Localization of Calcium in Presynaptic Nerve Terminals

Ultrastructural techniques and electron probe microanalysis were used to determine whether or not the smooth endoplasmic reticulum (SER) within presynaptic nerve terminals is a Ca-sequestering site . The three-dimensional structure of the SER was determined from serial sections of synaptosomes. The SER consists of flattened cisterns that may branch and are frequently juxtaposed to mitochondria . To investigate intraterminal Ca sequestration, synaptosomes were treated with saponin to disrupt the plasmalemmal permeability barrier. When these synaptosomes were incubated in solutions containing Ca, ATP, and oxalate, electrondense Ca oxalate deposits were found in intraterminal mitochondria, SER cisterns, and large vesicular profiles . Saponin-treated synaptosomes that were incubated in the presence of mitochondrial poisons contained electron-dense deposits within SER cisterns and large vesicular profiles, but very rarely in mitochondria. Similar deposits were observed within saponin-treated synaptosomes that were not postfixed with OS04, and within saponin-treated synaptosomes that were prepared for analysis by freeze-substitution . Electron-probe microanalyses of these deposits confirmed the presence of large concentrations of Ca . When oxalate was omitted from the incubation solutions, no electron-dense deposits were present in saponin-treated synaptosomes . In other control experiments, either the Ca ionophore A23187 or the Ca chelator EGTA was added to the incubation media; electron-dense deposits were very rarely observed within the intraterminal organelles of these saponin-treated synaptosomes . The data indicate that presynaptic nerve terminal SER is indeed a Ca-sequestering organelle . Neurotransmitter release at chemical synapses appears to be critically dependent upon the level of free intraterminal calcium ([Ca 2 +] i ) (35) . The entry of Ca during presynaptic depolarization, and the consequent rise in [Ca2+ li, is the immediate trigger for evoked release (cf. 36, 40) . The main sites of J . CELT BIOLOGY © The Rockefeller University Press 0021-9525/80/05/0228/14 $1 .00 Volume 85 May 1980 228-241 on A ril 4, 2017 D ow nladed fom Published May 1, 1980