MELATONIN EFFECTS ON INHIBITION OF THIRST AND FEVER INDUCED BY LIPOPOLYSACCHARIDE IN RAT

In 24 h water deprived rats we have evaluated the effects of melatonin on the inhibition of thirst and on fever induced by Escherichia coli lipopolysaccharide. Intraperitoneal (i.p) injection of lipopolysacchar ide (0.32, 0.64 and 0.96 mg/kg) alone induced, a dose-dependent and si gnificant inhibition of water intake as well as fever. In addition, li popolysaccharide at the same concentrations increased urinary prostagl andins and serum cytokines levels. On the contrary, lipopolysaccharide treatment had no effects on cerebral brain nitric oxide synthase acti vity. All lipopolysaccharide effects were reverted by a prior, concomi tant and subsequent i.p. treatment with melatonin (2, 4 and 6 mg/kg), whereas they were still present when melatonin was injected in combina tion with the melatonin receptor antagonist luzindole (15, 30 and 60 m g/kg, i.p.). We suggest that melatonin could exert its dipsogenic effe cts through a reduction of the free radical nitric oxide (NO.) whereas it may reduce body temperature by preventing an excessive formation o f prostaglandins and cytokines. (C) 1997 Elsevier Science B.V.