Chapter 12: Pharmacology of Acetylcholine

1. Introduction/Overview

Acetylcholine (ACh) represents the prototypical neurotransmitter of the cholinergic system, a fundamental signaling pathway in both the central and peripheral nervous systems. Its discovery by Otto Loewi in 1921, who termed it “Vagusstoff,” marked a seminal moment in neuropharmacology, establishing the chemical basis of synaptic transmission. The pharmacology of acetylcholine extends beyond the endogenous molecule to encompass a broad array of synthetic and natural agents that mimic, antagonize, or modulate its synthesis, release, and degradation. These agents constitute critical therapeutic tools in numerous clinical domains, including ophthalmology, neurology, anesthesiology, and gastroenterology. A thorough understanding of acetylcholine pharmacology is therefore indispensable for rational therapeutic decision-making and for anticipating the effects of drugs that influence cholinergic tone.

Clinical Relevance and Importance

The clinical importance of cholinergic pharmacology is profound and multifaceted. Agents that enhance cholinergic signaling are first-line treatments for conditions such as myasthenia gravis, Alzheimer’s disease, and glaucoma. Conversely, drugs that block cholinergic receptors are vital in managing conditions like Parkinson’s disease tremor, overactive bladder, and as pre-anesthetic medications to reduce secretions. Furthermore, the toxicology of organophosphate and carbamate insecticides, as well as nerve agents, is directly rooted in their irreversible inhibition of acetylcholinesterase, leading to a cholinergic crisis. Mastery of this topic enables clinicians to effectively utilize these drugs, manage their adverse effects, and treat cases of poisoning.

Learning Objectives

• Describe the synthesis, storage, release, and termination of action of endogenous acetylcholine.
• Classify drugs acting on the cholinergic system based on their mechanism of action and receptor specificity.
• Explain the molecular and cellular mechanisms of action of cholinergic agonists and antagonists at muscarinic and nicotinic receptors.
• Outline the pharmacokinetic properties, major therapeutic applications, and significant adverse effect profiles of key cholinergic and anticholinergic drugs.
• Identify major drug-drug interactions and special population considerations relevant to cholinergic pharmacotherapy.

2. Classification

Drugs affecting the cholinergic system are classified primarily by their mechanism of action and receptor selectivity. The primary categorization distinguishes between agents that potentiate cholinergic effects (cholinomimetics or parasympathomimetics) and those that inhibit them (anticholinergics or parasympatholytics).

Direct-Acting Cholinergic Agonists

These agents bind directly to and activate acetylcholine receptors. They are subdivided based on receptor selectivity:

• Muscarinic Agonists (Muscarinics): These selectively activate muscarinic acetylcholine receptors (mAChRs). Examples include pilocarpine, bethanechol, and cevimeline. They are further categorized as alkaloids (e.g., pilocarpine, muscarine) or synthetic esters (e.g., bethanechol).
• Nicotinic Agonists (Nicotinics): These selectively activate nicotinic acetylcholine receptors (nAChRs). Examples include nicotine, suxamethonium (succinylcholine), and varenicline. Their action is typically non-selective across autonomic ganglia and neuromuscular junctions unless specifically designed.
• Non-Selective Cholinergic Agonists: These activate both muscarinic and nicotinic receptor families. Acetylcholine itself and its synthetic ester, methacholine, fall into this category, though methacholine shows some muscarinic preference.

Indirect-Acting Cholinergic Agonists (Anticholinesterases)

These agents inhibit the enzyme acetylcholinesterase (AChE), which hydrolyzes acetylcholine, thereby increasing the concentration and duration of action of endogenous ACh at all cholinergic synapses. Classification is based on the nature of their interaction with AChE:

• Reversible Inhibitors: Form transient, non-covalent complexes with AChE. Duration of action ranges from minutes to hours.
  • Short-acting: Edrophonium.
  • Intermediate-acting: Neostigmine, pyridostigmine, physostigmine.
  • Long-acting: Donepezil, rivastigmine, galantamine.
• Irreversible Inhibitors: Form stable, covalent bonds with the serine hydroxyl group in the AChE active site, leading to prolonged inhibition lasting days to weeks. Recovery requires synthesis of new enzyme. This class includes organophosphate compounds (e.g., echothiophate, parathion, sarin).

Anticholinergic Agents (Cholinergic Receptor Antagonists)

These agents block the action of acetylcholine at its receptors.

