Chorda tympani nerve transection, but not amiloride, increases the KCl taste detection threshold in rats.

Water-restricted rats were trained to press one lever after KCl presentation and the other lever after distilled water. Water reinforcement was given after each correct response, and a time-out followed each incorrect response. Rats were trained and tested on KCl stimuli of varying concentrations. Threshold was defined as the KCl concentration corresponding to 1/2 the maximum asymptote of performance for each rat. The geometric mean KCl detection threshold for all rats was 0.033 M KCl. Rats that had the chorda tympani nerve (CT) bilaterally transected showed an average increase in KCl threshold of approximately 0.60 log10 units, whereas sham-operated rats showed no change. Control rats retested with 100 microM amiloride added to all KCl concentrations and water displayed no change in threshold. These results suggest that although the CT contributes significantly to the rat's sensitivity to KCl, amiloride-sensitive taste transduction pathways do not.

Chorda tympani transection and selective desalivation differentially disrupt two-lever salt discrimination performance in rats.

Water-restricted rats were trained to press 1 of 2 levers if a sampled stimulus was NaCl and the other lever if the stimulus was KCl (0.05, 0.1, or 0.2 M). Responses were reinforced with water. After training, the average rate of correct responses was 90%. Performance was unchanged following sham surgery. Chorda tympani (CT) transection reduced average discrimination performance to 67.7% correct, and extirpation of the sublingual and submaxillary salivary glands reduced average performance to 80% correct. Although selective desalivation moderately reduced discriminability, a disrupted salivary environment does not explain the effects of CT transection. More likely, the discrimination deficit in CT-transected rats reflects a loss of critical taste input conveyed by the CT about salts.

Amiloride disrupts NaCl versus KCl discrimination performance: implications for salt taste coding in rats.

Amiloride, an epithelial sodium channel blocker, suppresses the responsiveness of narrowly tuned sodium-responsive taste afferents when orally applied in the rat. Broadly tuned salt-responsive taste afferents, which respond to sodium and nonsodium salts and acids, are relatively unaffected by the drug. We used amiloride treatment to examine the consequences of the specific removal of input from narrowly tuned sodium-responsive afferents on taste discrimination. Five water-restricted rats were trained in a gustometer to press one lever after licking NaCl and another lever after licking KCl across a range of concentrations (0.05, 0.1, and 0.2 M). Correct responses were rewarded with brief water access, and incorrect responses were punished with a time-out. After training, animals averaged about 90% correct responses and maintained competent performance during subsequent control sessions. Amiloride was then placed in all solutions at a given concentration (1-100 microM) for single test sessions. Control sessions were interposed between amiloride sessions. At high amiloride concentrations, overall responding was reduced to 50% correct and progressively improved as the drug concentration was lowered. The sigmoidal dose-response functions corresponded quantitatively with electrophysiological findings. Performance deficits occurred primarily with NaCl and were concentration dependent; performance during KCl trials was relatively undisturbed by amiloride adulteration. At high amiloride concentrations, rats treated NaCl as if it were KCl. Given that amiloride is tasteless to the rat, these results provide convincing evidence of the importance of narrowly tuned afferents in the discrimination between sodium and nonsodium salts and suggest that this is a general coding principle in the gustatory system.