Role of Pineal and Melatonin in the Avian Circadian and Photoperiodic Systems

The pineal gland is an important component of the multioscillatory avian circadian timekeeping system. The other principal clock components reside in the retinae of the eyes and the hypothalamus. The best known output signal from the pineal gland is melatonin, which is a lipophilic molecule. The presence of melatonin is however not limited to the organisms having a pineal gland. Melatonin is present from plants to protozoa to humans. Melatonin seems to have been evolutionarily conserved as an adaptive molecule of darkness of the daily day-night environment. In birds, the major physiological roles of pineal melatonin are in its involvement in the daily and seasonal timekeeping as well as photoperiodic time measurement. Birds use daily rhythm in melatonin secretion to decode the time-of-day as well as the time-of-year information. Besides, melatonin performs other physiological roles, namely in the immune function, free radical scavenging, etc. Avian pineal (melatonin) directly regulates several circadian behaviors, but intriguingly not the circadian rhythm-mediated photoperiodic induction of gonadal development. Melatonin, however, may act as an endocrine modulator of seasonal reproduction. In this article, we describe briefly the avian timekeeping system and then discuss the potential roles of pineal gland and melatonin in daily and seasonal timing of physiology in birds, particularly in songbirds.