NEURAL RESPONSES TO BITTER COMPOUNDS IN RATS

To determine whether the idiosyncratic distribution of transduction me chanisms for bitter tastants in rat taste receptor cells (TRCs) could be inferred from the neural activity they evoke, single neuron respons es to ten bitter-tasting compounds were recorded from rat glossopharyn geal (n = 30) and chorda tympani (n = 22) neurons. Responses to severa l 'bitter' alkaloids were obtained: 10 mM quinine-HCl, 50 mM caffeine, and 1 mM each nicotine, yohimbine, and strychnine, plus a number of n on-alkaloid bitter-tasting compounds: 0.1 M KCl, 0.01 M MgCl2, and 1 m M each phenylthiocarbamide (PTC), L-tyrosine, and denatonium benzoate. To obtain some distinctions with other stimuli NaCl(0.1 M), HCl (pH 2 .0), and capsaicin (10 mu M) were also tested. It was found that indiv idual neurons in both glossopharyngeal and chorda tympani nerves diffe red in their relative sensitivities to the various bitter stimuli. To determine relationships among these stimuli, the differences in the ev oked responses between each stimulus pair were summarized in a multi-d imensional scaling space. In these analyses neither nerve showed any o bvious similarity between the placements of quinine and the other bitt er stimuli. Such data suggest that first-order gustatory neurons can d iscriminate among the above bitter stimuli, For glossopharyngeal neuro ns, some similarity to quinine was found only for nicotine and denaton ium, and for chorda tympani neurons, some similarity to quinine was fo und only for KCl and MgCl2. Of the bitter compounds tested, quinine ev oked the greatest response from glossopharyngeal neurons. We propose t his arises because quinine can activate TRCs by more transduction mech anisms than other bitter stimuli. The results from these studies were summarized in a qualitative model for the coding of bitter tastants wh ere the variety of transduction mechanisms for bitters are distributed among various TRCs to account for the heterogeneous responses among t he neurons. (C) 1997 Elsevier Science B.V.