Voltage imaging reveals the emergence of population activity in the spinal cord

One of the central questions in neural development is how individual neurons assemble functional networks. To address this, it is essential to elucidate how coordinated activity emerges during development. However, tracking the functional maturation of neuronal populations over time remains challenging, as it requires long-term, non-invasive monitoring of membrane potential dynamics. Here, we developed a voltage imaging approach for zebrafish embryos using genetically encoded voltage indicators (GEVIs), enabling fast, direct and cell-type-specific measurements of membrane potentials from defined neuronal populations in a non-invasive manner. Using this approach, we detected coordinated voltage changes in spinal motor neurons with high spatiotemporal resolution. Depolarization and hyperpolarization events were observed at the population, single-cell, and subcellular levels. Notably, long-term voltage imaging revealed the early emergence and progressive maturation of membrane potential dynamics, characterized by increased firing rate, coupling strength and axonal outgrowth. This optical approach constitutes a significant advancement in the study of neural development, providing a powerful tool for investigating the spatiotemporal dynamics of neuronal populations in vivo.