Cochlear fibrocytes play important roles in normal hearing as well as in several types of sensorineural hearing loss attributable to inner ear homeostasis disorders. Recently, we developed a novel rat model of acute sensorineural hearing loss attributable to fibrocyte dysfunction induced by a mitochondrial toxin. In this model, we demonstrate active regeneration of the cochlear fibrocytes after severe focal apoptosis without any changes in the organ of Corti. To rescue the residual hearing loss, we transplanted mesenchymal stem cells into the lateral semicircular canal; a number of these stem cells were then detected in the injured area in the lateral wall. Rats with transplanted mesenchymal stem cells in the lateral wall demonstrated a significantly higher hearing recovery ratio than controls. The mesenchymal stem cells in the lateral wall also showed connexin 26 and connexin 30 immunostaining reminiscent of gap junctions between neighboring cells. These results indicate that reorganization of the cochlear fibrocytes leads to hearing recovery after acute sensorineural hearing loss in this model and suggest that mesenchymal stem cell transplantation into the inner ear may be a promising therapy for patients with sensorineural hearing loss attributable to degeneration of cochlear fibrocytes. Mammalian cochlear fibrocytes of the mesenchymal nonsensory regions play important roles in the cochlear physiology of hearing, including the transport of potassium ions to generate an endocochlear potential in the endolymph that is essential for the transduction of sound by hair cells.1-3 It has been postulated that a potassium recycling pathway toward the stria vascularis via fibrocytes in the cochlear lateral wall is critical for proper hearing, although the exact mechanism has not been definitively determined.2 One candidate model for this ion transport system consists of an extracellular flow of potassium ions through the scala tympani and scala vestibuli and a transcellular flow through the organ of Corti, supporting cells, and cells of the lateral wall.4,5 The fibrocytes within the cochlear lateral wall are divided into type I to V based on their structural features, immunostaining patterns, and general location.5 Type II, type IV, and type V fibrocytes resorb potassium ions from the surrounding perilymph and from outer sulcus cells via the Na,KATPase. The potassium ions are then transported to type I fibrocytes, strial basal cells, and intermediate cells through gap junctions and are secreted into the intrastrial space through potassium channels. The secreted potassium ions are incorporated into marginal cells by the Na,K-ATPase and the Na-K-Cl co-transporter, and are finally secreted into the endolymph through potassium channels.Degeneration and alteration of the cochlear fibrocytes have been reported to cause hearing loss without any other changes in the cochlea in the Pit-Oct-Unc (POU)-domain transcription factor Brain-4 (Brn-4)-deficient mouse 6 and the otospiralin-deficient mouse.3 Brn-4 is the gene responsible f...

Type I and type II spiral ganglion neurons (SGN) innervate the inner and outer hair cells of the cochlea, respectively. This neural system is established by reorganization of promiscuous innervation of the hair cells, immediately before hearing is established. The mechanism for this synaptic reorganization is unresolved but probably includes regulation of trophic support between the hair cells and the neurons. We provide evidence that P2X receptors (ATP-gated ion channels) contribute such a mechanism in the neonatal rat cochlea. Single-cell quantitative RT-PCR identified the differential expression of two P2X receptor subunits, splice variant P2X 2-3 and P2X 3 , in a 1:2 transcript ratio. Downregulation of this P2X 2-3/3 receptor coincided with maturation of the SGN innervation of the hair cells. When the P2X 2-3 and P2X 3 subunits were co-expressed in Xenopus oocytes, the resultant P2X receptor properties corresponded to the SGN phenotype. This included enhanced sensitivity to ATP and extended agonist action. In P4 spiral ganglion explants, activation of the P2X receptor signaling pathway by ATP␥S or ␣,␤MeATP inhibited BDNF-induced neurite outgrowth and branching. These findings indicate that P2X receptor signaling provides a mechanism for inhibiting neurotrophin support of SGN neurites when synaptic reorganization is occurring in the cochlea.

Acute mitochondrial dysfunction in the cochlea is likely to result in hearing loss as a consequence of local energy shortage, similar to ischemia- or noise-induced hearing loss. To establish an animal model of acute cochlear mitochondrial dysfunction, we applied a mitochondrial toxin, 3-nitropropionic acid (3-NP) in the rat cochlea. Rats treated with 500mM 3-NP exhibited permanent threshold shifts in acoustic brainstem response while the same volume of 300mM 3-NP caused temporary threshold shifts. Histological examination in the permanent threshold shift model revealed severe degeneration of fibrocytes within spiral ligament and spiral limbus, indicating these cells are vulnerable to acute mitochondrial dysfunction. This model represents a novel tool for investigating the pathophysiology of acute cochlear mitochondrial dysfunction.

Ca2+ signalling is central to cochlear sensory hair cell physiology through its influence on sound transduction, membrane filter properties and neurotransmission. However, the mechanism for establishing Ca 2+ homeostasis in these cells remains unresolved. ] i was restored. Ca 2+ entry was confirmed by confocal fluorescence imaging, and rapidly recruited secondary charybdotoxin-and apamin-sensitive K Ca currents. Dual activation by the G protein-coupled receptor (GPCR)-phospholipase C-diacylglycerol (DAG) second messenger pathway was confirmed using the analogue 1-oleoyl-2-acetyl-sn-glycerol (OAG). Ion substitution experiments showed that the putative TRPC Ca 2+ entry current was selective for Na + > K + with a ratio of 1 : 0.6. The Ca 2+ entry current was inhibited by the TRPC channel blocker 2-aminoethyl diphenylborate (2APB) and the tyrosine kinase inhibitor, erbstatin analogue. We conclude that TRPC Ca 2+ entry channels, most likely incorporating TRPC3 subunits, support cochlear hair cell Ca 2+ homeostasis and GPCR signalling.