Evaluating social defeat as a model for psychopathology in adult female rodents

Activity-Dependent Synaptic Plasticity in the Medial Prefrontal Cortex of Male Rats Underlies Resilience-Related Behaviors to Social Adversity

Individuals considered resilient can overcome adversity, achieving normal physical and psychological development, while those deemed vulnerable may not. Adversity promotes structural and functional alterations in the medial prefrontal cortex (mPFC) and hippocampus. Moreover, activity-dependent synaptic plasticity is intricately linked to neuronal shaping resulting from experiences. We hypothesize that this plasticity plays a crucial role in resilience processes. However, there is a notable absence of studies investigating this plasticity and behavioral changes following social adversity at different life stages. Consequently, we evaluated the impact of social adversity during early postnatal development (maternal separation [MS]), adulthood (social defeat [SD]), and a combined exposure (MS + SD) on behavioral outcomes (anxiety, motivation, anhedonia, and social interaction). We also examined cFos expression induced by social interaction in mPFC and hippocampus of adult male rats. Behavioral analyses revealed that SD-induced anhedonia, whereas MS + SD increased social interaction and mitigated SD-induced anhedonia. cFos evaluation showed that social interaction heightened plasticity in the prelimbic (PrL) and infralimbic (IL) cortices, dentate gyrus (DG), CA3, and CA1. Social interaction-associated plasticity was compromised in IL and PrL cortices of the MS and SD groups. Interestingly, social interaction-induced plasticity was restored in the MS + SD group. Furthermore, plasticity was impaired in DG by all social stressors, and in CA3 was impaired by SD. Our findings suggest in male rats (i) two adverse social experiences during development foster resilience; (ii) activity-dependent plasticity in the mPFC is a foundation for resilience to social adversity; (iii) plasticity in DG is highly susceptible to social adversity.

The rostral ventromedial medulla modulates pain and depression-related behaviors caused by social stress

ABSTRACT

The rostral ventromedial medulla (RVM) is a crucial structure in the descending pain modulatory system, playing a key role as a relay for both the facilitation and inhibition of pain. The chronic social defeat stress (CSDS) model has been widely used to study stress-induced behavioral impairments associated with depression in rodents. Several studies suggest that CSDS also causes changes related to chronic pain. In this study, we aimed to investigate the involvement of the RVM in CSDS-induced behavioral impairments, including those associated with chronic pain. We used chemogenetics to activate or inhibit the RVM during stress. The results indicated that the RVM is a vital hub influencing stress outcomes. Rostral ventromedial medulla activation during CSDS ameliorates all the stress outcomes, including social avoidance, allodynia, hyperalgesia, anhedonia, and behavioral despair. In addition, RVM inhibition in animals exposed to a subthreshold social defeat stress protocol induces a susceptible phenotype, facilitating all stress outcomes. Finally, chronic RVM inhibition-without any social stress stimulus-induces chronic pain but not depressive-like behaviors. Our findings provide insights into the comorbidity between chronic pain and depression by indicating the involvement of the RVM in establishing social stress-induced behavioral responses associated with both chronic pain and depression.

The cycle of stress: A systematic review of the impact of chronic psychological stress models on the rodent estrous cycle

Stress is known to impair reproduction through interactions between the hypothalamic-pituitary-adrenal (HPA) and hypothalamic-pituitary-gonadal (HPG) axes. However, while it is well accepted that stress can alter estrous cycle regularity, a key indicator of female's HPG axis function, effects of different types of psychological stress have been inconsistent. This systematic review evaluated the impact of rodent models of psychological stress on estrous cyclicity, while reporting biological parameters pertaining to HPA or HPG axis function assessed within these studies. We performed a systematic database search and included articles that implemented a psychological stress model in rodents and reported estrous cyclicity for at least two cycles after initiation of stress. Of the 32 studies included, 62.5% reported post-stress alterations to estrous cyclicity, with Chronic Mild Stress (CMS) models showing the most conclusive effects. Twenty-five studies measured HPG or HPA axis markers, with cycle disruptions being commonly observed in parallel with altered estradiol and increased corticosterone levels. Our review highlights gaps in reporting estrous cyclicity assessments and makes recommendations to improve comparability between studies.

The prelimbic prefrontal cortex mediates the development of lasting social phobia as a consequence of social threat conditioning

Social phobia is highly detrimental for social behavior, mental health, and productivity. Despite much previous research, the behavioral and neurobiological mechanisms associated with the development of social phobia remain elusive. To investigate these issues, the present study implemented a mouse model of social threat conditioning in which mice received electric shock punishment upon interactions with unfamiliar conspecifics. This resulted in immediate reductions in social behavior and robust increases in defensive mechanisms such as avoidance, freezing, darting, and ambivalent stretched posture. Furthermore, social deficits lasted for prolonged periods and were independent of contextual settings, sex variables, or particular identity of the social stimuli. Shedding new light into the neurobiological factors contributing to this phenomenon, we found that optogenetic silencing of the prelimbic (PL), but not the infralimbic (IL), subregion of the medial prefrontal cortex (mPFC) during training led to subsequent forgetting and development of lasting social phobia. Similarly, pharmacological inhibition of NMDARs in PL also impaired the development of social phobia. These findings are consistent with the notion that social-related trauma is a prominent risk factor for the development of social phobia, and that this phenomenon engages learning-related mechanisms within the prelimbic prefrontal cortex to promote prolonged representations of social threat.

