Download Neurotox I

TOXC707 Advanced Toxicology (2007)
Neurotoxicology:
Overview:
Factors unique to nervous system
Examples of neurotoxicants
Cells of the Nervous System
◄ neurons
◄ neuroglia
(90% of cells)
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oligodendrocytes (Schwann cells in PNS)
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astrocytes
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microglia
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gliosis is a marker of gross CNS toxicity
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Increased expression of Glial Fibrillary Acidic Protein- (GFAP)
Dopamine: A Brain Neuromodulator
Frontal Cortex
Gyrus Cinguli
Corpus Callosum
Basal Ganglia
Nuc.
Accumben
s
Olfactory Tubercle
Hypothalamus
}
Sub. Nigra
Tegmentum
Pituitary
Medial Forebrain Bundle
Entorhinal Cortex
Midbrain
Types of neuronal connections
Axons
Axo-axonal
Synapse
Axo-somatic
Synapse
Perikarya
Dendro-dendritic
Synapse
Axo-dendritic
Synapse
Dendrites
Major Components of
Peripheral Nervous System
Peripheral neuron
Three views of myelinating Schwann cell
Nucleus
Schwann cell
cytoplasm
Unwrapped
Compact
membrane
(Myelin)
Longitudinal
Cross-section
Cytoplasmic
channel
(SchmidtLantermann)
Processes of demyelination and remyelination
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
Dr. Mailman’s Pet Peeves
◄ Neurotoxin
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A toxic compound of natural origin
◄ Neurotoxicant
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A toxic compound
◄ Putative
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“Generally regarded as such; supposed” (American
Heritage)
“Generally thought to be or to exist, whether or not this is
really true” (Cambridge)
Does not mean hypothesized or speculated.
Consequences of neuronal characteristics
◄ axonal
transport sensitive to toxicants
◄ hexanes
cause cross-linking of neurofilaments
◄ diabetic
neuropathy
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.
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Beard (1889) – loss of neuronal populations in fish (RohonBeard Neurons)
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Collin (1906) – death of many sensory and motor neurons
in the chick embryo
~50% of Post-mitotic neurons
die during normal development
Clarke, Rogers & Cowan J. Comp. Neurol. 167: 125 (1976)
Apoptotic neuronal death in
the developing substantia nigra
R. Burke. Cell Tiss. Res. 2004
Victor Hamburger:
Peripheral Targets Regulate Cell Death
led to NGF discovery
Transcriptional regulation of apoptotic cell death
Summary
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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 BH3domain proteins.
Impact of neurodevelopment on toxicology
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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.
Toxicant Access and Metabolism
Unique aspects of the nervous system for
neurotoxicology: Blood-brain barrier
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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.
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Tight junctions between adjacent capillary endothelial cells
Processes 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
Breaching the barrier
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Generalizations for healthy brain
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Large molecules (large peptides and proteins) are excluded
Polar molecules are excluded; nonpolar lipid-soluble molecules can
penetrate more easily
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Specific transport systems may facilitate toxicant passage
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e.g., increased absorption of dimethyl mercury vs. inorganic mercury
(Minamata disease)
e.g., MPP+ (toxic metabolite of MPTP) does not cross the BBB
e.g., elemental mercury forms complex with cysteine and is recognized by
amino acid transporters as methionine
Alterations in BBB
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substances that alter membrane function (organic solvents)
brain edema
bacterial 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.
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2,4,5-trihydroxyphenylalanine is activated
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MPTP is activated
Unique aspects of the nervous system for
neurotoxicology: Plasticity
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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:
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Desensitization
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Sensitization
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Up- and down-regulation
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Long-term potentiation and other types of synaptic plasticity
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Sprouting
Neurotransmission
Neurotransmission
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Relies on separation of positive and negative charges across
membrane
Ionic gradient depends on ATP-linked Na+/K+ pump
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at rest, interior more negatively charged
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following sufficient stimulus in dendritic region, unidirectional impulse
flow along axon occurs
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role of ion channels
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voltage gated sodium
voltage-gated potassium channels
Electrochemical neurotransmission vs. electrical transmission
Synapse
◄ Specialized
structure for releasing and sensing small
amounts of neurotransmitters
◄ Neurotransmitters
◄ Importance
vs. neuromodulators
to toxicologists
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toxicants may act directly at synaptic loci
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toxicants may indirectly alter synaptic function
Synaptic Structure
Synaptic targets for toxicants
◄ Neurotransmitter
synthesis
◄ Neurotransmitter
storage
◄ Neurotransmitter
inactivation or degradation
◄ Neurotransmitter
receptor binding
◄ Receptor-linked
◄ Pumping
second messenger events
or transport of ions
◄ Downstream
synthesis)
cellular function (e.g., nucleic acid
Mechanisms of toxicity:
Receptors
Receptors and Signal Transduction
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E2
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Toxicants acting directly on receptors
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morphine and codeine
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mescaline
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LSD (ergot-contaminated grain and medieval European cities and the Salem witch
trials??).
