Social conflict models: can they inform us about human psychopathology?

Chronic stress alters the density and morphology of microglia in a subset of stress-responsive brain regions

The current study, in parallel experiments, evaluated the impact of chronic psychological stress on physiological and behavioural measures, and on the activation status of microglia in 15 stress-responsive brain regions. Rats were subjected, for 14 days, to two 30 min sessions of restraint per day, applied at random times each day. In one experiment the effects of stress on sucrose preference, weight gain, core body temperature, and struggling behaviour during restraint, were determined. In the second experiment we used immunohistochemistry to investigate stress-induced changes in ionized calcium-binding adaptor molecule-1 (Iba1), a marker constitutively expressed by microglia, and major histocompatibility complex-II (MHC-II), a marker often expressed on activated microglia, in a total of 15 stress-responsive nuclei. We also investigated cellular proliferation in these regions using Ki67 immunolabelling, to check for the possibility of microglial proliferation. Collectively, the results we obtained showed that chronic stress induced a significant increase in anhedonia, a decrease in weight gain across the entire observation period, a significant elevation in core body temperature during restraint, and a progressive decrease in struggling behaviour within and over sessions. With regard to microglial activation, chronic stress induced a significant increase in the density of Iba1 immunolabelling (nine of 15 regions) and the number of Iba1-positive cells (eight of 15 regions). Within the regions that exhibited an increased number of Iba1-positive cells after chronic stress, we found no evidence of a between group difference in the number of MHC-II or Ki67 positive cells. In summary, these results clearly demonstrate that chronic stress selectively increases the number of microglia in certain stress-sensitive brain regions, and also causes a marked transition of microglia from a ramified-resting state to a non-resting state. These findings are consistent with the view that microglial activation could play an important role in controlling and/or adapting to stress.

Support for religio-political aggression among teenaged boys in Gaza: Part I: psychological findings

Politically aggressive militant groups usually rely on support from a larger community, although evidence suggests that only some members of that larger community support that aggression. A major subtype of political aggression is that associated with religious differences--or Religio-Political Aggression (RPA). Little previous research has explored demographic or psychological factors that might distinguish supporters from non-supporters of RPA. In an exploratory study, we investigated whether factors previously associated with aggression might correlate with support for RPA in the case of the Israeli/Palestinian conflict. During the second intifada, fifty-two 14-year-old Palestinian boys in Gaza completed self-report measures of life events, emotional status, and political attitudes. Teenaged boys who reported family members having been wounded or killed by the Israeli Defense Forces (IDF) expressed greater support for RPA (t(50) = -2.30, P = .026). In addition, boys who felt their group was treated unjustly reported greater support for RPA compared with those who did not (t(50) = -2.273, P = .027). Implications of these preliminary data are discussed.

Repeated social defeat selectively increases δFosB expression and histone H3 acetylation in the infralimbic medial prefrontal cortex

Exposure to social stress has been linked to the development and maintenance of mood-related psychopathology; however, the underlying neurobiological changes remain uncertain. In this study, we examined numbers of δFosB-immunoreactive cells in the forebrains of rats subjected to 12 episodes of social defeat. This was achieved using the social conflict model whereby animals are introduced into the home cage of older males ("residents") trained to attack and defeat all such "intruders"; importantly, controls were treated identically except that the resident was absent. Our results indicated that the only region in which δFosB-positive cells were found in significantly higher numbers in intruders than in controls was the infralimbic medial prefrontal cortex (mPFC). This same effect was not apparent using another psychological stressor, noise stress. Cells of the infralimbic mPFC also displayed evidence of chromatin remodeling. We found that exposure to repeated episodes of social defeat increased numbers of cells immunoreactive for histone H3 acetylation, but not for histone H3 phosphoacetylation, in the infralimbic mPFC. Collectively, these findings highlight the importance of the infralimbic mPFC in responding to social stress-a finding that provides insight into the possible neurobiological alterations associated with stress-induced psychiatric illness.

