Hyperpolarization-Activated Conductance's in the Peripheral Vestibular System

The Hcn gene family has been implicated to function at multiple loci along the peripheral vestibular pathway. The Hcn family consists of four members, Hcn1-4, which encode subunits that form homoor heterotetrameric ion channels. HCN channels are activated by hyperpolarization and generate a current known as I h . I h is unusual as it activates at potentials negative to -50 mV, is carried by both Na + and K + , and can be modulated directly by cyclic nucleotides. Here, I characterize HCN channels and I h at each level of the peripheral vestibular pathway. I begin with the role of HCN channels in mechanosensation, where I show that they are not required for mechanotransduction in vestibular or auditory hair cells. I show that while Hcn subunits are expressed in hair cells, there is little evidence to suggest localization in the stereocilia bundle. Genetic deletion of Hcn1, 2, or both Hcn1 and 2 as well as dominant-negative suppression by a mutant form of HCN2 had no effect on the mechanoelectrical current. I show next that the voltage-dependent current I h is present in vestibular hair cells, and that it increases in conductance up to ~4.2 nS throughout the first postnatal week. I present evidence that homomeric HCN1 is sufficient to generate hair cell I h . Loss of HCN1 results in a 10-15 mV decrease in rebound following hyperpolarization, which I show decreases VsEP amplitudes and balance ability. Finally, I show robust evidence for I h in both the cell bodies and dendritic terminals of the vestibular afferent neurons. Dendritic I h has a conductance of over 7 nS and is mediated by HCN1 and 2 while I h in the afferent cell bodies has a conductance of 3.5 nS and is mediated by HCN1, 2, and 4. I show that the dendritic terminals are spontaneously active with a resting firing rate of ~10 spikes per second. The firing rate can be increased through activation of the mechanotransduction iii channel or through modulation of I h with cAMP. I conclude that HCN channels in the afferent terminals help determine spontaneous firing rate and regularity, key components of the peripheral vestibular signaling process.

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