Differential effects of IL-1ra on sickness behavior and weight loss induced by IL-1 in rats

Peripheral and central injections of recombinant human interleukin-1β (IL-1β) have been shown to decrease social exploration and to induce body weight loss in rats. To characterize the receptor mechanisms of these effects, we used as a tool a specific antagonist of the receptors of IL-1, IL-1ra. Intraperitoneal (i.p.) administration of IL-1ra (8 mg/kg) blocked the effect of i.p. injection of IL-1β (4 μg/rat) on social behaviour but not on body weight. Central administration of IL-1ra (60 μg/rat, i.c.v.) abrogated the effects of centrally administered IL-1β (30 ngn/rat, i.c.v.) on both social behaviour and body weight. Central injection of IL-1ra (4 μg/rat, i.c.v.) also attenuated the effects of i.p. administered IL-1β (4 μg/rat) on social behaviour but not on body weight. These results suggest that the effects of IL-1β on social behavior are mediated centrally and that its effect on the loss of body weight involves different receptor mechanisms.     

Interleukin (IL)-6 and IL-1 beta act synergistically within the brain to induce sickness behavior and fever in rats

Pro-inflammatory cytokines interleukin (IL)-6 and IL-1β can act in the brain (centrally) to cause fever. Sickness behaviors which accompany fever also appear to involve the central action of IL-1β. We injected species-homologous rat IL-6 and IL-1β directly into the brains of conscious rats to examine the effect of these cytokines on fever, and two behaviors affected by sickness, voluntary wheel-running and food intake. Male Sprague–Dawley rats selected for their predisposition to spontaneously run on running wheels were used in the experiment. Each rat was anaesthetized and had a temperature-sensitive radiotransmitter implanted intra-abdominally, and a 23-gauge stainless steel guide cannula inserted stereotaxically over the lateral cerebral ventricle. Rats were randomly assigned to receive intracerebroventricular injections of three doses of either IL-1β or IL-6 (100 ng, 1 ng or 0.1 ng IL-1β and 200 ng, 20 ng or 2 ng IL-6), or one of three different combinations of IL-1β and IL-6. Rats receiving either IL-1β or IL-6 showed a dose-dependent increase in body temperature and decrease in wheel-running (ANOVA, p < 0.0001). Only rats receiving the highest dose of IL-1β significantly decreased food intake and body mass compared to rats receiving vehicle (ANOVA, p < 0.001). Doses of IL-1β and IL-6 which, when injected on their own were non-pyrogenic and did not affect food intake and body mass, induced fever and anorexia when they were co-injected centrally. These results show that species-homologous rat IL-6 and IL-1β can act directly within the brain to decrease voluntary activity and suggest they also can act synergistically to induce anorexia and fever.     

Evidence for a different sensitivity to various central effects of interleukin-1 β in mice

Interleukin 1 (IL-1) induces a series of metabolic and endocrine effects. Activation of the hypothalamus-pituitary-adrenal axis, inhibition of food and water intake, elevation of serum interleukin-6 (IL-6) concentration and hypoglycemia are some of the effects induced by IL-1. The purpose of this study was to compare the sensitivity of these effects following central and peripheral administration of IL-1β. Different doses of IL-1β (0.1–1000 ng/mouse) were centrally (ICV) or peripherally (IP) injected to male mice two hours prior to sacrifice. The ICV administration was more efficacious than the IP injection in elevating serum corticosterone and IL-6 concentrations, whereas no difference was evident in the IL-1β-induced hypoglycemia. Central IL-1β administration was also more potent than IP injection in inhibiting overnight food and water intake. A dose-dependent effect was evident in all these cases. In summary, our data compare effects elicited by central or peripheral administration of different doses of IL-1β. This comparison suggests that the IL-1β stimulation of serum corticosterone and IL-6 and inhibition of food and water intake are events more centrally mediated than the IL-1β-induced hypoglycemia.     

