K+ Currents in Isolated Vestibular Afferent Calyx Terminals

Abstract

Vestibular hair cells transduce mechanical displacements of their hair bundles into an electrical receptor potential which modulates transmitter release and subsequent action potential firing in afferent neurons. To probe ionic mechanisms underlying sensory coding in vestibular calyces, we used the whole-cell patch-clamp technique to record action potentials and K + currents from afferent calyx terminals isolated from the semicircular canals of Mongolian gerbils. Calyx terminals showed minimal current at the mean zero-current potential (−60 mV), but two types of outward K + currents were identified at potentials above −50 mV. The first current was a rapidly activating and inactivating K + current that was blocked by 4-aminopyridine (4-AP, 2.5 mM) and BDS-I (up to 250 nM). The time constant for activation of this current decreased with membrane depolarization to a minimum value of ∼1 ms. The 4-AP-sensitive current showed steady-state inactivation with a half-inactivation of approximately −70 mV. A second, more slowly activating current (activation time constant was 8.5±0.7 ms at −8 mV) was sensitive to TEA (30 mM). The TEA-sensitive current also showed steady-state inactivation with a half-inactivation of −95.4±1.4 mV, following 500-ms duration conditioning pulses. A combination of 4-AP and TEA blocked ∼90% of the total outward current.In current clamp, single Na + -dependent action potentials were evoked following hyperpolarization to potentials more negative than the resting potential. 4-AP application increased action potential width, whereas TEA both increased the width and greatly reduced repolarization of the action potential.

Hyperpolarization-Activated Current (I h) in Vestibular Calyx Terminals: Characterization and Role in Shaping Postsynaptic Events

“…We therefore tested the effects of removing I h on the zero current potential, action potential, and EPSPs in calyces. In ruptured and perforated patch clamp recordings from vestibular primary neurons, spontaneous action potentials are not usually observed, however, action potentials have been evoked in whole-cell current clamp in calyces (Rennie and Streeter 2006;Dhawan et al 2010) and vestibular ganglion somata (Limón et al 2005;Risner and Holt 2006;Iwasaki et al 2008;Kalluri et al 2010). We evoked action potentials by delivering repeated hyperpolarizing current injections in current clamp and also observed spontaneous EPSPs between action potentials (Fig.…”

“…Calyx endings and hair cells were isolated as described previously (Rennie and Streeter 2006;Dhawan et al 2010). Calyx endings remained attached to type I hair cells.…”

“…Gating variables, solved using the Rush-Larson approximation (Hines 1984), for each channel are used to control the dynamics of activation and inactivation of each channel to scale maximum g i . Steady-state activation, inactivation, time constants, maximal conductance, and reversal potential for each conductance are summarized in Tables 1 and 2 and are based on whole-cell calyx recordings from previously published (Rennie and Streeter 2006;Dhawan et al 2010) and the current work; some represent minor modifications to prior formulations (Athanasiades et al 2000). Conductances are assumed to be uniformly distributed throughout the calyx.…”

“…Calyx terminals express voltage-dependent outward K + currents sensitive to 4-aminopyridine, tetraethylammonium and KCNQ channel blockers (Hurley et al 2006;Rennie and Streeter 2006;Dhawan et al 2010) and a K(Ca) current carried by small conductance (SK) channels (Meredith et al 2011). In addition to the previously described inward Na + and outward K + conductances found in calyx terminals, prolonged voltage steps to hyperpolarized potentials also revealed the presence of a slowly-activating inward current (Meredith et al 2011), which was further investigated here.…”

“…Intrinsic membrane properties of afferent neurons may play an important role in determining firing pattern and ionic currents described in vestibular ganglion somata include voltage-and calcium-dependent K + currents (Chabbert et al 2001a;Limón et al 2005;Risner and Holt 2006;Iwasaki et al 2008;Kalluri et al 2010), sodium (Chabbert et al 1997) and calcium currents Chambard et al 1999;Autret et al 2005), and a mixed cation hyperpolarization-activated current (I h ) (Chabbert et al 2001b). Vestibular ganglion preparations lack peripheral dendritic terminals, but ionic conductances have also been described in the calyceal terminations of vestibular afferents (Hurley et al 2006;Rennie and Streeter 2006;Dhawan et al 2010;Meredith et al 2011). Recent immunocytochemical work confirms the localization of several types of ion channel to the calyx .…”