Prolonged increase of corticosterone secretion by chronic social stress does not necessarily impair immune functions

A mechanistic evaluation of the potential for octamethylcyclotetrasiloxane to produce effects via endocrine modes of action

This review is a hypothesis driven, mechanistic evaluation of the potential for octamethylcyclotetrasiloxane (D4) to produce any effects via endocrine modes of action. D4 is a volatile, lipophilic liquid used in the production of high molecular weight dimethylsiloxane polymers. These are used in a variety of industrial, medical, cleaning, and personal care products, and they may contain low levels of residual D4. Low concentrations of D4 are found in the environment and there is potential for low level human exposure. All of the measured environmental and workplace levels of D4 fall below no observed effect levels (NOEL). Most of the effects of high dose D4 involve the female reproductive system. In the mature intact female rat following chronic high dose exposure, D4 may cause inhibition of mating and ovulation, decreased live litter sizes, small increases in the estrogen to progesterone ratio primarily through decreases in progesterone, and increases in uterine hyperplasia. When endogenous estrogens are very low, high dose D4 causes increases in some uterine parameters. To assess whether these high dose effects can be attributed to an endocrine mode of action, endpoints are ranked for relevance and strength, consistent with published concepts. When sufficient information is available the level of activity of D4 for producing the observed effect is compared with that of potent endocrines. The conclusions reached are that all of the effects of D4 fall well short of any established criteria for D4 to be capable of producing any adverse effect via an endocrine mode of action.

The co-chaperone Fkbp5 shapes the acute stress response in the paraventricular nucleus of the hypothalamus of male mice

Disturbed activation or regulation of the stress response through the hypothalamic-pituitary-adrenal (HPA) axis is a fundamental component of multiple stress-related diseases, including psychiatric, metabolic, and immune disorders. The FK506 binding protein 51 (FKBP5) is a negative regulator of the glucocorticoid receptor (GR), the main driver of HPA axis regulation, and FKBP5 polymorphisms have been repeatedly linked to stress-related disorders in humans. However, the specific role of Fkbp5 in the paraventricular nucleus of the hypothalamus (PVN) in shaping HPA axis (re)activity remains to be elucidated. We here demonstrate that the deletion of Fkbp5 in Sim1+ neurons dampens the acute stress response and increases GR sensitivity. In contrast, Fkbp5 overexpression in the PVN results in a chronic HPA axis over-activation, and a PVN-specific rescue of Fkbp5 expression in full Fkbp5 KO mice normalizes the HPA axis phenotype. Single-cell RNA sequencing revealed the cell-type-specific expression pattern of Fkbp5 in the PVN and showed that Fkbp5 expression is specifically upregulated in Crh+ neurons after stress. Finally, Crh-specific Fkbp5 overexpression alters Crh neuron activity, but only partially recapitulates the PVN-specific Fkbp5 overexpression phenotype. Together, the data establish the central and cell-type-specific importance of Fkbp5 in the PVN in shaping HPA axis regulation and the acute stress response.

Chronic social instability in adult female rats alters social behavior, maternal aggression and offspring development

We investigated the consequences of chronic social instability (CSI) during adulthood on social and maternal behavior in females and social behavior of their offspring in a rat model. CSI consisted of changing the social partners of adult females every 2-3 days for 28 days, 2 weeks prior to mating. Females exposed to CSI behaved less aggressively and more pro-socially towards unfamiliar female intruders. Maternal care was not affected by CSI in a standard testing environment, but maternal behavior of CSI females was less disrupted by a male intruder. CSI females were quicker to attack prey and did not differ from control females in their saccharin consumption indicating, respectively, no stress-induced sensory-motor or reward system impairments. Offspring of CSI females exhibited slower growth and expressed more anxiety in social encounters. This study demonstrates continued adult vulnerability to social challenges with an impact specific to social situations for mothers and offspring.

The exercise-glucocorticoid paradox: How exercise is beneficial to cognition, mood, and the brain while increasing glucocorticoid levels

Exercise is known to have beneficial effects on cognition, mood, and the brain. However, exercise also activates the hypothalamic-pituitary-adrenal axis and increases levels of the glucocorticoid cortisol (CORT). CORT, also known as the "stress hormone," is considered a mediator between chronic stress and depression and to link various cognitive deficits. Here, we review the evidence that shows that while both chronic stress and exercise elevate basal CORT levels leading to increased secretion of CORT, the former is detrimental to cognition/memory, mood/stress coping, and brain plasticity, while the latter is beneficial. We propose three preliminary answers to the exercise-CORT paradox. Importantly, the elevated CORT, through glucocorticoid receptors, functions to elevate dopamine in the medial prefrontal cortex under chronic exercise but not chronic stress, and the medial prefrontal dopamine is essential for active coping. Future inquiries may provide further insights to promote our understanding of this paradox.

Immune responses of eastern fence lizards (Sceloporus undulatus) to repeated acute elevation of corticosterone

Prolonged elevations of glucocorticoids due to long-duration (chronic) stress can suppress immune function. It is unclear, however, how natural stressors that result in repeated short-duration (acute) stress, such as frequent agonistic social encounters or predator attacks, fit into our current understanding of the immune consequences of stress. Since these types of stressors may activate the immune system due to increased risk of injury, immune suppression may be reduced at sites where individuals are repeatedly exposed to potentially damaging stressors. We tested whether repeated acute elevation of corticosterone (CORT, a glucocorticoid) suppresses immune function in eastern fence lizards (Sceloporus undulatus), and whether this effect varies between lizards from high-stress (high baseline CORT, invaded by predatory fire ants) and low-stress (low baseline CORT, uninvaded) sites. Lizards treated daily with exogenous CORT showed higher hemagglutination of novel proteins by their plasma (a test of constitutive humoral immunity) than control lizards, a pattern that was consistent across sites. There was no significant effect of CORT treatment on bacterial killing ability of plasma. These results suggest that repeated elevations of CORT, which are common in nature, produce immune effects more typical of those expected at the acute end of the acute-chronic spectrum and provide no evidence of modulated consequences of elevated CORT in animals from high-stress sites.