Abstract Figure

Chronic non-discriminatory social defeat stress during the perinatal period induces depressive-like outcomes in female mice

Depression is more prevalent in women than in men. Perinatal stress is one of the main risk factors for depression in women. However, there is no suitable female model for perinatal depression that uses the social defeat stress (SDS) paradigm. The standard chronic SDS protocol, which is the most useful method for developing a depression-like model, is effective only in male mice. Thus, this study aimed to characterize a novel SDS method for producing a perinatal depression-like model mouse. We induced chronic SDS in perinatal female mice, wherein chronic non-discriminatory SDS (ND-SDS) was used to induce substantial stress in female mice. The female mice were placed in aggressive ICR mouse cages with sentinel male mice under ND-SDS conditions. Stressed female mice subjected to ND-SDS during the perinatal period efficiently exhibited stress-susceptible phenotypes, such as a social avoidance phenotype and anhedonic behavior, whereas stressed female mice subjected to SDS did not show depressive-like behaviors. These results indicate that chronic ND-SDS in perinatal females could be used to develop a female perinatal depression-like model that can be used to study women's health.

Sex differences in dominance relationships in Syrian hamsters

Dominance relationships are identified by changes in agonistic behavior toward specific individuals. While there are considerable individual and species differences in dominance relationships, sex differences are poorly understood in rodent models because aggression among female rodents is rare. The aim of this study was to characterize sex differences in the formation and maintenance of dominance relationships in same-sex pairs of male and female Syrian hamsters. We pooled data from multiple projects in our lab to evaluate dominance interactions in 68 male dyads and 88 female dyads. In each project, animals were matched with a partner similar in age, sex, and estrous cycle and we exposed animals to daily social encounters for two weeks in a resident-intruder format. We found that female hamsters were quicker to attack and attacked at higher rates compared to males regardless of dominance status. In addition, resident female hamsters were quicker to attack and attacked at higher rates than intruder females, but aggression in males did not depend on residency status. Female subordinates were quicker to submit and fled at higher rates from their dominant counterparts compared to male subordinates. Intruder subordinate females were quicker to submit and fled at higher rates than resident subordinate females. Females were also more resistant than males to becoming subordinate in that they fought back more consistently and were more likely to reverse their dominance status. These findings indicate that dominance relationships are less stable in females compared to males and that residency status has a larger impact on agonistic behavior in females than males. Overall, differences in how males and females display territorial aggression can lead to sex differences in the establishment and maintenance of dominance relationships.

Social stress under binge-like alcohol withdrawal in adolescence: evidence of cannabidiol effect on maladaptive plasticity in rats

BACKGROUND

Alcohol binge drinking may compromise the functioning of the nucleus accumbens (NAc), i.e. the neural hub for processing reward and aversive responses.

METHODS

As socially stressful events pose particular challenges at developmental stages, this research applied the resident-intruder paradigm as a model of social stress, to highlight behavioural neuroendocrine and molecular maladaptive plasticity in rats at withdrawal from binge-like alcohol exposure in adolescence. In search of a rescue agent, cannabidiol (CBD) was selected due to its favourable effects on alcohol- and stress-related harms.

RESULTS

Binge-like alcohol exposed intruder rats displayed a compromised defensive behaviour against the resident and a blunted response of the stress system, in addition to indexes of abnormal dopamine (DA)/glutamate plasticity and dysfunctional spine dynamics in the NAc. CBD administration (60 mg/kg) was able to: (1) increase social exploration in the binge-like alcohol exposed intruder rats, at the expenses of freezing time, and in control rats, which received less aggressive attacks from the resident; (2) reduce corticosterone levels independently on alcohol previous exposure; (3) restore DA transmission and (4) facilitate excitatory postsynaptic strength and remodelling.

CONCLUSIONS

Overall, the maladaptive behavioural and synaptic plasticity promoted by the intersection between binge-like alcohol withdrawal and exposure to adverse social stress can be rescued by a CBD détente effect that results in a successful defensive strategy, supported by a functional endocrine and synaptic plasticity. The current data highlight CBD's relevant therapeutic potential in alcohol- and stress-related harms, and prompt further investigation on its molecular targets.

