Single calcium channel (CaV1.3) activity recorded from mouse cochlear inner hair cells

Voltage-gated Ca2+ channels expressed in inner hair cells (IHCs) of the mammalian cochlea play a number of key physiological roles in their normal development and sound transduction (Housley et al. 2006, J Memb Biol 209:89-118). Ca2+ influx into IHCs occurs mainly (>90%) through Cav1.3 L-type Ca2+ channels (Platzer et al. 2000, Cell 102:89-97; Brandt et al 2003, J Neurosci 23:10832-40). Although some of the macroscopic biophysical properties of CaV1.3 Ca2+ channels are known, their elementary properties have yet to be determined in mammalian IHCs. Single Ca2+ channel activity was recorded at body temperature from immature IHCs in cell-attached configuration using acutely dissected mouse organs of Corti. Voltage-dependent Ca2+ channels were investigated using Ba2+ in the patch pipette solution as the main charge carrier and Bay K 8644 to resolve the channel openings. Cell-attached recordings confirmed the presence of L-type Ca2+ channels, which showed a slope conductance of 39.0 pS and 17.6 pS in 70 mM and 5 mM Ba2+, respectively. The mean half maximum open probability (V1/2) significantly shifted from about ñ22 mV in 70 mM Ba2+ to ñ41 mV in 5 mM Ba2+. The voltage threshold for Ca2+ channel activation was also significantly less hyperpolarized in 70 mM Ba2+ than in 5 mM Ba2+. The maximum and the minimum open probability were: 0.120 and 0.005 in 5 mM Ba2+ and 0.042 and 0.002 in 70 mM Ba2+. The present results are consistent with mammalian IHCs expressing a homogeneous population of voltage-dependent L-type Ca2+ channels containing the alpha1D (CaV1.3) subunit. The hyperpolarized activation of these Ca2+ channels indicate that they are likely to be active at the presumed resting membrane potential of IHCs, thus supporting spontaneous action potential activity characteristic of pre-hearing IHCs (Marcotti et al. 2003, J Physiol 552:743-761).