Subordinate Male Rats Research Articles

Allopregnanolone Promotes Success in Food Competition in Subordinate Male Rats

Background/Aims: Allopregnanolone or 3α-hydroxy-5α-pregnan-20-one (AlloP) is normally sedative and anxiolytic, but can under provoking circumstances paradoxically induce aggressive behavior. Therefore, it is of particular interest to determine if there is a relationship between an anxiolytic effect and aggressive behavior following AlloP administration. Method: Male Wistar rats were housed in triads comprising of 1 young rat (35 days) and 2 older rats (55 days), with the intent of producing a social hierarchy. The triads were sampled for total serum testosterone and submitted to a social challenge in the form of a food competition test (FCT), where the rats competed for access to drinking sweetened milk. At baseline, the younger rats were identified as subordinates. To test for the behavioral effect of AlloP, the subordinate rats were given intravenous AlloP injections of 0.5 and 1 mg/kg. To assess the optimal AlloP effect, 6 intervals (5, 10, 15, 20, 30 and 40 min) between injection and the FCT were used. In separate studies, AlloP was also given by subcutaneous and intraperitoneal administration at 10 and 17 mg/kg. Results: AlloP (1 mg/kg, i.v.) increased drinking time and aggressive behavior in subordinate rats, with a positive correlation between these behaviors. The subcutaneous injection (17 mg/kg) also increased drinking time in subordinate animals. Serum testosterone concentration was higher in dominant compared to subordinate rats, and correlated with drinking time and weight. Conclusions: AlloP increased drinking time and aggressive behavior, and the correlation indicates a relationship between an anxiolytic effect and aggressive behavior.

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.

Anabolic androgenic steroid affects social aggression and fear-related behaviors in male pair-housed rats

This study examines the effect of chronic administration of the anabolic androgenic steroid nandrolone decanoate (ND) on dominant and subordinate male rats in a pair-housed condition. Pair-housed rats were assessed for dominance status based on their behavior and alterations in body weights. Throughout the study the rats were allowed limited social interactions on a daily basis. At all other times, a Plexiglas divider kept the rats separated, allowing olfactory and visual contact between the cage mates while preventing significant physical contact. One week into the study all subjects were subcutaneously implanted with a pellet that continuously infused either ND (15 mg/kg/day) or placebo for 21 days. Following the pellet implant, behavioral tests including reassessment for dominance status, and a conditioned fear test were conducted over a period of approximately 2 months to investigate possible long-term changes. The main finding is that during the allowed social interactions, the dominant ND-pretreated rats spent more time on highly aggressive behaviors than the dominant placebo-treated rats. In addition, the probability for highly aggressive behaviors was maintained for the ND-treated rats throughout the study, whereas it was decreased for the placebo-treated rats. The ND-treated subordinate rats showed less fear in a potential threatening situation compared to placebo-treated controls. These findings support the relatively long-term behavioral changes that have been seen in humans after abuse of ND and other anabolic androgenic steroid compounds.

Changes in 3beta-HSD, 11beta-HSD type 1, and apoptotic proteins in rat testis induced by psychosocial stress

Stress in socially subordinate male rats, associated with aggressive attacks by dominant males, was studied in a group-housing context called visible burrow system (VBS). These data show that subordinate males have reduced serum testosterone and higher corticosterone relative to dominant and singly housed control males (Hardy et al. 2002: Biol. Reprod., 67, 1750–1755). The aim of this report is to describe the morphological and histochemical changes in the testes of dominant and subordinate males on day 4, 7, and 14 of a 14-day housing period in the VBS. Testes from 25 control, 39 subordinate and 26 dominant rats were processed.

Social stress and T cell maturation in male rats: transient and persistent alterations in thymic function

The present study demonstrates that social stress in subordinate male rats resulting from a 24-h confrontation with a dominant male opponent caused not only transient but also persistent alterations in the function and the cellular composition of the thymus. The investigation of regulation processes maintaining tissue homeostasis in the thymus showed a significant increase in apoptosis, a decrease in cellular proliferation and a decrease in thymic export as indicated by a reduction in the number of recent thymic emigrants (RTE) in the peripheral blood. The proliferative response of mature thymocytes to Concanavalin A (ConA) was also significantly reduced in socially defeated rats. Adrenalectomy (ADX) completely abolished the stress-associated alterations in thymic function whereas a decrease in total thymocyte numbers was still evident in defeated animals treated with the glucocorticoid type-II receptor antagonist RU486. The significant reduction in total thymocytes and the diminished proliferative response to ConA persisted for at least seven days after stressor cessation. An enhanced proliferative activity at this point of time may represent a beginning regeneration of thymic tissue after stress. Taken together, our studies show that social stress profoundly disturbs tissue homeostasis in the thymus and suggest that adrenal-derived hormones play a central role in mediating this effect. The strong positive relationship between total thymocyte numbers and peripheral RTE numbers and the long-lasting atrophy of the thymus in defeated animals may suggest negative consequences for the heterogeneity of naïve T cells in the peripheral T cell pool after social stress.

