Indirect Cholinergic Agonists

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Questions and Answers

How do indirect-acting parasympathomimetic drugs increase acetylcholine levels in the synapse?

  • By promoting the synthesis of acetylcholine.
  • By enhancing the release of acetylcholine from presynaptic neurons.
  • By inhibiting the breakdown of acetylcholine. (correct)
  • By directly stimulating acetylcholine receptors.

Which of the following best describes the mechanism of action of reversible cholinesterase inhibitors?

  • They temporarily bind to cholinesterase, preventing acetylcholine breakdown. (correct)
  • They permanently bind to and inactivate cholinesterase.
  • They prevent the release of acetylcholine into the synapse.
  • They increase the production of cholinesterase.

A patient presents with muscle weakness and is suspected of having myasthenia gravis. Which short-acting medication is most suitable to diagnose this condition?

  • Neostigmine
  • Edrophonium (correct)
  • Physostigmine
  • Pyridostigmine

Why is physostigmine able to cross the blood-brain barrier, while neostigmine cannot?

<p>Physostigmine is a tertiary amine, while neostigmine is a quaternary ammonium compound. (C)</p> Signup and view all the answers

Which condition is least likely to be treated with neostigmine?

<p>Glaucoma (B)</p> Signup and view all the answers

A patient with Alzheimer's disease is prescribed a cholinesterase inhibitor. Which of the following is most likely the drug being prescribed, given its ability to cross the blood-brain barrier?

<p>Donepezil (C)</p> Signup and view all the answers

Which of the following cholinesterase inhibitors has the longest duration of action?

<p>Echothiophate (D)</p> Signup and view all the answers

Why is physostigmine used as an antidote in atropine poisoning?

<p>It increases acetylcholine levels in the synapse, overcoming atropine's blockade. (B)</p> Signup and view all the answers

How do organophosphates cause toxicity?

<p>By causing an irreversible inhibition of cholinesterase. (C)</p> Signup and view all the answers

Following exposure to an organophosphate insecticide, a farmer presents with excessive salivation, blurred vision, and difficulty breathing. What single intervention is most important?

<p>Providing mechanical ventilation. (B)</p> Signup and view all the answers

Which of the following is a key difference between true cholinesterase (acetylcholinesterase) and pseudo cholinesterase (butyrylcholinesterase)?

<p>True cholinesterase specifically metabolizes acetylcholine, while pseudocholinesterase metabolizes a broader range of esters. (A)</p> Signup and view all the answers

Why is the 'aging' of cholinesterase important in organophosphate poisoning, and why does pralidoxime need to be administered before aging occurs?

<p>Aging increases the affinity of the organophosphate for cholinesterase, making reactivation by pralidoxime impossible. (A)</p> Signup and view all the answers

In organophosphate poisoning, which manifestations are directly related to excessive stimulation of muscarinic receptors?

<p>Diarrhea, urination, miosis, bradycardia, bronchospasm, emesis, lacrimation, salivation, and sweating (A)</p> Signup and view all the answers

A patient exposed to an organophosphate compound exhibits muscle weakness and paralysis. What is the primary mechanism leading to these neuromuscular effects.

<p>Excessive stimulation of nicotinic receptors at the neuromuscular junction, leading to depolarization blockade. (D)</p> Signup and view all the answers

Which of the following best describes a 'cholinergic crisis' and its most life-threatening consequence?

<p>Overstimulation of cholinergic receptors due to excessive acetylcholine; leading to respiratory failure. (D)</p> Signup and view all the answers

A researcher is studying the effects of a new drug on acetylcholine levels in the brain. Which type of cholinesterase inhibitor would be most suitable for increasing acetylcholine levels specifically within the central nervous system?

<p>A tertiary amine cholinesterase inhibitor (D)</p> Signup and view all the answers

In treating organophosphate poisoning, why is it crucial to administer both atropine and pralidoxime, rather than just one of these medications?

