Combinations of clozapine and phencyclidine: effects on drug discrimination and behavioral inhibition in rats.

Phencyclidine (PCP) produces psychotomimetic effects in humans that resemble schizophrenia symptoms. In an effort to screen compounds for antipsychotic activity, preclinical researchers have investigated whether these compounds block PCP-induced behaviors in animals. In the present study, the atypical antipsychotic clozapine was tested in combination with an active dose of PCP in two-lever drug discrimination and mixed signalled-unsignalled differential-reinforcement-of-low-rates (DRL) procedures. PCP produced distinctive effects in each task: it substituted for the training dose in PCP discrimination and it increased the number of responses with short (<3 s) interresponse times as well as increasing overall response rates in the DRL schedule. Acute dosing with clozapine failed to alter the behavioral effects of PCP in either procedure even when tested up to doses that produced pharmacological effects alone. These results suggest that acute dosing with clozapine would not affect behaviors most closely associated with PCP intoxication. Further, they bring into question the utility of using PCP combination procedures in animals to screen for antipsychotic potential. Since chronic dosing is required for therapeutic efficacy of antipsychotics, future studies should focus on investigation of chronic dosing effects of these drugs in combination with PCP.