Effects of light and an alpha-2-adrenergic agonist on serotonin N-acetyltransferase activity in chick pineal gland.

Pineal serotonin N-acetyltransferase (NAT) is the enzyme that catalyzes the production of N-acetylserotonin from serotonin and is the rate limiting step in the biosynthesis of melatonin in the chick pineal gland. Chick pineal NAT activity is decreased by light and by noradrenergic agents that act at the alpha-2-adrenergic receptor. Light-induced inhibition of nocturnal NAT activity can be demonstrated by exposing 4-day-old chicks to light, or by exposing pineal gland explants cultured in vitro either to light or to UK 14,304 (an alpha-2-adrenergic agonist). The inhibition by either light or UK 14,304 is preceded by a transient, but consistent rise in NAT activity. The paradoxical, transient light-induced rise in enzyme activity is mimicked by UK 14,304 which suggests a similarity in mechanisms of light and adrenergic signal transduction associated with the regulation of NAT activity.