A role for CRH and the sympathetic nervous system in stress-induced immunosuppression

Neuropeptides in Cancer: Friend and Foe?

Neuropeptides are small regulatory molecules found throughout the body, most notably in the nervous, cardiovascular, and gastrointestinal systems. They serve as neurotransmitters or hormones in the regulation of diverse physiological processes. Cancer cells escape normal growth control mechanisms by altering their expression of growth factors, receptors, or intracellular signals, and neuropeptides have recently been recognized as mitogens in cancer growth and development. Many neuropeptides and their receptors exist in multiple subtypes, coupling with different downstream signaling pathways and playing distinct roles in cancer progression. The consideration of neuropeptide/receptor systems as anticancer targets is already leading to new biological and diagnostic knowledge that has the potential to enhance the understanding and treatment of cancer. In this review, recent discoveries regarding neuropeptides in a wide range of cancers, emphasizing their mechanisms of action, signaling cascades, regulation, and therapeutic potential, are discussed. Current technologies used to manipulate and analyze neuropeptides/receptors are described. Applications of neuropeptide analogs and their receptor inhibitors in translational studies and radio-oncology are rapidly increasing, and the possibility for their integration into therapeutic trials and clinical treatment appears promising.

Hypothalamic circuitry underlying stress-induced insomnia and peripheral immunosuppression

The neural substrates of insomnia/hyperarousal induced by stress remain unknown. Here, we show that restraint stress leads to hyperarousal associated with strong activation of corticotropin-releasing hormone neurons in the paraventricular nucleus of hypothalamus (CRHPVN) and hypocretin neurons in the lateral hypothalamus (HcrtLH). CRHPVN neurons directly innervate HcrtLH neurons, and optogenetic stimulation of LH-projecting CRHPVN neurons elicits hyperarousal. CRISPR-Cas9-mediated knockdown of the crh gene in CRHPVN neurons abolishes hyperarousal induced by stimulating LH-projecting CRHPVN neurons. Genetic ablation of Hcrt neurons or crh gene knockdown significantly counteracts restraint stress-induced hyperarousal. Single-cell mass cytometry by time of flight (CyTOF) revealed extensive changes to immune cell distribution and functional responses in peripheral blood during hyperarousal upon optogenetic stimulation of CRHPVN neurons simulating stress-induced insomnia. Our findings suggest both central and peripheral systems are synergistically engaged in the response to stress via CRHPVN circuitry.

Effects of long-term administration of Senna occidentalis seeds on the hematopoietic tissue of rats

Senna occidentalis (S. occidentalis) is a toxic leguminous plant that contaminates crops and has been shown to be toxic to several animal species. All parts of the plant are toxic, but most of the plant's toxicity is due to its seeds. Despite its toxicity, S. occidentalis is widely used for therapeutic purposes in humans. The aim of the present work was to investigate, for the first time, the effects of the chronic administration of S. occidentalis seeds on hematopoietic organs, including the bone marrow and spleen. Fifty male Wistar rats were divided into five groups of 10 animals. Rats were treated with diets containing 0% (control), 0.5% (So0.5), 1% (So1), or 2% (So2) S. occidentalis seeds for a period of 90 days. Food and water were provided ad libitum, except to pair-fed (PF) group which received the same amount of ration to those of So2 group, however free of S. occidentalis seeds. It was verified that rats treated with 2% S. occidentalis seeds presented changes in hematological parameters. The blood evaluation also showed a significant decrease of the Myeloid/Erythroid (M/E) ratio. Chronic treatment with S. occidentalis promoted a reduction in the cellularity of both the bone marrow and spleen. Additionally, we observed changes in bone marrow smears, iron stores and spleen hemosiderin accumulation. Histological analyses of bone marrow revealed erythroid hyperplasia which was consistent with the increased reticulocyte count. These findings suggest that the long-term administration of S. occidentalis seeds can promote blood toxicity.

