Effects of Tetrachloroethylene on Visual and Cognitive Function in Rats

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1 Effects of Tetrachloroethylene on Visual and Cognitive Function in Rats Philip J. Bushnell William K. Boyes Vernon A. Benignus Elaina M. Kenyon Wendy M. Oshiro For the National Research Council Committee on Tetrachloroethylene January 29, 2009

2 Outline of the Presentation Goals of the Project Experiments o Animal model o Pharmacokinetics o Sensory function Visual evoked potentials (Boyes et al., 2008) o Cognitive function Behavioral signal detection (Oshiro et al., 2008) Dose-Response Modeling o Human exposure context Conclusions o 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

3 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 exposure o Dose-response models for quantitative comparisons across studies, endpoints, compounds and high-dose to low-dose Study acute-chronic extrapolation for systemic toxicity of HAPs Develop methods and models to study life-stage susceptibility

4 Animal Model Adult male Long-Evans (hooded) rat o Kinetics 3.5 months, maintained at 350 g body weight o Electrophysiology (Visual function) days, 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

5 PBPK Model for Perchloroethylene Inhalation Exhalation Lung Brain Blood flow-limited Slow metabolism Venous Blood RPTG SPTG Arterial Blood Physiological parameters specific for L-E rat Body weight Activity Fat Blood (C BL ) Brain (C BR ) Liver GI Tract Vmax, KM metabolism

6 PBPK Model Predictions ppm x 60 min Shaded area shows duration of exposure Boyes et al., 2008

7 PBPK Model Predictions ppm x 60 min Shaded area shows duration of exposure Boyes et al., 2008

8 PBPK Model Predictions 50 and 500 ppm (Data from Dallas et al. 1994) Shaded area shows duration of exposure Boyes et al., 2008

9 Assessment of Visual Function: Visual Evoked Potentials (VEPs) Electrophysiological measure of visual system function o Pattern VEP steady-state amplitude o Apical measure (retina to cortex) o Can be related to perception of visual contrast, which is important for pattern perception Neurological substrates well understood Measurable in humans and in experimental animals

10 Schematic of Exposure System for VEPs Boyes et al., 2003

11 Procedure to Record Visual Evoked Potentials 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

12 VEPs in Rats and Humans Rats and humans show similar spatial-temporal profiles Boyes, 1994

13 Effects of Perchloroethylene on Visual Evoked Potentials in Rats Boyes et al., 2008

14 Effect of Perchloroethylene on VEP F2 Amplitude 20 As a Function of Time 20 As a Function of Cxt 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. F2 Amplitude (μv) F2 Amplitude (μv) Time (min) As a Function of Brain PCE F2 Amplitude (μv) 0 ppm E ppm E ppm E ppm E ppm E1 0 ppm E2 250 ppm E2 500 ppm E ppm E2 noise E1 noise E2 F2 Amplitude (μv) e+5 2e+5 3e+5 4e+5 5e+5 6e+5 Concentration x Time (ppm-min) As a Function of Brain PCE AUC Brain Perchloroethylene (mg/l) Brain Perchloroethylene AUC (mg-hr/l) Boyes et al., 2008

15 Summary of Effects of Perchloroethylene on VEPs in Rats PCE reduced F2 amplitude as a function of inhaled concentration and duration of exposure o Rats awake and mobile after exposure o No one dose metric was statistically more predictive than the others o Maximum effect occurred at about 75% below baseline o Reduction occurred at relatively low internal doses Similarities with other solvents o F2 amplitude reduction o Concentration in brain closely related to magnitude of effect o Rapid onset Differences from other solvents o Higher potency o Lower efficacy o No best dose metric

16 Cognitive Function: Signal Detection Behavior Test designed from human literature to assess sustained attention o More like vigilance than selective attention o Temporal uncertainty, prolonged attending to a simple event Signal detection task o 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

