FUNCTIONAL-CHARACTERIZATION OF HUMAN GAMMA-AMINOBUTYRIC ACID(A) RECEPTORS CONTAINING THE ALPHA-4 SUBUNIT

The alpha subunits are an important determinant of the pharmocology of gamma-aminobutyric acid(A) (GABA(A)) receptors With respect to agonis ts, antagonists, and modulatory compounds, particularly the benzodiaze pines. The alpha 4 subunit is the least abundant subunit in the brain and the most similar in deduced primary amino acid sequence to the alp ha 6 subunit. We demonstrate that the human alpha 4 subunit forms a fu nctional receptor when expressed with beta gamma 2, demonstrating some properties similar to alpha 6 beta gamma 2 and some properties more a kin to alpha 1 beta gamma 2. It also exhibited some properties that we re unlike any other alpha subunit-containing receptor. GABA affinity s eemed to be identical to that of the alpha 1 beta 1 gamma 2 receptor; however, The partial agonists 4,5,6,7-tetrahydroisoxazolo-[5,4-c]pyrid in-3-ol and piperidine-4-sulfonic acid showed lower efficacy than at e ither alpha 1 beta 1 gamma 2 or alpha 6 beta 1 gamma 2. Benzodiazepine pharmacology of alpha 4-containing receptors was similar to that of a lpha 6-containing receptors with the exception of dimethoxy-4-ethyl-be ta-carboline-3-carboxylate, which behaved as a partial inverse agonist , Pentobarbital potentiated alpha 4 beta 1 gamma 2 receptor GABA respo nses to a level comparable with alpha 6 beta 1 gamma 2 (similar to 700 % of EC(50)); however, unlike alpha 6 beta 1 gamma 2 receptors, it did not elicit any direct activation of the receptor. Propofol also poten tiated alpha 4 beta 1 gamma 2 GABA responses but to a level more compa rable to that of alpha 1 beta 1 gamma 2, suggesting that these compoun ds act via different sites, Unlike other subunit combinations, propofo l did not elicit a direct activation of the receptor. These results su ggest that the mechanism for direct activation of the GABA(A) receptor by pentobarbital and propofol is absent on alpha 4-containing recepto rs, Furosemide, which noncompetitively inhibits the GABA(A) receptor, showed 700-fold selectivity for alpha 6 beta 3 gamma 2 receptors over alpha 1-, alpha 2-, alpha 3-, and alpha 5-containing receptors and exh ibited selectivity for alpha 4 beta 3 gamma 2 receptors (> 50-fold), T hese experiments reveal a unique pharmacology for alpha 4-containing r eceptors with some similarities to both alpha 6- and alpha 1-containin g receptors.