Chronic social defeat alters behaviors and neuronal activation in the brain of female Mongolian gerbils

Chronic social defeat has been found to be stressful and to affect many aspects of the brain and behaviors in males. However, relatively little is known about its effects on females. In the present study, we examined the effects of repeated social defeat on social approach and anxiety-like behaviors as well as the neuronal activation in the brain of sexually naïve female Mongolian gerbils (Meriones unguiculatus). Our data indicate that repeated social defeats for 20 days reduced social approach and social investigation, but increased risk assessment or vigilance to an unfamiliar conspecific. Such social defeat experience also increased anxiety-like behavior and reduced locomotor activity. Using ΔFosB-immunoreactive (ΔFosB-ir) staining as a marker of neuronal activation in the brain, we found significant elevations by social defeat experience in the density of ΔFosB-ir stained neurons in several brain regions, including the prelimbic (PL) and infralimbic (IL) subnuclei of the prefrontal cortex (PFC), CA1 subfields (CA1) of the hippocampus, central subnuclei of the amygdala (CeA), the paraventricular nucleus (PVN), dorsomedial nucleus (DMH), and ventrolateral subdivision of the ventromedial nucleus (VMHvl) of the hypothalamus. As these brain regions have been implicated in social behaviors and stress responses, our data suggest that the specific patterns of neuronal activation in the brain may relate to the altered social and anxiety-like behaviors following chronic social defeat in female Mongolian gerbils.

Adolescent binge-like alcohol exposure dysregulates NPY and CGRP in rats: Behavioural and immunochemical evidence

Alcohol binge drinking during adolescence impacts affective behaviour, possibly impinging on developing neural substrates processing affective states, including calcitonin gene-related peptide (CGRP) and neuropeptide Y (NPY). Here, we modelled binge-like alcohol exposure in adolescence, by administering 3.5 g/kg alcohol per os, within 1 h, to male adolescent rats every other day, from postnatal day 35 to 54. The effects on positive and negative affective behaviour during abstinence were explored including: consummatory behaviour and weight gain; social behaviour in the modified social interaction test; thermal nociception in the tail-flick test; psychosocial stress coping in the resident-intruder paradigm. Moreover, CGRP and NPY levels were evaluated in functionally relevant brain regions. Our data shows that binge-like intermittent alcohol administration during adolescence decreased weight gain, social preference and motivation, nociception, and active psychosocial stress coping during abstinence. In addition, intermittent alcohol-exposed rats displayed increased expression of CGRP and NPY in the prefrontal cortex and nucleus accumbens; decreased NPY levels in the amygdala; opposite changes in CGRP levels in the hypothalamus and brainstem. Overall, our data shows that adolescent binge-like alcohol exposure, through the allostatic load of alternate intoxication and withdrawal, produces long-term consequences in sensory and affective processes and dysregulated complementary neuropeptidergic systems. Thus, neuropeptide-targeted interventions hold promising potential for addressing negative affect during prolonged withdrawal in young subjects.

Vicarious Social Defeat Increases Conditioned Rewarding Effects of Cocaine and Ethanol Intake in Female Mice

Stress is a critical factor in the development of mood and drug use disorders. The social defeat model is not appropriate for female rodents due to their low level of aggression. Therefore, a robust female model of social stress needs to be developed and validated. The aim of the present study was to unravel the long-lasting effects of vicarious social defeat (VSD) on the conditioned rewarding effects of cocaine and ethanol intake in female mice. Although VSD seems to be a good model for inducing behavioral and physiologic endophenotypes induced by stress, there are no studies to date that characterize the effect of VSD on cocaine or alcohol use. The results confirm that VSD females showed an increase in corticosterone levels after a vicarious experience while also displaying an increase in anxiety- and anhedonic-like behaviors. Three weeks after the last VSD, vicariously defeated female mice showed an increased developed preference for a non-effective dose of cocaine in the conditioned place preference (CPP) paradigm and showed an increase in ethanol intake. Our results suggest that female mice vicariously experience a state of distress through the social observation of others suffering from adverse events, confirming the use of VSD as a valid model to study the response to social stress in females. The fact that VSD in females induced a comparable behavioral phenotype to that observed in physically defeated males could indicate a relationship with the higher rate of psychopathologies observed in women. Notwithstanding, more studies are needed to dissect the neurobiological and behavioral peculiarities of the female response to social stress.

Are cognitive aspects of defense a core feature of anxiety and depression?

Anxiety and depression are highly prevalent behavior disorders, particularly in women. Recent preclinical work using animal models has been suboptimal in predicting the efficacy of drugs targeted at these conditions, suggesting a potential discrepancy between such models and the human disorders. Notably female animals tend to be equal to, or less responsive than, males in these tasks. A number of analyses suggest that mammalian defense patterns are complex: In addition to relatively discrete and immediate fight, flight, and freezing responses, a risk assessment pattern may occur in response to threat stimuli or situations with ambiguous elements. This pattern combines defensiveness with a number of cognition-linked behaviors such as sensory attention and orientation, approach, contact, and investigation of the potential threat. Studies measuring elements of this pattern suggest that female rats, and perhaps female mice, show higher levels than equivalent males. Higher female involvement may also occur in tasks involving learning/generalization/extinction of defensiveness to conditioned stimuli. Such findings are consonant with recent analyses of "female survival strategies" based on differential adaptiveness of cognitive components of defensiveness in females, due to the necessity of female care of offspring until they are independent. These data suggest the value of additional behavioral and functional analyses of cognitive aspects of defensive behavior; contributing to both an understanding of their underlying mechanisms, and providing more sensitive measures of drug responsivity for use with animal models.

