Neurotoxicity:

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Neurotoxicity
Toxicology of the Nervous System
John J Woodward, PhD Department of
Neurosciences IOP471N woodward_at_musc.edu www.peopl
e.musc.edu/woodward
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Historical Events

• 1930s Ginger-Jake Syndrome
• During prohibition, an alcohol beverage was
  contaminated with TOCP (triortho cresyl
  phosphate) causing paralysis in 5,000 with 20,000
  to 100,000 affected.
• 1950s Mercury poisoning
• Methylmercury in fish in Japan cause death and
  severe nervous system damage in infants and
  adults (Minimata disease).

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• Central Nervous System (CNS)
• Brain Spinal Cord
• Peripheral Nervous System (PNS)
• Afferent (sensory) Nerves Carry sensory
  information to the CNS
• Efferent (motor) Nerves Transmit information to
  muscles or glands

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Cells of the Nervous System

• Neurons
• Signal integration/generation direct control of
  skeletal muscle (motor axons)
• Supporting Cells (Glia cells)
• Astrocytes (CNS blood brain barrier)
• Oligodendrocytes (CNS myelination)
• Schwann cells (PNS myelination)
• Microglia (activated astrocytes)

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Cellular Events in Neurodevelopment
Underlying Cellular Biology

• Events
• Division
• Migration
• Differentiation
• Neurogenesis
• Formation of synapses
• Myelination
• Apoptosis

Active throughout childhood adolescence
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• Development of GABA and Glutamate Synapses in
  Primate Hippocampus
• GABA synapses develop on contact
• Glutamate synapses develop but require a
  developed spine to become active
• GDPs dominate early developmental neuronal
  activity and disappear prior to birth (primates)
  or during early neonatal life (rodents)

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Why is the Brain Particularly Vulnerable to
Injury?

• Neurons are post-mitotic cells
• High dependence on oxygen
• Little anaerobic capacity
• Brief hypoxia/anoxia-neuron cell death
• Dependence on glucose
• Sole energy source (no glycolysis)
• Brief disruption of blood flow-cell death
• High metabolic rate
• Many substances go directly to the brain via
  inhalation

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Blood Supply to the Brain
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Blood-brain Barrier

• Anatomical Characteristics
• Capillary endothelial cells are tightly joined
  no pores between cells
• Capillaries in CNS surrounded by astrocytes
• Active ATP-dependent transporter moves
  chemicals into the blood
• Not an absolute barrier
• Caffeine (small), nicotine
• Methylmercury cysteine complex
• Lipids (barbiturate drugs and alcohol)
• Susceptible to various damages

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BBB can be broken down by

• Hypertension high blood pressure opens the BBB
• Hyperosmolarity high concentration of solutes
  can open the BBB.
• Infection exposure to infectious agents can open
  the BBB.
• Trauma, Ischemia, Inflammation, Pressure injury
  to the brain can open the BBB.
• Development the BBB is not fully formed at
  birth.

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What causes neurotoxicity?

• Wide range of causes
• Chemical
• Physical

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Toxicants and Exposure

• Inhalation (e.g. solvents, nicotine, nerve gases)
• Ingestions (e.g. lead, alcohol, drugs such as
  MPTP)
• Skin (e.g. pesticides, nicotine)
• Physical (e.g. load noise, trauma)

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NEURONS
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CELL MEMBRANE AND MEMBRANE PROTEINS

• Ion Channels
• Important for establishing resting membrane
  potetial
• Synaptic transmission/nerve conduction

• Voltage-sensitive

• Ligand-gated

Sodium channel
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Types of Neurotoxic Injury
Normal
Axonopathy
Transmission
Neuronopathy
Myelinopathy
Neuron
Myelin
Axon
Synapse
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Types Of Neurotoxicity

• Neuronopathy
• Cell Death. Irreversible cells not replaced.
• MPTP, Trimethyltin
• Axonopathy
• Degeneration of axon. May be reversible.
• Hexane, Acrylamide, physical trauma
• Myelinopathy
• Damage to myelin (e.g. Schwann cells)
• Lead, Hexachlorophene
• Transmission Toxicity
• Disruption of neurotransmission, toxins, heavy
  metals, organophosphate pesticides, DDT, drugs
  (eg., cocaine, amphetamine, alcohol)

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Ion Channels are Targets for a Variety of Toxins,
Chemicals and Therapeutic Compounds
Natural Toxins Snake, insect,plant toxins (cobra
venom, scorpion, curare) Environmental
Chemicals Heavy metals, industrial
solvents (lead, benzene, aromatic hydrocarbons)
Therapeutic Drugs Anesthetics,
Benzodiazepines (lidocaine, halothane,
valium) Drugs of abuse (Ketamine, alcohol,
inhalants)
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Neurotoxicology

• Heavy Metals
• Lead environmental exposure (paint, fuels)
• Mercury exposure via diet (bioaccumulation in
  fish)

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Historical Sources of Lead Exposure

• Ancient/Premodern History
• Lead oxide as a sweetening agent
• Lead pipes (plumbing)
• Ceramics
• Smelting and foundries

