Aim: 

Voltage-dependent L-type Ca²⁺ channels (Ca_V1.3) are expressed at the inner hair cells (IHCs) ribbon synapse, the primary sensory receptors of the mammalian cochlea, and trigger the afferent transmission in response to spontaneous Ca²⁺ action potentials during pre-hearing stages of development. The elementary properties of these Ca²⁺ channels have yet to be determined.

Methods: 

Single Ca²⁺ channel activity was recorded from mouse apical-coil IHCs (postnatal day 5–9) near body temperature using the "cell-attached" configuration of the patch-clamp technique. The patch pipette solution contained 5 mM of Ba²⁺ as the ion carrier and Bay K 8644 to resolve the single channel openings.

Results: 

L-type Ca²⁺ channel activity was already detectable at membrane voltages around –70 mV. The single channel slope conductance was 18 pS. The open probability (P_o) was strictly voltage-dependent and increased with depolarization, reaching a maximum value of about 0.23. The percentage of null-sweeps was significantly high (on average >80%) at all membrane potentials. The average single channel Ba²⁺ current showed slow inactivation.

Conclusions: 

The results are consistent with cochlear IHCs expressing Ca_V1.3 Ca channels. Since the voltage activation threshold is well in the range of the presumed IHCs resting membrane potential, the resulting Ca²⁺ inflow would support both the spontaneous Ca²⁺ action potential activity characteristic of pre-hearing IHCs (Marcotti et al. 2003, J Physiol 52:743–761), and the spontaneous neurotransmitter release onto the auditory afferent nerve fibers.

Supported by MIUR, Rome, Italy, and The Royal Society, The Wellcome Trust and Deafness Research, UK.