Health

Cholinergic Antagonists

Cholinergic Antagonists 

 

Introduction

  • Cholinergic antagonist are agents that bind to cholinoceptors (muscarinic or nicotinic) and prevent the effects of cholinergic agonists like of acetylcholine (ACh) on them.

Classification

On the basis of blocking behavior, cholinergic antagonist are classified as follows :-
1. Anti-Muscarinic Agents

  • Agents that are selective blockers of muscarinic receptors are known as antimuscarinic agents / parasympatholytics / anticholinergic agents (a misnomer, as they antagonize only muscarinic receptors).
  • In addition, these drugs block the few exceptional sympathetic neurons that are cholinergic, such as those innervating the salivary and sweat glands.
  • They are most clinically useful of cholinergic antagonist.
  • Because they do not block nicotinic receptors, the anticholinergic drugs (more precisely, antimuscarinic drugs) have little or no action at skeletal neuromuscular junctions (NMJs) or autonomic ganglia.
  • E.g.
    Natural Alkaloids
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    Semi-Synthetic Derivatives
    [wpsm_comparison_table id=”20″ class=””]
    Synthetic Compounds
    [wpsm_comparison_table id=”21″ class=””]
    Note :- A number of antihistamines and antidepressants (mainly tricyclic antidepressants) also have antimuscarinic activity.

2. Ganglionic Blockers

  • Ganglionic blockers specifically act on the nicotinic receptors of both parasympathetic and sympathetic autonomic ganglia. Some also block the ion channels of the autonomic ganglia.
  • These drugs show no selectivity toward the parasympathetic or sympathetic ganglia and are not effective as neuromuscular antagonists. Thus, these drugs block the entire output of the autonomic nervous system at the nicotinic receptor.
  • Ganglionic blockade is rarely used therapeutically, but often serves as a tool in experimental pharmacology.
  • E.g.
     – Nicotine

3. Neuro-Muscular Blockers

  • Neuro-Muscular Blockers are drugs which block cholinergic transmission between motor nerve endings and the nicotinic receptors on the skeletal muscle.
  • They possess some chemical similarities to ACh, and they act either as antagonists (nondepolarizing type) or as agonists (depolarizing type) at the receptors on the endplate of the NMJ.
  • Neuromuscular blockers are clinically useful during surgery to facilitate tracheal intubation and provide complete muscle relaxation at lower anesthetic doses, allowing for more rapid recovery from anesthesia and reducing postoperative respiratory depression.
  • E.g.
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Therapeutic usage

Cholinergic antagonists have various clinical usage which are as follows:

  • Useful in producing mydriasis and cycloplegia for retinal examination.
  • Used in iritis and uveitis.
  • Used for prevention of synechiae after surgery.
  • Used as an antispasmodic agent.
  • Used as an antisecretory agent.
  • Used to treat morning sickness and motion sickness.
  • Used for peptic ulcer, gastritis, gastrointestinal hypermotility, irritable bowel syndrome, nervous dyspersia
  • Used to treat bradycardia of varying etiologies.
  • Used as adjunct to treat Parkinson’s disease, Parkinsonian syndromes, including antipsychoticinduced extrapyramidal symptoms.
  • Used as bronchodilator for maintenance treatment of bronchospasm associated with chronic obstructive pulmonary disease (COPD) & asthma.
  • Used as vesico-selective agent to treat overactive bladder.
  • Used for pre-anaesthetic medication and during anaesthesia.
  • Used for the treatment of organophosphate (insecticides, nerve gases) poisoning, of overdose of clinically used anticholinesterases such as physostigmine, and in some types of mushroom poisoning (certain mushrooms contain cholinergic substances that block cholinesterases).
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