Introduction to �Autonomic and Cholinergic Pharmacology

Jose Paciano B.T. Reyes, MD, FPNA

Associate Professor

Department of Pharmacology

UP College of Medicine

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“Nature thought it prudent to remove these important phenomena* from the caprice of an ignorant will”

- Claude Bernard

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*breathing, regulation of the heart rate, lactation, eating, sleeping

French Physiologist mid 1800s

• Use of the scientific method in medicine
• Research in the function of the pancreas and liver
• Vasomotor system

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"The stability of the internal environment [the milieu intérieur] is the condition for the free and independent life."

Diseases implicated with the ANS

• Hypertension
• Asthma
• Fainting
• Ventricular tachycardia
• Bladder / Bowel incontinence

Somatic and Autonomic�Nervous System

Sympathetic and �Parasympathetic ANS

Sympathetic and �Parasympathetic ANS

Parasympathetic

Craniosacral (CN III, VII, IX, X, S3-4)‏

Parasympathetic

Craniosacral (CN III, VII, IX, X, S3-4)‏

Sympathetic

Thoracolumbar (T1-T12, L1-L5)‏

Neurotransmitter Chemistry

• Release Acetylcholine
• All preganglionic efferent autonomic & somatic motor
• All parasympathetic postganglionic
• Some postganglionic sympathetic

• Release Norepinephrine
• Most postganglionic sympathetic

Nerve terminal

Axon

Postsynaptic cell

Ca²⁺

A

Na⁺

Choline

B

AcCoA + Choline

ChAT

ACh

Ach

ATP, P

Ach

ATP, P

ACh

Cholinoceptors

Achesterase

Choline

Acetate

Presynaptic

receptors

A

Nerve terminal

Axon

Postsynaptic cell

Na⁺

Tyrosine

B

Tyrosine

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hydroxylase

Dopamine

NE

ATP, P

NE

ATP, P

NE

Ca²⁺

Adrenoreceptors

Presynaptic

receptors

1

2

Tyr

DOPA

Diffusion,

Metabolism

Relationship between �Sympathetic & Parasympathetic ANS

SITE

Arterioles………………

Veins…………………..

Heart…………………..

Iris……………………...

Ciliary muscle………...

PREDOMINANT TONE

Adrenergic

Adrenergic

Cholinergic

Cholinergic

Cholinergic

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Relationship between �Sympathetic & Parasympathetic ANS

SITE

GIT…………………………...

Urinary bladder……………

Salivary glands……………

Sweat glands

General……………..........

Palms and soles….........

PREDOMINANT TONE

Cholinergic

Cholinergic

Cholinergic

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Cholinergic

Adrenergic

Relationship between �Sympathetic & Parasympathetic ANS

Produce Opposite Effects

• Visceral smooth muscle of the gut
• Bladder
• Heart

Produce Similar Effects

• Salivary glands

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Only One Division of ANS

Sympathetic

• Sweat glands
• Most blood vessels
• Resistance arteries

Parasympathetic

• Bronchial smooth muscle

Cholinergic Receptors�

Nerve terminal

Axon

Postsynaptic cell

Ca²⁺

A

Na⁺

Choline

B

AcCoA + Choline

ChAT

ACh

Ach

ATP, P

Ach

ATP, P

ACh

Cholinoceptors

Achesterase

Choline

Acetate

Presynaptic

receptors

Acetylcholine Synthesis

Acetylcholine synthesis:

• Choline enters neuron via carrier mediated transport (rate limiting step)‏

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• Acetylation of choline with AcetylCoA as source of acetyl groups, involves choline acetyltransferase

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• Ach is packaged into synaptic vesicles at high concentration by carrier-mediated transport

Cholinergic Transmission

• Ach release occurs by Ca²⁺-mediated exocytosis; 100-500 vesicles released at the neuromuscular junction
• Ach acts on nAChR to open cation channels to produce rapid depolarization
• At ‘fast’ cholinergic synapses, Ach is hydrolysed < 1 ms by acetylcholinesterase
• Transmission mediated by mAChR is slower

