A muscarinic agonist 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.
M1-type muscarinic acetylcholine receptors play a role in cognitive processing. In Alzheimer disease (AD), amyloid formation may decrease the ability of these receptors to transmit signals, leading to decreased cholinergic activity. As these receptors themselves appear relatively unchanged in the disease process, they have become a potential therapeutic target when trying to improve cognitive function in patients with AD.
A number of muscarinic agonists have been developed and are under investigation to treat AD. These agents show promise as they are neurotrophic, decrease amyloid depositions, and improve damage due to oxidative stress. Tau-phosphorylation is decreased and cholinergic function enhanced. Notably several agents of the AF series of muscarinic agonists have become the focus of such research:. AF102B, AF150(S), AF267B. In animal models that are mimicking the damage of AD, these agents appear promising.
The agent xanomeline has been proposed as a potential treatment for schizophrenia.
In the form of pilocarpine, muscarinic receptor agonists have been used medically for a short time.
|Comparison of cholinergic agonists |
|Substance||Receptor specificity||Hydrolysis by acetylcholinesterase||Comments|
|Carbachol||++||+++||-||Used in the treatment of glaucoma|
|Methacholine||+++||+||++||Used to diagnose bronchial hyperreactivity, a hallmark of asthma and COPD.|
|Bethanechol||+++||-||-||Used in bladder and gastrointestinal hypotonia.|
|Muscarine||+++||-||-||Natural alkaloid found in certain mushrooms.
Cause of one form of mushroom poisoning
|Nicotine||-||+++||-||Natural alkaloid found in the tobacco plant.|
|Pilocarpine||++||-||-||Used in glaucoma.|
|Oxotremorine||++||+||-||Used in research to induce
symptoms of Parkinson's disease.
The targets for muscarinic agonists are the muscarinic receptors: M1, M2, M3, M4 and M5. These receptors are GPCRs coupled to either Gi or Gq subunits.