GABAp subunits confer a bicuculline-insensitive component to GFAP+ cells of cerebellum

GABA-A receptors mediating synaptic or extrasynaptic transmission are molecularly and functionally distinct, and glial cells are known to express a plethora of GABA-A subunits. Here we demonstrate that GFAP+ cells of the granular layer of cerebellum express GABAp subunits during early postnatal development, thereby conferring peculiar pharmacologic characteristics to GABA responses. Electron microscopy revealed the presence of GABAp in the plasma membrane of GFAP+ cells. In contrast, expression in the adult was restricted to Purkinje neurons and a subset of ependymal cells. Electrophysiological studies in vitro revealed that astrocytes express functional receptors with an EC50 of 52.2 ± 11.8 μM for GABA. The evoked currents were inhibited by bicuculline (100 μM) and TPMPA (IC50, 5.9 ± 0.6 μM), indicating the presence of a GABAp component. Coimmunoprecipitation demonstrated protein-protein interactions between GABAp1 and GABA?1, and double immunofluorescence showed that these subunits colocalize in the plasma membrane. Three populations of GABA-A receptors in astrocytes were identified: classic GABA-A, bicuculline-insensitive GABAp, and GABA-A-GABAp hybrids. Clusters of GABA-A receptors were distributed in the perinuclear space and along the processes of GFAP+ cells. Time-lapse microscopy showed GABAp2-GFP accumulation in clusters located in the soma and along the processes. The clusters were relatively immobile, with mean displacement of 9.4 ± 0.9 μm and a net distance traveled of 1-2 μm, owing mainly to directional movement or simple diffusion. Modulation of GABAp dynamics may be a novel mechanism of extrasynaptic transmission regulating GABAergic control of GFAP+ cells during early postnatal development.