Effects of separated pair housing of female C57BL/6JRj mice on well-being

In laboratory animal facilities, it is a common code of practice to house female mice in groups. However, some experimental conditions require to house them individually, even though social isolation may impair their well-being. Therefore, we introduced a separated pair housing system and investigated whether it can refine single housing of adult female C57BL/6JRj mice. Individually ventilated cages (IVC) were divided by perforated transparent walls to separate two mice within a cage. The cage divider allowed visual, acoustic, and olfactory contact between the mice but prevented interindividual body-contact or food sharing. Short- and long-term effects of the separated pair housing system on the well-being of the mice were compared with single and group housing using a range of behavioral and physiological parameters: Nest building behavior was assessed based on the complexity of nests, the burrowing performance was measured by the amount of food pellets removed from a bottle, and trait anxiety-related behavior was tested in the free exploratory paradigm. For the evaluation of the ease of handling, interaction with the experimenter's hand was monitored. Social interaction with unknown conspecifics and locomotor activity were investigated in a test arena. Moreover, body weight and stress hormone (metabolites) were measured in feces and hair. After the mice spent a day under the respective housing conditions, concentrations of fecal corticosterone metabolites were higher in separated pair-housed mice, and they built nests of a higher complexity when compared to single-housed mice. The latter effect was still observable eight weeks later. In week 8, separated pair-housed mice showed less locomotor activity in the social interaction arena compared to mice from the other housing systems, i.e., single and group housing. Regardless of the time of testing, pair housing improved the burrowing performance. Separated pair-housed mice were more difficult to catch than group-housed mice. Hair corticosterone, progesterone, and dehydroepiandrosterone concentrations changed with increasing age independently of the housing system. There were no effects of the housing systems on trait anxiety-related behavior in the free exploratory paradigm, voluntary interaction with the experimenter's hand, and body weight. Overall, the transfer to the separated pair housing system caused short-term stress responses in female C57BL/6JRj mice. Long-term effects of separated pair housing were ambiguous. On one hand, separated pair housing increased nesting and burrowing behavior and may therefore be beneficial compared to single housing. But on the other hand, locomotor activity decreased. The study underlined that the effects of the housing conditions on physiological and behavioral parameters should be considered when analyzing and reporting animal experiments.

Rescue of histone hypoacetylation and social deficits by ketogenic diet in a Shank3 mouse model of autism

Human genetic sequencing has implicated epigenetic and synaptic aberrations as the most prominent risk factors for autism. Here we show that autistic patients exhibit the significantly lower histone acetylation and elevated HDAC2 expression in prefrontal cortex (PFC). The diminished histone acetylation is also recaptured in an autism mouse model with the deficiency of the Shank3 gene encoding a synaptic scaffolding protein. Treating young (5-week-old) Shank3-deficient mice with a 4-week ketogenic diet, which can act as an endogenous inhibitor of class I HDACs via the major product β-hydroxybutyrate, elevates the level of histone acetylation in PFC neurons. Behavioral assays indicate that ketogenic diet treatment leads to the prolonged rescue of social preference deficits in Shank3-deficient mice. The HDAC downstream target genes encoding NMDA receptor subunits, GRIN2A and GRIN2B, are significantly reduced in PFC of autistic humans. Ketogenic diet treatment of Shank3-deficient mice elevates the transcription and histone acetylation of Grin2a and Grin2b, and restores the diminished NMDAR synaptic function in PFC neurons. These results suggest that the ketogenic diet provides a promising therapeutic strategy for social deficits in autism via the restoration of histone acetylation and gene expression in the brain.

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.

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.

Defining Valid Chronic Stress Models for Depression With Female Rodents

Women are twice as likely to experience depression than men, yet until recently, preclinical studies in rodents have focused almost exclusively on males. As interest in sex differences and sex-specific mechanisms of stress susceptibility increases, chronic stress models for inducing depression-relevant behavioral and physiological changes in male rodents are being applied to females, and several new models have emerged to include both males and females, yet not all models have been systematically validated in females. An increasing number of researchers seek to include female rodents in their experimental designs, asking the question "what is the ideal chronic stress model for depression in females?" We review criteria for assessing female model validity in light of key research questions and the fundamental distinction between studying sex differences and studying both sexes. In overviewing current models, we explore challenges inherent to establishing an ideal female chronic stress model, with particular emphasis on the need for standardization and adoption of validated behavioral tests sensitive to stress effects in females. Taken together, these considerations will empower female chronic stress models to provide a better understanding of stress susceptibility and allow the development of efficient sex-specific treatments.

Immunization with a heat-killed bacterium, Mycobacterium vaccae NCTC 11659, prevents the development of cortical hyperarousal and a PTSD-like sleep phenotype after sleep disruption and acute stress in mice

STUDY OBJECTIVES

Sleep deprivation induces systemic inflammation that may contribute to stress vulnerability and other pathologies. We tested the hypothesis that immunization with heat-killed Mycobacterium vaccae NCTC 11659 (MV), an environmental bacterium with immunoregulatory and anti-inflammatory properties, prevents the negative impacts of 5 days of sleep disruption on stress-induced changes in sleep, behavior, and physiology in mice.

