Cytokines and food intake: the relevance of the immune system to the student of ingestive behavior

Inflammatory Cytokines and Antipsychotic-Induced Weight Gain: Review and Clinical Implications

Antipsychotic medications (APs), particularly second-generation APs, are associated with significant weight gain in schizophrenia patients. Recent evidence suggests that the immune system may contribute to antipsychotic-induced weight gain (AIWG) via AP-mediated alterations of cytokine levels. Antipsychotics with a high propensity for weight gain, such as clozapine and olanzapine, influence the expression of immune genes, and induce changes in serum cytokine levels to ultimately down-regulate neuroinflammation. Since inflammatory cytokines are normally involved in anorexigenic responses, reduced inflammation has been independently shown to mediate changes in feeding behaviours and other metabolic parameters, resulting in obesity. Genetic variation in pro-inflammatory cytokines is also associated with both general obesity and weight change during AP treatment, and thus, may be implicated in the pharmacogenetics of AIWG. At this time, preliminary data support a cytokine-mediated model of AIWG which may have clinical utility in developing more effective metabolic monitoring guidelines and prevention measures. However, further research is still needed to clearly elucidate the validity of this immune model. This article reviews the evidence implicating inflammatory cytokines in AIWG and its potential clinical relevance.

The taste of sickness: Lipopolysaccharide-induced finickiness in rats

Decrease in food intake is one of the most documented non-specific symptoms of inflammatory processes. However, attention has been mainly focused on quantitative analysis. The present paper reports studies undertaken to test the possible contribution of changes in taste processes in inflammatory-induced alteration of feeding behavior. In a first experiment, the effects of lipopolysaccharide-induced sickness were assessed on preference for saccharin and aversion for quinine in rats using the two-bottle test paradigm. In a second experiment, effect of lipopolysaccharide (LPS) on the behavioral reactivity to palatable, unpalatable and mixed solutions was analyzed using the taste-reactivity paradigm. Our results show that LPS decreased total fluid intake but did not change taste responses to unpalatable or palatable substances. However, LPS increased aversive reactions and decreased hedonic responses to mixed taste. These LPS-induced changes are interpreted as an increase in finickiness and are discussed in regard to their potential role in the adaptation of individuals to sickness.

Pharmacokinetics and feeding responses to muramyl dipeptide in rats

N-acetyl-muramyl-L-alanine-D-isoglutamine or muramyl dipeptide (MDP) is the minimally active subunit of bacterial peptidoglycan. During a systemic infection, the involvement of MDP has been demonstrated in food intake depression by the macrophage hydrolysis of Gram-positive bacteria. Under normal conditions, mammals are constantly exposed to the release of endogenous MDP from degraded gut flora and that of exogenous MDP from the diet. However, MDP digestion and absorption in the gastrointestinal tract are not fully understood, and their physiological significance needs to be clarified. After gavage (1.5 mg/kg), very low levels of MDP were found in the systemic circulation of rats and feeding patterns were not altered. In contrast, after the intraperitoneal injection of a similar dose, a depression in food intake was observed. The rats reduced their meal frequency and constant feeding rate, showing signs of satiety. The behavioral satiety sequence (BSS) was modified by behavioral changes, similar to those which appear during sickness, such as an increase in resting and a reduction in grooming. Our data suggest that the hypophagic effect of MDP may result from satiety and sickness behavior.

Peripheral signals conveying metabolic information to the brain: short-term and long-term regulation of food intake and energy homeostasis

Numerous peripheral signals contribute to the regulation of food intake and energy homeostasis. Mechano- and chemoreceptors signaling the presence and energy density of food in the gastrointestinal (GI) tract contribute to satiety in the immediate postprandial period. Changes in circulating glucose concentrations appear to elicit meal initiation and termination by regulating activity of specific hypothalamic neurons that respond to glucose. Other nutrients (e.g., amino acids and fatty acids) and GI peptide hormones, most notably cholecystokinin, are also involved in short-term regulation of food intake. However, the energy density of food and short-term hormonal signals by themselves are insufficient to produce sustained changes in energy balance and body adiposity. Rather, these signals interact with long-term regulators (i.e., insulin, leptin, and possibly the orexigenic gastric peptide, ghrelin) to maintain energy homeostasis. Insulin and leptin are transported into the brain where they modulate expression of hypothalamic neuropeptides known to regulate feeding behavior and body weight. Circulating insulin and leptin concentrations are proportional to body fat content; however, their secretion and circulating levels are also influenced by recent energy intake and dietary macronutrient content. Insulin and leptin concentrations decrease during fasting and energy-restricted diets, independent of body fat changes, ensuring that feeding is triggered before body energy stores become depleted. Dietary fat and fructose do not stimulate insulin secretion and leptin production. Therefore, attenuated production of insulin and leptin could lead to increased energy intake and contribute to weight gain and obesity during long-term consumption of diets high in fat and/or fructose. Transcription of the leptin gene and leptin secretion are regulated by insulin-mediated increases of glucose utilization and appear to require aerobic metabolism of glucose beyond pyruvate. Other adipocyte-derived hormones and proteins that regulate adipocyte metabolism, including acylation stimulating protein, adiponectin, diacylglycerol acyltransferase, and perilipin, are likely to have significant roles in energy homeostasis.

