| Light absorption by visual pigment within photoreceptors is the initial step leading to the perception of light. This event also triggers the photochemical destruction (bleaching) of the pigment and leads to adaptation to subsequent visual stimuli. Regeneration of the pigment and recovery of sensitivity (dark adaptation) normally occur during darkness. Regeneration occurs in two steps---noncovalent binding of the chromophore 11-cis retinal to opsin followed by the covalent attachment of the two molecules. Aside from the ultimate recovery of sensitivity, very little is known about the physiological consequences of the reactions involved in the reconstitution of the visual pigment.;Three retinoids---beta-ionone, 11-cis 13-demethyl retinal, and 11-cis retinal were used to investigate physiological effects of the noncovalent and covalent interaction between chromophore and opsin. The effect of these retinoids on the level of phototransduction in retinal rods and cones was studied.;In bleach-adapted rods, treatment with a retinoid causes a decrease in light sensitivity and dark current, and acceleration of the photoresponse and the rates of guanylyl cyclase and cGMP phosphodiesterase. Together, these observations indicate that the noncovalent binding of retinoids in the chromophore pocket of rod opsin activates phototransduction. In the case of 11-cis 13-demethyl retinal and 11-cis retinal the activation of transduction is transient. Thus, the noncovalent binding of 11-cis retinal to opsin in the process of pigment regeneration leads to activation of the complex accompanied by a decrease in light sensitivity. The subsequent covalent attachment of retinal to opsin completely inactivates opsin and causes recovery of sensitivity. The transient desensitization of the rod in the process of dark adaptation could in part explain why rods dark-adapt much more slowly than cones. This newly found phenomenon might also provide insight in the mechanism of some retinal disorders.;Noncovalent binding of retinoids to cone opsin produces effects opposite to its effects in rods and causes partial recovery of sensitivity. This distinction indicates a fundamental difference in the interaction of rod and cone opsins with retinal and may have implications for the different physiology of the two types of photoreceptors. |
