Neurotoxic Effects of Neurotoxic Effects of SolventsSolvents
William BoyesWilliam Boyes
Neurotoxicology DivisionNeurotoxicology DivisionNational Health and Environmental National Health and Environmental
Effects Research LaboratoryEffects Research LaboratoryOffice of Research and Development, Office of Research and Development,
[email protected]@epa.gov
Chemical StructuresChemical Structures
Benzene Toluene
Trichloroethylene (TCE)
Tetrachloroethylene
(Perchloroethylene) or “Perc”
SolventsSolvents
LipophilicLipophilic Organic chemical compositionOrganic chemical composition Distribute to body lipid-rich tissuesDistribute to body lipid-rich tissues Easily cross lipid membranes & barriersEasily cross lipid membranes & barriers
VolatilityVolatility Inhalation is a significant route of exposureInhalation is a significant route of exposure Delivery to tissues depends upon the Delivery to tissues depends upon the
blood:tissue partition coefficientblood:tissue partition coefficient
Solvent NeurotoxicitySolvent Neurotoxicity
CNS depressionCNS depression EuphoriaEuphoria Sensory, cognitive, Sensory, cognitive,
motor deficitsmotor deficits Reversible after Reversible after
exposureexposure Causes accidents or Causes accidents or
injuriesinjuries Good animal modelsGood animal models Several mechanistic Several mechanistic
targetstargets
Organic brain disorderOrganic brain disorder Originally observed in Originally observed in
Scandinavian paintersScandinavian painters Sensory, cognitive, Sensory, cognitive,
motor deficitsmotor deficits Years of high level Years of high level
exposureexposure No good animal No good animal
modelsmodels Mechanisms unknownMechanisms unknown
Acute Chronic
Volatile Organic Volatile Organic CompoundsCompounds
Widely used in Widely used in industry and industry and commercecommerce
Emission SourcesEmission Sources Factories & chemical Factories & chemical
plants plants Smaller shopsSmaller shops Mobile sourcesMobile sources Indoor air sourcesIndoor air sources Consumer productsConsumer products Pesticide “inerts”Pesticide “inerts” Drinking waterDrinking water Hazardous waste sitesHazardous waste sites
Uses include:Uses include: Organic solventsOrganic solvents Cleaning & degreasingCleaning & degreasing GasolineGasoline Paints & gluesPaints & glues Dry cleaningDry cleaning Printing Printing Paint strippers, nail Paint strippers, nail
polish removerpolish remover Microelectronic Microelectronic
manufacturemanufacture
Upset EmissionsUpset Emissions
UpsetsUpsets Startup, maintance, Startup, maintance,
shutdownshutdown In some cases, upset In some cases, upset
releases exceeded releases exceeded annual releases several annual releases several thousand foldthousand fold
7,533 upset events 7,533 upset events reported to Texas in reported to Texas in 20042004
Some facilities report Some facilities report upset events on average upset events on average every other dayevery other day
Public Citizen, 2005
Small Gasoline EnginesSmall Gasoline Engines
2-cycle engines burn 2-cycle engines burn only about 60% of the only about 60% of the fuelfuel
The remainder is The remainder is emitted as a breathable emitted as a breathable hydrocarbon misthydrocarbon mist
Contains BTEXContains BTEX Benzene, toluene Benzene, toluene
ethylbenzene and xyleneethylbenzene and xylene Hundreds of other Hundreds of other
compounds in small compounds in small amountsamounts
The Washington Post, Aug. 14, 2002
Dry CleanersDry Cleaners
Perchloroethylene Perchloroethylene Detected in co-Detected in co-
located located apartmentsapartments Especially in NYCEspecially in NYC
Residents may Residents may show poor visual show poor visual function or other function or other problemsproblems
Acute Solvent Actions are a Acute Solvent Actions are a Function of Lipid SolubilityFunction of Lipid Solubility
Lipophilic nature Lipophilic nature measured as LogPmeasured as LogP Octanol/water partition Octanol/water partition
coefficientcoefficient Increased partitioning Increased partitioning
into brain and nerve into brain and nerve membranesmembranes
Seen as evidence for Seen as evidence for membrane fluidity membrane fluidity mechanism of actionmechanism of action
Goodman & Gilman, 1990
How are VOCs Causing How are VOCs Causing Acute Effects?Acute Effects?
