SNIFFING OUT SMELL: Odorant Detection, Coding, and Perception

Smell, the most ancient of all senses, is crucial for animal reproduction and survival. Humans can detect the most minute changes in odorant structures as a different perceived odor. Despite its significance, the mechanism of olfactory sensory perception remained unknown before 1991. Through research in my lab and others, we discovered a large multigene family containing approximately 1000 genes in mice and 500 in humans that encode seven transmembrane proteins in the olfactory epithelium (OE) responsible for odorant detection. These odorant receptors (ORs) are organized in four spatial zones. From the OE, identical ORs project to a distinct subset of glomeruli in the olfactory bulb. Genetic tracing from the olfactory bulb to the olfactory cortex (OC) demonstrated that the activation of a particular OR actives a specific region of the OC. These findings suggest odorant perception is predominantly computational coding. Additionally, three families of receptors in the vomeronasal organ have been found that are involved in pheromone detection. Although the pathway underlying olfaction is better understood, many questions remain. The mechanism by which only a single OR gene is expressed in each neuron is still unclear. Together these findings begin to elucidate one of the most important and conserved senses.