Pharmacological characterization of glycine-activated currents in HEK 293 cells expressing N-methyl-D-aspartate NR1 and NR3 subunits.

N-Methyl-D-aspartate (NMDA) receptors are important targets for drugs of abuse such as ethanol, toluene, and ketamine. Ligand-gated ion channels assembled from the NR1 and NR3 subunits have functional and pharmacological properties that are distinct from those of conventional NMDA receptors containing NR2 subunits. In the present study we used voltage-clamp electrophysiology to characterize excitatory glycine-activated receptors assembled from NR1, NR3A, and NR3B subunits expressed in human embryonic kidney (HEK) 293 cells. These glycine-activated receptors were not stimulated by glutamate or kainic acid and were resistant to magnesium block. A wide variety of NMDA receptor antagonists including d-2-amino-5-phosphonovaleric acid, ifenprodil, memantine, (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclo-hepten-5,10-imine hydrogen maleate (MK-801) or acamprosate did not inhibit glycine-activated NR1/NR3A/NR3B receptors. Likewise, these receptors were not affected by antagonists of inhibitory glycine receptors or glycine transporters. The NMDA receptor glycine site agonist, d-serine, partially activated NR1/NR3A/NR3B receptors, whereas the antagonist, 5,7-dichloro-kynurenic acid, inhibited receptor currents. Conversely, the antagonist, 7-chlorokynurenic acid, and the partial agonist, R-(+)-3-amino-1-hydroxy-2-pyrrolidinone (HA-966), potentiated glycine-stimulated currents of these receptors. NR1/NR3A/NR3B receptor currents were inhibited by 10 to 21% by ethanol and toluene but were relatively insensitive to ketamine. Ethanol inhibition was enhanced in receptors expressing the NR1(L819A) mutant, whereas those containing NR1(F639A) or NR1(M813A) showed no change relative to the wild-type NR1. The results of this study indicate that coexpression of NR1, NR3A, and NR3B subunits in HEK 293 cells results in glycineactivated receptors with novel functional and pharmacological properties.