A MAN smells rain and is transported back to a distant afternoon in Paris.
It鈥檚 the stuff of romantic novels but scientists have yet to discover how an
odour can dredge up a memory from the past. Almost nothing is known about how
the olfactory system is wired to the emotional and cognitive centres of the
brain.
Richard Axel of Columbia University, New York, wants to find out and is busy
building a picture of smell, starting from the ground and working up.
The mammalian olfactory system can recognise some 10 000 different odours.
When smelly molecules waft over the olfactory epithelium inside the nose, they
bind to receptors on neurons, triggering electrical impulses that travel to the
olfactory bulb in the brain. In 1991, Axel and Linda Buck, now at Harvard
Medical School in Boston, isolated the gene family that codes for all the
receptor proteins in the mammalian olfactory epithelium. They found about a
thousand genes and are now unravelling how the receptors they encode recognise
around ten thousand odours.
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Each neuron has only one type of receptor, and each receptor responds to a
single section of an odorous molecule. The unique identity of an odour is
represented by the combination of receptors it activates. Neurons with the same
receptors at their ends run to the same pair of nodes, or glomeruli, in the
olfactory bulb. The pattern of glomeruli that light up in response to a
particular odour thus defines its chemical composition. But how does the brain
read this map?
One approach to this question is to see how altering the map affects
perception. To do that, Axel needs to know how the map develops and what guides
neurons to their destinations. By swapping the DNA sequence encoding one
receptor for a different one, his group has already succeeded in diverting
neurons to different glomeruli in the bulb. The receptor itself clearly plays a
part in directing the immature neuron, he says. But that is not the whole story.
Experience of smell and the resulting activity of neurons will also be
important, says Axel.
A second approach is evolutionary. Every species has its own characteristic
odour map which holds clues to how olfaction evolved. So far, the Columbia team
has studied insects, fish and mammals. Last March, it identified odour receptor
genes in the fruit fly Drosophila (Cell, vol 96, p 725).
In mammals, the situation is complicated by a second olfactory system, the
vomeronasal organ, located behind the nostrils. This 鈥渟exual nose鈥� detects
pheromones and relays information separately to emotional centres in the
amygdala and hypothalamus. Axel and Buck have identified a smaller, distinct
family of genes that code for receptors here.
Controversy still rages over the role of the human sexual nose. Are we too at
the mercy of pheromones? For Axel, the answer is one part of a much bigger
mystery: how does a smell elicit a thought?
