Effects of selective D1 and D2 dopamine antagonists on the development of behavioral sensitization to apomorphine.

The objective of the present study was to determine whether the development of behavioral sensitization to apomorphine could be blocked by either D1 or D2 selective dopamine antagonists. In three experiments, male rats received 10-21 daily injections of a selective D1 (SCH 23390; 0 or 0.5 mg/kg IP) or D2 (sulpiride; 0, 30, or 100 mg/kg IP) antagonist followed by an apomorphine (0 or 1.0 mg/kg SC) injection. In two experiments, the rats were tested for locomotor activity in photocell arenas after the daily injections. In all experiments, the rats were tested for sensitization to apomorphine following the training phase. The results indicated that apomorphine produced a progressively greater increase in locomotor activity with each injection, and this apomorphine-induced increase in activity was completely blocked by both sulpiride and SCH 23390 treatments. However, although both sulpiride and SCH 23390 blocked apomorphine-induced activity, only SCH 23390 injections prevented the development of sensitization to apomorphine. That is, rats pretreated with sulpiride and apomorphine displayed significant sensitization when subsequently tested with a challenge dose of apomorphine alone. These findings suggest that the development of behavioral sensitization to apomorphine is related specifically to the stimulation of dopamine D1 receptors.