GABAA receptor cysteinyl mutants and the ginkgo terpenoid lactones bilobalide and ginkgolides

The terpenoid lactones from Ginkgo biloba, bilobalide and ginkgolides, have been shown to act as negative modulators at α1β2γ2L GABAA receptors. They have structural features similar to those of the chloride channel blocker picrotoxinin. Unlike picrotoxinin, however they are not known to produce convulsant effects. Using two-electrode voltage clamp electrophysiology, this study compared the effect of mutation of 2′, 6′ and 15′ pore facing M2 domain residues to cysteine on the action of picrotoxinin, bilobalide and ginkgolides at α1β2γ2L GABAA receptors expressed in Xenopus oocytes. Picrotoxinin was affected by mutation differently from the ginkgo terpenoid lactones. Although some of these compounds were affected by the mutation at same position and/or subunit, the changes in their potency were found to be dissimilar. The results suggest that the intracellular pore binding site for picrotoxinin, bilobalide, ginkgolide A, ginkgolide B and ginkgolide C is comprised of 2′β-6′β6′γ, 2′α2′β-6′α6′β, 2′α2′β2′γ-6′β6′γ, 2′α, 2′β2′γ-6′β and 2′α2′β, respectively. Unlike bilobalide and ginkgolides, the inhibitory action of picrotoxinin was not affected by mutations at 15′ position. It is proposed that 15′α15′β, 15′β, 15′α15′β and 15′α15′β15′γ forms an extracellular pore binding site for bilobalide, ginkgolide A, ginkgolide B and ginkgolide C, respectively. The lack of convulsant effects of bilobalide, and ginkgolide A and B may be associated in part with their different binding locations within the chloride channel.