Interleukin-1 beta induced corticosterone elevation and hypothalamic NE depletion is vagally mediated

Area postrema removal abolishes stimulatory effects of intravenous interleukin-1beta on hypothalamic-pituitary-adrenal axis activity and c-fos mRNA in the hypothalamic paraventricular nucleus

This study examined the role of the area postrema (AP) in transducing peripheral immune signals, represented by intravenous (i.v.) interleukin-1beta (IL-1), into neuroendocrine responses. The AP, a circumventricular organ with a leaky blood-brain barrier, lies adjacent to the nucleus of the solitary tract (NTS) in the medulla. The AP was removed by aspiration, and 2 weeks later, AP-lesioned or sham-lesioned rats were injected i.v. with 0.5 microg/kg IL-1 or sterile saline. After 30 min, brains were removed and analyzed for c-fos mRNA levels in various structures implicated in the hypothalamic-pituitary-adrenal axis response to peripheral cytokine challenge. The sham-lesioned animals responded to IL-1 with large elevations in adrenocorticotropic hormone (ACTH) and corticosterone levels in the plasma and c-fos mRNA levels in cells of the AP, NTS, central nucleus of the amygdala, bed nucleus of the stria terminalis, hypothalamic paraventricular nucleus (PVN), and meninges. Prior AP removal abolished the IL-1 -induced increases in ACTH and corticosterone in the plasma and c-fos mRNA levels in the NTS and PVN. However, AP removal had no effect on IL-1-induced increases in c-fos mRNA levels in the other areas examined. The selective AP lesion effects suggest that the AP and adjacent NTS play a pivotal role in transducing a circulating IL-1 signal into hypothalamic-pituitary-adrenal axis activation by a pathway that may be comprised of known anatomical links between the AP, NTS, and corticotropin-releasing hormone neurons of the PVN.

Indomethacin attenuates oxytocin and hypothalamic-pituitary-adrenal axis responses to systemic interleukin-1 beta

Systemic administration of the cytokine IL-1 beta produces a significant release of ACTH into the plasma and activation of hypothalamic oxytocin (OT) and corticotropin releasing factor (CRF) cells. However, the mechanism(s) by which systemic IL-1 beta induces these responses is not clear. In the present study, we have investigated the proposal that catecholamine cells of the ventrolateral medulla (VLM) and nucleus of the solitary tract (NTS) can relay circulating IL-1 signals via a prostaglandin-dependent mechanism to effect the HPA axis responses in the rat. Intra-arterial administration of IL-1 beta (1 pg/kg) to otherwise untreated animals produced a prominent release of ACTH into the plasma, substantial c-fos expression in paraventricular medial parvocellular (mPVN) corticotropin releasing factor (CRF) cells, supraoptic (SON) and paraventricular nucleus (PVN) OT cells, area postrema cells, NTS and VLM catecholamine cells and cells of the central amygdala. Pretreatment with the prostaglandin synthesis inhibitor, indomethacin (10 mg/kg body weight ia) 15 min before IL-1 beta administration (1 pg/kg ia) significantly reduced plasma ACTH release and c-fos expression in PVN and SON OT cells and MPVN CRF cells, in addition, the area postrema, A1 and C1 catecholamine cell groups of the VLM and A2 and C2 catecholamine cell groups of the NTS, all exhibited concomitant reductions in c-fos expression. Conversely indomethacin administration did not alter the IL1 beta-induced expression of c-fos in the central amygdala. These data suggest that central pathways involved in the IL-1 beta-induced activation of the HPA axis and OT cells are, at least in part, dependent upon prostaglandin synthesis. It is proposed that neurons in the area postrema, NTS and VLM might mediate this IL-1 beta-induced activation of hypothalamic CRF and OT cells and release of ACTH into the plasma.

Thermogenic and corticosterone responses to intravenous cytokines (IL-1beta and TNF-alpha) are attenuated by subdiaphragmatic vagotomy

The brain orchestrates changes in behavior and physiology as a consequence of peripheral immune activation and infection. These changes require that the brain receives signals from the periphery that an immunological challenge has occurred. Previous research has established that cytokines play a role in signalling the brain. What remains unclear, however, is how peripheral cytokines signal the central nervous system. A recent proposal is that cytokines signal the brain by stimulating peripheral nerves. The hypothesis states that following infection and the release of cytokines such as IL-1beta into local tissue or microvasculature, IL-1beta stimulates IL-1 receptors on vagal afferent terminals, or more likely on cells of vagal paraganglia. Vagal afferents, in turn, signal the brain. Previous work has demonstrated that transection of the vagus below the level of the diaphragm blocks or attenuates many illness consequences of intraperitoneally (i.p.) administered lipopolysaccharide (LPS) or IL-1beta. The present studies extend these findings by examining the effect of subdiaphragmatic vagotomy on illness consequences following intravenously (i.v.) administered IL-1beta and TNF-alpha. Subdiaphragmatic vagotomy attenuated both the fever response and corticosterone response produced by i.v. administered cytokines. This effect was dose dependent. The results add support to the hypothesis that vagal afferents are involved in peripheral cytokine-to-brain communication.

