Increase in efficiency and reduction in Ca 2+ dependence of exocytosis during development of mouse inner hair cells

Developmental changes in the coupling between Ca 2+ entry and exocytosis were studied in mouse inner hair cells (IHCs) which, together with the afferent endings, form the primary synapse of the mammalian auditory system. Ca 2+ currents ( I Ca ) and changes in membrane capacitance (Δ C m ) were recorded using whole‐cell voltage clamp from cells maintained at body temperature, using physiological (1.3 m m ) extracellular Ca 2+ . The magnitudes of both I Ca and Δ C m increased with maturation from embryonic stages until postnatal day 6 (P6). Subsequently, I Ca gradually declined to a steady level of about −100 pA from P13 while the Ca 2+ ‐induced Δ C m remained relatively constant, indicating a developmental increase in the Ca 2+ efficiency of exocytosis. Although the size of I Ca changed during development, its activation properties did not, suggesting the presence of a homogeneous population of Ca 2+ channels in IHCs throughout development. The Ca 2+ dependence of exocytosis changed with maturation from a fourth power relation in immature cells to an approximately linear relation in mature cells. This change applies to the release of both a readily releasable pool (RRP) and a slower secondary pool of vesicles, implying a common release mechanism for these two kinetically distinct pools that becomes modified during development. The increased Ca 2+ efficiency and linear Ca 2+ dependence of mature IHC exocytosis, especially over the physiological range of intracellular Ca 2+ , could improve the high‐fidelity transmission of both brief and long‐lasting stimulation. These properties make the mature cell ideally suited for fine intensity discrimination over a wide dynamic range.