Figure.
Action of botulinum toxin at cholinergic nerve terminals. The heavy (H) chain of the toxin binds selectively and irreversibly to high affinity receptors at the presynaptic surface of cholinergic neurones, and the toxin-receptor complex is taken up into the cell by endocytosis. The disulphide bond between the two chains is cleaved (by an unknown mechanism), and the toxin escapes into the cytoplasm. The light (L) chains of the seven serotypes interact with different proteins (synaptosomal associated protein (SNAP) 25, vesicle associated membrane protein (VAMP) and syntaxin) in the nerve terminals to prevent fusion of acetylcholine vesicles with the cell membrane and thereby impede its release. (Adapted from Moore,6 with permission)