A muscarinic receptor antagonist (MRA) is a type of anticholinergic agent that blocks the activity of the muscarinic acetylcholine receptor.
The muscarinic receptor is a protein involved in the transmission of signals through certain parts of the nervous system: the muscarinic receptor antagonists work to prevent this transmission from occurring.
Muscarinic antagonists reduce the activation of the parasympathetic nervous system.
The parasympathetic system activities: rest-and-digest, and includes slowing of the heart, an increased rate of digestion, narrowing of the airways, promotion of urination, and sexual arousal.
Muscarinic antagonists counter these parasympathetic activities.
Muscarinic antagonists also work elsewhere in both the central and peripheral nervous systems.
Muscarinic acetylcholine receptor antagonists:
Allergies, asthma, atrial fibrillation with bradycardia, motion sickness, Parkinson’s disease.
Muscarinic antagonist drugs are used in the treatment of low heart rate, overactive bladder, respiratory problems such as asthma and COPD, and neurological problems such as Parkinson’s disease and Alzheimer’s disease.
Many 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:
muscarinic, which respond to muscarine
nicotinic, which respond to nicotine
The two most commonly used anticholinergics, scopolamine and atropine, are belladonna alkaloids, and are naturally extracted from plants.
Muscarinic antagonists can be classified into either long-acting muscarinic receptor antagonists (LAMAs) or short-acting muscarinic receptor antagonists (SAMAs), depending on when maximum effect occurs and for how long the effect persists.
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 bloodâ€“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 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 (nigrostriatal pathway).
In parkinsonism there is decreased level of dopaminergic activity.
A 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.
Atropine causes tachycardia by blocking vagal effects on the sinoatrial node.
Acetylcholine hyperpolarizes the sinoatrial node, and thus increases the heart rate.
If atropine is given by intramuscular or subcutaneous injection, it causes initial bradycardia by acting on presynaptic M1 receptors and the presynaptic nerve releases more acetylcholine into the synapse that initially causes bradycardia.
Important muscarinic antagonists include: atropine, hyoscyamine, hyoscine butylbromide and hydrobromide, ipratropium, tropicamide, cyclopentolate, and pirenzepine.
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.