Different receptor mechanisms mediate the pyrogenic and behavioral effects of interleukin 1

Prior stressor exposure sensitizes LPS-induced cytokine production

Exposure to stressors often alters the subsequent responsiveness of many systems. The present study tested whether prior exposure to inescapable tailshock (IS) alters the interleukin (IL)-1beta, tumor necrosis factor (TNF)-alpha, or IL-6 response to an injection of bacterial endotoxin (lipopolysaccharide; LPS). Rats were exposed to IS or remained as home cage controls (HCC); 24 h later animals were injected i.p. with either 10 microg/kg LPS or equilvolume sterile saline. IS significantly increased plasma TNF-alpha, IL-1beta, and pituitary, hypothalamus, hippocampus, cerebellum IL-1beta 1 h, but not 2 h, after LPS, compared to controls. Additional animals were injected with LPS or saline 4, 10, or 21 days after exposure to IS and tail vein blood was collected and assayed for IL-1beta. An enhanced plasma IL-1beta response occurred 4 days after IS, but was gone by 10 days. These results suggest that exposure to IS sensitizes the innate immune response to LPS by resulting in either a larger or a more rapid induction of proinflammatory cytokines.

Involvement of the arcuate nucleus of the hypothalamus in interleukin-1-induced anorexia

Cytokine-mediated anorexia is a component of "sickness behavior" and presents a significant obstacle in the treatment of chronic illnesses. We hypothesized an involvement of the hypothalamic arcuate nucleus (ARH) in mediating the anorexic effects of a systemic interleukin-1 (IL-1) challenge based on its content of peptidergic neurons involved in feeding, its expression of IL-1 receptors and its sensitivity to systemic IL-1. IL-1 (6 microg/kg, i.v.) was found to induce Fos expression in both pro-opiomelanocortin- and neuropeptide Y-expressing neurons in and around the ARH. Contrary to expectations, rats that had sustained lesions of the arcuate nucleus, produced by neonatal monosodium glutamate treatment, displayed a more pronounced suppression (by 25%) of food intake than nonlesioned controls when treated with IL-1 after a 20 hr fast. To confirm and further characterize this unexpected result, a second ablation method was used in a similar paradigm. Animals bearing knife cuts designed to sever major ARH projections displayed an even more accentuated loss of appetite (by 60%, relative to controls) in response to systemic IL-1. This effect exhibited at least some degree of specificity, because the knife cuts did not alter either IL-1 effects on another centrally mediated acute phase response (fever) or the anorexia produced by an alternate agent, fenfluramine. These results fail to support the hypothesized ARH mediation of IL-1-induced anorexia and may suggest rather that the net output of this cell group may serve normally to restrain cytokine-induced reductions in food intake.

Fatigue in cholestatic liver disease--a perplexing symptom

Fatigue is an important symptom and a quality of life determinant in patients with cholestatic liver disease. The pathogenesis of fatigue is obscure, although alterations in central neurotransmission and peripheral muscle dysfunction have been incriminated. No effective treatment is available at present. The available literature on fatigue in cholestatic liver disease is reviewed.

Involvement of the arcuate nucleus of the hypothalamus in interleukin-1-induced anorexia

Cytokine-mediated anorexia is a component of "sickness behavior" and presents a significant obstacle in the treatment of chronic illnesses. We hypothesized an involvement of the hypothalamic arcuate nucleus (ARH) in mediating the anorexic effects of a systemic interleukin-1 (IL-1) challenge based on its content of peptidergic neurons involved in feeding, its expression of IL-1 receptors and its sensitivity to systemic IL-1. IL-1 (6 microg/kg, i.v.) was found to induce Fos expression in both pro-opiomelanocortin- and neuropeptide Y-expressing neurons in and around the ARH. Contrary to expectations, rats that had sustained lesions of the arcuate nucleus, produced by neonatal monosodium glutamate treatment, displayed a more pronounced suppression (by 25%) of food intake than nonlesioned controls when treated with IL-1 after a 20 hr fast. To confirm and further characterize this unexpected result, a second ablation method was used in a similar paradigm. Animals bearing knife cuts designed to sever major ARH projections displayed an even more accentuated loss of appetite (by 60%, relative to controls) in response to systemic IL-1. This effect exhibited at least some degree of specificity, because the knife cuts did not alter either IL-1 effects on another centrally mediated acute phase response (fever) or the anorexia produced by an alternate agent, fenfluramine. These results fail to support the hypothesized ARH mediation of IL-1-induced anorexia and may suggest rather that the net output of this cell group may serve normally to restrain cytokine-induced reductions in food intake.

Neural-immune gut-brain communication in the anorexia of disease

Peripheral administration of toxic bacterial products and cytokines have been used to model the immunological, physiological, and behavioral responses to infection, including the anorexia of disease. The vagus nerve is the major neuroanatomic linkage between gut sites exposed to peripheral endotoxins and cytokines and the central nervous system regions that mediate the control of food intake, and thus has been a major research focus of the neurobiological approach to understanding cytokine-induced anorexia. Molecular biological and neurophysiologic evidence demonstrates that peripheral anorectic doses of cytokines and endotoxins elicit significant increases in neural activation at multiple peripheral and central levels of the gut-brain axis and in some cases may modify the neural processing of meal-related gastrointestinal signals that contribute to the negative feedback control of ingestion. However, behavioral studies of the anorectic effects of peripheral cytokines and endotoxins have shown that neither vagal nor splanchnic visceral afferent fibers supplying the gut are necessary for the reduction of food intake in these models. These data do not rule out 1) the potential contribution of supradiaphragmatic vagal afferents or 2) a modulatory role for immune-stimulated gut vagal afferent signals in the expression of cytokine and endotoxin-induced anorexia in the intact organism.

Effects of serotonin synthesis blockade on interleukin-1 beta action in the brain of rats

The ability of the brain serotonergic system to mediate the effects of interleukin-1beta (IL-1beta) was investigated. Intracerebroventricular administration of IL-1beta induced a significant pyrogenic reaction, depression in social behaviour, loss of body weight and reduced food intake in rats. Pre-treatment with p-chlorphenylalanine, an inhibitor of serotonin synthesis, blocked the IL-1beta-induced decrease in food intake and loss of body weight, but failed to alter the temperature increase and the decrease in communicative activity.

