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ABSTRACT: There is current debate about the origin of mechanical amplification whereby outer hair cells generate force to augment the sensitivity and frequency selectivity of the mammalian cochlea. To distinguish contributions to force production from the mechanotransducer (MET) channels and somatic motility, we have measured hair bundle motion during depolarization of individual outer hair cells in isolated rat cochleas. Depolarization evoked rapid positive bundle deflections that were reduced by perfusion with the MET channel blocker dihydrostreptomycin, with no effect on the nonlinear capacitance that is a manifestation of prestin-driven somatic motility. However, the movements were also diminished by Na salicylate and depended on the intracellular anion, properties implying involvement of the prestin motor. Furthermore, depolarization of one outer hair cell caused motion of neighboring hair bundles, indicating overall motion of the reticular lamina. Depolarization of solitary outer hair cells caused cell-length changes whose voltage-activation range depended on the intracellular anion but were insensitive to dihydrostreptomycin. These results imply that both the MET channels and the somatic motor participate in hair bundle motion evoked by depolarization. It is conceivable that the two processes can interact, a signal from the MET channels being capable of modulating the activity of the prestin motor.
Journal of Neuroscience 04/2006; 26(10):2757-66. · 7.11 Impact Factor
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ABSTRACT: In the first step in auditory transduction, sound-induced vibrations of the stereociliary bundles on the sensory hair cells are converted into electrical signals by opening of mechanotransducer channels. Faithful transduction and hence auditory performance will be limited by the kinetic properties of these channels. We have measured the time course of mechanotransducer currents in turtle and rat auditory hair cells during rapid deflections of the hair bundle. Current activation in the turtle had a time constant that decreased 10-fold with stimulus amplitude to a limiting value of approximately 50 micros. Lowering the external Ca2+ concentration slowed both activation and adaptation time constants. Similar effects were seen in hair cells tuned to low and high frequencies, but the overall kinetics was slower in low-frequency cells. In rat outer hair cells, the time courses of both activation and adaptation were at least 10-fold faster. Although activation kinetics was too fast to characterize accurately, the adaptation time constants in the rat, like the turtle, were Ca2+ dependent and faster in hair cells tuned to higher frequencies. The results imply that mechanotransducer channels operate similarly in turtle and rat but are faster in the mammal to accommodate its higher frequency range of hearing. We suggest that the kinetics of channel activation and adaptation imposes a bandpass filter on transduction, with a center frequency matched to the frequencies detected by the hair cell, which may improve the signal-to-noise ratio near threshold.
Journal of Neuroscience 09/2005; 25(34):7831-9. · 7.11 Impact Factor
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ABSTRACT: Hair cells in the vertebrate cochlea are arranged tonotopically with their characteristic frequency (CF), the sound frequency to which they are most sensitive, changing systematically with position. Single mechanotransducer channels of hair cells were characterized at different locations in the turtle cochlea. In 2.8 mM external Ca2+, the channel's chord conductance was 118 pS (range 80-163 pS), which nearly doubled (range 149-300 pS) on reducing Ca2+ to 50 microM. In both Ca2+ concentrations, the conductance was positively correlated with hair cell CF. Variation in channel conductance can largely explain the increases in size of the macroscopic transducer current and speed of adaptation with CF. It suggests diversity of transducer channel structure or environment along the cochlea that may be an important element of its tonotopic organization.
Neuron 01/2004; 40(5):983-90. · 14.74 Impact Factor
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ABSTRACT: Outer hair cells are centrally involved in the amplification and frequency tuning of the mammalian cochlea, but evidence about their transducing properties in animals with fully developed hearing is lacking. Here we describe measurements of mechanoelectrical transducer currents in outer hair cells of rats between postnatal days 5 and 18, before and after the onset of hearing. Deflection of hair bundles using a new rapid piezoelectric stimulator evoked transducer currents with ultra-fast activation and adaptation kinetics. Fast adaptation resembled the same process in turtle hair cells, where it is regulated by changes in stereociliary calcium. It is argued that sub-millisecond transducer adaptation can operate in outer hair cells under the ionic, driving force and temperature conditions that prevail in the intact mammalian cochlea.
Nature Neuroscience 09/2003; 6(8):832-6. · 15.53 Impact Factor
