Circadian nature of rod outer segment disc shedding in the rat

Rod outer segment disc shedding has been examined in the rat to determine whether it follows a circadian rhythm. The burst of shedding that occurs soon after the onset of light repeats daily in continuous darkness for at least 12 days. The disc shedding rhythm becomes slightly longer than 24 hr in continuous darkness, with an estimated period of 24 hr and 8 min to 24 hr and 19 min. In rats maintained in cyclic light, the disc shedding rhythm can be re-entrained to a new light schedule following a phase shift in the environmental lighting. After a 3 hr phase delay, at least 4 weeks is required for re-entrainment. Rod outer segment disc shedding in the rat can be considered to follow a circadian rhythm because (1) it is a self-sustaining oscillation; (2) it appears to free-run with a period close to 24 hr in the absence of lighting cues; and (3) it can re-entrain to shifts in the light cycle.