Rhodopsin-based voltage imaging tools for use in muscles and neurons of Caenorhabditis elegans

Neuronal and other excitable cell activity is characterized by alteration in membrane voltage, while intracellular Ca²⁺ levels and transmitter release are affected downstream of electrical activity. Thus, the most direct way of monitoring neuronal activity is by membrane voltage. Electrophysiology is demanding for multiple cells or cell ensembles and difficult to use in live animals, thus imaging methods are desirable. Yet, genetically encoded voltage indicators fell behind Ca²⁺ indicators until recently, when microbial rhodopsins and derivatives were introduced as genetically encoded voltage indicators. We evaluated rhodopsin tools for voltage imaging in muscles and neurons of Caenorhabditis elegans, a prime animal model in neuro- and cell biology, showing robust performance and the ability to characterize genetic mutants.