Glutamate but not glycine agonist affinity for NMDA receptors is influenced by small cations.

NMDA receptor currents desensitize in an agonist-dependent manner when either the glutamate or glycine agonist is subsaturating. This may result from a conformational change in the NMDA receptor protein that lowers glutamate and glycine binding site affinity induced by co-agonist binding, channel opening, or ion permeation. We have used whole-cell voltage clamp of cultured hippocampal neurons with agonist paired-pulse protocols to demonstrate that glutamate and glycine dissociate 7.9- and 6.8-fold slower in the absence of their respective co-agonists than when their co-agonists are present. Paired-pulse and desensitization protocols were used to show that co-agonist binding and channel opening are sufficient to cause a reduction in glycine affinity, but extracellular sodium or magnesium binding was required in addition to conformational changes leading to channel opening to reduce glutamate binding-site affinity. Use of cesium or potassium as the major extracellular cation prevented the reduction of glutamate affinity. In addition, the use of choline-, sodium-, or cesium-based intracellular solutions did not alter desensitization characteristics, indicating that the site responsible for reduction of glutamate affinity is not in the intracellular domain. The fact that the reduction of glutamate affinity is dependent on certain small extracellular cations whereas the reduction of glycine affinity is insensitive to such cations indicates that conformational changes induced by the binding of glutamate are not completely paralleled by the conformational changes induced by glycine. Although glutamate and glycine are essential co-agonists, these data suggest that they have differential roles in the process of NMDA receptor activation.

Specificity of putative partial agonist, 1-aminocyclopropanecarboxylic acid, for rat N-methyl-D-aspartate receptor subunits.

The neuroprotective compound, 1-aminocyclopropanecarboxylic acid (ACPC), has been reported to act on the N-methyl-D-aspartate (NMDA) receptors simultaneously as a glycine binding site agonist and a glutamate binding site competitive antagonist. The complex kinetics of NMDA current changes measured by a whole-cell voltage clamp in rat hippocampal neurons resulting from application and removal of 1 mM ACPC in the continual presence of 15 microM NMDA confirm this hypothesis. Two-electrode voltage clamp on Xenopus oocytes expressing NR1-1a and either NR2A, NR2B or NR2C subunits yielded biphasic ACPC dose response curves with 15 microM NMDA. NR1-1a/NR2B and NR1-1a/NR2C subunit combinations yielded overlapping dose response curves with a maximal efficacy of approximately 80%; the maximal efficacy of ACPC for the NR1-1a/NR2A subunit combination was significantly lower at approximately 60%.