Different effects of dopamine antagonists on spontaneous and NMDA-induced motor activity in mice

The spontaneous motor activity of mice exposed to a new environment is characterized by an initial hyperactivity (exploratory period) followed by low levels of motor activity (habituation period). High doses of the dopamine D-1 receptor antagonist SCH 23390 (1 mg/kg SC) and the dopamine D-2 receptor antagonist raclopride (1 mg/kg SC) partially decreased motor activity during the exploratory period and did not modify motor activity during the habituation period or after reserpinization (5 mg/kg SC 20 h before motor activity recording). The systemic administration of a subconvulsant dose of N-methyl-D-aspartate (NMDA) (75 mg/kg IP) decreased motor activity during the exploratory period and increased motor activity during the habituation period. Both SCH 23390 and raclopride partially counteracted the Nh IDA-induced motor activation. Neither SCH 23390 nor raclopride counteracted the NMDA-induced motor activation in reserpinized mice. On the contrary, raclopride was found to potentiate the NMDA-induced motor activation in reserpinized animals. The present results suggest the existence of dopamine-dependent and dopamine-independent mechanisms involved in the motor activating effects of NMDA. Copyright (C) 1997 Elsevier Science Inc.