Development and function of the voltage-gated sodium current in immature mammalian cochlear inner hair cells

Inner hair cells (IHCs), the primary sensory receptors of the mammalian cochlea, fire spontaneous Ca(2+) action potentials before the onset of hearing. Although this firing activity is mainly sustained by a depolarizing L-type (Ca(V)1.3) Ca(2+) current (I(Ca)), IHCs also transiently express a large Na(+) current (I(Na)). We aimed to investigate the specific contribution of I(Na) to the action potentials, the nature of the channels carrying the current and whether the biophysical properties of I(Na) differ between low- and high-frequency IHCs. We show that I(Na) is highly temperature-dependent and activates at around -60 mV, close to the action potential threshold. Its size was larger in apical than in basal IHCs and between 5% and 20% should be available at around the resting membrane potential (-55 mV/-60 mV). However, in vivo the availability of I(Na) could potentially increase to >60% during inhibitory postsynaptic potential activity, which transiently hyperpolarize IHCs down to as far as -70 mV. When IHCs were held at -60 mV and I(Na) elicited using a simulated action potential as a voltage command, we found that I(Na) contributed to the subthreshold depolarization and upstroke of an action potential. We also found that I(Na) is likely to be carried by the TTX-sensitive channel subunits Na(V)1.1 and Na(V)1.6 in both apical and basal IHCs. The results provide insight into how the biophysical properties of I(Na) in mammalian cochlear IHCs could contribute to the spontaneous physiological activity during cochlear maturation in vivo.