Treger, J.S. et al. Biophys. J. 107, L09–L12 (2014).

Voltage sensing in humans has so far been impossible with fluorescent molecules such as organic dyes or genetically encoded voltage sensors. Treger et al. discovered that the dye indocyanine green (ICG), which is approved for use in humans, changes its fluorescence in response to voltage fluctuation. They showed that ICG can follow up to 100 action potentials per second in synthetic neurons. ICG reports voltage increases of 100 mV with a drop in fluorescence of about 2%. The researchers applied ICG to measure electrical activity in cultured neurons and rat hippocampal slices. Furthermore, they detected defects in synthetic neurons expressing a mutated sodium channel. Although ICG has not yet been applied to human tissue, conceivably this dye will be used to monitor electrical activity in humans in the future.