Interindividual Differences of Taste Sensitivity in Humans and

Chapter 20

Downloaded by IOWA STATE UNIV on April 19, 2017 | http://pubs.acs.org Publication Date: March 4, 2008 | doi: 10.1021/bk-2008-0979.ch020

Interindividual Differences of Taste Sensitivity in Humans and Hamsters: Multiple Receptor Sites for Single Organic Molecules Annick Faurion C N R S / I N R A , UMR mixte INRA-Université P A R I S 11 Sud NeuroBiologie Sensorielle, nopa, B325, Domaine de Vilvert, F-78352 Jouy en Josas, France

Measuring taste sensitivity in groups of human subjects for series of single molecules used as sapid stimuli and/or measuring taste nerve responses in hamsters reveals the strong discriminative power of the taste peripheral system. In the human, different molecular structures are always perceived as different tastes. Simultaneously, inter-individual differences of quantitative responses (sensitivity) are great, both in humans and in hamsters that are not inbred. These results were obtained under carefully controlled conditions, suppressing olfactory information and training subjects to each stimulus, which ensured a high level of intra-subject and intra-stimulus reproducibility. These data suggest that several weakly specific receptor sites code for each stimulus and these sites should be structurally, at least partially, different from subject to subject. Recent results of molecular biology point to the corollary interpretation that different receptors should bind and code every ligand. The inventory of nucleotide polymorphisms, which are the source of inter-individual receptor diversity, remains to be undertaken.

Sweeteners are numerous (hundreds) and display a large diversity of molecular structure, suggesting a variety of sweet tastes and a variety of mechanisms for detecting and discriminating sweet molecules. Inter-individual differences of sensitivity have been known and reported since 1935 by Blakeslee and Salmon (1) who determined thresholds the distributions of which were widespread in the population for most organic substances tested and up to 10 in concentration for 13

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© 2008 American Chemical Society Weerasinghe and DuBois; Sweetness and Sweeteners ACS Symposium Series; American Chemical Society: Washington, DC, 2008.


297 some of them. A second indication of inter-individual differences of sensitivity came from the discovery of the reduced sensitivity to phenyl-thiocarbamide (PTC) in some individuals by Fox in 1930 (2). This inter-individual difference was shown to be a heritable trait by Snyder (3) and further worked out by Kalmus (4) who demonstrated that only homozygous twins, but not siblings, had a similar sensitivity. Then, Lugg (5) showed, with several hundreds of subjects, a multimodal rather than bimodal distribution of thresholds. This should have prepared scientists to consider multiple factors for inter-individual differences of sensitivity. However, in the 1970s and even later, it was not yet clear that subjects perceived tastes differently and most investigators continued averaging subject responses. Moreover, it was not usual to consider that every molecule although grouped with other ones in one category (e.g., the sweet group etc.) would elicited a unique distinct taste. The first study showing that perceived intensities for sweet taste are significantly different depending on the individual and the molecule was published in 1980 (6). Aristotle (7) described a continuum for taste, which was a one-dimension continuum, where sweet (with no reference to sugar as sugar was not known yet) and bitter were at opposite ends; Linnaeus (1753) defined a series of "tastes" including "wet" and "mucous"; Chevreul (8) discriminated taste from olfaction. Authors progressively suppressed words from the taste descriptor list of Linnaeus and the "four tastes" (i.e., sweet, sour, salty and bitter) were the last descriptors which remained; Kiesow, in 1898 (9) described a bi-dimensional taste continuum behind the four words used as milestones. In 1914, Cohn (10) claimed he could categorize 4000 chemicals into only four categories. In opposition to the theory of Cohn, Henning in 1916 (11) replied in a subtle dissertation in favour of the continuous aspect of taste. He developed the idea that sapid stimuli could not be actually categorized but rather constituted a three dimensional continuous taste space. He localized the prototypic semantic references sweet, sour, salty, bitter as dots, landmarks or milestones within the continuous taste space constituted by chemicals. Due to a misinterpretation of his text illustrated by the image of the famous tetrahedron, he was further quoted, a contrario, as the father of the "four tastes theory". Other developments concerning the first theorization about four tastes in the 19 century may be found in Erickson (12). Pfaffinann (13) working in 1939 on single taste fibres in the rat chorda tympani taste nerve wrote: "there is evidence that certain other substances may also stimulate more than one fibre, so that i f a wide variety of agents were used, each fibre might be found to have a chemical spectrum which overlapped those of other fibres". Erickson, in 1963 (14), showed that the quantitative activity of each single neuron of a collection of neurons constituted a unique pattern, which was different for each of the tested stimuli. It is this very pattern of activity across neurons that codes for the taste, as a "signature" of each molecule. The array of neurons potentially coding for taste quality allows an infinite number of different patterns explaining the huge discriminating power of the taste system. The author th

