From the Guidelines
Botulinum toxin works by blocking the release of acetylcholine at the neuromuscular junction, preventing muscle contraction. The toxin, produced by Clostridium botulinum bacteria, is a zinc-endopeptidase protein that cleaves specific proteins involved in the release of the neurotransmitter acetylcholine 1. The sequence of botulinum neurotoxin activity at the neuromuscular junction includes heavy-chain binding to a neuronal cell followed by internalization by means of receptor-mediated endocytosis, translocation to the cytosol, and cleavage of the proteins involved in the release of acetylcholine 1.
Key Points
- The toxin enters the vascular circulation and is transported to peripheral cholinergic nerve terminals, including neuromuscular junctions, postganglionic parasympathetic nerve endings, and peripheral ganglia 1.
- The characteristic flaccid paralysis results from blocking acetylcholine transmission across the neuromuscular junction by inhibition of acetylcholine release from the presynaptic motor neuron terminal 1.
- Recovery, which takes weeks to months, occurs after sprouting of new nerve terminals 1.
Mechanism of Action
The mechanism of action of botulinum toxin involves the cleavage of specific proteins involved in the docking and fusion of acetylcholine-containing vesicles with the presynaptic membrane. Different serotypes (A-G) target different proteins; however, the exact protein targets for each serotype are not specified in the provided evidence 1.
Clinical Applications
The toxin's effects are localized to the injection site, dose-dependent, and reversible as nerve terminals regenerate over time. This mechanism makes it effective for treating conditions involving muscle overactivity such as dystonia, spasticity, and hyperhidrosis, as well as for cosmetic purposes to reduce facial wrinkles.
From the FDA Drug Label
The symptoms are consistent with the mechanism of action of botulinum toxin and may include asthenia, generalized muscle weakness, diplopia, ptosis, dysphagia, dysphonia, dysarthria, urinary incontinence, blurred vision, and breathing difficulties. The mechanism of action of botulinum toxin is not explicitly described in the provided text, but it can be inferred that it involves the inhibition of muscle activity, leading to symptoms such as muscle weakness and paralysis.
- Key points:
- Botulinum toxin affects muscle activity
- Symptoms consistent with its mechanism of action include muscle weakness and paralysis
- The exact mechanism is not directly stated in the label 2
From the Research
Mechanism of Action of Botulinum Toxin
The mechanism of action of botulinum toxin involves several key steps:
- Extracellular binding to glycoprotein structures on cholinergic nerve terminals 3, 4
- Intracellular blockade of the acetylcholine secretion 3, 4
- Blockade of presynaptic release of the neurotransmitter (acetylcholine) at the neuromuscular junction, resulting in reversible chemical denervation of the muscle fibre 5
- Inhibition of neuroexocytosis through proteolytic action on intracellular proteins such as SNAP-25, VAMP, and syntaxin 6
Effects on the Neuromuscular Junction
The effects of botulinum toxin on the neuromuscular junction include:
- Blockade of intrafusal muscle fibres with consecutive reduction of Ia/II afferent signals and muscle tone without affecting muscle strength (reflex inhibition) 3, 4
- Reduction of formalin-induced pain, possibly mediated through blockade of substance P, glutamate, and calcitonin gene-related peptide 3, 4
Molecular Basis of Inhibition
The molecular basis of inhibition by botulinum toxins involves:
- Cleavage of different intracellular proteins or the same target at distinct bonds, resulting in inhibition of exocytosis and release of acetylcholine 6
- Different durations of muscle relaxation depending on the serotype of the toxin and the cleavage site in SNAP-25 6
In Vitro Studies
In vitro studies have demonstrated the inhibitory effects of botulinum toxin on acetylcholine release from cultured neurons, including: