Relevance of astrocytic activation to reductions of astrocytic GABAA receptors.
Although astrocytes express gamma-aminobutyric acid subtype-A (GABAA) receptors in the mature brain, GABAA receptor expression in a cultivation state remains controversial. In this study, we investigated the alteration of astrocytic GABAA receptor expression in in vitro and in vivo studies to elucidate the relevance of astrocytic activation to reductions of astrocytic GABAA receptors. The GABA-evoked Cl- current (GABAA response) in cultured astrocytes was determined by recording in the whole-cell mode using a conventional patch-clamp technique under voltage-clamp conditions. The respective amplitudes of GABAA responses on days in vitro 1, 3-5, 7-10, and 12-15 were 1019+/-97, 512+/-76, 84+/-21, and 22+/-9 pA, respectively, suggesting that the GABAA response subsequently diminished with in vitro aging. In immunohistochemical and biochemical analyses, the expression of GABAA receptor beta-subunit decreased, whereas expressions of glial fibrillary acidic protein (GFAP) and S100B, hallmarks of astrocytic activation, increased dramatically in the cultured astrocytes with in vitro aging. With the use of [3H]SR95531, a GABAA-specific ligand, at 24 h after transient focal ischemia, binding was significantly reduced in the astrocytic fractions without affecting the synaptosomal fractions, and decreases in the mRNA expression level of GABAA receptor beta-subunits were concurrently observed. Interestingly, the loss of GABAA response in cultured astrocytes was mitigated by co-culturing with neurons or treatments with monoclonal S100B antibodies. These results indicate that astrocytic GABAA receptors are reduced with in vitro aging and cerebral ischemia, presumably through the overproduction of S100B in activated astrocytes.