AMILORIDE DISRUPTS NACL VERSUS KCL DISCRIMINATION PERFORMANCE - IMPLICATIONS FOR SALT TASTE CODING IN RATS

Amiloride, an epithelial sodium channel blocker, suppresses the respon siveness of narrowly tuned sodium-responsive taste afferents when oral ly 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 con sequences of the specific removal of input from narrowly tuned sodium- responsive afferents on taste discrimination. Five water-restricted ra ts 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 wate r 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 w as then placed in all solutions at a given concentration (1-100 mu M) for single test sessions. Control sessions were interposed between ami loride sessions. At high amiloride concentrations, overall responding was reduced to 50% correct and progressively improved as the drug conc entration was lowered. The sigmoidal dose-response functions correspon ded quantitatively with electrophysiological findings. Performance def icits occurred primarily with Nacl and were concentration dependent; p erformance during KCl trials was relatively undisturbed by amiloride a dulteration. At high amiloride concentrations, rats treated NaCl as if it were KCl. Given that amiloride is tasteless to the rat, these resu lts provide convincing evidence of the importance of narrowly tuned af ferents in the discrimination between sodium and nonsodium salts and s uggest that this is a general coding principle in the gustatory system .