Developmental expression of NMDA receptor subunits and NMDA-mediated toxicity in rat retina

Purpose. Glutamate receptors of the NMDA subtype mediate and modulate synaptic transmission in most parts the CNS. They are abundant in the inner retina. Excessive activation of NMDA receptors causes neuronal death, and this process is believed to be important in ophthalmic ischemic and neurodegenerative diseases. NMDA receptors are heteromeric assemblies of several subunits, which are a diverse family of distinct gene products and their splice variants. The pharmacological and physiological properties of the receptor depend upon the precise subunit composition. To determine if the subunit complement expressed by the retina differs during development and if this has toxicological consequences, we have determined the developmental expression of NMDA receptor subunits by western blotting and the sensitivity of the developing isolated rat retina to NMDA. Methods. Retinas from rats of several ages from newborn to adults were used. Highly specific monoclonal or affinity purified polyclonal antibodies raised against peptide- or fusion protein-based immunogens and highly specific for particular subunits were used in western blots. NR1-XXX, NR1-XX1, NR2A, and NR2B were measured. Isolated retinas were exposed to NMDA for 20 minutes, fixed for histology 90 minutes later, and scored by a blind observer for morphological evidence of damage. Results. Low amounts of NR1 were present at birth, NR2B appeared at ≈P7, and NR2A at ≈P14. Newborn and adult retinas were remarkably resistant to NMDA toxicity. There was a peak of sensitivity at ≈P9 during which many amacrine and ganglion cells show degenerative changes. Conclusions. At the earliest ages there are few NMDA receptors, and little toxicity. Peak toxicity coincides with the presence of NR1 and NR2B, and the toxicity of NMDA decreases as NR2A is expressed. We may hypothesize that cells expressing heteromeric NMDA receptors composed of NR1 and NR2B, but not NR2A, are especially vulnerable to NMDA toxicity.