The involvement of FK506-binding protein 51 (FKBP5) in the behavioral and neuroendocrine effects of chronic social defeat stress

Chronic stress is increasingly considered to be a main risk factor for the development of a variety of psychiatric diseases such as depression. This is further supported by an impaired negative feedback of the hypothalamic-pituitary-adrenal (HPA) axis, which has been observed in the majority of depressed patients. The effects of glucocorticoids, the main hormonal endpoint of the HPA axis, are mediated via the glucocorticoid receptor (GR) and the mineralocorticoid receptor. The FK506-binding protein 51 (FKBP5), a co-chaperone of the Hsp90 and component of the chaperone-receptor heterocomplex, has been shown to reduce ligand sensitivity of the GR. This study aimed to investigate the function of FKBP5 as a possible mediator of the stress response system and its potential role in the development of stress-related diseases. Therefore, we assessed whether mice lacking the gene encoding FKBP5 (51KO mice) were less vulnerable to the adverse effects of three weeks of chronic social defeat stress. Mice were subsequently analyzed with regards to physiological, neuroendocrine, behavioral and mRNA expression alterations. Our results show a less vulnerable phenotype of 51KO mice with respect to physiological and neuroendocrine parameters compared to wild-type animals. 51KO mice demonstrated lower adrenal weights and basal corticosterone levels, a diminished response to a novel acute stimulus and an enhanced recovery, as well as more active stress-coping behavior. These results suggest an enhanced negative glucocorticoid feedback within the HPA axis of 51KO mice, possibly modulated by an increased sensitivity of the GR. This article is part of a Special Issue entitled 'Anxiety and Depression'.

Pituitary glucocorticoid receptor deletion reduces vulnerability to chronic stress

The incidence of chronic stress is frequently related to the development of psychiatric disorders like depression. The hypothalamic-pituitary-adrenal (HPA) axis is a major physiological system that mediates the stress response. Tight HPA axis regulation through negative feedback mechanisms is essential for health and environmental adaptation. This feedback regulation acts in part through the glucocorticoid receptor (GR) on several organizational levels, including the pituitary, the hypothalamus and the hippocampus. However, the precise role of the different anatomical structures, specifically the pituitary, in HPA axis regulation is yet largely unknown. Here, we show that a conditional pituitary GR knockout is not necessarily detrimental for the animal's ability to cope with chronic stress situations. Mice with a deletion of the GR at the pituitary (GR(POMCCre)) were subjected to 3 weeks of chronic social defeat stress. We analyzed both the behavioral and neuroendocrine phenotype as well as the central nervous system expression of genes involved in HPA axis function in these animals. Our results show a more resilient phenotype of GR(POMCCre) mice with respect to anxiety-related behavior and neuroendocrine parameters compared to stressed wild type animals. In light of the previously reported high corticosterone levels during postnatal development in GR(POMCCre) mice, our findings suggest that adverse early life events may have beneficial developmental effects on the organism to improve stress coping later in life.

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.

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.

Effects of housing condition on experimental outcome in a reproduction toxicity study

In most toxicity studies single housing is still preferred, as social stress is believed to have an effect on experimental outcome through interaction with the toxic compound or by increasing variation. There are also arguments that single housing will have a similar effect. In this study the qualitative and quantitative effects of single- and social housing of rats has been investigated on immune- and endocrine responses, histopathology and body- and organ weights in a one-generation endocrine disrupter study (OECD 415) in rats exposed to tetrabromobisphenol A (TBBPA). The results of this study show that differences in parameters between the housing conditions were rarely noted. Striking results of the study are that in several parameters significant differences were noted in the un-dosed control group in single versus group housed animals, meaning that TBBPA can mask or enhance housing effects, or vice versa. In one case single housing altered the effect of the toxic compound. Depending on the endpoints of the study, the type of housing condition must be taken into consideration as findings like these could have great implications for the interpretation and validity of results from toxicological assays and the number of animals needed to detect significant effects of toxic compounds.

Stress-induced modulation of skin immune function: two types of antigen-presenting cells in the epidermis are differentially regulated by chronic stress

BACKGROUND

Inflammatory skin diseases are exacerbated by psychiatric stress. Previous studies have shown that the activity of epidermal antigen-presenting cells (APCs), Langerhans cells (LCs) and keratinocytes (KCs), is affected by stress. Hapten application causes migration of LCs to draining lymph nodes (DLNs). Recently, we found that hapten application also activates epidermal cells (ECs) to mature potent APCs, and that the main APCs in these populations are KCs. Thus, DLN cells and ECs following hapten application are available for estimating the APC function of LCs and KCs in stress studies.

OBJECTIVES

To investigate the mechanism of exacerbation of skin inflammation by chronic stress by observing the effect of isolation stress transversally on the skin immune and neurohormonal systems.

METHODS

Contact sensitivity (CS) was elicited in BALB/c mice. The APC function of LCs and ECs following hapten application was assessed by the CS-inducing activity in the recipient mice. Levels of neurohormonal transmitters and proinflammatory cytokines were measured by enzyme-linked immunosorbent assay. Cell surface molecules were detected using flow cytometry. Expression of mRNA for cytokines, neurohormonal receptors and a differentiation marker by ECs was determined by reverse transcription-polymerase chain reaction.

