Pineal Melatonin Content in Male Hamsters throughout the Seasonal Reproductive Cycle

The daily rhythm in pineal melatonin content has been characterized at 10-week intervals in male hamsters sequentially exposed to long photoperiods (14 hr light: 10 hr dark) for 10 weeks, short photoperiods (8 hr light: 16 hr dark) for 30 weeks, and then long photoperiods once again for 10 weeks. These photoperiodic manipulations induced the gonadal changes associated with the seasonal reproductive cycle experienced by this species. At the end of the initial long photoperiod, mean melatonin concentrations of 79-114 pg/gland were found throughout the day and the first 6 hr of darkness and a peak concentration of 949 pg/gland was found 8 hr after the onset of dark. Hamsters exposed to short photoperiods for either 10, 20, or 30 weeks had pineal melatonin concentrations ranging from 50 to 140 pg/gland throughout the day and the first 7 hr of darkness and peak concentrations ranging from 764 to 1025 pg/gland 13 hr after the light to dark transition. The last group of hamsters, which had been in long photoperiods for 10 weeks after exposure to 30 weeks of short photoperiods, had a melatonin concentration profile similar to that of the first group. If, in fact, pineal melatonin levels are related to the amount of indole secreted then the present results do not support the hypotheses which require dramatic alterations in the quantity of melatonin produced by the pineal gland to explain seasonal reproductive changes in hamsters. In particular, photorefractoriness in this species cannot be explained by exhaustion of the capacity of the pineal gland to produce melatonin.