Intersubunit interactions at putative sites of ethanol action in the M3 and M4 domains of the NMDA receptor GluN1 and GluN2B subunits

Background and Purpose The NMDA receptor is an important target of alcohol action in the brain. Recent studies in this laboratory have demonstrated that alcohol-sensitive positions in the intersubunit interfaces of the M3 and M4 domains of GluN1 and GluN2A subunits interact with respect to ethanol sensitivity and receptor kinetics and that alcohol-sensitive positions in the M domains of GluN2A and GluN2B subunits differ. In this study, we tested for interactions among alcohol-sensitive positions at the M domain intersubunit interfaces in GluN1/GluN2B NMDA receptors. Experimental Approach We used whole-cell patch-clamp recording in tsA201 cells expressing tryptophan substitution mutants at ethanol-sensitive positions in the GluN1 and GluN2B NMDA receptor subunits to test for interactions among positions. Key Results Six pairs of positions in GluN1/GluN2B significantly interacted to regulate ethanol inhibition: Gly⁶³⁸/Met⁸²⁴, Gly⁶³⁸/Leu⁸²⁵, Phe⁶³⁹/Leu⁸²⁵, Phe⁶³⁹/Gly⁸²⁶, Met⁸¹⁸/Phe⁶³⁷ and Val⁸²⁰/Phe⁶³⁷. Tryptophan substitution at Met⁸²⁴ or Leu⁸²⁵ in GluN2B did not alter ethanol sensitivity but interacted with positions in the GluN1 M3 domain to regulate ethanol action, whereas tryptophan substitution at Gly⁶³⁸, which is the cognate of an ethanol-sensitive position in GluN2A, did not alter ethanol sensitivity or interact with positions in GluN1. Two and three pairs of positions interacted to regulate glutamate steady-state and peak current EC₅₀, respectively, and one pair interacted with respect to macroscopic desensitization. Conclusions Despite highly-conserved M domain sequences and similar ethanol sensitivity in the GluN2A and GluN2B subunits, the manner in which these subunits interact with the GluN1 subunit to regulate ethanol sensitivity and receptor kinetics differs.