Social status differentiates rapid neuroendocrine responses to restraint stress

Male Anolis carolinensis that win aggressive interactions mobilize neuroendocrine responses to social stress more rapidly than defeated lizards. We initially examined temporal patterns of neuroendocrine response to restraint stress in lizards of unknown status, and then investigated whether winning males respond more rapidly to this non-social stressor. Size-matched male pairs interacted to establish social status, and then were returned to individual home cages for 3 days. Plasma and brains were collected from non-restrained dominants and subordinates, and from a non-interacting control group. Additional groups of dominants and subordinates underwent 90 s restraint stress, with plasma and brains collected either immediately or 300 s after restraint. In lizards of unknown social status restraint stimulated rapid monoaminergic responses in nucleus accumbens, hippocampus, amygdala, and locus ceruleus, with delayed responses seen in VTA and raphé. Non-restrained dominants and subordinates had lower levels of raphé serotonergic activity and lower hippocampal dopaminergic activity 3 days after interacting, compared to controls. Dominants had higher corticosterone levels, both immediately and 300 s after restraint, than either non-restrained dominants or restrained subordinates. Restraint induced higher raphé serotonergic activity in dominants. However, subordinates also showed rapid responses to restraint; exhibiting lower hippocampal dopamine (DA) levels than non-restrained subordinates. At 300 s after the stress, amygdalar serotonin levels increased in dominants, while subordinates showed higher amygdalar DA levels. These results suggest that stressful aggressive interactions will not only alter basal neurochemical activity, but also influence neuroendocrine responses to non-social stressors according to individual social status.     

Serotonergic response to social stress and artificial social sign stimuli during paired interactions between male Anolis carolinensis

Serotonergic activity is influenced by social status and manipulation of social signals. In the lizard Anolis carolinensis, eyespot formation, i.e. darkening of postorbital skin from green to black, appears during stressful and agonistic situations, forming first in males that become dominant. To assess the effect of eyespots on central serotonergic activity during social interaction, males were paired by weight and painted postorbitally with green or black paint. Manipulation of eyespot color influenced social interactions and status. All males that viewed an opponent with black painted eyespots became subordinate. In these subordinate animals, serotonergic activity was elevated in hippocampus, striatum, nucleus accumbens and locus ceruleus. In contrast, males that viewed opponents with hidden eyespots (painted green) and became dominant had increased serotonergic activity in hypothalamus, medial amygdala and raphé. Pre-painted eyespots produced results that distinguish dominant and subordinate relationships based on serotonergic activity not previously seen in unmanipulated pairs. Results from experiments using pairs are similar to those using mirrors for medial amygdala and locus ceruleus, but not hippocampus, nucleus accumbens or raphé. Decreased hypothalamic serotonin was associated with increased aggressive behavior. These results, when compared with previous studies, suggest some flexibility in central serotonergic systems, which may shape dominant and subordinate rank acquisition, and appear to be influenced by the completion of social role formation. Furthermore, social status and central serotonergic activity was influenced by a visual cue, the presence or absence of postorbital eyespots on an opponent.     

Dopaminergic activity modulation via aggression, status, and a visual social signal

Social interaction may elicit aggression, establish social rank, and be influenced by changes in central dopaminergic activity. In the lizard Anolis carolinensis, a sign stimulus (darkening of postorbital skin or eyespots) inhibits aggressive response from opponents, in part because it forms more rapidly in dominant males. The authors report that artificially hiding or darkening eyespots influences central dopaminergic activity, social status, and aggression during dyadic social interaction. All males that viewed an opponent with eyespots painted black became subordinate and exhibited elevated dopamine in raphe, lateral amygdala, and medial amygdala but decreased dopamine in septum and locus ceruleus. In contrast, males that viewed opponents with hidden eyespots (painted green) became dominant and had increased dopamine in striatum, nucleus accumbens, hypothalamus, and combined substantia nigra/ventral tegmental area.     

