Drugs acting on ACh release and breakdown

Question 1

How is acetylcholine synthesised?

Answer 1

Acetate and coenzyme A combine to form acetyl-CoA. This then combines with choline to form acetylcholine. The first step is catalysed by acetylcoA synthetase and the second step is catalysed by choline acetyltransferase.

Question 2

What is ACh broken down into?

Answer 2

Choline and acetate, catalysed by acetylcholinesterase.

Question 3

What is found in the plasma that can remove ACh?

Answer 3

Pseudocholinesterases that circulate in the plasma. They have lower activity than acetylcholinesterase but will mop up any “spill over”.

Question 4

What are the key ways in which ACh transmission can be inhibited?

Answer 4

Inhibition of synthesis, inhibition of transport and use of false transmitters, blocking neuronal depolarisation, blocking neuronal Ca2+ influx, inhibition of vesicular fusion.

Question 5

How can ACh synthesis be inhibited?

Answer 5

Hemicholinium - it is a competitive inhibitor of choline uptake.

Question 6

How can ACh transport be inhibited?

Answer 6

Vesamicol blocks ACh transport into synaptic vesicles.

Question 7

How can false transmitters be used to inhibit ACh transmission?

Answer 7

The false transmitter is transported and acetylated and then stored and released. It has no depolarising actions. An example is triethylcholine which is acetylated into acetyltriethylcholine (ATC).

Question 8

How is ACH released?

Answer 8

There is an action potential conducted along the motor nerve via Na+ influx. There is membrane depolarisation and an influx of calcium at the nerve terminal through voltage-gated calcium channels which causes vesicle fusion.

Question 9

What receptors does ACh bind to?

Answer 9

Nicotinic receptors.

Question 10

What can be used to block neuronal depolarisation?

Answer 10

Tetrodotoxin - it blocks voltage gated Na+ channels to block action potentials and decrease ACh release at NMJ.

Question 11

What does tetrodotoxin cause?

Answer 11

Muscular paralysis, respiratory failure and effects autonomic transmission.

Question 12

How can neuronal calcium influx be blocked?

Answer 12

Mg2+, streptomycin, neomycin inhibit neuronal Ca2+ entry.

Question 13

How can vesicular fusion be inhibited?

Answer 13

Neurotoxins - botulinum toxin and beta-bungarotoxin.

Question 14

What are the features of botulinum toxin?

Answer 14

It is produced by clostridium botulinum. Spores multiply in preserved food and can cause botulism (a serious form of food poisoning). It causes progressive parasympathetic and motor paralysis resulting in dry mouth, blurred vision and difficulty in swallowing and respiratory paralysis. It is irreversible once bound and has high mortality.

Question 15

What are the pharmacological features of botulinum toxins?

Answer 15

They are proteases that are transported into nerve terminals and cleave SNARE proteins that are required for vesicle fusion and release. They are persistent and cause block for months.

Question 16

What are the two types of botulinum toxin and how do they differ?

Answer 16

A and B. A cleaves synaptosome associated protein SNAP and B cleaves vesicle associated membrane protein VAMP. VAMP is also a target for tetanus toxin. These are both SNARE proteins.

Question 17

What can botulinum toxin be used for?

Answer 17

Botox, blepharospasm (persistent eyelid spasm), cervial dystonia (neck spasm), strabismus (crossed eyes), cerebral palsy, excessive sweating and migraines.

Question 18

What are the two types of bungarotoxin?

Answer 18

Alpha and beta.

Question 19

What effects do the two types of bungarotoxin have?

Answer 19

Alpha - irreversible binding to the nACh receptors at the NMJ and inhibits ACh binding and hence skeletal muscle response. Beta acts on presynaptic motor nerve terminals to block the release of ACh and has phospholipase PLA2 activity that may cause the destruction of the nerve terminal.

Question 20

What is an agent that increases ACh transmission?

