W8 - Neurotransmitters Systems I: Glutamate Flashcards
What are the criteria for a neurotransmitter?
Neurotransmitters are chemical messengers that transmit signals from a neuron to a target
cell across a synapse (i.e. neurotransmission).
I. The molecule must be synthesised and
stored in the pre-synaptic neuron
II. The molecule must be released by the pre-
synaptic axon terminal upon stimulation
III. The molecule must produce a response in
the post-synaptic cell
How can neurotransmitters be classified?
Neurons can be classified by the neurotransmitter that they use – these differences arise due to the differential expression of proteins involved in neurotransmitter synthesis, storage and release.
Acetylcholine (ACh)
Noradrenaline
Glutamate
GABA
Serotonin
Dopamine
What is an introduction to glutamate?
Glutamate is the major excitatory neurotransmitter in the central nervous system (CNS).
- It took a long time to realise that glutamate was a neurotransmitter – it is at the crossroad of multiple metabolic pathways eg. Krebbs cycle
- The excitatory role of glutamate in the brain and spinal cord was discovered in the 1950s. An experiment in 1954 showed injecting glutamate into the brain or carotid arteries in various mammals produce convulsions.
- Nearly all excitatory neurons in the CNS are
glutamatergic and it has been estimated that over half of all brain synapses release glutamate - It was not until the late 1970s that glutamate became recognised as the principle excitatory neurotransmitter in the CNS
How is glutamate synthesised and stored?
Whilst glutamate can be synthesised via glucose, the most prevalent precursor for glutamate synthesis is glutamine. The enzyme, glutaminase converts glutamine into glutamate. Glutaminase catalyses the substitution from an amine group to a carboxylic acid group.
Synthesised in the nerve terminals
Transported into vesicles by vesicular glutamate
transporters (VGLUT) -> Counter transport with H+ to drive glutamate entry into vesicles. The acidity inside the synaptic vesicles is maintained by the ATP driven hydrogen ion pumps. Movement of H+ ions down its conc grad drives entry of glutamate into vesicles. This means around 10^4 higher Glu in vesicles than in the cytosol.
This is a counter transport process with H+ ions that allows glutamate entry into the vesicles.
How does glutamate re-uptake and degradation take place?
Once it is released and binds with the post-synaptic receptors. The Glu activity now needs to be terminated.
Reuptake:
Neurons and glial contain Na+ dependent excitatory amino acid transporters (EAATs). These are a family of 5 different sodium ion dependent glutamate co transporters. They function to transport the Glu from the synaptic cleft back into the neurone or close cell for subsequent degradation.
Degradation:
Glutamate is transported into Glial cells via the EEAT is converted into glutamine via the action of the enzyme. Glu is then transported out of the glial cells by a second transporter( SN1) and then transported into neurons via the SAT2 transporter.
Glutamate
Glutamine synthetase
Glutamine
Glial cells
SN1 and SAT2
Neurons
SN1 = System N transporter (expressed on glial cells)
SAT2 = System A transporter 2 (expressed on neurons)
What are glutamate receptors?
IONOTROPIC
AMPA RECEPTORS
NMDA RECEPTORS
KAINATE RECEPTORS
METABOTROPIC
- Group I
- Group II
- Group III
How do Ionotropic glutamate receptors function anf what are the 3 main receptors?
Ionotropic glutamate receptors are named after agonists that activate them:
These two are responsible for the majority of excitatory neurotransmission in the brain:
-AMPA
(∝-amino-3-hydroxyl-5-methyl-4-isoxazole propionate)
-NMDA
(N-methyl-D-aspartate)
-Kainic acid
All three are glutamate gated ion channels that allow the passage of Na+ and the influx of K+.
NMDA allows Ca2+ ions in addition to the other two.
What AMPA receptors?
Four subunit types (plus alternate splice variants):
* GluA1
* GluA2
* GluA3
* GluA4
- Four orthosteric binding sites
- Two sites must be occupied for channel opening
- Current increases as more binding sites are occupied
Hetero-tetrameric - “Dimer of dimers”
Presence of GluA2 subunits prevents Ca2+ flow. If it is then substituted with GluA1,3 or 4, it is then permeable to calcium. GluA2 is protective against excitotoxicity from calcium
Most commonly:
* 2 GluA2 subunits
* 2 GluA1, 3 or 4
in various different combinations.
What are NMDA receptors?
