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

Central interleukin-1 (IL1) signaling is required for pharmacological, but not physiological, effects of leptin on energy balance

Hypothalamic IL1 is suggested to be a critical mediator of the central effects of the adipocyte hormone leptin on energy balance. We hypothesized that IL1 receptor signaling is required for exogenously administered leptin to cause anorexia and weight loss, but not for physiological effects of endogenous leptin signaling on energy balance. To test this hypothesis, we investigated whether chronic hypothalamic over-expression of an IL1 receptor antagonist (AdV-IL1ra) alters food intake and weight gain in normal rats. Our findings demonstrate that impaired IL1 signaling in the CNS did not cause excess weight gain over a period of 11 days (AdV-IL1ra +38.1+/-4.1 g vs. VEH +42.2+/-5.6g; p=0.6) and caused a slightly reduced daily food intake (AdV-IL1ra 29.0+/-1.1 g/day vs. VEH 33.0+/-1.6 g/day; p<0.05). Blocking central IL1 signaling also did not alter the re-feeding response to a prolonged fast, yet was entirely effective in preventing the anorexic effect of exogenously administered leptin (2 mg/kg ip, cumulative food intake at 18 h AdV-IL1ra 30.5+/-1.1 g vs. VEH 26.4+/-1.7 g, p<0.05) and prevented leptin-induced weight loss (AdV-IL1ra -0.1+/-1.3 g vs. VEH -2.7+/-1.9 g, p<0.05). Together these findings suggest that hypothalamic IL1 signaling is required for the pharmacological effects of leptin administration, but that impaired hypothalamic IL1 signaling does not alter the physiological regulation of energy balance.

Cytokine, sickness behavior, and depression

Sufficient evidence is now available to accept the concept that the brain recognizes cytokines as molecular signals of sickness. Clarifying the way the brain processes information generated by the innate immune system is accompanied by a progressive elucidation of the cellular and molecular components of the intricate system that mediates cytokine-induced sickness behavior. We are still far, however, from understanding the whole. Among the hundreds of genes that proinflammatory cytokines can induce in their cellular targets, only a handful has been examined functionally. In addition, a dynamic view of the cellular interactions that occur at the brain sites of cytokine production and action is missing, together with a clarification of the mechanisms that favor the transition toward pathology.

Regulation of food intake by inflammatory cytokines in the brain

A number of inflammatory cytokines are synthesized and released after activation of the immune system. In addition to other biological effects, these cytokines can potently inhibit food intake. Cytokine-mediated inhibition of food intake is of particular importance because excessive production of peripheral inflammatory cytokines is often associated with the cachexia-anorexia syndrome seen in some chronic diseases. The weight loss in cachexia is associated with an increase in morbidity and mortality. Understanding how cytokines regulate food intake may be crucial in enhancing quality of life and facilitating recovery in patients exhibiting cachexia. This review describes the main inflammatory cytokines that influence food intake and explores how peripheral cytokines communicate with hypothalamic nuclei to influence feeding.

Interleukin-1 beta-induced anorexia is reversed by ghrelin

Interleukins, in particular interleukin-1beta (IL-1beta), reduce food intake after peripheral and central administration, which suggests that they contribute to anorexia during various infectious, neoplastic, and autoimmune diseases. On the other hand, ghrelin stimulates food intake by acting on the central nervous system (CNS) and is considered an important regulator of food intake in both rodents and humans. In the present study, we investigated if ghrelin could reverse IL-1beta-induced anorexia. Intracerebroventricular (i.c.v.) injection of 15, 30 or 45 ng/microl of IL-1beta caused significant suppression of food intake in 20 h fasting animals. This effect lasted for a 24h period. Ghrelin (0.15 nmol or 1.5 nmol/microl) produced a significant increase in cumulative food intake in normally fed animals. However, it did not alter food intake in 20 h fasting animals. Central administration of ghrelin reduced the anorexic effect of IL-1beta (15 ng/microl). The effect was observed 30 min after injection and lasted for the next 24h. This study provides evidence that ghrelin is an orexigenic peptide capable of antagonizing IL-1beta-induced anorexia.

