13 Genetically encoded voltage indicators (GEVIs) based on microbial rhodopsins utilize the voltage-sensitive 14 fluorescence of the all-trans retinal (ATR) cofactor, while in electrochromic (eFRET) sensors, donor 15 fluorescence drops when the rhodopsin acts as depolarization-sensitive acceptor. We systematically 16 assessed Arch(D95N), Archon, and QuasAr, as well as the eFRET sensors MacQ-mCitrine and QuasAr-17 mOrange, in C. elegans. ATR-bearing rhodopsins reported on voltage changes in body wall muscles (BWMs) 18 and the pharynx, the feeding organ, where Arch(D95N) showed ca. 125 % F/F increase per 100 mV. The ATR 19 fluorescence is very dim, however, using the retinal analog dimethylaminoretinal (DMAR), it was boosted 20 250-fold. eFRET sensors provided sensitivities of 45 % to 78 % F/F per 100 mV, induced by BWM action 21 potentials (APs). All sensors reported differences in muscle depolarization induced by a voltage-gated Ca 2+ -22 channel mutant. Optogenetically evoked de-or hyperpolarization of motor neurons increased or eliminated 23 AP activity and caused a rise or drop in BWM sensor fluorescence. Last, we could analyze voltage dynamics 24 across the entire pharynx, showing uniform depolarization but compartmentalized repolarization of anterior 25 and posterior parts. Our work establishes all-optical, non-invasive electrophysiology in intact C. elegans. 26
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