Origine et évolution des perceptions gustatives chez les primates non humains et chez l'homme
Résumé
Our approach of the evolutionary background of taste perception is based on a comparison of the adaptive taste responses of primates, including humans, in relation to the composition of potential foods in various environments. In human subjects, taste recognition thresholds have been determined using a blind test, with solutions of sugars (fructose and sucrose), citric acid, sodium chloride, various tannins and quinine, and a test with propylthiouracil (PROP) to determine the status of taster vs. non-taster. Our data concern a total of 412 adults, including population samples of Europe, Africa, and Asia. The comparison with non human primates is based on the electrophysiological records of Hellekant et al., when soluble substances similar to those used in humans have been also used to elicit taste responses in marmosets (Callithrix jacchus), macaques (Macaca mulatta) and chimpanzees (Pan troglodytes). Using co-variation of taste thresholds in humans, we found, in the additive tree, a dichotomy of taste responses allowing discrimination of noxious vs. beneficent substances. Electrophysiological records in non-human primate species also show that primate sensory taste system is basically organised around two major clusters of taste fibres and their cortical projections. This comparison shows that the taste perception system in humans is not basically different from that of the other primates. The absence of correlation between the taste signals corresponding to, on the one hand, sugars, and, on the other hand, tannins and alkaloids such as quinine and caffeine, shows that no taste receptor (or very few) are common to these two sets of responses, respectively corresponding to nutritionally beneficent foodstuffs, and to potentially toxic or anti-nutritional foods in various environments. The gusto-facial reflexes allowing young primates to swallow sweet solutions and to avoid bitter or strong acids are involved in this dichotomy of the taste system. However, the preferences can be reversed if a palatable food includes a toxic compound and determines aversive conditioning. Similarly, the aversion for bitter or partly toxic solutions can also be reversed in a natural context where the ingestion of low amounts of tannins, saponins or alkaloids can provide a benefit, especially for curing intestinal parasites, as was shown by S. Krief in the chimpanzee. Some responses, especially to various salts and acids vary between species, without well-defined relationships to the major clusters. The interpretation of responses to salts in the extant primate species necessitates considering the origin of taste in aquatic and terrestrial vertebrates. The taste response to salt is present in fishes, and it has been persisting as a potential response of the receptors, although wild primates never experienced the salty taste. Accordingly, the salty taste does not result from a supposed adaptive process to meet sodium requirement. The salt was introduced in human civilisation as a taste exhauster with a great success, but this is a fortuitous result allowed by the weak specificity of taste receptors. Finally, there is no evidence of a trend towards separate ‘basic tastes' that appear as an arbitrary linguistic interpretation.
Nous avons mis en évidence, sur un échantillon de 412 sujets humains volontaires, une dichotomie des réponses gustatives à partir des co-variations des seuils de perception pour différentes substances. Les enregistrements des potentiels évoqués sur les fibres des nerfs de la gustation (d'après les données de Hellekant et al.) ont montré que le système de perception des primates s'organise également autour de deux grands ensembles permettant de discriminer certains nutriments des substances potentiellement toxiques. Nous montrons ainsi que le système de perception de l'homme ne diffère pas, globalement, de celui des autres primates. Néanmoins, certaines réponses, notamment pour différents types de sels et d'acides, sont assez variables d'une espèce à l'autre et souvent sans relation évidente avec les deux principaux ensembles de perceptions. Leur interprétation nécessite un examen des origines lointaines de ces perceptions chez les vertébrés aquatiques et terrestres. Cet examen des origines et du fonctionnement de l'appareil gustatif des primates nous permet de montrer que les “ goûts de base ” correspondent à un découpage arbitraire, par le langage, d'un vaste spectre de perceptions gustatives.
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