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1 Dept. Anatomy and Neurobiology, University of Tennessee College of Medicine, Memphis, Tennessee, USA
2 Dept. Functional, Morphological and Regulation Science, Div. Integrative Physiology, Faculty of Medicine, Tottori University, Yonago, Tottori, Japan
* To whom correspondence should be addressed. E-mail: chris{at}utmem.edu.
We examined the effect of the sweet transduction blocker gurmarin on taste responses recorded from neurons in the rat solitary nucleus (NST) to determine how gurmarin sensitivity is distributed across neuronal type. Initially, responses evoked by washing the anterior tongue and palate with 0.5 M sucrose, 0.1 M NaCl, 0.01 M HCl and 0.01 M quinine-HCl were recorded from 35 neurons. For some cells, responses to a sucrose concentration series (0.01 - 1.0 M) or an array of sweet-tasting compounds were also measured. Gurmarin (10 µg/ml, 2-4 ml) was then applied to the tongue and palate. Stimuli were reapplied after 10 - 15 min. Neurons were segregated into groups based on similarities among their initial response profiles using hierarchical cluster analysis (HCA). Results indicated that sucrose responses recorded from neurons representative of each HCA-defined class were suppressed by gurmarin. However, a disproportionate percentage of cells in each group displayed sucrose responses that were substantially attenuated following gurmarin treatment. Post-gurmarin sucrose responses recorded from neurons that composed 57% of class S, 40% of class N, and 33% of class H were suppressed by at least 50% relative to control. On average, attenuation was statistically significant only in class S and N neurons. Although the magnitude of gurmarin-induced response suppression did not differ across sucrose concentration, responses to different sweet-tasting compounds were differentially affected. Responses to NaCl, HCl or quinine were not suppressed by gurmarin. Results suggest that information from gurmarin-sensitive and -insensitive receptor processes converges onto single NST neurons.
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