NMJ and Pharmacology

Question 1

How is ACh synthesized? How are people with Alzheimer’s affected? Where does it go after synthesis?

Answer 1

• Choline transporter: membrane channel that transports choline into the cell
• Choine Acetyltransferase (ChAT): enzyme that combines AcCoA and choline to form ACh (happens in cytoplastm)
• Patietns with Alzheimer disease have reducted production of ChAT - thus decreased choline AcH production
• ACh vesicular transporter: ATP dependent transporter that immediately shuttles ACh from cytoplasm into storage vesicles after synthesis

Question 2

How is ACh released from the presynaptic cells?

Answer 2

• 1. Voltage-gated CA2+ channels: open upon depolarization and allow CA2+ to enter cell
     (2) Elevated Ca2+ levels promote the fusion of the vesicular membrane with the cell membrane and exocytosis of ACh occurs
     (3) The vesicle protein synaptobrevin, a member of the vesicle associated membrane protein (VAMP) family, assembles with the plasma membrane proteins synaptosome-associated proteins (SNAPs) and syntaxin 1 to form a core complex that initiates or drives the vesicle-plasma membrane fusion process (collectively known as the SNARE protein complex)
     (4) The exocytosis process releases ACh into the synaptic cleft where it can bind to acetylcholine receptors

Question 3

What does Botox - botulinium toxin - do?

Answer 3

cleaves SNARE proteins, resulting in ACh vesicles not being able to fuse - no ACh in the NMJ

Question 4

How is ACh destroyed?

Answer 4

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

Question 5

nACHRs: postjunctional

Answer 5

Activated by ACh and nicotine (agonists)

Ligand-gated ion channel (Na+, K+) - which allow ions to pass through the channel pore when activated (ionotropic)

• made of four distinct subunits assembled as a pentamer
• subunit composition allows for selectivity and specificity of drugs used to inhibit nACHR’s

Antagonists:

• Atracurium, Vecuronium, d-tubocurarine, pancuronium
• there are two binding sites on each gate: two ACh molecules bind to the sites - gates open and positively charged ions pass through the channel

–> results in muscle cell depolarization and an AP initiated

Question 6

nAChR’s prejunctional activation?

Answer 6

• prejunctional receptors are activated, which helps to mobilize more ACh to be released in the subsequent AP.

Mobilization of additional ACh for subsequent release

ACh vesicles move toward the synaptic membrane

*mAChR activation results in ACh-mediated inhibition of further ACh release

Question 7

tetrodotoxin

Answer 7

• puffer fish toxin
• inhibits voltage–gated Na+ channnels within motor neuron, prevents axonal conduction
• people usually die because their diaphragm (skeletal muscle) stops functioning, can’t breathe properly

Question 8

how do local anesthetics work?

Answer 8

inhibit voltage-gated Na+ channels thus preventing axonal conduction - thus blocks pain from reaching the brain

Question 9

Batrachotoxin

Answer 9

• poison dart frog poison
• causes an increase in permeability of Na+ channels inducing a persisten depolarization

Question 10

Botulinum toxin

Answer 10

• cleaves components of the SNARE complex (VAMP and SNAP proteins)

Question 11

What are 3 neuromuscular blocking agents?

Answer 11

• used for causing muscle paralysis during anesthesia induction
  1. curare alkaloids: nondepolarizing competitive nACHR antagonists (paralyze muscle)
  2. Succinylcholine: depolarizing nAChR agonist (paralyze muscle)
  3. snake alpha toxins : competitive irreversible inhibiter of nACHR (not used for anesthesia….)

Question 12

Cholinesterase inhibitor

Answer 12

Bind to AChE and block its enzymatic activity

Increase the concentration of ACh at the NMJ

ex. Clinical uses include dementia associated with Alzheimer or Parkinson disease, myasthenia gravis, nerve gas and organophosphate pesticide exposure, reversal of neuromuscular blockade during anesthesia

Serin gase/nerve ga also does this - and increases amount of ACh throughout the body, also activating muscarinic receptors –> leads to paralysis

Question 13

Dantrolene

Answer 13

Inhibits ryanodine receptors in the sarcoplamic reticulum and blocks release of Ca2+

Clinical uses include malignant hyperthermia, spasticity associated with upper motor neuron disorders