Movement of retinal along cone and rod photoreceptors

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Abstract
Single isolated photoreceptors can be taken through a visual cycle of light adaptation by bleaching visual pigment, followed by dark adaptation when supplied with 11–cisretinal. Light adaptation after bleaching is manifested by faster response kinetics and a permanent reduction in sensitivity to light flashes, presumed to be due to the presence of bleached visual pigment. The recovery of flash sensitivity during dark adaptation is assumed to be due to regeneration of visual pigment to pre-bleach levels. In previous work, the outer segments of bleached, light-adapted cells were exposed to 11–cisretinal. In the present work, the cell bodies of bleached photoreceptors were exposed. We report a marked difference between rods and cones. Bleached cones recover sensitivity when their cell bodies are exposed to 11–cisretinal. Bleached rods do not. These results imply that retinal can move freely along the cone photoreceptor, but retinal either is not taken up by the rod cell body or retinal cannot move from the rod cell body to the rod outer segment. The free transfer of retinal along cone but not along rod photoreceptors could explain why, during dark adaptation in the retina, cones have access to a store of 11–cisretinal which is not available to rods. Additional experiments investigated the movement of retinal along bleached rod outer segments. The results indicate that retinal can move along the rod outer segment, but that this movement is slow, occurring at about the same rate as the regeneration of visual pigment.