Stimulus Dependent Properties of Mammalian Cochlear Hair Cell Mechanoelectrical Transduction

Cochlear hair cell stereocilia move semi-independently, shaping the force transfer to mechanoelectrical transduction (MET) channels, as indicated by the MET current response. Semi-independent movement of stereocilia was evoked by stimulating inner hair cell (IHC) bundles from acutely dissected rat cochlea with stiff probes ranging in size from 1 to 10 mu m. MET current responses were recorded using whole-cell patch-clamp electrophysiology. Small probes directly displaced stereocilia they contacted, and recruited adjacent stereocilia depending on stimulus magnitude. We inferred that the recruitment of stereocilia resulted in less uniform and less synchronous movement. Step displacements using smaller probes resulted in smaller current responses (from 1 nA for large probes to 0.3 nA for small, p < .0001), slower rate of current activation, as measured from the linear portion (from 4 nA/ms to 1 nA/ms, p < .0001), slower time constants of adaptation, as measured from double exponential fits from peak to steady state current (fast component: from 0.6 to 1.2 ms, p = .004; slow component: from 8 ms to 12 ms, p = .001) and less complete adaptation (from 95% to 30%, p < .0001). These results indicate that the mechanical properties of less coherent bundles greatly affect force transfer to MET channels as indicated by the electrical response of the cell. Thus, outer hair cells (OHCs), with their bundles embedded in the tectorial membrane, may exhibit synchronous MET activation and therefore time-dependent adaptation where fast adaptation provides a high pass filter. Hair cells with free standing bundles, like inner hair cells (IHC), may exhibit more asynchronous MET activation and adaptation, in which case adaptation would not provide this additional filter.