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Hearing relies on faithful signal transmission by cochlear inner hair cells (IHCs) onto auditory fibres over a wide frequency and intensity range. Exocytosis at IHC ribbon synapses is triggered by Ca 2+  inflow through Ca V 1.3 (L-type) Ca 2+  channels. We investigated the macroscopic (whole-cell) and elementary (cell-attached) properties of Ca 2+  currents in IHCs positioned at the middle turn (frequency ∼2 kHz) of the adult gerbil cochlea, which is their most sensitive hearing region. Using near physiological recordings conditions (body temperature and a Na +  based extracellular solution), we found that the macroscopic Ca 2+  current activates and deactivates very rapidly (time constant below 1 ms) and inactivates slowly and only partially. Single-channel recordings showed an elementary conductance of 15 pS, a sub-ms latency to first opening, and a very low steady-state open probability ( P   o : 0.024 in response to 500-ms depolarizing steps at ∼−18 mV). The value of  P   o  was significantly larger (0.06) in the first 40 ms of membrane depolarization, which corresponds to the time when most Ca 2+  channel openings occurred clustered in bursts (mean burst duration: 19 ms). Both the  P   o  and the mean burst duration were smaller than those previously reported in high-frequency basal IHCs. Finally, we found that middle turn IHCs are likely to express about 4 times more Ca 2+  channels per ribbon than basal cells. We propose that middle-turn IHCs finely-tune Ca V 1.3 Ca 2+  channel gating in order to provide reliable information upon timing and intensity of lower-frequency sounds.

Fine Tuning of CaV1.3 Ca2+ Channel Properties in Adult Inner Hair Cells Positioned in the Most Sensitive Region of the Gerbil Cochlea