Social stress in laboratory rats: hormonal responses and immune cell distribution

Persistent neuroendocrine and behavioral effects of a novel, etiologically relevant mouse paradigm for chronic social stress during adolescence

Chronic stress is widely regarded as a key risk factor for a variety of diseases. A large number of paradigms have been used to induce chronic stress in rodents. However, many of these paradigms do not consider the etiology of human stress-associated disorders, where the stressors involved are mostly of social nature and the effects of the stress exposure persist even if the stressor is discontinued. In addition, many chronic stress paradigms are problematic with regard to stress adaptation, continuity, duration and applicability. Here we describe and validate a novel chronic social stress paradigm in male mice during adolescence. We demonstrate persistent effects of chronic social stress after 1 week of rest, including altered adrenal sensitivity, decreased expression of corticosteroid receptors in the hippocampus and increased anxiety. In addition, pharmacological treatments with the antidepressant paroxetine (SSRI) or with the corticotropin-releasing hormone receptor 1 antagonist DMP696 were able to prevent aversive long-term consequences of chronic social stress. In conclusion, this novel chronic stress paradigm results in persistent alterations of hypothalamus-pituitary-adrenal axis function and behavior, which are reversible by pharmacological treatment. Moreover, this paradigm allows to investigate the interaction of genetic susceptibility and environmental risk factors.

Stress hormones and sociality: integrating social and environmental stressors

In cooperatively breeding species, reproductive decisions and breeding roles may be influenced by environmental (food resources) or social factors (reproductive suppression of subordinates by dominants). Studies of glucocorticoid stress hormones in cooperatively breeding species suggest that breeding roles and hormone levels are related to the relative costs of dominance and subordination, which are driven primarily by social interactions. Few studies, however, have considered how environmental factors affect glucocorticoid levels and breeding roles in cooperative breeders, even though environmental stressors modulate seasonal glucocorticoid release and often influence breeding roles. I examined baseline and stress-induced levels of the glucocorticoid corticosterone (CORT) across 4 years in the plural breeding superb starling, Lamprotornis superbus, to determine whether (i) environmental factors (namely rainfall) directly influence breeding roles or (ii) environmental factors influence social interactions, which in turn drive breeding roles. Chronic baseline and maximal stress-induced CORT changed significantly across years as a function of pre-breeding rainfall, but dominant and subordinate individuals responded differently. Pre-breeding rainfall was also correlated directly with breeding roles. The results are most consistent with the hypothesis that environmental conditions influenced the relative costs of dominance and subordination, which in turn affected the degree and intensity of social interactions and ultimately reproductive decisions and breeding roles.

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.

Social instability in adolescence alters the central and peripheral hypothalamic-pituitary-adrenal responses to a repeated homotypic stressor in male and female rats

There has been little research on effects of chronic stressors on neuroendocrine function in adolescence despite increasing evidence of enduring effects of stressors during this period on behaviour in adulthood. We previously reported that social stress (SS: daily 1 h isolation and new cage partner for 16 days) in adolescence altered locomotor responses to psychostimulants in adulthood. Here, we investigated neuroendocrine responses over the duration of the procedure that may underlie the enduring effects of SS. SS rats were compared to rats undergoing daily isolation only (ISO) and controls (CTL) to determine responses to acute and repeated isolation with and without social instability. At 30 days of age (first isolation), higher plasma corticosterone and corticotrophin-releasing hormone (CRH) mRNA expression in the paraventricular nucleus (PVN) of the hypothalamus and in the central nucleus of the amygdala (CeA) were found in males caged with a new partner (SS) after isolation than those returned to their original partner (ISO). On day 45, SS males and females showed less habituation (higher bioactive levels of corticosterone based on plasma corticosterone and corticosteroid binding globulin levels) to the 16th episode of isolation than did ISO. SS and ISO had higher baseline expression of CRH mRNA in the PVN on day 45 than did CTL, and only CTL had increased levels after isolation. CRH mRNA expression in the CeA increased to a first isolation in CTL and to a 16th isolation in SS but not in ISO males. Modest differences in social interactions were observed between SS and ISO when returned to their cages after isolation. The results suggest that mild social stressors in adolescence impede neuroendocrine adaptation to homotypic stressors. The resultant increase in exposure to glucocorticoids over adolescence may alter ongoing brain development and increase vulnerability to psychopathology.

The Pharmacokinetic Properties of Methamphetamine in Rats with Previous Repeated Exposure to Methamphetamine: The Differences between Long-Evans and Wistar Rats

Repeated treatment with methamphetamine (METH) causes long-term behavioral changes, so-called behavioral sensitization (BS), in humans as well as experimental animals. However, there are no reports as to whether repeated METH treatment can establish BS in stress-sensitive Long-Evans (LE) rats. Thus, we investigated the effect of repeated METH treatment (5 mg/kg x 5 days) on the establishment of BS in LE rats. Wistar (WIS) rats were used as a reference. In LE rats, repeated METH treatment failed to cause BS although it did enhance METH-induced hyperlocomotion in WIS rats. The levels of METH in brain dialysate and the ratio of the area under the concentration-time curve area in plasma to that in brain dialysate was increased in repeated METH-treated WIS rats as reported previously, but not in repeated METH-treated LE rats. METH increases plasma corticosterone (CORT) in both strains. However, the intensity of increment of CORT by repeated METH was lower in LE rats than that in WIS rats. Repeated METH treatment decreased the expression of METH-transposable and CORT-sensitive transporter, organic cation transporter 3 (OCT3), in the brain of WIS rats. However, the intensity of the decrement of OCT3 with repeated METH treatment was similar between both strains. Taken together, these results suggest that the lack of establishment of BS in LE rats might have been caused by the unchanged brain penetration of METH after repeated METH administration, and that the differential CORT response to METH is an important strain difference.