Effects of septal damage and ovariectomy on feeding, drinking and body weight

GEISELMAN, P. J. AND C. R. ALMLI. Effects of septal damage and ovariectomy on feeding, drinking and body weight. BRAIN RES. BULL. 3(5) 457–466, 1978.—Septal destruction and ovariectomy each influenced food intake and body weight differentially. Animals sustaining septal damage ingested significantly more food than the other groups, and septal hyperphagia persisted for as long as 109 days. Ovarian hyperphagia did not occur under conditions of constant illumination. Septal destruction exerted essentially no effect on body weight, while ovariectomy substantially increased body weight. Sequential surgical manipulations provided further evidence that the ovaries and the septum influence food intake and body weight via independent mechanisms. Results indicated that the septal and ovarian effects on water intake are not mediated via independent mechanisms. Septal and ovarian hyperdipsia were found to be very robust effects occurring regardless of the lighting regimen. It was further demonstrated that ovarian hyperdipsia is not secondary to food intake but rather is primary hyperdipsia.     

Neuropeptide Y facilitates activity-based-anorexia

The hypothesis that treatment with neuropeptide Y (NPY) can increase running activity and decrease food intake and body weight was tested. Female rats with a running wheel lost more weight than sedentary rats and ran progressively more as the availability of food was gradually reduced. When food was available for only 1h/day, the rats lost control over body weight. Correlatively, the level of NPY mRNA was increased in the hypothalamic arcuate nucleus. This phenomenon, activity-based-anorexia, was enhanced by intracerebroventricular infusion of NPY in rats which had food available during 2h/day. By contrast, NPY stimulated food intake but not wheel running in rats which had food available continuously. These findings are inconsistent with the prevailing theory of the role of the hypothalamus in the regulation of body weight according to which food intake is a homeostatic process controlled by "orexigenic" and "anorexigenic" neural networks. However, the finding that treatment with NPY, generally considered an "orexigen", can increase physical activity and decrease food intake and cause a loss of body weight is in line with the clinical observation that patients with anorexia nervosa are physically hyperactive and eat only little food despite having depleted body fat and up-regulated hypothalamic "orexigenic" peptides.     

Intraventricular GLP-1 reduces short- but not long-term food intake or body weight in lean and obese rats

Glucagon-like-peptide-1 (7–36) amide (GLP-1), when infused into the third ventricle (IVT), reduces short-term food intake. In the present experiments, we assessed whether IVT administration of GLP-1 could influence long-term food intake and body weight of lean Long Evans rats and of fatty Zucker (fa/fa) rats. In Experiment 1, we replicated the observation that 10 μg GLP-1, given IVT, reduces one and 2 h food intake, and extended the observation to fatty Zucker rats. However, in both rat strains, 24 h food intake and body weight were unchanged by this acute treatment. In Experiment 2, GLP-1 (30 μg/day) was infused IVT continuously for 4 days via an osmotic mini-pump. This treatment also had no effect on food intake or body weight in either Long–Evans or fatty Zucker rats. A control experiment verified that the GLP-1 remained biologically active over the duration of the infusion period. In a final experiment, Long-Evans rats were restricted to two 2 h periods of access to food each day for 6 days. Prior to each of these access periods, rats received either 15 μg of GLP-1 IVT or a vehicle control injection. While GLP-1 significantly reduced food intake on the first day of treatment, this effect of GLP-1 rapidly disappeared such that it was reduced on the second day and absent on the third day; and there was no effect on body weight at any time. Collectively, the present experiments do not support the hypothesis that GLP-1, acting in the CNS, is an important regulator of long-term food intake and body weight.     

The role of brain cytokines in mediating the behavioral and neuroendocrine effects of intracerebral Mycoplasma fermentans

