Excitatory Amino Acids and Excitotoxicity Lecture (Dr. Karius)

Question 1

Excitatory Amino Acids

Answer 1

1) Glutamate
2) Aspartate

**There are OHT Inotropic and Metabotropic Receptors that can be activated by EAA

Question 2

Glutamate

Answer 2

• From ALPHA-KETOGLUTARATE

- Metabolic and NT pools Strictly SEPARATED!!!!!!

Question 3

Aspartate

Answer 3

• From OXALOACETATE
• Documented as NT in Visual Cortex and Pyramidal Cells
• Often found with GLUTAMATE

Question 4

Inotropic Receptors for EAA

Answer 4

1) NMDA

2) Non- NMDA Receptors
a) AMPA
b) Kainate

Question 5

NMDA Receptors

Answer 5

• Activated by the Exogenous agent N-METHYL-D-ASPARTATE
• Also Glutamate and Aspartate
• When activated, allows CA++ INFLUX!!!!!!!!!!!!
• Has Multiple Modulatory Sites:
  a) Glycine Binding Site
  b) Mg++ Binding Site
  c) PCP Binding Site
• Activation leads to EPSP
  a) SLOW ONSET
  b) PROLONGED Duration

Question 6

Glycine Binding Site on NMDA Receptors

Answer 6

• Glycine serves as a CO-AGONIST!!!!!!!
• Presence of Glycine required for EAA to have Effect
• Glycine on own CANNOT OPEN CHANNEL!!!!!

Question 7

Magnesium Binding Site on NMDA Receptors

Answer 7

• INSIDE CHANNEL!!!!
• Mg++ blocks the Channel
  a) CHANNEL MUST OPEN
  b) Cell must be DEPOLARIZED for Mg++ to Leave

Question 8

PCP Binding Site on NMDA Receptor

Answer 8

• INSIDE CHANNEL (Internal to Mg++ site)

- Blocks the Channel

Question 9

Activation fo NMDA Receptors leads to EPSP’s in the POST-Synaptic Cell

Answer 9

The EPSP’s Show:
1) LONGER Latency (Time to remove Mg++)

2) Longer Duration (Ca++ Slower)

Question 10

Non-NMDA Receptors

Answer 10

• Like the NMDA Receptor, almost exclusively POST-SYNAPTIC EXPRESSION
• SODIUM INFLUX (Some: Very small amount of Calcium too)
• Two Subtypes:
  a) AMPA
  b) KAINATE
• Activation leads to TYPICAL EPSP
• Often Co-Localized at SAME SYNAPSE with NMDA Receptor!!!!!!!!!!
  (The Non-NMDA Receptor allows for an Influx of Sodium which Depolarizes the Cell, thus removing the Mg++ Inhibition and therefore allowing and INFLUX of Ca++)

**BENZODIAZEPINE INHIBITS response to NT on the AMPA Non- NMDA Receptor!!!!!!!!

Question 11

Metabotropic Receptors

Answer 11

• Both PRE and POST-SYNAPTIC Location

a) Pre-Synaptic: CONTROL Neurotransmitter Release!!!!!!

Question 12

Functions of EAA

Answer 12

1) Non-NMDA Receptors:
- Primary AFFERENTS
- PREMOTOR (Upper MN)

2) NMDA Receptors:
- LONG TERM Changes in Synaptic Strength
- Learning
- Memory

3) METABOTROPIC Receptors:
- Learning
- Memory
- Motor Systems

Question 13

Getting Rid of EAA

Answer 13

1) NEURONS and GLIA:
- Uptake Systems
a) Na+ Dependent Secondary Active Transport
b) High AFFINITY

2) GLIA:
- Convert to GLUTAMINE
- Release into ECF

***Neurons takes Glutamine up and convert it back to GLUTAMATE

Question 14

EAA and Nitric Oxide

Answer 14

NMDA Receptors:
- Influx of Ca++

• Ca++ binds to CALCINEURIN!!!!!!!!!!!!!!
• Activates NITRIC OXIDE SYNTHASE (NOS)
• NOS catalyzes the reaction which creates NO from ARGININE!!!!!!

