Selective effects of a 4-oxystilbene derivative on wild and mutant neuronal chick a7 nicotinic receptor

We assessed the pharmacological activity of triethyl-(β-4-stilbenoxy-ethyl) ammonium (MG624), a drug that is active on neuronal nicotinic receptors (nicotinic AChR). Experiments on the major nicotinic AChR subtypes present in chick brain, showed that it inhibits the binding of [125I]-αBungarotoxin (αBgtx) to the α7 subtype, and that of [3H]-epibatidine (Epi) to the α4β2 subtype, with Ki values of respectively 106 nM and 84 μM. MG624 also inhibited ACh elicited currents (IACh) in the oocyte-expressed α7 and α4β2 chick subtypes with half-inhibitory concentrations (IC50) of respectively 109 nM and 3.2 μM. When tested on muscle-type AChR, it inhibited [125I]-αBgtx binding with a Ki of 32 μM and ACh elicited currents (IACh) in the oocyte-expressed α1β1γδ chick subtype with an IC50 of 2.9 μM. The interaction of MG624 with the α7 subtype was investigated using an α7 homomeric mutant receptor with a threonine-for-leucine 247 substitution (L247T α7). MG624 did not induce any current in oocytes expressing the wild type α7 receptor, but did induce large currents in the oocyte-expressed L247T α7 receptor. The MG624 elicited current (IMG624) has an EC50 of 0.2 nM and a Hill coefficient nH of 1.9, and is blocked by the nicotinic receptor antagonist methyllycaconitine (MLA). These binding and electrophysiological studies show that MG624 is a potent antagonist of neuronal chick α7 nicotinic AChR, and becomes a competitive agonist following the mutation of the highly conserved leucine residue 247 located in the M2 channel domain.