Glutamate Flashcards
Summarise the function of neurotransmitters
From the vesicles fuse to the membrane and are released via exocytosis and diffuse across the synaptic cleft
What is neurotransmission?
The fundamental process that drives information transfer between neurons and their targets
What are neurotransmitters?
Chemical messengers that transmit signals from a neuron to a target cell across a synapse
Criteria for a neurotransmitter
- To be synthesised and stored in the presynaptic neuron
- Released by the presynaptic axon terminal upon stimulation
- Produce a response in the postsynaptic cell
Why are neurotransmitters synthesied and stored in the presynaptic neuron?
To be released fast so a readily available pool is needed
What are the two types of neurotransmitter?
Ionotropic and Metabotropic
What are the major central neurotransmitters?
- Acetylcholine that acts on nicotinc receptors and muscarinic receptors
- Glutamate
- GABA
- Glycine
- Monoamines
What is glutamate?
A major excitatory neurotransmitter in the central nervous system
How is glutamate formed?
Glutamine is converted to glutamate via the enzyme glutaminase that is phosphate activated.
What enzyme converts glutamine to glutamate?
Glutaminase
Where is glutamate synthesised?
In the pre-synaptic nerve terminals
What transporter puts glutamate into vesicles?
vesicular glutamate transporter (v-glut)
How does VGLUT work?
It counter transports with H+ to drive glutamate entry into vesicles.
How much glutamate is stored in the vesicles compared to cytosol?
Estimated that there is 10(4) higher concentration of glutamate in vesicles than in the cytosol
What are the 3 ionotropic glutamate receptors?
AMPA receptors activated by AMPA
NMDA receptors activated by NMDA
Kainate receptors activated by kainic acid
What is the influx and efflux of the ions in AMPA, NMDA and kainate receptors?
Influx
AMPA - Na+
NMDA - Na+ Ca2+
Kainate - Na+
Efflux
AMPA - K+
NMDA - K+
Kainate - K+
Mechanism of Action Potential
- Resting state: At -70mV in a neuron, 3Na+/2Na+ pump.
- Depolarisation: Activation of a stimulus results in depolarisation of the neuron
- Rising phase of the action potential: Na+ channels open and more sodium enters the cell.
- Falling phase of the action potential: Na+ close and K+ open; K+ efflux from the cell. Membrane potential becomes more negative
- Undershoot: K+ channels are too slow to close therefore, there is an action potential that arrives at the presynaptic neuron. This triggers the release of calcium from VGCC (on pre-synaptic neuron). This triggers the release of synaptic vesicles containing neurotransmitters by exocytosis
What type of receptor is AMPA?
A glutamate ionotropic receptor
Four subunits of AMPA
GluA1
GluA2
GluA3
GluA4
Common subunits of AMPA
2 GluA2 subunits bind and then a mixture of either 2GluA1, 3 or 4 will bind.
What type of structure does the AMPA receptor form?
A hetero-tetrameric structure - dimer of dimers.
How many binding sites does the AMPA receptor have?
It has four orthosteric binding sites.
How many binding sites need to be occupied to open the AMPA channel?
When two of the binding sites are occupied. The current increases as more binding sites are occupied.
What prevents Ca2+ flow from entering the cell?
Presence of GluA2 subunits prevents Ca2+ flow from entering the cell, this is a protective measure of excitotoxicity.
What type of receptor is NMDA?
Ionotropic
What are the three subunits of NMDA receptors?
GluN1 (or NR1)
GluN2 (or NR2)
GluN3 (or NR3)
What are the most common subunits for NMDA receptors?
2 GluN1 subunits and 2 GluN2 (or GluN3)
What type of subunits are the GluN3 subunits?
They are inhibitory to the NMDA receptor function
What type of ion channels are the NMPA receptors?
Ligand and voltage gated
Which ligands bind to the NMPA receptor and which subunits do they bind to?
Glutamate binds to the GluN2 subunit.
Glycine or D-serine binds to the GluN1 subunit
How are the NMPA receptor also voltage gated?
The ligand binds to the receptor and when there is a depolarisation event, the Mg2+ block which is at the resting potential membrane moves to allow the ions through.
How many binding sites need to be occupied for the channel to open?
All sites must be occupied.