RESULTS

Acute stress (2-day isolation) suppressed induction of CS, while chronic stress (30-day isolation) markedly enhanced induction of CS. DLN cells from chronically stressed mice contained increased numbers of LCs and exhibited enhanced APC function for inducing CS. In contrast, the APC function of KCs from these mice was markedly suppressed. Serum corticosterone levels were enhanced in acute stress, while substance P (SP) levels were enhanced in chronic stress. Corticotrophin-releasing hormone receptor-1 mRNA expression in ECs was enhanced in acute stress, while SP receptor (i.e. neurokinin-1 receptor) mRNA expression in ECs was enhanced in chronic stress. Production and mRNA expression of the proinflammatory cytokines interleukin-1 alpha and tumour necrosis factor-alpha by ECs following hapten application was markedly suppressed in chronic stress. Expression by ECs of E-cadherin, which adheres LCs and KCs homophilically, was suppressed in chronic stress. In addition, these cells exhibited impaired differentiation, i.e. suppressed spontaneous proliferation and enhanced mRNA expression for transglutaminase-3.

CONCLUSIONS

Chronic isolation stress may enhance CS responses by upregulation of the APC activity of LCs and the SP system. However, dysregulation of KC function and differentiation by chronic stress suggests that KCs may not contribute to the enhancement of the CS response positively. These complex changes suggest that chronic isolation stress in mice may provide a possible model system for studying the mechanism of exacerbation of skin inflammation by stress.

Voluntary exercise protects against stress-induced decreases in brain-derived neurotrophic factor protein expression

Exercise is increasingly recognized as an intervention that can reduce CNS dysfunctions such as cognitive decline, depression and stress. Previously we have demonstrated that brain-derived neurotrophic factor (BDNF) is increased in the hippocampus following exercise. In this study we tested the hypothesis that exercise can counteract a reduction in hippocampal BDNF protein caused by acute immobilization stress. Since BDNF expression is suppressed by corticosterone (CORT), circulating CORT levels were also monitored. In animals subjected to 2 h immobilization stress, CORT was elevated immediately following, and at 1 h after the cessation of stress, but remained unchanged from baseline up to 24 h post-stress. The stress protocol resulted in a reduction in BDNF protein at 5 and 10 h post-stress that returned to baseline at 24 h. To determine if exercise could prevent this stress-induced reduction in BDNF protein, animals were given voluntary access to running wheels for 3 weeks prior to the stress. Stressed animals, in the absence of exercise, again demonstrated an initial elevation in CORT (at 0 h) and a subsequent decrease in hippocampal BDNF at the 10 h time point. Exercising animals, both non-stressed and stressed, demonstrated circulating CORT and hippocampal BDNF protein levels that were significantly elevated above control values at both time points examined (0 and 10 h post-stress). Thus, the persistently high CORT levels in exercised animals did not affect the induction of BDNF with exercise, and the effect of immobilization stress on BDNF protein was overcome. To examine the role of CORT in the stress-related regulation of BDNF protein, experiments were carried out in adrenalectomized (ADX) animals. BDNF protein was not downregulated as a result of immobilization stress in ADX animals, while there continued to be an exercise-induced upregulation of BDNF. This study demonstrates that CORT modulates stress-related alterations in BDNF protein. Further, exercise can override the negative effects of stress and high levels of CORT on BDNF protein. Voluntary physical activity may, therefore, represent a simple non-pharmacological tool for the maintenance of neurotrophin levels in the brain.

Effect of metabolic cage housing on immunoglobulin A and corticosterone excretion in faeces and urine of young male rats

Six 8-week-old Sprague-Dawley rats were studied for 9 days divided into three periods of 3 days each: before transferral to metabolism cages, during metabolic cage housing and after return to their home cages. Faeces were collected daily when the animals were housed in their home cages and every 6 h when the animals were housed in metabolic cages during which time urine was also collected every 6 h. The rate of weight gain was slightly reduced during the 3 days in metabolic cages and the animals produced significantly larger amounts of faeces when housed in metabolic cages than when housed in their home cages. The total faecal excretion of corticosterone (nanograms excreted per hour per kilogram body weight) and immunoglobulin A (IgA) (milligrams excreted per hour per kg body weight) quantified by enzyme-linked immunosorbent assays (ELISAs) exhibited a clear diurnal rhythm in the metabolic cage. Urinary excretions of corticosterone and IgA also followed a clear diurnal cycle. The mean daily amounts of corticosterone excreted were not significantly affected by cage change and by housing in metabolic cages. However, the excretion of faecal IgA was significantly reduced during the 3 days after the period in metabolic cages. Taken together the results indicate that metabolic cage housing is mildly stressful for young adult male rats.

Cytokine production by spleen cells after social defeat in mice: activation of T cells and reduced inhibition by glucocorticoids

Social disruption (SDR) is an effective model of social stress associated with an enhanced inflammatory reactivity of the immune system. The aim of the present study was to further describe SDR effects on cytokine production by spleen cells, testing selectively monocyte and T cell functions as a result of this stressor. For this purpose, splenocytes from control mice (C) and mice socially stressed for 7 days (SDR) were cultured in the presence of lipopolysaccharide (LPS) or concanavalin A (Con A). Splenocyte proliferation, cytokine production and sensitivity of spleen cells to corticosterone were assessed in vitro. The humoral response to keyhole limpet hemocyanin (KLH) immunization was assessed. SDR induced splenomegaly and enhanced splenocyte basal proliferation. The pro-inflammatory influence of SDR was confirmed by an increased release of interleukin-6 (IL-6) by LPS-stimulated cultures and by a reduced sensitivity of spleen cells to the anti-inflammatory effect of corticosterone. The mechanism increasing cytokine production in response to LPS was cytokine specific, since among inflammatory cytokines, IL-6 but not interferon-gamma (IFN-gamma) was enhanced by stress. In stressed mice, the increase in IL-6 and IFN-gamma and the decrease in IL-10 release in Con A-stimulated cultures indicate that SDR did not modify the Th1/Th2 cytokine balance but globally activated T cells. Plasma anti-KLH antibody levels were similar in both groups. Wounded and non-wounded mice presented similar responses to stress. This study shows that social disruption stress enhances the reactivity of cells from both the acquired and innate immune systems.