Stress and visceral pain: from animal models to clinical therapies

Epidemiological studies have implicated stress (psychosocial and physical) as a trigger of first onset or exacerbation of irritable bowel syndrome (IBS) symptoms of which visceral pain is an integrant landmark. A number of experimental acute or chronic exteroceptive or interoceptive stressors induce visceral hyperalgesia in rodents although recent evidence also points to stress-related visceral analgesia as established in the somatic pain field. Underlying mechanisms of stress-related visceral hypersensitivity may involve a combination of sensitization of primary afferents, central sensitization in response to input from the viscera and dysregulation of descending pathways that modulate spinal nociceptive transmission or analgesic response. Biochemical coding of stress involves the recruitment of corticotropin releasing factor (CRF) signaling pathways. Experimental studies established that activation of brain and peripheral CRF receptor subtype 1 plays a primary role in the development of stress-related delayed visceral hyperalgesia while subtype 2 activation induces analgesic response. In line with stress pathways playing a role in IBS, non-pharmacologic and pharmacologic treatment modalities aimed at reducing stress perception using a broad range of evidence-based mind-body interventions and centrally-targeted medications to reduce anxiety impact on brain patterns activated by visceral stimuli and dampen visceral pain.

Stress-related alterations of visceral sensation: animal models for irritable bowel syndrome study

Stressors of different psychological, physical or immune origin play a critical role in the pathophysiology of irritable bowel syndrome participating in symptoms onset, clinical presentation as well as treatment outcome. Experimental stress models applying a variety of acute and chronic exteroceptive or interoceptive stressors have been developed to target different periods throughout the lifespan of animals to assess the vulnerability, the trigger and perpetuating factors determining stress influence on visceral sensitivity and interactions within the brain-gut axis. Recent evidence points towards adequate construct and face validity of experimental models developed with respect to animals' age, sex, strain differences and specific methodological aspects such as non-invasive monitoring of visceromotor response to colorectal distension as being essential in successful identification and evaluation of novel therapeutic targets aimed at reducing stress-related alterations in visceral sensitivity. Underlying mechanisms of stress-induced modulation of visceral pain involve a combination of peripheral, spinal and supraspinal sensitization based on the nature of the stressors and dysregulation of descending pathways that modulate nociceptive transmission or stress-related analgesic response.

Stress and visceral pain: from animal models to clinical therapies

Epidemiological studies have implicated stress (psychosocial and physical) as a trigger of first onset or exacerbation of irritable bowel syndrome (IBS) symptoms of which visceral pain is an integrant landmark. A number of experimental acute or chronic exteroceptive or interoceptive stressors induce visceral hyperalgesia in rodents although recent evidence also points to stress-related visceral analgesia as established in the somatic pain field. Underlying mechanisms of stress-related visceral hypersensitivity may involve a combination of sensitization of primary afferents, central sensitization in response to input from the viscera and dysregulation of descending pathways that modulate spinal nociceptive transmission or analgesic response. Biochemical coding of stress involves the recruitment of corticotropin releasing factor (CRF) signaling pathways. Experimental studies established that activation of brain and peripheral CRF receptor subtype 1 plays a primary role in the development of stress-related delayed visceral hyperalgesia while subtype 2 activation induces analgesic response. In line with stress pathways playing a role in IBS, non-pharmacologic and pharmacologic treatment modalities aimed at reducing stress perception using a broad range of evidence-based mind-body interventions and centrally-targeted medications to reduce anxiety impact on brain patterns activated by visceral stimuli and dampen visceral pain.

Depression and immunity: inflammation and depressive symptoms in multiple sclerosis

An increasing body of evidence suggests that patients who have major depressive disorder show alterations in immunologic markers including increases in proinflammatory cytokine activity and inflammation. Inflammation of the central nervous system is a pathologic hallmark of multiple sclerosis (MS). Patients affected by this disease also show a high incidence of depression. Accumulating evidence from animal studies suggests that some aspects of depression and fatigue in MS may be linked to inflammatory markers. This article reviews the current knowledge in the field and illustrates how the sickness behavior model may be applied to investigate depressive symptoms in inflammatory neurologic diseases.

Depression and immunity: inflammation and depressive symptoms in multiple sclerosis

