Maintenance of opsin density in photoreceptor outer segments of retinoid-deprived rats

Dietary deficiency in the retinoid precursors of the visual pigment chromophore 11-cis-retinal eventually results in selective degeneration of the photoreceptor cells of the vertebrate retina. Early effects of retinoid deficiency are depletion of rhodopsin from the retina and vesiculation of the photoreceptor outer segment disc membranes. Experiments were conducted to determine whether these early changes were accompanied by an alteration of the opsin content of the disc membranes. After being fed a retinoid-deficient diet containing retinoic acid for 26 weeks, the rhodopsin content of rat retinas was reduced by over 85%. Both the diameters and the lengths of the outer segments decreased significantly. However, immunocytochemical and freeze-fracture analyses indicated that retinoid deficiency did not lower opsin density in the outer-segment disc membranes. These findings indicate that in the rat, opsin synthesis and disc assembly are coordinated processes that remain coupled despite reduced availability of the vitamin A chromophore. The fact that disc size decreases and disc synthesis eventually ceases in retinoid-deprived rats indicates that specific retinoids are essential for disc morphogenesis. The mechanism by which these retinoids regulate disc assembly remains to be determined.