Discriminative-stimulus effects of clozapine in squirrel monkeys: comparison with conventional and novel antipsychotic drugs.

The effects of conventional and novel atypical antipsychotic drugs were compared to clozapine in squirrel monkeys that discriminated I.M. injections of clozapine (1.0 mg/kg) from saline in a two-lever drug discrimination procedure. Clozapine (0.03-3.0 mg/kg) produced dose-related increases in responding on the clozapine-associated lever with full substitution at the training dose in all monkeys. Dose-related increases in responding on the clozapine-associated lever and full substitution also were observed with structural analogues of clozapine including perlapine and fluperlapine (0.1-3.0 mg/kg), seroquel (0.1-5.6 mg/kg), and JL 5, JL 8 and JL 18 (0.1-3.0 mg/kg). Other clozapine analogues, including olanzapine, amoxapine, loxapine and clothiapine, and conventional antipsychotic drugs, including phenothiazines such as chlorpromazine and thioridazine, produced some clozapine-associated responding up to the highest doses that could be studied, but did not substitute for clozapine. Olanzapine did produce full clozapine-lever responding following pretreatment with the dopamine D2-receptor agonist (+)-PHNO (0.003-0.01 mg/kg). Putatively atypical antipsychotics that are structurally unrelated to clozapine including risperidone (0.003-0.1 mg/kg), sertindole (0.03-1.0 mg/kg) and remoxipride (0.1-5.6 mg/kg) similarly failed to substitute for clozapine up to the highest doses. The present results indicate that some, but not all, structural analogs of clozapine have clozapine-like discriminative-stimulus effects and that novel antipsychotic drugs which purportedly have clozapine-like clinical efficacy may not produce its interoceptive stimulus effects.