The efferent-mediated suppression of otoacoustic emissions in awake guinea pigs and its reversible blockage by gentamicin.

The physiology of the medial efferent olivocochlear system involves suppressive interactions of contralateral sounds on ipsilateral sound-evoked responses, but its role is largely unknown to date. Medial efferents act at the level of cochlear outer hair cells via cholinergic synapses and might affect their mechanical activity, thereby modulating auditory sensitivity. The aim of the present work was to obtain noninvasive measurements of distortion-product otoacoustic emissions (DPOEs), which reflect outer hair cell function, in order to establish the characteristics of medial efferent-induced suppression in awake, restrained guinea pigs. A clear suppression of DPOEs was induced by continuous contralateral white noise presented at 20-70 dB SPL, in the absence of any confounding effect of anesthesia, middle-ear muscles, or acoustic cross talk. Recently, acute injection of a high dose of the aminoglycoside antibiotic gentamicin (150 mg/kg) was reported to alter the suppressive effect of contralateral noise on eighth nerve-compound action potentials, presumably by blocking efferent synapses to outer hair cells. This hypothesis was confirmed with DPOEs for which a single injection of gentamicin at the same dose abolished suppression after about 1-2 h, whereas no change in basal levels was observed. Complete recovery was obtained after 48 h. This experiment may provide an easy, noninvasive tool for studying auditory function with and without functioning efferents.