Molecular pharmacology of gamma-aminobutyric acid type A receptor agonists and partial agonists in oocytes injected with different alpha, beta, and gamma receptor subunit combinations.

Using systematic combination of alpha 1, alpha 3, and alpha 5 with beta 1, beta 2, and beta 3, together with gamma 1, gamma 2, and gamma 3, we have investigated the contributions of the various alpha, beta, and gamma subunits to the pharmacology of gamma-aminobutyric acid (GABA)A agonists. We have characterized GABA, (RS)-dihydromuscimol, piperidine-4-sulfonic acid, and 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol with recombinant human GABAA receptors expressed in Xenopus oocytes. Our observations indicate that the alpha subunit is the major determinant of efficacy for partial GABAA agonists. When alpha 1 and alpha 3 or alpha 1 and alpha 5 are coexpressed, the alpha 1 subunit determines the maximum efficacy, whereas the affinity is determined by the entire combination of subunits. Thus, the results of the present study demonstrate that the pharmacology of GABAA agonists is dependent on the subunit composition of the GABAA receptor complex. Functional GABAA receptors containing two different alpha subunits show pharmacological profiles distinctly different from those of receptors containing a single alpha subtype, indicating that two different alpha subunits can be coexpressed in one functional GABAA receptor complex.