Cultured hippocampal pyramidal neurons express two kinds of GABA _A receptors

We combined a study of the subcellular distribution of the α1, α2, α4, β1, β2/3, γ2, and δ 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 α1, α2, β2/3, and γ2 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, α4, β1, and δ 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 Zn²⁺, 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 α1, α2, β2/3, and γ2 subunits were contained in clustered receptors. The second kind was distributed diffusely in the extrasynaptic membrane. The α4, β1, and δ 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 α4, β1, and δ subunits.