Leptin and the regulation of food intake, energy homeostasis and immunity with special focus on periparturient ruminants

The biology of leptin has been studied most extensively in rodents and in humans. Leptin is involved in the regulation of food intake, energy homeostasis and immunity. Leptin is primarily produced in white adipose tissue and acts via a family of membrane bound receptors, including an isoform with a long intracellular domain (OB-Rb), and many isoforms with short intracellular domains (Ob-Rs). OB-Rb is predominantly expressed in the hypothalamic regions involved in the regulation of food intake and energy homeostasis. The other isoforms are distributed ubiquitously and are found in most peripheral tissues in far greater abundance than OB-Rb. The effects of leptin on food intake and energy homeostasis are central and are mediated via a network of orexigenic neuropeptides (neuropeptide Y, galanin, galanin-like peptide, melanin-concentrating hormone, orexins, agouti-related peptide) and anorexigenic neuropeptides (corticotropin-releasing hormone, pro-opiomelanocortin, alpha-melanocyte stimulating hormone and cocaine- and amphetamine-regulated transcript). In addition, leptin acts directly on immune cells to stimulate hematopoesis, T-cell immunity, phagocytosis, cytokine production, and to attenuate susceptibility to infectious insults. Emerging data in ruminants suggest that leptin is dynamically regulated by many factors and physiological states. Thus, leptin is secreted in a pulsatile fashion, but without a marked diurnal rhythm. A positive relationship between adiposity and plasma leptin concentration exists in growing and lactating ruminants. The concentration of plasma leptin increases during pregnancy, starts to decline 1--2 wk before parturition, and reaches a nadir in early lactation. The reduction of plasma leptin at parturition is likely to promote centrally mediated adaptations required in periods of energy deficit, but could have negative effects on immune cell function. Future research is needed in ruminants to address the roles played by leptin and the central nervous system in orchestrating metabolism during the periparturient period and during infectious diseases.

Integration of metabolism and intake regulation: a review focusing on periparturient animals

There has been great interest in dry matter intake regulation in lactating dairy cattle to enhance performance and improve animal health and welfare. Predicting voluntary dry matter intake (VDMI) is complex and influenced by numerous factors relating to the diet, management, housing, environment and the animal. The objective of this review is to identify and discuss important metabolic factors involved in the regulation of VDMI and their integration with metabolism. We have described the adaptations of intake and metabolism and discussed mechanisms of intake regulation. Furthermore we have reviewed selected metabolic signals involved in intake regulation. A substantial dip in VDMI is initiated in late pregnancy and continues into early lactation. This dip has traditionally been interpreted as caused by physical constraints, but this role is most likely overemphasized. The dip in intake coincides with changes in reproductive status, fat mass, and metabolic changes in support of lactation, and we have described metabolic signals that may play an equally important role in intake regulation. These signals include nutrients, metabolites, reproductive hormones, stress hormones, leptin, insulin, gut peptides, cytokines, and neuropeptides such as neuropeptide Y, galanin, and corticotrophin-releasing factor. The involvement of these signals in the periparturient dip in intake is discussed, and evidence supporting the integration of the regulation of intake and metabolism is presented. Still, much research is needed to clarify the complex regulation of VDMI in lactating dairy cows, particularly in the periparturient animal.

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.

