PHYSIOLOGY AND PATHOPHYSIOLOGY OF PHOTORECEPTORS

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Retinal degeneration is a major cause of blindness in our elderly population as well as the tens of thousands of younger Americans afflicted with inherited retinal degenerations such as retinitis pigmentosa (RP). Photoreceptor degeneration is a final common pathway resulting in loss of vision for many insults to the eye, including many mutations of rhodopsin or other proteins of the phototransduction cascade. In retinal degenerations caused by mutations in rod-specific genes, it is equally important to comprehend why the normal cone photoreceptors also die bringing patients from night blindness to near total blindness. We propose experiments on two newly characterized animal models of autosomal dominant retinitis pigmentosa, the transgenic pig carrying mutant rhodopsin. This grant will answer three important questions: (1) How do the P347L and P347S rhodopsin mutations alter normal phototransduction and rod signaling? (2) How do electrophysiological recordings of single cell photoresponses compare to those same responses derived by indirect methods with the electroretinogram (ERG)? This project will document the ERG's capacity as a tool used to probe photoreceptor function. Massive loss of rod photoreceptors in these animals and in patients with RP somehow kills the cone photoreceptors as well. If cone function could be rescued, a substantial portion of human visual behavior would remain intact. (3) What are the pathologic changes in the physiology of cone photoreceptors associated with retinal degeneration due to rhodopsin mutations in the pig? The past decade has seen tremendous advances in the understanding of the biochemistry and molecular biology of phototransduction, yet little is known about photoreceptors pathophysiology. We will investigate the single cell photocurrents with the suction electrode technique to examine the changes that take place throughout the course of the retinal degeneration and loss of vision. Several biophysical parameters of the rods and cones will be measured at 3 to 5 stages over a period in which all the rods and half the cones are lost. We will also examine the photoreceptor responses to flickering light which is predicted to be a sensitive indicator of cell health. Statistical analyses will determine major and minor effects. We will establish a quantitative physiological database for photoreceptor function in a degenerating retina coordinated with ERG evaluations of the retina at the same stages of disease. This data base will be useful in judging therapeutic intervention.