Botulinum neurotoxin A attenuates release of norepinephrine but not NPY from vasoconstrictor neurons.

We examined effects of botulinum neurotoxin A (BoNTA) on sympathetic constrictions of the vena cava and uterine artery from guinea pigs to test the role of soluble NSF attachment protein receptor (SNARE) proteins in release of the cotransmitters norepinephrine (NE) and neuropeptide Y (NPY). Protein extracts of venae cavae and uterine arteries showed partial cleavage of synaptosomal associated protein of 25 kDa (SNAP-25) after treatment in vitro with BoNTA (50-100 nM). The rising phase of isometric contractions of isolated venae cavae to field stimulation at 20 Hz, mediated by NE acting on alpha-adrenoceptors, was reduced significantly by 100 nM BoNTA. However, sustained sympathetic contractions mediated by NPY were not affected by BoNTA. In uterine arteries, noradrenergic contractions to 1-Hz stimulation were almost abolished by BoNTA, and contractions at 10 Hz were reduced by 50-60%. We conclude that SNARE proteins are involved in exocytosis of NE from synaptic vesicles at low frequencies of stimulation but may not be essential for exocytosis of NPY and NE from large vesicles at high stimulation frequencies.