Changes in the conductance of the neuronal nicotinic acetylcholine receptor channel induced by magnesium

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Abstract
We have investigated the effect of magnesium on the single-channel conductance of neuronal nicotinic acetylcholine receptors (nAChRs) in nerve growth-factor treated rat pheochromocytoma (PC12) cells. The patch-clamp technique was used to record single-channel currents from cell attached and excised, outside-out patches in the presence of various internal and external Mg$^{2+}$ concentrations. Mg$^{2+}$ reduced the single-channel conductance in a concentration-dependent manner with an IC$_{50}$ of 9.2 mM for external Mg$^{2+}$ (inward conductance) and 0.69 mM for internal Mg$^{2+}$ (outward conductance). Both estimated and measured conductances for divalent cation-free CsCl solutions were around 60 pS. We also find that divalent cations are not involved in the inward rectification of whole-cell ACh-induced currents in these cells. Our results imply that the amino acids screened by divalent cations sense electric fields only weakly and are presumably outside the lipid bilayer. They also suggest that the density and the number of charges (or both) differ on either side of the ion pore.