Calcium channel blockers and dantrolene differentially regulate the production of interleukin-12 and interferon-gamma in endotoxemic mice

Recent studies suggested that transmitters released from the sympathetic nerve terminals can modulate various inflammatory responses by occupation of receptors on immune cells. These neurotransmitters act via alteration of intracellular concentration of second messengers. For instance, intracellular calcium as a second messenger plays an important role in the regulation of immune responses. Endotoxemia has been shown to be associated with an increase in cytosolic free calcium concentration ([Ca2+]i). Previously we have demonstrated that the calcium channel blockers verapamil and diltiazem, as well as dantrolene, an agent that blocks the release of calcium from its cytoplasmic stores, inhibits tumor necrosis factor-a (TNF-alpha) and augments interleukin-10 (IL-10) plasma levels in endotoxemic BALB/c mice. Here we investigated the effects of verapamil, diltiazem, and dantrolene on lipopolysaccharide (LPS)-evoked production of interleukin-12 (IL-12) and interferon-gamma (IFN-gamma) in BALB/c, C57BL/6 IL-10+/+, and the IL-10 deficient C57BL/6 IL-10(0/0) mice. Intraperitoneal (i.p.) pretreatment with dantrolene (20 mg/kg), but not verapamil (10 mg/kg, i.p.) or diltiazem (20 mg/kg, i.p.) suppressed the LPS-induced (80 mg/kg, i.p.) plasma levels of IL-12 and IFN-gamma in BALB/c mice. Similarly to the BALB/c mice, dantrolene increased IL-10 plasma levels in C57BL/6 IL-10+/+ mice. On the other hand, dantrolene suppressed IL-12 and IFN-gamma production in both the C57BL/6 IL-10+/+ and C57BL/6 IL-10(0/0) mice. These data show that calcium entry blockers and dantrolene differentially regulate IL-12 and IFN-gamma production. Furthermore, dantrolene inhibits the IL-12 and IFN-gamma response independently of the increased release of IL-10.

Neuroendocrine and immune system interactions in stress and depression

Based on the extensive characterization of communication pathways between the nervous system and the immune system, there has been increasing interest in the impact of the nervous system on the development and expression of disorders involving the immune system and the contribution of the immune system to psychiatric disease. A vast literature has documented the impact of various stressors on a wide array of immune partners, and the specific neuroendocrine pathways involved have been elucidated. The impact of cytokines on neuroendocrine function and behavior has also been determined, and the relevance of cytokines to the pathophysiology of depression has become an exciting new research frontier. This article provides a foundation for integrating neuroendocrine-immune interactions into the formulation of neuropsychiatric and immunologic disease.

Effect of endotoxin and interleukin-1beta on corticotropin-releasing-factor and prostaglandin release by rat brainstem slices

This study investigated the effects of lipopolysaccharide (LPS) and interleukin-1beta (IL-1beta) on corticotropin releasing factor (CRF) and prostaglandin E2 (PGE2) release by brainstem slices in vitro. First, we characterized our experimental model and demonstrated that high potassium stimulates CRF release from rat brainstem slices in a calcium dependent way. The direct stimulation of brainstem slices with IL-1beta (3-25 pM) did not modify basal or potassium-stimulated CRF release, although IL-1beta at the dose of 25 pM increased PGE2 production. Peripheral injection (i.p.) of LPS (1-10 microg/kg) or IL-1beta (1-10 microg/kg) evoked a dose-related potentiation of the ex-vivo release of CRF and PGE2 from brainstem slices. However, central (i.c.v.) administration of LPS (10-500 ng/rat) potentiated the release of CRF and PGE2 only at the dose of 500 ng/rat, whereas IL-1beta (1-100 ng/rat) failed to modify significantly the ex vivo production of both CRF and PGE2. The results of the present study provide evidence that peripheral, rather than central, endotoxin and IL-1beta administration induce the activation of brainstem CRF and PGE2, supporting the hypothesis that peripheral cytokine signalling to the CNS is mediated by stimulation of peripheral afferents.

