PROCESSING OF TASTE MIXTURES BY BRAINSTEM NEURONS

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Although the gustatory components of food almost always occur as complex mixtures, most research in taste, particularly the neurophysiological work, has employed simple stimuli, representing the four basic taste qualities of sweet, salty, sour and bitter. Mixtures of substances with different tastes or the interaction of similar-tasting stimuli has received relatively little attention by electrophysiologists. The studies described in this proposal are designed to investigate the neurophysiology of homogeneous (same quality) and heterogeneous (different quality) mixtures of taste stimuli. The responses of single neurons in the parabrachial nuclei (PbN) of the hamster will be recorded extracellularly. Stimuli will be concentration series of sucrose NaCl, citric acid, and quinine hydrochloride (QHCl) and their mixtures (heterogeneous) and sucrose, fructose, xylitol, and Na-saccharin and their mixtures (homogeneous). Three heterogeneous mixtures will be studied: sucrose - NaCl, sucrose - QHCl, and NaCl - citric acid. Homogeneous mixture experiments will include sucrose - fructose, sucrose - Na-saccharin, and sucrose - xylitol mixtures. These stimulus pairs were selected on the basis of known mixture interactions in human psychophysical studies or in electrophysiological studies on the chorda tympani nerve. For a given cell, concentration series for each of a pair of stimuli will be presented, followed by several mixture series, in which different concentrations of one of the components will be added to the concentration series of the other. Thus, several concentrations of a pair of stimuli will be combined factorially so that the responses of individual PbN neurons can be assessed relative to predictions made from several models of stimulus-receptor interactions, which have been used in both electrophysiological and psychophysical studies of taste mixtures. The quantitative nature of the mixture interaction for each pair of stimuli will be compared across neuron types in the PbN, defined by their response profiles to stimuli representing the four taste qualities. Deviations from these models or differences in the response to a given mixture pair between neuron types will suggest a role for neural interactions in the mixtures. The mutual suppression between heterogeneous mixtures, which has been noted in human psychophysical studies, will be correlated with the occurrence of inhibitory responses, which are more frequently seen in these third-order gustatory neurons than in more peripheral cells. Interactions among stimuli showing mixture suppression or synergism in the PbN will be studied in single fibers of the chorda tympani nerve to further assess the contribution of synaptic processing to mixture phenomena.