Intracerebral administration of Mycoplasma fermentans (MF), a small microorganism that has been found in the brain of some AIDS patients, induces behavioral and neuroendocrine alterations in rats. To examine the role of tumor necrosis factor-α (TNFα) and interleukin-1 (IL-1) in mediating these effects we measured MF-induced expression of TNFα and IL-1β mRNA in various brain regions, and the effects of TNFα synthesis blockers and IL-1 receptor antagonist (IL-1ra) on MF-induced sickness behavior and adrenocortical activation. Intracerebroventricular (i.c.v.) administration of heat-inactivated MF induced the expression of both TNFα and IL-1β mRNA in the cortex, dorsal hippocampus, amygdala, and hypothalamus. Pre-treatment of rats with either TNFα synthesis blockers, pentoxifylline or rolipram, or with IL-1ra did not attenuate MF-induced anorexia, body weight loss, and suppression of social behavior. However, simultaneous administration of both pentoxifylline and IL-1ra markedly attenuated MF-induced anorexia and body weight loss, but had no effect on the suppression of social behavior. Pre-treatment with pentoxifylline, but not with IL-1ra, significantly attenuated MF-induced corticosterone (CS) secretion. Together, these findings indicate that both TNFα and IL-1 participate, in a complementary manner, in mediating some of the behavioral effects of MF, whereas only TNFα, but not IL-1, is involved in mediating MF-induced adrenocortical activation. We suggest that cytokines within the brain are involved in mediating at least some of the neurobehavioral and neuroendocrine abnormalities that may be produced by MF in AIDS patients.     

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.     

Weight loss in rats treated with intracerebroventricular cobalt protoporphyrin is not specific to the neuropeptide Y system

Cobalt protoporphyrin (CoPP) reduces food intake and body weight following intracerebroventricular (i.c.v.) administration in rats. We injected 0.2 μmol CoPP per kg body weight i.c.v. and monitored body weight and daily food intake for 7 days. The body weight and 24 h food intake of CoPP-treated animals was significantly lower than that of vehicle-treated animals in all studies (P < 0.01) from day 2 to day 7. The 2 h feeding response (CoPP vs. vehicle-treated) to 10 μg neuropeptide Y (NPY) (4.0 vs. 7.1 g;P < 0.05), the 1 h feeding response to 10 μg galanin (1.3 vs. 3.2 g;P < 0.05) and 30 μg norepinephrine (0.6 vs. 1.9 g;P < 0.05) in CoPP-treated animals were all reduced compared to the vehicle-treated group. In addition there was no change in hypothalamic NPY mRNA in CoPP-treated animals. I.c.v. COP decreases sensitivity to exogenous NPY, galanin and norepinephrine. The effect of COP is not specific to NPY as previously described.     

Reduction of food intake and body weight by chronic intraventricular insulin infusion

This study examined the effect of chronic infusions of insulin in one of three doses (5, 7.5 or 10 mU/day) into the third ventricle, on food and water intake and body weight in the rat. Solutions were infused via osmotic minipumps at a rate of 1 microliter/hour for seven days. The two highest doses of insulin produced a dose-related suppression of food intake and weight loss, which was greater than the effect produced by 5 mU/day or a control infusion of Ringers solution. The effect of 5 mU/day on food and water intake and body weight was similar to the effect of the control infusion. All groups treated with insulin decreased food intake during the day and night, although only differences in nighttime food intake were statistically significant. Ten mU/day also produced a significantly greater reduction in water intake than each of the other solutions. Weight loss in the animals infused with insulin could not be explained by a decrease in caloric intake alone. Food intake returned to normal in all groups by the end of a seven day post-infusion period, with recovery being slowest among the animals receiving the highest doses of insulin. All animals recovered body weight at approximately the same rate. These results provide further evidence for the view that brain insulin plays a role in the regulation of food intake and body weight.     

Microinjection of the α1-agonist methoxamine into the paraventricular hypothalamus induces anorexia in rats

Adrenergic receptors within the paraventricular hypothalamus (PVN) play a prominent role in the control of food intake: stimulation of α2-adrenoceptors induces food intake whereas stimulation of α1-adrenoceptors suppresses food intake. This study further examines the role of PVN α1-adrenoceptors hy examining the effects on food and water intake of the α1-adrenergic agonist methoxamine (100, 200, 400 nMol) microinjected into the rat paraventricular hypothalamus. Methoxamine suppressed food intake but not water intake. Doses of 100, 200, and 400 nMol methoxamine suppressed food intake by 47%. 64%, and 96%, respectively. These results further confirm the hypothesis that administration of α1-agonists into the PVN acts to significantly suppress food intake; an action that is in opposition to the facilitory effects of α2-adrenergic agonists on food intake.     