Models of depression

The incidence of depressive illness is high in the United States and worldwide, and the inadequacy of currently available drug treatments contributes to the significant health burden associated with depression. A basic understanding of the underlying disease processes in depression is lacking, and therefore, recreating the disease in animal models is not possible. Currently used models of depression attempt to produce quantifiable correlates of human symptoms in experimental animals. The models differ in the degree to which they produce features that resemble a depressive-like state, and models that include stress exposure are widely used. Paradigms that employ acute or subchronic stress exposure include learned helplessness, forced swim test, and tail suspension test, which employ relatively short-term exposure to inescapable or uncontrollable stress and can reliably detect antidepressant drug response. Longer-term models include chronic mild stress models, early-life stress models, and social conflict models, which may more accurately simulate processes that lead to depression. These models each have varying degrees of face, construct, and predictive validity for depression and contribute differently to our understanding of antidepressant processes.

Darwinian dynamics of depression

Darwin's theory of sexual selection offers a challenge to psychology and psychiatry. We select each other, and have been doing so since social life first evolved. But who is selected and what happens to those who are not selected? What social structures have evolved to contain the unselected? What behaviours have evolved to manage the selection process? How do the selected relate to the unselected and what behaviours have evolved to manage this asymmetry in social relations? What mental states have evolved to characterize the selected and the unselected? These questions should be kept in mind when we observe and study the social structures, behaviours and mental states that we see displayed before us in all the variety of nature. It is suggested that a significant amount of current psychiatric disorder, especially depressive states and both social and generalized anxiety disorder, have evolved because they managed the processes of being unselected and de-selected, and maintained the unselected in that social role without loss of life or physical incapacity, and enabled the unselected to contribute to general social well-being.

Gonadal hormones modulate the display of conditioned defeat in male Syrian hamsters

It has been widely reported that gonadal hormones influence the display of aggression in Syrian hamsters; conversely, much less is known about whether gonadal hormones modulate submissive/defensive behaviors in these animals. Following social defeat, male hamsters no longer display normal territorial aggression but instead display submissive/defensive behavior in the presence of a smaller opponent, a phenomenon we have termed conditioned defeat (CD). The purpose of the present study was to examine the effect of gonadal hormones on the display of CD in male hamsters. In Experiment 1, males were castrated or sham-operated. The castrated males were significantly more submissive following social defeat relative to their intact counterparts. The increased submissive behavior in the castrated males during CD testing was particularly surprising, given the fact that they were attacked significantly less during CD training. In Experiment 2a, males were castrated and given hormone replacement. Castrated males treated with testosterone or dihydrotestosterone displayed significantly less submissive behavior following social defeat than did those treated with cholesterol or estradiol. Finally, in Experiment 2b, there was no effect of hormone replacement on aggressive behavior in non-defeated hamsters suggesting that the decrease in submissive behavior in males treated with dihydrotestosterone or testosterone is specific to being previously defeated. Taken together the data indicate that the presence of androgens reduces the display of submission in defeated male hamsters. More importantly, these findings suggest that androgens may have a protective effect against the development of depression-like or anxiety-like behaviors following exposure to an ethologically relevant stressor.

Genetic, epigenetic and environmental impact on sex differences in social behavior

The field of behavioral neuroendocrinology has generated thousands of studies that indicate differences in brain structure and reactivity to gonadal steroids that produce sex-specific patterns of social behavior. However, rapidly emerging evidence shows that genetic polymorphisms and resulting differences in the expression of neuroactive peptides and receptors as well as early-life experience and epigenetic changes are important modifiers of social behavior. Furthermore, due to its inherent complexity, the neurochemical mechanisms underlying sex differences in social behavior are usually studied in a tightly regulated laboratory setting rather than in complex environments. Importantly, specific hormones may elicit a range of different behaviors depending on the cues present in these environments. For example, individuals exposed to a psychosocial stressor may respond differently to the effects of a gonadal steroid than those not exposed to chronic stress. The objective of this review is not to re-examine the activational effects of hormones on sex differences in social behavior but rather to consider how genetic and environmental factors modify the effects of hormones on behavior. We will focus on estrogen and its receptors but consideration is also given to the role of androgens. Furthermore, we have limited our discussions to the importance of oxytocin and vasopressin as targets of gonadal steroids and how these effects are modified by genetic and experiential situations. Taken together, the data clearly underscore the need to expand research initiatives to consider gene-environment interactions for better understanding of the neurobiology of sex differences in social behavior.

