Chronic psychosocial stress persistently alters autonomic function and physical activity in mice

Re-wilding model organisms: Opportunities to test causal mechanisms in social determinants of health and aging

Social experiences are strongly associated with individuals' health, aging, and survival in many mammalian taxa, including humans. Despite their role as models of many other physiological and developmental bases of health and aging, biomedical model organisms (particularly lab mice) remain an underutilized tool in resolving outstanding questions regarding social determinants of health and aging, including causality, context-dependence, reversibility, and effective interventions. This status is largely due to the constraints of standard laboratory conditions on animals' social lives. Even when kept in social housing, lab animals rarely experience social and physical environments that approach the richness, variability, and complexity they have evolved to navigate and benefit from. Here we argue that studying biomedical model organisms outside under complex, semi-natural social environments ("re-wilding") allows researchers to capture the methodological benefits of both field studies of wild animals and laboratory studies of model organisms. We review recent efforts to re-wild mice and highlight discoveries that have only been made possible by researchers studying mice under complex, manipulable social environments.

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

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

Juvenile exposure to doxorubicin alters the cardiovascular response to adult-onset psychosocial stress in mice

Childhood cancer survivors have a high risk for premature cardiovascular diseases, mainly due to cardiotoxic cancer treatments such as doxorubicin (DOX). Psychosocial stress is a significant cardiovascular risk factor and an enormous burden in childhood cancer survivors. Although observational studies suggest that psychosocial stress is associated with cardiovascular complications in cancer survivors, there is no translationally relevant animal model to study this interaction. We established a "two-hit" model in which juvenile mice were administered DOX (4 mg/kg/week for 3 weeks), paired to a validated model of chronic subordination stress (CSS) 5 weeks later upon reaching adulthood. Blood pressure, heart rate, and activity were monitored by radio-telemetry. At the end of CSS experiment, cardiac function was assessed by echocardiography. Cardiac fibrosis and inflammation were assessed by histopathologic analysis. Gene expressions of inflammatory and fibrotic markers were determined by PCR. Juvenile exposure to DOX followed by adult-onset CSS caused cardiac fibrosis and inflammation as evident by histopathologic findings and upregulated gene expression of multiple inflammatory and fibrotic markers. Intriguingly, juvenile exposure to DOX blunted CSS-induced hypertension but not CSS-induced tachycardia. There were no significant differences in cardiac function parameters among all groups, but juvenile exposure to DOX abrogated the hypertrophic response to CSS. In conclusion, we established a translationally relevant mouse model of juvenile DOX-induced cardiotoxicity that predisposes to adult-onset stress-induced adverse cardiac remodeling. Psychosocial stress should be taken into consideration in cardiovascular risk stratification of DOX-treated childhood cancer survivors.

Neurobiological Bases of Alcohol Consumption After Social Stress

The urge to seek and consume excessive alcohol is intensified by prior experiences with social stress, and this cascade can be modeled under systematically controlled laboratory conditions in rodents and non-human primates. Adaptive coping with intermittent episodes of social defeat stress often transitions to maladaptive responses to traumatic continuous stress, and alcohol consumption may become part of coping responses. At the circuit level, the neural pathways subserving stress coping intersect with those for alcohol consumption. Increasingly discrete regions and connections within the prefrontal cortex, the ventral and dorsal striatum, thalamic and hypothalamic nuclei, tegmental areas as well as brain stem structures begin to be identified as critical for reacting to and coping with social stress while seeking and consuming alcohol. Several candidate molecules that modulate signals within these neural connections have been targeted in order to reduce excessive drinking and relapse. In spite of some early clinical failures, neuropeptides such as CRF, opioids, or oxytocin continue to be examined for their role in attenuating stress-escalated drinking. Recent work has focused on neural sites of action for peptides and steroids, most likely in neuroinflammatory processes as a result of interactive effects of episodic social stress and excessive alcohol seeking and drinking.

Social isolation is a direct determinant of decreased home-cage activity in mice: A within-subjects study using a body-implantable actimeter

NEW FINDINGS

What is the central question of this study? It is generally recognized that social isolation is associated with physical inactivity; however, is social isolation a direct determinant of decreased physical activity? What is the main finding and its importance? We conducted a within-subjects experiment with the aid of a body-implantable actimeter. Our results clearly demonstrated that social isolation decreased home-cage activity in mice. This might have resulted from increased immobility and decreased vigorous activity, suggesting that avoiding social isolation is important to preventing physical inactivity.

ABSTRACT

An inactive lifestyle can negatively affect physiological and mental health. Social isolation is associated with physical inactivity; however, it remains uncertain whether social isolation is a direct determinant of decreased physical activity. Hence, we assessed whether social isolation decreases home-cage activity using a within-subjects design and examined the effects of social isolation on hippocampal neurogenesis in mice. This study used a body-implantable actimeter called nanotag®, which enabled us to measure home-cage activity despite housing the mice in groups. We first examined the influence of the intraperitoneal implantation of nanotag® on home-cage activity. Although nanotag® implantation decreased home-cage activity temporarily, 7 days post-implantation, it recovered to the same level as that of control (non-implanted) mice, suggesting that implantation of nanotag® does not have a negative influence on home-cage activity if mice undergo a 1-week recovery period after implantation. In the main experiment, after the 1-week baseline measurement performed while in group housing, the mice were placed in a group or in isolation. Home-cage activity was measured for an additional 4 weeks. Home-cage activity in isolated mice during the dark period decreased by 26% from pre-intervention to the last week of intervention. Furthermore, the reduction in the number of 5-minute epochs during which the activity count exceeded 301 (an index of vigorous activity) was significantly larger for isolated mice. Contrary to expectations, social isolation did not impair hippocampal neurogenesis. Our results demonstrate that social isolation is a direct determinant of decreased physical activity, possibly because of reduced vigorous physical activity. This article is protected by copyright. All rights reserved.

Social Stress and Aggression in Murine Models

Throughout life, animals engage in a variety of social interactions ranging from the affiliative mother-offspring interaction and juvenile play to aggressive conflict. Deprivation of the appropriate social interaction during early development is stressful and disrupts the development of appropriate social behaviors and emotional responses later in life. Additionally, agonistic encounters can induce stress responses in both dominant and subordinate individuals. This review focuses on the social stress that escalates aggressive behavior of animals and discusses the known neurobiological and physiological mechanisms underlying the link between social stress and aggression. Social instigation, a brief exposure to a rival without physical contact, induces aggressive arousal in dominant animals and escalates aggressive behaviors in the following agonistic encounter. Furthermore, the experience of winning an aggressive encounter is known to be as rewarding as addictive drugs, and the experience of repeatedly winning induces addiction-like behavioral and neurobiological changes and leads to abnormal aggressive behaviors. Social isolation stress in early development from neonatal to juvenile and adolescent periods also affects aggressive behavior, but these effects largely depend on the strain, sex, and species as well as the stage of development in which isolation stress is experienced. In conclusion, understanding neurobiological mechanisms underlying the link between social stress and aggression will provide an important insight for the development of more effective and tolerable treatments for maladaptive aggression in humans.

