Pentazocine-like discriminative stimulus effects of morphine are blocked by the dopamine D-1 antagonist SCH23390, but not by the dopamine D-2 antagonist sulpiride.

The effects of selective dopamine antagonists on the pentazocine-like discriminative stimulus effects of morphine were investigated in the rat trained to discriminate 3.0 mg/kg of pentazocine from vehicle. Morphine (3.0 mg/kg), a mu-selective agonist, produced stimulus effects in common with pentazocine (3.0 mg/kg), indicating that the stimulus effects are mediated through mu opioid receptors. The pentazocine-like stimulus effects of morphine (3.0 mg/kg) were clearly inhibited by the dopamine D-1 antagonist SCH23390 (0.03-0.56 mg/kg) but not by the dopamine D-2 antagonist sulpiride (20.0-80.0 mg/kg). The results suggest that the pentazocine-like discriminative stimulus effects of morphine are mediated via dopamine D-1 receptors.