Mechanotransduction and hyperpolarization-activated currents contribute to spontaneous activity in mouse vestibular ganglion neurons

The hyperpolarization-activated, cyclic nucleotide-sensitive current, I-h, is present in vestibular hair cells and vestibular ganglion neurons, and is required for normal balance function. We sought to identify the molecular correlates and functional relevance of I-h in vestibular ganglion neurons. I-h is carried by channels consisting of homo-or heteromeric assemblies of four protein subunits from the Hcn gene family. The relative expression of Hcn1-4 mRNA was examined using a quantitative reverse transcription PCR (RT-PCR) screen. Hcn2 was the most highly expressed subunit in vestibular neuron cell bodies. Immunolocalization of HCN2 revealed robust expression in cell bodies of all vestibular ganglion neurons. To characterize I-h in vestibular neuron cell bodies and at hair cella-fferent synapses, we developed an intact, ex vivo preparation. We found robust physiological expression of I-h in 89% of cell bodies and 100% of calyx terminals. I-h was significantly larger in calyx terminals than in cell bodies; however, other biophysical characteristics were similar. I-h was absent in calyces lacking Hcn1 and Hcn2, but small I-h was still present in cell bodies, which suggests expression of an additional subunit, perhaps Hcn4. To determine the contributions of hair cell mechanotransduction and I-h to the firing patterns of calyx terminals, we recorded action potentials in current-clamp mode. Mechanotransduction currents were modulated by hair bundle defection and application of calcium chelators to disrupt tip links. I-h activity was modulated using ZD7288 and cAMP. We found that both hair cell transduction and I-h contribute to the rate and regularity of spontaneous action potentials in the vestibular afferent neurons. We propose that modulation of I-h in vestibular ganglion neurons may provide a mechanism for modulation of spontaneous activity in the vestibular periphery.