Single neuron gustatory responses of the gerbil chorda tympani to a variety of stimuli (recorded by a new method)

Cracking taste codes by tapping into sensory neuron impulse traffic

Insights into the biological basis for mammalian taste quality coding began with electrophysiological recordings from "taste" nerves and this technique continues to produce essential information today. Chorda tympani (geniculate ganglion) neurons, which are particularly involved in taste quality discrimination, are specialists or generalists. Specialists respond to stimuli characterized by a single taste quality as defined by behavioral cross-generalization in conditioned taste tests. Generalists respond to electrolytes that elicit multiple aversive qualities. Na(+)-salt (N) specialists in rodents and sweet-stimulus (S) specialists in multiple orders of mammals are well characterized. Specialists are associated with species' nutritional needs and their activation is known to be malleable by internal physiological conditions and contaminated external caloric sources. S specialists, associated with the heterodimeric G-protein coupled receptor T1R, and N specialists, associated with the epithelial sodium channel ENaC, are consistent with labeled line coding from taste bud to afferent neuron. Yet, S-specialist neurons and behavior are less specific than T1R2-3 in encompassing glutamate and E generalist neurons are much less specific than a candidate, PDK TRP channel, sour receptor in encompassing salts and bitter stimuli. Specialist labeled lines for nutrients and generalist patterns for aversive electrolytes may be transmitting taste information to the brain side by side. However, specific roles of generalists in taste quality coding may be resolved by selecting stimuli and stimulus levels found in natural situations. T2Rs, participating in reflexes via the glossopharynygeal nerve, became highly diversified in mammalian phylogenesis as they evolved to deal with dangerous substances within specific environmental niches. Establishing the information afferent neurons traffic to the brain about natural taste stimuli imbedded in dynamic complex mixtures will ultimately "crack taste codes."

Neural responses to bitter compounds in rats

To determine whether the idiosyncratic distribution of transduction mechanisms for bitter tastants in rat taste receptor cells (TRCs) could be inferred from the neural activity they evoke, single neuron responses to ten bitter-tasting compounds were recorded from rat glossopharyngeal (n = 30) and chorda tympani (n = 22) neurons. Responses to several 'bitter' alkaloids were obtained: 10 mM quinine-HCl, 50 mM caffeine, and 1 mM each nicotine, yohimbine, and strychnine, plus a number of non-alkaloid bitter-tasting compounds: 0.1 M KCl, 0.01 M MgCl2, and 1 mM 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 microM) were also tested. It was found that individual neurons in both glossopharyngeal and chorda tympani nerves differed in their relative sensitivities to the various bitter stimuli. To determine relationships among these stimuli, the differences in the evoked responses between each stimulus pair were summarized in a multi-dimensional scaling space. In these analyses neither nerve showed any obvious similarity between the placements of quinine and the other bitter stimuli. Such data suggest that first-order gustatory neurons can discriminate among the above bitter stimuli. For glossopharyngeal neurons, some similarity to quinine was found only for nicotine and denatonium, and for chorda tympani neurons, some similarity to quinine was found only for KCl and MgCl2. Of the bitter compounds tested, quinine evoked the greatest response from glossopharyngeal neurons. We propose this arises because quinine can activate TRCs by more transduction mechanisms than other bitter stimuli. The results from these studies were summarized in a qualitative model for the coding of bitter tastants where the variety of transduction mechanisms for bitters are distributed among various TRCs to account for the heterogeneous responses among the neurons.

Chorda tympani responses under lingual voltage clamp: implications for NH4 salt taste transduction

Rat chorda tympani (CT) responses to NH4Cl, ammonium acetate (NH4Ac), and ammonium hippurate (NH4Hp) were obtained during simultaneous current and voltage clamping of the lingual field potential. Although functional and developmental similarities for gustation have been reported for NH4+ and K+ salts, we report here that significant differences are discernible in the CT responses to both salts. Unlike neural responses to KCl, those to NH4Cl are voltage sensitive, enhanced by submucosa negative and suppressed by positive voltage clamp. In this regard, NH4Cl responses are qualitatively similar to NaCl responses; however, the magnitude of NH4Cl voltage sensitivity is significantly less than that of NaCl. The concentration dependence of the CT response to NH4Cl manifests a biphasic nonlinear relationship not observed with KCl or NaCl. Below 0.3 M, the CT response increases as if to approach a saturation value. However, beyond 0.3 M an inflection appears in the CT-concentration curve because of an abrupt increase in CT responses. This kinetic profile is Cl-dependent and is correlated with an increase in transepithelial conductance that displays similar NH4Cl concentration dependence. The biphasic relation to salt concentration is not observed when acetate or hippurate is substituted for Cl-. As with Na+ and K+ salts, less mobile anions than Cl- (Ac- and Hp-) lower the CT responses. However, like Na+ salts, but in contrast to K+ salts, the onset kinetics of CT responses to NH4Ac or NH4Hp remained rapid, even under positive voltage-clamp conditions. Amiloride (100 microM) partially suppresses CT responses within the concentration range of 0.05-0.3 M (48-20% suppression). Amiloride also suppresses the voltage sensitivity of NH4Cl CT responses, but does not eliminate the sensitivity as it does for Na+ salts. In conclusion, the data suggest that taste transduction for NH4 salts is mediated over two NH+ conduction pathways in the taste bud. This is especially evident with NH4Cl, where the CT-concentration curves show two distinct kinetic regimes. Below 0.3 M the saturation with increasing concentration, clamp voltage response dependence, and amiloride sensitivity suggest an apical membrane transduction conductance. Above 0.3 M, the high anion dependence of the response and its amiloride insensitivity indicate participation of the paracellular pathway in transduction.

Contribution of chorda tympani and glossopharyngeal nerves to taste preferences of rat for amino acids and NaCl

To learn how the gustatory nerves convey information about the nutritionally dependent taste preference, intake of amino acid solutions and saline in rats with bilateral chorda tympani (CTX) and/or glossopharyngeal neurotomy (GPX) was determined during the feeding of a control diet (C) and a L-lysine (Lys) deficient diet (LD). Intact rats preferred L-arginine (Arg) more in C and Lys more in LD. The CTX group did not select nor ingest Lys in LD, and its intake of Arg was also low in C. The GPX group did not substantially alter its preference under both diets, while it did show an increase in total liquid intake. The preference changes in the CTX + GPX group appeared as combined effects of the CTX and the GPX groups. In an additional study, the preference for Lys shifted to higher concentrations and the total consumption of Lys increased in LD. The present data suggest that the chorda tympani nerves possibly function as discriminators of the nutritional information by altering the taste preference, and that the glossopharyngeal nerves may convey other functional taste information, such as aversive tastes, and sensory aspects of osmotic regulation. In addition, it is revealed that the animals have ability to search for a nutrient deficient in their body, and to ingest it to a level that at least nullifies the deficiency.