Neuroendocrine tests during treatment with neuroleptic drugs I. Plasma prolactin response to haloperidol challenge

The plasma prolactin (PRL) response to haloperidol 2 or 4 mg i.m. was studied in 18 schizophrenic men during their routine treatment with neuroleptic drugs. A substantial rise of the PRL level above the treatment baseline occurred in all but four of the 20 tests showing that the PRL elevation induced by treatment was not maximal. The challenge was ineffective only in patients receiving very high daily doses of medication. The increment was inversely correlated to the daily dose of medication but unrelated to plasma haloperidol concentrations during the test. Chronic schizophrenics who were receiving long term treatment and had low basal PRL levels did not show tolerance to the prolactin stimulating effect of haloperidol. That prolactin rose during the test in patients who had improved during their current treatment indicates that the degree of dopamine receptor blockade required for therapeutic effects is below that which produces a maximal PRL response.