RESUMEN
In vertebrates, the circadian rhythm in the activity of serotonin N-acetyltransferase [arylalkylamine N-acetyltransferase (AA-NAT); EC 2.3.1.87] drives the daily rhythm in circulating melatonin. We have discovered that expression of the AA-NAT gene in the rat pineal gland is essentially turned off during the day and turned on at night, resulting in a more than 150-fold rhythm. Expression is regulated by a photoneural system that acts through an adrenergic-cAMP mechanism in pinealocytes, probably involving cAMP response element-binding protein phosphorylation. Turning off AA-NAT expression appears to involve de novo synthesis of a protein that attenuates transcription. A approximately 10-fold night/day rhythm in AA-NAT messenger RNA occurs in the retina, and AA-NAT messenger RNA is also detected at low levels in the brain.