Micromechanical effects in the cochlea of tetracaine

Local anesthetics applied in the tympanic cavity have earlier been shown to affect the gross receptor potentials in reducing the cochlear microphonics and increasing the positive summating potential. To study the effects of t his drug on the mechanical responses in the cochlea, vibrations were measur ed using laser heterodyne interferometry in an isolated in vitro temporal b one preparation from the guinea pig. Measurements were made at a set of fre quencies in the fourth cochlear turn from the Hensen's cells and the outer hair cells in response to sound applied to the ear. The tuning curves of th e fundamental and the second harmonic components of the vibratory responses were plotted. When 2 mM tetracaine was applied, the high frequency slope o f the second harmonic curve shifted down in frequency, this caused the freq uency of the maximum of second harmonic tuning to shift down. These changes were reversible when tetracaine was washed out. Observations were also mad e in the temporal bone preparation in vitro with a confocal microscope. Flu orescent probes were used to label various structures in the organ of Corti . Optical sections were obtained by tilting the organ permitting a view fro m the side like a radial section through the organ. Images were acquired be fore, during and after application of tetracaine and were later analyzed wi th a computer program. Simultaneously, cochlear microphonics and the summat ing potential were obtained to monitor the electrical response of the prepa ration. Although the cochlear microphonics and summating potential decrease d when 2 mM tetracaine was applied, structural changes were not measurable in the organ of Corti. The decrease was reversible when tetracaine was wash ed out. It is concluded that tetracaine affected the high frequency part of the non-linear second harmonic component, possibly by lowering the stiffne ss of the stereocilia bundle or the body of the outer hair cells. (C) 1999 Elsevier Science B.V. All rights reserved.