The control of goal-directed actions by nutrient-specific appetites and rewards

There is evidence that appetites for specific nutrients can guide foraging behaviour and aid in dietary regulation through associative learning processes that link stimuli to nutrient-specific outcomes. However, most, if not all, examples of such behaviour can be interpreted as being stimulus-bound habits, i.e., reflexive responses induced by environmental stimuli. The control of identified goal-directed actions by nutrient-specific appetites has not been directly assessed. To address this question, we trained rats to press a lever for a high protein reward (whey protein shake) and another lever for a high carbohydrate reward (polycose solution). They were then tested under extinction conditions in which both levers were available following the extended exposure to meals that were high in protein or carbohydrate. When otherwise food-deprived rats had been selectively satiated on protein immediately prior to test, they pressed more on the lever they learned had produced polycose, whereas they pressed the lever they learned had produced whey protein more if they had instead been satiated on carbohydrate. Crucially, the same pattern emerged whether the satiety manipulation was achieved using the same nutrient sources that rats had earned during training (i.e., whey or polycose) or with foods high in the relevant nutrients, indicating that these behaviours were under goal-directed control and sensitive to nutritional state. These results show that actions can be motivated by the nutritional relevance of the instrumental outcome to specific appetites, a relationship that may guide natural foraging decisions.