They were once thought to simply They were once thought to simply dissolve lipid cell membranes dissolve lipid cell membranes
New evidence suggests more New evidence suggests more selection disruption of nerve selection disruption of nerve membrane ion channel proteinsmembrane ion channel proteins
But which ones?But which ones?
Patch-Clamp ElectrophysiologyPatch-Clamp Electrophysiology
V = IRV = IRTo equipment with lots To equipment with lots of wires, knobs, of wires, knobs, switches, buttons and switches, buttons and lights !lights !
VVII
• Glutamate•The most common CNS excitatory neurotransmitter
•NMDA receptor•One type of glutamate receptor •located in the visual system
• NMDA receptor activity is inhibited by toluene
•(in vitro studies, Cruz et al., 1998)
NMDA/gly NMDA/gly NMDA/gly
Taken from Cruz et al., 1998
Compound Ion Channel
Solvents NMDA Receptor nAChR GABAA Receptor Glycine Receptor VSCC*
Toluene Inhibits Inhibits Potentiates Potentiates Inhibits/Potentiates
Trichloroethylene ND ND Potentiates Potentiates Inhibits/Potentiates
Perchloroethylene ND Inhibits ND ND Inhibits/Potentiates
1,1,1-Trichloroethane Inhibits ND Potentiates Potentiates Inhibits
Ethanol Inhibits Inhibits/Potentiates Potentiates Potentiates Inhibits
Halothane Inhibits Inhibits Potentiates Potentiates Inhibits
Solvents & Ion Channel Solvents & Ion Channel FunctionFunction
Bushnell et al., 2005
Chronic Solvent Chronic Solvent Encephalopathy?Encephalopathy?
Scandinavian painters Scandinavian painters and other workersand other workers
Chronic exposure Chronic exposure a variety of a variety of
impairments of mood impairments of mood and intellectual function and intellectual function leading eventually to leading eventually to dementiadementia
Early studies Early studies confounded confounded Poorly matched controlsPoorly matched controls Poor documentation of Poor documentation of
exposure historyexposure history
More recent studies More recent studies showshow Increased reaction Increased reaction
timestimes Poor visual functionPoor visual function Impaired auditory Impaired auditory
thresholdsthresholds Impaired motor skillsImpaired motor skills Impaired performance Impaired performance
of cognitive and of cognitive and memory tasksmemory tasks
TolueneToluene Present in paints, glues, gasoline and many other Present in paints, glues, gasoline and many other
productsproducts Subject of over 40 EPA Maximum Achievable Subject of over 40 EPA Maximum Achievable
Control Technology (MACT) / residual risk Control Technology (MACT) / residual risk assessmentsassessments
The substance of choice for glue sniffersThe substance of choice for glue sniffers
CH3
Toluene
COOH
Benzoic acid
COOGly
Hippuric acid
glycine
0 1 2 3 4 5 6-0.1
0.0
0.1
0.2
0.3REPORTED DATA
PREDICTED EFFECT
95% CONFIDENCE LIMITS
DRAFT. DO NOT CITE.