METHODS

In a 2 × 2 × 2 experimental design, male C57BL/6N mice were given injections of either MV or vehicle on days -17, -10, and -3. On days 1-5, mice were exposed to intermittent sleep disruption, whereby sleep was disrupted for 20 h per day. Immediately following sleep disruption, mice were exposed to 1-h social defeat stress or novel cage (control) conditions. Object location memory (OLM) testing was conducted 24 h after social defeat, and tissues were collected 6 days later to measure inflammatory markers. Sleep was recorded using electroencephalography (EEG) and electromyography (EMG) throughout the experiment.

RESULTS

In vehicle-treated mice, only the combination of sleep disruption followed by social defeat (double hit): (1) increased brief arousals and NREM beta (15-30 Hz) EEG power in sleep immediately post-social defeat compared to baseline; (2) induced an increase in the proportion of rapid-eye-movement (REM) sleep and number of state shifts for at least 5 days post-social defeat; and (3) induced hyperlocomotion and lack of habituation in the OLM task. Immunization with MV prevented most of these sleep and behavioral changes.

CONCLUSIONS

Immunization with MV ameliorates a stress-induced sleep and behavioral phenotype that shares features with human posttraumatic stress disorder.

Resilience to fear: The role of individual factors in amygdala response to stressors

Resilience to stress is an adaptive process that varies individually. Resilience refers to the adaptation, or the ability to maintain or regain mental health, despite being subject to adverse situation. Resilience is a dynamic concept that reflects a combination of internal individual factors, including age and gender interacting with external factors such as social, cultural and environmental factors. In the last decade, we have witnessed an increase in the prevalence of anxiety disorders, including post-traumatic stress disorder. Given that stress in unavoidable, it is of great interest to understand the neurophysiological mechanisms of resilience, the individual factors that may contribute to susceptibility and promote efficacious approaches to improve resilience. Here, we address this complex question, attempting at defining clear and operational definitions that may allow us to improve our analysis of behavior incorporating individuality. We examine how individual perception of the stressor can alter the outcome of an adverse situation using as an example, the fear-conditioning paradigm and discuss how individual differences in the reward system can contribute to resilience. Given the central role of the endocannabinoid system in regulating fear responses and anxiety, we discuss the evidence that polymorphisms in several molecules of this signaling system contribute to different anxiety phenotypes. The endocannabinoid system is highly interconnected with the serotoninergic and dopaminergic modulatory systems, contributing to individual differences in stress perception and coping mechanisms. We review how the individual variability in these modulatory systems can be used towards a multivariable assessment of stress risk. Incorporating individuality in our research will allow us to define biomarkers of anxiety disorders as well as assess prognosis, towards a personalized clinical approach to mental health.

Enduring effects of adolescent ketamine exposure on cocaine- and sucrose-induced reward in male and female C57BL/6 mice

Ketamine has shown promising antidepressant efficacy for adolescent treatment-resistant depression. However, the potential enduring consequences of ketamine exposure have not been thoroughly evaluated. Thus, we examined if juvenile ketamine treatment results in long-lasting changes for the rewarding properties of sucrose and cocaine in adulthood, across three separate experiments. In Experiment 1, adolescent male and female C57BL/6 mice received ketamine (20 mg/kg) for 15 consecutive days (Postnatal Day [PD] 35-49). Twenty-one days later (PD70; adulthood) we examined their behavioral responsivity to sucrose (1%) on a two-bottle choice design, or cocaine (0, 5, 10 mg/kg) using the conditioned place preference (CPP) test. We found that juvenile ketamine-pretreatment increased preference for sucrose and environments paired with cocaine in male, but not female, adult mice. This long-term outcome was not observed when male and female mice received ketamine as adults (PD70-84) and tested for sucrose and cocaine preference 21-days later (Experiment 2). Similarly, in Experiment 3, no long-lasting differences in these measures were observed when adolescent male mice were exposed to concomitant ketamine and social stressors (PD35-44), namely the social defeat or vicarious defeat stress paradigms-procedures that mediated a depression-related phenotype (along with a ketamine antidepressant-like response). Collectively, we demonstrate that in the absence of physical or psychological stress, adolescent ketamine exposure increases later life preference for the rewarding properties of sucrose and cocaine in a sex- and age-specific manner. As such, this preclinical work provides awareness for the potential long-term behavioral consequences associated with juvenile ketamine exposure.

Cymbopogon citratus aqueous leaf extract attenuates neurobehavioral and biochemical changes induced by social defeat stress in mice

Objective

Psychosocial stress has been implicated in the genesis of psychiatric disorders such as memory deficits, depression, anxiety and addiction. Aqueous leaf extract of Cymbopogon citratus (CYC) otherwise known as lemongrass tea has antidepressant, anxiolytic and anti-amnesic effects in rodents. This study was designed to evaluate if C. citratus could reverse the neurobehavioral and biochemical derangements induced by social defeat stress (SDS) in the resident/intruder paradigm.

