Distinct allosteric paths mediate a Ca²⁺-dependent increase in NMDA receptor sensitivity to open-channel blockers

When active, NMDA receptors pass Ca²⁺-rich excitatory currents that are essential for the normal development and function of the central nervous system. In turn, fluctuations in extracellular Ca²⁺ levels, as observed during synaptic activity and pathological states, affect the NMDA receptor gating kinetics and conductance. Here, we used patch-clamp electrophysiology, kinetic analyses, and mutagenesis to evaluate how changes in the ambient Ca²⁺ concentration affect the sensitivity of recombinant NMDA receptors to open-channel blockers. NMDA receptor currents are characteristically sensitive to voltage-dependent block by Mg²⁺, which endows them physiologically with coincidence detection. This regulatory mechanism is shared with ketamine and memantine, two synthetic compounds that are clinically effective for treating depression and Alzheimer's disease, respectively. We found that extracellular Ca²⁺ increased the sensitivity of NMDA receptors to block by Mg²⁺ and memantine but not by ketamine. Further, the effect of Ca²⁺ on block by memantine required intracellular Ca²⁺ and functional calmodulin, whereas the effect of Ca²⁺ on block by Mg²⁺ required the extracellular residue GluN1-D658. We conclude that extracellular Ca²⁺ fluctuations modulate the sensitivity of NMDA receptors to open-channel blockers by discrete mechanisms, which may explain the distinct clinical profiles of NMDA receptor blockers.