Nicotinic Cholinergic Receptors

• Ligand-gated ion channels, coupled directly to cation channels
• Mediate fast excitatory synaptic transmission

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• Related in structure and sequence to receptors of GABA, glycine, 5HT₃, glutamate

Nicotinic Cholinergic Receptors

Muscle type

• Ligand gated ion channels
• Muscle receptor has a pentameric structure (ααβγε) made up of 4 distinct subunits

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Neuronal types

• Ligand gated ion channels
• Neuronal receptors are more diverse
• 8 subtypes of the α - subunit
• 4 subtypes of the β - subunit

Muscarinic Cholinergic Receptors

• G-protein linked
• 7 transmembrane domains
• 3rd cytoplasmic loop is coupled to G-proteins
• Regulate the production of intracellular second messengers

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Sites of Drug Action

1. action potential in nerve presynaptic nerve fiber

2. synthesis of transmitter

3. storage

4. metabolism

5. release

6. reuptake

7. degradation

8. receptor of transmitter

9. receptor-induced increase or decrease in ionic conductance

Cholinergic Agonists

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Direct Acting

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Indirect Acting

Direct-acting �Cholinoceptor Agents

Choline esters

• acetylcholine, methacholine, carbachol, bethanechol‏
• relatively insoluble in lipids
• contain a quarternary ammonium group
• poorly absorbed and hydrolyzed in the GI tract
• poorly distributed in the CNS
• presence of the β-methyl group (methacholine, bethanechol) reduces potency at the nicotinic receptor

Direct-acting �Cholinoceptor Agents

Alkaloids

• tertiary amines (pilocarpine, nicotine, lobeline) well absorbed from most sites of administration
• muscarine, a quarternary amine, less completely absorbed from the GI tract
• chiefly excreted in the kidneys, enhanced with acidification of the urine

Direct-acting �Cholinoceptor Agents

Direct-acting �Cholinoceptor Agents

Direct-acting�Cholinoceptor Agents

Direct-acting Cholinoceptor Agents

Direct-acting Cholinoceptor Agents

Drugs that Inhibit Cholinesterase

• Peripherally acting drugs

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• Centrally acting drugs

A

Nerve terminal

Axon

Muscle

Na⁺

Choline

B

AcCoA + Choline

ChAT

ACh

Ach

ATP, P

Ach

ATP, P

ACh

Ca²⁺

Nicotinic Receptors

Achesterase

Choline

Acetate

Presynaptic

receptors

Peripherally-acting �Cholinesterase Inhibitors

Enzymes inhibited

• Acetylcholinesterase (ACh)‏
• Bound to basement membrane synaptic cleft
• Specific for acetylcholine and related esters
• Buterylcholinesterase (BCh)‏
• Widespread
• Broader substrate specificity

Peripherally-acting �Cholinesterase Inhibitors

Structurally, they are:

• Simple alcohols with a quarternary ammonium group (edrophonium)‏
• Carbamic acid esters of alcohols bearing tertiary (neostigmine, pryridostigmine) or quarternary ammonium groups (physostigmine)
• Organic derivatives of phosphoric acid (organophosphates)‏

Peripherally-acting �Cholinesterase Inhibitors

1. Quarternary alchohols such as edrophonium bind reversibly to the binding site

2. Carbamate esters (neostigmine and physostigmine) undergo a two-step hydrolysis that allow for the formation of a carbomylated covalent bond

• Organophosphates form a phosphorylated covalent bond, “aging” of which strengthens the phosphorus-enzyme bond.

