NEUROTOXICITY ‘Primer’

Damage may be specific to a given cell type, a given region, or a specific function.

Central Nervous System (Brain and Spinal Cord)

Peripheral Nervous System (Peripheral nerves, motor and sensory)

NEUROTRANSMITTERS

Norepinephrine, serotonin, acetyl choline, dopamine, gamma amino butyric acid, GABA

Agents that Disrupt Transmission

Blocking agents - botulinus (bacterial toxin), tetrodotoxin (puffer fish)

Depolarizing - DDT, pyrethrin (chrysanthemum derived insecticide)

Stimulants - xanthines, strychnine, picrotoxin

Depressants - alcohol, barbiturates, VOC’s (CCl₄)

Receptor antagonists - atropine, scopolamine, belladonna alkaloids

Anticholinesterase - carbamates, organophosphates (insecticides)

Neuromuscular blockers - curare, succynyl choline (pharmacologic agents)

Blood Brain Barrier

Nonpolar lipid soluble compounds penetrate BBB. In the immature brain the BBB is less effective than in the adult, therefore, lead is more likely to affect CNS in children and peripheral nerves in adults.

Zonula Occludentes - tight junctions between capillary endothelial cells.

Glial Cells - 3 types of cells

1. astrocytes - nurse cells of neurons – in gray matter
2. Oligodendrocytes - similar to Schwann cells of peripheral nerves - spirals of myelin about neurons. These allow for saltatory (rapid) conduction of nervous impulse in white matter.
3. Microglia - phagocytic cells

Much of capillary endothelium is invested with astrocytic processes (in some areas capillaries are not wrapped in glial cells – area postrema of the 4th ventricle and median eminence of hypothalamus.

The Endothelial cells of the CNS normally lack pynocytotic vesicles (vesicles may appear in ischemia, irradiation edema, and hypertension).

Extracellular space acts as a barrier – low oncotic pressure and ordered fibrillar mucoprotein structure of interstitium.

Choroid Plexus – may actively transport certain toxicants

AGENTS CAUSING STRUCTURAL (PHYSICAL OR ANATOMIC) DAMAGE

Carbon Monoxide

Cyanide

Diphtheria Toxin

Hexachlorophene

Lead

Tellurium

Thallium

AGENTS ASSOCIATED WITH PERMANENT FUNCTIONAL LESIONS

Acetyl pyridine

DDT

Mercury

Manganese

Selective damage occurs as a result of:

1. differences in ease of penetration across barriers
2. differences in blood flow
3. differences in metabolic rates
4. differences in metabolic function

White matter is less vascularized than gray matter. White matter, myelinated cells, require less oxygen and are more resistant to a number of toxicants.

Small cells of the cerebellum and visual cortex are preferentially killed by methyl mercury (they have less cytoplasm and less rough endoplasmic reticula to bind methyl mercury), resulting in a syndrome akin to Cerebral Palsy after in utero exposure. Minimata Japan and Canadian native Americans.

Certain large cells with high metabolic rates are particularly susceptible to anoxia, damage to each of these tissues results in movement disorders.

hippocampal pyramidal cells

cerebellar Purkinje cells

motor cells of ventral horn of spinal cord

Anoxia is something of a prototype for CNS injury as many neurotoxicants mimic anoxia to a certain degree.

Anoxia - Neurons have little capacity for anaerobic metabolism.

Anoxic anoxia - Primary oxygen lack (otherwise adequate blood supply),

Respiratory failure, or

Decreased cardiac output, or

Decreased oxygen carrying capacity, or

Carbon monoxide or Methemoglobin.

Ischemic anoxia - Decreased arterial pressure,

Decreased cardiac output, or

Vascular occlusion.

Cytotoxic anoxia - Interference of cellular metabolism in the presence of adequate supply of both blood and oxygen. May result from hypoglycemia or from metabolic inhibitors:

cyanide - used in electroplating, metallurgy, etc.

Azide - explosive crystals used in Airbags.

