Intraventricular injection of melatonin inhibits naloxone-induced, but not NMDA- or LHRH-induced LH release in ovariectomized estrogen-primed rats.

The present study was aimed to examine the possible functional relationship between melatonin and hypothalamic transmitters, endogenous opioids and excitatory amino acids in controlling gonadotropin secretion in ovariectomized estrogen-primed rats. An intravenous injection of naloxone (mu opioid receptor antagonist), N-methyl-D-aspartate (NMDA; NMDA receptor agonist) or luteinizing hormone-releasing hormone (LHRH) significantly elevated serum luteinizing hormone (LH) concentrations within 10 min. An intraventricular treatment with melatonin, which did not affect the basal LH concentration by itself, significantly suppressed the effect of naloxone. However, the same melatonin treatment did not inhibit the NMDA-induced or LHRH-induced LH secretion. These results support the hypothesis that melatonin has a suprapituitary site of action to inhibit LHRH release, and suggest that the site of its action may be located downstream to that of naloxone action and upstream to that of NMDA in the hypothalamic LHRH neuronal pathway.