Stress/aggressiveness-induced immune changes are altered in adult rats submitted to neonatal malnutrition

BACKGROUND/AIMS

Neonatal malnutrition induces metabolic and endocrine changes that have beneficial effects on the neonatal in the short term but, in the longer term, these alterations lead to maladaptations. We investigated the effect of neonatal malnutrition on immune responses in adult rats submitted or not to an aggressiveness test.

METHODS

Male Wistar rats were distributed to one of two groups according to their mothers' diet during lactation: the well-nourished group (group C, n = 42, receiving 23% of protein) and the malnourished group (group MN, n = 42, receiving 8% of protein). After weaning, all rats received normoproteic diet. Ninety days after birth, each group was subdivided into three subgroups: control rats (n = 14, respectively), aggressive rats (n = 14, respectively) and rats receiving foot shock (FS; n = 14, respectively). Plasma corticosterone concentration was measured after FS sessions. Leukocyte counts and humoral immunity were evaluated.

RESULTS

In neonatal malnourished animals, FS-induced stress reduced plasma corticosterone concentration. Intraspecific aggressiveness induced alterations in leukocyte counts and antibody titers 7 and 15 days after immunization. Neonatal malnourished animals showed no changes in the immune parameters evaluated.

CONCLUSIONS

Expression of intraspecific aggressiveness activates the immune system. Neonatal malnutrition seems to have a long-lasting effect on components of both neuroendocrine and immune functions.

Dehydroepiandrosterone alleviates copulatory disorder induced by social stress in male rats

INTRODUCTION

Social stress induces sexual dysfunction and reduces serum testosterone (T) level in rats. Stressful events exert an influence on a variety of behaviors and physiology through hormonal changes. The mechanism of stress-induced sexual dysfunction is unknown.

AIM

To investigate the role of dehydroepiandrosterone (DHEA) in copulatory behavior induced by social stress in rats.

METHODS

Stress-induced male rats were subjected to social stress in which the males lived in a wire-mesh siege located in a colony of male and female rats and were exposed daily to a brief defeat by the colony of males for five consecutive days. After the stress period, copulatory behavior and serum concentrations of DHEA and T were measured.

MAIN OUTCOME MEASURES

The effects of DHEA, T, and NE-100, a selective sigma 1 receptor antagonist, on copulatory behavior following social stress were examined.

RESULTS

The males exhibited a marked suppression of copulatory behavior (elongation of intromission and ejaculation latencies). Serum concentrations of DHEA and T were significantly lower than those in nonstressed control males. Another three groups of social stressed males were injected daily with DHEA, T, or DHEA + NE-100 during the stress period. Injections of DHEA attenuated the stress-induced suppression of copulatory behavior, whereas T had no effect. The combined treatment of NE-100 made DHEA ineffective at restoring copulatory behavior.

CONCLUSIONS

These results indicate that DHEA, but not its conversion to T, alleviates the suppressive effect of social stress on copulatory behavior via sigma 1 receptors. We suggest that the decreased endogenous DHEA is involved in copulatory disorders induced by social stress in rats.

Sympathetic but not sensory denervation stimulates white adipocyte proliferation

White adipocyte proliferation is a hallmark of obesity, but it largely remains a mechanistic mystery. We and others previously demonstrated that surgical denervation of white adipose tissue (WAT) triggers increases in fat cell number, but it is unknown whether this was due to preadipocyte proliferation or maturation of existing preadipocytes that allowed them to be counted. In addition, surgical denervation severs not only sympathetic but also sensory innervation of WAT. Therefore, we tested whether sympathetic WAT denervation triggers adipocyte proliferation using 5-bromo-2'-deoxyuridine (BrdU) as a marker of proliferation and quantified BrdU-immunoreactive (ir) cells that were co-labeled with AD-3-ir, an adipocyte-specific membrane protein marker. The unilateral denervation model was used for all experiments where Siberian hamster inguinal WAT (IWAT) was unilaterally denervated, the contralateral pad was sham denervated serving as a within-animal control, and then BrdU was injected systemically for 6 days. When IWAT was surgically denervated, severing both sympathetic and sensory nerves, tyrosine hydroxylase (TH)-ir, a sympathetic nerve marker, and calcitonin gene-related peptide (CGRP)-ir, a sensory nerve marker, were significantly decreased, and BrdU+AD-3-ir adipocytes were increased approximately 300%. When IWAT was selectively sensory denervated via local microinjections of capsaicin, a sensory nerve-specific toxin, CGRP-ir, but not TH-ir, was decreased, and BrdU+AD-3-ir adipocytes were unchanged. When IWAT was selectively sympathetically denervated via local microinjections of 6-hydroxy-dopamine, a catecholaminergic-specific toxin, TH-ir, but not CGRP-ir, was significantly decreased, and BrdU+AD-3-ir adipocytes were increased approximately 400%. Collectively, these data provide the first direct evidence that sympathetic nerves inhibit white adipocyte proliferation in vivo.

