Pineal gland biorhythms: N-acetyltransferase in chickens and rats

Circadian rhythms have been identified in chemical factors involved in the metabolism of serotonin in the pineal gland. The rhythms are reviewed with respect to their characteristics and regulation by environmental lighting in vertebrates. Special emphasis has been given to the rhythm in pineal N-acetyltransferase activity in rats and chickens. This rhythm is not only apparent in a light-dark cycle in some birds and mammals, it also persists in constant dark. In constant light the rhythm is lost. The action of light is via the eyes in rats but extraretinal photoreceptors are involved in chickens. The details of the N-acetyltransferase activity rhythm include the presence of a refractory period for the rise in dark-time enzyme activity and the rapid turnoff of high dark-time activity by light. The N-acetyltransferase clock can be reset with a period of constant light in the chicken. Neural regulation of the rhythm in rats involves the suprachiasmatic nucleus and the superior cervical ganglion; in chickens, the ganglion is not necessary for the rhythm in light-dark cycles and the suprachiasmatic nucleus has not been studied.