title: Exocytosis, mediatophore, and vesicular Ca2+/H+ antiport in rapid neurotransmission.
abstract: In rapid synapses, neurotransmitter quanta are emitted in less than 100 mus, often at a high frequency. Using fast cryofixation of synapses, we found a very brief (2-3 ms) change affecting intramembrane particles in presynaptic membrane. Vesicle openings also occurred but after a significant delay. The particle change is most probably linked to mediatophore, a proteolipid of 220 kDa. Mediatophore aggregates were demonstrated in active zones of the presynaptic membrane. Reconstituted in liposomes, Xenopus oocytes, and neuroblastoma cells, mediatophore releases acetylcholine in a Ca(2+)-dependent and quantal manner, mimicking physiological release. In restricted presynaptic "nanodomains," Ca(2+) concentration explosively reaches a high level and then vanishes with a time constant of 300-400 micros. Among the processes contributing to the fast phase of Ca(2+) buffering, a vesicular Ca(2+)/H(+) antiport plays a major role. Energized by the Vesicular-ATPase-dependent proton gradient, the antiport has a low affinity for Ca(2+). We inactivated the Ca(2+)/H(+) antiport using bafilomycin A1, which annihilates the proton gradient. As a result, the postsynaptic potential was increased in duration for about 3 ms, an effect caused by persistence of transmitter release. A similar change was obtained by replacing extracellular Ca(2+) by strontium, which inhibits the antiport. The antiport function, therefore, is to abbreviate the presynaptic Ca(2+) signal, making transmitter release briefer. This allows transmission to operate at high frequency. Following a brief period of stimulation, calcium transiently accumulates in synaptic vesicles where it is exchanged against transmitter. Calcium is subsequently cleared from the terminal, most probably by exocytosis.
authors.affiliation: Neurosciences Fondamentales, Université de Genève, Centre Médical Universitaire, Geneva, Switzerland. a_y.dunant@bluewin.ch
authors.forename: Yves
authors.initials: Y
authors.lastname: Dunant
authors.forename: J Miguel
authors.initials: JM
authors.lastname: Cordeiro
authors.forename: Paula P
authors.initials: PP
authors.lastname: Gonçalves
mesh_terms: D000818:Animals; D017920:Antiporters / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism; D027682:Cation Transport Proteins / Q000378:metabolism*; D005089:Exocytosis; D009419:Nerve Tissue Proteins / Q000378:metabolism*; D011950:Receptors, Cholinergic / Q000378:metabolism; D009435:Synaptic Transmission
chemical_list: D017920:Antiporters; D027682:Cation Transport Proteins; D009419:Nerve Tissue Proteins; D011950:Receptors, Cholinergic; C027614:calcium-hydrogen antiporters; C050299:mediatophore; D002118:Calcium