Gender difference in basal and stress levels of peripheral blood leukocytes in laboratory rats

The aim of the present study was to investigate gender differences in numbers and function of blood immune cells in stressed and non-stressed laboratory rats. Psychosocial stress in adult male or female rats was induced by social confrontation of an intruder rat with a resident opponent for 2 h. Behavioral analysis indicated that intruders of both sexes were clearly defeated and had markedly higher plasma corticosterone concentrations than unstressed home cage controls at the end of the confrontation. Lower numbers of CD4, CD8, and B cells as well as a reduced proliferative response of lymphocytes to ConA were observed in stressed groups of either sex. However, some important gender differences were also observed. Stressed males had higher granulocyte numbers than controls, while granulocyte numbers remained unchanged in stressed females. Similarly, stressed males had higher phagocytic activity than stressed females. Second, there was a gender difference in some basal values. Female controls had lower NK cell numbers than control males. Interestingly, NK numbers in stressed males decreased considerably, reaching the same low levels as in (stressed and control) females. In addition, females exhibited higher basal corticosterone concentrations than males. To summarise, these data do not indicate a superior blood cellular immune function in female rats, neither for the control nor the stress condition. However, the data clearly suggest that male and female rats should not be considered as a uniform group with respect to their immunological response to stress.

Gonadal hormone modulation of hippocampal neurogenesis in the adult

Gonadal hormones modulate neurogenesis in the dentate gyrus (DG) of adult rodents in complex ways. Estradiol, the most potent estrogen, initially enhances and subsequently suppresses cell proliferation in the dentate gryus of adult female rodents. Much less is known about how estradiol modulates neurogenesis in the adult male rodent; however, recent evidence suggests that estradiol may have a moderate effect on cell proliferation but enhances cell survival in the DG of newly synthesized cells but only when estradiol is administered during a specific stage in the cell maturation cycle in the adult male rodent. Testosterone likely plays a role in adult neurogenesis, although there have been no direct studies to address this. However, pilot studies from our laboratory suggest that testosterone up-regulates cell survival but not cell proliferation in the DG of adult male rats. Progesterone appears to attenuate the estradiol-induced enhancement of cell proliferation. Neurosteroids such as allopregnalone decrease neurogenesis in adult rodents, while pregnancy and motherhood differentially regulate adult neurogenesis in the adult female rodent. Very few studies have investigated the effects of gonadal hormones on male rodents; however, studies have indicated that there is a gender difference in the response to hormone-regulated hippocampal neurogenesis in the adult. Clearly, more work needs to be done to elucidate the effects of gonadal hormones on neurogenesis in the DG of both male and female rodents.

The sectored foraging field: a novel design to quantify spatial strategies, learning, memory, and emotion

Although Norway rats are naturally gregarious, males typically live alone at some point during adulthood. Different social ecologies often require different learning strategies and also modulate response to stressors and gonadal development. To measure effects of the social environment on the interaction between cognition and emotion during aging, we focused on a natural learning context and devised the sectored foraging field, a progressively difficult spatial navigation task. Here, we describe how this apparatus and protocol permits multiple learning strategies in a minimally stressful environment, enabling finely graded analyses of cognition and emotionality. Male Sprague-Dawley rats living alone throughout adulthood adopted a sex-typic discernible spatial strategy. In contrast, males housed in group contexts utilized an algorithmic kinesthetic strategy, repeating the same motor action until they found food. Removal of food and distal, but not local cues, elicited anxious alertness, particularly in group-housed males. Cognitive performance of group-housed rats subsequent to food and cue removal was significantly impaired, yet enhanced in isolates.

Tissue-specific alterations in the glucocorticoid sensitivity of immune cells following repeated social defeat in mice

Endogenous glucocorticoids (GC) play an important role in the termination of the inflammatory response following infection and tissue injury. However, recent findings indicate that stress can impair the anti-inflammatory capacities of these hormones. Lipopolysaccharide (LPS)-stimulated splenocytes of mice that were repeatedly subjected to social disruption (SDR) stress were less sensitive to the immunosuppressive effects of corticosterone (CORT) as demonstrated by an increased production of pro-inflammatory cytokines and enhanced cell survival. Myeloid cells expressing the marker CD11b were shown to play a key role in this process. Here we investigated the role of the bone marrow as a potential source of the GC-insensitive cells. The study revealed that LPS-stimulated bone marrow cells, in the absence of experimental stress, were virtually GC-resistant and retained high levels of cell viability after treatment with CORT. Recurrent exposure to the acute stressor over a period of 2, 4 or 6 days led to an increase in the GC sensitivity of the bone marrow cells. This increase in GC sensitivity was associated with enhanced mRNA expression of granulocyte-macrophage colony-stimulating factor (GM-CSF), an increase in the number of myeloid progenitors, and a decrease in the proportion of mature CD11b+ cells. The changes in the cellular composition of the bone marrow were accompanied by an increase in splenic CD11b+ cell numbers. Simultaneous assessment of the GC sensitivity in bone marrow and spleen revealed a significant negative correlation between both tissues suggesting that social stress causes the redistribution of GC-insensitive myeloid cells from the bone marrow to the spleen.

Pregnancy and social stress in female rats: Influences on blood leukocytes and corticosterone concentrations

The consequences of pregnancy and social stress on blood immune cells and on plasma corticosterone concentrations were assessed in Long Evans rats. Normal pregnancy in control females was characterized by a progressive increase in corticosterone concentration and increasing numbers of granulocytes. In contrast, CD4 T, CD8 T, and B cell numbers as well as the proliferative response of lymphocytes decreased as pregnancy progressed. Stress was induced in pregnant females by social confrontation for 2 h daily with a female resident opponent over a period of 2 months. Corticosterone concentrations were substantially higher in pregnant stressed than in pregnant control rats. Furthermore, the numbers of monocytes, NK and B cells were lower in stressed females, and there was a strong trend towards suppressed lymphocyte proliferation. Interestingly, pregnant females did not show granulocytosis in response to the stressor. In sum, the social stress paradigm in females appears to be a good model for the investigation of the interactions between stress, pregnancy and the immune system. It also provides an excellent platform for studies on prenatal stress under relatively naturalistic conditions.

