Some failures of intragastric NaCl infusions to support flavor preference learning

Rats can acquire conditioned flavor preferences based on the postingestive delivery of energy-rich nutrients. Here, we tried to condition flavor preferences to the postingestive delivery of the nutrient, sodium. Three experiments were conducted. In one, sodium-depleted rats were allowed to choose between two arbitrary flavors for 8 days. Ingestion of one of the flavors initiated infusions of one of two volumes of 150 mM NaCl, whereas ingestion of the other flavor had no effect. The rats showed no preference for the flavor paired with NaCl infusion. In another, sodium-depleted rats were given two pairs of 24-h one-bottle training trials. Ingestion of the flavor presented during one trial initiated infusions of water or one of four concentrations of NaCl (75, 150, 300 or 450 mM NaCl). Ingestion of the flavor presented on the other trial in each pair had no effect. In a subsequent two-bottle choice between the two flavors, the rats were either indifferent to (water, 75 and 150 mM) or avoided (300 or 450 mM) the infusion-paired flavor. In the third study, replete rats were trained as in the previous experiment with 150 mM NaCl as the infusate and tested after sodium deficiency was induced by furosemide. These animals avoided the flavor paired with NaCl infusion. We conclude that rats cannot associate an arbitrary taste with the beneficial effects of postingestive delivery of NaCl, or if they can, this occurs only under rare circumstances.