MELATONIN INHIBITS NALOXONE-INDUCED LUTEINIZING-HORMONE RELEASE IN OVARIECTOMIZED ESTROGEN-PRIMED RATS

The present study aimed to examine the effect of melatonin on naloxone -induced luteinizing hormone (LH) secretion in ovariectomized estrogen -primed rats, A single intracerebroventricular (ICV) injection of nalo xone (mu opioid receptor blocker, 15 mu g) or an intravenous (IV) inje ction of LH-releasing hormone (LHRH, 50 ng/kg) elicited a transient an d significant increase in the serum LH concentration within 10 min, Wh ile an ICV injection of 100 ng melatonin by itself did not change the basal LH release, it almost completely inhibited the naloxone-induced LH release, Melatonin (10 ng) also significantly reduced the effect of naloxone, However, an ICV injection of 100 ng melatonin did not affec t the LHRH-induced LH release, In separate experiments, the effect of melatonin on naloxone-induced pulsatile LH secretion was studied in es trogen-treated rats, A continuous IV infusion of naloxone (20 mg/kg/h) induced LH pulses in rats treated ICV with saline. An ICV administrat ion of 100 ng melatonin, which by itself did not affect basal LH secre tion, significantly reduced the frequency, but not the amplitude, of L H pulses induced by the naloxone infusion. These results show that mel atonin has a suprapituitary site of action to inhibit naloxone-induced LH release, and suggest that melatonin has an effect in inhibiting th e activity of the hypothalamic LHRH pulse generator, either directly o r indirectly, in female rats.