Molecular mechanisms of sound amplification in the mammalian cochlea.

Mammalian hearing depends on the enhanced mechanical properties of the basilar membrane within the cochlear duct. The enhancement arises through the action of outer hair cells that act like force generators within the organ of Corti. Simple considerations show that underlying mechanism of somatic motility depends on local area changes within the lateral membrane of the cell. The molecular basis for this phenomenon is a dense array of particles that are inserted into the basolateral membrane and that are capable of sensing membrane potential field. We show here that outer hair cells selectively take up fructose, at rates high enough to suggest that a sugar transporter may be part of the motor complex. The relation of these findings to a recent candidate for the molecular motor is also discussed.

Cochlear function: hearing in the fast lane.

The cochlea amplifies sound over a wide range of frequencies. Outer hair cells have been thought to play a mechanical part in this amplification, but it has been unclear whether they act rapidly enough. Recent work suggests that outer hair cells can indeed work at frequencies that cover the auditory range.

A sugar transporter as a candidate for the outer hair cell motor.

Forces developed by cochlear outer hair cells (OHCs) are responsible for the sharp tuning that underlies sensitivity and frequency selectivity in the ear. OHCs exhibit a voltage-dependent motility involving a 'motor' protein embedded in the basolateral membrane. The motor has so far resisted molecular identification. Here we provide evidence that it may be related to a fructose transporter. We show that OHCs are able to transport this sugar selectively and that the sugar alters electrical properties of the OHC motor. These data can be combined into an integrated model of a sugar carrier, that makes the novel prediction, demonstrated here, that such 'neutral' transporters can be voltage dependent.

Transient expression of an inwardly rectifying potassium conductance in developing inner and outer hair cells along the mouse cochlea.

Inwardly rectifying K+ currents in inner and outer hair cells (IHCs, OHCs) were studied during post-natal development of the mouse cochlea. Hyperpolarizing steps from a holding potential of -64 mV induced a rapidly activating current in both cell types. This current showed strong inward rectification around the K+ equilibrium potential and, at potentials negative to -130 mV, partial inactivation. The activation range varied with extracellular K+ concentration. External application of Ba2+ and Cs+ reversibly blocked the elicited current. The results are consistent with the presence of an IK1-type inwardly rectifying potassium conductance in these cells. The maximum current was 60% larger in IHCs than in OHCs. In OHCs, but not IHCs, the amplitude of IK1 varied significantly with the cells' position along the cochlea. IK1 was maximal in cells located in the most basal region of the cochlea and its amplitude decreased in the apical coil. IK1 disappeared upon functional maturation: in OHCs at the end of the first postnatal week, and in IHCs at the onset of auditory function 12 days after birth. The current is active at the resting potential of the cells and plays a role in regulating the spiking behaviour characteristic of developing hair cells.

A quantitative comparison of mechanoelectrical transduction in vestibular and auditory hair cells of neonatal mice.

Vestibular hair cells (VHCs) and cochlear outer hair cells (OHCs) of neonatal mice were stimulated by a fluid jet directed at their stereociliary bundles. Relations between the force exerted by the jet, bundle displacement, and the resulting transducer current were studied. The mean maximum transducer conductance in VHCs (2.6 nS) was about half that of the OHCs (5.5 nS), with the largest recorded values being 4.1 nS and 9.2 nS, respectively. In some OHCs activity of a single, 112 pS transducer channel was observed, allowing an estimate of the maximum number of channels: up to 36 in VHCs and 82 in OHCs, corresponding to about one transducer channel per tip link. The VHC bundles required about 330 nm of tip displacement to activate 90% of the maximum transducer conductance, compared to 150 nm for the OHC bundles. This corresponded to 2 deg of rotation about their pivots for both, due to the greater length of the VHC bundles. The VHC bundles' translational stiffness was one-seventh of that of the OHCs. Conversion to rotational stiffness almost abolished this difference. Rotation of the hair bundle rather than translation determines the gating of the transducer channels, independent of bundle height or origin of the cells.

Intracellular calcium variations evoked by mechanical stimulation of mammalian isolated vestibular type I hair cells.

The variations of intracellular free calcium concentration ([Ca2+]i) were recorded on-line from guinea-pig isolated vestibular sensory cells using a fura-2 fast fluorescent photometry system, during mechanical displacements of the hair bundle. Repetitive displacements of the hair bundle towards the kinocilium (positive stimulation 7 degrees, 300 ms, 2Hz for 10 s), revealed [Ca2+]i variations detectable only in the cuticular plate. [Ca2+]i increased from 105 to 145 nM. Single mechanical displacements of the hair bundle (7 degrees, 200 ms, 0.5 Hz) evoked increases of [Ca2+]i from 50 +/- 23 nM to 139 +/- 79 (n = 12). In the opposite direction, the mechanical stimulations (8 degrees, 400 ms, 0.5 Hz) evoked a decrease of [Ca2+]i from 68 +/- 17 nM to 37 +/- 12 nM (n = 8). The variations of [Ca2+]i detected in the cuticular plate during positive displacements of the hair bundle were reversibly abolished in the presence of 100 microM gentamicin and they could not be evoked in 0.1 mM calcium in the external medium. From these experiments, it has been concluded that the [Ca2+]i variations recorded in the cuticular plate were due to a limited entry of calcium ions through transduction channels localized in the hair bundle. The typical kinetics of variations of [Ca2+]i evoked during positive displacements of the hair bundle should account for the presence of strong calcium regulation systems in the hair bundle and cuticular plate.