Principles of psychoneuroendocrinology

The goal of this article is to describe some of the central nervous system circuits involved in the regulation of the hypothalamopituitary-adrenocortical (HPA) axis, with an emphasis on animal models believed to mimic the human experience of emotional stress. First, the basic constitutive elements of the HPA axis that control glucocorticoid secretion are reviewed. A description of the neural systems assumed to regulate the activity of the HPA axis, both anatomically and functionally, follows. It is argued that hypothalamic, septal and bed nucleus of the stria terminalis neurons are involved in the regulation of the HPA axis by situations eliciting emotional responses.

Cytokines and fever

Fever is induced in response to the entrance of pathogenic microorganisms into the body and is thought to be mediated by cytokines. Because these pathogens most commonly invade the body through its natural barriers and because body temperature is regulated centrally, these mediators are presumed to be produced peripherally and transported by the bloodstream to the brain, to act. It is generally considered that their febrigenic messages are further modulated there by prostaglandin E2 (PGE2). However, the detailed mechanism by which these cytokines signal the brain and activate the febrile response is not yet clear. Indeed, the specific role of each cytokine has been difficult to establish due to complex interactions among them. Furthermore, recent evidence suggests that different pyrogens may induce different cytokines; for example, i.v. LPS (a model of systemic bacterial infection) induces large increases in IL-6, but only small rises in IL-1 and TNF alpha plasma levels. Moreover, their appearance lags the fever onset. We recently found that subdiaphragmatic vagotomy, decomplementation, and blockade of Kupffer cells suppress the febrile response of guinea pigs to i.v. LPS, and that i.v. LPS rapidly stimulates the release of norepinephrine (NE) and, hence, of PGE2 in their preoptic-anterior hypothalamus (POA, the brain region containing the thermoregulatory controller). Based on these and other data in the literature, we hypothesize that LPS fever may be initiated as follows: i.v., LPS-->complement-->Kupffer cells-->cytokines?-->vagal afferents -->n. tractus solitarius?-->A1/A2 cell groups?-->ventral noradrenergic bundle? -->POA-->NE-->PGE2-->fever.

The role of the vagus nerve in cytokine-to-brain communication

Peripheral interleukin-1 beta (IL-beta) and inflammatory stimuli that induce the synthesis and release of IL-1 beta produce a variety of central nervous system responses. Most proposals designed to explain how peripheral IL-1 beta influences the CNS have focused on blood-borne routes of communication. We will review data that indicate that at least some of the CNS response to peripheral IL-1 beta are instead mediated by a neural route of communication between the periphery and the CNS. IL-1 beta activates afferent vagal fibers that terminate in the nucleus tractus solitarius, and communication via the vagus is responsible for much of the hyperalgesia, fever, anorexia, taste aversions, increased levels of plasma corticosteroid, and brain norepinephrine changes produced by intraperitoneal injections of IL-1 beta and LPS. Data extending this analysis to TNF-alpha and intravenous routes will be described.

Altered neuroimmunoendocrine communication during a condition of chronically increased brain corticotropin-releasing hormone drive

Presently, it is clear that the brain, immune system, and endocrine system build a complex network of interactions at various levels. Inflammation, which may be regarded as a stressful challenge, initiates apart from immunological, autonomic, and neuroendocrine responses also profound behavioral (e.g., immobility, social disinterest) changes. Key mediators herein are corticotropin-releasing hormone (CRH) and cytokines, such as interleukin-1 beta (IL-1 beta). Currently, the behavioral changes, collectively termed sickness behavior, are thought to be adaptive responses to support the body's efforts to fight the infection. Using in vivo microdialysis and biotelemetry in freely moving animals, we have studied the monoaminergic circuits in the brain implicated in the regulation of physiological and behavioral responses to a peripheral inflammatory challenge (see also chapter of Linthorst and Reul in this volume). To expand our insight into the relationship between hypersecretion of CRH and physiological and behavioral abnormalities associated with stress-related disorders, a series of experiments was conducted with long-term centrally CRH-infused rats. These rats showed reduced body weight gain, decreased food intake, elevated plasma ACTH and corticosterone levels, thymus involution and immunosuppression, but, paradoxically, enhanced IL-1 beta mRNA expression in spleen macrophages. After a peripheral endotoxic challenge on the seventh day of treatment, the CRH-infused rats produced aberrant (i.e., blunted and/or delayed) HPA axis, fever, behavioral, and hippocampal serotonergic responses. However, endotoxin-induced plasma IL-1 and IL-6 bioactivities were significantly enhanced in these animals. The data show that chronically elevated central CRH levels as occurring during chronic stress result in defective central nervous system and immune system responses to an acute (inflammatory) challenge. These observations provide evidence that chronic CRH hypersecretion is an important factor in the etiology of stress-related disorders.

