Influence of Exogenous Melatonin on Melatonin Secretion and the Neuroendocrine Reproductive Axis of Intact Male Rats during Sexual Maturation*

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Abstract
The influence of daily sc administration of melatonin (5–100 μg/day) on sexual development of prepubertal and pubertal male rats was studied in vivo. Adult animals were also studied. When melatonin was injected daily into young animals starting at day 20 of age, dose-dependent reductions in plasma testosterone, testis and seminal vesicles weights, plasma FSH and LH levels, and pituitary GnRH receptor number were observed at day 40 or 45 of age when animals were killed. In contrast, prepubertal animals (5–20 days old) showed no significant responses to similar treatment with melatonin, whereas in adult animals (70–90 days old), melatonin elicited only a small decrease in plasma testosterone concentration. Melatonin analogs such as AT-acetyl-serotonin and 5-hydroxytryptophol administered daily from 20–45 days of age did not produce any effect. Chronic melatonin administration from 20–50 days of life did not alter the occurrence of the nocturnal rise of circulating plasma melatonin, but did enhance its amplitude. Our results demonstrate that exogenous melatonin can inhibit or delay sexual maturation in the male rat if administered between 20 and 40 days of age and suggest that this inhibitory action is exerted at the hypothalamic and/or pituitary level.

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