Toxicological Sciences 57, 131-140 (2000)
Copyright © 2000 by the Society of Toxicology
Neurotoxicology |
PCBs, Thyroid Hormones, and Ototoxicity in Rats: Cross-Fostering Experiments Demonstrate the Impact of Postnatal Lactation Exposure
* Neurotoxicology Division, MD-74B, National Health and Environmental Effects Research Laboratory, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina 27711; and
The University of Albany, School of Public Health, Albany, New York
Received March 6, 2000; accepted May 24, 2000
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Previous research has demonstrated the sensitivity of the developing rat to the hypothyroxinemic and ototoxic effects of perinatal exposure to Aroclor 1254 (A1254). We tested the hypothesis that postnatal exposure via lactation is the major cause of the ototoxicity by cross fostering animals at birth. Primiparous rats (22–24/dose) received 0 or 6 mg/kg A1254 (po in corn oil) from gestation day (GD) 6 to postnatal day (PND) 21. On the day of birth, half of the treated litters and half of the control litters were cross-fostered, resulting in the following groups: Ctrl/Ctrl (controls); A1254/A1254 (perinatal exposure); A1254/Ctrl (prenatal exposure only); and Ctrl/A1254 (postnatal exposure only). We assessed offspring at a number of ages for: serum thyroid hormone concentrations, liver and brain concentrations of PCBs, body weight, mortality, age of eye opening, auditory startle amplitudes, and auditory thresholds for 1 kHz and 40 kHz tones. Circulating thyroxine (T4) concentrations were sharply reduced at GD 21 in the A1254-exposed group, and on PND 3, 7, 14, and 21 in the A1254/A1254 and the Ctrl/A1254 groups. Smaller decreases in T4 were observed in the A1254/Ctrl group on PND 3, 7, and 14. PCB concentrations in the liver on PND 21 were sharply elevated in the A1254/A1254 and Ctrl/A1254 groups. Much smaller increases were seen in the A1254/Ctrl group. Age of eye-opening and startle amplitudes were unaffected by treatment. A1254 exposure caused permanent hearing deficits (20 dB increase) at the low frequency (1 kHz) in the A1254/A1254 and Ctrl/A1254 groups. The present findings demonstrated that the critical period for the ototoxicity of developmental A1254 exposure is within the first few postnatal weeks in the rat. This effect is consistent with the greater degree of postnatal hypothyroxinemia resulting from the greater magnitude of exposure that occurs postnatally via lactation.
Key Words: polychlorinated aromatic hydrocarbons; polychlorinated biphenyls; thyroid-axis development; circulating T4 concentrations; lactation exposure; maternal exposure.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Polyhalogenated aromatic hydrocarbons (PHAHs), including chlorinated and brominated dibenzofurans, and polychlorinated biphenyls (PCBs) are ubiquitous in the environment and accumulate in biological tissues to levels that are associated with a variety of health effects in humans and other animals (Giesy and Kannan, 1998
; Safe, 1994). During development, PCBs and dioxins adversely impact a variety of endocrine systems (Birnbaum, 1995; Brouwer et al., 1998, 1999; Colborn et al, 1993; Gray, 1998; Li and Hansen, 1998; Safe et al., 1998). Thyroid-axis development appears to be particularly vulnerable to disruption by PCBs (Collins and Capen, 1980; Corey et al., 1996; Goldey et al., 1995b, 1998; Meserve et al., 1992; Morse et al., 1993, 1996; Ness et al., 1993; Crofton and Rice, 1999; Seo et al., 1995) and PCB metabolites (Darnerud et al., 1996; Schuur et al., 1998; Sinjari and Darnerud, 1998). Related environmental contaminants, such as dioxin, hexachlorobenzene, polybrominated biphenyls, and chlorinated diphenyl ethers also cause a reduction in circulating thyroxine (T4) concentrations following developmental exposure (Collins and Capen, 1980; Gupta et al., 1983; Lans et al., 1990; Meserve et al., 1992; Rosiak et al., 1997; Seo et al., 1995; Zhou et al., 2000). In addition, some studies of mothers and their offspring have demonstrated that humans may also be sensitive to small decreases in T4 and increases in TSH caused by PCBs and dioxins (Fiolet et al., 1997; Koopman-Esseboom et al., 1994; Langer et al., 1998; Nagayama et al., 1998). The findings in these studies are not entirely consistent in that increases in T4 and no effects on T4 have also been reported (Ilsen et al., 1996; Longnecker et al., 2000; Pluim et al., 1993).