Hypothalamic integration of immune function and metabolism

The immune and neuroendocrine systems are closely involved in the regulation of metabolism at peripheral and central hypothalamic levels. In both physiological (meals) and pathological (infections, traumas and tumors) conditions immune cells are activated responding with the release of cytokines and other immune mediators (afferent signals). In the hypothalamus (central integration), cytokines influence metabolism by acting on nucleus involved in feeding and homeostasis regulation leading to the acute phase response (efferent signals) aimed to maintain the body integrity. Peripheral administration of cytokines, inoculation of tumor and induction of infection alter, by means of cytokine action, the normal pattern of food intake affecting meal size and meal number suggesting that cytokines acted differentially on specific hypothalamic neurons. The effect of cytokines-related cancer anorexia is also exerted peripherally. Increase plasma concentrations of insulin and free tryptophan and decrease gastric emptying and d-xylose absorption. In addition, in obesity an increase in interleukin (IL)-1 and IL-6 occurs in mesenteric fat tissue, which together with an increase in corticosterone, is associated with hyperglycemia, dyslipidemias and insulin resistance of obesity-related metabolic syndrome. These changes in circulating nutrients and hormones are sensed by hypothalamic neurons that influence food intake and metabolism. In anorectic tumor-bearing rats, we detected upregulation of IL-1beta and IL-1 receptor mRNA levels in the hypothalamus, a negative correlation between IL-1 concentration in cerebro-spinal fluid and food intake and high levels of hypothalamic serotonin, and these differences disappeared after tumor removal. Moreover, there is an interaction between serotonin and IL-1 in the development of cancer anorexia as well as an increase in hypothalamic dopamine and serotonin production. Immunohistochemical studies have shown a decrease in neuropeptide Y (NPY) and dopamine (DA) and an increase in serotonin concentration in tumor-bearing rats, in first- and second-order hypothalamic nuclei, while tumor resection reverted these changes and normalized food intake, suggesting negative regulation of NPY and DA systems by cytokines during anorexia, probably mediated by serotonin that appears to play a pivotal role in the regulation of food intake in cancer. Among the different forms of therapy, nutritional manipulation of diet in tumor-bearing state has been investigated. Supplementation of tumor bearing rats with omega-3 fatty acid vs. control diet delayed the appearance of tumor, reduced tumor-growth rate and volume, negated onset of anorexia, increased body weight, decreased cytokines production and increased expression of NPY and decreased alpha-melanocyte-stimulating hormone (alpha-MSH) in hypothalamic nuclei. These data suggest that omega-3 fatty acid suppressed pro-inflammatory cytokines production and improved food intake by normalizing hypothalamic food intake-related peptides and point to the possibility of a therapeutic use of these fatty acids. The sum of these data support the concept that immune cell-derived cytokines are closely related with the regulation of metabolism and have both central and peripheral actions, inducing anorexia via hypothalamic anorectic factors, including serotonin and dopamine, and inhibiting NPY leading to a reduction in food intake and body weight, emphasizing the interconnection of the immune and neuroendocrine systems in regulating metabolism during infectious process, cachexia and obesity.

Influence of feeding status on neuronal activity in the hypothalamus during lipopolysaccharide-induced anorexia in rats

Fasting attenuates disease-associated anorexia, but the mechanisms underlying this effect are not well understood. In the present study, we investigated the extent to which a 48 h fast alters hypothalamic neuronal activity in response to the anorectic effects of lipopolysaccharide in rats. Male rats were fed ad libitum or fasted, and were injected with i.p. saline or lipopolysaccharide (250 microg/kg). Immunohistochemistry for Fos protein was used to visualize neuronal activity in response to lipopolysaccharide within selected hypothalamic feeding regulatory nuclei. Additionally, food intake, body weight, plasma interleukin-1 and leptin levels, and the expression of mRNA for appetite-related neuropeptides (neuropeptide Y, proopiomelanocortin and cocaine-amphetamine-regulated transcript) were measured in a time-related manner. Our data show that the pattern of lipopolysaccharide-induced Fos expression was similar in most hypothalamic nuclei whatever the feeding status. However, we observed that fasting significantly reduced lipopolysaccharide-induced Fos expression in the paraventricular nucleus, in association with an attenuated lipopolysaccharide-induced anorexia and body weight loss. Moreover, lipopolysaccharide reduced fasting-induced Fos expression in the perifornical area of the lateral hypothalamus. Lipopolysaccharide-induced circulating levels of interleukin-1 were similar across feeding status. Finally, fasting, but not lipopolysaccharide, affected circulating level of leptin and appetite-related neuropeptides expression in the arcuate nucleus. Together, our data show that fasting modulates lipopolysaccharide-induced anorexia and body weight loss in association with neural changes in specific hypothalamic nuclei.

