The sensisivity of catecholamine release to botulinum toxin C 1 and E suggests selective targeting of vesicles set into the readily releasable pool

The impact of syntaxin and SNAP‐25 cleavage on [3H]noradrenaline ([3H]NA) and [3H]dopamine ([3H]DA) exocytotic release evoked by different stimuli was studied in superfused rat synaptosomes. The external Ca2+‐dependent K+‐induced [3H]catecholamine overflows were almost totally abolished by botulinum toxin C1 (BoNT/C1), which hydrolyses syntaxin and SNAP‐25, or by botulinum toxin E (BoNT/E), selective for SNAP‐25. BoNT/C1 cleaved 25% of total syntaxin and 40% of SNAP‐25; BoNT/E cleaved 40% of SNAP‐25 but left syntaxin intact. The GABA uptake‐induced releases of [3H]NA and [3H]DA were differentially affected: both toxins blocked the former, dependent on external Ca2+, but not the latter, internal Ca2+‐dependent. BoNT/C1 or BoNT/E only slightly reduced the ionomycin‐evoked [3H]catecholamine release. More precisely, [3H]NA exocytosis induced by ionomycin was sensitive to toxins in the early phase of release but not later. The Ca2+‐independent [3H]NA exocytosis evoked by hypertonic sucrose, thought to release from the readily releasable pool (RRP) of vesicles, was significantly reduced by BoNT/C1. Pre‐treating synaptosomes with phorbol‐12‐myristate‐13‐acetate, to increase the RRP, enhanced the sensitivity to BoNT/C1 of [3H]NA release elicited by sucrose or ionomycin. Accordingly, cleavage of syntaxin was augmented by the phorbol‐ester. To conclude, our results suggest that clostridial toxins selectively target exocytosis involving vesicles set into the RRP.