<p>Atropine reverses muscarinic effects, while pralidoxime reactivates cholinesterase. (D)</p> Signup and view all the answers

A patient is prescribed pyridostigmine for the treatment of myasthenia gravis. Which of the following explains why pyridostigmine is preferred over neostigmine in the long-term management of this condition?

<p>Pyridostigmine is more selective for the NMJ and has a longer duration of action. (A)</p> Signup and view all the answers

How would the use of succinylcholine be affected in a patient with genetically low levels of pseudocholinesterase?

<p>The duration of action of succinylcholine would be prolonged, increasing the risk of prolonged paralysis. (C)</p> Signup and view all the answers

After receiving a dose of succinylcholine during surgery, a patient experiences prolonged muscle paralysis. Which of the following factors would make you suspect a deficiency in pseudocholinesterase as the cause?

<p>A family history of similar reactions to anesthesia. (A)</p> Signup and view all the answers

Which of the following is the most likely mechanism by which donepezil improves cognitive function in patients with Alzheimer's disease?

<p>By increasing acetylcholine levels in the brain, thereby enhancing cholinergic neurotransmission. (C)</p> Signup and view all the answers

Eye drops containing echothiophate are occasionally used to treat glaucoma. By what mechanism does this medication reduce intraocular pressure?

<p>By constricting the pupil and increasing the outflow of aqueous humor (D)</p> Signup and view all the answers

Which of the following is a primary consideration when deciding between using physostigmine and neostigmine?

<p>Whether the desired effect is needed in the central nervous system. (A)</p> Signup and view all the answers

A patient with myasthenia gravis has been stable on pyridostigmine but is now scheduled for surgery. Which of the following considerations is most important regarding their medication?

<p>The anesthesiologist should be informed about the pyridostigmine use, as it can prolong the effects of certain muscle relaxants. (A)</p> Signup and view all the answers

Following an accidental exposure to an organophosphate insecticide, a farmworker is brought to the emergency department. Besides atropine and pralidoxime, which additional medication might be considered as part of the initial management?

<p>Diazepam to control seizures. (A)</p> Signup and view all the answers

A patient with Alzheimer's disease is started on donepezil. Which of the following side effects might be anticipated?

<p>Diarrhea and bradycardia (A)</p> Signup and view all the answers

Which treatment is LEAST likely to be effective for a patient showing signs of organophosphate poisoning?

<p>Administer pralidoxime AFTER aging of the enzyme (B)</p> Signup and view all the answers

A patient is receiving physostigmine to treat an overdose of atropine. What potential adverse effect should the nurse monitor for most closely?

<p>Bronchospasm (D)</p> Signup and view all the answers

A patient with a known genetic deficiency in pseudocholinesterase requires anesthesia for an emergency surgical procedure. Which of the following agents should the anesthesiologist avoid?

<p>Succinylcholine (C)</p> Signup and view all the answers

A researcher is evaluating the effects of different cholinesterase inhibitors on cognitive function. Which drug would be most appropriate for this research, given its ability to cross the blood-brain barrier and improve cholinergic neurotransmission in the brain?

<p>Donepezil (B)</p> Signup and view all the answers

Which of the following is a common component of the treatment strategy in organophosphate toxicity?

<p>Gastric lavage (stomach wash) to remove the poison. (C)</p> Signup and view all the answers

Which of the following clinical scenarios would indicate the use of pyridostigmine?

<p>To improve muscle strength in myasthenia gravis (A)</p> Signup and view all the answers

Which of the following is the main feature of organophosphate poisoning that leads to respiratory failure?

<p>The excessive secretion and bronchospasm resulting from overstimulation of muscarinic receptors, inhibiting proper respiration (B)</p> Signup and view all the answers

Donepezil is prescribed to treat what?

<p>Alzheimer's Disease (A)</p> Signup and view all the answers

Which drug is used as an antidote in atropine poisoning?

