The brain is a master in energy regulation. Despite its enormous computational power, it uses the amount of energy comparable to that of a light bulb. Therefore, it is very interesting and important to get to know closely the mechanisms governing energy regulation in the brain. Transcranial direct current stimulation (tDCS) is a non-invasive stimulation technique that allows for the modulation of cortical excitability in humans. Anodal transcranial direct current stimulation over the primary motor cortex (M1) has been reported to increase the firing rates of the neurons and modulate inhibitory neurotransmitter gamma-aminobutyric acid (GABA) concentration, and also leads to increased consumption of high-energy phosphates such as adenosine triphosphate (ATP), Phosphocreatine (PCr), and inorganic phosphates (Pi). To date, little is known about the nature and duration of these anodal tDCS-induced effects. We have investigated long-term effects of anodal tDCS over M1 on GABA, ATP and PCr in humans. Repeated magnetic resonance spectroscopy (MRS) was employed to measure relative GABA, ATP and PCr concentration in M1 for approximately 64-67 minutes. Two separate studies were conducted, and in both studies, 1 mA anodal/sham tDCS was applied with the active electrode over left M1 and the reference electrode over the right supra-orbital region. In the first study, 32 subjects were recruited and anodal/sham tDCS was applied for 10 minutes. Pre and post-tDCS MRS scans were performed to acquire ONLY the GABA signal using proton MRS at 3T Siemens PRISMA Scanner. GABA signals showed no change over time in the sham tDCS group, whereas anodal tDCS resulted in a significant early decrease within 25 minutes after tDCS and then a significant late decrease after 66 minutes, which continued until the last test measurement. In the second study, 44 subjects were recruited, and anodal/sham tDCS was applied for 20 minutes. Pre and post-tDCS MRS scans were performed to acquire GABA signal using proton MRS and ATP, PCr and Pi signals using phosphorus MRS at 3T Siemens PRISMA scanner. The GABA concentration increased significantly following anodal tDCS and remained increased until the end of the measurement. Similarly, both ATP/Pi and PCr/Pi ratios increased after anodal tDCS, but the increase in the energy phosphate was non-significant following anodal tDCS. This study shows the feasibility of measuring not just long-term GABA changes but also energy phosphate changes in M1 following anodal tDCS using proton and phosphorus MRS at 3T MR Scanner. The combined approach of proton and phosphorus MRS, as shown in our study, might help us to better understand the neurochemical mechanism underlying energy regulation, neuroplasticity, and the corresponding energy metabolism in healthy and diseased brains.
Harshal Patel (Thu,) studied this question.