effects of tetrachloroethylene on visual and cognitive

Office of Research and DevelopmentNational Health and Environmental Effects Research Laboratory January 29, 2009

Philip J. BushnellWilliam K. BoyesVernon A. BenignusElaina M. KenyonWendy M. Oshiro

For the National Research Council Committee on Tetrachloroethylene

Effects of Tetrachloroethylene on Visual and Cognitive Function in Rats

Office of Research and DevelopmentNational Health and Environmental Effects Research Laboratory

Outline of the Presentation

Goals of the Project Experimentso Animal modelo Pharmacokineticso Sensory function – Visual evoked potentials (Boyes et al., 2008)o Cognitive function – Behavioral signal detection (Oshiro et al., 2008)Dose-Response Modelingo Human exposure contextConclusionso Rats and humans are both sensitive to PCE

• On tests of similar neurological processes• At similar internal doses

o Rat data support the weight of evidence for adverse effects in humans exposed to PCE


Goals of the Project

Develop methods to predict acute ‘systemic’ toxicity of hazardous air pollutants (HAPs) in humans, using:

o Experimental animal models to demonstrate causality in support of associations observed in humans

o PBPK models for extrapolating across durations, species and routes of exposureo Dose-response models for quantitative comparisons across studies, endpoints,

compounds and high-dose to low-dose Study acute-chronic extrapolation for systemic toxicity of HAPsDevelop methods and models to study life-stage susceptibility


Animal Model

Adult male Long-Evans (hooded) rato Kinetics

• 3.5 months, maintained at 350 g body weighto Electrophysiology (Visual function)

• 60 – 90 days, 300 – 450 g, free-fed• Surgical implantation of electrodes in skull over visual cortex

o Behavior (Cognitive function)• 6 – 9 months, maintained at 350 g • Trained extensively to perform a visual signal detection task


– Blood flow-limited– Slow metabolism– Physiological

parameters specific for L-E rat

– Body weight– Activity

– Blood (CBL)– Brain (CBR)

PBPK Model for Perchloroethylene

SPTG

Fat

RPTG

Brain

Arte

rial B

lood

Vmax, KM

Ven

ous

Blo

od

metabolism

Lung

Liver GI Tract

SPTG

Fat

RPTG

Brain

Arte

rial B

lood

Vmax, KM

Ven

ous

Blo

od

metabolism

Lung

Liver GI TractLiverLiver GI TractGI Tract

Inhalation Exhalation


PBPK Model Predictions -- 500 ppm x 60 min

Boyes et al., 2008

Shaded area shows duration of exposure


PBPK Model Predictions -- 1500 ppm x 60 min

Boyes et al., 2008



PBPK Model Predictions – 50 and 500 ppm(Data from Dallas et al. 1994)

Boyes et al., 2008



Assessment of Visual Function:Visual Evoked Potentials (VEPs)

Electrophysiological measure of visual system functiono Pattern VEP – steady-state amplitudeo Apical measure (retina to cortex)o Can be related to perception of visual contrast, which is important for pattern perception

Neurological substrates well understoodMeasurable in humans and in experimental animals


Schematic of Exposure System for VEPs

Boyes et al., 2003


• Awake rat watches TV patterns

• Brain waves are recorded from implanted electrode over visual cortex

• The contrast of the visual pattern cycles on and off at ~5Hz (F1)

• VEPs are recorded and averaged

• Spectral analysis of the VEP shows response amplitude at the stimulation frequency and higher harmonics – especially at F2

• F2 is a measure of the integrated neural activity in visual cortex driven by the visual stimulus

Procedure to Record Visual Evoked Potentials


VEPs in Rats and Humans

Rats and humans show similar spatial-temporal profiles

Boyes, 1994


Effects of Perchloroethylene on Visual Evoked Potentials in Rats

Boyes et al., 2008


As a Function of Time

Time (min)0 20 40 60 80 100 120 140

F2 A

mpl

itude

( μV)

0

5

10

15

20As a Function of Cxt

Concentration x Time (ppm-min)0 1e+5 2e+5 3e+5 4e+5 5e+5 6e+5

F2 A

mpl

itude

( μV)

