| 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 homo- or hetero- tetrameric ion channels. HCN channels are activated by hyperpolarization and generate a current known as Ih. Ih 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 Ih 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 Ih 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 Ih. 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 Ih in both the cell bodies and dendritic terminals of the vestibular afferent neurons. Dendritic Ih has a conductance of over 7 nS and is mediated by HCN1 and 2 while Ih 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 channel or through modulation of Ih 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. |
