Melatonin Inhibits the Central Sympatho-adrenomedullary Outflow in Rats

Central effects of melatonin on the sympatho-adrenomedullary outflow were investigated in urethane-anesthetized rats. In the intact animals, intracerebroventricularly (i.c.v.) administered interleukin-1beta (IL-1beta) (100 ng/animal) slightly, but significantly, elevated the plasma level of noradrenaline (NA), but not the level of adrenaline (Ad). Melatonin (100 microg/animal, i.c.v.) did not modulate the effects of IL-1beta on plasma levels of catecholamines. In the pinealectomized animals, however, the same dose of IL-1beta markedly elevated plasma levels of both Ad and NA, and the elevation of 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-operated control animals. Furthermore, the IL-1beta-induced elevations of plasma catecholamines in these pinealectomized animals were attenuated by i.c.v. administered melatonin. These results suggest that melatonin plays an inhibitory role in the central regulation of sympatho-adrenomedullary outflow in rats.