Amiloride blocks acid responses in NaCl-best gustatory neurons of the hamster solitary nucleus

Abstract

Biophysical studies of isolated taste receptor cells show that one mechanism of Na+ salt transduction involves the inward movement of Na+ through amiloride-blockable ion channels on the apical receptor cell membrane, which leads to a direct depolarization. Hamster taste receptor cells with amiloride-blockable Na+ responses also show an amiloride-sensitive H+ current. Thus one mechanism for the transduction of acid taste involves the amiloride-sensitive channel. We investigated the effects of amiloride on responses to acids in neurons of the nucleus of the solitary tract (NST) of the hamster. The responses of 47 NST neurons were recorded extracellularly while the anterior tongue was stimulated with solutions representing the four taste qualities (NaCl, sucrose, HCl, quinine), which were used to characterize each cell on the basis of its best stimulus. The effects of amiloride on responses to 10 mM HCl, 10 mM citric acid, 100 mM NaCl, and 100 mM sucrose were then investigated. Stimuli were presented alone for 30 s (control trials) and also presented for 10 s, followed by a mixture of the stimulus with 10 microM amiloride for 10 s, followed by the stimulus alone again for 10 s (amiloride trials). The effects of amiloride were assessed by comparing the responses of cells with the stimulus + amiloride with that of the stimulus alone. In neurons classified as NaCl-best, amiloride reversibly blocked responses to NaCl, HCl, and citric acid. In HCl-best neurons, amiloride had no effect on responses to any of these stimuli. In sucrose-best neurons, amiloride blocked the response to NaCl but not to sucrose or to either acid. These results support the hypothesis that acids are transduced by at least two different receptor mechanisms in the hamster, amiloride sensitive and amiloride insensitive. At the NST, these inputs are tightly maintained in two separate populations of neurons. Sucrose-best neurons, which show amiloride effects on NaCl but not acids, appear to receive converging inputs from both amiloride-sensitive (N-best) and amiloride-insensitive (H-best) chorda tympani nerve fibers.