THE GAMMA-SUBUNITS OF THE NATIVE GABA(A) BENZODIAZEPINE RECEPTORS

Subunit-specific antibodies to all the gamma subunit isoforms describe d in mammalian brain (gamma(1), gamma(2S), gamma(2L), and gamma(3)) ha ve been made. The proportion of GABA(A) receptors containing each gamm a subunit isoform in various brain regions has been determined by quan titative immunoprecipitation. In all tested regions of the rat brain, the gamma(1) and gamma(3) subunits are present in considerable smaller proportion of GABA(A) receptor than the gamma(2) subunit. Immunocytoc hemistry shows that gamma(1) immunoreactivity concentrates in the stra tum oriens and stratum radiatum of the CA1 region of the hippocampus. In the dentate gyrus, gamma(1) immunoreactivity concentrates on the ou ter 2/3 of the molecular layer coinciding with the localization of the axospinous synapses of the perforant pathway. In contrast, gamma(3) i mmunoreactivity concentrates on the basket cells and other GABAergic l ocal circuit neurons of the hilus. These cells are also rich in gamma( 2S). In the cerebellum, gamma(1) immunolabeling was localized on the B ergmann glia. The gamma(2S) and gamma(2L) subunits are differentially expressed in Various brain regions. Thus the gamma(2S) is highly expre ssed in the olfactory bulb and hippocampus whereas the gamma(2L) is ve ry abundant in inferior colliculus and cerebellum, particularly in Pur kinje cells, as immunocytochemistry, in situ hybridization and immunop recipitation techniques have revealed. The gamma(2S) and gamma(2L) coe xist in some brain areas and cell types. Moreover, the gamma(2S) and g amma(2L) subunits can coexist in the same GABA(A) receptor pentamer. W e have shown that this is the case in some GABA(A) receptors expressed in cerebellar granule cells. These GABA(A) receptors also have alpha and beta subunits forming the pentamer. Immunoblots have shown that th e rat gamma(1), gamma(2S), gamma(2L) and gamma(3) subunits are peptide s of 47, 45, 47 and 44 kDa respectively. Results also indicate that th ere are aging-related changes in the expression of the gamma(2S) and g amma(2L) subunits in various brain regions which suggest the existence of aging-related changes in the subunit composition of the GABA(A) re ceptors which in turn might lead to changes in receptor pharmacology. The results obtained with the various gamma subunit isoforms are discu ssed in terms of the high molecular and binding heterogeneity of the n ative GABA(A) receptors in brain.