Dinitrophenol

hydrogen sulfide - product of anaerobic digestion of protein

malonitrile

In contrast to anoxic anoxia and ischemic anoxia, in which neurons are most sensitive, it is the oligodendrocyte that is most susceptible to cytotoxic anoxia.

Effects of Anoxia

decreased intracellular pH

increased intracellular lactate (anaerobic glycolysis)

clumping of nuclear chromatin

loss of mitochondrial granules

cell swelling (increased intracellular sodium)

shrinkage of cytoplasm

nuclear pyknosis (increased clumping)

loss of Nissl substance (rough endoplasmic reticulum)

calcium leak

cell death

Prolonged anoxia may result in damage to basal ganglia and substantia nigra and subsequent Parkinsonism (tremor, rigidity, Bradykinesia).

Cerebral edema from anoxia may reflect swelling of astrocytes.

Some work is being done on the possible protective effect of magnesium or calcium channel blockers on inhibiting calcium leakage.

Some toxicants damage the microtubular substructure necessary for transport of nutrients. Vinca alkaloids (and some slow viruses) cause neurofibrillary tangles similar to those seen in Alzheimer’s’ - (double stranded helices). Aluminum causes single stranded helices of microtubules.

Neuro degenerative diseases are often associated with alterations in metabolism and intracellular storage of a number of metals, therefore, it is difficult to tell if elevated levels of a metal reflect the cause of a disease or a result of the disease.

FUNCTIONAL TOXICITY

Functions: motor, sensory, integrative, emotional

Normal function may be restored after considerable damage (redundancy of function, adaptation).

Changes in visual-motor performance, intellect/cognition, and emotional state may be early-sensitive indicators of neurologic injury.

NEUROBEHAVIORAL OR NEUROPSYCHIATRIC ASSESSMENTS

Standardized testing using the World Health Organization neurobehavioral Core testing battery in epidemiologic setting allows for detection of subtle, subclinical neurotoxic effects of occupational or environmental toxicants.

Sensory functions - sight, hearing, temperature, touch, pain, smell.

Sensory neuropathies commonly associated with parasthesias numbness, tingling, or hypersensitivity to touch.

Organic mercury, lead, organophosphates, TOCP are all associated with peripheral neuropathies. Cortical blindness and deafness may result from lead or mercury encephalopathy - particularly in children.

Tests - Visual fields (may be constricted)

Audiometry

Visual, auditory and somatosensory evoked potentials

Nerve conduction velocities

Odor recognition (scratch and sniff kits or essence oils)

Dichotic listening (different words in either ear)

Vibratory Threshold (Optacon)

Thermal Threshold

Motor Functions

Weakness, paralysis, dysfunctional movement, gait disturbance NCV/EMG

Agents associated with movement disorders

MPTP (demeral analogue contaminant) Parkinsonism.

Carbon monoxide - Parkinsonism

Organophosphates - Parkinsonism

Manganese - Parkinsonism

Mercury - distinctive intention tremor

Acrylamide - sensory and cerebellar ataxia

Aromatics (toluene, xylene, glue) - cerebellar ataxia

Integrative Functions

Listed as three; learning is a characteristic of all three.

1. Symbol Formation - memory or learning

Visual Maze

Short and Long Term Memory - hippocampus and mammillary body

2. Sensory-Motor Integration

Reaction times

Card sorting

Finger tapping and Pegboard speeds

Balance board

Body weight in laboratory animals

Carbon monoxide

carbon disulfide

3. Emotional Responses

Limbic systems (hippocampus) has a low seizure threshold and may "control" emotion.

Erethrism (inorganic mercury) - irritability, apprehension, lability, tremor and salivation

Childhood hyperkinesis (hyperactivity) has been associated with lead exposure in exploring toddlers and infants and as a result of in utero, maternal exposures; studies by Dr. Herbert Needleman.

Emotional lability may be a sequellum of a variety of toxicants, including V.O.C.’s, toluene, xylene, etc., in painters.

