Pharmacology of the neuromuscular junction

Question 1

What is the neuromuscular junction receptor?

Answer 1

Nicotinic ACh receptor (nAChR), ligand-gated ion channel.
3 B2 subunits and 2 a4 subunits, 2 binding sites.
If 2 molecules of Ach bind have opening of channel.

Question 2

What happens to acetylcholine in the neuromuscular junction?

Answer 2

ACh is synthesised and loaded into vesicles in the presynaptic terminal.
Exocytosis when action potential arrives, ACh is relesaed into synaptic cleft, and acts on postsynaptic nAChR, or degraded by acetylcholinesterase to form choline and acetate.

Question 3

What happens to choline in the neuromuscular junction?

Answer 3

Choline enters back into the presynaptic terminal through a choline carrier.
Choline Acetyltransferase (CAT) converts it to ACh.
ACh can leak out through the choline carrier, or be packaged into vesicles.

Question 4

What is the presynaptic nAChR?

Answer 4

The presynaptic nAChR is stimulated by ACh, by autofeedback.
It is usually negative feedback to prevent more ACh being formed.
In the neuromuscular junction, it is positive, so more ACh is synthesised and released.

Question 5

What are potential targets for drugs by presynaptic modulation?

Answer 5

Synthesis of Acetylcholine through CAT from acetyl CoA and Choline, but no blockers for clinical use.
Block the carrier that transports ACh into vesicle, using Vesamicol.
Choline transporter, blocked by hemicholinium, not used clinically.
Exocytosis of vesicles, blocked by toxins.

Question 6

What is botulinum toxin?

Answer 6

Botox.
The 1st subunit recognises and binds to the presynaptic membrane.
The 2nd subunit has peptidase activity and goes inside the neurone to cleave exocytosis proteins.

Question 7

What is the use of botulinum toxin?

Answer 7

Blocks cholinergic synapses, which causes muscle paralysis (e.g. ones for expanding chest walls) and can cause death.
Can be a wrinkle-remover if a local injection.
Local injection in neck can be used to reduce some headaches, by removing tension in the neck.

Question 8

What is the treatment for botulinum poisoning?

Answer 8

4-aminopyridine blocks voltage gated K+ channels.
Pre-synaptic mechanism leads to increased ACh release.
This decreases K+ efflux during the repolarisation phase of action potential, which leads to prolonged depolarisation of an action potential.

Question 9

What is botulinum poisoning treatment with Ca2+.

Answer 9

Presynaptically, neurotransmitter release is related to VGCaC.
So increased neurotransmitter release increases Ca2+ influx ,which triggers exocytosis and release of neurotransmitter (ACh).

Question 10

What is immunological treatment for botulinum poisoning?

Answer 10

Antitoxin - an antibody against the botulinum toxin.
Binds to first subunit
Prevents second subunit going inside the neurone.

Question 11

What drugs act post-synaptically?

Answer 11

Add on to general anaesthesia.
Act at the nAChR on the post-synaptic terminal.
Can be:
Non-depolarising blockers - antagonists.
Depolarising blockers - agonists.

Question 12

What is tubocurarine?

Answer 12

A non-depolarising blocker, not used anymore.
It has 2 binding sites so it can bind to nAChR at both ACh binding sites, and block ACh binding.

Question 13

What are non-depolarising blockers?

Answer 13

Pancuronium, Vecuronium.
Block nAChR, decreases end plate potential, as less ACh can bind.
There is less stimulation, so the depolarisation is not enough to open voltage gated channels.
This causes paralysis.

Question 14

What are the uses of non-depolarising blockers?

Answer 14

Allows a lower amount of general anaesthetic to be used to make the procedure safer.
Recovery requires liver metabolism of the drugs, or direct renal excretion, so slow recovery.

Question 15

How do depolarising blockers work?

Answer 15

Suxamethonium is 2 ACh molecules joined together, can bind to both binding sites on nAChRs.
Causes endplate depolarisation, and causes action potentials and muscle contraction.
But it causes uncoordinated, fine contractions (fasciculations).

Question 16

How does Suxamethonium cause paralysis?

Answer 16

Suxamethonium is not broken down by acetylcholinesterase in the synaptic cleft.
This causes prolonged depolarisations and paralysis because of inactivation of VGNaC.
Phase 1 block.

Question 17

What are the uses of suxamethonium?

Answer 17

Short period of paralysis
Intubation - 10 minutes of paralysis in throat
Plasma cholinesterase breaks down cholinesterase, and Suxamethonium.
Rapid recovery as it is broken down by enzymes.

Question 18

What are the side effects of suxamethonium?

Answer 18

Bradycardia - activation of ACh muscarinic receptors in heart, slows heart rate.
Increased K+ release via nAChRs.
Nicotinic channel allows K+ efflux, but too much nicotinic activation causes too much K+ efflux.
The extracellular concentration inreases beyong physiological limits.

Question 19

How is suxamethonium bad for burns?

Answer 19

Denervated muscle, causes a big increase in nicotinic receptors post synapse.
Suxamethonium can stimulate the receptors, so more K+ comes out.
But there is an increase in K+ in plasma - hyperkalemia, causes cardiac arrest, as action potential canot be triggered due to high extracellular K+.

Question 20

What are intraocular pressure affects of suxamethonium?

Answer 20

Little contractions squeexes the eyeballs, which increases intraocular pressure in the eye.

Question 21

What is phase 2 block?

Answer 21

Due to inactivation of receptor.
Prolonged paralysis is produced.

Question 22

What are cholinesterase inhibitors?

Answer 22

e.g. neostigmine
Acetylcholinesterase inhibitors to prevent breakdown of ACh once released.
Also inhibit plasma cholinesterase
Enhances synaptic function - ACh lasts longer in synapse.

Question 23

What else do cholinesterase inhibitors do?

Answer 23

Also affects other synapses (not NMJ) where Ach is transmitter - autonomic ganglia and postganglionic parasympathetic nerves.

Question 24

How do cholinesterase inhibitors differ?

Answer 24

Drug binds weakly - ionic interaction, blocks it for few minutes e.g. edrophonium
Neostigmine forms covalent bonds, strong, blocks enzyme for hours.

Question 25

What are organophosphates?

Answer 25

Irreversible phosphorylation of enzyme by organophosphates.
Used as a chemical weapon, or pesticides.
Reactivated by pralidoxime if given early.

Question 26

What are the uses of cholinesterase inhibitors?

Answer 26

Reverse the effects of non-depolarising NMJ blockers at the end of an operation, reverses paralysis.
It blocks acetylcholinesterase so ACh remains in the synapse longer, and can outcompete the non-depolarising drug for the receptor.

Question 27

What is the effect of cholinesterase inhibitors with suxamethonium?

Answer 27

Makes the effects (paralysis) of depolarising blockers worse.
ACh acquires Suxamethonium like action
and is not broken down by acetylcholinesterase.

Question 28

What is myasthenia gravis?

Answer 28

Muscle weakness due to failure of NMJ transmission.
Autoantibodies against nAChRs.
Can cause death.

Question 29

How can drugs be used against myasthenia gravis?

Answer 29

Increase levels of ACh to overcome the lack of nicotinic receptors.
Strengthens synaptic communication.

Question 30

How is myasthenia gravis managed?

Answer 30

Edrophonium is short duration, can be used as a diagnostic test of drug efficacy.
Neostigmine has medium duration - muscarinic side effects.
Pyridostigmine - better oral absorption, long duration, less powerful.