Distribution of Na,K-ATPase α subunits in rat vestibular sensory epithelia.

The afferent encoding of vestibular stimuli depends on molecular mechanisms that regulate membrane potential, concentration gradients, and ion and neurotransmitter clearance at both afferent and efferent relays. In many cell types, the Na,K-ATPase (NKA) is essential for establishing hyperpolarized membrane potentials and mediating both primary and secondary active transport required for ion and neurotransmitter clearance. In vestibular sensory epithelia, a calyx nerve ending envelopes each type I hair cell, isolating it over most of its surface from support cells and posing special challenges for ion and neurotransmitter clearance. We used immunofluorescence and high-resolution confocal microscopy to examine the cellular and subcellular patterns of NKAα subunit expression within the sensory epithelia of semicircular canals as well as an otolith organ (the utricle). Results were similar for both kinds of vestibular organ. The neuronal NKAα3 subunit was detected in all afferent endings-both the calyx afferent endings on type I hair cells and bouton afferent endings on type II hair cells-but was not detected in efferent terminals. In contrast to previous results in the cochlea, the NKAα1 subunit was detected in hair cells (both type I and type II) but not in supporting cells. The expression of distinct NKAα subunits by vestibular hair cells and their afferent endings may be needed to support and shape the high rates of glutamatergic neurotransmission and spike initiation at the unusual type I-calyx synapse.