Next Lesson - Adrenergic Pharmacology
Core
Acetylcholine Life Cycle
Cholinergic pharmacology (the pharmacology of drugs acting on acetylcholine signalling) becomes manageable when every drug is placed on one transmitter cycle. In the nerve terminal, choline acetyltransferase joins choline to acetyl-CoA (the cell's carrier of acetyl groups) to make acetylcholine (ACh). The vesicular acetylcholine transporter, VAChT, concentrates ACh inside synaptic vesicles. An arriving action potential opens voltage-gated calcium channels; calcium then triggers vesicle fusion driven by SNARE proteins (the proteins that pull a vesicle into the cell membrane) and quantal exocytosis (release in fixed packets, one vesicle at a time). Released ACh crosses only a tiny gap, the synaptic cleft, before binding nicotinic or muscarinic receptors (the two families of ACh receptor).
Termination is unusually fast because acetylcholinesterase (AChE) hydrolyses ACh (splits it with water) into acetate and choline within the cleft. The transmitter itself is not simply taken back intact. High-affinity choline transport (a carrier that binds choline strongly, even at low concentrations) returns choline to the terminal for reuse. This sequence creates five conceptual intervention points: choline uptake, synthesis, storage, release and breakdown. Most clinically familiar drugs act at receptors or AChE. Knowing the whole cycle explains two further patterns. Inhibiting storage or release decreases every downstream cholinergic response. Inhibiting AChE increases ACh wherever the inhibitor can reach an active cholinergic synapse.
Experimental tools make the sequence tangible. Hemicholinium limits high-affinity choline uptake, vesamicol blocks VAChT, and botulinum toxins cleave selected SNARE proteins and prevent vesicle fusion. These are not interchangeable “anticholinergics” (drugs that block the effects of ACh): one restricts the recycling of choline, the raw material for ACh, one empties the future vesicle pool, and one prevents already filled vesicles from releasing. Their effects emerge at different speeds because terminals can temporarily draw on existing choline, ACh free in the cytoplasm or stored vesicles. A receptor agonist can still activate a postsynaptic cell (the cell receiving the signal) even when ACh release upstream is blocked. Testing with an agonist in this way shows whether an experimental defect lies before the receptor.
Follow ACh from recycled choline to receptor activation, then place each drug at its point of action.
SimpleMed original educational diagram
Locate the Receptor
Nicotinic receptors are ligand-gated cation channels (ion channels that open when a ligand such as ACh binds and let positive ions through), so they produce rapid depolarisation. Neuronal nicotinic (Nn) receptors sit on postganglionic neurons in both sympathetic and parasympathetic ganglia and on chromaffin cells, the hormone-secreting cells in the middle (medulla) of the adrenal gland. Muscle nicotinic (Nm) receptors sit at the skeletal neuromuscular junction. A drug at Nn can therefore disturb both autonomic divisions; a drug selective for Nm changes skeletal-muscle transmission without directly blocking muscarinic organ responses.
Muscarinic M1-M5 receptors are G-protein-coupled receptors (receptors that signal through G proteins inside the cell). M1 is prominent in neural and enteric (gut-wall nerve network) signalling; M4 and M5 are mainly emphasised in central pathways. At preclinical level, M2 and M3 are the receptors most useful for predicting drug effects outside the brain. Preganglionic autonomic fibres release ACh onto Nn in both divisions. Most postganglionic parasympathetic fibres release ACh onto muscarinic receptors at the organ. Somatic motor fibres release ACh onto Nm. The memorable exception is the sympathetic postganglionic supply to eccrine (ordinary, watery) sweat glands: it is cholinergic and acts mainly through M3. This lesson uses that map pharmacologically; the pathway anatomy remains in Autonomic Nervous System: Introduction.
Location also separates speed from direction. Nicotinic channel opening is always a fast excitatory event at the receptor, although the downstream pathway can later increase or decrease an organ's activity. Muscarinic receptors signal more slowly and can either excite or inhibit according to subtype and cell machinery. Calling ACh simply “excitatory” or “parasympathetic” therefore loses the information needed to predict a drug response.
Location predicts consequence: Nn links autonomic neurons, Nm drives skeletal muscle, and muscarinic receptors control target organs.
SimpleMed original educational diagram
M2 and M3 Signalling
M2 and M4 preferentially couple to Gi/o (an inhibitory G protein). In cells of the sinoatrial node (the heart's pacemaker) and the atrioventricular (AV) node, M2 activation reduces the activity of adenylyl cyclase (the enzyme that makes cyclic AMP, or cAMP, an intracellular messenger) and so lowers cAMP, and G-protein subunits open G-protein-gated inwardly rectifying potassium (GIRK) channels. Hyperpolarisation (the membrane becoming more negative) slows pacemaker firing and AV conduction. The useful prediction is not that every muscarinic receptor excites: cardiac M2 activation restrains electrical activity.
