Photoreceptor Outer Segment Fatty Acid Profiles of Cone- or Rod-Enriched Trout Retinas

Abstract

Abstract: : Purpose: Docosahexaenoic acid (DHA) is important to rod photoreceptor function. Its abundance in the outer segment (OS) is sensitive to diet, light, and disease. However, its relative abundance in rods and cones has not been demonstrated in a single species. The albino rainbow trout is an appropriate model to compare the fatty acid profiles of rods and cones because, when exposed to full daylight, rods but not cones lose their OS. Methods: Albino trout raised indoors in cyclic light were exposed to full daylight in an outdoor raceway for 30 days in mid-August, while some albinos remained indoors as unexposed controls. Retinas and retinal pigment epithelium (RPE) were dissected, separated and frozen on dry ice for later biochemical analysis. Some eyes were placed in cold fixative and prepared for histological analysis of semi-thin retinal slices. OS were retrieved from discontinuous sucrose gradients at the 1.11 and 1.13 g/ml interface. Fatty acid profiles were determined by gas chromatography; protein profiles were determined by SDS-PAGE. Multivariant ANOVA with post-hoc Neuman-Keuls tests determined statistical significance (p≤0.05; n≥3). Results: Histological analysis revealed a nearly complete elimination of ROS in the light-exposed albino retinas (>80% of the OS were COS), whereas retinas from unexposed albino trout had <10% COS. Protein profiles of OS from the cone-enriched retinas were very similar to those of the unexposed albinos. Fatty acid analysis of the RPE showed no significant differences between the exposed and the unexposed retinas, but there was ~20% more DHA in OS fractions from unexposed (ROS-enriched) albinos than from exposed (COS-enriched) albinos. Conclusions: The OS of light-exposed albino retinas are primarily derived from cones and their DHA content is significantly less than that of OS from unexposed (ROS-enriched) retinas. Whether this represents a loss of DHA due to light-exposure or that cones have less DHA than rods remains to be determined. We propose that a lower DHA content supports cone function in a duplex retina exposed to potentially damaging light.

Keywords: lipids • retinal degenerations: cell biology • photoreceptors