Responses of single hamster parabrachial neurons to binary taste mixtures: mutual suppression between sucrose and QHCl.

1. Although taste experience typically arises from a mixture of gustatory stimuli, nearly all previous neurophysiological studies of the mammalian central gustatory system have focused on responses to single chemical stimuli. To begin to systematically examine CNS responses to taste mixtures, we recorded the extracellular activity of single third-order neurons in the hamster PbN to anterior tongue stimulation with binary mixtures of sucrose and QHCl. In experiment 1, neurons were tested with four concentrations of sucrose (0.001, 0.01, and 1.0 M) presented alone and mixed with 0.1 M QHCl. In experiment 2, neurons were tested with four concentrations of QHCl (0.00032, 0.0032, 0.032, and 0.1 M) presented alone and mixed with 1.0 M sucrose. 2. The response to each binary mixture was compared with the response to the more effective component (MEC) presented alone, and those that differed by more than a selected criterion (based on response variability) were identified. Of all mixture responses, 37% (59/158) involved mixture suppression (mixture response < MEC response), only 4% (6/158) were greater than the MEC, and 59% (94/158) were classified as not different than the response to the MEC. Most neurons that displayed mixture suppression did so at several mixture concentrations. 3. Sucrose suppression (mixture response < sucrose response) was prevalent among neurons most responsive to sucrose and for the mixtures that contained the stronger sucrose concentrations. Among neurons that displayed sucrose suppression, the magnitude of suppression was significantly correlated with sucrose response magnitude but not with QHCl response magnitude. These and other factors suggest that a neuron's capacity to display sucrose suppression to sucrose+QHCl mixtures is related to its sucrose sensitivity. 4. QHCl suppression (mixture response < QHCl response) was less prevalent than sucrose suppression, and the neurons that displayed QHCl suppression were almost exclusively a subset of those that displayed sucrose suppression to the same or different mixtures. This finding and the observation that one-third of all mixture responses involved mutual suppression (response to the mixture less than that to either component alone), suggest an association between the factors underlying sucrose suppression and QHCl suppression. 5. The across-neuron patterns (ANPs) of taste responses, which are thought to represent taste quality, were compared for mixtures and components. In general, the ANP for each mixture was similar to (significantly correlated with) the ANP of the more stimulatory component. However, for the mixture that evoked the greatest sucrose suppression, the mixture ANP was more similar to the ANP of the less stimulatory component.(ABSTRACT TRUNCATED AT 400 WORDS)