PHOTORECEPTORS REGULATING CIRCADIAN BEHAVIOR - A MOUSE MODEL

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 un affected. Using c-fos expression within the suprachiasmatic nuclei (SC N) as a marker of neural activation of the circadian entrainment pathw ay, we have found identical levels of Fos in the SCN of rd/rd and +/mice in response to retinal illumination. On the basis of action spect rum studies, and measurements of photopigment retinoids using high-pre ssure liquid chromatography, we believe that the photopigment mediatin g circadian responses to light is based upon an opsin, and that 11-cis -retinaldehyde is the photopigment chromophore. Preliminary measuremen ts of mouse rod opsin, blue cone, and green-red cone opsin messenger R NA in retinally degenerate mice suggest that none of these opsins is e xclusively 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, al ternatively, is performed by an unrecognized class of photoreceptive c ell within the mammalian retina.