Olfactory cross-adaptation

Olfactory receptors have been a subject of great interest (9). Much that has been postulated was done by analogy to the sense of sight in which there are a limited number of receptor types and, as a consequence, only three primary colors. Thus attempts have been made to recognize primary odors that can combine to produce all of the odors that can be perceived. Evidence for this includes rough correlations of odors with chemical stmctural types and the existence in some individuals having specific anosmias. Cross-adaptation studies, in which exposure to one odorant temporarily reduces the perception of a chemically related one, also fit into this hypothetical framework. Implicit in this theory is the idea that there is a small number of well-defined odor receptors, so that eventually the shape and charge distribution of a specific receptor can be learned and the kinds of molecular stmctures for a specified odor can be deduced. 

Structure-odour correlations rely on the measurement of odour. It must be remembered that odour is not a physical property. Odour exists in the mind. We must not assume that it exists in the receptor also. The fact that cross-adaptation can occur between chemically different materials with similar odours, and also between chemically similar materials with different odours, suggests that it does not. There is no evidence that the receptors are tuned to specific odours or even odour types. It is quite likely that the recognition process is based on physical and/or chemical properties of odorants and that odour only exists when the brain puts an interpretation onto a pattern of signals from the olfactory nerves. 

It should be mentioned that data on cockroach antennal (U3) and maxillar palp (jtU) olfactory sensilla show that different receptor cells display consistently different sensitivities towards the same ranges of n-alcohols (e.g. so called pentano and heptanol receptors). Additionally, the existence of vertebrate olfactory receptor cells which display different sensitivities for the same alcohols can be concluded from single-unit adaptation and cross adaptation studies ( ). Although the effects could be due to different protein receptor species, they can also be explained on the basis of different lipid compositions in the receptor cells in question. 

Baylin, F. and Moulton, D.G. Adaptation and cross-adaptation to odor stimulation of olfactory receptors in the tiger salamander. J. Gen. Physiol., 1979, 7l, 37-55 ... 

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