Hair Bundle Stimulation Mode Modifies Manifestations of Mechanotransduction Adaptation

Abstract

Sound detection in auditory sensory hair cells depends on the deflection of the stereocilia hair bundle which opens mechano-electric transduction (MET) channels. Adaptation is hypothesized to be a critical property of MET that contributes to the auditory system's wide dynamic range and sharp frequency selectivity. Our recent work using a stiff probe to displace hair bundles showed that the fastest adaptation mechanism (fast adaptation) does not require calcium entry. Using fluid-jet stimuli, others obtained data showing only a calcium-dependent fast adaptation response. Because cochlear stereocilia do not move coherently and the hair cell response depends critically on the magnitude and time course of the hair bundle deflection, we developed a high-speed imaging technique to quantify this deflection in rat cochlear hair cells. The fluid jet delivers a force stimulus, and force steps lead to a complex time course of hair bundle displacement (mechanical creep), which affects the hair cell's macroscopic MET current response by masking the time course of the fast adaptation response. Modifying the fluid-jet stimulus to generate a hair bundle displacement step produced rapidly adapting currents that did not depend on membrane potential, confirming that fast adaptation does not depend on calcium entry. MET current responses differ with stimulus modality and will shape receptor potentials of different hair cell types based on their in vivo stimulus mode. These transformations will directly affect how stimuli are encoded.

Decades-old model of slow adaptation in sensory hair cells is not supported in mammals

“…First, we confirmed that slow adaptation is modulated by both Ca 2+ and the application of adenosine triphosphatase (ATPase) inhibitors. In rat cochlear hair cells, the hair bundle exhibits a mechanical creep when stimulated with a step-like force using the fluid jet (12), similar to the creep observed in bullfrog saccular hair cells (7). We found that slow adaptation and the hair-bundle creep exhibited similar time constants, but to our surprise, manipulation of the magnitude of slow adaptation had no effect on the magnitude of creep.…”

“…When delivering a 50-ms step-like force with a fluid jet (Fig. 1B), the outer hair cell (OHC) receptor current peaks and then decays with a time constant of ~25 ms, which is an indication of the slow adaptation process (12). We quantified the magnitude of adaptation as the ratio of peak to steady-state current.…”

“…1, C and E). The current decay with fluid-jet stimuli was best fit with a single exponential equation (12). In cells that had >15% adaptation magnitude for the ~50% peak current (I max ) step, time constants were 14.8 ± 5.4 ms (n = 7), 21.3 ± 10.2 ms (n = 14), and 29.3 ± 13.3 ms (n = 3) [P = 0.09, one-way analysis of variance (ANOVA)] for 0.1, 1, and 10 mM BAPTA, respectively.…”

“…The second hallmark of adaptation is the change in the position of the activation curve along the displacement axis during a sustained stimulus ( fig. S1B), which we term activation curve shifts (2,10,12). To confirm adaptation's Ca 2+ dependence with activation curve shifts, we performed multi-pulse experiments to track the activation curve shifts during a sustained-force step ( fig.…”

“…S1, A to C). We plotted activation curves using the true displacement of the hair bundle extracted from our recently developed high-speed imaging method (12). Activation curve shifts were larger for 0.1 mM versus 10 mM BAPTA for negative potentials at 10 and 50 ms after the adapting step onset ( fig.…”