Morphine potentiates the impairing effects of neuroleptics on two-way active conditioned avoidance response in male mice.

The dopaminergic and opioid systems have effects on the conditioned avoidance response (CAR), although the possible interaction between these systems on this behaviour has not been studied. The effects of morphine (12.6 mg/kg), haloperidol (0.075 mg/kg), sulpiride (20 mg/kg) and risperidone (0.1 mg/kg) alone as well as morphine combined with these dopamine (DA) antagonists on the acquisition and performance of the CAR were explored in mice. Morphine increased avoidances but this seemed secondary to a rise in activity levels. All DA antagonists impaired CAR in the acquisition phase but only haloperidol disrupted performance. The combination of morphine plus neuroleptics impaired acquisition and performance of CAR. These results suggest that morphine disrupts the learning of CAR and that the classical neuroleptic haloperidol profoundly impairs acquisition and performance of CAR to a greater degree than atypical neuroleptics such as sulpiride and risperidone. Finally, it is concluded that morphine potentiates the impairing effects of DA antagonists on CAR.