Scientists have long wondered how we manage to remember smells despite the fact that each olfactory neuron in the epithelium only survives for about 60 days, to be replaced by a new cell. In most of the body, neurons die without any successors. But as the olfactory neurons die, a layer of stem cells beneath them constantly generates new olfactory neurons to maintain a steady supply.
"The riddle was, how can we remember smells when these neurons are constantly turning over and the new crop has to form new synapses?" says Buck. "Now we know the answer: Memories survive because the axons of neurons that express the same receptor always go to the same place."
And so some stages of olfaction are beginning to yield to researchers. But many mysteries remain. For example, what happens to information about smells after it has made its way from the olfactory bulb to the olfactory cortex? How is it processed there? How does it reach the higher brain centers, in which information about smells is linked to behavior?
Some researchers believe that such questions can best be answered by studying the salamander. This lizard-like creature's nasal cavity is a flattened sac. "You can open it up more or less like a book" to examine how its olfactory neurons respond to odors, says John Kauer, a neuroscientist at Tufts Medical School and New England Medical Center in Boston, Massachusetts, who has been working on olfaction since the mid 1970s.
Salamanders will make it possible to analyze the entire olfactory systemfrom odorant receptors to cells in the olfactory bulb, to higher levels of the brain, and even to behavior, Kauer thinks. His research group has already trained salamanders to change their skin potentialthe type of behavioral response that is measured in lie detector testswhenever they perceive a particular odor.
To study the entire system non-invasively, Kauer uses arrays of photodetectors that record from many sites at once. He applies special dyes that reveal voltage changes in the membranes of cells. Then he turns on a videocamera that provides an image of activity in many parts of the system.
"We think this optical recording will give us a global view of what all the components do when they operate together," says Kauer. He hopes that "maybe 10 years from now, or 20 years from now, we'll be able to make a very careful description of each step in the process."
This would be amazing progress for a sensory system that was virtually unexplored five years ago. Axel and Buck's discoveries have galvanized the study of olfaction, and scientists now flock to this field, aroused by the possibility of success, at last, in solving its mysteries.
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