Interleukin-1β-induced brain injury in the neonatal rat can be ameliorated by α-phenyl-n-tert-butyl-nitrone

To examine the possible role of inflammatory cytokines in mediating perinatal brain injury, we investigated effects of intracerebral injection of interleukin-1beta (IL-1β) on brain injury in the neonatal rat and the mechanisms involved. Intracerebral administration of IL-1β (1 μg/kg) resulted in acute brain injury, as indicated by enlargement of ventricles bilaterally, apoptotic death of oligodendrocytes (OLs) and loss of OL immunoreactivity in the neonatal rat brain. IL-1β also induced axonal and neuronal injury in the cerebral cortex as indicated by elevated expression of β-amyloid precursor protein, short beaded axons and dendrites, and loss of tyrosine hydroxylase-positive neurons in the substantia nigra and the ventral tegmental areas. Administration of α-phenyl-n-tert-butyl-nitrone (PBN, 100 mg/kg i.p.) immediately after the IL-1β injection protected the brain from IL-1β-induced injury. Protection of PBN was linked with the attenuated oxidative stress induced by IL-1β, as indicated by decreased elevation of 8-isoprostane content and by the reduced number of 4-hydroxynonenal or malondialdehyde or nitrotyrosine-positive cells following IL-1β exposure. PBN also attenuated IL-1β-stimulated inflammatory responses as indicated by the reduced activation of microglia. The finding that IL-1β induced perinatal brain injury was very similar to that induced by lipopolysaccharide (LPS), as we previously reported and that PBN was capable to attenuate the injury induced by either LPS or IL-1β suggests that IL-1β may play a critical role in mediating brain injury associated with perinatal infection/inflammation.     

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-1β (IL-1β) 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-1β 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-1β (1 μg/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-1β 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-1β significantly elevated CORT secretion above basal levels and, as previously reported for mice, induced hypothermia (about 2°C). In the second experiment, we assessed mice receiving IL-1β (0.25; 0.5 or 1 μg/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-1β 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-1β. Overall these data support the notion that sickness behavior induced by IL-1β administration represents an organized behavioral strategy and is not an aspecific response to an illness-type of condition.     

Intermittent, chronic fenfluramine administration to rats repeatedly suppresses food intake despite substantial brain serotonin reductions

The mechanisms by which fenfluramine suppresses food intake and body weight have been linked to its ability to enhance transmission across serotonin synapses in brain. This drug initially lowers body weight and suppresses food intake, yet after repeated administration food intake soon returns to normal and body weight no longer decreases. Fenfluramine also causes rapid and prolonged reductions in brain serotonin concentrations, which might account for its loss of appetite suppression. This possibility has been evaluated in rats by assessing if intermittent, chronic fenfluramine administration could suppress food intake during each treatment period, and if so, whether such an effect occurs in the presence of reduced brain serotonin levels. Rats were injected once daily with 10 mg/kg D,L-fenfluramine for 5 days, and then received no injections for the next 5 days. Control rats received only vehicle injections. This 10-day sequence was repeated five more times. During each period of fenfluramine administration, daily food intake dropped markedly the first 1-2 days of treatment, but returned to pretreatment values by day 5. Daily food intake was normal or slightly above normal during non-injection periods. Body weight dropped modestly during each period of fenfluramine administration, and rose during each subsequent period when injections had ceased. Serotonin concentrations and synthesis rates in several brain regions were markedly reduced at early, middle, and late periods of the experiment. Despite the long-term reduction in brain serotonin pools produced by fenfluramine, the drug continues to reduce food intake and body weight. Several possible interpretations of these findings are considered, based on the multiple mechanisms through which this drug has been proposed to modify synaptic serotonin transmission.     

