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 neuronal families or by different stimuli from one neuronal population occurs through identical mechanisms is unknown. We studied the release of [H-3]noradrenaline, [H-3]acetylcholine and [3H]dopamine induced by different stimuli from superfused rat brain synaptosomes pretreated with tetanus toxin or botulinum toxin F, known to block exocytosis by cleaving VAMP/synaptobrevin. The external Ca2+-dependent [H-3]transmitter overflows evoked by KCl were similarly inhibited by tetanus toxin or botulinum toxin F; the toxins cleaved similar amounts of synaptosomal synaptobrevin, as determined by western blot analysis, suggesting prevalent involvement of synaptobrevin-II. GABA uptake-mediated release of the three [H-3]transmitters was that differentially 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]dopamine 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; the GABA-evoked releases of [H-3]acetylcholine and [H-3]dopamine and those elicited by ionomycin were insensitive to cadmium. The results suggest that tetanus toxin and botulinum toxin F selectively affect exocytosis linked to activation of voltage-sensitive Ca2+ channels; the Ca2+-dependent, exocytotic-like release induced by stimuli not leading to activation of voltage sensitive Ca2+ channels seems insensitive to these clostridial toxins. (C) 1999 IBRO. Published by Elsevier Science Ltd.