Generalities and Classification
Anticholinergic drugs are active ingredients that exert an antagonistic type of activity against acetylcholine receptors.
In this regard, anticholinergic drugs can be subdivided substantially into two macro-groups, such as:
- Muscarinic antagonists (since they act at the level of muscarinic-type acetylcholine receptors);
- Nicotinic antagonists (since they interact with nicotinic acetylcholine receptors).
Muscarinic antagonists
As mentioned, muscarinic antagonists - or antimuscarinics, if you prefer - are anticholinergic drugs that perform their activity by interacting with muscarinic-type acetylcholine receptors.
Basically, there are five different types of muscarinic receptors:
- M1, present mainly at the level of glands, brain and sympathetic ganglia;
- M2, especially located at the level of smooth muscles, heart and brain;
- M3, mostly present in smooth muscle (such as gastrointestinal), brain and glands;
- M4 ; present in the brain;
- M5 ; located mainly in the eyes and brain.
The forefather of this type of anticholinergic drugs is atropine, a molecule able to non-selectively antagonize all five types of muscarinic receptors present in our body.
However, in the market there are antimuscarinic drugs formulated in such a way as to carry out their action in a selective manner, only in relation to certain receptor types.
Active substances such as scopolamine, ipratropium, tiotropium, solifenacin, darifenacin, oxybutynin and triesifenidyl belong to the class of antimuscarinic drugs.
Applications and therapeutic uses
Depending on the type of muscarinic receptor with which anticholinergic drugs interact, different biological responses can be obtained.
In general, it can be stated that the responses to muscarinic antagonists include:
- Decreased contractility of the smooth muscles of the gastrointestinal tract and urinary tract (in particular, this action is performed by the type M3 receptor antagonists);
- Bronchodilation (due to the antagonization of M3 receptors present at the bronchial level);
- Reduction of gastric secretion and glandular secretions, including salivary and mucociliary secretion;
- Pupil dilation (mydriasis).
Antimuscarinic agents, therefore, can be used in the treatment of various diseases and disorders, such as:
- Gastrointestinal spasms;
- Overactive bladder;
- Nausea and vomiting (caused mainly by motion sickness);
- Tremors, muscle rigidity and sialorrhea, which characterize Parkinson's disease and parkinsonism;
- Symptoms associated with diseases such as asthma or chronic bronchitis (bronchoconstriction).
The mydriatic action of antimuscarinics, on the other hand, is used in the eye area during ophthalmic examinations and medical examinations.
Side effects
Of course, the type of side effects and the intensity with which they occur can vary greatly from individual to individual, also depending on the type of active ingredient used, the way the drug is administered and the sensitivity that each patient presents. towards the same drug.
However, the main side effects ascribed to use - especially if prolonged - of muscarinic antagonists are:
- Dry mouth;
- Mydriasis (adverse effect which, as mentioned, is sometimes used in the eye area);
- Difficulty urinating (an undesirable effect which, as we have seen, in some cases is useful to counteract the symptoms of overactive bladder);
- Blurring of vision;
- photophobia;
- Constipation;
- Drowsiness;
- Confusion;
- Tachycardia (quite rare side effect).
Nicotinic antagonists
Nicotinic antagonists are anticholinergic drugs that act at the level of nicotinic acetylcholine receptors.
The active ingredients belonging to this class of anticholinergic drugs are mainly used as muscle relaxants. These drugs, therefore, are mostly used to promote surgical interventions or during the execution of some types of endoscopic examinations.
Classification and mechanism of action
Anticholinergic drugs that antagonize nicotinic acetylcholine receptors exert a neuromuscular blocking type activity; for this reason they are often referred to as peripheral action muscle relaxants.
The anticholinergics in question, in fact, bind to the nicotinic receptors present at the level of the neuromuscular plaque, thus causing a decrease in the acetylcholine signal and, consequently, a relaxation of the musculature.
More precisely, these drugs can be divided into:
- Depolarising peripheral muscle relaxants, a group to which succinylcholine belongs;
- Non-depolarising peripheral muscle relaxants, a group to which active ingredients such as atracurium, rocuronium and vecuronium belong.
Side effects
Also in this case, the type of undesirable effects and the intensity with which they occur can vary between patients, depending both on the type of active ingredient chosen and on the sensitivity of each individual towards the drug employee.
However, many of these anticholinergic drugs can cause side effects, such as:
- bronchospasm;
- Asthma;
- Hypotension;
- Cardiovascular disorders;
- rashes;
- Itch;
- Urticaria.
In any case, for more detailed information on the uses, the mechanism of action and the adverse effects of this type of anticholinergic drugs, please refer to the article on this site: Miorelaxants.
