Channelrhodopsin2 Current During the Action Potential: “Optical AP Clamp” and Approximation

The most widely used optogenetic tool, Channelrhodopsin2 (ChR2), is both light- and voltage-sensitive. A light-triggered action potential or light-driven perturbations of ongoing electrical activity provide instant voltage feedback, shaping ChR2 current. Therefore, depending on the cell type and the light pulse duration, the typically reported voltage-clamp-measured ChR2 current traces are often not a good surrogate for the ChR2 current during optically-triggered action potentials. We discuss two experimental methods to reveal ChR2 current during an action potential: an "optical AP clamp" and its approximation employing measured current-voltage curve for ChR2. The methods are applicable to voltage- and light-sensitive ion currents operating in excitable cells, e.g. cardiomyocytes or neurons.