There is strong evidence that depression involves alterations in multiple aspects of immunity that may contribute to the development or exacerbation of a number of medical disorders and also may play a role in the pathophysiology of depressive symptoms. Accordingly, aggressive management of depressive disorders in medically ill populations or individuals at risk for disease may improve disease outcome or prevent disease development. On the other hand, in light of data suggesting that immune processes may interact with the pathophysiologic pathways known to contribute to depression, novel approaches to the treatment of depression may target relevant aspects of the immune response. Taken together, the data provide compelling evidence that a psychoimmunologic frame of reference may have profound implications regarding the consequences and treatment of depression. In addition, this approach may be used to investigate the possibility that peripheral and central production of cytokines may account for neuropsychiatric symptoms in inflammatory diseases. This article summarizes evidence for a cytokine-mediated pathogenesis of depression and fatigue in MS. The effects of central inflammatory processes may account for some of the behavioral symptoms seen in patients who have MS that cannot be explained by psychosocial factors or CNS damage. This immune-mediated hypothesis is supported by indirect evidence from experimental and clinical studies of the effect of cytokines on behavior, which have found that both peripheral and central cytokines may cause depressive symptoms. Emerging clinical data from patients who have MS support an association of central inflammation (as measured by MRI) and inflammatory markers with depressive symptoms and fatigue. Based on the literature reviewed in this article, subtypes of MS fatigue and depression may exist that are caused by different pathogenetic mechanisms, including inflammation and CNS damage as well as psychosocial factors or predisposition. The existence of these subtypes could have important clinical implications. For example, an inflammatory depression may require different therapeutic approaches than a reactive depression in MS. Future research should aim to characterize these subtypes better with the goal of optimizing treatment.

Role of beta-endorphin, corticotropin-releasing hormone, and autonomic nervous system in mediation of the effect of chronic ethanol on natural killer cell cytolytic activity

BACKGROUND

We have recently shown that alcohol feeding suppresses natural killer (NK) cell cytolytic activity partly by decreasing the function of hypothalamic beta-endorphin (beta-EP) neurons. The neuronal mechanism by which hypothalamic beta-EP communicates with the spleen to regulate the action of ethanol on NK cells is not known. In the present study, we evaluated the roles of beta-EP neurons, corticotropin releasing hormone (CRH) neurons, and the autonomic nervous system (ANS) in regulation of the ethanol effect on splenic NK cell cytolytic function.

METHODS

Male rats were fed an ethanol-containing liquid diet or control diets. These rats were used to determine the hormone release from the paraventricular nuclei (PVN) of the hypothalamus or used to determine the splenic NK cell cytolytic function after PVN administration of CRH or intraperitoneal (i.p.) administration of a ganglionic blocker chlorisondamine. The release of hormones from the PVN was measured using the push-pull perfusion method. Splenic cytolytic activity was determined using the 4-hour (51)Cr release assay against YAC-1 lymphoma target cells.

RESULTS

Alcohol feeding decreased the amount of beta-EP but increased the amount of CRH in the push-pull perfusate (PPP) samples collected from the PVN. When exogenous beta-EP was perfused into the PVN, it suppressed the release of endogenous CRH found in PPP samples of the PVN. Conversely, perfusion of an opiate antagonist naltrexone into the PVN increased the levels of endogenous CRH in PPP samples of the PVN. In addition, administration of exogenous beta-EP in the PVN stimulated the cytolytic function of NK cells, an action that was antagonized by CRH as well as by ethanol. Corticotropin-releasing hormone and ethanol alone also had an inhibitory action on NK cells. Finally, the ganglionic blocker used prevented the effect that ethanol, beta-EP, and CRH had on NK cells. These data suggest that ethanol inhibits the function of NK cells partly by suppressing the influence of the beta-EP-CRH-ANS signal to the spleen.

Beta-endorphin modulation of interferon-gamma, perforin and granzyme B levels in splenic NK cells: effects of ethanol

The effects of ethanol and beta-endorphin (beta-EP) on productions of cytolytic factors granzyme B, perforin and IFN-gamma in splenic rat NK cells were determined. Intracranial administration of beta-EP increased protein and mRNA levels of cytolytic factors in NK cells. Chronic ethanol feeding reduced the basal and beta-EP-induced levels of cytolytic factors in NK cells. In vitro treatment of beta-EP on NK cells increased the levels of perforin, granzyme B and IFN-gamma and their mRNA transcripts, whereas ethanol pre-treatment prevented beta-EP effects on cytolytic factors in these cells. These results suggest that beta-EP and ethanol interact to regulate NK cell functions.

Stress and the aging immune system

Immune functioning decreases with normal aging and with stress. Social and psychological stressors are a part of daily life and the source of life changing events. Across the lifespan, individuals encounter numerous stressors with effects that accrue at sundry rates due to differential stress exposure, differential stress buffering, differential stress reactivity, differential stress duration (recovery), and differential restorative processes. Research on stress in older adults provides evidence that these processes contribute to effects that mimic, exacerbate, and possibly accelerate the effects of aging on immunity.