The influence of social environmental condition on the production of stress-induced 22 kHz calls in adult male Wistar rats

Adult rats emit 22 kHz ultrasonic vocalizations (USVs) in response to aversive stimuli, and these sounds are suggested to have communicative information among conspecifics. It is conceivable that social environment during development of rats has relevance to the emission of 22 kHz USVs. To examine the effects of social environment after weaning on production of stress-induced USVs, we compared the amount of emission of USVs among three groups of rats reared under different conditions after weaning. One group of rats was housed individually, and the other two groups were housed in pairs, in which social hierarchy of the pair was determined by social dominance-subordination relationships. The USVs were induced by acute mild somatic stimuli on the back and neck. Individually reared rats emitted much fewer USVs than pair-reared rats. In addition, socially subordinate rats emitted more USVs compared with socially dominant ones. These results suggest that not only social interaction but also the status in social hierarchy may play an important role in the process of the development of USVs induced by somatic stimuli.

Coping with competitive situations in humans

The analysis of effects of competitive situations in our species may contribute to acquiring deeper knowledge about the effects of social stress and its relationship with different pathologies. The latest studies indicate that the neuroendocrine response to competition depends more on subjective factors related to the cognitive evaluation of the situation than on the outcome itself. Findings suggest that when subjects cope with a competition, they assess it in such a way that it activates a psychobiological coping response. The pattern of this response may correspond to a predominant active or passive coping strategy, the choice ultimately depending on factors such as the importance of the competition for the subject, the subject's involvement or perceived possibilities of control of outcome or success (e.g. past experience in similar competitions, judge or rank of the opponent), among others. More important than winning or losing is the coping pattern displayed by the subject, which determines the hormonal changes experienced when facing competition and its outcome.

Hepatic corticosteroid-binding globulin (CBG) messenger RNA expression and plasma CBG concentrations in young pigs in response to heat and social stress1,2

Plasma cortisol, porcine corticosteroid-binding globulin (pCBG), hepatic CBG expression, and other physiological and behavioral measures of stress were studied in pigs in response to elevated temperature in conjunction with establishing a social hierarchy. Twenty-four crossbred pigs were weaned at 25 d of age (three or six pigs from six sows) and housed in littermate groups at 23 +/- 2 degrees C. At 57 d of age (d 0), animals were weighed and placed under general anesthesia for collection of blood (10 mL) and liver (approximately 100 mg) samples. On d 1, three unacquainted pigs of similar BW (23 +/- 1 kg) from different litters were allotted to each of eight nursery pens within two environmentally controlled rooms (12 pigs per room). From d 1 to 7, one room was maintained at 23 +/- 2 degrees C (CON) and the other at 33 +/- 2 degrees C (HEAT). Both rooms were kept at 23 +/- 2 degrees C from d 8 to 14. Animals were videotaped for 72 h beginning on d 1 and 8 to document behavioral changes in response to room temperature. The social hierarchy of pigs within each pen was based on fight activity recorded on d 1 to 3. Blood and liver tissue were collected again on d 7 and 14. The ADG for HEAT pigs increased (P < 0.05) over d 8 to 14 compared with d 1 to 7. In contrast to CON pigs, HEAT pigs displayed increased (P < 0.01) drinking but decreased feeding and lying in contact with other pigs from d 1 to 3, and similar drinking and feeding but increased (P < 0.01) lying with contact behaviors from d 8 to 10. With the exception of subordinate pigs exhibiting less (P < 0.05) frequent standing/walking behavior than the dominant or intermediate pigs on d 1 to 3, frequency of behaviors for both recorded time periods did not differ among pigs due to social status, regardless of treatment. The concentration of plasma haptoglobin in HEAT pigs on d 7 compared with d 0 increased (467 vs. 763 mg/L; P < 0.05), whereas cortisol and pCBG decreased (274 vs. 235 nmol/L and 11.4 vs. 9.9 mg/L, respectively; P < 0.05) as a result of treatment. The free cortisol index (total cortisol/pCBG) was greater (P < 0.05) in HEAT pigs on d 14 than on d 0 or 7. Hepatic CBG mRNA level was not affected by treatment. On d 14, HEAT pigs had plasma cortisol, pCBG, and haptoglobin concentrations similar to those of CON pigs. These results indicate that measured behavioral and physiological responses were not related to social status, and decreased circulating levels of cortisol and pCBG in pigs following a 7-d exposure to elevated temperature may not be determined by hepatic CBG mRNA expression.

Social disruption stress exacerbates alpha-galactosylceramide-induced hepatitis in mice

OBJECTIVE

Psychosocial stress has been suggested as a possible aggravating factor in liver diseases, however, the underlying mechanism has yet to be clarified. Recently, our research revealed that electric foot-shock stress aggravated NK1.1 Ag(+) T cell-dependent alpha-galactosylceramide (alpha-GalCer)-induced hepatitis in mice via a mechanism mediated by endogenous glucocorticoids. In this study, we examined whether or not such aggravation could be applied to a psychosocially stressful situation, e.g. social disruption stress.

METHODS

Male wild-type C57BL/6 (B6) or B6 hepatitis virus type B surface antigen transgenic (HBs-tg) mice, a hepatitis B virus carrier mouse model, were exposed 3 times in 1 week to social disruption stress in which an 8-month-old aggressive male intruder was placed into their home cage (5 mice per group) for 2 h. Twelve hours after the final exposure to the stress, the wild-type and HBs-tg mice were intravenously injected with alpha-GalCer.

RESULTS

The stress-exposed wild-type mice exhibited significantly reduced thymus weight loss compared with the control animals. Moreover, this stress regimen led to a significant increase in serum alanine aminotransferase levels in both the wild-type and the HBs-tg mice, although the increase in the HBs-tg mice was higher than that in the wild-type mice.

