Toward a pharmacology of odor receptors and the processing of odor images.

Odor molecules may be considered as molecular ligands which bind to receptors in the olfactory sensory neurons to give rise to the sensory response. Binding studies in whole sensory epithelia suggest that the receptors also bind muscarinic cholinergic antagonists. Preliminary electrophysiological evidence indicates that muscarinic and beta adrenergic antagonists block odor-elicited membrane currents in single isolated salamander sensory neurons. These results support the idea that models developed for analyzing ligand binding by members of the 7 transmembrane domain family of membrane receptors may apply rather closely to olfactory transduction. We suggest that sensory neurons express single receptor types with differing degrees of affinity for different ligands. We further suggest that glomeruli in the olfactory bulb function as labeled lines for particular sets of odor ligand determinants, and that interglomerular circuits bind together similar glomeruli and enhance contrast between dissimilar glomeruli. The odor image laid down in the sensory neuron population is thus subjected to abstracting and enhancement at the glomerular stage, prior to being transmitted for further processing in the deeper layers of the olfactory bulb and in the olfactory cortex.