What are anticholinergics and how do they work
Anticholinergics are substances that occupy cholinergic receptors, preventing acetylcholine from binding to them and producing an effect. Depending on the receptor localization, a distinction is made between muscarinic antagonists (acting on muscarinic receptors) and central anticholinergics operating in the brain.
Among muscarinic antagonists, pirenzepine stands out — a drug that must be distinguished from gastric antisecretory agents (proton pump inhibitors and H2-blockers). Although formally it also reduces secretion, it does so by blocking muscarinic receptors rather than through a direct effect on the proton pump or histamine receptors.
Central anticholinergics in parkinsonism
In the treatment of parkinsonism, central anticholinergics hold a distinct place among antiparkinsonian agents. A representative drug is trihexyphenidyl.
This group acts alongside other mechanisms for correcting dopaminergic deficiency:
- dopamine receptor agonists (bromocriptine);
- glutamate NMDA receptor antagonists (amantadine);
- peripheral decarboxylase inhibitors (carbidopa, benserazide) — used in combination with levodopa.
Central anticholinergics reduce the relative cholinergic activity, which is enhanced against the background of dopamine deficiency in parkinsonism.
Combined muscarinic antagonists: Aeron
A classic example of a combined muscarinic antagonist is Aeron. Its composition is a mixture of camphorate salts of the alkaloids scopolaminum and hyoscyaminum.
The mechanism is combined central and peripheral muscarinic blockade. Due to this, Aeron is used for:
- prevention and treatment of motion sickness (sea and airsickness);
- relief of acute vertigo attacks, including in Meniere's disease.
Restrictions are standard for atropine-like agents: glaucoma is an absolute contraindication due to the risk of increased intraocular pressure, and side effects include dry mouth and thirst.
Interaction with anticholinesterase agents
A separate clinically important case is the combination of atropine (muscarinic antagonist) and neostigmine (anticholinesterase agent). A partial antagonism occurs between them.
Atropine eliminates all muscarinic effects of neostigmine: bronchospasm, bradycardia, miosis, increased glandular secretion, and GI motility. However, atropine does not eliminate one effect of neostigmine — the increase in skeletal muscle tone, as this is mediated via nicotinic receptors, which atropine does not affect.
This property is utilized in decurarization: neostigmine accumulates acetylcholine at the neuromuscular synapse, which competitively displaces the nondepolarizing muscle relaxant from the nicotinic receptor and restores impulse transmission. Atropine is administered concurrently to neutralize the unwanted muscarinic effects of neostigmine.
Atropine blocks muscarinic receptors — meaning it only eliminates effects mediated through muscarinic receptors (bronchi, heart, pupil, glands, GI tract). Muscle tone is mediated through nicotinic receptors, which atropine cannot reach.
Class and exam questions
How does Aeron differ from regular atropine?
Aeron is a combined preparation of scopolamine and hyoscyamine salts with combined central and peripheral muscarinic blocking action, used specifically for motion sickness and vertigo, unlike standalone atropine.
Why doesn't atropine abolish all effects of neostigmine?
Atropine blocks only muscarinic receptors, whereas the increase in skeletal muscle tone from neostigmine is mediated via nicotinic receptors, which atropine does not affect — hence partial, rather than complete, antagonism.
Which anticholinergic is used in parkinsonism?
Trihexyphenidyl is a representative of the central anticholinergic group used in the combination therapy of parkinsonism.
Why are anticholinergics contraindicated in glaucoma?
They increase intraocular pressure, which is hazardous in glaucoma and can exacerbate the course of the disease.
What else to study on this topic
- Clinical classification of cholinergic agents in general (muscarinic and nicotinic agonists, anticholinergics)
- Short-acting muscarinic antagonists in ophthalmology
- Principles of treating toadstool poisoning (the antidote role of atropine)
- Comparative characteristics of muscle relaxants and their interaction with anticholinesterase agents
- Difference of pirenzepine from other gastric antisecretory agents
- Sympatholytics as a separate group of vegetotropic drugs
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