ACTIVATION OF N-METHYL-D-ASPARTATE RECEPTORS BY GLYCINE - ROLE OF AN ASPARTATE RESIDUE IN THE M3-M4 LOOP OF THE NR1 SUBUNIT

Glutamate and glycine are coagonists that act at distinct sites to act ivate N-methyl-D-aspartate (NMDA) receptors. In the NR1 subunit of the NMDA receptor, mutation of D732 to glutamate (D732E), asparagine (D73 2N), alanine (D732A), or glycine (D732G) reduced the potency of glycin e by >4000-fold, but these mutations had no effect on sensitivity to g lutamate. Mutations at NR1(D732) also changed sensitivity to the glyci ne-site agonists D-serine and D-alanine, reducing the potencies and, i n some cases, the efficacies of these compounds. Thus, D-serine was a full agonist at the glycine site of receptors containing NR1(D732N) an d NR1(D732A), a partial agonist at receptors containing NR1(D732G), an d a competitive antagonist at receptors containing NR1(D732E). Mutatio ns at NR1(D732) had no effect or produced an increase in sensitivity t o the glycine-site antagonists 6,7-dichloroquinoxaline-2,3-dione and 5 ,7-dichlorokynurenic acid. These mutations did not affect the reversal potential, voltage-dependent block by extracellular Mg2+ block by ife nprodil, or stimulation by spermine at NR1/NR2B receptors. NR2 subunit s containing mutations at NR2A(D731) and NR2B(D732), which correspond to NR1(D732), did not produce functional receptors when coexpressed wi th NR1. Residue D732 in NR1 may be close to a glycine binding site on the NMDA receptor and may directly affect the properties of this site or be critical for coupling of glycine binding to channel activation.