Melatonin inhibits the central sympatho-adrenomedullary outflow in rats

Central effects of melatonin on the sympatho-adrenomedullary outflow were i nvestigated in urethane-anesthetized rats. In the intact animals, intracere broventricularly (i.c.v.) administered interleukin-lp (IL-1 beta) (100 ng/a nimal) slightly, but significantly, elevated the plasma level of noradrenal ine (NA), but not the level of adrenaline (Ad). Melatonin (100 mu g/animal, i.c.v.) did not modulate the effects of IL-1 beta on plasma levels of cate cholamines. In the pinealectomized animals, however, the same dose of IL-1 beta markedly elevated plasma levels of both Ad and NA, and the elevation o f Ad was more potent than that of NA. In these pinealectomized animals, the serum level of melatonin was significantly lower than that in the sham-ope rated control animals. Furthermore, the IL-1 beta-induced elevations of pla sma catecholamines in these pinealectomized animals were attenuated by i.c. v. administered melatonin. These results suggest that melatonin plays an in hibitory role in the central regulation of sympatho-adrenomedullary outflow in rats.