Cue and reward evoked dopamine activity is necessary for maintaining learned Pavlovian associations

Associating natural rewards with predictive environmental cues is crucial for survival. Dopaminergic (DA) neurons of the Ventral Tegmental Area (VTA) are thought to play a crucial role in this process by encoding reward prediction errors that have been hypothesized to play a role in associative learning. However, it is unclear whether this signal is still necessary after animals have acquired a cue-reward association. In order to investigate this we have trained mice to learn a Pavlovian cue-reward association. After learning, mice show robust anticipatory and consummatory licking behavior. As expected calcium activity of VTA DA neurons goes up for cue presentation as well as reward delivery. Optogenetic inhibition during the moment of reward delivery disrupts learned behavior, even in the continued presence of reward. This effect is more pronounced over trials and persists on the next training day. Moreover, outside of the task licking behavior and locomotion are unaffected. Similarly to inhibitions during the reward period, we find that inhibiting cue-induced dopamine signals robustly decreases learned licking behavior, indicating that cue-related DA signals are a potent driver for learned behavior. Overall we show that inhibition of either of these dopamine signals directly impairs the expression of learned associative behavior. Thus, continued dopamine signaling in a learned state is necessary for consolidating Pavlovian associations

Significance statement: Dopamine neurons of the Ventral Tegmental Area have long been suggested to be necessary for animals to associate environmental cues with rewards that they predict. Here we use timelocked optogenetic inhibition of these neurons to show that the activity of these neurons is directly necessary for performance on a Pavlovian conditioning task, without affecting locomotor per se. These findings provide further support for the direct importance of second-by-second dopamine neuron activity in associative learning.