Chorda tympani and lingual nerve responses to astringent compounds in rodents.

A wide variety of compounds in foods and beverages produce astringent sensations when introduced into the oral cavity. There is controversy, however, whether "astringency," with its associated puckering and drying sensations, is a fundamental taste quality or is a tactile sensation. To address this issue, electrophysiological recordings were made from the gerbil chorda tympani nerve and the rat lingual nerve. The chorda tympani nerve transmits taste information from the anterior 2/3 of the tongue, whereas the lingual nerve transmits tactile, thermal and pain sensations from the anterior 2/3 of the tongue. The astringent compounds tested were: tannic acid, tartaric acid, gallic acid, aluminum ammonium sulfate and aluminum potassium sulfate. Tannic acid, tartaric acid, and gallic acids were tested at concentrations up to 120 mM over a pH range from approximately 2 to 6. The aluminum salts were tested at concentrations up to 160 mM only at low pH's. All compounds rapidly (and at lower concentrations, reversibly) stimulate the chorda tympani nerve in a concentration-dependent manner at all pH's tested. The rapidity and reversibility of the chorda tympani responses suggest that astringent-tasting compounds interact directly with taste cells rather than indirectly by precipitating salivary proteins. At pH 6, tannic acid, tartaric acid, and gallic acid all elicit robust chorda tympani responses, implying that the ionized forms of these compounds produce taste sensations. None of these compounds stimulate lingual nerves over the same concentration and pH ranges used in the chorda tympani experiments.(ABSTRACT TRUNCATED AT 250 WORDS)

Trigeminal nerve responses in the rat elicited by chemical stimulation of the tongue.

Epithelial and neural mechanisms underlying trigeminal chemoreception were investigated by recording lingual nerve responses to chemical stimulation of the tongue. The chloride salts, NaCl, KCl, NH4Cl, and CaCl2, each elicited distinctly different, integrated whole-nerve responses (thresholds, 0.5-2.0 M). Incubation of the tongue with lanthanum (2.5 mM), which reduces the permeability of epithelial tight junctions, reversibly attenuated these salts responses. It did not affect neural responses to mechanical and thermal stimuli. Incubation with other established transport inhibitors--amiloride, tetra-ethyl ammonium or tetrodotoxin, had no effect on the salt responses. Acetic and hydrochloric acids (thresholds, 0.1-1.0 M), and MgCl2 and BaCl2 (greater than or equal to 0.5 M), also elicited distinctive responses. Other salts, MgSO4, Na isethionate and LaCl3 (greater than or equal to 1.0 M), and also ethanol (4 M), capsaicin (100 mM), nicotine (29.6 mM) and dextrose (0.5-2.5 M), did not elicit responses. These results indicate that ions of selected salts can diffuse through the tight junctions of the lingual epithelium to activate the trigeminal nerve, and suggest that both the cation and the anion may be important in determining if the nerves are activated and the wave-form of the responses.

Electrophysiological responses to non-electrolytes in lingual nerve of rat and in lingual epithelia of dog.

Epithelial and neural mechanisms underlying the trigeminal chemoreception of non-electrolytes were investigated in whole-nerve recordings from lingual nerve and in Ussing-chamber studies of isolated lingual epithelia. The non-electrolytes included menthol, amyl acetate, phenethyl alcohol, toluene, methanol, ethanol, propanol, butanol, hexanol and octanol. They produced different lingual nerve responses: methanol and ethanol only increased ongoing activity; longer-chain alcohols initially increased but then suppressed activity below baseline; phenethyl alcohol and toluene only suppressed activity. Their threshold concentrations for lingual nerve responses, with the exception of menthol, were proportional to the octanol:water partition coefficients of the stimuli. The threshold concentration for menthol was significantly lower than predicted by this coefficient. Calculation of the free energy of transfer from the threshold concentrations for the n-alcohols suggests that they undergo partition into a hydrophobic environment such as is found in lipid bilayers. Lanthanum chloride, which inhibited lingual nerve responses to hydrophilic compounds, presumably by blocking their diffusion across tight junctions, did not inhibit responses to these non-electrolytes. At high concentrations, hexanol acted as an anaesthetic in that the lingual nerve no longer responded to thermal and chemical stimuli whereas ethanol, which only increased lingual nerve activity, did not inhibit those responses. Epithelial transport, as indicated by the short-circuit current (Isc) measured across tongues bathed in symmetrical solutions of Krebs-Henseleit buffer, was reversibly inhibited by ethanol, hexanol, octanol, phenyl ethanol and menthol. The stimulus concentration necessary to inhibit 50% of the Isc decreased with increasing octanol:water partition coefficient.(ABSTRACT TRUNCATED AT 250 WORDS)