SINGLE-CHANNEL EVIDENCE FOR GLYCINE AND NMDA REQUIREMENT IN NMDA RECEPTOR ACTIVATION

N-Methyl-D-aspartate (NMDA) receptor dose-response relationships that are based on macroscopic currents suggest that NMDA and a different ag onist molecule, glycine, must together activate the channel. Since sin gle-channel recordings have a much higher resolution than whole-cell c urrents, they provide a highly sensitive test for the absolute require ment of NMDA channel opening for glycine. Rapid application of 10-300 mu M NMDA to outside-out patches from cultured cortical neurons evoked substantial single-channel activity in the absence of added glycine. However, in the presence of a high affinity and highly selective glyci ne-site antagonist, 5,7-dichlorokynurenate (DCK), NMDA failed to evoke any openings on its own. Channel openings could not be produced by sa turating concentrations of NMDA (up to 1 mM) but were evoked when glyc ine was added to the test solution. Glycine alone (up to 100 mu M) was similarly ineffective in the continuous presence of D(-)-2-amino-5-ph osphonovaleric acid (D-APV), an NMDA-site antagonist. Reversal of anta gonist blockade by the appropriate ligand (glycine or NMDA) and the no rmal appearance and duration of channel openings evoked in the presenc e of either antagonist ruled out open channel block. These single-chan nel data confirm the hypothesis that both NMDA and glycine are coagoni sts of the NMDA receptor. Furthermore, the coagonist requirement incre ases the potential targets for therapeutic drugs aimed at blocking the pathologies resulting from overactivation of NMDA receptors.