5 - Acetylcholine Action

Acetylcholinesterase (AChE)

the principal enzyme for inactivating acetylcholine and terminating its action at synaptic junctions

Where is acetylcholinesterase located?

neurons, neuroeffector junctions, some other tissues

Activity of Ach in the skeletal neuromuscular junction

binds to the nicotinic Nm receptors, mediates skeletal muscle depolarization

What effect does black widow spider venom have on the Nm junction?

increased Ach release causing painful muscle cramps and spasms

What effect does botulinum toxin have on the Nm junction?

inhibits the release of Ach, causing temporary chemical denervation causing muscle weakness or flaccid paralysis

Protein fasciculin effect on AChE

a family of closely related peptides isolate from mamba venom toxins; inhibits AChE, causing flaccid paralysis

What is the rate limiting step of cholinesterase?

choline reuptake

Physostigmine

• only reversible cholinesterase inhibitor that can cross the blood-brain barrier

• antidote for Atropa belladonna poisoning

Neostigmine mechanism

• reversible acetylcholinesterase inhibitor

• does not cross BBB

• treatment for myasthenia gravis, reverses non-depolarizing neuromuscular blockade, postoperative ileus and urinary retention

Physostigmine derivative: neostigmine

antidote for neuromuscular blocking agents like tubocurarine

Edrophonium

a diagnostic tool for myasthenia gravis

Pyridostigmine

nerve agent pretreatment; protects AChE from irreversible inhibition

Reversible indirect cholinesterase inhibitors

• physostigmine

• neostigmine

• edrophonium

• pyridostigmine

Irreversible indirect cholinesterase inhibitors

• organophosphate

• parathion

• malathion

Oranophosphorus pesticides

originally developed for use as chemical warfare nerve agents; blocks AChE

Organophosphate mechanism of action

binds to AChE, leading to excess ACh in the synapse; results in excessive parasympathetic drive

Malathion & Parathion association

agricultural pesticide exposure

Diazinon

causes muscarinic and nicotinic overstimulation causing Bronchorrhea and fasciculations

Muscarinic receptors

• GPCRs activated by ACh

• mediate parasympathetic and CNS signaling

• produce slower, modulatory effects compared to nicotinic receptors

Choline esters

• acetylcholine

• methacholine

• carbachol

• bethanechol

Natural alkaloids

• muscarine

• nicotine

• pilocarpine

• arecoline

Bethanechol

treatment or urinary retention, exerts its effects via directly stimulating the muscarinic receptors (M1, M2, M3, M4, M5)

Carbachol

mimics ACh, stimulating muscarinic and nicotinic receptors

Methacholine

selectively stimulates muscarinic receptors, used to assist in diagnosis of asthma

How do choline esters differ in breakdown by AChE?

• acetylcholine - very susceptible

• methacholine - 3x less susceptible

• bethanechol - not susceptible

Cholinomimetic alkaloids

mainly muscarinic agonists:

• natural alkaloids:

  • muscarine

  • pilocarpine

  • arecholine

• synthetic alkaloid

  • oxotremorine

Pilocarpus jaborandi

• crosses the BBB

• treats glaucoma

Pilocarpine

sweat test; considered the gold standard for diagnosing cystic fibrosis

Arecoline common name

betelnut

Atropine

high affinity for muscarinic receptors, where it binds competitively, preventing ACh from binding to those sites

Differences between atropine vs. scopolamine

• greater permeation of scopolamine through the BBB

• atropine has limited CNS effects, given in preference to scopolamine

Oxybutynin (Ditropan)

M3 antagonism promotes detrusor contractions

Tiotropium

inhibits M3 receptors, has longer duration than ipratropium in treatment of asthma

Which enzyme breaks down acetylcholine in the synaptic cleft?

acetylcholinesterase

Which drug crosses the BBB?

a. neostigmine

b. pyridostigmine

c. edrophonium

d. physostigmine

d. physostigmine

Which drug is historically used to diagnose myasthenia gravis?

a. neostigmine

b. pyridostigmine

c. edrophonium

d. physostigmine

c. edrophonium

Which drug was used in the military as a pretreatment for exposure to the chemical nerve agent Soman?

a. neostigmine

b. pyridostigmine

c. edrophonium

d. physostigmine

b. pyridostigmine

Which finding best indicates Diazinon poisoning?

a. mydriasis

b. dry skin

c. bronchorrhea & fasciculations

c. bronchorrhea & fasciculations

Which agent is NOT an irreversible AChE inhibitor?

a. parathion

b. diazinon

c. malathion

d. propoxur

d. propoxur

Which compound is an irreversible AChE inhibitor?

