Vigor encoding in the ventral pallidum

Abstract

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 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’s 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.

SIGNIFICANCE STATEMENT

The ventral pallidum (VP) connects the nucleus accumbens and other upstream structures with downstream motor structures, providing a conduit by which motivation is translated to action. Prior studies have shown that the firing responses of VP neurons to cues that elicit motivated behavior reflect parameters related to the vigor of the motor response, but have not fully analyzed how firing could be related to a broader range of movement parameters. Here, we show that the speed and efficiency of locomotor approach to reward, as well as proximity to the reward-associated movement target, influence the firing of VP neurons independent of representation of other motor parameters. These results suggest a specific role for VP neurons in invigorating locomotor approach to reward-associated locations.