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This Article Full Text Full Text (PDF) All Versions of this Article: 586/2/495    most recent jphysiol.2007.142059v1 Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Florian, J. Articles by Ziemann, U. Search for Related Content PubMed PubMed Citation Articles by Florian, J. Articles by Ziemann, U. Related Collections Neuroscience

¹ Motor Cortex Group, Department of Neurology, Johann Wolfgang Goethe-University, D-60528 Frankfurt am Main, Germany
² Department of Rehabilitation Medicine, Tongji Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan 430030, People's Republic of China

Inhibitory circuits are crucial in modulating corticospinal output in the primary motor cortex (M1). Relatively little is known about how these inhibitory circuits interact. Here we measured three forms of inhibition in M1 by paired-pulse transcranial magnetic stimulation: short-interval intracortical inhibition (SICI), long-interval intracortical inhibition (LICI) and short-interval interhemispheric inhibition (SIHI). We specifically tested their interactions under pharmacological challenge with a single oral dose of diazepam, a positive allosteric modulator of the -aminobutyric acid type A receptor (GABA_AR), or baclofen, a specific agonist at the GABA type B receptor (GABA_BR). Motor evoked potentials were recorded bilaterally from the first dorsal interosseous muscle in eight right-handed healthy volunteers. Diazepam enhanced SICI, and baclofen produced a trend towards enhanced LICI, corroborating the view that SICI reflects inhibition mediated by the GABA_AR, and LICI very likely reflects inhibition mediated by the GABA_BR. The pharmacology of SIHI was inconclusive and warrants further investigation. Findings strongly suggest that SICI, LICI and SIHI recruit three distinct inhibitory circuits in the human M1. The interactions between SIHI and SICI, LICI and SIHI, and LICI and SICI were all negative, that is SIHI suppressed SICI, and LICI suppressed both SIHI and SICI. Diazepam partially restored SICI in the presence of LICI, while all other interactions remained unaffected by diazepam or baclofen. It will be argued that the negative interactions between SIHI and SICI, LICI and SIHI, and LICI and SICI are most likely due to presynaptic GABA_BR-mediated autoinhibition.

(Received 31 July 2007; accepted after revision 5 November 2007; first published online 8 November 2007)
Corresponding author U. Ziemann: Department of Neurology, Johann Wolfgang Goethe-University, Schleusenweg 2-16, D-60528 Frankfurt am Main, Germany. Email: u.ziemann{at}em.uni-frankfurt.de