Trends of reproductive hormones in male rats during psychosocial stress: role of glucocorticoid metabolism in behavioral dominance.

Stress in socially subordinate male rats, associated with aggressive attacks by dominant males, was studied in a group-housing context called the visible burrow system (VBS). It has been established that subordinate males have reduced serum testosterone (T) and higher corticosterone (CORT) relative to dominant and singly housed control males. The relationship of the decreased circulating T levels in subordinate males to changes in serum LH concentrations has not been evaluated previously. Since decreases in LH during stress may cause reductions in Leydig cell steroidogenic activity, the present study defined the temporal profiles of serum LH, T, and CORT in dominant and subordinate males on Days 4, 7, and 14 of a 14-day housing period in the VBS. The same parameters were followed in serum samples from single-housed control males. Leydig cells express glucocorticoid receptors and may also be targeted for direct inhibition of steroidogenesis by glucocorticoid. We hypothesize that Leydig cells are protected from inhibition by CORT at basal concentrations through oxidative inactivation of glucocorticoid by 11beta-hydroxysteroid dehydrogenase (11betaHSD). However, Leydig cell steroidogenesis is inhibited when 11betaHSD metabolizing capacity is exceeded. Therefore, 11betaHSD enzyme activity levels were measured in Leydig cells of VBS-housed males at the same time points. Significant increases in LH and T relative to control were observed in the dominant animals on Day 4, which were associated with the overt establishment of behavioral dominance as evidenced by victorious agonistic encounters. Serum LH and T were lower in subordinate males on Day 7, but T alone was lower on Day 14, suggesting that lowered LH secretion in subordinates may gradually be reversed by declines in androgen-negative feedback. Serum CORT levels were higher in subordinate males compared to control at all three time points. In contrast, oxidative 11betaHSD activity in Leydig cells of dominant males was higher relative to control and unchanged in subordinates. These results suggest the following: 1) failure of Leydig cells of subordinate males to compensate for increased glucocorticoid action during stress, by increasing 11betaHSD oxidative activity, potentiates stress-mediated reductions in T secretion; and 2) an inhibition of the reproductive axis in subordinate males at the level of the pituitary.

Defense changes in stress nonresponsive subordinate males in a visible burrow system

Dominant and subordinate male rats housed in a visible burrow system (VBS), and male controls each housed with a female, were run in a series of tests evaluating their response to threatening stimuli. Subordinates were later assessed as stress responsive (SRS) or nonresponsive (NRS) on the basis of plasma corticosterone response to restraint stress. In the tests using mildly threatening stimuli (handling, open field), NRS had longer latencies to show a righting response and reduced activity compared to SRS or (handling, open field) to dominants. There were no differences among the VBS groups in tests with more intense threat stimuli (cat odor, cat presentation). These results suggest that stressful social experience may produce a shift toward more passive and immobile forms of defense and that the experience of subordination may interact with individual differences characteristics of rats to exacerbate this shift. The hypothalamic–pituitary–adrenal (HPA) axis changes of NRS, manifested as reduced plasma corticosterone response to restraint, and previous findings of sharply reduced testosterone in these animals, along with their passive defensive behaviors, form a pattern that is suggestive of a biobehavioral “reactive” (as opposed to “proactive”) coping style, and suggest that these behavioral and endocrine variables may show related changes even when altered by individual experience.

Chronic social stress: Changes in behavioral and physiological indices of emotion

Subordination constitutes a natural and often chronic stress condition for males of virtually all species of social mammals. Subordinate male rats of mixed-sex groups maintained in visible burrow systems show high-magnitude changes in adrenal and gonadal steroid hormones and, in a subpopulation, disruptions in hypothalamic pituitary adrenal axis function. Changes in brain serotonin systems have been extensively documented in both colony subordinates and colony dominants, with other regional neurotransmitter system changes also indicated. The group experience seems to change a variety of behaviors both inside the colony situation and in tests run outside the colonies and may disrupt the relationship found in controls between hypothalamic pituitary adrenal response to an acute stressor and defensive behavior. Comparisons of these patterns of stress response with those of other chronic stress models such as learned helplessness and chronic mild stress suggest that, in terms of adrenal and gonadal steroid hormone response, subordination constitutes a more severe stressor. Some involvement of the serotonin system has been demonstrated for both subordination and more traditional models, and other systems may prove to be involved in both. However, a central feature of both learned helplessness and chronic mild stress paradigms is a specific criterion behavior or behaviors, emphasized as models for particular psychopathologies. Thus, learned helplessness and chronic mild stress studies have tended to emphasize the effects of pharmacological agents on these models, whereas work with subordination has been focused on analysis of the biobehavioral stress process itself.

