Intense visible light can damage retinal photoreceptor cells by photochemic
al or thermal processes, leading to cell death, The precise mechanism of li
ght-induced damage is unknown; however, oxidative stress is thought to be i
nvolved, based on the protective effect of antioxidants on the light-expose
d 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 si
ngle-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. Trea
tment 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 im
plicated 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 at
tack on DNA by activated oxygen species preceding enzymatic degradation.