Mammals can perceive and discriminate myriad volatile chemicals as having a distinct odor. Odorants are initially detected by odorant receptors (ORs) on olfactory sensory neurons (OSNs) in the nose. In the mouse, each OSN expresses one of _1000 different OR genes. Although OSNs and their expressed ORs constitute the fundamental units of sensory input to the brain, a comprehensive understanding of how they encode odor identities is still lacking. To gain a broader and more detailed understanding of odorant recognition and odor coding at this level, we tested the responses of 3000 mouse OSNs to 125 odorants with diverse structures and perceived odors. These studies revealed extraordinary diversity, but also bias, in odorant recognition by the OSN, and thus OR, repertoire. They indicate that most OSNs are narrowly tuned to detect a subset of odorants with related structures and often related odors, but that the repertoire also includes broadly tuned components. Strikingly, the vast majority of odorants activated a unique set of OSNs, usually two or more in combination. The resulting combinatorial codes varied in size among odorants and sometimes contained both narrowly and broadly tuned components. While many OSNs recognized multiple odorants, some appeared specific for a given pheromone or other animalassociated compound, or for one or more odorants with a particular odor quality, raising the possibility that signals derived from some OSNs and ORs might elicit an innate behavior or convey a specific odor quality.
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