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.

Effect of variable moderate chronic stress on ligature-induced periodontal disease in Wistar rats

The aim of this investigation was to study the impact of stress on ligature-induced periodontal disease in rats by means of a variable moderate chronic stress model. Thirty male Wistar rats were randomly assigned to six groups. Control groups received only ligatures around the second maxillary molars, while experimental groups were exposed to stress in additional. Stress was imposed by means of flashing light, isolation, rat blood smelling, new environment exposure, immobilization in cold temperature and immobilization at room temperature. Stress was applied randomly, thereby diminishing adaptation of the animals to the model. The animals were killed after 29, 43, and 57 days. The distance between the cementum-enamel junction and the alveolar bone (CEJ-AB) was measured. Alveolar bone loss was statistically different between stressed and control animals, whereas differences were not observed between experimental periods. The mean CEJ-AB distance in animals exposed to stress was 154.50 microm smaller than the corresponding distance in the controls. It might be concluded that variable moderate chronic stress decreased alveolar bone loss in a ligature-induced periodontal disease model.

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.

Quantitative aspects of stress-induced immunomodulation

Recent studies indicate that neuroendocrine-immune interactions can cause sufficient immunosuppression to adversely affect human health, but quantitative relationships between stress-related hormones or neurotransmitters and immune function have not been well documented. The mechanisms of stress-induced immunomodulation cannot be fully understood solely by identifying the hormones, neurotransmitters, and cytokines involved. Quantitative relationships and interactions must also be understood. Depending on the nature and duration of the stressor and the immunological parameter under investigation, stress responses can enhance, have no effect, or suppress immunological parameters. These quantitative relationships have implications with regard to safety assessment of drugs and chemicals and with regard to potential development of pharmacological interventions to ameliorate some of the immunosuppressive effects of stress. This review describes selected studies that relate the quantity and duration of exposure to stress-related neuroendocrine mediators to modulation of the immune system. These studies provide a useful starting point, but they also illustrate how much work remains to achieve a fully integrated qualitative and quantitative understanding of stress-induced immunomodulation.

Social status in mice: behavioral, endocrine and immune changes are context dependent

The aim of the present study was to investigate the effect of social status on the endocrine, immune and behavior response of male mice. We found that in mice reared in a group of siblings since weaning, no difference exists between dominants and subordinates in basal corticosterone level, in behavior in the open-field test (OFT) and in a series of immune parameters. These results suggest that living with siblings is not a stressful condition for either dominant or subordinate mice. Therefore, group-housed siblings can be regarded as a valid control group in social stress studies. When mice were subjected to chronic psychosocial stress for 21 days, four types of social outcome occurred: residents becoming dominants, intruders becoming subordinates, residents becoming subordinates and intruders becoming dominants. Interestingly, the behavioral profile in the OFT revealed a status-dependent effect, with resident dominants (RD) and intruder dominants (InD) showing the highest locomotor and exploratory activity, whereas the corticosterone level was higher than control for all four categories. In addition, a context-dependent effect emerged at the immune level: resident subordinates (RS) had a reduced splenocyte proliferation and IL-4 and IL-10 production. Mice in all the other three social ranks showed no immune alterations. Therefore, the loss of an individual's social rank position seems a promising field of study to investigate the psychological impact of stressful events.

Social stress paradigms in male mice: Variations in behavior, stress and immunology

Male OF1 strain mice were allocated, after 2 weeks of individual housing, to cohabitating (6 or 16 days), fixed dyadic interaction pairs (6 or 16 daily encounters) or control groups (6 or 16 days). These different social stress situations were assessed for their effects on splenic contents of NE, IL-1 and IL-2 and serum levels of corticosterone. Spleen NE contents showed no significant variations, but serum corticosterone titers were generally higher in interacting pairs and subordinates. Splenic IL-2 did not respond in the same way to the treatments as IL-1. The differences in splenic interleukin contents could not be simply related to observed changes in serum corticosterone levels. Different mechanisms appear to regulate changes in glucocorticoids and the measured cytokines. These physiological phenomena do not simply reflect in the animal's social status (dominant or submissive). The intensity and duration of the agonistic behavior displayed as well as the interaction experience accumulated may account for the observed differences between the paradigms.

Plasma corticosterone and immune reactivity in restrained female C3H mice

Psychological stressors are known to stimulate the hypothalamic-pituitary-adrenal axis and the sympathetic nervous system resulting in the release of corticosterone and catecholamines respectively. They have also been reported to induce cytokine production. All these molecules affect various immune parameters and can alter overall immune competence of the individual. The purpose of this investigation was to study the regulation of the production of corticosterone during stress and its possible effects on immune reactivity. In a first series of experiments, the possible regulation of corticosterone production by interleukin (IL)-1beta and peripheral catecholamines during restraint was assessed using a pharmacological approach in mice. Plasma IL-1beta concentrations remained at basal after 1-h restraint and the stress-induced increase of plasma corticosterone was not modified by a peripheral injection of an IL-1 receptor antagonist (IL-1ra). By contrast, chemical sympathectomy potentiated the restraint-induced increase in plasma corticosterone concentration, this potentiation being reversed by IL-1ra. In a second series of experiments, the role of corticosterone in stress-immune relationships was studied in adrenalectomized mice subjected to restraint and immunized with sheep erythrocytes. Non-specific immunity, i.e. proliferation of splenocytes and thymocytes and plasma levels of IL-1beta, as well as specific immunity, i.e. antibody production and delayed hypersensitivity, were not altered after 2-h restraint. Adrenalectomy failed to induce immune effects in stressed animals, except that delayed hypersensitivity was stronger in adrenalectomized animals, revealing that the high levels of corticosterone produced during stress have an anti-inflammatory activity. The present data show that the stress-induced production of corticosterone was modulated by both peripheral catecholamines and IL-1beta. However, this production of corticosterone was unable to modulate immune reactivity except delayed hypersensitivity.

