Mucus alters odors

It’s getting to be cold season, and that means lots of stuffy, sniffly noses filling the streets and subways. In the throes of a bad cold, snot is your worst enemy: nasal congestion keeps you from smelling, tasting and breathing. But new research published in The Journal of Neuroscience on December 1 shows that mucus does more than keep our noses from doing their job on sick days; even on healthy days it might fundamentally change how we perceive odors.

Every scent we smell is related to a different molecule. Esters, for example, which are characterized by two oxygen atoms bonded to a carbon atom, have fruity smells that can change based on the different carbon chains attached to them. Two straight chains of carbons will give you a pineapple-y smell, but a more complex web of carbons smells like banana.

For each of these volatile molecules that makes its way into your nose, there’s an olfactory receptor that binds to it, sending a signal to your brain that you’re smelling something sweet or acrid or minty. It might make sense that mucus can mess with what you smell because it blocks those receptors. But it turns out that the snot in your nose may actually physically change scent molecules before they bind to odor receptors.

A group of biochemists from the University of Tokyo behind the new research found that enzymes in mouse mucus snip some volatile odorants into smaller pieces, creating different molecules that have different scents. When the researchers boiled the mucus, the molecular splicing didn’t occur because the enzymes had been destroyed.

Having observed how enzymes in extracted mucus change odiferous molecules in test tubes, the researchers then decided to see how the rodents’ brains perceived those odors. They sprayed different enzyme inhibitors up their subjects’ nostrils—effectively imitating a mucus-less nose—and monitored the activation of the glomerular layer of the olfactory bulb, part of the brain that processes signals from olfactory receptors. The activation pattern was different between the inhibitor-treated and normal mice, which suggests that inactivating the mucus enzymes changed the way scents were perceived.

While this is all just rodent research so far, there is reason to believe that human noses work similarly—and therefore, snot may be more important to perceiving scents than we might have thought.