HIOMT drives the photoperiodic changes in the amplitude of the melatonin peak of the Siberian hamster

In the pineal, melatonin (Mel) is synthesized from serotonin by arylalkylam ine-N-acetyltransferase (AA-NAT) and hydroxyindole-O-methyltransferase (HIO MT). Although it is clear that AA-NAT drives the daily rhythm in Mel synthe sis, the mechanisms involved in the photoperiodic changes of the amplitude of the Mel peak, as observed in the Siberian hamster, remain to be determin ed. We investigated the characteristics of AA-NAT and HIOMT in Siberian ham sters kept either under a short (SP) or a long photoperiod (LP). The amplit ude of the nocturnal peak of Mel was about two times higher under SP than u nder LP, whereas AA-NAT activity was about two times smaller under SP. In c ontrast, a twofold increase of HIOMT activity was observed under SP compare d with LP. No change in the affinity of the enzymes for their substrates wa s observed between the two photoperiods. Our data strongly suggest that the photoperiodic variations in the amplitude of the nocturnal peak of Mel are driven by HIOMT, thereby promoting an important physiological role for thi s enzyme in the seasonal regulation of Mel production.