Lack of evidence for a role of serotonin in interleukin-1-induced hypophagia

Interleukin-1 (IL-1) administration depresses food intake in rodents. IL-1 is known to increase the metabolism of serotonin, which is known to affect feeding behavior. Thus, serotonin is an obvious candidate for a mediator of the hypophagic response to IL-1. Therefore, we tested the ability of serotonergic agonists and antagonists to alter the hypophagic responses to IL-1 and bacterial lipopolysaccharide (LPS). Hypophagia was assessed in ad lib-fed mice by recording the intake of sweetened milk in a 30-min period. Acute intraperitoneal administration of mouse IL-1beta reliably decreased milk intake. This hypophagic response was not affected by any of the serotonin antagonists tested, including 5-HT(1A) (WAY100135 and propranolol), 5-HT(1B) (GR127935), 5-HT(2) (ritanserin, ketanserin, SB206553, and RS102221), mixed 5-HT(1/2) (methysergide and metergoline), and 5-HT(3) (tropisetron) receptor antagonists. The 5-HT(1A) agonists (8-OH-DPAT and ipsapirone) and a 5-HT(1B) agonist (CGS12066B) known to decrease the activity of serotonergic neurons, also had no effect. Mice pretreated with 5,7-dihydroxytryptamine to deplete brain serotonin ate less, but, nevertheless, displayed similar hypophagic responses to mIL-1beta or LPS. The results suggest that serotonin is not involved in the decrease in short-term milk intake induced by mIL-1beta or LPS in mice that have been fed ad lib.

The roles of IL-1, IL-6, and TNFalpha in the feeding responses to endotoxin and influenza virus infection in mice

Influenza infection or administration of bacterial endotoxin (lipopolysaccharide, LPS) results in diminished feeding and loss of body weight. It has been suggested that these effects may be mediated by cytokines, such as interleukin-1 (IL-1), interleukin-6 (IL-6), and/or tumor necrosis factor-alpha (TNFalpha). To assess the potential role of these cytokines, we tested the ability of the naturally occurring IL-1-receptor antagonist (IL-1ra), a monoclonal antibody to mouse IL-6 (IL-6mAb), and a TNF binding protein fragment (TNFbp) to antagonize hypophagia induced by intraperitoneally (ip) injected mouse IL-1beta or LPS or by inoculation with influenza virus. Feeding was assessed by measuring the daily intake of food pellets and sweetened milk in a 30-min period. The hypophagia induced by mIL-1beta or LPS was not affected by pretreatment with IL-6mAb. The effects of IL-1beta were blocked by IL-1ra but unaffected by TNFbp. TNFbp and IL-1ra given separately both exhibited a tendency to attenuate LPS-induced hypophagia. The effectiveness of TNFbp plus IL-1ra treatment was similar to that of the individual antagonists. However, combined treatment with TNFbp, IL-1ra, and IL-6mAb almost completely prevented the depressing effect of LPS on milk intake. The antagonists were also tested in influenza virus-inoculated mice. IL-1ra was delivered chronically by osmotic minipumps and was supplemented by treatment with TNFbp and IL-6mAb. The treatments slightly attenuated the effects of the virus on milk intake 48 h after the inoculation and delayed the decrease in body weight. However, over the entire course of the experiment, the treatment produced very small, statistically nonsignificant, attenuations of the depressions in milk and food pellet intake. Similar results were obtained with TNFbp alone or the combination of IL-6mAb and TNFbp. The results suggest that IL-1beta, TNFalpha, and IL-6 contribute to the hypophagia induced by LPS. However, antagonism of all three cytokines was not sufficient to prevent the decreases in feeding and loss of body weight induced by influenza virus infection.

CRF-deficient mice respond like wild-type mice to hypophagic stimuli

Corticotropin-releasing factor (CRF) has been implicated in physiological processes associated with stress, including changes in feeding behavior. Intracerebroventricular (ICV) administration of CRF and urocortin have been shown to depress feeding, and antagonism of CRF receptors has been reported to attenuate hypophagic responses to many treatments, suggesting that brain CRF may mediate these responses. We have now studied feeding behavior of mice lacking the CRF gene (CRFko), comparing them to wild-type (CRFwt) mice. Feeding was assessed in nondeprived mice by measuring the intake of sweetened milk in a 30-min period and the food pellet intake over 24 h. ICV administration of CRF or urocortin (1 microg, but not lower doses) depressed milk and food pellet intake in normal mice. Physical restraint for 30 min, or administration of mouse interleukin-1beta (mIL-1beta, 100 ng, IP), lipopolysaccharide (LPS, 1 microg, IP), or the serotonergic agonist (d-fenfluramine, 4 mg/kg, IP) reliably reduced milk intake. LPS also reduced food pellet intake. The responses to restraint, IL-1, LPS, and fenfluramine were indistinguishable between the CRFwt and CRFko mice. These results suggest that CRF is not essential for the reduction in sweetened milk intake that occurs following restraint, LPS, IL-1, or d-fenfluramine administration to mice.

