Regulation of Pineal Melatonin in the Syrian Hamster

One purpose of this study was to determine if pineal melatonin is altered by chronic exposure to a lighting schedule which induces gonadal atrophy in the male Syrian hamster. Pineal melatonin in reproductively competent males was found to exhibit a daily rhythm; night values were 10-fold higher than day values. Exposure of males to a lighting cycle with a short photoperiod (10-h light, 14-h dark) caused gonadal atrophy but did not alter either the amplitude of the increase or the duration of the period melatonin was elevated. However, a difference in the phasing of the pineal melatonin rhythm relative to the time of day was seen between these groups. Although lights were turned on at the same time for both groups, pineal melatonin just before this time was basal in the animals exposed to the short photoperiod and maximal in animals exposed to the longer photoperiod (14-h light, 10-h dark). The second purpose of this study was to investigate the acute regulation of pineal melatonin. The nocturnal increase in melatonin was found to be blocked by light and the normally high night values occurring in the dark decreased rapidly when animals were exposed to light. In vivo adrenergic stimulation with isoproterenol resulted in a 5- fold increase in pineal melatonin. These results indicate a daily rhythm in hamster pineal melatonin exists which is influenced by environmental lighting; the rhythm may be regulated by an adrenergic mechanism. It was also found that the large changes in pineal melatonin in the hamster were not associated with similar changes in N-acetyltransferase or hydroxyindole-Omethyltransferase activities.