Imaging mitral/tufted glomeruli in the mouse olfactory bulb using the genetically encoded voltage indicator ArcLight

In the mammalian olfactory bulb, each olfactory receptor neuron type maps to receptor-specific channels called glomeruli. These input axons interact with the apical dendrites of mitral/tufted cells, which are projection neurons that send axons to the olfactory cortex. Therefore, each glomerulus reflects the input-output relationship for a different olfactory receptor type, which is transformed by a complex synaptic network. While prior 2-photon Ca2+ imaging experiments have shown that the glomerular output is heterogeneous with respect to how odor concentration information is processed, the nature of voltage dynamics remains unclear. Therefore, we used the genetically encoded voltage indicator (GEVI) ArcLight to image the glomerular output in vivo in anesthetized mice. We found that ArcLight could resolve both excitatory and suppressive odor-evoked signals from mitral/tufted glomeruli. ArcLight expression remained stable over multi-week imaging sessions and successfully captured odor- and concentration-specific patterns of activation across a large concentration range. Importantly, this approach allowed us to resolve heterogeneous concentration-response sensitivities and respiratory-coupled dynamics. We also demonstrate that several newer GEVIs can similarly measure odor-evoked signals using epifluorescence imaging. Our study establishes ArcLight and other emerging GEVIs as powerful tools for dissecting how sensory information is encoded and transformed through the mouse olfactory bulb.