Nature and expression of dihydropyridine-sensitive and -insensitive calcium currents in hair cells of frog semicircular canals

Elementary properties of CaV1.3 Ca(2+) channels expressed in mouse cochlear inner hair cells

Mammalian cochlear inner hair cells (IHCs) are specialized to process developmental signals during immature stages and sound stimuli in adult animals. These signals are conveyed onto auditory afferent nerve fibres. Neurotransmitter release at IHC ribbon synapses is controlled by L-type Ca_V1.3 Ca²⁺ channels, the biophysics of which are still unknown in native mammalian cells. We have investigated the localization and elementary properties of Ca²⁺ channels in immature mouse IHCs under near-physiological recording conditions. Ca_V1.3 Ca²⁺ channels at the cell pre-synaptic site co-localize with about half of the total number of ribbons present in immature IHCs. These channels activated at about −70 mV, showed a relatively short first latency and weak inactivation, which would allow IHCs to generate and accurately encode spontaneous Ca²⁺ action potential activity characteristic of these immature cells. The Ca_V1.3 Ca²⁺ channels showed a very low open probability (about 0.15 at −20 mV: near the peak of an action potential). Comparison of elementary and macroscopic Ca²⁺ currents indicated that very few Ca²⁺ channels are associated with each docked vesicle at IHC ribbon synapses. Finally, we found that the open probability of Ca²⁺ channels, but not their opening time, was voltage dependent. This finding provides a possible correlation between presynaptic Ca²⁺ channel properties and the characteristic frequency/amplitude of EPSCs in auditory afferent fibres.

Potassium currents in the hair cells of vestibular epithelium: position-dependent expression of two types of A channels

The complement of voltage-dependent K+ currents was investigated in hair cells of the frog crista ampullaris. The currents were recorded in transversal slices of the peripheral, intermediate and central regions of the crista by applying the patch clamp technique to cells located at different positions in the slices. Voltage-clamp recordings confirmed that cells located in each region have a distinctive complement of K+ channels. Detailed investigation of the currents in each region revealed that the complement of K+ channels in intermediate and central regions showed no variations among cells, whereas peripheral hair cells differed in the expression of two classes of A-type currents. These currents showed different kinetics of inactivation as well as steady-state inactivation properties. We termed these currents fast I(A) and slow I(A) based on their inactivation speed. The magnitude of both currents exhibited a significant gradient along the transversal axis of the peripheral regions. Fast I(A) magnitude was maximal in cells located in the external zone of the crista slice and decreased gradually to become very small in the median zone (centre) of the section, while the gradient of slow I(A) magnitude was reversed. A-type currents appear to act as a transient buffer that opposes hair cell depolarization induced by positive current injections. However, fast I(A) is partially active at the cell resting potential, while slow I(A) can be recruited only following large hyperpolarizations. Thus, two types of A currents are differentially distributed in vestibular hair cells and have different roles in shaping receptor potential.

Ca2+ currents and voltage responses in Type I and Type II hair cells of the chick embryo semicircular canal

Type I and Type II hair cells, and Type II hair cells located in different zones of the semicircular canal crista, express different patterns of voltage-dependent K channels, each one specifically shaping the hair cell receptor potential. We report here that, close to hatching, chicken embryo semicircular canal Type I and Type II hair cells express a similar voltage-dependent L-type calcium current (I(Ca)), whose main features are: activation above -60 mV, fast activation kinetics, and scarce inactivation. I(Ca) should be already active at rest in Zone 1 Type II hair cells, whose resting membrane potential was on average slightly less negative than -60 mV. Conversely, I(Ca) would not be active at rest in Type II hair cells from Zone 2 and 3, nor in Type I hair cells, since their resting membrane potential was significantly more negative than -60 mV. However, even small depolarising currents would activate I(Ca) steadily in Zone 2 and 3 Type II hair cells, but not in Type I hair cells because of the robust repolarising action of their specific array of K(+) currents. The implications of the present findings in the afferent discharge are discussed.