Immunity in barren and enriched housed pigs differing in baseline cortisol concentration

It was shown in a recent study [De Jong IC, Prelle IT, Van de Burgwal JA, Lambooij E, Korte SM, Blokhuis HJ, Koolhaas JM. Effects of environmental enrichment on behavioral responses to novelty, learning and memory and the circadian rhythm in cortisol in growing pigs. Physiol Behav, in press.] that barren housed pigs (small pens, no substrate) have a blunted circadian rhythm of salivary cortisol as compared to enriched housed pigs (large pens with daily fresh bedding). In the light period, enriched housed pigs showed significantly higher concentrations of cortisol in saliva than barren housed pigs, whereas in the dark period, cortisol concentrations were low in both enriched and barren housed pigs. In the present study, the immunological consequences of the difference in baseline salivary cortisol concentration in the light period were evaluated. It appeared that leukocyte and lymphocyte distributions, and in vitro lymphocyte proliferation following ConcanavalineA (ConA) stimulation in the assay using purified lymphocytes were not affected. However, barren and enriched housed pigs did show a different proliferation response to ConA in the whole blood assay. At day 2 of culture, proliferation was higher in barren housed pigs than in enriched housed pigs, whereas at day 4 of culture, proliferation was higher in enriched housed pigs than in barren housed pigs. Lymphocyte proliferation at day 2 of culture in the whole blood assay correlated negatively with plasma cortisol levels, which might thus explain the higher proliferation in barren housed pigs at day 2 of culture. The in vivo humoral and cellular (delayed type hypersensitivity, DTH) immune response to KLH was not affected by housing conditions. We conclude that, although baseline salivary cortisol concentrations differ between enriched and barren housed pigs, immune function appears to be relatively unaffected.

Effect of an auditory stress on peritoneal and alveolar cells in C57 BL/6J mice of advanced age

The aim of this study was to analyse the effects of auditory stress on peritoneal and alveolar macrophages in C57 BL/6J mice of advanced age, and to compare the results to those obtained in old mice submitted to a sham stress, and to those observed in young mice submitted to the same auditory stress. We used a chemiluminescence assay to measure the production of free oxygen radicals (FOR) by macrophages. Eight 22 month-old mice were exposed to a sound stress of 110 dB for three consecutive nights; nine were submitted to a sham stress. The results were compared to those obtained in young (8 week-old) mice, 21 submitted to noise stress, and 17 controls. The corticosterone level was not increased after stress in any group. FOR production in old mice was significantly higher than that in young mice. Stress did not induce significant changes in FOR production by alveolar cells in young mice; however, the FOR production by alveolar cells was significantly higher in the stressed group than in the control group of old mice. These results show that noise stress is associated with modifications of macrophage functions that are influenced by cell localization, the behaviour of alveolar and peritoneal macrophages of old mice being clearly different in our experimental model.

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

Previous experiments with chronically coexisting groups of Long-Evans rats indicated differences in many aspects of blood cellular immunity between winner and loser rats. The present study investigated the specific hormonal response patterns of winners and losers in relation to changes in numbers of blood immune cells. At the beginning of a 7 day period of chronic confrontation, a partition wall was removed between two neighboring rat groups, each containing a male-female pair. Fights for dominance between the males resulted in fast establishment of stable dominance relationships. At day 7 of the confrontation, winner males showed stable concentrations of CBG (corticosteroid-binding globulin) and even reduced titers of total CORT (corticosterone). In contrast, a marked decrease in CBG and unaffected total CORT concentrations were determined in loser males. Increased norepinephrine (NE) and epinephrine (E) titers were evident only in losers. In addition, reduced testosterone titers were observed in the bitten loser male subgroup. All male subgroups lost body mass with most pronounced reductions in loser males. Confrontation caused a marked granulocytosis, especially in loser males. NE concentrations in loser males correlated with the percentage of granulocytes. Numbers of CD4 T-cells were lowered in all loser males and in non-biting winners. In not-bitten losers also a reduced number of CD8 T-cells was determined. Interestingly, higher pre-confrontational NE titers were detected in future bitten loser and future biting winner males relatively to not-bitten losers and non-biting winners. The present report indicates that differential hormonal response patterns may play an important role in some of the immunological differences observed between winner and loser males under stressful social conditions.

The effect on opioid peptides in the rat brain, after chronic treatment with the anabolic androgenic steroid, nandrolone decanoate

In recent years, an increase in abuse of anabolic androgenic steroids (AAS) has been seen among individuals not directly connected to sports. Clinical evidence suggests that abuse of these steroids may result in profound changes in personality, expressed by depressive symptoms, irritability and increased aggression. It is still unknown whether these alterations are related to changes in any particular transmitter system or whether they are persistent or reversible. In this study we focused on AAS effect on the endogenous dynorphin and enkephalin system in the brain. Male rats were given intramuscular injections of the AAS nandrolone decanoate (15 mg/kg), once daily for 2 weeks. The levels of the opioid peptide immunoreactivities (ir) were assessed by radioimmunoassay in two groups immediately after the treatment and in two other groups after additional 3 weeks without any drug treatment (recovery period). The result indicates that chronic AAS treatment increased the activity in the dynorphin B- and Met-enkephalin-Arg(6)Phe(7)-ir in the hypothalamus, striatum and periaqueductal gray (PAG) compared to controls. In addition, the steroid induced an imbalance between the dynorphin and the enkephalin opioid system in the nucleus accumbens, hypothalamus and PAG. This imbalance remained after the recovery period. Since increased peptide activity was found in brain regions regulating emotions, dependence, defensive reactions and aggression, it was suggested that the actual endogenous opioid systems are involved in previously reported AAS-induced changes in these behaviours.