Vagotomy blocks the induction of interleukin-1beta (IL-1beta) mRNA in the brain of rats in response to systemic IL-1beta

There is considerable interest in the mechanisms by which systemic cytokines signal the CNS to elicit centrally controlled biological actions. This study determined the effects of intraperitoneal injections of interleukin-1beta (IL-1beta) on IL-1beta mRNA and IL-1 receptor accessory protein (IL-1RAP) mRNA production in rat liver and brain using the reverse transcription-PCR. Saline or IL-1beta (0.5 microg/kg) was injected intraperitoneally in subdiaphragmatically vagotomized and sham-operated (SHAM) rats. All injections were performed at dark onset, and rats were killed 2 hr after the injection. In SHAM rats, IL-1beta increased IL-1beta mRNA levels in the liver, hypothalamus, hippocampus, and brainstem. Subdiaphragmatic vagotomy blocked the IL-1beta-induced increase in IL-1beta mRNA in the brainstem and hippocampus and significantly attenuated the increase in the hypothalamus. Vagotomy did not affect IL-1beta-induced IL-1beta mRNA production in the liver. IL-1RAP mRNA was highly expressed in each region examined; however, no significant differences in IL-1RAP mRNA production were found in any region after IL-1beta injection. The current results indicate that the vagus nerve is involved in transmitting cytokine signals to the brain and suggest that the induction of brain cytokines is a critical step in the pathway by which vagal-mediated signals result in centrally controlled symptoms of the acute phase response.

Effects of interferon-gamma, interleukin-1 beta, and tumor necrosis factor-alpha on the serotonin metabolism in the nucleus raphe dorsalis of the rat

The effects of the cytokines interferon (IFN)-gamma, interleukin (IL)-1, and tumor necrosis factor (TNF)-alpha on the serotoninergic transmission in the nucleus raphe dorsalis (NRD) were studied after peripheral and central application. The studies were performed in the freely moving rat using differential pulse voltammetry with multicarbon fibre electrodes to study the extracellular levels of the serotonin (5-HT) metabolite 5-hydroxyindoleacetic acid (5-HIAA). The extracellular 5-HIAA levels were not changed in the NRD after peripheral application of rat recombinant IFN-gamma, but elevated by the cytokines IL-1 beta and TNF-alpha. After intracerebroventricular (i.c.v.) application the cytokines IFN-gamma, IL-1 beta and TNF-alpha stimulated the serotoninergic transmission in the NRD. Our data suggest that the effect of peripherally elevated cytokine concentrations on the serotonin metabolism in the NRD of the rat is cytokine-dependent. In this respect the T-cell and NK-cell cytokine IFN-gamma acts clearly different when compared to the mainly macrophage-derived cytokines IL-1 beta and TNF-alpha, and plays a different role in the communication between immune and central nervous system.

Somnogenic and pyrogenic effects of interleukin-1beta and lipopolysaccharide in intact and vagotomized rats

The vagus nerve appears to serve a role in mediating peripheral immunologic influences on CNS processes. Previous work demonstrates that subdiaphragmatic vagotomy prevents or attenuates many of the behavioral and physiological responses to exogenous interleukin-1 (IL-1) or lipopolysaccharide (LPS). We determined whether the somnogenic effects of IL-1 and LPS were altered in vagotomized rats, and whether the effects of vagotomy on IL-1- and LPS-induced alterations in sleep would vary as a function of circadian phase. The data indicate that vagotomy does not influence the normal circadian patterns of sleep and wakefulness in untreated rats, or modify the pyrogenic or somnogenic effects of intracerebroventricular administration of IL-1. However, in unchallenged animals vagotomy reduces basal brain temperatures, increases delta wave amplitudes during slow-wave sleep (SWS), and induces a reduced rate of weight gain, gastric distension, and adrenal hypoplasia. Vagotomy attenuates the febrile effects of IL-1 during both light and dark phases, attenuated IL-1-induced sleep enhancement during the dark phase, and attenuated IL-1-induced increases in delta wave amplitudes within SWS during the light period. In LPS-treated rats, vagotomy attenuates the febrile and SWS responses to LPS after administration at light onset, but not after administration at dark onset. These results indicate that subdiaphragmatic vagotomy attenuates several of the somnogenic and pyrogenic effects of IL-1beta and LPS, although the effectiveness of the vagal transection in modulating these responses is influenced by circadian factors.

