Nicotinic Acetylcholine Receptor Channel Electrostatics Determined by Diffusion-enhanced Luminescence Energy Transfer

The electrostatic potentials within the pore of the nicotinic acetylcholine receptor (nAChR) were determined using lanthanide-based diffusion-enhanced fluorescence energy transfer experiments. Freely diffusing Tb3+ -chelates of varying charge constituted a set of energy transfer donors to the acceptor, crystal violet, a noncompetitive antagonist of the nAChR. Energy transfer from a neutral Tb3+ -chelate to nAChR-bound crystal violet was reduced 95% relative to the energy transfer to free crystal violet. This result indicated that crystal violet was strongly shielded from solvent when bound to the nAChR. Comparison of energy transfer from positively and negatively charged chelates indicate negative electrostatic potentials of -25 mV in the channel, measured in low ionic strength, and -10 mV measured in physiological ionic strength. Debye-Hückel analyses of potentials determined at various ionic strengths were consistent with 1-2 negative charges within 8 A of the crystal violet binding site. To complement the energy transfer experiments, the influence of pH and ionic strength on the binding of [3H]phencyclidine were determined. The ionic strength dependence of binding affinity was consistent with -3.3 charges within 8 A of the binding site, according to Debye-Hückel analysis. The pH dependence of binding had an apparent pKa of 7.2, a value indicative of a potential near -170 mV if the titratable residues are constituted of aspartates and glutamates. It is concluded that long-range potentials are small and likely contribute little to selectivity or conductance whereas close interactions are more likely to contribute to electrostatic stabilization of ions and binding of noncompetitive antagonists within the channel.