Light exposure induces ubiquitin conjugation and degradation activities in the rat retina

To evaluate the consequences of light exposure on retinal ubiquitin (Ub) conjugation and degradation.

Two-month-old Long Evans pigmented rats were exposed to constant light (180 foot-candles) or were left in complete darkness for 18 hours. Rats used for cyclic light and diurnal rhythm experiments were removed from their light cycles at different times (24-hour clock): 0700 (before the light was turned on), 1000 (3 hours into the light cycle), 1000D (continued in the dark cycle), 1900 (before the light was turned off), 2200 (3 hours into the dark cycle), and 2200L (continued in the light cycle). The retinas were examined for Ub conjugation, adenosine triphosphate-Ub-dependent degradation, levels of Ub messenger RNA, and localization of Ub immunocytochemistry.

There was a statistically significant increase in Ub conjugation and degradation in retinas isolated from light-exposed animals compared with degradation in retinas of dark-adapted animals. However, no significant differences were observed in the levels of Ub messenger RNA from cyclic light, or light-exposed or dark-adapted retinas, suggesting that light-stress-induced changes do not reflect increased transcriptional activity. The daily variations observed in Ub conjugation and degradation suggest that these processes are probably the result of a circadian rhythm. Results of immunohistochemical studies revealed that Ub and its conjugates were uniformly distributed throughout the retinal cell layers in light- and dark-adapted retinas. However, in light-exposed retinas, a strong positive immunoreactivity was observed in the inner retina, specifically in horizontal and ganglion cells.

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 protein that plays an important role in protecting cells under stress conditions.