Cytokine-induced sickness behavior: mechanisms and implications

Sickness behavior refers to a coordinated set of behavioral changes that develop in sick individuals during the course of an infection. At the molecular level, these changes are due to the brain effects of proinflammatory cytokines such as interleukin-1 (IL-1) and tumor necrosis factor alpha (TNFalpha). Peripherally released cytokines act on the brain via a fast transmission pathway involving primary afferent nerves innervating the bodily site of inflammation and a slow transmission pathway involving cytokines originating from the choroid plexus and circumventricular organs and diffusing into the brain parenchyma by volume transmission. At the behavioral level, sickness behavior appears to be the expression of a central motivational state that reorganizes the organism priorities to cope with infectious pathogens. There is evidence that the sickness motivational state can interact with other motivational states and respond to nonimmune stimuli probably by way of sensitization and/or classical conditioning. However, the mechanisms that are involved in plasticity of the sickness motivational state are not yet understood.

Cytokine-induced sickness behavior: where do we stand?

Sickness behavior refers to the coordinated set of behavioral changes that develop in sick individuals during the course of an infection. At the molecular level, these changes are due to the effects of proinflammatory cytokines, such as interleukin-1 (IL-1) and tumor necrosis factor-alpha (TNFalpha), in the brain. Peripherally released cytokines act on the brain via a fast transmission pathway involving primary afferent nerves innervating the body site of inflammation and a slow transmission pathway involving cytokines originating from the choroid plexus and circumventricular organs and diffusing into the brain parenchyma by volume transmission. At the behavioral level, sickness behavior appears to be the expression of a central motivational state that reorganizes the organism's priorities to cope with infectious pathogens. There is clinical and experimental evidence that activation of the brain cytokine system is associated with depression, although the exact relationship between sickness behavior and depression is still elusive.

Neural and humoral pathways of communication from the immune system to the brain: parallel or convergent?

The first studies carried out on the mechanisms by which peripheral immune stimuli signal the brain to induce fever, activation of the hypothalamic-pituitary-adrenal axis and sickness behavior emphasized the importance of fenestrated parts of the blood-brain barrier known as circumventricular organs for allowing blood-borne proinflammatory cytokines to act on brain functions. The discovery in the mid-1990s that subdiaphragmatic section of the vagus nerves attenuates the brain effects of systemic cytokines, together with the demonstration of an inducible brain cytokine compartment shifted the attention from circumventricular organs to neural pathways in the transmission of the immune message to the brain. Since then, neuroanatomical studies have confirmed the existence of a fast route of communication from the immune system to the brain via the vagus nerves. This neural pathway is complemented by a humoral pathway that involves cytokines produced at the level of the circumventricular organs and the choroid plexus and at the origin of a second wave of cytokines produced in the brain parenchyma. Depending on their source, these locally produced cytokines can either activate neurons that project to specific brain areas or diffuse by volume transmission into the brain parenchyma to reach their targets. Activation of neurons by cytokines can be direct or indirect, via prostaglandins. The way the neural pathway of transmission interacts with the humoral pathway remains to be elucidated.

Cholecystokinin receptors do not mediate the suppression of food-motivated behavior by lipopolysaccharide and interleukin-1 beta in mice

During the course of an infection, profound metabolic and behavioral changes are observed. The resulting decrease in food intake can be reproduced by administration of lipopolysaccharide (LPS) or the proinflammatory cytokines (e.g., interleukin-1 [IL-1] and tumor necrosis factor it induces. To test the possibility that cholecystokinin (CCK) mediates anorexia induced by IL-1 beta and LPS, mice trained to poke their noses in a hole to obtain a food reward according to a fixed ratio (1 reward per 20 actions) were pretreated with the CCK-A receptor antagonist L364,718 (at 1 mg/kg) or with the CCK-B receptor antagonist L365,260 (50 microg/kg) before being injected with LPS (100 microg/kg) or IL-1 beta (20 microg/kg). All injections were given via the intraperitoneal (i.p.) route. In spite of its ability to block the effects of exogenous CCK-8 on food-motivated behavior in mice, the CCK-A receptor antagonist did not block the depressive actions of LPS and IL-1 beta on food-motivated behavior. The CCK-B receptor antagonist was not more effective at blocking. These results do not support a role for CCK in the anorexic effect of LPS and IL-1 beta.

Cytokines and the anorexia of infection: potential mechanisms and treatments

Anorexia during infection is thought to be mediated by immunoregulatory cytokines such as interleukins 1 and 6 and tumor necrosis factor. This article reviews the potential mechanisms of action by which these cytokines are thought to suppress food intake during infection and examines the proposition that blocking of cytokine activity might be one approach to improving food intake of the infected host.

Interleukin-6 increases sensitivity to the locomotor-stimulating effects of amphetamine in rats

Interleukin (IL)-6 mediates brain-immune interactions, influences the survival of postnatal mesencephalic and basal forebrain cells, influences mesocorticolimbic dopamine and serotonin neurotransmission, and is linked with various central nervous system disorders. In the present study, single injections of IL-6 (1 or 2 microg/Long-Evans rat, i.p.) induced modest elevations of locomotor activity. The locomotor increases were not augmented by repeated intermittent injections of IL-6 (five daily injections; 1 microg/rat), however. Nonetheless, repeated IL-6 treatment increased sensitivity to the locomotor-stimulating effects of 1.0 and 0.5 mg/kg amphetamine, when tested 5, 7, or 14 days following interruption of the cytokine treatment. The ability of acute IL-6 injections to alter locomotor activity and the ability of repeated IL-6 injections to produce long-lasting sensitization to the locomotor-stimulating effects of amphetamine suggest an interaction of this cytokine with the mesolimbic dopamine system, a system implicated in aspects of schizophrenia, addiction, and movement disorders. The fact that IL-6 caused a lasting change in responsiveness to amphetamine implies a mechanism by which immunogenic stimuli can alter brain circuitry, changing its sensitivity to seemingly unrelated subsequent stimuli or events.

Multiple functions of polypeptides mediated by distinct domains interacting with different receptors

There exist multiple functions of polypeptide molecules. Both a polypeptide molecule interacting with one receptor, and distinct domains of the molecule interacting with different receptors could induce different intracellular signal transduction to elicit multiple functions. This review highlights the distinct domains of the polypeptide molecule interacting with different receptors to elicit multiple functions. It includes distinct domains, different receptor mechanisms, and different signal transduction of the polypeptide molecule.

