Pharmacology · Cholinergic Pharmacology

Muscarinic Agonists and Acetylcholinesterase Inhibitors

Drug classes, mechanisms, clinical uses, crisis recognition, and interactions

Direct-acting muscarinic agonists differ by two key properties: quaternary amines (bethanechol, carbachol, methacholine) carry a permanent positive charge that blocks CNS penetration, while tertiary amines (pilocarpine, cevimeline) cross the blood-brain barrier; pilocarpine is used for open-angle glaucoma and Sjogren syndrome xerostomia by directly activating M3 receptors — resistance to AChE hydrolysis gives it a longer duration than ACh itself.
AChE inhibitors fall into two mechanistic classes: carbamylation (neostigmine, pyridostigmine, physostigmine, rivastigmine) — the drug transfers a carbamyl group to the enzyme active site, blocking it for minutes to hours before regeneration; and ionic binding (edrophonium only) — non-covalent electrostatic binding lasting only minutes, making edrophonium diagnostically useful but therapeutically impractical.
Quaternary AChE inhibitors (neostigmine, pyridostigmine, edrophonium) cannot cross the blood-brain barrier and produce peripheral effects only — neostigmine is coadministered with glycopyrrolate when reversing neuromuscular blockade to suppress muscarinic side effects; pyridostigmine is the preferred oral long-term agent for myasthenia gravis.
Distinguishing myasthenic crisis from cholinergic crisis is essential: myasthenic crisis (too little drug) causes worsening weakness with normal secretions and normal heart rate; cholinergic crisis (too much drug) produces weakness plus copious secretions, bradycardia, and miosis — treatment is opposite in each case (more drug vs. hold all drug and give atropine).
The three AChE inhibitors for Alzheimer disease differ in their drug interaction profiles: donepezil and galantamine are CYP2D6/3A4 substrates with significant interaction risk; rivastigmine undergoes no CYP metabolism and is preferred when polypharmacy is a concern; all three are directly opposed by concurrent anticholinergic drugs (diphenhydramine, oxybutynin, tricyclic antidepressants), which must be reviewed and discontinued when AChE inhibitor therapy begins.

Abbreviations: AChE = acetylcholinesterase · ACh = acetylcholine · CNS = central nervous system · BBB = blood-brain barrier · CYP = cytochrome P450 · NMB = neuromuscular blockade · MG = myasthenia gravis

Direct-Acting Muscarinic Agonists

Mechanism

Bind and activate muscarinic receptors directly — no dependence on ACh release or AChE inhibition. Modified to resist enzymatic hydrolysis. Quaternary compounds carry a permanent positive charge: no BBB crossing, no CNS effects. Tertiary amines cross membranes: CNS penetration possible.

Quaternary

Bethanechol

Use: non-obstructive urinary retention; neurogenic bladder
No nicotinic activity; no CNS effects
Avoid: obstruction, asthma, peptic ulcer disease

Tertiary

Pilocarpine

Uses: open-angle glaucoma; acute angle-closure glaucoma; xerostomia (Sjogren, radiation)
Crosses BBB
Adverse effect: diaphoresis

Quaternary

Carbachol

Muscarinic and nicotinic activity
Use: intraocular miosis during cataract surgery; second-line glaucoma

Quaternary

Methacholine

Diagnostic only
Use: bronchoprovocation challenge for airway hyperresponsiveness in suspected asthma
No therapeutic role

Tertiary

Cevimeline

M1 and M3 selective
Use: Sjogren xerostomia
Longer duration than pilocarpine; less diaphoresis

AChE Inhibitor Mechanisms

Core Mechanism

Block AChE → ACh accumulates at every cholinergic synapse → amplified and prolonged muscarinic and nicotinic effects. Quaternary agents (neostigmine, pyridostigmine, edrophonium): no BBB crossing; peripheral effects only. Tertiary agents (physostigmine, donepezil, rivastigmine, galantamine): cross BBB; CNS effects.