Social stability attenuates the stress in the modified multiple platform method for paradoxical sleep deprivation in the rat

The instrumental methods to induce paradoxical sleep (PS) deprivation are stressful. The modified multiple platform method (MMPM), in which animals are placed with new cohorts inside the water tanks, results in augmented ACTH and corticosterone (CORT) responses. We hypothesised that this increased response could be attributed to social instability. In addition, we tested a new environmental control, a grid (GR) placed on the tank floor. Animals were submitted to the MMPM for 4 days as socially unstable (UG--coming from several cages) or stable groups (SG--coming from one cage), placed either on narrow platforms or on the grid. All UG animals presented higher ACTH levels than their SG counterparts, including home-cage controls. CORT levels of manipulated animals were higher than controls only in the stable group. UG animals presented heavier adrenals than their SG counterparts. Only adrenals from SG animals placed on the grid were similar to cage controls. SG rats lost less weight than UG animals. While UG animals ate the same amount of chow as home-cage controls, SG animals ate more than control and UG animals. These results suggest that the stress of the MMPM can be attenuated in stable groups. The introduction of a grid on the tank floor may serve an adequate environmental control as far as stress-related variables are considered.

Defeat is a major stressor in males while social instability is stressful mainly in females: towards the development of a social stress model in female rats

Social stress models appear useful in elucidating the interrelationship between stress, mood disorders, and drug efficacy. However, reliable social stress models for females are virtually lacking. The aim of this study was to determine stress-related consequences of (a) defeat in aggressive encounters and (b) social instability, in male and female rats. Defeat in male and female subjects was induced by aggressive male residents and female residents made aggressive by surgery (mediobasal hypothalamic lesion [MBHL]), respectively. Aggressiveness of resident males and resident MBHL females was remarkably similar. Alternating isolation and mixed-sex crowding phases with membership rotation were used to induce social instability. Aggression was kept low in the latter paradigm by manipulating crowding group composition. Defeat stress reduced weight gain, and increased both adrenals and plasma corticosterone in males. Only adrenal weight was affected in females. Social instability reduced weight gain, and induced thymus involution, adrenal hypertrophy and elevated plasma corticosterone levels in females. Only weight gain and thymus weights were affected in males. It is concluded that defeat stresses males more than females, while social instability is more stressful for females than for males, if aggressive contacts are low. It is suggested that the social instability model is a good model of social stress in females.

Anterior pituitary response to stress: time-related changes and adaptation

A wide array of physical and psychological stressors alter the secretion of anterior pituitary hormones. However, both the qualitative and the quantitative features of the stressors as well as its duration markedly influence the final endocrine response. In addition, among all anterior pituitary hormones, only ACTH and prolactin levels appear to reflect the intensity of the stress experienced by the animals. Although physical stressors show a somewhat specific neuroendocrine profile, the response of the pituitary-adrenal (PA) and sympathomedulloadrenal axes are common to almost all stressors. After an initial stimulatory effect of stress, an inhibition of all anterior pituitary hormones, except ACTH, can be found provided the stressor is intense enough. The mechanisms responsible for this biphasic response to stress are likely to be located at sites above the pituitary. When the animals are repeatedly exposed to the same stressor, some behavioural and physiological consequences of stress exposure are reduced, suggesting that the animals become adapted to the stimulus. This process has been also termed habituation. Among all the pituitary hormones, only ACTH and prolactin levels are reduced as a consequence of repeated exposure to the same (homotypic) stressor, although some negative results have been reported. However, it has been recently reported that subtle changes in the characteristics of the stressors or in their regularity can greatly influence adaptation, and these factors might explain failure to find adaptation of ACTH and prolactin in some works. Habituation of ACTH and prolactin, when observed, appears to be specific for the chronically applied stressor so that the potentiality of the PA axis and prolactin to respond to a novel (heterotypic) stressor can be preserved. In the case of the PA axis, an intact or potentiated response to a novel stressor is observed in spite of presumably negative feedback exerted by daily stress-induced glucocorticoid release and the high resting levels of glucocorticoids. This phenomenon has been termed as facilitation and can be unmasked alternating stress. Although with the exception of the PA axis, developmental aspects of anterior pituitary response to stress have been poorly studied, available data suggest that dramatic changes occur in some hormones during weaning, with some, but less profound, change thereafter. Responsiveness to stressors appears to mature with age, but developmental patterns differ among the various anterior pituitary hormones.

Adrenal axis activation by chronic social stress fails to inhibit gonadal function in male rats

Stress in males via the hypothalamic-pituitary-adrenal (HPA) axis may set into motion varied physiological alterations, including dysfunction of the hypothalamic-pituitary-gonadal (HPG) axis. However, the influence of the HPA on the HPG axis may not always be inhibitory. Presence or absence of stimuli of sexual significance that typically activates the HPG axis may alter the influence of the adrenal axis on gonadal axes. In this project, we used male rats and chronic social stimulation that included brief or extended periods with female rats to examine HPA-HPG axes interactions. In experiment 1, we used intact males and a 'chronic social stress' paradigm developed in our previous research that induces social instability by daily changing the membership of group-housed males with females. Thymus weight was reduced and corticosterone levels were marginally increased by chronic social stress, indicating a HPA axis hyperactivity. The HPG axis was also activated as shown by the increased weight of the androgen-sensitive sex structures. These results indicate that when these two axes are stimulated together, neither interferes with nor suppresses activities of the other. Implants of corticosterone pellets to adrenalectomized animals that maintained constant, high corticosterone levels failed to reverse the gonadal hyperactivity induced by sexual stimulation. In a second experiment, we studied the influence of different intensity of sexual stimulations on HPA-HPG axes interactions. Increased corticosterone levels and adrenal weight, indicating a HPA hyperactivity, failed to inhibit HPG hyperactivity as measured by the increased sexual organs weight, whatever the sexual intensity of the stimulation. This work demonstrates that the gonadal axis is freed from suppression when sexual stimulation occurs together with stress. The general conclusion is that the nature of complex social settings is important in determining interactions between the two neuroendocrine axes.

