Peripheral Gustatory Processing
Recent convergence of several lines of experimental evidence ranging from molecular biology to behavior has helped to expose some of the basic mechanisms of the gustatory system. Three integrated behavioral, neurophysiological and genetic studies are proposed on golden hamsters (Mesocricetus auratus) and laboratory mice aimed at delineating the functioning of their sweet (attracting) and bitter (avoiding) taste systems.
(1) Studies of mixtures address the nature, specificity and component inhibition in peripheral mixture processing by testing the ability of hamsters to recognize components in sucrose-quinine, dulcin-quinine and sucrose-quinine-NaCl mixtures. With a quasi-natural conditioned taste aversion (CTA) paradigm, hamsters trained to avoid components, by pairing drinking with LiCl injection, are behaviorally tested on binary and ternary mixtures. Chorda tympani and glossopharyngeal nerve recordings are used to test the limits of peripheral stimulation and inhibition on behavior; and, for Na+–quinine suppression, stimulus, nerve, and neuron specificity.
(2) A definition of a bitter taste quality for hamsters is sought with behavioral and neural assays of aversive stimuli with diverse chemical (e.g., ionic vs. non-ionic) and biological (e.g., exposure-enhanced potency) properties.
(3) Studies of genetic variation in sweet and bitter taste domains in multiple inbred strains of golden hamsters and mice seek sources of intake variation with behavioral assays and nerve recordings. Unitary bitter and unitary sweet hypotheses are tested by measuring thresholds for CTA learning and generalizing to multiple stimuli in mice selected for genetic variation in taste preferences and avoidances.
The mouse behavioral experiments compare all stimuli at equal CTA saliency to avoid confounding stimulus concentration and intensity. Interpretation of mouse data is greatly facilitated by recent advances in definition of gustatory molecular receptors. By comparing data from hamsters and mice, we hope to establish common features that can be used to describe essential taste phenomena. Understanding taste function is a prerequisite for redressing taste disorders that can lead to serious problems with nutrition and homeostasis. Also, elimination of taste disorders would improve the quality of life that depends on enjoyment of food and drink, a universal human need contributing much to human sociality.
Human Taste and Odor Coding Mechanisms
Taste and smell evolved to identify stimulus quality within an ever-changing mix of compounds in nature. To develop an understanding of stimulus coding, new experimental strategies are required that challenge human chemosensory quality identification with dynamic, complex stimuli.
(1) Quality identification in mixtures. One series of experiments tests the ability of humans to identify characteristic odors (e.g., vanilla, rose, almond, caramel) and tastes (e.g., sugar, salt, acid, quinine) of single mixture components. Identification of component quality is difficult in binary mixtures and impossible in quaternary mixtures. An efficient experimental design presents stimulus pairs, each for a few seconds; all the compounds in the first stimulus are also in the second, which contains one extra compound that is expected to be identified. Components of the first stimulus are ambient stimuli in the second and even more difficult to identify. The experiments establish whether a simple combination of mixture suppression, rapid adaptation and extra-stimulus emergence occurs in controlled settings to mimic the common waxing and waning of odor and taste qualities in natural settings.
(2) Regional taste discrimination. In a second series, regional taste discrimination is studied by having human subjects identify taste quality and rate perceived sweet, salty, sour, and bitter for stimuli applied to oral regions in receptive fields of cranial nerve (CN) VII, and CN IX. Taste buds in the receptive fields of the two peripheral taste nerves play distinct roles in gustatory systems of other species, and for electrolyte tastes may be represented differently in the two receptive fields in humans. Two inhibitors of electrolyte tastes, chlorhexidine gluconate, an antiseptic, and weak cathodal currents, reversibly perturb taste quality identification of salts such as Na2SO4, sodium acetate, KCl and CaCl2, which represent compounds that are perceived as salty, sour or bitter. Like the change in NaCl taste from salty to bitter in rodents with CN VII receptive fields blocked by amiloride, tastes elicited from CN VII and IX receptive fields are expected to be differentially affected by inhibitors. This would mean that humans may make distinct use of information carried by the glossopharyngeal and chorda tympani nerves. Results of the quality identification and regional discrimination experiments contribute to the understanding of human taste and odor coding strategies.