BRIGHT ENVIRONMENTAL LIGHT ACCELERATES RHODOPSIN DEPLETION IN RETINOID-DEPRIVED RATS

Purpose. Dietary deficiency in the retinoid precursors of the visual p igment chromophore 11-cis retinal eventually results in selective dege neration of the photoreceptor cells of the vertebrate retina. An early effect of retinoid deficiency is depletion of chromophore from the ph otoreceptor outer segments. Experiments were conducted to determine wh ether the rate of chromophore depletion was affected by the intensity of environmental light. Methods. Rats were maintained on diets either containing of lacking retinoid precursors of 11-cis retinal for up to 30 weeks. Animals in both dietary groups were exposed to either bright (90 lux) or dim (5 lux) cyclic light for the duration of the experime nt. At various time intervals retinal rhodopsin content and photorecep tor densities were determined in animals from each treatment group. Re sults. Bright light greatly accelerated the depletion of rhodopsin fro m the retina. Rhodopsin was almost completely depleted from the retina s of the retinoid-deficient animals raised under bright light for 25 w eeks, whereas the dim-light-reared animals fed the retinoid-deficient diet still had significant amounts of retinal rhodopsin even after 30 weeks. Bright light alone moderately depressed retinal rhodopsin level s in animals fed the diet containing a vitamin A precursor of 11-cis r etinal. Among rats fed the latter diet, retinal rhodopsin content in t he animals kept in bright cyclic light was maintained throughout the e xperiment at about 70% of the amount of rhodopsin in rats housed in di m cyclic light. The light-related rhodopsin depletion in the retinoid- deprived rats was accompanied by photoreceptor cell death. After 30 we eks of treatment, photoreceptor cell densities remained similar in all treatment groups except the retinoid-deprived group housed under brig ht cyclic light. In the latter group, photoreceptor cell densities in the central retinas were reduced by an average of more than 50% after 30 weeks. Retinoid deficiency and bright light exposure alone each res ulted in a reduction in rod outer segment size. An even greater reduct ion in outer segment size was observed in vitamin A-deprived animals h oused under bright cyclic light. Conclusion. These findings indicate t hat light accelerates the depletion of retinoids from the retina and t he accompanying photoreceptor cell degeneration.