Ligand-binding properties of an unusual nicotinic acetylcholine receptor subtype on isolated outer hair cells from guinea pig cochlea

Acetylcholine receptors on isolated guinea pig cochlear outer hair cells (OHC) were characterized by radioligand binding. Equilibrium binding of [ I25]α-bungarotoxin revealed a K D of 62 ± 2 nM, B max = 7.2 ± 1.8 × 10 7 binding sites/OHC, and a slowly reversible dissociation rate constant, k −1 = 2.2 ± 0.01 × 10 −4 min −1. L-[ 3H]Nicotine bound reversibly (estimated K rmD ≈ 230 nM and B max ≈ 5 × 10 7) with kinetic rate constants of association k 1 = 6.2 ± 0.06 x 10 4 min −1 nM −1 and dissociation k −1 = 0.23 ± 0.003 min −1. [ 3H]Strychnine bound to OHC with a K D of 35 ± 6 nM and B max = 2.6 ± 0.5 × 10 7, and binding increased 3–4 fold after membrane depolarization with 56.2 mM [K +], suggesting additional binding sites. Binding, seen only at > nM concentrations, of [ 3H]3-quinuclidinyl benzilate ( K D = 11.5 ± 5 nM; B max = 2.5 ± 0.6 × 106) was competitively inhibited by the muscarinic antagonists atropine and 4-DAMP (IC 50 of 6.1 ± 0.5 and 6.5 ± 0.4 nM). The OHC receptor is thus an atypical nicotinic acetylcholine receptor subtype with unusual pharmacological properties.