1. Toxicology of the Nervous System
Neurotoxicity:
Toxicology of the Nervous System
John J Woodward, PhD
Department of Neurosciences
IOP471N

2. 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).

3. 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

4. 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)

5. Cellular Events in Neurodevelopment
Underlying Cellular Biology
Cellular Events in Neurodevelopment
Events:
Division
Migration
Differentiation
Neurogenesis
Formation of synapses
Myelination
Apoptosis
Active throughout
childhood &
adolescence
Cell division, migration, differentiation, synapse formation, synapse pruning, apoptosis, and myelination occur during brain development, though the timing of the sequence of events differs somewhat in various portions of the brain. The sequence is genetically programmed, but mediated by a variety of neurotrophic biochemical compounds, including neurotransmitters, and subject to disruption by environmental influences. Interference with any stage of this process may alter subsequent stages and result in permanent impairments.
Neurotoxic compounds can interfere with critical processes in these events, making the developing brain uniquely susceptible to exposure. Extensive studies of a few well-known neurodevelopmental toxicants, including lead, mercury, alcohol, and nicotine, reveal multiple mechanisms by which these compounds disrupt normal brain development. These include alterations in levels of neurotransmitters or other neurotrophic compounds and impairment of cell division, migration, differentiation, synapse formation, and apoptosis.

6. 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)

7. 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

8. Blood Supply to the Brain

9. Blood-brain Barrier Anatomical Characteristics Not an absolute barrier
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

10. 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.

11. What causes neurotoxicity?
Wide range of causes
Chemical
Physical

12. 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)

13. NEURONS

14. CELL MEMBRANE AND MEMBRANE PROTEINS
Ion Channels
Important for establishing resting membrane potetial
Synaptic transmission/nerve conduction
Voltage-sensitive
Ligand-gated
Sodium channel

15. Types of Neurotoxic Injury
Normal
Axonopathy
Transmission
Neuronopathy
Myelinopathy
Neuron
Myelin
Axon
Synapse

16. 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)

17. 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)

18. Neurotoxicology Heavy Metals
Lead – environmental exposure (paint, fuels)
Mercury – exposure via diet (bioaccumulation in fish)

19. 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

20. Nervous Systems Effects
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)

21. 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

22. Environmental Sources of Mercury
Natural Degassing of the earth
Combustion of fossil fuel
Industrial Discharges and Wastes
Incineration & Crematories
Dental amalgams
CF bulbs

23. 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+)

24. 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.

25. 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

26. Excitatory synapse of brain Required to generate action potentials
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

27. 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

28. Both Native and Recombinant NMDA Receptors Can Cause Excitotoxicity
Neurons
Transfected CHO cells

29. 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

30. 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)

31. 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

32. Synaptic and non-synaptic NMDA Receptors Increase Calcium
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

33. 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

34. 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)

35. Glutamate and Human Brain Trauma

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