Etiology of distinct membrane excitability in pre- and posthearing auditory neurons relies on activity of Cl⁻ channel TMEM16A

The developmental rehearsal for the debut of hearing is marked by massive changes in the membrane properties of hair cells (HCs) and spiral ganglion neurons (SGNs). Whereas the underlying mechanisms for the developing HC transition to mature stage are understood in detail, the maturation of SGNs from hyperexcitable prehearing to quiescent posthearing neurons with broad dynamic range is unknown. Here, we demonstrated using pharmacological approaches, caged-Ca ²⁺ photolysis, and gramicidin patch recordings that the prehearing SGN uses Ca ²⁺-activated Cl ⁻ conductance to depolarize the resting membrane potential and to prime the neurons in a hyperexcitable state. Immunostaining of the cochlea preparation revealed the identity and expression of the Ca ²⁺-activated Cl ⁻ channel transmembrane member 16A (TMEM16A) in SGNs. Moreover, null deletion of TMEM16A reduced the Ca ²⁺-activated Cl ⁻ currents and action potential firing in SGNs. To determine whether Cl ⁻ ions and TMEM16A are involved in the transition between pre- and posthearing features of SGNs we measured the intracellular Cl ⁻ concentration [Cl ⁻] ᵢ in SGNs. Surprisingly, [Cl ⁻] ᵢ in SGNs from prehearing mice was ∼90 mM, which was significantly higher than posthearing neurons, ∼20 mM, demonstrating discernible altered roles of Cl ⁻ channels in the developing neuron. The switch in [Cl ⁻] ᵢ stems from delayed expression of the development of intracellular Cl ⁻ regulating mechanisms. Because the Cl ⁻ channel is the only active ion-selective conductance with a reversal potential that lies within the dynamic range of SGN action potentials, developmental alteration of [Cl ⁻] ᵢ, and hence the equilibrium potential for Cl ⁻ (E Cₗ), transforms pre- to posthearing phenotype. Significance One of the major issues in auditory neuroscience is the mechanism by which the developing hair cells and spiral ganglion neurons (SGNs) transition from prehearing characteristics to posthearing features. For decades it was thought that spontaneous action potentials in SGNs emanate from spontaneously firing prehearing hair cells. Here, we demonstrate that developing SGNs use Ca ²⁺-activated Cl ⁻ conductance to depolarize the resting membrane potential and to prime the neurons in a hyperexcitable prehearing state. Moreover, SGNs undergo global changes in intracellular Cl ⁻ homeostasis to alter their coding properties during development. Our findings address the endogenous origin of spontaneous activity in SGNs, transcend auditory-n