Muscarinic receptors were named as such because they are more sensitive to muscarine than to nicotine. Their counterparts are nicotinic acetylcholine receptors (nAChRs), receptor ion channels that are also important in the autonomic nervous system. Many drugs and other substances (for example pilocarpine and scopolamine) manipulate these two distinct receptors by acting as selective agonists or antagonists.
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. Muscarinic agonists activate muscarinic receptors while nicotinic agonists activate nicotin receptors. Both are direct-acting cholinomimetics; they produce their effects by binding to and activating cholinergic receptors. Final proof of the structure was given by Jellinek (61) in 1957 with the help of X-ray diffraction analysis. These new findings set into motion research not only on the pharmacology of muscarine, but also on that of muscarine-like substances that are structurally related to acetylcholine.