Coping with defeat: acute glucocorticoid and forebrain responses to social defeat vary with defeat episode behaviour

Individuals vary in the way in which they cope with stressful situations. It has been suggested that 'active' coping behaviour, characterised by aggression and territorial control, is more effective in moderating the stress associated with social defeat than 'passive' coping behaviour, as characterised by immobility, decreased reactivity, and low aggression. We used the rodent 'resident/intruder' paradigm to determine whether individual differences in coping behaviour modulate the acute adrenocortical response to social defeat. During the 10 min conflict episode, behaviours displayed by the intruder were recorded and subsequently scored. Intruders that engaged in large numbers of fights and/or frequently used physical structures to block the resident's approach (a behaviour referred to as 'guarding'), displayed smaller corticosterone responses to defeat than other intruders. Corticosterone responses to defeat were unrelated to a measure of coping style preferences (defensive burying test) obtained prior to the defeat encounter. We further chose to investigate the neurobiological basis of this observation by comparing the patterns of defeat-induced neuronal activation in the forebrains of intruders that displayed high versus low numbers of defensive behaviours during the defeat episode. The results of this analysis indicated that 'low fight' and 'low guard' intruders, i.e. those that achieved a fight or a guard score below the 20th percentile, had significantly higher numbers of Fos-positive neurons in forebrain regions such as the medial prefrontal cortex and the amygdala than did control animals exposed to an empty resident's cage. In summary, the present data suggest that 'active' coping behaviour is associated with both a smaller adrenocortical response and a lower level of 'neural activation' following social defeat. This outcome differs from that of earlier studies, a difference that we suggest is due to the fact that the present study is the first to assess coping on the basis of behaviour actually displayed during the conflict interaction.

Social defeat stress activates medial amygdala cells that express type 2 corticotropin-releasing factor receptor mRNA

Defeat is a social stressor involving subordination by a threatening conspecific. Type 2 corticotropin-releasing factor receptors (CRF(2)) are abundant in brain regions implicated in defeat responses and are putative stress-related molecules. The present study sought to determine whether neuroactivation and CRF(2) expression co-occurred at brain region or cellular levels following acute defeat. Male "intruder" Wistar rats were placed into the cage of an aggressive "resident" Long-Evans rat (n=6). Upon defeat, intruders (n=6) were placed in a wire-mesh chamber and were returned to the resident's cage for an additional 75 min. Controls (n=6) were handled and returned to their home cage for the same duration. Coronal brain sections were stained for an immediate early gene product, Fos, as a neuronal activation marker. Combined immunohistochemistry with in situ hybridization was performed on a subset of brain sections from defeated intruders to visualize Fos immunoreactivity and CRF(2) mRNA jointly. Defeated rats had fivefold, sevenfold, and 10-fold more Fos-positive cells than controls in the arcuate, ventromedial nucleus of the hypothalamus, and medial amygdala post-defeat. Significant colocalization of CRF(2) mRNA and Fos-positive cells was observed in the posterior medial amygdala but not in the arcuate nucleus or ventromedial hypothalamus. The results indicate CRF(2) receptor-positive neurons in the posterior medial amygdala are involved in the neural response to social defeat.

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 status and shaming experiences related to adolescent overt aggression at school

Feelings of rejection and humiliation in interpersonal interaction are strongly related to aggressive behavior. The aim of this study was to investigate the association between social status, shaming experiences, gender and adolescent aggressive behavior by using a status-shaming model. A population-based sample of 5,396 adolescents aged from 15 to 18 completed a questionnaire that asked questions regarding psychosocial background, shaming experiences, social status of family, peer group and school and involvement in physical or verbal aggression at school. Shaming experiences, i.e. being ridiculed or humiliated by others, were strongly related to aggressive behavior. Social status and shaming were related in the prediction of aggressive behavior, suggesting that a person's social status may influence the risk for taking aggressive action when subjected to shaming experiences. Medium social status seemed to have a protective function in the association between shaming experiences and aggression. This study confirms the importance of further evaluation of the role of perceived social status and shaming experiences in the understanding of aggressive behavior. Moreover, the results indicate the need for different kinds of status measures when investigating the associations between status and behavior in adolescent populations. The results may have important implications for the prevention of bullying at school as well as other deviant aggressive behavior among adolescents.

