Neurotransmitters 1 Flashcards
Fast, reliable, and bidirectional mode of communication throughout the CNS
Electrical synapses/gap junctions
Electrical synapses/gap junctions allow what type of molecules to pass from one cell into another? What type of passage?
Ions and small molecules (<1000 Da); Diffusion
What proteins form the electrical synapse/gap junction? Be specific in the makeup.
- Connexins, ~20 of them
- 6 connexins form 1 connexon
- 2 connexons align to form a channel
What are electrical synapses important in? List 4.
- Embryonic stem cells
- Retina
- Auditory system
- Cortex
What regulates an electrical synapse? List 4.
- Cytosolic pH
- Cytosolic calcium
- Phosphorylation
- Voltage
This is caused by mutations in one connexin
The most common form of inherited deafness in Caucasian populations (DFNB1)
What do electrical synapses allow?
Allow groups of similar neurons to be synchronized e.g. hormone-secreting neurons in hypothalamus - get burst of hormone secretion; neocortical interneurons; cells of adrenal medulla
Explain the membrane potentials in pre and post-synaptic neurons
- Potential in downstream neuron won’t go as high
- Signal will be degraded
- Signal is less intense, more spread out
- Not faithfully transmitter. Quick, but can’t keep doing this forever
Do electrical synapses have a synaptic delay?
Yes, ~0,1 msec
What is the predominant mode of signaling?
Chemical synapses
Which type of synapse (electrical or chemical) is faster?
Electrical synapses. (Chemical are not quite as fast)
2 key features of a chemical synapse
- Synaptic vesicles
2. Receptors on post-synaptic ending
What is the major different in contact of pre and post synaptic membranes in chemical and electrical synapses?
Electrical synapse-connected by desmosomes, no synaptic cleft, touching each other
Chemical synapse-no direct contact with each other, 2-4x bigger membrane space, contain synaptic cleft ~20 nm
In vesicular release for major neurotransmitters, an action potential propagates down an axon and does what to the presynaptic terminal?
Depolarizes it
What is the consequence of depolarization of the presynaptic terminal in vesicular release?
Opens voltage gated Ca2+ channels
What is the difference in intracellular and extracellular concentration of Ca2+ ?
[Ca2+] outside = mM (10^-3)
[Ca2+] inside = uM (10^-6)
much smaller inside
What initiates fusion of synaptic vesicles with plasm membrane in vesicular release?
Influx of Ca2+. After depolarization, calcium voltage gated channels open and calcium ions enter down their electrochemical gradient. (Equilibrium potential for Ca = ~100 mV)
After fusion of synaptic vesicles with plasma membrane, vesicle content is released into this very small region
Synaptic cleft
What type of receptors on post-synaptic cells do released transmitters act on?
Ligand gated ion channels or receptors that use G proteins (GPCRs)
True or false: Transmitters can only act on receptors on the post-synaptic cell
FALSE. They can also act on receptors on the pre-synaptic terminal
What inactivates transmitter? List 4.
Na+ dependent reuptake, degradation and/or diffusion, or glial metabolism
Vesicular membrane must be retrieved. What two events facilitate this?
Exocytosis followed by endocytosis
Pre-synaptic vesicles contain more than one kind of vesicle. Release from these types has a different Ca2+ dependence and occurs at a different place
Large dense core vesicles (LDCVs)
Calcium is a crucial player of vesicle release. What are two of its sources?
- Usually comes across the synaptic plasma membrane to start release
- Can also be liberated from intracellular space
3 benefits of chemical synapses
- Amplification of signal
- Integration of inputs
- Makes use of different receptors
How do different receptors create a variety of post-synaptic effects?
Ligand gated ion channel-response is RAPID
Other receptors link to signaling pathways that alter morphology, receptor localization, gene transcription- response is SLOWER
Major mode of release
Vesicular release
Most prevalent vesicle, lined up at synapse
Small clear synaptic vesicles (SSVs)
These vesicles are less numerous and not as localized
Large dense core vesicles (LDCVs)
What type of chemical mediators are released by diffusion (non vesicular release)
Lipid mediators like anandamide/endocannabinoids, neurosteroids, and gasses like NO
What type of chemical mediators are released by transport?
Hydrophilic molecules (via carriers or pores)
There are over a hundred different neurotransmitters known. They are split into what two groups?
Smal molecules and peptides
List 5 biogenic amines that fall under small molecules group of neurotransmitters
- Dopamine
- Epinephrine
- Norepinephrine
- Serotonin
- Histamine
List 4 amino acids that fall udner small molecules group of neurotransmitters
- Glycine
- Glutamate
- Aspartate
- GABA
What store small molecule transmitters?
Small clear synaptic vesicles (SSVs)
What store peptides?
Large dense core vesicles (LDCVs)
Which neurotransmitter is present in both types of vesicles?
ATP
Which type of neurotransmitter group is most abundant?