a. parathion

b. diazinon

c. malathion

d. propoxur

c. malathion

Which agent selectively stimulates muscarinic receptors?

a. acetylcholine

b. methacholine

c. carbachol

d. nicotine

b. methacholine

Which agent is resistant to AChE and stimulates both muscarinic and nicotinic receptors?

a. acetylcholine

b. methacholine

c. carbachol

d. nicotine

c. carbachol

Which drug is used to treat postoperative urinary retention?

a. acetylcholine

b. bethanechol

c. nicotine

d. muscarine

b. bethanechol

Which drug is used in bronchial challenge testing in the diagnosis of asthma?

a. acetylcholine

b. bethanechol

c. nicotine

d. methacholine

d. methacholine

Which alkaloid is used to treat glaucoma?

a. muscarine

b. nicotine

c. pilocarpine

d. arecoline

c. pilocarpine

What is the main purpose of the pilocarpine sweat test?

diagnose cystic fibrosis

Chewing betel nut produces parasympathetic effects due to which compound?

a. muscarine

b. nicotine

c. arecoline

d. methacholine

c. arecoline

What is atropine’s mechanism of action?

competitive muscarinic antagonism

Scopolamine is most commonly used for:

a. copd maintenance

b. motion sickness

c. overactive bladder

d. peptic ulcer disease

b. motion sickness

Oxybutynin improves overactive bladder by:

a. beta-3 agonism

b. nicotinic blockade

c. M3 muscarinic antagonism

d. AChE inhibition

c. M3 muscarinic antagonism

What is the key difference between Tiotropium vs. Ipratropium?

a. lower receptor selectivity

b. longer duration of action

c. used only acutely

d. more systemic effects

b. longer duration of action

Acetylcholine receptor activation & clinical association

muscarinic + nicotinic; rapidly degraded

Methacholine receptor activation & clinical association

muscarinic-selective; bronchial challenge

Carbachol receptor activation & clinical association

muscarinic + nicotinic; glaucoma

Bethanechol receptor activation & clinical association

muscarinic-selective; urinary retention

Muscarine receptor activation & clinical association

muscarinic; mushroom poisoning

Pilocarpine receptor activation & clinical association

muscarinic; glaucoma, sjögren’s syndrome

Arecoline receptor activation & clinical association

muscarinic; CNS stimulation (betel nut)

Why is Bethanechol preferred over acetylcholine for urinary retention?

it is resistant to acetylcholinesterase & is muscarinic-selective, making it effective for urinary retention without nicotinic side effects

A patient receives Bethanechol post-operatively. Which organ system is primarily targeted & what is the most likely adverse effect?

targets bladder smooth muscle, can cause bradycardia or diarrhea

Asthma Provocation (Methacholine):

• activated receptor

• effect on airways

• diagnostic use

muscarinic (M3)

bronchoconstriction

reveals airway hyperreactivity

Glaucoma therapy (Pilocarpine) effect on:

• pupil size

• aqueous humor outflow

• key adverse effect

miosis

increased outflow

sweating

What is a mixed receptor drug and what is an example? Why is systemic use limited?

a drug that activates both muscarinic & nicotinic receptors & is used in the eye

Carbachol, nicotinic side effects limit systemic use

Tyrosine hydroxylase:

• substrate

• product

• location

tyrosine → L-DOPA

cytosol

DOPA carboxylase:

• substrate

• product

• location

L-DOPA → dopamine

cytosol

Dopamine β-hydroxylase

• substrate

• product

• location

dopamine → norepinephrine

synaptic vesicle

PNMT

• substrate

• product

• location

norepinephrine → epinephrine

adrenal medulla

Which enzyme converts tyrosine → L-DOPA?

tyrosine hydroxylase

Which enzyme converts L-DOPA → dopamine?

DOPA decarboxylase

Which enzyme converts dopamine → norepinephrine?

dopamine β-hydroxylase

Which enzyme converts norepinephrine → epinephrine?

PNMT

How will dopamine, norepinephrine, and epinephrine be effected by tyrosine hydroxylase deficiency?

↓dopamine, ↓norepinephrine, ↓epinephrine

How will dopamine, norepinephrine, and epinephrine be effected by DOPA decarboxylase deficiency?

↓dopamine, ↓norepinephrine, ↓epinephrine

How will dopamine, norepinephrine, and epinephrine be effected by DBH deficiency?

↑dopamine, ↓norepinephrine, ↓epinephrine

L-DOPA mechanism

synthesis

Carbidopa mechanism

prevents breakdown of L-DOPA

MAO-B inhibitor mechanism?

reduces breakdown

COMT inhibitor mechanism

reduces breakdown

Dopamine agonist mechanism

prolongs signaling (bypasses synthesis)