CONCLUSION

These findings demonstrated that, similar to electric foot-shock stress, social disruption stress exacerbated alpha-GalCer-induced hepatitis.

Social factors and individual vulnerability to chronic stress exposure

The stress-response is adaptive in the short-term, but it can be maladaptive if sustained levels of its mediators are chronically maintained. Furthermore, not all individuals exposed to chronic stress will progress to disease. Thus, understanding the causes of individual differences and the consequences of variation in vulnerability is of major importance. The aim of this review is to shed light on this issue by presenting a new naturalistic model of chronic psychosocial stress in male mice. Resident/intruder pairs of mice lived in continuous sensory contact and physically interacted daily. Four categories were identified: Resident Dominant, Resident Subordinate (RS), Intruder Dominant, and Intruder Subordinate. Behavior, autonomic and immune functions, hypothalamic-pituitary-adrenocortical responses, brain cytokine expression and cardiac histology were investigated in stress-exposed mice. Certain stress-induced alterations were present in all mice independent of their social status, while others clearly differentiated dominants from subordinates. RS mice showed a unique profile of alterations suggesting that the loss of relevant resources, such as the territory, is the key factor determining why only certain stress-exposed individuals ultimately show malignancy and psychopathologies.

Effects of social stress on blood leukocyte distribution: the role of α- and β-adrenergic mechanisms

Social stress in mammals has repeatedly been shown to cause substantial alterations in the distribution pattern of immune cells in the peripheral blood. The studies described here investigated the role of adrenergic mechanisms in mediating stressor-induced changes in blood leukocyte numbers using a social confrontation procedure in the rat. Experimental manipulations were carried out to eliminate the stress-associated release of adrenal hormones or to block the binding of endogenous catecholamines to alpha- and beta-adrenergic receptors. Adrenalectomy completely abolished the stressor-induced decreases in circulating numbers of T helper cells (CD4+), cytotoxic T cells (CD8+) and B cells but was ineffective in preventing neutrophil granulocytosis, monocytosis and an increase in natural killer (NK) cells. Treatment with the alpha-adrenergic antagonist phentolamine (PHE) was highly effective in preventing granulocytosis and partially blocked lymphopenia, but failed to abolish monocytosis and an increase in NK cells. Treatment with the beta-adrenergic antagonists propranolol (PROP) or nadolol (NAD) entirely blocked the increases in monocytes and NK cells. In addition, beta-adrenergic blockade also significantly reduced neutrophilia, with PROP being more effective than NAD. The results presented here provide evidence that catecholamines play an important role in the redistribution of blood leukocytes during social stress. In particular, the mobilization of cells of the innate immune response seems to be regulated by adrenergic mechanisms.

Effects of repeated social stress on leukocyte distribution in bone marrow, peripheral blood and spleen

Leukocyte trafficking between the various body compartments has an important surveillance function that ensures the detection of antigen and enables the immune system to initiate a rapid and effective response. Repeated social defeat of group-housed male mice induced by daily, acute encounters with an aggressive conspecific substantially altered leukocyte trafficking and led to a gradual redistribution of immune cells in bone marrow, peripheral blood and spleen. Recurrent exposure to the stressor over a period of 2, 4 or 6 consecutive days was associated with cell mobilization and increased myelopoiesis in the bone marrow that was paralleled by an accumulation of neutrophils and monocytes in circulation and spleen. Substantial depletion of B cells in bone marrow and blood was associated with an increase in splenic B cells indicating a redirection of this cell type to the spleen. In contrast, T cells were markedly reduced in these immune compartments. The recruitment of CD11b+ leukocytes (i.e., monocytes/macrophages and neutrophils) from the bone marrow to the spleen might play a critical role in the development of functional glucocorticoid resistance in the murine spleen that was reported in context with repeated social defeat.

Differential responses of circulating prolactin, GH, and ACTH levels and distribution and activity of submaxillary lymph node lymphocytes to calorie restriction in male Lewis and Wistar rats

OBJECTIVES

Calorie restriction has been associated with anorexia in growing individuals, but the mechanisms involved are not known. Also, the effects of carbohydrates and lipid restriction in growing individuals were not studied. The aim of this study was to determine whether 66% calorie restriction (lipids and carbohydrates) differentially affects growing rats of the Wistar or Lewis strains.

METHODS

Growing male Wistar and Lewis rats were subjected to 66% calorie restriction for 4 weeks. Plasma prolactin, growth hormone (GH), and adrenocorticotropic hormone (ACTH) levels were measured by specific radioimmunoassays. Likewise, lymphocytes from submaxillary lymph nodes were aseptically obtained to study absolute number of lymphocytes, cell surface markers (CD4+, CD8+, CD4+CD8+, B, and T cells), and proliferation.

RESULTS

The body weight gain was lower in Lewis than in Wistar rats and was blunted in both strains by calorie restriction. Wistar and Lewis rats exhibited differential patterns of plasma prolactin, ACTH, and GH levels as well as proliferative capacities of T and B cells and their distribution in the submaxillary lymph nodes. Calorie restriction increased plasma prolactin and ACTH levels in Lewis as compared with Wistar rats. However, the plasma GH levels were diminished in both strains of rats by calorie restriction. All changes in plasma prolactin ACTH and GH levels seemed to correlate with the modifications observed in the distributions of T and B subsets in the submaxillary lymph nodes as well as in their proliferative capacity.

CONCLUSIONS

Calorie restriction differentially modifies the secretory patterns of prolactin, GH, and ACTH in Lewis and in Wistar rats. These changes may explain, at least in part, the associated modifications in the proliferative capacity of submaxillary lymph node lymphocytes and in their distribution.

