Muscarinic ACh receptors (mAChRs) are GPCRs — distinct from nicotinic (nicotinic ach receptor pharmacology). Five subtypes with conserved Gq vs Gi coupling pattern: M1, M3, M5 → Gq → PLC → IP3 + DAG → Ca²⁺ mobilization (excitatory). M2, M4 → Gi → ↓cAMP + ↑GIRK K⁺ channels (inhibitory). Tissue distribution: M1 = CNS (cortex, hippocampus — cognitive function); M2 = cardiac (SA + AV node — bradycardia; vagal heart-rate slowing); M3 = smooth muscle (bronchial, GI, bladder, vascular endothelium NO release) + glands (salivary, lacrimal); M4 = CNS (striatum); M5 = CNS (VTA — implicated in addiction). Therapeutic challenges: most clinically-used "anticholinergics" lack subtype selectivity → broad side effects (dry mouth, blurred vision, urinary retention, constipation, cognitive impairment in elderly = "anticholinergic burden" — dementia association in long-term use). Subtype-targeted programs are difficult — orthosteric binding pocket is highly conserved. PAMs for M1 (xanomeline/trospium combo — KarXT, FDA-approved 2024 for schizophrenia) achieve selectivity via less-conserved allosteric sites. M3-selective inhaled bronchodilators (tiotropium, umeclidinium, aclidinium) minimize systemic load. Urinary M3 antagonists (oxybutynin, tolterodine, solifenacin, darifenacin, trospium, fesoterodine) for overactive bladder. M2-selective drugs not clinically used (cardiac vagal effect dangerous). Cross-links: acetylcholine axis (ACh synthesis + AChE), autonomic balance (parasympathetic output is M-receptor-mediated), bladder detrusor pharmacology (M3 urinary).
Organ Systems
nervous
cardiovascular
digestive
respiratory
Pathway Steps
ACh release at parasympathetic terminal → muscarinic receptor binding (M1–M5) — via all 5 subtypes bind ACh with similar orthosteric affinity; tissue + subtype determines effect. Muscarinic receptors are the metabotropic (GPCR) arm of cholinergic signaling, activated by ACh from parasympathetic and some CNS terminals. The five subtypes M1–M5 share ACh but differ in G-protein coupling and tissue distribution — the basis for the broad, system-wide effects of muscarinic drugs.
M1 / M3 / M5 (Gq) → PLC → IP3 + DAG → Ca²⁺ + PKC — via excitatory; smooth-muscle contraction, glandular secretion, cortical excitation. The odd-numbered receptors M1/M3/M5 couple to Gq, activating PLC to make IP3 and DAG — raising intracellular Ca²⁺ and activating PKC. This excitatory/secretory branch drives smooth-muscle contraction and glandular secretion, the classic “rest-and-digest” parasympathetic outputs.
M2 / M4 (Gi) → ↓cAMP + ↑GIRK K⁺ → hyperpolarization — via inhibitory; cardiac bradycardia (M2 SA/AV node); striatal M4. The even-numbered M2/M4 couple to Gi, lowering cAMP and opening GIRK potassium channels to hyperpolarize the cell. In the heart, M2 slows rate and conduction — the vagal brake that atropine blocks — making this the inhibitory branch of muscarinic signaling.
M3 endothelial → eNOS → NO → vasodilation — via paradoxical vasodilator effect of ACh in vivo (in vitro M3 = constriction). On vascular endothelium, M3 raises Ca²⁺ to stimulate eNOS, generating nitric oxide that relaxes the underlying smooth muscle — vasodilation. This is why infused muscarinic agonists dilate vessels despite M3 contracting other smooth muscle: the effect requires an intact endothelium.
M3 bronchial smooth muscle → bronchoconstriction — via COPD / asthma anticholinergic target (tiotropium etc.). In airway smooth muscle, M3 (Gq) drives bronchoconstriction — the target of inhaled antimuscarinic bronchodilators (ipratropium, tiotropium) in COPD and asthma. Vagal cholinergic tone is a major reversible component of airway narrowing.
M1 CNS (cortex / hippocampus) → cognitive function + memory — via KarXT (xanomeline-trospium) leverages this for schizophrenia treatment. In cortex and hippocampus, M1 supports learning and memory — why the cholinergic deficit of Alzheimer’s is treated with cholinesterase inhibitors and why antimuscarinic (“anticholinergic”) drugs impair cognition. M1-selective agonists and PAMs have long been pursued as cognitive enhancers.