Mice and men express the same olfactory preferences

Humans and mice are attracted by the same odors. This has been revealed for the first time by a team of French researchers in the "Neurosciences sensorielles, comportement, cognition" Unit (CNRS / Université Lyon 1).

Published on January 16, 2009 in the journal Plos One, their work confirms that olfactory preferences are not solely determined by experience or culture, but also by the structure of the odorant molecule. It will undoubtedly enable a clearer understanding of the neuronal mechanisms coding for olfactory perception. More immediately, it may be possible to predict human olfactory preferences based on those observed in the mouse.

In humans, odors strongly influence numerous compartments of daily living, such as sexual activity, social relations or food intake. Some are pleasant, others unpleasant, and induce attraction or repulsion, respectively. This positive or negative hedonic value of an odor is very markedly affected by the experience and culture of the individual. For example, if we consider camembert cheese, its odor attracts many French people but may be repulsive to an individual from another culture.

And what if olfactory preferences involved an innate characteristic? They would then be dictated by the chemical structure and physical properties (1) of the odorant molecule. To answer this question, Nathalie Mandairon and Moustafa Bensafi, CNRS scientists in Anne Didier's team in the "Neurosciences sensorielles, comportement, cognition" laboratory measured the olfactory preferences of humans and mice in response to a series of odors (2). And indeed, although the odor "value" is predetermined by the structure of the odorant molecule, the latter still needs to contain information that will induce choice. If this is the case, then humans and mice faced with the same odor should react in the same way.

In mice, the researchers used the time spent by the animal in exploring a given odor as their index of preference. The human subjects were asked to reach their decision and attribute a "score" ranging from 1 to 9, from the most unpleasant to the most pleasant. At the same time, the duration of sniffing, which tended to be longer when the odor was more pleasant, was also recorded.

The first conclusion was that humans and mice were attracted or repelled by the same odors. Geraniol, a floral odor, was one that was preferred by both species. In contrast, guaiacol, which corresponds to a smell of smoke or burning, was one of the least appreciated. This result demonstrates the conservation of olfactory preferences between these two mammalian species. In addition, the scientists confirmed that this hedonic judgment was closely linked to the structure of the odorant molecule, which thus partly predetermines our olfactory preferences.

No-one had previously suggested so strongly that the neuronal mechanisms coding for olfactory preference were situated at the initial levels of the processing of sensory information. Until now, it had been supposed that anything related to olfactory "judgments" was mainly processed at a high level within the integrative structures of the brain. These findings thus raise hopes of a clearer understanding of these mechanisms and how they function. In the shorter term, they suggest that the behavior of a mouse might predict human olfactory preferences, which could then open the way to practical applications; for example, in the agri-food industry.

(1) Structure implies a series of physicochemical characteristics that describe the odorant molecule.
(2) As odorants are pure entities, they do not necessarily evoke a food.

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