AGENTS CAUSING ANOXIA

Barbiturates - Severe poisoning often resolves without neurologic deficit (may stabilize cell membranes and reduce metabolism).

Carbon Monoxide - Much more commonly associated with sequelae - causes both ischemic and cytoxic damage. Damage to blood barrier may result in autoimmune state and eventual delayed toxicity. Basal ganglia and subthalamus behavioral changes, confusion, ataxia, rigidity (Parkinsonism).

Cyanide - Affects cortical gray matter and white matter. Inhibits cytochrome oxidase and produces cytoxic anoxia. Cyanide salts are used in gold electroplating and in precious metal reclamation (x-ray silver). When cyanide salts are exposed to acids, volative cyanide gas is generated, a "gas chamber."

Azide - Inhibits cytochrome oxidase, results in hyperkinesis. Azide crystals supply the explosive force in automobile airbags.

Nitrogen Trichloride - Caused an outbreak of "running fits" in dogs fed feed tainted by nitrogen trichloride (contaminated wheat).

Hydrogen Sulfide - Inhibits cytochrome chain; causes cerebral and pulmonary edema. Paralyzes olfactory nerve, therefore, distinctive rotten egg smell may not be noted at toxic concentrations. Sewage pits, wastewater treatment, petroleum industry, Lake Nyos Cameroon disaster.

AGENTS DAMAGING MYELIN

Myelin - Damage may be central, or peripheral, or both. Myelin "insulates" nerve fibers, allowing for rapid, saltatory conduction.

Triethyltin - Highly toxic to oligodendroglia.

Hexachlorophene - Bactericide, causes severe vacuolization in peripheral nerve myelin and in white matter of cerebellum. Bebe incident in France, contaminated talcum powder.

Isoniazid, INH - Causes spongy degeneration and vacuolization of cerebellum and peripheral nerves. Pyridoxine protects against peripheral damage.

Lead - Causes segmental demyelination of peripheral nerves with axonal sparing (similar to Guillan-Barre). Lead toxicity may be selective to motor fibers, particularly of extensor muscles (wristdrop). Upper extremities often affected more than lower extremities. Demyelination results in decreased nerve conduction velocity, NCV, by surface electrodes. Toxicity in children is more likely to be in CNS.

Thallium - depilatory - alopecia - Causes ataxia and painful paresthesias.

Tellurium - Similar to lead, toxicity is much greater in young. Causes neuropathy and behavioral changes.

AGENTS CAUSING PERIPHERAL AXONOPATHIES

Alcohol - Causes distal axonopathy as well as encephalopathy. May reflect failure to axonal nutrient transport. Resembles thiamine deficiency.

Acrylamide - Vinyl monomer resembles TOCP. Causes polyneuritis, with sensory changes, weakness and both sensory (peripheral) and cerebellar (central) ataxia.

Carbon Disulfide - Solvent in rayon industry, and a grain fumigant. Predominant effects are psychosis, Parkinsonian tremor and polyneuritis. Results in encephalopathy with behavioral changes and peripheral neuropathy. (CS₂ has also been associated with atherosclerotic diseases, both cardiovascular and cerebrovascular.) Several cases of relatively acute neurotoxicity have been reported in the grain transport industry, ships holds.

Hexanedione - (N hexane and N butyl ketone are metabolized to a common toxic metabolite, 2,5 hexanedione) Solvent used in glues, cleaning agents, shoe production and increasingly in fabric manufacturing. Causes swelling of large axons. Increased neurofillaments is one of the earliest changes. Giant axonal swelling is followed by myelin thinning.

Organophosphates - 1930 "ginger jake" paralysis affecting 50,000 in U.S. Delayed toxicity documented with TOCP, leptofos, mipafox, diisopropyl-fluorophosphate. Injury may be independent of cholinesterase inhibition. Red blood cell cholinesterase may stay depressed for up to two months after poisoning, baseline levels are critical for interpreting changes. Leukocyte, lymphocyte or platelet neurotoxic esterase assay may be of value in determining the cause of a peripheral neuropathy.