Acute-phase responses in transgenic mice with CNS overexpression of IL-1 receptor antagonist

The interleukin-1 (IL-1) receptor antagonist (IL-1ra) is an endogenous antagonist that blocks the effects of the proinflammatory cytokines IL-1alpha and IL-1beta by occupying the type I IL-1 receptor. Here we describe transgenic mice with astrocyte-directed overexpression of the human secreted IL-1ra (hsIL-1ra) under the control of the murine glial fibrillary acidic protein (GFAP) promoter. Two GFAP-hsIL-1ra strains have been generated and characterized further: GILRA2 and GILRA4. These strains show a brain-specific expression of the hsIL-1ra at the mRNA and protein levels. The hsIL-1ra protein was approximated to approximately 50 ng/brain in cytosolic fractions of whole brain homogenates, with no differences between male and female mice or between the two strains. Furthermore, the protein is secreted, inasmuch as the concentration of hsIL-1ra in the cerebrospinal fluid was 13 (GILRA2) to 28 (GILRA4) times higher in the transgenic mice than in the control animals. To characterize the transgenic phenotype, GILRA mice and nontransgenic controls were injected with recombinant human IL-1beta (central injection) or lipopolysaccharide (LPS, peripheral injection). The febrile response elicited by IL-1beta (50 ng/mouse icv) was abolished in hsIL-1ra-overexpressing animals, suggesting that the central IL-1 receptors were occupied by antagonist. The peripheral LPS injection (25 micrograms/kg ip) triggered a fever in overexpressing and control animals. Moreover, no differences were found in LPS-induced (100 and 1,000 micrograms/kg ip; 1 and 6 h after injection) IL-1beta and IL-6 serum levels between GILRA and wild-type mice. On the basis of these results, we suggest that binding of central IL-1 to central IL-1 receptors is not important in LPS-induced fever or LPS-induced IL-1beta and IL-6 plasma levels.

Augmented interleukin-1beta-induced depression of locomotor activity in cholestatic rats

"Sickness behaviors" such as lethargy, fatigue, and malaise occur commonly in patients with cholestatic liver diseases and contribute significantly to the morbidity associated with these diseases. However, the cause of these symptoms is unknown. Interleukin-1beta (IL-1beta) released within the brain has been implicated in the genesis of a number of "sickness behaviors," including malaise and lethargy. Therefore, we investigated whether experimental cholestatic liver disease caused by bile duct resection (BDR) in rats is associated with enhanced central sensitivity to IL-1beta-induced "sickness behaviors." The central infusion of IL-1beta at a dose that produced an insignificant decrease in locomotor activity in control rats produced a striking reduction in locomotor activity in cholestatic rats. The anorectic response to central IL-1beta infusion was similar in cholestatic and noncholestatic animals and did not parallel our locomotor activity findings. Therefore, cholestatic liver injury is characterized by augmented central responsiveness to IL-1beta with respect to a decrease in locomotor activity. These findings may explain, at least in part, the high incidence of symptoms such as fatigue, malaise, and lethargy that occur in cholestatic patients and may open novel future avenues for their treatment.

Interleukin-2 and -6 induce behavioral-activating effects in mice

Interleukin (IL)-1, IL-2 and IL-6 influence central monoamine activity in a cytokine-specific manner. We demonstrated that whereas IL-2 increased hypothalamic and hippocampal norepinephrine (NE) utilization, and DA turnover in the prefrontal cortex, IL-6 induced profound elevations of serotonin (5-HT) and mesocortical dopamine (DA) activity in the hippocampus and prefrontal cortex [S. Zalcman, J.M. Green-Johnson, L. Murray, D.M. Nance, D.G. Dyck, H. Anisman, A. H. Greenberg, Cytokine-specific central monoamine alterations following IL-1, -2 and -6 administration, Brain Res. 643 (1994) 40-49]. IL-1, in contrast, induced a wide range of central monoamine alterations. We presently report that these cytokines also differentially influence behavior. Profound reductions in non-ambulatory and ambulatory exploration were induced in BALB/c mice following IL-1 administration. In contrast, IL-2-treated mice displayed significant increases in the time spent engaged in non-ambulatory exploration, digging, rearing (particularly the number of free rears), and in the investigation of a novel stimulus (i.e., increased number and duration of stimulus contacts). IL-6-treated mice, moreover, exhibited significant increases in the time spent engaged in ambulatory exploration, digging and rearing (particularly the number of free rears, which tended to be of short duration). Modest increases in locomotion and grooming were also observed in IL-6-treated animals. Plasma corticosterone levels did not vary significantly as a function of IL-6 treatment. Hence, cytokine-specific behavioral-activating effects were induced following administration of IL-2 and IL-6. We suggest that these effects have adaptive significance and relevance to sickness behavior; however, pathological outcomes (e.g., schizophrenia, anxious-like states, anxious depression, motor abnormalities) could develop should these cytokines be overproduced or dysregulated.

Molecular basis of sickness behavior

Peripheral and central injections of lipopolysaccharide (LPS), a cytokine inducer, and recombinant proinflammatory cytokines such as interleukin-1 beta (IL-1 beta) induce sickness behavior in the form of reduced food intake and decreased social activities. Mechanisms of the behavioral effects of cytokines have been the subject of much investigation during the last 3 years. At the behavioral level, the profound depressing effects of cytokines on behavior are the expression of a highly organized motivational state. At the molecular level, sickness behavior is mediated by an inducible brain cytokine compartment that is activated by peripheral cytokines via neural afferent pathways. Centrally produced cytokines act on brain cytokine receptors that are similar to those characterized on peripheral immune and nonimmune cells, as demonstrated by pharmacologic experiments using cytokine receptor antagonists, neutralizing antibodies to specific subtypes of cytokine receptors, and gene targeting techniques. Evidence exists that different components of sickness behavior are mediated by different cytokines and that the relative importance of these cytokines is not the same in the peripheral and central cytokine compartments.

Immune and endocrine regulation of food intake in sick animals

To understand why sick animals do not eat, investigators have studied how the immune system interacts with the central nervous system (CNS), where motivation to eat is ultimately controlled. The focus has been on the cytokines secreted by activated mononuclear myeloid cells, which include interleukin-1 beta (IL-1 beta), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF-alpha). Either central or peripheral injection of recombinant IL-1 beta, IL-6, and TNF-alpha reduce food-motivated behavior and food intake in rodents. Moreover, these cytokines and their receptors are present in the endocrine system and brain, and antagonism of this system (i.e., the cytokine network) has been shown to block or abrogate anorexia induced by inflammatory stimuli. Recent studies indicate that the same cytokines act on adipocytes and induce secretion of leptin, a protein whose activity has been neuroanatomically mapped to brain areas involved in regulating food intake and energy expenditure. Therefore, many findings converge to suggest that the reduction of food intake in sick animals is mediated by inflammatory cytokines, which convey a message from the immune system to the endocrine system and CNS. The nature of this interaction is the focus of this short review.

