Cell and tissue researchJul 12, 2002

Daily rhythms and internal clock in the bird’s pineal gland

Circadian Biology Weekly Brief ↗PubMed ↗DOI ↗

Updated Mar 29, 2026

Abstract

The avian pineal gland exhibits a striking circadian rhythm in melatonin synthesis and release.

The avian pineal gland is relatively autonomous and contains its own circadian clock.
In addition to melatonin production, the avian pineal has photoreceptors and an entire circadian system within itself.
Avian pinealocytes are directly responsive to light, suggesting they play a critical role in regulating circadian rhythms.
Potential photopigments such as melanopsin and cryptochromes are being evaluated for their roles in melatonin production.
Several clock genes similar to those in Drosophila and mouse are expressed in the avian pineal gland.
The regulation of the enzyme arylalkylamine N-acetyltransferase (AA-NAT), crucial for melatonin synthesis, is being studied.

Simplified

The avian pineal gland, like that of mammals, displays a striking circadian rhythm in the synthesis and release of the hormone melatonin. However, the pineal gland plays a more prominent role in avian circadian organization and differs from that in mammals in several ways. One important difference is that the pineal gland in birds is relatively autonomous. In addition to making melatonin, the avian pineal contains photoreceptors and a circadian clock (thus, an entire circadian system) within itself. Furthermore, avian pineals retain their circadian properties in organ or dispersed cell culture, making biochemical components of regulatory pathways accessible. Avian pinealocytes are directly photosensitive, and novel candidates for the unidentified photopigments involved in the regulation of clock function and melatonin production, including melanopsin, pinopsin, iodopsin, and the cryptochromes, are being evaluated. Transduction pathways and second messengers that may be involved in acute and entraining effects, including cyclic nucleotides, calcium fluxes, and protein kinases, have been, and continue to be, examined. Moreover, several clock genes similar to those found in Drosophila and mouse are expressed, and their dynamics and interactions are being studied. Finally, the bases for acute and clock regulation of the key enzyme in melatonin synthesis, arylalkylamine N-acetyltransferase (AA-NAT), are described. The ability to study entrainment, the oscillator itself, and a physiological output in the same tissue at the same time makes the avian pineal gland an excellent model to study the bases and regulation of circadian rhythms.

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Daily rhythms and internal clock in the bird’s pineal gland

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