Blood Toluene (mg/liter)
Be
ha
vio
ral
Eff
ec
t (p
rop
ort
ion
of
ba
seli
ne
)
Toluene & Acute Behavioral ChangesToluene & Acute Behavioral Changese.g. Choice Reaction Timee.g. Choice Reaction Time
Meta-analysis of 6 studies, Benignus et al., in press
Toluene AbuseToluene Abuse
Neuropathic effects in humansNeuropathic effects in humans Following repeated solvent abuseFollowing repeated solvent abuse Very high dose levelsVery high dose levels Cerebellar damageCerebellar damage Cerebral atrophyCerebral atrophy Multiple symptoms of dementiaMultiple symptoms of dementia Confounded by hypoxia and other exposuresConfounded by hypoxia and other exposures
Toluene at High Doses Causes Toluene at High Doses Causes Outer Haircell Damage in Outer Haircell Damage in
ChochleaChochlea
Johnson (1993)Johnson (1993)
Ethanol (Alcohol)Ethanol (Alcohol)
CNS depressantCNS depressant Legal definition of inebriation based on BACsLegal definition of inebriation based on BACs
Often 0.1% (100 mg/100 ml)Often 0.1% (100 mg/100 ml) Acute exposureAcute exposure
EuphoriaEuphoria Loss of inhibitions / poor judgmentLoss of inhibitions / poor judgment Loss of balance & motor coordinationLoss of balance & motor coordination Impaired vision & visual/motor functionImpaired vision & visual/motor function Ataxia, nausea, vomitingAtaxia, nausea, vomiting Unconsciousness Unconsciousness
CH3—CH2—OH
Ethanol MetabolismEthanol Metabolism
CH3—CH2—OH
EthanolEthanol
CH3—CHO
AcetaldehydeAcetaldehydeNAD
Alcohol Alcohol dehydrogenasedehydrogenase
CatalaseCatalaseHH22OO22
+ NADH
Microsomal ethanol Microsomal ethanol oxidizing system oxidizing system
(MEOS) = CYP2E1(MEOS) = CYP2E1
NADPH
10-30% of 10-30% of metabolized metabolized
ethanolethanol
Rate of metabolism Rate of metabolism is ~ 15-20 mg%/hris ~ 15-20 mg%/hr
< 10%
< 20%
Ethanol MetabolismEthanol Metabolism
CH3—CH2—OH
EthanolEthanol
CH3—CHO
AcetaldehydeAcetaldehydeNAD
Alcohol Alcohol dehydrogenase dehydrogenase
(ADH)(ADH)+ NADH
CH3—COO-
Acetic acidAcetic acid
Acetaldehyde dehydrogenase Acetaldehyde dehydrogenase (ALDH)(ALDH)
NAD
+ NADH
ADH
• males > females
ALDH isozymes
• inactive variant in %50 Asians
•Variant form in Native Americans
• inhibited by Disulfiram
Chronic Ethanol ToxicityChronic Ethanol Toxicity
AlcoholismAlcoholism High % of calories from alcoholHigh % of calories from alcohol Thiamine deficientThiamine deficient Wernicke's encephalopathyWernicke's encephalopathy
Damage to multiple brain areasDamage to multiple brain areas Impaired cognition, motor functionImpaired cognition, motor function
Korsakoff's psychosisKorsakoff's psychosis Acute & ChronicAcute & Chronic
100,000 premature deaths / year in U.S.100,000 premature deaths / year in U.S.
Fetal Alcohol Syndrome Fetal Alcohol Syndrome (FAS)(FAS)
Characterized by:Characterized by: Mental retardationMental retardation MicrocephalyMicrocephaly IrritabilityIrritability Reduced birth weightReduced birth weight Poor muscle coordinationPoor muscle coordination Cranio/Facial anomaliesCranio/Facial anomalies Mechanism poorly understoodMechanism poorly understood
Methanol (Wood Alcohol)Methanol (Wood Alcohol)
Clinical signs (people)Clinical signs (people) Multiphasic syndromeMultiphasic syndrome
Early – like ethanol (Central nervous system Early – like ethanol (Central nervous system depression, weakness, headache, vomiting)depression, weakness, headache, vomiting)
Mid- asymptomatic period (12-24 hr)Mid- asymptomatic period (12-24 hr) Late - Severe metabolic acidosis, optic disc Late - Severe metabolic acidosis, optic disc
edema, and bilateral necrosis of the putamenedema, and bilateral necrosis of the putamen Other adverse effects of methanol in humans Other adverse effects of methanol in humans
include minor skin and eye irritationinclude minor skin and eye irritation Formic acid is the toxic metabolite of methanol. Formic acid is the toxic metabolite of methanol.