Chronic cholestasis in rats induces anhedonia and a loss of social interest

Central fatigue commonly occurs in patients with primary biliary cirrhosis (PBC) and correlates closely with depression, and cholestatic rats exhibit central fatigue. Therefore, we undertook a series of experiments in both rats with cholestasis caused by bile duct resection (BDR) and sham-resected controls (15 days after surgery) to determine if experimental cholestasis is associated with symptoms of depression that can be modeled in rats, namely anhedonia (loss of pleasure) and the loss of social interest. BDR rats exhibited significant anhedonia compared with sham controls as indicated by a loss in their preference for consuming a saccharin solution, a highly desirable drink for rats. Furthermore, social interest was examined by determining the time BDR or sham rats spent investigating a juvenile rat in an open-field apparatus compared with the time spent on nonsocial behaviors. BDR rats exhibited significantly reduced time spent in social investigation and significantly more time in nonsocial behaviors than did sham rats. Major depression in humans is often associated with elevated circulating glucocorticoid levels and impaired glucocorticoid feedback. Therefore, we measured these parameters in BDR and sham rats and found a striking elevation in circulating glucocorticoid levels in BDR compared with sham animals. However, elevated circulating glucocorticoid levels in BDR rats suppressed normally in response to exogenous dexamethasone, indicating intact glucocorticoid feedback control at the pituitary level in BDR rats. In summary, we have identified behaviors in cholestatic rats that are consistent with those seen in depression.

Interleukin-1beta-converting enzyme-deficient mice resist central but not systemic endotoxin-induced anorexia

Interleukin-1beta (IL-1beta) mediates many of the behavioral responses to infection and inflammation, and IL-1beta-converting enzyme (ICE) processes intracellular IL-1beta, leading to its maturation and secretion. Here we demonstrate that intracerebroventricular injections of lipopolysaccharide (LPS) produced a greater reduction in both food intake and food-motivated behavior in wild-type compared with ICE-deficient (ICE -/-) mice. This defect occurred although ICE -/- mice were able to fully respond to intracerebroventricular injections of IL-1beta. In contrast, ICE -/- mice remained fully responsive to intraperitoneal injections of LPS. These results indicate that brain, but not peripheral, IL-1beta plays a critical role in the depression in food intake that occurs during inflammation.

Production of digoxigenin-labelled RNA probes and the detection of cytokine mRNA in rat spleen and brain by in situ hybridization

Non-radioactive in situ hybridization is a sensitive method for determining the site of production for secretory molecules such as cytokines. We report here on the central and peripheral induction of proinflammatory cytokines by endotoxin, and outline procedures for the generation and application of rat-specific digoxigenin (Dig)-labelled RNA probes for the localization of mRNA by in situ hybridization. Rats were injected either intravenously (i.v.) or intracerebroventricularly (i.c.v.) with vehicle or lipopolysaccharide (LPS) and sacrificed at various time intervals post-injection. Rats were then perfused with 4% paraformaldehyde and the spleens and brains were removed and cryoprotected in 30% sucrose. Dig-labelled, rat-specific, antisense and sense RNA probes were generated by in vitro transcription from PCR-derived templates. Positive staining with all the antisense probes was cytoplasmic, whereas the sense probes showed no staining. Numerous tumor necrosis factor alpha (TNF-alpha) and interleukin-1 beta (IL-1beta) mRNA positive cells were observed in the marginal zone and in the red pulp of the spleen after iv LPS injections, whereas sections from saline-treated animals showed minimal cytokine mRNA expression. Cells positive for TNF-alpha and IL-1beta mRNA were detectable in the brain after i.c.v. injections of LPS, but not after icv injection of vehicle. An antisense probe for c-fos was utilized in these studies as a positive control for our procedure due to its anatomically specific expression in the rat brain after LPS. In conclusion we have demonstrated that in situ hybridization with Dig-labelled RNA probes is an efficient, sensitive and reliable tool to localize cytokine mRNA production in rat tissue.

An assessment of the effects of central interleukin-1beta, -2, -6, and tumor necrosis factor-alpha administration on some behavioural, neurochemical, endocrine and immune parameters in the rat

Despite a vast amount of research into the actions of cytokines within the central nervous system, the pharmacological role and/or physiological function of the various cytokines within the central nervous system is still not fully understood. The present study evaluated the effects of intracerebroventricular administration of interleukin-1beta, -2, -6 (20 ng) and tumour necrosis factor-alpha (40 ng) on elevated plus maze behaviour, monoamine levels in the hypothalamus, hippocampus and amygdala, plasma corticosterone and catecholamine concentrations and Concanavalin A-induced splenic lymphocyte proliferation in the rat. Both interleukin-1beta and tumour necrosis factor-alpha induced "anxiogenic-like" effects on the elevated plus maze, whereas interleukin-2 and interleukin-6 did not. However only interleukin-1beta led to endocrine variations often associated with stress and anxiety. Cytokine specific alterations in monoamine levels were evident in the hypothalamus and hippocampus, while neurotransmitter concentrations in the amygdala were not significantly altered by cytokine treatment. In addition, interleukin-1beta reduced Concanavalin A-induced lymphocyte proliferation, whereas the other cytokine treatments failed to significantly alter this response. These results demonstrate that in some, but not all, respects interleukin-1beta administration produced "stress like" effects on behaviour, monoamine neurotransmitters, hypothalamic pituitary adrenal axis activity and immune function, while the other cytokines produced less consistent effects on these parameters. It is noteworthy that although interleukin-1beta and tumour necrosis factor-alpha provoked an anxiogenic response in the elevated plus maze test of anxiety, neither cytokine significantly altered amygdaloid noradrenergic or serotonergic activity, as many previous studies have implicated increased amygdaloid noradrenergic and/or serotonergic activity in the pathophysiology of anxiety.

Cytokines and sickness behavior

Peripheral and central injections of interleukin-1 (IL-1) and lipopolysaccharide (LPS) induce the expression of proinflammatory cytokines in the brain and have profound depressing effects on spontaneous and learned behaviors. These effects are mediated by vagal afferents, because they are abrogated by section of the vagus nerves at the subdiaphragmatic level in rats and mice. Vagotomy does not interfere with the synthesis and release of proinflammatory cytokines at the periphery, because plasma and tissue levels of interleukin-1 of vagotomized animals are similar to those of sham-operated animals. Furthermore, the consequences of vagotomy on the host behavioral response to peripheral cytokines are specific to the intraperitoneal route of administration of cytokines because vagotomized animals are still able to respond to IL-1 injected intravenously, subcutaneously, and into the lateral ventricle of the brain. Finally, substance P and cholecystokinin do not appear to play a key role in the transmission of the immune message to the brain because pretreatment by capsaicin or by specific antagonists of CCKA and CCKB receptors does not alter the behavioral effects of LPS and IL-1. All these findings point to the role of neural afferents for transmitting the immune message from the periphery to the brain.

