A novel cholinergic receptor mediates inhibition of chick cochlear hair cells

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Abstract
The central nervous system provides feedback regulation at several points within the peripheral auditory apparatus. One component of that feedback is inhibition of cochlear hair cells by release of acetylcholine (ACh) from efferent brainstem neurons. The mechanism of hair cell inhibition, and the character of the presumed cholinergic receptor, however, have eluded understanding. Both nicotinic and muscarinic, as well as some non-cholinergic ligands can affect the efferent action. We have made whole-cell, tight-seal recordings from short (outer) hair cells isolated from the chick's cochlea. These are the principal targets of cochlear efferents in birds. ACh hyperpolarizes short hair cells by opening a cation channel through which Ca$^{2+}$ enters the cell and subsequently activates Ca$^{2+}$-dependent K$^{+}$ current (Fuchs & Murrow 1991, 1992). Both curare and atropine are effective antagonists of cholinergic inhibition at 3 $\mu $M, whereas trimethaphan camsylate and strychnine block at 1 $\mu $M. The normally irreversible nicotinic antagonist, $\alpha $-bungarotoxin, reversibly blocked the hair cell response, as did $\kappa $-bungarotoxin. The half-blocking concentration for $\alpha $-bungarotoxin was 26 nM. It is proposed that the hair cell AChR is a ligand-gated cation channel related to the nicotinic receptor of nerve and muscle.