LIGHT EXPOSURE INDUCES UBIQUITIN CONJUGATION AND DEGRADATION ACTIVITIES IN THE RAT RETINA

Purpose. To evaluate the consequences of light exposure on retinal ubi quitin (Ub) conjugation and degradation. Methods. Two-month-old Evans pigmented rats were exposed to constant light (180 foot-candles) ore w ere left in complete darkness for 18 hours. Rats used for cyclic light and diurnal rhythm experiments were removed from their light cycles a t different times (24-hour clock): 0700 (before the light was turned o n), 1000 (3 hours into the light cycle), 1000D (continued in the dark cycle), 1900 (before the light was turned off), 2200 (3 hour into the dark cycle), and 2200L (continued in the light cycle). The retinas wer e examined for Ub conjugation, adenosine triphosphate-Ub-dependent deg radation, levels of Ub messenger RNA, and localization of Ub immunocyt ochemistry. Results. There was a statistically significance increase i n Ub conjugation and degradation in retinas isolated from light-expose d animals compared with degradation in retinas of dark-adapted animals . However, no significant differences were observed in the levels of U b messenger RNA from cyclic light, or light-exposed or dark-adapted re tinas, suggesting that light-stress-induced changes do not reflect inc reased transcriptional activity. The daily variations observed in Ub c onjugation and degradation suggest that these processes are probably t he result of a circadian rhythm. Results of immunohistochemical studie s revealed that Ub and its conjugates were uniformly distributed throu ghout the retinal cell layers in light- and dark-adapted retinas. Howe ver, in light-exposed retinas, a strong positive immunoreactivity was observed in the inner retina, specifically in horizontal and ganglion cells. Conclusions. These results suggest that light exposure may play a role in inducing Ub-conjugating activity in certain retinal cells. Furthermore, the results support the hypothesis that Ub is a stress pr otein that plays an important role in protecting cells under stress co nditions.