• Muscarinic Antagonists (Antimuscarinics): Competitive inhibitors at mAChRs. Examples include atropine, scopolamine, ipratropium, oxybutynin, and glycopyrrolate.
• Nicotinic Antagonists:
  • Ganglionic Blockers: Non-depolarizing agents like hexamethonium and trimethaphan that block nAChRs in autonomic ganglia.
  • Neuromuscular Blocking Agents (NMBAs): Block nAChRs at the skeletal muscle neuromuscular junction.
    • Depolarizing: Suxamethonium (succinylcholine).
    • Non-depolarizing: Atracurium, rocuronium, vecuronium.

3. Mechanism of Action

Endogenous Acetylcholine: Synthesis and Signaling

Acetylcholine is synthesized within cholinergic nerve terminals from acetyl coenzyme A (Acetyl-CoA) and choline via the enzyme choline acetyltransferase (ChAT). The rate-limiting step is the uptake of choline into the neuron via the high-affinity sodium-dependent choline transporter (CHT1). Synthesized ACh is then packaged into synaptic vesicles by the vesicular acetylcholine transporter (VAChT). Upon neuronal depolarization and calcium influx, vesicles fuse with the presynaptic membrane, releasing ACh into the synaptic cleft. The action of ACh is terminated primarily by rapid hydrolysis into acetate and choline by acetylcholinesterase, which is densely localized on the postsynaptic membrane. The liberated choline is subsequently taken back into the presynaptic neuron for re-synthesis.

Receptor Interactions and Signal Transduction

Acetylcholine exerts its effects by binding to and activating two distinct families of receptors: muscarinic and nicotinic. These differ fundamentally in structure, mechanism, and distribution.

Muscarinic Acetylcholine Receptors (mAChRs)

Muscarinic receptors are metabotropic, or G-protein-coupled receptors (GPCRs). Five subtypes (M₁–M₅) have been cloned, each with unique distributions and signaling pathways.

• M₁, M₃, M₅ Receptors: Coupled to G_q/11 proteins. Activation leads to stimulation of phospholipase C (PLC), which hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP₂) into inositol trisphosphate (IP₃) and diacylglycerol (DAG). IP₃ mobilizes intracellular calcium, while DAG activates protein kinase C (PKC). This pathway mediates excitatory responses such as glandular secretion, smooth muscle contraction, and neuronal excitation.
• M₂ and M₄ Receptors: Coupled to G_i/o proteins. Their activation inhibits adenylyl cyclase, reducing intracellular cyclic AMP (cAMP) levels. The βγ subunits of G_i/o can also directly activate inwardly rectifying potassium channels (GIRKs) and inhibit voltage-gated calcium channels. This results in inhibitory effects, most notably slowing of the heart rate (negative chronotropy) and reduced force of contraction (negative inotropy) in the sinoatrial and atrioventricular nodes.

Nicotinic Acetylcholine Receptors (nAChRs)

Nicotinic receptors are ligand-gated ion channels (ionotropic receptors). They are pentameric structures composed of various combinations of α, β, γ, δ, and ε subunits, forming a central cation-permeable pore. Two main classes exist:

• Muscle-type nAChRs: Found at the neuromuscular junction (NMJ), with the adult form having the stoichiometry (α1)₂β1δε. Binding of two ACh molecules causes a conformational change that opens the channel, allowing an influx of Na⁺ and Ca²⁺ and an efflux of K⁺. The net depolarization generates an end-plate potential, which, if sufficient, triggers a muscle action potential and contraction.
• Neuronal-type nAChRs: Found in autonomic ganglia, the central nervous system (CNS), and the adrenal medulla. They are typically homomeric (e.g., α7) or heteromeric (e.g., α4β2). Their activation leads to neuronal depolarization and modulation of neurotransmitter release. The α4β2 and α7 subtypes are particularly implicated in cognition, reward, and attention.

Molecular Mechanisms of Pharmacological Agents

Direct Agonists mimic ACh by binding to the orthosteric site of the receptor, stabilizing the active conformation. Their efficacy and selectivity are determined by their chemical structure and affinity for receptor subtypes. Anticholinesterases bind to the catalytic site of AChE. Reversible inhibitors like neostigmine form a carbamylated enzyme intermediate that hydrolyzes slowly, temporarily inactivating the enzyme. Irreversible organophosphates form a stable phosphorylated enzyme that is resistant to hydrolysis. Competitive Antagonists like atropine bind the orthosteric site without activating the receptor, preventing ACh binding. Neuromuscular blocking agents act at the NMJ: non-depolarizing agents (e.g., rocuronium) competitively block ACh binding, while depolarizing agents (suxamethonium) act as persistent agonists, causing initial fasciculations followed by a sustained depolarization that inactivates voltage-gated sodium channels, leading to paralysis.