Plasma catecholamines and social behavior in male vervet monkeys (Cercopithecus aethiops sabaeus).

Many investigations in humans indicate that epinephrine, norepinephrine and their ratio may correlate with such traits as social competence, academic achievement, and aggression. However, the socioeconomic, dietary, and environmental confounds accompanying most human studies complicate their interpretation. Social status, aggression, and other social behaviors can be reliably assessed in nonhuman primates under conditions controlling for crucial environmental factors. If interpretation of human studies is correct, dominant and subordinate male vervet monkeys should exhibit distinctive patterns of catecholamine secretion. To test this possibility, seventeen adult male monkeys living in six stable social groups were observed for 6 months. Based on their success in agonistic events, subjects were categorized as dominant or subordinate. Alpha scores were calculated from empirically derived factors to provide a noncategorical measure of dominant behavioral style. Plasma epinephrine and norepinephrine samples obtained from anesthetized subjects did not differ between dominant and subordinate males. Alpha scores, however, distinguished high from low norepinephrine/epinephrine ratio groups. These findings are consistent with studies in humans linking high epinephrine, low norepinephrine, and social competence.     

Temporal patterns of limbic monoamine and plasma corticosterone response during social stress

Dominant and subordinate males respond differently to the stress of social interaction. After an hour of social interaction, subordinate male Anolis carolinensis have elevated serotonergic activity in hippocampus, but dominant males do not. In other species, and using other stressors, the activation of hippocampal serotonergic activity is much more rapid than one hour. To elucidate early stress responsiveness, adult male A. carolinensis were divided into four groups: isolated controls, and pairs of males sampled after 10, 20 or 40 minutes of aggressive interaction. Development of dominant-subordinate relationships was determined by behavior and by the celerity of eyespot darkening. Serotonergic activity in the hippocampus, nucleus accumbens and amygdala was elevated rapidly and equally in both dominant and subordinate males, as were plasma corticosterone concentrations. Serotonergic activity remained elevated through 40 minutes in hippocampus and nucleus accumbens. Only subordinate males had elevated corticosterone levels at 40 minutes. Social status does not impede socially induced stress responses. Rather, rapid regulation of serotonergic stress responses appears to be a mediating factor in determining both behavioral output and social status. Temporal expressions of monoaminergic and endocrine stress responses are distinctive between males of dominant and subordinate social status. Such temporal patterns of transmitter and glucocorticoid activity may reflect neurocircuitry adaptations that result in behavior modified to fit social status.     

Coping with social conflict, and short-term changes of plasma cortisol titers in familiar and unfamiliar environments

Each of 27 male guinea pigs was confronted with a strange opponent a) in its familiar home cage (CHC) and b) in the unfamiliar cage of the opponent (CSC) and was c) exposed to an unfamiliar open-field area without a conspecific present (OFA). The behavior displayed during confrontations was recorded in detail and plasma-cortisol-titers (PCT) were measured immediately before and after each test. The main findings were: 1) PCT increased significantly in response to the tests carried out in unfamiliar environments (OFA and CSC) compared with the response to the home-cage confrontation. 2) In both confrontation tests "offensive" males could be distinguished from "defensive" males according to marked differences in their agonistic behavior. Offensive males approached and threatened their opponents. Defensive males retreated and did not perform elements of threat behavior. 3) Offensive males showed significantly lower increases in PCT than defensive males in the home-cage confrontations. Increases in PCT did not differ significantly between the two categories of male in the strange-cage confrontations. The results show a) that different intensities of adrenocortical responses depend on the behavioral coping pattern and b) that this behavioral effect can be masked by environmental factors.     

