Alcohols inhibit N-methyl-D-aspartate receptors via a site exposed to the extracellular environment

N-Methyl-D-aspartate (NMDA) receptors are important CNS target sites of alcohols, but the site and mechanism of action of alcohols on NMDA receptors remains unclear. In CHO-K1 cells transfected with NR1/NR2B NMDA receptor subunits, ethanol inhibited NMDA-activated current with an IC(50) of 138 mM. Truncation of the intracellular C-terminal domain of the NR1 subunit (NR1T) did not alter ethanol sensitivity when combined with the NR2B subunit, but a similar truncation of the NR2B subunit (NR2BT) slightly enhanced ethanol sensitivity of receptors formed from coexpression with either NR1 or NR1T subunits. 1-Pentanol applied externally inhibited NMDA receptors with an IC(50) of 9.9 mM, but intracellular application of 1-pentanol (25 mM) did not alter NMDA receptor inhibition by externally applied ethanol or 1-pentanol. In addition, the amplitude of NMDA-activated current did not decrease during the time required for 1-pentanol (25 mM) to diffuse throughout the cytoplasm. Ethanol did not inhibit NMDA receptors when bath-applied in cell-attached patches or when applied to the cytoplasmic face of inside-out membrane patches. These results appear to be best explained by an action of alcohols on the NMDA receptor-channel protein, at a site located in a domain exposed to, or only accessible from, the extracellular environment.