Occlusion of synaptic plasticity by gating

Introduction: Intracortical Inhibition is discussed as gating mechanism for the induction of synaptic plasticity. This study investigates if the occlusion of synaptic plasticity induced by paired associative stimulation (PAS) by prior very low frequency repetitive transcranial magnetic stimulation (rTMS) is mediated by enhanced intracortical inhibition

Methods: We performed rTMS on 10 healthy volunteers (mean age 25±2 years). RTMS consisted of 250 stimuli at a frequency of 0.1Hz. Before and after this intervention we measured amplitudes of motor evoked potentials (MEP), resting motor threshold (MT), intracortical facilitation (ICF), cortical silent period (CSP) duration as well as short-interval (SICI) and long-interval intracortical inhibition (LICI). In addition, we performed PAS after 0.1Hz rTMS with 10 healthy volunteers (mean age 25±3 years). PAS consisted of 200 pairs with 25ms inter-stimulus interval at 0.25Hz.

Results: MEP amplitudes, MT, ICF and CSP duration remained unchanged after rTMS. SICI and LICI, however, were significantly enhanced after 250 stimuli of 0.1Hz rTMS (SICI at 3ms interstimulus interval (ISI): 55.5% to 68.2%, p=0.037, LICI at 100ms ISI: 61.9% to 88.8%, p=0.014). After conditioning with 0.1Hz rTMS, PAS no longer significantly increased MEP amplitudes.

Conclusion: 0.1Hz rTMS led to a significant enhancement of intracortical inhibition and to an occlusion of synaptic plasticity induced by PAS. This study shows for the first time a connection between a stimulation induced increase in intracortical inhibition and an occlusion of stimulation induced synaptic plasticity and thus supports the previously described concept of gating (Ziemann & Siebner, 2008).

Literature: Ziemann, U. & Siebner, H.R. (2008) Modifying motor learning through gating and homeostatic metaplasticity. Brain Stimulation, 1, 60–66.