P 26. Long lasting intracortical inhibition and facilitation in the human primary motor cortex

Abstract

Introduction Intracortical inhibitory processes in the primary motor cortex (M1) play an important role in both the preparation and execution of motor tasks. Long-interval intracortical inhibition (LICI), measured with paired-pulse transcranial magnetic stimulation (TMS), is generally thought to reflect a single, long-lasting inhibitory process mediated by GABA B receptor activity. Recently, however, preliminary evidence has emerged to suggest that independent LICI processes with different time courses exist Chu et al., 2008. Furthermore, there is one report of long-lasting facilitation in response to paired-pulse TMS; the original work of Valls-Sole and colleagues Valls-Sole et al., 1992 showed that the conditioned MEP was facilitated when paired stimuli of sub-or near-threshold intensities were delivered. Objectives Here, we were interested in Chu et al., 2008 investigating whether LICI measured with an inter-stimulus interval (ISI) of 100ms and LICI measured with an ISI of 150ms are independent processes, and Valls-Sole et al., 1992 characterising long-lasting facilitation evident with subthreshold CS intensities and a suprathreshold TS intensity. Materials and methods Recruitment curves were obtained by delivering paired-pulse TMS with varying conditioning stimulus (CS) intensities. Single pulse test stimuli (TS) and paired-pulse stimuli were delivered to the left M1. CS intensity ranged from 50% to 100% resting motor threshold (rMT), increasing in increments of 5% rMT, TS intensity was set at an intensity that evoked a MEP of approximately 1mV, and ISIs were set to 100ms and 150ms. Results Significant inhibition of the conditioned MEPs was evident at a lower CS intensity when the ISI was set to 100ms (LICI 100 ) than 150ms (LICI 150 ). Significant facilitation of the conditioned MEP was evident at a range of subthreshold CS intensities, from 75% to 90% rMT, when ISI was set to 100ms. Conclusions The emergence of LICI 100 at a lower CS intensity than LICI 150 provides support for the suggestion that independent LICI processes with different time courses exist. It is plausible that LICI 100 and LICI 150 are mediated by pre-and postsynaptic GABA B receptor activity respectively (1); this suggestion is supported by evidence from animal studies showing peak activation of presynaptic GABA B receptors at 100ms and postsynaptic GABA B receptors at 150ms Davies et al., 1990. The identification of independent LICI processes with different time courses might have important implications for studies examining the role of GABA B inhibition in motor function and plasticity. The current results also provide evidence for a long-lasting corticospinal facilitation or disinhibition, however, at present, the mechanism (s) underlying this process remains unknown.