Pralidoxime is a drug that splits the phosphorus-enzyme bond and is most effective before “aging” has occurred

Peripherally-acting �Cholinesterase Inhibitors

1. Short-acting anticholinesterase

Edrophonium

• Binds to the anionic site of the enzyme
• Ionic bond is formed, action is brief
• Used mainly for diagnostic purposes

Peripherally-acting �Cholinesterase Inhibitors

2. Medium-duration anticholinesterases

Neostigmine, Pyridostigmine, Physostigmine

• Possess strongly basic groups which bind to the anionic site of the enzyme (carbamyl instead of acetyl)‏
• Hydrolysed by acetylcholinesterase but at a very slow rate
• Slow recovery of the carbamylated enzyme
• Action of drug is long lasting

Peripherally-acting �Cholinesterase Inhibitors

3. Irreversible anticholinesterases

Organophosphates

• Pentavalent phosphorous compounds, contains a labile group (fluoride) or an organic group
• Group is released, residue of molecule is covalently bonded to the enzyme
• Inactive phoshorylated enzyme is very stable
• Can cause a severe type of peripheral nerve demyelination

Peripherally-acting �Cholinesterase Inhibitors

Peripherally-acting �Cholinesterase Inhibitors

Effects on autonomic cholinergic synapses

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• Enhancement of cholinergic activity at the parasympathetic postganglionic synapse
• Large doses block ganglia

• Increased secretions
• Increased peristaltic movement
• Bronchoconstriction
• Bradychardia
• Hypotension
• Pupillary constriction
• Fixation of accommodation
• Fall in intraocular pressure

Peripherally-acting �Cholinesterase Inhibitors

Amplifies endogenous acetylcholine

Postoperative and neurogenic ileus and urinary retention

Neostigmine

Amplifies effect of ACh

Glaucoma

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Physostigmine, Echothiaphate

Amplifies endogenous acetylcholine; increase strength

Myasthenia gravis, reversal of neuromuscular blockade

Neostigmine, pyridostigmine, edrophonium

Activates pupillary sphincter and ciliary muscles of the eye

Glaucoma

Carbachol, Pilocarpine

Activates bowel and bladder smooth muscle

Postoperative and neurogenic ileus and urinary retention

Bethanechol

Action

Clinical Applications

Drug

Peripherally-acting �Cholinesterase Inhibitors

Toxicity of indirect acting agents:

Diarrhea

Urination

Miosis

Bronchoconstriction

Excitation

Lacrimation

Salivation

Centrally-acting �Cholinesterase Inhibitors

• Orally active
• Enters the CNS readily
• Used for the treatment of Alzheimers Disease

Tacrine Donezepil

Rivastigmine Galantamine

Cholinergic Antagonists

Presynaptic

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Postsynaptic

Mechanisms for pharmacologic block of cholinergic transmission

• Inhibition of choline uptake

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• Inhibition of ACh release

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• Block of postsynaptic receptors or ion channels

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• Persistent postsynaptic depolarization

A

Nerve terminal

Axon

Postsynaptic cell

Na⁺

Choline

B

AcCoA + Choline

ChAT

ACh

Ach

ATP, P

Ach

ATP, P

ACh

Ca²⁺

Cholinoceptors

Achesterase

Choline

Acetate

Presynaptic

receptors

Presynaptic �Cholinergic Inhibitors

Hemicholinium

block transport of choline

ganglion blocker

Triethylcholine

block transport of choline

forms false neurotransmitter

Vesamicol

blocks transport into vesicles

*useful experimental tools, no clinical applications

Presynaptic �Cholinergic Inhibitors

Botulinum toxin

• Inhibit acetylcholine release
• Peptidases as active component
• Causes progressive parasympathetic and motor paralysis
• Acts as a ganglion blocker

β-bungarotoxin

• Cobra family venom*
• Phospholipase as active ingredient
• Similar effect to botulinum

