Distribution of a1, a4, g2, and y subunits of GABAA receptors in hippocampal granule cells

GABAA receptors are pentamers composed of subunits derived from the a, h, g, y, u, e, and k gene families. a1, a4, g2, and y subunits are expressed in the dentate gyrus of the hippocampus, but their subcellular distribution has not been described. Hippocampal sections were double-labeled for the a1, a4, g2, and y subunits and GAD65 or gephyrin, and their subcellular distribution on dentate granule cells was studied by means of confocal laser scanning microscopy (CLSM). The synaptic versus extrasynaptic localization of these subunits was inferred by quantitative analysis of the frequency of colocalization of various subunits with synaptic markers in high-resolution images. GAD65 immunoreactive clusters colocalized with 26.24F0.86% of the a1 subunit immunoreactive clusters and 32.35F1.49% of the g2 subunit clusters. In contrast, only 1.58F0.13% of the a4 subunit immunoreactive clusters and 1.92F0.15% of the y subunit clusters colocalized with the presynaptic marker GAD65. These findings were confirmed by studying colocalization with immunoreactivity of a postsynaptic marker, gephyrin, which colocalized with 27.61F0.16% of the a1 subunit immunoreactive clusters and 23.45F0.32% of the g2 subunit immunoreactive clusters. In contrast, only 1.90F0.13% of the a4 subunit immunoreactive clusters and 1.76F0.10% of the y subunit clusters colocalized with gephyrin. These studies demonstrate that a subset of a1 and g2 subunit clusters colocalize with synaptic markers in hippocampal dentate granule cells. Furthermore, all four subunits, a1, a4, g2, and y, are present in the extrasynaptic locations. D 2004 Elsevier B.V. All rights reserved. Theme: Neurotransmitters, modulators, transporters, and receptors Topic: GABA receptors