Excitotoxicity in the Brain

Causes of Excitotoxicity

1. Presynaptic
   1. Increased/prolonged action potential firing (e.g. in status epilepticus)
   2. A malfunction in Calcium entry into the presynaptic cell (Voltage-gated calcium channels)
   3. Excess glutamate release (altered response to calcium/ altered vesicle-membrane action)
2. Postsynaptic
   1. Increased affinity for glutamate at binding site
   2. Increased receptor density
   3. Deficient cation selectivity
      1. Under certain conditions, AMPARs (normally only sodium permeable) become calcium permeable to calcium.  This mainly happens when there is a change in the structure(/presence) of the GluR2 subunit

Ischaemia

Stroke is becoming increasingly more common.

During ischaemia, the loss of ATP production causes non-functioning of glutamate re-uptake and a build up of extracellular glutamate.  If ATP levels deplete too much, tranporter proteins may even act in reverse to pump more glutamate out of the cell.  This causes high levels of NMDAR activation, which has been shown to be of benefit in ischaemic tissue (preventing axonal damage) in the short term.  However, in the longer term, activation of NMDAR’s can cause neuronal death.

 

Agonists/Antagonists of Glutamate transmission

Domoic Acid

Domoic acid is a glutamate analog and potent receptor agonist of AMPA and kainate receptors (20x moreso than kainate at the KA receptor).  It is not readily removed from the synapse by transporters, making it extremely toxic.  The activation of AMPA/KA receptors causes an influx of calcium into the cell, in turn causing glutamate release and activation of the NMDA receptor causing more calcium influx (excitotoxicity).

It is produced in certain algae and thus enters the food chain.  It is known to cause amnesic shellfish poisoning: a condition causing short-term memory loss, brain damage and, rarely, death.

β-Methylamino-L-alanine

This is a neurotoxin produced by cyanobacteria.  It seems to be selective for motor neurons and activates AMPA/KA receptors, as well as inducing over-production of oxygen free-radical species and excitotoxicity.

It has been implicated in rare forms of amyotrophic lateral sclerosis (motor neuron disease)/parkinsonian-dementia complex in a tribe of Guam who eat fruit bats (that eat seeds containing this compound).

Glutamate Receptors

Ionotropic Glutamate Receptors

There are 3 main types of ionotropic receptor for glutamate: NMDA, AMPA and kainate, so named after their original agonists.

NMDA receptors are assembled from seven types of subunit (GluN1, GluN2A, GluN2B, GluN2C, GluN2D, GluN3A, GluN3B).  The subunits that make up AMPA receptors (GluA1-4) and kainate receptors (GluK1-5), are closely related to, but distinct from, GluN subunits.  The subunit arrangement of a receptor can have important effects on its function as well as its structure, e.g. AMPA can possess two versions of GluA2- one which makes the AMPA receptor permeable to calcium, the other which makes it impermeable.

AMPA receptors facilitate fast excitatory transmission (fast EPSP- excitatory post-synaptic potential), while NMDA receptors facilitate slow transmission (slow EPSP).  NB Conventionally, fast EPSPs are usually produced via the opening of ion channels and take just several ms to work, whereas slow EPSP’s are usually G-protein coupled receptor mediated and take longer and may require more stimulation.  In the NMDAR (a ligand-gated ion channel)- this is not the case.  In fact, the NMDAR seems to show slow kinetics.  Also, under resting conditions, an Mg(2+) ion acts to block the NMDA receptor from opening.  It is released once the cell membrane is persistently depolarised.  As a result, NMDARs do not contribute to basal levels of excitation.  On top of all of this, NMDA receptors require the binding of glycine as well as glutamate/NMDA to open.  The complexity of the NMDA receptor may be protective against glutamate mediated neurotoxicity.

Ionotropic receptors are found in the cortex, basal ganglia and sensory pathways.  NMDA and AMPA receptors are generally found together throughout these areas whereas kainate receptors are a little more selective.

Metabotropic Glutamate Receptors

There are 8 different types of these G-protein coupled glutamate receptors (mGlu(1-8)).  They are arranged as homodimers connected by a disulfide bridge.

They are widely distributed throughout the CNS in neurons and glial cells.  They regulate excitability and transmission via molecular signalling.  Group 1 receptors generally are found postsynaptically and generally increase excitability by raising intracellular calcium levels.  Group 2 and 3 receptors are pre-synaptic and generally reduce cellular excitability.

Glutamate

L-Glutamate is the main excitatory neurotransmitter in the CNS.   It is found throughout the CNS.

Metabolism and Release

Glutamate in the CNS comes mainly from glucose via the Kreb’s cycle or from glutamine syntesised by glial cells.  The metabolism of glutamate is directly linked to that of GABA (the primary inhibitory neurotransmitter).  This makes targeting drugs difficult as to affect one will undoubtedly affect the other.

Green arrows represents pathways in GABAergic neurons and orange arrows represent pathways in glutaminergic neurons. see http://www.pnas.org/content/102/15/5588/F1.expansion.html

Glutamate is stored in synaptic vesicles and released by Ca(2+)-dependent exocytosis.  Released glutamate is taken up by Na+/H+/K+ dependent transporters.  From there it is stored in synaptic vesicles once more.  Its action is terminated either by reuptake into adjacent glial cells (main route), which can convert it to inactive glutamine, or reuptake into the transmitting neuron to be recycled.

Extracellular concentrations of glutamate are kept low by transporter proteins in the plasma membrane (ATP-mediated) and then cytosolic concentrations are also kept low by storing glutamate in vesicles via ATP mediated vesicular transporters.

Glutamate is the main excitatory neurotransmitter in the brain and is crucial for normal synaptic communication.  However, too much glutamate is known to be damaging and sometimes lethal to cells- a process known as excitotoxicity.