CODING THE SWEET TASTE IN THE NUCLEUS OF THE SOLITARY TRACT - DIFFERENTIAL ROLES FOR ANTERIOR TONGUE AND NASOINCISOR DUCT GUSTATORY RECEPTORS IN THE RAT

1. A variety of chemicals that humans describe as sweet drive neurons in the nucleus of the solitary tract (NST) of the rat more vigorously when applied to the taste receptors associated with the nasoincisor ducts (NID) than when applied to taste receptors on the anterior tongue (AT). 2. The differential effects of sweet stimuli applied to the AT and NID also are evident in the set of across-neuron correlations produced by these stimuli. The psychophysical similarity among the sweet stimuli is better accounted for by responses to stimulation of the NID than by responses to stimulation of the AT (mean correlation between pairs of sweet stimuli = + 0.70 for the NID, + 0.44 for the AT). 3. Disaccharides or polysaccharides of glucose, i.e., maltose (0.3 M) and Polycose (0.1 M), are poor stimuli on the NID, evoking responses only 17.8 and 26.7% as great as the response elicited by sucrose (0.3 M), an optimal stimulus for this receptor subpopulation. This suggests that Polycose and maltose interact with receptor sites distinct from those with an affinity for sweet stimuli. Polycose and maltose also are ineffective stimuli on the AT, evoking responses only 11.8 and 4.9% as large as the response evoked by an optimal stimulus for this receptor subpopulation, a mixture of electrolytes (0.3 M NaCl, 0.03 M HCl, and 0.01 M quinine HCl). 4. The relative effectiveness of the sweet sugars in driving NST neurons (sucrose > fructose > glucose) correlates with their order of effectiveness in generating preference behavior in the rat. This is true for both the AT and the NID, but across individual cells the ordering is more consistent for NID responses. 5. The relative effectiveness of sucrose, glycine, and Na saccharin differs for the AT and NID. Sucrose is the most effective stimulus for the NID but the least effective stimulus for the AT. In the case of Na saccharin, the difference is probably due to the stimulatory effect of the Na+ ion and reflects the greater prominence of these receptors on the AT. The differential effects of sucrose and glycine, however, support the hypothesis that there are multiple types of sweet receptors and that the distribution of these receptors is different on the AT and NID.