Phencyclidine- and Dizocilpine-Induced Hyperlocomotion Are Differentially Mediated

Abstract

The dopamine (DA) D₂ agonist quinpirole and the D₂ receptor antagonists, haloperidol, raclopride, and remoxipride, were examined for their ability to block the locomotion induced by the noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonists phencyclidine (PCP) and dizocilpine, both given in equipotent doses. Quinpirole, given in a “DA D₂ autoreceptor selective” dose (0.01 mg/kg), failed to influence the motor stimulation by PCP. On the other hand, the locomotor response induced by dizocilpine was significantly reduced by quinpirole. The three DA receptor antagonists blocked dose dependently the motor stimulation produced by both the low (2 mg/kg) and the high dose (3 mg/kg) of PCP. Haloperidol and remoxipride also blocked dose dependently and fully the stimulation produced by the low dose (0.1 mg/kg) of dizocilpine, whereas raclopride partially reduced the effect. The motor stimulation produced by the high doses of dizocilpine (0.2 mg/kg) and PCP (3 mg/kg) was reduced by haloperidol and raclopride only in cataleptogenic doses. Remoxipride, in contrast, fully blocked the effects of both PCP (3 mg/kg) and dizocilpine (0.2 mg/kg) in noncataleptogenic doses. These data suggest that different mechanisms of action may account for the motor stimulatory effects of PCP and dizocilpine. At the presynaptic level, PCP and dizocilpine may differ in the way they act on “regulatory” NMDA receptors controlling neuronal activity in midbrain neurons, and at the postsynaptic level they may interact with subtypes of NMDA receptors differentially coupled to subpopulations of D₂ receptors.