Weerasinghe and DuBois; Sweetness and Sweeteners ACS Symposium Series; American Chemical Society: Washington, DC, 2008.


298 furthermore considered this pattern constituted both the entry signal identified with the stimulus and the "read out" (15) of the taste system for signalling the sapid molecule. In spite of these first timid and sparse scientific contributions indicating a complex variety of the taste perceptions among human beings, - and human beings never agree on the denomination of tastes - the layman's vocabulary continues to be limited to the use of four words for the description of hundreds of sweet compounds, thousands of bitter compounds and even those compounds which elicit tastes that do not match with any of sweet, sour, salty or bitter. The huge cognitive power of words maintained a majority of tenants in favour of four unique basic, primary tastes, versus authors in favour of a continuous variety of taste sensations possibly experienced. Such a radical divergence of opinions strongly motivated us to look further into this controversy. Rather than subjective qualitative descriptions, we used quantitative evaluation of perceived intensities to compare subjects' sensations elicited by a wide range of taste stimuli. We probed the question would these stimuli nicely range into cultural semantic categories or would physiological data significantly depart from language as in a world of unlimited and unarticulated sensations? Should we discover many varieties of sensations for each subject and a diversity of sensations across various subjects, we would contribute to understanding receptor mechanisms, at the very level of the interface between the external medium and the inside of our self. We expected that inter-individual differences in sensitivity for stimuli molecules may lead to an understanding of the number and nature of receptor sites in individuals for every molecule. In the mean time, the food industry has been struggling at the replacement of sucrose by non-caloric sweeteners for nutritional or cost-reduction reasons. Although the motivation was there, the conclusion 30 years later is that this aim has not yet been reached and no one knows how to replace sucrose harmoniously. Is taste really such a simple sense? The aim of this chapter is to summarize some aspects of a wealth of data presently still under examination for looking for multiple receptor sites with the recently available molecular biology approach. Three main lines directed our research, which were: 1. 2.

approaching taste receptors by measuring the effect of tastants on the living organism, quantifying similarities/dissimilarities between molecules from the point of view of their interacting properties with the taste receptor system (i.e., biological similarities). For that purpose, we made use of the interindividual differences of quantitative sensitivity: • in the human, using quantitative measurement of individual sensitivity to each molecule • in the animal, quantifying chorda tympani taste nerve responses in each animal, after selecting a species with genetics variability instead of inbred animals


299 3.

identifying structural and energy properties on the van der Waals surface of molecules to map receptor sites (Froloff: 16,17).

In the design of our studies aiming at the evaluation of inter-individual differences of perceived intensities, or, of differences of nerve response amplitudes, great care was taken to ensure individual reproducibility. Thus, great care was given to the development of experimental conditions which are reproducible over time.


