نبذة مختصرة : Background Electroconvulsive therapy (ECT) constitutes one of the most effective antidepressant treatment strategies in major depression (MDD). Despite its common use and uncontested efficacy, its mechanism of action is still insufficiently understood. Previously, we showed that ECT is accompanied by a global decrease of serotonin-1A receptors in MDD; however, further studies to investigate the involvement of the serotonergic system in the mechanism of action of ECT are warranted. The monoamine oxidase A (MAO-A) represents an important target for antidepressant treatments and was found to be increased in MDD. Here, we investigated whether ECT impacts on MAO-A levels in treatment-resistant patients (TRD). Methods 16 TRD patients (12 female, age 45.94 ± 9.68 years, HAMD 25.12 ± 3.16) with unipolar depression according to DSM-IV were scanned twice before (PET1 and PET2, to assess test-retest variability under constant psychopharmacotherapy) and once after (PET3) completing a minimum of eight unilateral ECT sessions using positron emission tomography and the radioligand [¹¹C]harmine to assess cerebral MAO-A distribution volumes (VT). Age- and sex-matched healthy subjects (HC) were measured once. Results Response rate to ECT was 87.5%. MAO-A VT was found to be significantly reduced after ECT in TRD patients (−3.8%) when assessed in 27 a priori defined ROIs (p < 0.001). Test-retest variability between PET1 and PET2 was 3.1%. MAO-A VT did not significantly differ between TRD patients and HC at baseline. Conclusions The small effect size of the significant reduction of MAO-A VT after ECT in the range of test-retest variability does not support the hypothesis of a clinically relevant mechanism of action of ECT based on MAO-A. Furthermore, in contrast to studies reporting elevated MAO-A VT in unmedicated depressed patients, MAO-A levels were found to be similar in TRD patients and HC which might be attributed to the continuous antidepressant pharmacotherapy in the present sample.
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