NH3- and CO2-induced suppression of taste nerve responses in clawed toads and eels.
We investigated the effects of intracellular pH values (pHi) on taste nerve responses of clawed toads and eels. (1) CO2, NH3 or trimethylamine reversibly suppressed the taste nerve responses of clawed toads to various amino acids, CaCl2 and caffeine. IC50 values of the suppression of the responses to 0.1 mM L-proline were as follows: approximately 10 mM for CO2, approximately 0.3 mM for NH3, approximately 0.2 mM for trimethylamine. (2) Cross-adaptation experiments showed that L-proline, caffeine and CaCl2 stimulated different receptor sites from each other, indicating the suppressive effect was non-specific. (3) Although CO2, NH3 or trimethylamine yielded the charged molecules, HCO3-, CO3(2-), NH4+ or trimethylammonium on hydration, none of these charged species suppressed taste responses. (4) NH3 increased the threshold concentration of L-proline by e-fold per 0.37 mM NH3 and decreased the maximum response to L-proline with increasing NH3 concentration. (5) The taste nerve responses of eels to 0.1 mM L-arginine, a potent stimulus on eel taste receptors, were similarly suppressed by NH3 with an IC50 value of approximately 0.3 mM. (6) These results indicated that these uncharged species changed pHi, which suppressed the taste responses. CO2-induced acidosis and NH3- or trimethylamine-induced alkalosis are likely to inhibit activities of ion channels or enzymes involved in taste transduction mechanisms.