Properties of the single calcium channels CaV1.3 in mouse cochlear inner hair cells

Inner hair cells (IHCs) of the adult mammalian cochlea respond to sound stimuli with small and graded receptor potentials. Before the onset of hearing immature IHCs fire spontaneous Ca2+ action potentials. Both types of response lead to Ca2+ inflow through voltage-gated Ca2+ channels that causes neurotransmitter release at IHC ribbon synapses onto afferent fibres (Moser & Beutner 2000). More than 90% of the Ca2+ inflow occurs through the Cav1.3 L-type Ca2+ channels (Platzer et al. 2000), the single channel kinetics of which have yet to be determined. METHODS: Single Ca channel activity was recorded in the “cell-attached” configuration (patch-clamp technique) from the mouse immature IHCs near body temperature. Voltage steps were applied from the holding potential of –70 mV. The patch pipette solution contained 5 or 70 mM of Ba2+ as the ion carrier and Bay K 8644 to better resolve the single channel openings. RESULTS: L-type Ca channels showed a slope conductance of 39 pS and 17.6 pS in 70 and 5 mM Ba2+ respectively. The channel showed two open time constants at all voltages (at –30 mV the time constants were 0.4 and 4.9 ms with 5 mM Ba2+, and 0.4 and 1.5 ms with 70 mM Ba2+). Only the slower time constant was clearly voltage-dependent. Three closed states were found, of which only the longest was clearly voltage-dependent. Time-to-first opening distributions were fit by two time constants of which one was very fast (<1 ms). CONCLUSIONS: IHCs L-type Ca channels show multiple gating states, some of which are voltage-dependent. The kinetics properties of the single Ca2+ channels recorded in the immature IHCs are consistent with the ribbon synapses sustaining a fast and precise release onto the afferent terminal (Goutman and Glowatzki 2007).