Muscarin

Muscarin, also called muscarine, is a naturally occurring quaternary ammonium alkaloid that acts as a potent muscarinic acetylcholine receptor agonist. The name derives from its historical association with muscarine-containing mushrooms and the early study of cholinergic toxins.

Natural occurrence and sources: Muscarine is found in several mushroom species, most notably in the genera Inocybe and Clitocybe, with variable concentrations depending on species and specimen. It is also present in smaller amounts in a few other fungi. Ingestion can produce a cholinergic mushroom poisoning, sometimes called mushroom poisoning with muscarinic symptoms.

Mechanism of action: Muscarine binds selectively to muscarinic acetylcholine receptors (M1–M5), mimicking acetylcholine and stimulating the parasympathetic nervous system. As a quaternary ammonium compound, it has limited ability to cross the blood–brain barrier, so its effects are primarily peripheral, including increased glandular secretions and smooth-muscle activity.

Clinical features: Onset is typically within 15 to 60 minutes after ingestion and can include sweating, salivation, lacrimation, bronchial secretions, bronchoconstriction, miosis, bradycardia, hypotension, abdominal cramps, nausea, vomiting, and diarrhea. Severe cases may require medical intervention.

Treatment and management: Management focuses on supportive care and reversal of muscarinic effects. Atropine, a muscarinic antagonist, is the primary antidote used to alleviate symptoms. Additional measures include airway support, fluid management, and monitoring. Decontamination with activated charcoal may be considered if presentation is recent.

Research and clinical relevance: Muscarine is used in pharmacology as a tool to study muscarinic receptors. It is not used therapeutically due to its toxicity; clinically useful muscarinic agonists, such as bethanechol and cevimeline, are used instead for specific indications.