Coexistence of two beta subunit isoforms in the same gamma-aminobutyric acid type A receptor

Three novel subunit-specific antisera to the beta(1), beta(2), and beta(3) subunits of rat gamma-aminobutyric acid type A (GABA(A)) receptors have been used to study the native receptor in the rat brain. Affinity-purified anti-beta(1), anti-beta(2), and anti-beta(2) antibodies recognized in immunoblots protein bands of 57, 55, and 57 kDa, respectively. Quantitative immunoprecipitation of solubilized GABA(A) receptors from various rat brain regions showed that the beta(3), subunit was the most abundant isoform in cerebellum (in 96% of the GABA(A) receptors) and cerebral cortex (64%) but that it was the least abundant isoform in hippocampus (44%). The beta(3) subunit was found most abundant in hippocampus (64%) followed by cerebral cortex (48%) and cerebellum (33%). The beta(1) subunit was present in a very small proportion of the cerebellar GABA(A) receptors (3%), but it was present in a high proportion of the GABA(A) receptors from the hippocampus (49%) and cerebral cortex (32%). Quantitative receptor immunoprecipitation or immunopurification followed by immunoblotting experiments have revealed the existence of colocalization of two different beta subunit isoforms in a significant proportion of the brain GABA(A) receptors. Thus, in the rat cerebral cortex 33% of the GABA(A) receptors have both beta(2) and beta(3) subunits, and 19% of the receptors have both beta(2) and beta(3) subunits. The extent of colocalization of beta subunit isoforms varied among brain regions, being highest in hippocampus and lowest in cerebellum. These and other results taken together suggest that the number of alpha, beta, and gamma subunits (stoichiometry) in the brain GABA(A) receptor pentamers might not be unique. It might vary depending on receptor type.