Estimating the Ca²⁺ Permeability of NMDA Receptors with Whole-Cell Patch-Clamp Electrophysiology

In the mammalian central nervous system (CNS), fast excitatory transmission relies primarily on the ionic fluxes generated by ionotropic glutamate receptors (iGluRs). Among iGluRs, NMDA receptors (NMDARs) are unique in their ability to pass large, Ca²⁺-rich currents. Importantly, their high Ca²⁺ permeability is essential for normal CNS function and is under physiological control. For this reason, the accurate measurement of NMDA receptor Ca²⁺ permeability represents a valuable experimental step in evaluating the mechanism by which these receptors contribute to a variety of physiological and pathological conditions. In this chapter, we provide a theoretical and practical overview of the common methods used to estimate the Ca²⁺ permeability of ion channels as they apply to NMDA receptors. Specifically, we describe the principles and methodology used to calculate relative permeability (P_Ca/P_Na) and fractional permeability (P_f), along with the relationship between these two metrics. With increasing knowledge about the structural dynamics of ion channels and of the ongoing environmental fluctuations in which channels operate in vivo, the ability to quantify the Ca²⁺ entering cells through specific ion channels remains a tool essential to delineating the molecular mechanisms that support health and cause disease.