Visual and circadian responses to light in aged retinally degenerate mice.

The progression of photoreceptor degeneration in retinally degenerate (rd) mice commences early in postnatal development resulting in the complete loss of rods by 60-70 days of age followed by the more protracted loss of cones. We have previously shown that rd mice 80 days of age are capable of phase shifting their circadian locomotor rhythms in response to brief pulses of light and these animals show the same sensitivity as wild-type (+/+) controls. If surviving cones mediate these circadian responses, then one would expect the sensitivity of the circadian system in rd mice to decline with age and parallel the loss of cones. We demonstrate that aging rd mice (80-767 days of age) remain capable of photically regulating circadian locomotor rhythms in a manner indistinguishable from +/+ mice. Circadian responses to light do not parallel cone cell degeneration in rd mice. In contrast to the circadian responses to light, old (> 210 days of age) rd mice show no visually-evoked behavioral or electroretinogram (ERG) responses.