SOLAR PRUNING OF RETINAL RODS IN ALBINO RAINBOW-TROUT

Morphology of the central retina and scotopic visual sensitivity were compared in juvenile albino and normally pigmented rainbow trout livin g under natural and reduced daylight. Outdoor albinos avoided exposing their eyes to direct sunlight, whereas normals were indifferent to it . After 4 months outdoors (similar to 10,000 lux in albinos, similar t o 100,000 lux in normals), albinos had severely truncated or missing r od outer segments (ROS) and some missing rod ellipsoids, but normal nu mbers of photoreceptor nuclei and fully intact cones. Albino estimated ROS volume was only 7.1% of normal in July, but increased to 20% by t he following February, mainly via an increase in numbers of ROS. Howev er, in albinos moved indoors October 7 and exposed to 10-30 lux ambien t daylight, both the number and length of ROS increased significantly, with estimated ROS volume reaching 95% of normal by 34 days. Albinos generally had more phagosomes (similar to 3 X normal) and more macroph ages (similar to 2 X normal) in their outer retina. An optomotor refle x was used to define the effect of ROS volume on the ability to respon d visually during dark adaptation. In July, albinos and normals from o utdoor raceways (3 months) or indoor raceways (35 days) showed equal s ensitivity after first being placed in darkness, but after 1 h in dark ness, outdoor albinos with 6% of normal ROS volume were 2.0 log units less sensitive than indoor or outdoor normals, whereas indoor albinos with 53% of normal ROS volume were only 0.7 log units less sensitive. This verifies that most rod cell bodies of albino trout can persist wi thout functional ROS in indirect sunlight, and can regrow functional o uter segments in dim daylight. This finding is distinct from the exten sive retinal light damage observed in albino rats exposed to more mode rate cyclic light, in which entire rod cells degenerate early on.