Differential Synaptic Localization of Two Major $\gamma $-aminobutyric Acid Type A Receptor $\alpha $ Subunits on Hippocampal Pyramidal Cells

Hippocampal pyramidal cells, receiving domain specific GABAergic inputs, express up to 10 different subunits of the $\gamma $-aminobutyric acid type A (GABA$_{\text{A}}$) receptor, but only 3 different subunits are needed to form a functional pentameric channel. We have tested the hypothesis that some subunits are selectively located at subsets of GABAergic synapses. The $\alpha $1 has been found in most GABAergic synapses on all postsynaptic domains of pyramidal cells. In contrast, the $\alpha $2 subunit was located only in a subset of synapses on the somata and dendrites, but in most synapses on axon initial segments innervated by axo-axonic cells. The results demonstrate that molecular specialization in the composition of postsynaptic GABA$_{\text{A}}$ receptor subunits parallels GABAergic cell specialization in targeting synapses to a specific domain of postsynaptic cortical neurons.