Effect of Melatonin on the Reproductive Systems of Male and Female Syrian Hamsters: A Diurnal Rhythm in Sensitivity to Melatonin¹²

Abstract

Hamsters were maintained on a long photoperiod (14L:10D) and were injected once daily with melatonin (10-25 mug) or sesame oil. Males which received melatonin during the afternoon (e.g., 6.5-13.75 h after lights-on) showed regressed testes and decreased levels of serum LH and FSH after several weeks of treatment. Injections of the oil vehicle or injections of melatonin given in the morning (3 h after lights-on) had no detectable effect on testicular size or on serum gonadotropins. Females which received melatonin during the afternoon became acyclic after several weeks of treatment and showed a diurnal pattern of LH secretion. The acyclic females required 4-6 weeks to resume estrous cyclicity following termination of the melatonin injections. The effects of melatonin on gonadal function and on serum gonadotropin concentrations in both sexes were similar to the previously observed effects of prolonged exposure to short photoperiods. These results indicate that chronic daily injections of melatonin can depress reproductive function in hamsters and that the effectiveness of the injections is dependent upon the time of day at which they are administered.

Plasma melatonin profiles of Romney Marsh sheep in natural photoperiod and in acutely extended darkness

“…In sheep and some other species the secretion of melatonin has generally been reported to follow the duration of the dark period (Rollag & Niswender, 1976;Kennaway et al, 1977;Lincoln & Short, 1980;Arendt et al, 1981) and this mechanism is believed to underlie the seasonal repro¬ ductive response given the longer period of melatonin secretion in winter than summer (Maywood et al, 1990). However, another major hypothesis is centred around the possibility that there is a phase of the circadian cycle sensitive to the presence of melatonin (Tamarkin et al, 1976;Rollag et al, 1978) …”

What is the role of melatonin within the anterior pituitary?

“…Removal of the pineal gland or ablation of circadian input to the pineal abolish both the normal pattern of melatonin synthesis and seasonal responses (Lincoln et al 1989). Conversely, exogenous melatonin delivered by injection or infusion can mimic the seasonal effects of changing photoperiod (Elliott et al 1989, Maywood et al 1990, Bartness et al 1993, Tamarkin et al 1977.…”

Evidence for prolactin-releasing and release-inhibiting effects of melatonin, serotonin and arginine vasotocin.

“…Hence, the PRL releasing action of the pineal gland would be ascribed to these substances. However, since the antigonadotropic action of melatonin was not observed except when there was an intact pineal gland and when melatonin was injected in the afternoon (Tamarkin et al 1976), we hypothesized that melatonin would not play a stimulatory role on PRL secretion under certain experimental conditions. In the present study, 50µg melatonin lowered serum PRL levels of PX rats in both OVX-EP and postpartum groups.…”