Non-Muscle Myosin II Modulates the Properties of Spontaneous Oscillation in Bullfrog Saccular Hair Cells

Inner-ear hair cells detect mechanical stimuli such as sound and gravity and convert them into electrical signals. Because these inputs are minute, an active amplification mechanism known as the active process is essential for sensory transduction. In non-mammalian vestibular hair cells, spontaneous oscillations, which are considered as a mechanical hallmark of the active process, have been observed. The spontaneous oscillations are influenced by hair-cell stiffness and tip-link tension, both of which are critical components of the active process. Recent studies have demonstrated that inhibition of non-muscle myosin II (NMII) expressed on the apical surface of the vestibular hair cells alters the hair-bundle stiffness in American bullfrogs; however, its effect on the spontaneous oscillations in the hair cells has not yet been fully investigated. In this study, we analyzed the effects of blebbistatin (10 μM), an ATPase inhibitor of NMII, on the spontaneous oscillations in the saccular hair cells of American bullfrogs. Hair-bundle displacement was measured with a photodiode, followed by quantitative mathematical analyses. Following blebbistatin treatment, the dominant frequency of the spontaneous oscillations and the quality factor (Q value), representing spectral sharpness, tended to decrease, while the full-width at half maximum of the power spectrum increased significantly (p = 0.047, Wilcoxon’s signed-rank test with Holm correction). Furthermore, displacement histograms of the spontaneous oscillations were approximated using a two-component Gaussian mixture model to quantify separation (Δμ), distribution width (σ), and occupancy asymmetry (Δw). Both Δμ and σ showed a trend towards increase. The absolute value of Δw (|Δw|) tended to decrease, indicating a transient reduction in left–right occupancy asymmetry. Notably, spontaneous oscillations were not abolished by blebbistatin, and the bimodal structure of the positional distribution was preserved. Taken together, these findings suggest that NMII does not directly drive the spontaneous oscillations in the saccular hair cells, but rather modulate the frequency characteristics and stability of the active process through its effect on hair-bundle stiffness.