Extraocular phototransduction and circadian timing systems in vertebrates

It is widely accepted that, for organisms with eyes, the daily regulation o f circadian rhythms is made possible by light transduction through those or gans. Yet, it has been demonstrated repeatedly in recent years that ocular light receptors that mediate vision, at least in mammals, are not the same photoreceptors involved in circadian regulation. Moreover, it has been reco gnized for many years that circadian regulation can occur in organisms with out eyes. In fact, extraocular circadian phototransduction (EOCP) appears t o be a phylogenetic rule for the vast majority of species. EOCP has been re ported in every nonmammalian species studied to date. In mammals, however, the story is very different. This paper presents findings from studies that have examined specifically the capacity for EOCP in vertebrate species. In addition, the literature addressing noncircadian aspects of extraocular ph ototransduction is briefly discussed. Finally, possible mechanisms underlyi ng EOCP are discussed, as are some of the implications of the presence, or absence, of EOCP across phylogeny.