Inhibition Type 1

Carbamylation

Neostigmine, pyridostigmine, physostigmine, rivastigmine
Drug transfers a carbamyl group to AChE active site
Enzyme blocked for minutes to hours before regeneration
Called "reversible" — enzyme eventually recovers

Inhibition Type 2

Ionic Binding

Edrophonium only
Non-covalent electrostatic binding to active site
No covalent bond formed — effects resolve within minutes
Ultrashort duration → diagnostic use only

Peripheral AChE Inhibitors

Drug	Duration	Primary Uses	Key Point
Neostigmine	Moderate (hours)	MG; NMB reversal; Ogilvie syndrome	Coadminister glycopyrrolate for NMB reversal to block muscarinic AEs; poor oral absorption
Pyridostigmine	Longer than neostigmine	MG (preferred oral long-term agent)	Sustained-release for overnight dosing; dose-limiting AEs are muscarinic (cramping, diarrhea)
Edrophonium	Minutes only (ionic binding)	Diagnostic test for MG	Ultrashort duration is the diagnostic feature; atropine must be on hand during test

Myasthenic Crisis vs. Cholinergic Crisis

Too little drug / disease progression

Myasthenic Crisis

Worsening weakness, bulbar and respiratory
Secretions normal or minimal
Heart rate normal or elevated
Pupils normal
No GI symptoms
Treatment: respiratory support; plasma exchange or IVIG; resume drug when stable

Too much drug / excess ACh

Cholinergic Crisis

Worsening weakness (depolarizing block)
Copious secretions — bronchorrhea, hypersalivation
Bradycardia
Miosis
Diaphoresis; GI cramping and diarrhea
Treatment: hold all AChE inhibitors; atropine for muscarinic symptoms

CNS AChE Inhibitors — Alzheimer Disease

Toxicology Use

Physostigmine

Tertiary amine — crosses BBB
Reverses CNS anticholinergic toxidrome (delirium, agitation, hallucinations)
Short duration; may need repeated dosing
Contraindicated in tricyclic antidepressant overdose (seizures and fatal arrhythmia risk from cardiac sodium channel blockade)

Alzheimer Disease

Class Properties

Target: cholinergic deficit in basal forebrain projection to cortex and hippocampus
Modest symptomatic benefit only — no disease modification
Class AEs: nausea, diarrhea (muscarinic), bradycardia at high doses

Alzheimer Agent

Donepezil

AChE-selective
Once-daily dosing (long duration)
CYP2D6 and 3A4 substrate — interaction risk with inhibitors
Bedtime dosing reduces GI AEs

Alzheimer Agent

Rivastigmine

AChE and BuChE inhibitor
No CYP metabolism — preferred in high-polypharmacy patients
Transdermal patch reduces GI AEs
Also approved for Parkinson disease dementia

Alzheimer Agent

Galantamine

AChE inhibitor + nicotinic receptor allosteric modulator
Allosteric modulation enhances nicotinic response to endogenous ACh
CYP2D6 and 3A4 substrate — same interaction risk as donepezil

Drug Interactions — Three Categories

Category 1

CYP Pharmacokinetics

Donepezil and galantamine: CYP2D6 and 3A4 substrates
Inhibitors (fluoxetine, azole antifungals, clarithromycin): raise levels → more cholinergic toxicity
Inducers (rifampin, carbamazepine): lower levels → reduced efficacy
Rivastigmine avoids this — no CYP metabolism

Category 2

Additive Bradycardia

Digoxin + AChE inhibitor: additive slowing of SA node via M2 → monitor heart rate
Beta-blockers + AChE inhibitor: additive bradycardia
Risk greatest in elderly patients and those with reduced cardiac reserve

Category 3

Pharmacodynamic Antagonism

Anticholinergic drugs directly oppose AChE inhibitors for Alzheimer disease
Offenders: diphenhydramine, oxybutynin, tolterodine, tricyclic antidepressants, certain antipsychotics
Review and discontinue anticholinergics when AChE inhibitor therapy begins

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