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Neurotoxicology:

Factors unique to nervous system

TOXC707 Advanced Toxicology (2006)

Cells of the Nervous System

◄ neurons◄ neuroglia (90% of cells)

oligodendrocytes (Schwann cells in PNS)astrocytes microgliagliosis is a marker of gross CNS toxicity

Increased expression of Glial Fibrillary Acidic Protein- (GFAP)

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Dopamine: A Brain Neuromodulator

Frontal Cortex

Gyrus Cinguli

Sub. Nigra

Corpus Callosum

TegmentumEntorhinal Cortex

Basal Ganglia

Olfactory Tubercle

Nuc. Accumbens

Medial Forebrain Bundle

Hypothalamus

Pituitary

Midbrain}

Types of neuronal connections

Axo-somaticSynapse

Axons

Dendro-dendriticSynapse

Axo-axonalSynapse

Dendrites

Axo-dendriticSynapse

Perikarya

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Major Components of Peripheral Nervous System

Peripheral neuron

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Three views of myelinating Schwann cell

Nucleus

Compact membrane (Myelin)

Cytoplasmic channel(Schmidt-Lantermann)

Schwann cell cytoplasm

Unwrapped

Longitudinal

Cross-section

Processes of demyelination and remyelination

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Susceptibility to neurotoxicants

◄ High metabolic rate and electrical excitability are dependent on membrane integrity and aerobic metabolism

◄ Extended length of axons poses logistical problems associated with transport from cell bodies to terminal fields

◄ Metabolism of some neurotransmitters may produce oxidative stress (e.g., dopamine)

◄ Inability to replace dead or dying cells

Consequences of neuronal characteristics

◄ axonal transport sensitive to toxicants◄ hexanes cause cross-linking of neurofilaments◄ diabetic neuropathy

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Neurodevelopmental Toxicology

Unique aspects of the nervous system for neurotoxicology: Neurodevelopment

◄ Massive loss of neurons during vertebrate development has been known for more than a century.

Beard (1889) – loss of neuronal populations in fish (Rohon-Beard Neurons)Collin (1906) – death of many sensory and motor neurons in the chick embryo

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Clarke, Rogers & Cowan J. Comp. Neurol. 167: 125 (1976)

~50% of Post-mitotic neurons die during normal development

Apoptotic neuronal death in the developing substantia nigra

R. Burke. Cell Tiss. Res. 2004

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Victor Hamburger: Peripheral Targets Regulate Cell Death

led to NGF discovery

Transcriptional regulation of apoptotic cell death

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Summary

◄ There is massive death of neurons, neuroprogenitors, and oligodendroglia in normal vertebrate development.

◄ This is largely regulated by access to limiting supplies of exogenous survival-promoting trophic factors.

◄ Survival is promoted largely by activation of Akt as well as Erks, and involves blockade of death pathways at multiple points.

◄ Developmental neuron death is transcription dependent. ◄ Induction of death involves multiple pro-apoptotic signaling

pathways, some of which converge on induction of BH3-domain proteins.

Impact of neurodevelopment on toxicology

◄ The effects of toxicological insults may be temporally delayed, being expressed as a variety of alterations in development.

◄ The effects of toxicant exposure will be markedly affected not only by dose/concentration, but also by timing.

◄ Insults by the same dose/concentration at different times during development may result in markedly different sequelae.

◄ Extrapolation from animal models present an even greater challenge than usual because of species differences in developmental patterns.

◄ This will be discussed later re. Fetal Alcohol Syndrome and solvents.

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Toxicant Access and Metabolism

Unique aspects of the nervous system for neurotoxicology: Blood-brain barrier

◄ The choroid plexus separates the blood from the cerebrospinal fluid, whereas the blood-brain barrier limits the influx of circulating substances into the immediate brain interstitial space.

◄ Blood brain barrier limits influx of circulating substances from capillaries into interstitial space

◄ Brain capillaries, unlike those in other tissues, are not fundamentally porous.

Tight junctions between adjacent capillary endothelial cellsProcesses from adjacent cells (astrocytic end feet).A microperoxidase (molecular mass 1800 daltons) that is readily transverses capillaries in other tissues will not pass through capillaries in the brain.

