An outcome of the intricate Kϩ regulation in the cochlear duct is the endocochlear potential (EP), ϳ80 mV, the "battery" that runs hair-cell transduction; however, the detailed molecular mechanisms for the generation of the EP remain unclear. We provide strong evidence indicating that the intermediate cells (ICs) of the stria vascularis (StV) express outward K ϩ current that rectifies inwardly at positive potentials. The channel belongs to the ether-a-go-go-related gene (erg) family of K ϩ channels. We cloned an ERG1a channel in the mouse inner ear (MERG1a). The cellular distribution of MERG1a in the cochlea displayed the highest levels of immunoreactivity in the ICs and modest reactivity in the marginal cells as well as in several extrastrial cells (e.g., hair cells). Functional expression of the StV-specific MERG1a channel reveals a current that activates at relatively negative potentials (approximately Ϫ50 mV) and shows rapid inactivation reflected as inward rectification at depolarized potentials. The current was sensitive to the methanesulfonanilide drug E-4031 (IC 50 , ϳ165 nM) and the recombinant peptide rBeKm-1 (IC 50 , ϳ16 nM), and the single-channel conductance in symmetrical K ϩ was ϳ14 pS. The site of expression of MERG1a and its functional phenotype (e.g., modulation of the current by external K ϩ ) make it one of the most likely candidates for establishing the high throughput of K ϩ ions across ICs to generate EP. In addition, the property of the channel that produces marked K ϩ extrusion in increased external K ϩ may be important in shaping the dynamics of K ϩ cycling in the inner ear.

The enzyme nitric oxide (NO) synthase, that produces the signaling molecule NO, has been identified in several cell types in the inner ear. However, it is unclear whether a measurable quantity of NO is released in the inner ear to confer specific functions. Indeed, the functional significance of NO and the elementary cellular mechanism thereof are most uncertain. Here, we demonstrate that the sensory epithelia of the frog saccule release NO and explore its release mechanisms by using self-referencing NO-selective electrodes. Additionally, we investigated the functional effects of NO on electrical properties of hair cells and determined their underlying cellular mechanism. We show detectable amounts of NO are released by hair cells (>50 nM). Furthermore, a hair-cell efferent modulator acetylcholine produces at least a threefold increase in NO release. NO not only attenuated the baseline membrane oscillations but it also increased the magnitude of current required to generate the characteristic membrane potential oscillations. This resulted in a rightward shift in the frequency-current relationship and altered the excitability of hair cells. Our data suggest that these effects ensue because NO reduces whole cell Ca(2+) current and drastically decreases the open probability of single-channel events of the L-type and non L-type Ca(2+) channels in hair cells, an effect that is mediated through direct nitrosylation of the channel and activation of protein kinase G. Finally, NO increases the magnitude of Ca(2+)-activated K(+) currents via direct NO nitrosylation. We conclude that NO-mediated inhibition serves as a component of efferent nerve modulation of hair cells.

son GP, Yamoah EN. Molecular identity and functional properties of a novel T-type Ca 2ϩ channel cloned from the sensory epithelia of the mouse inner ear.