What type of structure is the NMDA receptor?
Heterotetrameric
What is synaptic plasticity?
The ability of the neuron to change and adapt
What is long term potentiation?
A type of synaptic plasticity which is the strengthening of the connection between two neurons.
What is important for synaptic plasticity?
Glutamate
Why is glutamate important for synaptic plasticity?
- It binds to the receptors causing an influx of sodium into the cell.
- It removes the Mg block, allowing sodium and calcium into the cell.
- Leads to receptor trafficking meaning more AMPA receptors are added to the membrane
- More sodium influx which can activate CamKII.
- CamKII phosphorylates the receptors to increase sodium influx.
- This increases the ionic conductance of these receptors therefore, more Na+ and Ca2+ is entering the cell.
- This activates long-term potentiation where over time there is synaptic strengthening.
- NMDA receptors will not work without the activation of sodium from the influx from the AMPA receptors.
- This process underlies learning and memory essentially.
- Increased EPSP triggers the action potential.
What is receptor trafficking?
When more receptors are added to the membrane causing a high influx of ions
What are glutamate kainate receptors?
Originally thought to be AMPA receptors, before being discovered as Kainite receptors.
What are the five subunit sites of Kainate receptors?
GluK1 GluK2 GluK3 KA1 KA2
What type of structure do kainate receptors form?
Tetrameric
- GluK1 - 3 can form homomers or heteromers
- GluK4 and 5 only heteromers with GluK1-3 subunits
What type of channel is the Kainate receptor?
It is a ligand gated ion channel.
- Glutamate binds for the channel to open but this mechanism is not well understood. Limited distribution in the brain. Function is less understood than AMPARs and NMDARs
What are metabotropic receptors?
G-protein coupled receptors
Structure of metabotropic receptors
- Venus flytrap domain
- Cysteine rich domain
- C-terminal domain
- 7-transmembrane domain
What are the subtypes of metabotropic glutamate receptors?
Group 1 containing mGlu 1 and 5
Group 2 containing mGLu2 and 3
Group 3 containing mGlu4, 6 7 and 8
Where are the group 1 glutamate receptors located and what is there mechanism of action?
Located post-synaptically. Coupled to Gq leading to an elevation in Ca2+.
Where are group 2 glutamate receptors located and what is there mechanism of action?
Located pre-synaptically. coupled to Gi/o which inhibits adenylyl cyclase inhibiting ATP to cAMP reducing cAMP formatino
Where are group 3 glutamate receptors located?
Located pre-synaptically.
What type of dimers do metabotropic glutamate receptors form?
Homomers
Heteromers e.g. mGlu1 and mGlu5
Heteromers e.g. mGlu2 and serotonin (5-HT2A)
Which transporters reuptake glutamate?
Excitatory amino acid transporter (EAAT)
Where are AMPA, MGlu2, MGlu8 and MGlu5 found on the synapse?
AMPA - post synaptically
MGlu2 - pre-synaptically
MGlu5 - postsynaptically
MGlu8 - pre-synaptically
What is excitotoxicity?
The pathological process by which excessive excitatory stimulation can lead to neuronal damage and death.
What happens during glutamate-mediated excitotoxicity?
- VGLUT transporters will stop working and they are important for getting NT into vesicles.
- This means NT glutamate will build up in the cytosol.
- The EAATs will reverse their function. Normally, glutamate will move from a high concentration in the synaptic cleft to the cytosol.
- However, in this situation there will be movement of the neurotransmitter from the cytosol into the synaptic cleft.
- When there is excess in the cytosol, it will bind to AMPA receptors to remove the magnesium block and leading to calcium influx via the NMDA receptors.
- There is excessive calcium release into the neuron.
What happens if there is excessive Ca2+?
Mitochondrial damage
Oxidative stress
Apoptosis
Which diseases is excitotoxicity linked to?
Stroke
Autism
Alzheimer’s disease
What is the link between glutamate mediated excitotoxicty and Alzheimer’s disease?
Cause shrinkage in the hippocampus which is important in memory.
This is caused by cell death.
What drug is used to alleviate the symptoms of Alzheimer’s?
Memantine is an antagonist which blocks the NMDA receptor stopping the excess release of calcium. By targeting the NMDA receptors, the symptoms can be alleviated.
