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NH3- and CO2-induced suppression of taste nerve responses in clawed toads and eels
1Dept. of Biochemical Engineering and Science, Kyushu Institute of Technology, Fukuoka, Japan. yoshii@bse.kyutech.ac.jp
Abstract:
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.
Insights
Changes in intracellular pH (pHi) by CO2, ammonia, or trimethylamine suppress taste nerve responses in toads and eels. This suppression is linked to altered ion channel or enzyme activity in taste transduction.
Area of Science:
- Neuroscience
- Sensory Biology
- Physiology
Background:
- Intracellular pH (pHi) plays a crucial role in cellular functions.
- Taste perception relies on complex signaling pathways involving ion channels and enzymes.
Purpose of the Study:
- To investigate the impact of altered intracellular pH on taste nerve responses in clawed toads and eels.
- To determine the specific mechanisms by which chemical stimuli affect taste transduction.
Main Methods:
- Electrophysiological recordings of taste nerve responses in amphibians and fish.
- Application of carbon dioxide (CO2), ammonia (NH3), and trimethylamine to alter pHi.
- Cross-adaptation experiments to identify receptor specificity.
Main Results:
- CO2, NH3, and trimethylamine reversibly suppressed taste nerve responses to various stimuli in clawed toads.
- The suppressive effects were non-specific, affecting different receptor sites.
- Charged species (bicarbonate, carbonate, ammonium) did not suppress responses, indicating the effect is due to the uncharged molecules altering pHi.
- NH3 altered the threshold and maximum response of L-proline in toads.
- Similar suppression was observed in eels, with NH3 affecting responses to L-arginine.
Conclusions:
- Uncharged molecules like CO2, NH3, and trimethylamine suppress taste responses by altering intracellular pH (pHi).
- CO2-induced acidosis and NH3/trimethylamine-induced alkalosis likely inhibit key ion channels or enzymes involved in taste transduction.
- These findings elucidate a novel mechanism regulating taste sensitivity through pHi modulation.