The expression of GABAAβ subunit isoforms in synaptic and extrasynaptic receptor populations of mouse dentate gyrus granule cells

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Abstract
The subunit composition of GABAA receptors influences their biophysical and pharmacological properties, dictates neuronal location and the interaction with associated proteins, and markedly influences the impact of intracellular biochemistry. The focus has been on α and γ subunits, with little attention given to β subunits. Dentate gyrus granule cells (DGGCs) express all three β subunit isoforms and exhibit both synaptic and extrasynaptic receptors that mediate ‘phasic’ and ‘tonic’ transmission, respectively. To investigate the subcellular distribution of the β subunits we have utilized the patch‐clamp technique to compare the properties of ‘tonic’ and miniature inhibitory postsynaptic currents (mIPSCs) recorded from DGGCs of hippocampal slices of P20–26 wild‐type (WT), β2−/−, β2N265S (etomidate‐insensitive), α1−/− and δ−/− mice. Deletion of either the β2 or the δ subunit produced a significant reduction of the tonic current and attenuated the increase of this current induced by the δ subunit‐preferring agonist 4,5,6,7‐tetrahydroisoxazolo[5,4‐c]pyridin‐3‐ol (THIP). By contrast, mIPSCs were not influenced by deletion of these genes. Enhancement of the tonic current by the β2/3 subunit‐selective agent etomidate was significantly reduced for DGGCs derived from β2N265S mice, whereas this manipulation had no effect on the prolongation of mIPSCs produced by this anaesthetic. Collectively, these observations, together with previous studies on α4−/− mice, identify a population of extrasynaptic α4β2δ receptors, whereas synaptic GABAA receptors appear to primarily incorporate the β3 subunit. A component of the tonic current is diazepam sensitive and is mediated by extrasynaptic receptors incorporating α5 and γ2 subunits. Deletion of the β2 subunit had no effect on the diazepam‐induced current and therefore these extrasynaptic receptors do not contain this subunit. The unambiguous identification of these distinct pools of synaptic and extrasynaptic GABAA receptors should aid our understanding of how they act in harmony, to regulate hippocampal signalling in health and disease.