ENVIRONMENTAL-CONTROL OF THE SEASONAL-VARIATIONS IN THE DAILY PATTERNOF MELATONIN SYNTHESIS IN THE EUROPEAN HAMSTER, CRICETUS-CRICETUS

Nocturnal patterns of pineal melatonin concentrations were measured at hourly intervals in the European hamster, Cricetus cricetus, maintain ed under different natural or experimental environmental conditions. T here were pronounced variations in the night peak of pineal melatonin both in the duration and the amplitude of the melatonin peak and in th e onset and decline of melatonin synthesis. The duration of the melato nin peak increased proportionnally with increased dark period. The amp litude increased abruptly from LD 16/8 to LD 15/9 and remained constan t in all other photoperiods. The onset of synthesis started 6:00 hours after the onset of darkness in LD 16/8, 15/9, and 14/10, while it sta rted 4:00 hours after dark onset in shorter photoperiods (LD 12/12 and 10/14). This result is opposite to that observed in the rat. The decl ine of synthesis was delayed as darkness increased and was directly re lated to lights on in long photoperiods, while it was endogenous in sh ort photoperiods. Temperature, under a long photoperiod, also seems to be implicated in the regulation of the amplitude of the melatonin pea k. (C) 1997 Academic Press.