Regulation of Melatonin Release and N-acetyltransferase Activity in Ovine Pineal Cells

Abstract A suspension culture of ovine pineal cells was developed to investigate the regulation of melatonin synthesis and release. Dosedependent stimulation of melatonin release by a series of adrenergic agonists yielded a typical beta-adrenergic profile. Serotonin N-acetyltransferase (NAT), the rate-limiting enzyme in melatonin biosynthesis, was also stimulated by a beta-adrenergic mechanism. However, NAT activity appeared less sensitive than melatonin release to beta-adrenergic stimulation. No evidence was obtained for a contribution of alpha(1)-adrenergic receptors to the regulation of NAT activity and melatonin release. Activation of adenylate cyclase by forskolin or addition of a cyclic AMP analogue increased both melatonin release and NAT activity. In contrast, the Ca(2+) ionophore A(23187) stimulated melatonin release without a detectable increase in NAT activity. Together, the present data argue for a beta-adrenergic regulation of both NAT activity and melatonin release in ovine pinealocytes. The evidence also suggests that two intracellular mechanisms may control melatonin release in ovine pinealocytes: a cyclic AMP-dependent mechanism, associated with an increase in NAT activity and a Ca(2+)-dependent mechanism, independent of NAT activity.