INTRAGASTRIC GLUCOSE BUT NOT FRUCTOSE CONDITIONS ROBUST FLAVOR PREFERENCES IN RATS

Prior work indicates that glucose and fructose differ in their posting estive reinforcing effects. The present study investigated this phenom enon by training rats to associate the intake of flavored water with i ntragastric (IG) infusions of 16% sugar solutions. In experiment 1, ra ts had one flavor [conditional stimulus (CS)] paired with IG sugar inf usions (CS+; e.g., cherry) and another flavor paired with IG water (CS -; e.g., grape) 23 h/day; Chow was available ad libitum. In subsequent choice tests, rats infused with glucose displayed a strong preference (89%) for the CS+ flavor, whereas rats infused with fructose showed o nly a small and nonreliable CS+ flavor preference (62%). When next tra ined to associate one flavor (e.g., orange) with IG glucose and anothe r flavor (e.g., strawberry) with IG fructose, rats in both groups deve loped a significant preference (81%) for the glucose-paired flavor. In experiment 2, food-deprived rats were trained 2 h/day to associate a CS+ flavor with IG sugar and a CS- flavor with IG water infusions. The glucose-reinforced rats displayed a near-total preference (95%) for t he CS+ flavor, whereas fructose-reinforced rats showed a much smaller CS+ preference (67%). The preference findings indicate that the postin gestive consequences of glucose are much more reinforcing than those o f fructose. It appears that food conditioning is mediated- by chemospe cific actions of nutrients rather than their general satiating or ener gy repleting effects.