The α9α10 nicotinic acetylcholine receptor is permeable to and is modulated by divalent cations

The native cholinergic receptor that mediates synaptic transmission between olivocochlear fibers and outer hair cells of the cochlea is permeable to Ca²⁺ and is thought to be composed of both the α9 and the α10 cholinergic nicotinic subunits. The aim of the present work was to study the permeability of the recombinant α9α10 nicotinic acetylcholine receptor to Ca²⁺, Ba²⁺ and Mg²⁺ and its modulation by these divalent cations. Experiments were performed, by the two-electrode voltage-clamp technique, in Xenopus laevis oocytes injected with α9 and α10 cRNA. The relative divalent to monovalent cation permeability was high (∼10) for Ca²⁺, Ba²⁺ and Mg²⁺. Currents evoked by acetylcholine (ACh) were potentiated by either Ca²⁺ or Ba²⁺ up to 500 μM but were blocked by higher concentrations of these cations. Potentiation by Ca²⁺ was voltage-independent, whereas blockage was stronger at hyperpolarized than at depolarized potentials. Mg²⁺ did not potentiate but it blocked ACh-evoked currents (IC₅₀=0.38 mM). In the absence of Ca²⁺, the EC₅₀ for ACh was higher (48 μM) than that obtained with 1.8 mM Ca²⁺ (14.3 μM), suggesting that potentiation by Ca²⁺ involves changes in the apparent affinity of the α9α10 receptor for ACh.