Continuing damage to rat retinal DNA during darkness following light exposure

The damaging effects of visible light on the mammalian retina can be detect ed as functional, morphological or biochemical changes in the photoreceptor cells. Although previous studies have implicated short-lived reactive oxyg en species in these processes, the termination of light exposure does not p revent continuing damage, To investigate the degenerative processes persist ing during darkness following light treatment, rats were exposed to 24 h of intense visible light and the accumulation of DNA damage to restriction fr agments containing opsin, insulin 1 or interleukin-6 genes was measured as single-strand breaks (ssb) on alkaline agarose gels. With longer dark treat ments all three DNA fragments showed increasing DNA damage. Treatment of ra ts with the synthetic antioxidant dimethylthiourea prior to light exposure reduced the initial development of alkali-sensitive strand breaks and allow ed significant repair of all three DNA fragments. The time course of double -strand DNA breaks was also examined in specific genes and repetitive DNA, Nucleosomal DNA laddering was evident immediately following the 24 h light treatment and increased during the subsequent dark period. The increase in the intensity of the DNA ladder pattern suggests a continuation of enzymati cally mediated apoptotic processes triggered during light exposure, The pro tective effects of antioxidant suggests that the light-induced DNA degradat ive process includes both early oxidative reactions and enzymatic processes that continue after cessation of light exposure.