Effects of Melatonin on the Reproductive System in Intact and Pinealectomized Male Hamsters Maintained under Various Photoperiods*

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Abstract
Male hamsters were maintained under various photoperiods and were given daily treatment with melatonin. The objective of these studies was to learn more about the possible interactions between photoperiod, biological rhythms, and responses of the reproductive system to melatonin. Melatonin was administered three times daily by sc injection, with the injections spaced 3 h apart. This regimen of treatment caused testicular regression in pinealectomized hamsters maintained in continuous illumination as well as in animals kept on long (14 h light, 10 h dark) or short (10 h light, 14 h dark) photoperiods. When the injections were administered during the light phase in the long photoperiod, pinealectomized hamsters showed a greater response than did intact animals. Superior cervical ganglionectomized males responded similarly to pinealectomized animals. Continuous release implants of melatonin blocked the response to three times per day injections when the dosage was 25 μg melatonin/injection. The results indicate that exogenous melatonin can cause testicular regression in pinealectomized hamsters irrespective of photoperiod. The data also suggest that the mode of melatonin administration is very important, as continuously elevated levels of melatonin (e.g. implants) may render the animal relatively insensitive to daily injections of the compound. This observation may provide a clue to the mode of action of endogenous melatonin, since a marked daily rhythm in melatonin synthesis seems to be characteristic for a wide variety of vertebrate species.