Evidence for calcium-dependent vesicular transmitter release insensitive to tetanus toxin and botulinum toxin type F

Whether exocytosis evoked by a given releasing stimulus from different neur onal families or by different stimuli from one neuronal population occurs t hrough identical mechanisms is unknown. We studied the release of [H-3]nora drenaline, [H-3]acetylcholine and [3H]dopamine induced by different stimuli from superfused rat brain synaptosomes pretreated with tetanus toxin or bo tulinum toxin F, known to block exocytosis by cleaving VAMP/synaptobrevin. The external Ca2+-dependent [H-3]transmitter overflows evoked by KCl were s imilarly inhibited by tetanus toxin or botulinum toxin F; the toxins cleave d similar amounts of synaptosomal synaptobrevin, as determined by western b lot analysis, suggesting prevalent involvement of synaptobrevin-II. GABA up take-mediated release of the three [H-3]transmitters was that differentiall y sensitive to the toxins: only the release of [H-3]noradrenaline, which is dependent on external Ca2+, but not of [H-3]acetylcholine and [H-3]dopamin e was blocked. Neither toxin affected the [H-3]transmitter overflows evoked by the Ca2+ ionophore ionomycin. Cadmium blocked the K+-evoked release of all [H-3]transmitters and the GABA-evoked release of [H-3]noradrenaline; th e GABA-evoked releases of [H-3]acetylcholine and [H-3]dopamine and those el icited by ionomycin were insensitive to cadmium. The results suggest that tetanus toxin and botulinum toxin F selectively af fect exocytosis linked to activation of voltage-sensitive Ca2+ channels; th e Ca2+-dependent, exocytotic-like release induced by stimuli not leading to activation of voltage sensitive Ca2+ channels seems insensitive to these c lostridial toxins. (C) 1999 IBRO. Published by Elsevier Science Ltd.