11-cis retinal restores visual function in vitamin A-deficient Manduca

Abstract

Larvae of the tobacco hornworm moth Manduca sexta were reared on either a carotenoid-supplemented or a carotenoid-deficient diet. The former yields fortified adults with normal visual function, whereas visual sensitivity and rhodopsin content are reduced by 2−4 log units in the compound eyes of the deprived moths reared on the latter. We characterized the retinoids of fortified retinas and investigated the recovery of visual function in deprived moths that were provided with retinaldehyde as a source of photopigment chromophore. Retinoids were identified and measured by high-performance liquid chromatography (HPLC). Fortified retinas contained mainly 3-hydroxyretinaldehyde (R₃); 11-cis R₃ predominated in dark-adaptation, all-trans in light-adaptation, indicating that R₃ is the photopigment chromophore. No retinoids could be measured in deprived eyes. Retinaldehyde (R₁) was delivered to the retinas of deprived moths by “painting” solutions of 11-cis or all-trans R₁ in dimethylsulfoxide (DMSO) on the corneal surfaces of the compound eyes or on the head capsule between the eyes. 11-cis R₁ induced rapid recovery: during 3 days, sensitivity rose to within a log unit of that measured from fortified animals. By 7 days, sensitivity was close to normal. Although rhodopsin and P-face particle densities of photoreceptor membranes increased, neither rose to the levels found in fortified animals. All-trans R₁ induced only a slight increase in sensitivity that could have resulted from some nonspecific isomerization of the all-trans to the 11-cis isomer; we found no evidence for a retinal isomerase that functions in darkness. Small amounts of R₃ were measured in recovering retinas, indicating some conversion of R₁ to R₃. However, the chromophore of most of the rhodopsin that was synthesized must have been R₁. It is possible that rhodopsin did not reach normal levels in the retina even after a week of recovery because the normal chromophore R₃ was not provided. Although the rhodopsin that initially formed in recovering moths may have resulted from the association of the chromophore with pre-existing opsin, the extent of eventual recovery indicates that opsin synthesis was stimulated by 11-cis R₁.