Is the avian circadian system a neuroendocrine loop?

Abstract

Avian circadian organization is a result of a complex interaction of photoreceptive and oscillatory components. The known components include the pineal gland, the lateral eyes, the suprachiasmatic nuclei (SCN), and extraocular brain photoreceptors. The pathways by which these components integrate circadian rhythmicity suggest a neuroendocrine loop in which the SCN inhibits pineal and ocular oscillators during the course of subjective day via a multisynaptic neuronal pathway which includes the superior cervical ganglia (SCG). During the night, the pineal in turn inhibits SCN activity via its secretion of the hormone melatonin into the blood circulation. This neuroendocrine loop, it is proposed, synchronizes multiple oscillators within each component and maintains the stability and precision of the system.