M1, M3 and M5 preferentially couple to Gq/11 (a stimulatory G protein). M3 activates phospholipase C (an enzyme), producing inositol trisphosphate (IP3, an intracellular messenger) and raising intracellular calcium. In most target tissues that means glandular secretion or smooth-muscle contraction. In the eye this gives miosis (pupil constriction) and near accommodation (focusing on near objects). In the airways it gives bronchoconstriction (airway narrowing) and secretion. In the gut it increases motility and secretion. In the bladder, contraction of the detrusor (the bladder wall muscle) favours emptying. The endothelium (the lining of blood vessels) is the key exception. Endothelial M3 calcium activates nitric oxide synthase (the enzyme that makes nitric oxide); nitric oxide diffuses to adjacent vascular smooth muscle and relaxes it through cyclic GMP (cGMP). The receptor is still Gq-coupled, but the observed tissue response is relaxation because the signalling cell and responding muscle are different.
This pathway logic is more reliable than memorising “odd receptors excite, even receptors inhibit”. M2 can also regulate transmitter release and smooth muscle outside the heart, while M3 expression and tissue architecture shape the final response. The coupling rule is a first prediction, not a claim that every receptor has one effect in every cell.
M2 usually slows through Gi and potassium current; M3 raises calcium, with endothelial nitric oxide creating a vascular exception.
SimpleMed original educational diagram
Direct Agonists and Enzyme Inhibitors
Direct muscarinic agonists bind the receptor themselves. Bethanechol favours muscarinic effects and can promote detrusor and gastrointestinal smooth-muscle activity; pilocarpine drives secretion and, in the eye, miosis with contraction of the ciliary muscle (the muscle that changes the shape of the lens). Their adverse effects are extensions of the same receptor map: bradycardia (slow heart rate), sweating, salivation, abdominal cramping, diarrhoea, urinary urgency, bronchoconstriction and secretions. Bowel or urinary obstruction, marked bradycardia or reactive airway disease (asthma-like narrowing of the airways) can therefore make added muscarinic drive hazardous. These are mechanism predictions, not a complete prescribing checklist.
Indirect agonists inhibit AChE rather than stimulating one receptor subtype. Neostigmine and pyridostigmine prolong endogenous ACh at accessible cholinergic synapses, so both muscarinic and nicotinic responses can increase. That explains useful strengthening of neuromuscular transmission alongside unwanted bradycardia, secretions or gut activity. Physostigmine enters the central nervous system more readily than quaternary agents (permanently charged drugs that cross into the brain poorly) such as neostigmine; distribution therefore changes which synapses are amplified. The existing Parkinson's Disease and Myasthenia Gravis lesson retains disease-specific treatment detail.
Direct and indirect agonism also differ in dependence on nerve activity. A receptor agonist supplies a signal even when little ACh has just been released. An AChE inhibitor mainly magnifies transmitter present after endogenous impulses, so its pattern depends on active synapses, tissue access and whether the molecule reaches the brain. Neither drug class is automatically selective simply because one clinical use dominates its name.
Antimuscarinics
Antimuscarinics competitively prevent ACh from activating muscarinic receptors while leaving Nm and Nn transmission directly intact. Atropine removes the restraint that the vagus nerve places on the heart through M2 receptors and reduces muscarinic secretions. Ipratropium and tiotropium reduce airway M3-mediated bronchoconstriction; tropicamide relaxes the iris sphincter and ciliary muscle; oxybutynin reduces detrusor drive. The same logic predicts class effects: dry mouth, reduced sweating, mydriasis (dilated pupils) with blurred near vision, tachycardia (fast heart rate), reduced gut motility and urinary retention. Central penetration can add confusion and cognitive impairment, especially in susceptible older people. NICE guideline NG123 advises against immediate-release oxybutynin for older women who may be at higher risk of a sudden deterioration in their physical or mental health.
Risk follows physiology. Blocking detrusor contraction can worsen urinary retention; reducing gut movement can worsen obstruction; mydriasis can trigger acute angle-closure glaucoma (a sudden rise in eye pressure) in people whose eyes have narrow drainage angles; reducing sweating can impair heat loss. A drug may be selected for one organ, but receptor distribution creates effects elsewhere. “Anticholinergic” is sometimes used loosely, so state the actual target: an antimuscarinic does not directly block nicotinic weakness at the neuromuscular junction or autonomic ganglion.
