Stephen Kent

Sickness behavior as a new target for drug development

Sickness behavior refers to the nonspecific symptoms (anorexia, depressed activity, loss of interest in usual activities, disappearance of body-care activities) that accompany the response to infection. Increasing evidence suggests that these symptoms are part of an organized defense response to antigenic challenge and that they are mediated by the neural effects of cytokines such as interleukin 1. An understanding of the mechanisms involved in these effects should permit development of new drugs aimed at decreasing sickness or promoting recovery processes.

Mechanisms of sickness-induced decreases in food-motivated behavior.

Interleukin-1 beta (IL-1 beta) is a cytokine released by activated macrophages and monocytes, which mediates many of the local and systemic responses to inflammation. Interleukin-1 beta induces anorexia in rats when administered peripherally or centrally. An endogenous antagonist for the IL-1 type I receptor has been characterized and cloned (IL-1ra). We have used this protein to ascertain the site of action for the anorexic effects of IL-1 beta. Male rats were food restricted and trained on an operant schedule for food reinforcement. Administration of recombinant human IL-1 beta (4 micrograms i.p. or 40 ng i.c.v.) induced profound decreases in operant responding, with maximal effects 1-4 h post-injection. Interleukin-1ra pretreatment (2.4 mg i.p. or 24 micrograms i.c.v.) completely blocked these effects when administered by the same route. In contrast, i.c.v. Il-1ra only partially blocked the effects of i.p. IL-1 beta, and i.p. IL-1ra was unable to block the effects of i.c.v. IL-1 beta. Interleukin-1ra did not affect responding by itself. These results suggest that IL-1 beta acts as both peripheral and central IL-1 receptors to reduce food motivated behavior. To determine the central site of action of IL-1 beta, small quantities of IL-1 beta (5 and 30 ng) were infused into the ventromedial hypothalamus of male rats. Both doses produced profound decreases in responding; the magnitude and time course of these effects were nearly identical to those observed after i.c.v. administration. These results suggest that the VMH may serve as a central site of action for the depressive effects of IL-1 beta on food intake. There is much controversy over the pathways of communication from the immune system to the brain. To test the hypothesis that the peripheral immune stimulus is transmitted to the brain via a neutral communication pathway, mice were injected with lipopolysaccharide at a behaviorally active dose (10 micrograms i.p.). This treatment increased the concentrations of substance P, neurokinin A, and calcitonin gene-related peptide in mouse spinal cord in a prostaglandin-dependent manner. Maximal increases in neuropeptide content were observed 1 h post-injection. Finally, subdiaphragmatic vagotomy was found to attenuate the reduction in food-motivated behavior induced by both IL-1 beta and lipopolysaccharide in mice.

Reduction in food and water intake induced by microinjection of interleukin-1β in the ventromedial hypothalamus of the rat

Interleukin-1 (IL-1) is a cytokine which is released during immune activation and mediates some of the hosts responses to infection and inflammation. Increasing evidence suggests that it also has a role as an intrinsic neuromodulator in the central nervous system. We report here that microinjections of 5 and 30 ng (286 fmol and 1.71 pmol) of recombinant human IL-1 beta in the ventromedial nucleus of the hypothalamus (VMH) of adult male rats time- and dose-dependently induce anorexia and weight loss in two experimental paradigms: rats allowed free-access to food and water and food-restricted rats trained to press a lever for food on a fixed ratio 10 schedule. IL-1 beta (5 ng) diminished food and water consumption by 45 and 30%, respectively, and decreased body weight for at least 24 h postinjection in rats fed ad lib. These effects were more severe and lasted at least 48 h after infusion of the larger dose of 30 ng. The IL-1 beta-induced anorexia and weight loss were neither as large nor as long-lasting in food-restricted rats. Operant responding for food was decreased 2-4 h postinfusion of 5 ng IL-1 beta and 2-8 h after 30 ng IL-1 beta, but in both cases returned to baseline within 24 h. Body weight was decreased compared to saline injections from 4 to 24 h postinfusion. Nevertheless, when allowed to eat ad lib for the 24 h immediately following the behavioral testing, body weight returned to control (5 ng) or near control levels (30 ng).(ABSTRACT TRUNCATED AT 250 WORDS)

Behavioral Effects of Cytokines: An Insight into Mechanisms of Sickness Behavior

Publisher Summary This chapter provides an overview of behavioral effects of cytokines. Pyrogenic cytokines are somnogenic and induce lethargia, behavioral depression, and anorexia. Although interleukin-1 (IL-1) and tumor necrosis factor (TNF) share many biological properties, they do not necessarily induce the same experience of sickness. Proinflammatory cytokines decrease general activity. In general, peripheral and central injections of cytokines induce the same range of behavioral effects. Chronic administration of rhIL-1β via a subcutaneously implanted osmotic mini pump results in the development of tolerance to the anorexic and weight-depressing effects of IL-1 within a few days. Cytokines have the ability to induce the synthesis and release of other cytokines and themselves in a cascade-dependent manner. This cascade serves to amplify the local effects of cytokines into a global effect due to multiplicity of targets and the overlapping biological effects of cytokines. IL-1 and other proinflammatory cytokines can significantly interfere with spontaneous or acquired behavior by their action on several peripheral cellular targets.

