1. Neurotoxic Effects of Solvents
William Boyes
Neurotoxicology Division
National Health and Environmental Effects Research Laboratory
Office of Research and Development, EPA

2. Chemical Structures Benzene Toluene
Tetrachloroethylene (Perchloroethylene) or “Perc”
Trichloroethylene (TCE)

3. Solvents Lipophilic Volatility Organic chemical composition
Distribute to body lipid-rich tissues
Easily cross lipid membranes & barriers
Volatility
Inhalation is a significant route of exposure
Delivery to tissues depends upon the blood:tissue partition coefficient

4. Solvent Neurotoxicity
Acute
Chronic
CNS depression
Euphoria
Sensory, cognitive, motor deficits
Reversible after exposure
Causes accidents or injuries
Good animal models
Several mechanistic targets
Organic brain disorder
Originally observed in Scandinavian painters
Sensory, cognitive, motor deficits
Years of high level exposure
No good animal models
Mechanisms unknown

5. Volatile Organic Compounds
Uses include:
Organic solvents
Cleaning & degreasing
Gasoline
Paints & glues
Dry cleaning
Printing
Paint strippers, nail polish remover
Microelectronic manufacture
Widely used in industry and commerce
Emission Sources
Factories & chemical plants
Smaller shops
Mobile sources
Indoor air sources
Consumer products
Pesticide “inerts”
Drinking water
Hazardous waste sites

6. Upset Emissions Upsets
Startup, maintance, shutdown
In some cases, upset releases exceeded annual releases several thousand fold
7,533 upset events reported to Texas in 2004
Some facilities report upset events on average every other day
Public Citizen, 2005

7. Small Gasoline Engines
2-cycle engines burn only about 60% of the fuel
The remainder is emitted as a breathable hydrocarbon mist
Contains BTEX
Benzene, toluene ethylbenzene and xylene
Hundreds of other compounds in small amounts
The Washington Post, Aug. 14, 2002

8. Dry Cleaners Perchloroethylene Detected in co-located apartments
Especially in NYC
Residents may show poor visual function or other problems

9. Acute Solvent Actions are a Function of Lipid Solubility
Lipophilic nature
measured as LogP
Octanol/water partition coefficient
Increased partitioning into brain and nerve membranes
Seen as evidence for membrane fluidity mechanism of action
Goodman & Gilman, 1990

10. How are VOCs Causing Acute Effects?
They were once thought to simply dissolve lipid cell membranes
New evidence suggests more selection disruption of nerve membrane ion channel proteins
But which ones?

11. Patch-Clamp Electrophysiology
To equipment with lots of wires, knobs, switches, buttons and lights !
V = IR V I

12. 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

13. Solvents & Ion Channel Function
Bushnell et al., 2005

14. Chronic Solvent Encephalopathy?
Scandinavian painters and other workers
Chronic exposure
a variety of impairments of mood and intellectual function leading eventually to dementia
Early studies confounded
Poorly matched controls
Poor documentation of exposure history
More recent studies show
Increased reaction times
Poor visual function
Impaired auditory thresholds
Impaired motor skills
Impaired performance of cognitive and memory tasks

15. Toluene Present in paints, glues, gasoline and many other products
Subject of over 40 EPA Maximum Achievable Control Technology (MACT) / residual risk assessments
The substance of choice for glue sniffers
Toluene
Benzoic acid
Hippuric acid
CH3
COOH
COOGly
glycine

16. Toluene & Acute Behavioral Changes e.g. Choice Reaction Time
Meta-analysis of 6 studies, Benignus et al., in press

17. Toluene Abuse Neuropathic effects in humans
Following repeated solvent abuse
Very high dose levels
Cerebellar damage
Cerebral atrophy
Multiple symptoms of dementia
Confounded by hypoxia and other exposures

18. Toluene at High Doses Causes Outer Haircell Damage in Chochlea
Johnson (1993)

19. Ethanol (Alcohol) CNS depressant
CH3—CH2—OH
CNS depressant
Legal definition of inebriation based on BACs
Often 0.1% (100 mg/100 ml)
Acute exposure
Euphoria
Loss of inhibitions / poor judgment
Loss of balance & motor coordination
Impaired vision & visual/motor function
Ataxia, nausea, vomiting
Unconsciousness

20. Ethanol Metabolism CH3—CH2—OH CH3—CHO + NADH NAD Ethanol Acetaldehyde
Alcohol dehydrogenase
CH3—CH2—OH
CH3—CHO
+ NADH
NAD
Ethanol
Acetaldehyde
< 10%
Catalase
H2O2
10-30% of metabolized ethanol
NADPH
Rate of metabolism is ~ mg%/hr
< 20%
Microsomal ethanol oxidizing system (MEOS) = CYP2E1

21. Alcohol dehydrogenase (ADH)
Ethanol Metabolism
Alcohol dehydrogenase (ADH)
CH3—CH2—OH
CH3—CHO
+ NADH
NAD
Ethanol
Acetaldehyde
NAD
Acetaldehyde dehydrogenase
(ALDH)
ADH
males > females
ALDH isozymes
inactive variant in %50 Asians
Variant form in Native Americans
inhibited by Disulfiram
CH3—COO-
+ NADH
Acetic acid

22. Chronic Ethanol Toxicity
Alcoholism
High % of calories from alcohol
Thiamine deficient
Wernicke's encephalopathy
Damage to multiple brain areas
Impaired cognition, motor function
Korsakoff's psychosis
Acute & Chronic
100,000 premature deaths / year in U.S.

23. Fetal Alcohol Syndrome (FAS)
Characterized by:
Mental retardation
Microcephaly
Irritability
Reduced birth weight
Poor muscle coordination
Cranio/Facial anomalies
Mechanism poorly understood

24. Methanol (Wood Alcohol)
CH3—OH
Clinical signs (people)
Multiphasic syndrome
Early – like ethanol (Central nervous system depression, weakness, headache, vomiting)
Mid- asymptomatic period (12-24 hr)
Late - Severe metabolic acidosis, optic disc edema, and bilateral necrosis of the putamen
Other adverse effects of methanol in humans include minor skin and eye irritation
Formic acid is the toxic metabolite of methanol.
Accounts for the metabolic acidosis and blindness seen in people following methanol poisoning

25. Methanol Metabolism (Primates)
Alcohol dehydrogenase
CH3—OH
CH2O
+ NADH
NAD
Methanol
Formaldehyde
NAD
Formaldehyde dehydrogenase
Tetrahydrofolate
CO2
HCOO-
+ NADH
Low
Formic acid

26. Methanol Metabolism (Rodents)
Catalase
CH3—OH
CH2O
Methanol
Formaldehyde
NAD
Formaldehyde dehydrogenase
Tetrahydrofolate
CO2
HCOO-
+ NADH
HIGH
Formic acid
Folate deficiency increases the sensitivity of methanol in rodents. Intraretinal metabolism may be important.

27. Methanol Visual Toxicity
Eells et al., PNAS 2003

28. n-Hexane and Methyl n-butyl ketone
Neurotoxicity
Sensorimotor polyneuropathy
Sensory numbness and paresthesia
Distal nerves affected first
Clinical signs often delayed for months
Axonal swelling and secondary demyelination
2,5-hexanedione is common toxic metabolite

29. Spencer & Schaumburg, 2000

30. Peripheral Neuropathy

32. Sources & 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 , 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), , 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 , 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.