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For Immediate Release
Contact: Ahmed Barakat, [email protected]

News — PHILADELPHIA (October 26, 2023) – A research team from the  demonstrated, using a novel quantitative analysis, that the neuronal response to odors undergoes some modifications in the nose. Such early processing of olfactory signals is a result of interactions between olfactory receptor neurons and the neighboring trigeminal neurons.

Olfactory and trigeminal neurons represent two nerve systems that extend into the nose. Together, the two systems carry the signals that allow our bodies to remain on the lookout for all kinds of molecules we inhale.

Olfactory nerve signals, on the one hand, enable our brains to determine the odor identity of molecules. On the other hand, trigeminal signals produce a variety of sensations such as cooling, tingling, and irritation. Most odorants, however, can activate a signaling response in both the olfactory and trigeminal nerve systems. 

Monell researchers have now shown that trigeminal signals can alter the signals of the olfactory system even before they reach the brain. The recent study, published in the , developed objective measurements aiming to discern the intricate relationship between the two nerve systems.

“The mechanisms behind the alteration of olfactory signals by the trigeminal system are still poorly understood,” said Monell Research Associate and first author . “We are addressing this problem by investigating the olfactory epithelium, the region of the nose where olfactory and trigeminal nerves co-localize and where the olfactory and trigeminal signals are generated.”

The research team conducted a series of physiological measurements of the nerve signals within the olfactory epithelium of two groups of mice. The first group included healthy mice while the other group included mice who lack the ability to produce trigeminal responses to certain odorants.

“By comparing the signals within the olfactory epithelium of the two types of mice, we could quantify changes in the activity of olfactory receptor neurons in the presence of a trigeminal signal,” said Genovese. 

To measure the olfactory activity, the team used electro-olfactograms (EOGs), a method that records nerve signals produced within the olfactory epithelium. EOG results demonstrated a decreased olfactory response to 2-phenylethanol, an odorant with a rose-like smell, in the presence of trigeminal activity induced by odorants.

“The findings reveal that the trigeminal system has a profound impact on the olfactory response from its earliest stage,” said Monell Member and senior author , highlighting the complex interplay between the two systems.

The study also quantified the odorant-induced activity of individual trigeminal neurons. These measurements, carried out by Monell Research Associate , allowed the team to develop a trigeminal scoring system. The scores describe the level of trigeminal activity induced by odorants, and they make it possible to infer how much the olfactory signals are impacted. 

“The stronger the trigeminal response to an odorant, the greater the inhibition of the olfactory response,” explained Genovese. “However, the scoring system is currently based on the limited set of odorants used in the study.”

The researchers emphasize the importance of further investigations into the modulation of olfactory signals by trigeminal activity. “Understanding how trigeminal activation alters odor perception could have significant implications in various fields, including fragrance development, food science, and the treatment of smell loss,” said Genovese.

The work was supported by NIH NIDCD grants R21DC018358, R01DC016598, R03DC012413, R01DE028979, and 1R01DC016647.

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The  is an independent nonprofit basic research institute based in Philadelphia, Pennsylvania. Founded in 1968, Monell‘s mission is to improve health and well-being by advancing the scientific understanding of taste, smell, and related senses, where our discoveries lead to improving nutritional health, diagnosing and treating disease, addressing smell and taste loss, and digitizing chemosensory data.