Odor Molecules Can Change Inside the Nose

Odor molecules entering the nose do not necessarily reach receptors unchanged. They may be retained by the mucus covering the upper nasal cavity or transformed into other molecules by enzymes.

A 2023 study combined analysis of mucus collected from people, perceptual odor tests, and receptor experiments in cultured cells. It examined the mucus’s ability to capture and chemically transform odorants, as well as age-related differences in the amount of mucus.

What does nasal mucus do before an odor reaches its receptors?

Olfactory sensory neurons are located in an area near the top of the nose called the olfactory cleft, where the surface is covered with mucus. This mucus contains proteins, enzymes, inorganic elements, and other components.

The study aimed to determine whether these components actually capture or transform odor molecules and how those functions relate to perception. It also examined whether the quantity and properties of mucus vary with age and might help explain changes in the sense of smell.

The participants were 30 Japanese adults without obvious nasal or sinus inflammation or sensory complaints. Under endoscopic guidance, small absorbent pads were placed for five minutes in the olfactory cleft and the lower-lying inferior nasal meatus to collect mucus. Saliva was also used for comparison.

Smell tests measured how dilute three odorants could become before participants could no longer detect them: ambrettolide, phenylethyl alcohol, and tert-butyl mercaptan.

A highly variable fluid containing proteins and copper

The amount of mucus collected from the olfactory cleft varied widely between individuals, from 7 to 144 mg per side, with a median of about 37 mg. Protein concentration ranged from 6.2 to 20.8 mg/mL and tended to be higher in people from whom less mucus was collected.

Olfactory cleft mucus tended to have higher concentrations of proteins and elements including magnesium, iron, and copper than mucus from the inferior nasal meatus or saliva. Glutathione, which is involved in antioxidant activity, was also detected.

Copper is known to assist certain receptors that respond to sulfur-containing odorants. In this study’s cell experiments, adding copper also increased the response of a receptor called OR2T11. However, copper concentration in human mucus was not significantly associated with sensitivity to tert-butyl mercaptan.

Even when a component has a demonstrated function in cells, its amount alone may not explain individual differences in human sensitivity. The form in which copper is present and differences in receptors may also matter.

Mucus could capture odorants, but that ability was not associated with sensitivity

The researchers assessed odorant capture by adding fragrance compounds to mucus and measuring how much entered the air above it. After validating the method using a pig protein with known binding properties, they tested human mucus.

When ambrettolide, muscone, and phenylethyl alcohol were mixed with olfactory cleft mucus, less entered the air than from the comparison fluids. Limited sample quantities prevented enough replicates for statistical testing in this initial comparison. In a more detailed examination of ambrettolide, however, retention was still evident even with mucus diluted tenfold.

The researchers also confirmed that ambrettolide remained in the mucus, showing that the reduced release was not simply due to decomposition. Retention capacity was associated with protein concentration.

However, people with greater retention capacity were not more sensitive to ambrettolide. The authors also consider the possibility that this capture helps remove odor molecules from around sensory neurons, preventing prolonged adaptation or the accumulation of harmful substances.

Enzymes in mucus transformed odor molecules

Next, fragrance compounds were added to collected mucus, and the original molecules and products were analyzed. For example, p-cresyl acetate was converted to p-cresol, and benzaldehyde to benzyl alcohol. Octanal produced octanol and octanoic acid.

Susceptibility to conversion differed between molecules. Several esters were broken down, whereas compounds such as linalyl acetate, whose structure makes reaction sterically difficult, were not converted under these conditions. Nasal mucus is selective rather than processing every molecule in the same way.

In a comparison focused on p-cresyl acetate, conversion after five minutes in tenfold-diluted mucus ranged from 5% to 45% between individuals. An enzyme called CES-1 was a candidate contributor, but a solution containing that enzyme alone did not reproduce the mucus’s reactions, and activity remained in mucus even after inhibitors were added. Multiple enzymes therefore appear to be involved.

Does conversion inside the nose affect perception?

To investigate this, the researchers used adaptation in a test with eight people. After smelling the same odor for a while, people tend to perceive it as weaker afterward.

Participants rated the intensity of p-cresol, smelled another stimulus for two minutes, and then rated p-cresol again. They perceived p-cresol as weaker not only after adapting to p-cresol itself but also after smelling its precursor, p-cresyl acetate. The control odorant, muscone, did not produce the same change.

The researchers also examined whether the two molecules might stimulate the same receptor. Screening 378 human olfactory receptors in cells showed that OR9Q2, which was sensitive to p-cresol, did not respond to p-cresyl acetate.

Together, chemical conversion in collected mucus, sensory adaptation, and the difference in receptor responses support the explanation that p-cresyl acetate is converted to p-cresol inside the nose and that this contributes to perception.

The authors also examined the cooling sensation when smelling menthyl acetate and responses of the cooling receptor TRPM8. Menthyl acetate did not activate TRPM8 in cells, whereas its conversion product, menthol, did, offering additional evidence for conversion within the nose. This concerns cooling, a somatosensory sensation, rather than odor quality itself. The study did not directly track, over time inside the nose, how much of a molecule changes during a single inhalation.

Considering age and mucus quantity through a cell model

Older participants tended to yield less olfactory cleft mucus, with higher protein concentrations and enzyme activity. The authors suggest that reduced water content may concentrate the components. Of the three odorants tested, sensitivity was associated with age for ambrettolide but not significantly so for the other two.

A separate cell experiment examined whether a smaller amount of mucus might affect receptor function. The researchers reduced the volume of culture medium covering cells with olfactory receptors and delivered odorants through the gas phase. Reducing the medium from 50 µL to 20 µL reduced the maximum responses of two receptor types to approximately 33% and 35% of their original values.

This model shows that the volume of liquid covering cells can influence their responses. However, culture medium is not human mucus. Even considered alongside the observations in 30 people, the experiment does not establish that adding water to the nose improves smell as a treatment. It provides a specific hypothesis for investigating age and sensitivity through the mucus environment.

Summary

Human olfactory cleft mucus could retain odor molecules and convert some into other molecules. Neither capture capacity nor copper concentration explained individual differences in sensitivity in this study, but adaptation and receptor experiments supported the involvement of chemical conversion in perception.

Older participants tended to yield smaller amounts of mucus, and the amount of liquid covering cells affected receptor responses. The nose is not simply an entrance through which odors pass: the environment encountered before molecules reach receptors is itself part of the experience of smell.

The paper identifies seven of its eight authors as Kao employees. Alongside support from the company, it discloses patents and applications concerning a p-cresol receptor, methods of applying water to nasal mucus, and gas-phase stimulation of receptors.

References

Shirai T, et al. (2023). Functions of human olfactory mucus and age-dependent changes. Scientific Reports, 13, 971. Original article and open-access full text