Melatonin prevents decrease in plasma PRL and LH levels in male hamsters exposed to a short photoperiod.

Treatment of male golden hamsters with melatonin injections can cause a decrease in plasma gonadotropin and Prl levels and induce testicular regression. However, administration of melatonin via subcutaneously implanted Silastic capsules can prevent short photoperiod from causing the testes to regress. In order to explain this effect of melatonin capsules, we examined plasma Prl, LH and FSH levels in pinealectomized and sham-operated hamsters in which empty or melatonin-filled Silastic capsules had been implanted. After implantation, the hamsters were transferred to a short photoperiod (5 h L:19 h D) for 9 weeks. Both pinealectomy and melatonin completely prevented the decline in plasma Prl and LH levels and in the weight of the testes and the seminal vesicles. Moreover, testicular weight and plasma Prl and LH levels were higher in pinealectomized animals given melatonin capsules than in pinealectomized animals given empty capsules. In order to obtain some indication whether melatonin may affect testicular function directly, we have examined the influenced of melatonin on the production of testosterone by hamster testes in response to hCG in vitro. Addition of 0.2, 10 or 500 ng of melatonin per ml of incubation medium had no effect on testosterone production in this system. It is concluded that melatonin capsules prevent regression of the male reproductive system in short photoperiod, most likely by preventing the decline in plasma Prl and LH levels and that these effects of melatonin are not mediated through or dependent on the pineal.