Contributions of the GABA_AReceptor α6 Subunit to Phasic and Tonic Inhibition Revealed by a Naturally Occurring Polymorphism in the α6 Gene

Abstract

GABA A receptors (GABARs) are heteromultimeric proteins composed of five subunits. The specific subunit composition determines critical properties of a GABAR such as pharmacological sensitivities and whether the receptor contributes to synaptic or extrasynaptic forms of inhibition. Classically, synaptic but not extrasynaptic GABARs are thought to respond to benzodiazepines, whereas the reverse has been suggested for ethanol. To examine the effects of subunit composition on GABAR function in situ, we took advantage of two naturally occurring alleles of the rat gene for GABAR subunit ␣6 (Gabra6 100R and Gabra6 100Q). Depending on their subunit partners, these two variants of ␣6 can lead to differential sensitivities to benzodiazepines and ethanol. An examination of synaptic and extrasynaptic GABA-mediated currents in cerebellar granule cells from Gabra6 100R/100R and Gabra6 100Q/100Q rats uncovered marked alleledependent differences in benzodiazepine sensitivity. Unexpectedly, we found that the benzodiazepines flunitrazepam and diazepam enhanced extrasynaptic inhibition mediated by ␦ subunit-containing GABARs in Gabra6

A new meaning for “Gin & Tonic”: tonic inhibition as the target for ethanol action in the brain

“…2). This is in contrast to the effects of benzodiazepines on tonic currents mediated by δ subunit-containing GABA A Rs, which contrary to a well established benzodiazepine-insensitive pharmacology, become potentiated by benzodiazepines when activated only by low GABA concentrations (Santhakumar et al, 2006). A point mutation in position 100 (Q/R) of the α6 subunit markedly affects benzodiazepine and ethanol sensitivities (Hanchar et al, 2005;Santhakumar et al, 2006).…”

Decrease in Tonic Inhibition Contributes to Increase in Dentate Semilunar Granule Cell Excitability after Brain Injury

“…For patch-clamp recordings, slices (300 m) were transferred to a submerged recording chamber and perfused with oxygenated aCSF at 33 Ϯ 1°C. Whole-cell voltage-clamp and currentclamp recordings from dentate granule cells and neurons in the IML were performed using infrared differential interference contrast visualization techniques (Stuart et al, 1993;Santhakumar et al, 2006) with a Nikon Eclipse FN-1 microscope, using a 40ϫ water-immersion objective. Recordings were obtained using MultiClamp 700B (Molecular Devices).…”

“…or gabazine (SR95531, 10 M, Sigma-Aldrich), was measured as described previously (Santhakumar et al, 2006;Glykys and Mody, 2007) using custom macros in IgorPro7.0 software (WaveMetrics).…”

“…Individual sIPSCs were detected using custom software in IgorPro7.0 (Santhakumar et al, 2006(Santhakumar et al, , 2010. Events were visualized, and any "noise" that spuriously met trigger specifications was rejected.…”

Molecular and synaptic organization of GABAA receptors in the cerebellum: Effects of targeted subunit gene deletions

“…In granule cells, a major fraction of GABA A receptors is associated with the d subunit (29,30), forming extrasynaptic receptors with a high affinity for GABA (31,32) and diffusely distributed on the soma and dendrites (12,22,28). The presence of the d subunit, which does not co-assemble with the c2 subunit, is determinant for the location of GABA A receptors relative to postsynaptic sites and for their functional properties.…”