Pharmacology of Acetylcholine and Receptors

Autonomic Nervous System

• The autonomic nervous system is composed of cholinergic and adrenergic drugs.
• Cholinergic drugs act on receptors activated by acetylcholine.
• Adrenergic drugs act by either stimulating or blocking receptors of the autonomic nervous system.
• Cholinergic drugs, described in the chapter, act on receptors activated by acetylcholine.
• Cholinergic and adrenergic drugs, both act by stimulating or blocking receptors of the autonomic nervous system.

Cholinergic Agonists and Parasympathetic System

• Cholinergic drugs, act on receptors activated by acetylcholine.
• Cholinergic and adrenergic drugs, either stimulate or block receptors in the autonomic nervous system.
• Cholinomimetic (cholinergic) drugs have direct and indirect acting categories, including subgroups like cholinergic esters, alkaloids, organophosphates, carbamates, and edrophonium.

Parasympathetic System - The Cholinergic Neuron

• Preganglionic fibers terminate in the adrenal medulla, autonomic ganglia (parasympathetic and sympathetic), and postganglionic fibers of the parasympathetic nervous system; these use acetylcholine as neurotransmitter.
• Cholinergic neurons innervate somatic system muscles and central nervous system (CNS) important area.
• Alzheimer's disease patients have significantly less cholinergic neurons in the temporal lobe and entorhinal cortex. Acetylcholinesterase inhibitors are used to treat it.

Parasympathetic System - Neurotransmission

• Neurotransmission in cholinergic neurons is a six-step process: synthesis, storage, release, binding to receptor, degradation, and recycling of choline.
• Choline, transported from extracellular fluid to cytoplasm of cholinergic neuron; an energy-dependent carrier system. Inhibited by hemicholinium.
• Acetyl CoA, derived from mitochondria via Krebs cycle/fatty acid oxidation, is crucial for acetylcholine synthesis.

Parasympathetic System - Storage of Acetylcholine

• Acetylcholine is stored in presynaptic vesicles via active transport (coupled to proton efflux).
• The mature vesicle contains both acetylcholine and adenosine triphosphate (ATP).

Parasympathetic System - Release of Acetylcholine

• Action potential, propagated by voltage-sensitive sodium channels arrival at nerve ending. Increases intracellular calcium levels.
• Fusion of synaptic vesicles with cell membrane releases acetylcholine contents into synaptic space.
• Release can be blocked by botulinum toxin; while black widow spider venom causes all acetylcholine in synaptic vesicles to release into synaptic gap.

Binding to the Receptor

• Acetylcholine diffuses across synaptic space to bind to postsynaptic receptors (muscarinic or nicotinic) on target cell (or presynaptic receptors on releasing neuron).
• Postsynaptic cholinergic receptors on effector organs fall into muscarinic and nicotinic classes.

Degradation of Acetylcholine

• Signal at post-junctional effector site is rapidly terminated. Acetylcholinesterase catalyzes acetylcholine degradation into choline and acetate in synaptic cleft.

Recycling of Choline

• Choline is recaptured by sodium-coupled, high-affinity uptake system; transported back into neuron. Acetylation back into acetylcholine; stored until next action potential release.
• Butyrylcholinesterase or 'pseudocholinesterase' is found in plasma. Not significant role in acetylcholine termination effects at synapse.

Cholinergic Receptors

• Two families of cholinoceptors (muscarinic and nicotinic receptors). Different affinities for agents acting like acetylcholine (parasympathomimetics).
• Muscarinic receptors recognize muscarine, an alkaloid present in some poisonous mushrooms. Show weak affinity for nicotine. Five subclasses (M1, M2, M3, M4, M5). M1, M2, and M3 characterized functionally.
• M1 receptors also found on gastric parietal cells; M2 on cardiac cells and smooth muscle; M3 on bladder, exocrine glands, and smooth muscle. Drugs with muscarinic actions mainly stimulate muscarinic receptors; high concentrations can also act on nicotinic receptors.

Mechanisms of Acetylcholine Signal Transduction

• M1 or M3 receptor activation leads to Gq protein conformational change; activates phospholipase C. Hydrolysis of phosphatidylinositol-(4,5)-bisphosphate-P2.
• Results in diacylglycerol and inositol; increases intracellular Ca2+. This action can stimulate/inhibit enzymes; or cause hyperpolarization, secretion, or contraction.
• M2 subtype activation; stimulated by G protein (Gi); inhibits adenylyl cyclase; increases K+ conductance. Reduces rate and force of cardiac contraction.

