Inorganic, monovalent cations compete with agonists for the transmitter binding site of nicotinic acetylcholine receptors

The properties of adult mouse recombinant nicotinic acetylcholine receptors activated by acetylcholine (ACh⁺) or tetramethylammonium (TMA⁺) were examined at the single-channel level. The midpoint of the dose-response curve depended on the type of monovalent cation present in the extracellular solution. The shifts in the midpoint were apparent with both inward and outward currents, suggesting that the salient interaction is with the extracellular domain of the receptor. Kinetic modeling was used to estimate the rate constants for agonist binding and channel gating in both wild-type and mutant receptors exposed to Na⁺, K⁺, or Cs⁺. The results indicate that in adult receptors, the two binding sites have the same equilibrium dissociation constant for agonists. The agonist association rate constant was influenced by the ionic composition of the extracellular solution whereas the rate constants for agonist dissociation, channel opening, and channel closing were not. In low-ionic-strength solutions the apparent association rate constant increased in a manner that suggests that inorganic cations are competitive inhibitors of ACh⁺ binding. There was no evidence of an electrostatic potential at the transmitter binding site. The equilibrium dissociation constants for inorganic ions (Na⁺, 151 mM; K⁺, 92 mM; Cs⁺, 38 mM) and agonists (TMA⁺, 0.5 mM) indicate that the transmitter binding site is hydrophobic. Under physiological conditions, about half of the binding sites in resting receptors are occupied by Na⁺.