We reported previously that perinatal exposure of pregnant rats to a commercial PCB mixture, Aroclor 1254 (A1254) resulted in low-frequency hearing loss in adult offspring (Crofton et al., 2000; Goldey and Crofton, 1998; Goldey et al., 1995b; Herr et al., 1996). We hypothesized that this ototoxicity resulted from A1254-induced hypothyroxinemia during development (Goldey and Crofton, 1998). Evidence for this hypothesis included a correlation between the severity of functional auditory damage and the degree of postnatal thyroid hormone depletion (Goldey et al., 1995a, b) and amelioration of the hearing loss following postnatal thyroxine replacement (Goldey and Crofton, 1998). Furthermore, this hearing deficit was shown to be caused by loss of hair cells in the area of the cochlea, which develops postnatally and transponds low-frequency sounds (Crofton et al., 2000). To date, auditory function in children of exposed mothers has not been tested.
The auditory system of the rat is vulnerable to disruption of thyroid hormones during the late prenatal and early postnatal periods (Puel and Uziel, 1987; Uziel, 1986), a time that corresponds to ongoing cochlear development (Rubel, 1978, 1984). The early postnatal period is also a time of peak effects of PCBs on thyroid hormones (Morse et al., 1993, 1996; Goldey et al., 1995b, 1998). We hypothesized that postnatal exposure to PCBs via lactation is the major route of exposure responsible for the decreases in circulating thyroid hormone concentrations during the early postnatal period and the corollary outcome of ototoxicity. We tested this hypothesis using a cross-fostering design that assessed a number of endpoints previously shown to be sensitive to developmental A1254 exposure (e.g., total serum T4 and T3 concentrations, eye opening, body weights, startle amplitudes, and auditory thresholds). In addition, PCB concentrations in fetal and postnatal liver tissue were assessed at multiple ages as a measure of delivered dose.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Animals and exposure.
Primiparous Long-Evans rats (Charles River, Portage, MI) were obtained on gestation day (GD) 2. Rats were housed individually, in standard plastic hanging cages (45 x 24 x 20 cm) with sterilized pine shavings as bedding, in an AAALAC-approved animal facility. All experiments were approved in advance by the National Health and Environmental Effects Research Laboratory`s animal-care committee of the U.S. Environmental Protection Agency. Animal rooms were maintained on a 12:12-h photoperiod (L:D, 0600:1800). Food (Purina Lab Chow, Barnes Supply Co., Durham, NC) and tap water were provided ad libitum.
The dams were administered either 0 or 6 mg/kg Aroclor 1254 (AccuStandard, Inc., Lot# 124–191) in corn oil via gavage (1.0 ml/kg) from GD 6 to postnatal day (PND) 21. Dams were assigned to treatment groups by balancing body weight gain from GD 2 to GD 6. A total of 71 dams were assigned as follows: 11 litters were used for the fetal time point on GD 21 (n = 5 controls and n = 6 A1254), and the remaining litters were used for postnatal time points and behavioral testing (n = 29 for control and n = 31 for A1254). On the day of birth, approximately half of the treated litters and half of the control litters were cross-fostered, resulting in the groups outlined in Table 1. The remaining litters were left with their original dams.
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All litters were randomly culled to 10 pups on postnatal day (PND) 3, with 2 males and 2 females within each litter identified by foot tattoos. Non-tattooed animals (1 male and 1 female per litter) were randomly selected from each litter for blood and liver collection on PND 3, 7, 14, and 21 (Fig. 1
). Litter size was kept similar to the degree possible within 1 or 2 pups throughout the pre-weaning period. Tattooed animals were maintained after weaning for the behavioral assessments. Tissue samples for the GD 21 and PND 3 (culled animals only) time points were pooled by litter (i.e., on GD 21, all blood from one litter was pooled into one sample and all liver into another; on PND 3 all culled animals were pooled). Liver tissue was removed and frozen at –80°C until analysis. On PND 21, brains were rapidly removed and the frontal cerebral cortex was removed and frozen at –80°C until analysis.
