Photoentrainment in mammals: A role for cryptochrome?

There is growing evidence in support of the hypothesis that, in mammals, ph otoreceptive tasks are segregated into those associated with creating a det ailed visual image of the environment and those involved in the photic regu lation of temporal biology. The hypothesis that this segregation extends to the use of different photoreceptors remains unproven, but published report s from several mammalian species that circadian photoentrainment survives a degree of retinal degeneration sufficient to induce visual blindness sugge st that this may be so. This has lead to speculation that mammals might emp loy a dedicated 'circadian photoreceptor' distinct from the rod and cone ce lls of the visual system. The location and nature of this putative circadia n photoreceptor has become a matter of conjecture. The latest candidates to be put forward as potential circadian photopigments are the mammalian cryp tochrome proteins (CRY1 and 2), putative vitamin-B2 based photopigments. To date, published experimental evidence falls short of a definitive demonstr ation that these proteins form the basis of circadian photoreception in mam mals. Consequently, this review aims to assess their suitability for this t ask in Light of what we know regarding the biology of the cyrptochromes and the nature of mammalian photoentrainment.