Synaptic and nonsynaptic localization of GABAAreceptors containing the α5 subunit in the rat brain

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Abstract
The α5 subunit of the GABAAreceptors (GABAARs) has a restricted expression in the brain. Maximum expression of this subunit occurs in the hippocampus, cerebral cortex, and olfactory bulb. Hippocampal pyramidal cells show high expression of α5 subunit‐containing GABAARs (α5‐GABAARs) both in culture and in the intact brain. A large pool of α5‐GABAARs is extrasynaptic and it has been proposed to be involved in the tonic GABAergic inhibition of the hippocampus. Nevertheless, there are no studies on the localization of the α5‐GABAARs at the electron microscope (EM) level. By using both immunofluorescence of cultured hippocampal pyramidal cells and EM postembedding immunogold of the intact hippocampus we show that, in addition to the extrasynaptic pool, there is a pool of α5‐GABAARs that concentrates at the GABAergic synapses in dendrites of hippocampal pyramidal cells. The results suggest that the synaptic α5‐GABAARs might play a role in the phasic GABAergic inhibition of pyramidal neurons in hippocampus and cerebral cortex. J. Comp. Neurol. 499:458–470, 2006.