Tms protocols for assessment of GABA_B function integrity

Transcranial magnetic stimulation (TMS) is a safe and easy to use technique for evaluation of cortico-cortical circuits. This chapter will discuss the TMS protocols for assessment of the integrity of the TMS induced inhibition mediated by gamma-Aminobutyric acid (GABA)B receptors. This includes cortical silent period (SP), long interval intracortical inhibition (LICI) and interhemispheric inhibition (IHI). SP is defined as the transient disruption of electrical muscle activity following a suprathreshold single-pulse TMS-induced motor evoked potential (MEP). To assess LICI and IHI, paired-pulse TMS (ppTMS) (inter-stimulus interval (ISI) between 50-250 ms) and dual-site TMS (ISI between 6-50 ms) are used respectively. In both techniques two identical suprathreshold pulses are used. For LICI both stimuli are applied over primary motor cortex (M1) contralateral to the target muscle through one coil, however, in IHI the first stimulus is applied over conditioned M1 and the second stimulus applied over the target M1. Following a brief outline of each technique, the chapter will present a detailed step-by-step description of the procedure for assessment of SP, LICI and IHI before and after therapeutic interventions and also follow-up assessments. The chapter will then present the physiological basis and the applications for each technique.