Three subunit types (plus alternate splice variants):
* GluN1 (or NR1)
* GluN2 (or NR2)
* GluN3 (or NR3)
Hetero-tetrameric - “Dimer of dimers”
Most commonly:
* 2 GluN1 subunits
* 2 GluN2 (or GluN3)
→ GluN3 subunits are non-functional so are inhibitory to NMDA receptor function
Ligand and voltage-gated:
* Ligands: Glutamate (binds to GluN2) and glycine or D-serine (binds to GluN1)
* All sites must be occupied for channel opening
* Voltage: Mg2+ block at resting membrane potential
What are Kainate receptors?
Five subunit types (plus alternate splice variants):
* GluK1 (GluR5)
* GluK2 (GluR6)
* GluK3 (GluR7)
* GluK4 (KA1)
* GluK5 (KA2)
Tetrameric:
* GluK1-3 can form homomers or heteromers
* GluK4 & 5 only heteromers with GluK1-3 subunits
Ligand-gated ion channel:
* Glutamate binding required for channel opening – not particularly well understood
* Limited distribution in the brain compared to AMPA/NMDA receptors
What are metabotropic glutamate receptors?
The metabotropic receptor comprise a large extracellular domain for neurotransmitter binding - a Venus Flytrap Domain (7 transmembrane structure) and an intracellular C-terminal domain.
8 sub-types:
mGlu1
mGlu2
mGlu3
mGlu4
mGlu5
mGlu6
mGlu7
m Glu8
Group 1 - 1,5 -Gq coupled PIP2 -> DAG and IP3IP3R activation on ER↑Ca2+Synaptic plasticity. These are predominantly post-synaptic.
Group 2 - 2,3
Group 3 - 4,6,7,8
-> Gi/o coupled Θ adenylyl cyclase↓cAMP formation Inhibit NT release. Where as these two are predominantly pre-synaptic.
G-Protein Coupled Receptor (GPCR):
Dimers:
* Homomers
* Heteromers e.g. mGlu1 and mGlu5
* Heteromers e.g. mGlu2 and 5-HT2A
What is excitatory neurotransmitters and depolarisation?
Excitatory neurotransmitters (e.g. glutamate) can lead to neuronal membrane depolarisation
– displacement of a membrane potential towards a more positive value.
Hyper polarisation involves the displacement of a membrane potential towards a more negative value. This inhibits action potential firing by increasing the stimulus required to fire that action potential.
Depolarisation involves the displacement of a membrane towards a more positive value. This is required to meet the threshold to fire a nerve impulse or action potential.
What is the relationship between AMPA, NMDA and kainate receptors and EPSCs?
The excitatory post-synaptic current (EPSC) represents the flow of ions, and change in
current, across a post-synaptic membrane.
- EPSCs lead to the generation of excitatory post synaptic potentials (EPSPs), which depolarises the cell increasing the likelihood of firing an action potential
- EPSCs produced by the NMDA receptor and kainate receptor are slower and last longer than those produced by AMPA receptors
Accordingly, AMPA receptors are the
primary mediators of excitatory neurotransmission in the brain.
What are Glutamate-mediated excitotoxicity?
Excitotoxicity is the pathological process by which excessive excitatory stimulation can lead to neuronal damage and death.
EXCESSIVE CA2+ CAN CAUSE:
* Mitochondrial damage
* Oxidative stress
* Apoptosis
EXCITOTOXICITY LINKED TO
* Stroke
* Alzheimer’s disease.
If the transporters are not functioning correctly, glutamate can accumulate in the cytosol of presynaptic neurones. Normally, EEATs work on the basis that they take high concentrations of glutamate from the synaptic cleft to lower concentrations in the cytosol. BUT, if there is a build up of glutamate in the pre-synaptic neurones, EEATs can reverse in their functions and begin to pump glutamate out of the neurone into the synaptic cleft, leading to glutamate being released without an action potential - aka a stimulus.
What are the Glutamate-mediated exitotoxicity in Alxheimer’s disease?
Neurodegenerative disease characterised by neuronal cell death in the hippocampus - associated with memory, subsequently neuronal death throughout the cerebral cortex.
Memantine is a low-affinity NMDA receptor antagonist that blocks the NMDA receptor ion channel to reduce glutamate mediated neurotoxicity
These can be used to treat moderate to severe Alzheimer’s given it is a NMDA receptor antagonist. It blocks excessively open NMDA receptor ion channels thereby reducing glutamate mediated access to toxicity and therefore reducing neuronal cell death.