• Modern History
• Gasoline (leaded)
• Ceramics
• Crystal glass
• Soldering
• pipes
• tin cans
• car radiators
• House paint

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Nervous Systems Effects
Lead Neurotoxicity

• Developmental Neurotoxicity
• Reduced IQ
• Impaired learning and memory
• Life-long effects
• Related to effects on calcium permeable channels
  (NMDA, Ca channels)

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Mechanisms of Damage to the Nervous System by Lead

• Central
• Cerebral edema
• Apoptosis of neuronal cells
• Necrosis of brain tissue
• Glial proliferation around blood vessels
• Peripheral
• Demyelination
• Reversible changes in nerve conduction velocity
  (?NCV)
• Irreversible axonal degeneration

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Environmental Sources of Mercury

• Natural Degassing of the earth
• Combustion of fossil fuel
• Industrial Discharges and Wastes
• Incineration Crematories
• Dental amalgams
• CF bulbs

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Toxicity of Mercury

• Different chemical forms inorganic, metallic,
  organic (
• Organic mercury (methylmercury) is the form in
  fish bioaccumulates to high levels
• Organic mercury from fish is the most significant
  source of human exposure
• Brain and nervous system toxicity
• High fetal exposures mental retardation,
  seizures, blindness
• Low fetal exposures memory, attention, language
  disturbances

Hg0
Hg2
CH3Hg)
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MeHg Consumption Limits
US EPA 0.1 ug/kg-day US FDA 1 ppm (mg/kg) in
tuna
Consuming large species such as tuna and
swordfish even once a week may be linked to
fatigue, headaches, inability to concentrate and
hair loss, all symptoms of low-level mercury
poisoning. In a study of 123 fish-loving
subjects, the researchers found that 89 had
blood levels of methylmercury that exceeded the
EPA standard by as much as 10 times. How Much
Tuna Can You Eat Each Week? A safe level would be
approximately 1oz for every 20lb of body weight.
So for a 125lb (57kg) person, 1 can of tuna a
week maximum.
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Excitotoxicity-Glutamate Mediated Cell Death

• Experimental Observations
• Glutamate induces a delayed cell death in neurons
• This cell death requires extracellular calcium
  and is blocked by antagonists of NMDA receptors
• Hypothesis Prolonged or inappropriate
  activation of NMDA receptors underlies glutamate
  excitotoxicity of neurons

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Glutamate Synapses
Excitatory synapse of brain Required to generate
action potentials Both AMPA and NMDA receptors
are critical for normal brain function NMDA-hi
Ca permeability
Glutamate synapse
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Overview of Glutamate and Excitotoxicity
Glutamate activates two types of ion channels
(AMPA and NMDA) Cell Death is associated with
excessive calcium entry through NMDA receptors
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Both Native and Recombinant NMDA Receptors Can
Cause Excitotoxicity
Neurons
Transfected CHO cells
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NMDA-induced Excitotoxicity is NR2 Subunit
Dependent in Recombinant Expression Systems
NMDARS require two NR1 subunits and two NR2
subunits -NR2 family-NR2A, 2B, 2C, 2D -NR2A, NR2B
high excitotoxicity potential -NR2C, NR2D lower
excitotoxicity potential
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Calcium and Excitotoxicity
Glutamate-mediated apotosis in spinal motor
neurons is blocked by calpain inhibitors
Expose cells to 10 µM Glu in absence or presence
of calpeptin Monitor apoptosis (left panel) or
membrane potential (right panel)
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The Calcium That Triggers Excitotoxicity is
Source-Dependent

• Calcium entry via NMDA receptors can trigger
  neuronal cell death
• Calcium entry through other channels (eg. VSCC)
  does not
• Location of NMDA receptors is also important,
  synaptic versus extrasynaptic

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Mitochondrial Dysfunction Resulting from Calcium
Overload is Source-Specific
Calcium Mito Vm

• Synaptic and non-synaptic NMDA Receptors Increase
  Calcium
• L-type calcium channel increase calcium
• Synaptic NMDA receptors and L-type channels do no
  affect mitochondrial function
• Extrasynaptic NMDA receptors disrupt
  mitochondrial function and are linked to
  excitotoxicity

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Glutamate Excitoxicity in Oligodendrocytes

• Historically, oligos were thought to lack NMDA
  receptors
• More recent studies demonstrate NMDA and non-NMDA
  currents in oligos
• These receptors may be activated by injury or
  ischemic conditions that result in the release of
  glutamate
• Loss of oligo processes may underlie myelin
  degeneration associated with many diseases such
  as cerebral palsy, spinal cord injury and
  multiple sclerosis

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Glutamate Excitoxicity in Oligodendrocytes

• Oxygen-glucose deprivation (OGD)-model of
  ischemic damage
• Leads to loss of oligo processes
• This is prevented by blockers of NMDA receptors
  (MK801)

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Glutamate and Human Brain Trauma
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Glutamate in Human Brain Following Stroke
Glutamate
Threonine
Glutamate levels remain high after
stroke Threonine, a structural amino acid, is
measured as a control