Encoding odorant mixtures by human olfactory receptors

Perceptual properties of odorant mixtures are assumed to result from interactions occurring at different stages of odour processing. Peripheral mechanisms have been proposed to play an important role in the processing of mixture information. To test this hypothesis, we investigated the responses of several human olfactory receptors (ORs) to aldehyde binary mixtures containing various ratios of either octanal and citronellal or octanal and methional. Receptor responses were measured by calcium imaging using HEK293 cells transfected with ORs. Responses to octanal + citronellal mixtures were almost always found to be lower than the sum of single compound responses (hypo-addition). For one OR (OR1A1), octanal was even found to extinguish the receptor response altogether, suggesting strong competition mechanisms. In contrast, OR activation by octanal + methional mixtures was hyper-additive. Interestingly, the results revealed that (i) the interaction pattern was largely influenced by the nature and ratio of the mixed odorants whatever the OR and (ii) the interaction pattern depended on the OR response patterns to single odorants. These findings provide some insight into the phenomena that may occur at the periphery of the olfactory system. Moreover, since OR responses are partially reconcilable with human integrated odour responses, this highlights the critical role of peripheral events in the encoding of natural odours. Copyright © 2016 John Wiley & Sons, Ltd.