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.

Behavioral effects of peripheral interleukin-1 administration in adult CD-1 mice: specific inhibition of the offensive components of intermale agonistic behavior

Peripheral administration of interleukin-1beta (IL-1beta) in rodents reduces exploratory behavior in a novel environment while decreasing social investigation of a juvenile conspecific. In this study we wanted to test the effects of peripherally administered IL-1beta on another aspect of the mouse social repertoire, namely intraspecific fighting towards an adult male intruder. In the first experiment, sickness behavior induced by IL-1beta (1 microg/mouse) in adult CD-1 mice was assessed by direct observation of behavioral changes following placement into a novel environment. Three hours after injection, subjects were individually introduced for 20 min in a cage with clean sawdust and a number of behavioral items recorded. Blood samples were collected at the end of the testing session. Body temperature was measured right before, 1 h and 3.5 h following injection. In IL-1beta treated mice, exploration (assessed by measuring duration and frequency of Wall Rearing and Rearing behaviors) was nearly totally suppressed, while duration and frequency of behaviors such as Grooming, Bar Holding, and Digging were also markedly reduced. Administration of IL-1beta significantly elevated CORT secretion above basal levels and, as previously reported for mice, induced hypothermia (about 2 degrees C). In the second experiment, we assessed mice receiving IL-1beta (0.25; 0.5 or 1 microg/mouse or saline solution) in a social context. Three hours after injection, subjects were placed into a neutral cage for 20 min with a non-injected adult male conspecific and aggressive behavior scored. Overall, IL-1beta administration affected the social repertoire of treated mice in a dose-dependent fashion. Specifically, agonistic components of aggressive behavior were nearly totally suppressed, while the defensive elements, such as Upright Defensive posture, Upright Submissive posture, Crouching, or Flee were not affected by IL-1beta. Overall these data support the notion that sickness behavior induced by IL-1beta administration represents an organized behavioral strategy and is not an aspecific response to an illness-type of condition.

Role of central IL-1 in regulating peripheral IGF-I during endotoxemia and sepsis

Inflammatory cytokines may mediate the host response to infection via central nervous system, endocrine, and/or paracrine/autocrine signaling mechanisms. Previous studies have shown that intravenous administration of interleukin (IL)-1 beta alters the concentration of the anabolic hormone insulin-like growth factor (IGF)-I in plasma and various tissues. The purpose of the present study was to determine 1) whether the intracerebroventricular injection of IL-1 beta can influence peripheral IGF-I levels in control animals and 2) whether the central administration of a IL-1 receptor antagonist (IL-1ra) can prevent the changes in peripheral IGF-I induced by endotoxin [lipopolysaccharide (LPS)] or sepsis produced by cecal ligation and puncture. In the first experiment, injection of IL-1 beta (100 ng/rat) decreased IGF-I levels in plasma, liver, and gastrocnemius muscle 28-36% by 1.5 h in conscious fasted rats. IGF-I levels remained reduced at 3 h, but returned to baseline by 6 h. IGF-I content was not altered in soleus, kidney, spleen, intestine, or whole brain after IL-1 beta. In the second series of experiments, LPS injected intravenously decreased IGF-I levels in plasma, liver, and gastrocnemius at 1.5 h, and levels were even further reduced at 3 and 6 h in these tissues (59, 57, and 48%, respectively). Moreover, the IGF-I content was also decreased in soleus (30-35%) and increased in kidney (2- to 3-fold) after LPS. In the third experiment, changes in IGF-I levels in plasma and tissues, similar to those seen in LPS-treated rats, were detected 24 h after induction of peritonitis. Intracerebroventricular infusion of IL-1ra did not alter any of the changes in IGF-I produced by either LPS or sepsis, although it did attenuate the concomitant changes in growth hormone levels. These data suggest that, although central IL-1 beta is capable of modulating peripheral IGF-I levels, central administration of IL-1ra was unable to modulate the changes in peripheral IGF-I in blood and tissues produced by either endotoxemia or peritonitis.

Potential for the effects of anabolic steroid abuse in the immune and neuroendocrine axis

Some of the effects that high-dose anabolic steroid abuse have and could have on the interactions between the immune and neuroendocrine systems are reviewed. Considering the past demonstrations on the actions of normal steroids on endocrine and immune responses, it is apparent that pharmacologically high doses of both normal and derivatized androgens (anabolic steroids) could have a significant effect. Indeed, some of the pathologies attributed to anabolic steroid abuse point to disturbances in the intimate connection between neuroendocrine and immune function and interaction. We attempt to review both the direct and indirect effects of this abuse, not only on this interaction but also on certain immune functions in particular.

The proinflammatory cytokine network: interactions in the CNS and blood of rhesus monkeys

Proinflammatory cytokines [interleukin (IL)-1 and -6 and tumor necrosis factor-alpha] function within a complex network, stimulating the release of one another, as well as other cytokine agonists and antagonists. These interactions have not been as widely studied in vivo. Therefore, the following studies measured cytokines in blood and cerebrospinal fluid (CSF) from juvenile rhesus monkeys after intravenous administration of cytokines. IL-1 alpha and IL-1 beta were equally effective in elevating blood levels of IL-6. In contrast, IL-1 beta was the only cytokine that significantly elevated IL-6 levels in the CSF. Interestingly, both IL-1 and IL-6 increased levels of IL-1 receptor antagonist in the blood and comparably stimulated the release of cortisol. A second study confirmed that the IL-1-induced IL-6 in CSF was brain derived and not a result of diffusion from blood. This research extends studies of the cytokine cascade to the central nervous system (CNS), highlighting the brain response to peripheral activation.

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.