methylxanthines
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derivative of peyote cactus
mescaline is believed to cause central actions via interactions with serotonin receptors
ergot alkaloids
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alkaloids of the opium poppy that causes acute analgesic, antitussive, euphoric,
emetic/antiemetic effects
caffeine; theophylline are found in coffee and tea
Adenosine receptor ligands plus phosphodiesterase inhibitors
reserpine – VMAT2 ligand
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blocks vesicular monamine transporter in dopamine and serotonin neurons
initial effect is massive release
later effect is long term depletion
Cholinergic toxins
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Snake neurotoxins
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a-bungarotoxin
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Belladonna alkaloids
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atropine; scopolamine
derived from “deadly nightshade” (Belladonna sp.)
competitively blocks muscarinic cholinergic receptors
also used as antidote for muscarinic agonists/ACh overactivity
nicotine
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blocks nicotinic acetylcholine receptor (binds irreversibly)
nAChR agonist
Cholinesterase inhibitors
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solanine and chaconine (Solanum sp., tomoato, potato)
Huperzine A
physostigmine (eserine) from Calabar bean
Clostridium Indirect Actions
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Tetanus
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Botulism
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Cl. tetani produces 70,000 KDa protein called tetanospasmin
Blocks 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.
Cl. botulinum produces a series of neurotoxins
Bind to presynaptic cholinergic nerve terminals
Gas gangrene
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Cl. perfringens
Amino acid receptors
◄ strychnine
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blocks glycine receptors in spinal cord predominantly
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effects due to blockade of normal inhibitory influence of
glycine receptor complex
◄ monosodium
glutamate (MSG)
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sodium salt of amino acid glutamate
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can be actively transported into brain
Ion channel ligands
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Alteration in sodium channel activity
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tetrodotoxin (isolated from puffer fish) and saxitoxin (dinoflaggelate
phytoplankton)
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veratridine
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steroidal alkaloid (found in Veratrum and Zygadenus species) depolarizes
nerve membranes.
grayanotoxins (plant alkaloids from leaves of Ericaceae family)
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binds to voltage-dependent sodium channel and blocks increases in
conductance
disrupts generation of action potentials
causes reversible increase in Na+ channel permeability
Ouabain
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inhibits Na+K+ ATPase by high affinity binding to a site on the enzyme
interferes with maintenance of electrical potential across membrane
Agents that disrupt calcium homeostasis
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In addition to effects on Na+ channels, pyrethroid insecticides
target Ca2+/Mg2+ ATPase and calmodulin
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inhibition of these enzymes increases intracellular calcium
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excessive 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
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actions may derive from competition for binding sites on various types
of calcium binding proteins
Agents that alter intracellular signaling
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Mercury has ubiquitous effects
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e.g., interferes with synthesis of tubulin and other proteins
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mechanism my be its ability to couple to cysteine and other thiolcontaining groups, promoting binding to many proteins
Aluminum
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competes with iron for cellular uptake due to similar coordination
chemistry
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participates in redox cycling and oxygen radical formation
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promotes aggregation of certain proteins
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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
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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)
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Ischemic hypoxia (block of blood supply to brain)
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carbon monoxide
any agent causing cardiovascular failure (digitalis glycosides)
Cytotoxic hypoxia (interference with cellular respiration)
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cyanide
azide
Agents that affect membranes
◄ Organic
and inorganic lead damage membranes
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probably occurs via disruption of ion channels
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results in ultrastructural damage to mitochondria,
breakdown of active transport, damage to myelincontaining 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 release
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latrotoxin (black widow venom) releases vesicle-bound
neurotransmitters
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amphetamine, methamphetamine, ephedrine release
catecholamines
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methylmercury neurotransmitter release occurs secondary
to altered calcium homeostasis
◄ inhibition
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of neurotransmitter reuptake or metabolism
organophosphates inhibit acetylcholinesterase
Agents that interfere with oxidative
phosphorylation
◄ Classical
inhibitors of oxidative phosphorylation
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dinitrophenol
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cyanide
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hydrogen 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 effects
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Hexachlorophene
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Isoniazid
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Tellurium
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Organotins
Protection from oxidative damage
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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 glia
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e.g., glutathione is distributed ubiquitously--chelates transition metals
and prevents redox cycling events
glutathione peroxidase and superoxide dismutase are present in
astrocytes
catalase is found in oligodendrocytes
The cytochrome P450 isozymes found in brain purported to
have a role in Parkinson’s disease
Discussion
Translational Medicine (Buzzzzzzz):
How can we assess neurotoxicity?