Abstract
Abstract: :
Purpose: Oxidative stress is widely accepted to play a major role in age–related diseases including multiple retinopathies associated with loss of photoreceptor cells (PR). In the retina, protection from oxidative damage is conferred, in part, by metallothioneins (MT) that are metal–binding proteins that remove free radicals. Here we examined the action of hyperbaric oxygen (HBO)–induced oxidative stress in normal and metallothionein–knockout (MT–KO) mouse retinas to establish a direct link between oxidative stress, PR viability and MT function. Methods: Six month old normal and MT–KO mice were treated either in an HBO chamber exposed to 3 atm of 100% oxygen for 3 hours (in the dark), 3 times per week on alternate days, for up to 5 weeks, or maintained normally in room air (controls). Retinas were fixed and retinal sections examined by light microscopy. The number of PR nuclei in selected regions was counted and the outer nuclear layer thickness measured. Results: In both normal and MT–KO retinas, a significant loss of 50–60% of PR nuclei occurred in the central retinal regions following 5 weeks of HBO treatment, compared to control retinas kept at room air. In general, PR cell loss decreased in the peripheral retina in both superior and inferior hemispheres. In peripheral regions, however, a striking difference in PR sensitivity to HBO was noted 1) between normal and MT–KO retinas and 2) between superior and inferior hemispheres. In the superior peripheral regions, MT–KO retinas exposed to HBO for 5 weeks exhibited a 30% loss of PR nuclei compared to MT–KO retinas not exposed to HBO, whereas normal retinas showed no HBO–induced PR cell loss. Conversely, HBO treatment produced an opposite effect in the inferior peripheral retina; here, normal retinas showed a 30% loss of PR cells after 5 weeks of HBO, compared to normal retinas maintained in room air, while MT–KO retinas exhibited no such PR cell loss following HBO treatment, but rather, showed an unexpected increase in the number of PR nuclei compared to MT–KO retinas not exposed to HBO. Conclusions:The present results establish a direct effect of HBO–induced oxidative stress on photoreceptor viability. This effect is different between normal and MT–knockout mice suggesting a direct role for MTs in photoreceptor maintenance and/or protection. These results provide insight into those oxidative stress mechanisms that underly photoreceptor damage, whose elucidation should prove useful towards understanding retinal disease.
Keywords: photoreceptors • oxidation/oxidative or free radical damage • retinal degenerations: cell biology