The hypothalamic arcuate nucleus has an essential role in mediating the homeostatic responses of the thyroid axis to fasting by altering the sensitivity of prothyrotropin-releasing hormone (pro-TRH) gene expression in the paraventricular nucleus (PVN) to feedback regulation by thyroid hormone. Because agouti-related protein (AGRP), a leptin-regulated, arcuate nucleus-derived peptide with ␣-MSH antagonist activity, is contained in axon terminals that terminate on TRH neurons in the PVN, we raised the possibility that ␣-MSH may also participate in the mechanism by which leptin influences pro-TRH gene expression. By double-labeling immunocytochemistry, ␣-MSH-IR axon varicosities were juxtaposed to ϳ70% of pro-TRH neurons in the anterior and periventricular parvocellular subdivisions of the PVN and to 34% of pro-TRH neurons in the medial parvocellular subdivision, establishing synaptic contacts both on the cell soma and dendrites. All pro-TRH neurons receiving contacts by ␣-MSH-containing fibers also were innervated by axons containing AGRP. The intracerebroventricular infusion of 300 ng of ␣-MSH every 6 hr for 3 d prevented fasting-induced suppression of pro-TRH in the PVN but had no effect on AGRP mRNA in the arcuate nucleus. ␣-MSH also increased circulating levels of free thyroxine (T4) 2.5-fold over the levels in fasted controls, but free T4 did not reach the levels in fed controls. These data suggest that ␣-MSH has an important role in the activation of pro-TRH gene expression in hypophysiotropic neurons via either a mono-and/or multisynaptic pathway to the PVN, but factors in addition to ␣-MSH also contribute to the mechanism by which leptin administration restores thyroid hormone levels to normal in fasted animals.

Inflammation and microbial infection produce symptoms, including fever, anorexia, and hypoactivity, that are thought to be mediated by endogenous proinflammatory cytokines. Melanocortins are known to act centrally to suppress effects on fever and other sequelae of proinflammatory cytokine actions in the central nervous system, but the roles of melanocortins in anorexia and hypoactivity occurring during the acute phase response are unknown. The present study was designed to determine the effects of exogenous and endogenous α-melanocyte stimulating hormone (α-MSH) on lipopolysaccharide (LPS)-induced anorexia in relation to their effects on fever. Rats were fasted overnight to promote feeding behavior, then injected intraperitoneally with LPS (100 μg/kg ip), followed 30 min later by intracerebroventricular injection of either α-MSH or the melanocortin receptor subtype 3/subtype 4 (MC3-R/MC4-R) antagonist SHU-9119. Food intake, locomotor activity, and body temperature (Tb) were monitored during the ensuing 24-h period. Each of two intracerebroventricular doses of α-MSH (30 and 300 ng) potentiated the suppressive effects of LPS on food intake and locomotion, despite the fact that the higher dose alleviated LPS-induced fever. In control rats that were not treated with LPS, only the higher dose of α-MSH significantly inhibited food intake, and Tb and locomotor activity were unaffected. To assess the roles of endogenous central melanocortins, LPS-treated rats received intracerebroventricular SHU-9119 (200 ng). Central MC3-R/MC4-R blockade did not affect Tb or food intake in the absence of LPS treatment, but it reversed the LPS-induced reduction in 24-h food intake and increased LPS-induced fever without altering the LPS-induced suppression of locomotion. Taken together, the results suggest that exogenous and endogenous melanocortins acting centrally exert divergent influences on different aspects of the acute phase response, suppressing LPS-induced fever but contributing to LPS-induced anorexia and hypoactivity.