Different receptor mechanisms mediate the effects of endotoxin and interleukin-1 on female sexual behavior

Activation of the immune system by lipopolysaccharide (LPS) produces physiological, neuroendocrine and behavioral effects, some of which are mediated by cytokine production. We have previously shown that the cytokine interleukin-1 (IL-1) inhibits sexual behavior in female, but not male rats, while producing a comparable suppression of locomotion in both sexes. The present study examined the effects of LPS on sexual behavior and locomotion of male and female rats, and the involvement of IL-1 receptors in mediating the effects of IL-1 and LPS on females' behavior. Peripheral (i.p.) administration of LPS (50 or 250 microg/kg) significantly decreased sexual behavior in females, up to 6 h after administration, while it had no effect on male sexual behavior. However, locomotor activity, measured in the open-field test, was similarly reduced by LPS in both males and females. Pretreatment with the IL-1 receptor antagonist (IL-1ra) either i.p. (10 mg/kg) or intracerebroventricularly (i.c.v.) (50 microg/rat) did not prevent the inhibition of female sexual behavior and locomotion induced by either i.p. (50 microg/kg) or i.c.v. (200 or 400 ng/rat) administration of LPS, respectively. However, identical doses of IL-1ra significantly reversed the effects of IL-1beta, administered either i.p. (5 microg/kg) or i.c.v. (50 ng/rat), respectively. These results demonstrate that both LPS and IL-1beta produce marked inhibition of sexual behavior in female, but not in male rats. However, IL-1 receptors are not required for the effects of LPS on sexual behavior in female rats.

Involvement of interleukin-1 receptor mechanisms in development of arterial hypotension in rat heatstroke

Rats, under urethan anesthesia, were exposed to a high ambient temperature (42 degrees C) to induce heatstroke and to assess the hemodynamic changes associated with heatstroke. Compared with normothermic controls, rats with heatstroke showed higher values of colonic temperature, heart rate, and plasma levels of interleukin (IL)-1 but lower values of R wave amplitude, P-R and Q-T intervals, systolic wave amplitude, diastolic and dicrotic wave duration, mean arterial pressure, stroke volume, and cardiac output. Animals injected intravenously with an IL-1-receptor antagonist at the time of heatstroke induction were protected from some of the cardiovascular effects of heatstroke, such as depressed ventricular depolarization, decreased stroke volume, decreased cardiac output, and arterial hypotension. The hemodynamic changes associated with heatstroke could be mimicked by IL-1beta administration. Other cardiovascular parameters such as total peripheral vascular resistance were unaffected by heatstroke induction or IL-1beta treatment. The results indicate that a selective decline in stroke volume or ventricular depolarization resulting from increased plasma levels of IL-1 may be an important mechanism signaling arterial hypotension or circulatory failure in rat heatstroke.

Relative contributions of peripheral and central sources to levels of IL-1 alpha in the cerebral cortex of mice: assessment with species-specific enzyme immunoassays

The peripheral administration or release of cytokines is associated with central nervous system (CNS) effects that are often due to the actions of cytokines behind the blood-brain barrier (BBB). It is not known whether the majority of cytokine behind the BBB is derived from blood or is released from the CNS in response to peripheral signals. We addressed this question for interleukin-1 alpha (IL-1 alpha) by infusing human IL-1 alpha (humIL-1 alpha) into mice and measuring humIL-1 alpha and murine IL-1 alpha (murIL-1 alpha) in cerebral cortex and serum with specific, sensitive enzyme immunoassays. In adult mice receiving 50 micrograms/kg-24 h of humIL-1 alpha subcutaneously for 48 h, brain and blood samples contained humIL-1 alpha but no murIL-1 alpha. This shows that in our study blood-borne IL-1 alpha did not self-stimulate its release in blood or brain. The presence of humIL-1 alpha in brain could only have originated from blood, where it was administered; the brain/blood ratio of 0.126 ml/g indicates that at steady state, brain levels reach about 12% of blood levels. In neonatal mice, both murIL-1 alpha and humIL-1 alpha were detected in brain and blood after the acute subcutaneous injection of humIL-1 alpha. However, the vast majority of immunoactivity in blood and brain was humIL-1 alpha. These results show that most of the IL-1 alpha appearing in response to circulating IL-1 alpha in areas of the CNS behind the BBB is due to passage across the BBB and not to release from stores endogenous to the CNS.

Central interleukin-1 receptors as mediators of sickness

These data establish that cytokines, such as IL-1, can act on specific receptors within the brain to induce many symptoms of sickness. A number of inflammatory stimuli in the periphery can activate both the transcription and translation of IL-1 within the central nervous system. It will now be important to determine if similar central IL-1 pathways are activated during SLE and whether these central inflammatory cytokines are involved in the neurologic complications that often accompany this disease.

Defect in interleukin-1beta secretion prevents sickness behavior in C3H/HeJ mice

To examine the role of interleukin-1beta (IL-1beta) in mediating sickness, we studied the effects of lipopolysaccharide (LPS) and IL-1beta on social behavior in endotoxin-responsive C3H/HeOuJ (OuJ) mice and endotoxin-resistant C3H/HeJ (HeJ) mice. Whereas LPS (1, 10 and 100 microg) depressed social behavior and body weight compared to saline in OuJ mice, in HeJ mice it did not. To determine if the refractoriness of HeJ mice to the behavioral effects of LPS was related to secretion of IL-1beta, in a second study, HeJ and OuJ mice were injected IP with LPS (10 microg) and plasma concentration of IL-1beta was determined postinjection. At 4 h postinjection, the plasma concentration of IL-1beta was increased by LPS in OuJ mice, but not in HeJ mice. The increase in plasma IL-1beta in OuJ mice corresponded to the maximal depression in social behavior. To further verify that HeJ mice are refractory to the behavioral effects of LPS because they fail to respond and produce cytokines, the social behavior of HeJ and OuJ mice injected IP with recombinant murine IL-1beta (0, 50, 100, or 200 ng) was compared. As anticipated, exogenous IL-1beta depressed social behavior similarly in endotoxin-responsive OuJ mice and endotoxin-resistant HeJ mice. These data indicate that a genetic mutation in HeJ mice that prevents LPS-induced synthesis of cytokines also renders HeJ mice refractory to the behavioral effects of LPS.

Immunologically induced electrophysiological dysfunction: implications for inflammatory diseases of the CNS and PNS

During inflammation of the central or peripheral nervous system, a high number of immunologically active molecules, including bacterial or viral products as well as host-derived cytokines, are released. Patients suffering from inflammatory CNS or PNS diseases often develop transient symptoms with a rapid recovery, which obviously cannot be accounted for by immunologically induced tissue damage. These observations led to the hypothesis that immunologically active molecules can affect directly the electrophysiological functions of neurons and glial cells. Evidence for this hypothesis came from in vitro studies showing that cytokines, such as interleukins or tumor necrosis factors, arachidonic acid and its metabolites, interfere with electrophysiological properties of neurons or glial cells. These molecules affect ion currents, intracellular Ca2+ homeostasis, membrane potentials, and suppress or enhance the induction and maintenance of long-term potentiation. Similarly, virus proteins from human immunodeficiency virus type I were found to alter intracellular Ca2+ concentrations of neurons and astrocytes by modulating either transmitter receptors and channels or membrane transporters. Cerebrospinal fluid from MS patients contains factors which increase Na+ current inactivation and thereby reduce neuronal excitability. Immunoglobulins in sera of patients suffering from multifocal motor neuropathy and from acquired neuromyotonia interfere with nerve fibers, inducing alterations of conduction. Increased knowledge of these mechanisms will help to explain the pathogenesis of neurological symptoms and may provide a rationale for new therapeutic strategies.

