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 mu M) inhibited glycine-induced currents of homo-oligomeric alpha 1, alpha 2 and alpha 3 GlyRs, with the highest potency being found at the alpha 1 GlyR (IC50 value = 0.61 +/- 0.1 mu M). Coexpression of the alpha subunits with the beta subunit resulted in a shift of the IC50 value of GB to nanomolar values, indicating selectivity of GB for beta subunit containing GlyRs. We also analyzed the mechanism of GB inhibition and the effect of point mutations introduced into the alpha 1 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 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. (c) 2005 Elsevier Ltd. All rights reserved.