Posthearing Ca²⁺ currents and their roles in shaping the different modes of firing of spiral ganglion neurons

Whereas prehearing spiral ganglion neurons (SGNs) rely faithfully on outputs from spontaneously active developing hair cells, the electrical phenotypes of posthearing neurons are shaped by distinct rapid and graded receptor potentials from hair cells. To date, technical difficulties in isolation of fragile posthearing neurons from the rigid bony labyrinth of the inner ear have hindered analyses of the electrical phenotype of SGNs. Therefore, we have recently developed new strategies to isolate posthearing mouse SGNs for functional analyses. Here, we describe the coarse and fine properties of Ca²⁺ currents, which sculpt the firing properties of posthearing SGNs. Murine SGNs express multiple Ca²⁺ channel currents to enable diverse functions. We have demonstrated that suppression of Ca²⁺ currents results in significant hyperpolarization of the resting membrane potential (rmp) of basal SGNs, suggesting that Ca²⁺ influx primesrmpfor excitation. In contrast, removal of external Ca²⁺has modest effects onrmpof apical SGNs. The blockade of Ca²⁺currents with a mixture of specific blockers attenuates spontaneously active SGNs. Paradoxically, different subtypes of Ca²⁺ currents, such as R-type currents, may activate resting outward conductances since blockage of the current results in depolarization of rmp. In keeping with whole-cell current data, single-channel records revealed multiple diverse Ca²⁺ channels in SGNs. Additionally, there were differential expressions of distinct Ca²⁺ current densities in the apicobasal contour of the adult cochlea. This report provides invaluable insights into Ca²⁺-dependent processes in adult SGNs.