Extraretinal photoreception and circadian systems in nonmammalian vertebrates

Nonmammalian vertebrates have been known to have photoreceptors in their pineal complex and deep brain as well as the retina. The extraretinal photoreceptors are believed to be essential for seasonal reproduction and regulation of circadian physiology. In this review, we discuss the relation between extraretinal photoreceptors and circadian systems in some species of nonmammalian vertebrates in terms of the pineal as the photoreceptor and oscillator for circadian rhythms, the hypothalamus as a photoreceptor and the extraretinal photoreceptors as the multi oscillator system for circadian behavioral rhythms. Although immunohistochemical results suggest the existence of rod and cone-type photoreceptors in the pineal, photoreception in the pineal is not always necessary for circadian systems in nonmammalian vertebrates. Circadian systems are involved in avian photoperiodic gonadal responses, and these responses are induced via deep brain photoreceptors. In some animals, deep brain photoreceptors are stained by antisera against photoreceptor proteins (rhodopsin, iodopsin and transducins). The relationship between the oscillators and photoreceptors is different among species. Physiological significance of this diversity is discussed from evolutional and developmental points of view.