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Body weights of dams were recorded daily from GD 3 to PND 21. Offspring body weights were recorded twice per week during lactation, weekly until PND 70, and once more on PND 101. During the pre-weaning period, pups were counted and sexed on PND 0 (day of birth), PND 3, 7, 14, and 21. Postnatal mortality was recorded as the total percentage of pups/litter that did not survive until PND 21 (excluding pups killed for tissue sampling). Eye opening was recorded daily from PND 11 through 17. The ratio of pups within a litter with at least one eye open vs. total number of pups in the litter was recorded each day.
Thyroid hormones.
Serum concentrations of total thyroxine (T4) and triiodothyronine (T3) were measured as previously described (Goldey and Crofton, 1998; Goldey et al., 1995b). TSH was not measured because previous reports failed to find any effect of developmental A1254 exposure on this hormone (Goldey et al., 1995b; Morse et al., 1996). Trunk blood (from fetus on GD 20, pups on PND 3, 7, 14, and 21, and dams on GD 20 and PND 21) was allowed to clot on ice for a minimum of 30 min. Serum was collected via centrifugation of clotted samples and stored at –80°C for later analyses using radioimmunoassay kits (Diagnostic Products Corp., Los Angeles). To increase the sensitivity of the assays for the GD 20 fetal time point, an additional 25 µl of fetal serum was added to assay tubes and 25 µl of T4 free serum was added to the calibration curve tubes to correct for total volume. All samples were run in duplicate. The intra- and inter-assay variations were below 10% for both hormones.
PCB congener specific analysis.
All the tissue samples were frozen on dry ice, and stored at –80°C until analysis of PCBs, using high resolution gas chromatography with electron capture detection (Bush et al., 1985, 1989). Briefly, PCB congeners were extracted by grinding the tissue in a mixture of hexane:acetone (1:1). The extract was cleaned up by passing through a chromatography column containing calibrated 4% deactivated FlorisilR with a layer of sodium sulfate on top. The column was eluted with 50 ml of hexane and the eluate containing PCBs was concentrated to 1 ml. The concentrated samples were analyzed on a Hewlett-Packard 5890 gas chromatograph equipped with a 63Ni electron capture detector with a 30-m fused silica Ultra II (DB-5) capillary column (5% phenylmethylsilicone coating, 0.33 mm film thickness, 0.25 mm internal diameter). Approximately 113 PCB congeners were identified and verified using a separate Apiezon L capillary column (Bush et al., 1985). The microprocessor of the gas chromatograph was calibrated with a previously characterized solution of Aroclors 1221, 1016, 1254, and 1260 (200 ng/ml of each) fortified with HCB, DDE, and mirex at 5, 10, and 10 ng/ml, respectively, and 3,3',4,4'-tetrachlorobiphenyl at 20 ng/ml (Bush et al., 1985; Schulz et al., 1989).
Quality control for the analysis was ensured by running replicates of a sample in every batch of 10 samples to ensure the consistency of sample extraction. The samples were spiked with DDE to standardize retention times and check percent recovery of the extraction process. The average minimum detectable level (MDL) (p < 0.05 that the reported level is zero) (Type 1 error) for a sample weighing, 1 g was 0.02 ng/g. The results of PCB analysis were expressed as ppb PCB, based on tissue wet-weight.
Behavioral testing.
One male and one female per litter were repeatedly tested as previously described (Goldey et al., 1995b) for startle habituation on PND 28 and 65 and auditory thresholds for 1 and 40 kHz tones on PND 85–95.
Acoustic startle responses were measured using the method of Ruppert et al. (1984). Testing was conducted in sound-attenuated chambers, each containing a wire mesh plastic-framed test cage, mounted on a load-cell/force-transducer assembly. After 5 min of acclimation, each animal received 50 trials (eliciting stimulus = 120-dB, 40-ms burst of white noise). Peak response amplitude on each trial was recorded, from which an average startle response was calculated for each animal.
