Examination of the neuroendocrine basis for the social conflict-induced enhancement of immunity in mice

Microbial Endocrinology: An Ongoing Personal Journey

The development of microbial endocrinology is covered from a decidedly personal perspective. Specific focus is given to the role of microbial endocrinology in the evolutionary symbiosis between man and microbe as it relates to both health and disease. Since the first edition of this book series 5 years ago, the role of microbial endocrinology in the microbiota-gut-brain axis is additionally discussed. Future avenues of research are suggested.

Neuroimmunological response of beluga whales (Delphinapterus leucas) to translocation and a novel social environment

This study assessed changes in phagocyte function and activation of the sympatho-adrenal medullary and hypothalamo-pituitary adrenal axes of beluga whales (Delphinapterus leucas) in response to translocation and introduction to a novel social environment. Transported belugas exhibited increases in epinephrine (E), norepinephrine (NE), and cortisol levels in response to the translocation process. In response to the introduction of the transported belugas, resident belugas exhibited an increase in E and NE but not cortisol. Moreover, the increase in E and NE shown by the transported belugas was significantly greater than the increase exhibited by the resident belugas. Resident belugas exhibited a concomitant decrease in neutrophil and monocyte phagocytosis associated with the introduction of the transported belugas. In contrast, transported belugas exhibited an attendant increase in phagocytosis and respiratory burst activity immediately following transport. Differences in phagocyte response may derive from differences in hormonal milieu, stressor modality and/or intensity, or phagocyte priming. Investigating the complex interactions between types of stressors, neuroendocrine response, and immunocompetence will lead to a better understanding of the impacts of environmental challenges, including anthropogenic perturbations, on the health of cetacean populations.

Behavior: a relevant tool for brain-immune system interaction studies

Neuroimmunomodulation describes the field focused on understanding the mechanisms by which the central nervous system interacts with the immune system, potentially leading to changes in animal behavior. Nonetheless, not many articles dealing with neuroimmunomodulation employ behavior as an analytical endpoint. Even fewer papers deal with social status as a possible modifier of neuroimmune phenomena. In the described sets of experiments, we tackle both, using a paradigm of social dominance and subordination. We first review data on the effects of different ranks within a stable hierarchical relationship. Submissive mice in this condition display more anxiety-like behaviors, have decreased innate immunity, and show a decreased resistance to implantation and development of melanoma metastases in their lungs. This suggests that even in a stable, social, hierarchical rank, submissive animals may be subjected to higher levels of stress, with putative biological relevance to host susceptibility to disease. Second, we review data on how dominant and submissive mice respond differentially to lipopolysaccharide (LPS), employing a motivational perspective to sickness behavior. Dominant animals display decreased number and frequency in several aspects of behavior, particularly agonistic social interaction, that is, directed toward the submissive cage mate. This was not observed in submissive mice that maintained the required behavior expected by its dominant mate. Expression of sickness behavior relies on motivational reorganization of priorities, which are different along different social ranks, leading to diverse outcomes. We suggest that in vitro assessment of neuroimmune phenomena can only be understood based on the behavioral context in which they occur.

Neuroendocrine, behavioral and macrophage activity changes induced by picrotoxin effects in mice

The relevance and property of studies related to stress effects on immune function are undisputable. All studies conducted on stress-immune relationships, however, provide from physical and/or psychological stressors. Indeed, as far as it is of our knowledge brain-innate immune responses were not analyzed after anxiogenic-like drugs use. The present experiment was then undertaken to analyze the effects of picrotoxin (0.3, 0.6 and 1.0mg/kg doses) on behavior, macrophage activity, serum corticosterone and noradrenaline (NE) levels and turnover in the brain of adult mice. Results showed that picrotoxin treatment in mice: (1) decreased motor and rearing activities in an open-field; (2) decreased the number of entries into the plus-maze open-arms and decreased the time spent in the exploration of the plus-maze open-arms; (3) decreased both motor activity and the level of holes exploration in the hole-board; (4) increased the levels of serum corticosterone in dose-dependent way; (5) increased noradrenaline (NE) and MHPG levels and NE turnover in the hypothalamus; and (6) increased Staphylococcus aureus and PMA-induced macrophage oxidative burst. However, and contrary to that reported after physical or psychological stress, this drug induced no effects on macrophage phagocytosis and NE levels and turnover in the frontal cortex. The present results are thus showing that picrotoxin induces some but not all neuro-innate immunity changes previously reported for inescapable foot-shock and psychological stressors in mice. These facts suggest that this chemical stressor triggers CNS pathways that might be somehow different from those fired by inescapable foot-shock and psychological stressors, leading to different neuro-innate immune responses.

