Proton-Mediated Block of Ca2+ Channels during Multivesicular Release Regulates Short-Term Plasticity at an Auditory Hair Cell Synapse

Synaptic vesicles release both neurotransmitter and protons during exocytosis, which may result in a transient acidification of the synaptic cleft that can block Ca2+ channels located close to the sites of exocytosis. Evidence for this effect has been reported for retinal ribbon-type synapses, but not for hair cell ribbon synapses. Here, we report evidence for proton release from bullfrog auditory hair cells when they are held at more physiological, in vivo-like holding potentials (V-h = -60 mV) that facilitate multivesicular release. During paired recordings of hair cells and afferent fibers, L-type voltage-gated Ca2+ currents showed a transient block, which was highly correlated with the EPSC amplitude (or the amount of glutamate release). This effect was masked at V-h = -90mV due to the presence of a T-type Ca2+ current and blocked by strong pH buffering with HEPES or TABS. Increasing vesicular pH with internal methylamine in hair cells also abolished the transient block. High concentrations of intracellular Ca2+ buffer (10 mM BAPTA) greatly reduced exocytosis and abolished the transient block of the Ca2+ current. We estimate that this transient block is due to the rapid multivesicular release of similar to 600 - 1300H(+) ions per synaptic ribbon. Finally, during a train of depolarizing pulses, paired pulse plasticity was significantly changed by using 40 mM HEPES in addition to bicarbonate buffer. We propose that this transient block of Ca2+ current leads to more efficient exocytosis per Ca2+ ion influx and it may contribute to spike adaptation at the auditory nerve.