Role of IL-1alpha and IL-1beta in ischemic brain damage.

The cytokine interleukin-1 (IL-1) has been strongly implicated in the pathogenesis of ischemic brain damage. Evidence to date suggests that the major form of IL-1 contributing to ischemic injury is IL-1beta rather than IL-1alpha, but this has not been tested directly. The objective of the present study was to compare the effects of transient cerebral ischemia [30 min middle cerebral artery occlusion (MCAO)] on neuronal injury in wild-type (WT) mice and in IL-1alpha, IL-1beta, or both IL-1alpha and IL-1beta knock-out (KO) mice. Mice lacking both forms of IL-1 exhibited dramatically reduced ischemic infarct volumes compared with wild type (total volume, 70%; cortex, 87% reduction). Ischemic damage compared with WT mice was not significantly altered in mice lacking either IL-1alpha or IL-1beta alone. IL-1beta mRNA, but not IL-1alpha or the IL-1 type 1 receptor, was strongly induced by MCAO in WT and IL-1alpha KO mice. Administration (intracerebroventricularly) of recombinant IL-1 receptor antagonist significantly reduced infarct volume in WT (-32%) and IL-1alpha KO (-48%) mice, but had no effect on injury in IL-1beta or IL-1alpha/beta KO mice. These data confirm that IL-1 plays a major role in ischemic brain injury. They also show that chronic deletion of IL-1alpha or IL-1beta fails to influence brain damage, probably because of compensatory changes in the IL-1 system in IL-1alpha KO mice and changes in IL-1-independent mediators of neuronal death in IL-1beta KO mice.

Importance of brain IL-1 type II receptors in fever and thermogenesis in the rat.

Interleukin-1 (IL-1) acts centrally to induce fever and thermogenesis in rodents. The central actions of IL-1 alpha and IL-1 beta apparently involve different mechanisms, and the effects of IL-1 beta are not consistent with interaction with a type I (IL-1RI) 80-kDa receptor. In the present study the involvement of the type II IL-1 receptor (IL-1RII) was tested in the rat by examining the effects of central injection of a monoclonal antibody (ALVA-42), which blocks the IL-1RII. Pretreatment of rats with ALVA-42 (6 micrograms icv) inhibited the thermogenic and pyrogenic responses to intracerebroventricular injection of 5 ng (but not 50 ng) of IL-1 beta in conscious rats but did not significantly modify responses to IL-1 alpha. ALVA-42 also failed to modify the responses to peripherally administered IL-1 beta (1 microgram) but significantly attenuated the pyrogenic and thermogenic responses to peripheral (125 micrograms) or central (1 microgram) injection of endotoxin. These data indicate that IL-1RII mediates the central effects of a low dose of IL-1 beta, but not IL-1 alpha, on fever and thermogenesis in the rat. They also imply that responses to endotoxin are due, at least in part, to the activation of IL-1RII by IL-1 beta released within the brain and that effects of peripherally injected IL-1 beta involve different mechanisms, probably associated with IL-1RI.

Thermogenic actions of tryptophan in the rat are mediated independently of 5-HT.

Serotonin (5-HT) has been implicated in the central control of energy balance, via inhibition of food intake and stimulation of thermogenesis. Its rate of synthesis in brain is dependent on the availability of its precursor amino acid, tryptophan. The objective of the present study was therefore to investigate the thermogenic actions of tryptophan and to determine whether these actions are mediated by 5-HT. Central or peripheral injections of 5-HT (i.c.v.; 0.5-40 micrograms), 5-hydroxytryptophan (5-HTP) (i.c.v.; 20 micrograms) or tryptophan (i.p.; 20 mg/kg, i.c.v.; 12-60 micrograms) significantly increased resting oxygen consumption (VO2 by approximately 15-20%) in conscious rats, without apparent effects on physical activity. Small increases (5-7%) in VO2 were also observed following peripheral injections of aspartate or glycine (20 mg/kg) but not taurine, whilst central injections of tyrosine or leucine (15-18 micrograms) significantly increased VO2 by 15%. We have previously reported that the thermogenic and anorexic actions of 5-HT are mediated by corticotropin-releasing factor (CRF). In the present study, the thermogenic actions of 5-HTP, like those of 5-HT, were significantly reduced by pretreatment (5 min before) with the CRF antagonist alpha-helical CRF9-41 (25 micrograms, i.c.v.) or a polyclonal antibody to CRF. However, the thermogenic actions of tryptophan were not significantly modified by pretreatment with either the 5-HT antagonist, methysergide (20 micrograms, i.c.v.) or with the CRF antagonist or antibody and thus appear to act through different mechanisms to 5-HT.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Central actions of CRF on thermogenesis are mediated by pro-opiomelanocortin products.

