Circadian photoreception in vertebrates

To be adaptively useful, internal circadian clocks must be entrained (synchronized) to daily rhythms in the external world. The entraining process adjusts the period of the internal clock to 24 hours and its phase to a value that determines the organism's temporal niche (e.g., diurnal and nocturnal). For most vertebrates, the dominant environmental synchronizer is light. All vertebrates employ specialized photoreceptor cells to perceive synchronizing light signals, but mammals and nonmammalian vertebrates do this differently. Mammals concentrate circadian photoreceptors in the retina, employing rods, cones, and a subset of retinal ganglion cells that are directly photosensitive and contain an unusual photopigment (melanopsin). Nonmammalian vertebrates use photoreceptors located deep in the brain and in the pineal gland as well as others in the retina. Such photoreceptor extravagance is difficult to explain. It seems likely that the different photoreceptor classes in this elaborate sensory system may have specialized roles in entrainment. There is some evidence that this is in fact the case. Furthermore, this nonvisual "circadian" photoreceptive system also controls acute behavioral responses to light (masking), pupillary constriction, and photoperiodic regulation of reproductive state. We review some of the early work on birds and describe new findings that indicate specific roles for retinal rods, cones, and photosensitive retinal ganglion cells in mammals.