ANS 26- Drugs acting at neuromuscular junctions - modulators of acetylcholine

3 strategies to modulate NMJ transmission,Three layer to receptor to muscle,how the strategies interact

Why do we block neuromuscular junctions?

• surgical paralysis - relax muscle for intubation etc

• critical care - facilitates mechanical ventilation

• treat disease- myasthenia gravis ( boost ACH)

• antidotes - reverse the inhibition or organophosphate poisoning

Why is NMJ a good drug target?

• It has a single well-characterised receptor

• high safety margin

• distinct molecular targets

• effects are immediate and measurable

note - the drugs must overcome 3x the safety margin for silent transmission,which is why there are multiple strategies

What are the three strategies to modulate NMJ?

1. Competitive antagonism- drug binds to ACh receptor, does not activate, prevents ACh access - channel not open

2. Depolarising agonism - drug binds to receptor and activates it, persistent depolarisation = Na channel failure

3. Enzyme inhibition- drugs inhibit ache,ach accumulates in cleft,uses to reverse strategy 1

Describe strategy 1 - the first layer - the receptor

• the target is the nicotinic acetylcholine receptor

• two ach binding sites at alphas - both must be occupied for it to open

• two quaternary ammoniums - span the ach simultaneously,binding to the same sites as ach = competitive

• no conformational change - channel stays closed

Describe strategy 1 - the second layer - the cell biology

With competitive antagonism :

• the binding sites are occupied,ach can’t bind

• reduced number of functional receptors - smaller EPP as 70-80% receptors blocked

• EPP isn’t above threshold = no AP

why does the 70-80% matter ?

• The huge safety margin means- onset takes 1-2 min, block has to pass the safety margin, partial reversal can lead to residual block

• Fade is the key part of the competitive block

• tetanic stimulation depletes ach stores ,less ach + same antagonist == block deepens ,twitch fades less visibly

Strategy 1 - layer 3 - cell to muscle

• no muscle ap so no muscle contraction sequence, etc

Layers integrated - strategy one

Describe strategy 2 ( depolarising) - layer 1 - an agonist that paralyses

• the drug is two ACh molecules joined together

• double agonist - opens channel, wrong shape for AChE

• not destroyed, left in the cleft - persists at the receptor

at the receptor:

• binds to both alpha sites, triggers a conformational change

• channel opens na enters, and end plate depolarises ——continues until desensitised

Strategy 2 - cell to muscles

What happens in strategy 2 when the cell biology fails?

Describe strategy 3 - Enzyme inhibition

• doesn't touch the receptor, the drug inhibits ACHE production in the cleft

• ACh accumulates - more agonist available

used for :

• reverse strategy 1 ( more Ach outcompetes the antagonist )

• Treat myasthenia gravis ( boots weak transmission )

• Reverse organophosphate poisoning (regenerate AChE)

Example - drug ,picture

Neostigmine = competitive AChE inhibitor:
• Same kinetic signature as any competitive enzyme inhibitor:
• Km ↑ (apparent affinity for ACh decreases)
• Vmax unchanged (raise [ACh] enough and you reach the same maximum)
• AChE blocked → ACh accumulates in cleft

What is the muscarinic problem?

The problem:
• AChE is at every cholinergic synapse
• Neostigmine doesn't know which synapse it's at
• Boosting ACh at NMJ → also boosting ACh at muscarinic
• GPCRs everywhere

What is the solution to this muscarinic problem ?

Co-administer a muscarinic antagonist:
• Blocks M₁/M₂/M₃ GPCRs throughout the body
• Competitive antagonism at muscarinic receptors
• Doesn't affect nicotinic receptors at the NMJ
• Clean separation: nicotinic helped (good), muscarinic blocked (good)

What are the three strategy interactions/links ?

Strategy interactions (the reversal logic):
Strategy 3 reverses Strategy 1
• Both competitive — boost agonist, win competition
Strategy 3 worsens
• Strategy 2 Phase I
• More agonist = deeper depolarisation block

Take home messages