Olfactory transduction and olfactory receptor molecules.

(A) Odorants in the mucus bind directly (or are shuttled via odorant binding proteins) to one of many receptor molecules located in the membranes of the cilia. This association activates an odorant-specific G-protein (Golf) that, in turn, activates an adenylate cyclase, resulting in the generation of cyclic AMP (cAMP). One target of cAMP is a cation-selective channel that, when open, permits the influx of Na+ and Ca2+ into the cilia, resulting in depolarization. The ensuing increase in intracellular Ca2+ opens Ca2+-gated Cl- channels that provide most of the depolarization of the olfactory receptor potential. The receptor potential is reduced in magnitude when cAMP is broken down by specific phosphodiesterases to reduce its concentration. At the same time, Ca2+ complexes with calmodulin (Ca2+-CAM) and binds to the channel, reducing its affinity for CAMP. Finally, Ca2+ is extruded through the Ca2+/Na+ exchange pathway.

(B) The generic structure of putative olfactory odorant receptors. These proteins have seven transmembrane domains, plus a variable cell surface region and a cytoplasmic tail that interacts with G-proteins. As many as 1000 genes encode proteins of similar inferred structure in several mammalian species, including humans. Each gene presumably encodes an odorant receptor that detects distinct sets of odorant molecules. (Adapted from Menini, 1999.)