<p>Physostigmine (A)</p> Signup and view all the answers

What is the mechanism of action of Pralidoxime?

<p>Reactivates cholinesterase (A)</p> Signup and view all the answers

Flashcards

Cholinesterase (ChE)

Enzymes that break down acetylcholine.

True Cholinesterase (ChE)

Found in ganglia, neuromuscular junctions and the CNS, it metabolizes acetylcholine.

Pseudo Cholinesterase (ChE)

Found in plasma and the liver, it metabolizes acetylcholine and succinylcholine.

Cholinesterase Inhibitors

Drugs that inhibit cholinesterase, increasing acetylcholine levels.

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Reversible Cholinesterase Inhibitors

Cholinesterase inhibitors that bind reversibly to the enzyme.

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Irreversible Cholinesterase Inhibitors

Cholinesterase inhibitors forming a stable bond, rendering the enzyme inactive.

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Examples of Reversible Cholinesterase Inhibitors

Examples include physostigmine, neostigmine, pyridostigmine, and donepezil.

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Examples of Irreversible Cholinesterase Inhibitors

An example is organophosphate compounds.

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Physostigmine

A natural plant alkaloid that is well-absorbed from the GIT and can cross the CNS.

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Neostigmine Nm receptors

It is a direct Nm receptor at NMJ.

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Uses of Physostigmine

Used to treat glaucoma and as an antidote to atropine poisoning.

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Neostigmine

Synthetic, poorly absorbed from the GIT, and cannot cross the CNS.

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Uses of Neostigmine

Used for myasthenia gravis, post-operative urine retention, and as an antidote to D-tubocurarine.

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Pyridostigmine

Longer duration of action than neostigmine and used for myasthenia gravis treatment.

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Edrophonium

Very short acting (5 min) and used for diagnosing myasthenia gravis.

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Donepezil, Rivastigmine

Crosses the blood-brain barrier, increases acetylcholine, and treats Alzheimer's disease.

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Organophosphorous Compounds

Drugs like echothiophate, insecticides like parathion & malathion, and nerve gases like sarin & soman.

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Characteristics of Organophosphorous Compounds

Highly lipid soluble, rapidly absorbed via all routes, and has rapid CNS penetration.

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Irreversible Inhibition of Cholinesterase

Leads to increased acetylcholine at synapses, causing severe muscarinic and nicotinic symptoms.

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Aging of ChE enzyme

Complete enzyme inhibition as the bond gets stronger.

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Manifestations of Organophosphate Poisoning

Includes diarrhea, urination, miosis, bradycardia, bronchospasm, lacrimation, salivation, and sweating.

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CNS Effects of Organophosphate Poisoning

Excitation of CNS, including hallucinations, convulsions, and coma.

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Cause of death in organophosphate poisoning

Respiratory failure due to blocked airway, paralyzed respiratory muscles, and inhibited respiratory center.

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Initial steps for treating organophosphate toxicity

Ensuring a patent airway, artificial respiration, stomach wash, and intravenous normal saline.

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Atropine for organophosphate toxicity

Antagonizes the peripheral and central muscarinic manifestations.

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Pralidoxime (Cholinesterase Re-activator)

Dephosphorylates cholinesterase, but is effective only before aging of the enzyme.

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Diazepam for organophosphate toxicity

Used to control convulsions, calm the patient.

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Diagnosis of myasthenia gravis

Edrophonium

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Drugs has the longest duration of AChE inhibition

Echothiophate

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Study Notes

Indirect Cholinergic Agonists

  • Indirect cholinergic agonists are a class of drugs.
  • These drugs can be classified.
  • They have a mechanism of action.
  • They have several uses.
  • Also relevant is organophosphorus poisoning.

