Vesicular neurotransmitter release Flashcards
what is NT release a highly specialised form of?
what are the 2 types of resolution?
what are the small packets called?
what is release triggered by?
what is the speed of the transduction of an electrical stimulus into vesicular release?
membrane fusion
spatial and temporal
quanta of NT
calcium-mediated fusion of vesicles
less than 1ms
what did Katz discover about the amount of NT in a vesicle?
it was in defined quanta
- e.g. 10, 20, or 30 molecules
but not 12, or 27
what do small synaptic vesicles contain?
what is a key feature of these transmitters?
describe the process of these SSVs
small molecule transmitters
e.g. dopamine, GABA
synthetic machinery resides in nerve terminal
- vesicles synthesised in cell soma
- trafficked up to nerve terminal
- filled with small molecule transmitter
what happens when SSVs reach the nerve terminal?
- vesicles fuse with membrane
- release quanta into synaptic cleft
- vesicular machinery is re-used
- vesicles are recycled and re-packaged with small molecules
what type of response are small molecule transmitters involved in?
fast acting systems
- fast excitation and inhibition
what do large dense-core vesicles (LDCVs) carry?
where is the synthetic machinery for these localised?
peptides
up to 90 amino acids
cell soma
what is different between SSVs and LDCVs?
LDCVs are loaded at the cell soma
LDCVs are not recycled
LDCVs = 250nm SSVs = 50nm
what is the postsynaptic density?
highly complex structure just inside post-synaptic membrane
contains 100s of proteins involved in post-synaptic signalling and the coordination of post-synaptic events
describe the process from action potential to ion channel opening on postsynaptic membrane
- action potential propagation in presynaptic neurone
- depolarisation opens Ca2+ channel
- Ca2+ entry into synaptic knob
- triggers release of NT by exocytosis
- binding of NT to postsynaptic receptor
- opening of specific ion channels
why are there differences in NT release rates at different synapses?
give an example of a fast and slow synapse
response mechanism to a Ca2+ trigger
no. of vesicles in a release-ready state
rate and efficiency of replenishment
fast = inner ear synapse
= 1000s vesicles/ms
slow = fast cerebellar connection
= 3 vesicles/ms
what are the 4 stages of NT release?
where are loaded SVs targeted too?
- docking
- priming
- fusion
- recycling
active zones
describe docking
describe priming
membrane of vesicles tightly associates with plasma membrane
(others vesicles are in a reserve pool)
creation of a competent readily releasable pool of vesicles
describe fusion
describe recycling
active fusion of vesicle membrane with plasma membrane to release vesicular contents when the local calcium reaches a threshold
recycling of vesicular membrane by endocytosis to form a new vesicle for transmitter filling
what are the 2 forms of exocytosis?
full exocytosis
- fusion with membrane to release all contents
kiss and run
- vesicle partly fuses and releases of some of the contents
- then reseals
what is synapsin?
a protein associated with the vesicle membrane
determines the release of vesicles from the reserve pool
-> anchors vesicles onto cytoskeleton at the reserve pool
what is synaptobrevin (VAMPs)?
critical in fusion events
what is synaptotagmin?
controls sensitivity of vesicles to local rise in ca2+
-> triggers fusion event
what is Rab3 and what’s its role?
a small GTPase
- regulated by GTP-binding and hydrolysis
GTP-bound Rab3 (active) binds Rabphilin and the plasma membrane protein RIM
GDP-bound Rab3 (inactive) dissociates from vesicle
what does fusion require?
what transport was this discovered to be essential for?
N-ethylmaleimide (NEM) sensitive factor (NSF)
together with SNAPs (soluble NSF attachment proteins)
ER-Golgi and intra-Golgi membrane transport
what are SNAREs?
what do they include?
soluble-NSF-attachment proteins receptors
syntaxin
SNAP-25
synaptobrevin/VAMP
describe the role of the fusion complex
VAMP attaches vesicle to fusion particle
SNAP25 + Syntaxin attach fusion particle to presynaptic membrane
what is the functional role of NSF and SNAPs?
regulation of SNARe disassembly
what do all membrane fusion secretory events involve?
formation of a helical coiled coil complex assembled from SNAREs
which SNAREs are responsible for synaptic vesicle fusion?
Syntaxin 1
= presynaptic plasma membrane (integral)
- tSNARE
SNAP25
= presynaptic plasma membrane (Anchored)
- tSNARE
Synaptobrevin 2
= vesicular membrane (integral)
- vSNARE
what do synaptic SNARES form?
a complex of 4 parallel helices
- Syntaxin and Synaptobrevin contribute 1 motif each
and SNAP25 2 motifs
how do clostridial toxins effect SNARE complexes?
tetanus and botulinum toxin:
act at Synaptobrevin
-> induces muscle paralysis
how does Munc-18 act?
binds syntaxin 1
-> catalyses the step wise zippering of the SNAREs
designed to bind 4 helix bundles
-> pulls complex together
what is the effect of having the 4 coiled coil complex?
gives rise to an inward force on the 2
membranes
-> pulls them together
= gives rise to a single fused membrane with a pore through the middle
what happens when the vesicles first dock at the membrane?
enter a primed state
-> puts them into a hemifuse position
(undergo a partial fusion with nerve terminal membrane and held in that state until they switch to full fusion)
how are vesicles held in a hemifused state?
Complexins clamps the SNARE complexes
- arrests fusion
until synaptotagmi 1 can displace complexing from SNAREs in the presence of Ca2+
describe the structure of Synaptotagmin 1
integral membrane protein on vesicles with 2 C-terminal C2 domains
calcium and phospholipid binding domains
how does synaptotagmin work?
put a Ca2+-sensitive inhibitor constraint on exocytosis
when Ca2+ enters
-> synaptotagmin knocks complexin off