Who's the Boss? Assessing Convergent Validity of Aggression Based Dominance Measures in Male Laboratory Mice, Mus Musculus

Aggression among group housed male mice continues to challenge laboratory animal researchers because mitigation strategies are generally applied at the cage level without a good understanding of how it affects the dominance hierarchy. Aggression within a group is typically displayed by the dominant mouse targeting lower ranking subordinates; thus, the strategies for preventing aggression may be more successful if applied specifically to the dominant mouse. Unfortunately, dominance rank is often not assessed because of time intensive observations or tests. Several correlates of dominance status have been identified, but none have been directly compared to home cage behavior in standard housing. This study assessed the convergent validity of three dominance correlates (urinary darcin, tube test score, preputial gland to body length ratio) with wound severity and rankings based on home cage behavior, using factor analysis. Discriminant validity with open field measures was assessed to determine if tube test scores are independent of anxiety. Cages were equally split between SJL and albino C57BL/6 strains and group sizes of 3 or 5 (N = 24). Home cage behavior was observed during the first week, and dominance measures were recorded over the second. After controlling for strain and group size, darcin and preputial ratio had strong loadings on the same factor, which was a significant predictor of home cage ranking showing strong convergent validity. Tube test scores were not significantly impacted by open field data, showing discriminant validity. Social network analysis revealed that despotic power structures were prevalent, aggressors were typically more active and rested away from cage mates, and the amount of social investigation and aggression performed by an individual were highly correlated. Data from this study show that darcin and preputial ratio are representative of home cage aggression and provide further insight into individual behavior patterns in group housed male mice.

Animal models for the study of depressive disorder

Depressive disorder is one of the most widespread forms of psychiatric pathology, worldwide. According to a report by the World Health Organization, the number of people with depression, globally, is increasing dramatically with each year. Previous studies have demonstrated that various factors, including genetics and environmental stress, contribute to the risk of depression. As such, it is crucial to develop a detailed understanding of the pathogenesis of depressive disorder and animal studies are essential for identifying the mechanisms and genetic disorders underlying depression. Recently, many researchers have reported on the pathology of depression via various models of depressive disorder. Given that different animal models of depression show differences in terms of patterns of depressive behavior and pathology, the comparison between depressive animal models is necessary for progress in the field of the depression study. However, the various animal models of depression have not been fully compared or evaluated until now. In this paper, we reviewed the pathophysiology of the depressive disorder and its current animal models with the analysis of their transcriptomic profiles. We provide insights for selecting different animal models for the study of depression.

Toward Understanding the Sex Differences in the Biological Mechanism of Social Stress in Mouse Models

Significant sex differences in terms of prevalence, symptomatic profiles, severity, and comorbidities of psychiatric disorders are quite common. Women have been shown to be more vulnerable to stress and are nearly twice as likely as men to develop stress-related disorders such as depression and anxiety. Therefore, understanding sex differences with respect to the neurobiological mechanisms underlying stress-related disorders is important for developing more efficient pharmacological interventions for women. However, most preclinical studies on stress-related disorders have focused heavily on male rodents. Here, recent developments in the study of repeated social defeat stress models in female mice are summarized. Our findings suggest that a variety of factors need to be considered when employing this model.

Individual baseline behavioral traits predict the resilience phenotype after chronic social defeat

Chronic social defeat (CSD) has been widely used as a psychosocial stress model in mice, with the magnitude of CSD-induced social avoidance as the major behavioral hallmark of the resilient and susceptible groups. Despite significant progress in the study of the neurobiology of resilient and susceptible mice, the nature and ethological relevance of CSD-induced social avoidance and social approach, particularly measured using a CD1 mouse, needs conceptual clarification. Based on the findings of a recent study revealing substantial individuality in genetically homogeneous inbred mice, we investigated whether certain baseline individual characteristics of male C57BL/6J mice predict the resilient outcome after CSD. We focused on two well-studied individual traits that seem to have heritable underpinnings—approach to novelty and avoidance of harm, which are essential for the expression of the exploratory drive. Our results showed that the exploration levels and the approach to novelty and harm were different before and after CSD in resilient and susceptible mice. Before the stress, resilient mice had higher horizontal activity in a novel environment, shorter approach latencies, and higher exploration times for social and non-social targets than susceptible mice. However, susceptible mice performed better in the passive avoidance task than resilient mice as they were more successful in learning to avoid potential adversity by suppressing the spontaneous exploratory drive. Our findings challenge the validity of the current selection criteria for the susceptible and resilient groups and encourage comprehensive assessment of both baseline and stress-induced individual behavioral signatures of mice.

Social defeat: Vagal reduction and vulnerability to ventricular arrhythmias

Previously, a sub-population of defeated anesthetized rats (Dlow) was characterized by persistent low blood levels of brain-derived neurotrophic factor (BDNF) at day 29 and autonomic alteration at day 30 after social challenge, while the other population (Dhigh) was similar to non-defeated (ND) animals. The aims of this study were to determine the time-course of autonomic dysfunction in awake animals, and whether Dhigh and/or Dlow were vulnerable to cardiac events. Defeated animals were exposed to four daily episodes of social defeats from day 1 to day 4. At day 30, anesthetized Dlow displayed decreased experimental and spontaneous reflex responses reflecting lower parasympathetic efficiency. In addition, Dlow but not Dhigh were characterized by left ventricular hypertrophy at day 30. Telemetric recordings revealed that Dlow had increased low frequency-to-high frequency ratio (LF/HF) and diastolic (DBP) and systolic (SBP) blood pressure, associated with decreased HF and spontaneous baroreflex responses (BRS) from day 3 to day 29. LF/HF, DBP and SBP recovered at day 5, and HF and BRS recovered at day 15 in Dhigh. Ventricular premature beats (VPBs) occurred in Dlow and Dhigh animals from day 5. Time course of VBP fluctuations in Dhigh mirrored that of HF and BRS, but not that of LF/HF, DBP and SBP. These results suggest that a psychosocial stress associated to low serum BDNF levels can lead to vulnerability to persistent autonomic dysfunction, cardiac hypertrophy and ventricular ectopic beats. The parasympathetic recovery seen in Dhigh may provide protection against cardiac events in this population.