Corticotrophin-releasing factor (CRF) causes central activation of thermogenesis. The aim of this study was to investigate whether this action is mediated by ACTH or other peptides derived from the ACTH precursor pro-opiomelanocortin (POMC) within the CNS. Central (intracerebroventricular) injection of rat CRF caused dose-dependent increases in resting oxygen consumption (VO2) in conscious rats (maximal 26 +/- 5% at 2 nmol CRF). These responses were significantly attenuated by pretreatment (i.c.v.) with either a monoclonal antibody raised to gamma 1MSH or with naloxone which antagonises beta-endorphin (beta-EP) actions. The increases were not affected by pretreatment with monoclonal antibodies to ACTH or the N-terminal of POMC. Central injections of gamma 1-melanocyte-stimulating hormone (MSH) or beta-EP caused dose-dependent increases in VO2 (maximal at 0.5-1.5 pmol) and these were markedly inhibited by pretreatment with the anti-gamma 1-MSH antibody or naloxone respectively. Injection of ACTH or alpha MSH did not significantly affect VO2 at doses up to 2 nmol. These data indicate that the central actions of CRF on thermogenesis may be mediated, at least in part, by release of gamma MSH and/or beta-EP.

Impaired effects of interleukin-1 beta on fever and thermogenesis in genetically obese rats.

Interleukin-1 beta (IL-1 beta) is an endogenous peptide which induced fever (1.8 degrees C rise in colonic temperature) when injected interacerebroventricularly (i.c.v., 80ng human recombinant IL-1 beta) into conscious lean rats. IL-1 beta also stimulated resting oxygen consumption (VO2) by 38 percent, in vitro thermogenic activity (mitochondrial GDP binding) of brown adipose tissue (BAT) by almost two-fold, and blood flow to brown fat (assessed from the distribution of radiolabelled microspheres) by nine-fold in lean animals. Genetically obese Zucker rats showed only small increases in temperature (0.5 degrees C), VO2 (15 percent) and blood flow to BAT (less than two-fold), and no change in GDP binding, but exhibited normal thermogenic responses to i.c.v. injection of corticotrophin releasing factor (GRF). The results indicate that the obese Zucker is insensitive to the central effects of interleukin; this may explain the reduced febrile responses to endotoxin which have previously been reported.

Opposing effects of activation of central GABAA and GABAB receptors on brown fat thermogenesis in the rat.

Baclofen (a GABAB agonist) stimulates body temperature, metabolic rate and brown adipose tissue (BAT) in the rat through a central action, but no effects of gamma-aminobutyric acid (GABA) itself on these parameters were observed. In the present study, it was found that the central effects of (+/-)baclofen (0.5-2.0 micrograms injection i.c.v.) on the temperature (1.2 degrees C increase) and metabolic rate (44-76% increase) of brown adipose tissue were inhibited by previous treatment with the GABAA agonist, muscimol (0.05 micrograms). Injection of GABA alone (12 micrograms) did not significantly affect these parameters, but in the presence of the GABAA antagonist bicuculline (2.5 micrograms), GABA significantly increased the temperature (0.3 degrees C) and oxygen consumption (22%) of brown fat. (-)Baclofen was found to be approximately 50-times more effective in stimulating the temperature of brown adipose tissue than (+/-)baclofen. The results indicate that activation of central GABAB receptors stimulates the activity and hence metabolic rate of brown adipose tissue. However, activation of the GABAA receptors opposes the effects of GABAB stimulation on the thermogenesis of brown fat.

Activation of brown fat thermogenesis in response to central injection of corticotropin releasing hormone in the rat.

Injection of CRF-41 (2-5 nmol) into the third ventricle, or the paraventricular nucleus of anaesthetized rats caused a marked rise in the temperature of the interscapular brown adipose tissue (BAT) depot (peak rise 0.8-1 degree C) which was inhibited by prior intravenous injection of propranolol. There was also a significant increase in the mitochondrial proton conductance pathway of brown adipose tissue, assessed from the binding of guanosine diphosphate (GDP) to mitochondria isolated from the interscapular (89% above control) and perirenal and para-aortic depots (130%). Acute surgical sympathectomy of interscapular brown adipose tissue immediately prior to injection of CRF significantly attenuated the increase in mitochondrial GDP-binding. Hypophysectomized (HYPX) rats showed a large (180%) increase in GDP-binding of brown adipose tissue 7 days after surgery and this was almost completely prevented by denervation of the interscapular depot prior to hypophysectomy. Acute injection of morphine also reduced the GDP-binding in hypophysectomized, but not in control rats. These data demonstrate that central injection of CRF stimulates thermogenesis in brown adipose tissue, probably by modifying sympathetic outflow. The activation of brown adipose tissue following hypophysectomy was also dependent on the sympathetic innervation and could be due to an increase in release of CRF.