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Aminoglycoside antibiotics

*also contains α-bungarotoxin which blocks postsynaptic acetylcholine receptors

A

Nerve terminal

Axon

Muscle

Na⁺

Choline

B

AcCoA + Choline

ChAT

ACh

Ach

ATP, P

Ach

ATP, P

ACh

Ca²⁺

Muscarinic Receptors

Achesterase

Choline

Acetate

Presynaptic

receptors

Muscarinic antagonists

• Atropine derived from Atropa belladona (deadly nightshade) and Datura stramonium (jimsonweed)‏
• Scopolamine from Hyoscyamus niger (henbane)‏
• Chemistry:
• Natural alkaloids eg. Atropine, Scopolamine
• Tertiary ammonium eg. Pirenzipine, Tropicamide
• Quarternary ammonium eg. Ipratropium, Benztropine
• Many antihistamine and antidepressant drugs have similar structures to tertiary ammonium analogs and thus also have significant antimuscarinic effects

Cholinoceptor Blockers

Mechanism of Action:

Reversible blockade of receptors

• Most sensitive to atropine: salivary, bronchial and sweat glands tissues
• Intermediate sensitivity: smooth muscles and cardiac tissue
• Least sensitive: gastric parietal cells
• Atropine is highly selective for muscarinic receptors
• Synthetic anti-muscarinic drugs (eg. Quarternary ammoniums) less specific and may have significant ganglion-blocking actions (ganglions contain nicotine receptors)‏

Cholinoceptor Blockers

Absorption, distribution and metabolism:

• Natural alkaloids (atropine/scopolamine),

tertiary ammonium antimuscarinic agents well absorbed form the gut and across the conjunctival membrane

widely distributed after absorption

• Quarternary derivatives

poorly absorbed in the gut

not taken up in the central nervous system.

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About 60% of atropine is excreted unchanged in the urine and the effect declines rapidly in all organs except the eye.

Cholinoceptor Blockers

Cholinoceptor Blockers

Cholinoceptor Blockers

Mnemonic for atropine toxicity

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“Blind as a bat”

“Dry as a bone”

“Red as a beet”

“Mad as a hatter”

“Hot as hell”

Drugs Affecting Autonomic Ganglia

Ganglion stimulants

• Not used clinically but as experimental tools
• Cause complex peripheral responses associated with generalized stimulation of autonomic ganglia

• Tachycardia
• Increase in BP
• Variable effects on GI motility and secretions
• Increase bronchial, salivary, sweat secretions

Drugs Affecting Autonomic Ganglia

Ganglion stimulants

• Most nicotinic receptor agonists affect both ganglionic and motor endplate receptors, but show selectivity

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• Nicotine: tertiary amine, tobacco
• Lobeline: tertiary amine, lobelia
• Dimethylphenylpiperazinium (DMPP): synthetic compound selective for ganglionic receptors

Drugs Affecting Autonomic Ganglia

Ganglion-blocking drugs

Mechanisms:

1. Interference with acetylcholine release

botulinum toxin, hemicholinium, magnesium ion

2. Prolonged depolarization

nicotine

3. Interference with postsynaptic action of acetylcholine, blockade of nicotinic receptors / ion channel

hexamethonium, trimetaphan, tubocurarine

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Drugs Affecting Autonomic Ganglia

Ganglion-blocking drugs

block all autonomic ganglia and enteric ganglia

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• Hypotension
• Loss of cardiovascular reflexes (postural hypotension)‏
• Inhibition of secretions
• Gastrointestinal paralysis
• Impaired micturition

Ganglion Blocking Drugs

Ganglion Blocking Drugs

Acetylcholine

CH₃

CH₃

Hexamethonium*

*hexamethonium is a ganglionic blocker

decamethonium is a neuromuscular blocker

Acetylcholine

Decamethonium*

*decamethonium is a neuromuscular blocker

hexamethonium is a ganglionic blocker

CH₂-CH2-CH2-CH₂-CH₂-⁺N-CH₃

CH₃

CH₃

CH₃-⁺N-CH₂-CH₂-CH₂-CH₂-CH₂

CH₃

CH₃

Introduction to Autonomic and Cholinergic Pharmacology

Autonomic Nervous System

Cholinergic Receptors

Direct Acting Agonists

Cholinesterase Inhibitors

Cholinoceptor Antagonists

Drugs Affecting Autonomic Ganglia

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