It has recently been reported that both physical and psychological stress elevate plasma inter-leukin (IL)-6 levels independently of endotoxemia, tissue damage, or inflammation. However, the mechanism of plasma IL-6 elevation in these models is poorly understood. In the present study, plasma IL-6 levels were measured using the IL-6-dependent murine hybridoma subclone B9 cell line, which is commonly used by other investigators. We first demonstrated that an immobilization (IM) stress, a typical physicopsychological stress, increased plasma IL-6 levels. Then the contribution of the hypothalamic-pituitary-adrenal (HPA) axis and the central and peripheral catecholaminergic systems in IM-induced plasma IL-6 elevation were examined because these mechanisms play important roles in host defense against stress. Blood samples were collected through an indwelling jugular venous catheter before, during, and after IM; the number of samples taken serially from each animal was 12-13. Blood cells were resuspended in a saline solution and injected into the animals through the same catheter after each blood collection in order to prevent loss of blood volume. After initiation of restraint, plasma IL-6 levels significantly increased at 60 min and peaked at 90 min in the animals immobilized for either 30 or 120 min. The peak levels of IM-induced plasma IL-6 in the animals immobilized for 120 min (1,905 ± 414 U/ml) were significantly higher than those in the animals subjected to 30 min IM (837 ± 95 U/ml; p < 0.05). In the hypophysectomized (Hypox) or adrenalectomized (ADX) rats, the peak levels of IM-induced (60 min) plasma IL-6 were considerably higher than in sham-operated rats (Hypox: 33,281 ± 4,983 U/ml at 150 min; ADX: 52,020 ± 19,231 U/ml at 120 min). In addition, in order to determine the possible involvement of endogenous catecholamines in IM-induced plasma IL-6 elevation, 6-hydroxydopamine (6-OHDA) was injected into the lateral cerebral ventricle (i.cv., 100 µg/rat) or jugular vein (i.v., 100 mg/kg) of the rats, and the animals were exposed to IM stress 1 week (i.cv.) or 3 days (i.v.) after injection. Both i.cv. and i.v. injections of 6-OHDA markedly attenuated the plasma IL-6 response to IM as compared to the respective vehicle-injected group, whereas the plasma adrenocorticotropic hormone response to IM stress was reduced only by pretreatment with an i.cv. injection of 6-OHDA. These data suggest that (1) neither the pituitary nor the adrenal gland is a major source of increased plasma IL-6 induced by IM stress; (2) the HPA axis exhibits a suppressive regulatory role in the IM-induced IL-6 response; (3) both the central and peripheral catecholamines play critical roles in causing IL-6 elevation induced by IM stress, and (4) the involvement of peripheral catecholamines in elevating plasma IL-6 by IM stress is independent of HPA axis activation. Moreover, to determine whether splenocytes are the source of IM-induced plasma IL-6, splenectomized (Splex) rats were examined for their plasma IL-6 response to IM stress. The peak IL...

Systemically administered α-melanocyte-stimulating hormone (α-MSH) inhibits endotoxin (lipopolysaccharide; LPS)- or interleukin (IL)-1-induced fever and adrenocortical activation, but the sites of these actions and the mechanisms involved are unknown. The aims of this study were, first, to determine whether melanocortin receptors (MCR) located within the central nervous system mediate the suppressive effects of peripherally administered α-MSH on LPS-induced fever and activation of the pituitary-adrenal axis and, second, to determine whether systemic α-MSH suppresses the LPS-induced rise in plasma IL-6 levels, potentially contributing to its antipyretic effect. Male rats received Escherichia coli LPS (25 μg/kg ip). Core body temperatures (Tb) were determined hourly by radiotelemetry (0–8 h), and blood was withdrawn via venous catheters for plasma hormone immunoassays (0–2 h) and IL-6 bioassay (0–8 h). α-MSH (100 μg/kg ip) completely prevented the onset of LPS-induced fever during the first 3–4 h after LPS and suppressed fever throughout the next 4 h but did not affect Tb in afebrile rats treated with intraperitoneal saline rather than LPS. Intraperitoneal α-MSH also suppressed the LPS-induced rise in plasma IL-6, ACTH, and corticosterone (CS) levels. Intracerebroventricular injection of SHU-9119, a potent melanocortin-4 receptor (MC4-R)/MC3-R antagonist, completely blocked the antipyretic effect of intraperitoneal α-MSH during the first 4 h after LPS but had no effect on α-MSH-induced suppression of LPS-stimulated plasma IL-6 and CS levels. Taken together, the results indicate that the antipyretic effect of peripherally administered α-MSH during the early phase of fever is mediated by MCR within the brain. In contrast, the inhibition of LPS-induced increases in plasma CS and IL-6 levels by intraperitoneal α-MSH appears to be mediated by a different mechanism(s), and these effects do not contribute to its antipyretic action.