Dopamine agonists increase perseverative instrumental responses but do not restore habit formation in a rat model of Parkinsonism.

Dopamine (DA) deafferentation of the dorsolateral striatum has been shown to prevent habit development, leaving instrumental behavior under action-outcome control that is persistently sensitive to modification of the motivational value of the reward. The present experiment further explored the basis of this dysfunction by examining the ability of intrastriatal DA agonist injections (D1 SKF 38393 or D2/D3 Quinpirole) during overtraining of a signaled instrumental task to restore habit formation in rats subjected to bilateral 6-hydroxydopamine (6-OHDA) lesions of the nigrostriatal dopaminergic pathway. Overtraining was followed by a test of goal sensitivity by satiety-specific devaluation of the reward. The results confirmed the impaired shift in performance from action to habit in control lesioned rats. However, lesioned rats repeatedly injected with quinpirole D2/D3 agonist showed an increase in non-rewarded instrumental responses (intertrials periods) during overtraining, suggesting the development of perseverative behavior. Following the procedure of devaluation, quinpirole D2/D3 agonist treatment, and to a lesser extent SKF 38393 D1 agonist, caused the persistence of sensitivity to reward devaluation, indicating clear goal-directed behavior despite extended training. This absence of restoration of habit formation by DA agonist treatment is discussed in the light of DA agonist effects in Parkinson patients.