Alterations in regulatory behaviors induced by medial septal lesions and superior cervical ganglionectomy

Following medial septal (MS) lesions peripheral sympathetic fibers, orginating from the superior cervical ganglia (SCG), grow into the hippocampus and habenula. To assess their effect on regulatory behaviors, body weight, and food and water consumption were studied under ad libitum and pharmacological stress conditions, after MS lesions, superior cervical ganglionectomy (Gx) or MS lesion + ganglionectomy (MSGx). Twenty-two animals completed the study: control (n = 7), MS lesion (n = 6), Gx (n = 6), MSGx (n = 4). No differences were observed preoperatively. Postoperatively, body weight fell but over time all groups gained weight. However, animals with MSGx were lighter than MS or Gx animals (which were equivalent), which in turn were lighter than controls (P < 0.0001). Hypophagia was observed in the Gx and MSGx animals when compared to the MS and control groups (P < 0.05), while hyperdipsia was seen in the MS and Gx groups (P < 0.001). Administration of both 1 M NaCl and isoproterenol (25 μg/kg) increased drinking in all animals (P < 0.001), with the MSGx group consuming significantly less than all others (P < 0.025). Food intake increased following 2-deoxy-d-glucose (500 mg/kg) (P < 0.0001), while epineprine (120 μg/kg) treatment produced anorexia only in the MS group (P < 0.05). Hyperthermia was found in the Gx and MSGx groups. The results of this study suggest that both MS region and SCG contribute to the maintenance of normal regulatory behaviors, with combined loss of these neural systems resulting in severe disturbances, both qualitatively and quantitatively different from either MS lesion or Gx. Although the MS lesion group clearly regulated better than the MSGx group, it is unclear whether this is due to ingrowth or just the presence of the SCG.     

Mechanisms of tumor necrosis factor-α (TNF-α) hyperalgesia

Activation of immune cells by pathogens induces the release of a variety of proinflammatory cytokines, including IL-1β and TNF-α. Previous studies using IL-1β have demonstrated that this cytokine can alter brain function, resulting in a variety of ‘illness responses’ including increased sleep, decreased food intake, fever, etc. We have recently demonstrated that i.p. IL-1β also produces hyperalgesia and that this hyperalgesia (as well as most illness responses) is mediated via activation of subdiaphragmatic vagal afferents. The present series of studies were designed to provide an initial examination of the generality of proinflammatory cytokine-induced hyperalgesia by examining the effects of i.p. TNF-α on pain responsivity. These studies demonstrate that: (a) i.p. TNF-α produces dose-dependent hyperalgesia as measured by the tailflick test, (b) this hyperalgesia is mediated via the induced release of IL-1β, (c) hyperalgesia is mediated via activation of subdiaphragmatic vagal afferents, and (d) the effects of subdiaphragmatic vagotomy cannot be explained by a generalized depression of neural excitability.     

Hypothalamic orexin, OX1, αMSH, NPY and MCRs expression in dopaminergic D2R knockout mice

In 5-month-old male and female dopamine receptor 2 (D2R) knockout mice food intake per animal was unaltered while food per g BW was increased. We wished to evaluate the effect of D2R disruption on different components of energy balance and food intake regulation. We determined hypothalamic orexin precursor (PPO) expression, its receptor OX1, serum leptin levels, hypothalamic leptin receptor (OBR), circulating and pituitary α MSH levels, as well as central MC3 and MC4 receptors and NPY mRNA in wildtype and D2R knockout mice (KO).Loss of D2R caused a marked increase in serum prolactin levels, to higher levels in females compared to male KO mice. On the other hand, it produced a female-specific increase in circulating αMSH, and hypothalamic αMSH content, while neurointermediate αMSH content was decreased in both sexes. No differences were found in hypothalamic NPY, MC3R or MC4R concentration. Hypothalamic PPO mRNA expression was significantly decreased only in female KOs, while OX1 mRNA was not different between genotypes. Serum leptin levels were also similar in both genotypes.Our results show that in female and not in male mice disruption of the D2R produces two potentially anorexigenic events: an increase in serum and hypothalamic αMSH, and a decrease in hypothalamic orexin expression. Very high prolactin levels, which are orexigenic, probably counterbalance these effects, so that food intake is slightly altered. In males, on the other hand, hypothalamic PPO, and serum or hypothalamic αMSH are not modified, and increased prolactin levels may account for increased food intake per g BW. These results suggest a sexually dimorphic participation of the D2R in food intake regulation.     

