Differential contribution of syntaxin 1 and SNAP-25 to secretion in noradrenergic and adrenergic chromaffin cells

We used botulinum neurotoxins (BoNT) to examine whether differences in the secretory activity of noradrenergic and adrenergic chromaffin cells are related to differences in the exocytotic machinery of these two types of bovine adrenal medulla cells. Cleavage of syntaxin and SNAP-25 by BoNT/C1 decreased in a dose-dependent way the release of both noradrenaline and adrenaline, but noradrenaline release was more sensitive to BoNT/C1. Cleavage of SNAP-25 by BoNT/A also had a larger inhibitory effect on noradrenaline release than on adrenaline release. Neither BoNT/C1 nor BoNT/A affected the intracellular Ca2+ responses induced by K+-depolarisation, and the extent of the inhibition of K+-evoked catecholamine release by selective blockers of voltage-gated Ca2+ channels was not affected by BoNT/C1. Therefore, our data do not support the hypothesis of a regulatory effect of syntaxin or SNAP-25 on the activity of Ca2+ channels. The lower sensitivity of adrenaline release to BoNT was not due to a reduced ability of the toxins to enter or to cleave their protein targets in adrenergic cells, since immunoblot analysis showed the cleavage of a larger fraction of syntaxin 1A in adrenergic cells, as compared to the cleavage in noradrenergic cells. The immunoblot analysis also showed larger amounts of syntaxin 1A in noradrenergic chromaffin cells than in adrenergic cells. Thus, in spite of a greater cleavage of syntaxin 1A in adrenergic cells by BoNT/C1, adrenaline release was less sensitive to BoNT/C1, suggesting that the release process in noradrenergic cells might be more dependent on syntaxin 1A and SNAP-25, as compared to adrenergic cells.