Chronic social stress: Changes in behavioral and physiological indices of emotion

Subordination constitutes a natural and often chronic stress condition for males of virtually all species of social mammals. Subordinate male rats of mixed-sex groups maintained in visible burrow systems show high-magnitude changes in adrenal and gonadal steroid hormones and, in a subpopulation, disruptions in hypothalamic pituitary adrenal axis function. Changes in brain serotonin systems have been extensively documented in both colony subordinates and colony dominants, with other regional neurotransmitter system changes also indicated. The group experience seems to change a variety of behaviors both inside the colony situation and in tests run outside the colonies and may disrupt the relationship found in controls between hypothalamic pituitary adrenal response to an acute stressor and defensive behavior. Comparisons of these patterns of stress response with those of other chronic stress models such as learned helplessness and chronic mild stress suggest that, in terms of adrenal and gonadal steroid hormone response, subordination constitutes a more severe stressor. Some involvement of the serotonin system has been demonstrated for both subordination and more traditional models, and other systems may prove to be involved in both. However, a central feature of both learned helplessness and chronic mild stress paradigms is a specific criterion behavior or behaviors, emphasized as models for particular psychopathologies. Thus, learned helplessness and chronic mild stress studies have tended to emphasize the effects of pharmacological agents on these models, whereas work with subordination has been focused on analysis of the biobehavioral stress process itself. Aggr. Behav. 24:307–321, 1998. © 1998 Wiley-Liss, Inc.

The Effect of Social Stress on Hippocampal Gene Expression

The housing of male and female rats in a visible burrow system in which a dominance hierarchy is established and interactions between animals can be monitored provides an effective method by which the behavioral and neuroendocrine effects of social stress can be examined. In this social system, subordinate male rats exhibit plasma corticosterone levels that appear to be chronically elevated relative to the dominant rats in the colonies and compared to singly housed controls. Previous studies have indicated that in the hippocampus adrenal steroids negatively regulate the expression of the mRNAs encoding the glucocorticoid receptor (GR), the mineralocorticoid receptor (MR), and the growth-associated protein GAP-43, while the preproenkephalin (ppENK) mRNA is positively regulated by glucocorticoids. This study examined the effect of social stress and the resulting sustained increase in steroid levels in the subordinate animals on gene expression in the hippocampus. The results indicated that GR, MR, and GAP-43 mRNAs were decreased in subordinate rats relative to controls, in the CA1 region of the hippocampus, while ppENK mRNA was unaffected. No statistically significant differences in mRNA expression were observed in other hippocampal subfields. These findings suggest that the social stress encountered by the subordinate rats in this burrow environment elicits changes in hippocampal gene expression that are consistent with their regulation by adrenal steroids.

Influence of dominance on the development of play fighting in pairs of male Syrian golden hamsters (Mesocricetus auratus)

In the highly social rat, male juvenile and adult subordinates initiate more playful contacts with dominant pairmates than vice versa. This study examined the effect of dominance on playful contacts in the relatively asocial golden hamster. Pairs of male hamsters were reared together from weaning, and their play was filmed in the juvenile (28-36 days) and the young adult (60-70 days) stages of development. By the adult stage, it became clear that one pairmate was dominant over the other. The dominant pairmate launched all aggressive attacks (i. e., bites to the lower flanks and rump), and the subordinate pairmate performed all the submissive gesturing (e. g., tail up submissive posture). Playful contact, which in this species involves gentle nibbling of the posterior cheeks, was more frequently launched by the dominant than by the subordinate. This was not only true at the adult stage, but also at the juvenile stage, before dominance-subordination relationships were sharply polarized. Therefore, it would appear that in the relatively asocial hamster, the subordinates tend to avoid playful contact with dominants. This is markedly different to rats, where the subordinates actively seek out and engage dominants in play. This contrast further supports our hypothesis that subordinate male rats use play as a means of maintaining familiarity with dominants. © 1993 Wiley-Liss, Inc.

