Ca_v1.3 (α1D) Ca²⁺ Currents in Neonatal Outer Hair Cells of Mice

Abstract

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 μm nifedipine and increased to 178.5 ± 57.8 % (n= 14) by 5 μm Bay K 8644. A dose‐response curve for nifedipine revealed an IC50 of 2.3 μm, 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.

Alternative Splicing of the CaV1.3 Channel IQ Domain, a Molecular Switch for Ca2+-Dependent Inactivation within Auditory Hair Cells

“…Second, weak CDI could help prolong Ca 2ϩ entry that would sustain activity-dependent transcription underlying outer hair cell development (Platzer et al, 2000;Brandt et al, 2003;Glueckert et al, 2003). During the development of hearing, cochlear hair cells undergo major changes in potassium (Kros et al, 1998;Marcotti et al, 2003) and calcium channel expression (Beutner and Moser, 2001;Michna et al, 2003), as well as dramatic rearrangement of efferent and afferent innervation (Liberman and Simmons, 1985;Sobkowicz et al, 1986Sobkowicz et al, , 2004Simmons and Liberman, 1988a,b;Simmons et al, 1992Simmons et al, , 1996Simmons, 1994Simmons, , 2002Fuchs et al, 2003;Bergeron et al, 2005). Hence, the early presence of Ca V 1.3 IQ⌬ channels, as well as the loss of OHCs shortly after the onset of hearing in Ca V 1.3 knock-out mice (Platzer et al, 2000), cohere with a contribution of these channels to activity-dependent gene expression underlying development.…”

“…Ca V 1.3 channels exhibit strikingly rapid (de)activation, which permits high-fidelity spike encoding of acoustic stimuli (Zidanic and Fuchs, 1995;Helton et al, 2005). Most intriguingly, the inactivation of Ca V 1.3 channels by elevated intracellular Ca 2ϩ [Ca 2ϩ -dependent inactivation (CDI)] appears weak to absent (Platzer et al, 2000;Michna et al, 2003;Song et al, 2003), in a manner believed to be important for continued perception of sustained auditory signaling (Lewis and Hudspeth, 1983;Glossmann et al, 1987). In contrast, the unmistakable CDI manifest by prototypic L-type channels (Ca V 1.2) (Yue and Marban, 1990) emphasizes the specialization of Ca V 1.3 properties for hair-cell function.…”

Deletion of the Ca2+-activated potassium (BK) α-subunit but not the BKβ1-subunit leads to progressive hearing loss

“…But the data presented here already strongly suggest that human slo1 must be considered an important susceptibility factor for progressive deafness analogous to KCNQ4 channel mutations (33). This paper is dedicated to Alfons Rüsch.…”

“…A possible explanation is that K ϩ ions entering through the transduction channels lack a major exit route out of the OHCs, thus leading to chronic depolarization. The depolarization may cause induction of apoptosis via sustained Ca 2ϩ influx through voltage-activated Ca 2ϩ channels Ca v 1.3 (33). The depolarization also reduces the driving force that may explain the observed reduction in DPOAEs.…”

Molecular Nature of Anomalous L-Type Calcium Channels in Mouse Cerebellar Granule Cells

“…Total RNA kit (Omega Bio-Tek, Winooski, VT) according to the manufacturer instructions. Total cochlea RNA (P4 mice) was isolated as described by Michna et al (2003). RNA integrity was checked by the presence of clear 28S and 18S rRNA bands after loading 1-2 g of RNA on a denaturating gel (1%).…”