Adrenocorticotropin stress response but not glucocorticoid-negative feedback is altered by prenatal morphine exposure in adult male rats

The present study was designed to investigate the effects of prenatal morphine exposure on the hypothalamic-pituitary-adrenal (HPA) axis-regulated stress responses by measuring restraint stress-induced changes in the adrenocorticotropic hormone (ACTH) and corticosterone (CORT) levels. In experiment 1, plasma levels of ACTH and CORT in prenatally morphine-, saline-exposed and control male rats were determined before and at several times after restraint stress. There were no statistically significant differences in plasma ACTH and CORT levels before restraint stress between the groups. However, prenatal morphine exposure dampened the stress-induced increase and spontaneous recovery of ACTH levels after the restraint stress. There were no differences in plasma CORT levels between the three groups either before or at any time after restraint stress. Experiment 2 was designed to investigate the sensitivity of negative feedback of glucocorticoids using the dexamethasone (DEX) suppression test. DEX was administered at different doses (0.001, 0.01, 0.1 and 1.0 mg/kg) and ACTH and CORT plasma levels were measured before and at several times after restraint stress in prenatally morphine- and saline-exposed males. DEX pretreatment eliminated the differences observed in ACTH responses to stress in morphine- and saline-exposed males. DEX pretreatment dose dependently suppressed the restraint stress-induced increased plasma ACTH concentration. In plasma CORT levels, DEX pretreatment dose dependently suppressed the restraint stress-induced increased plasma CORT concentration regardless of prenatal drug exposure. Thus, the present study demonstrates that prenatal morphine exposure alters the ACTH and CORT responses to stress but not the sensitivity of negative feedback of glucocorticoids.

Importance of fighting in the immune effects of social defeat

Social defeat involves a clear physical component in the form of fight-induced injuries. The impact of body injuries on the immune response is not yet well known. In this study we compared the endocrine and immune responses to two types of social defeat in mice, one limiting the occurrence of skin injuries (mild social stress, MSS), and the other not (social disruption stress, SDR). In the two situations, six defeats were applied within 1 week. Plasma corticosterone and IL-6 levels were measured in blood samples taken after social defeat. Reactivity to LPS and sensitivity to corticosterone (CS) of spleen cells was assessed by measuring the in vitro production of cytokines (IL-6, IFN-gamma and IL-10) in response to LPS under a range of increasing concentrations of CS. The two types of stressors induced a similar plasma corticosterone response, but SDR mice showed significantly higher plasma IL-6 than MSS mice. Splenocytes from SDR but not from MSS mice produced more IL-6 and IL-10 in response to LPS and presented an altered responsiveness to CS in comparison to control mice. We conclude that the procedure involving fights and skin injuries was able to modulate the immune response in the spleen, whereas the procedure preventing the occurrence of fights did not. The increased immune reactivity observed in the fight-associated procedure could result from either a stronger psychological stress or a direct immune activation through the wounds.

The hypothalamic-pituitary-adrenal axis and viral infection

The hypothalamic-pituitary-adrenal (HPA) axis plays an important immunomodulatory role during viral infection. Activation of the HPA axis ultimately leads to elevated plasma levels of glucocorticoid (GC) hormones with the ability to mediate adaptive behavioral, metabolic, cardiovascular and immune system effects. In this review, we focus on the modulation of anti-viral immunity and viral pathogenesis by the HPA axis.

Chronic psychosocial stress-induced down-regulation of immunity depends upon individual factors

The effect of chronic stress on immune functions is strongly biased by individual factors. Mice were subjected to a new model of chronic psychosocial stress in which four different subcategories of stressed animals may be identified: Resident Dominants (RD), Resident Subordinates (RS), Intruder Dominants (InD), and Intruder Subordinates (InS). After 7 days of stress, mice were immunized with keyhole limpet hemocyanine (KLH). Their immune functions were investigated 14 days later with stress continuing trough. Importantly, RS mice, which are mice losing territory ownership, were the more affected, having lower IgG, proliferation, and IL-2. RD and InD showed lower IgG while InS showed no immune alteration. In conclusion, loss of resources could be a key factor in determining individual vulnerability to stressful events.

Social stress affects migration of blood T cells into lymphoid organs

The effect of social confrontation stress on the normal distribution of blood T cells into lymphoid organs and some other body tissues was studied. Social stress was induced by placing a male Fischer 344 (F334) rat into the home cage of a resident opponent. 51Cr-labeled blood T cells isolated from syngeneic donor, were intravenously injected into recipients immediately before confrontation. The accumulation of 51Cr-labeled T cells in the spleen, mesenteric and cervical lymph nodes 24 h after injection was about 30% lower in socially stressed than in control males. Substantially higher localization was observed in the bone marrow of socially stressed males.

IFN-gamma production in rabbits: behavioral and endocrine correlates

Freely interacting male rabbits were studied to establish the effect of exogenous testosterone on interferon-gamma (IFN-gamma) production in peripheral blood mononuclear cells (PBMCs) and to evaluate if this effect is related to season, social rank, plasma corticosterone and glucocorticoid receptors (GcR) in PBMCs. Dominance behavior increases after testosterone propionate (TP) administration only in rank 1 animals, while submission behavior increases after TP only in rank 4 animals, indicating a reinforcing effect of TP on the behavior. Corticosterone and IFN-gamma production are higher and GcR binding capacity is lower in spring than in autumn, suggesting that seasonal fluctuations in the immune system may be related to the pattern of secretion of immunomodulatory hormones. In autumn, corticosterone decreases after TP treatment and increases after social interaction, while GcR binding capacity decreases after TP treatment and social interaction. IFN-gamma production decreases in spring and increases in autumn after TP treatment plus social interaction, indicating that the modulating action of testosterone is related to the current immune status. The relationship between dominance, testosterone and the immune system in spring is suggested by the finding that GcR binding capacity after TP treatment is directly related to social rank, as confirmed by the positive correlation with dominance behavior frequency. The dominance index is positively correlated with GcR binding capacity and negatively with IFN-gamma production before TP treatment, indicating that high receptor activity in immunocompetent cells and low immunoreactivity could be prerequisites for dominance behavior. The immunosuppressive effect of corticosterone and the mechanism of down-regulation on GcR are confirmed by the negative correlations with IFN-gamma production and GcR binding capacity.

