What is the mechanism of action of botulinum toxin at the neuromuscular junction?

• A. Blocks action at the neuromuscular junction and prevents release of acetylcholine.
• B. Stimulates release of acetylcholine into the synaptic cleft.
• C. Blocks reuptake of dopamine.
• D. Inhibits mitochondrial respiration in muscle cells.

Answer: (A)

Botulinum toxin acts at the neuromuscular junction to block communication from nerve to muscle by preventing acetylcholine release. It does this by cleaving SNARE proteins needed for vesicle fusion, so acetylcholine-containing vesicles cannot fuse with the presynaptic membrane and the neurotransmitter is not released into the synaptic cleft. Without acetylcholine, the motor endplate isn’t depolarized, there’s no action potential in the muscle, and the muscle becomes temporarily paralyzed. This presynaptic blockade of acetylcholine release distinguishes it from ideas like stimulating dopamine, inhibiting mitochondrial respiration, or increasing acetylcholine release, which do not describe the toxin’s action at the NMJ.