Conditioned locomotion induced by unilateral intrastriatal administration of Apomorphine: D2 receptor activation is critical but not the expression of the unconditioned response

The present study examined the role of D1 and D2 receptors in the conditioning of Apomorphine-induced locomotor behavior. A Pavlovian conditioning protocol was used in which rats received 5 daily intrastriatal Apomorphine treatments paired or unpaired to an open-field environment followed, 2 days later, by a saline test for conditioning. In the conditioning induction phase, the intrastriatal Apomorphine treatment increased locomotor activity expressed as an increased number of sectional crossings and rearings. In the conditioning test, the Apomorphine-paired group had significantly higher locomotor activity than the unpaired and vehicle groups, consistent with the development of a conditioned locomotor response. The concomitant blockade of D1 and D2 receptors with D1 (SCH23390) and D2 (Sulpiride) antagonists prevented the Apomorphine-induced behavioral response during the induction phase and in the conditioning test. Pretreatment with the D1 antagonist SCH 23390 also blocked the Apomorphine-induced behavioral response during the induction phase but did not block the Apomorphine conditioned response. Pretreatment with the selective D2 antagonist Sulpiride blocked the Apomorphine behavioral response during the induction phase and in the conditioning test. Altogether, these results indicate that antagonism of either the D1 or D2 receptors in the dorsal striatum can block Apomorphine-induced locomotor activation but that D2 but not D1 antagonism can prevent the development of the Apomorphine conditioned response. Altogether, these findings indicate a key role for the D2 receptor site in the mediation of Apomorphine conditioned behavior; and, in addition, that Apomorphine conditioned locomotor response can develop without the expression of the locomotor stimulant response during the induction phase of conditioning.