Anti-Muscarinic Agents Study Notes

Anti-Muscarinic Agents Study Notes

1. Overview

• Anti-muscarinic agents are antagonists at muscarinic acetylcholine (ACh) receptors, opposing parasympathetic activity (parasympatholytic).

• Key effect: Block parasympathetic actions → “fight against rest & digest” responses.

2. Physiology of Muscarinic Receptors

A. Central Nervous System (CNS)

• Basal Ganglia: Balance between dopamine (DA) and ACh regulates movement.

  • Diseases with ↓ dopamine (e.g., Parkinson's) → ACh dominance → tremor.

• Vomiting (Medulla – Chemoreceptor Trigger Zone / Emesis Center): Muscarinic receptors mediate vomiting reflex.

B. Eye

• Pupil: Muscarinic stimulation → constriction; blockade → dilation (mydriasis).

• Ciliaris muscle: Muscarinic stimulation → contraction → aqueous humor drainage & accommodation; blockade → impaired drainage, blurred vision.

• Parasympathetic nerve: CN III (oculomotor)

C. Glands

• Salivary & lacrimal glands: Muscarinic stimulation → ↑ secretion; blockade → dry mouth & eyes.

• Cranial nerves: CN VII (facial), CN IX (glossopharyngeal)

D. Heart

• SA & AV nodes: Muscarinic (M2) stimulation → ↓ HR & cardiac output; blockade → ↑ HR & output.

E. Lungs

• Bronchioles (smooth muscle): Muscarinic stimulation → bronchoconstriction; blockade → bronchodilation.

F. Gastrointestinal (GI) & Liver

• ↑ Motility, secretions, bile release; blockade → constipation, decreased digestion.

G. Bladder (S2–S4 Sacral Nerves)

• Muscarinic stimulation → detrusor contraction → urination; blockade → urinary retention.

H. Sweat Glands

• Sympathetic cholinergic fibers: Acetylcholine acts on muscarinic receptors → sweating; blockade → ↓ sweating → hyperthermia risk.

3. Mechanism at Muscarinic Receptors

• M2 & M4: Inhibitory (G_i protein) → ↓ cAMP, ↑ K⁺ efflux → hyperpolarization (heart: ↓ HR, ↓ conduction).

• M1, M3, M5: Stimulatory (G_q/G_s proteins) → ↑ intracellular signaling → contraction, secretion, motility.

• Anti-muscarinics block both stimulatory & inhibitory effects depending on receptor location.

4. Drug Categories & Key Uses

Drug	Target / Effect	Key Clinical Uses
Atropine	M2 (heart), M3 (eye/glands)	↑ HR (bradycardia), mydriasis, reduce secretions, antidote for cholinergic crisis
Scopolamine	CNS / emetic center	Motion sickness, postoperative nausea/vomiting
Benztropine / Trihexyphenidyl	CNS / basal ganglia	Parkinson’s disease (tremor), antipsychotic-induced extrapyramidal symptoms
Oxybutynin / Tolterodine / Solifenacin	Bladder (M3)	Overactive bladder, urinary incontinence
Dicyclomine / Hyoscyamine	GI (M3)	IBS-D, reduce GI spasms & diarrhea
Glycopyrrolate	Peripheral glands	Reduce drooling, pre-op secretion control
Ipratropium / Tiotropium	Lungs (M3)	COPD/asthma: bronchodilation (ipratropium = short-acting, tiotropium = long-acting)

Tertiary vs Quaternary Amines

• Tertiary: Lipophilic → CNS penetration (e.g., scopolamine, benztropine, trihexyphenidyl)

• Quaternary: Hydrophilic → limited CNS penetration → mainly peripheral effects (e.g., glycopyrrolate, ipratropium)

5. Anti-Muscarinic Effects

• CNS: Delirium, confusion, agitation, seizures (high doses)

• Eye: Mydriasis, blurred vision, ↑ intraocular pressure

• Glands: Dry mouth, dry eyes, decreased secretions

• Heart: Tachycardia, ↑ cardiac output

• Lungs: Bronchodilation

• GI: ↓ motility → constipation

• Bladder: Urinary retention

• Sweating: ↓ → hyperthermia risk

6. Contraindications / Cautions

• Narrow-angle glaucoma (↑ IOP)

• Bowel obstruction, ileus, severe constipation

• Urinary retention, BPH

7. Anticholinergic Toxicity

• Causes: Excess anti-muscarinics, tricyclic antidepressants, first-generation antihistamines, antipsychotics, belladonna plant ingestion.

• Symptoms: Delirium, hallucinations, mydriasis, dry mouth/eyes, tachycardia, hyperthermia, constipation, urinary retention

• Antidote: Physostigmine (cholinesterase inhibitor)

8. Key Clinical Applications / Exam Points

• Cholinergic crisis / organophosphate poisoning: Atropine

• Motion sickness: Scopolamine

• Bradycardia: Atropine

• COPD / asthma: Ipratropium (short-acting), Tiotropium (long-acting)

• Parkinsonism / EPS: Benztropine, Trihexyphenidyl

• Overactive bladder / urinary incontinence: Oxybutynin, Tolterodine, Solifenacin

• GI spasm / IBS-D: Dicyclomine, Hyoscyamine.