Modulation of behavior by expected reward magnitude depends on dopamine in the dorsomedial striatum.

Reward-predictive cues are important to guide behavioral responding. In a series of experiments, we sought to characterize the role of dopamine in the dorsomedial striatum in modulation of reward-directed responding by visual cues. Different groups of rats subjected to infusion of 6-hydroxydopamine or vehicle into the posterior part of the dorsomedial striatum (pDMS) were tested in three experiments. In experiment 1, rats were examined in an operant task demanding a lever release response. In intact rats, reaction times of responding were reliably shorter on cued large reward trials than on cued small reward trials. Results showed that pDMS dopamine depletion impaired reward-dependent modulation of reaction times, if visual cues predict large versus small reward, but not if visual cues predict reward versus no reward. These observations suggest that dopamine signaling in the pDMS contributes to a process through which reward-directed responses become guided by cues associated with distinct reward magnitudes. Experiment 2 revealed that pDMS dopamine depletion did not compromise the acquisition of a conditional visual discrimination task in an operant box that required learning a rule of the type "if the cue light is bright press left lever for reward, if dim press right lever". Furthermore, experiment 3 showed that pDMS dopamine depletion did not impair the acquisition of a cross maze task that required learning a visual cue discrimination strategy to obtain food reward. Together results of experiments 2 and 3 indicate that dopamine signaling in the pDMS does not subserve stimulus discrimination per se and stimulus-response learning.