Accounts for the metabolic acidosis and blindness Accounts for the metabolic acidosis and blindness seen in people following methanol poisoning seen in people following methanol poisoning
CH3—OH
Methanol Metabolism Methanol Metabolism (Primates)(Primates)
CH3—OH
MethanolMethanol
CH2O
FormaldehydeFormaldehydeNAD
Alcohol Alcohol dehydrogenasedehydrogenase
+ NADH
HCOO-
Formic acidFormic acid
Formaldehydealdehyde dehydrogenaseNAD
+ NADHCO2
TetrahydrofolateTetrahydrofolate
LowLow
Methanol Metabolism Methanol Metabolism (Rodents)(Rodents)
CH3—OH
MethanolMethanol
CH2O
FormaldehydeFormaldehyde
CatalaseCatalase
HCOO-
Formic acidFormic acid
Formaldehydealdehyde dehydrogenaseNAD
+ NADHCO2
TetrahydrofolateTetrahydrofolate
HIGHHIGH
Folate deficiency increases the sensitivity of methanol in rodents. Intraretinal metabolism may be important.
Methanol Visual ToxicityMethanol Visual Toxicity
Eells et al., PNAS 2003
n-Hexane and Methyl n-n-Hexane and Methyl n-butyl ketonebutyl ketone
NeurotoxicityNeurotoxicity Sensorimotor polyneuropathySensorimotor polyneuropathy
Sensory numbness and paresthesiaSensory numbness and paresthesia Distal nerves affected firstDistal nerves affected first Clinical signs often delayed for 6-12+ monthsClinical signs often delayed for 6-12+ months Axonal swelling and secondary demyelinationAxonal swelling and secondary demyelination
2,5-hexanedione is common toxic 2,5-hexanedione is common toxic metabolitemetabolite
Spencer & Schaumburg, 2000
Peripheral NeuropathyPeripheral Neuropathy
Sources & ReadingsSources & Readings
Anthony, DC., Montine, T.J., Valentine W.M., and Graham, D.G. Toxic responses of the nervous system. In Casarett and Doull’s Toxicology: the Basic Science of Poisons, Sixth Edition. McGraw-Hill Medical Publishing Division, New York, pp 535-563, 2001
Bushnell, P.J., Shafer, T.J., Bale, A.S., Boyes, W.K., Simmons, J.E., Eklund, C. and Jackson, T.L. Developing an exposure-dose-response model for organic solvents: overview and progress on in vitro models and dosimetry. Environmental Toxicology and Pharmacology, 19: 607–614, 2005.
Benignus, V.A., Bushnell, P.J. and Boyes, W. K. Toward cost-benefit analysis of acute behavioral effects of toluene in humans. Risk Analysis, 25 (2), 447-456, 2005.
Bruckner, J.V. and Warren, D.A., W.K. Toxic effects of solvents and vapors. In Casarett and Doull’s Toxicology: the Basic Science of Poisons, Sixth Edition. McGraw-Hill Medical Publishing Division, New York, pp 869-916, 2001
Schreiber, J S; Hudnell, H K; Geller, A M; House, D E; Aldous, K M; Force, M E; Langguth, K W; Prohonic, E J; Parker, J C (2002) Apartment residents’ and day care workers’ exposures to tetrachloroethylene and deficits in visual contrast sensitivity. Environ Health Perspect 110:655–664.
Arlien-Søborg, P. (1992). Solvent Neurotoxicity. CRC Press, Boca Raton FL.
Spencer PS., and Schaumburg H.H. Experimental and Clinical Neurotoxicology, 2nd Edition, Oxford University Press, 2000.