Central administration of a nitric oxide precursor abolishes both the hypothalamic serotonin release and the hypophagia induced by interleukin-1beta in obese Zucker rats

Serotonin-induced anorexia has long been recognized as an important part of the CNS mechanisms controlling energy balance. More recently, interleukin-1beta and nitric oxide have been suggested to influence this control, possibly through modulation of hypothalamic serotonin. The present work aimed at investigating the interaction of these systems. We addressed whether 5-HT is affected during IL-1beta-induced anorexia in obese Zucker rats and the influence of the central NO system on this IL-1beta/5-HT interaction. Using microdialysis, we observed that an intracerebroventricular injection of 10 ng IL-1beta significantly stimulated 5-HT extracellular levels in the VMH, with a peak variation of 102+/-41% above baseline. IL-1beta also significantly reduced the 4-h feeding by 33% and the 24-h feeding by 42%. Contrarily, these effects were absent when IL-1beta was injected 2 h after the i.c.v. administration of 20 microg of the NO precursor L-arginine. The results suggest that, in obese Zucker rats, activation of the serotonergic system in the medial hypothalamus participates in IL-1beta-induced anorexia. Since L-arginine, probably through NO stimulation, abolished both the anorexia and the serotonergic activation, it can be proposed that the NO system, either directly or indirectly, counteracts IL-1beta anorexia. The hypothalamic serotonergic system is likely to mediate this NO effect.

Homeostatic alterations induced by interleukin-1β microinjection into the orbitofrontal cortex in the rat

The present experiments were designed to elucidate the effect of direct orbitofrontal cortical administration of interleukin-1beta (IL-1beta) on the homeostatic regulation. Short- and long-term food intakes (FI), water intakes and body temperature (BT) were measured before and after a bilateral microinjection of IL-1beta (with or without paracetamol /P/ pretreatment) into the orbitofrontal cortex (OBF) of Wistar rats, and the effects were compared with those found in vehicle-treated and i.p. injected IL-1beta, IL-1beta+P or control animals. In addition, blood glucose levels (BGLs), along a glucose tolerance test, and plasma concentrations of insulin, leptin, cholesterol, triglycerides and urate were determined in cytokine treated and control rats. Short-term FI was suppressed after orbitofrontal cortical or peripheral application of IL-1beta. In the long-term FI, however, there was no significant difference among the groups. Cytokine microinjection into the OBF, similar to the i.p. administration, was also followed by a significant increase in BT. Pretreatment with P failed to influence the anorexigenic and hyperthermic effects of the centrally administered IL-1beta. The sugar load led to a diabetes-like prolonged elevation of BGL in the IL-1beta treated animals. Following cytokine administration, plasma levels of insulin and that of triglycerides were found decreased, whereas that of uric acid increased. The present findings confirm that the OBF is one of the neural routes through which IL-1beta exerts modulatory effect on the central homeostatic regulation.

The regulation of feeding and metabolic rate and the prevention of murine cancer cachexia with a small-molecule melanocortin-4 receptor antagonist

Cachexia is metabolic disorder characterized by anorexia, an increased metabolic rate, and loss of lean body mass. It is a relatively common disorder, and is a pathological feature of diseases such as cancer, HIV infection, and renal failure. Recent studies have demonstrated that cachexia brought about by a variety of illnesses can be attenuated or reversed by blocking activation of the melanocortin 4 subtype receptor (MC4-R) within the central nervous system. Although the potential use of central MC4-R antagonists for the treatment of cachexia was supported by these studies, utility was limited by the need to deliver these agents intracerebroventricularly. In the current study, we present a series of experiments demonstrating that peripheral administration of a small molecule MC4-R antagonist can effectively stimulate daytime (satiated) food intake as well as decrease basal metabolic rate in normal animals. Furthermore, this compound attenuated cachexia and preserved lean body mass in a murine cancer model. These data clearly demonstrate the potential of small molecule MC4-R antagonists in the treatment of cachexia and underscore the importance of melanocortin signaling in the development of this metabolic disorder.

