Mechanoelectrical transduction and adaptation in hair cells of the mouse utricle, a low-frequency vestibular organ.

Hair cells of inner ear organs sensitive to frequencies above 10 Hz adapt to maintained hair bundle deflections at rates that reduce their responses to lower frequencies. Mammalian vestibular organs detect head movements at frequencies well below 10 Hz. We asked whether hair cells of the mouse utricle adapt, and if so, whether the adaptation was similar to that in higher frequency organs such as the frog saccule. Whole-cell transduction currents were recorded from hair cells in the epithelium of the mouse utricle. Hair bundles were deflected by a fluid jet or a stiff probe. The transduction currents evoked by step deflections adapted over 10-100 msec. The mean operating range was 1.5 micron (deflection of the tip of the bundle), approximately threefold larger than in frog saccule. Taller and more compact bundles of the mouse utricle account for this difference. As in frog saccular hair cells, adaptation shifted the current-deflection (I(X)) relation along the deflection axis. These adaptive shifts had time constants of 10-20 msec and reached 60-80% of stimulus amplitude. The adaptive shift and voltage-dependent bundle movement are consistent with the motor model of adaptation. When the fluid jet was used, adaptation also broadened the I(X) relation and reduced the maximum current. Adaptation attenuated the transduction currents evoked by sinusoidal bundle deflections below 5 Hz, within the frequency range of the utricle, but because it was incomplete, substantial responses remained. Moreover, the adaptive shift mechanism preserves sensitivity even in the presence of large stimuli that would otherwise saturate transduction.