Intracochlear application of acetylcholine alters sound-induced mechanical events within the cochlear partition

Activation of olivocochlear (OC) efferent fibers has been suggested to alter micromechanical events occurring within the cochlear partition, possibly through an effect of the efferent neurotransmitter (acetylcholine; ACh) on outer hair cells (OHCs). Based on the widely-accepted assumption that otoacoustic emissions reflect OHC activity, we investigated the in vivo influence of ACh on OHCs by studying alterations in emission amplitude with local ACh application. Distortion product otoacoustic emissions (DPOAEs) were measured in anesthetized guinea pigs before, during, and after intracochlear application of ACh (250 μM) with the cholinesterase inhibitor, eserine (20 μM). Perfusion of ACh/eserine was associated with a desensitizing reduction in DPOAE amplitude of approximately 4.4 dB. This reduction was intensity-dependent, with greater and more consistent reductions observed for DPOAEs elicited by low- than by moderate-intensity primaries. The response reduction was not seen during consecutive ACh perfusions performed without an intervening artificial perilymph wash, and was effectively blocked in the presence of pharmacologic antagonists of OC efferent activity (curare, 50 μM, strychnine, 50 μM). Finally, a similar alteration in DPOAE amplitude was never seen during perfusion of the control (artificial perilymph) solution alone. It is argued that these results support the hypothesis that OC efferent activation can alter sound-induced cochlear mechanical events.