Environmental stress mediates changes in neuroimmunological interactions

Combinations of environmental stress coordinately increase toxicological assaults on health, dependent on the genetics of the exposed organism. Multiple gene variances between individuals influence the risks associated with environmental exposures, and environmental stress presents in multiple forms including chemical, physical, and psychological stresses. Combined chemical, physical, and psychological stresses are suggested as exacerbating the initiation and/or duration of illnesses, and many of the detrimental outcomes on health are posited to relate to changes in neuroendocrine immune circuitry. However, most human epidemiological or experimental animal studies have not considered the combination of chemical, physical, and psychological stress on health status. Current consideration is being given to "real world" exposures for assessment of health risk, but this mainly relates to evaluation of chemical mixtures. In addition to concomitant chemical exposures having agonistic and/or antagonistic interactions, the physical and psychological status of the individual can influence exposure outcomes. An individual's psychosocial environment is likely to be important in epidemiological investigations. Neuroimmunology is a burgeoning discipline, and neurotoxicology and immunotoxicology studies should consider the bidirectional regulatory mechanisms between these organ systems and the potential long-term influences of psychological stress. This mini-review discusses some intriguing data from animal and human studies, which address the regulatory pathways between the neural, endocrine, and immune systems, with emphasis on psychological stress.

Role of vagus nerve signaling in CNI-1493-mediated suppression of acute inflammation

CNI-1493 is a potent anti-inflammatory agent, which deactivates macrophages and inhibits the synthesis of proinflammatory mediators. The objective of the present study was to identify the role of the central nervous system (CNS) and efferent vagus nerve signaling in CNI-1493-mediated modulation of acute inflammation in the periphery. CNI-1493 was administered either intracerebroventricularly (i.c.v., 0.1-1,000 ng/kg) or intravenously (i.v., 5 mg/kg) in anesthetized rats subjected to a standard model of acute inflammation (subcutaneous (s.c.) injection of carrageenan). I.c.v. CNI-1493 significantly suppressed carrageenan-induced paw edema, even in doses at least 6-logs lower than those required for a systemic effect. Bilateral cervical vagotomy or atropine blockade (1 mg/kg/h) abrogated the anti-inflammatory effects of CNI-1493 (1 microg/kg, i.c.v. or 5 mg/kg, i.v.), indicating that the intact vagus nerve is required for CNI-1493 activity. Recording of the efferent vagus nerve activity revealed an increase in discharge rate starting at 3-4 min after CNI-1493 administration (5 mg/kg, i.v.) and lasting for 10-14 min (control activity=87+/-5.4 impulses/s versus CNI-1493-induced activity= 229+/-6.7 impulses/s). Modulation of efferent vagus nerve activity by electrical stimulation (5 V, 2 ms, 1 Hz) of the transected peripheral vagus nerve for 20 min (10 min before carrageenan administration and 10 min after) also prevented the development of acute inflammation. Local administration of the vagus nerve neurotransmitter, acetylcholine (4 microg/kg, s.c.), or cholinergic agonists into the site of carrageenan-injection also inhibited acute inflammation. These results now identify a previously unrecognized role of efferent vagus nerve activity in mediating the central action of an anti-inflammatory agent.

Urocortin, a member of the corticotropin-releasing factor family, in normal and diseased heart

In the present study we investigated the form of expression, action, second messenger, and the cellular location of urocortin, a member of the corticotropin-releasing factor (CRF) family, in the heart. Urocortin mRNA, as shown by quantitative RT-PCR analysis, is expressed in the cultured rat cardiac nonmyocytes (NMC) as well as myocytes (MC) in the heart, whereas CRF receptor type 2beta (CRF-R2beta), presumed urocortin receptor mRNA, is predominantly expressed in MC compared with NMC. Urocortin mRNA expression is higher in left ventricular (LV) hypertrophy than in normal LV, whereas CRF-R2beta mRNA expression is markedly depressed in LV hypertrophy compared with normal LV. Urocortin more potently increased the cAMP levels in both MC and NMC than did CRF, and its effect was more potent in MC than in NMC. Urocortin significantly increased protein synthesis by [(14)C]Phe incorporations and atrial natriuretic peptide secretion in MC and collagen and increased DNA synthesis by [(3)H]prolin and [(3)H]Thy incorporations in NMC. An immunohistochemical study revealed that urocortin immunoreactivity was observed in MC in the normal human heart and that it was more intense in the MC of the human failing heart than in MC of the normal heart. These results, together with the recent evidence of urocortin for positive inotropic action, suggest that increased urocortin in the diseased heart may modulate the pathophysiology of cardiac hypertrophy or failing heart, at least in part, via cAMP signaling pathway.

