Lecture 19: Synaptic Vesicle Release and NMJ Pharmacology

List the steps of the synthesis and recycling of vesicles and vesicular content.

1. vesicle and peptide neurotransmitter precursors and enzymes are synthesized in the cell and are released from the Golgi

2. vesicles travel through the axon on MT tracks via fast axonal transport; peptide neurotransmitters are already in some vesicles

3. a nonpeptide neurotransmitter is synthesized in the nerve terminal and transported into a vesicle

Where are peptide NTs synthesized?

cell body

Where are peptide NTs loaded into and transported?

loaded into vesicles and transported down the axon

Where are non-peptide NTs synthesized?

the presynaptic terminal?

Where are non-peptide NTs loaded?

vesicles

What do dense-core secretory granules contain?

pre-synthesized peptides or peptide precursors when leaving the cell body

What is the diameter of dense-core secretory granules?

100-200 nm diameter (large)

How are dense-core secretory granules distributed in the axon terminal?

randomly

How do clear synaptic vesicles travel down the axon?

with no NT inside

What is the diameter of clear synaptic vesicles?

40-50 nm

What do clear synaptic vesicles take up?

non peptide NT synthesized locally in the nerve terminal

Where are clear synaptic vesicles docked?

at active zones

Where are presynaptic ACh vesicles formed?

in the cell body

Where are presynaptic ACh vesicles transporterd?

the presynaptic terminal via fast axonal transport

Where is ACh synthesized?

the presynaptic terminal

What is the enzyme needed for ACh synthesis?

ChAT

What are the two modes for loading ACh into vesicles?

• vacuolar-type H+-ATPase

• ACh-H exchanger

What does vacuolar-type H+-ATPase load into vesicles?

H+

How does the ACh-H exchanger move ACh into vesciles?

uses the proton electrochemical gradient

What does vacuolar-type H+ pump/ATPase catalyze?

the inward movement of H+ into vesicles

What happens after the vacuolar-type H+ pump catalyzes the inward movement of H+?

the resulting pH and voltage gradients energize NT uptake via neurotransmitter transporters

Neurotransmitter transporters are specific for different NTs. What is the vesicular GABA transporter? What is the vesicular glutamate transporter?

• GABA transporter: vGAT

• glutamate transporter: vGLUT

Define quantal size.

• the size (amplitude) of individual release events

• amount of NT packaged per vesicle

• number of postsynaptic receptors

Define quantal content.

• size (amplitude) of action potential-dependent release events

• number of vesicles released per action potential

• change in readily releasable pool size and/or vesicular release probability

True or False: The molecular model of the synaptic vesicle is showing synaptobrevin only.

true

What are SNARE proteins?

molecular machinery for membrane-membrane fusion

What are some SNARE proteins?

• synaptobrevin (vSNARE)

• syntaxin (tSNARE)

• SNAP-25 (tSNARE)

What is the function of synaptotgamin?

Ca2+ sensor for exocytosis

What is Rab3? What is its function?

a GTP-binding protein that regulates synaptic release via GTP binding/hydrolysis

List the steps for vesicle endocytosis and recycling.

1. Vesicle docks near membrane

2. SNARE complexes form (vSNARE and tSNAREs) to pull membranes together

3. Entering Ca2+ binds to synaptotgamin

4. Ca2+-bound synaptotgamin binds to SNARE complex and catalyzes membrane fusion

5. Synaptotgamin dissociates from SNARE complex as intracellular Ca2+ concentration levels fall

6. a-SNAP and NSF use ATP hydrolysis to disassemble SNAREs

7. vesicle membrane and synaptobrevin endocytosed and recycled

What type of disease is Myasethenia Gravis?

autoimmune disease

What is the cause of MG? What are the symptoms?

• cause: antibodies attack nAChRs in skeletal muscle NMJs

• symptoms: ptosis (droopy eyelid), difficulty speaking and swallowing, and extremity weakness

What does EAMG stand for?

experimental autoimmune myasethenia gravis

True or False: In animals with MG, there is a lack of AP in muscle despite normal AP in nerve.

true

Cholinergic NMJ synapses are targets for toxins and drugs. List some of these targets.

• neuronal Na+ channel (TTX and STX)

• Ca2+ channel (w-conotoxin)

• muscle Na+ channel (TTX, STX, and mu-conotoxin)

• AChR channel (curare and bungarotoxin)

• K+ channel (dendrotoxin)

• ACh release (tetanus and botulinum toxin)

• acetylcholinesterase (physostigmine and DFP)

What does the enzymatic action of AChE result in?

the immediate termination of ACh actions at the NMJ

True or False: AChE is either soluble or bound to the cell membranes and/or ECM.

true

What happens to the choline left over after ACh breakdown?

choline is taken back up into the presynaptic nerve terminal via Na+-coupled uptake

List the process of AChE inhibitors.

1. AChE inhibitors

2. ACh remains in synaptic cleft longer and nAChRs continue to be bound by ACh and activated

3. slower depletion of ACh by diffusion away from the junctional folds

4. increased EPP amplitude and prolonged duration

What is botulism?

weakness and paralysis of skeletal muscle

What does botulinum toxin cause?

the inhibition of cholinergic motor neuron endings

What is tetanus?

the general increase in muscle tension and rigidity

What does tetanus toxin do? What does this cause?

prevents synaptic transmission from spinal cord inhibitory neurons—causing increased stimulation of skeletal muscle contraction

What do both toxins do?

cleave SNARE proteins required for synaptic vesicle exocytosis