PURPOSE. To investigate the potential role of caspase-3 in blue light-induced apoptosis in photoreceptor cells.METHODS. Cyclic light-raised Sprague-Dawley rats were exposed to 400 to 480 nm light for 6 hours at an irradiance of 0.64 W/m(2). The rats were then kept in darkness for recovery and killed at scheduled time points from 0 hours up to 24 hours after exposure to light. The unexposed rats were used as the control. Apoptosis was marked with in situ terminal dUTP nick end labeling (TUNEL). Caspase-3 expression was explored using immunohistochemistry, Western blot analysis, real-time RT-PCR, and enzyme activity assay.RESULTS. Exposure to blue light resulted in photoreceptor cell apoptosis, mostly in the superior temporal area of the retina. TUNEL-positive cells were highest during 8 to 16 hours of recovery in darkness. Procaspase-3 protein was constitutively expressed in the rat retina and was apparently upregulated after exposure to light, with expression peaking at 8 to 16 hours and subsiding at 24 hours. The upregulation was further supported by enhanced transcription of caspase-3 in the postexposure retina. Meanwhile, increased cleavage of caspase-3 into its active fragments of 17 and 19 kDa was detected after exposure to light, peaking at 16 hours. Activation of caspase-3 was subsequently located in the photoreceptor cells and predominantly in the superior part of the temporal quadrant. This coincided with elevated caspase-3-like activity at 16 hours after exposure to light.CONCLUSIONS. Procaspase-3 protein is temporally upregulated in the retina after in vivo exposure to blue light, and the upregulation is coupled to activation of caspase-3 and concomitant induction of apoptosis in the photoreceptor cells.