In neuroscience, it is fundamental to understand how sensory stimuli are translated into neural activity at the entry point of sensory systems. In the olfactory system, odorants inhaled into the nasal cavity are detected by ~1,000 types of odorant receptors (ORs) that are expressed by olfactory sensory neurons (OSNs). Since each OSN expresses only one type of odorant receptor, the odor-evoked responses reflect the interaction between odorants and the expressed OR. The responses of OSN somata are often measured by calcium imaging and electrophysiological techniques; however, previous techniques require tissue dissection or cell dissociation, rendering it difficult to investigate physiological responses. Here, we describe a protocol that allows us to observe odor-evoked responses of individual OSN somata in the mouse olfactory epithelium in vivo. Two-photon excitation through the thinned skull enables highly-sensitive calcium imaging using a genetically encoded calcium indicator, GCaMP. Recording of odor-evoked responses in OSN somata in freely breathing mice will be fundamental to understanding how odor information is processed at the periphery and higher circuits in the brain.
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