Acetylcholine

Acetylcholine is a neurotransmitter used in the autonomic nervous system, both as an internal transmitter for the sympathetic nervous system and as the final product released by the parasympathetic nervous system. It also plays an important role in regulating the inflammatory response.

Function
Acetylcholine is required for the generation of muscular force. In the central nervous system, acetylcholine modulates arousal and temperature regulation. It also may play a role in central fatigue. During exercise, levels of acetylcholine drop.

Immune system
Acetylcholine plays a role in innate immunity through nicotinic acetylcholine receptors and in the adaptive immune response via M3 muscarinic acetylcholine receptors (M3R).

Muscarinic receptors
Knockout mice, that is mice lacking the gene that encodes for M3R, had impaired response to bacterial infection, while normal mice given a muscarinic agonist (to increase the activity of M3R) had enhanced production of IL-13 and IFN-γ. Another study used a muscarinic agonist and an antagonist (reduce activity) and found that the antagonist suppressed the immune response while the agonist exaggerated it.

The vagus nerve speaks directly to the immune system via acetylcholine.

Mast cells
Several studies suggest a relationship between autonomic nervous system dysfunction and mast cell activation via acetylcholine.

One study found that acetylcholine via muscarinic receptors strongly inhibited the release of histamine in mucosal mast cells. The activity of acetylcholinesterase, the enzyme that breaks down acetylcholine, was found to be significantly increased in 64% of patients experiencing flares of ulcerative colitis.

Autoimmunity
Autoantibodies to acetylcholine receptors, alpha subunit have been found in patients with myasthenia gravis. These cross react with herpesvirus glycoprotein D. Antibodies to HSV-1 and acetylcholine receptor antigens crossreact.

Chronic fatigue syndrome
In 2015, a large German study found 29% of ME/CFS patients had elevated autoantibodies to M3 and M4 muscarinic acetylcholine receptors, as well as ß2 adrenergic receptors. A 2016 Australian study found that ME/CFS patients had significantly greater numbers of single nucleotide polymorphisms associated with the gene encoding for M3 muscarinic acetylcholine receptors.

Anecdotally, some ME/CFS patients have tried Mestinon, an aceytlcholinesterase inhibitor that increases circulating acetylcholine and is used to treat myasthenia gravis, with some success.

Learn more

 * Scientists uncover new role for neurotransmitter that helps fight infection, Imperial College London News, 24 February 2015
 * The Cholinergic Anti-inflammatory Pathway: A Missing Link in Neuroimmunomodulation, Molecular Medicine, 2003 May-Aug; 9(5-8): 125–134.