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.

Variations in behavior, innate immunity and host resistance to B16F10 melanoma growth in mice that present social stable hierarchical ranks

The present study analyzed the effect of social stable hierarchical dominance/submissive relationships in C57BL/6 mice on behavior, innate immunity, serum corticosterone levels and host resistance to B16F10 melanoma growth. Adult mice (90 days old) kept in pairs since weaning, were analyzed for dominant/submissive ranking in three consecutive days according to the presence or absence of fighting and/or anticipatory submissive responses. Only the pairs of mice where dominant/submissive relationships were clearly stated were employed. Results showed that submissive mice presented in relation to dominants: (1) decreased time spent in the central open-field area; (2) decreased number of entries into the open arms and decreased time spent in the exploration of the open arms of the plus maze; (3) increased time spent in exploration of the plus-maze closed arms; (4) decreased number of entries and in the time spent in the exploration of the third part of the plus-maze open arms; (5) increased number of B16F10 metastasis in the lungs; (6) decreased NK cell cytotoxicity measured in vitro in the peripheral blood and spleen; (7) decreased basal but not in S. aureus induced oxidative burst in both neutrophils and monocytes and (8) similar basal serum levels of corticosterone. The present behavioral findings show that submissive mice, within a stable social hierarchy, present anxiety like-responses a fact that would make than more prone to stressful stimuli. This condition would be responsible for the decreases presently observed on basal neutrophil oxidative burst, NK cell activity and resistance to B16F10 tumor growth. Together the obtained data show that mice that present stable hierarchical relationships display neuro-immune alterations comparable to those reported in mice under a situation of chronic social stress.

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.

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.

Stress, leukocyte trafficking, and the augmentation of skin immune function

Delayed type hypersensitivity (DTH) reactions represent cell-mediated immune responses that exert important immunoprotective (resistance to viruses, bacteria, and fungi) or immunopathologic (allergic or autoimmune hypersensitivity) effects. We have used the skin DTH response as an in vivo model to study neuro-endocrine-immune interactions. We hypothesized that just as an acute stress response prepares the cardiovascular and musculoskeletal systems for fight or flight, it may also prepare the immune system for challenges (e.g., wounding) that may be imposed by a stressor (e.g., an aggressor). Studies showed that acute (2 hours) stress experienced before primary or secondary cutaneous antigen exposure induces significantly enhanced skin DTH. This enhancement involves innate as well as adaptive immune mechanisms. Adrenalectomy eliminates the stress-induced enhancement of DTH. Acute administration of physiological concentrations of corticosterone and/or epinephrine to adrenalectomized animals enhances skin DTH. Compared with those in controls, DTH sites from acutely stressed or hormone-injected animals show significantly greater erythema and induration, numbers of infiltrating leukocytes, and levels of cytokine gene expression. In contrast to acute stress, chronic stress is immunosuppressive. Chronic exposure to corticosterone or acute exposure to dexamethasone significantly suppresses skin DTH. These results suggest that during acute stress, endogenous stress hormones enhance skin immunity by increasing leukocyte trafficking and cytokine gene expression at the site of antigen entry. Elucidation of mechanisms mediating a stress-induced enhancement of skin immune function is important because such immunoenhancement can have protective (wound healing, resistance to infection) or pathological (allergic or autoimmune hypersensitivity) consequences.

