title: Proton-mediated block of Ca2+ channels during multivesicular release regulates short-term plasticity at an auditory hair cell synapse.
abstract: Synaptic vesicles release both neurotransmitter and protons during exocytosis, which may result in a transient acidification of the synaptic cleft that can block Ca(2+) 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 (Vh = -60 mV) that facilitate multivesicular release. During paired recordings of hair cells and afferent fibers, L-type voltage-gated Ca(2+) currents showed a transient block, which was highly correlated with the EPSC amplitude (or the amount of glutamate release). This effect was masked at Vh = -90 mV due to the presence of a T-type Ca(2+) 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 Ca(2+) buffer (10 mm BAPTA) greatly reduced exocytosis and abolished the transient block of the Ca(2+) current. We estimate that this transient block is due to the rapid multivesicular release of ∼600-1300 H(+) 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 Ca(2+) current leads to more efficient exocytosis per Ca(2+) ion influx and it may contribute to spike adaptation at the auditory nerve.
authors.affiliation: The Vollum Institute, Oregon Health & Science University, Portland, Oregon 97239.
authors.forename: Soyoun
authors.initials: S
authors.lastname: Cho
authors.affiliation: The Vollum Institute, Oregon Health & Science University, Portland, Oregon 97239 vongersd@ohsu.edu.
authors.forename: Henrique
authors.initials: H
authors.lastname: von Gersdorff
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D020746:Calcium Channels, L-Type / Q000378:metabolism*; D005089:Exocytosis / Q000502:physiology; D005260:Female; D006198:Hair Cells, Auditory / Q000378:metabolism*; D008297:Male; D009473:Neuronal Plasticity / Q000502:physiology*; D009924:Organ Culture Techniques; D011522:Protons; D011892:Rana catesbeiana; D013569:Synapses / Q000502:physiology*; D055366:Synaptic Potentials / Q000502:physiology; D013572:Synaptic Vesicles / Q000378:metabolism*
chemical_list: D020746:Calcium Channels, L-Type; D011522:Protons; D002118:Calcium
keywords: auditory; calcium current; electrophysiology; exocytosis; hair cells; protons