The Moderating Role of Coping Style on Chronic Stress Exposure and Cardiovascular Reactivity Among African American Emerging Adults

Chronic stress exposure may contribute to dysregulation of cardiovascular functions and increase CVD risk among African Americans. This study investigated the direct and interactive effects of chronic stress exposure and coping styles on cardiovascular reactivity to acute stress. A sample of African American emerging adults (n = 277) completed a battery of self-report assessments and underwent the Trier Social Stress Test (TSST) across two time points. Prior chronic stress exposure was negatively associated with heart rate (HR) reactivity among females at 1-month follow-up. Task-oriented coping was positively associated with HR reactivity, while avoidance-oriented coping showed a negative association. Higher use of emotion-oriented coping moderated the relationship between chronic stress exposure and HR reactivity, resulting in more robust reactivity. Among females, but not males, lower use of avoidance-oriented coping moderated the relationship between prior chronic stress exposure and HR reactivity, also resulting in more robust reactivity. Prior chronic stress exposure and the use of maladaptive coping strategies may confer negative impacts on cardiovascular reactivity, particularly among African American females. Using adaptive coping styles may mitigate these effects and improve cardiovascular reactivity. These findings provide preliminary support for psychosocial determinants of health within a controlled laboratory experiment and highlight important gender differences to consider in prevention efforts for African American cardiovascular health disparities.

Stimulation of brain cannabinoid CB1 receptors can ameliorate hypertension in spontaneously hypertensive rats

Excessive activation of the sympatho-adrenomedullary system plays a pathogenic role in triggering and sustaining essential hypertension. We previously reported that, in normotensive rats, intracerebroventricularly (i.c.v.) administered neuropeptides, corticotropin-releasing factor and bombesin induced activation of the sympatho-adrenomedullary system, and that brain cannabinoid CB1 receptors negatively regulated this activation. In this study, we investigated the effects of brain CB1 receptor stimulation on blood pressure and the sympatho-adrenomedullary outflow in spontaneously hypertensive rats (SHRs), commonly used animal models of essential hypertension, and in Wistar-Kyoto (WKY) rats, normotensive controls of SHRs. In 18-week-old SHRs and WKY rats under urethane anaesthesia (1.0 g/kg, i.p.), SHRs exhibited significantly higher systolic, mean and diastolic blood pressures and plasma noradrenaline and adrenaline, and a lower heart rate than WKY rats. Single administration of arachidonyl 2'-chloroethylamide (ACEA, CB1 agonist, 1.4 µmol/animal, i.c.v.) significantly but partially reduced mean and diastolic blood pressures and the plasma level of noradrenaline in SHRs compared to vehicle (N,N-dimethylformamide)-treated SHRs. These ACEA-induced reductions were abolished by central pretreatment with rimonabant (CB1 antagonist, 300 nmol/animal, i.c.v.), which alone showed no significant effect on blood pressures or plasma noradrenaline and adrenaline levels of SHRs. On the other hand, ACEA had no significant effect on blood pressure or plasma noradrenaline and adrenaline levels in WKY rats. These results suggest that stimulation of brain CB1 receptors can ameliorate hypertension accompanied by enhanced sympathetic outflow without affecting blood pressure under normotensive conditions.

Social stress is lethal in the mdx model of Duchenne muscular dystrophy

Background

Duchenne muscular dystrophy (DMD) is caused by the loss of dystrophin. Severe and ultimately lethal, DMD progresses relatively slowly in that patients become wheelchair bound only around age twelve with a survival expectancy reaching the third decade of life.

Methods

The mildly-affected mdx mouse model of DMD, and transgenic DysΔMTB-mdx and Fiona-mdx mice expressing dystrophin or utrophin, respectively, were exposed to either mild (scruffing) or severe (subordination stress) stress paradigms and profiled for their behavioral and physiological responses. A subgroup of mdx mice exposed to subordination stress were pretreated with the beta-blocker metoprolol.

Findings

Subordination stress caused lethality in ∼30% of mdx mice within 24 h and ∼70% lethality within 48 h, which was not rescued by metoprolol. Lethality was associated with heart damage, waddling gait and hypo-locomotion, as well as marked up-regulation of the hypothalamus-pituitary-adrenocortical axis. A novel cardiovascular phenotype emerged in mdx mice, in that scruffing caused a transient drop in arterial pressure, while subordination stress caused severe and sustained hypotension with concurrent tachycardia. Transgenic expression of dystrophin or utrophin in skeletal muscle protected mdx mice from scruffing and social stress-induced responses including mortality.

Interpretation

We have identified a robust new stress phenotype in the otherwise mildly affected mdx mouse that suggests relatively benign handling may impact the outcome of behavioural experiments, but which should also expedite the knowledge-based therapy development for DMD.

Funding

Greg Marzolf Jr. Foundation, Summer's Wish Fund, NIAMS, Muscular Dystrophy Association, University of Minnesota and John and Cheri Gunvalson Trust.

Development of an Exercise Training Protocol to Investigate Arteriogenesis in a Murine Model of Peripheral Artery Disease

Exercise is a treatment option in peripheral artery disease (PAD) patients to improve their clinical trajectory, at least in part induced by collateral growth. The ligation of the femoral artery (FAL) in mice is an established model to induce arteriogenesis. We intended to develop an animal model to stimulate collateral growth in mice through exercise. The training intensity assessment consisted of comparing two different training regimens in C57BL/6 mice, a treadmill implementing forced exercise and a free-to-access voluntary running wheel. The mice in the latter group covered a much greater distance than the former pre- and postoperatively. C57BL/6 mice and hypercholesterolemic ApoE-deficient (ApoE^−/−) mice were subjected to FAL and had either access to a running wheel or were kept in motion-restricting cages (control) and hind limb perfusion was measured pre- and postoperatively at various times. Perfusion recovery in C57BL/6 mice was similar between the groups. In contrast, ApoE^−/− mice showed significant differences between training and control 7 d postoperatively with a significant increase in pericollateral macrophages while the collateral diameter did not differ between training and control groups 21 d after surgery. ApoE^−/− mice with running wheel training is a suitable model to simulate exercise induced collateral growth in PAD. This experimental set-up may provide a model for investigating molecular training effects.

The first multi-zoo application of an allostatic load index to western lowland gorillas (Gorilla gorilla gorilla)

Vertebrate stress responses are highly adaptive biological functions, maximizing survival probability in life-threatening situations. However, experiencing repeated and/or chronic stressors can generate physiological dysregulation and lead to disease. Because stress responses are multi-systemic and involve a wide range of physiological functions, identifying responses to stressors is best accomplished using integrated biomarker models. Allostatic load, defined as the physiological dysregulation that accumulates over the lifespan due to stressful experiences, is one such model. Allostatic load is measured using allostatic load indices, which are composites of biomarkers from multiple somatic systems. Previously, we reported the use of a 7-biomarker allostatic load index (albumin, CRH, cortisol, DHEA-S, glucose, IL-6, TNF-α) in western lowland gorillas housed at a single zoo. Herein, this index is expanded to examine allostatic load responses to lifetime stressors in gorillas from two additional zoos (n = 63) as well as two pooled samples. The index was created using quartile cut-points for each biomarker. Significant associations were observed between multiple predictor variables and allostatic load, including sex, age, number of stressful events (anesthetic events, zoo transfers, agonistic interactions with wounding, pregnancies), and rearing history (mother-reared, nursery-reared, wild-caught). Additionally, allostatic load was associated with indicators of morbidity (creatinine, cholesterol, triglycerides), age at death, and mortality risk. These results are consistent with those reported in human allostatic load research, suggesting allostatic load indices have potential as an investigative and clinical tool for gorillas and other great apes.

