Gentamicin blocks both fast and slow effects of olivocochlear activation in anesthetized guinea pigs

The medial olivocochlear (MOC) efferent system, which innervates cochlear o uter hair cells, suppresses cochlear responses. MOC-mediated suppression in cludes both slow and fast components, with time courses differing by three orders of magnitude. Pharmacological studies in anesthetized guinea pigs su ggest that both slow and fast effects on cochlear responses require an init ial acetylcholine activation of alpha-9 nicotinic receptors on outer hair c ells and that slow effects require additional intracellular events downstre am from those mediating fast effects. Gentamicin, an aminoglycoside antibio tic, has been reported to block fast effects of sound-evoked OC activation following intramuscular injection in unanesthetized guinea pigs, without ch anging slow effects. In the present study, we show that electrically evoked fast and slow effects in the anesthetized guinea pig are both blocked by e ither intramuscular or intracochlear gentamicin, with similar time courses and/or dose-response curves. We suggest that sound-evoked slow effects in u nanesthetized animals are fundamentally different from electrically evoked slow effects in anesthetized animals, and that the former may arise from ef fects of the lateral OC system.