Recognition of competitors by male golden hamsters

Golden hamsters, like many animals, form dominant/subordinate relationships after aggressive encounters. We examined whether behavioral responses by males that won or lost fights would differ toward familiar and unfamiliar male stimulus animals. In Experiment 1, male winners or losers of fights explored an arena containing a confined stimulus animal that was either familiar or novel and had either won or lost a fight. Compared to dominant males, losers spent less time in proximity to stimulus males and investigated them less. Losers also displayed higher levels of stretch-attend postures (indicative of risk assessment) than winners, and they showed more escape and locomotion in response to familiar winners than to unfamiliar winners, indicating recognition of the male that they had lost to. In Experiment 2, losers scent marked less to the odors of a familiar winner than to those of an unfamiliar winner. Thus, male hamsters appear to use familiarity with a former opponent's odors to adaptively regulate their responses to variations in social threat.     

Endocrine and lymphoproliferative response changes produced by social stress in mice

Daily dyadic resident-intruder encounters and uninterrupted cohabitation in pairs were used to assess the impact of different durations (5 and 15 days) of dominance and subordination experiences on splenic lymphoproliferative responses in male OF1 strain mice. HPA axis activity was assessed by measuring serum corticosterone levels, whereas splenic norepinephrine (NE) content provided a sympathetic activity index. Corticosterone levels in subordinate subjects were generally higher than in their control or dominant counterparts in both treatment paradigms. Corticosterone levels in dominant subjects were lower than in their control counterparts in both. Increasing the duration of treatments generally decreased such titers, especially so in subordinate subjects. No differences were detected in splenic NE content. Animals subjected to social interaction generally showed greater proliferation than their control counterparts. This effect was more pronounced in subordinates than dominants and after longer- rather than short-duration treatments. There was no inverse relation between proliferative responses and the subject's corticosterone levels. While corticosterone may have a general immunomodulating effect, other mediators apparently account for the effects produced by these social stress paradigms on splenic proliferative response.     

Dominance status predicts response to nonsocial forced movement stress in the green anole lizard (Anolis carolinensis)

We used changes in body color and eyespot formation, two somatic indices of stress controlled mainly by catecholamine activity, to compare the reactions of dominant and subordinate male green anole lizards (Anolis carolinensis) to a nonsocial stressor, forced movement. Individual males were pretested by subjecting them to 10 min of forced movement induced by chasing them around their home cage with a slender wooden stick. Stress responses were assayed via changes in body color (progressive darkening from green to brown indicating increasing stress) and expression of a black postorbital eyespot (which appears with increasing catecholaminergic stress responses). Lizards were paired and allowed to form stable dominant/subordinate relationships for 2 weeks. After that period of stable social status, dominants and subordinates were separated and subjected to the same forced-movement stress. There was no difference between experimental groups in the pretest. After assuming positions in the dominance hierarchy, however, dominant males showed reduced somatic indicators of stress and were quicker to recover from the stress. The data suggest that animals that assumed the dominant position decreased their stress response relative to the pretest, while animals that assumed the subordinate position increased their stress response relative to the pretest. The results indicate that dominant social status may have advantages beyond the realm of social interactions by enhancing an individual's ability to tolerate other, nonsocial stressful events.     

Chronic social stress in the visible burrow system modulates stress-related gene expression in the bed nucleus of the stria terminalis

Prolonged social subjugation produces physiological indices of chronic stress in rats. In the current study, we examined the impact of social stress on glutamic acid decarboxylase (GAD) isoforms, corticotropin-releasing hormone (CRH) and vasopressin mRNA expression in forebrain stress circuitry, using the visible burrow system model of dominance-subordination. Subordinate male rats develop behavioral and neuroendocrine changes consistent with exposure to chronic stress, including marked loss of body weight and elevation of basal plasma corticosterone relative to dominant rats. Forebrain GAD65, GAD67, CRH and vasopressin mRNA expression in central stress-regulatory circuits were examined by in situ hybridization. Elevated CRH mRNA was observed in the oval nucleus of the bed nucleus of the stria terminalis (BST) of subordinate males. In contrast, GAD67 expression was decreased in the interfascicular nucleus of the BST in both the subordinate and dominant rats compared to non-burrow control rats. No changes in CRH, GAD or vasopressin were observed in amygdaloid nuclei, other BST nuclei or in the hypothalamic paraventricular nucleus. Collectively, these data suggest that exposure to the visible burrow system attenuates BST GAD expression regardless of social status, whereas the enhanced physiological responses to social stress seen in subordinates may be associated with enhanced CRH expression in the oval nucleus of the BST.     

