The agonistic action of pentobarbital on GABAA beta-subunit homomeric receptors.

Murine gamma-aminobutyric acid type A (GABAA) receptor beta 1, beta 2, and beta 3 subunits were expressed in Xenopus oocytes and studied using the two electrode voltage clamp technique. Although all three beta-subunits were unresponsive to GABA when expressed as homomers, the intravenous general anaesthetics pentobarbital, etomidate and propofol induced currents in beta 2 and beta 3 homomers. The pentobarbital-induced currents in beta 3 homomers showed a dose dependence with an ED50 of 89 +/- 8.9 microM and a Hill coefficient of 0.94 +/- 0.08. Zinc (50 microM) blocked (61.1 +/- 5.6% of control) and 200 microM lanthanum potentiated (139 +/- 8.6% of control) the pentobarbital-induced current. This current was also blocked by picrotoxin but was insensitive to the GABAA receptor antagonist bicuculline. These observations indicate that the full expression of the agonistic action of GABA requires the presence of an alpha-subunit, in contrast to the agonistic action of intravenous general anesthetics, where the presence of a beta2 or beta 3-subunit is sufficient. The difference in the agonistic action of intravenous anaesthetics among these highly homologous beta-subunits suggests that the beta-subunit homomeric receptors may be useful to further define the molecular sites of action of intravenous general anaesthetics and other functional domains on GABAA receptors.