Solaris: a panel of bright and sensitive hybrid voltage indicators for imaging membrane potential in cultured neurons

Dynamic changes in the membrane potential underlie neuronal activities. Fluorescent voltage indicators allow optical recording of electrical signaling across a neuronal population with cellular precision and at millisecond-level temporal resolution. Here we report the design and characterization of a chemigenetic hybrid voltage indicator, Solaris, in which a circularly permuted HaloTag is inserted into the first extracellular loop of Acetabularia rhodopsin. Solaris is compatible with fluorogenic HaloTag ligands JF525, JF549, JF552, JF585, and JF635. The most sensitive conjugate, Solaris585, has more than 2-fold higher voltage sensitivity than the spectrally similar Voltron2585 (ΔF/F = -28.1 ± 1.3% versus -12.3 ± 0.7% per action potential in cultured neurons). Solaris585 supports the measurement of optogenetically evoked spike activity or dual-color imaging in conjunction with green-emitting calcium or glutamate indicators. Solaris indicators are also applicable to fluorescence lifetime imaging, which probes the absolute membrane potential. This new hybrid voltage indicator is a valuable tool for imaging neuronal electrophysiological activities in cultured cells with substantially improved dynamic range compared to previous hybrid indicators. ### Competing Interest Statement The authors have declared no competing interest.