Altered NaCl taste responses precede increased NaCl ingestion during Na⁺ deprivation

It has been suggested that Na⁺ deficiency alters the sensitivity of taste receptors, thereby rendering NaCl solutions more palatable or preferred and more likely to be ingested. Increased ingestion of concentrated NaCl solutions by rats during dietary Na⁺ deprivation occurs only after approximately 8-10 days. To determine whether changes in gustatory responses mediate the deprivation-induced NaCl ingestion (salt appetite), we evaluated taste responses to a range of NaCl concentrations before, during, and after dietary Na⁺ deprivation. Rats were trained to lick rapidly in short-duration (10 s) tests by mixing NaCl solutions in a dilute sucrose solution. This method elicited consistent, interpretable rates of licking, even of normally avoided NaCl concentrations, without the necessity of depriving the rats of water. The licking rate increased after dietary Na⁺ deprivation of only 2 days, increased further after 5 days of Na⁺ deprivation and, after 10 days, was not different from that after 2 days. These results suggest that a change in the response to NaCl taste, as evidenced by increased rates of licking during short-access tests, occurred after 2 days of dietary Na⁺ deprivation. In contrast, a significant increase in the 24-h ingestion of a concentrated NaCl solution occurred only after approximately 1 week of maintenance on Na⁺-deficient chow. Thus, it is unlikely that a delayed change in the response to NaCl taste to more palatable or preferred underlies the delayed increase in 24-h NaCl intake during dietary Na⁺ deprivation.