◄ Carrier-mediated transport systems exist for entry of certain required molecules (e.g., hexoses, carboxylic acids, amino acids (separate ones for neutral, basic, and acidic amino acids), amines, and inorganic ions

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Breaching the barrier

◄ Generalizations for healthy brain Large molecules (large peptides and proteins) are excludedPolar molecules are excluded; nonpolar lipid-soluble molecules can penetrate more easily

e.g., increased absorption of dimethyl mercury vs. inorganic mercury (Minamata disease)e.g., MPP+ (toxic metabolite of MPTP) does not cross the BBB

Specific transport systems may facilitate toxicant passagee.g., elemental mercury forms complex with cysteine and is recognized by amino acid transporters as methionine

◄ Alterations in BBBsubstances that alter membrane function (organic solvents)brain edemabacterial meningitis

Unique aspects of the nervous system for neurotoxicology: Toxicant metabolism

◄ Although some xenobiotic metabolic capacity exists in brain, the relative concentration is low compared to the liver or other tissues.

◄ Detoxification mechanisms in CNS have much lower capacity and diversity than in periphery.

◄ Can be important for specific toxicants. 2,4,5-trihydroxyphenylalanine is activatedMPTP is activated

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Unique aspects of the nervous system for neurotoxicology: Plasticity

◄ The nervous system has a unique capacity to accommodate to change.

◄ These changes may sometimes mask, or even be caused by, neurotoxic insult.

◄ Interesting phenomena include:DesensitizationSensitizationUp- and down-regulationLong-term potentiation and other types of synaptic plasticitySprouting

Neurotransmission

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Neurotransmission

◄ Relies on separation of positive and negative charges across membrane

◄ Ionic gradient depends on ATP-linked Na+/K+ pumpat rest, interior more negatively chargedfollowing sufficient stimulus in dendritic region, unidirectional impulse flow along axon occurs role of ion channels

voltage gated sodium voltage-gated potassium channels

◄ Electrochemical neurotransmission vs. electrical transmission

Synapse

◄ Specialized structure for releasing and sensing small amounts of neurotransmitters

◄ Neurotransmitters vs. neuromodulators◄ Importance to toxicologists

toxicants may act directly at synaptic locitoxicants may indirectly alter synaptic function

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Synaptic Structure

1

4

2

Na+

transmitter synthesis Precursor

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6

3

Second messenger events

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PresynapticCell

PostsynapticCell

ImpulseFlow

etc.

mRNA

Nucleus

(in perikaryon)

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adenylate cyclaseG protein

GTPcAMPATP

GDPGTPcAMPATP

GDP

Synaptic targets for toxicants

◄ Neurotransmitter synthesis◄ Neurotransmitter storage◄ Neurotransmitter inactivation or degradation◄ Neurotransmitter receptor binding◄ Receptor-linked second messenger events◄ Pumping or transport of ions◄ Downstream cellular function (e.g., nucleic acid

synthesis)

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Mechanisms of toxicity:Receptors

Receptors and Signal Transduction

2 Ion

R R

ligand1

αβ γ

α

β E2

R E1

ligand

γ

ligand

nucleus

R

R

3 ligand

E

R R

4

P

P

P

P

R R

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Receptor-active toxicants

◄ morphine and codeinealkaloids of the opium poppy that causes acute analgesic, antitussive, euphoric, emetic/antiemetic effects

◄ mescaline derivative of peyote cactusmescaline is believed to cause central actions via interactions with serotonin receptors

◄ ergot alkaloidsLSD (ergot-contaminated grain and medieval European cities and the Salem witch trials??).

◄ methylxanthinescaffeine; theophylline are found in coffee and teaAdenosine receptor ligands plus phosphodiesterase inhibitors

◄ reserpine – VMAT2 ligandblocks vesicular monamine transporter in dopamine and serotonin neuronsinitial effect is massive releaselater effect is long term depletion

Cholinergic agents

◄ α-bungarotoxinblocks nicotinic acetylcholine receptor (binds irreversibly)

◄ belladonna alkaloids (atropine)derived from a group of alkaloids from “deadly nightshade”(Belladonna)competitively blocks muscarinic cholinergic receptor (antidote for muscarinic agonists/ACh overactivity, but in high doses can itself produce toxicity)

◄ nicotine nAChR agonist

◄ Snake neurotoxins: α-bungarotoxin

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Clostridium Indirect Actions

◄ TetanusCl. tetani produces 70,000 KDa protein called tetanospasminBlocks inhibitory synaptic input on spinal motor neurons, resulting in spastic paralysis.moved through nerve cells via retrograde axonal transport until it binds, or is fixed, to gangliosides in the brain stem or cord.Ricin also retrogradely transported.