Hormones, lymphohemopoietic cytokines and the neuroimmune axis

The classical distinction between hormones and cytokines has become increasingly obscure with the realization that homeostatic responses to infection involve coordinated changes in both the neuroendocrine and immune systems. The hypothesis that these systems communicate with one another is supported by the ever-accruing demonstrations of a shared molecular network of ligands and receptors. For instance, leukocytes express receptors for hormones and these receptors modulate diverse biological activities such as the growth, differentiation and effector functions. Leukocyte lineages also synthesize and secrete hormones, such as insulin-like growth factor-I (IGF-I), in response to both growth hormone (GH) and also to cytokines such as tumor necrosis factor-alpha (TNF-alpha). Since hormones share intracellular signaling substrates and biological activities with classical lymphohemopoietic cytokines, neuroendocrine and immune tissues share a common molecular language. The physiological significance of this shared molecular framework is that these homeostatic systems can intercommunicate. One important example of this interaction is the mechanism by which bacterial lipopolysaccharide, by eliciting a pro-inflammatory cytokine cascade from activated leukocytes, modulate pituitary GH secretion as well as other CNS-controlled behavioral and metabolic events. This article reviews the cellular and molecular basis for this communication system and proposes novel mechanisms by which neuroendocrine-immune interactions converge to modulate disease resistance, metabolism and growth.

Interleukin (IL) 1beta, IL-1 receptor antagonist, IL-10, and IL-13 gene expression in the central nervous system and anterior pituitary during systemic inflammation: pathophysiological implications

The pathophysiology of systemic inflammation and sepsis involves peripheral organs, causing gastrointestinal, renal, and cardiovascular alterations, as well as the central nervous system (CNS), affecting sleep, temperature regulation, behavior, and neuroendocrine function. The molecular basis of the CNS effects of systemic inflammation are not fully elucidated. Here we show that the CNS responds to systemic inflammation with pronounced IL-1beta gene expression and limited IL-1 receptor antagonist (IL-1ra), IL-10, and IL-13 gene expression. This pattern occurs throughout the CNS, including areas such as the subfornical organ, pineal gland, neurohypophysis, and hypothalamus. In contrast, in the anterior pituitary, we found limited IL-1beta gene expression but marked induction of the mRNA encoding for the secreted isoform of IL-1ra, secreted IL-1ra. We conclude that the central manifestations of peripheral inflammation are mediated by endogenous brain IL-1beta synthesized during systemic inflammation in the context of limited central cytokine counter regulation of IL-1. As IL-1beta is a potent stimulus for inducible nitric oxide synthase expression and activity, these findings explain our previous observation that systemic inflammation promotes inducible nitric oxide synthase gene expression in the brain and the spillover of NO metabolites into cerebrospinal fluid. The CNS transcription of the HIV-1 replication factor IL-1beta in the context of limited transcription of the IL-1 replication inhibitors IL-1ra, IL-10, and IL-13 might help explain the negative impact of systemic inflammation on the clinical course of AIDS. In addition, we propose that IL-1ra may be secreted by the anterior pituitary as a systemic anti-inflammatory hormone that is released in response to IL-1beta originated from multiple sources.

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.

Adenovirus-mediated over-expression of interleukin-1 receptor antagonist reduces susceptibility to excitotoxic brain injury in perinatal rats

In seven-day-old rats, intracerebral injection of N-methyl-D-aspartate transiently stimulates expression of Interleukin-1 beta messenger RNA. To evaluate the role of Interleukin-1 beta in the pathogenesis of excitotoxic injury, we sought to determine if Interleukin-1 receptor antagonist, an endogenous competitive inhibitor of Interleukin-1 beta, could attenuate N-methyl-D-aspartate-induced injury. To induce sustained over-expression of Interleukin-1 receptor antagonist in the brain, a recombinant adenovirus encoding Interleukin-1 receptor antagonist was administered by intracerebroventricular injection into three-day-old rats. Increased brain concentrations of Interleukin-1 receptor antagonist two to six days later were documented by assays of tissue homogenates and by immunocytochemistry. To evaluate the impact of Interleukin-1 receptor antagonist on N-methyl-D-aspartate neurotoxicity, three-day-old animals received intracerebroventricular injections of either adenovirus encoding Interleukin-1 receptor antagonist or a control adenovirus encoding beta-galactosidase, followed four days later by right intrastriatal injections of N-methyl-D-aspartate (10 nmol/0.5 microliter), a dose that typically elicits excitotoxic injury in the ipsilateral striatum and adjacent hippocampus, or saline. Animals were killed five days later, and brain damage was quantitated by measurement of bilateral cross-sectional areas of the striatum and anterior hippocampus. In three independent experiments, in N-methyl-D-aspartate-lesioned animals, both striatal and hippocampal injuries were reduced in animals that had been infected with adenovirus that encoded Interleukin-1 receptor antagonist, in comparison with littermates infected with the control adenovirus (right striatal volume loss ranged from 16 to 24%, compared with 54-65% volume loss in control). There was no striatal atrophy in adenovirus-infected saline-injected animals. These results provide strong support for the hypothesis that Interleukin-1 beta is a mediator of excitotoxic brain injury in perinatal rats.

Biological responses to administered interferon in alcoholics

To elucidate the influence of chronic alcohol intake on interferon (IFN) therapy in patients with various diseases, 3 million units of natural IFN (nHuIFN-alpha Ly) was administered once to alcoholics, nonalcoholic chronic hepatitis C (CH-C) patients, and healthy subjects. The serial changes of serum activity of 2'-5' oligoadenylate synthetase, serum levels of beta 2-microglobulin, C-reactive protein (CRP), and body temperature were monitored. The increase in the activity of 2'-5' oligoadenylate synthetase, the response of serum-reactive protein, and body temperature after IFN administration were significantly lower in the alcoholics than those in CH-C group, and healthy subjects (control group). The serum beta 2-microglobulin level was already higher in the alcoholics before IFN administration than those of the control group and the CH-C group. These findings suggest that physiological responses to exogenous IFN in alcoholics are generally decreased. Hence, for the maximum effect of IFN therapy, it is important for patients to abstain from alcohol intake and for doctors to carefully consider the timing of initiation.

