Interaction of morphine and haloperidol on agonistic and motor behaviors of male mice

To further clarify the interaction between opioid and dopaminergic systems, the effects of simultaneous administration of morphine hydrochloride (1.25 or 2.5 mg/kg) and haloperidol (0.1 mg/kg) on aggressive behavior of male mice were explored. Isolated male mice (experimental animals) were confronted in a neutral area with anosmic, group-housed consepecifics (standard opponents) 30 min after injection of both compounds, and aggression was evaluated by estimation of times allocated to 11 different behavioral categories. In the first experiment (which functioned as a pilot study), the two doses of morphine were explored. In the second one, incorporating a more complete experimental design, only the lowest morphine dose was used and the animals were preselected by a previous aggression test. In attack behavior, morphine added to haloperidol counteracted, at least partially, the antiaggressive effect of the neuroleptic. In contrast, the impairing effects of haloperidol on motor activity were increased by the addition of morphine. These results show that the behavioral effects of dopaminergic antagonists are modulated by opioid influences and that opiates and dopaminergic agents interact in a different manner on motor and on aggressive behaviors.