Cholinoceptor Antagonists Flashcards
Where are nicotinic receptors found?
In ALL autonomic ganglia
At neuromuscular junctions (NMJs)
Where are muscarinic receptors found?
At parasympathetic effector organs and on sweat glands
What are the few clinically useful nicotinic receptor antagonists called and how do they block the receptor?
Ganglion Blockers These block the ion channel itself instead of the receptor, thus preventing the ions from moving through the pore
Give two examples of ganglion blocking drugs.
Hexamethonium
Trimethaphan
What is hexamethonium?
It is a ganglion blocker that was the first anti-hypertensive It has a generalised action and had loads of side-effects
What is trimethaphan and when is it used?
The only ganglion-blocking drug that is still in clinical use.
Highly polar and cannot cross BBB. (Does not cause NMJ block either).
It is short acting but very potent and used when a controlled hypotension is needed in surgery.
What does ‘use-dependent block’ mean?
The drugs work most effectively when the ion channels are open. This means that the more agonist is present at the receptor, the opportunity the antagonist has to block the channel, thus the more useful and effective the drugs can be (E.G. Noiceceptors are frequently opened)
What determines the effect of ganglion blockade in a tissue?
It depends on which limb of the autonomic nervous system predominates in the particular tissue (at the time e.g. at rest)
Which tissues are sympathetic dominated at rest?
Vasculature
Kidneys
What is the overall effect of ganglion blockade in terms of loss of sympathetic dominance?
Hypotension. The sympathetic-mediated vasoconstriction is taken away and the ability of the kidneys to increase renin secretion and increase sodium and water reabsorption is also taken away
Which tissues are parasympathetic dominated at rest?
Lungs – causes bronchoconstriction
Eyes – maintains partial pupillary constriction
Bladder, ureters and GI tract
Exocrine functions
What would the effect of ganglion blockage be on these parasympathetic dominated tissues?
Bronchodilation Pupil dilation (blurred vision)
Bladder dysfunction
Loss of GI motility and secretions
Decrease in exocrine secretion
How do receptor blockade antagonists have their effect?
These are irreversible – they bind covalently and prevent the ion channels from opening
In what types of chemicals are nicotinic receptor blockade antagonists found?
Toxins and venoms
Give an example of a nicotinic receptor blockade antagonist.
Alpha-bungarotoxin (common krait snake venom)
What are the targets of muscarinic receptor antagonists?
Parasympathetic effector organs and sweat glands
What effect do muscarinic receptor antagonists have on the CNS?
The parasympathetic nervous system is important in the CNS in terms of attention, memory and certain sleep pathways. At low doses atropine can cause mild restlessness At low doses hyoscine can be a good sedative At high doses, both drugs can cause CNS agitation
Give four examples of muscarinic receptor antagonists.
Atropine
Hyoscine
Tropicamide
Ipratropium Bromide
What is tropicamide used for?
It is used to dilate the pupil to observe the retina (it is used to examine the eye)
What is an important use of muscarinic receptor antagonists with regards to surgery? Why is it useful in this circumstance?
Anaesthetic premedication It causes dilation of the airways so it is easier to intubate the patient
It reduces secretions thus reducing the risk of aspiration
It also knocks out the effect of the parasympathetic nervous system in decreasing heart rate and contractility (because general anaesthetics will decrease heart rate and contractility anyway)
What can hyoscine be used to treat? Explain how.
Motion Sickness. Muscarinic receptors are important in relaying information from the labyrinth in the inner ear to the vomiting centres. Muscarinic receptor antagonists can reduce the flow of information from the labyrinth to the brain thus reducing the nausea.
What degenerative disorder of the central nervous system can be treated by muscarinic receptor antagonists? Explain how.
Parkinson’s Disease. In Parkinson’s disease, many of the nigro-striatal dopamine neurones are lost (these are important in the fine control of movement) Musarinic receptors have a negative effect on this dopamine signalling so by blocking the muscarinic receptors (knocking out the M4 receptors) you can remove this inhibitory effect and allow the remaining dopaminergic neurones to fire at the maximum rate.
Explain the use of muscarinic antagonists in treating asthma and COPD.
Ipratropium Bromide is used to treat asthma and COPD as it removes the parasympathetic mediated bronchoconstriction.
Note: It is highly polar and therefore cannot diffuse out, ensuring a localised effect
Explain the role of muscarinic antagonists in treating irritable bowel syndrome.
Muscarinic antagonists will reduce smooth muscle contraction, gut motility and gut secretions thus relieving the symptoms of IBS.
State some general unwanted side-effects of muscarinic antagonists.
Hot as hell (decreased sweating affects thermoregulation)
Dry as bone (due to reduced exocrine secretions)
Blind as a bat (due to effects on the accommodation ability of the ciliary muscle – cycloplegia)
Mad as a hatter (high doses will cause CNS agitation, restlessness, confusion etc.)
How do you treat muscarinic receptor antagonist poisoning (e.g. atropine poisoning)?
Give an anticholinesterase e.g. physostigmine
Describe how botulinum toxin causes paralysis.
It binds to the SNARE complex and prevents the fusion of the vesicles, containing acetylcholine, with the presynaptic membrane thus preventing the release of acetylcholine from the nerve terminal. This leads to muscle paralysis
State the overall effects of ganglion blocking drugs on a subject at rest.
Hypotension Pupil dilation Bronchodilation Bladder dysfunction Decreased GI tone Decreased GI secretions
What are the effects of Atropine and Hyoscine on the CNS?
Atropine - Little effect at normal dose, Overdose Restlessness/Agitation
Hyoscine - Normal dose Sedation, Toxic dose - CNS depression or paradoxical excitation (pain associated) Maybe higher selectivity for M1 receptor and more lipophilic
What other disorders can centrally acting anti-muscarinics treat?
Dystonia and tremor
