Neuronal activity regulates GABAA receptor subunit expression in organotypic hippocampal slice cultures

The postnatal expression of GABA(A) receptor subunit mRNAs in the rat brain, including the hippocampus, exhibits a unique temporal and regional developmental profile in vivo, which may be altered by external stimuli. Using the in situ hybridization technique we have now studied the in vitro expression of alpha1,alpha2, alpha 4, alpha 5, beta 1, beta 3, gamma 2, and gamma 3 subunit mRNAs of GABA(A) receptors in organotypic hippocampal slices cultured for 7 days. To find out whether neuronal activity regulates the subunit expression, a subset of cultures was chronically treated either with a GABA(A) receptor antagonist picrotoxin, or by a non-N-methyl-D-aspartate (non-NMDA)-receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX). In untreated control cultures, the expression pattern of the subunits varied regionally, the most abundantly expressed subunits being alpha 2 and alpha 5 in all subregions. All studied subunits were expressed in CA3a/b and CA1, whereas in CA3c and in granule cells of the dentate gyrus (DG) no signal of alpha 4 and gamma 3 was detected. The drug treatment differently affected the regional subunit expression. In picrotoxin-treated cultures, the expression of alpha1, alpha 5 and gamma 2 mRNAs was significantly increased in pyramidal cell layers, and in DNQX-treated cultures the expression of alpha2 mRNA in CA3c and DG, and that of beta1 in DG. Changes in the expression of GABA(A) receptor subunit mRNAs in treated cultures suggest that neuronal activity can regulate their regional expression in vitro. Since the expression profile in untreated control cultures closely resembled that observed earlier in vivo, organotypic hippocampal slice cultures could serve as a good model system to study the regulatory mechanisms of receptor expression under well-controlled experimental conditions in the developing hippocampus.