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

Abstract

Fast inhibitory synaptic transmission is primarily mediated by synaptically released gamma-aminobutyric acid (GABA) acting on postsynaptic GABA(A) receptors. GABA acting on GABA(A) 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 GABA(A) 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 (I(hold)). Low concentrations of SR95531, a competitive GABA(A) receptor antagonist, abolished mIPSCs but had no significant effect on I(hold). The benzodiazepine midazolam produced an inward shift in I(hold) by augmenting tonic GABA(A) 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 alpha1 and alpha5 subunit mRNA in layer V neurons. L-655708, an alpha5 subunit-specific inverse agonist, reduced tonic currents in layer V but not in layer II/III neurons, whereas zolpidem, an alpha1-subunit agonist, exerted equivalent effects in both layers. These data suggest that the alpha1 GABA(A) receptor subunit is generally involved in tonic inhibition in pyramidal neurons of the neocortex, whereas the alpha5 subunit is specifically involved in layer V neurons.

Postsynaptic GABABReceptors Enhance Extrasynaptic GABAAReceptor Function in Dentate Gyrus Granule Cells

“…One salient difference between DGGCs and CA1 pyramidal cells is that tonic inhibition in CA1 pyramidal cells is mediated by ␣5 subunit-containing GABA A receptors, whereas ␦ subunit-containing receptors are dominant in DGGCs (Caraiscos et al, 2004;Glykys et al, 2008). In layer 2/3 pyramidal neurons there is little, if any, functional expression of ␦ subunits (although ␦ subunit mRNA is present) (Yamada et al, 2007). Thus, we speculate that signals produced by postsynaptic GABA B receptors may preferentially affect ␦ subunits.…”

Differential Tonic GABA Conductances in Striatal Medium Spiny Neurons

“…Because ␣5-subunit-containing GABA A receptors mediate tonic inhibition in other brain regions (Caraiscos et al, 2004;Scimemi et al, 2005;Yamada et al, 2007) and this subunit is expressed in the striatum (Pirker et al, 2000;Schwarzer et al, 2001), we studied the action of a selective inverse agonist of these receptors. Although micromolar concentrations of L655,708 have been used in the majority of brain slice physiology studies (Caraiscos et al, 2004;Scimemi et al, 2005;Yamada et al, 2007), pharmacological analysis of recombinant GABA A receptors (Quirk et al, 1996) would suggest that nanomolar concentrations should be as efficacious.…”

“…Although micromolar concentrations of L655,708 have been used in the majority of brain slice physiology studies (Caraiscos et al, 2004;Scimemi et al, 2005;Yamada et al, 2007), pharmacological analysis of recombinant GABA A receptors (Quirk et al, 1996) would suggest that nanomolar concentrations should be as efficacious. Indeed, we found that 50 nM L655,708 was as efficacious in antagonizing tonic currents in D 2 ϩ MSNs as the higher concentrations (10 M).…”

“…Therefore, we sought to investigate whether tonic GABA currents in MSNs is mediated by ␣5-subunit-containing GABA A receptors as has been shown in hippocampal pyramidal neurons (Caraiscos et al, 2004;Scimemi et al, 2005) and somatosensory cortex layer V pyramidal cells (Yamada et al, 2007). Indeed, L655,708, an inverse agonist with high selectivity for GABA A receptors containing the ␣5 subunit (Quirk et al, 1996) at concentrations ranging from 50 nM to 10 M, reduced tonic GABA currents in the D 2 ϩ MSNs (Fig.…”

The Development of Phasic and Tonic Inhibition in the Rat Visual Cortex

“…The amplitude of tonic inhibition was analyzed as the difference between the holding currents measured before and after the application of the GABAA receptor antagonist bicuculline (10 μM) while the membrane potential was clamped at −75 mV. The holding current was calculated from 100 msec epochs containing no obvious spontaneous synaptic events, taken every four seconds during an 80-second period [24].…”