Lecture 4.1: Neurotransmitter Release Flashcards
APs generated after summation of inputs travel down ________ to the _____ and trigger ______
APs generated after summation of inputs travel down the axon to presynaptic sites and trigger neurotransmitter release
The neurotransmitter particles are stored in _______ located ____
The NT particles are stored in small vesicles located at the presynaptic site
Arrival of the AP to the presynapse triggers the opening of ________ (then explain the mechanism that follows)
Arrival of the AP to the pre-synapse triggers the opening of voltage gated calcium channels adjacent to the vescicles, calcium entry induces vesicle fusion with the plasma membrane producing a “fusion pore” and then NTs are released
What kind of proteins are involved in vesicle fusion?
SNARE proteins
TopHat Question:
What is the function of the AP in the presynaptic terminal?
Opening of Calcium channels
Explain how the brain couples synaptic activity with vesicle recycling
The brain times this really well, it couples NT release with vesicle recycling… it adapts the vesicle recycling that is already going on within cells
Graph shown below on how brain couples synaptic activity with vesicle recycling
Membrane fusion occurs in all cells….
What is the constituitive process in all cells?
What is different about the fusion of vesicles to the plasma membrane during NT release?
Membrane fusion occurs in all cells:
- Consituitive process in all cells
- Synthesis and trafficking of proteins through the ER, Golgi, cell division, etc
- Fusion of vesicles to plasma membrane during NT release is different because it must be tightly controlled, so it only occurs when triggered, usually by an AP
Explain the overview of the vesicle fusion that happens of NTs after an AP:
- Some of these vesicles, the “readily releasable pool” are docked at the ________ ( in the _____ zone), via _____ proteins
- Empty vesicles are _____, during this step the SNARE protein _________
- Some of these vesicles, the ‘readily releasable pool’ are docked in the plasma membrane (in the ACTIVE ZONE), via SNARE proteins (snarepin)
- Empty vesicles are endocytosed to be recycled and refilled, during this step the SNAREpin disengages and docks a new synaptic vesicle
“Different from constituitive membrane fusion, fusion of vesicles to the plasma membrane during NT release must be tightly controlled so it only occurs when triggered, usually by an AP”
Explain the two proteins that help this process:
- complexin
- synaptotagmin
Complexin acts as the brakes: it supressess consituitive fusion
Synaptotagmin senses calcium in the immediate vicinity of the vesicle and when Ca reaches a threshod, it triggers fusion
Depolarization activates Voltage Gated calcium channels to trigger NT release:
- Depolarization to ____ activates calcium channels, these channels _____ inactivate rapidly, therefore they ________
- Depolarization can result from an ____ or from _____
Depolarization to approx 20 mV activates calcium channels; these channels do NOT inactivate rapidly, therefore they remain open for as long as the membrane is sufficiently depolarized
- Depolarization can result from an AP or from graded depolarization in non-spiking neurons
Depolarization activates Voltage gated calcium channels to trigger NT release:
Long lasting APs allow more _____ and more ____
The ACTIVE Zone contains what?
Long lasting APs allow more calcium entry and more NT release
The ACTIVE zone contains:
- calcium channels
- a specialized stretch of plasma membrane and cytoskeletal components that keep the vesicles tethered close to the PM
Explain what synaptic delay is
Synaptic delay is the time elapsed between the arrival of the presynaptic AP and the post-synaptic response. This delay includes the time needed for the depolarization to allow calcium entry through calcium channels (most of the delay), diffusion of the NT across the cleft and activation of the post synaptic receptor.
Patients with Lambert Eaton syndrome make antibodies directed at ______
What part of the CNS gets affected?
Diagnosis method?
Treatment?
Lambert-Eaton Syndrome patients make antibodies directed against a subset of voltage-gated calcium channels critical to triggering the release of NTs.
Spontaneous release of NTs do occur. Since the antibodies do not cross the blood brain barrier, only peripheral terminals are affected.
Most affected are terminals releasing ACh.. causing motor weakness.
Diagnosis is an increamental res[pse to repeated stimulation (stress test)
Treatment: diaminopyridine (a K channel blocker)
Synaptic vesicles are held close to the PM by complexes of SNARE proteins:
Define v-Snares and t-SNAREs
Which ones does NT release depend on
Where are they anchored?
SNARE proteins make up the snare pin:
v-SNARES are located in synaptic vesicle membrane
t-SNARES are located in target membrane (pm)
NT release depends on two t-SNARESs and one v-SNARE:
SNAP-25 and syntaxin (t-SNARES)
Synaptovrevin/VAMP (v-SNARE)
SNAP-25 and syntaxin are anchored at PM and form a bundle/ snare pin
VAMP is anchored at the vesicular membrane
Name the v-SNAREs and t-SNARES that help with NT release
v-SNARE: synaptobrevin or “VAMP”
t-SNARE: SNAP-25 and syntaxin
Explain the mechanism by which the SNARE protein complex assists in NT release
DRAW it out to help yourself
- In trans-SNARE configuration, the SNARE pin holds the vesicle near the PM but physically seperated
- When calcium triggers fusion, the SNARE bundle zips up into cis-SNARE configuration, resulting in vesicle fusion with the PM, through the fusion pore and subsequent NT diffusion into the cleft.
TETANUS:
Where does tetanus come from?
How does the toxin work?
Treatment?
Tetanus:
Tetanus toxin, from Clostridium tetanii cleaves SNARE proteins, synaptobrevin.
Transported retrogradedly, specifically attacks interneurons, preventing NT release and releasing inhibitory control…. leading to muscle contraction, lockjaw, etc
Causes severe pain, broken bones
Vaccine is available (need a booster every ten years)
Explain the differences between Botox, Tetanus and Latrotoxin (specific spider venom)
Botox and Tetanus both block NT release
Latrotoxin induces NT release, bypassing calcium entry
Botox causes flaccid paralysis and autonomic symptoms, Tetanus causes lethal spastic paralysis, and Latrotoxin causes elevated BP…GI issues, SOB and sweating
Botox affects peripheral cholinergic terminal
Tetanus affects CNS inhibitrory interneurons of the spinal cord
Latrotoxin affects presynapses
Explain “Kiss and Run” vs full collapse
The vesicle fusion described earlier in this chapter is the full collapse
Kiss and run is a more transient type of release that occurs in synapses that fire at high frequencies (first postulated by Bruno Ceccarelli)
In kiss and run, the fusion pore forms, releases a fraction of the NT and quickly closes and the vesicle is retrived, there is never full vesicle collapse
Explain the difference between the RRP, the recycling pool and the reserve pool of vesicles in the synapse
What are the percentages of each?
- About 2% of the total vesicle population is docked and readily releasable (RRP)
- Recycled vesicles constitute the recycling pool (10-20% of total population) and are interspersed with the whole reserve pool (80-90%)
- Vesicles are released via clathirin coated mechanisms and refilled with NT after fusion
- When there is high synaptic acticity, the recycling pool is depleted and vesicles from the reserve pool are recruited
Small vesicles contain small molecule NTs
What do Dense-core vesicles (DVCs) contain?
What is the requirement for those to get released?
- Small vesicles contain small molecule NTs
- Dense-core vesicles (DCV) contain peptides, ATP and small NT molecules; these vesicles are not so readily releasable
- The large vesicles require bulk calcium concentrations - not oo high- to reach DCVs, this occurs mainly during high frequency stimulation (trains of APs)
Single APs favor ______ release whereas trains of APs recruit ______ release
Single APs favor small vesicle release, whereas trains of AP also recruit DCV release
