Non-Cancer Toxicology

Introduction to Neurotoxicology

Introduction to the Health Effects of Chemicals

Neurotoxicology

Steven G. Gilbert, PhD, DABT &

www.asmalldoseof.orgwww.toxipedia.org

Non-Cancer Toxicology

A Small Dose of Toxicology Introduction to Neurotoxicology

Introduction

“You cannot reach your full genetic potential with

a damaged nervous system.”

S.G. Gilbert


An adverse change in the chemistry, structure or function of the nervous system during development or at maturity, following exposure to a chemical or physical agent.

What is Neurotoxicity


Even minor changes in the structure or function of the nervous system may have profound consequences for neurological, behavioral, and related body functions.

Nervous System Sensitivity


Developmental alterationsImmunological impairmentNeurobehavioral effectsKidney/liver dysfunctionMutagenesisReproductive impairmentMetabolic inhibition

Examples of non-cancer toxicity


Toxicant Delivery to target tissue

Interaction with target molecule

Cell dysfunctionand/or Injury

Tissuedamage

TOXICITY

Exposure

General Mechanisms of Non-Cancer Toxicity


Interference with receptor-ligand interactions Interference with membrane functions Interference with cellular energy production Binding to macromolecules Perturbation of calcium regulation Toxicity from selective cell loss Non-lethal genetic alterations

Are there general mechanisms?


Rough Endoplasmic Reticulum

LysosomesPeroxisomes Mitochondria

MembraneProtein

Nucleus Golgi

Ribosomes

Animal Cell


CellCell

Lipid bilayerLipid bilayer

ChannelChannelPumpPump

TransportTransportProteinProtein

Hydrophilic headHydrophilic head

Hydrophobic tailsHydrophobic tails

Interference with Membrane Function


Neuro Insulation


Ancient Awareness

“LEAD MAKES THE MIND GIVE WAY”

Dioscorides - GREEK 2ND BC


“The upsurge of interest in recent years in academia, industry, and government on the effects of toxic chemicals on the nervous system has created a new discipline of neurotoxicology.”

Peter S. Spencer & Herbert H. Schaumberg, in Experimental and Clinical Neurotoxicology, 1980

Current Awareness


1930’s – Ginger-Jake Syndrome• During prohibition, an alcohol beverage was

contaminated with TOCP (triorthocresyl phosphate) causing paralysis in 5,000 with 20,000 to 100,000 affected.

1950’s – Mercury poisoning• Methylmercury in fish cause death and sever

nervous system damage in infants and adults.

Historical Events


Case Studies

Lead – damages developing brain

Alcohol – Fetal alcohol syndrome

MPTP – similar to Parkinson’s disease


Lead In Homes


Nervous Systems Effects

Developmental Neurotoxicity Reduced IQ Impaired learning and memory

Life-long effects

Lead Neurotoxicity


Alcohol (ethanol)

CH

H

H

OH

Ethyl Alcohol

C

H

H


Vulnerability of Developing Nervous System

FAS – Fetal Alcohol SyndromeFAE – Fetal Alcohol Effects

What is a save level of consumption during pregnancy?

Alcohol


Vulnerability of Developing Nervous SystemFAS – Fetal Alcohol SyndromeFAE – Fetal Alcohol Effects

ALCOHOL

What is a safe level of ethanol consumption during pregnancy?

http://www.fetalalcohol.com/what-is-fase.htm http://www.alumni.ca/~syed4s0/discussion.htm

A Small Dose of Toxicology Introduction to NeurotoxicologyFAS Child


MPTP

N

CH3

1-methyl-4-phenyl-1,2,3,6-tetrahydrophyridine


• 1980s – Designer Drug• Caused effects similar to

Parkinson’s disease• Damaged neurons that

secrete dopamine

MPTP Effects


• MPTP – 1-methyl-4-phenyl-1,2,3,6 tetrahydropyridine: a contaminant in

“synthetic heroin”

• Parkinson’s Disease-like symptoms in drug addicts

• Loss of neurons in the substantia nigra which synthesize and secrete dopamine

• MPTP-->MPP+ ---> oxidative stress

MPTP

A Small Dose of Toxicology Introduction to Neurotoxicologyhttp://www.swmed.edu/stars/resources/neurodisslides.html

MPTP – Real People

A Small Dose of Toxicology Introduction to Neurotoxicologyhttp://www.swmed.edu/stars/images/neurodisslides/Slide26.JPG

A Small Dose of Toxicology Introduction to Neurotoxicologyhttp://www.swmed.edu/stars/images/neurodisslides/Slide29.JPG

PD Brain


• CNS – Central Nervous System• PNS – Peripheral Nervous System• Blood brain barrier• Neuronal cells• Neurotransmitters & receptors• 10-100 billion cells with 1015

connections

Nervous System Biology


Nervous System – CNS & PNS

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


Nervous System

Nervous System

CNSCentral Nervous System

PNSPeripheral Nervous System

Autonomic Somatic

Sympathetic Parasympathetic


Central Nervous System

Central Nervous System (CNS)(Brain and Spinal Cord)

Peripheral Nervous System(PNS)

