Neurons of the Ventral Tegmental Area Encode Individual Differences in Motivational "Wanting" for Reward Cues

It has been argued that the dopaminergic system is involved in the attribution of motivational value to reward predictive cues as well as prediction error. To evaluate, dopamine neurons were recorded from male rats performing a Pavlovian approach task containing cues that have both "predictive" and "incentive" properties. All animals learned the predictive nature of the cue (illuminated lever entry into cage), but some also found the cue to be attractive and were motivated toward it ("sign-trackers," STs). "Goal-trackers" (GTs) predominantly approached the location of reward receptacle. Rats were implanted with tetrodes for neural electrophysiological recordings in the ventral tegmental area. Cells were characterized by spike waveform shape and firing rate. Firing rates and magnitudes of responses in relation to Pavlovian behaviors, cue presentation, and reward delivery were assessed. We identified 103 dopamine and 141 nondopamine neurons. GTs and STs both showed responses to the initial lever presentation (CS1) and lever retraction (CS2). However, higher firing rates were sustained during the lever interaction period only in STs. Further, dopamine cells of STs showed a significantly higher proportion of cells responding to both CS1 and CS2. These are the first results to show that neurons from the VTA encode both predictive and incentive cues, support an important role for dopamine neurons in the attribution of incentive salience to reward-paired cues, and underscore the consequences of potential differences in motivational behavior between individuals.SIGNIFICANCE STATEMENT This project serves to determine whether dopamine neurons encode differences in cued approach behaviors and incentive salience. How neurons of the VTA affect signaling through the NAcc and subsequent dopamine release is still not well known. All cues that precede a reward are predictive in nature. Some, however, also have incentive value, in that they elicit approach toward them. We quantified the attribution of incentive salience through cue approach behavior and cue interaction, and the corresponding magnitude of VTA neural firing. We found dopamine neurons of the VTA encode strength of incentive salience of reward cues. This suggests that dopamine neurons specifically in the VTA encode motivation.