Abstract
Purpose::
To examine the long-term effects of acute light exposure in the retina, and retinal pigment epithelial (RPE) and choroidal layers.
Methods::
Albino rats injected with either the protective antioxidant phenyl-N-tert-butylnitrone (PBN) or saline 30 min prior to exposure with 5 k lux white fluorescent light for 6 h were kept for up to 3 months. Electroretinograms were recorded and the outer nuclear layer (ONL) and the choroidal thickness were measured. The expression of rod, cone, and RPE cell-specific markers was detected by Western blotting, and apoptosis was analyzed by TUNEL staining. Oxidative stress was analyzed by immunohistochemistry of 4-hydroxynonenal (HNE)-modified proteins. Retinal and choroidal ultrastructure was observed by transmission electron microscopy. Choroidal circulation was determined by in vivo staining of the choroidal layer by trypan blue.
Results::
In saline-injected animals, TUNEL- and 4-HNE-labeling in the ONL, RPE, and choroid was greater 24 h and 7 days after light exposure, and ERG amplitudes, ONL and choroidal thickness, and the expression of rhodopsin and RPE65 were lower 7 days after light exposure and later, compared to PBN-injected animals. In saline-injected animals, the expression of mid-wavelength opsin and the presence of cone nuclei in the ONL, and the choroidal circulation were preserved 7 days after light exposure, but started to decrease by 1 month and continued to decrease for 3 months following 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 light exposure. Delayed loss of cone cells and remaining rod cells, and reduction in choroidal circulation, were counteracted by PBN treatment.
Conclusions::
Although cone cells are resistant to cell damage induced by acute photo-oxidative stress, progressive slow loss of cone cells continued for up to 3 months after light exposure. Impaired choroidal circulation may be involved in the mechanism of delayed photoreceptor cell death. Therefore, preserving choroidal circulation may provide a novel target for retarding loss of cone and rod cells in patients with retinal degeneration such as retinitis pigmentosa.
Keywords: photoreceptors • choroid • oxidation/oxidative or free radical damage