Endothelial cells of the rat brain vasculature express cyclooxygenase-2 mRNA in response to systemic interleukin-1 beta: a possible site of prostaglandin synthesis responsible for fever

We previously showed that intraperitoneal injection of lipopolysaccharide induced cyclooxygenase-2 (COX-2) mRNA in as yet unidentified cells of blood vessels and leptomeninges in the rat brain and proposed a possible role of these cells as the source of prostaglandin E2 in the genesis of fever (Cao et al., Brain Res., 697 (1995) 187-196). In the present study, to proceed further with this line of research, we addressed the following two questions: first, does a pyrogenic dose of interleukin-1 beta (IL-1 beta), an endogenous pyrogen, induce COX-2 mRNA in the brain blood vessels and leptomeninges? Secondly, if it does, what type of cells are positive for COX-2 mRNA? Intraperitoneal injection of recombinant human IL-1 beta (30 micrograms/kg) induced fever in rats and an in situ hybridization study revealed that faint but significant COX-2 mRNA signals appeared in the blood vessels and leptomeninges at 1.5 h after the injection (the early rising phase of fever). The mRNA signals increased in number and intensity at 4 h (early plateau phase), decreased at 6.5 h (early recovery phase), and completely disappeared by 10 h after the injection (late recovery phase). The COX-2 mRNA positive cells in the blood vessels were likely to be the endothelial cells since the corresponding cells in the adjacent mirror-imaged section also expressed mRNAs for intracellular adhesion molecule-1 and the type-I interleukin-1 receptor, although those in the leptomeninges still remained unidentified. These results imply that circulating IL-1 beta acts on its receptor on the endothelial cells of the brain vasculature to induce COX-2 mRNA, which is possibly responsible for the elevated level of PGE2 seen during fever.

The role of TNF-alpha in fever: opposing actions of human and murine TNF-alpha and interactions with IL-beta in the rat

1. The role of tumour necrosis factor-alpha (TNF-alpha) in fever is controversial. Some studies have indicated that TNF-alpha acts as a cryogen to inhibit fever, while others suggest that TNF-alpha is an endogenous pyrogen which mediates fever. The majority of studies in experimental animals supporting a cryogenic action have been conducted using human (h)TNF-alpha, which has been shown to bind only to one (p55) of the two TNF-alpha receptors in rodents. 2. The aim of the present investigation was to study the role of TNF-alpha in fever by comparing effects of hTNF-alpha, which binds only to the p55 receptor, with those of murine (m) TNF-alpha, which binds to both p55 and p75 TNF-alpha receptors, and to investigate the relationship between TNF-alpha and interleukin-1 (IL-1), an important endogenous pyrogen. 3. Injection of hTNF-alpha (0.3-10 micrograms kg-1, i.p.) had no effect on core temperature in conscious rats (measured by remote radiotelemetry), whereas mTNF-alpha (3 micrograms kg-1) induced fever which was maximal 1 h after the injection (38.2 +/- 0.2 degrees C compared to 37.3 +/- 0.1 degrees C in controls). Intracerebroventricular (i.c.v.) administration of either form of TNF-alpha elicited dose-dependent fever at doses higher than 0.12 microgram kg-1. 4. Peripheral injection of hIL-1 beta (1 microgram kg-1) resulted in fever (38.3 +/- 0.2 degree C compared to 37.2 +/- 0.1 degrees C in controls at 2 h), which was significantly attenuated (P < 0.01) by co-administration of a sub-pyrogenic dose of hTNF-alpha (1 microgram kg-1), but was unaffected by co-administration of mTNF-alpha (0.1 or 0.3 microgram kg-1, i.p.). In contrast, intracerebroventricular (i.c.v.) co-administration of a sub-pyrogenic dose (0.12 microgram kg-1) of hTNF-alpha did not attenuate fever induced by intraperitoneal (i.p.) injection of IL-1 beta, and sub-pyrogenic dose (0.12 microgram kg-1, i.c.v.) of mTNF-alpha significantly prolonged the febrile response to IL-1 beta. Pretreatment of animals with anti-TNF-alpha antiserum (i.c.v.) did not affect the febrile response to systemic IL-1 beta. 5. Animals injected i.p. with a pyrogenic dose of mTNF-alpha developed fever (38.2 +/- 0.2 degrees C compared to 37.3 +/- 0.1 degrees C in controls 2 h after the injection) that was completely abolished by peripheral administration of IL-1ra (2 mg kg-1, P < 0.001), while i.c.v. administration of IL-1ra (400 micrograms/rat) did not affect mTNF-alpha-induced fever. 6. These data indicate that endogenous TNF-alpha is probably a pyrogen and that previous results suggesting cryogenic actions of TNF-alpha resulted from the use of a heterologous protein in the rat. The markedly contrasting effects of mTNF-alpha and TNF-alpha could result from different interactions with the two TNF-alpha receptor subtypes. The data also suggest that fever induced by exogenous TNF-alpha is mediated via release of IL-1 beta in peripheral tissues, but not in the brain.

Central interleukin-1 partially mediates endotoxin-induced changes in glucose metabolism

The purpose of the present study was to determine whether intracerebral interleukin (IL)-1 mediates the endotoxin [lipopolysaccharide (LPS)]-induced increase in glucose flux. To accomplish this goal, a specific receptor antagonist for IL-1 (IL-1ra) or artificial cerebrospinal fluid was infused into the lateral ventricle via an intracerebroventricular cannula before, and for 4 h after, the intravenous injection of LPS. Whole body glucose flux was measured in conscious unrestrained rats using [3-3H]glucose. LPS increased both the plasma glucose concentration and the rate of glucose production (95 and 80%, respectively). In contrast, intracerebroventricular infusion of IL-1ra (2 mg/kg + 2 mg-kg-1.h-1) attenuated by approximately 50% the LPS-induced changes in glucose metabolism. IL-1ra also blunted the increase in plasma catecholamines, but not the elevation in glucagon and corticosterone concentrations, observed after LPS. Intracerebroventricular infusion of IL-1ra greatly reduced the LPS-induced hyperlactacidemia but did not alter the increase in muscle pyruvate dehydrogenase activity. An intravenous infusion of a 10-fold greater dose of IL-1ra, however, did not antagonize the LPS-induced increase in glucose flux. These data indicate that a major portion of the stimulation of glucose flux, as well as the increase in plasma catecholamines in response to LPS, is mediated by IL-1 within the central nervous system.