Different receptor mechanisms mediate the pyrogenic and behavioral effects of interleukin 1.

Interleukin 1 (IL-1) is a cytokine released during immune activation that mediates the host's response to infection and inflammation. Peripheral and central injections of IL-1 induce fever and sickness behavior, including decreased food motivation and reduced interest in social activities. To determine the receptor mechanisms responsible for these effects, rats were injected with IL-1 receptor antagonist (IL-1ra), an endogenous cytokine that acts as a pure antagonist of IL-1 receptors. IL-1ra blocked the increased body temperature and oxygen consumption induced by injection of recombinant human IL-1 only when both cytokines were administered i.p. In contrast, i.p. or intracerebroventricular administration of IL-1ra blocked the depressive effect of IL-1 beta on food-motivated behavior and social exploration when this cytokine was administered by the same route as the antagonist. In addition, intracerebroventricular IL-1ra blocked the reduction in social exploration produced by i.p. IL-1 beta but had only partial antagonist effects on the decrease in food-motivated behavior induced by i.p. IL-1 beta. In each case, the dose of IL-1ra was 100- to 1000-fold in excess of the biologically active dose of IL-1. These results suggest that the receptor mechanisms that mediate the behavioral and pyrogenic effects of IL-1 are heterogeneous.

Inhibition of central actions of cytokines on fever and thermogenesis by lipocortin-1 involves CRF.

In the present studies, the mechanisms underlying the inhibitory actions of lipocortin-1 on the pyrogenic and thermogenic properties of cytokines were investigated. Central (icv) injection of corticotropin-releasing factor (CRF, 4.7 micrograms) or the recombinant cytokines interleukin (IL)-1 alpha (50 ng), IL-1 beta (5 ng), IL-6 (20 ng), IL-8 (20 ng), or tumor necrosis factor-alpha (TNF-alpha, 1 microgram) in conscious rats produced significant increases in resting oxygen consumption (VO2, 13-26%) and colonic temperature (0.7-1.6 degrees C) within 2 h postinjection. Administration (icv) of a recombinant fragment (NH2-terminus, 1-188 amino acids) of human lipocortin-1 (1.2 micrograms) produced small increases in VO2 (< 5%) and body temperature (< 0.3 degrees C). Pretreatment (-5 min) with lipocortin-1 significantly attenuated the thermogenic and pyrogenic effects of centrally injected IL-1 beta (80% inhibition), IL-6 (60%), IL-8 (80%), or CRF (60%). However, pretreatment with lipocortin-1 failed to modify the actions of IL-1 alpha or TNF-alpha. We have previously demonstrated that the pyrogenic and thermogenic effects of IL-1 beta, IL-6, and IL-8 are dependent on the central actions of CRF, whereas IL-1 alpha and TNF-alpha act independently of CRF. Fever and thermogenesis induced by all of these cytokines (with the exception of IL-8) can also be prevented by administration of a cyclooxygenase inhibitor. The data presented here suggest that the potent antipyretic effects of lipocortin-1 may result from inhibition of the release or actions of CRF rather than modulation of eicosanoid synthesis.

Interleukin-6 is a centrally acting endogenous pyrogen in the rat.

Intracerebroventricular (i.c.v.) injection of human recombinant interleukin-6 (IL-6; 20-100 ng) caused significant increases in colonic temperature and resting oxygen consumption (VO2) in conscious rats. These effects were prevented by pretreatment with a cyclooxygenase inhibitor (flurbiprofen, 1 mg/kg, i.p.) or a corticotrophin-releasing factor antagonist (alpha-helical CRF9-41, 25 micrograms, i.c.v.). Higher doses of IL-6 (i.c.v.) caused only small changes in VO2 and temperature, and very high doses given intravenously (i.v.) (4 micrograms/kg) were required to stimulate these parameters. Central injection of anti-rat IL-6 antibody inhibited the effects of interleukin-1 beta (i.c.v.) or endotoxin injection (i.p.) on colonic temperature and VO2 in conscious rats. These data indicate that IL-6 is an important endogenous pyrogen that acts within the central nervous system.

Thermogenic effects of the antiglucocorticoid RU-486 in the rat: involvement of corticotropin-releasing factor and sympathetic activation of brown adipose tissue.

Acute injection (sc) of the antiglucocorticoid RU-486 (5-10 mg/kg) stimulated oxygen consumption (VO2) and brown adipose tissue (BAT) activity (mitochondrial GDP binding) in the rat. A peak effect was seen 60-80 min after injection. The rise in VO2 was prevented by prior injection of the beta-adrenergic antagonist propranolol, and the effect on BAT was abolished by surgical denervation of the sympathetic supply to the tissue. Central injection (cerebroventricular) of a much lower dose (3-8 micrograms/kg) of RU-486 also stimulated VO2, and this effect was inhibited by a CRF receptor antagonist [alpha-helical CRF-(9-41)]. Peripheral injection of RU-486 also elicited acute thermogenic responses in older (greater than 12 months) rats and in genetically obese (Zucker) rats. In lean animals, daily injection of RU-486 inhibited weight gain and stimulated BAT without affecting food intake. The thermogenic effects of RU-486 appear to be due to central stimulation of sympathetic outflow and may involve CRF release. The data support previous studies on the effects of adrenalectomy and CRF on thermogenesis in the rat.