Daily melatonin injection mimic the short day-induced increase in negative feedback effects of testosterone on gonadotropin secretion in hamsters

One of the mechanisms underlying short day-induced testicular regression in golden hamsters is an increase in hypothalamic-pituitary sensitivity to the negative feedback effects of gonadal steroids on gonadotropin secretion. Since pinealectomy abolishes this effect of short days, and daily afternoon melatonin injections given to hamsters maintained on long days lead to testicular regression, we investigated whether melatonin may exert its antigonadal effect by increasing hypothalamic-pituitary sensitivity to negative feedback effects of testosterone on luteinizing hormone (LH) and follicle-stimulating hormone (FSH) release. Intact and long-term castrated male golden hamsters were either exposed to 6L:18D for 14 weeks, or maintained on 14L:10D for 14 weeks and injected daily with either oil or 25 μg melatonin 4 h before lights off. During Weeks 10-14, the castrated hamsters received various doses of testosterone by subcutaneous implantation of either a 2, 4, 8 or 20 mm-long testosterone-filled Silastic capsule. Both exposure to 6L:18D and daily melatonin injections led to testicular regression within 6-8 weeks in the intact hamsters. In castrated hamsters on a 14L:10D cycle receiving oil injections, serum LH and FSH were suppressed only by the 20 mm testosterone capsule. In castrated hamsters on a 14L:10D cycle receiving melatonin injections, and in castrated hamsters exposed to 6L:18D, serum LH and FSH were suppressed to levels close to or below the limits of the assays by even the 2 mm capsule, as well as all other doses of testosterone administered. This study demonstrates that melatonin, like exposure to short days, can cause an increase in hypothalamic-pituitary sensitivity to the negative feedback effects of gonadal steroids. These results, along with those of other studies, suggest that short day-induced testicular regression may be due to antigonadotropic actions of melatonin.