Depression, stress and immunological activation: the role of cytokines in depressive disorders

Traditionally, both stress and depression have been associated with impaired immune function and increased susceptibility to infectious and neoplastic disease. However over the last number of years a large body of evidence suggests that major depression is associated with signs of immunological activation. Moreover it has been suggested that cytokine hypersecretion may be involved in the aetiology of depressive disorders. The present article reviews the evidence from both clinical and experimental studies which implicates immunological activation and particularly hypersecretion of cytokines in the onset and maintenance of depressive illness. Both clinical and experimental studies indicate that stress and depression are associated with increased circulating concentrations of cytokines such as IL-1beta, IL-6 and gamma-IFN and positive acute phase proteins, and hyperactivity of the HPA-axis. In addition, it has been reported that immunological activation induces "stress-like" behavioural and neurochemical changes in laboratory animals. Although for many years it has been suggested that stress acts a predisposing factor to depressive illness, the precise mechanisms by which stress-induced depressive symptoms occur are not fully understood. Nevertheless, behavioural changes due to stress have often been explained in terms of changes in neurotransmitter function in the brain. In the present article increased cytokine secretion is implicated as a mechanism whereby stress can induce depression.

TNF-alpha-induced corticosterone elevation but not serum protein or corticosteroid binding globulin reduction is vagally mediated

Immune activation leads to production of mediators such as cytokines, which act to induce both brain-mediated and peripheral defense processes. We used intraperitoneal administration of the cytokine tumor necrosis factor-alpha (TNF-alpha) to investigate whether defense processes induced by this cytokine are mediated by vagal afferents and/or interleukin-1 (IL-1) receptors. Because some effects of TNF-alpha are mediated, at least in part, by the brain [plasma corticosterone (CORT) elevation] and some are mediated by peripheral organs [reduction of serum protein and corticosteroid binding globulin (CBG)], we also investigated whether the effects of vagotomy are specific to those defense processes mediated by the brain. Both vagotomy and IL-1 receptor antagonist attenuated serum CORT elevation, but had no effects on serum protein or CBG reduction. These results support the idea that vagal afferents provide a true immune-to-brain pathway that may include IL-1 receptors.

Infection-induced anorexia: active host defence strategy

In association with fever production, decreased food consumption is the most common sign of infection. This effect is often regarded as an undesirable manifestation of sickness. However, evidence suggests that just as many behaviours have now been shown to modify immunocompetence, infection-induced anorexia is a behaviour systematically organised for pathogen elimination. That is, anorexia is an active defence mechanism that is beneficial for host defence. This review details the mechanism of infection-induced anorexia, placing it within the framework of the intricately organised acute phase response--the host response to infection. Furthermore, the evolutionary, behavioural, metabolic and immunological consequences of infection-induced anorexia are outlined, each providing evidence for the beneficial nature of this response. The evidence suggests that food restriction is one of the important behavioural strategies that organisms have evolved for the fight against pathogenic invasion. Nevertheless, such benefits require fine homeostatic control, as chronic undernutrition has deleterious consequences for host defence.

Reduced preference for sucrose in autoimmune mice: a possible role of interleukin-6

In a continuous one-bottle sucrose intake test, 4-month-old autoimmune MRL-Ipr mice show a shift to the right along the X-axis of the concentration-intake function, compared to congenic MRL +/+ controls. Using a brief (60-min) and a continuous (48-h) two-bottle test, the present report examines potential factors that could account for the reduced responsiveness to a palatable stimulus. Study 1 examines whether preference for sucrose is associated with age, changes in food/water intake, or impaired renal function. Reduced preference for sucrose was observed in 5-6-week-old MRL-Ipr males, although food/water intake or blood creatinine levels did not differ from control values. Immunosuppressive treatment abolished this deficit, suggesting a role of immune factor(s). Study 2 tests the hypothesis that chronic upregulation of the neuroactive cytokine interleukin-6 (IL-6), reported to occur from 3 weeks of age in young MRL-Ipr mice, reduces preference for sucrose. Sustained administration of IL-6 was produced by infecting healthy MRL +/+, C3H.SW and Balb/C mice with adenovirus vector carrying cDNA for murine IL-6. This resulted in high serum IL-6 levels over 5 days, a rapid decline in preference for sucrose and low blood glucose levels. The results from Study 1 indicate that impaired sensitivity to sucrose in MRL-Ipr mice can be detected before autoimmune disease is florid in MRL-Ipr mice. The results from Study 2 are consistent with altered motivation/emotional states after infection, and point to sustained IL-6 production as an early mechanism in behavioral alterations during chronic autoimmune/inflammatory conditions.