Muscarinic Agonists and Antagonists

• Pirenzepine; a tricyclic anticholinergic drug; has greater selectivity for inhibiting M1 muscarinic receptors, especially in gastric mucosa.
• At therapeutic doses; pirenzepine does not cause many side effects; but rapid infusion causes reflex tachycardia due to blockade of M2 receptors in the heart.

Darifenacin

• Competitive muscarinic receptor antagonist; Greater affinity for M3 than other muscarinic receptors.
• Used in the treatment of overactive bladder.

Nicotinic Receptors

• Receptors, in addition to binding acetylcholine, recognize nicotine but have weak affinity for muscarine.
• Binding of two acetylcholine molecules triggers conformational change allowing sodium ion entry; results in effector cell depolarization.
• Nicotinic receptors are located in CNS, adrenal medulla, autonomic ganglia, and neuromuscular junction.
• Two main types (Nm and Nn). Nm found at neuromuscular junction causing skeletal muscle contraction; Nm causing depolarization in autonomic ganglia.

Cholinergic Receptors Summary

• Nicotine (or acetylcholine) initially stimulates then blocks nicotinic receptors.
• Nicotinic receptors are found within CNS, adrenal medulla, autonomic ganglia, and neuromuscular junctions; two types (Nm and Nn).
• Nm receptors located at neuromuscular junction, cause skeletal muscle contraction; Nn receptors cause depolarization in autonomic ganglia.
• Hexamethonium blocks ganglionic receptors.
• Tubocurarine blocks neuromuscular junction receptors.

Direct-Acting Cholinergic Agonists

• Cholinergic agonists (parasympathomimetics) mimic effects of acetylcholine.
• Classified into choline esters and naturally occurring alkaloids.
• Choline esters include acetylcholine, carbachol and bethanechol;
• Alkaloids include pilocarpine.
• Direct-acting drugs generally have longer durations of action than acetylcholine.

Acetylcholine

• A quaternary ammonium compound; cannot penetrate membranes.
• Neurotransmitter for parasympathetic and somatic nerves, affecting autonomic ganglia.
• Has both muscarinic and nicotinic activities.
• It acts to decrease heart rate and cardiac output (negative chronotropy)

Acetylcholine Actions (Continued)

• Effects mimic vagal stimulation.
• Causes vasodilation and lowers blood pressure; indirect mechanism.
• Activates M3 receptors on endothelial cells, causes nitric oxide production.
• Nitric oxide causes smooth muscle relaxation.
• Atropine blocks muscarinic receptors, preventing vasodilation.

Other Actions of Acetylcholine

• In gastrointestinal tract; stimulates intestinal secretions, motility, and salivary secretion.
• Increases tone of detrusor urina muscle, causing urine expulsion.
• Stimulates ciliary muscle contraction, accommodating near vision; constricts pupillae sphincter, aiding miosis.

Bethanechol

• Structurally similar to acetylcholine.
• Does not require hydrolysis by acetylcholinesterase (addition of carbonic acid).
• Inactivated by hydrolysis by other esterases.
• Stronger muscarinic activity than acetylcholine.

Bethanechol (Continued)

• Used to stimulate atonic bladder; specifically postpartum or postoperative, non-obstructive urinary retention.
• Causes increased intestinal motility and tone.
• Stimulates detrusor muscles; relaxes bladder trigon/sphincter for urine expulsion.
• May be used to treat neurogenic atony and megacolon.

Adverse Effects of Bethanechol, Carbachol, and Pilocarpine

• Sweating, salivation, flushing, and decreased blood pressure.
• Nausea, abdominal pain, and bronchospasm.
• Carbachol causes muscarinic and nicotinic effects.
• Pilocarpine has low potency but uncharged, can penetrate CNS; used in ophthalmology and a miotic.

Carbachol

• Carbamylcholine is an ester of carbamic acid; less susceptible to acetylcholinesterase.
• Has both muscarinic and nicotinic action..
• Biotransformed much more slowly by other esterases than bethanechol/pilocarpine; can last over an hour.
• Cardiovascular and gastrointestinal profound effects; initially stimulate then depress those systems.

