Making Sense of Scents

Science News, April 10, 1999 by John Travis

"If you raise the concentration, you add new receptors to the code," explains Buck. In the case of bromooctanoic acid, varying concentrations of the odorant were recognized by between two and eight different types of odorant receptors.

The exquisite talent of the mammalian nose to discriminate different compounds is also reflected in Buck's results. "A slight change in the structure of an odor can lead to dramatic changes in smell," she notes. For instance, the rosy, orangy smell of octanol contrasts sharply with the rancid, sweaty odor of octanoic acid, although the molecules differ only in a side chain of atoms. As expected, Buck and her colleagues found that the two odorants triggered overlapping, yet distinct, arrays of receptors.

In the next year or two, predicts Reed, researchers may completely unravel the codes used to identify the world's smells. "We could scan all 1,000 or so mammalian odorant receptors to find the ones that are best at giving a response to [any given] odor," he says.

From such data, the researchers hope to discern eventually how the shapes of odorant receptors determine which odorants they recognize. Moreover, the investigators plan to examine whether the code for some odors can consist of a single receptor. Perhaps detecting rotten food has been so important that mammals evolved a specific receptor for just that odor, muses Buck.

Then, there's the musk issue, one of personal interest to her. Many people have anosmias--defects in the ability to smell certain odors. Buck's inability to sense musk is a common one; more than 10 percent of people share the problem. She hopes to test the hypothesis that such anosmias result from a lack of certain odorant receptors.

While Buck can accept not being able to enjoy the scent of some perfumes, she won't rest until she knows why.

RELATED ARTICLE: Insect odor receptors fly into view

Whether they are malaria-carrying mosquitoes homing in on human flesh, bees searching for flowers, or crop-devouring pests seeking their next meal, insects depend heavily on their sense of smell. Thus, insect olfaction is a topic of intense scientific, medical, and economic interest.

"There is a lot of motivation to learn how insect olfaction works--in the hope of finding ways to intrude into the control system and to get harmful insects to stop doing their bad behaviors," says John G. Hildebrand of the University of Arizona in Tucson. He studies the olfactory system of moths, which are major agricultural pests, for example.

Understanding insect olfaction may also provide insight into the sense of smell in higher animals. "What we learn about olfaction in flies and moths will guide us in our efforts to understand olfaction in our own kind," says Hildebrand.

Yet insect olfactory investigators have been at a major disadvantage when compared with their vertebrate-studying counterparts. Despite years of effort by more than a dozen research groups, no one had identified any insects' odor-receptor molecules. Two research teams now report unearthing these elusive proteins in Drosophila melanogaster, the common fruit fly studied in many laboratories.