4. Pharmacokinetics

The pharmacokinetic profiles of cholinergic drugs vary widely depending on their chemical nature, influencing their route of administration, duration of action, and clinical utility.

Absorption and Distribution

Quaternary ammonium compounds (e.g., bethanechol, neostigmine, glycopyrrolate) are permanently charged, resulting in poor oral bioavailability and minimal penetration of the blood-brain barrier (BBB). They are often administered parenterally or topically. In contrast, tertiary amines (e.g., atropine, scopolamine, physostigmine) and natural alkaloids are uncharged at physiological pH, facilitating good absorption from the gastrointestinal tract and significant CNS penetration. Lipid solubility is a key determinant of distribution; for instance, the highly lipophilic scopolamine is effective in transdermal patches for motion sickness.

Metabolism and Excretion

Esters of choline (like ACh itself) and many synthetic analogs are rapidly hydrolyzed in the plasma and liver by non-specific esterases, contributing to their short duration of action. Anticholinesterases are metabolized through various pathways: pyridostigmine undergoes hepatic hydrolysis, while donepezil is extensively metabolized by cytochrome P450 enzymes (CYP2D6 and CYP3A4). Renal excretion of the parent drug or metabolites is a major route of elimination for many agents, including neostigmine, pyridostigmine, and atropine. Consequently, renal impairment can significantly prolong their effects. The elimination half-life (t_1/2) ranges from minutes (edrophonium, suxamethonium) to several hours (atropine, ipratropium) or even days (once-daily donepezil).

Dosing Considerations

Dosing is highly indication-specific and must account for pharmacokinetic properties. For example, pilocarpine is administered as eye drops (1-4%) for glaucoma due to local effect, while for xerostomia it is given orally. The dosing of reversible anticholinesterases in myasthenia gravis is titrated to symptom control, often requiring multiple daily doses (e.g., pyridostigmine every 4-6 hours) due to their intermediate duration. Long-acting agents like donepezil permit once-daily dosing for Alzheimer’s disease. The onset and duration of neuromuscular blockers are critical in anesthesia; suxamethonium provides rapid-onset, short-duration paralysis, while rocuronium has a slower onset but intermediate duration.

5. Therapeutic Uses/Clinical Applications

Ophthalmology

Cholinergic agonists are cornerstone treatments for glaucoma. Pilocarpine, a direct muscarinic agonist, and echothiophate, an irreversible anticholinesterase, are used to induce miosis (pupil constriction) and contraction of the ciliary muscle. This action opens the trabecular meshwork, facilitating aqueous humor outflow and reducing intraocular pressure. Anticholinesterases are also used diagnostically and to reverse the mydriasis (pupil dilation) caused by antimuscarinic agents.

Neurology

• Myasthenia Gravis: Reversible anticholinesterases (pyridostigmine, neostigmine) are first-line symptomatic therapy. They increase ACh at the NMJ, improving muscle strength. Edrophonium is used in the Tensilon test for diagnosis.
• Alzheimer’s Disease and Other Dementias: Central-acting, reversible anticholinesterases (donepezil, rivastigmine, galantamine) are approved to modestly improve cognitive function, activities of daily living, and behavioral symptoms. They are believed to augment cholinergic transmission in the cerebral cortex and hippocampus.
• Reversal of Neuromuscular Blockade: At the end of surgery, anticholinesterases (neostigmine, pyridostigmine) are administered with an antimuscarinic agent (glycopyrrolate or atropine) to reverse the effects of non-depolarizing NMBAs.

Anesthesiology and Critical Care

Antimuscarinic agents are used as pre-anesthetic medications to reduce airway secretions (atropine, glycopyrrolate) and to prevent bradycardia during surgery. Suxamethonium is employed for rapid-sequence intubation due to its fast onset. Anticholinesterases are used for reversal of non-depolarizing blockade as described.

Gastroenterology and Urology

Direct muscarinic agonists like bethanechol were historically used to stimulate gastrointestinal motility and bladder emptying in conditions like postoperative ileus and neurogenic bladder, though their use has declined due to side effects. Conversely, antimuscarinics (oxybutynin, tolterodine, solifenacin) are first-line therapy for overactive bladder, reducing detrusor muscle contractions. Dicyclomine is used for irritable bowel syndrome.