Physiological and health consequences of social status in zebrafish (Danio rerio)

Social status affects access to food, mates and shelter and has consequences for the physiology of individuals and their health status. In the zebrafish (Danio rerio), an emerging model for studies into animal behavior, the possible consequences of social hierarchy to an individual's physiology and health are unknown. To address this, in this species we assessed the effects of social interaction (for periods of 1-5 days) on growth, stress, immune function and reproductive condition. Wide-ranging differences in physiology occurred between the social ranks, some of which were sex-related and time-dependent. In both sexes, dominant fish were larger than subordinates and dominant males had a higher growth rate during the trials. Subordinates had higher plasma cortisol and in males higher telencephalic corticotrophin-releasing hormone, neuropeptide y and glucocorticoid receptor gene expression. Splenic cytokine expression suggested differences in immune status between ranks in both sexes and hematocrit was elevated in subordinate males. In both sexes, dominants and subordinates differed in the expression of genes for various gonadal sex steroid receptors and steroidogenic enzymes and in dominant females the ovary was larger relative to body mass compared with in subordinates. Dominant males had higher plasma 11-ketotestosterone than subordinates and there was an increase in the number of spermatids in their testes over the duration of the study that was not seen in subordinate males. The wide-ranging physiological differences seen between dominant and subordinate zebrafish as a consequence of their social status suggest negative health impacts for subordinates after prolonged durations in those hierarchies.     

Seasonal changes in chronic social interactions and physiological states in female rat-like hamsters (Tscheskia triton)

We examined the behavioral characteristics and physiological states related to solitary lifestyle and dominant-subordinate relationships in female rat-like hamsters (Tscheskia triton), formerly known as Cricetulus triton. Wild adult hamsters were captured and caged singly in the laboratory during the non-breeding and breeding seasons. The experimental hamsters were subjected to 5 min staged dyadic encounters every day for 28 consecutive days by pairing two unfamiliar and weight matched females in a neutral arena. Aggressive behavior, defense and flank marking were quantified everyday within the first week and once each week during the last 3 weeks. Animals were then autopsied and their physiological and reproductive state assessed. Our results suggested that dominant-subordinate relationships could be established especially in non-breeding conditions, where the dominant displayed higher aggression and flank marking, and lower defense than its opponent. The breeding females followed this pattern except there was no difference in aggressive behavior, between the partners. The repeated encounters did not appear to reduce aggression or lead to amiable behavior or bonding. At the end of the experiment breeding females exhibited higher levels of serum estradiol, progesterone and corticosterone than non-breeding females. Both dominant and subordinate females in non-breeding condition had atrophied ovaries and uteri, whereas both dominant and subordinate females in breeding condition had hypertrophied ovaries and uteri. Non-breeding females possessed heavier and thicker flank glands than breeding females did. Dominant females displayed longer or thicker flank glands than subordinate did. Thus, our data suggest that the behavioral traits observed in our experiment support the solitary lifestyle of adult female rat-like hamsters and physiological state show some differences between social ranks or in both seasons.     

