Introduction 2 25 1, 2, 4, 6, 11, 12, 13, 14 1 1, 11, 12 13 6 2 4 14 32 16 38 12 30 16 1 32 38 11 2 15 6 4 2+ 9 24 12 + 2+ 2+ 24 36 28 21 26 35 36 28 21 26 2+ 22 17 8 27 2+ 2+ 29 2+ 35 13 2+ 5 10 12 Materials and methods Materials 3 −1 3 −1 Cell culture and transfection 24 L −1 −1 3 10 Immunoblotting of SNARE proteins 18 3 3 23 3 2 2 3 2 3 + + 3 Determination of cyclic AMP 33 2 2 2A 12 14 33 2 2 2 2A 33 Statistics n p Results 12 5 Cleavage of SNARE proteins by the expression of the BoNT/C1 light chain 5 11 11 5 1 1 1 Fig. 1 WT BT2 BT7 Stx S25 Sb a b Stx/Sb n c Stx/Sb n p  Inhibition of transmitter release by the expression of the BoNT/C1 light chain 10 3 2 2 Fig. 2 3 WT BT2 BT7 3 a 3 n b 3 n c 3 n d 3 n p p p  + + 2 + 2 2+ 7 + + 2 2 Changes in the regulation of cAMP synthesis via P2Y receptors due to the expression of the BoNT/C1 light chain 12 33 24 2A 3 Fig. 3 WT BT2 BT7 3 a b c d cAMP a n b n c n d n p p p  3 12 14 3 + 3 12 Discussion 22 27 12 5 3 11 37 7 31 11 37 20 20 34 3 3 10 12 2A 24 12 12 12 33 12 2+ 19 12 12 12 2+ 2+ 2+ 24 + + 2+ Taken together, the present results show that the expression of the BoNT/C1 light chain impairs vesicular transmitter release from PC12 cells, but does not alter the autocrine/paracrine activation of P2Y receptors by spontaneously released nucleotides. Thus, the spontaneous nucleotide release from neuroendocrine cells may occur independently of vesicle exocytosis. The depolarization-evoked nucleotide release, in contrast, relies predominantly on exocytotic mechanisms.