Circadian organization and the role of the pineal in birds.

All organisms exhibit significant daily rhythms in a myriad of functions from molecular levels to the level of the whole organism. Significantly, most of these rhythms will persist under constant conditions, showing that they are driven by an internal circadian clock. In birds the circadian system is composed of several interacting sites, each of which may contain a circadian clock. These sites include the pineal organ, the suprachiasmatic nucleus (SCN) of the hypothalamus, and, in some species, the eyes. Light is the most powerful entraining stimulus for circadian rhythms and, in birds, light can affect the system via three different pathways: the eyes, the pineal, and extraretinal photoreceptors located in the deep brain. Circadian pacemakers in the pineal and in the eyes of some avian species communicate with the hypothalamic pacemakers via the rhythmic synthesis and release of the hormone melatonin. Often the hypothalamic pacemakers are unable to sustain persistent rhythmicity in constant conditions in the absence of periodic melatonin input from the pineal (or eyes). It has also been proposed that pineal pacemakers may be unable to sustain rhythmicity in constant conditions without periodic neural input from the SCN. Significant variation can occur among birds in the relative roles that the pineal, the SCN, and the eyes play within the circadian system; for example, in the house sparrow pacemakers in the pineal play the predominant role, in the pigeon circadian pacemakers in both the pineal and eyes play a significant role, and in Japanese quail ocular pacemakers play the predominant role.