Opsins and mammalian photoentrainment
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Research over the past decade has provided overwhelming evidence that photoreception in the vertebrate eye is not confined to the rod and cone photoreceptors. It appears that photoreceptor cells within the inner retina provide irradiance information to a wide variety of different photosensory tasks including photoentrainment, pupillary constriction and masking behaviour. Action spectra in mice lacking all rod and cone photoreceptors (rd/rd cl) have demonstrated the existence of a previously uncharacterised, opsin/vitamin-A-based photopigment with peak sensitivity at 479 nm (opsin photopigment/OP479). The review addresses the question: has the gene encoding OP479 already been isolated, and if not, what type of gene should we be seeking and where in the eye might this gene be expressed? On the basis of available data, the gene that encodes OP479 remains unidentified, and two broad possibilities exist. On the assumption that OP479 will be like all of the other vertebrate photopigments (ocular and extraocular) and share a close phylogenetic relationship based upon amino acid identity and a conserved genomic structure, then the gene encoding OP479 has yet to be isolated. Alternatively, there may have been a separate line of photopigment evolution in the vertebrates that has given rise to the melanopsin family. If true then the mammalian melanopsin gene may encode OP479. Only when melanopsin and other candidates for OP479 have been functionally expressed, and shown to encode a photopigment that matches the action spectrum of OP479, can firm conclusions about the identity of the non-rod, non-cone ocular photoreceptor of mammals be made.
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