Developmental alterations in the biophysical properties of Cav1.3 Ca2+ channels in mouse inner hair cells

Prior to hearing onset, spontaneous action potentials activate voltage-gated Ca(v)1.3 Ca2+ channels in mouse inner hair cells (IHCs), which triggers exocytosis of glutamate and excitation of afferent neurons. In mature IHCs, Ca(v)1.3 channels open in response to evoked receptor potentials, causing graded changes in exocytosis required for accurate sound transmission. Developmental alterations in Ca(v)1.3 properties may support distinct roles of Ca(v)1.3 in IHCs in immature and mature IHCs, and have been reported in various species. It is not known whether such changes in Ca(v)1.3 properties occur in mouse IHCs, but this knowledge is necessary for understanding the roles of Ca(v)1.3 in developing and mature IHCs. Here, we describe age-dependent differences in the biophysical properties of Ca(v)1.3 channels in mouse IHCs. In mature IHCs, Ca(v)1.3 channels activate more rapidly and exhibit greater Ca2+-dependent inactivation (CDI) than in immature IHCs. Consistent with the properties of Ca(v)1.3 channels in heterologous expression systems, CDI in mature IHCs is not affected by increasing intracellular Ca2+ buffering strength. However, CDI in immature IHCs is significantly reduced by strong intracellular Ca2+ buffering, which both slows the onset of, and accelerates recovery from, inactivation. These results signify a developmental decline in the sensitivity of CDI to global elevations in Ca2+, which restricts negative feedback regulation of Ca(v)1.3 channels to incoming Ca2+ ions in mature IHCs. Together with faster Ca(v)1.3 activation kinetics, increased reliance of Ca(v)1.3 CDI on local Ca2+ may sharpen presynaptic Ca2+ signals and improve temporal aspects of sound coding in mature IHCs.