Cultured Hippocampal Pyramidal Neurons Express Two Kinds of GABA_AReceptors

Abstract

We combined a study of the subcellular distribution of the ␣1, ␣2, ␣4, ␤1, ␤2/3, ␥2, and ␦ subunits of the GABA A receptor with an electrophysiological analysis of GABA A receptor currents to determine the types of receptors expressed on cultured hippocampal pyramidal neurons. The immunocytochemistry study demonstrated that ␣1, ␣2, ␤2/3, and ␥2 subunits formed distinct clusters of various sizes, which were colocalized with clusters of glutamate decarboxylase (GAD) immunoreactivity at rates ranging from 22 to 58%. In contrast, ␣4, ␤1, and ␦ subunits were distributed diffusely over the cell soma and neuronal processes of cultured neurons and did not colocalize with the synaptic marker GAD. Whole-cell GABA A receptor currents were moderately sensitive to GABA and were modulated by diazepam. The whole-cell currents were also enhanced by the neurosteroid allopregnanolone (10 nM). Tonic currents, measured as changes in baseline current and noise, were sensitive to Zn 2ϩ , furosemide, and loreclezole; they were insensitive to diazepam. These studies suggest that two kinds of GABA A receptors are expressed on cultured hippocampal neurons. One kind of receptor formed clusters, which were present at GABAergic synapses and in the extrasynaptic membrane. The ␣1, ␣2, ␤2/3, and ␥2 subunits were contained in clustered receptors. The second kind was distributed diffusely in the extrasynaptic membrane. The ␣4, ␤1, and ␦ subunits were contained in these diffusely distributed receptors. The properties of tonic currents recorded from these neurons were similar to those from recombinant receptors containing ␣4, ␤1, and ␦ subunits.

Multiple and Plastic Receptors Mediate Tonic GABAAReceptor Currents in the Hippocampus

“…Indeed, the most likely subunit combination mediating the tonic current in CA1 pyramidal cells is ␣4␦ (Liang et al, 2004). These findings are consistent with the findings in cultured hippocampal neurons (Mangan et al, 2005) but, at first hand, appear at odds with the finding of Caraiscos et al (2004) that the tonic current in CA1 pyramidal cells is primarily mediated by ␣5GABA A Rs. The cited study was, however, performed on hippocampal neuronal cultures treated with the GABA transaminase inhibitor vigabatrin for 24 h before recording and thus in conditions of raised [GABA] o .…”

Enhancement of GABA Release through Endogenous Activation of Axonal GABAAReceptors in Juvenile Cerebellum

“…First, we find that gabazine does not reduce the holding current (even at doses up to 50 M), suggesting that ambient GABA levels are too low to significantly activate either somatodendritic or axonal receptors. Second, previous reports of tonic GABA A R-induced currents indicate the involvement of high-affinity receptors either containing the ␦ subunit (Brickley et al, 2001;Mangan et al, 2005;Glykys et al, 2007) or containing only ␣ and ␤ subunits (Mortensen and Smart, 2006). The high affinity of these receptors is presumably required to sense the very low concentration of ambient GABA.…”

Alterations in GABA-related transcriptome in the dorsolateral prefrontal cortex of subjects with schizophrenia

“…60,61 In contrast, GABA A receptors containing the d subunit, which is often coassembled with the a4 subunit in the forebrain, are selectively localized to extrasynaptic sites. [60][61][62] These extrasynaptic receptors, which have a high sensitivity to GABA and thus can be activated by ambient GABA molecules in extracellular space, mediate tonic inhibition which reduces the effects of synaptic inputs over time.…”

“…60,61 In contrast, GABA A receptors containing the d subunit, which is often coassembled with the a4 subunit in the forebrain, are selectively localized to extrasynaptic sites. [60][61][62] These extrasynaptic receptors, which have a high sensitivity to GABA and thus can be activated by ambient GABA molecules in extracellular space, mediate tonic inhibition which reduces the effects of synaptic inputs over time. 62,63 Given the predominant localization of the a1, g2 and d subunits to dendrites, 64,65 the highly correlated expression deficits for these transcripts suggest coordinated downregulation of GA-BA A receptors mediating phasic and tonic inhibition in the dendritic domain of DLPFC pyramidal neurons in schizophrenia.…”