Conditioned enhancement of flavor evaluation reinforced by intragastric glucose. II. Taste reactivity analysis.

Rats learn to prefer a flavor that is paired with intragastric (IG) glucose infusion, which may represent a learned shift in the hedonic evaluation or palatability of the flavor. The present study used the taste reactivity (TR) test to infer nutrient-conditioned changes in flavor palatability. Rats were first conditioned in 20-h/day and then 30-min/day sessions to associate a flavored saccharin solution (the CS+) with IG infusions of 16% glucose, and a different flavored saccharin solution (the CS-) with IG water infusions. They strongly preferred the CS+ to the CS- in two-bottle intake tests. When next tested with brief intraoral (IO) infusions of the CS flavors, the rats exhibited significantly more hedonic TR responses to the CS+ flavor than the CS- flavor, which indicates an enhanced hedonic evaluation. A subsequent series of two-bottle tests established that the CS- flavor was equally preferred to 3% fructose, whereas the CS+ flavor was equally preferred to 16% fructose. In addition, the difference between the TR responses to 3% and 16% fructose paralleled the difference between the TR responses to CS- and CS+. These findings support the idea that pairing flavored saccharin with the postingestive effects of glucose produces a learned shift in palatability comparable to that produced by increasing the concentration of a sugar solution.