Delayed loss of cone and remaining rod photoreceptor cells due to impairment of choroidal circulation after acute light exposure in rats

PURPOSE. To examine the long-term effects of acute photooxidative stress in the retina, retinal pigment epithelium (RPE), and choroid. METHODS. Albino rats injected with either the protective antioxidant phenyl-N-tert-butylnitrone (PBN) or saline 30 minutes before exposure to 5 klx white fluorescent light for 6 hours were kept for up to 3 months in 5 lux cyclic light. Electroretinograms were recorded, and the outer nuclear layer (ONL) and the choroidal thickness and area were measured after hematoxylin-eosin (H&E) staining. The expression of rod, cone, and RPE cell markers was detected by Western blotting, and apoptosis was analyzed by TUNEL staining. Oxidative stress was analyzed by immunohistochemistry against 4-hydroxynonenal (4-HNE)-modified proteins. Retinal and choroidal ultrastructures were observed by transmission electron microscopy (TEM). Choroidal circulation was analyzed by in vivo staining of the choroidal layer by trypan blue. RESULTS. In the saline-injected animals, TUNEL- and 4-HNE-labeling in the ONL, RPE, and choroid were higher 24 hours and 7 days after fight exposure, and ERG amplitude, ONL and choroidal thickness and area, and rhodopsin and RPE65 expression were lower 7 or more days after fight exposure than in phenyl-N-tert-butylnitrone (PBN)-injected animals. In the saline-injected animals, the expression of mid-wavelength opsin and the presence of cone cells in the ONL and the choroidal circulation were preserved for 7 days after light exposure but started to decrease by 1 month and continued to decrease for 3 months after light exposure. An increase in TUNEL-positive cells was observed in the ONL at the inferior peripheral retina, just behind the iris, by 3 months after fight exposure. Delayed loss of cone cells, remaining rod cells, and choroidal circulation were counteracted by PBN treatment. CONCLUSIONS. Although cone cells are resistant to cell damage induced by acute photooxidative stress, progressive loss of cone. cells continued for up to 3 months after fight exposure. Impaired choroidal circulation is likely to be involved in the mechanism of delayed photoreceptor cell death after light exposure. Preserving choroidal circulation may provide a novel target for preserving the cone and the remaining rod cells in patients with retinal degeneration such as retinitis pigmentosa.