Modulation of the immune response by changing neuromediator systems activity under stress

The possible correction of the immune response affecting the mechanisms of neuroimmunomodulation in the animal model of immobilization stress is considered. Immobilization (3 h on the back) of CBA mice caused a suppression of the immune response. The number of plaque-forming cell (PFC) on the 4th day as well as rosette-forming cell (RFC) number on the 5th day of the immune reaction to sheep red blood cells (SRBC) were found to be reduced as compared to the control. Immunoinhibition in the stressed mice was reversed by the depletion of the cerebral serotonin (5-HT) with p-chlorophenylalanine (PCPA) 2 days before immobilization at a dose of 500 mg/kg. Activation of postsynaptic D-1 and D-2 dopamine (DA) receptors with apomorphine administered 30 min before stress attenuated stress-induced immunosuppression as well. Thus, the immunosuppression elicited by a stressor can be modified by drugs influencing the 5-HT and DAergic systems. It is suggested that the stress-provoked alterations of the immune response can be a consequence of changing neurochemical pattern of the brain and the disturbances of the mechanisms of psychoneuroimunomodulation.

In the mouse, the corticoid stress response depends on lateralization

The influence of brain/behavioral lateralization on the neuroendocrine stress response was studied in the mouse. Using a paw preference test in a food reaching task, mice were classified as left-pawed, ambidextrous or right-pawed. Plasma levels of corticosterone (CS) were measured in basal conditions, 4 h after an intraperitoneal injection of lipopolysaccharide (LPS) or after a short period of restraint. In unstressed control mice, plasma levels of corticosterone were higher in left-pawed animals as compared to ambidextrous. LPS increased plasma levels of CS to similarly high levels, around 600 ng/ml, in the three experimental groups. By contrast after 1 h of restraint, the increased CS levels, lower to those observed after LPS injection, were higher in left-pawed mice as compared to right-pawed animals. These results are the first demonstration that activation of the hypothalamic-pituitary-adrenal axis observed during the stress response to a physical stimulus may be related to lateralization.

Short and long restraint differentially affect humoral and cellular immune functions

The aim of this work was to examine the effect of different periods of restraint on the humoral and cellular immune functions in adult male rats. Short restraint stress (2 h over 2 consecutive days) enhanced the primary serum antibody response to sheep red blood cells. The enhancement of this humoral response was dependent on the restraint period, since long restraint stress (6 h over 4 days) failed to modify this response. Short and long restraint decreased both the number of lymphocytes and the T-lymphocyte response to Con A stimulation in the peripheral blood. Neither 2 h over 2 days nor 6 h over 4 days modified the splenic lymphoproliferative response to Con A stimulation, but restraint stress progressively decreased the number of mononuclear splenic cells. Both periods of restraint significantly increased plasma concentration of corticosterone, however plasma prolactin levels were significantly lower after 4 days of restraint but not after short restraint (2 h over 2 days). These results indicate that although some immune functions can be increased after acute or short stress, long stress has an immunosuppressive effect, above all on the cellular immunity which is more susceptible to this effect than the humoral response.

Self-reported stressors, symptom complaints and psychobiological functioning-II: Psychoneuroendocrine variables

The present study examined resting endocrinological functioning and endocrine responsivity to new challenges as a function of self-reported stress load and symptomatology. Following a baseline period, four groups of male subjects (low-load/low-symptoms; low-load/high-symptoms; high-load/low-symptoms; high-load/high-symptoms) were exposed to stressful films, followed by a rest period. Blood samples were drawn after each film and after the rest condition, and urinary samples were collected during two nights preceding the experimental session. Neuroendocrine variables measured in plasma included adrenaline, noradrenaline, ACTH, cortisol, growth hormone, prolactin, and testosterone. The urinary samples were assayed for noradrenaline and adrenaline (in relation to creatinin). High-symptom subjects had significantly higher plasma levels of noradrenaline and overnight urinary adrenaline levels, whereas their cortisol levels tended to be lower as compared to the low-symptom group. The plasma noradrenaline/cortisol ratio was higher among the high-symptom subjects. However, upon controlling for neuroticism and life style factors (smoking and alcohol consumption), all but the effects on cortisol failed to meet significance criteria. Higher stress load was associated with higher plasma adrenaline responses during the laboratory session, irrespective of neuroticism or life-style measures. These results therefore suggest that in addition to measuring exposure to real-life stressors, it is also necessary to measure outcomes, such as symptoms, and to be aware of the effects of neuroticism and life-style when attempting to understand which specific psychosocial factors effect psychoendocrinological functioning.

