Photopigments and circadian systems of vertebrates.

In the retinal degeneration (rd) mouse the absence of rod cells and the 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 data suggest that cone cells lacking outer segments are sufficient to maintain normal circadian responses to light, or some unidentified photoreceptor within the retina. An action spectrum for circadian responses to light in rd/rd mice, and molecular analysis of retinally degenerate mice and blind mole rat eyes, suggests the involvement of a mid-to-long wavelength sensitive cone opsin in photoentrainment. Extraocular photoreceptors of non-mammalian vertebrates are currently being analyzed in order to identify functional and evolutionary similarities between visual and non-visual photoreceptor systems.