Different effects of a single and repeated administration of clozapine on phencyclidine-induced hyperlocomotion and glutamate releases in the rat medial prefrontal cortex at short- and long-term withdrawal from this antipsychotic.

Clozapine is a prototype of atypical antipsychotics that has a profile not only to block D(2)/5-HT(2A) receptors but also to enhance N-methyl-D-aspartate (NMDA) receptor-mediated glutamatergic neurotransmission. This study hypothesized different effects between a single and repeated administration of clozapine on NMDA receptor-mediated neurotransmission, and examined effects of these treatments of clozapine on a non-competitive NMDA receptor antagonist, phencyclidine (PCP)-induced hyperlocomotion and acute increases in glutamate levels in the medial prefrontal cortex (mPFC), after short- and long-term withdrawal from this antipsychotic. Locomotor activity and extracellular levels of glutamate were measured by an infrared sensor and in vivo microdialysis respectively. A single administration of clozapine attenuated PCP-induced hyperlocomotion and blocked PCP-induced increases in glutamate levels in the mPFC at 48 hours, but not 11 days after the injection of clozapine. Repeated administration of clozapine attenuated PCP-induced hyperlocomotion not only at 48 hours, but also 11 days after the last injection of clozapine, with blocking PCP-induced increases in glutamate levels in the mPFC. Both a single and repeated administration of clozapine had no effect on methamphetamine (METH)-induced hyperlocomotion at 48 hours or 11 days after the treatment of clozapine. Considering fast dissociation of clozapine from dopamine D(2) receptors and no effect of a single or repeated administration of clozapine on METH-induced hyperlocomotion, the attenuated PCP-induced hyperlocomotion by a single and repeated clozapine treatments cannot be explained by clozapine occupancy of dopamine D(2) receptors. Repeated but not a single administration of clozapine inhibited a 5-HT(2A/2C) agonist, DOI-induced increases in the mPFC 11 days after the last injection of clozapine. These findings suggest that subchronically treated clozapine-induced long-lasting downregulation of 5-HT(2A) receptors may block the enhanced PCP-induced neurochemical and behavioral changes.