A point mutation in the a9 nicotinic acetylcholine receptor subunit prolongs efferent inhibition to cochlear hair cells and attenuates sound-induced acoustic injury

TARANDA J; MAISON, S.F; BALLESTERO J; KATZ E; VETTER D; BOULTER J; LIBERMAN, M.C.; FUCHS P; ELGOYHEN AB

Sanger Wellcome Center, Cambridge, UK

Congreso; Conference Meeting, Nicotinic Acetylcholine Receptors; 2008

Gene knock-out mice helped establish that both a9 and a10 subunits are necessary to form the nicotinic acetylcholine receptor (nAChR) mediating synaptic transmission between medial efferent olivocochlear (OC) fibers and outer hair cells of the cochlea. However, in a9 and a10 knock-outs, lack of OC function was evident only when this feedback system was activated by shocks. The knockouts showed no alterations in cochlear function when assessed by baseline auditory brainstem responses or distortion product otoacoustic emissions, thus little was revealed about the function of this feedback system. In this report we used a complementary strategy to probe the role of a9-containing nAChRs in cochlear physiology. We generated a line of knock-in mice which harbor a leucine for threonine substitution at position 9’ (L9’T) of the second transmembrane domain of the a9 protein. This mutation rendered a9-containing receptors hypersensitive to acetylcholine with slower desensitization kinetics. In addition, it produced a 4 to 5-fold prolongation of efferent synaptic currents recorded in hair cells of cochlear explants. In vivo, shock-evoked inhibition of cochlear responses was dramatically enhanced and lengthened. Interestingly, baseline auditory thresholds were improved by pharmacologically blocking hair cell nAChRs with strychnine, revealing a tonic inhibition in cochleae of homozygous knock-in mice. Finally, acoustic over-exposure caused less permanent threshold elevation in the knock-in mice compared to their wild-type littermates. Thus, a9-containing nAChRs are sufficient to underlie two effects ascribed to the medial OC efferent innervation of hair cells: inhibition of cochlear sensitivity and protection from sound-induced permanent acoustic trauma.