MOVEMENT OF RETINAL ALONG CONE AND ROD PHOTORECEPTORS

Single isolated photoreceptors can be taken through a visual cycle of light adaptation by bleaching visual pigment, followed by dark adaptat ion when supplied with 11-cis retinal. Light adaptation after bleachin g 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 dar k 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-cis retinal. In the present wor k, the cell bodies of bleached photoreceptors were exposed. We report a marked difference between rods and cones. Bleached cones recover sen sitivity when their cell bodies are exposed to 11-cis retinal. Bleache d 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 out er 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-cis retinal which is not availabl e 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 .