Subdiaphragmatic vagotomy does not prevent fever following intracerebroventricular prostaglandin E2: further evidence for the importance of vagal afferents in immune-to-brain communication

Brain-mediated sickness responses can be blocked by subdiaphragmatic vagotomy, suggesting that vagal afferents signal peripheral inflammation or infection. This study tested whether subdiaphragmatic vagotomy disrupts sickness responses by interrupting effector pathways. If this explanation is correct, intracerebroventricular prostaglandin E2-induced fever should be blocked by this procedure. Fever was unaffected by subdiaphragmatic vagotomy, thus these data provide support for the conclusion that vagal afferents signal the brain during immune activation.

Effect of three different modes of alcohol administration on the activity of the rat hypothalamic-pituitary-adrenal axis

The present study compared the effect of different modes of alcohol administration on the activity of the hypothalamic-pituitary-adrenal (HPA) axis. In a first series of studies, we investigated the influence of the intraperitoneal (ip) and intragastric (ig) effect of acutely administered alcohol. Over a 3-hr period, alcohol induced dose-related increases in adrenocorticotropic hormone (ACTH) and alcohol levels in the circulation. There was a good correlation between blood ACTH and alcohol levels (ip treatment, R = 0.84; ig treatment, R = 0.79). Measurement of steady-state mRNA of the immediate early gene NGFI-B, taken as an index of neuronal activation in the paraventricular nucleus (PVN) of the hypothalamus of rats administered 3 g of alcohol/kg, indicated significant (p < 0.01) increases between 60 and 180 min after both ip and ig alcohol injection, with peak stimulation at 1 and 2 hr, respectively. Although NGFI-B mRNA levels had returned to control level in the parvicellular portion of the PVN of animals administered the drug ip 4 hr earlier, they were still significantly (p < 0.01) elevated 4 hr after ig treatment. A second series of studies used rats fed an alcohol diet containing 6.4% alcohol (w/v), or pair-fed. The rats were tested during the 4th or 6th night of treatment. Despite blood alcohol levels ranging between 0.060 and 0.140% w/v, there was no significant rise in plasma ACTH/corticosterone levels in animals fed the drug, and no detectable NGFI-B mRNA in their PVN. Collectively, these results indicate that both the ip and the ig acute injection of alcohol induced dose-related increases in plasma ACTH levels. PVN neuronal activation was also observed. In contrast, the alcohol diet had no effect on HPA hormone levels or PVN expression of NGFI-B.

Effects of area postrema lesion and abdominal vagotomy on interleukin-1 beta-induced norepinephrine release in the hypothalamic paraventricular nucleus region in the rat

Peripherally administered interleukin-1 beta (IL-1 beta) has been shown to increase extracellular norepinephrine (NE) concentration in the paraventricular nucleus (PVN) of the hypothalamus. The present study was carried out using an in vivo microdialysis technique in conscious rats in order to examine the possible involvement of the area postrema (AP) and the abdominal vagal afferent nerves in this effect. Extracellular NE concentrations in the PVN region were measured by high performance liquid chromatography with electrochemical detection. In AP-lesioned or abdominal-vagotomized rats, the NE increase was significantly attenuated compared to that in sham-operated rats; this reduction was greater in abdominal-vagotomized rats than in AP-lesioned rats. The results suggest that the AP as well as the abdominal vagal afferent nerves is involved in intraperitoneal (i.p.) administered IL-1 beta-induced NE release in the PVN region.

Blockade of lipopolysaccharide-induced fever by subdiaphragmatic vagotomy in guinea pigs

It is generally believed that fever is mediated by certain cytokines produced by immune cells activated by exogenous pyrogens, e.g., lipopolysaccharides (LPS), released into the circulation and transported to the brain There, the cytokines are thought to stimulate prostaglandin (PG) E2 production within the organum vasculosum laminae terminalis region. PGE2 then may act as a febrigenic mediator locally or in the surrounding preoptic area (POA). However, whereas the increases in preoptic PGE2 and body (core) temperature (Tc) following the intravenous (i.v.) administration of LPS correlate temporally, cytokine levels in blood lag both these increases. From recent data in the literature, we have conjectured that a possible, alternative communication pathway between the i.v. LPS-activated immune system and brain PGE2 may be provided by the vagi. To test this possibility, we measured the levels of PGE2 in the extracellular fluid of the POA (collected by microdialysis) of conscious, subdiaphragmatically vagotomized or sham-operated guinea pigs following LPS administration (2 micrograms/kg; i.v.); controls received pyrogen-free saline (PFS). The effluents from the microdialysis probes were collected over 30-min periods throughout the experiments and the samples analyzed by radioimmunoassay; Tc was monitored continuously using thermocouples inserted 5 cm into the colon. LPS induced a biphasic fall in Tc and failed to increase preoptic PGE2 levels in the vagotomized guinea pigs (n = 10), whereas in their sham-operated controls (n = 10) it induced increases in both preopitc PGE2 and Tc within 15 min after its injection; PFS (n = 13) had no effect on either variable. We postulate that peripheral immune cell-derived signals may be transmitted via the vagi to the medulla. From other data, we suggest further that they may be conveyed from here via the ventral noradrenergic bundle to the POA region, where the released norepinephrine induces the local synthesis of PGE2 and, hence, fever onset.

