Mechanisms of Ischemic Brain Damage

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Mechanisms of Ischemic Brain Damage

• Jenn Mejilla

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2 Hypothesis of Brain Ischemia

• Calcium Hypothesis
• Excitotoxic Hypothesis

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Calcium Hypothesis

• Massive Ca2 entry into cells leads to cell death
• Ca2 catalyzed the breakdown of structural
  components of cells (membrane lipids and
  cytoskeletal proteins).
• Agonist-receptor interactions at the motor end
  plate caused necrosis of the target, innervated
  by cholinergic fibers.

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When this general hypothesis was applied to the
nervous system, it was assumed that calcium
entering dendritic cells, caused necrosis of
selectively vulnerable neurons by ischemia or
hypoxia, hypoglycemic coma, and status
epilepticus. Calcium was assumed to enter cells
by way of voltage-sensitive calcium channels,
which are abundant at the basal dendrites of
cells with a tendency to epileptogenic firing.
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Calcium Metabolism

• Presynaptic depolarization causes Ca2 to enter
  the cytoplasm of the presynaptic endings
• Followed by release of glutamate. This activates
  two types of ionotropic glutamate receptors- AMPA
  and NMDA.
• (AMPA amino-3-hydroxy-5-methol-4-isoazole
  propionic acid)
• (NMDA N-methyl D- aspartate)

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• When glutamate activates the AMPA receptor, a
  channel is opened that
• allows the passage of Na, K and H. When Na
  enters down its electrochemical gradient, it
  depolarizes the membrane. This allows the influx
  of Ca2 by way of any voltage-sensitive calcium
  channels that may be localized to the
  postsynaptic membranes of the dendrites and cell
  body (eg. L and T types)
• In addition, it relieves the Mg2 block of the
  NMDA gated channel, allowing Ca2 to enter this
  high-conductance, unselective cation channel.
• The excitatory event is terminated by reuptake of
  glutamate into presynaptic vesicles and into
  glial cells.

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• Ca2 entry via NMDA receptors has special
  pathophysiologic significance
• NMDA receptor-gated channel has a high calcium
  conductance
• The channels or calcium ions they conduct are in
  contact with cell
• structures that are vulnerable to the increase
  in intracellular Ca2.
• When Ca2 ions enter cells by way of NMDA
  receptor-gated
• channels, they are more prone to trigger the
  production of ROS
• , reactive oxygen species, such as H2O2,
  O2-, OH.
• Postynaptic calcium influx stimulates neuronal NO
  synthase, allowing
• for the simultaneous appearance of O2- and
  NO in postsynaptic
• structures.

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Excitotoxic Hypothesis

• Described in 1981
• Excitatory amino acid-related toxicity led to
  neuronal cell death in tissue slices or primary
  neuronal cell cultures.
• It was initially argued that glutamate activation
  of AMPA receptors leads to an influx of Na, Cl-
  and water- which causes osmolytic cell damage.
• Later, results showed that the osmolytic damage
  was reversible, but the influx of calcium caused
  a delayed type of damage.

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• It is now clear that a single Ca2 exposure can
  lead to secondary compromise of Ca2, suggesting
  a delayed failure of calcium regulation.

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Glutamate and Calcium Triggered Events

• Enhanced Lipolysis
• Altered Phosphorylation of Proteins
• Enhanced production of reactive oxygen and
  reactive nitrogen species.

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Enhanced Lipolysis

• Ischemia leads to lipolysis because ATP and
  cytidine triphosphate are no longer present to
  catalyze the resynthesis of phospholipids, once
  they are broken down, and because calcium
  activates enzymes, degrading phospholipids to
  biologically active compounds such as FFAs and
  lysophospholipids.
• FFAs and lysophospholipids are mediators of
  membrane dysfunction b/c they make act as
  ionophores and uncoupling agents.

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• Once reperfusion is initiated, the oxidative
  meta-
• bolism of arachidonic acid accumulated during
  the ischemia leads to the formation of
  cyclooxygenase and lipooxygenase products- active
  in triggering inflammatory responses.

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Altered Phosphorylation of Proteins

• Ca2 is an important modulator of the
  phosphorylation state of many proteins. When
  proteins are phosphorylated and dephosphorylated,
  their functions are altered. So, when calcium
  concentration is transient,particularly when Ca2
  is excessive and sustained, membrane function and
  metabolic activities alteration can cause harmful
  effects.

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Production of ROS and NOS

• Ischemia with reperfusion leads to the production
  of ROS . These free radicals give rise to lipid
  peroxidation, protein oxidation and DNA damage.
• Oxygen radicals and NO, together, exert toxicity
• NO has important role in brain ischemia.
• 3 types
• n-NOS and e-NOS (calcium dependent and
  constitutively expressed)
• i-NOS (expressed by activated macrophages and
  neutrophils)

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Neuronal NOS is involved in synaptic signalling
however, under ischemic conditions, it mediates
cell death. The same is true for
i-NOS. Therefore, the production of NO by the
calcium- Dependent n-NOS may be detrimental
because it Allows additional and toxic ROS to be
formed.
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Dissolution of the Cytoskeleton

• Increase in intracellular Ca2 activates
  proteases that break down neurofilaments and
  contribute to the disassembly of microtubules.
• This breakdown cause serious problems in
  intracellular communication, which depends on the
  integrity of the cytoskeleton as well as cause
  damage to the mitochondria of cells.