Melatonin inhibition of reproduction in the male hamster: Its dependency on time of day of administration and on an intact and sympathetically innervated pineal gland

The daily s.c. injection of 25 μg melatonin (MEL) in oil into adult male hamsters at 7 p.m. (lights on 6 a.m. to 8 p.m.) for 50 days caused involution of the testes, coagulation of glands and seminal vesicles and depression in pituitary prolactin (Prl) levels. Similar injections of MEL given at9 a.m. completely failed to cause regression of the sex organs or a depression in pituitary Prl levels. Injections of MEL in the p.m. were completely ineffective in inhibiting either the growth of the gonads and adnexa or the pi-tuitary Prl levels if the animals had been pinealectomized. Likewise, superior cervical ganglionectomy, decentralization of the superior cervical ganglia and anterior hypothalamic deaH'cretation, procedures which interfere with the sympathetic nerve supply to the pineal gland, negated the ability of p.m. MEL injections to inhibit reproduction in male hamsters. The results indicate that daily MEL injections are capable of suppressing reproductive physiology in male hamsters, but only when the indole is injected late in the light period, in this case, 13 h after lights on. The findings also illustrate that daily p.m. MEL injections can inhibit reproduction only in animals that have an intact and sympathetically innervated pineal gland.