Anticholinergic drugs are muscarinic or nicotinic, having different medical uses.
This article explains what anticholinergic drugs are and everything you need to know about them: what they are for, the different types, what contraindications and side effects they have, etc.
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What are anticholinergic drugs?
Anticholinergic drugs are substances with activity on acetylcholine receptors . Acetylcholine is a specific neurotransmitter of different synapses of the nervous system, places of transmission of neuronal signals.
These anticholinergic substances act antagonistically, that is, they block the mechanism by which the body secretes acetylcholine.
Depending on the way in which they do it, or rather, on the receptor they block, they are classified according to two main types of anticholinergic drugs: muscarinic and nicotinic.
1. Muscarinic antagonist drugs
Muscarinic antagonists are named as they act on muscarinic receptors . Atropine is the molecule that regularly antagonizes muscarinic acetylcholine receptors in the body, and these drugs perform the same function on them.
There are five such receptors, located in different parts of the body:
- M1 receptors : brain, sympathetic ganglia, and glands
- M2 receptors : brain, smooth muscle, and heart
- M3 receptors : brain, glands, and smooth muscle
- M4 receptors : brain
- M5 receptors : brain and eyes
Unlike atropine, antimuscarinic drugs exert their antagonistic function selectively. This means that they only act on certain types of the above receptors.
The following active substances stand out: tiotropium, scopolamine, ipratropium bromide, darifenacin, oxybutynin, solifenacin and trihexyphenidyl.
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2. Pharmacos antagonistas nicotínicos
Nicotinic antagonists act on nicotinic acetylcholine receptors . These medications are usually given to patients who are due to undergo surgical interventions. They are also sometimes used in some types of exploratory examinations such as endoscopic examinations.
The blockade that occurs in this type of anticholinergic is neuromuscular, and that is why it is used to carry out interventions. Smooth muscle moves involuntarily, and is responsible for different internal functions such as mechanical movements that help digest food.
These drugs, therefore, allow muscle relaxation with peripheral action by preventing the reuptake of acetylcholine by muscarinic receptors. These are divided into: muscle relaxants that have peripheral depolarizing action and those that do not:
- Nicotinic antagonists with depolarizing action : Like acetylcholine, these drugs can depolarize muscle fibers by causing contractile proteins (myosin and actin) to contract the muscle. Active ingredients such as succinylcholine, suxamethonium and decamethonium stand out
- Nicotinic antagonists with depolarizing action : peripheral non-depolarizing antagonists do not cause stimulation, so the mechanism is different from that of muscle contraction. They achieve their function by directly blocking the ionotropic activity of cholinergic receptors.
Among the former, active ingredients such as succinylcholine, suxamethonium and decamethonium stand out. On the other hand, among those that do not have a peripheral depolarizing action, atracurium, vecuronium and rocuronium stand out.
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What are anticholinergic drugs used for?
Anticholinergic drugs have different uses related to muscle contractility , but not only. The two large groups of anticholinergic drugs are differentiated, so their uses are explained below according to the type of active ingredient being discussed.
The main uses attributed to these two types of drugs are specified below. As we will see, there are notable differences, with muscarinics being widely consumed by the population and nicotinics only in very specific situations.
Functions of Muscarinic Anticholinergic Drugs
Muscarinic anticholinergic drugs are used for different purposes . To begin with, they stand out for their ability to decrease the contractility of the smooth muscles of the gastrointestinal tract and the urinary tract. In this way they help treat diseases and disorders related to gastrointestinal spasms or overactive bladder.
These drugs have the ability to dilate parts of the body such as bronchi and pupils, so they serve to improve the symptoms of asthmatics and are exploited in performing ophthalmological examinations. They also decrease gastric secretion and various glands, such as the salivary glands.
In addition, they also calm nausea and vomiting caused by dizziness and tremors and muscle stiffness associated with Parkinson’s disease.
Roles of Nicotinic Anticholinergic Drugs
Nicotinic anticholinergic drugs are mainly used in surgical interventions . They are used to facilitate work in this type of medical intervention, as well as during the execution of some types of endoscopic examinations.
Therefore, there are not as many uses as in the case of muscarinic anticholinergic drugs. Nicotinic anticholinergic drugs are rarely prescribed, so they are not sold in pill form.
Administration is normally intravenous, and its main function is to relax smooth muscle in surgical contexts. This muscle moves even if the person does not want it to, and it is used to perform functions such as breathing, digesting, etc.
Side effects
One of the most important side effects of these drugs is that they can cause gastrointestinal problems . They have a certain degree of toxicity for the organs of the digestive system, which leads to symptoms such as dry mouth, constipation or cramps in the abdominal area of the body.
Neurological toxicity is also notable, and may affect the level of consciousness of the person taking anticholinergics. Mental confusion, vertigo, syncope, and drowsiness may manifest.
The elderly population is the most sensitive to its effects, being able to show insomnia, irritability and dyskinesia. Long-term consumption of these substances has also been linked to a greater possibility of symptoms of dementias such as Alzheimer’s.
Older people are therefore more susceptible to the neurotoxicity of these drugs. This is due to physiological changes linked to aging, such as increased permeability of the blood-brain barrier. However, these drugs are widely and necessarily used, as they help fight other health problems.
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Bibliographic references
- Boustani, M., Campbell, N., Munger, S., et al. (2008). Impact of anticholinergics on the aging brain: a review and practical application. Aging Health, 311-320.
- Brunton, LL and Chabner, BA (2011). Goodman & Gilman’s The pharmacological basis of therapeutics (12th ed.). New York (USA): McGraw Hill .
- Stolerman, I. P. (2010). Encyclopedia of psychopharmacology. Berlin: Springer
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