Social experience and social context alter the behavioral response to centrally administered oxytocin in female Syrian hamsters

The type of social behavior displayed by an individual is profoundly influenced by its immediate social environment or context and its prior social experience. Although oxytocin is important in the expression of social behavior in several species, it is not known if social factors alter the ability of oxytocin to influence behavior. The purpose of the present study was to test the hypothesis that social experience and social context alter the ability of oxytocin to regulate flank marking (a form of scent marking) in female Syrian hamsters. Oxytocin was microinjected into the medial preoptic anterior hypothalamic continuum (MPOA-AH) of socially experienced, dominant female hamsters which were then tested with either a subordinate partner, with a novel partner, or alone. Oxytocin induced flank marking in a dose-dependent manner but only when the experienced dominant hamsters were tested with their familiar, subordinate partners. Oxytocin did not induce flank marking when injected into socially naive female hamsters that were tested with an opponent or alone. In males, by contrast, oxytocin induced flank marking in dominant hamsters when they were tested with their subordinate partner or alone. These data support the hypothesis that social experience and social context interact to regulate the ability of oxytocin to stimulate flank marking by its actions in the MPOA-AH in female hamsters.     

Development of social familiarity in ewes

We assessed the development of individual discrimination resulting from direct contact/familiarization in ewes. Unfamiliar ewes were introduced during 6, 24 or 72 h in groups of already familiarized ewes. At the end of this contact period, the development of social recognition with the animal that has been introduced was assessed using two different types of tests: a Y-maze preference test and a delayed paired close encounters test where tested ewes are successively and randomly interacting with the familiar animal and an unfamiliar conspecific. The results of both tests showed that ewes developed a recognition of the familiar animal in comparison to a completely unfamiliar female. However, this preference was evidenced after 24 h of contact when using the paired close encounters test whereas it appears only after 72 h when using the Y-maze test, suggesting that the paired close encounters test is a more sensitive methodology to assess the development of social familiarization. The importance of estrogens, in the formation of social familiarization was also evaluated. To this end, social recognition in the paired close encounters test was compared between ovariectomized animals receiving estrogen implants or not. Despite significant high levels of estradiol in estrogen implanted females, no major differences in recognition appeared between groups, suggesting that in our conditions estrogens do not have major influence on social familiarization.     

Social stress and corticosterone regionally upregulate limbic N-methyl-D-aspartatereceptor (NR) subunit type NR(2A) and NR(2B) in the lizard Anolis carolinensis

Social aggression in the lizard Anolis carolinensis produces dominant and subordinate relationships while elevating corticosterone levels and monoaminergic transmitter activity in hippocampus (medial and mediodorsal cortex). Adaptive social behavior for dominant and subordinate male A. carolinensis is learned during aggressive interaction and therefore was hypothesized to involve hippocampus and regulation of N-methyl-d-aspartate (NMDA) receptors. To test the effects of social stress and corticosterone on NMDA receptor subunits (NR), male lizards were either paired or given two injections of corticosterone 1 day apart. Paired males were allowed to form dominant-subordinate relationships and were killed 1 day later. Groups included isolated controls, dominant males, subordinate males and males injected with corticosterone. Brains were processed for glutamate receptor subunit immunohistochemistry and fluorescence was analyzed by image analysis for NR(2A) and NR(2B) in the small and large cell divisions of the medial and mediodorsal cortex. In the small granule cell division there were no significant differences in NR(2A) or NR(2B) immunoreactivity among all groups. In contrast, there was a significant upregulation of NR(2A) and NR(2B) subunits in the large pyramidal cell division in all three experimental groups as compared with controls. The results revealed significantly increased NR(2A) and NR(2B) subunits in behaving animals, whereas animals simply injected with corticosterone showed less of an effect, although they were significantly increased over control. Upregulation of NR(2) subunits occurs during stressful social interactions and is likely to be regulated in part by glucocorticoids. The data also suggest that learning social roles during stressful aggressive interactions may involve NMDA receptor-mediated mechanisms.     

