This proposal addresses critically important unresolved issues concerning the antioxidant capacities of vertebrate photoreceptor cells and inner retinal neurons. A common factor in the loss of photoreceptor cells appears to be reactive oxygen species generated in excess of that normally seen by these cells. The main goal of this proposal is to understand why photoreceptor cells are more vulnerable to metabolic or oxidative chemical stress than cells in the inner retinal layers. While it is commonly believed that reduced glutathione (GSH) is present in all retinal cells, several reports show that GSH is not immunologically detectable in rod and cone photoreceptor cells. However, GSH is clearly seen in inner retinal neurons and MOiler cells, and in the pigment epithelium. Our working hypothesis is that the exceptional vulnerability of photoreceptor cells to toxic chemicals is linked to a specific deficiency in GSH.
The specific aims of this proposal test the following hypothesis: that a deficiency of photoreceptor cell GSH is responsible for the detrimental effects of toxic chemicals that cause the death of these cells, while the presence of GSH in inner retinal cells reduces their vulnerability to the same chemicals. We will evaluate this hypothesis by using whole rat retinas, serial sections of rat retinas, and bovine rod outer segments in electrophysiological and metabolic experiments designed to assess the linkage between photoreceptor cell viability and the GSH-redox cycle. Measurements will be made of glucose-dependent metabolism, NADPH generation, GSH content, antioxidant enzymes, morphology, and receptor potentials. We will also determine the extent to which N-acetylcysteine and glutathione ethyl ester, membrane permeant GSH-mimetics, protect photoreceptor cells exposed to a metabolic poison (iodoacetate) or an oxidizing chemical (ferrous ion). The two models of chemical stress that have been chosen for study each produce selective death of photoreceptor cells. Despite differences in the initiating damaging reactions for each stress, it is the guiding hypothesis of this proposal that the basis for the damage resides in a deficiency in GSH, and thus, a deficiency in the overall antioxidant and detoxication status of photoreceptor cells. The outcomes of the proposed work will improve our understanding of why photoreceptor cells are so vulnerable to oxidative and detrimental chemical reactions. These outcomes should help in the design of therapeutic treatments for photoreceptor degenerations that are linked to an inadequate antioxidative defense mechanism.
The initiation of the visual process depends upon healthy photoreceptor cells. Yet, these cells are highly susceptible to a variety of damaging conditions. This proposal addresses the mechanisms underlying their susceptibility and explores possible therapeutic protective treatments.