Peptides, far outnumber small molecule transmitters
List and describe the three main features major neurotransmitters share
- Localization (substance must be present at presynaptic site, usually made by that neuron)
- Release (must be released in response to presynaptic depolarization, release dependent on extracellular Ca2+)
- Receptor (on postsynaptic cell, mimicry, given exogenously, get same affect with same amount of substance)
List the 3 approaches for inactivation and which transmitters employ them
- Enzymatic degradation- ACh, peptides
- Reuptake- Dopa, norepi, epi, serotonin, GABA, glycine, glutamate
- Diffusion- Important for all
List the three important catecholamines
Dopamine, epinephrine, norepinephrine
Why use vesicles? (3)
- Concentration of transmitter- makes it high enough to affect receptor following release into synaptic cleft
- Separates transmitter pool from metabolic pool and from degradative enzymes- Glu, Gly, GABA, ACh, catchecholamines
- Essential part of biosynthetic pathway for some transmitters (peptides, norepi and epi)
What is the importance of having vesicles recycle multiple times?
If synaptic vesicles simply fused with presynaptic membrane, the terminal would enlarge and supply of vesicles would be depleted.
Where are synaptic vesicles proteins synthesized?
ER and Golgi, cannot be replenished quickly
The recycling process of synaptic vesicles is fast. ~1 min. List the steps (5)
- Budding from endosome - exocytosis
- Docking - exocytosis
- Priming
- Fusion of calcium (1msec)
- Budding with endoome- endocytosis (10-20 sec)
Vesicles nearby, but not at the terminal
Reserve pool
Some reserve pool vesicles are even further away form terminal, often tethered to actin cytoskeleton by this molecule
Synapsin (on surface)
This type of vesicle is siting right next to plasma membrane at active zone.
Docked vesicles
This type of vesicle is not ready to be released quickly
Docked vesicles
An ATP-dependent, multi-step biochemical change has occurred so that this type of vesicle can response to an increase in intracellular Ca2+
Primed vesicle
Form the “readily releasable pool” of vesicles, quickly fusing with the plasma membrane
Primed vesicle
Occurs when calcium concentration is elevated by opening of calcium channels, energetically very unfavorable
Fusion (of membranes)
Studies on mutations affecting membrane trafficking in yeast identified these common set of proteins
SNARE complex
What forms the SNARE complex? [# of helices]
- VAMP/synaptobrevin (vesicular or V-SNAREs) [1]
- Syntaxin [1] and SNAP-25 [2] (target or T-SNAREs on presynaptic membrane)
Before docking, the trans-SNARE complex is not assembled. What proteins organize the trans-SNARE complexes?
Sec/Munc Proteins - act as break to control
This protein acts as a clamp, controlling the “zipping” together of the SNARE complex helices
Complexins (Ca enters and binds synaptotagmin, causes release of complexin, zippering is completed)
The zipping together of the SNARE complex provides enough energy to force the bilayer fusion of what?
The synaptic vesicle and plasma membrane lipid bilayers
True or false: The SNARE complex is Ca2+ sensitive
FALSE. It is not Ca2+ sensitive
Ca2+ binding protein and sensor; triggers Ca2+ dependent SV release
Synaptotagmin
What binds calcium channels directly?
Syntaxin - is at the site of greatest Ca2+ concentration during action potnetial
Ca2+ influx is through what type of channels?
Voltage gated channels (triggers release in less than a msec)
What does Ca2+ binding to synaptotagmin allow?
Final zippering of helices
These proteases disrupt the process of vesicle fusion by cleaving VAMP/synaptobrevin, syntaxin, or SNAP-25
Clostridial toxins/Botulinum toxins
Prevents release of inhibitory transmitters from interneurons by cleaving synaptobrevin, causes over excitation of skeletal muscle, tetanic contractions
Tetanus toxin
Prevents ACh release at neuromuscular junction, causes flaccid paralysis
Botulinum toxin B
After release of neurotransmitter, the SNARE complex needs to be taken apart. What set of proteins disassemble it?
NSF (N-ethylmaleimide sensitive factor) and SNAPs (soluble NSF accessory proteins, alpha, beta, and gamma)
Unraveling the cis-SNARE complex is ATP- independent/dependent
ATP-DEPENDENT. NSF uses 3-6 ATPs to disrupt each SNARE complex.
The average neuron has 10^6-7 vesicles. How many are at any given terminal?
50 vesicles
This synaptic vesicle protein generates electrochemical gradient
Proton pump or V-ATPase
These are examples of vesicular transmitter transporters (3)
VGLUT, VMAT, AChT
Acetylcholine (ACh) is the neurotransmitter at these receptors throughout the autonomic nervous system and brain
- Nicotinic AChR (ligand-gated ion channels)
- Skeletal neuromuscular junction
- Muscarinic AChR (7 transmembrane domain, second-messenger mediated)
The only enzyme unique to ACh synthesis; soluble, cytoplasmic enzyme; not rate limiting
Choline Acetyltransferase (ChAT)
What limits ACh synthesis?
Choline availability
Neurons do not make choline. Choline comes from what two sources?
Hydrolysis of ACh or phosphatidylcholine
What is rate limiting step in ACh synthesis?
Choline uptake at plasma membrane
Cholinergic neurons have a high affinity for choline via what type of uptake system?
