Andrew S. Abler

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.

Inhibitory Effects of Cycloheximide and Flunarizine on Light-Induced Apoptosis of Photoreceptor Cells

The pathogenesis of photic retinopathy has been actively investigated for many years. Although the exact pathogenic mechanism involved in light-induced photoreceptor degeneration remains unknown, certain hypotheses were made based on previous animal studies [1-8]. Free radical formation and lipid peroxidation are among the most widely accepted hypotheses regarding the pathogenesis of photic retinopathy [1-5]. In addition, possible roles for protein synthesis and alteration of intracellular Ca2+ concentration in light-induced photoreceptor cell death have been suggested [6–8]. Protein synthesis inhibitors, such as cycloheximide and Ca2+ channel overload blockers, such as flunarizine, were both demonstrated to have ameliorative effects on retinal photic injury [6–8]. These findings provided supportive evidence of the possible involvement of protein synthesis and alteration of intracellular Ca2+ concentration in retinal photic injury. However, the mechanism whereby these two factors ameliorated light-induced photoreceptor cell death was not determined.