Glycine site-directed agonists reverse the actions of ethanol at the N-methyl-D-aspartate receptor

Ethanol has been shown to inhibit N-methyl-D-aspartate (NMDA)-stimulated calcium influx into cerebellar granule cells grown in culture. Because NMDA-mediated responses are modulated by a number of substances, we investigated the effects of several of these agents on ethanol-induced inhibition of calcium flux. Ethanol (50 mM) inhibited NMDA-dependent Ca2+ influx by approximately 50%. The percentage of inhibition remained constant with increasing NMDA concentrations (5-250 microM). Increasing Mg2+ concentrations in the assay medium inhibited NMDA-stimulated calcium influx but the EC50 for Mg2+ was unchanged in the presence of ethanol. Glycine at concentrations of 0.3-100 microM potentiated the effects of NMDA. Glycine at concentrations in excess of 10 microM decreased ethanol-mediated inhibition of NMDA-stimulated calcium influx. D-Serine was shown to have effects similar to those of glycine, whereas L-serine was significantly less active in potentiating NMDA-stimulated activity and reversing the ethanol-induced inhibition of calcium influx. N-Methylglycine and L-leucine were ineffective in potentiating NMDA actions but high concentrations (1 mM) of N-methylglycine attenuated ethanol-induced inhibition, whereas L-leucine (1 mM) had no effect. High concentrations of N-methylglycine were shown to reduce glycine-induced enhancement at the NMDA receptor, whereas L-leucine did not affect the glycine response. Glycine did not affect kainate-stimulated calcium influx and did not alter the small amount of inhibition produced by ethanol in the response of the cells to kainate. The results demonstrate that the in vivo actions of ethanol on the NMDA systems of brain may be dependent on glycine concentrations at these receptor sites.