Effects of different forms of environmental enrichment on behavioral, endocrinological, and immunological parameters in male mice

This study investigated effects of different forms of environmental enrichment on behavioral, endocrinological, and immunological parameters in male mice. For this purpose, animals of the inbred strain CS were kept in groups of four males under three different housing conditions: (A) nonstructured Makrolon type III laboratory cages ("standard-housing" = S); (B) equivalent laboratory cages that were enriched with a box and a scaffolding ("enriched-housing" = E); and (C) spacious terraria that were structured richly ("super-enriched-housing" = SE). Both forms of enrichment caused a sharp rise in aggressive behavior, though play behavior was increased in E and SE mice, too. Levels of sociopositive behaviors in S and SE mice were higher than those in E mice. Plasma corticosterone concentrations and adrenal tyrosine hydroxylase activities were significantly increased in male mice kept in both forms of enriched cages, indicating an activation of the pituitary-adrenocortical and the adrenomedullary systems. The behavioral and endocrinological differences were partly reflected by immunological parameters: SE mice had levels of IgG1 and ratios of IFN-gamma/IL-10 and IL-2/IL-10 significantly lower than those of S mice. Ratios of IgG2a/IgG1 were significantly higher in SE mice. The absolute percentages of CD8 cells in E-mice were significantly lower than those in S mice. Despite the elevated levels of stress hormones under both forms of enriched housing, the behavioral parameters also indicate positive effects of the enrichment, especially on SE animals. Obviously, an environmental enrichment is beneficial for male mice as long as the spatial conditions are generous enough to allow coping with the increased aggression brought about by the enrichment.

Social stress alters splenocyte phenotype and function

Social stress of group-housed male mice induced a state of functional glucocorticoid (GC) resistance in splenocytes. The following studies examined the effects of paired-fighting (PF) stress on immune cell distribution and function in spleens of male mice. Following six daily PF stress sessions, splenic monocytes and neutrophils increased and lymphocytes decreased. PF also altered the distribution of CD62L and CD11b positive monocytes. Additionally, PF augmented proliferation and lowered the sensitivity of LPS-stimulated splenocytes to the antiproliferative effects of corticosterone, suggesting that PF induced a state of GC resistance in splenocytes. Together, these findings indicate that social stress altered phenotype and function of splenic immune cells. These findings may have implications for the healing of bite wounds that are often associated with social stress in rodents.

Social disruption-induced glucocorticoid resistance: kinetics and site specificity

Social disruption (SDR) of male mice has been shown to induce a state of functional glucocorticoid (GC) resistance in splenocytes. The present study demonstrated that GC resistance developed following repeated, but not acute exposure to SDR. GC resistance was long-lasting and persisted for at least 10 days after stress. In contrast, SDR did not alter cytokine secretion from peritoneal mononuclear cells treated with corticosterone. These findings suggest that SDR-induced GC resistance may be restricted to specific sites such as the spleen.

Social stress induces glucocorticoid resistance in macrophages

Stress-induced levels of plasma glucocorticoid hormones are known to modulate leukocyte function. These experiments examined the effects of a social stressor on the responsiveness of peripheral immune cells. Male mice experienced six evening cycles of social disruption (SDR), in which an aggressive male intruder was placed into their home cage for 2 h. Although circulating corticosterone was elevated in SDR mice, they had enlarged spleens and increased numbers of splenic leukocytes. Splenocytes from SDR and control mice were cultured with lipopolysaccharide and corticosterone. Cells from SDR mice exhibited decreased sensitivity to the antiproliferative effects of corticosterone, suggesting that the peripheral immune cells were resistant to glucocorticoids. In addition, SDR cells produced more interleukin (IL)-6. To determine which cell population was affected, we used antibody-labeled magnetic beads to deplete splenocyte suspensions of B cells or macrophages. Depletion of macrophages from SDR cultures, but not depletion of B cells, abolished both the corticosterone resistance and enhanced IL-6 secretion. These findings demonstrate that a psychosocial stressor induced glucocorticoid resistance in mouse splenic macrophages.

Social defeat as a stressor in humans

Studies on social defeat in humans, and their similarities with studies on social defeat in animals are reviewed. Studies on social defeat in humans typically are conducted as a branch of social psychology, most often focusing on bullying in schools and in workplaces. Victims of bullying are known to suffer from depression, anxiety, sociophobia, loss of self-esteem, psychosomatic diseases, and other behavioral symptoms. On the other hand, animal studies on social defeat, usually based on the rodent resident--intruder paradigm, present findings related to physiological rather than to behavioral consequences of defeat. The two branches use different terminology, e.g., "dominant" and "subordinate" (animal studies) and "bully" and "victim" (human studies). It is suggested that the two fields could benefit from a mutual exchange in theory and methodology.

