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M. Glosmann, L. Peichl; Cone Damage and Ectopic Cone Opsin in the Albino Rat Retina Induced by Moderate Cyclic Light. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1343.
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To assess the effects of a moderate light increase under cyclic lighting conditions on shortwave-sensitive (S) cone opsin expression in the retina of pigmented and albino rats.
Pigmented (Brown Norway) and albino (Wistar) rats were raised under cyclic dim light (12 h 5 lx light, 12 h dark). Young adult individuals of both strains were exposed to bright (160 lx) or dim (5 lx, control group) cyclic white light for 4 weeks. Changes in S opsin expression were assessed using in situ hybridization and immunohistochemistry on retinal wholemounts and transverse cryostat sections. S cone morphology was evaluated using S opsin immunostaining. PKC-alpha and recoverin immunostaining served to identify bipolar cell classes.
After cyclic exposure to 160 lx for 4 weeks Wistar but not Brown Norway rats displayed various stages of S cone degeneration. Structural damage ranged from reduction of outer and inner segments, sprouting of axon collaterals, deterioration of nuclei, to total disintegration of the photoreceptor. No morphological changes were observed for middle-to-longwave-sensitive cones. Bipolar cells adjacent to degenerating S cone pedicles stained positive for S opsin protein but in contrast to S cones did not express S opsin transcript. These bipolar cells showed uniform narrow-field morphology and appeared to exclusively contact the adjacent S cone pedicle. Their somata were located in the outer INL and their axon terminals ramified in the inner third of the IPL (ON bipolar cell morphology). S opsin positive bipolar cells were negative for PKC-alpha and recoverin.
Albino rat S cones structurally degenerate as a consequence of increased light intensity. It appears unlikely that degenerating S cones trigger S opsin expression in adjacent bipolar cells. Rather, the absence of S opsin transcript in the bipolar cells suggests a cone-to-bipolar cell transfer of the S opsin protein. Further studies will determine whether the bipolar cells are a novel type of blue cone bipolar cell that has escaped previous classification studies or whether their distinct morphology reflects secondary degeneration.
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