Summary Monitoring neuronal electrical activity using fluorescent protein-based voltage sensors has been limited by small response magnitudes and slow kinetics of existing probes. Here we report the development of a novel fluorescent protein voltage sensor, named ArcLight, and derivative probes that exhibit large changes in fluorescence intensity in response to voltage changes. ArcLight consists of the voltage-sensing domain of Ciona intestinalis voltage-sensitive phosphatase and super ecliptic pHluorin that carries the point mutation A227D. The fluorescence intensity of ArcLight A242 decreases 35% in response to +100mV depolarization when measured in HEK 293 cells, which is more than five times larger than the signals from previously reported fluorescent protein voltage sensors. We show that the combination of signal size and response speed of these new probes, for the first time, allows the reliable detection of single action potentials and excitatory potentials in individual neurons and dendrites.

To visualize odorant representations by receptor neuron input to the mouse olfactory bulb, we loaded receptor neurons with calcium-sensitive dye and imaged odorant-evoked responses from their axon terminals. Fluorescence increases reflected activation of receptor neuron populations converging onto individual glomeruli. We report several findings. First, five glomeruli were identifiable across animals based on their location and odorant responsiveness; all five showed complex response specificities. Second, maps of input were chemotopically organized at near-threshold concentrations but, at moderate concentrations, involved many widely distributed glomeruli. Third, the dynamic range of input to a glomerulus was greater than that reported for individual receptor neurons. Finally, odorant activation slopes could differ across glomeruli, and for different odorants activating the same glomerulus. These results imply a high degree of complexity in odorant representations at the level of olfactory bulb input.

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