I have been itching to post about this work since David DiGregorio presented it at a meeting at Janelia last year. His group’s results, Submillisecond Optical Reporting of Membrane Potentials In Situ Using a Neuronal Trace Dye, were published in the Journal of Neuroscience last week. Their method of optical voltage sensing is the first one that looks like its ready for “prime-time” action outside of the labs of developers of these sorts of techniques. It has sufficient speed (<1 ms resolution), sensitivity (25% dF/F per 100mV), and limited membrane perturbation to see single action potentials, without dramatically altering the shape of these currents.
Like previous methods, Bradley et al. use voltage-dependent membrane partitioning of dipicrylamine (DPA), a charged small molecule, to quench a fluorophore via FRET. Previously, high-concentrations of DPA were required to have a reasonable signal change, which caused toxicity, increased membrane capacitance and slowed voltage transients. By using DiO, a lipophilic neuronal tracer, as the fluorophore, the DPA concentration could be reduced to 1uM, while retaining sufficient optical sensitivity for action potential detection.