A red-emitting, genetically encoded indicator for two-photon voltage recording in vivo

Genetically encoded voltage indicators (GEVIs) enable minimally invasive, cell-type-specific optical measurements of neuronal membrane potential with millisecond temporal resolution. Red-shifted GEVIs are especially advantageous because they permit spectral multiplexing with complementary sensors and enable all-optical circuit interrogation in combination with blue-light-activated opsins. Despite these advantages, existing red GEVIs remain poorly suited for in vivo use due to limited performance under two-photon (2P) excitation, the predominant modality for deep-tissue imaging. Here, we introduce VADER1, a red GEVI that overcomes this limitation and enables reliable spike detection in vivo under 2P illumination. Under 2P excitation, VADER1 supports extended voltage imaging with both random-access and resonant-scanning microscopy, enables recordings from neurons as deep as cortical layer 5, and allows dual-color imaging with calcium indicators. By filling a critical spectral gap, VADER1 enables integrated optical measurements of fast electrical activity alongside other neural signals and establishes a foundation for two-photon all-optical electrophysiology.