Molecular mechanisms of actions of interleukin-6 on the brain, with special reference to serotonin and the hypothalamo-pituitary-adrenocortical axis

Biological activities of the multifunctional cytokine, interleukin-6 (IL-6) include stimulation of B cell proliferation, immunoglobulin production, and initiation of the acute-phase response. IL-6 affects the CNS in that it activates the hypothalamo-pituitary-adrenocortical (HPA) axis and increases brain tryptophan and serotonin metabolism. IL-6 has been proposed as an important mediator of interaction between the neuroendocrine and immune systems. The peripheral and central effects of IL-6 are presumably mediated through its membrane receptor (IL-6R). IL-6, IL-6R and their respective mRNAs have been detected in several brain regions. Although the functions of cytokines overlap considerably, each displays its own characteristic properties. Expression of IL-6 in the brain has been observed in several CNS disorders, some of which have been associated with disorders of serotonin metabolism. It is proposed that interactions between IL-6 and brain serotonin is a complex process which involves corticotropin-releasing factor (CRF) and opioid peptides. It is likely that the molecular mechanisms underlying the actions of IL-6 on the HPA axis and its other brain functions involve the integrated effects of glutamate, Ca2+, 3',5'-cyclic AMP, protein kinase C, and other metabolic pathways.

Endotoxin- and interleukin-1-induced hypophagia are not affected by adrenergic, dopaminergic, histaminergic, or muscarinic antagonists

Endotoxin (lipopolysaccharide, LPS) and interleukin-1 (IL-1) administration induce hypophagia in rodents. Both IL-1 and LPS are known to activate cerebral norepinephrine and serotonin metabolism, and IL-1 affects that of acetylcholine and histamine. Each of these neurotransmitters has been implicated in feeding behavior. Therefore, the ability of specific antagonists of the above neurotransmitter systems to counteract feeding responses to peripherally injected mIL-1beta and LPS was studied. Feeding was assessed in nondeprived mice by measuring the intake of sweetened milk in a 30-min period, as well as daily food pellet intake. LPS and mIL-1beta reliably reduced milk intake, and often reduced food pellet intake and body weight. Treatment of the mice with peripherally administered alpha-adrenergic (phentolamine or prazosin) or 3-adrenergic antagonists (propranolol), either alone or in combination, did not significantly alter the hypophagic responses to mIL-1beta or LPS. Mice in which cerebral norepinephrine was depleted with DSP-4 or 6-hydroxydopamine also displayed the usual hypophagia in response to mIL-1beta and LPS. The hypophagic responses to mIL-1beta and LPS were not affected by the histaminergic antagonists, pyrilamine (H1), cimetidine (H2), thioperamide (H3), or the histamine-depleting agent, alpha-fluoromethylhistidine, nor by the muscarinic cholinergic antagonist, scopolamine. The responses to mIL-l1 were also unaffected by the dopamine receptor antagonist, haloperidol, the opioid receptor antagonist, naloxone, and the NO synthase inhibitor, L-NAME. These results suggest that adrenergic, dopaminergic, histaminergic, cholinergic, opioid or nitric oxide systems are not essential for the hypophagia induced by IL-1, and that multiple redundant pathways may be involved in illness-related hypophagia.

Stress-associated immune modulation: relevance to viral infections and chronic fatigue syndrome

The frequent association of an active viral infection with the symptoms of CFS led researchers to hypothesize that chronic fatigue syndrome (CFS) is induced by a virus. Results of these studies indicated that despite clinical support for this hypothesis, there were no clear data linking viruses to CFS. In this overview, we will explore the interrelation of the immune, endocrine, and central nervous systems, and the possibility that stress and/or the reactivation/replication of a latent virus (such as Epstein Barr virus) could modulate the immune system to induce CFS. Relevant research conducted in the developing field of psychoneuroimmunology will be reviewed, with a particular focus on cytokine synthesis, natural killer (NK) cell activity, and T-lymphocyte function, as they relate to CFS.

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.

