Characterization of the serotonin transporter in lymphocytes and platelets of schizophrenia patients treated with atypical or typical antipsychotics compared to healthy individuals

A rapidly growing body of data suggests that abnormalities in serotonergic function might be involved in the pathophysiology of schizophrenia and that serotonergic mechanisms play a role in the therapeutic effects of antipsychotics. The activity of the serotonin transporter (5-HTT), as determined by [³H]5-HT uptake to blood lymphocytes, was measured in 38 medicated schizophrenia patients (15 of them treated with typical antipsychotics and 23 treated with atypical antipsychotics) and 15 healthy control subjects. In addition, the pharmacodynamic characteristics of platelet 5-HTT were assessed by [³H]citalopram binding. There were no significant differences in the density (B_max) of platelet [³H]citalopram binding sites between the three groups. Similarly, the dissociation constant (K_d) values were indistinguishable. There were no significant differences in the maximal uptake velocity (V_max) of [³H]5-HT to fresh lymphocytes between the three groups. The affinity (K_m) values of 5-HT to the 5-HTT were significantly higher in schizophrenia patients treated with typical antipsychotics compared with control subjects. The K_m values in schizophrenia patients treated with atypical antipsychotics were significantly lower compared with those observed in the group of schizophrenia patients treated with typical antipsychotics; however, they were comparable to values in the control group. The high values of K_m associated with typical antipsychotic treatment may be relevant to the high risk of developing extrapyramidal side effects (EPS). The role of the various components of the serotonergic system in the etiopathology of schizophrenia and the mechanisms by which antipsychotics achieve their therapeutic effects need to be further evaluated.