A muscarinic receptor antagonist is an agent that reduces the activity of the muscarinic acetylcholine receptor. Most of them are synthetic, but scopolamine and atropine are belladonna alkaloids, and are naturally extracted.
Effects
Effect on central nervous system
Muscarinic antagonists stimulate the central nervous system (CNS) as a result of stimulation of medulla and higher cerebral centers. At low doses of atropine, it causes slight restlessness; at high or toxic doses it causes restlessness, agitation, and hallucinations. In atropine poisoning, marked excitation and agitation leads to hyperactivity and increase in body temperature through inhibition of sweating. Scopolamine in therapeutic doses causes CNS depression characterized by amnesia, fatigue and reduction in rapid eye movement sleep. Hyoscine has anti-emetic activity, so is used for 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.
Effect on heart
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 which is overcome by MRA and thus increases the heart rate. If atropine is given by intramuscular or subcutaneous, it causes initial bradycardia. This is because by i.m/s.c it acts on presynaptic M1 receptors (autoreceptors). Intake of acetylcholine in axoplasm is prevented and the presynaptic nerve releases more acetylcholine into the synapse which initially causes bradycardia.
In the atrioventricular node, the resting potential is abbreviated which facilitates conduction. This is seen as a shortened PR-interval on an electrocardiogram.
It has an opposite effect on blood pressure. Tachycardia and stimulation of the vasomotor center causes an increase in blood pressure. But due to feed back regulation of the vasomotor center, there is fall in blood pressure due to vasodilation.
Important1 muscarinic antagonists include atropine, hyoscine, ipratropium, tropicamide, cyclopentolate and pirenzepine.
Comparison table
See also
References
- ^ Rang, H. P. (2003). Pharmacology. Edinburgh: Churchill Livingstone. ISBN 0-443-07145-4. Page 147
- ^ a b c Unless else specified in table boxes, then ref is: Rang, H. P. (2003). Pharmacology. Edinburgh: Churchill Livingstone. ISBN 0-443-07145-4. Page 147
External links
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