PHOTOPIGMENTS AND CIRCADIAN SYSTEMS OF VERTEBRATES

In the retinal degeneration (rd) mouse the absence of rod cells and th e progressive loss of cones does not result in a decrease in circadian phase shifting responses to light. By contrast, rd/rd mice are unable to perform simple visual tasks. In addition, rodless transgenic mice, and mice homozygous for the retinal degeneration slow (rds) mutation, show unattenuated circadian responses to light. Collectively these da ta suggest that cone cells lacking outer segments are sufficient to ma intain normal circadian responses to light, or some unidentified photo receptor within the retina. An action spectrum for circadian responses to light in rd/rd mice, and molecular analysis of retinally degenerat e mice and blind mole rat eyes, suggests the involvement of a mid-to-l ong wavelength sensitive cone opsin in photoentrainment. Extraocular p hotoreceptors of non-mammalian vertebrates are currently being analyze d in order to identify functional and evolutionary similarities betwee n visual and non-visual photoreceptor systems.