Changes in Heart Rate and Its Regulation by the Autonomic Nervous System Do Not Differ Between Forced and Voluntary Exercise in Mice

Most exercise studies in mice have relied on forced training which can introduce psychological stress. Consequently, the utility of mouse models for understanding exercise-mediated effects in humans, particularly autonomic nervous system (ANS) remodeling, have been challenged. We compared the effects of voluntary free-wheel running vs. non-voluntary swimming on heart function in mice with a focus on the regulation of heart rate (HR) by the ANS. Under conditions where the total excess O₂ consumption associated with exercise was comparable, the two exercise models led to similar improvements in ventricular function as well as comparable reductions in HR and its control by parasympathetic nervous activity (PNA) and sympathetic nervous activity (SNA), compared to sedentary mice. Both exercise models also increased HR variability (HRV) by similar amounts, independent of HR reductions. In all mice, HRV depended primarily on PNA, with SNA weakly affecting HRV at low frequencies. The differences in both HR and HRV between exercised vs. sedentary mice were eliminated by autonomic blockade, consistent with the similar intrinsic beating rates observed in atria isolated from exercised vs. sedentary mice. In conclusion, both forced and voluntary exercise induce comparable ventricular physiological remodeling as well as HR reductions and HR-independent enhancements of HRV which were both primarily dependent on increased PNA.

New and noteworthy

-No previous mouse studies have compared the effects of forced and voluntary exercise on the heart function and its modulation by the autonomic nervous system (ANS).-Both voluntary free-wheel running and forced swimming induced similar improvements in ventricular contractile function, reductions in heart rate (HR) and enhancements of HR variability (HRV).-HR regulation in exercised mice was linked to increased parasympathetic nerve activity and reduced sympathetic nerve activity.- HRV was independent of HR and depended primarily on PNA in both exercised and sedentary mice.- Complete cardiac autonomic blockade eliminated differences in both HR and HRV between exercised and sedentary mice.

Eye region surface temperature reflects both energy reserves and circulating glucocorticoids in a wild bird

Body temperature of endotherms shows substantial within- and between-individual variation, but the sources of this variation are not fully understood in wild animals. Variation in body temperature can indicate how individuals cope with their environment via metabolic or stress-induced effects, both of which may relate to depletion of energy reserves. Body condition can reflect heat production through changes to metabolic rate made to protect energy reserves. Additionally, changes in metabolic processes may be mediated by stress-related glucocorticoid secretion, which is associated with altered blood-flow patterns that affect regional body temperatures. Accordingly, both body condition and glucocorticoid secretion should relate to body temperature. We used thermal imaging, a novel non-invasive method of temperature measurement, to investigate relationships between body condition, glucocorticoid secretion and body surface temperature in wild blue tits (Cyanistes caeruleus). Individuals with lower body condition had lower eye-region surface temperature in both non-breeding and breeding seasons. Eye-region surface temperature was also negatively correlated with baseline circulating glucocorticoid levels in non-breeding birds. Our results demonstrate that body surface temperature can integrate multiple aspects of physiological state. Consequently, remotely-measured body surface temperature could be used to assess such aspects of physiological state non-invasively in free-living animals at multiple life history stages.

Establishment of a repeated social defeat stress model in female mice

Numerous studies have employed repeated social defeat stress (RSDS) to study the neurobiological mechanisms of depression in rodents. An important limitation of RSDS studies to date is that they have been conducted exclusively in male mice due to the difficulty of initiating attack behavior directed toward female mice. Here, we establish a female mouse model of RSDS by inducing male aggression toward females through chemogenetic activation of the ventrolateral subdivision of the ventromedial hypothalamus (VMHvl). We demonstrate that females susceptible to RSDS display social avoidance, anxiety-like behavior, reduction of body weight, and elevated levels of circulating interleukin 6. In contrast, a subset of mice we term resilient only display anxiety-like behaviors after RSDS. This model allows for investigation of sex differences in the neurobiological mechanisms of defeat‒induced depression‒like behaviors. A robust female social defeat model is a critical first step in the identification and development of novel therapeutic compounds to treat depression and anxiety disorders in women.

REAL-TIME SUBJECTIVE ASSESSMENT OF PSYCHOLOGICAL STRESS: ASSOCIATIONS WITH OBJECTIVELY-MEASURED PHYSICAL ACTIVITY LEVELS

Psychosocial stress may be a factor in the link between physical activity and obesity. This study examines how the daily experience of psychosocial stress influences physical activity levels and weight status in adults. This study reports temporally ordered relationships between sedentary, light, and moderate-to-vigorous physical activity levels and real-time reports of subjective psychosocial stress levels. Adults (n=105) wore an accelerometer and participated in an ecological momentary assessment (EMA) of stress by answering prompts on a mobile phone several times per day over 4 days. Subjective stress was negatively related to sedentary activity in the minutes immediately preceding and immediately following an EMA prompt. Light activity was positively associated with a subsequent EMA report of higher stress, but there were no observed associations between stress and moderate-to-vigorous activity. Real-time stress reports and accelerometer readings for the same 4-day period showed no association. Nor were there associations between real-time stress reports and weight status.

Social defeat protocol and relevant biomarkers, implications for stress response physiology, drug abuse, mood disorders and individual stress vulnerability: a systematic review of the last decade

INTRODUCTION

Social defeat (SD) in rats, which results from male intraspecific confrontations, is ethologically relevant and useful to understand stress effects on physiology and behavior.

METHODS

A systematic review of studies about biomarkers induced by the SD protocol and published from 2002 to 2013 was carried out in the electronic databases PubMed, Web of Knowledge and ScienceDirect. The search terms were: social defeat, rat, neurotrophins, neuroinflammatory markers, and transcriptional factors.

RESULTS

Classical and recently discovered biomarkers were found to be relevant in stress-induced states. Findings were summarized in accordance to the length of exposure to stress: single, repeated, intermittent and continuous SD. This review found that the brain-derived neurotrophic factor (BDNF) is a distinct marker of stress adaptation. Along with glucocorticoids and catecholamines, BDNF seems to be important in understanding stress physiology.

CONCLUSION

The SD model provides a relevant tool to study stress response features, development of addictive behaviors, clinic depression and anxiety, as well as individual differences in vulnerability and resilience to stress.