Carbachol (Continued)

• Causes epinephrine release from adrenal medulla due to nicotinic activity.
• Rare therapeutic use, mostly in ophthalmology for glaucoma, due to its potency, non-selectivity and prolonged duration of action.
• Atropine useful to treat overdoses/side effects.

Pilocarpine

• A tertiary amine; stable in the presence of acetylcholinesterase.
• Less potent than acetylcholine but can cross the blood-brain barrier and is uncharged.
• Primarily used in ophthalmology, especially for glaucoma treatment, due to its rapid drop in intraocular pressure effect.
• Has side effects like sweating, salivation, gastrointestinal upset.

Pilocarpine (Continued)

• Produces rapid miosis and ciliary muscle contraction in eye.
• Causes accommodation spasm, making vision fixed at one distance.
• Used to treat glaucoma, preferable over bethanechol.

Indirect-Acting Cholinergic Agonists (Anticholinesterases)

• These agents inhibit acetylcholinesterase, cause acetylcholine accumulation in the synaptic cleft.
• Such as physostigmine, is a naturally occurring tertiary amine useful in treating atropine poisoning.

Physostigmine

• Substrate for acetylcholinesterase; forms carbamoylated intermediates. Results in the potentiation of cholinergic activity throughout the body.
• Acts on muscarinic and nicotinic sites in the autonomic nervous system and neuromuscular junction.
• Moderate duration of action (2-4 hours), considered an intermediate-acting agent. Can enter and stimulate cholinergic sites in the CNS.
• Used to treat overdoses of certain drugs with anticholinergic properties (e.g., atropine, phenothiazines); and myasthenia gravis (autoimmune disease).

Physostigmine (Continued)

• Has a wide range of effects due to its actions on muscarinic and nicotinic receptors of autonomic nervous system; neuromuscular junction.
• More potent than bethanechol but can cause significant side effects.

Pyridostigmine and Ambenomium

• Other cholinesterase inhibitors used in the chronic treatment of myasthenia gravis.
• Considered intermediate-acting agents (3-8 hours); longer duration than neostigmine.
• Similar adverse effects to neostigmine.

Demecarium

• Another cholinesterase inhibitor; useful in treating chronic open angle/or closed angle glaucoma, especially after an irredectomy.
• Also used to diagnose and treat accommodative esotropia.

Edrophonium

• Rapidly absorbed; short duration of action (10–20 minutes).
• Similar actions to neostigmine, but only acts on cholinergic receptors, not central nervous system..
• Used in diagnosis of myasthenia gravis.
• Used to treat overdoses of anticholinergic drugs. Side effects may include cholinergic crisis if too much drug administered. Administration method must be careful.

Tacrine, Donepezil, Rivastigmine, and Galantamine

• Anticholinesterase drugs used to treat Alzheimer's disease; increase acetylcholine levels in CNS.
• Effective in treating a deficiency of cholinergic neurons seen in Alzheimer's disease.
• May hinder disease progression; though not a cure.
• Gastrointestinal complaints are primary adverse effects.

Indirect-Acting Cholinergic Agonists (Anticholinesterases—Irreversible)

• Echothiophate; an organophosphate compound. Binds covalently to acetylcholinesterase; irreversible inhibition results in a long-lasting rise in acetylcholine.
• Developed as nerve agents in warfare.
• Related compounds like parathion, used as insecticides; extremely toxic.

Mechanism of Action (Echothiophate)

• Echothiophate binds to the serine-OH group in the active site of acetylcholinesterase; covalent.
• Resulting in a long-lasting increase in acetylcholine at sites where it is released.

Actions of Echothiophate

• Causes generalized cholinergic stimulation; paralysis of motor functions (breathing difficulties); and seizures. Causes intense miosis.

Therapeutic Uses of Echothiophate

• Used for chronic open-angle glaucoma.
• Effects last for up to a week after a single dose. Possible cataracts risks may limit use.
• Atropine in high dosage can reverse many of the muscarinic and some few central effects of echothiophate.

