L 19 Mechanism of NT Release Flashcards
what is a NT?
chemical substance
synthesized in neurons
released at synapse following depolarization
binds to its respective receptor
elicits a response in the postsynaptic cell
what are the 3 main categories of NT in the nervous system?
- small molecule transmitters
- neuroactive peptide
- gaseous NTs
what are some small molecule NT
ACh glutamate GABA glycine doapine NE E serotonin histamine
what are some neuropeptides
orexins (hypocretins) - vasopressin + oxytocin
what are some gaseous NTs
Carbon monoxide
Nitric oxide
why don’t NO and CO fit all the definitions of NTs?
- not stored in vesicles
- don’t have calcium dependent release
- doesn’t interaction with postsynaptic receptors
when is NO generated?
whenever it is needed - NO synthase needed for NO synthesis- nNOS is found in neurons
where are the enzymes for small molecule NT synthesized?
cytosol
where are the enzymes for small molecule NT stored?
clear vesicles bound to the cytoskeleton and await release at the active zone
where are neuropeptides synthesized?
soma
where are the neuropeptides stored?
rough ER - enzymes + prepropeptides => pro peptides Golgi - packaged into large dense core vesicles Anterograde transport (kinesin) terminal - propeptides cleaved into vesicles - neuropeptides stored in viscle on cutoskeleton
what does filing the synaptic vesicles depend on
est. gradient of H+ ions across the vesicular membrane
vesicular membrane has an antiport exhanging system - ex. exchanging dopamine for H+ ions
what are the two populations of vesicles?
clear vesicles - small molecule NT (ACh, glutamate)
Large dense cored vesicles - peptides (5-HT, NA)
what does a rise in [Ca2+] in terminal induce?
indirect phosphorylation of synapsin => untethers vesicle from the cytoskeleton
what does a local rise in [Ca2+] in the active zone facilitate?
opening of the inserted fusion protein allowing NT to leave the vesicle and enter the synaptic cleft
what is the falling phase?
removal mech. restoring the [Ca2+] to its normal low value
what is clearance of Ca2+?
Ca2+ ATPase and Ca2+/Na+ exchange systems extrude Ca2+ against its concentration gradient
where is calcium stored?
in sER, mitochondria, and other intracellular calcium stores
what is the role of synapsin
tethering the vesicle to the cytoskeleton
what is the role of Rab proteins
move vesicles towards the active zones for exocytosis
what is the role of SNARE
docking the vesicle with the nerves terminal membrane
what does the vesicular membrane have
v-SNARE (synaptobrevin and synaptotagmin)
what does the nerve membrane have
t-SNARE (syntaxin and neurexin)
synaptobrevin (v-SNARE) binds to?
syntaxin (t-SNARE)
synaptotagmin (v-SNARE) binds to?
neurexin (t-SNARE)
what is the role of synaptophysin?
binds to membrane cholesterol and forms the fusion pore in the nerve terminal membrane allowing transmitters to be released!
what are the steps in exocytosis of NT
mobilization
trafficking
docking
fusion
what promotes fusion
synaptobrevin + synaptotagmin + calcium + “complexin” cofactor
tSNARE complex = SNAP 25 + syntaxin
what does synaptophysin regulate?
quantal size of transmitter release in the nerve terminal membrane!
what happens at low nerve stimulation
rise in [Ca2+] occurs near the active zones ==> exocytosis of small clear vesicles containing small molecule transmitter
what happens at high nerve stimulation?
rise in [Ca2+] spreads far ==> exocytosis of large dense core vesicle containing peptides
the same terminal can be used to release…
both transmitter types - peptides and small molecule transmitters
all transmitters (except cholinergic transmitters) are ___ taken up back into nerve terminals, some are even absorbed by glial cells
rapidly
how is the small clear vesicular membrane recovered?
slowly by endocytosis
what is endocytosis responsible for the uptake of?
material (also viruses) from the extracellular fluid
what is endocytosis of the small clear vesicular membrane mediated by?
clathrin - coats the vesicle
once the vesicles are internalized, the vesicles lose their clathrin coats and fuse with the endosomes which form new vesicles to be ?
refilled with NTs
how are dense core vesicles retrieved?
endocytosis – transported to soma by retrograde transport (dynein) for refilling with NTs
define quantum
a packet of transmitter located in one synaptic vesicle
define quantal size
the number of transmitter molecules in a quantum
quantal size of 7,000 molecules for synap. vesicles in motor n. ending
quantal size of 4,000 molecules for glutamate
define quantum content
number of quanta released by a synaptic bouton under the influence of one invading impulse
ACh is released from the motor nerve terminals in …
uniform packets = quanta- universal mechanism
each vesicle containing a small molecule transmitter stores how many molecules?
