Many theories have been proposed in an attempt to explain how humans perceive odours. Those that are still taken seriously can be grouped into two classes, which I label recognition and vibration. In both of these, it is proposed that an odorant molecule comes into contact with a receptor and that the nature of the contact is such that a nerve impulse is generated by the cell containing the receptor. In the recognition theories, the contact is postulated to be that of molecular recognition of a substrate (the odorant) by a protein (the receptor), whilst in the vibration theories it is postulated that the receptor in some way senses a specifiation or group of vibrations in the substrate.

One theory of lesser importance is the radiation theory, first postulated by Aristotle in the fourth century вс. This theory proposes that odorous substances emit radiation, which is detected by the olfactory receptor. In 1947, W. R. Miles and L. H. Beck claimed that bees could detect the odour of honey through a container that was transparent to the far infrared spectrum, but their results have not been repeated. The radiation theory is not given serious consideration nowadays. Other theories that have also been rejected include M. M. Mozell’s chromatographic theory (1970), the thermodynamic activity theory, first proposed by P. Gavaudan (1948) and the membrane penetration theory of J. T. Davies (1971). Interestingly, these three all relate to the solubility and/or volatility of the odorant molecule. These properties determine the transport of the molecule from source to receptor and so are important in the overall process though not necessarily in the actual event that generates the signal. Further details of these and other theories are given in the review of Paul Laffort (1994).

The recognition theory was first postulated by Epicurus. In the fourth century вс, he proposed that odorous materials gave off minute particles, which he called atoms, which were detected by the nose. He postulated that smooth, rounded ‘atoms’ gave rise to sweet odours and sharp, pointed ones to irritant and acidic odours. The best-known modern recognition theory is that of John Amoore (1970) of the US Department of Agriculture in California. He postulated that there is a limited number of receptor types, each of which recognizes a particular molecular shape and, when triggered, generates the signal for a primary odour. The primary odours serve a similar purpose in olfaction to that served by the primary colours of vision. Thus, the enormous variety of odours that we can recognize and describe occurs because of the blending of these primary signals, just as the many hues of colour are composed of appropriate mixtures of red, blue and green. [For a concise account of the current state of knowledge of colour vision, see the article by Hideki Kandori (1995), of Kyoto University.]

Initially, Amoore sought the primary odours by looking for the words that were most commonly used to describe odours. This led him to postulate first seven primary odours, viz. ethereal, camphoraceous, musky, floral, minty, pungent and putrid. He studied the chemical and steric properties of typical odorants of each class and proposed shaped receptors for the first five and the generation of charged species for the last two, positive for pungent and negative for putrid. Later on, he postulated that, if these primary odours existed, then specific anosmias should correspond to them. Specific anosmia is the inability of one group of subjects to detect a particular odour. By examining the reactions of a large number of subjects, Amoore attempted to identify anosmias and hence primary odours. This led to an increase in the proposed number of primary odours since, for example, of the four commonest anosmias (musk, sandalwood, ambergris and urine), only one was included in his original set (Amoore, 1970).

There are two other recognition theories which deserve mention. The first is that of M. G. J. Beets (1968), who proposed that the functional group in an odorant serves to align it with the receptor and that the profile of steric bulk thus presented to the receptor is the key determinant of odour. The electron topological approach has been championed by a group of workers at the Academy of Science in Kishinev in Moldavia, principally P. F. Vlad, I. B. Bersuker, M. Yu. Gorbachev and A. S. Dimoglo. In this theory, the recognition involves the electrons of the frontier orbitals of the odorant and the proponents postulated a series of odour triangles. For each of smoke, meat, ambergris and musk, they defined the dimensions of a triangle of atoms and the electronic properties of the orbitals of those atoms, which are postulated to be required for that odour type. In their most recent work on musk, ambergris and sandalwood, they described two molecular fragments of specific composition which must be present in a specific relationship to each other in a molecule for it to possess the given odour.

The vibration theory was first proposed by G. M. Dyson in 1937. He suggested that the receptor in the nose could detect vibrations of the odorant molecules and that patterns of firing of vibrationally tuned detectors could be interpreted as odours by the brain. His theory was taken up by Robert Wright (principally funded by the British Colum­bia Research Council), who spent a great deal of time in the 1960s and 1970s searching for correlations between infrared spectra and odour. He claimed to have found such correlations, but one of the weaknesses of his theory was that there was no suggestion of how a receptor might sense vibration (Wright, 1982). Such a mechanism has now been provided by Turin (1996). He postulates the presence of an electric potential gap in a protein, with nicotinamide adenine dinucleotide (NAD) and zinc ions providing the ‘electrodes’. Electrons cannot cross the gap unless an odorant molecule is placed between the ‘electrodes’. To cross the gap the electron must lose energy and this it does by tunnelling through the orbitals of the odorant molecule and exciting vibrational modes in it as a result. Thus, Turin has moved the search for correlations from infrared to inelastic electron tunnelling spectra.

All of these theories of chemoreception have sprung up largely in the absence of hard biological information and are based on ideas of how it might work. They are based on observations of the total process, which is a dangerous thing to do since there are many stages between the
evolution of an odour from its source and the consciousness of odour in the brain. The progress at each stage involves an interplay of physical and/or chemical and/or physiological and/or psychological para­meters. Models based on the above theories usually adopt a very mechanistic approach and largely ascribe discrimination to one stage in the process, viz. the odorant-receptor interaction.

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