The effect of escapable versus inescapable social defeat on conditioned defeat and social recognition in Syrian hamsters

Male Syrian hamsters are naturally aggressive animals that reliably defend their home territory against intruding conspecifics. Hamsters that lose agonistic encounters subsequently exhibit a striking change in their agonistic behavior, however, expressing no aggression and instead becoming highly submissive, a behavioral change that we have termed conditioned defeat. We have generally employed an inescapable defeat training protocol when studying conditioned defeat. The purpose of the present study was to determine if conditioned defeat is an epiphenomenon of the inescapable defeat experience by comparing the behavior of hamsters exposed to inescapable versus escapable defeat. In the conditioned defeat model, defeated hamsters subsequently generalize their submission and social avoidance to a novel, non-aggressive opponent, suggesting that hamsters subjected to inescapable defeat may not form a specific memory of their aggressive opponent. Thus, a secondary purpose of the present study was to determine whether hamsters subjected to our defeat protocol have the ability to recognize a familiar opponent following defeat. Our results provide evidence that conditioned defeat is not solely a by-product of inescapable defeat because all experimental animals, regardless of the type of defeat, expressed conditioned defeat during testing. We also found that animals experiencing an inescapable defeat avoided a familiar aggressor significantly more than they did an unfamiliar aggressor, demonstrating that these animals have the ability to recognize their previous attacker. Thus, we maintain that a variety of social defeat models, and conditioned defeat in particular, represent generalizable and ethologically valid models with which to study the effects of social stress on physiology and behavior.     

Rapid neuroendocrine responses evoked at the onset of social challenge

At the onset of agonistic social challenge, individuals must assess the degree of threat the opponent represents in order to react appropriately. We aimed to characterize the neuroendocrine changes accompanying this period of initial social assessment using the lizard Anolis carolinensis. Conveyance of aggressive intent by male A. carolinensis is facilitated by rapid postorbital skin darkening (eyespot), whereas eyespot presence inhibits opponent aggression. By manipulating this visual signal, we also investigated whether differing neuroendocrine changes were evoked by initial presentation of varying levels of social threat. Subjects were painted postorbitally either with black paint (high threat level), green paint (low threat level) or water (controls). Painted animals were presented with a mirror and sampled immediately upon exhibiting aggressive intent towards the reflected simulated opponent, but before producing behaviors such as motor pattern-based displays. Control animals (blank surface presented) were sampled at times derived from averaging response times of painted subjects. Brains and plasma were analyzed for monoamine activity and catecholamine levels using electrochemical HPLC. Social threat evoked increases in plasma catecholamine levels indistinguishable from those caused by brief environmental disturbance. However, brief social challenge caused distinct rapid increases in amygdala and nucleus accumbens (NAc) dopamine and serotonin levels. Amygdalar changes were associated with general social threat presence, but NAc monoamines were affected by both threat level and subject motivation to engage in confrontation. This suggests that specific rapid activity changes in key forebrain limbic nuclei differ according to the degree of social threat perceived at the start of the interaction.     

Alterations in core body temperature, locomotor activity, and corticosterone following acute and repeated social defeat of male NMRI mice

Repeated social defeat of male NMRI mice, coupled with the stress of continuously living opposite a dominant animal, induces a citalopram-reversible increase in anxiety. The experiments reported in the present paper were performed in an attempt to further validate this paradigm by studying the effects of acute and repeated social defeat on corticosterone and the circadian rhythms of core body temperature and locomotor activity, measured by telemetry. Acute social defeat induced a large (controls: 37.14+/-0.29 degrees C; subordinates: 39.79+/-0.33 degrees C) increase in core body temperature and corticosterone (controls: 30.14+/-2.70 ng/ml; subordinates: 89.62+/-9.25 ng/ml). Repeated social defeat (24 defeats) induced a chronic elevation in core body temperature across 24-h (controls: 36.62+/-0.04 degrees C; subordinates: 37.11+/-0.16 degrees C) in subordinate animals and a very large increase in corticosterone (controls: 28.60+/-1.27 ng/ml; subordinates: 441.52+/-8.86 ng/ml). These results illustrate that the chronic social defeat procedure described in this paper induces a state of chronic stress in the subordinate animals. Further studies are warranted to ascertain if the chronic hyperthermia and increases in corticosterone observed in the subordinate animals could be attenuated by chronic antidepressant treatment, thus further extending the predictive validity of this model.     