Auditory thresholds were tested using reflex modification audiometry (RMA). Testing was conducted in the apparatus described above for the acoustic startle response. Auditory thresholds for 5- and 40-kHz tones were determined using the reflex modification audiometry procedure of Young and Fechter (1983) as modified by Crofton et al. (1990). After a 10-min acclimation period each rat was tested for 240 trials per frequency (eliciting stimulus = 120-dB, 40-msec burst of white noise; prepulse stimulus = 40-ms tone of variable intensity; interstimulus interval = 90 ms). RMA thresholds (auditory thresholds) were defined as the prepulse intensity above which the response was inhibited (Crofton, 1992 and Crofton et al., 1990 for details).
Data analyses.
ANOVAs and repeated measures ANOVAs were used where appropriate (SAS, 1989). Gender was nested under litter (to control for possible "litter effects") where a male and female from the same litter were tested (behavioral tests). Treatment and age were between subject factors for the hormone analyses. Age (for body weights) and startle habituation block were within subject-repeated variables. Body weights for the dams used for the GD 21 time points were analyzed separately. Auditory thresholds were estimated using a nonlinear regression procedure (see Crofton, 1992; Crofton et al., 1990 for details). Significant interactions were followed by step-down ANOVAs, as appropriate. Mean contrast comparisons between each dosage group and the vehicle-control group were made using Tukey's studentized range test (SAS, 1989). An alpha level of 0.05 was used for all tests. For the nonlinear analyses, failure to achieve a significant fit led to retesting of that animal at that frequency. Data from animals that failed to fit after 2 tests were not used. The total number of animals that did not fit for each treatment and frequency were: Ctrl/Ctrl = 3 for both frequencies; A1254/A1254 = 2 for both frequencies; A1254/Ctrl = 1 animal at 1 kHz and 2 animal for both frequencies; and Ctrl/A1254 = none.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Maternal Body Weight
Measurement of maternal body weight throughout gestation and lactation revealed no effect of A1254 treatment (Fig. 2
.). Repeated measures analyses failed to reveal any main effect of treatment [F(1, 48) = 0.00, p < 0.9643], nor any treatment by time interaction [F(34, 15) = 1.34, p < 0.2792]. There was a main effect of time [F(34, 15) = 101.16, p < 0.0001). There were also no effects of treatment or interactions with time for the gestational-only exposed dams used for tissue collection on GD 20 (data not shown). Data from nonpregnant animals were excluded from all analyses. The number of nonpregnant animals was not related to treatment (n = 2 for controls, n = 3 for A1254). An additional 5 litters were not usable due to loss of all pups within one day of cross fostering (n = 2 Ctrl/Ctrl; n = 0 A1254/A1254; n = 1 A1254/Ctrl; n = 2 Ctrl/A1254). This resulted in the following final numbers of litters for postnatal use: Ctrl/Ctrl = 11; A1254/A1254 = 14; A1254/Ctrl = 13; and Ctrl/A1254 = 12.
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Offspring Development
Developmental exposure to A1254 resulted in transient deficits in offspring body weights (Fig. 3
). Perinatal exposure to A1254 caused decreased body weight gain relative to controls from PND 3 to PND 14 and again from PND 28 to PND 35. The decrease relative to controls was 10% on PND 3, increasing to 18% on PND 10, and decreasing to 16% on PND 14, 12% on PND 28, and 10% on PND 35. The decreases of 10 and 9% on PND 17 and 21 were not statistically significant compared to same-age controls (p > 0.05). Prenatal-only exposure in the A1254/Ctrl group yielded a similar decrease in body weight gain on PND 3 of 10% (p < 0.05), with no significant differences at any later ages. Except for PND 28, where there was an 8.4% decrease compared to controls (p < 0.05), there was no effect of postnatal-only exposure to A2154 on body weights of offspring. As expected, the largest effect on body weight was due to age-related growth. These results were substantiated by a significant treatment by age interaction [F(3, 98) = 6.60, p < 0.0001] and main effects of treatment on PND 3–14, 28, and 35 [all F(3, 50) > 3.34, p < 0.0266]. While there was a significant main effect of gender on body weight, and a significant gender by age interaction, there was no interaction of gender with treatment, nor was there an age by gender by treatment interaction (all p > 0.05).
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A1254 exposure did not have an effect on the age of eye opening (Fig. 4
). There was no main effect of treatment [F(3, 46) = 0.30, p < 0.8224], nor was there a significant age by treatment interaction [F(9, 107) = 1.17, p < 0.3194]. There was a main effect of age [F(4, 43) = 541.91, p < 0.0001]. Gender effects were not assessed for eye opening as data were collected per litter.
