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 Ca(2+)-dependent K+ depolarization-induced noradrenaline secretion. With botulinum toxin type E, <10% of intact SNAP-25 remained and K(+)-evoked release of glutamate and GABA was inhibited. The large component of noradrenaline release evoked within 120 s by inclusion 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 phosphatidylinositol 4-kinase inhibitor. Based on these collective findings, we suggest that the ionophore-induced elevation of [Ca2+]i culminates in the disassembly of complexes containing nonproteolyzed SNAP receptors protected from the toxins that can then contribute to neuroexocytosis.
Ashton
A C
AC
Department of Biochemistry, Imperial College, London, England.
Dolly
J O
JO
D000818:Animals; D001905:Botulinum Toxins / Q000494:pharmacology*; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D004357:Drug Synergism; D005089:Exocytosis / Q000187:drug effects*; D018698:Glutamic Acid / Q000378:metabolism; D007425:Intracellular Membranes / Q000378:metabolism*; D007476:Ionophores / Q000494:pharmacology; D008565:Membrane Proteins / Q000187:drug effects; D051379:Mice; D009474:Neurons / Q000502:physiology; D009498:Neurotoxins / Q000494:pharmacology*; D009638:Norepinephrine / Q000037:antagonists & inhibitors / Q000378:metabolism; D009994:Osmolar Concentration; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D050600:SNARE Proteins; D013574:Synaptosomes / Q000187:drug effects / Q000502:physiology*; D050827:Syntaxin 1; D013997:Time Factors; D033921:Vesicular Transport Proteins; D005680:gamma-Aminobutyric Acid / Q000378:metabolism
D007476:Ionophores; D008565:Membrane Proteins; D009498:Neurotoxins; D050600:SNARE Proteins; C494214:Stx1a protein, mouse; C494213:Stx1a protein, rat; D050827:Syntaxin 1; D033921:Vesicular Transport Proteins; D000001:Calcimycin; D018698:Glutamic Acid; D005680:gamma-Aminobutyric Acid; D001905:Botulinum Toxins; D011188:Potassium; D002118:Calcium; D009638:Norepinephrine