Visual pigment loss after light-induced shedding of rod outer segments

Abstract

The amount of visual pigment in the eyes of adult frogs kept in darkness for about 5 weeks increases by approximately one-third. There is a comparable fall in the amount of vitamin A stored in the pigment epithelium (RPE), due to a steep deeline in the all- trans isomer. The proportion in the 11- cis configuration, however, increases from 15% to nearly 50%. Similarly, tadpoles kept in complete darkness for a week have more visual pigment than those exposed to a diurnal 12 hr light/dark cycle. The difference is correlated with rod outer segment (ROS) length which also increases in the dark. When dark animals are exposed to light for 2 hr, massive shedding of fragments from the tips of the ROS results in a decrease in ROS length. The engulfment of these shed ROS fragments results in an increase in the phagosome content of the RPE. After 2·5 hr in darkness the bleached visual pigment in these phagosomes does not regenerate, and any unbleached material present appears to be degraded. Both effects may be attributed to the action of lysosomal enzymes following phagocytosis by the RPE. During the 24 hr after shedding, the ROS elongate at a rate suggesting that renewal has been accelerated by a factor of 3–5 fold. The amount of visual pigment in the dark-adapted eye recovers in parallel with this increase in length.