Classification by site of action and mechanism
The action of cholinomimetics is mediated through the stimulation of specific receptors or protection of the neurotransmitter from degradation:
- M-cholinergic receptors are located on the postsynaptic membrane of organs with parasympathetic innervation and presynaptically to regulate release.
- N-cholinergic receptors ($N_M$) are located in the neuromuscular junctions of skeletal muscles.
- Direct M-cholinomimetics bind directly to receptors (the standard is muscarine, as well as pilocarpine, carbachol, aceclidine, bethanechol).
- Indirect stimulants inhibit acetylcholinesterase, increasing the amount of acetylcholine in the synapse.
Characteristics of direct M-cholinomimetics
Drugs in this group differ in selectivity and pharmacological properties:
- Selective M-cholinomimetics include muscarine (the reference substance, used in toxicology, does not cross the blood-brain barrier), pilocarpine, aceclidine, and cevimeline.
- Mixed M-, N-cholinomimetics are acetylcholine and carbachol (carbachol).
- Pilocarpine and aceclidine are widely used in ophthalmology, including for glaucoma.
- Bethanechol is used as a gastrointestinal motility stimulant.
Anticholinesterase agents (indirect cholinomimetics)
The mechanism of action is based on the blockade of the enzyme acetylcholinesterase. According to the chemical stability of the bond, they are divided into reversible and irreversible:
- Representatives include physostigmine, armin, neostigmine (neostigmine methylsulfate), and ipidacrine (neuromidin).
- Based on their structure, carbamic acid derivatives (temporary bond with the enzyme) and organophosphorus compounds (OPCs, which form a practically irreversible bond due to phosphorylation) are distinguished.
GI motility stimulants and cholinergic agents
In modern medical practice, cholinergic agents for motility stimulation (M-cholinomimetics and anticholinesterase drugs) are rarely used. Although they increase smooth muscle tone, they do not provide sufficient propulsive activity for the effective movement of intestinal contents.
Direct agents act directly on the receptor (Pilocarpine, Bethanechol); indirect agents act by protecting acetylcholine (Anticholinesterase agents).
Class and exam questions
Why is muscarine not used as a medicinal product?
Muscarine is the reference non-selective M-cholinomimetic that stimulates all M-receptor subtypes. It does not cross the blood-brain barrier and is studied exclusively in the context of toxicology.
What is the difference between direct and indirect cholinomimetics?
Direct cholinomimetics bind to and activate cholinergic receptors themselves. Indirect agents (anticholinesterase drugs) block the enzyme acetylcholinesterase, increasing the concentration of endogenous acetylcholine in the synapse.
What are the features of anticholinesterase agents according to the type of bond with the enzyme?
They are divided into reversible-action compounds with a temporary bond (e.g., carbamic acid derivatives) and irreversible-action compounds with a stable bond, such as organophosphorus compounds.
What else to study on this topic
- Localization and subtypes of N-cholinergic receptors ($N_N$ in autonomic ganglia)
- Pharmacological effects and clinical use of anticholinesterase agents
- Comparison of M-cholinomimetics and dopamine $D_2$-receptor antagonists in gastroenterology
- Classification of gastrointestinal motility stimulants
- The role of reversible and irreversible cholinesterase inhibitors in medical practice
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