◄ BotulismCl. botulinum produces a series of neurotoxinsBind to presynaptic cholinergic nerve terminals

◄ Gas gangrene Cl. perfringens

Amino acid receptors

◄ strychnineblocks glycine receptors in spinal cord predominantlyeffects due to blockade of normal inhibitory influence of glycine receptor complex

◄ monosodium glutamate (MSG)sodium salt of amino acid glutamatecan be actively transported into brain

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Ion channel ligands

◄ Alteration in sodium channel activitytetrodotoxin (isolated from puffer fish) and saxitoxin (dinoflaggelate phytoplankton)

binds to voltage-dependent sodium channel and blocks increases in conductancedisrupts generation of action potentials

veratridine steroidal alkaloid (found in Veratrum and Zygadenus species) depolarizes nerve membranes.

grayanotoxins (plant alkaloids from leaves of Ericaceae family)causes reversible increase in Na+ channel permeability

◄ Ouabaininhibits Na+K+ ATPase by high affinity binding to a site on the enzymeinterferes with maintenance of electrical potential across membrane

Agents that disrupt calcium homeostasis

◄ In addition to effects on Na+ channels, pyrethroid insecticides target Ca2+/Mg2+ ATPase and calmodulin

inhibition of these enzymes increases intracellular calciumexcessive intracellular calcium is linked to a variety of deleterious effects

◄ Among other effects, a variety of heavy metals (lead, mercury, aluminum) are associated with increased intracellular calcium

actions may derive from competition for binding sites on various types of calcium binding proteins

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Agents that alter intracellular signaling

◄ Mercury has ubiquitous effectse.g., interferes with synthesis of tubulin and other proteinsmechanism my be its ability to couple to cysteine and other thiol-containing groups, promoting binding to many proteins

◄ Aluminumcompetes with iron for cellular uptake due to similar coordination chemistryparticipates in redox cycling and oxygen radical formationpromotes aggregation of certain proteins

has been linked to pathogenesis of Alzheimer’s disease (AD)presence of aluminum in neurofibrillary tangles may be a consequence, not a cause of AD

Agents that cause hypoxia

◄ Any agent that derives CNS of oxygen is neurotoxic◄ Neuronal subpopulations with very metabolic activity are

particularly susceptible (e.g., hippocampus, neocortex)◄ Sequelae of hypoxia are similar to excitotoxicity◄ Anoxic hypoxia (compromised oxygen supply to brain despite

adequate blood flow)carbon monoxide

◄ Ischemic hypoxia (block of blood supply to brain)any agent causing cardiovascular failure (digitalis glycosides)

◄ Cytotoxic hypoxia (interference with cellular respiration)cyanideazide

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Agents that affect membranes

◄ Organic and inorganic lead damage membranes probably occurs via disruption of ion channelsresults in ultrastructural damage to mitochondria, breakdown of active transport, damage to myelin-containing membranes

◄ Copper can participate in formation of reactive oxygen species and lipid peroxidation

◄ Solvents and vapors are lipid soluble and can alter membrane fluidity

Indirect effects on neurotransmission

◄ activation of neurotransmitter releaselatrotoxin (black widow venom) releases vesicle-bound neurotransmittersamphetamine, methamphetamine, ephedrine release catecholaminesmethylmercury neurotransmitter release occurs secondaryto altered calcium homeostasis

◄ inhibition of neurotransmitter reuptake or metabolismorganophosphates inhibit acetylcholinesterase

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Agents that interfere with oxidative phosphorylation

◄ Classical inhibitors of oxidative phosphorylationdinitrophenolcyanidehydrogen sulfide

◄ Lead, mercury and other metals indirectly compromise oxidative phosphorylation by mitochondrial insult

◄ MPTP directly inhibits oxidative phosphorylation

Agents that damage myelin

◄ Some demyelinating agents do not cross BBB and demyelinate only in periphery

◄ Other agents of capable of CNS and PNS effectsHexachloropheneIsoniazidTelluriumOrganotins

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Protection from oxidative damage

◄ Brain has highest rate of oxidative activity of any organ◄ Endogenous oxygen-derived radicals are thought to be

important in pathogenesis of many neurodegenerative diseases◄ Both neurons and glia contain protective mechanisms; neurons

benefit from secreted enzymes manufactured in gliae.g., glutathione is distributed ubiquitously--chelates transition metals and prevents redox cycling eventsglutathione peroxidase and superoxide dismutase are present in astrocytescatalase is found in oligodendrocytes

◄ The cytochrome P450 isozymes found in brain purported to have a role in Parkinson’s disease