N -methyl- D -aspartate (NMDA) induced apoptosis in adult rabbit retinas

Previously we showed that apoptosis is involved in N -methyl- D -aspartate (NMDA) induced excitotoxicity in adult rat retinas. Since rabbits have a higher endogenous level of glutamate in the retina and very different retinal structures, it is not clear if apoptosis is similarly involved in adult rabbit retinas after intravitreal injection of NMDA. In this study, we used ultrastructural features, TdT-mediated biotin-dUTP nick end labeling (TUNEL) and two caspase inhibitors to examine whether apoptosis is involved in NMDA-induced excitotoxicity in adult rabbit retinas. At 18 hr after an intravitreal injection of 400 nmoles NMDA, typical apoptotic features in degenerative cells in the retinal ganglion cell layer (RGCL) and the inner nuclear layer (INL) were noted by electron microscopy. TUNEL positive nuclei were detected in these layers as early as 4 hr showing maximal numbers at 18 hr. At 7 days, significant loss of nuclei from the RGCL was noted at the visual streak, the superior and the inferior retinas. These losses were abolished by simultaneous administration of MK-801 and ameliorated by YVAD, a caspase-1 inhibitor, but not by IETD, a caspase-8 inhibitor. These results indicated that, similar to adult rat retinas, apoptosis is involved in NMDA receptor-mediated excitotoxicity in rabbit retinas and that specific caspases may play important roles.