Potassium-selective channelrhodopsins can exert hyper- or depolarizing effects in excitable cells of Caenorhabditis elegans , depending on experimental condition

One of the most frequent applications of optogenetic tools is for depolarization and stimulation of excitable cells such as neurons and muscles. Equally important, but less frequently used, are inhibitory tools that suppress activity through cellular hyperpolarization. These tools often rely on chloride conductance. Yet, in vivo , re- and hyperpolarization is typically mediated by potassium. In recent years, light-gated ion channels with a high preference for potassium were identified (Kalium channelrhodopsins, KCRs), and their inhibitory potential described in different organisms. Here, we characterized Hc KCR1 and WiChR, in cholinergic neurons and muscles of Caenorhabditis elegans . Hyperpolarization of these cell types both induces muscle relaxation and, consequently, an elongation of the animals. Thus, we analyzed body length before, during, and after illumination, to assess KCR effectiveness, and to benchmark stimulation parameters like light intensity and duration. For Hc KCR1 in cholinergic neurons, continuous illumination at high light intensities (1-4.5 mW/mm ² ) evoked only a transient elongation, while stimulation at 0.1 mW/mm ² could maintain inhibition for the duration of the stimulus in some transgenic strains. For animals expressing WiChR in body wall muscle cells or cholinergic neurons, we again observed brief hyperpolarization during continuous illumination, however, still during the stimulus, this changed to body contraction, corresponding to depolarization. This effect was long lasting, and required dozens of seconds for reversion, but could be reduced by pulsed illumination and fully avoided by less efficient channel activation using green or orange light. Hence, KCRs can be applied to hyperpolarize C. elegans cells, but require optimized illumination parameters.