Genetic, cellular, and functional evidence for ca²⁺ inflow through ca_v1.2 and ca_v1.3 channels in murine spiral ganglion neurons

Spiral ganglion neurons (SGNs) of the eighth nerve serve as the bridge between hair cells and the cochlear nucleus. Hair cells use Ca_v1.3 as the primary channel for Ca ²⁺ inflow to mediate transmitter release. In contrast, SGNs are equipped with multiple Ca ²⁺ channels to mediate Ca ²⁺-dependent functions. We examined directly the role of Ca_v1.3 channels in SGNs using Ca_v1.3-deficient mice (Ca_v1.3-/-). We revealed a surprising finding that SGNs functionally express the cardiac-specific Ca_v1.2, as well as neuronal Ca_v1.3 channels. We show that evoked action potentials recorded from SGNs show a significant decrease in the frequency of firing in Ca_v1.3 ^-/- mice compared with wild-type (Ca_v1.3+/+) littermates. Although Ca_v1.3 is the designated L-type channel in neurons, whole-cell currents recorded in isolated SGNs from Ca_v1.3 ^-/- mice showed a surprising remnant current with sensitivity toward the dihydropyridine (DHP) agonist and antagonist, and a depolarization shift in the voltage-dependent activation compared with that in the Ca_v1.3 ^+/+ mice. Indeed, direct measurement of the elementary properties of Ca ² channels, in Ca_v1.3 ^+/+ neurons, confirmed the existence of two DHP-sensitive single-channel currents, with distinct open probabilities and conductances. We demonstrate that the DHP-sensitive current in Ca_v1.3 ^-/- mice is derived from Ca_v1.2 channel activity, providing for the first time, to our knowledge, functional data for the expression of Ca_v1.2 currents in neurons. Finally, using shRNA gene knockdown methodology, and histological analyses of SGNs from Ca_v1.2 ^+/- and Ca_v1.3 ^+/- mice, we were able to establish the differential roles of Ca_v1.2 and Ca_v1.3 in SGNs.