Experimental Pre-Selection of Testants Stimuli were chosen according to their ability to elicit taste in humans and/or hamsters. First, 182 candidate molecules were rapidly tested on laboratory staff members and on hamsters and Wistar rats (with chorda tympani taste nerve recordings in these rodents). Among the 182 stimuli extracted from the literature (see Beets (18) for a review), 123 elicited taste in the human subjects. Included were 7 PTC - PROP related chemicals containing the -N-C=S- functional group. In the hamster, 92 compounds elicited responses in the chorda tympani taste nerve. We observed that the hamster is a PROP non-taster, giving only minute neural responses to concentrations at the limits of solubility for four PROP type compounds. The Wistar rat, also found to be a PROP non-taster, was tested for 49 substances and found responsive to 22. Our aim was to collect rectangular matrices of data, where subjects are represented as rows and stimuli as columns for both humans (psychophysics) and hamsters (electrophysiological recording of the uncut C T taste nerve). In such a matrix of data, each figure located at {line x, column y} is a statistically validated evaluation, after training, of the sensitivity of one given subject for one given compound. This collection of data was used to calculate the correlation between paired stimuli across subjects, i.e. to evaluate A e covariance of the effect of compounds on the taste system, indicating their relative similarity from the point of view of their interaction with the peripheral receptors. Independently, the covariance of results on paired subjects was also evaluated looking for a quantitative measurement of inter-individual differences of taste sensitivity. When working at supra-threshold levels, concentrations were chosen below half maximum, or below the inflection point of a dose/response curve. The body of data developed includes findings on 101 tastants in studies on 178 human subjects and 108 hamsters, which were partitioned in several experimental blocks. In the Human, Thresholds were evaluated for 43 Stimuli, in "Experiment I" including 19 Stimuli and 61 Subjects and in "Experiment I F including 19 other Stimuli and 31 Subjects. 38 stimuli and 19 Subjects were common to both studies. At Supra-Threshold Levels, "Experiment I" gathered 21 Stimuli and up to 71 Subjects, "Experiment II" included 20 Stimuli and 46 Subjects; a matrix of 26 Stimuli and 27 Subjects was common to both studies.


300 In the Hamster Chorda Tympani Recordings were performed in Experiment I including 49 Hamsters and 41 stimuli and in Experiment II including 59 hamsters and 51 stimuli. 54 stimuli as a whole were studied in humans and 70 in the hamster. These data were collected between 1985 and 1991 and were partially published (19).

The High Discriminative Power of the Taste System: Evaluation of Human Taste Sensitivity (Detection Threshold and Supra-Threshold Sensitivity)


Experimental procedures: In humans, the protocol employed was based on a paired comparison task associated to a staircase procedure. The exact protocol of Dixon (20) was used including the calculation of a p taken as the definition of threshold. A modification was brought so as to take account of a geometric progression of the stimulus concentration instead of an arithmetic progression as originally employed by Dixon. Several caveats were worked out: 50

Starting concentration: This indication comes from Dixon himself: the starting test concentration should be at the level of the expected result; hence, we averaged results from the subject's previous session, which gave the starting concentration of the next session. Another caveat from Dixon is also that the ratio of concentrations presented should be approximately equal to the standard deviation of the data. Randomization: Several tests were run simultaneously for each subject, inter-mingling the concentrations presented for different tests so that presentations were randomized for the subject. Sterile solutions: In preliminary studies, it appeared that threshold evaluation could give reproducible data, provided no bacteria would grow up in the solutions. Subjects are earnestly looking at threshold level for any tiny perception, which should correspond to the stimulus diluted in the solvent, not to any other source of stimulation for taste or olfactory systems. Therefore, the experiments included preparing solutions in sterile conditions (UV illuminated water and sterilized glassware) and keeping them sterile throughout the experimental session. Solutions were made each day and checked every evening, after experimental sessions, on P C A culture medium. Data obtained with accidentally contaminated solutions were discarded.


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Temperature control at 0.1 °C: It also appeared that subjects could possibly associate the "higher" temperature to the stimulus, then "recognize" the stimulus from the reference owing to the difference of temperature instead of taste. It was therefore necessary to control differences in stimuli temperatures in every pair. This could be technically achieved at the level of 0.1 °C, which was not sufficient since the trigeminal temperature sense is so sensitive that subjects are able to make out differences of temperature as low as a few hundredths of a Celsius degree. To escape this bias, the (computer controlled) solutions delivery was organized by programming so that residual temperature differences

Interindividual Differences of Taste Sensitivity in Humans and

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