Neurotransmitters Flashcards
What factors define any given NT system?
- NT molecule
- synthetic machinery
- packaging
- reuptake and degradation mechanisms
- action
What are the 3 criteria for a molecule to be a NT?
- synthesis & storage in the presynaptic neuron
- Released by presynaptic terminal in response to ↑[Ca++]
- When applied, elicits/mimics the post-synaptic cell response
What is immunocytochemistry?
Process for localising molecules to cells (identifying proteins)
How can we localise classic NT vs neuropeptides?
Neuropeptides - we can look for the NT themselves
Classic NTs - can look for their synthesis enzymes
Can only look for proteins
Briefly describe the aim/process of immunohistochemistry
Using the immune system to create antibodies against a particular sequence of Amino Acids.
* inject NT candidate
* purify antibodies from the blood
* fix a brain tissue section
* flood with antibodies & they will bind to the AA sequence
What is in situ hybridization?
Detect cells when synthesis of a protein/peptide is taking place - by detecting mRNA that codes for that AA sequence
How can NT release be studied?
Using a brain slice kept alive in vitro, stimulate synapses, collect & measure released chemicals.
Can also now use optogenetics
How can synaptic mimicry be studied?
stimulate the neuron & record the results. Then release a drug & record the results.
What are methods for identifying receptor subtypes?
- Ligand-binding methods - radioligand can show where the drug is binding
- Molecular analysis - can identify by their molecular/subunit composition
What is Dale’s principle and is it true?
A neuron has only one neurotransmitter.
It is true for classic NTs
Describe ACh synthesis in the presynaptic terminal
Choline + Acetyl CoA —ChAT–> ACh + CoA
ChAT = choline acetyltransferase [enzyme]
Describe ACh degradation in the synaptic cleft
ACh –AChE–> Choline + Acetic acid
AChE = Acetylcholinesterase [enzyme]
Where does Choline come from for ACh synthesis?
Choline transporters uptake choline from the synaptic cleft (left there by ACh breakdown by AChE)
Describe the pathway of catecholaminergic NT synthesis & identify the common precursor
Common precursor - Tyrosine
Pathway:
Tyrosine
↓
L-Dopa
↓
Dopamine
↓
Norepinephrine
↓
Epinephrine
What are the catecholamine NTs?
- Dopamine
- Norepinephrine
- Epinephrine
What determines which NT is synthesised in a catecholaminergic neuron?
The enzymes - needs extra enzymes to get further down the pathway between tyrosine & epinephrine
What is the precursor to serotonin?
Tryptophan
How are the amino acid NTs related?
All part of the same synthesis pathway:
Glutamate
↓
Glycine
↓
GABA
Which enzyme synthesises GABA from Glutamate?
GAD (glutamic acid decarboxylase)
Describe ionotropic receptors
- fast transmission
- sensitive to chemicals/voltage
- regulate the flow of large currents
- differentiate between ions (specificity) but not as specific as Na/K channels etc
- 5 protein subumits (similarity in structure between different ionotropic receptor types)
Describe AMPA receptors
- glutamate-gated channels
- cause EPSPs
- allows Na+ (inwards) & K+ (outwards) flow
Describe NMDA receptors
- glutamate-gated channel
- causes EPSP
- Allows Na+ & Ca++ (inwards) & K+ (outwards)
- has Mg++ gate in place at resting Vm
- voltage & chemically dependent - requires glutamate binding PLUS strong depolarisation to remove Mg++ gate to open channel
What is the general function of GABA-gated and glycine-gated channels?
major inhibitory ionotropic receptors
Describe a GABA_A receptor
- Cl- channel
- inhibitory
- requires GABA to bind to open channel
What are the 3 steps in gPCR transmission?
- NT binds to gPCR
- activation of G Protein
- Activation of effector systems
What are the 3 steps in gPCR transmission?
- NT binds to gPCR
- activation of G Protein
- Activation of effector systems
What is the basic structure of gPCRs?
single polypeptide with 7 transmembrane α-helices
List some commonly mentioned gPCRs
- muscarinic receptors (M_1, M_2 etc)
- GABA_B receptors
- Dopamine receptors (D1, D2, D3 etc)
- NE receptors (α1, α2, β1, β2, β3)
- opiod receptors (µ etc)
Where are inactive G proteins found?
floating around in the membrane (occasionally bumping into receptors)
What are the 3 subunits of a G -protein, and how do they split?
α
β
γ
split into Gα and Gβγ functional units
What are the 3 subunits of a G -protein, and how do they split?
α
β
γ
split into Gα and Gβγ functional units
What are the 5 steps of G protein action?
- G protein subunits together with α bound to GDP (inactive state)
- G protein associates with activated gPCR, GDP exchanged for GTP
- Gα dissociates from Gβγ, they act on other cellular systems
- Gα inactivated when GTP converted to GDP
- Gα recombines with Gβγ → reenters inactive state
What is an example of a fast, localised G protein response?
G protein subunit acting on an ion channel
What is a slow G protein response?
G protein subunit activating secondary messenger cascades (activation of downstream enzymes)
How can different G Proteins have different effects when activated by the same NT?
Different G protein types - ie Gs is stimulatory, Gi is inhibitory.
How can different G Proteins have different effects when activated by the same NT?
Different G protein types - ie Gs is stimulatory, Gi is inhibitory.
Compare the activity of α2 and β receptor effects
Both activated when NE binds to gPCR.
α2 = Gi receptor → inhibits Adenylyl cyclase
β = Gs receptor → stimulates Adenylyl cyclase
Describe the phospholipase C (PLC) second messenger cascade and the singificance of the end result
- Activated G Protein stimulates PLC
- Activated PLC splits PIP2 into DAG & IP3
- DAG stimulated Protein Kinase C (PKC)
- IP3 stimulates release of Intracellular Ca++
End result: This is another way to ↑[Ca++]
What is the function of signal cascades?
signal amplification - ie 1 NT binding can then result in many channels being opened later (rather than 1:1 ratio of directly binding ion channel)
What is signal divergence?
1 NT → lots of different responses (more than 1 receptor type activated)
What is signal convergence?
Many NT → all eliciting the same response
(different NTs with the same effect)
