Sodium taste during sodium appetite

Sodium appetite appears to be an excellent model to study the neural mechanisms of motivation. In this issue of Chemical Senses, experiments by St John (2016) challenge 2 hypotheses for how a systemic sodium deficit guides an animal to find and ingest more Na⁺ ions in the environment. Both hypotheses deal with modifications of the sensory neural code produced by Na⁺+ ions on the tongue. One envisions a change in the Na⁺+ signal amplitude. A reduction could make the strong Na⁺+ signals less aversive; an increase, weak signals more noticeable. The other hypothesis requires no changes in the identity or amplitude of the Na⁺+ signal, but a shift in its hedonic tone toward sweetness or reward. The results of the 3 behavioral experiments render both explanations unlikely but fail to suggest alternatives.