0

5

10

15

20

As a Function of Brain PCE

Brain Perchloroethylene (mg/l)

0 50 100 150 200 250 300

F2 A

mpl

itude

( μV)

0

5

10

15

20

0 ppm E1 1000 ppm E12000 ppm E13000 ppm E14000 ppm E10 ppm E2 250 ppm E2500 ppm E21000 ppm E2noise E1noise E2

As a Function of Brain PCE AUC

Brain Perchloroethylene AUC (mg-hr/l)

0 50 100 150 200 250F2

Am

plitu

de ( μ

V)0

5

10

15

20

Effect of Perchloroethylene on VEP F2 Amplitude

Effect of PCE on the VEP F2 amplitude in rats as a function of four dose metrics.

No one metric predicted the effect better than did the others.

Boyes et al., 2008


Summary of Effects of Perchloroethylene on VEPs in Rats

PCE reduced F2 amplitude as a function of inhaled concentrationand duration of exposure

o Rats awake and mobile after exposureo No one dose metric was statistically more predictive than the otherso Maximum effect occurred at about 75% below baselineo Reduction occurred at relatively low internal doses

Similarities with other solventso F2 amplitude reductiono Concentration in brain closely related to magnitude of effecto Rapid onset

Differences from other solventso Higher potencyo Lower efficacyo No best dose metric


Cognitive Function:Signal Detection Behavior

Test designed from human literature to assess sustained attentiono More like vigilance than selective attention o Temporal uncertainty, prolonged attending to a simple event

Signal detection tasko Humans and rats respond similarly to factors affecting attention

• Signal intensity• Trial rate• Detection vs discrimination

o Sensitive to many CNS-active drugs and solvents


Behavioral Exposure and Test System

Bushnell et al., 2007


Signal Detection Task

Choice OutcomeTrial Type

Signal

Blank

Hit

Miss

"Signal"

"Signal"

"Blank"

"Blank" CorrectRejection

FalseAlarm

Extend Levers

Signal Period


Visual Signal Detection Behaviorin Humans and Rats

Bushnell et al., 2003

Human Females

Signal Intensity (Arbitrary units)1 2 3 4 5 6

0.0

0.2

0.4

0.6

0.8

1.0

P(hit)

P(fa)

4 Tpm7 Tpm10 Tpm

Rat Males

1 2 3 4 5 6 70.0

0.2

0.4

0.6

0.8

1.0

P(hit)

P(fa)

4 Tpm7 Tpm10 Tpm

Human Males

1 2 3 4 5 6

Mea

n Pr

opor

tion

of R

espo

nses

0.0

0.2

0.4

0.6

0.8

1.0

P(hit)

P(fa)

4 Tpm7 Tpm10 Tpm


Hits and False Alarms

Signal Intensity (lux)0.3 0.7 2.1 5.2 0 500 1000 1500

Prop

ortio

n of

Res

pons

es (M

ean

± SE

M)

0.0

0.2

0.4

0.6

0.8

1.0

Concentration ofPCE (ppm)

Hits

False Alarms

**

**

0 ppm 500 ppm1000 ppm1500 ppm

Effects of PCE on Visual Signal

Detection Behavior in

Rats

Oshiro et al., 2008a


Effects of Perchloroethylene on Signal Detection Behavior

0 200 400 600 800 1000 1200 1400 16000.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0 20 40 60 80 100 120 140 1600.5

0.6

0.7

0.8

0.9

1.0

0 20 40 60 80 100 1200.5

0.6

0.7

0.8

0.9

1.0

0 200 400 600 800 1000 1200 1400 16000.5

0.6

0.7

0.8

0.9

1.0

0.2 0.4 0.6 0.8 1.0

Mea

n R

espo

nse

Tim

e (s

ec ±

SEM

)

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0 20 40 60 80 100 120 140 1600.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0 20 40 60 80 100 1200.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8 0 ppm 500 ppm1000 ppm1500 ppm

0.2 0.4 0.6 0.8 1.0

Mea

n N

umbe

r (±

SEM

)

