A comparison of continuous versus intermittent light exposure on apoptosis

Purpose. We recently found that continuous light exposure at a moderate intensity triggered apoptosis of photoreceptor cells. Since intermittent light exposure is known to cause more severe retinal damage than is continuous light exposure, we sought to determine if intermittent light exposure also triggered apoptosis of photoreceptor cells. Methods. Lewis albino rats were reared, for 2 weeks, in cyclic light and dark adapted for 24 hr before light exposure. Rats were exposed to intermittent light or continuous light for 6 or 9 hr, respectively. Light-exposed rats were killed by lethal injection at three timepoints: immediately after light exposure, after 6 hr of dark recovery following light exposure and after 24 hr of dark recovery following light exposure. Retinal damage after light exposure was evaluated by morphology, morphometry, the terminal transferase-mediated biotin dUTP nick end labeling (TUNEL) technique for identification of nicked/cleaved nuclear DNA and agarose gel electrophoresis of retinal DNA. Results. Evaluation of morphology confirmed that intermittent light exposure caused more photoreceptor cell damage than did continuous light exposure of the same duration and intensity. The TUNEL technique showed that photoreceptor nuclei contained nicked or cleaved DNA after either intermittent or continuous light exposure, although more TUNEL-positive nuclei were observed after intermittent exposure. Agarose gel electrophoresis of retinal DNA showed internucleosomal DNA fragmentation, which is associated with apoptosis in samples from intermittent light exposure. Conclusions. These data demonstrated that intermittent light exposure triggered apoptosis in more photoreceptor cells than did continuous light exposure of the same intensity and duration.