Emotional Stress and Cardiovascular Complications in Animal Models: A Review of the Influence of Stress Type

Emotional stress has been recognized as a modifiable risk factor for cardiovascular diseases. The impact of stress on physiological and psychological processes is determined by characteristics of the stress stimulus. For example, distinct responses are induced by acute vs. chronic aversive stimuli. Additionally, the magnitude of stress responses has been reported to be inversely related to the degree of predictability of the aversive stimulus. Therefore, the purpose of the present review was to discuss experimental research in animal models describing the influence of stressor stimulus characteristics, such as chronicity and predictability, in cardiovascular dysfunctions induced by emotional stress. Regarding chronicity, the importance of cardiovascular and autonomic adjustments during acute stress sessions and cardiovascular consequences of frequent stress response activation during repeated exposure to aversive threats (i.e., chronic stress) is discussed. Evidence of the cardiovascular and autonomic changes induced by chronic stressors involving daily exposure to the same stressor (predictable) vs. different stressors (unpredictable) is reviewed and discussed in terms of the impact of predictability in cardiovascular dysfunctions induced by stress.

A Stress-Related Peptide Bombesin Centrally Induces Frequent Urination through Brain Bombesin Receptor Types 1 and 2 in the Rat

Stress exacerbates symptoms of bladder dysfunction including overactive bladder and bladder pain syndrome, but the underlying mechanisms are unknown. Bombesin-like peptides and bombesin receptor types 1 and 2 (BB1 and BB2, respectively) in the brain have been implicated in the mediation/integration of stress responses. In this study, we examined effects of centrally administered bombesin on micturition, focusing on their dependence on 1) the sympathoadrenomedullary system (a representative mechanism activated by stress exposure) and 2) brain BB receptors in urethane-anesthetized (1.0-1.2 g/kg, i.p.) male rats. Intracerebroventricularly administered bombesin significantly shortened intercontraction intervals (ICI) at both doses (0.1 and 1 nmol/animal) without affecting maximal voiding pressure. Bombesin at 1 nmol induced significant increments of plasma noradrenaline and adrenaline levels, which were both abolished by acute bilateral adrenalectomy. On the other hand, adrenalectomy showed no effects on the bombesin-induced shortening of ICI. Much lower doses of bombesin (0.01 and 0.03 nmol/animal, i.c.v.) dose-dependently shortened ICI. Pretreatment with either a BB1 receptor antagonist (BIM-23127; d-Nal-cyclo[Cys-Tyr-d-Trp-Orn-Val-Cys]-Nal-NH2; 3 nmol/animal, i.c.v.) or a BB2 receptor antagonist (BEA; H-d-Phe-Gln-Trp-Ala-Val-Gly-His-Leu-NHEt; 3 nmol/animal, i.c.v.), respectively, suppressed the BB (0.03 nmol/animal, i.c.v.)-induced shortening of ICI, whereas each antagonist by itself (1 and 3 nmol/animal, i.c.v.) had no significant effects on ICI. Bombesin (0.03 nmol/animal, i.c.v.) significantly reduced voided volume per micturition and bladder capacity without affecting postvoid residual volume or voiding efficiency. These results suggest that brain bombesin and BB receptors are involved in facilitation of the rat micturition reflex to induce bladder overactivity, which is independent of the sympathoadrenomedullary outflow modulation.

Role of peripheral vascular resistance for the association between major depression and cardiovascular disease

Major depression and cardiovascular diseases are 2 of the most prevalent health problems in Western society, and an association between them is generally accepted. Although the specific mechanism behind this comorbidity remains to be elucidated, it is clear that it has a complex multifactorial character including a number of neuronal, humoral, immune, and circulatory pathways. Depression-associated cardiovascular abnormalities associate with cardiac dysfunctions and with changes in peripheral resistance. Although cardiac dysfunction in association with depression has been studied in detail, little attention was given to structural and functional changes in resistance arteries responsible for blood pressure control and tissue perfusion. This review discusses recent achievements in studies of depression-associated abnormalities in resistance arteries in humans and animal experimental models. The changes in arterial structure, contractile and relaxing functions associated with depression symptoms are discussed, and the role of these abnormalities for the pathology of major depression and cardiovascular diseases are suggested.

Increased atrial arrhythmia susceptibility induced by intense endurance exercise in mice requires TNFα

Atrial fibrillation (AF) is the most common supraventricular arrhythmia that, for unknown reasons, is linked to intense endurance exercise. Our studies reveal that 6 weeks of swimming or treadmill exercise improves heart pump function and reduces heart-rates. Exercise also increases vulnerability to AF in association with inflammation, fibrosis, increased vagal tone, slowed conduction velocity, prolonged cardiomyocyte action potentials and RyR2 phosphorylation (CamKII-dependent S2814) in the atria, without corresponding alterations in the ventricles. Microarray results suggest the involvement of the inflammatory cytokine, TNFα, in exercised-induced atrial remodelling. Accordingly, exercise induces TNFα-dependent activation of both NFκB and p38MAPK, while TNFα inhibition (with etanercept), TNFα gene ablation, or p38 inhibition, prevents atrial structural remodelling and AF vulnerability in response to exercise, without affecting the beneficial physiological changes. Our results identify TNFα as a key factor in the pathology of intense exercise-induced AF.

Central bombesin possibly induces S-nitrosylation of cyclooxygenase-1 in pre-sympathetic neurons of rat hypothalamic paraventricular nucleus

AIMS

Cyclooxygenase (COX) can be activated by nitric oxide-induced (NO-induced) conversion of cysteine thiol group of COX into S-nitrosothiol. We previously reported the involvement of brain COX/NO synthase (NOS) in centrally administered bombesin-, a stress-related neuropeptide, induced secretion of rat adrenal noradrenaline and adrenaline. To examine a possible involvement of the NO-induced modification of COX in bombesin-induced response, we investigated whether bombesin induces close proximity of COX-1 and neuronal NOS (nNOS) or S-nitroso-cysteine in pre-sympathetic spinally projecting neurons in the rat hypothalamic paraventricular nucleus (PVN), a regulatory center of adrenomedullary outflow.

MAIN METHODS

In twelve-week-old male Wistar rats, pre-sympathetic spinally projecting neurons in the PVN were labeled with a retrograde tracer Fluoro-Gold (FG). After intracerebroventricular administration of bombesin, we performed double immunohistochemical analysis for Fos and COX-1 or nNOS in FG-labeled PVN neurons. We also performed a fluorescent in situ proximity ligation assay (PLA) for visualizing of close proximity (<40 nm) of COX-1 with nNOS or S-nitroso-cysteine.

KEY FINDINGS

Bombesin significantly increased the number of Fos-immunoreactive cells in FG-labeled PVN neurons with COX-1 or nNOS immunoreactivity. 7-Nitroindazole, a selective nNOS inhibitor, abolished Fos-immunoreactivity induced by bombesin in COX-1-immunoreactive FG-labeled PVN neurons. Bombesin also induced PLA-positive signals indicating close proximity of COX-1/nNOS and COX-1/S-nitroso-cysteine in FG-labeled PVN neurons.

SIGNIFICANCE

Centrally administered bombesin possibly induces S-nitrosylation of COX-1 through close proximity of COX-1 and nNOS in pre-sympathetic spinally projecting PVN neurons, thereby activating COX-1 during the bombesin-induced activation of central adrenomedullary outflow in the rat.

