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

Abstract

Summary: A Ca2+ current transient block (ICaTB) 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 ICaTB 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 ICaTB at other ribbon synapses, ICaTB 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 ICaTB, whereas enhancing release synchronization by raising Ca2+ influx or temperature increases ICaTB. This suggests that ICaTB is produced by fast multivesicular proton-release events. We propose that ICaTB may function as a submillisecond feedback mechanism contributing to the auditory nerve’s fast spike adaptation during sound stimulation. : Mature inner hair cells in the cochlea contain compact active zones where clustered Ca2+ channels are tightly coupled to the sites of vesicle fusion. Vincent et al. show that vesicle exocytosis leads to the release of protons into the synaptic cleft, which transiently blocks a subset of Ca2+ channels. Keywords: Ca2+ channels, pH buffering, protons, inner hair cells, ribbon synapses, exocytosis, otoferlin, auditory nerve fiber