Membrane impermeant antioestrogens discriminate between ligand- and voltage-gated cation channels in NG108-15 cells

Native 5-HT3 and AChR ligand-gated cation channels can be inhibited (blocked) by the non-steroidal antioestrogentamoxifen. However, the exact site and mechanism of inhibition by tamoxifen on these channels remain unclear. We haveinvestigated the action of the membrane impermeant quaternary derivative, ethylbromide tamoxifen (EBT), on native ligandgated5-HT3 receptor channels and voltage-gated K+ channels in NG108-15 cells using whole cell patch clamp. ExtracellularEBT inhibited whole cell cationic currents of 5-HT3 receptors with IC50 of 0.22 þ 0.4 WM (nH = 1.05 þ 0.2). The channel blockwas characterised by voltage independent and use independent behaviour (similar to that of tamoxifen). EBT was unable toinhibit voltage-gated K+ currents in NG108-15 cells. This was in contrast to the inhibition by tamoxifen which, at similarconcentrations, accelerated the apparent inactivation of these outward K+ currents. The inhibition of 5-HT3 receptors by amembrane impermeant derivative of tamoxifen supports the view that the binding site for antioestrogens is extracellular andthe inhibition is not mediated through genomic/transcriptional activity