Long-term variations in cyclic light intensity and dietary vitamin A intake modulate lipofuscin content of the retinal pigment epithelium.
Experiments were conducted to determine whether the intensity of cyclic light exposure to the retina over a long period of time affects retinoid-dependent accumulation of lipofuscin in the retinal pigment epithelium (RPE). Albino rats were maintained from weaning on diets either containing (+A) or lacking (-A) retinyl palmitate, which can be metabolized to the retinoids involved in the visual cycle. Animals in each dietary group were divided between bright (L) and dim (D) cyclic light treatments. Thus, the experiments employed the following four treatment groups: +A/D, +A/L, -A/D, and -A/L. After 6, 12, and 15 months from the start of the treatments, animals in each group were killed for quantitative determination of: 1) retinal photoreceptor densities; 2) RPE lipofuscin content; and 3) RPE lipofuscin fluorescence intensity. Animals in the L groups had a lower volume of RPE lipofuscin than those in the D groups fed the same diet. Among the -A rats, this reduced lipofuscin volume could be attributed to a light-enhanced depletion of vitamin A from the retina and an accompanying loss of photoreceptor cells. In the +A animals, however, there were no differences in photoreceptor densities between the D and L groups. In the -A rats, the volume of RPE lipofuscin decreased between 6 and 15 months of age, whereas it increased in the +A animals. In contrast, lipofuscin fluorescence intensity increased between 6 and 15 months of age in all four treatment groups. However, in the +A rats, the fluorescence intensity was lower in the L than in the D group at all three ages. In the -A groups, light level had no effect on lipofuscin fluorescence intensity. At all three ages, fluorescence intensity was lower in the -A animals than in +A rats. Thus, at light intensities below those that induce acute retinal degeneration, long-term exposure to higher intensity light inhibits the age-related increase in RPE lipofuscin volume. A decrease in the volume of RPE lipofuscin after the retina is depleted of vitamin A suggests that lipofuscin is turned over, and that RPE lipofuscin content is determined by a balance between the rates at which lipofuscin is formed and at which it is eliminated from the RPE. An age-related increase in lipofuscin-specific fluorescence intensity after vitamin A depletion from the retina suggests that lipofuscin fluorophores may continue to form slowly from retinoids that have been modified such that they can no longer enter the visual cycle.
University of Missouri School of Medicine, Mason Eye Institute, Columbia 65212, USA. KatzM@health.missouri.edu
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D000818:Animals; D002452:Cell Count; D004032:Diet; D008027:Light; D008062:Lipofuscin / Q000378:metabolism*; D008297:Male; D008856:Microscopy, Fluorescence; D020419:Photoreceptor Cells, Vertebrate / Q000166:cytology / Q000187:drug effects / Q000528:radiation effects; D010857:Pigment Epithelium of Eye / Q000187:drug effects / Q000378:metabolism* / Q000528:radiation effects; D051381:Rats; D011916:Rats, Inbred F344; D014801:Vitamin A / Q000378:metabolism* / Q000494:pharmacology
D008062:Lipofuscin; D014801:Vitamin A