Sodium channel diversity in the vestibular ganglion: NaV1.5, NaV1.8, and tetrodotoxin-sensitive currents

Firing patterns differ between subpopulations of vestibular primary afferent neurons. The role of sodium (Na-V) channels in this diversity has not been investigated because Na-V currents in rodent vestibular ganglion neurons (VGNs) were reported to be homogeneous, with the voltage dependence and tetrodotoxin (TTX) sensitivity of most neuronal Na-V channels. RT-PCR experiments, however, indicated expression of diverse Na-V channel subunits in the vestibular ganglion, motivating a closer look. Whole cell recordings from acutely dissociated postnatal VGNs confirmed that nearly all neurons expressed Na-V currents that are TTX-sensitive and have activation midpoints between -30 and -40 mV. In addition, however, many VGNs expressed one of two other Na-V currents. Some VGNs had a small current with properties consistent with Na(V)1.5 channels: low TTX sensitivity, sensitivity to divalent cation block, and a relatively negative voltage range, and some VGNs showed Na(V)1.5-like immunoreactivity. Other VGNs had a current with the properties of Na(V)1.8 channels: high TTX resistance, slow time course, and a relatively depolarized voltage range. In two Na(V)1.8 reporter lines, subsets of VGNs were labeled. VGNs with Na(V)1.8-like TTX-resistant current also differed from other VGNs in the voltage dependence of their TTX-sensitive currents and in the voltage threshold for spiking and action potential shape. Regulated expression of Na V channels in primary afferent neurons is likely to selectively affect firing properties that contribute to the encoding of vestibular stimuli.