Low platelet MAO activity in chronic schizophrenics: a long-term effect of neuroleptic treatment?

Platelet MAO activity was determined, using 14C-tryptamine and 14C-beta-phenylethylamine as substrates, in two groups of schizophrenic patients and a normal control population. The first patient group consisted of 75 schizophrenics who had been medication-free for 2 weeks and had not been exposed to neuroleptic drugs for at least 2 months before the off-drug period. The second patient group comprised 55 schizophrenics who were on treatment with haloperidol for at least 2 months. The enzyme activity was found to be significantly decreased in both drug-free (using only tryptamine as substrate, P less than 0.05) and haloperidol-treated (P less than 0.001) chronic schizophrenics as compared with normal controls, and to be significantly lower (P less than 0.05) in haloperidol-treated than in off-drug chronic schizophrenics. An in vitro study confirmed the lowering effect of haloperidol on MAO activity. It is suggested that low platelet MAO values observed in chronic schizophrenics might be in part an effect of neuroleptic treatment.