Cholinergic Antagonists and Their Types

Cholinergic Antagonists

• Cholinergic antagonists bind to cholinoceptors and prevent the effects of acetylcholine (ACh) and other cholinergic agonists.
• There are three types of cholinergic antagonist drugs: antimuscarinic agents, ganglionic blockers, and neuromuscular-blocking agents.

Antimuscarinic Agents

• Antimuscarinic agents block muscarinic receptors, causing inhibition of muscarinic functions.
• They do not block nicotinic receptors, and therefore have little or no action at skeletal neuromuscular junctions (NMJs).

Atropine (Antimuscarinic Agent)

• Atropine is an alkaloid with a high affinity for muscarinic receptors.
• It binds competitively and prevents ACh from binding to muscarinic receptors.
• Atropine acts both centrally and peripherally, with general actions lasting about 4 hours.
• Effects of atropine:
  • CNS: confusion, delirium
  • GI: decrease motility and acid secretions (without interfering with hydrochloric secretion)
  • Heart rate: increase at high doses, decrease at low doses
  • Body secretions: decrease saliva, bronchial secretions, and sweat (leading to elevated body temperature)
  • Eye: mydriasis (cycloplegic), allowing for measurement of refractive errors without interference by the accommodative capacity of the eye

Therapeutic Uses of Atropine

• As a mydriatic and cycloplegic: used topically for producing mydriasis and cycloplegia, lasting 7-10 days
• As a preanaesthetic medication:
  • Preventing vagal bradycardia during anaesthesia
  • Preventing laryngospasm by decreasing respiratory secretions
  • Acting as an antisecretory agent to block secretions in the upper and lower respiratory tracts before surgery
• As an antispasmodic: useful in dysmenorrhoea, intestinal and renal colic
• In poisoning:
  • Life-saving drug in organophosphorous poisoning
  • Drug of choice in some types of mushroom poisoning
  • Used in curare poisoning with neostigmine to counteract the muscarinic effects of neostigmine
• As a vagolytic: used to treat sinus bradycardia and partial heart block due to increased vagal activity

Side Effects of Atropine

• Blurred vision
• Decreased secretions
• Hyperthermia
• Constipation
• Urinary retention
• Delirium
• Hallucinations