Damage to rat retinal DNA induced in vivo by visible light

Intense visible light can damage retinal photoreceptor cells by photochemical or thermal processes, leading to cell death. The precise mechanism of light-induced damage is unknown; however, oxidative stress is thought to be involved, based on the protective effect of antioxidants on the light-exposed retina. To explore the in vivo effects of light on retinal DNA, rats were exposed to intense visible light for up to 24 h and the time courses of single-strand breaks in restriction fragments containing the opsin, insulin 1 and interleukin-6 genes were measured. All three gene fragments displayed increasing single-strand modifications with increasing light exposure. Treatment with the antioxidant dimethylthiourea prior to light exposure delayed the development of net damage. The time course of double-strand DNA damage was also examined in specific genes and in repetitive DNA. The appearance of discrete 140-200 base-pair DNA fragments after 20 h of light exposure implicated a nonrandom, possibly enzymatic damaging mechanism. The generation of nucleosome core-sized DNA fragments, in conjunction with single-strand breaks, suggests two phases of light-induced retinal damage, with random attack on DNA by activated oxygen species preceding enzymatic degradation.