A Nature Methods article from the cohen lab shows how a microbial light-driven proton pump, Archeorhodopsin-3 (Arch), usually used to silence neuronal activity, can be used to efficiently image the membrane potential of individual neurons. Like many other microbial rhodopsins, Arch is weakly fluorescent. When excited using green light (550 nm), Arch emits light in the far red (680 nm). This fluorescence process is poorly efficient (quantum yield < 0.001) but sensitive to the membrane potential. When expressed in mammalian neurons in vitro, the fluorescence change of Arch in response to steps of potential was extremely fast (< 0.5 ms) with a high signal-to-noise ratio, allowing the detection of single action potentials in single trials. However, the fact that Arch generates an outward proton current when illuminated seriously challenges the relevance of its use as a voltage sensor. A mutated form of Arch (D95N) which does not generate any current was also tested. Arch(D95N) retains the ability to report single action potentials but with a significantly slower response time (40 ms).
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