Mechanisms of ion selectivity and ion-pore interactions in a Ligand-Gated Ion Channel

The elementary response of the nicotinic receptor (nAChR) is the rapid and selective transmembrane flow of small cations. We examined the amplitude and fluctuations of unitary ionic currents through wild-type and mutant nAChRs using single-channel electrophysiology combined with site-directed mutagenesis. Conductance and power spectral analyses showed that both current amplitude and open channel power spectra depend strongly on the permeant cation and specific transmembrane structural determinants. In wild-type receptors, current amplitude and power spectra vary systematically with the ion’s hydrated radius, reflecting size limitations of the pore and differences in ion–pore interactions. Additionally, for each permeant ion current-voltage relationships showed ohmic conductance and lack of rectification, indicating a functionally symmetric ion pore. Mutations at a transmembrane salt bridge, a membrane-facing cysteine, and a selectivity-filter residue each altered the current amplitude, power spectra, and rectification in distinct ways. These results highlight the importance of ion-pore interactions in determining the amplitude, dynamics, and rectification of ionic currents through the nAChR that contribute to neuronal excitability.