Circadian dependency of nocturnal immediate-early protein induction in rat retina

Daily rhythms in the mammalian retina are regulated by an endogenous circadian clock. Previously it was found that neuronal elements of the rat retina respond to light:dark (L:D) transitions with cell-specific changes in expression of the c-fos gene. Using a pan-Fos antibody to probe Western blots of rat retina, we have now shown that darkness is associated with a 60-fold increase in c-Fos protein, whereas levels of FosB and Fos-related antigens are invariant. The induction of c-Fos exhibits circadian dependency; accumulation of c-Fos protein was significantly enhanced, by a factor of 2.5-fold, when darkness onset was coincident with the established L:D transition. c-Fos exhibited only a low amplitude circadian rhythm in the absence of L:D cycles. Similar results were obtained for another immediate early gene (IEG) protein, Egr-1. These findings show that IEG induction in the rodent retina exhibits circadian clock dependency.