10

20

30

40

50

60

0 200 400 600 800 1000 1200 1400 160010

20

30

40

50

60

0 20 40 60 80 100 120 140 16010

20

30

40

50

60

0 20 40 60 80 100 12010

20

30

40

50

60


0.2 0.4 0.6 0.8 1.0

Mea

n P(

Cor

rect

) (±

SEM

)

0.5

0.6

0.7

0.8

0.9

1.0


Inhaled Conc (CInh) and Time (t)

AUC for Inhaled Conc (CInh x t)

Internal Dose (CBr)

AUC for Internal Dose (CBr x t)

Exposure Duration(hr)

Cumulative Inhaled dose (ppm-hr)

Estimated CBr(mg/L)

Cumulative Brain [PCE] (mg-hr/L)

Accuracy (Proportion correct)

Response Time(sec)

Number of Trials Completed

Oshiro et al. 2008b


Summary of Effects of Perchloroethylene on Signal Detection Behavior in Rats

PCE reduced accuracy, increased response time, and lowered trial completion as a function of magnitude of exposure

o Rats awake and mobile throughout exposureo PCE in brain was more predictive than the other dose metrics

Similarities with other solventso All effects in same direction as other solventso Concentration in brain best metric

Differences from other solventso Low efficacy on accuracy (like effect of PCE on VEP amplitude)


Relevance of Rat Data to Human Hazard:An Initial Look Using Data of Altmann 1990

Convert exposure C and t to concentration in braino Rat: Use PBPK model to estimate CBR for each exposure conditiono Human: Convert measured CBL to estimated CBR using a Brain:Blood partition coefficient of 13.5 (from Thrall et al., 2002)

• After 4 hr exposure to PCE,• 10 ppm → 0.33 mg/L CBL → 4.5 mg/L CBR

• 50 ppm → 1.1 mg/L CBL → 15.0 mg/L CBR


EVEP= (VEPb – VEPi) / VEPb

The Effect of PCE on F2 Amplitude in Relation to Human Effective Doses

– F2 amplitude converted to effect magnitude

– Dose shown as estimated PCE concentration in the brain– Shaded bar shows range of internal doses in human volunteers, as estimated CBR

– This effect occurs in rats at internal doses that are effective in humans

Range of CBR values estimated from Altmann et al. 1990

Modified from Boyes et al., 2008


P(Correct)

Predicted CBR (mg/L)0 20 40 60 80 100 120 140 160

Mea

n Pr

opor

tion

of R

espo

nses

(± S

EM)

0.5

0.6

0.7

0.8

0.9

1.0

0 ppm500 ppm1000 ppm1500 ppm

Reponse Time

Predicted CBR (mg/L)0 20 40 60 80 100 120 140 160

Mea

n R

espo

nse

Tim

e (s

ec ±

SEM

)

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0 ppm500 ppm1000 ppm1500 ppm

Effects of Perchloroethylene on Signal Detection Behavior

Oshiro et al., 2008b

Altmann et al. 1990 Altmann et al. 1990


Assumptions and Limitations

Assumptions: CBR is proper dose metric for acute effects of PCE Human and rat brains are equally sensitive to PCEMeasures of function assess analogous processes across species• VEP vs contrast sensitivity • Signal detection vs choice reaction time

Limitation:o The results do not address the issue of persistent effects fromchronic exposures


Further Use of Rat Data to EstimateHazard for Humans

Use PBPK models to o Simulate exposure scenarios to generate or improve estimates of CBR in studies of humans exposed to PCEo Predict the degree of change in internal dose that will cause an adverse effect in humansUse Dose-response models too Compare sensitivity to PCE across specieso Generate quantitative dose-equivalence relationships between PCE and other solvents (and ethanol)


Conclusions

Acute neurophysiological and behavioral effects of PCE in ratso Can be measured by methods based on tests of human neurological

dysfunctiono Are similar to effects in reported in humanso Are similar to effects of other volatile organic compoundso Can be predicted well by concurrent internal doses (concentration in the

brain and blood)o Effects can be measured in rats at internal doses that appear to be

effective in humansThe acute neurological effects in rats support the weight of evidence

for adverse effects in humans exposed to PCE

effects of tetrachloroethylene on visual and cognitive

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