The N-methyl-D-aspartate receptor is an important mediator of the behavioral effects of ethanol in the central nervous system. Previous studies have demonstrated sites in the third and fourth membrane-associated (M) domains of the N-methyl-Daspartate receptor NR2A subunit that influence alcohol sensitivity and ion channel gating. We investigated whether two of these sites, Phe-637 in M3 and Met-823 in M4, interactively regulate the ethanol sensitivity of the receptor by testing dual substitution mutants at these positions. Ethanol is unusual among the major drugs of abuse in that it acts only at high concentrations (in the millimolar range) and that it acts on multiple targets in the central nervous system. For the greater part of the last century, ethanol was generally believed to produce its effects on central nervous system function via nonspecific actions on neuronal lipids, but it is now well accepted that the biologically important actions of ethanol are due to its interactions with proteins (1, 2). Of these proteins, the N-methyl-D-aspartate (NMDA) 2 receptor is among the most important target sites of ethanol in the central nervous system. At relevant concentrations, ethanol inhibits ionic current (3), synaptic potentials (4), Ca 2ϩ influx (5, 6), and neurotransmitter release (7) mediated by NMDA receptors. Studies of the mechanism of this inhibition have shown that it does not involve competitive inhibition at the glutamate or glycine binding sites (7-12) or interaction with sites for other allosteric modulators (8,12) or open channel block (13,14) but that it involves changes in NMDA receptor gating, notably mean open time and opening frequency (13,14). Thus, ethanol appears to inhibit NMDA receptors via low affinity interactions with sites that regulate ion channel gating. Although sites in the intracellular C-terminal domain may modulate both ethanol sensitivity of the NMDA receptor (15) and ion channel gating (16 -19), this domain does not contain the site of ethanol action, since removal of this region of the protein does not decrease ethanol inhibition of the receptor (20).In a previous study, Ronald et al. (21) demonstrated that a phenylalanine residue (Phe-639) in the third membrane-associated (M) domain of the NMDA receptor NR1 subunit influences alcohol sensitivity and shows some characteristics of a site of alcohol action. A previous study from this laboratory (22) identified a methionine residue in the M4 domain of the NMDA receptor NR2A subunit that also influences alcohol sensitivity and that fulfills some of the criteria for a site of alcohol action. The methionine in M4, however, also profoundly affects the gating behavior of the ion channel (23). We have recently shown (24) that the cognate position of NR1(Phe-639) in the NR2A subunit, Phe-637, also regulates alcohol sensitivity as well as desensitization and agonist potency. Studies in ␥-aminobutyric acid A and glycine receptors have demonstrated residues in transmembrane domains 2 and 3 that form sites of alcohol and anesthetic act...