Cochlear hair cells respond with phenomenal speed and sensitivity to sound vibrations that cause submicron deflections of their hair bundle. Outer hair cells are not only detectors, but also generate force to augment auditory sensitivity and frequency selectivity. Two mechanisms of force production have been proposed: contractions of the cell body or active motion of the hair bundle. Here, we describe recently identified proteins involved in the sensory and motor functions of auditory hair cells and present evidence for each force generator. Both motor mechanisms are probably needed to provide the high sensitivity and frequency discrimination of the mammalian cochlea.

Hearing loss is most often the result of hair-cell degeneration due to genetic abnormalities or ototoxic and traumatic insults. In the postembryonic and adult mammalian auditory sensory epithelium, the organ of Corti, no hair-cell regeneration has ever been observed. However

Sound stimuli are detected in the cochlea by opening of hair cell mechanotransducer (MT) channels, one of the few ion channels not yet conclusively identified at a molecular level. To define their performance in situ, we measured MT channel properties in inner hair cells (IHCs) and outer hair cells (OHCs) at two locations in the rat cochlea tuned to different characteristic frequencies (CFs). The conductance (in 0.02 mM calcium) of MT channels from IHCs was estimated as 260 pS at both low-frequency and mid-frequency positions, whereas that from OHCs increased with CFs from 145 to 210 pS. The combination of MT channel conductance and tip link number, assayed from scanning electron micrographs, accounts for variation in whole-cell current amplitude for OHCs and its invariance for IHCs. Channels from apical IHCs and OHCs having a twofold difference in unitary conductance were both highly calcium selective but were distinguishable by a small but significant difference in calcium permeability and in their response to lowering ionic strength. The results imply that the MT channel has properties possessed by few known candidates, and its diversity suggests expression of multiple isoforms.

The distribution and colocalization of gamma-aminobutyric acid (GABA)- and glycine-like immunoreactivity in the cochlear nuclear complex of the guinea pig have been studied to produce a light microscopic atlas. The method used was based on post-embedding immunocytochemistry in pairs of 0.5-micron-thick plastic sections treated with polyclonal antibodies against conjugated GABA and glycine respectively. Immunoreactive cells, presumably short axon neurones, predominated in the dorsal cochlear nucleus, with mostly single-GABA-labelled cells in the superficial layer, double-labelled in the middle, and single-glycine-labelled in the deep layers. A few large single-glycine-labelled cells, interpreted as commissural neurons, occurred in the ventral nucleus. Scattered double-labelled cells, probably Golgi cells, were seen in the granule cell domain. Immunolabeled puncta of all three staining categories occurred in large numbers throughout the complex, apposed to somata and in the neuropil, showing a differential distribution onto different types of neuron. Three immunolabeled tracts were noted: the tuberculoventral tract, the commissural acoustic stria, and the trapezoidal descending fibres. Most of the fibres in these tracts were single-labelled for glycine, although in the last mentioned tract single-GABA- and double-labelled fibres were also found. Some of the immunolabeled cell types described here are proposed as the origins of the similarly labelled puncta and fibres on the basis of known intrinsic connections.

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Sound stimuli elicit movement of the stereocilia that make up the hair bundle of cochlear hair cells, putting tension on the tip links connecting the stereocilia and thereby opening mechanotransducer (MT) channels. Tmc1 and Tmc2, two members of the transmembrane channel–like family, are necessary for mechanotransduction. To assess their precise role, we recorded MT currents elicited by hair bundle deflections in mice with null mutations of Tmc1, Tmc2, or both. During the first postnatal week, we observed a normal MT current in hair cells lacking Tmc1 or Tmc2; however, in the absence of both isoforms, we recorded a large MT current that was phase-shifted 180°, being evoked by displacements of the hair bundle away from its tallest edge rather than toward it as in wild-type hair cells. The anomalous MT current in hair cells lacking Tmc1 and Tmc2 was blocked by FM1-43, dihydrostreptomycin, and extracellular Ca2+ at concentrations similar to those that blocked wild type. MT channels in the double knockouts carried Ca2+ with a lower permeability than wild-type or single mutants. The MT current in double knockouts persisted during exposure to submicromolar Ca2+, even though this treatment destroyed the tip links. We conclude that the Tmc isoforms do not themselves constitute the MT channel but are essential for targeting and interaction with the tip link. Changes in the MT conductance and Ca2+ permeability observed in the absence of Tmc1 mutants may stem from loss of interaction with protein partners in the transduction complex.

Hair cells of the mammalian cochlea are specialized for the dynamic coding of sound stimuli. The transduction of sound waves into electrical signals depends upon mechanosensitive hair bundles that project from the cell's apical surface. Each stereocilium within a hair bundle is composed of uniformly polarized and tightly packed actin filaments. Several stereociliary proteins have been shown to be associated with hair bundle development and function and are known to cause deafness in mice and humans when mutated. The growth of the stereociliar actin core is dynamically regulated at the actin filament barbed ends in the stereociliary tip. We show that Eps8, a protein with actin binding, bundling, and barbed-end capping activities in other systems, is a novel component of the hair bundle. Eps8 is localized predominantly at the tip of the stereocilia and is essential for their normal elongation and function. Moreover, we have found that Eps8 knockout mice are profoundly deaf and that IHCs, but not OHCs, fail to mature into fully functional sensory receptors. We propose that Eps8 directly regulates stereocilia growth in hair cells and also plays a crucial role in the physiological maturation of mammalian cochlear IHCs. Together, our results indicate that Eps8 is critical in coordinating the development and functionality of mammalian auditory hair cells.

The cyto- and fibre-architecture of the cochlear nuclear complex of the guinea-pig has been studied in serial sections using Nissl, Golgi and combined cell-myelin staining of normal material, and a silver degeneration method after cochlear ablation. The nuclear subdivisions and major cell types can be recognised on the basis of those found in the cat, but there are some differences between the two species in the precise distribution and morphology of the neurons. The rostrodorsal part of the anteroventral cochlear nucleus (AVCN) contains predominantly spherical bushy cells, but these cannot be readily divided into large and small types as in the cat. Globular bushy cells are seen in the caudal region of the AVCN, but the majority occur in the posteroventral cochlear nucleus (PVCN), in an area extending from the nerve root right up to the boundary of the dorsal cochlear nucleus (DCN). The octopus cells constitute a distinct region in the most dorsomedial part of the PVCN underneath the DCN. Giant cells are seen scattered around the nerve root region. Multipolar and small cells are seen throughout the non-granular regions of the ventral cochlear nucleus (VCN) except for the octopus cell area, but occur mainly in the more rostral regions of the PVCN. Small cells occur in greatest abundance in the thin cap area at the dorsal edge of the VCN below a superficial granule cell layer. The latter covers the dorsolateral surface of the VCN, and a lamina of granule cells partially separates the PVCN from the DCN. The DCN can be divided into four layers. The outermost molecular layer (layer 1) is separated from the deeper regions by a prominent layer of granule cells (layer 2) which also contains the pyramidal cells. Molecular layer stellate cells are seen in layer 1 and a staggered row of cartwheel neurons is found at the boundary between layers 1 and 2. Layer 3 contains the basal dendrites of the pyramidal cells and some small (vertical) cells, and is innervated by the descending branches of the cochlear nerve. The deepest layer 4, which contains multipolar cells and giant cells, does not appear to receive this direct cochlear input.