Centrally administered bombesin activates COX-containing spinally projecting neurons of the PVN in anesthetized rats

The paraventricular nucleus (PVN) of the hypothalamus has a heterogenous structure containing different types of output neurons that project to the median eminence, posterior pituitary, brain stem autonomic centers and sympathetic preganglionic neurons in the spinal cord. Presympathetic neurons in the PVN send mono- and poly-synaptic projections to the spinal cord. In the present study using urethane-anesthetized rats, we examined the effects of centrally administered bombesin (a homologue of the mammalian gastrin-releasing peptide) on the mono-synaptic spinally projecting PVN neurons pre-labeled with a retrograde tracer Fluoro-Gold (FG) injected into T8 level of the spinal cord, with regard to the immunoreactivity for cyclooxygenase (COX) isozymes (COX-1/COX-2) and Fos (a marker of neuronal activation). FG-labeled spinally projecting neurons were abundantly observed in the dorsal cap, ventral part and posterior part of the PVN. The immunoreactivity of each COX-1 and COX-2 was detected in FG-labeled spinally projecting PVN neurons in the vehicle (10 μl of saline/animal, i.c.v.)-treated group, while bombesin (1 nmol/animal, i.c.v.) had no effect on the number of these immunoreactive neurons for each COX isozyme with labeling of FG. On the other hand, the peptide significantly increased the number of double-immunoreactive neurons for Fos and COX-1/COX-2 with FG-labeling in the PVN (except triple-labeled neurons for FG, COX-2 and Fos in the dorsal cap of the PVN), as compared to those of vehicle-treated group. These results suggest that centrally administered bombesin activates spinally projecting PVN neurons containing COX-1 and COX-2 in rats.

Stress triggered changes in expression of genes for neurosecretory granules in adrenal medulla

With acute stress, the release of adrenomedullary catecholamines is important for handling the emergency situation. However, when chronic or repeated, stress alters the allostatic load and leads to a hyperadrenergic state, resulting in the development or worsening of a wide range of diseases. To help elucidate the mechanism, we examined the effects of single and repeated immobilization stress on gene expression of components of neurosecretory vesicles in the adrenal medulla. Male Sprague-Dawley rats were exposed to immobilization stress once for 2 h (1× IMO) or daily for six consecutive days (6× IMO). Compared to unstressed controls, 1× IMO elevated gene expression of vesicular monoamine transporter 2 (VMAT2). In response to 6× IMO, not only was VMAT2 mRNA still elevated, but chromogranin A (CgA) and chromogranin B (CgB) mRNAs were also increased two to three-fold above basal levels. To investigate the possible role of the hypothalamic-pituitary-adrenal axis in the induction of VMAT2, PC12 cells were treated with the synthetic glucocorticoid dexamethasone, which was found to elevate VMAT2 mRNA expression. The findings suggest that following repeated stress, elevations of various components of neurosecretory vesicles in the adrenal can facilitate more efficient utilization of the well-characterized heightened catecholamine levels.

Behavioural consequences of two chronic psychosocial stress paradigms: anxiety without depression

Chronic stress, in particular chronic psychosocial stress, is a risk factor in the aetiology of various psychopathologies including anxiety- and depression-related disorders. Therefore, recent studies have focussed on the development of social-stress paradigms, which are believed to be more relevant to the human situation than non-social-stress paradigms. The majority of these paradigms have been reported to increase both anxiety- and depression-related behaviour in rats or mice. However, in order to dissect the mechanisms underlying anxiety or depression, animal models are needed, which specifically induce one, or the other, phenotype. Here, we study both short- (1d after stressor termination) and long-term (4d or 7d after stressor termination) behavioural and physiological consequences of two well-validated chronic psychosocial stress models: social-defeat/overcrowding (SD/OC) and chronic subordinate colony housing (CSC). We demonstrate that SD/OC and CSC result in different physiological alterations: SD/OC more strongly affecting body-weight development, whereas CSC more strongly affects adrenal and pituitary morphology. Both stressors were shown to flatten circadian locomotor activity immediately after stress termination, which normalized 7d later in SD/OC group but reversed to hyperactivity during the dark phase in the CSC group. Importantly, neither stress paradigm resulted in an increase in depression-related behaviour as assessed using the forced swim test, tail suspension test and saccharin preference test at any time-point. However, both stress paradigms lead to an anxiogenic phenotype; albeit with different temporal profiles and not towards a novel con-specific (social anxiety). CSC exposure elevates anxiety-related behaviour immediately after stressor termination, which lasts for at least 1 wk. In contrast, the anxiogenic phenotype only develops 1 wk after SD/OC termination. In conclusion, both models are unique for uncovering the molecular underpinnings of anxiety-related behaviour without conflicting depression-based alterations.

Repeated Short-term (2h×14d) Emotional Stress Induces Lasting Depression-like Behavior in Mice

Chronic behavioral stress is a risk factor for depression. To understand chronic stress effects and the mechanism underlying stress-induced emotional changes, various animals model have been developed. We recently reported that mice treated with restraints for 2 h daily for 14 consecutive days (2h-14d or 2h×14d) show lasting depression-like behavior. Restraint provokes emotional stress in the body, but the nature of stress induced by restraints is presumably more complex than emotional stress. So a question remains unsolved whether a similar procedure with "emotional" stress is sufficient to cause depression-like behavior. To address this, we examined whether "emotional" constraints in mice treated for 2h×14d by enforcing them to individually stand on a small stepping platform placed in a water bucket with a quarter full of water, and the stress evoked by this procedure was termed "water-bucket stress". The water-bucket stress activated the hypothalamus-pituitary-adrenal gland (HPA) system in a manner similar to restraint as evidenced by elevation of serum glucocorticoids. After the 2h×14d water-bucket stress, mice showed behavioral changes that were attributed to depression-like behavior, which was stably detected >3 weeks after last water-bucket stress endorsement. Administration of the anti-depressant, imipramine, for 20 days from time after the last emotional constraint completely reversed the stress-induced depression-like behavior. These results suggest that emotional stress evokes for 2h×14d in mice stably induces depression-like behavior in mice, as does the 2h×14d restraint.

Vulnerability to chronic subordination stress-induced depression-like disorders in adult 129SvEv male mice

Exposure to stressful life events is intimately linked with vulnerability to neuropsychiatric disorders such as major depression. Pre-clinical animal models offer an effective tool to disentangle the underlying molecular mechanisms. In particular, the 129SvEv strain is often used to develop transgenic mouse models but poorly characterized as far as behavior and neuroendocrine functions are concerned. Here we present a comprehensive characterization of 129SvEv male mice's vulnerability to social stress-induced depression-like disorders and physiological comorbidities. We employed a well characterized mouse model of chronic social stress based on social defeat and subordination. Subordinate 129SvEv mice showed body weight gain, hyperphagia, increased adipose fat pads weight and basal plasma corticosterone. Home cage phenotyping revealed a suppression of spontaneous locomotor activity and transient hyperthermia. Subordinate 129SvEv mice also showed marked fearfulness, anhedonic-like response toward a novel but palatable food, increased anxiety in the elevated plus maze and social avoidance of an unfamiliar male mouse. A direct measured effect of the stressfulness of the living environment, i.e. the amount of daily aggression received, predicted the degree of corticosterone level and locomotor activity but not of the other parameters. This is the first study validating a chronic subordination stress paradigm in 129SvEv male mice. Results demonstrated remarkable stress vulnerability and establish the validity to use this mouse strain as a model for depression-like disorders.

