Comparison of dopamine receptor antagonists on hyperlocomotion induced by cocaine, amphetamine, MK-801 and the dopamine D1 agonist C-APB in mice

Rationale: Direct or indirect stimulation of dopamine receptors increases locomotor activity in mice. Determining the role played by D-1 and D-2 dopamine receptors in the mediation of this activity can be difficult due to the wide variety of experimental paradigms used to investigate these phenomena. Objectives: This study set out to compare the role of selective antagonism of dopamine D-1 and D-2 receptors on the hyperactivity induced by a range of stimulants. Methods: Mice were habituated to perspex locomotor activity boxes (30x30x30 cm) and activity was measured via photobeam interrupts. Results: Haloperidol and clozapine both reduced the hyperactivity induced by MK-801. Haloperidol did so only at a dose that also decreased spontaneous activity (0.1 mg/kg), whereas clozapine reduced MK-801-induced hyperactivity at a dose that bad no effect on spontaneous activity (1.25 mg/kg). The D-1 antagonist SCH23390 (0.01 mg/kg) reduced hyperlocomotion induced by amphetamine (2.5 mg/kg), cocaine (10 mg/kg) and C-APB (1.0 mg/kg) at doses that did not consistently alter spontaneous activity, whereas the selective D-2 antagonist raclopride only attenuated the hyperlocomotion induced by amphetamine, cocaine and C-APB at doses in excess of the minimum dose required to attenuate spontaneous locomotor activity significantly. The latency to peak levels of hyperlocomotion induced by MK-801 (0.3 mg/kg) was delayed by SCH23390 (0.1 mg/kg) but peak levels of activity were not reduced. Conclusions: The results of the present study suggest that selective blockade of D-1 receptors suppresses amphetamine and cocaine-induced hyperactivity in mice but not MK-80-induced locomotor activity.