The extraretinal photoreceptors of non-mammalian vertebrates play an important role in the regulation of temporal physiology. Both the regulation of circadian clocks and the photoperiodic response of many animals depend upon the photic information provided by these receptors. Since their discovery at the beginning of this century, and despite their importance, extraretinal photoreceptors have remained poorly understood. Until recently, their cellular location within the central nervous system, and the nature of the photopigments they use, remained a mystery. Antibodies directed against rod or cone photopigment proteins have been used in immunocytochemical procedures to localize extraretinal photoreceptors. However, findings have been confusing. The use of molecular approaches has led to the identification of several new photopigment gene families. Significantly, these genes are not expressed in the rods and cones of the retina, but in many sites within the central nervous system. Moreover, molecular approaches have proved useful in clarifying some of the earlier immunocytochemical results. Collectively, the recent findings show that non-mammalian vertebrates possess multiple extraocular photoreceptors that may express novel, rod or even cone photopigments. The future challenge is to link these photoreceptors with circadian and photoperiodic physiology.