Endocrine and lymphoproliferative response changes produced by social stress in mice

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

Effects of physical and psychological stressors on behavior, macrophage activity, and Ehrlich tumor growth

The present study analyzed the effects of physical and psychological stressors on behavior, immune function, and serum corticosterone in mice. Adult mice were submitted once daily, for 6 days to one of the following conditions: escapable (ES) or inescapable (IS) footshocks (0.2 mA) signaled by a tone cue or to a psychological stressor (PS) generated through the use of a communication box; in this box, mice received no footshock but were exposed to responses delivered by IS mice. Results showed that IS and PS: (1). decreased locomotor activity observed in an open-field; (2). decreased number of entries into the open arms and decreased time spent in the exploration of the open arms of the plus-maze; (3). decreased macrophage spreading and phagocytosis; (4). increased macrophage H(2)O(2) release; and (5). increased growth of the ascitic form of Ehrlich tumor. Behavioral and/or immunological changes were not observed after ES; this absence of effects, however, might not be attributed solely to footshock controllability since mice of groups ES and IS differed with respect to the psychological setting used and the amount of shock they received. An increase of serum corticosterone concentrations was also observed in the stressed mice of all groups; this increment was higher in animals of group IS. These data provide evidence that inescapable footshock and psychological stressors alter, at the same time and in mice, stress levels, macrophage activity, and Ehrlich tumor growth. They also show that ES and PS induced similarly elevated serum corticosterone concentrations, but significantly differ in the immunological and behavioral outcomes they produced in mice. These findings suggest that another factor besides HPA axis activation might be responsible for behavioral and immunological consequences of IS and PS in mice. It is proposed that the final neural link between behavioral and immunological changes observed after physical and psychological stressors might involve catecholaminergic systems within the central nervous system and/or sympathetic autonomic nerve fibers and also opioid peptides.

A genetic model of stress displays decreased lymphocytes and impaired antibody responses without altered susceptibility to Streptococcus pneumoniae

Stress pathways affect immune function, the most notable of these pathways being activation of the hypothalamic-pituitary-adrenal (HPA) axis. Although HPA activation has generally been relegated to an immunosuppressive role, recent evidence suggests that stress and HPA activation can be immunoenhancing in certain situations. To investigate specific effects of stress on immune function, we used a genetic model of chronic stress wherein transgenic mice overexpress corticotropin-releasing hormone (CRH), a primary mediator of the stress response. In these mice, CRH is overproduced in the brain, leading to chronic activation of the HPA axis. We found that CRH-transgenic mice have decreased leukocyte numbers in lymphoid compartments, with preferential loss of B lymphocytes. They also exhibit decreased Ab production and impaired isotype switching in response to immunization with a thymus-dependent Ag, phosphocholine-keyhole limpet hemocyanin. Despite these deficits, immunization protected CRH-transgenic and wild-type mice equally well against lethal challenge with Streptococcus pneumoniae, an encapsulated Gram-positive bacterium known to require Ab-mediated opsonization for clearance. While IgG responses are severely depressed in these mice, IgM titers are only modestly decreased. This fairly robust IgM response may be sufficient to protect against S. pneumoniae. Additionally, while total leukocyte numbers are decreased in these mice, neutrophil numbers are increased. This increase in number of neutrophils may compensate for the depressed IgG response, allowing adequate host defense during chronic stress.

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 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.

Human aggression and enumerative measures of immunity

OBJECTIVE

A pattern of clinical, behavioral, and experimental findings suggests that individual differences in aggressive behavior may be related to immunologic processes. We evaluated two conflicting models of the relationship: 1) A positive association stems from an adaptive mechanism protecting aggressive individuals from increased exposure to immune stimuli and 2) a negative association is due to potential immunosuppressive effects of high testosterone levels.

METHODS

We investigated the models using enumerative measures of cellular and humoral immunity in a sample of 4415 men aged 30 to 48 years who were interviewed and underwent a medical examination.

RESULTS

Analysis revealed positive (and curvilinear) associations between aggressive behavior and enumerative measures of helper/inducer and suppressor/cytolytic T lymphocytes and B lymphocytes. The aggression-immunity relationship was independent of testosterone level, age, current health status, and negative health behaviors and was most pronounced for helper/inducer T cells. There was no evidence of a negative association between testosterone and any immune measure.

CONCLUSIONS

In a large sample of men, individual differences in aggressive behavior were positively associated with enumerative measures of cellular immunity.

