Effects of odorant mixtures on olfactory receptor cells

The general mechanisms by which chemical stimuli may influence the firing frequency of olfactory neurons were briefly described. They include specific mechanisms mediated by receptor molecules and nonspecific mechanisms involving general properties of the chemicals and of cells. It is difficult to imagine that odorant mixtures influence receptor cells by mechanisms that are fundamentally different from those by which homogeneous chemicals act. It is argued that even under the best experimental conditions the presentation of odorants usually or always involves exposing the receptor cells to more than one additional molecular species compared to the unstimulated condition. This is because odorants invariably have contaminants that may be of potency such that their contribution to the odor is large even though their contribution to the number of molecules in the stimulus stream is small. Furthermore, the partition coefficients of the major and minor components are unlikely to be identical; therefore, their relative concentrations in the aqueous environment of the receptor cells can differ greatly from that in the gas phase. Finally, metabolic transformations of odorants in the olfactory mucosa can result in the exposure of receptor cells to mixtures of odorant and metabolites, with the mixture composition varying with time. Finally, some pitfalls in analyzing the effects of odorant mixtures are discussed. At the very least, it is necessary to determine the relation of concentration to response for each odorant in the mixture in order to interpret results in terms of interactions. Even with such data caution must be used, especially in attaching significance to reductions in the apparent maximal responses to one odorant induced by the presence of the other.