Some subordinates are more equal than others: Play fihgting amongst adult subordinate male rats

Adult male rats living together form dominance relationships, with one dominant and the remainder adopting subordinate roles. In previous studies, it was shown that in adult male pairs, the subordinate rat initiates more playful contacts and retains a more juvenile response to the playful contacts by the dominant. In this experiment, triads were used to examine the play between subordinate males. The subordinates directed fewer playful contacts to each other than to the dominant rat, and there was a symmetrical play relationship between the subordinates. After the dominant was removed from the colony, one subordinate became the dominant. Playful interactions amongst these pairs increased, with the subordiante intiating more playful contacts than the dominant. Furthermore, from a similarly low frequency of juvenile-type response to playful contact to each other when in triads, the subordinate in the dyads increased its frequency of juvenile responses to the dominant partner. This supports the hypothesis that the playful behavior of subordinate male rats towards the dominant is an adaptive response, playful behavior of subordinate male rats towards the dominant is an adaptive response, serving a “friendship maintenance” function. Finally, when in triads, one subordinate was more playful with the dominant than the other subordinate. It was the least playful subordinate that was the most likely to become the dominant. This sugests that within a colony, not all subordinates are the same. © 1993 Wiley-Liss, Inc.

Juvenilized play fighting in subordinate male rats

As pairs of male juvenile sibling rats that are housed together become sexually mature, they develop a dominance-subordinance relationship. These dominance relationships appear to be reflected in the play fighting of the pairmates both as juveniles and as young adults, in that the seemingly subordinate partner initiates more playful attacks at both ages. However, as adults, even though it is the subordinate that initiates more playful attacks, it is the subordinate that is pinned on his back by the partner most often. Dominant pairmates were found to switch to defensive patterns typically found in adult males. In contrast, the subordinates, when contacted on the nape, were more likely to retain the juvenile pattern of turning over to supine. Therefore, the subordinate pairmate of an adult pair of male siblings both initiates more playful attacks and defends itself in a more juvenile manner than its dominant partner, and this leads to it being pinned more frequently. This pattern of behavior by subordinate rats is suggested to function as a friendship maintenance mechanism permitting co-existence in multimale colonies.

Responses of dominant and subordinate male rats to the odors of male and female conspecifics

The resident-intruder paradigm was used to examine the effects of social dominance and individual recognition on odor preferences and urine-marking in male rats. Resident males were significantly more aggressive than intruders and spent more time investigating the odors of familiar intruders. Resident males urine-marked most over the odors of females and familiar intruders while intruders marked least over the odor of the familiar resident. Intruders did not avoid investigating nor marking over the odors of familiar resident males or other conspecifics. These results suggest that individual odors of male rats may be more salient than a general odor of dominance, and that the dominant males increase their investigation and marking over the odors of familiar subordinates but not unfamiliar subordinates. The importance of olfactory learning during aggressive interactions is discussed.

Serotonin, but not dopamine, metabolites are increased in selected brain regions of subordinate male rats in a colony environment

Subordinate male laboratory rats maintained in mixed-sex groups in a Visible Burrow System habitat show a complex pattern of stress-related changes including enhanced defensive behavior, early mortality and increased voluntary ethanol consumption. Analysis of serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) levels indicated that 5-HT levels do not differ between colony subordinates, colony dominants, and singly-housed control animals. However, 5-HIAA levels were higher in subordinates than either dominants or control animals in the preoptic area, amygdala, hippocampus, and spinal cord, and, were higher than dominants only, in entorhinal cortex. Subordinates' regional 5-HIAA/5-HT ratios were reliably higher than those of dominant or control animals in midbrain and spinal cord and reliably higher than dominants only, in hypothalamus. Dopamine (DA) and dihydroxyphenylacetic acid (DOPAC) levels and DA/DOPAC ratios were affected neither in hypothalamus nor midbrain. These findings suggest that a consistent increase of 5-HIAA levels in selected brain regions of subordinate rats may represent a biological substrate for a well-characterized pattern of alterations in defensive behaviors for these animals.

Inhibition of shock-elicited ultrasonic vocalization by opioid peptides in the rat: A psychotropic effect

Following three series of electric footshocks (10 shocks/day), one out of three rats in most cages were brought to emit ultrasonic vocalization for several minutes after a single shock. The characteristics of shock-elicited ultrasound were pure tone pulses of a frequency between 22 and 28 kHz, with duration longer than 300 msec. The same type of ultrasound is produced by subordinate male rats during agonistic behavior. The intracerebroventricular injection of beta-endorphin, dynorphin, methionine-enkephalin or leucine-enkephalin attenuated the shock-elicited ultrasonic vocalization. Psychotropic drugs such as diazepam and chlorpromazine also attenuated the shock-elicited ultrasonic vocalization. A test utilizing ultrasonic vocalization in rodents can provide useful data for studying the psychotropic properties of neuropeptides.