Transcranial Direct Current Stimulation in Major Depression

Transcranial direct current stimulation (tDCS), a noninvasive neuromodulation technique, applied to the left dorsolateral prefrontal cortex (DLPFC) can reduce depressive symptoms and improve cognitive control in major depressive disorder (MDD). Such findings suggest modulation of top down prefrontal-limbic circuits, which are functionally distinct from ventro-limbic networks and include reciprocally connected DLPFC and dorsomedial anterior cingulate cortex (dACC). However, substantial variation in tDCS response is observed in MDD. This may be due to imprecise stimulation protocols and suboptimal engagement of the neural circuits mediating antidepressant response. Methods that optimize electrode placement and account for individual variation in anatomy and that map current flow directly in the brain may inform the mechanisms and potential clinical utility of tDCS. A new tDCS technique, high definition (HD) tDCS, offers more focal stimulation than conventional tDCS (C-tDCS). The degree to which C-tDCS or HD-tDCS engage dorsal prefrontal-limbic neural circuits is unknown, yet is vital for understanding, confirming and subsequently improving possible therapeutic effects. Innovative MRI techniques that are able to map tDCS currents in vivo and that track changes in regional cerebral blood flow occurring with tDCS over time can provide direct evidence of neural effects. Based on a) theoretical modeling of tDCS current flow, b) studies showing hypo-metabolism, decreased CBF or activity in dorsal prefrontal-limbic networks, c) modulation of these regions with treatment, and, c) our prior results showing significant relationships in between change in dACC rCBF and clinical response to electroconvulsive therapy (ECT), an established brain stimulation treatment, we will test for the tDCS engagement and modulation of the DLPFC and dACC using tDCS current mapping performed in vivo and perfusion MRI. MRI-guided neuronavigation will be used to optimize and standardize electrode placement for DLPFC stimulation. In this trial we will test for the target engagement of the DLPFC and dACC by comparing C-tDCS, HD-tDCS and sham tDCS applied to the left DLPFC in patients with moderate to severe MDD before and after they complete 12 daily 20-minute sessions of C-tDCS, HD-tDCS or Sham tDCS (n=20 randomized to each group). In-vivo electric current mapping performed at different current intensities (0-2 mA) for 20-30 minutes, and change in regional cerebral blood flow (rCBF) measured before and after a 12-day tDCS trial will determine acute and longer-term modulation of DLPFC and dACC circuitry for each tDCS modality respectively.