Conditioned enhancement of flavor evaluation reinforced by intragastric glucose: I. Intake acceptance and preference analysis.

Rats acquire strong preferences for flavors that are paired with intragastric (IG) glucose infusions. The present study assessed the magnitude of this nutrient-conditioned change in flavor evaluation by comparing the strength of the rats' conditioned preference to their unconditioned preference for the sweet taste of sugar. Food-restricted rats were first trained (20 h/day) to associate one flavor (CS+) with IG 16% glucose infusions and another flavor (CS-) with IG water infusions. Then, in a series of 30 min/day two-bottle tests, the CS+ and CS- were separately offered in a choice vs. fructose of varying concentrations. The CS- was preferred to 2% fructose, but not 4% fructose. The CS+ was preferred to 2%, 4%, and 8% fructose and was isopreferred to 16% fructose. This CS+ vs. fructose preference profile was replicated and found to be independent of deprivation state. The intake-stimulating efficacy of the CS+ flavor and of fructose taste was also compared in one-bottle sham-feeding tests. Intakes of CS+ and of 16% fructose increased by similar amounts in the first sham-feeding test. However, with repeated testing, fructose intake continued to increase, whereas CS+ intake declined. This profile suggests extinction of the conditioned satiation response to fructose and extinction of the conditioned acceptance response to the CS+, respectively. These findings provide novel evidence for a robust learned shift in flavor evaluation reinforced by the postingestive actions of glucose.