Ionotropic receptor with a probable role in the modulation of auditory stimuli. Agonist binding induces a conformation change that leads to the opening of an ion-conducting channel across the plasma membrane. The channel is permeable to a range of divalent cations including calcium, the influx of which may activate a potassium current which hyperpolarizes the cell membrane. In the ear, this leads to a reduction in basilar membrane motion, altering the activity of auditory nerve fibers and reducing the range of dynamic hearing. This may protect against acoustic trauma. May also regulate keratinocyte adhesion. Can form homo- or heterooligomeric channels in conjunction with CHRNA10. The native outer hair cell receptor may be composed of CHRNA9-CHRNA10 heterooligomers. Interacts with the alpha-conotoxin RgIA (PubMed:25740413). Detected in the nasal epithelium, in the outer hair cells of the cochlea, in the pars tuberalis of the hypophysis, and in the developing muscle of the tongue. Also expressed in the neurons of dorsal root ganglia. The heterooligomeric receptor composed of CHRNA9 and CHRNA10 has an atypical pharmacological profile, binding several non-nicotinic ligands including strychnine (a glycine receptor antagonist) and atropine (a muscarinic acetylcholine receptor antagonist). Belongs to the ligand-gated ion channel (TC 1.A.9) family. Acetylcholine receptor (TC 1.A.9.1) subfamily. Alpha-9/CHRNA9 sub-subfamily.