🧠 Acetylcholine and Cholinergic Drugs for USMLE Step 1: Direct & Indirect Agonists, Receptor Actions, and Toxicities

Acetylcholine (ACh) is a cornerstone neurotransmitter in both the central and peripheral nervous system, and the target of numerous drugs and toxins.

On Step 1, you’ll encounter it in:

• Pharmacology

• Toxicology

• Neuro and GI systems

Understanding the types of receptors, drug classes, and clinical applications will help you reason through complex vignettes.

🔬 ACh Receptors: Muscarinic vs Nicotinic

Receptor Type	Location	Mechanism	Effects
Muscarinic (M1–M5)	Parasympathetic target organs	G-protein coupled	SLUDGE effects
Nicotinic (Nn, Nm)	Ganglia, NMJ, adrenal medulla	Ligand-gated ion channel	Sympathetic activation, muscle contraction

🧠 Step 1 loves receptor type-to-organ and drug matchups

💊 Cholinergic Agonists

Drug	Type	Use
Bethanechol	Direct	Urinary retention
Pilocarpine	Direct	Glaucoma, dry mouth
Neostigmine	Indirect	Myasthenia, ileus
Physostigmine	Indirect	Anticholinergic toxicity

☠️ Cholinergic Toxicity

SLUDGE-M Mnemonic

• Salivation

• Lacrimation

• Urination

• Diarrhea

• GI cramps

• Emesis

• Miosis, Muscle fasciculations

Organophosphates (e.g., parathion, sarin gas)

• Irreversibly inhibit AChE → toxic ACh buildup

🧪 Step 1 Case Clue: Farmer exposed to pesticide with pinpoint pupils, wheezing, bradycardia

💉 Antidote: Atropine + Pralidoxime

Drug	Mechanism	Notes
Atropine	Muscarinic antagonist	Treats SLUDGE; no effect on nicotinic symptoms
Pralidoxime	Regenerates AChE	Effective before enzyme aging in organophosphate poisoning

🧠 Step 1 Clinical Clue

A patient treated for myasthenia with neostigmine presents with diarrhea, salivation, and miosis. How do you distinguish cholinergic crisis from MG worsening?

✅ Give edrophonium

• If improves: MG exacerbation

• If worsens/no change: Cholinergic crisis

🧠 Mnemonics

• “Hot as a hare, dry as a bone, blind as a bat…” → Atropine toxicity

• “DUMBELS” for organophosphate symptoms

  • Diarrhea, Urination, Miosis, Bradycardia, Emesis, Lacrimation, Salivation

🎯 Final Review Table

Drug Class	Examples	Main Uses
Direct agonists	Bethanechol, Pilocarpine	Stimulate bladder, saliva, and eye drainage
AChE inhibitors	Neostigmine, Donepezil	Myasthenia gravis, Alzheimer’s
Anticholinergics	Atropine, Scopolamine	Bradycardia, motion sickness, GI cramping

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