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Am J Physiol Regul Integr Comp Physiol 278: R1339-R1345, 2000;
0363-6119/00 $5.00
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Vol. 278, Issue 5, R1339-R1345, May 2000

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

Christophe Ribelayga, Paul Pévet, and Valérie Simonneaux

Neurobiologie des Fonctions Rythmiques et Saisonnières, Université Louis Pasteur, Unité Mixte de Recherche-Centre National de la Recherche Scientifique 7518, F-67000 Strasbourg, France

In the pineal, melatonin (Mel) is synthesized from serotonin by arylalkylamine-N-acetyltransferase (AA-NAT) and hydroxyindole-O-methyltransferase (HIOMT). Although it is clear that AA-NAT drives the daily rhythm in Mel synthesis, the mechanisms involved in the photoperiodic changes of the amplitude of the Mel peak, as observed in the Siberian hamster, remain to be determined. We investigated the characteristics of AA-NAT and HIOMT in Siberian hamsters kept either under a short (SP) or a long photoperiod (LP). The amplitude of the nocturnal peak of Mel was about two times higher under SP than under LP, whereas AA-NAT activity was about two times smaller under SP. In contrast, a twofold increase of HIOMT activity was observed under SP compared with LP. No change in the affinity of the enzymes for their substrates was 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 this enzyme in the seasonal regulation of Mel production.

hydroxyindole-O-methyltransferase; arylalkylamine-N-acetyltransferase; photoperiodism; seasonal rhythms; circadian rhythms


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