Can Mixing Scents Suppress Signals in the Nose? What Indole Reveals About Odorant Receptors

Indole is a molecule found in both floral scents and fecal odors. In perfumery, it contributes floral and animalic nuances, but the impression it creates varies with its concentration and the ingredients around it.

A 2020 study used this molecule to investigate how mixing odorants frequently suppresses signals at the level of receptors in the nose. Odor information was being reshaped not only after reaching the brain, but also at its point of entry.

Looking at odor mixtures through receptor inhibition

Odor molecules act on odorant receptors in the nose’s olfactory sensory neurons. One receptor can respond to several molecules, and one molecule can stimulate several receptors. These combinations encode odor information.

Natural scents, however, contain many molecules. If the activation produced by each molecule were simply added together, increasing the number of ingredients should drive many receptor responses toward their upper limits, making differences harder to represent.

Patrick Pfister and colleagues focused on antagonism: the ability of one molecule to interfere with receptor activation by another. How common is it? Are there patterns in which receptors it affects? Can closely related receptors differ? They addressed these questions through cell experiments and mathematical modeling.

Observing indole-responsive cells one by one

The researchers began with olfactory sensory neurons isolated from mouse noses. They compared calcium responses to indole alone and to indole combined with another odorant. Optical measurements of calcium changes reveal how strongly a cell responds.

Of 36 structurally diverse odorants tested, 32 completely suppressed the response in at least some indole-responsive cells. This did not mean that every cell stopped responding uniformly: the strength of inhibition depended on the added molecule and the cell being observed.

Inhibition was not restricted to molecules closely resembling indole. Some structurally dissimilar compounds also produced strong inhibition. Tests of three selected compounds at different concentrations showed a tendency for higher concentrations to inhibit more cells and produce stronger inhibition.

The researchers also checked whether the effect might be an artifact of separating the cells. Using a different microscopy method on preparations that retained the structure of the olfactory epithelium and its connections to the olfactory bulb, they observed similar effects with representative inhibitory compounds.

Which receptors account for the inhibition?

A reduced cellular response alone cannot establish whether the effect originates at a receptor or elsewhere inside the cell. The team therefore isolated six olfactory sensory neurons with known response patterns and analyzed RNA from each cell.

This identified three mouse odorant receptors that respond to indole: Olfr740, Olfr741, and Olfr743. When these receptors were expressed in cultured cells and tested with the same odorants, the ranking of sensitivities closely reproduced that seen in the original sensory neurons.

Moving the phenomenon observed in nasal neurons into a system expressing selected receptors made it possible to investigate the receptors’ contribution. The study then expanded to include closely related receptors.

430 of 800 compounds inhibited receptor responses

The large-scale screen examined ten indole-responsive receptors from the same gene family. The inhibition screen tested 800 compounds, while the screen for activation by individual compounds tested 700.

The results showed that 430 of the 800 compounds inhibited the response to indole in at least one receptor. In the activation screen, 328 of 700 compounds activated at least one receptor. Because the two screens had different denominators, simply comparing their compound counts cannot establish which phenomenon is more common. Nevertheless, the screen demonstrates that inhibition was far from a rare exception.

Of the 430 inhibitory compounds, 149 acted on only one of these ten receptors, while 281 acted on several. More detailed concentration tests also found compounds that inhibited one receptor but activated another.

A molecule does not have a fixed role as an inhibitor: its effect can change with the receptor and concentration. Like activation, inhibition follows combinatorial patterns.

Related receptors differ, helping shape mixture signals

Even receptors with similar gene sequences differed in their sensitivity to a compound and in whether it activated or inhibited them. A molecule’s ability to bind and the signal produced after binding can vary as separate properties.

The authors suggest that differences in inhibitory molecules, not just activating molecules, also matter when considering receptor evolution. Having several related receptors may increase the range of patterns available to represent mixtures.

In the mathematical analysis, a model in which molecules compete to bind receptors fitted the experimental concentration responses well. When the measurements were incorporated into this model, adding more components did not drive the receptor population’s responses toward saturation as strongly as simple summation did.

This computational result suggests that inhibition could regulate signals and preserve room to represent differences even in complex mixtures. The study did not behaviorally test how many different odor mixtures people or mice can distinguish.

What would it take to connect this with deodorization in people?

The main subjects here were mouse olfactory sensory neurons, a specific group of mouse receptors, and responses in cultured cells. This was not a study in which people smelled mixtures and researchers measured a percentage reduction in unpleasantness.

The odorants were dissolved in liquid and applied to cells. Those concentrations cannot be directly equated with concentrations in air or in the human nose. The authors also discuss differences between liquid and gas phases and whether the selected receptor family and compound collection are representative of the broader system.

Moreover, inhibiting a receptor signal does not remove the molecule responsible from the surrounding space. The value of this research lies not in demonstrating a finished deodorizing product’s efficacy, but in its extensive investigation of how mixture information changes at the point of detection.

Summary

Using indole as a starting point, this study found widespread, combination-dependent inhibition of sensory-neuron and receptor responses by other odorants. In the ten-receptor screen, 430 of 800 compounds produced inhibition, and even related receptors showed different patterns of effects.

The competitive-binding model suggested that this inhibition could help prevent signal saturation in complex mixtures. Odor mixing is not simply a matter of adding individual smells and then combining them in the brain. Both activation and inhibition at the nose’s entry point shape the information that reaches it.

At the time of the research, most authors were Firmenich employees. The work received internal Firmenich funding and support from the Simons Foundation. Published patent applications relating to parts of the research were also disclosed.

References

• Pfister, P., et al. (2020). Odorant Receptor Inhibition Is Fundamental to Odor Encoding. Current Biology, 30(13), 2574–2587.e6. DOI and original article. Public repository record.