Light-induced apoptosis in the neonatal mouse retina and superior colliculus

PURPOSE. Apoptosis occurs naturally in the rodent retina and superior colliculus (SC) during the neonatal period. The authors used mice to demonstrate the dependency of this apoptosis on the light stimulation and the developmental period. METHODS. A number of apoptotic cells were counted in the retina and SC from a group of newborn mice reared in constant darkness (DD group), a group reared in normal light and dark conditions (LD group), and a group reared in constant darkness up to P7 and then transferred to normal condition (DD-to-LD group). Terminal deoxynucleotidyl transferase-mediated biotin-dUTP nick end labeling (TUNEL) was used for visualization of the apoptotic cells. RESULTS. In the LD group, apoptotic cells significantly increased in the retinal nuclear layers, including both the outer and inner nuclear layers, the retinal ganglion cell layer, and SC at postnatal day 1 (P1) and postnatal day 2 (P2). The number of apoptotic cells in the ganglion cell layer and SC reached the maximum level at P1. In contrast, in the DD group, an increase in the number of apoptotic cells was not observed. At P9, no significant increase in the number of apoptotic cells was observed in the outer nuclear layer, ganglion cell layer, and SC either in the LD, DD, or DD-to-LD groups, but the LD and DD-to-LD groups showed a significant increase in the inner nuclear layer compared to the DD group. CONCLUSIONS. Apoptosis during the neonatal period in the mouse visual system is induced by a light stimulus. This apoptosis was not induced after P7 in the retinal ganglion cell layer and SC, even if excessive cells survived.