A functional comparison of the antagonists bicuculline and picrotoxin at recombinant GABAA receptors

Allosteric modulation of GABAA receptor function by a number of ligands has been shown to be dependent on the subunit composition of the receptor complex. In this respect, modulation of GABAA receptors by the antagonists bicuculline and picrotoxin was examined in Xenopus laevis oocytes expressing recombinant GABAA receptors composed of combinations of murine alpha 1, beta 1, gamma 2S and gamma 2L subunits. Bicuculline and picrotoxin reduced GABA-activated responses mediated by GABAA receptors composed of alpha 1 beta 1, alpha 1 beta 1 gamma 2S and alpha 1 beta 1 gamma 2L subunits in a dose-dependent manner. GABA equilibrium concentration-response curves for each receptor construct were shifted to the right by increasing concentrations of bicuculline in a competitive manner, whereas picrotoxin induced a slight lateral shift as well as a depression of the maximum response consistent with a mixed/non-competitive inhibitory mechanism. GABA concentration-response curves in the absence and presence of bicuculline were subjected to Schild analysis, which revealed similar pKB values of approximately 5.9 for alpha 1 beta 1, alpha 1 beta 1 gamma 2S and alpha 1 beta 1 gamma 2L receptor constructs. Concentration inhibition curves were used to estimate IC50 for picrotoxin were relatively unaffected by the GABAA receptor isoforms used in this study, and in particular, by the absence of the gamma 2 subunit in the alpha 1 beta 1 GABAA receptor complex. The similarity of the pKBs reported in this study to those previously reported using native neuronal preparations, which are likely to represent heterogeneous GABAA receptor populations, further indicates the lack of dependence on receptor subunit composition for the inhibitory action of bicuculline.