Methods

Intruder male mice were divided into five groups (n = 7): group 1 received saline (10 mL/kg, p.o.; non-stress control), group 2 also received saline (10 mL/kg, p.o.; SDS control) while groups 3–5 had C. citratus (50, 100 and 200 mg/kg, p.o.) daily for 14 d. The SDS was carried out 30 min after each treatment from day 7 to day 14 by exposing each intruder mouse in groups 2–5 to a 10 min confrontation in the home cage of an aggressive resident counterpart. The neurobehavioral features (spontaneous motor activity-SMA, anxiety, memory, social avoidance and depression were then evaluated. The concentrations of nitrite, malondialdehyde and glutathione as well as acetylcholinesterase activity in the brain tissues were also determined.

Results

C. citratus (50, 100 and 200 mg/kg) attenuated hypolocomotion, heightened anxiety, depressive-like symptom, memory deficit and social avoidance induced by SDS. The altered levels of oxidative stress and acetyl-cholinesterase in SDS-mice were positively modulated by C. citratus.

Conclusion

The results of this study suggest that C. citratus might mitigate psychosocial stress-induced neurologic diseases in susceptible individuals.

A novel social fear conditioning procedure alters social behavior and mTOR signaling in differentially housed adolescent rats

Vulnerabilities to fear-related disorders can be enhanced following early life adversity. This study sought to determine whether post-weaning social isolation (PSI), an animal model of early life adversity, alters the development of social fear in an innovative model of conditioned social fear. Male and female Sprague-Dawley rats underwent either social rearing (SR) or PSI for 4 weeks following weaning. Rats were then assigned to groups consisting of either Footshock only, Social conditioned stimulus (CS) only, or Paired footshock with a social CS. Social behavior was assessed the next day. We observed a novel behavioral response in PSI rats, running in circles, that was rarely observed in SR rats; moreover, this behavior was augmented after Paired treatment in PSI rats. Other social behaviors were altered by both PSI and Paired footshock and social CS. The mammalian target of rapamycin (mTOR) pathway was assessed using immunohistochemistry for phosphorylated ribosomal protein S6 (pS6) in subregions of the prefrontal cortex (PFC) and amygdala. Paired treatment produced opposite effects in the PFC and amygdala in males, but no differences were observed in females. Conditioned social fear produced alterations in social behavior and the mTOR pathway that are dependent upon rearing condition and sex.

Different effects of chronic social defeat on social behavior and the brain CRF system in adult male C57 mice with different susceptibilities

Chronic social defeat stress (CSDS) has been found to produce different impacts on anxiety-like behaviors, spatial cognitive function and memory in rodents with different susceptibilities. However, the impacts of chronic social defeat on social behaviors in adult male mice with different susceptibilities to social defeat and the underlying mechanisms in the brain remain unclear. In the present study, we found that ten days of social defeat reduced the tendency of susceptible adult male C57 mice to approach an unfamiliar individual and increased their avoidance of an unfamiliar CD-1 mouse but had no effects on resilient individuals. In addition, CSDS enhanced anxiety-like behavior in susceptible animals, but produced no effects in the resilient group. Meanwhile, CSDS increased the number of corticotropin-releasing factor (CRF)-positive neurons in the paraventricular nucleus of the hypothalamus and CRF-R2-positive neurons in the accumbens nucleus shell in both resilient and susceptible animals. CSDS increased the number of CRF-R1-positive neurons and CRF-R1 mRNA expression in the prelimbic cortex (PrL) and the number of CRF-R2-positive neurons in the basolateral amygdala, but reduced the number of CRF-R2-positive neurons and mRNA expression in the PrL in susceptible animals. Therefore, the different effects of CSDS on sociability and anxiety-like behavior in mice with different susceptibilities may be associated with region- and type-specific alterations in CRF receptor levels. These findings help us understand the underlying mechanism by which social stress affects emotion and social behavior and provides an important basis for the treatment of disorders of social and emotional behavior caused by social stress.

The serotonin system in the hippocampus CA3 involves in effects of CSDS on social recognition in adult female mandarin voles (Microtus mandarinus)

Chronic social defeat stress (CSDS) exacerbated the development of stress-related psychiatric disorders, and the social recognition dysfunction is the core feature of many psychiatric disorders. However, the effects of CSDS on female social recognition and the underlying neural mechanisms remain unclear. Using highly aggressive adult female mandarin voles (Microtus mandarinus) as animal model, the aim of this work is to investigate the effects of CSDS on social recognition in adult female rodents and the neurobiological mechanisms underlying these effects. Our results indicate the CSDS disrupted the normal social recognition in adult female voles. Meanwhile, defeated voles exhibited increased neural activity in the DG, CA1 and CA3 of the hippocampus. Furthermore, CSDS reduced levels of serotonin (5-HT) and serotonin 1A receptors (5-HT_1AR) in the CA3. We also discovered that microinjection of 8-OH-DPAT into the CA3 effectively reversed the social recognition deficits induced by CSDS, and an infusion of WAY-100635 into the CA3 of control female voles impaired social recognition. Moreover, targeted activation of the 5-HT neuron projection from the DRN to CA3 by long-term administration of CNO significantly prevented the CSDS induced social recognition deficits. Taken together, our study demonstrated that CSDS induced social recognition deficits in adult female voles, and these effects were mediated by the action of 5-HT on the 5-HT_1AR in the hippocampus CA3. The projection from the DRN to CA3 may be involved in social recognition deficits induced by CSDS.