Neuroimmunomodulation via limbic structures--the neuroanatomy of psychoimmunology

During the last 20 years, mutual communications between the immune, the endocrine and the nervous systems have been defined on the basis of physiological, cellular, and molecular data. Nevertheless, a major problem in the new discipline "Psychoneuroimmunology" is that controversial data and differences in the interpretation of the results make it difficult to obtain a comprehensive overview of the implications of immunoneuroendocrine interactions in the maintenance of physiological homeostasis, as well as in the initiation and the course of pathological conditions within these systems. In this article, we will first discuss the afferent pathways by which immune cells may affect CNS functions and, conversely, how neural tissues can influence the peripheral immune response. We will then review recent data, which emphasize the (patho)physiological roles of hippocampal-amygdala structures and the nucleus accumbens in neuroimmunomodulation. Neuronal activity within the hippocampal formation, the amygdaloid body, and the ventral parts of the basal ganglia has been examined most thoroughly in studies on neuroendocrine, autonomic and cognitive functions, or at the level of emotional and psychomotor behaviors. The interplay of these limbic structures with components of the immune system and vice versa, however, is still less defined. We will attempt to review and discuss this area of research taking into account recent evidences for neuroendocrine immunoregulation via limbic neuronal systems, as well as the influence of cytokines on synaptic transmission, neuronal growth and survival in these brain regions. Finally, the role of limbic structures in stress responses and conditioning of immune reactivity will be commented. Based on these data, we propose new directions of future research.

Systemic capsaicin pretreatment fails to block the decrease in food-motivated behavior induced by lipopolysaccharide and interleukin-1beta

The physiological and behavioral disturbances observed during an infection can be reproduced by systemic administration of proinflammatory cytokines (e.g., interleukin (IL)-1, IL-6, tumor necrosis factor-alpha) or lipopolysaccharide (LPS), a potent inducer of these cytokines. It is now well established that these molecules induce their effects by acting centrally, however, the mechanisms by which they reach central structures are not clear. We have earlier proposed that the humoral immune message is converted to a central neural activation by the action of cytokines on peripheral terminations of afferent neurons. Subdiaphragmatic vagotomy abolishes several effects of peripherally injected IL-1beta and LPS (e.g., decreased food-motivated behavior and social exploration, central expression of cytokines). To further define the nature of the peripheral fibers implicated in this phenomenon, we used a potent sensory neurotoxin, capsaicin, to selectively destroy C-fiber afferents. Adult rats were injected I.P. with a total dose of 25 mg/kg capsaicin in a series of 10 injections over a 48-h period. Adult mice were injected I.P. with a total dose of 75 mg/kg in a series of seven injections over a 7-day period. Although capsaicin treatment altered visceral chemosensory function, corneal and pain sensitivity, vagal-mediated anorexic effects of cholecystokinin, and depleted levels of substance P in the thoracic spinal cord, it was completely ineffective in blocking the decrease in food-motivated behavior induced by IL-1beta (4 microg/rat I.P. in rats) and LPS (250 microg/kg I.P. in rats and 400 microg/kg I.P. in mice). Thus, other afferents besides capsaicin-sensitive C-fibers appear to be involved in the transduction of cytokine effects during inflammatory and infectious events.

Vagal paraganglia bind biotinylated interleukin-1 receptor antagonist: a possible mechanism for immune-to-brain communication

Interleukin-1 beta is a proinflammatory cytokine released by activated immune cells. In addition to orchestrating immune responses to infection, interleukin-1 beta is a key mediator of immune-to-brain communication. Interleukin-1 beta and endotoxin (which releases IL1 beta from immune cells) cause centrally mediated illness responses such as fever, aphagia, etc. These effects are blocked by intraperitoneal IL1 receptor antagonist (IL1ra), suggesting critical involvement of peripheral IL1 receptors. Centrally mediated illness responses are also blocked by vagotomy, suggesting that IL1 beta directly or indirectly activates vagal afferents. To test for IL1 beta binding whole vagus (abdominal, laryngeal, and thoracic) and sections of hepatic vagus and liver hilus were incubated with biotinylated IL1ra and processed for avidin-biotin complex (ABC) or avidin-FITC histochemistry. Glomus cells of vagal paraganglia were labeled in all regions of the vagus. Biotinylated IL1ra also labeled smooth muscle and endothelial cells of blood vessels and lymphoid tissues. No label was present in omission or competition controls. These data suggest that centrally mediated illness responses result from IL1 activation of vagal paraganglia.

