Researchers have taken a major step forward in understanding how the nose perceives and differentiates between the estimated 10,000 different smells to which it is exposed.
A Columbia University team reported Friday in the journal Cell that it had isolated a family of 18 genes that are the blueprints for the exquisitely sensitive receptors in the nose that signal the presence of odors. The team's results suggest that there may be 100 to 200 more such genes.
"It's a bombshell," said molecular biologist Charles Wysocki of the Monell Chemical Senses Center in Philadelphia. "It's something that was not expected for many years."
"This is a wonderful piece of work," added Columbia neurobiologist Eric Kandel.
The research is expected to provide new insight not only into human physiology, but also into the control of insects and animals, whose behavior is regulated in part by scents, called pheromones, transmitted through the air. Disruption of the insects' ability to perceive the scents, for example, could lead to new methods for pest control that would not harm other organisms.
Proteins produced from the genes are located on the exterior of nerve cells high in the nose and are found nowhere else in the body, said molecular geneticists Linda Buck and Richard Axel, co-authors of the Cell paper. When these protein receptors bind odorous molecules from the air, they send a signal to the brain, which combines signals from all the different receptors to identify the scent.
Human sensory systems have long been viewed as a doorway into understanding the brain, Buck said, and the discovery of the olfactory genes should lead to new insight into information processing by the brain. "If you could identify the receptors for odorants," she said, "you would have a way of knowing what was sent" by individual sensor cells in the nose to the brain. "And if you could follow the connections into the brain . . . you might see patterns of connections that would allow you to see the logic of the connections."
One clear conclusion from Buck and Axel's research is that the olfactory system is much more complicated than the visual system. The eyes contain only three different types of light receptors, which process all visual information. Experts noted Friday that it was only when researchers had begun to identify those three receptors and the genes that serve as blueprints for them that scientists began to understand how the visual system operated.
Now, the experts concluded, the isolating of similar receptor genes for the olfactory system should allow the same amount of progress to be made in understanding the sense of smell.