Effects of lipopolysaccharide on food-motivated behavior in the rat are not blocked by an interleukin-1 receptor antagonist

To investigate the role of interleukin-1 (IL-1) in the decrease in food-motivated behavior after peripheral administration of lipopolysaccharide (LPS), rats trained to press a lever for food on a fixed ratio 10 schedule were pre-treated with a recombinant human IL-1 receptor antagonist (IL-1ra). This endogenous cytokine has been shown to block most of the inflammatory and immune effects of IL-1 both in vitro and in vivo. Intraperitoneal (i.p.) injection of LPS (400 micrograms/kg) decreased operant responding for food to 30-60% of baseline for 1-4 h. Response rates gradually recovered, but were still below control levels 8 and 24 h post-injection. Neither i.p. (8 mg/kg) nor intracerebroventricular (288 micrograms/kg) administration of IL-1ra blocked the effects of peripherally administered LPS on food-motivated behavior. These results suggest that the effects of LPS on this behavior are not mediated by the release of IL-1.

Cytokine Actions on Behavior

Sickness behavior refers to a coordinated set of behavioral changes that develop in sick individuals during the course of an infection. These changes are due to the effects of proinflammatory cytokines such as interleukin-1 (IL-1) and tumor necrosis factor alpha (TNF-α) on brain cellular targets and represent the expression of a well organized central motivational state. Based on the results of pharmacological and biochemical experiments, it is now apparent that sickness behavior is mediated by cytokines which are temporarily expressed in the brain in response to peripheral cytokines. Centrally released cytokines act on brain receptors which are identical to those characterized on immune cells. Primary afferent nerves represent the main communication pathway between peripheral and central cytokines. The sickness inducing effects of cytokines are downregulated by a number of endogenous neuropeptides and hormones, including vasopressin and glucocorticoids.

Systemic Capsaicin Pretreatment Fails to Block the Decrease in Food-Motivated Behavior Induced by Lipopolysaccharide and Interleukin-1β

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.

EFFECTS OF EXCITATORY AMINO ACIDS ON THE HYPOTHALAMIC-PITUITARY-ADRENAL AXIS OF THE NEONATAL RAT

Most stimuli that elicit a response by the hypothalamic-pituitary-adrenal (HPA) axis of adult rats fail to do so in infant rats aged 4-14 postnatal days (pnd). This interval is termed the stress hyporesponsive period (SHRP). The present study examined the development of the HPA response to the excitatory amino acids (EAAs), N-methyl-D-aspartic acid (NMDA) and kainic acid (KA), at 3 ages (i.e., pnd 6, 12, 18) during or immediately after the SHRP. Results indicate that intraperitoneal (i.p.) administration of 2.5 mg/kg KA or 5 mg/kg NMDA is capable of inducing age- and time-dependent elevations of ACTH and CORT, with KA being the more potent of the two EAAs. In contrast to other stimuli which are capable of eliciting an HPA response during the SHRP, NMDA and KA appear to possess more potent effects at earlier ages. Administration of lower doses of these EAAs did not elicit an HPA response. Pretreatment with 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 0.69 mg/kg i.p.), a KA receptor-specific antagonist, attenuated the effects of KA. These results suggest that KA exerts its effects via the KA receptor and that this receptor appears to be mature at both pnd 12 and 18. In contrast, pretreatment with D,L-2-amino-5 phosphonovaleric acid (APV; 7.5 mg/kg i.p.), an NMDA receptor-specific antagonist, was only effective at pnd 18 suggesting that the NMDA receptor is not yet mature at pnd 12. Finally, EAAs induce age- and time-dependent behavioral modifications (i.e., hindpaw scratching and hyperlocomotion). These effects, however, appear to only contribute to, but not cause, the endocrine responses.

Effect of intracerebroventricular administration of vasopressin on stress-induced hyperthermia in rats.

Vasopressin has been reported to be an endogenous antipyretic peptide. The present study assessed whether this peptide has similar effects on stress-induced hyperthermia. Infusion of 3 ng of vasopressin into the lateral ventricle prior to a 40-min restraint stress reduced significantly the hyperthermic response of rats to this stress, compared to saline-injected controls. Half of the vasopressin-injected animals showed an immediate hypothermic response, with a significant reduction in body temperature of 0.34 degree C or more within 10 min; however, the effect of vasopressin on stress-induced hyperthermia remained significant after exclusion of these animals from the analysis. Administration of a V1 receptor antagonist prior to the stress did not affect the hyperthermic response, which may suggest that the hyperthermic response had reached maximal (ceiling) levels. Administration of vasopressin, or of the V1 receptor antagonist immediately after the stress, did not affect defervescence, suggesting that vasopressinergic systems are not implicated in the defervescence process. Thus, the results show that ICV administration of vasopressin reduces stress-induced hyperthermia. The mechanisms underlying the effects remain to be elucidated.

8 – Behavioural effects of cytokines

Publisher Summary nThis chapter describes the nonspecific symptoms of infectious diseases that include fever, and profound psychological and behavioural changes. Sick individuals experience weakness, malaise, listlessness, and an inability to concentrate. They also show hypersomnia, anorexia, adipsia, depressed activity, and loss of interest in usual activities, including social contacts. These nonspecific changes are collectively termed as sickness behaviour. Modern medicine tends to consider psychological and behavioral symptoms of sickness as the result of the debilitation process that occurs during infection. The neglect of fever and related symptoms is a relatively recent trend in human medicine. The fever response that is displayed by both warm-blooded and cold-blooded animals during the course of an infection is not just a symptom of disease but an adaptive reaction that has evolved to enable individuals to efficiently fight microbial pathogens. Fever is the only physiological state during which most cellular proliferation is stopped with the exception of proliferation of immune cells. In addition, reduction in plasma iron and zinc during fever decreases the availability of these vital elements for bacterial growth.