Distribution of Voltage‐Gated Sodium Channels and Scaffolding Proteins on Vestibular Calyx Ending Delineates the Axon Initial Segment

The amniote inner ear contains an unusual type of hair cell and a unique postsynaptic calyx terminal with specialized ion channel expression and afferent transmission mechanisms. The calyceal afferent terminal enwraps the hair cell and leads to a heminode. It has morphological and functional microdomains with distinct complements of potassium channels and scaffolding proteins. Stimulation of hair cells gives rise to postsynaptic potentials in the membrane facing the hair cell that propagate along the outer face of the calyx and parent axon to the heminode, giving rise to spikes with timing and response properties that vary with location (epithelial zone) and afferent morphology (calyx-only vs. dimorphic with additional bouton terminals). Heminodes of calyx-only afferents lie within the epithelium, placing the calyces themselves closer to the heminode. We report that diverse voltage-gated sodium (Na_V) channel proteins (including Na_V1.1-1.3, 1.5. 1.6, 1.8, and 1.9), HCN (hyperpolarization-activated cyclic nucleotide-gated) channels, and associated scaffolding proteins (ankyrins, βIV-spectrin, and ezrin) are differentially deployed across calyx microdomains, and specific complements of proteins also vary with innervation zone in vestibular epithelia. Our results suggest the calyx outer surface plays a role analogous to an axon initial segment in central neurons, and that systematic variation in Na_V pore-forming subunits underlies differences in firing properties of vestibular afferents in different epithelial zones.