Sense of Smell Wins Nobel Prize (cont.)
Each odorant receptor first activates a G protein, to which it is coupled. The G protein in turn stimulates the formation of cAMP (cyclic AMP). This messenger molecule activates ion channels, which are opened and the cell is activated. Axel and Buck showed that the large family of odorant receptors belongs to the G protein-coupled receptors (GPCR).
All the odorant receptors are related proteins but differ in certain details, explaining why they are triggered by different odorous molecules. Each receptor consists of a chain of amino acids that is anchored into the cell membrane and traverses it seven times. The chain creates a binding pocket where the odorant can attach. When that happens, the shape of the receptor protein is altered, leading to G protein activation.
Each Olfactory Cell is Unique
Buck's research group examined the sensitivity of individual olfactory receptor cells to specific odorants. By means of a pipette, they emptied the contents of each cell and showed exactly which odorant receptor gene was expressed in that cell. In this way, they could correlate the response to a specific odorant with the particular type of receptor carried by that cell.
Most odors are composed of multiple odorant molecules, and each odorant molecule activates several odorant receptors. This leads to a combinatorial code forming an "odorant pattern" - somewhat like the colors in a patchwork quilt or in a mosaic. This is the basis for our ability to recognize and form memories of approximately 10,000 different odors.
Activating the Brain
Axel and Buck independently showed that receptor cells carrying the same type of receptor converge their processes into the same glomerulus, and Axel's research group used sophisticated genetic technology to demonstrate the role of the receptor in this process. The convergence of information from cells with the same receptor into the same glomerulus demonstrated that also glomeruli exhibit remarkable specificity.
Conscious of a Smell
Last Editorial Review: 10/4/2004