Melatonin rhythms: Trekking toward the heart of darkness in the chick pineal

Chick pineal cells make melatonin rhythmically, even in culture. Light pulses have two effects on these cells: acute suppression of melatonin synthesis and phase shifts (entrainment) of the underlying pacemaker. The two effects use different mechanistic pathways: the first goes through cAMP, and the second goes through the clock. Both converge on serotonin N-acetyltransferase, whose gene has recently been cloned, with cAMP acting on mRNA levels, but primarily on enzyme activity, and the clock acting primarily on mRNA levels. Aspects of calcium regulation, not yet well-understood, may impinge on both pathways. These cells also have a novel calcium channel and a novel photopigment. (C) 1996 Academic Press Ltd.