Serotonin Signaling Trough Prelimbic 5-HT1A Receptors Modulates CSDS-Induced Behavioral Changes in Adult Female Voles

BACKGROUND

Most previous studies have focused on the effects of social defeat in male juvenile individuals. Whether chronic social defeat stress in adulthood affects female emotion and the underlying mechanisms remains unclear.

METHODS

Using highly aggressive adult female mandarin voles (Microtus mandarinus), the present study aimed to determine the effects of chronic social defeat stress on anxiety- and depression-like behaviors in adult female rodents and investigate the neurobiological mechanisms underlying these effects.

RESULTS

Exposure of adult female voles to social defeat stress for 14 days reduced the time spent in the central area of the open field test and in the open arms of the elevated plus maze and lengthened the immobility time in the tail suspension and forced swimming tests, indicating increased anxiety- and depression-like behaviors. Meanwhile, defeated voles exhibited increased neural activity in the prelimbic cortex of the medial prefrontal cortex. Furthermore, chronic social defeat stress reduced serotonin projections and levels of serotonin 1A receptors in the medial prefrontal cortex-prelimbic cortex. Intra-prelimbic cortex microinjections of the serotonin 1A receptor agonist 8-OH-DPAT reversed the alterations in emotional behaviors, whereas injections of the serotonin 1A receptor antagonist WAY-100635 into the prelimbic cortex of control voles increased the levels of anxiety- and depression-like behaviors.

CONCLUSIONS

Taken together, our results demonstrated that chronic social defeat stress increased anxiety- and depression-like behaviors in adult female voles, and these effects were mediated by the action of serotonin on the serotonin 1A receptors in the prelimbic cortex. The serotonin system may be a promising target to treat emotional disorders induced by chronic social defeat stress.

Social defeat stress: Mechanisms underlying the increase in rewarding effects of drugs of abuse

Social interaction is known to be the main source of stress in human beings, which explains the translational importance of this research in animals. Evidence reported over the last decade has revealed that, when exposed to social defeat experiences (brief episodes of social confrontations during adolescence and adulthood), the rodent brain undergoes remodeling and functional modifications, which in turn lead to an increase in the rewarding and reinstating effects of different drugs of abuse. The mechanisms by which social stress cause changes in the brain and behavior are unknown, and so the objective of this review is to contemplate how social defeat stress induces long-lasting consequences that modify the reward system. First of all, we will describe the most characteristic results of the short- and long-term consequences of social defeat stress on the rewarding effects of drugs of abuse such as psychostimulants and alcohol. Secondly, and throughout the review, we will carefully assess the neurobiological mechanisms underlying these effects, including changes in the dopaminergic system, corticotrophin releasing factor signaling, epigenetic modifications and the neuroinflammatory response. To conclude, we will consider the advantages and disadvantages and the translational value of the social defeat stress model, and will discuss challenges and future directions.

Social Defeat Stress, Sex, and Addiction-Like Behaviors

Social confrontation is a form of social interaction in animals where two conspecific individuals confront each other in dispute over territory, during the formation of hierarchies, and during breeding seasons. Typically, a social confrontation involves a prevailing individual and a yielding individual. The prevailing individual often exhibits aggressive postures and launches attacks, whereas the yielding individual often adopts postures of defeat. The yielding or defeated animals experience a phenomenon known as social defeat stress, in which they show exaggerated stress as well as autonomic and endocrine responses that cause impairment of both the brain and body. In laboratory settings, one can reliably generate social defeat stress by allowing a naïve (or already defeated) animal to intrude into a home cage in which its resident has already established a territory or is nursing. This resident-intruder paradigm has been widely used in both males and females to study mechanisms in the brain that underlie the stress responses. Stress has profound effects on drug reward for cocaine, methamphetamine, alcohol, and opioids. Particularly, previous experiences with social defeat can exaggerate subsequent addiction-like behaviors. The extent of these addiction-like behaviors depends on the intensity, duration, frequency, and intermittency of the confrontation episodes. This chapter describes four types of social defeat stress: acute, repeated, intermittent, and chronic. Specifically, it focuses on social defeat stress models used in laboratories to study individual, sex, and animal strain differences in addiction-like behaviors.

A Novel Method for Chronic Social Defeat Stress in Female Mice

Historically, preclinical stress studies have often omitted female subjects, despite evidence that women have higher rates of anxiety and depression. In rodents, many stress susceptibility and resilience studies have focused on males as one commonly used paradigm-chronic social defeat stress-has proven challenging to implement in females. We report a new version of the social defeat paradigm that works in female mice. By applying male odorants to females to increase resident male aggressive behavior, we find that female mice undergo repeated social defeat stress and develop social avoidance, decreased sucrose preference, and decreased time in the open arms of the elevated plus maze relative to control mice. Moreover, a subset of the female mice in this paradigm display resilience, maintaining control levels of social exploration and sucrose preference. This method produces comparable results to those obtained in male mice and will greatly facilitate studying female stress susceptibility.

