Neurotoxicity:
Toxicology of the Nervous System
John J Woodward, PhD
Department of Neurosciences
IOP471N
woodward@musc.edu
www.people.musc.edu/~woodward
Historical Events
 1930’s – 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.
 1950’s – Mercury poisoning
•
Methylmercury in fish in Japan cause death and
severe nervous system damage in infants and
adults (Minimata disease).
 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
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)
Underlying Cellular Biology
Cellular Events in Neurodevelopment
Events:
 Division
 Migration
 Differentiation
 Neurogenesis
 Formation of synapses
 Myelination
 Apoptosis
Active
throughout
childhood &
adolescence
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)
6
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
Blood Supply to the Brain
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
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.
What causes neurotoxicity?
Wide range of causes
Chemical
Physical
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)
NEURONS
CELL MEMBRANE AND MEMBRANE PROTEINS
Ion Channels
•Important for establishing resting membrane potetial
•Synaptic transmission/nerve conduction
•Voltage-sensitive
Sodium channel
•Ligand-gated
Types of Neurotoxic Injury
Normal
Neuron
Myelin
Axon
Synapse
Axonopathy
Transmission
Neuronopathy
Myelinopathy
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)
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)
Neurotoxicology
 Heavy Metals
 Lead – environmental exposure (paint, fuels)
 Mercury – exposure via diet (bioaccumulation in fish)
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
Lead
Neurotoxicity
Nervous Systems Effects





Developmental Neurotoxicity
Reduced IQ
Impaired learning and memory
Life-long effects
Related to effects on calcium permeable
channels (NMDA, Ca++ channels)
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
Environmental Sources of Mercury
• Natural Degassing of the earth
• Combustion of fossil fuel
• Industrial Discharges and Wastes
• Incineration & Crematories
• Dental amalgams
• CF bulbs
Toxicity of Mercury
• Different chemical forms – inorganic, metallic,
organic ( Hg0
Hg2+
CH3Hg+)
• 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
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.
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
Glutamate Synapses
Excitatory
synapse of brain
Required to
generate action
potentials
Both AMPA and
NMDA receptors
are critical for
normal brain
function
Glutamate synapse
NMDA-hi Ca++
permeability
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
Both Native and Recombinant NMDA Receptors Can
Cause Excitotoxicity
Neurons
Transfected CHO cells
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
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)
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
Calcium
Mitochondrial Dysfunction
Resulting from Calcium
Overload is Source-Specific
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
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
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)
Glutamate and Human Brain Trauma
Glutamate in Human Brain Following Stroke
Glutamate
Threonine
Glutamate levels remain high after stroke
Threonine, a structural amino acid, is measured as a control