High-Frequency Repetitive Transcranial Magnetic Stimulation Regulates Astrocyte Activation by Modulating the Endocannabinoid System in Parkinson's Disease

Reactive astrogliosis and the over-production of proinflammatory factors are key pathogenetic processes in Parkinson's disease (PD). Repetitive transcranial magnetic stimulation (rTMS), a promising noninvasive technique in treating PD, has been shown to alleviate neuroinflammation. However, high-frequency (HF) and low-frequency (LF) rTMS, which one produces better therapeutic and anti-inflammatory effects, and the underlying mechanism have yet to be determined. The efficacies of HF, LF, and sham rTMS on the survival of dopaminergic (DA) neurons, improvement of motor function, and downregulation of proinflammatory factors were compared in 6-hydroxydopamine (6-OHDA) rat model. Then we investigated the role of endocannabinoid (eCB) system in the inhibition of astrocyte activation between HF vs LF rTMS. The results showed that HF rTMS daily for 4 weeks produced stronger anti-inflammatory and neuroprotective effects. ECB receptor 2 (CB2R) but not receptor 1 (CB1R) expressions were substantially elevated in the GFAP-positive reactive astrocytes of the rat brains with 6-OHDA or LPS insults. Increased anandamide (AEA) and 2-arachidonoylglycerol (2-AG) were also observed. Interestingly, the elevated CB2R, AEA and 2-AG, and the increased GFAP expression could be all significantly suppressed by HF rTMS, but not by LF rTMS. This effect was also confirmed in cell culture. Of note, selective agonism of CB2R was able to reverse HF rTMS-mediated activation of extracellular regulated kinase1/2 (Erk1/2) and suppression of GFAP expression, while selective antagonism of CB2R sustained these effects. This study indicates that the modulation of eCB/CB2R is a potential mechanism for the greater effectiveness of HF rTMS on the inhibition of astrogliosis.