A food-borne outbreak presents with descending flaccid paralysis and cranial nerve palsies. The responsible toxin blocks neuromuscular transmission. What is its precise molecular action?
- A It blocks voltage-gated sodium channels on the motor axon terminal
- B It cleaves SNARE proteins on the presynaptic vesicle and plasma membrane, preventing acetylcholine release ✓
- C It competitively antagonizes nicotinic receptors on the postsynaptic membrane
- D It accelerates acetylcholine hydrolysis by synaptic acetylcholinesterase
Correct answer: B. It cleaves SNARE proteins on the presynaptic vesicle and plasma membrane, preventing acetylcholine release
Explanation
Botulinum toxin is a zinc endopeptidase that enters the motor nerve terminal and cleaves SNARE proteins such as SNAP-25, syntaxin and synaptobrevin, so synaptic vesicles cannot fuse with the presynaptic membrane and acetylcholine is never released. Postsynaptic receptors and acetylcholinesterase are intact, which distinguishes it from curare (option C) and anticholinesterase poisoning (option D) respectively.
Reference: Katzung's Basic and Clinical Pharmacology, 16th ed.
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