Exposure and distribution change the balance. An inhaled antagonist is delivered toward the airway, a short-acting ocular preparation is used locally, and a lipid-soluble systemic drug can reach central receptors. Local delivery reduces off-target effects but cannot rule them out. Always combine receptor selectivity with route, concentration and access to the relevant compartment.
Neuromuscular Blockers
At the neuromuscular junction, motor-neuron ACh opens postsynaptic Nm channels. The endplate (the muscle membrane directly under the nerve ending) depolarises, reaches threshold and triggers a muscle action potential. Non-depolarising blockers such as rocuronium competitively occupy Nm receptors without opening them. Fewer channels respond to ACh, the endplate potential fails to reach threshold, and flaccid (limp) paralysis follows. Raising ACh in the synaptic cleft with an AChE inhibitor such as neostigmine can oppose non-depolarising blockade, but it also raises muscarinic signalling elsewhere, so glycopyrronium or atropine (antimuscarinics) is given with it. Sugammadex, a modified gamma cyclodextrin rather than an AChE inhibitor, is another option for reversing rocuronium or vecuronium.
Suxamethonium (succinylcholine) is different: it is an Nm agonist that depolarises the endplate. Initial activation can cause fasciculation (brief muscle twitches), but persistent depolarisation prevents the membrane from resetting; with continued exposure, desensitisation (the receptors stop responding) contributes to transmission failure. Both classes produce paralysis without unconsciousness, analgesia (pain relief) or amnesia (loss of memory). An AChE inhibitor such as neostigmine does not reverse suxamethonium and can prolong its action. Practical selection, monitoring and reversal belong in Anaesthetics; the pharmacological distinction here is competitive blockade versus depolarising activation followed by failure.
Non-depolarising drugs prevent Nm activation; suxamethonium activates first, then traps the endplate in transmission failure.
SimpleMed original educational diagram
Organophosphate Poisoning
Organophosphates phosphorylate AChE (attach a phosphate group to it, which inactivates it), allowing ACh to accumulate at muscarinic, nicotinic and central synapses. Muscarinic excess commonly produces salivation, lacrimation (tear production), sweating, bronchorrhoea (excess airway secretions), bronchoconstriction, bradycardia, gastrointestinal hyperactivity, urination and miosis. Nicotinic excess can cause fasciculation followed by weakness and respiratory-muscle paralysis. Central effects can include agitation, confusion, seizures and depressed consciousness. Respiratory failure can therefore combine secretions and bronchospasm (airway muscle spasm) with weak breathing muscles and reduced drive to breathe from the brain.
The treatment logic has separate jobs. Immediate decontamination, airway support, oxygenation and ventilation address exposure and physiology. Atropine competitively blocks muscarinic receptors, so it treats bronchial secretions, bronchoconstriction and other muscarinic effects. It does not restore Nm transmission directly. Atropine is given early by intravenous injection and repeated until the skin becomes flushed and dry, the pupils dilate and bradycardia is abolished. Pralidoxime can remove the phosphate group and reactivate AChE before the inhibited enzyme undergoes ageing (a chemical change in the enzyme, nothing to do with the patient's age), which makes reactivation much harder or impossible. The BNF lists pralidoxime as an adjunct to atropine in moderate or severe poisoning, and trial evidence that oximes (the drug class that includes pralidoxime) improve outcomes remains uncertain. Timing matters mechanistically, but this is not a dose protocol. Suspected poisoning needs an urgent 999 call or emergency department care. Clinicians can get specialist advice from the National Poisons Information Service, through TOXBASE (its online database) or its 24-hour telephone line.
Support breathing and give atropine early. Atropine blocks muscarinic excess, while pralidoxime targets unaged phosphorylated AChE.
SimpleMed original educational diagram
The Three-Question Method
For any unfamiliar cholinergic problem, ask three questions in order. First, where is the synapse: autonomic ganglion, parasympathetic target, sympathetic sweat gland, neuromuscular junction or central pathway? Second, which receptor or enzyme is involved: Nn, Nm, muscarinic subtype or AChE? Third, is ACh signalling being added, prolonged, blocked or converted into persistent depolarisation? The phenotype then follows from ordinary physiology.
This method prevents common mix-ups between drug groups. AChE inhibition is broader than a selective muscarinic agonist. Atropine can dry secretions while nicotinic weakness persists. An Nm blocker paralyses skeletal muscle without directly removing consciousness. M3 activation usually contracts smooth muscle but relaxes vessels indirectly through endothelial nitric oxide. Locate, name, predict: that sequence turns a long drug list into a small set of reusable mechanisms and prepares the contrast with Adrenergic Pharmacology.
Reviewed by: Dr. Marcus Judge
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