Photoreceptors Regulating Circadian Behavior: A Mouse Model

Abstract
Our recent studies have examined circadian photoreception in mice with hereditary retinal disorders (rd/rd and rds/rds). Despite the loss of visual function in these mice, circadian responses to light remain unaffected. Using c-fos expression within the SCN as a marker of neural activation of the circadian entrainment pathway, we find identical levels of Fos in the SCN of rd/ rd and +/+ mice in response to retinal illumination. On the basis of action spectrum studies, and measurements of photopigment retinoids using HPLC, we believe the photopigment mediating circadian responses to light is based upon an opsin, and that 11-cis-retinaldehyde is the photopigment chromophore. Preliminary measurements of mouse rod opsin, blue cone, and green/red cone opsin mRNA in retinally degenerate mice suggest that none of these opsins are exclusively used to mediate circadian responses to light. Collectively our data suggest that circadian photoreception can be maintained by a very small number of rod or cone cells without outer segments, or alternatively, is performed by an unrecognized class of photoreceptive cell within the mammalian retina.

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