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Cerebral Cortex Advance Access published online on September 22, 2006
Cerebral Cortex, doi:10.1093/cercor/bhl087
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© The Author 2006. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org

Article

Molecular Basis for the GABAA Receptor-Mediated Tonic Inhibition in Rat Somatosensory Cortex

Junko Yamada 1 *, Tomonori Furukawa 2, Shinya Ueno 2, Sumii Yamamoto 2, and Atsuo Fukuda 3

1 Department of Biological Information Processing, Graduate School of Electronic Science and Technology, Shizuoka University, Hamamatsu, Shizuoka 432-8011, Japan
2 Department of Physiology, Hamamatsu University School of Medicine, Hamamatsu, Shizuoka 431-3192, Japan
3 Department of Biological Information Processing, Graduate School of Electronic Science and Technology, Shizuoka University, Hamamatsu, Shizuoka 432-8011, Japan; Department of Physiology, Hamamatsu University School of Medicine, Hamamatsu, Shizuoka 431-3192, Japan

* To whom correspondence should be addressed.
Junko Yamada, E-mail: djyamad{at}ipc.shizuoka.ac.jp


  Abstract

Fast inhibitory synaptic transmission is primarily mediated by synaptically released {gamma}-aminobutyric acid (GABA) acting on postsynaptic GABAA receptors. GABA acting on GABAA receptors produces not only phasic but also tonic inhibitions by persistent activation of extrasynaptic receptors. However, the mechanistic characteristics of tonic inhibition in the neocortex are not well-understood. To address this, we studied pharmacologically isolated GABAA receptor-mediated currents in neocortical pyramidal neurons in rat brain slices. Bath application of bicuculline blocked miniature inhibitory postsynaptic currents (mIPSCs) and produced an outward shift in baseline holding current (Ihold). Low concentrations of SR95531, a competitive GABAA receptor antagonist, abolished mIPSCs but had no significant effect on Ihold. The benzodiazepine midazolam produced an inward shift in Ihold by augmenting tonic GABAA receptor-mediated currents, which were significantly greater in layer V neurons than in layer II/III. Single-cell reverse transcriptase-polymerase chain reaction (RT-PCR) revealed a relatively higher expressions of {alpha}1 and {alpha}5 subunit mRNA in layer V neurons. L-655708, an {alpha}5 subunit-specific inverse agonist, reduced tonic currents in layer V but not in layer II/III neurons, whereas zolpidem, an {alpha}1-subunit agonist, exerted equivalent effects in both layers. These data suggest that the {alpha}1 GABAA receptor subunit is generally involved in tonic inhibition in pyramidal neurons of the neocortex, whereas the {alpha}5 subunit is specifically involved in layer V neurons.

Keywords: extrasynaptic GABAA receptor; single-cell RT-PCR; spillover; tonic inhibition; {alpha}5 subunit.
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