Effects of chronic social defeat on social behaviors in adult female mandarin voles (Microtus mandarinus): Involvement of the oxytocin system in the nucleus accumbens

Chronic social defeat affects many aspects of behavior. Most previous studies have focused on effects on males and defeat during adolescence. The extents to which chronic social defeat can impact female social behavior in adulthood and the neural mechanisms of such effects are poorly understood. Using highly social and aggressive female mandarin voles (Microtus mandarinus), the present study found that chronic social defeat reduced social preference in adult females, and that the defeated voles exhibited a high level of freeze, self-grooming and defensive behavior, as well as reduced exploration, intimacy and aggression during social interactions. Furthermore, chronic social defeat reduced levels of oxytocin (OT) and OT receptors (OTR) in the shell region of the nucleus accumbens (NACC). Intra-NACC shell OT microinjections reversed alterations in social behavior induced by chronic social defeat, whereas injections of an OTR antagonist (OTR-A) blocked the effects of OT. Taken together, our data demonstrate that chronic social defeat suppresses measures of sociability, and that these effects are mediated by the action of OT on the OTR in the NACC. NACC OT may be a promising target to treat socio-emotional disorders induced by chronic social stress.

Sex differences in the neuro-immune consequences of stress: Focus on depression and anxiety

Women appear to be more vulnerable to the depressogenic effects of inflammation than men. Chronic stress, one of the most pertinent risk factors of depression and anxiety, is known to induce behavioral and affective-like deficits via neuroimmune alterations including activation of the brain's immune cells, pro-inflammatory cytokine expression, and subsequent changes in neurotransmission and synaptic plasticity within stress-related neural circuitry. Despite well-established sexual dimorphisms in the stress response, immunity, and prevalence of stress-linked psychiatric illnesses, much of current research investigating the neuroimmune impact of stress remains exclusively focused on male subjects. We summarize and evaluate here the available data regarding sex differences in the neuro-immune consequences of stress, and some of the physiological factors contributing to these differences. Furthermore, we discuss the extent to which sex differences in stress-related neuroinflammation can account for the overall female bias in stress-linked psychiatric disorders including major depressive disorder and anxiety disorders. The currently available evidence from rodent studies does not unequivocally support the peripheral inflammatory changes seen in women following stress. Replication of many recent findings in stress-related neuroinflammation in female subjects is necessary in order to build a framework in which we can assess the extent to which sex differences in stress-related inflammation contribute to the overall female bias in stress-related affective disorders.

Sex/gender influences on the nervous system: Basic steps toward clinical progress

A Commentary highlighting the progress that sex-based data and research have made in neuroscience and the complexities that research has revealed thus far. Basic and preclinical neuroscientific research that considers sex as a biological variable will continue to build on the foundation of knowledge that has been started by multiple predecessors. The expansion of knowledge in preclinical neuroscience that integrates the study of both sexes will have a significant role in informing clinical trial design. We applaud the efforts of the editors and authors who have contributed to this issue. Published 2016. This article is a U.S. Government work and is in the public domain in the USA.

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

Although Syrian hamsters are thought to be naturally solitary, recent evidence from our laboratory demonstrates that hamsters may actually prefer social contact. Hamsters increase their preference for a location associated with an agonistic encounter regardless of whether they have "won" or "lost". It has also been reported that social housing as well as exposure to intermittent social defeat or to a brief footshock stressor increase food intake and body mass in hamsters. By contrast, it has also been suggested that housing hamsters in social isolation causes anxiety-induced anorexia and reductions in body mass selectively in females. The purpose of this study was to determine the physiological consequences of housing hamsters in social isolation versus in social groups. Male and female hamsters were housed singly or in stable groups of 5 for 4weeks after which they were weighed and trunk blood was collected. In addition, fat pads and thymus and adrenal glands were extracted and weighed. Serum and fecal cortisol were measured using an enzyme-linked immunoassay. Housing condition had no effect on serum or fecal cortisol, but socially housed hamsters displayed modest thymus gland involution. Socially housed females weighed more than did any other group, and socially housed females and males had more fat than did socially isolated hamsters. No wounding or tissue damage occurred in grouped hamsters. Overall, these data suggest that Syrian hamsters tolerate both stable social housing and social isolation in the laboratory although social housing is associated with some alteration in stress-related and bioenergetic measures.

Social defeat models in animal science: What we have learned from rodent models

Studies on stress and its impacts on animals are very important in many fields of science, including animal science, because various stresses influence animal production and animal welfare. In particular, the social stresses within animal groups have profound impact on animals, with the potential to induce abnormal behaviors and health problems. In humans, social stress induces several health problems, including psychiatric disorders. In animal stress models, social defeat models are well characterized and used in various research fields, particularly in studies concerning mental disorders. Recently, we have focused on behavior, nutrition and metabolism in rodent models of social defeat to elucidate how social stresses affect animals. In this review, recent significant progress in studies related to animal social defeat models are described. In the field of animal science, these stress models may contribute to advances in the development of functional foods and in the management of animal welfare.