GABAergic Innervation Organizes Synaptic and Extrasynaptic GABAAReceptor Clustering in Cultured Hippocampal Neurons

Abstract

We have studied the effects of GABAergic innervation on the clustering of GABA_A receptors (GABA_ARs) in cultured hippocampal neurons. In the absence of GABAergic innervation, pyramidal cells form small (0.36 ± 0.01 μm diameter) GABA_AR clusters at their surface in the dendrites and soma. When receiving GABAergic innervation from glutamic acid decarboxylase-containing interneurons, pyramidal cells form large (1.62 ± 0.08 μm breadth) GABA_AR clusters at GABAergic synapses. This is accompanied by a disappearance of the small GABA_AR clusters in the local area surrounding each GABAergic synapse. Although the large synaptic GABA_AR clusters of any neuron contained all GABA_AR subunits and isoforms expressed by that neuron, the small clusters not localized at GABAergic synapses showed significant heterogeneity in subunit and isoform composition. Another difference between large GABAergic and small non-GABAergic GABA_AR clusters was that a significant proportion of the latter was juxtaposed to postsynaptic markers of glutamatergic synapses such as PSD-95 and AMPA receptor GluR1 subunit. The densities of both the glutamate receptor-associated and non-associated small GABA_AR clusters were decreased in areas surrounding GABAergic synapses. However, no effect on the density or distribution of glutamate receptor clusters was observed. The results suggest that there are local signals generated at GABAergic synapses that induce both assembly of large synaptic GABA_AR clusters at the synapse and disappearance of the small GABA_AR clusters in the surrounding area. In the absence of GABAergic innervation, weaker GABA_AR-clustering signals, generated at glutamatergic synapses, induce the formation of small postsynaptic GABA_AR clusters that remain juxtaposed to glutamate receptors at glutamatergic synapses.