Estimating the Ca²⁺ Block of NMDA Receptors with Single-Channel Electrophysiology

In vertebrate central neurons, NMDA receptors are glutamate- and glycine-gated ion channels that allow the passage of Na⁺ and Ca²⁺ ions into the cell when these neurotransmitters are simultaneously present. The passage of Ca²⁺ is critical for initiating the cellular processes underlying various forms of synaptic plasticity. These Ca²⁺ ions can autoregulate the NMDA receptor signal through multiple distinct mechanisms to reduce the total flux of cations. One such mechanism is the ability of Ca²⁺ ions to exclude the passage of Na⁺ ions resulting in a reduced unitary current conductance. In contrast to the well-characterized Mg²⁺ block, this "channel block" mechanism is voltage-independent. In this chapter, we discuss theoretical and experimental considerations for the study of channel block by Ca²⁺ using single-channel patch-clamp electrophysiology. We focus on two classic methodologies to quantify the dependence of unitary channel conductance on external concentrations of Ca²⁺ as the basis for quantifying Ca²⁺ block.