Dopamine receptor responsivity in schizophrenic patients before and after switch from haloperidol to risperidone.

The increases that occur in plasma prolactin (PRL) levels after i.m. administration of 5 mg of haloperidol (the HAL test) provide information about the responsivity of D2 dopamine receptors in the hypothalamus-hypophysis, and may be a measure of their occupancy during the neuroleptic treatment of schizophrenic patients. We studied these responses during treatment with haloperidol (doses 7.5-60 mg daily, mean = 20.6) in 12 male schizophrenic patients who did not have a satisfactory therapeutic response to the drug, and the test was repeated 6 weeks later, after the patients were switched to therapy with the atypical neuroleptic risperidone (8-16 mg daily, mean = 11.7). After the institution of risperidone treatment, the total score on the Brief Psychiatric Rating Scale (BPRS) fell by 38% (from a mean score of 47.2 to 29.3). BPRS subscale scores for positive, negative, and general symptoms were reduced by 38, 35, and 40%, respectively. Moderate PRL responses to the HAL test were found during haloperidol treatment and no responses at all during treatment with risperidone. Baseline PRL increased significantly from a mean of 35.0 (S.D. = 16.0) to a mean of 55.7 ng/ml plasma (S.D. = 19.6). This high potency of risperidone to increase PRL levels cannot be explained by the serotonergic blocking activity of the drug, and seems not to be restricted to its D2 receptor blocking capacity.