We can recognise around 10,000 different odours. Our ability to smell, known as 'olfaction', is a potent yet often neglected player in our sensory world, and a surprising 3 per cent of our genes are dedicated to fine-tuning its subtleties.

The Nobel Prize committee has now honoured two scientists who have done most to determine just how we recognize and differentiate the scents of roses, wines, or of good or bad meat. Their work also helps explain how an evocative smell can take us back to a poignant time in our lives.

Neuroscientists Richard Axel from Columbia University in New York and Linda Buck from the Fred Hutchinson Cancer Research Center in Seattle share this year's US$1.4-million Nobel Prize in Physiology or Medicine.

Exploiting state-of-the-art molecular techniques over the past two decades, they have developed a complete picture of how a scent is converted into a signal in the brain, where it is not only recognized, but remembered in association with accompanying emotions.

Making scents

Axel and Buck showed that each particular scent molecule activates a particular receptor on a particular cell in the lining of the nose. They identified the chain reaction that results from this activation, which involves a transducing 'G' protein and ion channels that open and close.

They also worked out the neural circuitry that passes the signal on to the higher parts of the brain, which deal with more complex matters, such as automatic recall of a childhood memory or, more pragmatically, deciding whether to discard a whiffy meal or move closer to a potential mate.

There are many different olfactory receptors, which belong to a more general family of proteins called G-protein receptors. But Axel and Buck showed that each cell in the lining of the nose contains just one sort of receptor.

This came as a surprise to the neuroscience community. But, as the pair went on to show, any receptor can be activated by a handful of related scent mole-

cules at different intensities. And most odours are composed of many molecules, which activate different receptor-bearing cells. The researchers revealed a combinatorial code, often likened to the colours on a patchwork quilt, that allows us to recognize, and form memories of, around 10,000 different odours.

Wide application

The general principles of their work apply to other sensory systems such as that of pheromones. These are molecules that affect social behaviour in animals and that are regulated by a different family of G-protein receptors.

"The work has really been a tour de force in molecular biology," says Jonathan Ashmore, a sensory physiologist at University College London. "The pair saw the problem through from beginning to the end with extraordinary determination.”

Buck is only the seventh woman ever to win the Nobel Prize in Physiology or Medicine.