A late phase of exocytosis from synaptosomes induced by elevated [Ca2+](i)is not blocked by Clostridial neurotoxins

Treatment of rat cerebrocortical synaptosomes with botulinum toxin types E and C1 or tetanus toxin removed the majority of intact SNAP-25, syntaxin 1A /1B, and synaptobrevin and diminished Ca2+-dependent K+ depolarization-indu ced noradrenaline secretion. With botulinum toxin type E, <10% of intact SN AP-25 remained and K+-evoked release of glutamate and GABA was inhibited. T he large component of noradrenaline release evoked within 120 s by inclusio n of the Ca2+ ionophore A23187 with the K+ stimulus was also attenuated by these toxins; additionally, botulinium neurotoxin type E blocked the first 60 s of ionophore-induced GABA and glutamate exocytosis. However, exposure to A23187 for longer periods induced a phase of secretion nonsusceptible to any of these toxins (>120 s for noradrenaline; >60 s for glutamate or GABA ). Most of this late phase of release represented exocytosis because of its Ca2+ dependency, ATP requirement, and sensitivity to a phosphatidylinosito l 4-kinase inhibitor. Based on these collective findings, we suggest that t he ionophore-induced elevation of [Ca2+](i) culminates in the disassembly o f complexes containing nonproteolyzed SNAP receptors protected from the tox ins that can then contribute to neuroexocytosis.