4000-10,000 molecules of NT
what is exocytosis governed by?
probability of release (p) and the number of vesicles available for release (n) = p x n
what happens to the value of p (probability of release ) when an increase in [Ca2+]?
increase
p (probability of release ) may be non-zero even in the absence of
stimulation of the nerve terminal
what is the standard way to lower p (probability of release ) at the NMJ
replace Ca2+ with Mg2+ - it blocks the voltage gated Ca2+ channels –> decrease EPP
when does EPP show quantal fluctuations?
when p has been reduced to a very low value
what are miniature end plate potentials (minEPP)
Vm at the end plate, in the absence of nerve stimulation, show spontaneous small transient depolarizations about 1mV in size - indicate that the motor nerve terminal is releasing ACh spontaneously
what are all EPPs
integral multiples of a unit size equal to that of a spontaneous minEPP
the release of NT is
quantal
what is the number of quanta released by a single impulse?
quantum constant
what is the mean quantum content formula
(mean size of EPP) / (mean size of minEPP)
what is the quantal hypothesis?
the presynaptic AP causes the release of the contents of an integral number of vesicles (1,2,3) and occasionally may fail to cause release of the contents of any vesicle
what does botulinum toxin act as
protease - an enzyme that breaks down proteins and peptides.
how does botulinum toxin work?
it binds to cholinergic nerve endings
enters and inactivated the docking mech. by breaking down synaptobrevin
prevents vesicular release of ACh irreversibly
what is botulinum toxin used to tx?
dystonias (irregular or troublesome clonic contraction of muscles)
cosmetic surgery to reduce facial wrinkles.
snake venom (beta-bungarotoxin) blocks what?
ACh release
how does snake venom (beta-bungarotoxin) block ACh release?
binds irreversibly to actin and possibly other cytoskeleton components in cholinergic nerve endings and blocks ACh release
= paralysis and can be fatal is subject is not ventilated
tetanus toxin blocks
glycine release
how does tetanus block glycine release?
tetanus toxin has a protease component
it enters PNC via a cut or wound and passes into CNS by retrograde axonal transport
it enters glycinergic interneurons which inhibit firing of motor neurons where it inactivated docking of vesicles by breaking down synaptobrevin
it stops vesicular release of glycine irreversibly
what are the symptoms of tetanus ?
powerful generalized muscle spasms
interference with respiration
what is the tx for tetanus
neuromuscular blocking agents combined with assisted ventilation
what does alpha-latrotoxin do?
depleted endings of cholinergic neurons
how does alpha-latrotoxin work
black widow spider - promotes the fusion of cholinergic vesicles by binding to the neurexin/synaptotagmin complex
it induces a massive release of vesicular ACh that motor nerve terminals become depleted of synaptic vesicles
what are the symptoms of alpha-latrotoxin poising?
muscle spasm followed by flaccid paralysis
ventilation may be necessary
what is the role of aminoglycoside antibiotics - neomycin and steptomycin
inhibit exocytosis of ACh at motor nerve terminals by blocking presynaptic calcium channels
in high concentrations - can block postsynaptic nAChR
which aminoglycoside antibiotic is more effective?
neomycin
how are the effects of aminoglycoside antibiotics reversed?
increase extracellular calcium
what does 4-aminopyridine do
K+ channel blocking drug prolongs the duration of the impulse and enhances Ca2+ entry into the motor nerve ending and raising the probabiltiy of ACh release and then the quantum content
how do neurotransmitters alter quantum release?
at certain axo-axonic synapses the presynaptic inhibitory ending releases a transmitters (GABA, seratonin, dopamine) which binds to receptors in the membrane of the second (postsynaptic) cells and reduces its transmitter release
such axo-axonic synapses are common on the terminals of afferent neurons in the spinal cord
what drugs alter quantum release?
aminoglycoside antibiotics
4-aminopyridine
neurotransmitters
what is hypocalcemia - what causes it and what is it?
hypoparathyroidism causes hypocalcemia - lower blood concentration of calcium ions
what are the symptoms of hypocalcemia?
tetany and paresthesias due to repetitive firing of AP in peripheral motor and sensory fibers
why does low [Ca2+] cause repetitive firing?
extracellular calcium is needed to neutralize the negative charges in the extracellular matrix so its in its absence, the outside becomes more negative which makes the membrane different from outside to inside will get close to threshold which will cause eventual unwanted depolarizations
motor nerve endings release about ___quanta of ACh in response to an invading impulse, but all other nerve endings usually release just ___ quanta
200
a few