Pulmonary Medicine

Ipratropium and tiotropium, inhaled quaternary antimuscarinics, are bronchodilators used in chronic obstructive pulmonary disease (COPD) and asthma. They block M₃ receptors in airway smooth muscle, inhibiting bronchoconstriction and reducing mucus secretion.

Other Applications

Scopolamine is effective for the prevention of motion sickness and postoperative nausea. Pilocarpine and cevimeline are used to treat xerostomia (dry mouth) associated with Sjögren’s syndrome or radiation therapy. Varenicline, a partial agonist at the α4β2 neuronal nAChR, is used as a smoking cessation aid.

6. Adverse Effects

The adverse effects of cholinergic drugs are largely predictable extensions of their pharmacological actions and are often described by the acronyms SLUDGE (Salivation, Lacrimation, Urination, Defecation, Gastrointestinal upset, Emesis) for muscarinic agonism and “Dry as a bone, blind as a bat, red as a beet, hot as a hare, mad as a hatter” for antimuscarinic toxicity.

Adverse Effects of Cholinomimetics

• Muscarinic Effects: Bradycardia, hypotension, increased glandular secretions (salivary, bronchial, sweat), bronchoconstriction, miosis, blurred vision (due to ciliary spasm), abdominal cramps, diarrhea, and urinary urgency.
• Nicotinic Effects: These are primarily seen with anticholinesterase overdose or poisoning. Effects include muscle fasciculations, weakness, paralysis (due to depolarizing blockade at the NMJ), tachycardia, and hypertension (due to ganglionic stimulation).
• Central Nervous System Effects: With agents that cross the BBB (e.g., physostigmine), anxiety, nightmares, seizures, and respiratory depression may occur. Irreversible inhibitors can cause profound and prolonged CNS toxicity.

Adverse Effects of Anticholinergic Agents

• Peripheral Effects: Dry mouth (xerostomia), blurred vision and photophobia (from mydriasis and cycloplegia), tachycardia, constipation, urinary retention, reduced sweating (which can lead to hyperthermia), and flushed, dry skin.
• Central Nervous System Effects: With tertiary amines, effects range from drowsiness, confusion, and amnesia to hallucinations, delirium, and coma in severe overdose. Scopolamine is particularly associated with sedation and amnesia.

Serious and Rare Adverse Reactions

Cholinergic crisis, a life-threatening condition of excessive cholinergic stimulation, can result from overdose of anticholinesterases (e.g., organophosphate poisoning). It is characterized by profound muscarinic and nicotinic effects, culminating in respiratory failure due to bronchoconstriction, excessive secretions, and muscle paralysis. Anticholinergic toxicity can progress to hyperthermia, seizures, cardiorespiratory collapse, and death. Some antimuscarinics for overactive bladder have been associated with an increased risk of cognitive decline and dementia in elderly patients with long-term use. Suxamethonium can cause malignant hyperthermia in susceptible individuals and dangerous hyperkalemia in patients with burns, denervation injuries, or major trauma.

7. Drug Interactions

Pharmacodynamic Interactions

• Additive Cholinergic Effects: Concomitant use of multiple cholinomimetics (e.g., a direct agonist with an anticholinesterase) can precipitate a cholinergic crisis.
• Additive Anticholinergic Effects: The combined use of drugs with antimuscarinic properties (e.g., tricyclic antidepressants, first-generation antihistamines, antipsychotics, and anti-Parkinson drugs) can lead to pronounced anticholinergic toxicity, especially in the elderly.
• Antagonistic Interactions: Antimuscarinics will directly oppose the effects of muscarinic agonists. Similarly, anticholinesterases will antagonize the effects of non-depolarizing NMBAs.
• Cardiovascular Interactions: Cholinomimetics that cause bradycardia may potentiate the effects of other negative chronotropes like beta-blockers, digoxin, and non-dihydropyridine calcium channel blockers. Antimuscarinics that cause tachycardia may counteract the effects of these same drugs.

Pharmacokinetic Interactions

• Enzyme Inhibition/Induction: Donepezil metabolism can be inhibited by strong CYP2D6 or CYP3A4 inhibitors (e.g., paroxetine, ketoconazole), potentially increasing its plasma concentration and toxicity risk. Rivastigmine is metabolized independently of the CYP450 system, minimizing such interactions.
• Altered Absorption: Antimuscarinics that slow gastrointestinal motility may delay the absorption of other orally administered drugs.

