Glutamate Flashcards
Glutamate
the major excitatory neurotransmitter in the CNS
@ crossroad of multiple metabolic pathways
(Almost all excitatory neutrons in the CNS are glutamatergic - estimated that over half of all brain synapses release glutamate)
Can lead to neuronal membrane depolarisation
Glutamine
Amino acid precursor of glutamate
Phosphate-activated glutaminase
Enzyme that converts glutamine to glutamate
Vesicular glutamate transporter (VGLUT)
Vesicular membrane transporter that mediates glutamate accumulation in presynaptic vesicles (counter-transported with H+)
AMPA receptor
A ligand-gated ionotropic glutamate receptor subtype selective to the synthetic molecule AMPA
Composed of 4 subunits: (four types + alternate splice variants)
- GluA1
- GluA2
- GluA3
- GluA4
They’re organised into a hetero-tetrameric structure. “Dimer of dimers”
Most commonly composed of:
2 GluA2 subunits
2 GluA1, 3 or 4
= gives rise to 4 orthosteric (site at which the andogenous ligand (glutamate) binds to) binding sites
- 2x sites must be occupied for ion channel opening to occur
- Current increases as more binding sites are occupied = influx of sodium ions
- presence of GluA2 subunits prevents calcium flow (require it being replaced by GluA1/3/4)
NMDA receptors
A ligand- and voltage-gated ionotropic glutamate receptor subtype selective to the synthetic molecule NMDA
3 subunits (+ alternate slice variants): - GluN1 (or NR1) - GluN2 (or NR2) - GluN3 (or NR3) Hetero-tetrameric (“dimer of dimers”)
Most commonly:
-2x GluN1
-2x GluN2 (or GluN3)
GluN3 subunits are non-functional so are inhibitory to NMDA receptor function
Both 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
Kainate receptor
A ligand-gated ionotropic glutamate receptor subtype selective to the synthetic molecule kainite
5 subunits (+ 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 - number required not well understood
Limited distribution in the brain compared to AMPA/NMDA receptors
Long-term potentiation (LTP)
A persistent increase in synaptic strength following high-frequency stimulation of a chemical synapse. Studies of LTP are often carried out in slices of the hippocampus, an important organ for learning and memory.
Excitatory amino acid transporter (EAAT)
A membrane transporter that mediates glutamate accumulation in the presynaptic bouton from the presynaptic cleft
Excitotoxicity
The pathological process by which neurons are damaged and killed by the overactivations of the NMDA and AMPA receptors.
E.g. of this: vesicular glutamate transporters VGLUT. If they’re not functioning properly, glutamate can accumulate in the cytosol of pre-synaptic neurons. If there’s a build up in the cytosol, EAATs can reverse their function and begin to pump glutamate out of the cytosol and into the synaptic cleft. This release of glutamate w/o an A.P. = leads to AMDA receptors and NMDA receptors activation = influx of Ca2+ into the post-synaptic neurone, in uncontrolled fashion.
Neurotransmission
The fundamental process that drives information transfer between neurones and their targets.
Neurotransmitter
Chemical messengers 5ha5 transmit signals from a neuron to a target cell, across a synapse.
How can neutrons 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.
Glutamate storage and synthesis
Glutamine -> glutaminase (phosphate- activated) -> glutamate
Amine group is substituted to carboxylic acid group
Where is glutamate synthesised?
Synthesised in the nerve terminals
VGLUT in detail
Once it is synthesised, glutamate is transported into vesicles by vesicular glutamate transporters (VGLUT)
At least 3 different VGLUT genes have been identified, with these different subtypes transporting glutamates into synaptic vesicles @ different types of glutamatergic pre synaptic nerve terminals.
Inside these vesicles = very acidic -> maintained by ATP and H+ ion pumps. Movement of H+ ions down its conc gradient, drives the entry of glutamate into vesicles. = approx. 10000x more glutamate in vesicles than in cytosol
= counter transport process w/ H+ ions to drive glutamate entry into vesicles.
Re-uptake
Both neurones and glial cells contain EAAT ( Excitatory amino acid transporters)
family of 5 different Na+ ion dependent glutamate co transporters ( function to transport glutamate from cleft back into the neurone/glial cell for degradation
Degradation
Start: Glumate (excitatory reaction)
glutamine synthetase
Finish: Glutamine
Start: Glial cells (transport)
SN1 + SAT2
Finish: Neurones
SN1 = system N transporter (expressed in glial cells) SAT2 = system A transporter 2 (expressed on neutrons)
Why is important that calcium ion flow can be prevented?
It is protective against excitotoxicity. GluA2 can be considered as protective of this potential excitotoxicity.
Metabotropic glutamate receptors
Comprise a large extracellular domain for neurotransmitter binding, which can be termed Venus flytrap domain, a characteristic 7 transmembrane domain structure and an intercellular c-terminal domain.
Total: 8 sub-types mGlu1 (group 1) mGlu2 (group 2) mGlu3 (group 2) mGlu4 (group 3) mGlu5 (group 1) mGlu6 (group 3) mGlu7 (group 3) mGlu8 (group 3)
Group 1 is Gq coupled = synaptic plasticity (mostly post synaptic)
Group 2 and 3 is Gi/o coupled = inhibit NT release (predominantly pre synaptic)
Form dimers:
Homomers
Heteromers e.g. mGlu1 and mGlu5
Heteromers e.g. mGlu2 and 5-HT 2A
Depolarisation
Displacement of a membrane potential towards a more positive value. Required to meet the threshold to fire a nerve impulse/ A.P.
Hyperpolarisation
Displacement of a membrane potential towards a more negative value. = inhibits A.P. firing = increasing the stimulus required to fire that A.P.
Excitatory Post-synaptic Current (EPSC)
Represents the flow of ions and the change in current, across a post-synaptic membrane.
Lead to the generation of EPSCs, which increase the likelihood of firing an A.P.
EPSC’s produced by the NMDA receptor and kainate receptor are slower and last longer than those produced by AMPA receptors. = AMPA receptors are primary mediators of excitatory neurotransmission in the brain.
What does excessive Ca2+ cause?
Neuronal damage due to: Mitochondrial damage, oxidative stress and apoptosis
What is glutamate related excitotoxicity linked to?
Stroke, Alzheimer’s disease
Alzheimer’s disease
Neurodegenerative disorder, characterised by neuronal cell death in the hippocampus. = subsequently neuronal cell death throughout the cerebral cortex.
NMDA over activation is showed to be linked w/ glutamate mediated neurotoxicity and shown to contribute to neuronal cell death.
Few select treatments for disease. E.g. Mematine is a low affinity NMDA receptor antagonist that blocks the NMDA receptor ion channel to reduce glutamate mediated neurotoxicity. (Drug that directly targets NMDA) - used to treat moderate to severe Alzheimer’s
Long Term Potentiation defined
Refers to the persistent strengthening of a synapse based upon repeated patterns of activity.
Underlies important processes, including both learning and memory- initial phase involves glutamatergic neurotransmission.
LTP mechanism of Action
Glutamate binds + activates AMPA receptors w/ Na+ flowing into the post-synaptic neurone and causes depolarisation.
NMDA receptors open, due to depolarisation removing the voltage gated Mg2+ ion block.
Ca2+ ions enter the cell, activate the post-synaptic protein kinases, such as calmodulin kinase II (CamMKII) and protein kinase C (PKC)
CaMKII and PKC trigger a series of reactions that lead to the insertion of new AMPA receptors into the post-synaptic membrane.
AMPA receptors increase the post-synaptic membranes sensitivity to glutamate and increases ion channel conductance.
This underlies the initial phase of LTP
