Occlusion of bidirectional plasticity by preceding low-frequency stimulation in the human motor cortex

Abstract

Low-frequency stimulation, which does not induce long-term potentiation (LTP) or long-term potentiation (LTD) by itself, suppresses consecutive LTP or LTD induction in vitro. We tested whether a similar interaction occurs in the human motor cortex. LTP- or LTD-like plasticity was induced using paired associative stimulation (PAS) with 25 and 10 ms interstimulus interval and conditioned by suprathreshold repetitive transcranial magnetic stimulation (rTMS) at a frequency of 0.1Hz. RTMS completely abolished the significant increase of motor-evoked potential (MEP) amplitudes after PAS(25 ms) (PAS(25 ms) only: 1.05+/-0.14 to 1.76+/-0.66 mV, p=0.001; rTMS+PAS(25 ms): 1.08+/-0.18 to 1.02+/-0.44 mV, n.s.) and also abolished the significant decrease of MEP amplitudes after PAS(10 ms) (PAS(10 ms) only: 1.00+/-0.14 to 0.73+/-0.32 mV; rTMS+PAS(10 ms): 1.15+/-0.35 to 1.25+/-0.43 mV, p=0.006). RTMS alone did not significantly alter MEP amplitudes but increased SICI and LICI. Low frequency stimulation increases intracortical inhibition and occludes LTP- and LTD-like plasticity in the human motor cortex. This finding supports the concept that metaplasticity in the human motor cortex follows similar rules as metaplasticity in in vitro experiments.