Stress-induced enhancement of skin immune function: A role for gamma interferon

Contrary to the widespread belief that stress is necessarily immunosuppressive, recent studies have shown that, under certain conditions, stress can induce a significant enhancement of a skin cell-mediated immune response [delayed-type hypersensitivity (DTH) or contact hypersensitivity]. Adrenal stress hormones and a stress-induced trafficking of leukocytes from the blood to the skin have been identified as systemic mediators of this immunoenhancement. Because gamma interferon (IFNgamma) is an important cytokine mediator of DTH, the studies described here were designed to examine its role as a local mediator of the stress-induced enhancement of skin DTH. The effect of acute stress on skin DTH was examined in wild-type and IFNgamma receptor-deficient (IFNgammaR-/-) mice that had previously been sensitized with 2,4-dinitro-1-fluorobenzene. Acutely stressed wild-type mice showed a significantly larger DTH response than nonstressed mice. In contrast, IFNgammaR-/- mice failed to show a stress-induced enhancement of skin DTH. Immunoneutralization of IFNgamma in wild-type mice significantly reduced the stress-induced enhancement of skin DTH. In addition, an inflammatory response induced by direct IFNgamma administration to the skin was significantly enhanced by acute stress. Our results suggest that IFNgamma is an important local mediator of a stress-induced enhancement of skin DTH. These studies are clinically relevant because, depending on the nature of the antigen, DTH reactions mediate numerous protective (e.g., resistance to viral, bacterial, parasitic, and fungal infections) or pathological (e.g., autoimmune reactions and contact sensitivity reactions such as that to poison ivy) immune responses.

Effects of social conflict on immune responses and E. coli growth within closed chambers in mice

Social conflict has been shown to affect the neuroendocrine stress response in rodents. The current study was designed to characterize the effects of social conflict on leukocyte subset distribution and function as well as in vivo bacterial growth. Male DBA/2 mice implanted or not implanted with a closed chamber containing Escherichia coli were repeatedly challenged by temporary placement in the territory of a dominant CF-1 mouse five times a day for 2 consecutive days. Nonstressed animals were similarly handled, but were not exposed to social conflict. Effects on immune responses and E. coli growth were analyzed 13 h after the last social conflict session. Social conflict alone was associated with an increase in plasma corticosterone concentration and decreases in thymocyte numbers and splenocyte ability to proliferate in vitro in the presence of lipopolysaccharide (p < 0.05). After social conflict, immature CD4+CD8+ thymocytes decreased, whereas mature T cells increased (p < 0.05). In the presence of E. coli, social conflict induced a significant increase in plasma concentration of interleukin-1beta, and a decrease in the number of thymocytes and the percentage of CD4+CD8+ T cells in the thymus (p < 0.05). In addition to the lymphocyte subpopulation changes observed with social conflict alone, the proportion of CD3+ and major histocompatibility complex (MHC) class II IAd+ cells were significantly higher in stressed mice implanted with a closed chamber containing E. coli (p < 0.05). Social conflict tended to favor E. coli growth in the closed chamber, indicating possible direct bacterial-neuroendocrine hormone interactions. Taken together, these results suggest that stress may modulate the host immune response by altering both bacterial growth and resistance to infection.

Anxiogenic effect of subclinical bacterial infection in mice in the absence of overt immune activation

Challenge of animals with infectious microorganisms is well documented to affect a number of behavioral measures through activation of immune-neural mechanisms. In the present study, the ability of an infectious microorganism to directly alter behavioral responses in the absence of an overt immunologic response was examined. Eight-week-old CF-1 male mice were infected orally with the Gram-negative pathogen Campylobacter jejuni in order to establish a subclinical infection that did not result in immune activation. Microbiological examination of cecal contents revealed the presence of C. jejuni in all infected, but not control, animals 1 and 2 days post-oral challenge. Measurement of interleukin-6 (IL-6) levels and peripheral blood leukocyte populations did not reveal the activation of an overt immune response in 1 or 2 day infected animals as compared to controls. Infected mice demonstrated altered levels of anxiety-like behaviors on the elevated plus-maze as compared to controls on Day 2, but not Day 1, as reflected by a significant decrease in exploratory and an increase in nonexploratory behaviors. The anxiogenic effect of a subclinical infection in the absence of an overt immunologic response suggests that the direct activation of neural pathways by microorganisms may play a role in behavior.

