Acetylcholine (ACh) is an organic compound that functions in the brain and body of many types of animals as a neurotransmitter. Its name is derived from its chemical structure: it is an ester of acetic acid and choline. Parts in the body that use or are affected by acetylcholine are referred to as cholinergic.
Muscarine, L-(+)-muscarine, or muscarin 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. Trace concentrations of muscarine are also found in Amanita muscaria, though the pharmacologically more relevant compound from this mushroom is the Z-drug-like alkaloid muscimol. A. muscaria fruitbodies contain a variable dose of muscarine, usually around 0.0003% fresh weight. This is very low and toxicity symptoms occur very rarely. Inocybe and Clitocybe contain muscarine concentrations up to 1.6%.
Muscarinic acetylcholine receptors (mAChRs) are acetylcholine receptors that form G protein-coupled receptor complexes in the cell membranes of certain neurons and other cells. They play several roles, including acting as the main end-receptor stimulated by acetylcholine released from postganglionic fibers. They are mainly found in the parasympathetic nervous system, but also have a role in the sympathetic nervous system in the control of sweat glands.
A muscarinic acetylcholine receptor agonist, also simply known as a muscarinic agonist or as a muscarinic agent, is an agent that activates the activity of the muscarinic acetylcholine receptor. The muscarinic receptor has different subtypes, labelled M1-M5, allowing for further differentiation.
A muscarinic acetylcholine receptor antagonist, also simply known as a muscarinic antagonist or as an antimuscarinic agent, is a type of anticholinergic drug that blocks the activity of the muscarinic acetylcholine receptors (mAChRs). The muscarinic receptors are proteins involved in the transmission of signals through certain parts of the nervous system, and muscarinic receptor antagonists work to prevent this transmission from occurring. Notably, muscarinic antagonists reduce the activation of the parasympathetic nervous system. The normal function of the parasympathetic system is often summarised as "rest-and-digest", and includes slowing of the heart, an increased rate of digestion, narrowing of the airways, promotion of urination, and sexual arousal. Muscarinic antagonists counter this parasympathetic "rest-and-digest" response, and also work elsewhere in both the central and peripheral nervous systems.
The human muscarinic acetylcholine receptor M5, encoded by the CHRM5 gene, is a member of the G protein-coupled receptor superfamily of integral membrane proteins. It is coupled to Gq protein. Binding of the endogenous ligand acetylcholine to the M5 receptor triggers a number of cellular responses such as adenylate cyclase inhibition, phosphoinositide degradation, and potassium channel modulation. Muscarinic receptors mediate many of the effects of acetylcholine in the central and peripheral nervous system. The clinical implications of this receptor have not been fully explored; however, stimulation of this receptor is known to effectively decrease cyclic AMP levels and downregulate the activity of protein kinase A (PKA).
The muscarinic acetylcholine receptor M1, also known as the cholinergic receptor, muscarinic 1, is a muscarinic receptor that in humans is encoded by the CHRM1 gene. It is localized to 11q13.
The muscarinic acetylcholine receptor, also known as cholinergic/acetylcholine receptor M3, or the muscarinic 3, is a muscarinic acetylcholine receptor encoded by the human gene CHRM3.
The muscarinic acetylcholine receptor M4, also known as the cholinergic receptor, muscarinic 4 (CHRM4), is a protein that, in humans, is encoded by the CHRM4 gene.
Vedaclidine (INN, codenamed LY-297,802, NNC 11-1053) is an experimental analgesic drug which acts as a mixed agonist–antagonist at muscarinic acetylcholine receptors, being a potent and selective agonist for the M1 and M4 subtypes, yet an antagonist at the M2, M3 and M5 subtypes. It is orally active and an effective analgesic over 3× the potency of morphine, with side effects such as salivation and tremor only occurring at many times the effective analgesic dose. Human trials showed little potential for development of dependence or abuse, and research is continuing into possible clinical application in the treatment of neuropathic pain and cancer pain relief.
Sabcomeline (Memric; SB-202,026) is a selective M1 receptor partial agonist that was under development for the treatment of Alzheimer's disease. It made it to phase III clinical trials before being discontinued due to poor results.
N-Desmethylclozapine (NDMC), or norclozapine, is a major active metabolite of the atypical antipsychotic drug clozapine.
Tazomeline (LY-287,041) is a drug which acts as a non-selective muscarinic acetylcholine receptor agonist. It was in clinical trials for the treatment of cognitive dysfunction such as that seen in Alzheimer's disease and schizophrenia, but development was apparently scrapped for unknown reasons. Another of the patented uses is for the treatment of "severe painful conditions".
CI-1017 is a muscarinic acetylcholine receptor agonist which is selective for and is approximately equipotent at the M1 and M4 receptors, with 20-30-fold lower affinity for the M2, M3, and M5 subtypes It is the (R)-enantiomer of the racemic compound PD-142,505.
VU-0238429 is a drug which acts as a selective positive allosteric modulator for the muscarinic acetylcholine receptor M5. It was the first selective ligand developed for the M5 subtype, and is structurally derived from older M1-selective positive allosteric modulators such as VU-0119498. Replacing the O-methyl- by a phenyl group further improves the receptor subtype selectivity.
Xanomeline/trospium chloride, sold under the brand name Cobenfy, is a fixed-dose combination medication used for the treatment of schizophrenia. It contains xanomeline, a muscarinic agonist; and trospium chloride, a muscarinic antagonist. Xanomeline is a functionally preferring muscarinic M4 and M1 receptor agonist. Trospium chloride is a non-selective muscarinic antagonist.
Emraclidine is an investigational antipsychotic for the treatment of both schizophrenia and Alzheimer's disease psychosis developed by Cerevel Therapeutics. As of August 2024, it is in phase 2 clinical trials.
The conditioned avoidance response (CAR) test, also known as the active avoidance test, is an animal test used to identify drugs with antipsychotic-like effects. It is most commonly employed as a two-way active avoidance test with rodents. The test assesses the conditioned ability of an animal to avoid an unpleasant stimulus. Drugs that selectively suppress conditioned avoidance responses without affecting escape behavior are considered to have antipsychotic-like activity. Variations of the test, like testing for enhancement of avoidance and escape responses, have also been used to assess other drug effects, like pro-motivational and antidepressant-like effects.
ML-007 is a selective muscarinic acetylcholine M1 and M4 receptor agonist which is under development for the treatment of schizophrenia, psychotic disorders, and dyskinesias. It is being developed in combination with a peripherally selective muscarinic acetylcholine receptor antagonist (also known as ML-007/peripherally acting anticholinergic or ML-007/PAC). The drug is taken by mouth.
Itameline is a non-selective muscarinic acetylcholine receptor agonist which was under development for the treatment of Alzheimer's disease and memory disorders but was never marketed. It has been referred to as a "nootropic".