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There were no apparent treatment-related effects of A1254 exposure on the number of pups born per litter or on pup survival. The average numbers of pups ± SD per litter were 12.4 ± 3.8, 12.6 ± 2.1, 13.3 ± 1.5, and 12.2 ± 2.1 for the Ctrl/Ctrl, A1254/A1254, A1254/Ctrl, and Ctrl/A1254 groups, respectively. Most litters had no mortality of pups from birth to weaning. However, there were 4 Ctrl/Ctrl litters with 10, 25, 100, and 100% mortality, 2 litters in the A1254/A1254 group with 10% mortality, 4 litters in the A1254/Ctrl group with 7, 7, 42, and 100% mortality, and 3 litters in the Ctrl/A1254 group with 20, 100, and 100% mortality. All of the litters with 100% mortality were from dams that appeared to have abandoned their pups within the first few postnatal days.
Thyroid Hormones
Maternal exposure to A1254 caused hypothyroxinemia in the dams, GD 20 fetuses, and early postnatal offspring (Fig. 5). Maternal T4 was depressed by 32% relative to controls on GD 20 (Fig. 5, right panel). On PND 22, maternal T4 was decreased 40% in the A1254/A1254 group and 39% in the Ctrl/A1254 group. There was no significant effect of A1254 on maternal T3 concentrations in any of the treated groups (p > 0.05, data not shown). For the GD 20 data, these results were supported by a significant hormone by treatment interaction on GD 20 [F(1, 18) = 9.27, p < 0.0070], a main effect of treatment on maternal T4 [F(1, 9) = 9.39, p < 0.0135], and no main effect of treatment on maternal T3 [F(1, 9) = 0.21, p < 0.6569]. For PND 22, there was a significant hormone by treatment interaction [F(3, 92) = 14.75, p < 0.0001], a main effect of treatment on maternal T4 [F(3, 46) = 14.72, p < 0.0001], and no main effect of treatment on maternal T3 [F(3, 46) = 0.63, p < 0.5966]. Mean contrast comparisons revealed the treatment-dependent differences discussed above and illustrated in Figure 5.
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Maternal A1254 exposure also produced marked hypothyroxinemia in offspring of exposed dams with the magnitude of the effect dependent on age and the timing of exposure. Perinatal exposure produced an age-dependent depression in circulating total serum T4 concentrations that reduced the normal increase in T4 seen between birth and PND 14. Relative to controls, this effect started with a 42% decrease at GD 20 and a 60% decrease at PND 3 (Fig. 5
, left panel). This effect increased to a 75% decrease on PND 7 and 14, with a maximal suppression of 82% on PND 21. Prenatal-only exposure, in the A1254/Ctrl group, caused a smaller postnatal decrease compared to the perinatal and postnatal exposure groups (40% on PND 3 and 7, and 24% on PND 14). This effect of prenatal A1254 diminished as the animals aged, so that there was no difference compared to the control group on 21. Postnatal-only exposure, as seen in the Ctrl/A1254 group, showed a 40% decrease on PND 3 relative to controls that increased rapidly to 60% on PND 7, 70% on PND 14 and 80% on PND 21. The only effects of A1254 exposure on T3 concentrations were significant decreases of 19 and 18% on PND 21 in the A1254/A1254 and Ctrl/A1254 groups, respectively (data not shown). The effects on thyroid hormones were confirmed by a significant hormone by age by treatment interaction [F(10,231) = 28.89, p < 0.0001]. There were significant age by treatment interactions for both T3 [F(10,152)=3.47, p < 0.0004] and T4 [F(10,169)=32.73, p < 0.0001]. Step-down ANOVAs for a main effect of treatment on T4 were significant for all ages (all p < 0.0003). Only on PND 21 was there a significant main effect of treatment on T3 (p < 0.0003). Mean contrast comparisons confirmed the group comparisons above and those illustrated in Figure 5.
