1989 年 94 巻 1 号 p. 7-15
The γ-aminobutyric acid (GABA) receptor has been classified into two receptor subtypes (GABAA and GABAAB receptors) based on their pharmacological properties. The GABAAA receptor in the central nervous system (CNS) has been found to be coupled structurally as well as functionally with the benzodiazepine receptor and Cl- channel. Purified GABAAA receptor from bovine brain consisted of both α and β subunits. The complementary DNAS encoding the GABAAA receptor α and β subunits have been cloned; and from their elucidated nucleotide sequences, the amino acid sequences of the subunits were deduced. The structure of both subunits, having four putative membrane domains, has been found to be similar to other ligand-gated receptors such as the nicotinic acetylcholine receptor α subunit and glycine receptor 48K subunit. Therefore, it has been suggested that these ligand-gated receptors comprise a superfamily. In addition, the presence of similarities in the nucleotide and deduced amino acid sequences of human brain GABAAA receptor with those of bovine brain has been noted. On the other hand, the GABAB receptor, which is insensitive to bicuculline but sensitive to baclofen, has been found to be pharmacologically distinct from the GABAAA receptor. The GABAB receptor in the brain has been found to be coupled with GTP-binding protein and generates the inhibitory transmission coupled with various intracellular effector systems such as adenylate cyclase and phosphoinositides turnover. The exact structure and function of the GABAB receptor in the CNS, however, remain to be clarified in future studies.