Effects of clozapine and haloperidol on baseline levels of vacuous jaw movements in aged rats.

Some studies have indicated that aged rats have elevated basal levels of vacuous jaw movements and these vacuous jaw movements are exacerbated by classic neuroleptic drugs like haloperidol, but the effects of the atypical antipsychotic clozapine on vacuous jaw movements in aged rats has not previously been studied. Aged rats were administered daily intraperitoneal injections of either haloperidol (0.04, 0.1 or 0.4 mg/kg), clozapine (0.4, 1.0, 4.0 mg/kg) or 0.3% tartaric acid vehicle for 22 days. On days 1, 8, 15 and 22 these rats were placed in an observation tube and vacuous jaw movements were recorded by two trained observers. Vacuous jaw movements were present in the aged rats receiving vehicle. Haloperidol produced a dose-dependent increase in vacuous jaw movements while clozapine produced a dose-dependent attenuation of vacuous jaw movements, relative to the vehicle-treated rats. These results indicate that screening for vacuous jaw movements may provide a useful behavioral assay for atypical antipsychotic drugs which do not produce extrapyramidal side effects and that clozapine's resistance to these side effects may extend to populations of elderly human patients.