Pharmacology of the Neuromuscular Junction Flashcards
What are the steps involved in NMJ neurotransmission?
- axonal conduction
- junctional transmission (cholinergic)
a) synthesis of acetylcholine (ACh)
b) storage of ACh
c) release of ACh
d) destruction of ACh - ACh signaling
- muscle contraction
What drug/toxin inhibits step 1 of junctional transmission, ACh synthesis?
What drug/toxin inhibits step 2 of junctional transmission, ACh storage?
What drug/toxin inhibits step 3 of junctional transmission, ACh release?
- ACh synthesis: hemicholinium
- ACh storage: vesamicol
- ACh release: botulinium toxin
How is ACh synthesized?
- choline transporter: membrane channel that transports choline into the cell
- choline acetyltransferase (ChAT): enzyme that combines acetyl coenzyme A (AcCoA) and choline to form ACh
(there are pharmaceutical agents that inhibit ChAT, but they are of little use because the uptake of choline is the rate-limiting step in ACh biosynthesis)
*patients w/ Alzheimer’s dz have reduced cerebral production of ChAT
How is ACh stored in vesicles?
- ACh vesicular transporter: ATP dependent transporter that immediately shuttles ACh into storage vesicles after ACh synthesis
- 1K-50K molecules of ACh per vesicle
- motor nerve terminal may contain over 300K vesicles
How is ACh released into the synaptic cleft?
- voltage-gated Ca2+ channels: open upon depolarization and allow Ca2+ to enter the cell
- Ca2+ promotes vesicle membrane fusion
- VAMP and SNAPs: vesicular and plasma membrane proteins that initiate vesicle-plasma membrane fusion and release of ACh
- roughly 125 vesicles rupture per AP
What is the molecular process of vesicle-membrane fusion in terms of ACh NT?
- vesicles w/ synaptotagmin (calcium sensor on vesicle membrane that triggers vesicle fusion and exocytosis) and synaptobrevin (aka VAMP), a v-SNARE, move to the nerve terminal membrane, which contains syntaxin and SNAP-25, both t-SNAREs
- n-sec 1 dissociates from syntaxin, allowing syntaxin and SNAP-25 to form a complex. The distal end of synaptobrevin begins to wind around the syntaxin/SNAP-25 complex, forming a ternary complex (SNARE complex)
- the 3 SNAREs continue to form a tight bundle of alpha-helices, drawing the vesicle and presynaptic membranes into close apposition
- entry of Ca2+ and its binding to synaptotagmin triggers fusion of vesicle and membrane
- alpha-SNAP and ATPase NSF bind to ternary SNARE complex and use the energy of ATP hydrolysis to disassemble the SNAREs
- w/ endocytosis of the vesicle, the synaptobrevin is effectively recycled. The syntaxin and SNAP-25 are now free for an additional cycle of vesicle fusion
How is ACh destroyed/recycled?
- acetylcholinesterase (AChE): enzyme that cleaves ACh into choline and acetate
- choline is recycled back into the motorneuron via the choline transporter
- endocytosis occurs at the nerve terminal to replenish the number of available vesicles
How does ACh signaling occur via different receptors?
- ACh activates two subsets of receptors: nicotinic (n) and muscarinic (m)
nAChRs:
a) activated by ACh and nicotine
b) ligand-gated ion channel
c) pre- and postjunctional
d) NMJ: Na+ increase causes muscle AP
mAChRs:
a) activated by ACh and muscarine
b) G-protein coupled receptor
c) pre- and postjunctional
d) NOT located on skeletal NMJ
nAChR:
- tissue location:
- function:
- agonists:
nAChR:
- tissue location: skeletal muscle
- function: contraction
- agonists: acetylcholine, nicotine
mAChR:
- tissue location:
- function:
- agonists:
mAChR:
- tissue location: smooth muscle
- function: contraction
- agonists: acetylcholine, muscarine
mAChR:
- tissue location: cardiac muscle (SA node, AV node, atrium, ventricle)
- function: decrease HR, conduction velocity, and contraction (slight)
- agonists: acetylcholine, muscarine
What is the general mechanism of action of mAChRs?
- mAChRs are GPCRs w/ 5 different substypes: M2 and M3 receptors predominate smooth muscle, M2 receptors predominate cardiac muscle
- metabotropic receptors that do not form an ion channel pore
- agonists binding activated intracellular G-proteins that trigger activation of intracellular signaling pathways
What is the general mechanism of action of nAChRs?
- nAChRs are ligand-gated ion channels that allow ions to pass through the channel pore when activated (ionotropic)
- fasted synaptic events in nervous system (miliseconds)
- ions are selected based on the charge of amino acids lining pore of channel
- negatively charged amino acids line pore of channels that pass positively charged ions and vice versa
- nAChRs specifically: aspartic acid and glutamic acid line pore (negative charge) so channel can be selective to Na+, Ca2+, and K+
skeletal musle
- main synaptic location:
- membrane response:
- molecular response:
- agonists:
- antagonists:
skeletal musle
- main synaptic location: skeletal neuromuscular junction (postjunctional)
- membrane response: excitatory, contraction
- molecular response: increased cation permeability (Na+, K+)
- agonists: ACh, nicotine, succinylcholine
- antagonists: d-tubocurarine, Atracurium, Vecuronium, Pancuronium
peripheral neuronal
- main synaptic location:
- membrane response:
- molecular response:
- agonists:
- antagonists:
peripheral neuronal
- main synaptic location: autonomic ganglia; adrenal medulla
- membrane response: excitatory, depolarization
- molecular response: increased cation permeability (Na+, K+)
- agonists: ACh, nicotine
- antagonists: mecamylamine
central neuronal
- main synaptic location:
- membrane response:
- molecular response:
- agonists:
- antagonists:
central neuronal
- main synaptic location: CNS
- membrane response: excitatory, pre-junctional control of ACh release
- molecular response: increased cation permeability (Na+, K+, Ca2+)
- agonists: ACh, nicotine
- antagonists: mecamylamine
What is the complete process of transmitting ACh across a NMJ?
