Terpene Trilactones from Ginkgo biloba Are Antagonists of Cortical Glycine and GABAA Receptors

Glycine and γ-aminobutyric acid, type A (GABAA) receptors are members of the ligand-gated ion channel superfamily that mediate inhibitory synaptic transmission in the adult central nervous system. During development, the activation of these receptors leads to membrane depolarization. Ligands for the two receptors have important implications both in disease therapy and as pharmacological tools. Terpene trilactones (ginkgolides and bilobalide) are unique constituents of Ginkgo biloba extracts that have various effects on the central nervous system. We have investigated the relative potency of these compounds on glycine and GABAA receptors. We find that most of the ginkgolides are selective and potent antagonists of the glycine receptor. Bilobalide, the single major component in G. biloba extracts, also reduces glycine-induced currents, although to a lesser extent. Both ginkgolides and bilobalide inhibit GABAA receptors, with bilobalide demonstrating a more potent effect. Additionally, we provide evidence that open channels are required for glycine receptor inhibition by ginkgolides. Finally, we employ molecular modeling to elucidate the similarities and differences in the structure of the terpene trilactones to account for the pharmacological properties of these compounds and demonstrate a striking similarity between ginkgolides and picrotoxinin, a GABAA and recombinant glycine α-homomeric receptor antagonist.