On the chronobiology of cohabitation

Social regulation of animal circadian rhythms may enable individuals in a population to temporally synchronize or segregate their activities within the community. Relatively little is known about the mechanisms for such interindividual temporal adaptations or how the circadian system might be involved. The literature suggests that actual prolonged cohabitation might lead to robust effects on the rhythmicity of cohoused individuals but that these effects are not easily reproduced by indirect or pulsatile social contacts. We have begun to study the conditions under which such cohabitation effects might be revealed in the laboratory, and we present and discuss initial data that cohousing pairs of golden hamsters can result in a persistent change in the free-running circadian period of one of the two hamsters of the pair. We believe that analyzing the societal level of temporal organization, and ultimately dissecting its underlying mechanisms, will enrich our understanding of the circadian clock and its role in establishing ecological communities.

A new comprehensive evolutionary model of depression and anxiety

Difference amplification was the process whereby the difference in fitness between two competing individuals in early man was magnified by the results of the competition. It arises from adaptive and maladaptive cycles (characterized by depression and anxiety) that are initiated by winning and losing agonistic encounters. Those who were most successful were likely to find mates that were also successful and vice versa. This would have contributed to well-endowed progeny and accelerated phylogenetic evolution. The adaptive and maladaptive cycles of the difference amplification model are also a feature of the social rank and attachment models. Ineffective operation of social rank and attachment systems is associated with anxiety and depression. This paper introduces the notion that the efficient operation of these two systems in hierarchical encounters accelerates the phylogenetic adaptation of the individual's genetic line. This suggests an adaptive function of attachment and social rank mechanisms that has not been previously described. Social rank, attachment and difference amplification should be viewed as different aspects of a comprehensive evolutionary model of depression and anxiety. This new model has psychotherapeutic implications.

Territory, Rank and Mental Health: The History of an Idea

We trace the development of ideas about the relation of mood to social rank and territory. We suggest that elevated mood enabled a person to rise in rank and cope with the increased activities and responsibilities of a leadership role, while depressed mood enabled a person to accept low rank and to forego the rewards associated with high rank. This led to the concept of a trio of agonist/investor strategy sets, each consisting of escalating and de-escalating strategies, one set at each of the three levels of the triune forebrain. Depressed mood can be seen as a de-escalating (appeasement) strategy at the lowest (reptilian) level; this should facilitate de-escalation at the highest (rational) level, but sometimes this rational level de-escalation is blocked (e.g., by stubbornness, courage, pride or ambition) and then clinical depression may ensue. These evolved psychobiological mechanisms survived the partial transition from agonistic to prestige competition. We discuss difficulties which have arisen with our ideas, and their implications for clinical work and research.

Social stress, immune functions and disease in rodents

The link between social factors, stress and health has been the focus of many interdisciplinary studies mostly because: (i) animals, including humans, often live in societies; (ii) positive and negative social relationships affect disease and well being; (iii) physiological alterations, which parallel social interactions also modulate immune and neuroendocrine functions. This review will focus on studies conducted on laboratory and wild rodents where social factors such as dyadic interactions, individual housing and differential group housing were investigated. The results obtained allow one to conclude that social factors in rodents are causally linked with immune disorders/disease susceptibility. In particular, lower lymphocyte proliferation and antigen-specific-IgG, granulocytosis and lymphopenia, as well as higher tumor induction and progression, are reliably associated with negative social events. Finally, due to the increasing utilization of social stress-based animal models the reliability of the concept of "social stress" and its evolutionary context are re-evaluated.

What's wrong with my mouse model?

Stress plays a key role in pathogenesis of anxiety and depression. Animal models of these disorders are widely used in behavioral neuroscience to explore stress-evoked brain abnormalities, screen anxiolytic/antidepressant drugs and establish behavioral phenotypes of gene-targeted or transgenic animals. Here we discuss the current situation with these experimental models, and critically evaluate the state of the art in this field. Noting a deficit of fresh ideas and especially new paradigms for animal anxiety and depression models, we review existing challenges and outline important directions for further research in this field.