Effects of exogenous and endogenous melatonin on gonadal function in hamsters

In this article we review the functions of the pineal gland and of pineal melatonin in regulating gonadal activity in three species of hamster, the golden (Syrian) hamster (Mesocricetus auratus), the Turkish (Brandt's) hamster (Mesocricetus brandti), and the Djungarian (Siberian, or hairy-footed) hamster (Phodopus sungorus sungorus). Some experimental treatments elicit the same response in all three species; this is especially true for the regulation of the nocturnal elevation of pineal melatonin content and release and for the gonadal response to timed melatonin injections in intact animals throughout the day. In other experimental paradigms, species differences are paramount; pinealectomy or subcutaneous implants of melatonin in intact individuals have different effects on the photoperiodic gonadal response in all species. Yet in all hamster species investigated, the pineal gland and its hormone melatonin constitute part of the central mechanism whereby environmental information (photoperiod) is transduced to neuroendocrine signals responsible for the functional integrity of the reproductive system.