Intracellular Ca2+ buffers can dramatically affect Ca2+ conductances in hair cells

The effects of endogenous and exogenous Ca(2+) buffers on Ca(2+) current kinetics have been investigated by patch clamp in hair cells mechanically isolated from frog semicircular canals. This preparation displays at least three different Ca(2+) channel types: transient currents flow through a drug-resistant channel ("R1"), while non-inactivating channels sustain a steady, plateau current comprised of a large L component and a small drug-resistant fraction ("R2"). In the perforated-patch condition a large and stable Ca(2+) current was recorded, with all three components. In whole-cell, a buffer-free pipette solution did not prevent a complete Ca(2+) response. The size of the transient and plateau current fractions were greatly reduced, but the ratio between the two fractions, as well as the activation, inactivation and deactivation kinetics, were substantially unmodified. Current amplitude partially recovered with 5 mM EGTA in the pipette solution. With 50 mM EGTA all the kinetic parameters were slowed down and the transient component, but not the plateau component, markedly increased in size. Response kinetics slowed down even more with 30 mM Cs-BAPTA and the Ca(2+) waveform was substantially modified. The transient component was very large and inactivated slowly; the remaining very small plateau fraction deactivated along a slow, single exponential time. Under this condition nifedipine (10 microM) produced a great reduction of the transient current, leaving plateau and deactivation phase unaltered. This suggests that only R2 channels were still active at the end of the test and that the minor remaining transient component flowed through slowly but completely inactivating R1 channels. These results confirm the presence of several channel types in semicircular canal receptors, at difference with cochlear hair cells, and highlight a dramatic alteration of L-type channel behavior when intracellular Ca(2+) buffers are sufficiently concentrated and fast to interfere with rapid and local changes in Ca(2+) levels.

L-Type Calcium Channels: The Low Down

L-type calcium channels couple membrane depolarization in neurons to numerous processes including gene expression, synaptic efficacy, and cell survival. To establish the contribution of L-type calcium channels to various signaling cascades, investigators have relied on their unique pharmacological sensitivity to dihydropyridines. The traditional view of dihydropyridine-sensitive L-type calcium channels is that they are high-voltage–activating and have slow activation kinetics. These properties limit the involvement of L-type calcium channels to neuronal functions triggered by strong and sustained depolarizations. This review highlights literature, both long-standing and recent, that points to significant functional diversity among L-type calcium channels expressed in neurons and other excitable cells. Past literature contains several reports of low-voltage–activated neuronal L-type calcium channels that parallel the unique properties of recently cloned CaV1.3 L-type channels. The fast kinetics and low activation thresholds of CaV1.3 channels stand in stark contrast to criteria currently used to describe L-type calcium channels. A more accurate view of neuronal L-type calcium channels encompasses a broad range of activation thresholds and recognizes their potential contribution to signaling cascades triggered by subthreshold depolarizations.

Cav1.3 (alpha1D) Ca2+ currents in neonatal outer hair cells of mice

Outer hair cells (OHC) serve as electromechanical amplifiers that guarantee the unique sensitivity and frequency selectivity of the mammalian cochlea. It is unknown whether the afferent fibres connected to adult OHCs are functional. If so, voltage-activated Ca2+ channels would be required for afferent synaptic transmission. In neonatal OHCs, Ca2+ channels seem to play a role in maturation since OHCs from Cav1.3-deficient (Cav1.3-/-) mice degenerate shortly after the onset of hearing. We therefore studied whole-cell Ca2+ currents in outer hair cells aged between postnatal day 1 (P1) and P8. OHCs showed a rapidly activating inward current that was 1.8 times larger with 10 mM Ba2+ as charge carrier (IBa) than with equimolar Ca2+ (ICa). IBa started activating at -50 mV with Vmax = -1.9 +/- 6.9 mV, V0.5 = -15.0 +/- 7.1 mV and k = 8.2 +/- 1.1 mV (n = 34). The peak IBa showed negligible inactivation (3.6 % after 300 ms) whereas the ICa (10 mM Ca2+) was inactivated by 50.7 %. OHC IBa was reduced by 33.5 +/- 10.3 % (n = 14) with 10 microM nifedipine and increased to 178.5 +/- 57.8 % (n = 14) by 5 microM Bay K 8644. A dose-response curve for nifedipine revealed an IC50 of 2.3 microM, a Hill coefficient of 2.7 and a maximum block of 36 %. Average IBa density in OHCs was 24.4 +/- 10.8 pA pF-1 (n = 105) which is only 38 % of the value in inner hair cells. Single cell RT-PCR revealed expression of Cav1.3 in OHCs. In OHCs from Cav1.3-/- mice, Ba2+ current density was reduced to 0.6 +/- 0.5 pA pF-1 (n = 9) indicating that > 97 % of the Ca2+ channel current in OHCs flows through Cav1.3.