Effects of NPY and NPY2–36 on body temperature and food intake following administration into hypothalamic nuclei

Our previous in vivo structure-activity studies suggested that the putative receptors mediating the effects of NPY and NPY_2–36 on food intake and body temperature are pharmacologically different [17]. In the present study, we examined and compared dose-related effects of NPY and NPY_2–36 on ad lib food intake and rectal temperature after administration into discrete hypothalamic nuclei of the rat. Results indicate that NPY and NPY_2–36 have opposite effects on body temperature to those of NPY when injected in the preoptic area (POA): hypothermia and hyperthermia, respectively. When administered in the paraventricular nucleus (PVN), both increased body temperature. When injected into the third ventricle (3V), NPY produced a biphasic effect: hypothermia at low doses and hyperthermia at high doses. Similar effects were obtained with NPY_2–36, but in an inverted dose-related fashion: hyperthermia at low and hypothermia at higher doses. In the arcuate nucleus (Arc), NPY induced a significant hypothermia whereas NPY_2–36 had no effect. Finally, neither peptide affected body temperature when injected into the ventromedial (VMH) and perifornical (PeF) nuclei. Both NPY and NPY_2–36 increased food intake after injection in all regions examined. In general, NPY was more potent and efficacious than NPY_2–36. The present results clearly dissociate the effects of NPY on food intake and body temperature. Furthermore, the data support the hypothesis that the putative receptors underlying the effects of NPY and NPY_2–36 on food intake are similar, whereas those mediating the effects on body temperature are pharmacologically different.     

Tumor necrosis factor and interleukin-1β: suppression of food intake by direct action in the central nervous system

Intracerebroventricular microinfusion of recombinant human tumor necrosis factor (rhTNF) and recombinant human interleukin-1β (rhIL-1β) suppressed food intake in rats. Central infusion of heat-inactivated rhTNF and rhIL-1β, bovine serum albumin, heparin or transforming growth factor-β had no such effect. Central infusion of rhIL-1β did not affect the dipsogenic response to central administration of angiotensin II. Peripheral administration of rhTNF and rhIL-1β in doses equivalent to or higher than those administered centrally had no effect. Electrophoretically applied rhTNF and rhIL-1β specifically suppressed the activity of glucose-sensitive neurons in the lateral hypothalamic area. Glucose-insensitive neurons were little affected. The results suggest that TNF and IL-1β act directly in the central nervous system to suppress feeding, and this effect may be operative during acute and chronic disease.     

Inhibition of interleukin-1β and prostaglandin E2 thermogenesis by glycyl-glutamine, a pro-opiomelanocortin-derived peptide

Interleukin-1β (IL-1β) and other cytokines produce fever by stimulating prostaglandin E₂ (PGE₂) synthesis in thermoregulatory regions of the preoptic area and anterior hypothalamus (POA/AH). Prostaglandin E₂ is thought to raise body temperature, at least in part, by stimulating β-endorphin release from pro-opiomelanocortin neurons that innervate the POA/AH. In this study, we investigated whether glycyl-glutamine (β-endorphin_30–31), an inhibitory dipeptide synthesized from β-endorphin post-translationally, inhibits IL-1β and PGE₂-induced hyperthermia. Hyperthermic sites were identified by microinjecting PGE₂ (3 fmol/1 μl) into the medial preoptic area (mPOA) of conscious, unrestrained rats. Interleukin-1β (1 U) injection into the same PGE₂ responsive thermogenic sites in the mPOA elicited a prolonged rise in colonic temperature (T_c) (+1.02±0.06°C) that persisted for at least 2 h. Glycyl-glutamine (3 nmol) co-injection into the mPOA inhibited IL-1β thermogenesis completely (T_c=−0.18±0.22°C). Glycyl-glutamine had no effect on body temperature when given alone to normothermic rats. Co-injection of individual amino acids, glycine and glutamine (3 nmol each amino acid), failed to influence IL-1β-induced thermogenesis, which indicates that Gly-Gln hydrolysis does not explain its inhibitory activity. Glycyl-glutamine (3 nmol) also prevented the rise in body temperature produced by PGE₂ (PGE₂=0.89±0.05°C; PGE₂ plus Gly-Gln=−0.16±0.14°C), consistent with evidence that PGE₂ mediates IL-1β-induced fever. These findings demonstrate that Gly-Gln inhibits the thermogenic response to endogenous pyrogens.