9/21 Pharm of NMJ

Question 1

Peripheral nervous system organization

Question 2

Steps involved in NMJ neurotransmission

1) Axonal conduction
2) Junctional transmission* (cholinergic) 1) Synthesis of acetylcholine (ACh, the primary NT in skeletal muscle) 2) Storage of ACh 3) Release of ACh 4) Destruction of ACh
3) ACh signaling
4) Muscle contraction

Question 3

Junctional transmission

the Ach systhesis is combination of which two?

BoTox impedes which step in the junctional transmission?

**Almost all steps in this process can be inhibited by pharmacological agents, but not agents are used clinically

Answer 3

Acetyl-CoA + choline

Release of ACh

Question 4

ACh synthesis

Choline transporter: membrane channel that transports choline into the cell

Choline acetyltransferase (ChAT): enzyme that combine acetyl coenzyme A (AcCoA) and choline to form ACh

*Patients with Alzheimer disease (AD) have reduced cerebral production of ChAT

Question 5

ACh storage

ACh vesicular transporter: ATP dependent transporter that immediately shuttles ACh into storage vesicles after ACh synthesis

1K-50K molecules of ACh per synaptic vesicle;

A motor nerve terminal may contain over 300K vesicles

Question 6

ACh release

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 action potential

BoTox impedes the release of ACh

Question 7

ACh destruction

Acetylcholinesterase (AChE): enzyme that cleaves ACh into choline and acetate

Choline is recycled back into the motor neuron via the choline transporter

Endocytosis occurs at the nerve terminal to replenish the number of available vesicles

Question 8

ACh signaling

ACh activates two subsets of receptors?

Answer 8

nicotinic (n) and muscarinic (m)

nAChRs •Activated by ACh and nicotine •Ligand-gated ion channel (Na+) •Pre- and postjunctional •NMJ: Na+ increase causes muscle action potential

mAChRs •Activated by ACh and muscarine •G-protein coupled receptor •Pre- and postjunctional •NOT located at skeletal NMJ

Question 9

AChRs: types and muscle distribution

Question 10

mAChRs are GPCRs

• Five different subtypes in mammals (M1-M5) •Activation leads to a series of intracellular events triggered by second messengers (metabotropic) • Cellular effects measured in seconds

Question 11

nAChRs are ligand-gated (Na+) ion channels

Ligand-gated ion channels (LGICs) allow ions to pass through the channel pore when activated (ionotropic)

Fastest synaptic events in the nervous system (miliseconds)

Question 12

LGIC selectivity of ions

• Ions are selected based on the charge of the amino acids lining the pore of the channel
• Negatively charged amino acids line the pore of channels that pass positively charged ions and vice versa nAChRs
• Aspartic acid and glutamic acid line pore (negative charge)
• Channel selective to Na+, Ca2+, and K+

Question 13

Characteristics of subtypes of nAChRs

Question 14

summary of NMJ

Question 15

Agents that affect the nerve AP

Tetrodotoxin

• Puffer fish poison (fugu, globefish, blowfish); not used clinically
• MOA: inhibition of voltage-gated Na+ channels blocks axonal conduction
• Symptoms include •Weakness •Dizziness •Paresthesias of the face and extremities •Loss of reflexes •Hypotension •Generalized paralysis •Death can occur due to respiratory failure and hypotension

Answer 15

Local anesthetics

• MOA: inhibition of voltage-gated Na+ channels inhibition axonal conduction
• Utilized for pain control during a variety of clinical procedures
• Lidocaine, bupivacaine, procaine

Question 16

Agents that affect vesicular ACh release

Botulinum toxin

• Botulism is caused by Clostridium botulinum, a heterogeneous group of gram-positive, rodshaped, 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 the release of ACh
• Botulism is classically described as the acute onset of bilateral cranial neuropathies associated with symmetric descending weakness
• No sensory deficits with the exception of blurred vision
• Foodborne botulism symptoms often include nausea, vomiting, abdominal pain, diarrhea, and dry mouth
• Clinical uses include temporary improvement in the appearance of lines/wrinkles of the face, prophylaxis of chronic migraine headache

Question 17

Agents that affect vesicular ACh release

Tetanus toxin

• Tetanus is a nervous system disorder characterized by muscle spasms that is caused by the toxin-producing anaerobe Clostridium tetani, which is found in the soil
• MOA: block fusion of synaptic vesicles by targeting synaptobrevin • After binding to the presynaptic membrane of the NMJ, tetanus toxin is internalized and transported retroaxonally to the spinal cord
• Spastic paralysis is caused by the toxin’s actions on the spinal inhibitory interneurons, blocking release of inhibitory neurotransmitters that normally serve to relax contracted muscle by inhibiting excitatory motor neurons • Generalized tetanus typically presents as spastic paralysis with symptoms that include trismus (lockjaw), autonomic overactivity (restlessness, sweating, tachycardia), stiff neck, board-like rigid abdomen, opisthotonus, dysphagia

Question 18

Agents that affect depolarization

Neuromuscular blocking drugs

• Both agonists and antagonists of the nAChR can prevent synaptic transmission
• Agonists activate the receptor to signal as a direct result of binding to it
• Antagonists bind to receptors but do not activate generation of a signal

Question 19

Curare alkaloids – d-tubocurarine is the prototype

• MOA: competes with ACh for the nAChR on the motor end plate, decreasing the size of the EPP (nondepolarizing competitive nAChR antagonist)
• Inhibition of ACh binding to the nAChR leads to flaccid paralysis of skeletal muscle • Used during anesthesia to relax skeletal muscle •Paralysis reversed by increasing ACh in the NMJ (AChE inhibitor)

Answer 19

Succinylcholine

• MOA: depolarizing neuromuscular blocker that binds to skeletal muscle nAChRs and initially causes depolarization (acts as an agonist); continued depolarization leads to receptor blockade and paralysis • Used as an induction agent for anesthesia •Paralysis reversed by termination of succinylcholine’s effects (i.e., time)

Question 20

Depolarizing vs. nondepolarizing

Question 21

Agents that inhibit AChE

Cholinesterase inhibitors

• Bind to AChE and block its enzymatic activity •Increase the concentration of ACh at the NMJ
• Clinical uses include dementia associated with Alzheimer or Parkinson disease, myasthenia gravis, nerve gas and organophosphate pesticide exposure, reversal of neuromuscular blockade during anesthesia

Question 22

Agents that affect muscle contraction

Dantrolene

• Inhibits ryanodine receptors in the sarcoplamic reticulum and blocks release of Ca2+
• Clinical uses include malignant hyperthermia, spasticity associated with upper motor neuron disorders

Question 23