Question 15

Neural Functions of NO

Answer 15

• Long term Potentiation and Memory

- Cardiovascular and Respiratory Control

Question 16

Nitric Oxide can be Very Toxic

Answer 16

• Leads to production of FREE RADICALS

- These KILL invading Bacteria (or other cells)

Question 17

EAA

Glutamate/ Aspartate/ Taurine

Answer 17

CENTRAL LOCATION:
- Widespread: Spinal Cord through to Cortex

FUNCTIONS:

• Sensory: Primary AFFERENTS
• Motor: Activation of Alpha Motoneuron
• Consciousness
• Learning
• Memory

IONOTROPIC RECEPTORS:

• NMDA: Ca++ Influx Modulatory Sites
• Non- NMDA: Sodium Influx; two Subtypes

METABOTRPIC RECEPTORS:
- Yes

OTHER:
- Creation of NO by NMDA Receptor Activation

Question 18

Excitotoxicity

Answer 18

• Proposed to explain continuing Neuronal Deatha after and Ischemic Event
• Based on possibility that OVER STIMULATION of EAA System can cause CELL DEATH even in Neurons that were not ISCHEMIC/ HYPOXIC/ ANOXIC

Question 19

Excitotoxicity

Answer 19

Strong evidence of Involvement in:

• Cerebral Ischemia/ Stroke
• Hypoxia or Anoxia
• Mechanical Trauma to CNS
• Hypoglycemia

Substantial evidence of Involvement in:
- EPILEPSY

Question 20

In the area most Directly affected by Ischemia (Anoxic Core):

Answer 20

• OXYGEN DEPRIVATION
• Cells unable to MEET METABOLIC NEEDS (DEPOLARIZATION of MEMBRANE)

1) **Within 4 MINS:
- ATP levels within Neuron to 0
- Na/ K ATPase Ceases
- Vm DEPOLARIZES!!!!!!!

2) High levels of EAA:
- EAA release EXCESSIVE
- EAA Re-uptake is Na+ Dependent

3) NMDA Receptor Activation:
- CALCIUM INFLUX!!!!!!!!!!!!!!!!!

Question 21

Increases Calcium Concentration initiates

Answer 21

1) Activation of Phospholipase A2
2) Activation of Calcineurin (Phosphatase)
3) Activation of Mu-Calpain (Protease)
4) Activation of Apoptotic Pathway

Question 22

Excessive activation of these Enzymes disrupts Normal Cellular Function:

Activation of PHOSPHOLIPASE A2

Answer 22

• Release of Arachidonae from Membrane:
  1) Causes PHYSICAL DAMAGE to MEMBRANE!!!!!

(Arachidonate acts as Ryanodine Receptor on ER)
2) RELEASE of CALCIUM from INTRACELLULAR STORES!!!!!!!!!!!!(Including E.R and Mitochondria)

3) ER:
- ‘UNFOLDED PROTEIN RESPONSE”: Stops making Protein
- Activation of eIF2Alpha-Kinase
- Mitochondria: Impaired Function

Question 23

Excessive activation of these Enzymes disrupts Normal Cellular Function:

Activation of Mu-CALPAIN (Protease)

Answer 23

PROTEOLYSIS:
1) SPECTRIN: More structural Damage to Cell

2) eIF4G: Eukaryotic Induction factor 4G—> Protein Syntehsis
3) Others: Metabolic Impairment

Question 24

Excessive activation of these Enzymes disrupts Normal Cellular Function:

Activation of CALCINEURIN

Answer 24

• **PHOSPHATASE:
• Among other things, ACTIVATES NOS
• INCREASES NO SYNTHESIS

Question 25

Disruption of Mitochondrial and ER

Answer 25

A) The Disruption of Mitochondrial and ER Function INCREASES Free Cytosolic CALCIUM

B) As Mitochondrial Membranes are Disrupted, APOPTOTIC Pathways are ACTIVATED

• Cytochrome C and Caspase 9———————————————————–»» ACTIVATION of CASPASE 3 (Proteolytic Enzyme, Apoptotic)

Question 26

Reperfusion Injury

Answer 26

• This Neuron is no longer “NORMAL”
• Much of this O2 being added will end up as a FREE RADICAL somewhere (PEROXIDES)
• Kinases take ATP —> ADP “ PO4
  a) PHOSPHORYLATIO, further modifying Enzyme Action

b) PHOSPHORYLATION of eIF2alpha Kinase leads to a DECREASE in Protein Synthesis and activates CASPASE 3, which INCREASES APOPTOTIC Signaling

Question 27

Nitric Oxide adds to the Cascade

Answer 27

• In HIGH QUANTITY, NO contributes to EDEMA by damaging Capillary Endothelial Cells!!!!

Question 28

Prevention of this Cycle:

Answer 28

• Difficult at best, to date, most experimentally successful treatment are Pre-treatments that focus on NMDA Receptors