Monoclonal antibodies (mAb) against a gamma-aminobutyric acid/benzodiazepine receptor complex (GABAA/BZR) were produced by using spleen cells from a mouse immunized with GABAA/BZR purified from bovine cerebral cortex. The mAb, most of which were of the IgG1 isotype could be divided into four groups (I-IV) specifying different antigenic structures. On immunoblots, group I mAb recognized exclusively the Mr 55,000 beta-subunit, while groups II and IV mAb recognized the Mr 50,000 alpha-subunit of bovine GABAA/BZR. Three of the four groups of mAb (I, III, and IV) crossreacted with both human and rat GABAA/BZR with the same subunit specificity as in bovine brain; the fourth group (II) crossreacted with human but not with the rat receptor. The binding sites for benzodiazepines as well as the high and low affinity GABA sites reside on the same structural complex as shown by immunoprecipitation. Ligand binding to these sites was not inhibited by mAb. Since quantitative immunoprecipitation of GABAA/BZR was achieved with mAb selective for either the alpha- or beta-subunit, both subunits occur in each individual receptor complex. The pattern of immunoblot staining suggests that the smaller alpha-subunit is not a processing product of the larger beta-subunit. Both alpha- and beta-subunits were present in all brain areas and species tested (rat cerebral cortex, cerebellum, and hippocampus; bovine cerebral cortex and cerebellum; human cerebral cortex). This suggests a uniform subunit composition of the receptor throughout the brain in contrast to earlier evidence for a heterogeneous subunit composition based on photoaffinity labeling.
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