous study showed that pair housing with a familiar male prevented an increase in anxiety caused by social defeat in male rats. The present study attempted to identify the aspects of social interactions with a familiar male that are needed for the emergence of such a pair-housing effect. In Experiment 1, the subject was repeatedly exposed to the cage and bedding used by a familiar pairmate, after two instances of social defeat. Mere exposure to the soiled cage and bedding did not prevent an increase in anxiety in the elevated plus-maze test performed two weeks after social defeat. In Experiment 2, the subject was separated from a familiar pairmate with a wire mesh partition, which allowed visual, auditory, and limited physical contact, in addition to olfactory contact with the pairmate. The separation with a wire mesh partition abolished the buffering effect of pair housing on anxiety. These results indicate that visual, auditory, and olfactory contact with a familiar male was not sufficient in reducing the anxiogenic effect of social defeat in male rats. It was suggested that full physical contact is necessary for the emergence of the buffering effect of pair housing on social stress.

The context specificity of anxiety responses induced by chronic psychosocial stress in rats: a shift from anxiety to social phobia?

The aim of the present study was to evaluate whether the anxiety-increasing effects of chronic psychosocial stress generalize to non-social (i.e. heterotypic) stressful situations. To investigate this issue, we repeatedly exposed rats to predictable or unpredictable psychosocial stress for 5 or 12 days and examined their anxiety in two markedly different contexts: the elevated plus maze and social interaction tests. Psychosocial stress and the social interaction test were administered under highly similar conditions, i.e. the two situations were homotypic. Psychosocial stress did not affect anxiety in the elevated plus-maze under any condition, but markedly increased anxiety in the social interaction test. In contrast, repeated restraint-a non-social stressor heterotypic to both the elevated plus maze and social interaction tests-increased plus-maze anxiety, demonstrating that anxiety in this test was sensitive to repeated restraint, and the effects were manifested in heterotypic situations. Thus, the anxiety-related effects of chronic psychosocial stress-unlike those of the chronic non-social stressor-were context-dependent. This is reminiscent of phobic anxiety, which manifests in specific situations only. In addition, behavior in the social interaction test showed changes that went beyond simple anxiogenesis. Socially stressed rats spent nearly 40% of total time in aggressive interactions. Based on recent data showing that social phobics are prone to violence under social pressure, and also based on the situation-dependent effects of the social stressor, we suggest that chronic psychosocial stress leads to a behavioral profile akin to social phobia.