Pharmacology · Cholinergic Pharmacology

Cholinergic Receptors and Acetylcholine Neurotransmission

Synthesis, storage, release, receptor pharmacology, and autonomic balance

The acetylcholine lifecycle runs from choline uptake (rate-limiting step, blocked by hemicholinium) → synthesis by choline acetyltransferase → vesicular storage → calcium-triggered exocytosis via SNARE proteins (blocked by botulinum toxin) → receptor activation → hydrolysis by acetylcholinesterase (choline recycled); butyrylcholinesterase is a plasma enzyme that metabolizes succinylcholine but plays no role in synaptic neurotransmission.
Muscarinic receptors M1–M5 are all GPCRs: M1 (Gq, cortex/ganglia — cognition, atropine causes delirium), M2 (Gi, heart — vagal slowing, atropine causes tachycardia), M3 (Gq, smooth muscle and glands — bladder contraction, bronchoconstriction, secretions), M4 (Gi, striatum — movement control, benztropine target in Parkinson disease); M3 selective blockers treat overactive bladder (darifenacin, solifenacin) and COPD (ipratropium, tiotropium).
Nicotinic receptors are ligand-gated ion channels: N-M at the neuromuscular junction is blocked non-depolarizingly by rocuronium/vecuronium (reversed by acetylcholinesterase inhibitors) or depolarizingly by succinylcholine (not reversible); N-N at all autonomic ganglia is blocked by hexamethonium/trimethaphan; the CNS alpha4-beta2 subtype mediates nicotine's rewarding effects and is the target of varenicline (partial agonist).
The two mirror-image clinical syndromes: cholinergic excess produces SLUDGE (salivation, lacrimation, urination, defecation, GI distress, emesis) plus bradycardia, bronchospasm, and neuromuscular fasciculations → paralysis; anticholinergic syndrome produces the mnemonic "hot as a hare, dry as a bone, red as a beet, blind as a bat, mad as a hatter" plus tachycardia and urinary retention — this is the signature of atropine overdose and is highest risk in elderly patients.
Cholinergic imbalance underlies three major diseases: Alzheimer disease (cholinergic neuron loss in basal forebrain → treat with acetylcholinesterase inhibitors donepezil/rivastigmine/galantamine), Parkinson disease (relative cholinergic excess in striatum from dopamine neuron loss → treat with muscarinic antagonists benztropine/trihexyphenidyl), and myasthenia gravis (autoimmune N-M receptor destruction → treat with acetylcholinesterase inhibitors neostigmine/pyridostigmine).

Abbreviations: ACh = acetylcholine · AChE = acetylcholinesterase · BuChE = butyrylcholinesterase · GPCR = G-protein-coupled receptor · SNARE = soluble NSF attachment protein receptor · N-M = neuromuscular nicotinic receptor · N-N = neuronal nicotinic receptor · COPD = chronic obstructive pulmonary disease

The Cholinergic Cycle — Synthesis to Termination

Choline uptake
rate-limiting step

→

Choline acetyltransferase
synthesis

→

Vesicular storage

→

Ca²⁺-triggered exocytosis

→

Receptor activation

→

AChE hydrolysis
choline recycled

Presynaptic block

Botulinum Toxin

Cleaves SNARE proteins required for vesicle fusion
Blocks ACh release → flaccid paralysis
Therapeutic: spasticity, dystonia, hyperhidrosis, overactive bladder

Synaptic cleft

AChE Inhibitors

Block ACh hydrolysis → amplify all cholinergic signals
Therapeutic: Alzheimer disease, myasthenia gravis, NMB reversal
Toxic: organophosphate poisoning

Butyrylcholinesterase (BuChE)

Plasma enzyme synthesized by the liver. Metabolizes succinylcholine. Genetic variant → prolonged neuromuscular blockade. No role in synaptic neurotransmission.

Muscarinic Receptor Subtypes — Gq (Odd) and Gi (Even)

M1 — Gq

Cortex & Ganglia

Cognition; memory
Ganglionic modulation
Blockade → delirium
Target: benztropine

M2 — Gi

Heart

SA node, AV node
Vagal slowing of heart rate
Atropine → tachycardia
Atropine for bradycardia

M3 — Gq

Smooth Muscle & Glands

Bladder detrusor contraction
Bronchoconstriction
Miosis; accommodation
Salivation; sweating

M4 — Gi

Striatum

Dopaminergic modulation
Movement control
Target: Parkinson disease tremor drugs

Selective Muscarinic Antagonists

M3 bladder: darifenacin, solifenacin (overactive bladder) · M3 airway: ipratropium, tiotropium (COPD, asthma) · M1/M4 CNS: benztropine, trihexyphenidyl (Parkinson disease tremor) · Non-selective: atropine, scopolamine (all five subtypes → full anticholinergic syndrome)

Nicotinic Receptor Subtypes — Ligand-Gated Ion Channels

N-M — Neuromuscular junction

Muscle-Type Receptor

Non-depolarizing block (rocuronium, vecuronium): competitive antagonism; reversed by AChE inhibitors or sugammadex
Depolarizing block (succinylcholine): activates then desensitizes; not reversed by AChE inhibitors
Myasthenia gravis: autoimmune destruction of N-M receptors → fatigable weakness

N-N — Autonomic ganglia

Neuronal-Type Receptor

Both sympathetic and parasympathetic ganglia
Hexamethonium, trimethaphan block all ganglia simultaneously
Result: orthostatic hypotension, tachycardia, mydriasis, dry mouth, urinary retention
Abandoned as antihypertensives

Alpha4-beta2 — CNS

Brain Nicotinic Receptor

Primary site of nicotine action in brain
Dopamine release in reward pathways
Varenicline: partial agonist → reduces craving and blocks nicotine reward

Two Mirror-Image Clinical Syndromes

Cholinergic excess

SLUDGE Toxidrome

Salivation, Lacrimation, Urination, Defecation, GI distress, Emesis
Bradycardia, bronchospasm, bronchorrhea
Fasciculations → flaccid paralysis; seizures
Cause: AChE inhibitor toxicity, organophosphate poisoning
Treatment: atropine; pralidoxime (if early)

Cholinergic deficit

Anticholinergic Syndrome

Hot as a hare (hyperthermia from anhidrosis)
Dry as a bone (dry mouth, dry skin)
Red as a beet (cutaneous vasodilation)
Blind as a bat (mydriasis, cycloplegia)
Mad as a hatter (delirium — CNS-penetrating agents)
Tachycardia, urinary retention, constipation
Risk: highest in elderly patients

Cholinergic Imbalance in Three Disease States

Disease

Alzheimer Disease

Loss of basal forebrain cholinergic neurons
Deficit in cortex and hippocampus
Correlates with cognitive decline
Treatment: AChE inhibitors (donepezil, rivastigmine, galantamine)

Disease

Parkinson Disease

Loss of dopaminergic neurons in substantia nigra
Relative cholinergic excess in striatum
Contributes to tremor and rigidity
Treatment: muscarinic antagonists (benztropine, trihexyphenidyl)

Disease

Myasthenia Gravis

Autoimmune destruction of N-M nicotinic receptors
Reduced safety margin at neuromuscular junction
Fatigable weakness that worsens with use
Treatment: AChE inhibitors (neostigmine, pyridostigmine)

Suggested References

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