Effects of neuroleptics displaying antidepressant activity on behavior of rats in the forced swimming test

Levomepromazine, thioridazine and cis-chlorprothixene, neuroleptics with antidepressant activity, trans-chlorprothixene, the therapeutically inactive isomer of chlorprothixene, clozapine, an atypical neuroleptic, and imipramine, a classical antidepressant, were studied in the forced swimming test in rats after single or chronic administration. Levomepromazine (1.5 mg/kg), clozapine (2.5 and 5.0 mg/kg) and imipramine (10 mg/kg) after single administration, 1 hr before the test, shortened the period of the immobility. After chronic administration only imipramine (10 mg/kg orally, twice daily, for 10 days) diminished the immobility. Levomepromazine, thioridazine, cis-chlorprothixene and trans-chlorprothixene (1.5 mg, orally, twice daily, for 10 days), 15-18 hr after the last dose did not influence the immobility, although the behavioral parameters in the open field test were not depressed. It is concluded that the forced swimming test is not a suitable pharmacological model for revealing antidepressant activities of certain neuroleptics that are useful in treating certain forms of human depression.