Expression patterns of gamma-aminobutyric acid type A receptor subunit mRNAs in primary cultures of granule neurons and astrocytes from neonatal rat cerebella.

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Abstract
Using a competitive polymerase chain reaction assay, we have quantitated the absolute amounts of mRNA encoding 14 distinct subunits of the gamma-aminobutyric acid type A (GABA(A)) receptor in primary cultures of rat cerebellar granule neurons and cerebellar astrocytes. We found that the total amount of GABA(A) receptor subunit mRNA in astrocytes was 2 orders of magnitude lower than in neuronal cells. Furthermore, granule cell cultures expressed all 14 different GABA(A) subunit mRNAs, while the astroglial cultures contained detectable amounts of all the subunits expressed by granule cells except the alpha6 and the gamma2L subunits. Of the alpha subunit family members, the alpha1, alpha5, and alpha6 mRNAs were prominent in granule cells, while the alpha1 and alpha2 mRNAs were abundant in astrocytes. Of the beta receptor subunit mRNAs, the beta1 and beta3 mRNAs were abundantly expressed in both cultures. The gamma2S and gamma2L mRNAs constituted the great majority of gamma subunit mRNAs in neurons, while the gamma1 subunit mRNA was the most abundant gamma subunit mRNA in astrocytes. When various allosteric modulators of GABA(A) receptors were tested electrophysiologically, methyl 6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate (DMCM) (DMCM) was the only one to modulate chloride currents elicited by GABA in a significantly different manner in granule cells (negative modulation) compared with astrocytes (positive modulation). The latter effect was previously observed in transiently expressed recombinant GABA(A) receptors containing a gamma1 instead of a gamma2 subunit. Our quantitative mRNA results suggest that an important molecular determinant responsible for the DMCM-positive modulatory effect on astroglial native GABA(A) receptors is the presence of the gamma1 subunit in the receptor assembly.