Transient capacitance changes recorded from vestibular type I hair cells are produced by <i>G</i>_K,L gating and do not involve neurotransmitter exocytosis

Head movements are detected and signalled to primary sensory neurons by vestibular types I and II hair cells. Signal transmission involves glutamate exocytosis from hair cells, which is triggered by Ca²⁺ inflow through voltage-gated Ca_V1.3 Ca²⁺ channels. In a previous study on mice, we reported a Ca²⁺-dependent exocytosis in both hair cell types, measured as a sustained change in cell membrane capacitance (ΔC_m) following cell depolarization, which was significantly smaller in type I than in type II hair cells. By contrast, only type I hair cells showed a large transient ΔC_m, which was still present in Ca_V1.3^-/- mouse type I hair cells. Here we investigated the nature of this transient ΔC_m. We found that it was unaffected by 10 mm intracellular EGTA, which blocked most of the sustained exocytosis in these cells, demonstrating its insensitivity to intracellular Ca²⁺. Moreover the amplitude of the transient ΔC_m correlated with the degree of activation of the low-voltage activated outward rectifying K⁺ conductance, G_K,L, expressed by type I, but not type II hair cells. Finally the sign and kinetics of the transient ΔC_m changed based on voltage steps activating or deactivating G_K,L. These findings are consistent with the transient ΔC_m arising from the mobilization of charges during the gating of K,L channels, while excluding fast transient neurotransmitter exocytosis. Its large amplitude can be explained by the high resistance of the calyceal synaptic cleft since it was significantly reduced in Caspr^-/- mice, which show a significantly larger synaptic cleft compared to wild type mice. KEY POINTS: Vestibular type I and type II hair cells signal head movement to the central nervous system. Signal transmission from both hair cell types relies on Ca²⁺-dependent glutamate exocytosis, measured here as a sustained change in cell membrane capacitance (ΔC_m). Type I hair cells exhibit also a large transient ΔC_m, whose nature has not been elucidated. In this study we found that the transient ΔC_m does not involve exocytosis, but it is generated by the gating of the low-voltage activated outward rectifying K⁺ conductance, specifically expressed in type I hair cells. Transient ΔC_m analysis (also carried out in mice lacking the core protein of the septate-like junction) conclusively demonstrates that type I hair cells, like type II ones, do not elicit a transient release of neurotransmitter. Knowledge of the basic mechanisms of vestibular signalling is crucial in the study of pharmacological treatment for vestibular disorders and in the drug side effects targeted there.