GABA _A receptor α4 subunits mediate extrasynaptic inhibition in thalamus and dentate gyrus and the action of gaboxadol

Abstract

The neurotransmitter GABA mediates the majority of rapid inhibition in the CNS. Inhibition can occur via the conventional mechanism, the transient activation of subsynaptic GABA A receptors (GABA A-Rs), or via continuous activation of high-affinity receptors by low concentrations of ambient GABA, leading to ''tonic'' inhibition that can control levels of excitability and network activity. The GABA A-R ␣4 subunit is expressed at high levels in the dentate gyrus and thalamus and is suspected to contribute to extrasynaptic GABAA-R-mediated tonic inhibition. Mice were engineered to lack the ␣4 subunit by targeted disruption of the Gabra4 gene. ␣4 Subunit knockout mice are viable, breed normally, and are superficially indistinguishable from WT mice. In electrophysiological recordings, these mice show a lack of tonic inhibition in dentate granule cells and thalamic relay neurons. Behaviorally, knockout mice are insensitive to the ataxic, sedative, and analgesic effects of the novel hypnotic drug, gaboxadol. These data demonstrate that tonic inhibition in dentate granule cells and thalamic relay neurons is mediated by extrasynaptic GABA A-Rs containing the ␣4 subunit and that gaboxadol achieves its effects via the activation of this GABA A-R subtype.ABA is the major inhibitory neurotransmitter in the mammalian CNS. Its primary target, GABA A receptors (GABA A -Rs), are pentameric complexes that function as ligandgated chloride ion channels. Two types of inhibitory neurotransmission are mediated via GABA A -Rs (1, 2). Phasic inhibition results from the activation of receptors at the synapse by intermittent release of high concentrations of GABA from presynaptic terminals. Tonic inhibition, in contrast, is mediated by the continuous activation of receptors located outside the synaptic cleft by low concentrations of ambient GABA. These ''extrasynaptic'' GABA A -Rs have a higher affinity for GABA and have faster channel deactivation rates (3, 4) and, more importantly, slower rates of desensitization (1-5), relative to the classical ''synaptic'' GABA A -Rs.There are a variety of subunit families that make up GABA ARs; a total of 19 distinct subunits have been cloned, ␣1-6, ␤1-3, ␥1-3, ␦, , , , and 1-3 (6). This diversity in subunit composition results in substantial anatomical, functional, and pharmacological heterogeneity. GABA A -Rs containing the ␣4 subunit are highly expressed in the thalamus and dentate gyrus, with lower levels in cortex, striatum, and other brain areas (7-9). GABA A -Rs containing ␣4 subunits often are found with the ␥2 or ␦ subunits, in combination with ␤ subunits; the ␣4␤␦ subtypes are proposed to be localized to extrasynaptic sites and contribute to tonic inhibition (5, 10-13). Other extrasynaptic receptor subtypes include ␣5␤3␥2 in hippocampal CA1 pyramidal cells (14) and ␣6␤␦ in cerebellar granule cells (15). Notably, the ␣4 subunit containing GABA A -Rs, especially ␣4␤␥2, are not exclusively extrasynaptic; some are found within dentate gyrus synapses and others are located perisynaptically, wher...

Altered Localization of GABAAReceptor Subunits on Dentate Granule Cell Dendrites Influences Tonic and Phasic Inhibition in a Mouse Model of Epilepsy

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α4βδ GABA A receptors are high-affinity targets for γ-hydroxybutyric acid (GHB)

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