1. It has been suggested that the high prevalence of
drug abuse in schizophrenics is related to chronic treatment with typical
neuroleptics and dopaminergic supersensitivity that develops as a consequence. Within this context, atypical
neuroleptics do not seem to induce this phenomenon. In the present study, we investigated the effects of acute administration or withdrawal from long-term administration of
haloperidol and/or
ziprasidone on
morphine-induced open-field behaviour in mice. 2. In the first experiment, mice were given a single injection of
haloperidol (1 mg/kg, i.p.) or several doses of
ziprasidone (2, 4 or 6 mg/kg, i.p.) and motor activity was quantified by the open-field test. The aim of the second experiment was to verify the effects of an acute injection of
haloperidol (1 mg/kg) or
ziprasidone (6 mg/kg) on 20 mg/kg
morphine-induced behaviours in the open-field test. In the third experiment, mice were treated with 1 mg/kg
haloperidol and/or 2, 4 or 6 mg/kg
ziprasidone for 20 days. Seventy-two hours after the last injection, mice were injected with 20 mg/kg, i.p.,
morphine and then subjected to the open-field test. Acute
haloperidol or
ziprasidone decreased spontaneous general activity and abolished
morphine-induced locomotor stimulation. 3. Withdrawal from
haloperidol or
ziprasidone did not modify
morphine-elicited behaviours in the open-field test. The results suggest that withdrawal from
neuroleptic treatments does not contribute to the acute effect of
morphine in schizophrenic patients.