Contraindications

Cholinomimetics are generally contraindicated in patients with asthma or chronic obstructive pulmonary disease (due to risk of bronchoconstriction), peptic ulcer disease, urinary or intestinal obstruction, and bradycardia or heart block. Antimuscarinics are contraindicated in narrow-angle glaucoma (as mydriasis can precipitate an acute attack), severe ulcerative colitis, myasthenia gravis (can exacerbate weakness), and obstructive uropathy. Suxamethonium is contraindicated in patients with a personal or family history of malignant hyperthermia, and in those with hyperkalemic conditions.

8. Special Considerations

Pregnancy and Lactation

Most cholinergic and anticholinergic drugs are classified as FDA Pregnancy Category C (risk cannot be ruled out). Use during pregnancy should be limited to situations where the potential benefit justifies the potential fetal risk. For example, pyridostigmine may be used cautiously in pregnant women with myasthenia gravis. Anticholinesterases may increase uterine tone. Small amounts of drugs like atropine and scopolamine are excreted in breast milk, but significant effects on the nursing infant are uncommon. However, anticholinergic agents may potentially reduce milk production.

Pediatric and Geriatric Considerations

Children may exhibit heightened sensitivity to both cholinergic and anticholinergic drugs. Paradoxical excitement (restlessness, hallucinations) is more common with antimuscarinics in children. Dosing must be carefully adjusted based on weight or body surface area. In geriatric patients, age-related declines in renal and hepatic function can prolong the elimination of many agents. The elderly are particularly susceptible to the CNS effects of anticholinergic drugs (confusion, delirium, falls) and the bradycardic effects of cholinomimetics. The Beers Criteria strongly advise against the use of certain anticholinergics in older adults due to these risks.

Renal and Hepatic Impairment

Renal impairment significantly affects the clearance of drugs that are primarily excreted unchanged in the urine, such as neostigmine, pyridostigmine, and atropine. Dose reduction and careful monitoring are essential to avoid toxicity. Hepatic impairment can affect the metabolism of drugs like donepezil and pilocarpine. In patients with liver disease, lower starting doses and slower titration are recommended. For drugs with dual routes of elimination (e.g., rivastigmine), the need for dose adjustment may be less pronounced.

9. Summary/Key Points

• Acetylcholine is the primary neurotransmitter of the parasympathetic nervous system, acting on muscarinic (GPCR) and nicotinic (ion channel) receptors.
• Cholinergic drugs are classified as direct agonists (muscarinic, nicotinic, non-selective), indirect agonists (reversible/irreversible anticholinesterases), and antagonists (antimuscarinics, ganglionic blockers, neuromuscular blockers).
• The therapeutic applications are diverse, spanning glaucoma (pilocarpine), myasthenia gravis (pyridostigmine), Alzheimer’s disease (donepezil), reversal of neuromuscular blockade (neostigmine), overactive bladder (oxybutynin), and COPD (tiotropium).
• Adverse effects are predictable extensions of receptor activation or blockade. Cholinomimetics cause SLUDGE symptoms and can lead to cholinergic crisis, while anticholinergics cause dry mouth, blurred vision, tachycardia, constipation, urinary retention, and CNS disturbances.
• Significant drug interactions occur primarily through additive pharmacodynamic effects (cholinergic or anticholinergic) and, for some agents, through pharmacokinetic mechanisms involving CYP450 enzymes.
• Special caution is required in geriatric patients due to increased susceptibility to CNS and cardiac adverse effects, and in patients with renal or hepatic impairment due to altered drug clearance.

Clinical Pearls

• When administering an anticholinesterase to reverse neuromuscular blockade, always co-administer an antimuscarinic agent (e.g., glycopyrrolate) to prevent severe bradycardia and other muscarinic side effects.
• In suspected cholinergic poisoning (e.g., organophosphate exposure), the antidotes are atropine (to block muscarinic effects) and pralidoxime (2-PAM), which reactivates phosphorylated acetylcholinesterase if given early.
• The cognitive impairment and delirium caused by anticholinergic drugs in the elderly are often misattributed to dementia or aging; a thorough medication review is essential.
• For inhaled antimuscarinics like ipratropium, advise patients to use a spacer and rinse their mouth after administration to minimize the risk of local side effects like dry mouth and to reduce systemic absorption.

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