Odorant receptor specificities and receptor combinatorials: implications for olfactory coding.

The perception and discrimination of thousands of different odorants by the vertebrate olfactory system results from the activation of specific olfactory neurons within the olfactory epithelium of the nose (reviewed in Buck, 2000; Firestein, 2001). Activity from these cells is then interpreted by the brain to discern the molecular identity of a given odorant stimulus. How is this process of molecular recognition accomplished? The first step in olfactory discrimination resides at the level of the interaction of odorous ligands with odorant receptors. How many different odorants can bind to an individual receptor, and how many different receptors are employed to identify a given ligand as a discrete odor percept? In one model, discrimination may occur by the use of relatively few receptors, each capable of interacting with numerous odorants. The discrimination of one odorant from another would then be achieved by combining or blending the inputs from these relatively non-selective, low affinity receptors. Alternatively, the olfactory system may employ a large number of receptors, each capable of binding to a more restricted subset of odorants.