Smell's Course Predetermined

<<p> The brain can differentiate thousands of scents, should they happen to waft into our noses and tickle sensory neurons located there. But how those neurons pass information to the brain, and how the brain processes it, has been a mystery. Now, two studies appearing in the 8 November issue of Nature suggest that many odor-specific neural connections are hardwired, which may explain why smells trigger instinctual behavior.

The sensation of a smell begins when an odor molecule binds to a receptor protein on sensory neurons in the nose. Those neurons branch into a part of the brain called the olfactory bulb. What happens next--when information from the bulb travels to the olfactory cortex--has been "terra incognita," says neuroscientist Lawrence Katz of Duke University Medical Center in Durham, North Carolina.

To trace connections between the bulb and the cortex, Linda Buck and colleagues at Harvard Medical School in Boston marked neurons that received signals from specific smell receptors. The neurons transferred the marker to connecting neurons in the olfactory bulb and cortex. The stained pathway was clearly not random, the team found--for each type of odor, the same pattern of connections appeared.

Meanwhile, Liqun Luo's team at Stanford University has filled in information about how neurons form the links that carry olfactory signals. In fruit flies, so-called "projection neurons" pick up signals in the bulb and connect to the fly's equivalent of the olfactory cortex. The researchers labeled projection neurons according to their time of birth and found that neurons make connections based on when they are born. The analysis provides "absolutely compelling" evidence that the projection neurons get their marching orders at birth, says Katz.

"The organization of this very complicated sensory system seems to be highly predetermined," says Leslie Vosshall of Rockefeller University in New York City. That makes sense to many researchers, because smells are tightly linked to many instincts. If a young mouse had to learn from experience that the smell of coyote urine means danger, Vosshall notes, it may not get a second chance to use that information.

Related sites

Linda Buck's home page
The Luo lab

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