Olanzapine and JL13 induce cross-tolerance to the clozapine discriminative stimulus in rats.

We have previously shown that chronic treatment with clozapine induces tolerance to the clozapine discriminative stimulus in rats. The studies reported here extended this work to assess whether chronic treatment with the clozapine-like antipsychotics olanzapine and 5-(4-methylpiperazin-1-yl)-8-chloro-pyrido[2,3-b][1,5] benzoxazepine fumarate (JL13) induced cross-tolerance to clozapine. Two groups of rats were trained to discriminate clozapine (5 mg/kg, intraperitoneal). Training was suspended and the rats were treated with either olanzapine or JL13 at high doses (5 and 20 mg/kg, respectively). These doses were administered twice daily. The clozapine generalization curve was computed three times - before chronic drug treatment, after 10 days of chronic treatment, and after 16 drug-free days. Both olanzapine and JL13 induced cross-tolerance to the clozapine stimulus, shown by significant 3.4 and 3.9 fold parallel shifts to the right in the clozapine generalization curves. Cross-tolerance was lost spontaneously during the drug-free days after treatment as clozapine sensitivity returned to baseline. We interpret these findings as indicative of the development of pharmacodynamic cross-tolerance to clozapine. Possible neuroadaptive mechanisms involved in such cross-tolerance are discussed. The paradigm outlined here allows refinement of antipsychotic drug discrimination assays to identify common chronic effects of such drugs.