Transient Down-regulation of NMDA Receptor Subunit Gene Expression in the Rat Retina Following NMDA-induced Neurotoxicity is Attenuated in the Presence of the Non-competitive NMDA Receptor Antagonist MK-801

Abstract

Excitotoxic challenge has been thought to directly target NMDA-receptive neurons to undergo cell death. Recent evidence suggests that NMDA induced cell death is a selective process and that the specificity may be determined by the subunit composition of the NMDA receptor. Using a rat retinal model, we examined the effects of NMDA induced neurotoxicity on the regulation of NMDA receptor subunit gene and protein expression levels to determine if excitotoxic challenge preferentially regulates one or more of the NMDA receptor subunits. Following NMDA insult, the mRNA levels for NR1com, NR2A, NR2B and, to a lesser extent, the NR2C subunit were substantially reduced within 24hr post-treatment (PT), and remained depressed for up to 48hr. Levels for NR2D, although initially suppressed as early as 6hr-PT, were least affected by NMDA insult and showed almost full recovery by 48hr. By 10 days, the levels of gene expression for all five subunits recovered to levels that were indistinguishable from sham treated and untreated retinas. Co-administration of MK-801 with NMDA suppressed the effects of NMDA-induced down-regulation of all five genes.Protein levels for NR1com, NR2A and NR2B were also monitored at select time points following NMDA-insult. By 2 days-PT, protein levels for the three subunits were dramatically reduced. By day 10, the levels of protein expression for NR1comand NR2B remained suppressed despite the rise in gene expression for these two subunits, whereas protein for NR2A showed a substantial rise in expression.Of the five genes assayed, NR2A and NR2B showed the greatest reduction in expression following NMDA treatment, suggesting that one or both of these subunit may signal events leading to neuronal cell death in the retina. Conversely, gene expression of the NR2D subunit was least affected by NMDA exposure. In view of the evidence that the NR2D subunit is expressed by rod bipolar cells in the rat and that these neurons do not die following NMDA insult, it appears that inclusion of this subunit into functional receptors may provide protection against NMDA-induced cell death. Although the significance of the transient down-regulation of four out of the five NMDA receptor subunits is still not fully understood, the recovery of expression of these genes by day 10-PT indicates that not all of the NMDA receptive neurons are susceptible to NMDA-induced cell death. The preferential down-regulation of the NR2A and NR2B receptor subunits may implicate these subunits as key players in mediating the excitotoxic signal in the retina and possibly elsewhere in the brain.