Abstract
Our group has shown that daily administration of melatonin (MT) markedly delays sexual maturation in the male rat (Endocrinol. 112,1578,1983 and 115,2303,1984). In this study, we have evaluated the level of tonic inhibition by opiates in normal 40-day old rats, and in rats with delayed sexual development induced by daily MT (100 ug) injection between 20 and 40 days. Naloxone (NAL) s.c. injection (2.5 mg/kg) produced a significant increase of plasma LH in normal rats, not seen in MT-treated rats. Injection of morphine sulphate (MS) or of the potent Met-Enkephalin analog FK-33-824 (FK) inhibited LH secretion in control rats. In MT-treated rats, the low plasma LH levels were not affected by opiates. Pretreatment with MS, or with the FK agonist prevented the NAL-induced rise of LH in rats not treated with MT. Plasma PRL levels were decreased after NAL both in untreated- and MT-treated rats. In keeping with the observation that MT no longer inhibits sexual functions in adult rats, LH response to NAL was normal in adult rats that have been treated for 20 days with MT. These results demonstrate that MT may potentiate or mimic the tonic inhibition of LH secretion exerted by endogenous opiates during sexual development. They reinforce the concept that modulation of opiate control is important for the progress of sexual development.
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