Role of the pineal in the circadian system of birds

Abstract

Pinealectomy of sparrows abolishes the free running rhythm of locomotor activity in constant darkness. Without their pineals, sparrows still entrain to light-dark cycles and show other signs that they retain part, but not all, of the circadian system. Interruption of either the neural input to the pineal or its neural output, or both, does not abolish the free running rhythm. Rhythmicity can be restored to a pinealectomized bird by implanting the pineal of a donor into the anterior chamber of its eye. A model of the circadian system controlling locomotor behavior is proposed to account for these facts. We suggest that the avian pineal contains a self-sustained oscillator, and as a consequence, produces a rhythmic hormonal output. Circadian fluctuations of this hormone entrain a damped oscillator located elsewhere, which in turn drives the locomotor activity. Each oscillator has separate access to environmental light cycles. The behavior of a pinealectomized bird is determined exclusively by its damped oscillator. It does not free run because it lacks the self-sustained oscillator which, however, can be restored by transplantation. The transplanted pineal continues its self-sustained circadian hormonal output which entrains the damped oscillator and restores the system to its normal state. © 1976 by the American Society of Zoologists.