Single Neurons in the Nucleus of the Solitary Tract (nst) Respond Selectively to Bitter Taste Stimuli

Molecular data suggest that receptors for all bitter ligands are coexpressed in the same taste receptor cells (TRCs) while physiological results indicate that individual TRCs respond only to a subset of bitter stimuli. It is also unclear to what extent bitter-responsive neurons are stimulated by non-bitter stimuli. To explore these issues, single neuron responses were recorded from the rat nucleus of the solitary tract (NST) during whole-mouth stimulation with a variety of bitter compounds: 10 µM cycloheximide, 7 mM propylthiouracil, 10 mM denatonium benzoate, and 3 mM quinine hydrochloride at intensities matched for behavioral effectiveness. Stimuli representing the remaining putative taste qualities were also tested. Particular emphasis was given to activating taste receptors in the foliate papillae innervated by the quinine-sensitive glossopharyngeal nerve. This method revealed a novel population of bitter-best (B-best) cells with foliate receptive fields and significant selectivity for bitter tastants. Across all neurons, multidimensional scaling depicted bitter stimuli as loosely clustered yet clearly distinct from non-bitter tastants. When neurons with posterior receptive fields were analyzed alone, bitter stimuli formed a tighter cluster. Nevertheless, responses to bitter stimuli were variable across B-best neurons, with cycloheximide the most, and quinine the least frequent optimal stimulus. These results indicate heterogeneity for the processing of ionic and non-ionic bitter tastants, which is dependent on receptive field. Further, they suggest that neurons selective for bitter substances could contribute to taste coding.