The role of cytokines in infection-related behavior

Infections are associated with a specific behavioral pattern that includes hypomotility, hypophagia, increased sleep, decreased libido, and decreased exploration. This behavioral response is considered adaptive and important for the survival of the animal. A similar behavioral pattern was observed following treatment with endotoxin (lipopolysaccharide [LPS]) and cytokines, such as interleukin-1 (IL-1). Because the secretion of these cytokines is induced by LPS and infections, it is possible that they mediate the behavioral responses to infection. We have studied ingestive behavior and locomotor activity in mice following infection with influenza virus, or injection of LPS, IL-1, or IL-6. A lethal dose of influenza virus, LPS, IL-1a and IL-1b each decreased the intake of sweetened condensed milk and 24-hour food pellet intake and decreased locomotor activity. Mouse IL-6 was ineffective. A sublethal dose of influenza virus decreased food pellet intake and locomotor activity, but did not significantly alter milk intake. Indomethacin prevented the behavioral responses to IL-1, and attenuated those to LPS, but had only a very small effect on those to influenza virus. Similar results were obtained with the IL-1-receptor antagonist (IL-1ra); it completely prevented the responses to IL-1, attenuated those to LPS, but, even after chronic high dose administration, attenuated the effects of influenza virus infection only slightly. Our results suggest that while IL-1 may play an important role in the responses to infection, IL-6 does not. Moreover, IL-1 cannot be the only factor contributing to the altered behavior of LPS-injected or influenza virus-infected mice.

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.

The role of cytokines in the behavioral responses to endotoxin and influenza virus infection in mice: effects of acute and chronic administration of the interleukin-1-receptor antagonist (IL-1ra)

Following infection with influenza virus, animals display decreased locomotor activity and feeding behavior and loss of body weight. It has been suggested that these effects may be mediated by cytokines, such as interleukin-1 (IL-1), interleukin-6 (IL-6) and tumor necrosis factor alpha (TNF-alpha), induced by the infection. To assess the potential role of IL-1, we tested the ability of a naturally occurring IL-1-receptor antagonist (IL-1ra) to antagonize the changes in feeding behavior induced by IL-1, endotoxin (lipopolysaccharide, LPS), and infection with influenza virus. Feeding behavior was assessed by measuring the daily intake of food pellets and sweetened milk in a 30-min period. Acute injection of IL-1 beta decreased milk intake, but mouse IL-6 and mouse TNF-alpha did not. However, TNF-alpha decreased food pellet intake slightly, especially when it was injected at the beginning of the dark phase. The reductions in milk intake induced by mouse IL-1 beta were largely prevented by IL-1ra pretreatment (100 micrograms/mouse i.p.). The LPS-induced reductions in milk intake were attenuated, but not blocked, by IL-1ra treatment (300 micrograms/mouse). LPS still induced significant decrements in the presence of the antagonist. In influenza virus-infected mice, IL-1ra was administered either by repeated subcutaneous (s.c.) injections, or by continuous s.c. infusion from osmotic minipumps. These IL-1ra treatments produced small, but statistically significant, attenuations of the depression in milk and food pellet intake in the virus-infected mice. In several experiments, IL-1ra treatment increased the survival of influenza virus-infected mice. Thus the attenuation of the hypophagia may have been caused by this IL-1ra-induced increase in survival. The results suggest that IL-1 contributes to sickness behavior induced by LPS and influenza virus infection, but it is not the only factor involved.

The physiology and brain mechanisms of feeding

This article is designed as an introduction to the major theoretical models in the field of regulation of eating behavior, and a selective review of relevant neurobiological data. We first critically consider the paradigm of homeostasis as it relates to body energy content, and argue that additional theoretical constructs will be needed to account for the complexity of eating behavior in both nonhumans and humans. We then summarize some of the methods available to the neuroscientist in this area, and address some of their limitations. We review treatments and potential mechanisms that increase food intake, including deprivation, antimetabolites, norepinephrine, and several peptides including neuropeptide Y. We next review treatments that decrease food intake, including a variety of humoral, gastrointestinal, and pancreatic factors, as well as examine central pathways of satiety. This includes a discussion of leptin and other potential anorectic agents. We conclude with a discussion of human obesity and anorexias, and prospects for pharmacotherapy of eating disorders. We emphasize throughout that most regions of the human brain probably make some contribution to feeding behavior, and so a focus on any one area of transmitter/hormone is an unrealistic approach both in basic and applied areas.