Abstract
Previous evidence indicates that transcranial direct stimulation (tDCS) is a neuromodulatory brain stimulation technique. Easy applicability, low side-effects and negligible costs facilitated its wide–spread application in efforts to modulate brain function, however neuronal mechanisms of tDCS are insufficiently understood. Hence, we investigated the immediate impact of tDCS on the brain’s glucose consumption in a continuous infusion protocol with the radioligand 2-[18F]fluoro-2-deoxy-D-glucose ([18F]FDG) and positron emission tomography (PET). This novel functional PET (fPET) method is capable to reliably detect area-specific and dynamic absolute glucose demand related to neuronal activity in a single molecular imaging session. Fifteen healthy subjects underwent tDCS at 0.5, 1 and 2 mA (mA) at the bilateral dorsolateral prefrontal cortex (dlPFC, cathodal right) for 10 min during functional [18F]FDG-PET lasting 70 min. Active stimulation compared to sham did not yield significant changes in glucose consumption at any tested stimulation intensity in this paradigm. Exploratory investigation of aftereffects provided hints for increased glucose consumption with a delay of 5 min at 1 mA in the right posterior temporal cortex. This is the first study investigating changes of glucose consumption in the brain during tDCS. The lack of immediately increased glucose consumption indicates that energy demanding processes in the brain such as glutamatergic signaling might not be immediately increased by tDCS. However, our results implicate the need of fPET investigations for medium-term and long-term effects.
Highlights
Transcranial direct current stimulation provides a simple and cost-effective neuromodulatory brain stimulation technique and is widely used in neuropsychological research for inducing changes in cortical excitability
We tested for post-stimulations effects of Transcranial direct current stimulation (tDCS) on regional cerebral metabolic rate of glucose (rCMRGlu) %SC by shifting the stimulation model term by 3 min and 5 min
Exploratory post-hoc t-tests yielded a significant difference of rCMRGlu %SC at 1 mA stimulation (t = 6.18, p = 0.03, FDR-corrected cluster level) in the right medial temporal cortex (MNI x, y, z = 56, −54, 20, cluster size = 116, see Fig. 2c)
Summary
Transcranial direct current stimulation (tDCS) provides a simple and cost-effective neuromodulatory brain stimulation technique and is widely used in neuropsychological research for inducing changes in cortical excitability. Clinical trials suggest efficacy in fibromyalgia, depression without drug resistance. For approved clinical usage, improvements of the existing method will be necessary whereby identification of a mechanism of action and its optimal utilization could be helpful. The neurophysiological effects of tDCS are attributed to changes in resting membrane potentials towards depolarization or hyperpolarization, whereby anodal tDCS is thought to increase excitability and cathodal tDCS should mediate decreases. The definite mechanism of action and – in the case of stimulation for antidepressant treatment over the dorsolateral prefrontal cortex (dlPFC) – the location of the elicited neurophysiological changes remains open.
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