Muscarinic antagonist
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).
Muscarinic antagonists, also called antimuscarinic agents, are drugs that block muscarinic acetylcholine receptors, thereby inhibiting the action of acetylcholine in the body, especially in the parasympathetic nervous system.
The muscarinic receptors are proteins involved in the transmission of signals through parts of the nervous system.
Muscarinic receptor antagonists work to prevent this nervous system transmission from occurring.
Notably, muscarinic antagonists reduce the activation of the parasympathetic nervous system.
The parasympathetic system functions include slowing of the heart, an increased rate of digestion, narrowing of the airways, promotion of urination, and sexual arousal.
Muscarinic antagonists counter this parasympathetic responses, and also work elsewhere in both the central and peripheral nervous systems.
Drugs with muscarinic antagonist activity are widely used in the treatment of low heart rate, overactive bladder, respiratory problems such as asthma and chronic obstructive pulmonary disease (COPD), and neurological problems such as Parkinson’s disease and Alzheimer’s disease.
Major examples of muscarinic antagonists are atropine, scopolamine, ipratropium, and tropicamide.
Muscarinic antagonists work by competitively inhibiting muscarinic receptors (M1–M5), and their effects depend on which organ system is influenced, such as causing pupil dilation (mydriasis), bronchodilation, increased heart rate, and decreased glandular secretions.
Long-acting muscarinic antagonists (LAMAs) are a class of bronchodilator medications primarily used to treat chronic obstructive pulmonary disease (COPD) and sometimes asthma.
These drugs work by blocking muscarinic receptors in the airways, which leads to bronchodilation and improved airflow.
A number of other drugs, such as antipsychotics and the tricyclic family of antidepressants, have incidental muscarinic antagonist activity which can cause unwanted side effects such as difficulty urinating, dry mouth and skin, and constipation.
Acetylcholine is a neurotransmitter whose receptors are proteins found in synapses and other cell membranes.
Acetylcholine receptors are classified into two groups based on this:
muscarinic, which respond to muscarine nicotinic, which respond to nicotine
Most muscarinic receptor antagonists are synthetic chemicals.
The two most commonly used anticholinergics, scopolamine and atropine, are belladonna alkaloids, and are naturally extracted from plants.
Muscarinic antagonist effects and muscarinic agonist effects counterbalance each other for homeostasis.
Certain muscarinic antagonists can be classified into either long-acting muscarinic receptor antagonists (LAMAs) or short-acting muscarinic receptor antagonists (SAMAs).
Scopolamine and atropine have similar effects on the peripheral nervous system.
Scopolamine has greater effects on the central nervous system (CNS) than atropine due to its ability to cross the blood–brain barrier.
At higher-than-therapeutic doses, atropine and scopolamine cause CNS depression characterized by amnesia, fatigue, and reduction in rapid eye movement sleep.
Scopolamine (Hyoscine) has anti-emetic activity and is, therefore, used to treat motion sickness.
Antimuscarinics are also used as anti-parkinsonian drugs.
In parkinsonism, there is imbalance between levels of acetylcholine and dopamine in the brain, involving both increased levels of acetylcholine and degeneration of dopaminergic pathways.
Thus, in parkinsonism there is decreased level of dopaminergic activity.
One method of balancing the neurotransmitters is through blocking central cholinergic activity using muscarinic receptor antagonists.
Atropine acts on the M2 receptors of the heart and antagonizes the activity of acetylcholine.
It causes tachycardia by blocking vagal effects on the sinoatrial node.
Acetylcholine hyperpolarizes the sinoatrial node; this is overcome by muscarinic receptor antagonists, and thus they increase the heart rate.
If atropine is given by intramuscular or subcutaneous injection, it causes initial bradycardia.
In the atrioventricular node, the resting potential is lowered, which facilitates conduction.
This is seen as a shortened PR-interval on an electrocardiogram.
It has an opposite effect on blood pressure, with tachycardia and stimulation of the vasomotor center causes an increase in blood pressure.
But, due to feedback regulation of the vasomotor center, there is a fall in blood pressure due to vasodilation.
Important muscarinic antagonists include atropine, hyoscyamine, hyoscine butylbromide and hydrobromide, ipratropium, tropicamide, cyclopentolate, pirenzepine and scopolamine.
Muscarinic antagonists such as ipratropium bromide can also be effective in treating asthma, since acetylcholine is known to cause smooth muscle contraction, especially in the bronchi.
LAMAs block muscarinic receptors (specifically M3 receptors) in bronchial smooth muscle.
This prevents acetylcholine from binding to these receptors, reducing bronchoconstriction and mucus secretion.
Long-acting designation means they provide bronchodilation for 12-24 hours with once or twice daily dosing.
Once-daily medications: Tiotropium (Spiriva) – The first LAMA, typically given as 18 mcg once daily via dry powder inhaler Glycopyrronium Umeclidinium Aclidinium
Clinical Uses of LAMAs
Primary indications: COPD maintenance therapy (first-line treatment) Asthma (as add-on therapy when ICS/LABA combinations are insufficient) Often combined with long-acting beta-2 agonists (LABAs) or inhaled corticosteroids
Advantages of LAMAs
Long duration of action improves medication adherence Effective bronchodilation Reduces COPD exacerbations Can be combined with other respiratory medications Generally well-tolerated
Common side effects include dry mouth, constipation, urinary retention, and potentially increased risk of pneumonia in some patients, and confusion, especially in elderly patients.
LAMAs should generally not be used as rescue medications and are contraindicated in those with certain conditions, such as glaucoma or risk of urinary retention.
They should be used cautiously in patients with narrow-angle glaucoma or urinary retention.
LAMAs have become a cornerstone of COPD management and represent an important advance in respiratory medicine due to their efficacy and convenient dosing schedule.
LAMAs results in reduced activation of “rest-and-digest” functions, such as slowed heart rate, increased secretions, and smooth muscle activity.
These agents are widely used in medicine to treat conditions including: Overactive bladder (e.g., oxybutynin, tolterodine, solifenacin, darifenacin) Respiratory issues like asthma and COPD (e.g., ipratropium, tiotropium) Bradycardia Motion sickness and nausea (scopolamine) Symptoms of Parkinson’s disease (benztropine, trihexyphenidyl).
Antihistamines
Chlorphenamine (chlorpheniramine) Cyproheptadine
Diphenhydramine
Doxylamine
Mequitazine
Terfenadine
Antidepressants
Amitriptyline
Bupropion
Citalopram
Desipramine
Doxepin
Escitalopram
Fluoxetine
Fluvoxamine
Imipramine
Nortriptyline
Paroxetine
Sertraline
Trazodone
Venlafaxine
Antipsychotics
Aripiprazole
Chlorpromazine
Clozapine
Haloperidol
Quetiapine
Risperidone
Ziprasidone
Long-acting muscarinic antagonists (LAMAs) are a class of inhaled bronchodilators that block muscarinic (acetylcholine) receptors in airway smooth muscle, leading to sustained bronchodilation.
In stable chronic obstructive pulmonary disease (COPD), LAMAs are considered first-line maintenance therapy for patients with persistent symptoms or those at risk for exacerbations.
Common agents include tiotropium, glycopyrrolate, umeclidinium, and aclidinium, typically administered once or twice daily via inhaler devices.
LAMAs as initial maintenance therapy for stable COPD, particularly for patients with moderate to severe disease or those with a history of exacerbations, due to their superior efficacy in reducing exacerbation rates and improving lung function compared to long-acting beta2-agonists (LABAs) alone.
LAMAs can be used as monotherapy or in combination with LABAs for patients with more severe symptoms or frequent exacerbations.