Lec 12- Excitotoxicity

Question 1

Excitotoxicity- definition

Answer 1

• Too much glutamate receptor activity causes neuronal damage and death

Question 2

Why is excitation dangerous

Answer 2

1. Na⁺ and Ca²⁺ entry through AMPAR +NMDAR
2. Depolarization leads to more Na⁺ entry via Na⁺ channel
3. Depolarization leads to more Ca²⁺ entry via VGCC
4. Too much Ca²⁺ builds up in the neuron
5. If stimulus persists- Ca²⁺ homeostasis is overwhelmed

• Essentially it is the level of Ca that enters into the cell

Question 3

Intracellular Ca²⁺ overload- Energetic stress

Answer 3

• Detrimental effects of sustained Ca²⁺ overload
  
  • Increased energy demand on neurons
  • Mitochondrial dysfunction

Question 4

Too much [Ca²⁺]_i activates Ca²⁺ activated enzymes

Answer 4

• Phospholipases- release of arachidonic acid => free radical production O₂^-
• NO synthases-=> free radical production O₂^-
• Proteases- Cytoskeleton breakdown=> free radical production O₂^-
• Endonucleases- DNA breakdown=>free radical production O₂^-
• ‘Suicide’ gene activation

Question 5

Free Radicals- Lipid Peroxidation

Answer 5

• -OH ion attacks hydrophobic tail creating peroxylipid damaging it
• Free radical will cause a chain reaction of lipid damage

Question 6

NO = apoptosis

Answer 6

• Ca enters and activates induceable nitric oxide synthase
• Nitric oxide is a reactive Oxygen species
  
  • This overwhelms the mitochondria which cause them to shut down
  • Causes Cytochrome C activation, causes caspase cascade to activate = apoptosis

Question 7

How did the notion of excitotoxicity emerge

Answer 7

• 1957, Lucas and Newhouse sc injection of mice w/glutamate causes retinal degeneration
• 1969, Olney finds that MSG injection into mice brain causes lesions
• 1969, Onley and Sharpe repeat in the primate model
• 1984, Rothman shows gamma-D-glutamyl glycine and EAAR inhibitor blocks excitotoxic death
• 1984, Olney and Schwarz show Kainic acid, glutamate, quinolinic acid, NMDA all kill neurones
• Then, in 1984, dramatic proof that exogenous glutamate receptor agonist can cause neuronal damage
• Seafood lovers in Canada at shellfish
• Exogenous versus Endogenous excitotoxicity

Question 8

Prince Edward Island

Answer 8

• 100 locals ate cultivated mussels
• 3 died, all were seriously ill
• or had seizures
• PMs showed massive damage to hippocampus, amygdala, entorhinal cortex
• Survivors had irreversible short-term memory deficits- dementia like symptoms

Question 9

The Culprit- Domoic acid

Answer 9

• Produced by Algal bloom pseudonitzschia
• Taken up be seaweed (Demoic acid)
• Mussels eat seaweed
• We (and birds, seals) eat mussels and get amnesic Shellfish Poisoning (ASP)- Short term memory loss, seizures, death

Question 10

Domoic acid- Mechanism

Answer 10

• Acute neurotoxcity, arising from the primary action of domoic acid on kainate receptors
• Kainic acid receptors= ionotropic receptors = excited = excitotoxicity
• Lots of Kainic acid receptors in the limbic system (hippocampus, amygdala etc) hence why we get dementia/memory/epileptic like symptoms

Question 11

The Birds

Answer 11

• 1961- Capitola- hundreds of birds go crazy from ingestion of domoic acid
• Sooty Seawater- normally non-aggressive small fish eating bird
• Hitchcock-fascinated by incident
• Included newspaper clippings of Capitola incident in the movie proposal
• 1963- The birds released

Question 12

Excitotoxins plant origin

Answer 12

• From Lathyrus sativus (chickpea)
• AMPAR agonist
• Produces Neurolathyrism (spasticity)

Question 13

Excitotoxins of plant origin- BMMA

Beta-N-methyl-amino-L-alanine

Answer 13

• From cycas circinalis (cycad)
• NMDAR + AMPAR Agonist
• Produces amyotrophic lateral sclerosis/ Parkinson’s dementia like-complex (ALS/PDC)
• ALS= motor neurone disease

Question 14

Summary of exogenous excitotoxins

Answer 14

• Exogenous excitotoxins activate ionotropic GluR
• Cause excessive Ca²⁺ accumulation
• Leads to neuronal death
• What about endogenous excitotoxicity

Question 15

Endogenous excitotoxicity

Answer 15

• 1984- Key year again
  
  • Benveniste shows that extracellular levels of glutamate increase during ischaemia
• Rothman shows that GluR antagonists protect cultured neurons against both anoxia and the toxicity of exogenous glutamate and aspartate
• Simon et al, show that antagonists at NMDAR reduce ischemia-induced neuronal damage in vivo
• Glutamate receptor antagonist against excitotoxicity

Question 16

The current dogma

Answer 16

• High extracellular glutamate underlies endogenous excitotoxicity
• Too simplistic why
• Excitotoxicity associated with PD, ALS, Epilepsy, stroke, dementias- cannot all be due to same simple mechanism