Answer 20

Delta-atracotoxin - it prolongs neuronal Na+ opening and prolongs action potentials. It stimulates spontaneous firing of neuron.

Question 21

What is foaming at the mouth caused by?

Answer 21

Cholinergic activation.

Question 22

How can ACh levels be increased?

Answer 22

Acetylcholinesterase inhibitors.

Question 23

What is acetylcholinesterase made up of?

Answer 23

Two distinct regions - an anionic site that binds the choline moiety of ACh and a catalytic site the breaksdown ACh to acetate and choline. ACetylserine is rapdily hydrolysed to return AchE to the active conformation.

Question 24

What are the different types of cholinesterase inhibitors?

Answer 24

Short acting anti-cholinesterases, medium duration anti-cholinesterases and irreversible anti-cholinesterases.

Question 25

What is an example of a short acting anticholinesterase and how does it act?

Answer 25

Edrophonium. It forms a reversible ionic bond with AChE at the anionic site and has very brief action. It is too short acting for clinical use.

Question 26

What is an example of a medium duration anticholinesterase and how does it work?

Answer 26

Neostigmine, physostigmine. They are competitive inhibitors at anionic and esteric sites and form carbamyl esters at the esteric site. The carbamylate-enzyme complex is much slower to hydrolyse.

Question 27

What is an example of an irreversible anticholinesterase?

Answer 27

Dyflos, Sarin, organophosphate compounds.

Question 28

What have organophosphate compounds be used for?

Answer 28

They are used as agricultural pesticides and are very volatile and are absorbed through the skin. They have also been used as war gases and pesticides.

Question 29

What is Novichuk?

Answer 29

A weapon - it is a dual agent, two non-toxic compounds combine to produce a reactive species.

Question 30

What is the mode of action of irreversible AChE inhibitors?

Answer 30

They covalently bind to the serine-OH group at the catalytic site. There is no spontaneous hydrolysis of the bound complex and recovery of enzyme activity is dependent on enzyme resynthesis. This takes weeks.

Question 31

How can cholinesterase be reactivated?

Answer 31

Pralidoxime. It brings an oxime group into close proximity with the phosphorylated enzyme and attracts phosphate groups from serine-OH and strips the bound organophosphate from the enzyme.

Question 32

What type of recovery does pralidoxime cause?

Answer 32

Recovery to around halfway between the inhibition and initial.

Question 33

What can anticholinesterases be used for?

Answer 33

Overcome non-depolarising block at the end of anaesthesia, treatment of myasthenia gravis, glaucoma treatment, overcome muscarinic block in poisoning, Alzheimer’s disease.

Question 34

What is myasthenia gravis?

Answer 34

Grave muscle weakness - contraction of skeletal muscle cannot be maintained and there is dropping of eyelids, limb muscles, diaphragm and neck muscles.

Question 35

How does the pharmacology of patients of myasthetnia gravis differ to normal?

Answer 35

Autoantibodies block nicotinic receptors and there is a prevented reaction between ACh and nAChR.

Question 36

How does treatment for myasthenia gravis work?

Answer 36

There is reduced action potentials, but after treatment with neostigmine there is increased action potentials and contraction.

Question 37

What are some of the treatments for myasthenia gravis?

Answer 37

Neostigmine and pyridostigmine, glucocorticoids, immunosuppresives, thymectomy. They improve muscle contraction but have some adverse effects such as muscle twitching and autonomic side effects (salivation, GI disturbances, bradycardia).

Question 38

What are the problems with neostigmine and pyridostigmine?

Answer 38

They improve muscle contraction but have some adverse effects such as muscle twitching and autonomic side effects (salivation, GI disturbances, bradycardia).

Question 39

What is 4-aminopyridine?

Answer 39

Fampridine - it blocks neuronal K+ channels and increases ACh release the NMJ.

Question 40

What can Fampridine be used for?

Answer 40

Multiple sclerosis to improve motor function. However it increases sensation of pain and causes increased muscle spasms. It is expensive so is not used too much.