Rodents missing a single protein can detect the weakest scents.
Researchers have created a mouse with a superior sense of smell. The animals should help shed light on the biology of odour detection, which could one day help humans crank up our ability to smell.
Debi Fadool from Florida State University in Tallahassee and her colleagues were intrigued by a strain of genetically modified mice that lack a protein called Kv1.3. This molecule is used in nerve communication, and is also found in odour-related brain regions. So the team tested the animals to see if their sense of smell was different from an ordinary mouse.
To their surprise, removing the protein actually had a beneficial effect on the mice. Those without Kv1.3 could find hidden smelly treats such as peanut-butter crackers twice as quickly as those with the protein.
When other odours, such as peppermint, were diluted down with water, the super-sniffers continued to out-perform their rivals - they detected scents that were ten thousand times weaker than those picked up by the average mouse1. That's a significant achievement for a creature that already has a super-sensitive snout.
Removing Kv1.3 is "a bit like turning up the gain on an amplifier", says smell researcher Lawrence Katz of Duke University Medical Center in North Carolina. It changes the sensitivity setting for odour detection.
The genetically modified animals were also able to distinguish between scented molecules that were chemically almost identical. This shows they are better than ordinary mice at discriminating between odours, says Fadool. Humans also differ in their ability to discriminate smells, sometimes mistaking the scent of pineapple for banana, for example. The Kv1.3 protein may be involved, says Fadool.
Drugs that alter the amount of Kv1.3 in humans might one day help to treat human anosmia - a group of medical conditions that leaves people unable to detect odours - says John Kauer of Tufts University School of Medicine, Boston. Kauer has built an artificial nose, an electronic device that can detect chemicals in the air.
"Anosmia is a serious issue," says Kauer. An estimated 4 million Americans suffer from disorders of taste or smell. Our sense of smell also weakens naturally with age, causing many elderly people to lose their appetite and become malnourished.
Even among those without disorders, sensitivity to smell varies widely between people. Women are generally ten times more sensitive to smell than men, and pregnant women in particular have a heightened sense of smell. Ultrasensitive humans might have less of the Kv1.3 protein, speculates Fadool.
The protein might work by increasing the number of glomeruli in the brain - specialized regions that focus signals from the nose and relay them on to the part of the brain that identifies smells.
The team found that mice lacking Kv1.3 have more glomeruli and that their smell-related nerve cells fire more frequently. No one is sure exactly what effect this might have on our sense of smell, but it makes sense to think that this might increase smell sensitivity, says Fadool.
Fadool, D. A. et al. Kv1.3 channel gene-targeted deletion produces "super-smeller mice" with altered glomeruli, interacting scaffold proteins and biophysics. Neuron, 41, 389 - 404, (2004).