Online Effects of Transcranial Direct Current Stimulation in Real Time on Human Prefrontal and Striatal Metabolites

Background Studies have reported that transcranial direct current stimulation (tDCS) can modulate human behaviors, symptoms, and neural activity; however, the neural effects during stimulation are unknown. Most studies compared the effects of tDCS before and after stimulation. The objective of our study was to measure the neurobiological effect of a single tDCS dose during stimulation. Methods We conducted an online and offline protocol combining tDCS and magnetic resonance spectroscopy (MRS) in 17 healthy participants. We applied anodal tDCS over the left dorsolateral prefrontal cortex (DLPFC) and cathodal tDCS over the right DLPFC for 30 minutes, one of the most common montages used with tDCS. We collected MRS measurements in the left DLPFC and left striatum during tDCS and an additional MRS measurement in the left DLPFC immediately after the end of stimulation. Results During stimulation, active tDCS, as compared with sham tDCS, elevated prefrontal N-acetylaspartate and striatal glutamate + glutamine but did not induce significant differences in prefrontal or striatal gamma-aminobutyric acid level. Immediately after stimulation, active tDCS, as compared with sham tDCS, did not significantly induce differences in glutamate + glutamine, N-acetylaspartate, or gamma-aminobutyric acid levels in the left DLPFC. Conclusions These observations indicate that tDCS over the DLPFC has fast excitatory effects, acting on prefrontal and striatal transmissions, and these effects are short lived. One may postulate that repeated sessions of tDCS might induce similar longer lasting effects of elevated prefrontal N-acetylaspartate and striatal glutamate + glutamine levels, which may contribute to its behavioral and clinical effects.