Glutamate Receptors: Learning, Memory,

Excitotoxicity & Seizures

Outline

Glutamate receptors & excitotoxicity: NMDA-R antagonist as therapeutics

Glutamate receptors & Learning, memory, long-term potentiation: Can increasing the effect of glutamate enhance cognition?

Glutamate Receptors

Glutamate: major excitatory neurotransmitter in brain AMPA receptor

agonist: glutamate, AMPA, Quisqualate; Antagonist: NBQX, CNQX

Na+ & K+ permeable, low Ca2+ permeability, not blocked by Mg2+

Can be activated at all membrane potentials; fast EPSP

NMDA receptor: agonist: glutamate, aspartate, NMDA; co-agonist: glycine;

antagonist AP-5; channel blocker: PCP, ketamine, memantine Na+, K+ & highly Ca2+ permeable, blocked by Mg2+

Can be activated only at depolarized potentials; slow EPSP

Calcium

Excitotoxicity

NMDA Receptors & Calcium

Excitotoxicity,Seizures

LearningMemory

Seizures

DiatomRed Tide

Glutamate

Domoic Acid

Seizures,Death

Excitoxicity

Ischemic Stroke:Clot prevents blood flow,O2 & glucose to brainAlso caused by: Head injury, Hypoglycemia Repeated intense seizures

(status epilepticus)

Energy-dependent reuptake of glutamate from synaptic cleft is reduced.

Glutamate causes cytoplasmic Ca2+,neurotoxicity & cell death.

Ca2+

TOOMUCH

CALCIUM

Mechanisms of Excitotoxicity & Cell Death

Glutamate: activates AMPA R depolarizes cell membrane permits Ca2+ entry through

NMDA R & voltage-activated calcium channels

Ca2+ entry: Overloads mitochondrial and

ER mechanisms to remove cytoplasmic Ca2+

Activates proteases & lipases Creates reactive oxygen

species, free radicals, damages lipids, proteins & DNA.

Pharmacological Treatment for Stroke: Aspirin: reduces clot formation, inhibits platelet

thromboxane production which reduces platelet aggregation

tPA (tissue plasminogen activator): clot buster; catalyzes breakdown of plasminogen to plasmin, which proteolyzes fibrin; administered within 3 hrs of stroke

Inhibit NMDA receptor?

NMDA Receptor Blockers:

Block of Excitotoxicity: Memantine as Treatment for Alzheimers Disease

Memantine binds in pore & blocks open NMDA Receptor channel

Unlike PCP and ketamine, it has a fast off rate and lower affinity, so doesnt block normal synaptic transmission, but does block excessive glutamate stimulation.

Excitotoxicity NMDA Receptors & Calcium

Excitotoxicity,Seizures

LearningMemory

Morris Water Maze

Or with NMDA antagonist

Learning, Memory & Long-term potentiation: The hippocampus is a brain region involved in learning Activation of NMDA receptors is required for learning

High Frequency Stimulation Causes Long Term Potentiation (LTP)

Calcium

LTD:Endocytosis of AMPA receptors from plasma membrane of dendritic spines

LTP:Insertion of AMPA receptors into plasma membrane of dendritic spines

AMPA R subunit

Green fluorescent protein

High Frequency Stimulation Causes Delivery of AMPA Receptors to Dendritic Spines

High Frequency Stimulation

AMPA R delivered to spine

Delivery of AMPA-R to Spines is Blocked by NMDA-R Antagonist (APV) AMPA Modulators as Therapeutics: Ionotropic receptors desensitize in

continued presence of agonist

Agonist binds to desensitized receptor with higher affinity, but receptor doesnt open

AMPA modulators ampakine or aniracitam slow desensitization

Enhancement of AMPA responses enhances attention, learning & memory; a new approach for cognition enhancers (nootropic drugs)

Glutamate Glutamate

+ Ampakine

Closed Open Desensitized

desensitization

What happens downstream of calcium influx through NMDA-R to cause LTP?

Calcium activates both Ca2+/calmodulin kinase II and protein kinase C

These kinases phosphorylate substrates, including AMPA-R, which increases AMPA-R insertion into the spine plasma membrane

How can downstream signaling pathways be studied?

Measure activation of CaMKIIusing fluorescent CaMKII reporter

At the same time, apply a caged glutamate agonist, and release the glutamate with a laser flash to locally increase [glutamate] in one dendritic spine

Summary Glutamate: major excitatory neurotransmitter in brain AMPA receptor

agonist: glutamate, AMPA, Quisqualate; Antagonist: NBQX, CNQX Na+ & K+ permeable, low Ca2+ permeability, not blocked by Mg2+

Can be activated at all membrane potentials; fast EPSP NMDA receptor:

agonist: glutamate, aspartate, NMDA; co-agonist: glycine; antagonist AP-5; channel blocker: PCP, ketamine

Na+, K+ & highly Ca2+ permeable, blocked by Mg2+

Can be activated only at depolarized potentials; slow EPSP Excitotoxicity: ischemia or trauma leads to glutamate, Ca2+ influx & cell death

tPA to treat stroke; NMDA antagonists as therapeutic for Alzheimers & potential therapeutic for stroke, trauma

Long-term potentiation: The hippocampus is a brain region involved in learning Activation of NMDA receptors is required for learning Train of stimuli activates AMPA R which depolarizes the membrane & relieves the Mg2+ block of

NMDA R. NMDA R allow Ca 2+ influx, which initiates signaling cascades that cause long term increase in

synaptic strength Redistribution of AMPA receptors into dendritic spines underlies LTP (insertion of AMPA R) and

LTD (retrieval of AMPA R) AMPA R positive modulators as therapeutics for cognition enhancers (nootropic drugs)