Reactivation of Acetylcholinesterase

• Pralidoxime can reactivate inhibited acetylcholinesterase; cannot cross into CNS.
• Drug binding to the phosphate group of an organophosphate allows regeneration and removal; this displaces the phosphate group, regenerating the enzyme.
• Effective when administered before irreversible binding occurs.
• At higher doses, pralidoxime may cause side effects that are similar to cholinesterase inhibitors.

Cholinergic Antagonists - Overview

• Cholinergic antagonists (anticholinergic drugs, parasympatholytics) are substances that bind with cholinoceptors; does not trigger intracellular responses.
• Ganglionic blockers have preference for nicotinic receptors of sympathetic/parasympathetic ganglia; clinically less used than anticholinergic drugs.
• Neuromuscular blocking agents affect efferent impulses to skeletal muscles. The agents are adjuvants used in surgery; increase safety of anesthesia.

Ganglionic Blockers - summary

• Ganglionic blockers specifically block nicotinic receptors of parasympathetic/sympathetic autonomic ganglia; not selective/effective as neuromuscular antagonists.
• Responses are unpredictable; lack of selectivity.

Nicotine

• Component of cigarette smoke.
• Not therapeutically beneficial; deleterious to health.
• Available in formats like patches, lozenges, gums; helps in reducing cravings in smokers.
• Depolarizes autonomic ganglia at low doses; then causes paralysis of autonomic ganglia at higher doses.
• Causes sympathetic and parasympathetic ganglionic effects; increased blood pressure, cardiac rate, increased peristalsis, and secretions.

Mecamylamine

• Potent, oral antihypertensive agent; attributed to a reduction in sympathetic tone, vasodilation, decreased cardiac output.
• Competitive nicotinic blockade of ganglia.
• Oral absorption is good compared to trimethaphan.

Neuromuscular Blocking Drugs

• Block cholinergic transmission; structural analogs of acetylcholine.
• Act as antagonists or agonists at nicotinic receptors on neuromuscular end plate.

Non-depolarizing (Competitive) Blockers

• Curare, early use in South America to paralyze prey; tubocurarine chemically purified, introduced into clinical practice.
• At low doses; block nicotinic receptors; prevent acetylcholine binding to depolarize muscle cell membrane, thus preventing muscular contraction. This can be reversed with cholinesterase inhibitors.
• At high doses; block ion channels in the end-plate which causes weakening of neuromuscular transmission; decreases the ability of cholinesterase inhibitors to reverse actions of non-depolarizing muscle relaxants.

Actions of Neuromuscular Blocking Drugs

• Sequence of paralysis may vary depending on the drug, but respiratory muscles paralyzed last.
• Succinylcholine—short acting; causes muscle fasciculations followed by paralysis; broken down rapidly by plasma cholinesterase. Short duration of action.

Therapeutic Uses of Neuromuscular Blocking Drugs

• Used in surgery as adjuvants; provide muscle relaxation; minimizes need for higher anesthesia doses
• Facilitates intubation
• Also used in electroconvulsive shock treatment.

Adverse Effects of Neuromuscular Blocking Drugs

• Hyperthermia. Halothane administration; especially in patients genetically predisposed; administration of succinylcholine. Rapid cooling and administration of dantrolene to block calcium release.
• Apnea. Succinylcholine administration; patients with genetic plasma cholinesterase deficiency, causing prolonged apnea due to diaphragm paralysis.

5-Adverse Effects (Neuromuscular Blocking Drug Specific)

• The depolarizing agent succinylcholine, affects sodium channels associated with nicotinic receptors; this causes depolarization of the receptor (Phase I).
• Subsequent binding causes a transient twitching of the muscle (fasciculation).
• Over time, continuous depolarization gives way to a gradual repolarization; which leads to resistance to further depolarization, and flaccid paralysis (Phase II); in contrast to non-depolarizing drugs.

Additional Notes

• Atropine; the antidote of many anticholinergic medications
• Inhaled ipratropium acts against asthma.
• Tropicamide and Cyclopentolate are used as ophthalmic solutions to mimic atropine effects.

Test your knowledge on the actions and effects of acetylcholine, particularly its role in muscarinic receptors and the gastrointestinal system. The quiz covers neurotransmission, related drugs, and the physiological functions influenced by acetylcholine. Delve into cholinergic mechanisms and their significance in both normal and pathological conditions.