The β subunit increases the ginkgolide B sensitivity of inhibitory glycine receptors

We investigated the effect of ginkgolide B (GB), a component of the extract from the leaves of the Ginkgo biloba tree, on recombinant glycine receptors (GlyRs) expressed in Xenopus oocytes by using voltage-clamp recording. GB (0.01-10 μM) inhibited glycine-induced currents of homo-oligomeric α1, α2 and α3 GlyRs, with the highest potency being found at the α1 GlyR (IC50 value = 0.61 ± 0.1 μM). Coexpression of the α subunits with the β subunit resulted in a shift of the IC50 value of GB to nanomolar values, indicating selectivity of GB for β subunit containing GlyRs. We also analyzed the mechanism of GB inhibition and the effect of point mutations introduced into the α1 subunit. Our results are consistent with a channel blocking effect, since (i) GB inhibited glycine currents non-competitively, and (ii) a point mutation in the pore forming M2 domain reduced GB potency. In conclusion, GB is a potent blocker of β subunit containing GlyR channels and hence can be used to discriminate homo- from hetero-oligomeric GlyRs. As hetero-oligomeric GlyRs are known to be synaptically localized, GB represents a channel blocker that may be employed to separate extrasynaptic from synaptic glycine currents.