Neural immunoregulation: emerging roles for nerves in immune homeostasis and disease

In this review, James Downing and Jaleel Miyan outline emerging evidence for neural mechanisms that contribute to specific categories of host defence. Involvement of direct innervation in the adaptive control of immunological responses complements an established view of neuroendocrine-immune modulation. The challenge remains to understand the integrative and homeostatic functions of 'hardwiring' of peripheral immune effector sites, its bearing on disorder and potential for therapeutic modification.

Do centrally administered neuropeptides access cognate receptors?: an analysis in the central corticotropin-releasing factor system

To determine the extent to which centrally administered corticotropin-releasing factor (CRF) activates neurons that express CRF receptors (CRF-Rs), we followed the kinetics and distribution (relative to those of CRF-Rs) of Fos induction seen in response to intracerebroventricular (icv) injection of the peptide (1-10 microg). CRF provoked widespread Fos expression: its strength was dose-related, it peaked at 2 hr after injection, and it was antagonized in a dose-dependent manner by coinjection of CRF-R antagonists. The activation pattern closely mimicked the distribution of CRF-R1 mRNA, in including widespread Fos induction throughout the cortical mantle, in cell groups involved in sensory information processing, and in the cerebellum and several of its major afferents and targets. Dual labeling revealed extensive correspondence of CRF-stimulated Fos-immunoreactivity (Fos-ir) and CRF-R1 mRNA at these and other loci. Unique sites of CRF-R2 expression were relatively unresponsive to CRF but were more so after icv administration of urocortin (UCN), a new mammalian CRF-related peptide. Both CRF and UCN elicited activational responses in cell groups that are involved in central autonomic control but that express neither CRF-R, including the central amygdaloid and paraventricular hypothalamic nuclei, and brainstem catecholaminergic cell groups. The results support an ability of CRF-related peptides in the ventricular system to access receptor-expressing cells directly but leave open questions as to the basis for the recruitment of central autonomic structures, many of which have been identified as stress-related sites of CRF action.

Plant adaptogens. III. Earlier and more recent aspects and concepts on their mode of action

Stimulus-response coupling systems responsible for defence and adaptation of organism to stressors are multi-target and very complicated pharmacological systems, including the neuroendocrine (stress) and immune system. The mode of action of adaptogens is basically associated with the stress-system (neuroendocrine-immune complex) and can be directed on the various targets of the system involved in regulation (activation and inhibition) of stimulus-response coupling. However, clinical studies performed according to the most modern standards are quite limited. On the other hand there is an extensive amount of clinical experience and also established use in self care etc. These aspects are planned to be dealt within a subsequent article which will be devoted to the application in three areas: self care, adjuvants in medicine and curative action in some diseases. At this stage, nevertheless, it seems possible to define some most important "stress-markers" for evaluation of efficiency of adaptogens in experimental and clinical pharmacological studies. They can be both activating (catecholamines, LT-s, cytokines, NO, etc.--"switch on" system--which activates energetic and other resources of the organism), and deactivating (corticosteroids and PGE2-endogenous mediators of cellular communications, which protect cells and whole organism from overreacting to the activating messengers--"switch off" system) stress-messengers. The balance between the activities of the "switch on" and "switch off" systems reflects the well being of the organism. It could be established on different levels of the homeostasis (heterostasis) with different levels of the sensitivity to stressors (Figure 8). The response of stress system--"reactivity" is different at the various levels of heterostasis and depends on adaptation--capacity of the organism (or a cell) to protect itself. In the process of adaptation to stressor's effects the basal levels mediators of switch on (e.g. NO) and switch of (e.g. cortisol) systems are increasing but their balance (the ratio) does not change. In other words, adaptogens increase the capacity of stress system to respond to external signals at the higher level of the equilibrium of activating and deactivating mediators of stress response. Consequently, plant adaptogens can be defined as "smooth" pro-stressors which reduce reactivity of host defense systems and decrease damaging effects of various stressors due to increased basal level of mediators involved in the stress-response. In further studies of adaptogens it seems important to find correlation between adaptogenic activity (a decrease in the "reactivity" of the organism--the basal level of activating and deactivating messengers: ILs, LTB4, NO, PGE2, cortisol, but not their ratio) and their therapeutic efficiency (symptomatic evaluation).

