Muscarinic antagonist

Muscarinic-Acetylcholine receptor antagonist
Drug class
Skeletal formula of Scopolamine, a nonselective antagonist of the muscarinic receptors
Class identifiers
Use	Allergies, Asthma, Bradycardia, Motion sickness, Parkinson's disease, etc.
ATC code	V
Biological target	Metabotropic acetylcholinergic receptors.
External links
MeSH	D018727

A muscarinic receptor antagonist (MRA) is a type of anticholinergic agent that blocks the activity of the muscarinic acetylcholine receptor. Acetylcholine (often abbreviated ACh) is a neurotransmitter whose receptor is a protein found in synapses and other cell membranes. Besides responding to their primary neurochemical, neurotransmitter receptors can be sensitive to a variety of other molecules. Acetylcholine receptors are classified into two groups based on this:

Most muscarinic receptor antagonists are synthetic chemicals; however, the two most commonly used anticholinergics, scopolamine and atropine, are belladonna alkaloids, and are naturally extracted.

Muscarinic antagonist effects and muscarinic agonist effects counterbalance each other for homeostasis.

Certain substances are known as long-acting muscarinic receptor antagonists (LAMAs).

Scopolamine and atropine have similar effects on the peripheral nervous system. However, 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 (nigrostriatal pathway). 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. In addition to increased activity to dopaminergic neurons from anticholinergics, antagonists of the muscarinic-acetylcholine receptors produce a sense of happiness and euphoria. This led to the discovery of various tricyclic antidepressants.