PCB Tissue Concentrations
Concentrations of total PCBs for liver and frontal cortex are illustrated in Figure 6. In the perinatal exposed group there was very sharp increase in liver concentration of PCBs following birth which peaked at around 70 ppm on PND 7 and declined to 30 ppm by PND 21 (Fig. 6, left panel). The prenatal-only group, A1254/Ctrl, had much lower concentrations on PND 3, approximately 4 ppm, and declined steadily to 99 ppb by PND 21. The postnatal-only exposure, Ctrl/A1254 group, had PCB concentrations that were only slightly less than those seen in the A1254/A1254 group. Interestingly, PCB concentrations in the unexposed group, Ctrl/Ctrl, rose sharply after birth and then steadily decreased through PND 21. PCB concentrations in the frontal cortex on PND 21 (Fig. 6, right panel) reflected the same pattern seen in the liver, i.e., postnatal exposure accounted for the bulk of the PCBs found in postnatal tissue on PND 21, and tissue concentrations in the prenatal-only group had declined to control levels. Due to limited sample sizes (n = 3 per treatment group per time point) no statistical analyses were conducted on these data.
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Auditory Startle Response
Data collected on both PND 28 and PND 70 did not reveal any treatment-related effects on the amplitude of the startle response, nor were there any effects on the habituation of the startle response (data not shown). As expected, there were higher startle amplitudes in the males compared to the females, and significantly greater habituation in the older animals. These results were confirmed by significant main effects of age [F(1, 32) = 123.74, p < 0.0001], test-block (habituation) [F(4, 29) = 24.30, p < 0.0001], and significant interactions of test-block by age [F(4, 29) = 18.12, p < 0.0001] and gender by age [F(1, 32) = 6.09, p < 0.0191]. There was no main effect of treatment [F(3, 32) = 1.11, p < 0.3069], nor were there any significant interactions of treatment with any other variables (all p > 0.2).
Auditory Thresholds
Results of cross fostering clearly demonstrate that the low-frequency hearing deficit seen after perinatal exposure to A1254 is the result of postnatal exposure (Fig. 7). Perinatal A1254 exposure caused a low-frequency specific hearing deficit, with a 19 dB increase in threshold for the 1-kHz tone. There was no effect of prenatal-only exposure on either the high- or low-frequency tones. The postnatal-only exposure demonstrated the same magnitude (21 dB) and frequency specificity of effect seen in the perinatal exposed group. These results are supported by a significant frequency by treatment interaction [F(3, 38) = 6.89, p < 0.0008], a significant main effect of treatment for 1 kHz [F(3, 42) = 13.72, p < 0.0001], and a nonsignificant main effect of treatment for 40 kHz [F(3, 42) = 0.53, p < 0.6636]. There was no main effect of gender, nor any interaction of gender with any other variables (all p > 0.1).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The goal of the present cross-fostering study was to compare the impact of prenatal versus postnatal exposure of rats to A1254. Data clearly demonstrated that postnatal exposure alone is responsible for the ototoxic effects previously reported using perinatal exposures (Crofton et al., 2000
; Goldey and Crofton, 1998; Goldey et al., 1995b; Herr et al., 1996). Furthermore, these cross-fostering experiments showed that prenatal-only exposure leads to limited postnatal hypothyroxinemia, which recovers by the end of lactation, whereas the hypothyroxinemia that occurs following postnatal-only exposure matches that seen with perinatal exposure within a few days after birth. These effects, both the hypothyroxinemia and the ototoxicity, correlate well with the time course of the kinetics of PCB transfer from the dam to the offspring, i.e., most of the exposure occurs via postnatal lactation (see Fig. 6).