Single neuron activity changes to interleukin-1β in the orbitofrontal cortex of the rat

The orbitofrontal cortex (OBF) is known to play important roles in various regulatory processes. Our preliminary behavioral studies showed homeostatic alterations after orbitofrontal cortical microinjection of interleukin-1beta (IL-1beta) in the rat. To elucidate whether the above alterations were due to direct neuronal action of the cytokine, extracellular single neuron activity was recorded in the OBF of anesthetized rats by means of tungsten fiber multibarreled glass microelectrodes during microelectrophoretic administration of IL-1beta. More than half (56%) of all cells tested changed in firing rate in response to IL-1beta. Approximately 90% of these cytokine-modulated neurons were also sensitive to microelectrophoretically applied d-glucose, that is, proved to be the elements of the central glucose-monitoring neural network. The present findings demonstrate that IL-1beta can exert direct modulatory role on neurons in the OBF.

Ciliary neurotrophic factor and leptin induce distinct patterns of immediate early gene expression in the brain

Ciliary neurotrophic factor (CNTF) and leptin decrease food intake and body weight. Lipopolysaccharide (LPS) is a potent exogenous pyrogen and produces anorexia via cytokine production. CNTF-, leptin-, and LPS-induced cytokines all act on type I cytokine receptors. However, it is not known if these cytokines engage similar central nervous system (CNS) pathways to exert their effects. To assess mechanisms by which these cytokines act, we examined the patterns of immediate early gene expression (SOCS-3, c-fos, and tis-11) in the brain following intravenous administration. CNTF and LPS induced gene expression in circumventricular organs; ependymal cells of the ventricles, meninges, and choroid plexus; and the arcuate nucleus of the hypothalamus. CNTF administration also induced fever and cyclooxygenase-2 mRNA expression. In contrast, we found no evidence of leptin-induced inflammation. CNTF and leptin are being assessed as potential therapeutic anti-obesity agents, and both potently reduce food intake. Our findings support the hypothesis that CNTF and leptin engage distinct CNS sites and CNTF possesses inflammatory properties distinct from leptin.

Minireview: From anorexia to obesity--the yin and yang of body weight control

Over the past decade, there has been a tremendous increase in the understanding of the molecular and neural mechanisms that control food intake and body weight. Yet eating disorders and cachexia are still common, and obesity cases are rising at alarming rates. Thus, despite recent progress, an increased understanding of the molecular and neural substrates that control body weight homeostasis is a major public health goal. In this review, we discuss the mechanisms by which metabolic signals interact with key behavioral, neuroendocrine, and autonomic regulatory regions of the central nervous system. Additionally, we offer a model in which hormones such as leptin and ghrelin interact with similar central nervous system circuits and engage them in such a way as to maintain an appropriate and tight regulation of body weight and food intake. Our model predicts that overstimulation or understimulation of these central pathways can result in obesity, anorexia, or cachexia.

Categorically distinct acute stressors elicit dissimilar transcriptional profiles in the paraventricular nucleus of the hypothalamus

The paraventricular hypothalamic nucleus (PVH) is a key site for integrating neuroendocrine, autonomic, and behavioral adjustments to diverse homeostatic challenges, including "physiological" (e.g., infection or hemorrhage) and "emotional" [e.g., restraint (RST) or footshock] stresses. Both types of challenges ultimately converge to activate common response systems represented in PVH, including the hypothalamo-pituitary-adrenal axis and the sympathoadrenal system. Oligonucleotide microarrays (U74A; Affymetrix, Santa Clara, CA) were used to compare and contrast gene expression profiles in the PVH elicited at 1 and 3 hr after acute exposure to representative physiological [intraperitoneal injection of 10 microg lipopolysaccharide (LPS)] and emotional (30 min RST) stressors. In general, the two challenges recruited relatively few genes in common, with the degree of overlap varying across functional classes of genes. The greatest degree of commonality was seen among signaling molecules and neuropeptides, whereas transcription factors upregulated by RST and LPS were largely distinct. Unexpectedly, RST induced a number of immune-related molecules, which were not regulated by LPS. Hybridization histochemical analyses localized a subset of responsive transcripts to the PVH and/or immediately adjoining regions. Immunerelated molecules in particular distributed broadly to vascular and other barrier-associated cell types. These global transcriptional profiles inform the search for early (transcription factors) and late (target genes) mechanisms in the modulation of PVH, and generalized CNS, responses to categorically distinct stressors.