Brain IL-1beta increases neutrophil and decreases lymphocyte counts through stimulation of neuroimmune pathways

Leukocytosis after cerebral injury is well described and may participate in the generation of cerebral damage. However, the mechanisms of brain-induced leukocytosis are still speculative. Since it is known that proinflammatory cytokines are involved in neuroimmunomodulation and since others and we have demonstrated high cytokine levels in the cerebrospinal fluid following injury, we supposed that brain cytokines may also influence leukocyte counts. In order to evaluate this hypothesis, we established an animal model using continuous intracerebroventricular (i.c.v.), intrahypothalamic (i.h.), or intravenous infusion of the proinflammatory cytokines tumor necrosis factor (TNF)-alpha and IL-1beta. Controls received vehicle solution. With this experimental paradigm we could show that i.c.v. and i.h. infusion of IL-1beta but not TNF-alpha dramatically increased neutrophil counts, whereas lymphocytes dropped. Blocking the hypothalamic-pituitary-adrenal (HPA) axis by hypophysectomy abolished the neutrophilia, whereas the lymphopenia remained unchanged. Furthermore, application of the beta2-adrenoreceptor antagonist propranolol prevented the decrease of lymphocytes and diminished the neutrophilia. All parameters normalized within 48 h after termination of infusion. So, our results demonstrate that brain IL-1beta can modify blood leukocyte counts through stimulation of both the sympathetic nervous system (SNS) and the HPA axis.

Pharmacokinetics, cortisol release, and hemodynamics after intravenous and subcutaneous injection of human corticotropin-releasing factor in humans

OBJECTIVE

Two clinical trials investigated the pharmacokinetics of human corticotropin-releasing factor (hCRF), resulting cortisol release, and associated hemodynamic changes.

METHODS

In a 3 x 3 Latin square design, subjects were randomized to receive a single dose of 5 microg x kg(-1) hCRF as a 10-minute intravenous infusion, a 180-minute infusion, and a subcutaneous injection in separate study sessions 7 days apart. Twelve additional subjects obtained a subcutaneous dose of either 300, 600, or 1200 microg hCRF on 3 consecutive days. Noncompartmental and compartmental pharmacokinetic analysis was performed. Hemodynamic response was characterized with use of pharmacodynamic models.

RESULTS

The volume of distribution at steady state was 9.81 +/- 3.0 and 15.61 +/- 2.9, and the clearance was 256 +/- 40 mL x min(-1) and 345 +/- 90 mL x min(-1) for the 10-minute and 180-minute intravenous infusion, respectively (P < .05). Corresponding elimination half-life was 45 +/- 7 minutes and 37 +/- 10 minutes. Two-compartment and 1-compartment models adequately described the 10-minute and 180-minute infusions, respectively. The bioavailability of hCRF after subcutaneous administration was 67% +/- 17%. Apparent clearance remained unchanged for different subcutaneous doses. Peak plasma cortisol concentrations were similar after subcutaneous and intravenous administration of hCRF. Repetitive administration of hCRF did not result in accumulation but produced a reduced plasma cortisol response. A sigmoidal model related plasma hCRF concentrations to increase in heart rate (maximum, 39 beats x min(-1)). The relationship between the modest decrease in diastolic blood pressure and plasma hCRF concentrations was linear.

CONCLUSION

The pharmacokinetics of intravenously administered hCRF were nonlinear, but apparent clearance was constant for various subcutaneous doses. An excellent bioavailability and preserved bioactivity make the subcutaneous route an attractive choice. Repetitive administration of hCRF probably caused tolerance of the cortisol response.

Regulation of stress-induced reduced myelopoiesis in rats

In this work we demonstrate that the stress-induced reduction in bone marrow granulocyte-macrophage colonies, reported previously from our laboratory, is prevented by both the inhibition of the hypothalamic-pituitary-adrenal axis (HPAA) and the blockage of opioid receptors. The inhibition of the HPAA was obtained through the administration of dexamethasone (1 mg/kg). The blockage of opioid receptors was done in two ways, by the administration of naltrexone (8 mg/kg) and induction of tolerance to morphine. On the other hand, no protection was observed in metyrapone treated rats. We suggest that the two physiological systems, opioid and HPAA, mediate the stress-induced myelosuppression and that these systems may function independently in this particular situation.