MODULATION OF SYMPATHETIC NEUROTRANSMISSION BY MELATONIN

Melatonin is of considerable interest for its regulatory influence on a variety of physiological processes including biological rhythms and neuroendocrine functions. We showed that melatonin potentiates sympath etic neurotransmission in the prostatic portion of the rat vas deferen s, by increasing contractions in response to noradrenaline and ATP rel eased by acetylcholine stimulation of presynaptic nicotinic receptors. Melatonin in vitro (100 pg/ml; for 4 h) increased the maximal acetylc holine-induced contraction only when the hypogastric ganglion was pres ent, and this effect was blocked by cycloheximide (100 mu g/ml). Melat onin also modulated the sympathetic trophic influence on smooth muscle , since it reduced [S-35]methionine incorporation into the vas deferen s in the hypogastric ganglion-vas deferens preparation. Thus, it is su ggested that the regulation of protein synthesis might be one of the m echanisms whereby melatonin modulates endogenous rhythms and synchroni zes them to the environmental light cycle.