Necrosis and apoptosis after retinal ischemia: Involvement of NMDA-mediated excitotoxicity and p53

PURPOSE. Accumulated evidence has shown that apoptosis and necrosis contrib ute to neuronal death after ischemia. The present study was performed to st udy the temporal and spatial patterns of neuronal necrosis and apoptosis af ter ischemia in retina and to outline mechanisms underlying necrosis and ap optosis. METHODS. Retinal ischemia was induced try increasing intraocular pressure t o a range of 160 mm Hg to 180 mm Hg for 90 minutes in adult rats. The patte rns of neuronal cell death were determined using light and electron microsc opy and were visualized by TdT-dUTP nick-end labeling (TUNEL). The mRNA exp ression profile of p53 was examined using reverse transcription-polymerase chain reaction (RT-PCR) and in situ hybridization histochemistry. Immunohis tochemistry was performed using anti-p53, anti-microtubule associated prote in-2, and anti-glial fibrillary acidic protein antibodies. RESULTS. Within 4 hours after ischemia, neurons in the inner nuclear cell l ayer (INL) and ganglion cell layer (GCL) underwent marked necrosis, made ap parent by swelling of the cell body and mitochondria, early fenestration of the plasma membrane, and irregularly scattered condensation of nuclear chr omatin. After 3 days, the INL and GCL neurons showed further degeneration t hrough apoptosis marked by cell body shrinkage, aggregation, and condensati on of nuclear chromatin. Apoptotic neurons were also observed sparsely in t he outer nuclear cell layer. Intravitreal injections of MK-801 prevented ea rly neuronal degeneration after ischemia. Of note, mRNA and protein levels of p53, the tumor suppressor gene known to induce apoptosis, were increased in the retinal areas undergoing apoptosis 1 to 3 days after ischemic injur y. CONCLUSIONS. Ischemia produces the N-methyl-D-aspartate-mediated necrosis a nd slowly evolving apoptosis of neurons in the retina. The latter may depen d on the expression of the p53 proapoptosis gene.