Clustered Ca²⁺ Channels Are Blocked by Synaptic Vesicle Proton Release at Mammalian Auditory Ribbon Synapses

A Ca²⁺ current transient block (I_CaTB) by protons occurs at some ribbon-type synapses after exocytosis, but this has not been observed at mammalian hair cells. Here we show that a robust I_CaTB occurs at post-hearing mouse and gerbil inner hair cell (IHC) synapses, but not in immature IHC synapses, which contain non-compact active zones, where Ca²⁺ channels are loosely coupled to the release sites. Unlike I_CaTB at other ribbon synapses, I_CaTB in mammalian IHCs displays a surprising multi-peak structure that mirrors the EPSCs seen in paired recordings. Desynchronizing vesicular release with intracellular BAPTA or by deleting otoferlin, the Ca²⁺ sensor for exocytosis, greatly reduces I_CaTB, whereas enhancing release synchronization by raising Ca²⁺ influx or temperature increases I_CaTB. This suggests that I_CaTB is produced by fast multivesicular proton-release events. We propose that I_CaTB may function as a submillisecond feedback mechanism contributing to the auditory nerve's fast spike adaptation during sound stimulation.