AGENTS AFFECTING NEUROMUSCULAR JUNCTION

Botulinus toxin - Irreversibly binds the axon terminal, prevents the release of acetyl choline (muscle appears denervated).

Tetrodotoxin - Puffer fish toxin (concentrated in liver cells) selectively blocks the sodium channels; affects both skeletal muscle and sensory nerves. Fugu, a sushi delicacy, commonly results in perioral numbness.

Saxitoxin - Dinoflagellate, Gonyaulax, tainted shellfish – resembles tetrodotoxin.

Batrachotoxin - South American frog.

DDT, lindane and other organochlorines - Depolarizes presynaptic nerve terminal causing repetitive firing and seizures.

Pyrethrins (Chrysanthemum derived, non-persistent insecticide) - Most effective at low temperature hence little mammalian toxicity. Affect sodium channel. Piperonyl butoxide is typically added as a synergistic toxin as it selectively inhibits an insect specific metabolic pathway.

Nicotine - Green tobacco illness, hyper-cholinergic state.

Domoic acid - A newly identified excitotoxin, isolated from Atlantic blue mussels from a Prince Edward Island estuary. As epidemic of a G.I. illness and encephalopathy with at least three deaths occurred in Canada in 1987. Domoic acid is felt to be a byproduct of an algae, Nitzchi pungens.

Enzyme Inhibitions, Endocrinopathies and Antidotes

Many toxic substances act as poisons by enzyme inhibition.

1. Cyanide (-C = N), hydrogen sulfide (H₂S), and several mushroom alkaloids bind to cytochrome oxidase enzymes (various, seeds, cassava, laetrile), resulting in cellular anoxia.
2. Metals and low M.W. halocarbons (methyl bromide) bind to sulfhydryl (-SH) groups of multiple cellular enzymes.

Dimercaprol (antidote) supplies an abundance of sulfhydryl groups:

HS - CH₂ - CH - CH₂OH

½

SH

(Parenthetically, as did bezoars or hairballs used in Medieval times as a protection from arsenic poisonings.)

Phosphorylating and carbamylating chemical bind to the esteratic serine site on cholinesterase by attaching phosphoryl or carbamyl groups.

S

(RO) º

P - O - Group

(RO)

O

º

H₃ - NH - C - O - Group

Clinically manifest by hypersecretory states and neurotoxicity.

4. Nitro- and chloro-phenols (herbicides and wood preservatives) inhibit phosphorylating enzymes involved in storing energy from cellular oxidation (those that convert ADP to ATP)

ADP + PO4

Phosphoryl Enzyme

ATP

Result is unrestricted heat production from cellular oxidations (uncoupled oxidative phosphorylation). Clinically manifest by hyperthermia and yellowed discoloration of skin.

5. Impairment of iodination of triiodothyronine (a precursor of thyroid hormone) by thiocyanate.

---

---

I- I- O

º

HO- - O - - CH₂ - CH - C - OH

½

---

---

| | NH₂

I I - SC º N

Explains goitrogenic action of Brassicae (cauliflower), kale, turnips, etc. Similar mechanisms involved in aminotriazole goitrogenicity and ethylene thiourea’s thyroid suppression.

6. Hexachlorobenzene and lead stimulate d amino levulinic acid synthetase involved in heme synthesis with resultant increases in d ALA in blood and urine.

O O

º º

Pyridox = N - CH₂ - C - OH + CoA - C - CH₂ - CH₂ - COOH

Activated Glycine Succinate

³

³

Synthetase Enzyme

O

º

H₂N - CH₂ - C - CH₂ - COOH

d amino levulinic acid

7. op’DDt, in gm/day dosage, inhibits adrenal steroid synthesis, concentrates in and eventually destroys the adrenal cortex.
8. op’DDt, at high dosage in rats, in estrogenic (enlarged uterus). Presumably caused by induction of estrogen-synthesizing enzymes of liver. This presumably explains the egg-shell toxicity experienced by certain raptor bird species.
9. Dipyridyl compounds (Paraquat and probably other chemicals) depress synthesis of dipalmitoyl lecithin in lung. Loss of surfactant activity predisposed to alveolar collapse resultant from decreased alveolar surface tension. Lipid peroxidation may also be involved. Similar to hyaline membrane disease of the newborn. (Paraquat is also profoundly fibrogenic, resulting in interstitial fibrosis and is nephrotoxic.)
10. Effects on critical enzymes undoubtedly play a part in the toxic actions of many chemicals. Some that are being studied are:

