Neurotoxic Effects of Solvents William Boyes Neurotoxicology Division

Neurotoxic Effects of Solvents William Boyes Neurotoxicology Division National Health and Environmental Effects Research Laboratory Office of Research and Development, EPA boyes. william@epa. gov

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

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

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

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

Upset Emissions n Upsets n n 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

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

Dry Cleaners n n Perchloroethylene Detected in colocated apartments n n Especially in NYC Residents may show poor visual function or other problems

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

How are VOCs Causing Acute Effects? n n n 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?

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

• Glutamate NMDA/gly Taken from Cruz et al. , 1998 • 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)

Solvents & Ion Channel Function Bushnell et al. , 2005

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

Toluene n n n 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 Benzoic acid Toluene CH 3 Hippuric acid COOH COOGly glycine

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

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

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

Ethanol (Alcohol) CH 3—CH 2—OH n n CNS depressant Legal definition of inebriation based on BACs n n Often 0. 1% (100 mg/100 ml) Acute exposure n n n Euphoria Loss of inhibitions / poor judgment Loss of balance & motor coordination Impaired vision & visual/motor function Ataxia, nausea, vomiting Unconsciousness

Ethanol Metabolism CH 3—CH 2—OH Ethanol Alcohol dehydrogenase NAD CH 3—CHO + NADH Acetaldehyde < 10% Catalase H 2 O 2 NADPH < 20% Microsomal ethanol oxidizing system (MEOS) = CYP 2 E 1 10 -30% of metabolized ethanol Rate of metabolism is ~ 15 -20 mg%/hr

Ethanol Metabolism CH 3—CH 2—OH Alcohol dehydrogenase (ADH) Ethanol NAD CH 3—CHO Acetaldehyde NAD ADH • males > females ALDH isozymes • inactive variant in %50 Asians • Variant form in Native Americans • inhibited by Disulfiram + NADH Acetaldehyde dehydrogenase (ALDH) CH 3—COOAcetic acid + NADH

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

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

Methanol (Wood Alcohol) n n CH 3—OH Clinical signs (people) n Multiphasic syndrome n Early – like ethanol (Central nervous system depression, weakness, headache, vomiting) n Mid- asymptomatic period (12 -24 hr) n Late - Severe metabolic acidosis, optic disc edema, and bilateral necrosis of the putamen n Other adverse effects of methanol in humans include minor skin and eye irritation Formic acid is the toxic metabolite of methanol. n Accounts for the metabolic acidosis and blindness seen in people following methanol poisoning

Methanol Metabolism (Primates) CH 3—OH Methanol Alcohol dehydrogenase NAD CH 2 O Formaldehyde NAD CO 2 + NADH Formaldehyde dehydrogenase Tetrahydrofolate HCOOLow Formic acid + NADH

Methanol Metabolism (Rodents) CH 3—OH Methanol Catalase CH 2 O Formaldehyde NAD CO 2 Formaldehyde dehydrogenase Tetrahydrofolate HCOOHIGH Formic acid + NADH Folate deficiency increases the sensitivity of methanol in rodents. Intraretinal metabolism may be important.

Methanol Visual Toxicity Eells et al. , PNAS 2003

n-Hexane and Methyl n-butyl ketone n Neurotoxicity n Sensorimotor polyneuropathy Sensory numbness and paresthesia n Distal nerves affected first n Clinical signs often delayed for 6 -12+ months n Axonal swelling and secondary demyelination n n 2, 5 -hexanedione is common toxic metabolite

Spencer & Schaumburg, 2000

Peripheral Neuropathy

Sources & Readings n Anthony, DC. , Montine, T. J. , Valentine W. M. , and Graham, D. G. Toxic responses of the nervous system. In Casarett and Doull’s n and Doull’s Toxicology: the Basic Science of Poisons, Sixth Edition. Mc. Graw-Hill Medical Publishing n n 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-doseresponse 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 n n n 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, 2 nd Edition, Oxford University Press, 2000.