The Utility of Animal Models in Understanding Links between Psychosocial Processes and Cardiovascular Health

A bidirectional association between mood disorders and cardiovascular disease has been described; however, the neurobiological mechanisms that underlie this link have not been fully elucidated. The purpose of this review is first to describe some of the important behavioral neurobiological processes that are common to both mood and cardiovascular disorders. Second, this review focuses on the value of conducting research with animal models (primarily rodents) to investigate potential behavioral, physiological, and neural processes involved in the association of mood disorders and cardiovascular disease. In combination with findings from human research, the study of mechanisms underlying mood and cardiovascular regulation using animal models will enhance our understanding of the association of depression and cardiovascular disease, and can promote the development of novel interventions for individuals with these comorbid conditions.

Stress revisited: a critical evaluation of the stress concept

With the steadily increasing number of publications in the field of stress research it has become evident that the conventional usage of the stress concept bears considerable problems. The use of the term 'stress' to conditions ranging from even the mildest challenging stimulation to severely aversive conditions, is in our view inappropriate. Review of the literature reveals that the physiological 'stress' response to appetitive, rewarding stimuli that are often not considered to be stressors can be as large as the response to negative stimuli. Analysis of the physiological response during exercise supports the view that the magnitude of the neuroendocrine response reflects the metabolic and physiological demands required for behavioural activity. We propose that the term 'stress' should be restricted to conditions where an environmental demand exceeds the natural regulatory capacity of an organism, in particular situations that include unpredictability and uncontrollability. Physiologically, stress seems to be characterized by either the absence of an anticipatory response (unpredictable) or a reduced recovery (uncontrollable) of the neuroendocrine reaction. The consequences of this restricted definition for stress research and the interpretation of results in terms of the adaptive and/or maladaptive nature of the response are discussed.

Behavior and pro-inflammatory cytokine variations among submissive and dominant mice engaged in aggressive encounters: moderation by corticosterone reactivity

Psychosocial stressors contribute to the pathophysiology of affective disorders and variations of cytokine functioning have been implicated in this process. The present investigation demonstrated, in mice, the impact of stressful aggressive encounters on activity levels, plasma corticosterone and cytokine concentrations, and on cytokine mRNA expression within the prefrontal cortex (PFC) and hippocampus. As glucocorticoids have been tied to cytokine variations, mice were subdivided into low or high corticosterone responders, defined in terms of circulating hormone levels 75 min post-confrontation. Interestingly, stressor-induced effects among low and high responders varied as a function of whether mice were submissive or dominant during the aggressive bout. Agonistic encounters elicited subsequent hyperactivity, particularly among low corticosterone responders and among dominant mice. Plasma levels of corticosterone and interleukin (IL)-6 concomitantly increased after aggressive encounters and varied with dominance status and with the low versus high corticosterone response. Among the low responders corticosterone and IL-6 increases were modest and only apparent among submissive mice, whereas among high responders these elevations were more pronounced and comparable in submissive and dominant mice. Aggressive episodes also increased IL-1β and IL-6 mRNA brain expression. The IL-1β rise was greater in the PFC and hippocampus of submissive mice that were low responders. Among high responders IL-1β and IL-6 increased in both groups, although in the PFC this effect was specific to dominant mice. The data are discussed in terms of their relevance to the impact of aggressive encounters on affective behaviors, and to the role that cytokines might play in this regard.

[Effect of social interaction on skin temperature in mice]

We investigated physiological and behavioral characteristics of socially stressed animals in a resident-intruder paradigm. ICR male mice (resident, n = 14) were exposed individually to a novel male conspecific (intruder, n = 14) in their homecage for 30 min. Along with behavioral analyses, the skin temperatures of both the resident and the intruder were measured simultaneously using a multipoint radiation thermometer. There were no significant differences between the resident and intruder in the amount of locomotion, flight and aggressive behaviors. The mean skin temperature of the residents during the interaction was higher than before the interaction. In addition, the skin temperatures of the intruders were consistently higher than the residents. The results suggest that social stress causes elevation in skin temperature as well as stress-induced hyperthermia in core temperature. Moreover, infrared radiation thermometers may provide an alternative means of measuring physiological parameters of two (or more) subjects simultaneously in the study of animal social behavior.

Neural regulation of endocrine and autonomic stress responses

The survival and well-being of all species requires appropriate physiological responses to environmental and homeostatic challenges. The re- establishment and maintenance of homeostasis entails the coordinated activation and control of neuroendocrine and autonomic stress systems. These collective stress responses are mediated by largely overlapping circuits in the limbic forebrain, the hypothalamus and the brainstem, so that the respective contributions of the neuroendocrine and autonomic systems are tuned in accordance with stressor modality and intensity. Limbic regions that are responsible for regulating stress responses intersect with circuits that are responsible for memory and reward, providing a means to tailor the stress response with respect to prior experience and anticipated outcomes.

Corticosterone differences rather than social housing predict performance of T-maze alternation in male CD-1 mice

This study examined the effects of social housing manipulations on bodyweight, corticosterone levels, and performance of T-maze alternation in male CD-1 mice. Males that adopted a dominant social rank were heavier than those that adopted a subordinate social rank. Dominant males also had lower corticosterone concentrations than the subordinates. However, there was little to suggest that these physiological indicators of social rank were moderated by housing condition. Indeed, statistical analysis confirmed that the difference in bodyweights was evident before males were socially housed. The mice showed high levels of spatial alternation on the T-maze from the start of testing so performance accuracy was high. Neither social rank nor housing condition had any clear categorical effect on T-maze performance. However, performance did fluctuate over successive blocks of testing and there was a negative association between accuracy on the T-maze and corticosterone levels (consistent with performance impairment because of elevated corticosterone). Therefore, under present conditions, individual differences in corticosterone were a better predictor of T-maze performance than social rank or housing condition. The results of the present study lend further support to the proposition that corticosterone levels measured non-invasively in urine may be used to predict diverse welfare outcomes for laboratory mice, from bodyweight to cognitive performance. Moreover, intrinsic physiological parameters rather than external influences, such as social housing, may have more influence on mouse behaviour.

