Researchers Shed Light on How Humans Distinguish Different Odors
Boston, MA--March 1, 1999--Humans can perceive thousands of distinct odors, even though the nose has only about 1,000 odorant receptors. Linda Buck, a Howard Hughes investigator, postdoctoral fellow Bettina Malnic, and ther colleagues in Japan, have shown how the olfactory system distinguishes so many different odorants with a limited number of receptors. Their findings, which appear in the March 5 Cell, also help explain why slight alterations in an odorant or a change in its concentration can cause a dramatic shift in its perceived odor.
Buck, associate professor of neurobiology at Harvard Medical School, Malnic and their colleagues developed a technique in the mouse to identify odorant receptors that recognize specific odorants. They showed that the olfactory system uses a combination of odorant receptors to recognize different odors. While a single receptor can recognize multiple odorants and a single odorant can be recognized by multiple receptors, different odorants are recognized by distinct combinations of receptors. This combinatorial coding scheme appears to permit the nose to discriminate a vast number of diverse odorants.
In addition, they showed that odorants nearly identical in structure are recognized by different, but often overlapping, sets of odorant receptors. Human studies have demonstrated that even a slight change in the structure of an odorant can cause a dramatic shift in its perceived odor. For example, when the hydroxyl group of octanol is replaced by a carboxyl group to make octanoic acid, its perceived odor changes from orange and rose-like to rancid and sweaty.
"This explains something that has puzzled people for a very long time," says Buck. "If you change the structure of an odor molecule--even slightly, its odor can undergo a profound change."
Buck's findings also explain another well known phenomenon of human perception: how the perceived quality of an odorant can differ with a change in its concentration. Indole, for example, has a putrid odor when concentrated but is perceived as floral when diluted.
"We call the set of receptors that recognize the odor its code," says Buck. "If you change the concentration of the odor, you can also get a change in its code."
Eventually, she and her colleagues hope to determine how signals derived from different odorant receptors are represented in the olfactory cortex and other areas of the brain and how individual components of an odor code are decoded to yield the perception of an odor.
Cell