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 at 9 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 pituitary Prl levels if the animals had been pinealectomized. Likewise, superior cervical ganglionectomy, decentralization of the superior cervical ganglia and anterior hypothalamic deafferetation, 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.

Blinding adult male golden hamsters was followed by atrophy, within 12 weeks, of the testes and accessory sex organs (seminal vesicles and coagulating glands) and by a significant reduction in pituitary prolactin levels. In experiment 1 blind hamsters received subcutaneously implanted melatonin-beeswax (1:24 mg) pellets at the following intervals: once per week, per 2, 3, 4, 6 weeks, or only one pellet during the 12-week experimental period. The melatonin-beeswax pellets, regardless of the frequency of implantation, overcame completely the inhibitory effects of blinding on reproduction and nearly completely the depressant action of light deprivation on pituitary prolactin levels. In the second study the melatonin-beeswax pellets were implanted subcutaneously into blind hamsters every 2 weeks. The pellets contained either 1 mg, 500, 100, 50, or 1 µg melatonin. With the exception of the 1-µg dosage, melatonin again negated almost totally the inhibitory action of darkness on the gonads and accessory organs and also, for the most part, prevented the drop in pituitary prolactin levels. Based on these studies, when melatonin is chronically administered subcutaneously in a beeswax pellet the minimal dosage of melatonin required to counteract the inhibitory effect of darkness on reproduction seems to be less than 3.6 µg/day. The effects of chronic melatonin treatment are similar to those of pinealectomy.

Intact and pinealectomized adult male hamsters kept in light:dark cycles of 14:10 (in hours) were given daily injections of either 25 µg melatonin, 5-methoxy-tryptophol, N-acetylserotonin, or 6-hydroxymelatonin at 19.00 hours (13 h after lights on). The injections were continued for 50 consecutive days. The daily melatonin injections significantly elevated body weights and depressed the growth of the testes and accessory sex organs and pituitary prolactin levels in intact hamsters. By comparison, in pinealectomized animals daily afternoon melatonin injections failed to alter body or testicular weights or pituitary prolactin levels. These findings indicate that for single injections of melatonin to be antigonadotrophic in the male hamster, the pineal must be intact. In intact animals, 5-methoxytryptophol and N-acetylserotonin slightly depressed the growth of the reproductive organs and pituitary prolactin levels, respectively. Again, these effects were negated by removal of the pineal gland. 6-Hydroxymelatonin was without influence on any of the parameters of reproduction that were measured.

No abstract

Male and female hamsters were exposed to natural photoperiodic and temperature conditions beginning on November 13 and groups of hamsters were necropsied either on January 15 or on April 10. Throughout the study, half of the hamsters (control hamsters) received subcutaneous implants of beeswax (25 mg) every other week while the other half received beeswax pellets containing 1 mg melatonin. Control beeswax-treated males necropsied in January had atrophic testes and accessory sex organs while the uteri of the control females were infantile in appearance.Melatonin treatment prevented the atrophic responses of the testes and accessory sex organs in males and the uteri in females. Melatonin-treated hamsters killed in January also had higher plasma levels of immunoreactive LH while in females both pituitary and plasma levels of LH were elevated after melatonin treatment. By April 10, the testes and accessory organs of control males and the uteri of control females had regenerated to the adult condition and these organs were comparable in size to those in melatonin-treated hamsters. The pituitary glands of control males contained slightly more LH and much more FSH than the pituitaries of melatonin-treated males. Melatonin-treated females necropsied in April had pituitary and plasma levels of LH and FSH similar to those in beeswax-treated females killed at the same time. The data show that chronic melatonin treatment prevents atrophy of the reproductive organs of male and female hamsters which is associated with the shortened days of the winter.