Dissociation of the role of nucleus accumbens dopamine in responding to reward-predictive cues and waiting for reward

The choice among behavioral options is influenced by the anticipated cost of working for the reward relative to the anticipated reward magnitude. Dopamine release in the nucleus accumbens (NAc) has been suggested to play an important role in cost/benefit computation. From a behavioral perspective, work involves two elements: the caloric expenditure of energy and the time required to complete the task. In many studies of the contribution of NAc dopamine to cost/benefit decisions, measures of work have conflated these separate elements. Here we describe a novel cued progressive delay task, an analog of the progressive ratio task that minimizes energy expenditure. In this task, rats obtain sucrose reward by entering a reward receptacle in response to a cue and remaining in the receptacle for a required wait that is increased after each successful trial. In an experiment in which the magnitude of reward was varied across sessions, the animals' fail point (maximum wait achieved) was correlated with the amount of reward delivered. Microinjection of D1 or D2 dopamine receptor antagonists into the NAc did not affect the length of time animals were willing to wait for reward (fail point), but did reduce the proportion of cues to which the animal responded. These results suggest that waiting for reward without increased caloric energy expenditure does not require NAc dopamine.