Neuroplastic modulation of inhibitory motor cortical networks by spaced theta burst stimulation protocols

Background: Continuous theta burst stimulation (cTBS) suppresses the excitability of motor networks responsible for generating motor evoked potentials (MEPs), and may also modulates the excitability of inhibitory motor networks. However, its effects on intracortical inhibition are modest in comparison to the effects on MEPs. The repeated, spaced, application of cTBS protocols results in more MEP suppression than seen with a single cTBS protocol, but whether this approach is also effective at modulating intracortical inhibition has not been tested. Objective: To determine whether the paired application of cTBS effectively modulates the excitability of intracortical inhibitory motor networks. Methods: Single and paired-pulse transcranial magnetic stimulation (TMS) were used to assess resting motor threshold (RMT), MEP amplitude, short-interval intracortical inhibition (SICI), and long-interval intracortical inhibition (LICI) before and during two time periods (0-10 and 30-40 min) following application of either a single or paired cTBS protocols. Results: Both the single and paired cTBS conditions induced a significant reduction in both MEP amplitudes and the level of SICI. While paired cTBS produced a significantly greater MEP suppression than single cTBS, the effects on SICI were similar. Neither single nor paired cTBS had an effect on RMT or LICI. Conclusions: Although the repeated application of cTBS protocols may be effective for enhancing modulation of the MEP-generating excitatory motor networks, these findings suggest that this approach offers little advantage when targeting intracortical inhibitory networks.