ASTROGLIAL CELLS EXPRESS LARGE AMOUNTS OF GABA(A) RECEPTOR PROTEINS IN MATURE BRAIN

GABA(A) receptors were characterized in cellular fractions isolated fr om adult bovine brain. The fraction enriched in cortical astrocytes is very rich in high-affinity binding sites for [H-3]flunitrazepam and o ther ''central-type'' benzodiazepine ligands. The amount of specific [ H-3]flunitrazepam binding was more than five times higher in the gliai fraction than in synaptosomal and perikaryal fractions. [H-3]flunitra zepam was displaced by low concentrations of clonazepam and other spec ific ligands for central GABA(A) receptors. Specific binding sites for GABA, flunitrazepam, barbiturates, and picrotoxin-like convulsants we re characterized. Allosteric interactions between the different sites were typical of central-type GABA(A) receptors. The presence of alpha- subunit(s), as revealed by [H-3]-flunitrazepam photoaffinity labeling, was demonstrated in all brain fractions at molecular mass 51-53 kDa. Photoaffinity labeling was highest in the glial fraction. However, in primary cultured astrocytes from neonate rat cortex, no photoaffinity labeling was detected. Information obtained from astrocytes in culture should thus be taken with caution when extrapolated to differentiated astroglial cells. Our results actually show that, in mature brain, mo st of the fully pharmacologically active GABA(A) receptors are extrasy naptic and expressed in astroglia.