Appetites in Action: On the role of Protein and Carbohydrate Rewards in the Goal-Directed Behaviour of the Rat

We and other animals have evolved various nutrient-specific appetites to guide feeding our responses to the sources of nutrition that we encounter in our environment. These appetites serve, generally, to ensure we do not consume too much of one nutrient while becoming deficient in others, which is important given specific nutrients are needed in different quantities and proportions to grow and support phenotypes that are key for determining health outcomes and Darwinian fitness. However, food items consumed in actively foraging species, like rats and humans, are seldom encountered haphazardly and are largely obtained through goal-directed actions acquired through instrumental conditioning: i.e., through the selective performance of actions that animals have learned result in one outcome or another. To allocate performance efficiently, motivation should be sensitive to the value of alternative outcomes, and, therefore, it is likely that the nutritional properties of these outcomes play an important role in determining such values. Experiments presented here studied the behaviour of rats as an animal model to understand if and how protein and carbohydrate components of food outcomes influence goal-directed behaviour. We trained rats to press levers earning these outcomes and then selectively manipulated the rats' appetite for protein vs. carbohydrate immediately before they were tested on the levers in a choice extinction test. Using this approach, we found that shifts in appetite affected performance in a manner that would be expected if it were driven largely by the nutritional relevance of each incentive to these appetites. We further showed that these nutrient-driven revaluation effects were not an immediate consequence of encoding specific action-outcome relationships alone and depended on rats learning about the value of a food outcome under a specific appetite through consummatory experience: For example, when shifted to a protein appetite, rats only shifted their performance to a lever previously associated with a protein outcome if they had previously learned the value of that outcome under different levels of protein appetite; i.e., choice on test depended on incentive learning. A similar form of incentive learning effect was found for carbohydrate outcomes when rats were shifted to a carbohydrate appetite. However, when an outcome was composed of a mixture of protein and carbohydrate its consumption interacted with appetite and choice performance in asymmetrical ways, revealing that press rates were more responsive to shifts in protein than carbohydrate appetite. The implications of these findings are discussed for nutritional phenomena, such as obesity, and regarding the role of these learning processes in decision making more generally.