The analgesic effects of peripheral and central administration of melatonin in rats

To explore the site and mechanism of the analgesic action of melatonin, the present study was designed to evaluate the analgesic effects of intraperit oneal (i.p.) and intracerebroventricular (i.c.v.) administration of melaton in, and to investigate the effect of i.c.v. naloxone on the analgesic effec t induced by i.p. melatonin in rats. Antinociception was determined by tail -flick latency to hot water at 50 degrees C. On i.p. administration, melato nin (30, 60 and 120 mg/kg) produced the antinociceptive effect in a dose-de pendent manner, with an A(50) of 72.8 mg/kg. Administered i.c.v., melatonin (0.25, 0.5 and 1 mg/kg) also resulted in dose-dependent antinociception, w ith an A(50) of only 0.693 mg/kg. Injected i.c.v, to rats, 10 mu g of nalox one antagonized significantly the antinociceptive effect induced by i.p. me latonin. It is concluded that melatonin has an analgesic effect in rats and the central nervous system (CNS) may be the primary site for melatonin to elicit the response, and the effect of melatonin is related to the central opioid system. (C) 2000 Elsevier Science B.V. All rights reserved.