Alteration in the production of free oxygen radicals and proinflammatory cytokines by peritoneal and alveolar macrophages in old mice and immunomodulatory effect of RU 41740 administration. Part I: Effect of short and repetitive noise stress

The aim of this study was to analyze the effects of stress on natural immunity in old mice, and the potential of an immunomodulating drug to correct stress-induced immune abnormalities. We analyzed both the alveolar (ALM) and peritoneal macrophage (PerM) oxidative responses and cytokine productions of TNF-alpha and IL-1alpha, in old mice after exposure to 3 days of noise stress, with and without treatment with RU 41740, an immunomodulating compound. Production of Free Oxygen Radicals (FOR) by ALM and PerM macrophages was evaluated using a luminol-dependent chemiluminescence method at the basal state and after stimulation. Serum corticosterone was also measured. Three groups of 22-24 month-old C57BL/6 mice were studied. Seven mice were treated with RU 41740 (10 mg kg(-1)) every day for 5 days and then exposed to sound stress (110 dB, 1000 Hz, for 3 nights). Ten mice were treated with saline, then submitted to a sham stress. Eight animals received neither treatment nor stress. There was no difference in corticosterone level between the three groups and between these groups and a control group nonstressed, noncannulated. After stimulation by fMetLeuPhe, production of FOR by ALM and PerM significantly increased after stress and returned to normal values after RU 41740 treatment (p < 0.001). The cytokine levels (TNF-alpha and IL-1alpha) decreased after stress and returned to normal levels after RU 41740 treatment. This study shows that, in aged mice, auditory stress is associated with modifications of macrophage functions which are different, depending on their localization and on the function under study. It confirms the immunomodulatory capacities of RU 41740 that was shown to counteract these effects of stress in elderly animals.

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.

Corticotropin-releasing hormone is involved in conditioned stimulus-induced reduction of natural killer cell activity but not in conditioned alterations in cytokine production or proliferation responses

Research from our laboratory has demonstrated that the presentation of an aversive conditioned stimulus produces pronounced suppression of several in vitro measures of immune status. The present study was designed to evaluate the role of central corticotropin-releasing hormone (CRH) in the mechanisms mediating these conditioned effects. The aversive conditioned stimulus was a distinct environment that had previously been associated with electric footshock. Lewis rats received intraventricular administration of either buffered saline or a dose of the CRH-selective receptor antagonist alpha-helical CRH(9-41) (0, 0.5, 5, or 50 micrograms) prior to exposure to the aversive conditioned stimulus or home cage control treatment. The aversive conditioned stimulus produced decreases in splenic natural killer cell activity, splenocyte responsiveness to the mitogens concanavalin A (ConA), phytohemagglutinin (PHA), lipopolysaccharide (LPS), and the combination of ionomycin and phorbol myristate acetate (PMA), blood leukocyte responsiveness to ConA and PHA, and the production of interleukin-2 and interferon-gamma by activated splenocytes. The conditioned stimulus also produced an increase in plasma levels of corticosterone. Pretreatment with alpha-helical CRH(9-14) completely blocked the conditioned stimulus-induced suppression of natural killer cell activity. The CRH antagonist had no attenuative effect on the conditioned suppression of splenocyte or blood leukocyte proliferation in response to mitogens, or the production of interleukin-2 or interferon-gamma by activated splenocytes. There was also no effect of alpha-helical CRH(9-14) on the conditioned stimulus-induced increase in plasma corticosterone. These findings suggest that conditioned stimulus-induced suppression of natural killer cell activity is mediated by a mechanism that involves activity at central CRH receptors, and that this conditioned modulation is independent of HPA activation. Furthermore, these results indicate that the mechanisms involved in conditioned stimulus-induced suppression of proliferative or cytokine production responses are distinct from those involved in the modulation of natural killer cell activity.