Cholinesterase Types

  • True cholinesterase is found in ganglia, the neuromuscular junction (NMJ), and the central nervous system (CNS).
  • Pseudo cholinesterase is located in plasma and the liver.
  • True cholinesterase specifically metabolizes acetylcholine (ACh).
  • Pseudo cholinesterase metabolizes both ACh and other drugs like succinylcholine.
  • Deficiency in true cholinesterase is lethal and regenerates in 2-3 months.
  • Deficiency in pseudo cholinesterase isn't lethal, and regenerates in 2-3 weeks.

Indirect-Acting Parasympathomimetic Drugs

  • These drugs are also know as cholinesterase inhibitors.
  • Reversible inhibitors include physostigmine, neostigmine, pyridostigmine, and donepezil.
  • Irreversible inhibitors include organophosphate compounds.

Physostigmine

  • Physostigmine is a natural plant alkaloid, specifically a tertiary amine.
  • It is well-absorbed from the gastrointestinal tract (GIT) and is able to cross into the CNS.
  • It is a reversible cholinesterase inhibitor, which increases endogenous ACh, leading to muscarinic and nicotinic effects.
  • The uses for physostigmine are glaucoma, administered as eyedrops, and as an antidote to atropine poisoning.

Neostigmine

  • Neostigmine is a synthetic quaternary ammonium compound.
  • It is poorly absorbed from the GIT and unable to cross into the CNS.
  • It is a reversible cholinesterase inhibitor, increasing endogenous ACh and causing muscarinic and nicotinic effects.
  • Neostigmine has a direct and strong effect on Nm receptors at NMJ.
  • Uses for neostigmine include treating myasthenia gravis, post-operative urine retention, post-operative paralytic ileus, and as an antidote to D-tubocurarine.

Neostigmine Substitutes

  • Pyridostigmine has a longer duration of action compared to neostigmine.
  • Pyridostigmine is more selective on the NMJ than neostigmine.
  • Pyridostigmine is used for the treatment of myasthenia gravis.
  • Edrophonium is a very short-acting drug, lasting only 5 minutes.
  • Edrophonium is more selective on the NMJ than neostigmine.
  • Edrophonium is used in the diagnosis of myasthenia gravis.

Central Cholinesterase Inhibitors

  • Donepezil and Rivastigmine are central cholinesterase inhibitors.
  • They can cross the blood-brain barrier.
  • Increases acetylcholine at central cholinergic synapses.
  • Used to treat Alzheimer's disease by improving cognitive functions.

Organophosphorus Compounds

  • These compounds include drugs like echothiophate (used in eye drops for glaucoma), insecticides such as parathion and malathion, and nerve gases like sarin and soman.
  • They are highly lipid soluble.
  • They are rapidly absorbed through all routes.
  • They penetrate the CNS rapidly.
  • They cause irreversible inhibition of cholinesterase by phosphorylating the enzyme, leading to increased endogenous ACh at synapses and severe muscarinic and nicotinic symptoms.
  • Aging of the cholinesterase enzyme involves complete enzyme inhibition where the bond gets stronger within 12 hours.

Manifestations & Treatments of Organophosphate Poisoning

  • Symptoms of organophosphate poisoning include diarrhea, urination, miosis, bradycardia, bronchospasm, lacrimation, salivation, sweating, emesis, excitation of the CNS (hallucinations, convulsions, & coma), along with skeletal muscle twitches and paralysis.
  • The main cause of death in organophosphate poisoning is respiratory failure, due to blocked airways, paralyzed respiratory muscles, and inhibited respiratory control.
  • Treatment involves ensuring a patent airway and providing artificial respiration.
  • Administer a stomach wash (skin wash).
  • Administer intravenous normal saline to raise blood pressure.
  • Atropine antagonizes peripheral and central muscarinic manifestations and is the main treatment.
  • Pralidoxime, a cholinesterase re-activator which dephosphorylates the ChE, is effective only before aging of the enzyme, within 12 hours.
  • Diazepam is administered to control convulsions.

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