Rhodopsin-mediated blue-light damage to the rat retina: Effect of photoreversal of bleaching

PURPOSE. Acute white-light damage to rods depends on the amount of rhodopsi n available fur bleaching during light exposure. Bleached rhodopsin is meta bolically regenerated through the visual cycle involving the pigment epithe lium, or photochemically by deep blue light through photoreversal of bleach ing. Because photoreversal is faster than metabolic re generation of rhodop sin by several orders of magnitude, the photon catch capacity of the retina is significantly augmented during blue-light illumination, which may expla in the greater susceptibility of the retina to blue light than to green lig ht. However, blue light can also affect function of several blue-light-abso rbing enzymes that may lead to the induction of retinal damage. Therefore, this study was conducted to test whether rhodopsin and its bleaching interm ediates play a role in blue-light-induced retinal degeneration. METHODS. Eyes of anesthetized rats and mice that did or did nor contain rho dopsin were exposed to green (550 +/- 10 nm) or deep blue (403 +/- 10 nm) l ight for up to 2 hours. Rats with nearly rhodopsinless retinas were obtaine d by bleaching rhodopsin in animals with inhibited metabolic rhodopsin rege neration-that is, under halothane anesthesia. In addition, Rpe(65-/-) mice that are completely without rhodopsin were used to test the susceptibility to blue-light damage of a rodent retina completely devoid of the visual pig ment. Effects of illumination on photoreceptor morphology were assessed 24 hours or 10 days thereafter by morphologic and biochemical methods. RESULTS. Exposure to blue light resulted in severe retinal damage and activ ation of the transcription factor AP-1 in rats. In contrast, green light ha d no effect. When rhodopsin was almost completely bleached by short-term gr een-light exposure while metabolic regeneration (but not photoreversal) was prevented by halothane anesthesia, blue-light exposure induced distinct le sions in rat retinas. When both metabolic rhodopsin regeneration and photor eversal of bleaching were almost completely inhibited, blue-light exposure caused only very moderate lesions. When mice without rhodopsin were exposed to blue light, no damage occurred, in contrast to wild-type control mice. CONCLUSIONS. Short time exposure to blue light has deleterious effects on r etinal morphology. Because damage was observed only in the presence of the visual pigment, blue-light-induced retinal degeneration is rhodopsin mediat ed. Absorption of blue light by other proteins is not sufficient to induce light damage. Photoreversal of bleaching, which occurs only in blue but not in green light, increases the photon-catch capacity of the retina and may thus account for the difference in the damage potential between blue and gr een light.