Carbon disulfide - CNS enzymology

Phosphine and phosphide - liver enzymology

Mushroom poisons - cellular respiration enzymes

Oxidant gases - lung enzymes

Chlorinated hydrocarbons - steroid synthetic enzymes

11. Certain halocarbons (DBCP, kepone) and heavy metals (lead) suppress spermatogenesis.
12. Nicotinamide antimetabolites (vacor) induce diabetes mellitus by destroying beta cells of pancreatic islets.

Antidotes

Antivenins

1. Antivenin - rattlesnake (crotalidae-polyvalent). Uses serum globulins from gradually desensitized horses active for rattlesnake copperhead, cottonmouth, bust master, and fer de lance snakes. Serum sickness often occurs 1 to 3 weeks after a heavy dose.
2. Antivenin - Black widow spider (latrodectus mactans). Serum from immunized horses, rarely indicated.
3. North American Coral Snake - Serum from immunized horses.

Chelators

4. Calcium EDTA - lead, cadmium, copper, zinc poisoning.

Calcium, ethylene diaminetetra acetic acid

5. Deferoxamine - iron, aluminum
6. Dimercaprol - BAL (British AntiLewisite agent) contains ligands that bind heavy metals in preference to the body’s sulfhydryl containing enzymes. Arsenic, lead encephalopathy, gold, mercury, antimony, zinc, copper, bismuth, chromium.

SH

³

HS - C H- CH₂ - OH

7. D Penicillamine - Oral chelation for copper, lead, mercury, arsenic and bismuth.

CH₃ NH₂ O

³ ³ º

H₃C - C - C - C - OH

³

SH

8. Succimer - DMSA (or 2,3-dimercaptosuccinic acid), a water soluble analog of dimercaprol for oral chelation of lead and mercury.
9. Atropine - Anticholinesterases, carbamates, mushrooms, antagonizes cholinergic stimuli at muscarinic receptors (not nicotinic - muscle weakness, respiratory depression).
10. Pralidoxime - 2 PAM or Protopam, on oxime, used concurrently with atropine for organophosphate toxicity;. Reactivates cholinesterase inactivated by O.P.’s by dephosphorilating the enzyme if administered within 24-48 hours of exposure.
11. Calcium Gluconate - hydrofluoric acid skin burns - chelates fluoride; reverses hyperkalemia, hypermagnesemia and muscle cramps after widow spider bite.
12. Cyanide antidote (also Hydrogen Sulfide) - cyanide binds Fe+3 ferric ion and blocks cytochrome oxidase.

Sodium Nitrite induces methemoglobinemia - methemoglobin pulls cyanide off at cytochrome, or created mesulfhemoglobin from hydrogen sulfide.

Sodium Thiosulfate increases renal excretion of cyanide, but not needed for hydrogen sulfide. In Europe, cyanocobalamin (vitamin B12) is used as an antidote for cyanide.

13. Methylene Blue - reduces methemoglobin to hemoglobin.
14. Methanol - Ethylene Glycol: Ethanol delays formation of formic acid and glycolic or glyoxylic acids by competing with alcohol dehydrogenase:
15. Methionine - repletes glutathione (used in Europe for acetaminophen).

N-Acetyl Cysteine (mucomyst) acts as a glutathione substitute.

H O

| º

H N - C - CH₃

| |

HS - C - C - C - OH

| | º

H H₂ O

16. Naloxone - counteracts opiate narcotics, (antagonist).