Social stress in laboratory rats: behavior, immune function, and tumor metastasis

This report summarizes data from social confrontations studies in laboratory rats dealing with the effects of psychosocial stress on immune functioning and tumor metastasis. The paper focuses on the physiological alterations observed in subdominant males after 2 days of continuous social confrontation. A significant loss of body mass and elevated plasma concentrations of adrenal hormones in subdominant males indicate a stressful social environment. Subdominant males showed lower numbers of blood CD4 and CD8 T cells as well as reduced activity levels of T cells and natural killer (NK) cells relative to control subjects. In order to evaluate the possible health impact of suppressed NK functioning, we used the MADB 106 tumor model. A 10-fold lower tumor clearance in subdominant males demonstrates suppression of the animals' capacity to prevent metastatic development. The relationship between individual behavior and immunological outcome is briefly discussed. Together, the study of male rats in social confrontations appears to be a good model to investigate stress-induced immune modulation and tumor metastasis under relatively naturalistic social conditions.

Animal models of social stress: effects on behavior and brain neurochemical systems

Social interactions serve as an evolutionarily important source of stress, and one that is virtually ubiquitous among mammalian species. Animal models of social stress are varied, ranging from a focus on acute, intermittent, or chronic exposure involving agonistic behavior, to social isolation. The relative stressfulness of these experiences may depend on the species, sex, and age of the subjects, and subject sex also appears to influence the value of hypothalamic--pituitary--adrenal (HPA) axis activity as a general criterion for stress response: higher glucocorticoid levels are typically found in dominant females in some species. Social stress models often produce victorious and defeated, or dominant and subordinate, animals that may be compared to each other or to controls, but the appropriateness of specific types of comparisons and the interpretations of their differences may vary for the different models. Social stress strongly impacts behavior, generally reducing aggression and enhancing defensiveness, both inside and outside the stress situation. Social and sexual behaviors may be reduced in subordinate animals, as is activity and responsivity to normally rewarding events. However, some components of these changes may be dependent on the presence of a dominant, rather than representing a longer-term and general alteration in behavior. Social stress effects on brain neurotransmitter systems have been most extensively investigated, and most often found in serotonin and noradrenergic systems, with changes also reported for other monoamine and for peptidergic systems. Morphological changes and alterations of neogenesis and of cell survival particularly involving the hippocampus and dentate gyrus have been reported with severe social stress, as have longer-term changes in HPA axis functioning. These findings indicate that social stress models can provide high magnitude and appropriate stressors for research, but additionally suggest a need for caution in interpretation of the findings of these models and care in analysis of their underlying mechanisms.

Social stress induces glucocorticoid resistance in subordinate animals

Introducing an aggressive intruder into a cage of mice (social disruption, SDR) resulted in intense fighting and defeat of the cage residents. Defeat was accompanied by elevated levels of serum corticosterone and nerve growth factor (NGF). Repeated exposure to an intruder induced a state of glucocorticoid resistance in peripheral immune cells. The present study sought to examine the behavioral factors that mediated the development of glucocorticoid resistance following SDR. Glucocorticoid resistance developed in animals that exhibited a subordinate behavioral profile, which consisted of a low tendency for social investigation and a high level of submissive behavior in response to the intruder's attacks. Glucocorticoid resistance was also linked to the presence of injuries due to fighting, but not to changes in systemic levels of either corticosterone or NGF. Since a submissive behavioral profile is associated with increased risk for injuries due to fighting, it may be that the development of glucocorticoid resistance is an adaptive mechanism that allows the inflammatory component of wound healing to occur in the presence of high levels of corticosterone. Together, these findings demonstrate that the outcomes of social stress may be modified by physiological changes associated with wounding, as well as by behavioral variables such as social status.

Seasonal regulation of membrane and intracellular corticosteroid receptors in the house sparrow brain

A number of studies have demonstrated seasonal regulation of the adrenocortical response to stress, or of corticosteroid binding globulins, but very few studies have examined seasonal regulation of corticosteroid receptor levels. As a result, there have been few attempts to produce an integrated picture of seasonal plasticity of the stress response. We measured baseline and stress-induced corticosterone (CORT), corticosteroid binding globulin and neuronal cytosolic and membrane corticosteroid receptor levels in male and female, wild-caught house sparrows (Passer domesticus) during three different seasons over the annual cycle (nesting, molting and winter). We identified three neuronal corticosteroid receptors in the house sparrow brain: two intracellular receptors and one membrane-associated receptor. Little is known about corticosteroid receptors in neuronal membranes of avian and mammalian species, but we found that the levels of membrane corticosteroid receptors varied seasonally, being lowest during the nesting season. Cytosolic corticosteroid receptor numbers (both low and high affinity receptors) also varied seasonally. In contrast to the membrane bound receptors, however, the numbers of low and high affinity cytosolic receptors were lowest during winter. In addition, mean levels of total basal and stress-induced CORT in the plasma varied seasonally. Both basal and stress-induced levels of total CORT were significantly higher during nesting than during winter or molt. Finally, corticosteroid binding globulin levels in plasma were also seasonally regulated, in a pattern similar to total CORT, so that estimated free CORT levels did not vary between seasons. These data indicate that multiple components of the stress response are seasonally regulated in birds obtained from wild populations. Interactions between these regulated components provide a basis for seasonal differences in behavioural and physiological responses to stress.

Long-term colony housing in Long Evans rats: immunological, hormonal, and behavioral consequences

The distribution pattern and the function of blood immune cells were investigated for 10 weeks in three mixed-sex colonies of Long Evans rats. After colony formation, a despotic dominance system was established between the males. This paper focuses on differences between subdominant colony and pair-housed control males. A reduced body mass development and hormonal status in subdominant males indicate stressful colony conditions. Subdominant males had lower numbers of CD4 and CD8 T cells, pronounced granulocytosis and reduced lymphocyte proliferation rates as compared with controls. The persistency of changes in subdominant males offers the opportunity to investigate the effects of long-term immuno-modulation on health.