Vigor Encoding in the Ventral Pallidum.

The ventral pallidum (VP) is the major downstream nucleus of the nucleus accumbens (NAc). Both VP and NAc neurons are responsive to reward-predictive stimuli and are critical drivers of reward-seeking behavior. The cue-evoked excitations and inhibitions of NAc neurons predict the vigor (latency and speed) of the cue-elicited locomotor approach response and encode the animal's proximity to the movement target, but do not encode more specific movement features such as turn direction. VP neurons also encode certain vigor parameters, but it remains unknown whether they also encode more specific movement features, and whether such encoding could account for vigor encoding. To address these questions, we recorded the firing of neurons in the VP of freely moving male rats performing a discriminative stimulus (DS) task. Similar to NAc neurons, VP neurons' cue-evoked excitations were correlated with the speed of the upcoming approach movement and the animal's proximity to the movement target at cue onset. Unlike NAc neurons, VP neurons' firing reflected the efficiency of the approach movement path but not the latency to initiate locomotion. VP cue-evoked excitations are unlikely to be directly influenced by NAc cue-evoked excitations because unilateral treatment of the NAc with a dopamine D1 receptor antagonist, a manipulation that reduces NAc neurons' cue-evoked excitations, did not alter ipsilateral VP cue-evoked excitations. These observations suggest that the two structures receive simultaneous activation by inputs conveying similar but not identical information, and work in parallel to set the vigor of the behavioral response.