| Salt stimuli can activate two transduction mechanisms in the rat's oral cavity. One mechanism appears to rely on taste receptor cells that contain sodium-selective ion channels on their surface. Passage through these channels can be blocked with the drug amiloride. The other mechanism is thought to be less selective, with associated fibers responding to potassium and ammonium as well as sodium salts. Most data suggest that the salt responsiveness of this population of fibers is not significantly attenuated with amiloride treatment, although some researchers have found evidence to the contrary. The two salt transduction pathways are commonly grouped according to their amiloride sensitivity as either amiloride-sensitive (AS) or amiloride-insensitive (AI). Activation of the AI pathway appears to be limited by anion size, with large anions like gluconate producing the greatest suppression. Previous research has indicated that the AS pathway is necessary and sufficient for normal sodium detection in the rat as well as necessary for sodium recognition. The current experiments were designed to determine whether the AS pathway was also sufficient for sodium recognition as well as to elucidate possible functional roles of the AI pathway regarding the perception of nonsodium salts. This was accomplished by observing the effects of physiological manipulations like gustatory nerve transection, amiloride treatment, and acute sodium depletion on the taste-guided behavior of highly-trained rats. Briefly, the sodium-specific AS pathway appears to be sufficient for sodium recognition in acutely depleted animals. In addition, AI receptor cells innervated by the chorda tympani (CT) nerve were found to be necessary for normal detection of ammonium chloride (NH4Cl) and AI cells innervated by the facial nerve were both necessary and sufficient to discriminate NH4Cl from KCl. This finding suggests that taste receptor cells innervated by the facial nerve could use separate AI transduction mechanisms with different selectivities for ammonium and potassium. This work also supports the hypothesis that amiloride does not significantly impair the perception of nonsodium salts, as well as the contention that the facial nerve may provide unique information about taste quality in spite of innervating only about 30% of the taste buds in the oral cavity. |