Autonomic Somatic

Sympathetic Parasympathetic

Afferent (sensory) Nerves(Carry sensory information to the CNS)

Efferent (motor) Nerves(Transmit information to muscles or glands)


Peripheral Nervous System

Peripheral Nervous System (PNS)• Efferent (motor) Nerves –

• Transmit information to muscles or glands Somatic Nervous System

• Stimulates Skeletal muscles Autonomic Nervous System

• Stimulates Glands and Organs (e.g. heart)• Sympathetic

- Adrenergic – stress response

• Parasympathetic - Cholinergic –

basic functions


Cells of the Nervous System

Neurons• Information conductors

Supporting Cells (Glia cells)• Astrocytes (CNS – blood brain barrier)

• Oligodendrocytes (CNS – link cells)

• Schwann cells (PNS – wrap cells)


Blood-brain Barrier

Not an absolute barrier• Caffeine (small)• Methylmercury cysteine complex• Lipids (brain is a ball of fat)

Anatomic Characteristics• Capillary endothelial cells are tightly joined –

no pores between cells• Capillaries in CNS surrounded by astrocytes• Low protein concentration in CNS fluid• Active ATP-dependent transporter – moves

chemicals into the blood


Neuronal Cells

Axon

Myelin (Schwann cell)

Synapse

Dendrite

Cell Body

Nucleus


DopamineTransmitter Cell(Excitatory Neuron)

DopamineReceptor Cell(Post-synaptic receptor)

DopamineReceptor

Synaptic Cleft

Synaptic Vesicles

Neurotransmission


Neuronal Transmission

+ + - + +

+ + - + +

+ + - + +

+ + - + +

- - + - -- - + - -

- - + - -- - + - -

- - - - -+ + + + +- - - - -+ + + + +

++

++

+ + +++ +

++ +-

--

- -- ----

-K+

K+

Na+

Cl-

-- -

-

-

--

-

- -

Inhibitory Synapse

Excitatory Synapse

+400

-40-70

+400

-40-70

Action Potential IPSP

EPSP

Action Potential

No Action Potential


• Inhalation (e.g. solvents, nicotine)

• Ingestions (e.g. lead, alcohol)• Skin (e.g. pesticides, nicotine)• Physical (e.g. load noise)

Exposure Issues


What causes neurotoxicity?

Wide ranged of agents – chemical and

physical


Types Of Neurotoxicity

Neuronopathy• Cell Death. Irreversible – cells not replaced.• MPTP, Trimethytin

Axonopathy • Degeneration of axon. Reversible.• Hexane, Acrylamide

Myelinopathy• Damage to myelin (e.g. Schwann cells)• Lead, Hexachlorophene

Transmission Toxicity• Disruption of neurotransmission• Organophosphate pesticides, Cocaine, DDT


Neurotoxic Injury

Neuron

Normal

Neuronopathy

Axonopathy

Myelinopathy

Transmission

Axon

Synapse

Myelin


Examples of Neurotoxicology

DiseasesParkinson's, Alzheimer's, MS, ALS..

EnvironmentalLead, Methylmercury, PCBs

OccupationalSolvents, Pesticides

Drugs - ClinicalVincristine, cisplatin

Drugs - SocialAlcohol, cocaine, nicotine


Cognitive Effects - memory, learning, confusion

Motor Effects- weakness, convulsion, paralysis

Sensory Effects- vision, auditory, touch, balance

Mood and Personality Effects- sleep, depression, irritability, excitability

General Effects- loss of appetite, fatigue

Neurotoxic Effects


Classification of Neurotoxicant

Temporary inhibition of nerve function• Agents which alter membrane

function• Agents with interfere with synaptic

transmission

Mechanism of action


Classification of Neurotoxicant

Permanent inhibition of nerve function• Agents which cause Anoxia

• Anoxic anoxia (e.g. CO2 asphyxiation)• Ischemic anoxia (e.g. blood clot)• Cytotoxic anoxia (e.g. cyanide)

• Agents which damage myelin formation• Oligodendroglia (CNS)• Schwann cells (PNS)

• Agents which damage peripheral axons• Agents which damage nerve cell body• Agents which cause localized CNS lesions

Mechanism of action


1. Autonomic function (lacrimation and salivation, piloerection and exophthalmus, urination and defecation, pupillary function, palpebral closure)

2. Convulsions, tremors, or abnormal motor movements3. Reactivity to general stimuli such as removal from the cage or handling

(no reaction to hyperreactivity)4. Arousal level (from coma to hyperalertness)5. Posture and gait abnormalities (home cage and open field)6. Forelimb and hindlimb grip strength7. Landing foot splay8. Sensorimotor responses to stimuli (a tail-pinch, tailflick, hot-plate,

acoustic startle response )9. Body weight10. Any unusual or abnormal behaviors, excessive or repetitive actions

(stereotypies), emaciation, dehydration, hypotonia or hypertonia, altered fur appearance, red or crusty deposits around the eyes, nose, or mouth, and any other observations that may facilitate interpretation of the data.

