Antidepressant and Related Neurobiological and Neurophysiological Effects of Add-On Transcranial Direct Current Stimulation in Major Depressive Disorder with Residual Symptoms: A Randomized, Double-Blind Clinical Trial Protocol

Major depressive disorder (MDD) is a prevalent and disabling condition. Transcranial direct current stimulation (tDCS) may improve symptoms by modulating neuroplastic and inflammatory mechanisms. This randomized, double-blind, placebo-controlled trial will recruit adult outpatients with MDD showing residual symptoms despite at least four weeks of stable SSRI treatment. Participants will be randomized to active or sham add-on tDCS while continuing their antidepressant regimen. The intervention will consist of 15 sessions over 3 weeks, targeting the left dorsolateral prefrontal cortex (anode F3, cathode F4) at 2 mA for 30 min per session. The primary outcome is the reduction of depressive symptoms measured by the Hamilton Depression Rating Scale-17 (HDRS), with remission defined as HDRS-17 ≤ 7. Secondary outcomes include cognitive performance (attention, executive functioning, memory), serum biomarkers (BDNF, VEGF, NGF, NRG1, angiogenin, IGF1, IL-6, TNF-α), cortical excitability assessed by transcranial magnetic stimulation (motor threshold, silent period, intracortical inhibition/facilitation), and cerebral hemodynamics by transcranial Doppler sonography (blood flow velocity, pulsatility, resistivity). Assessments will occur at baseline, post-treatment, and 3- and 6-month follow-ups. This trial aims to evaluate the efficacy of adjunctive tDCS in MDD with residual symptoms and its biological correlates, bridging clinical improvement with electrophysiological and neurovascular mechanisms.