Excitotoxicity Flashcards
What is the main excitatory neurotransmitter in the brain?
Glutamate
Define excitotoxicity
Cell death resulting from the toxic actions of excitatory amino acids
Give examples of amino acids which cause ecotoxicity
Cysteine Cysteine sulfonate Cysteic acid Homocysteine Glutamate Aspartate
Where are neurotransmitters synthesised?
Presynaptic terminal
Which ion influxes in response to depolarisation?
Calcium
Outline synaptic transmission
NT synthesised and stored in presynaptic terminal.
AP depolarises the presynaptic terminal.
Ca2+ influx.
Release of NT via exocytosis and diffusion to postsynaptic membrane.
Interacts with postsynaptic receptors.
NT degradation and reuptake
How is excess glutamate removed from the synapse?
Reuptake by ATP dependent transporters
Uptake into glial cells
Which channels open in the axon to allowed Na+ influx?
Voltage gated Na channels
What causes repolarisation of the membrane in synaptic transmission?
Outflow of K+
Describe the extracellular glutamate level (under normal circumstance)
Low
How is cytosolic glutamate transported into vesicles?
Vesicular ATP-dependent glutamate transporters
What happens to glutamate uptake during ischaemia?
Stop or reversal
What are the three main types of glutamate receptor?
Kainate
AMPAR
NMDAR
What are NMDARs permeable to?
Ca2+
Which ions block NMDARs?
Mg2+
NMDARs do not contribute to basal transmission true/false
True (as Mg2+ present to block receptors)
How are NMDARs activated?
Intense synaptic activity activates AMPARs to cause depolarisation and alleviation of Mg2+
How do NMDARs mediate excitotoxicity?
Due to high Ca2+ permeability
Incomplete desensitisation
What clinical scenarios are associated with excessive NMDAR activation?
Ischaemia
Chronic neurodegeneration
Which receptors are responsible for fast excitation?
AMPAR
Which receptors are responsible for slow excitation?
NMDAR
What is meant by a “dual component EPSC”?
Has both slow NMDAR and fast AMPAR mediated components
In which foods does domoic acid accumulate?
Shellfish
Which compound is associated with amnesic shellfish poisoning (ASP) ?
Domoic acid
What are the clinical implications of domoic acid poisoning?
Short term memory loss
Brain damage
Death
Where does neural necrosis occur with ASP?
Hippocampus
Amygdala
At which sites does domoic acid act as an agonist?
AMPARs
KARs
What does domoic acid induced activation of AMPA/KARs lead to?
Increased intracellular Ca2+ and therefore glutamate release
Kainate is more potent than domoic acid at KARs.
true/false
False
DA is 20x more potent that kainate
Domoic acid is readily removed by glutamate transporters
True/false
False
Not readily removed and is therefore toxic
Which amino acid does cyanobacteria produce?
Beta-methylamino-alanine (BMAA)
BMAA is the cause of which clinical condition?
ALS
How does BMAA kill motor neurons?
Activates AMPA-KARs
Boosts ROS production
NMDAR antagonists increase ischaemic damage in vivo
True/false
False
They reduce it
Glutamate toxicity is predominantly mediated through…..
NMDARs
Which is the most sensitive to glutamate: AMPAR, KAR, NMDAR
NMDAR
What is the EC50?
Concentration that elicits 50% response
Which pre-synaptic abnormalities may lead to excessive excitation?
Increased firing of APs
Mutation of VGCC
Altered req for Ca2+ influx
Which post-synaptic abnormalities lead to excessive excitation?
Increased affinity of glutamate binding site
Increased density of glutamate receptors
Deficient cation selectivity
Which subunits to AMPARs normally contain?
GluA2 subunits(R)
GluA2 (R) subunits are impermeable to calcium true/false
True
Which AMPAR subunits would be permeable to calcium?
GluA2(Q)
Compare apoptosis and necrosis
Apoptosis vs Necrosis
Indiv cells /// groups of cells
Physiological stimuli /// non-physiological stimuli
No inflammation /// inflammation
Shrinkage of cytoplasm and condensed nuclei /// swelling of both
Blebbing /// loss of membrane
Active /// passive
Which mediators does the mitochondria release which leads to apoptosis?
Cytochrome C
Caspase 9
Aif (apoptosis inducing factor)
What do mitochondria buffer within cells?
Ca2+
What does activation of NOS by sever insult lead to?
Increased mitochondrial Ca2+, superoxide formation and peroxynitrite (ONOO-) formation
Cellular damage caused by ONOO- leads to….
Activation of poly-ADP-ribose polymerase (PARS)
What does mitochondrial Ca2+ accumulation then cause?
Activation and of permeability transition pore (PTP)
Why does PARS activation lead to mitochondrial dysfunction?
Depletes ATP and NAD which are required for normal functioning
What does elimination of NMDARs cause in vivo?
Widespread apoptosis in developing neurons
Which curve do NMDAR responses follow?
hermetic-dose-response
Which NMDARs are pro-survival: synaptic or extra-synaptic?
Synaptic
Why are extra-synaptic NMDARs linked to pro-death signalling?
They shut down CREB functioning and trigger mitochondrial depolarisation
How does NMDAR activity promote survival through Akt?
NMDAR enhances PI3K mediated conversion to PIP2 to PIP3
PIP3 then activates Akt which reduces cell death and promotes growth
What are the main inhibitory targets for Akt?
GSK-3-beta
BAD
p53
Which CREB-target genes are implicated in NMDAR-dependent neuroprotection?
AID (Activity-dependent inhibitor of death)
NFAT (nuclear factor of activated T cells)
Neurons with higher basal NMDAR can withstand more/less oxidative stress?
More
What effect does synaptic activity have on mitochondrial fission?
Enhances
What effect does synaptic activity have on mitochondrial mobility?
Reduction
Where does synaptic activity localise mitochondria to?
Dendritic spines
Does synaptic activity increase or decrease BDNF expression?
Increase (this is neuroprotective)
Ca2+ efflux from neurons occurs via….
PMCA and NCX
plasma membrane Ca2+ ATPase and Na+-Ca2+ exchangers
Are calpains activated or inhibited by excessive NMDAR Ca2+ influx?
Activated
Which isoform of NCX do calpains cleave?
NCX3 (impaired function)
How do calpains affect PMCA?
Inactivate
Where do PMCAs normally get energy from?
ATP hydrolysis
Give examples of stress-activated protein kinases (SAPKs)
P38 MAPK
JNK
Which compound is involved in NMDAR dependent P38 MAPK activation in cerebellar neurons?
nNOS (neuronal nitric oxide synthase)
Compare synaptic and extrasynaptic NMDARs
Synaptic vs Extrasynaptic Ca2+ tolerated /// triggers cell death Activates ERK /// inactivates ERK Activates CREB dependent gene expression /// cause CREB dephosphorylation Activates PI3K/Akt /// doesn't
Describe the composition of NMDARs
One NR1 subunits
one or more NR2 subunits (NR2A/B/C/D)
With/out NR3 subunit
Which NR subunit types predominate in the hippocampus?
A and B
synaptic, extraysnaptic
What is memantine?
NMDAR antagonist
In spinal motor atrophy mice, daily administration of NMDA caused….
Improvements in motor behaviour and lifespan
Reduced motor neuron death
Huntington’s disease shows increased/decreased NMDAR?
Increased extrasynaptic
