GABAA receptor subtype specific enhancement of inhibition in human motor cortex

Inhibition is of fundamental importance to regulate activity in cortical circuits. Inhibition is mediated through a diversity of different interneurones and gamma-aminobutyric acid A receptor (GABA(A)R) subtypes. Here we employed paired-pulse transcranial magnetic stimulation (TMS) to measure short interval intracortical inhibition (SICI), a GABA(A)R-mediated inhibition in human motor cortex, to address the question of which GABA(A)R subtype is responsible for this form of inhibition. It has been shown that classical benzodiazepines (diazepam and lorazepam) have a non-selective affinity profile at different alpha-subunit-bearing subtypes of the GABA(A)R while zolpidem has a 10-fold greater affinity to the alpha 1-subunit-bearing GABA(A)R compared with those bearing the alpha 2- or alpha 3-subunit. We found that, in seven healthy subjects, a single oral dose of 20 mg of diazepam or 2.5 mg of lorazepam significantly increased SICI, whereas 10 mg of zolpidem did not change SICI. This dissociation occurred despite equal sedation by all three drugs, an alpha 1-subunit GABA(A)R-mediated effect. The findings strongly suggest that SICI is not mediated by the alpha 1-subunit-bearing subtype of the GABA(A)R but by those bearing either the alpha 2- or alpha 3-subunit. This study represents an attempt by means of TMS to identify GABA(A)R subtype-specific action at the systems level of human cortex, a highly relevant issue because the different alpha-subunit-bearing subtypes of the GABA(A)R are differently involved in benzodiazepine-mediated effects such as sedation, amnesia or anxiolysis, in developmental cortical plasticity, and in neurological disorders such as epilepsy.