Muscarine is a natural product found in certain mushrooms, particularly in Inocybe and Clitocybe species, such as the deadly C. dealbata. Mushrooms in the genera Entoloma and Mycena have also been found to contain levels of muscarine which can be dangerous if ingested. Muscarine has been found in harmless trace amounts in Boletus, Hygrocybe, Lactarius and Russula. Muscarine is only a trace compound in the fly agaric Amanita muscaria; the pharmacologically more relevant compound from this mushroom is muscimol.

Muscarine was first isolated from Amanita muscaria in 1869. It was the first parasympathomimetic substance ever studied and causes profound activation of the peripheral parasympathetic nervous system that may end in convulsions and death. Being a quaternary amine, muscarine is less completely absorbed from the gastrointestinal tract than tertiary amines, but it does cross the blood brain barrier. Muscarine poisoning is characterized by miosis, blurred vision, increased salivation, excessive sweating, lacrimation, bronchial secretions, bronchoconstriction, bradycardia, abdominal cramping, increased gastic acid secretion, diarrhea and polyuria. If muscarine reaches the brain it can cause tremor, convulsions and hypothermia. Cardiac ventricles contain muscarinic receptors that mediate a decrease in the force of contractions leading to a lower blood pressure. If muscarine is administered intravenously, muscarine can trigger acute circulatory failure with cardiac arrest. The symptoms of intoxication with mushrooms rich in muscarine, especially Inocybe, are very typical: The symptoms start early, after one-quarter to two hours, with headache, nausea, vomiting, and constriction of the pharynx. Then salivation, lacrimation, and diffuse perspiration set in, combined with miosis, disturbed accommodation, and reduced vision. Gastric and small bowel colic leads to diarrhea, and there is a painful urge for urination. Bronchoconstriction leads to asthmatic attacks and severe dyspnea, and bradycardia combined with marked hypotension and vasodilation results in circulatory shock. Death after 8 to 9 hours has been reported in about 5% of the cases, but can be avoided completely by prompt diagnosis and treatment with atropine.

Muscarinic agonists are used as drugs in treating glaucoma, postoperative ileus, congenital megacolon, urinary retention and xerostomia. Muscarine is used in patients with diseases that make them susceptible to parasympathetic stimulation, patients who have asthma or COPD or patients who have peptic ulcer disease.

Formal Chemical Name (IUPAC)
2,5-Anhydro-1,4,6-trideoxy-6-(trimethylammonio)-D-ribo-hexitol

References

http://en.wikipedia.org/wiki/Muscarine