NMDA receptor-mediated motor cortex plasticity after 20 Hz transcranial alternating current stimulation.

Wischnewski, M., M. Engelhardt, M.A. Salehinejad, D.J.L.G. Schutter, M.-F. Kuo, & M.A. Nitsche. 2019. NMDA receptor-mediated motor cortex plasticity after 20 Hz transcranial alternating current stimulation. Cerebral Cortex, 29 (7): 2924-31.
 
Summary: The authors conducted a double-blind, placebo-controlled, crossover design study of the effects of an NMDAR antagonist, dextromethorphan (DMO) on the tACS-induced changes in motor cortex excitability. Single- and paired-pulse (SICI & ICF) TMS was used to measure changes in MEP amplitudes. High-density tACS (4x1 montage, Center: C3; Surround: T7, F3, Cz, P3) was applied to the motor cortex in at 20 Hz. Normalized MEP amplitudes increased after 20 Hz tACS during the course of the placebo visit as expected, but not for the DMO visit, suggesting that NMDARs mediate the aftereffects observed in beta tACS protocols. However, neither placebo nor DMO impacted the excitability profile of ppTMS. 
 
Pros: 

• High-density tACS emphasizing regional focality
• Strong study design: placebo controlled, crossover. 
• Excluded subjects whose motor hotspot was not in the field of stimulation.

 
Open Questions:

• Will these findings hold up to an active sham condition?
• Are the findings specific to NMDARs?
• Would other neurotransmitter systems (e.g., GABA) demonstrate differential effects for ppTMS? 

 
Contributed by: Christopher Walker, PhD