Calcium-dependent inactivation of recombinant N-methyl-D-aspartate receptors is NR2 subunit specific

Intracellular Ca2+ can reversibly reduce the activity of native N-methyl-D-aspartate (NMDA) receptors in hippocampal neurons, a phenomenon termed Ca2+-dependent inactivation. We examined inactivation in heteromeric NMDA receptors expressed in human embryonic kidney (HEK) 293 cells using whole-cell recording. NR1-1a/2A heteromers showed reversible inactivation that was very similar to native NMDA receptors in cultured hippocampal neurons. Inactivation was dependent on the extracellular Ca2+ concentration and the degree of intracellular Ca2+ buffering. In 2 mM extracellular Ca2+, inactivation resulted in a 46.1 +/- 12.6% reduction in the whole-cell current during a 5-sec agonist application. Inactivation of NR1-1a/2A heteromers was unaffected by calcineurin inhibitors, staurosporine, or phalloidin. NR1-1a/2D heteromers also showed a similar degree of inactivation. In contrast, NR1-1a/2B and NR1-1a/2C heteromers showed no significant inactivation. At saturating concentrations of NMDA (1 mM), NR1-1a/2A heteromers also showed Ca- and glycine-independent desensitization, as seen in native hippocampal neurons. Ca(2+)- and glycine-independent desensitization was less pronounced in NR1-1a/2B heteromers and absent in NR1-1a/2C heteromers. Activation of NR1-1a/2C heteromers triggered intracellular Ca2+ transients similar to NR1-1a/2A heteromers as verified by combined Ca2+ imaging and whole-cell recording. Thus differences in Ca2+ permeability were not responsible for the lack of inactivation in NR1-1a/2C heteromers. Our results show that inactivation of recombinant NMDA receptors requires either the NR2A or NR2D subunit, whereas both inactivation and desensitization were absent in NR2C-containing receptors. The gating of inactivating NMDA receptors is more likely to be influenced by ongoing NMDA receptor activity and Ca2+ transients, perhaps consistent with the prominent expression of NR2A in hippocampus and cerebral cortex.