HCN1-4 subunits form N a+/K +-permeable ion channels that are activated by hyperpolarization and carry the current known as I h. I h has been characterized in vestibular hair cells of the inner ear, but its molecular correlates and functional contributions have not been elucidated.Weexamined HcnmRNAexpression and immunolocalization ofHCNprotein in the mouse utricle, a mechanosensitive organ that contributes to the sense of balance.Wefound thatHCN1is the most highly expressed subunit, localized to the basolateral membranes of type I and type II hair cells. We characterized I h using the whole-cell, voltage-clamp technique and found the current expressed in 84% of the cells with a mean maximum conductance of 4.4 nS. I h was inhibited by ZD7288, cilobradine, and by adenoviral expression of a dominant-negative form of HCN2. To determine whichHCNsubunits carried I h,we examined hair cells from mice deficient in Hcn1, 2, or both. Ih was completely abolished in hair cells of Hcn1 -/-mice and Hcn1/2 -/-mice but was similar to wild-type in Hcn2 -/-mice. To examine the functional contributions of I h, we recorded hair cell membrane responses to small hyperpolarizing current steps and found that activation of Ih evoked a 5-10 mV sag depolarization and a subsequent 15-20 mV rebound upon termination. The sag and rebound were nearly abolished in Hcn1-deficient hair cells. We also found that Hcn1-deficient mice had deficits in vestibular-evoked potentials and balance assays. We conclude that HCN1 contributes to vestibular hair cell function and the sense of balance.
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