Light-induced damage in the retina: Differential effects of dimethylthiourea on photoreceptor survival, apoptosis and DNA oxidation

In the rat, photoreceptor cell death from exposure to intense visible light can be prevented by prior treatment with antioxidants. In this study we su bjected albino rats raised in dim cyclic light and rats made more susceptib le to light damage by rearing in darkness to exposures of green light that led to similar losses of photoreceptor cells. Rhodopsin and photoreceptor D NA, indicators of the number of surviving photoreceptor cells, were determi ned at various times over a period of 14 days after light exposure. Fragmen tation of DNA was determined over a similar time course by neutral and alka line agarose gel electrophoresis. Apoptosis in retinal DNA was measured by quantitating the appearance of 180 base pair (bp) nucleosomal fragments. Ox idation of DNA was measured by electrochemical detection of the nucleoside 8-hydroxydeoxyguanosine (8-OHdG) after separation by high-performance chrom atography. For albino rats reared in dim cyclic light, 24 h of intense ligh t exposure resulted in the loss of 50% rhodopsin and photoreceptor cell DNA . In dark-reared rats, the losses were 40%, respectively, after only 3 h of intense light treatment. In both cases pretreatment with the antioxidant d imethylthiourea (DMTU) prevented rhodopsin and photoreceptor cell DNA loss. The kinetics of the light-induced apoptosis depended markedly on the reari ng environment of the rats. The DNA ladders appeared within 12 h of the ons et of intense light in the rats reared in dim cyclic light. In these rats t he 180 bp fragment was at two-thirds of its maximum intensity immediately a fter 24 h of light exposure and rc:ached the maximum 12 h later. Dimethylth iourea partially inhibited ladder formation in rats reared in dim cyclic li ght and delayed the time of appearance of the 180 bp maximum by 6 h. By con trast, in rats reared in darkness the 180 bp fragment was undetected immedi ately after 3 h of light exposure and reached its maximum 2 days later. Pre treatment with DMTU completely eliminated DNA ladders in these rats. Alkali ne gel electrophoresis revealed a pattern of single-strand DNA breaks, with relatively high molecular weight fragments, 6 h after light exposure of da rk-reared rats. Single-strand DNA breaks in cyclic light rats corresponded with the onset of apoptotic ladders, but peak values preceded by 12 h the p eak of DNA ladder formation. The quantity of 8-OHdG in retinal DNA remained close to control values in all samples with the exception of a peak of twi ce the control value 18 h after light exposure in the dark-reared rats and a value 60% higher 16 days after exposure in cyclic light animals. Dimethyl thiourea had no effect on the amount of oxidized purine in any of the sampl es. The differences between dark-reared rats and rats reared in dim cyclic light in the kinetics of DNA fragmentation and in their response to treatme nt with DMTU is consistent with previous observations of fundamental differ ences in retinal cell physiology in these animals. In dim light-reared rats , the pathway to apoptosis may be qualitatively different from the pathway to net photoreceptor loss in rats reared in darkness. The lack of effect of DMTU on 8-OHdG formation suggests that the oxidation of DNA bases is not a causal factor in light-mediated photoreceptor cell death.