Chemical Synaptic Transmission Flashcards
(41 cards)
What is the criteria for defining a neurotransmittter?
- It must be present at the synapse (along with appropriate synthetic pathways).
- It must be released in response to appropriate stimuli (pre-synaptic action potentials) through a Ca2+ dependent mechanism.
- Specific post-synaptic receptors must be present (agonists and antagonists have the same actions on the
post-synaptic effects of synaptically released and exogenously applied transmitter).
What is the neuromuscular junction (NMJ)?
aka motor endplate
The synapse between motorneurone and skeletal muscle
called motor endplate because the motorneurone terminal branches and makes several connections w muscle fibre
How is ACh made and where is it made and how is it transported?
Synthesied in the PNS nerve terminal from choline and acetyl CoA by choline acetyltransferase (ChAT)
Transported into vesicles by vesicular transport (high H+ concentration inside vesicle, transporter exchanging H+ for ACh)
Whats vesamicol?
Inhibits vesicular acetylcholine transporter
(VAChT) which loads ACh into vesicles
Whats immunocytochemistry?
Technique that uses antibodies raised against ChAT, then labels them with secondary antibody conjugated with a marker against primary antibody?
This allows microscopic identification of the location of ChAT-containing (cholinergic) neurones
What is end-plate potential (EPP)?
Voltage that depolarizes in the muscle fibres after motorneurone stimulation
How is Ca released? How does it promote contraction?
Action potential depolarizes muscle fibres, which depolarizes t-tubule, which triggers Ca release from sarcoplasmic reticulum via ryanodine receptor type 1, which interacts with troponin and stimulates muscle contraction
DHP receptors undergo conformational change during AP, and this interacts with ryanodine receptor to release Ca
What is different in cardiac muscle regarding Ca release?
Ca is released through L-type channel (DHP-receptor) FIRST to activate ryanodine receptors
What is tetrodoxin (TTX)?
It blocks voltage-gated Na+ channels in the motorneurone, which stops pre-synaptic action potential and the EPP
What is the safety factor?
Refers to the margin that an EPP exceeds AP threshold
Means that a single AP will ALWAYS trigger the release of enough ACh to elicit a post-junctional action potential and muscle contraction
What relationship does Ca have with ACh release?
Ca2+ entry triggers the exocytosis of vesicles releasing ACh into the synaptic cleft
No calcium = no transmitter release= no response
Blocking voltage-gated Ca2+ channels abolishes the post-synaptic potential (as does removing external Ca2+
What is quantal theory?
When EPP is reduced, motor neuronal stimulation elicits mini EPPs, caused by Ca vesicles (‘quanta’) releasing Ca into the synapse asynchronously
EPP is caused by hundreds of quanta releasing at the same time
How are neurotransmitters released via : Ca2+ -induced vesicular exocytosis?
Synaptobrevin interacts with SNAP-25 and syntaxin and draws the two membranes together. Ca binds to synaptotagmin and then interacts with the rest of the proteins and fuses the vesicle to the membrane
SNARE complexes form to intitally bring the two membranes together
SNAP-25 and syntaxin are on the synapse, and synaptobrevin is on the vesicle
What do botulinum toxins do?
Botulinum toxins are peptidases,specifically taken up by motoneurones, that cleave SNARE proteins preventing exocytosis / ACh release causing paralysis
What functions do clatherin and dynamin have in vesicle endocytosis?
Clathrin attaches to the membrane with the help of adapter proteins. As it polymerizes it causes curvature of the membrane. Ultimately a vesicle is cleaved off with the help of dynamin.
What is the function of Nicotinic Acetylcholine Receptors (nAChR)?
nicotine is a selective agonist that can bind with high affinity to the ACh binding site of these receptors
nAChR belong to the ligand-gated ion channel super-family.
At the NMJ, ACh binds nAChRs
nAChRs mediate the EPP
What is the End Plate Current (EPC?) What is reversal potential?
EPC is the current flow resulting from nAChR activation
Reversal potential (E.ACh) is the potential at which no EPC is observed
- E.ACh changes in the depolarizing direction if we increase extracellular K+
- E.ACh changes in the hyperpolarizing direction if we reduce extracellular Na+
because the normal resting potential is closer to EK+, it is predominantly Na+
flux that mediates the EPP.
What does acetylcholinesterase (AChE) do?
Hydrolyses ACh to choline and acetate
Important because it stops ACh from signalling
What does hemicholinium do?
Blocks high-affinity choline transporter (ChT), which carries choline into acetylcholine-synthesising neurons
## Footnote
Choline transporter is A high affinity transporter, that co-transports Na+, moves choline back into the motoneurone terminal, plays important role in re-cycling choline ready for new ACh synthesis
What do non-polarizing blockers do? Give two examples
- they are competitive, reversible, receptor antagonists, compete for binding site on nAChR
- Can be overcome by increased ACh conc (via anti-cholinesterase drugs)
- eg. tubocurarine and atracurium
Non-depolarizing blockers compete with ACh for the agonist binding site on the nAChR i.e. they are competitive, reversible, receptor antagonists. A key feature of the actions of non-depolarizing blockers is that they can be overcome by increasing the ACh concentration in the synaptic cleft. This can be achieved by administering an anti-cholinesterase drug.
Blocking AChE prevents ACh breakdown thereby increases ACh concentration at the NMJ. This increases the probability that a molecule of ACh will occupy the binding site on the receptor rather than a molecule of antagonist.
What do depolarizing drugs do? Give an example
- initially activate the nAChR, but ultimately causes a loss of electrical excitability at the NMJ
- Cannot be releived by anti-cholinesterases
- Suxamethonium (succinylcholine)
The sustained depolarization of the NMJ by suxamethonium leads to prolonged voltage-gated Na+ channel inactivation. Thus, an initial action potential is elicited in the muscle by the depolarization but subsequent action potentials cannot be produced because voltage-gated Na+ channels cannot recover from inactivation until the membrane potential has returned to its resting level. In addition, the prolonged presence of agonist leads to changes in nAChR conformation resulting in a receptor that does not respond to agonist. This is known as receptor desensitization
What does a-bungarotoxin do?
highly potent, selective and irreversible peptide inhibitor of muscle nAChR that also binds to the ACh binding site
Use of this toxin was instrumental in allowing sufficient receptor protein to be purified to enable the initial structural characterization of the muscle nAChR. We also saw the use of fluorescently labelled aBGTx to demonstrate the location of nAChR at the NMJ in the Chemical Synaptic Transmission 1 lecture.
Which is faster, GPCRs or ligand gated ion channels?
The ion channels are significantly faster.
Depending on the complexity of the signaling pathway the time course of the effects of GPCR activation can range from 100’s of milliseconds to seconds
What are the three GPCR groups?
A: Largest group, amine transmitters, peptides, purines, cannabinoids. Short extracellular N–terminus.
Ligand binds to TM helices (amines) / or extracellular loops (peptides)
B: Intermediate length extracellular N–terminus. N-terminus incorporates Ligand Binding Domain
(LBD).
C: metabotropic glutamate receptors, GABAB, Ca2+
-sensing receptors. Long extracellular N–terminus.
LBD in long N-terminus – (“Venus Fly Trap” domain).