Subdiaphragmatic vagotomy inhibits intra-abdominal interleukin-1 beta stimulation of adrenocorticotropin secretion

Although interleukin (IL)-1 beta activates the hypothalamic-pituitary-adrenal (HPA) axis, the mechanisms by which peripheral IL-1 beta acutely stimulates adrenocorticotropin (ACTH) secretion are not clear. Recently, the vagus has been implicated in mediating peripheral cytokine signalling of the brain. To investigate a possible central mechanism for peripheral cytokine stimulation of the HPA axis, we tested the hypothesis that the vagus mediates IL-1 beta activation of the HPA axis by an intra-abdominal stimulus. We studied the effect of subdiaphragmatic vagotomy on plasma ACTH stimulation in rats by intraperitoneal (i.p.) IL-1 beta. Adult male Sprague-Dawley rats underwent subdiaphragmatic vagotomy or sham surgery 1 week prior to study. Rats were killed 1 and 2 h after i.p. saline (control) and low- (4 micrograms/kg) and high-dose (20 micrograms/kg) IL-1 beta. Vagotomy markedly attenuated plasma ACTH secretion at 2 h after high-dose IL-1 beta stimulation and abolished plasma ACTH secretion at 2 h after low-dose IL-1 beta stimulation. At 1 h after low-dose IL-1 beta, stimulation of plasma ACTH in vagotomized animals was also markedly diminished compared to sham animals. However, vagotomy did not alter stimulation of plasma corticosterone at 1 or 2 h after low-dose IL-1 beta or at 2 h after high-dose IL-1 beta. In addition, vagotomy did not alter stimulation of plasma ACTH or corticosterone secretion by insulin-induced hypoglycemia. We conclude that: (1) the vagus plays an important role in stimulation of ACTH secretion by intra-abdominal (i.p.) IL-1 beta; (2) stimulation of corticosterone secretion by i.p. IL-1 beta is not altered by vagotomy; and (3) the inhibitory effect of vagotomy on activation of the HPA axis appears to be specific for immune stimulation by cytokines.

Endotoxin administration induced differential neurochemical activation of the rat brain stem nuclei

Lipopolisaccharide (LPS) is a potent activator of the immune system, but also activates the hypothalamic-pituitary-adrenocortical (HPA) axis and cerebral catecholamine systems. In the present study, the effect of peripheral LPS administration on catecholaminergic and serotonergic neurotransmission in discrete brainstem nuclei was examined. Two hours following systemic administration of LPS (1, 10, or 100 micrograms/kg) norepinephrine (NE) content in the locus coeruleous (LC) was significantly increased in a dose related manner. An increased dopamine (DA) turnover as reflected by the 3,4-dihydroxyphenylacetic (DOPAC) + Homovanillic acid (HVA)/DA ratio, [DO-PAC + HVA]/[DA], was also observed at the LC with the medium and high doses of LPS administered. Endotoxin caused the main effects in the nucleus of the tractus solitarii (NTS) in which (a) it was found NE content increased in a dose related fashion, (b) DA turnover index was elevated with 10 and 100 micrograms/kg LPS doses, and (c) levels of serotonin (5-HT) and its catabolite, 5-hydroxyindole-3-acetic acid (5-HIAA), were also significantly elevated following the injection of 10 or 100 micrograms/kg LPS. By contrast, a consistent lack of catecholaminergic and serotonergic responses to endotoxin treatment was observed at the level of midbrain Raphe nuclei (MRN). These results demonstrate that differential neurochemical changes may occur in the brainstem region with a rank order of activation by LPS that was NTS > LC > MRN, suggesting different neural substrate for central effects of peripheral immune activation.

Immune activation: the role of pro-inflammatory cytokines in inflammation, illness responses and pathological pain states

It has recently become accepted that the activated immune system communicates to brain via release of pro-inflammatory cytokines. This review examines the possibility that pro-inflammatory cytokines (interleukins and/or tumor necrosis factor) mediate a variety of commonly studied hyperalgesic states. We will first briefly review basic immune responses and inflammation. We will then develop the concept of illness responses and provide evidence for their existence and for the dramatic changes in neural functioning that they cause. Lastly, we will examine the potential roles that both pro-inflammatory cytokines and the neural circuits that they activate may play in the hyperalgesic states produced by irritants, inflammatory agents, and nerve damage. The possibility is raised that apparently diverse hyperalgesic states may converge in the central nervous system and activate similar or identical neural circuitry.