17 Behavioral Exposure and Test System Bushnell et al., 2007

18 Signal Detection Task Trial Type Signal Period Choice Outcome "Signal" Hit Signal "Blank" Miss Extend Levers Blank "Signal" False Alarm "Blank" Correct Rejection

19 Visual Signal Detection Behavior in Humans and Rats Human Males Human Females Rat Males Mean Proportion of Responses P(hit) 4 Tpm 7 Tpm 10 Tpm P(fa) P(hit) 4 Tpm 7 Tpm 10 Tpm P(fa) P(hit) 4 Tpm 7 Tpm 10 Tpm P(fa) Signal Intensity (Arbitrary units) Bushnell et al., 2003

20 Effects of PCE on Visual Signal Detection Behavior in Rats Proportion of Responses (Mean ± SEM) Hits * Hits and False Alarms False Alarms Signal Intensity (lux) Concentration of PCE (ppm) * 0 ppm 500 ppm 1000 ppm 1500 ppm * * Oshiro et al., 2008a

21 Effects of Perchloroethylene on Signal Detection Behavior Inhaled Conc (C Inh ) and Time (t) AUC for Inhaled Conc (C Inh x t) Internal Dose (C Br ) AUC for Internal Dose (C Br x t) Accuracy (Proportion correct) Mean P(Correct) (± SEM) ppm 500 ppm 1000 ppm 1500 ppm Response Time (sec) Mean Response Time (sec ± SEM) ppm 500 ppm 1000 ppm 1500 ppm Number of Trials Completed Mean Number (± SEM) ppm 500 ppm 1000 ppm 1500 ppm Exposure Duration (hr) Cumulative Inhaled dose (ppm-hr) Estimated C Br (mg/l) Cumulative Brain [PCE] (mg-hr/l) Oshiro et al. 2008b

22 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 exposure o PCE in brain was more predictive than the other dose metrics Similarities with other solvents o All effects in same direction as other solvents o Concentration in brain best metric Differences from other solvents o Low efficacy on accuracy (like effect of PCE on VEP amplitude)

23 Relevance of Rat Data to Human Hazard: An Initial Look Using Data of Altmann 1990 Convert exposure C and t to concentration in brain o Rat: Use PBPK model to estimate C BR for each exposure condition o Human: Convert measured C BL to estimated C BR 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 C BL 4.5 mg/l C BR 50 ppm 1.1 mg/l C BL 15.0 mg/l C BR

24 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 C BR This effect occurs in rats at internal doses that are effective in humans E VEP = (VEP b VEP i ) / VEP b Range of C BR values estimated from Altmann et al Modified from Boyes et al., 2008

25 Effects of Perchloroethylene on Signal Detection Behavior 1.0 P(Correct) 2.0 Reponse Time Mean Proportion of Responses (± SEM) ppm 500 ppm 1000 ppm 1500 ppm Mean Response Time (sec ± SEM) ppm 500 ppm 1000 ppm 1500 ppm 1.4 Altmann et al Altmann et al Predicted C BR (mg/l) Predicted C BR (mg/l) Oshiro et al., 2008b

26 Assumptions and Limitations Assumptions: C BR is proper dose metric for acute effects of PCE Human and rat brains are equally sensitive to PCE Measures 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 from chronic exposures

27 Further Use of Rat Data to Estimate Hazard for Humans Use PBPK models to o Simulate exposure scenarios to generate or improve estimates of C BR in studies of humans exposed to PCE o Predict the degree of change in internal dose that will cause an adverse effect in humans Use Dose-response models to o Compare sensitivity to PCE across species o Generate quantitative dose-equivalence relationships between PCE and other solvents (and ethanol)

28 Conclusions Acute neurophysiological and behavioral effects of PCE in rats o Can be measured by methods based on tests of human neurological dysfunction o Are similar to effects in reported in humans o Are similar to effects of other volatile organic compounds o 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 humans The acute neurological effects in rats support the weight of evidence for adverse effects in humans exposed to PCE