Molecular cloning of the arylalkylamine-N-acetyltransferase and daily variations of its mRNA expression in the Syrian hamster pineal gland

The arylalkylamine-N-acetyltransferase (AA-NAT) expressed in the vertebrate pineal gland catalyzes the N-acetylation of the serotonin into N-acetylser otonin and is considered to be the rate limiting enzyme of the pineal melat onin synthesis. Indeed, dramatic changes in its activity throughout the 24- h period drive the large day/night variations in plasma melatonin concentra tions. Recently, AA-NAT was cloned in the rat pineal. In this species, AA-N AT mRNA variations were demonstrated to be responsible of the well known AA -NAT activity and plasma melatonin circadian fluctuations. In the Syrian ha mster, the pineal melatonin secretion pattern is characterized by a late-ni ght short-duration peak of melatonin synthesis. We investigated whether thi s typical pattern could be due to a late-night delayed pineal AA-NAT mRNA e xpression. The first part of our study was dedicated to the molecular cloni ng of a Syrian hamster AA-NAT cDNA. A PCR-generated clone of 1045 bp encodi ng the AA-NAT has been isolated and sequenced. In situ hybridization using an AA-NAT cRNA probe revealed that the AA-NAT mRNA expression undergoes str ong daily fluctuations in the Syrian hamster pineal, with undetectable leve l in the second half of the Light period and a dramatic increase at night. After lights off, the AA-NAT mRNA expression requires 6-7 h to reach its ma ximum expression. This result thus suggests that the transcription of the A A-NAT mRNA in the Syrian pineal gland determines the lag period in pineal r esponsiveness and melatonin synthesis to darkness. (C) 1999 Elsevier Scienc e B.V. All rights reserved.