1. The effects of adaptation to diluted natural water (NW) and various salt solutions on the gustatory responses recorded from the palatine nerve in rainbow trout (Oncorhynchus mykiss) were studied. 2. The magnitude of the response to 1 mmol l-1 l-proline (l-Pro) decreased when the perfusing NW was diluted with artificial fresh water (AFW) that maintained concentrations of major cations. AFW suppressed the responses to l-Pro by about 70 %. 3. The responses to 1 mmol l-1 l-Pro, 0.1 mmol l-1 quinine­HCl (Q-HCl) and 10 nmol l-1 taurolithocholic acid (TLCA) were eliminated or reduced (to <10 %) by adapting the palate to distilled water (DW). The addition of 0.1–100 mmol l-1 salts (NaCl, KCl, CaCl2, MgCl2) and choline chloride restored the gustatory responses to about 50 % of those in NW. The addition of salts to NW had no effect on the gustatory responses. 4. The gustatory responses to 5 % CO2 were similarly reduced when the palate was adapted to solutions that contained no NW (DW, AFW, 10 mmol l-1 NaCl in DW). However, the reduction was independent of salt concentration, suggesting a different transduction mechanism for CO2. 4. Tetrodotoxin (1 µmol l-1) had no effect on the gustatory responses to l-Pro. 5. We conclude that NW is required and that cations alone are not sufficient to support maximal gustatory responses. The results suggest that an unknown substance(s) contained in NW plays an essential role in gustatory reception and that permeation of cations through the apical membrane of gustatory cells is not involved in gustatory transduction in rainbow trout.

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