GAMMA-AMINOBUTYRIC-ACID TYPE-A RECEPTORS IN THE RAT-BRAIN CAN CONTAINBOTH GAMMA-2 AND GAMMA-3 SUBUNITS, BUT GAMMA-1 DOES NOT EXIST IN COMBINATION WITH ANOTHER GAMMA-SUBUNIT

Antibodies specific for the gamma 1, gamma 2, and gamma 3 subunits of the gamma-aminobutyric acid (GABA)(A) receptor have been used to probe the composition of naturally occurring GABA(A) receptors in the rat b rain. Most GABA(A) receptors contain at least one of these three subun its. The percentage of each, determined by immunoprecipitation of [H-3 ]muscimol binding, was 11 +/- 1%, 59 +/- 3%, and 14 +/- 2% for gamma 1 , gamma 2, and gamma 3 subunits, respectively. Receptors containing ga mma 2 or gamma 3 subunits were labeled by benzodiazepine site ligands with high affinity, whereas gamma 1-containing receptors could be labe led only by [H-3]muscimol. Receptors immunoprecipitated by anti-gamma 2 or anti-gamma 3 antibodies were labeled with [H-3]Ro 15-1788 with si milar affinities (K-d for anti-gamma 2-immunoprecipitated receptors, 1 .9 nM; K-d for anti-gamma 3-immunoprecipitated receptors, 1.7 nM). Imm unoprecipitation or Western blot analysis of GABA(A) receptors solubil ized from rat cerebellar or whole-brain preparations indicated that ga mma 1 was not present coassembled with any other gamma subunit. Wester n blot analysis of receptors purified on alpha-specific immunoaffinity resins showed that gamma 1 was predominantly assembled with the alpha 2 subunit. Some GABA(A) receptors may contain more than one type of g amma subunit. Quantitative immunoprecipitation and Western blot analys is both indicated that gamma 2 and gamma 3 subunits can exist in the s ame receptor complex. A large proportion of GABA(A) receptors immunopu rified on a gamma 3 affinity resin also appeared to contain a gamma 2 subunit. In contrast, when receptors were purified on a gamma 2 affini ty resin a small proportion also appeared to contain a gamma 3 subunit . We conclude that most gamma 1-containing receptors have no other gam ma subunit in the same receptor complex but some GABA(A) receptors con tain both gamma 2 and gamma 3 subunits.