N-methyl-D-aspartate-mediated glutamate toxicity in the developing rabbit retina

Extracellular levels of endogenous glutamate are relatively high in the developing rabbit retina but nonetheless appear to promote cell survival and developmental processes at concentrations considered toxic in the adult. We wished to examine the development of retinal susceptibility to glutamate toxicity as well as the protective effects of two N-methyl-D-aspartate (NMDA) antagonists, 2-amino-5-phosphono-5-valeric acid (APV) and dextromethorphan (Dex), and the nitric oxide synthase (NOS) inhibitor, NG-methyl-L-arginine (metARG). One day in vitro retinal explants of adult and neonatal rabbits were incubated with various agonists and antagonists, and stained with trypan blue to visualize necrotic cells. The density of the necrotic cells was analyzed using the Zeiss Videoplan 2. Immature neurons were approximately 10-fold less sensitive to NMDA toxicity compared to the adult. Although both NMDA antagonists and metARG provided marked protection for adult retinal neurons against glutamate toxicity, the modest susceptibility of the immature neuron was blocked only by Dex and not APV or metARG. At least two factors may contribute to the ability of the neonatal retina to survive in the presence of high levels of endogenous extracellular glutamate. First, the 10-fold developmental increase in NMDA toxicity occurs simultaneously with a 12-15-fold downregulation of extracellular glutamate, probably through the actions of maturing Muller cells. Second, the NMDA/NO excitotoxic pathway may not be active at birth since an NOS inhibitor had little effect at this stage and our previous morphological data demonstrate that NOS-containing cells are not present in their mature configuration until the second postnatal week.