Comparison of the effects of clozapine, risperidone, and olanzapine on ketamine-induced alterations in regional brain metabolism.

The ability of subanesthetic doses of N-methyl-D-aspartate (NMDA) antagonists to induce positive, negative, and cognitive schizophrenia-like symptoms suggests that reduced NMDA receptor function may contribute to the pathophysiology of schizophrenia. An increasing body of evidence indicates that antipsychotic drugs, especially those with "atypical" properties, can antagonize the effects of NMDA antagonists in a variety of experimental paradigms. We demonstrated previously that clozapine, the prototype of atypical antipsychotics, but not haloperidol, the typical antipsychotic, blocked ketamine-induced alterations in brain metabolism. In this study, effects of clozapine were compared with two of the newer atypical antipsychotic drugs, risperidone and olanzapine, on ketamine-induced alterations in regional [(14)C]2-deoxyglucose (2-DG) uptake. A subanesthetic dose of ketamine (25 mg/kg) induced robust increases in 2-DG uptake in limbic cortical regions, hippocampal formation, nucleus accumbens, and basolateral amygdala. Pretreatment of rats with risperidone (0.3 mg/kg) before ketamine administration did not alter the effects of ketamine. These data suggest that novel pharmacological properties may contribute to the effects of clozapine in this model, in addition to the well characterized actions at D(2) and 5HT(2A) receptors. In contrast to the results with risperidone, olanzapine blocked ketamine-induced increases in 2-DG uptake. However, a higher dose of olanzapine (10 mg/kg) was required to completely block the effects of ketamine than would be expected if D(2) and 5HT(2) receptor blocking properties of the drug were solely responsible for its action. The results suggest that the ketamine challenge 2-DG paradigm may be a useful model to identify antipsychotic drugs with atypical characteristics and to explore mechanisms of atypical antipsychotic action.