ously donated by Dr. Matthew LaVail (University of California San Francisco, CA). The room light was kept on a 12 hr light/dark cycle using standard fluorescent lighting. During the light cycle, the illumination at the level of the cages was 100?00 lux. On the day of an experiment, a rat was euthanized with sodium pentobarbital (150 mg/kg, i.p.), and the eyes were removed and hemisected under norm