General Medical Information

Olfactory Transduction And Odor Coding

Richard L. Doty
Daniel A. Deems

Although, as noted above, a given receptor cell seems to express only one type of receptor derived from a single allele, each cell is electrophysiologically responsive to a wide, but circumscribed, range of stimuli. This implies that a single receptor accepts a range of molecular entities and that coding occurs via a complex cross-fiber patterning of responses. Most, if not all, of the olfactory receptor proteins are linked to the stimulatory guanine nucleotide-binding protein Golf. When stimulated, they activate the enzyme adenylate cyclase to produce the second messenger cAMP. Golf-induced cAMP diffuses through the cell and activates cellular depolarization via the opening of cyclic-nucleotide-gated ionic channels and Ca2+-dependent Cl– or K+ channels. The amount of adenylate cyclase activity produced by various odorants in a frog ciliary preparation is positively correlated with the magnitude of the frog’s electroolfactogram (EOG; a surface potential associated with the number of receptors activated) and with the perceived intensity of these same odorants to humans. Some odorants also activate cGMP, which is believed to play a role in the modulation of the sensitivity of olfactory receptor neurons, such as during adaptation. Although G proteins other than Golf (e.g., Gi2 and Go) have been identified in olfactory receptor cells, they appear not to be involved in early transduction events, likely assisting in such processes as axonal signal propagation, axon sorting, and target innervation.