Functional Observational Battery (FOB)


1. Count of rearing activity on the open field. 2. Ranking of righting ability. 3. Body temperature. 4. Excessive or spontaneous vocalizations. 5. Alterations in rate and ease of respiration,

e.g., rales or dyspnea. 6. Sensorimotor responses to visual or

proprioceptive stimuli.

Functional Observational Battery (FOB)


• Memory and learning tests (e.g. radial maze, Morris water maze)

• Conditioned stimulus/responses• Other supportive techniques:• Histopathology/Quantitative Stereology• EEG, EMG, Functional MRI:

CBV-fMRI of rat brain induced by electrical stimulation of both forepaws. T. Reese, A. Sauter, N. Beckmann, M. Rudin et al.Novartis Pharmaceutical, Basel, Switzerland

http://www.bruker-biospin.de/MRI/applications/bio29.html

Other measures


WHAT IS THIS?

N

NN

N

CH3

CH3

CH3

O

O

1

3

7


Dependence on oxygen• Little anaerobic capacity• CO – less available oxygen• Cyanide – inability to use oxygen

Dependence on glucose• Sole energy source

High metabolic rate

Physiological Sensitivity


H+

H+

H+

I III IV V

+++++++ +++++++++++++ +++ ++++++

- - - - - - - - - - - - - - - - - - - - - - - - - -- - -

NAD+NADH + H+

O2 + 4H+

2H2O

Matrix

Intermembrane Space

3 ATP

H+

Interference with Energy Production


Carbon monoxide, nitrites Affect oxygen delivery to tissues

Nitrophenols, organotins, mercurials Uncouple oxidative

phosphorylation Rotenone, antimycin A, cocaine

Inhibit electron transport

Energy Production Inhibitors


Binding to macromolecules

Interference with enzyme functionMethotrexate (DHFR inhibitor)Hydroxyurea (Ribonucleotide reductase)

Formation of lipid hydroperoxides, free radical generationCarbon tetrachloride, paraquat, ozone

Nucleic acids (DNA, RNA)Cyclophsophamide, MNU, MMS


Oxidative stress Depletion of GSH (DEM, BSO, metals) Oxidation of protein thiols (metals)

SS

Hg S

S

Hg

Active Inactive

Binding to macromolecules


Structure• Long cell requires extensive

intracellular transport

Blood-Brain BarrierDevelopmental stage

(lead and alcohol)

Physiological Sensitivity


Neurons CANNOT divide and replace themselves

Neurons CAN repair limited axonal damage

Most Recovery• Redundancy of Function• Plasticity of Organization

Reversibility of Damage


Classification of neurotoxicants by mechanism of action

Permanent inhibition of nerve function• Agents which cause Anoxia

• Anoxic anoxia• Ischemic anoxia• Cytotoxic anoxia

• Agents which damage myelin formation• Oligodendroglia (CNS)• Schwann cells (PNS)

• Agents which damage peripheral axons• Agents which damage nerve cell body• Agents which cause localized CNS lesions


Neurological and Behavioral Effects of Exposure to Toxic Substances

Motor Effects - Convulsions, weakness, tremor, twitching, lack of coordination, unsteadiness, paralysis, reflex abnormalities, activity changes

Sensory Effects - Equilibrium changes, vision disorders, pain disorders, tactile disorders, auditory disorders

Cognitive Effects - Memory problems, confusion, speech impairment, learning impairment

Mood and personality effects - Sleep disturbances, excitability, depression, irritability, restlessness, nervousness, tension, delirium, hallucinations

General effects - Loss of appetite, depression of neuronal activity, narcosis stupor, fatigue, nerve damage


ReceptorReceptor

LigandLigand

Cell MembraneCell Membrane

Signal ProteinSignal Protein

Positive ResponsePositive Response

Outside CellOutside Cell

Inside CellInside Cell

Ligand binds to receptorLigand binds to receptor

11

33

22

Normal Receptor-Ligand Interaction


ToxicantToxicant11

Toxicantinactivates

receptor

Toxicantinactivates

receptor

No ResponseNo Response

3322

Inactivation of Receptor by Toxicant


Toxicant1

No Response

32

Ligand

Toxicant out competes normal ligand

Ligand cannot bind receptor

Competition For Receptor


Who Is Vulnerable?

Young or OldMale or FemaleGenetics - Individual Diff.Species


Fetal Nervous System

Developing Nervous System

Mature Nervous System

Aging Nervous System

Vulnerability / Sensitivity


A Small Dose of ™ Neurotoxicity


Additional Information

Web Sites• U.S. National Institute of Health - National Institute

of Neurological Disorders and Stroke (NINDS). Online. Available HTTP: <http://www.ninds.nih.gov/> (accessed: 10 April 2003).

• Natural Resources Defense Council (NRDC). Online. Available HTTP: <http://www.nrdc.org/health/kids/cfqpa0599.asp> (accessed: 10 April 2003). NRDC site provides information on children’s health and neurotoxicology.

Other Chapters• Mercury, lead, pesticides


Authorship Information

For Additional Information ContactSteven G. Gilbert, PhD, DABT

E-mail: [email protected]: www.asmalldoseof.org

This presentation is supplement to “A Small Dose of Toxicology”

Documents

Non-Cancer Toxicology