- an AP in the motor neuron is propagated to the terminal buttom
- presence of an AP triggers the opening of voltage-gated Ca2+ channels
- Ca2+ triggers the release of ACh from vesicles
- ACh diffuses across the synaptic cleft and activates nAChRs
- nAChRs open leading to a relatively large influx of Na+ compared to a smaller efflux of K+
- local current flows between the depolarized end plate and adjacent membrane
- local flow opens voltage-gated Na+ channels
- influex of Na+ initiates an AP, which propagates throughout the muscular fiber
- ACh is destroyed by AChE, terminating the muscle cell’s response
Agents that affect nerve action potential:
- Tetrodotoxin
- local anesthetics
Agents that affect vesicular acetylcholine release:
- Botulinum toxin
- Tetanus toxin
Agents that affect depolarization:
(neuromuscular blocking drugs)
- Curare alkoids (d-tubocurarine)
- succinylcholine
Agents that inhibit acetylcholinesterase:
- acetylcholinesterase inhibitors
Agents that affect muscle action potential:
- Tetrodotoxin
Agents that affect muscle contraction:
- Dantrolene
(figure is a study tool and summary of agents that act on NMJ)
- puffer fish poison (fugu, globefish, blowfish); not used clinically
- MOA: inhibition of voltage-gated Na+ channels blocks axonal conduction
- sx: weakness, dizziness, paresthesias of face/extremities, loss of reflexes, hypotension, generalized paralysis, death can occur due to respiratory failure and hypotension
Tetrodotoxin
(affects nerve action potential and muscle action potential)
- MOA: inhibition of voltage-gated Na+ channels inhibit axonal conduction
- utilized for pain control during a variety of clinical procedures
- ex: Lidocaine, Bupivacaine, Procaine
local anesthetics
(affects nerve AP, Procaine also affects vesicular ACh release)
- toxin released by Clostridium botulinum, a heterogenous group of gram-positive, rod-shaped, spore-forming, obligate anaerobic bacteria; found on vegetables, fruits, seafood; exists in soil and marine sediment worldwide
- MOA: cleaves components of the core SNARE complex involved in exocytosis, preventing release of ACh
- sx: acute onset of bilateral cranial neuropathies a/w symmetric descending weakness, no sensory deficits w/ exception of blurred vision
- foodborn sx: nausea, vomiting, abd pain, diarrhea, dry mouth
- clinical uses: temporary improvement of appearance of lines/wrinkles on face, prophylaxis of chronic migraine headaches
botulinum toxin
(affects vesicular ACh release)
- nervous system disorder characterized by muscle spasms that are caused by toxin-producing anaerobe, Clostridium tetani, which is found in soil
- MOA: block fusion of synaptic vesicles by targeting synaptobrevin; after binding to presynaptic membrane of NMJ this toxin is internalized and transported retroaxonally to the spinal cord; spastic paralysis is caused by toxin’s actions on the spinal inhibitory interneurons, blocking release of inhibitory NT’s that normally serve to relax contracts muscle by inhibiting excitatory motor neurons
- sx: spastic paralysis, trismus (lockjaw), autonomic overactivity (restlessness, sweating, tachycardia), stiff neck, board-like rigid abdomen, opisthotonus, dysphagia
Tetanus toxin
(affects vesicular ACh release)
- neuromuscular blocking drug
- MOA: competes w/ ACh for the nAChR on the motor end plate, decreasing the size of the EPP (nondepolarizing competitive nAChR antagonist)
- inhibition of ACh binding to nAChR leads to flaccid paralysis of skeletal muscle
- used during anesthesia to relax skeletal muscle
- paralysis reversed by increasing ACh in NMJ (AChE inhibitor)
Curare alkaloids (d-tubocurarine)
(affect depolarization, neuromuscular blocking agent)
- neuromuscular blocking drug
- MOA: depolarizing neuromuscular blocker that binds to skeletal muscle nAChRs and initially causes depolarization (acts as an agonist; muscle fasiculations); continued depolarization leads to receptor blockade and paralysis
- used as induction agent for anesthesia
- paralysis reversed by termination of effects (i.e. time)
succinylcholine
(affects depolarization, neuromuscular blocking agent)
- bind to AChE and block its enzymatic activity
- increase the conc of ACh at the NMJ
- clinical uses: dementia a/w Alzheimer or Parkinson dz, myasthenia gravis, nerve gas and organophosphate pesticide exposure, and reversal of neuromuscular blockade during anesthesia
cholinesterase inhibitors
(inhibits AChE)
- inhibits ryanodine receptors in the sarcoplasmic reticulum and blocks release of Ca2+
- clinical uses: malignant hyperthermia, spasticity a/w upper motor neuron disorders
Dantrolene
(agent that affects muscle contraction)
