VISUAL PIGMENT BLEACHING IN ISOLATED SALAMANDER RETINAL CONES - MICROSPECTROPHOTOMETRY AND LIGHT ADAPTATION

Visual pigment bleaching desensitizes rod photoreceptors greatly in ex cess of that due to loss of quantum catch. Whether this phenomenon als o occurs in cone photoreceptors was investigated for isolated salamand er red-sensitive cones. In parallel experiments, (a) visual pigment de pletion by steps of bleaching light was measured by microspectrophotom etry, and (b) flash sensitivity was measured by recording light-sensit ive membrane current. In isolated cones, visual pigment bleaching perm anently reduced flash sensitivity significantly below that due to the reduction in quantum catch, and there was little spontaneous recovery of visual pigment. The ''extra'' desensitization due to bleaching was most prominent up to bleaches of approximately 80% visual pigment and reached a level approximately 1 log unit beyond that due to loss of qu antum catch. At higher bleaches, the effect of loss of quantum catch b ecame more important. Bleaching did not greatly reduce the maximum lig ht-suppressible membrane current. A 99% reduction of the visual pigmen t permanently reduced the circulating current by only 30%. Visual pigm ent bleaching speeded up the kinetics of dim flash responses. All elec trical effects of bleaching were reversed on exposure to 11-cis retina l, which probably caused visual pigment regeneration. Light adaptation in photopic vision is known to involve significant visual pigment dep letion. The present results indicate that cones operate with a maintai ned circulating current even after a large pigment depletion. It is sh own how Weber/Fechner behavior may still be observed in photopic visio n when the contributions of bleaching to adaptation are included.