Na+ dependent uptake system (CHT1)
Acetyl CoA combines with Choline in the first step of ACh synthesis. What is source of acetyl CoA?
Comes from pyruvate generated by glucose metabolism (used in cytosol by ChAT
This moves acetylcholine form cytosol into synaptic vesicle
Vesicular ACh Transporter (VAChT)
Charge of ACh
positive charge
Transport of ACh is driven by a _____ set up by the ______
Proton gradient, V-ATPase proton pump
V-ATPase proton pump uses energy from ATP to
acidify synaptic vesicles
Uphill/downhill transport of ACh is driven by uphill/downhill flux of protons
Uphill of ACh, downhill of Protons (1-2 H+ leave granule as 1 ACh+ enters)
How many molecules of ACh does each synaptic vessicle store?
6000 molecules
What is also stored in the same vesicles and released with ACh?
ATP
This exteremly fast enzyme is critical for inactivation of ACh
Acetylcholinesterase (AChE)
Sources of ACHE (3)
- Neurons
- Target cells (muscle)
- Glia
Choline generated by AChE is retrieved by
Plasma membrane choline transporter (CHT1) = exclusively cholinergic
CHT1 is stored with VAChT in synaptic vesicles; very little on cell surface until what event?
Exocytosis of vesicles containing ACh
____ gradient established by _____ drives choline retrieval
Na+ gradient; Na+/K+ ATPase
CHT1 resembles what type of transporters?
Na+ dependent glucose transporters, NOT other neurotransmitter transporters
These types of inhibitors are key ingredients in many pesticides (DTT) and nerve gases (e.g. sarin)
Acetylcholinesterase inhibitors (inhibit ACh inactivation)
What is a common result of many snake venoms?
AChR blockade
The major excitatory neurotransmitter
Glutamate
What percentage of neurons in CNS use glutamate as a neurotransmitter?
90%
Glutamate in synaptic vesicles is a small/large percentage of total Glu
Small: 1-20%
Asymmetrical contacts often on dendritic spines
Excitatory synapses
Glutamate is often used to make (4)
proteins, glutathione, GABA, glutamine
Glue does/does not pass the blood-brain barrier
Does not. needs to be made locally
Carbon backbone of brain glutamate comes from ____, which is converted into ______
Blood glucose, converted into alpha-ketoglutarate
To be used asa neurotransmitter, Glu must be concentrated in
Synaptic vesicles
Glutamate concentrations in cytosol vs. synaptic vesicle
Much higher in synaptic vesicle:
Cytosolic [Glu] = low mM
SV [Glu] = 60-250 mM
This transports Glu into SV
VGLUT (vesicular glutamate transporter). Glutamatergic neurons have VGLUT1 or VGLUT2.
Two important characteristics of VGLUT
- H+/glutamate antiporter
2. NOT Na+ dependent
Two factors that drive entry of Glu into SV
High concentration of H+ and positive charge in SC
VGLUTs are activated by _____ and inhibited by ______
Activated by Cl-, inhibited by ketone bodies (links Glu neurotransmission to metabolic state)
Inactivation of Glu is by this process
Reuptake into surrounding glia and pre and post synaptic neurons
Volume of synaptic clef is about 100 x larger/smaller than volume of a single SV
larger
Glu is removed from cleft by this family of transporters
EAAT or EATT 1-5 (excitatory amino aid transmitter transporters)
Certain EATTs are expressed in _____ and others in ____
Glial cells and nerve terminals (EATTs differ in structure from catecholamine transporters)
Concentration of glial EAATs surrounding glutamatergic synapse is very low/high
high- allows for rapid binding of Glu
Uptake of Glu is driven by?
Na+ gradient
Explain how EAATs are Glu symporters
3 Na+ and 1 H+ enter with Glutamate; 1 K+ exits cell; costs 1 ATP for each Glu taken up
Glial cells use this ATP-depdendent cytosolic enzyme to convert Glu into Gln and export it
Glutamine synthetase (NOT found in neurons)
Glutamine release from glial cell (astrocyte) is taken up by neuronal transporter and is driven by
Na+ gradient and membrane potential
This enzyme converts Glutamine that is taken up by pre-synaptic terminal to Glutamate
Glutaminase
This cycle accounts for almost half of the glutamate turnover
Glutamate-glutamine cycle
Disruption of this enzyme in astrocyte rapidly impairs glutamatergic neurotransmission
Glutamine synthetase
Each subunit (monomer) of EAAT functions independently/dependently
independently
Neurons generally use more than one/only one transmitter
MORE THAN ONE.
True or false: the transmitter used by a given neuron can change
True.
EAAT forms
trimers
In EAAT structure, glutamate sits between ______ and interacts with ______
Two hairpin loops, interacts with TMDs 7 and 8
Why may you see convulsant effects in ketogenic diet or fasting?
Ketone bodies (acetoacetate and beta hydroxybutyrate) compete with choride binding site on VGLUT. As result, glutamate can’t get transferred into vesicle, and thus less Glu is arriving at postsynaptic cell.
