Dopamine-dependent changes of cortical excitability induced by transcranial static magnetic field stimulation in Parkinson's disease

Abstract

Transcranial static magnetic field stimulation (tSMS) is a recent low-cost non-invasive brain stimulation technique that decreases cortical excitability in healthy subjects. The objective of the present study was to test the ability of tSMS to modulate cortical excitability in patients with Parkinson's disease. We performed a randomized double-blind sham-controlled cross-over study to assess cortical excitability before and immediately after tSMS (or sham) applied for 10 min to the more affected motor cortex of patients with Parkinson's disease. Cortical excitability was quantified by the amplitude of motor evoked potentials (MEPs) elicited by single-pulse transcranial magnetic stimulation (TMS). tSMS significantly decreased MEP amplitudes in patients OFF medication (after overnight withdrawal of dopaminergic drugs), but not ON medication (after an acute dose of levodopa). The between-patients variability of tSMS-induced changes was significantly greater ON medication. The variability ON medication could be partly explained by disease progression, i.e. the more advanced the patient, the more likely it was to observe a switch from inhibitory tSMS plasticity OFF medication to paradoxical facilitatory plasticity ON medication. These results suggest that tSMS induces dopamine-dependent changes of cortical excitability in patients with Parkinson's disease.

Figure 1

Effects of tSMS on cortical excitability in Parkinson’s disease. (A) Experimental protocol. Patients repeated the protocol twice, at least one week apart, exchanging the tSMS intervention (sham vs real). (B) Average MEP traces for a single patient, before (baseline) and immediately after (post) real and sham tSMS, OFF and ON medication. (C) Average cortical excitability changes, as measured by MEP amplitude (normalized to group-averaged baseline values for visualization purposes), induced by real and sham tSMS in patients OFF medication (blue) and ON medication (red). Error bars are SEM. **p < 0.01, *p < 0.05 (Dunnett) (D) Scatter plot of excitability changes induced by real tSMS (100*[0–6 min]/baseline-1, y-axis) vs baseline (x-axis) in patients OFF (blue) and ON (red) medication. (E) Scatter plot of relative increment of tSMS plasticity after levodopa (y-axis) vs UPDRS III OFF medication immediately before levodopa intake (x-axis). The best fitting straight line is plotted in gray. tSMS significantly decreased cortical excitability in patients OFF medication, but not ON medication. The between-patients variability of tSMS-induced changes was significantly greater ON compared to OFF medication. The variability ON medication could be partly explained by disease progression, i.e. the more advanced the patient, the more likely it was to observe a switch from inhibitory tSMS plasticity OFF medication to paradoxical facilitatory plasticity ON medication.