Significance of adrenal corticosteroid secretion for the food restriction-induced enhancement of alcohol drinking in the rat

Male Wistar rats with continuous access to 6% ethanol solution and water in their home cages were subjected to food restriction (FR). Reduction of body weight to 80% of normal was associated with a significant increase in ethanol drinking. It is known that the stress of FR gives rise to increased corticosterone secretion, and in line with these findings it was found that the weight of the thymus (whose size is inversely related to corticosterone levels) was reduced to 55% of normal in the present FR rats. Two subsequent experiments indicated that this adrenal activation contributed to the FR-induced enhancement of alcohol drinking. Firstly, adrenalectomized rats showed no evidence of enhanced alcohol drinking during food restriction, suggesting that adrenal corticosterone hypersecretion contributes to the enhanced ethanol consumption during FR. Secondly, treatment of FR rats with the enzyme inhibitor cyanoketone, which blocks stress-induced but not basal corticosterone secretion, at least partly prevented the FR-induced increase in ethanol drinking. These results add further evidence that sustained exposure to corticosterone facilitates ethanol consumption in the rat.

Impact of differential housing on humoral immunity following exposure to an acute stressor in rats

The purpose of this study was to examine the effect of differential housing on humoral immunity following exposure to an acute stressor. Forty male Sprague-Dawley adult rats were randomly assigned to either a singly housed or group-housed (five rats/cage) condition. Approximately 2 weeks after the start of the study, all animals were immunized with 1 ml of a 10% suspension of sheep red blood cells (SRBC) in saline. After the injections, half of the animals from each housing condition were subjected to an acute stressor (forced swim, 60 min/day for 3-5 days). Animals exposed to the acute stressor displayed adrenal gland hypertrophy and reduced thymus and spleen weights compared to the unstressed (control) animals. Both behavioral stimuli (housing and forced swim) demonstrated no effect on antibody production to SRBC. However, singly housed animals showed an increase in lymphocyte percentage, and corticosterone and glucose levels regardless of subsequent exposure to acute stress. Within a treatment condition, there were no significant correlations between the immune and endocrine measures. It was concluded that reduced social contact (i.e., individual housing) with subsequent exposure to an acute stressor does not appear to inhibit immunological responsiveness to an antigen.

Chronic social stress conditions differentially modify vulnerability to amphetamine self-administration

Using social instability and cohabitation with females as chronic stress, we observed that neuroendocrine systems were differentially activated according to the experimental design. We show here that amphetamine self-administration, a paradigm to study the reinforcing effects of psychostimulants, is also differentially affected by these conditions. Coexistence with females increases amphetamine self-administration and this effect is reduced when social instability is superimposed. On the other hand, locomotor response to amphetamine is not modified by either social factor, suggesting a specific involvement of a subset of dopaminergic neurons.

Modulation of the locomotor response to amphetamine by corticosterone

In the present experiments, we investigated the influence of chronic modifications of circulating levels of corticosterone on the locomotor response to amphetamine. Different groups of rats were adrenalectomized and implanted subcutaneously with pellets releasing different amounts of corticosterone (0-200 mg). A wide range of corticosterone concentrations was reached in order to saturate selectively either the type I (mineralocorticoid) or the type II (glucocorticoid) corticosteroid receptors. The locomotor response to d-amphetamine (1.5 mg/kg) was studied 10-14 days later. We found that adrenalectomy reduced the response to d-amphetamine by 33% and that a normal response was restored with pellets releasing physiological concentrations of corticosterone (50-mg pellets), and was potentiated in animals with pellets releasing high amounts of corticosterone mimicking chronic stress situations (200-mg pellets). The correlation between plasma corticosterone concentration, locomotor activity following d-amphetamine and thymus weight, which is a reliable indicator of glucocorticoid action, shows that the influence of the locomotor response to d-amphetamine administration is likely to be mediated via a type II receptor. Since the locomotor activating effect of peripheral administration of d-amphetamine has been shown to depend on the integrity of the dopaminergic innervation of the nucleus accumbens, the effect of d-amphetamine at different doses (0, 1, 3, 10 micrograms/microliter) injected directly into the nucleus accumbens was studied. The results demonstrated that removing the circulating corticosterone induced a similar decrease of the locomotor activity elicited by d-amphetamine injection in the nucleus accumbens. This response was restored in animals with the 50- and 200-mg pellets.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of chemical-induced stress responses in immunosuppression: a review of quantitative associations and cause-effect relationships between chemical-induced stress responses and immunosuppression

Although there is an increasing awareness that drugs and chemicals can modulate the immune system by indirect mechanisms, few compounds have been thoroughly evaluated in this regard. Several environmentally relevant chemicals induce stresslike responses, as indicated by elevated glucocorticoid levels. Comparable glucocorticoid levels induced by physical or psychological stressors are consistently associated with suppression of one or more immunological parameters. Thus, it seems likely that stress-related neuroendocrine mechanisms are important in immunosuppression by some environmental chemicals. Distinguishing direct and indirect (stress-related) mechanisms of immunosuppression is generally possible, and this could be done as a routine part of immunotoxicity assessment. Although it is clear that glucocorticoids can contribute to such immunosuppression, it is also clear that several other neuroendocrine mediators associated with stress responses can be immunomodulatory. Thus, correlation between glucocorticoid levels and immunosuppression does not conclusively demonstrate a cause-effect relationship. Demonstrating such relationships has been difficult, but it has been done in a few cases of drug-induced thymic hypoplasia by monitoring several parameters known to be affected by glucocorticoids and by measuring the ability of a glucocorticoid antagonist (RU 486) or adrenalectomy to block changes in these parameters. A similar strategy might be useful for evaluation of the role of glucocorticoids in drug- or chemical-induced suppression of a variety of immune functions, but the effects of RU 486 on neuroendocrine feedback circuits and the possibility of consequent immunological changes must be considered when the data are interpreted. This approach could also be applied to evaluation of the roles in chemical-induced immunosuppression of other neuroendocrine mediators for which antagonists or agents that block the synthesis or release of the mediator are available. However, it is likely that a comprehensive (and perhaps predictive) understanding of the relationship between chemically induced stress responses and immunosuppression will require more detailed and quantitative elucidation of the mechanisms and regulation of neuroendocrine-immune interactions.