PHARMACOLOGICAL CONTROL OF FACILITATORY I-WAVE INTERACTION IN THE HUMAN MOTOR CORTEX - A PAIRED TRANSCRANIAL MAGNETIC STIMULATION STUDY

A novel paired transcranial magnetic stimulation (TMS) paradigm with a suprathreshold first and a subthreshold second stimulus was used in h ealthy volunteers to investigate the acute effects of a single oral do se of various CNS-active drugs on short-interval motor evoked potentia l (MEP) facilitation. MEPs were recorded from the relaxed abductor dig iti muscle. Three peaks of MEP facilitation were consistently observed at interstimulus intervals of 1.1-1.5 ms, 2.3-2.7 ms, and 3.9-4.5 ms. The size of these MEP peaks was transiently suppressed by drugs which enhance gamma-aminobutyric acid (GABA) function in the neocortex (lor azepam, vigabatrin, phenobarbital, ethanol), while the GABA-B receptor agonist baclofen, anti-glutamate drugs (gabapentin, memantine), and s odium channel blockers (carbamazepine, lamotrigine) had no effect. The interstimulus intervals effective for the production of the MEP peaks remained unaffected by all drugs. The MEP peaks are thought to be due to a facilitatory interaction of I-(indirect) waves in the motor cort ex. Therefore, the present results indicate that the production of I-w aves is primarily controlled by GABA related neuronal circuits. The po tential relevance of this nan-invasive paired TMS protocol for the inv estigation of I-waves in patients with neurological disease will be di scussed. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.