Kinetic analysis of barium currents in chick cochlear hair cells

Abstract

Inward barium current (IBa) through voltage-gated calcium channels was recorded from chick cochlear hair cells using the whole-cell clamp technique. IBa was sensitive to dihydropyridines and insensitive to the peptide toxins omega-agatoxin IVa, omega-conotoxin GVIa, and omega-conotoxin MVIIC. Changing the holding potential over a -40 to -80 mV range had no effect on the time course or magnitude of IBa nor did it reveal any inactivating inward currents. The activation of IBa was modeled with Hodgkin-Huxley m2 kinetics. The time constant of activation, tau m, was 550 microseconds at -30 mV and gradually decreased to 100 microseconds at +50 mV. A Boltzmann fit to the activation curve, m infinity, yielded a half activation voltage of -15 mV and a steepness factor of 7.8 mV. Opening and closing rate constants, alpha m and beta m, were calculated from tau m and m infinity, then fit with modified exponential functions. The H-H model derived by evaluating the exponential functions for alpha m and beta m not only provided an excellent fit to the time course of IBa activation, but was predictive of the time course and magnitude of the IBa tail current. No differences in kinetics or voltage dependence of activation of IBa were found between tall and short hair cells. We conclude that both tall and short hair cells of the chick cochlea predominantly, if not exclusively, express noninactivating L-type calcium channels. These channels are therefore responsible for processes requiring voltage-dependent calcium entry through the basolateral cell membrane, such as transmitter release and activation of Ca(2+)-dependent K+ channels.

Functional Interaction of Auxiliary Subunits and Synaptic Proteins With CaV1.3 May Impart Hair Cell Ca2+Current Properties

“…Figure 1 shows examples of Ca 2ϩ current traces generated from hair cells. In agreement with earlier studies (Hudspeth and Lewis 1988;Martini et al 2000;Smotherman and Narins 1999;Zidanic and Fuchs 1995), the DHP-sensitive Ca 2ϩ current in hair cells has a rapid onset and inactivates slowly. Despite the fact that the channel has been identified as Ca V 1.3␣ 1 channel (Kollmar et al 1997a), the current profile of the expressed-channel in HEK 293 cells is in sharp contrast to that in hair cells.…”

“…The current activates and deactivates with fast time constants ( ϭ approximately 0.5 ms), shows weak inactivation, and activates at potentials approximately 20 mV more negative than the activation voltages of other DHP-sensitive currents (Hudspeth and Lewis 1988;Rodriguez-Contreras and Yamoah 2001;Zidanic and Fuchs 1995). These properties allow hair cells to respond to tonic and phasic changes in vestibular and auditory stimuli through Ca 2ϩ -mediated neurotransmitter release.…”

“…The rapid kinetics of synaptic transmission at the hair cell afferent nerve terminals require the influx of Ca 2ϩ , which is supplied mainly by the Ca V 1.3 channels (Zidanic and Fuchs 1995). The strategic placement of the channels with the exocytotic apparatus is crucial for fast neurotransmitter release, as observed in the synaptic terminals of the squid giant axon (Adler et al 1991).…”

Tonotopic Distribution of Short-Term Adaptation Properties in the Cochlear Nerve of Normal and Acoustically Overexposed Chicks

“…The amplitude of in vivo hair cell receptor potentials is constant during constant acoustic stimulation (Goodman et al 1982). In addition, the Ca 2+ currents that evoke neurotransmitter release display little inactivation over time in vitro (Zidanic and Fuchs 1995;Spassova et al 2001). Thus, the cochlear mechanics and hair cell membrane events that occur before exocytosis appear to produce a constant output during constant stimulation.…”

Association of β-catenin with the α-subunit of neuronal large-conductance Ca 2+ -activated K+ channels

“…Electrophysiological and molecular-biological analyses have shown that hair cells express L-type Ca V channels and largeconductance (BK or Maxi) K Ca channels (12)(13)(14)(15) that contain, respectively, ␣ 1D -subunits (16)(17)(18) and slowpoke (slo) ␣-and slo ␤-subunits (19)(20)(21)(22). In hippocampal neurons, both large-and small-conductance K ϩ channels occur in presynaptic regions and are associated, respectively, with L-and N-type Ca V channels (7,23).…”