Persistent and resurgent na currents in vestibular calyx afferents

Abstract

Vestibular afferent neurons convey information from hair cells in the peripheral vestibular end organs to central nuclei. Primary vestibular afferent neurons can fire action potentials at high rates and afferent firing patterns vary with the position of nerve terminal endings in vestibular neuroepithelia. Terminals contact hair cells as small bouton or large calyx endings. To investigate the role of Na currents (INa) in firing mechanisms, we investigated biophysical properties of INa in calyx-bearing afferents. Whole cell patch-clamp recordings were made from calyx terminals in thin slices of gerbil crista at different postnatal ages: immature [postnatal day (P)5–P8, young (P13–P15), and mature (P30–P45)]. A large transient Na current (INaT) was completely blocked by 300 nM tetrodotoxin (TTX) in mature calyces. In addition, INaT was accompanied by much smaller persistent Na currents (INaP) and distinctive resurgent Na currents (INaR), which were also blocked by TTX. ATX-II, a toxin that slows Na channel inactivation, enhanced INaP in immature and mature calyces. 4,9-Anhydro-TTX (4,9-ah-TTX), which selectively blocks Nav1.6 channels, abolished the enhanced INa in mature, but not immature, calyces. Therefore, Nav1.6 channels mediate a component of INaT and INaP in mature calyces, but are minimally expressed at early postnatal days. INaR was expressed in less than one-third of calyces at P6–P8, but expression increased with development, and in mature cristae INaR was frequently found in peripheral calyces. INaR served to increase the availability of Na channels following brief membrane depolarizations. In current clamp, the rate and regularity of action potential firing decreased in mature peripheral calyces following 4,9-ah-TTX application. Therefore, Nav1.6 channels are upregulated during development, contribute to INaT, INaP, and INaR, and may regulate excitability by enabling higher mean discharge rates in a subpopulation of mature calyx afferents.