A rose sniffed through a snotty nose may not smell so sweet. Enzymes in mice’s nasal mucus transform certain scents before the nose can detect them, a new study finds. The results, published December 1 in the Journal of Neuroscience, show that lowly mucus may feature prominently in the sense of smell.
“It is completely unexpected that snot would play a potential role in changing how we perceive odors,” says neuroscientist Leslie Vosshall at Rockefeller University in New York City. “Most people and most scientists pay no attention at all to mucus.”
But there’s more to mucus than what meets the nose: The thick goo that serves to lubricate the nose is teeming with proteins and protein-chopping enzymes. Some of these molecules are thought to catch smells and shuttle them to odor receptors in the nose. Other components may protect the body from toxic chemicals by chopping them up into less harmful pieces. But no one knew whether this chopping action had any effect on smell perception.
In the new study, Ayumi Nagashima and Kazushige Touhara of the University of Tokyo added particular odorants to tiny amounts of mucus sucked out of a mouse’s nose and tested the resulting chemical composition of the mix. After five minutes of sitting in mucus, about 80 percent of almond-smelling benzaldehyde was converted into benzyl alcohol (a scent found in some teas and plants) and the odorless benzoic acid. Inactive enzymes in boiled mucus couldn’t do this odor conversion, the team found.
Snot enzymes can cut up aldehydes and molecules with chemical features called acetyl groups, the researchers reported. Scents from these kinds of molecules are common in flowers, plants and animals (and perfumes: aldehydes feature prominently in Chanel No. 5’s formulation).
This odor transformation happened inside the mice’s noses and was reflected in their brains, too, the researchers found. Parts of the mouse brain called glomeruli get signals from mice’s smell-sensing nerve cells. When the researchers inactivated a scent-chopping enzyme in the mice’s noses (in effect, removing the effect of the mucus), the pattern of glomeruli activation changed, suggesting that the snot enzymes affect what the mouse smells.
Mice’s behavior confirmed this altered sense of smell. Mice were trained to associate sugar with a particular odor. Later, the mice were presented with two smells, one that usually comes with a treat and one that doesn’t. Normally, the mice spent more time nosing around the smell that came with a treat. But when mucus enzymes were inactivated, the mice spent less time sniffing around the treat-linked smell, suggesting that they could no longer recognize the odor.
That the mucus enzymes could act on the odors before the nose could detect them was unexpected, says neuroscientist Sigrun Korsching of the University of Cologne in Germany, “but the behavioral tests are convincing in that respect.”
Whether humans experience this same mucus effect is unclear. Some of the same mucus enzymes are also found in people, and preliminary data from other researchers suggest that nasal enzymes can change odorant quality for humans, Touhara says.
In addition to showing how mucus can change smells, the study answers a vexing mystery that has been lurking in the olfactory field — experiments in lab dishes give slightly different results than experiments in a live animal. Korsching says that the new study offers a convincing explanation for this difference — snot.