Polymorphisms in the serotonin reuptake transporter gene modify the consequences of social status on metabolic health in female rhesus monkeys

Individuals vary substantially in their vulnerability to physical and psychosocial stressors. The causes of such variation in susceptibility to stress are poorly understood, but are thought to relate in part to genetic factors. The present study evaluated the extent to which polymorphisms in the gene encoding the serotonin reuptake transporter (5HTTLPR or SERT) modulated physiologic responses to the imposition of psychosocial stress (social reorganization and subordinate social status) in female rhesus monkeys. Forty females, drawn from the middle ranking genealogies of several large social groups, were reorganized into eight groups containing 5 monkeys each; four groups were comprised entirely of animals homogeneous for the long promoter variant in the SERT gene (l/l), while the other four groups had monkeys with at least one allele of the short promoter variant (l/s or s/s). Females were sequentially introduced into these new groups in random order and dominance ranks were established within several days. During the ensuing 6 weeks, dominant monkeys exhibited elevated rates of aggression while subordinates displayed high rates of submission. Notably, females with the s-variant SERT genotype, collapsed across social status positions, exhibited the highest overall rates of both aggression and submission. Although neither social status nor SERT genotype influenced morning cortisol concentrations, glucocorticoid negative feedback was reduced significantly in subordinate compared to dominant females irrespective of genotype. All animals lost weight and abdominal fat across the experiment. However, decreases were greatest in subordinates, regardless of genotype, and least in dominant females with the l/l genotype. Serum concentrations of insulin, glucose, and ghrelin decreased significantly during the group formation process, effects that were independent of genotype or social status. In contrast, social status and genotype interacted to influence changes in serum concentrations of leptin and triiodothyronine (T3), as dominant, l/l females had the highest levels while subordinate s-variant females had the lowest levels. The order in which a female was introduced to her group generally predicted her eventual social rank. However, rank was additionally predicted by pre-experimental T3 and abdominal fat values, but only in the l/l animals. While these findings must be replicated with a larger sample size, the data suggest that the s-variant SERT genotype confers increased vulnerability to the adverse effects of psychosocial stress associated with subordinate status while the l/l genotype benefits the most from the absence of stress conferred by dominant social status. These findings suggest that genetic factors modify the responses of monkeys to social subordination and perhaps other psychosocial stressors.     

Comparison of arginine vasotocin immunoreactivity differences in dominant and subordinate green anole lizards

The neuropeptide arginine vasotocin (AVT) and its mammalian homologue arginine vasopressin (AVP) are believed to be involved in many social behaviors including territorial aggression. Testosterone (T) is also important for controlling territorial aggression, and it is believed to be involved in modulating AVT/AVP levels in the brain. In this study, male Anolis carolinensis were paired (n = 11 pairs) in a neutral cage and were allowed to establish a dominant-subordinate relationship for 10 days (experimental groups) or housed in a neutral cage with or without a female (control groups; each n = 4). On 10th day animals were sacrificed and their brain sections were processed for AVT immunohistochemistry and their serum was analyzed for testosterone levels. AVT immunoreactive (AVT-ir) cell numbers were counted in the anterior hypothalamus (AH), paraventricular nucleus (PN), posterior hypothalamus (PH), preoptic area (POA), and supra optic nuclei (SON). 2-way randomized block design was conducted to assess AVT-ir cell number differences between dominant and subordinate animals and Pearson's correlations were used to determine if a relationship existed between T levels and AVT-ir cell numbers. Dominant animals had more AVT-ir cells in the POA compared to subordinate animals, and subordinate animals had fewer AVT-ir cells in the POA compared to males housed either singly or with a female. There were no differences in AVT-ir cell numbers between dominant and subordinate animals in other areas. T levels were not correlated with the AVT-ir cell numbers in any area. Thus dominant animals have increased AVT-ir cell numbers compared to subordinate animals in a brain region known to be important in male sexual behavior. However, this difference is not related to differences in T.