Mechanisms underlying altered mood and cardiovascular dysfunction: the value of neurobiological and behavioral research with animal models

A bidirectional association between mood disorders and cardiovascular diseases has been described in humans, yet the precise neurobiological mechanisms that underlie this association are not fully understood. This article is focused on neurobiological processes and mediators in mood and cardiovascular disorders, with an emphasis on common mechanisms including stressor reactivity, neuroendocrine and neurohumoral changes, immune alterations, autonomic and cardiovascular dysregulation, and central neurotransmitter and neuropeptide dysfunction. A discussion of the utility of experimental investigations with rodent models, including those in rats and prairie voles (Microtus ochrogaster), is presented. Specific studies using these models are reviewed, focusing on the analysis of behavioral, physiological and neural mechanisms underlying depressive disorders and cardiovascular disease. Considered in combination with studies using human samples, the investigation of mechanisms underlying depressive behaviors and cardiovascular regulation using animal models will enhance our understanding of the association of depression and cardiovascular disease, and will promote the development of improved interventions for individuals with these detrimental disorders.

Metabolic consequences and vulnerability to diet-induced obesity in male mice under chronic social stress

Social and psychological factors interact with genetic predisposition and dietary habit in determining obesity. However, relatively few pre-clinical studies address the role of psychosocial factors in metabolic disorders. Previous studies from our laboratory demonstrated in male mice: 1) opposite status-dependent effect on body weight gain under chronic psychosocial stress; 2) a reduction in body weight in individually housed (Ind) male mice. In the present study these observations were extended to provide a comprehensive characterization of the metabolic consequences of chronic psychosocial stress and individual housing in adult CD-1 male mice. Results confirmed that in mice fed standard diet, dominant (Dom) and Ind had a negative energy balance while subordinate (Sub) had a positive energy balance. Locomotor activity was depressed in Sub and enhanced in Dom. Hyperphagia emerged for Dom and Sub and hypophagia for Ind. Dom also showed a consistent decrease of visceral fat pads weight as well as increased norepinephrine concentration and smaller adipocytes diameter in the perigonadal fat pad. On the contrary, under high fat diet Sub and, surprisingly, Ind showed higher while Dom showed lower vulnerability to obesity associated with hyperphagia. In conclusion, we demonstrated that social status under chronic stress and individual housing deeply affect mice metabolic functions in different, sometime opposite, directions. Food intake, the hedonic response to palatable food as well as the locomotor activity and the sympathetic activation within the adipose fat pads all represent causal factors explaining the different metabolic alterations observed. Overall this study demonstrates that pre-clinical animal models offer a suitable tool for the investigation of the metabolic consequences of chronic stress exposure and associated psychopathologies.

Social stress, therapeutics and drug abuse: preclinical models of escalated and depressed intake

The impact of ostensibly aversive social stresses on triggering, amplifying and prolonging intensely rewarding drug taking is an apparent contradiction in need of resolution. Social stress encompasses various types of significant life events ranging from maternal separation stress, brief episodes of social confrontations in adolescence and adulthood, to continuous subordination stress, each with its own behavioral and physiological profile. The neural circuit comprising the VTA-accumbens-PFC-amygdala is activated by brief episodes of social stress, which is critical for the DA-mediated behavioral sensitization and increased stimulant consumption. A second neural circuit comprising the raphe-PFC-hippocampus is activated by continuous subordination stress and other types of uncontrollable stress. In terms of the development of therapeutics, brief maternal separation stress has proven useful in characterizing compounds acting on subtypes of GABA, glutamate, serotonin and opioid receptors with anxiolytic potential. While large increases in alcohol and cocaine intake during adulthood have been seen after prolonged maternal separation experiences during the first two weeks of rodent life, these effects may be modulated by additional yet to be identified factors. Brief episodes of defeat stress can engender behavioral sensitization that is relevant to escalated and prolonged self-administration of stimulants and possibly opioids, whereas continuous subordination stress leads to anhedonia-like effects. Understanding the intracellular cascade of events for the transition from episodic to continuous social stress in infancy and adulthood may provide insight into the modulation of basic reward processes that are critical for addictive and affective disorders.

Kinetics and persistence of cardiovascular and locomotor effects of immobilization stress and influence of ACTH treatment

Stress triggers crucial responses, including elevated blood pressure and heart rate (HR), to handle the emergency and restore homeostasis. However, continuation of these effects following cessation of the stress is implicated with many stress-related disorders. Here, we examine the kinetics and persistence of cardiovascular and locomotor responses to single and repeated immobilization stress (IMO), with and without prior treatment with adrenocorticotropic hormone (ACTH). Radiotelemetry probes were implanted into male Sprague-Dawley rats to continually monitor mean arterial pressure (MAP), HR and locomotor activity. Rats were subjected to IMO for 2 h daily (10 a.m. to noon, 6 consecutive days). The first IMO induced the greatest change in MAP (about 30 mm Hg) and HR (about 200 bpm). Following each IMO, MAP and HR were elevated during the remaining light phase and in the subsequent dark phase, HR was lower than prior to IMO. We further examined whether elevation of ACTH to a level similar to IMO will elicit similar effects, and if it will alter subsequent responses to IMO. Injection of ACTH (13 IU/kg, s.c.) triggered a short-lived rise in MAP, and decreased HR. After six daily injections of ACTH and recovery time (8 days), rats were immobilized as above. The cardiovascular responses were similar during the IMO, but the ACTH-pretreated group displayed differences following cessation of the IMO. In addition, IMO led to a large reduction of locomotor activity during the dark (normally active) phase to levels similar to the light phase. Following the IMOs, locomotor activity recovered more slowly in the ACTH-pretreated group. The study revealed that IMO-triggered cardiovascular and locomotor responses are evident after termination of the stress. In addition, prior exposure to ACTH delayed recovery in cardiovascular and locomotor functions following cessation of stress.

Long-term behavioral and neuroendocrine alterations following chronic social stress in mice: implications for stress-related disorders

The period of adolescence is characterized by a high vulnerability to stress and trauma, which might result in long-lasting consequences and an increased risk to develop psychiatric disorders. Using a recently developed mouse model for chronic social stress during adolescence, we studied persistent neuroendocrine and behavioral effects of chronic social stress obtained 12 months after cessation of the stressor. As a reference, we investigated immediate effects of chronic stress exposure obtained at the end of the chronic stress period. Immediately after the 7 week chronic stress period stressed animals show significantly increased adrenal weights, decreased thymus weight, increased basal corticosterone secretion and a flattened circadian rhythm. Furthermore, stressed animals display an increased anxiety-like behavior in the elevated plus maze and the novelty-induced suppression of feeding test. Hippocampal mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR) mRNA levels were significantly decreased. To investigate persistent consequences of this early stressful experience, the same parameters were assessed in aged mice 12 months after the cessation of the stressor. Interestingly, we still found differences between formerly stressed and control mice in important stress-related parameters. MR expression levels were significantly lower in stressed animals, suggesting lasting, possibly epigenetic alterations in gene expression regulation. Furthermore, we observed long-term behavioral alterations in animals stressed during adolescence. Thus, we could demonstrate that chronic stress exposure during a crucial developmental time period results in long-term, persistent effects on physiological and behavioral parameters throughout life, which may contribute to an enhanced vulnerability to stress-induced diseases.