The data presented herein clearly demonstrate that the critical period for the ototoxicity resulting from developmental A1254 exposure is postnatal (Fig. 7). The degree of hearing deficit seen in the cross-fostered postnatal group was similar to the animals that were not cross-fostered and received perinatal exposure. These results also demonstrate that the fetal period is not a sensitive period for A1254-induced ototoxicity in the rat, since there was no detectable effect of A1254 on auditory function in the cross-fostered prenatal group compared to controls. The postnatal window of vulnerability to the ototoxic effects of A1254 is consistent with the ontogeny of the cochlea in the rat, the known periods of sensitivity to hypothyroidism, and the effects of A1254 on thyroid hormones. Cochlear development is tonotopic in that basal regions (which develop early) are sensitive to higher frequencies (e.g., 40 kHz) and apical regions (which mature later) are sensitive to lower frequencies (e.g., 1 kHz) (Müller, 1991a,b; Rubel, 1978, 1984). The cochlear sensitive period for hypothyroidism in rats varies from about GD 19 to PND 15 (Deol, 1973; Hebert et al., 1985; Uziel et al., 1985a,b, 1986). Hypothyroidism during this period results in permanent malformation of cochlear structures and functional hearing loss, and T4 replacement therapy during the postnatal period allows for more normal functional and anatomical development (Deol, 1973, 1976; Meza et al., 1996; Uziel et al., 1980, 1981, 1985a, 1985b). The fact that low-frequency hearing and apical hair cells are affected (Crofton et al., 2000; Goldey and Crofton, 1998; Goldey et al., 1995b; Herr et al., 1996) following the hypothyroxinemia induced by A1254, which is maximal during the second and third postnatal week (Goldey and Crofton, 1998; Goldey et al., 1995b; Morse et al., 1993), is reasonably consistent with the known basal-to-apical direction of postnatal cochlear ontogeny (Müller, 1991a,b; Rubel, 1978, 1984). Furthermore, recent work has demonstrated that postnatal thyroxine administration will partially alleviate the low-frequency loss caused by A1254 exposure (Goldey et al., 1998).
The present data also demonstrate that the timing of the hypothyroxinemia induced by developmental exposure to A1254 is dependent on the timing of exposure. Prenatal-only exposure resulted in prenatal hypothyroxinemia, as evidenced by the 42% depression in T4 on GD 20. However, this effect rapidly diminished and recovered to control levels during the postnatal period, when exposure was limited to only that A1254 still remaining in the offspring from fetal exposure. In contrast, postnatal-only exposure resulted in the rapid development of hypothyroxinemia that was similar to that found following perinatal exposure. These effects are consistent with two important features of developmental exposure to polyhalogenated hydrocarbons. The first is the kinetics of highly lipid-soluble chemicals during development. As seen in the present study (Fig. 6.) and in a number of other reports (Masuda et al., 1978; Takagi et al., 1986; Vodicnik and Lech, 1980), compounds like TCDD and PCBs are transferred to the fetus in limited quantities compared to the amount delivered via lactation. Thus, the time of greatest exposure to highly lipophilic chemicals during development in mammals is postnatal. Furthermore, the rapid decline in PCB tissue concentrations following a prenatal-only exposure is also consistent with other studies showing that the rapid decline in postnatal tissue concentrations is due to declines in milk concentrations (McCormack et al., 1979; Vodicnik and Lech, 1980) and dilution of tissue concentrations due to the rapid body weight gain of the offspring (Young et al., 1997). The second important feature of developmental exposure is the ontogeny and induction of hepatic uridine diphosphoglucuronosyl transferases (UDPGTs). Induction of hepatic UDPGT isoforms that glucuronidate thyroxine is known to be, at least partially, responsible for the declines in circulating T4 concentrations following exposure to PCBs (Barter and Klaassen, 1992, 1994; Bastomsky and Murthy, 1976; Visser et al., 1993; Volp, 1988). The greater effects of A1254 on postnatal T4 concentrations is consistent with the mostly postnatal exposure and with inducibility of UDPGT isoforms during the period right after birth (Lucier et al., 1976; Morse et al., 1993, 1996). Alternatively, data from in vitro studies suggest that PCBs and hydroxy metabolites of PCBs may displace T4 from serum transport proteins (Cheek et al., 1999; Chauhan et al., 2000; Lans et al., 1993). Whether or not the displacement of T4 from serum proteins plays an in vivo role during development remains to be determined.
The impact of the current A1254 exposure regimen to the dams appeared to be minimal. There was no apparent maternal toxicity associated with the 6 mg/kg/day dosage of A1254 as evidenced by a lack of difference in maternal body weights. There was, however, a mild hypothyroxinemia present in the exposed dams. There was a significant depression in T4 for both the fetal (32%) and weaning (39%) time points. This maternal hypothyroxinemia is consistent with previous observations (Morse et al., 1993, 1996) and is consistent with increased glucuronidation of T4 in maternal liver following A1254 exposure (Morse et al., 1993, 1996). The mild hypothyroxinemia, coupled with the lack of effect on maternal weight gain and no impact on fetal or postnatal survival, suggest minimal maternal toxicity compared to effects in offspring.
The lack of effect of any of the exposures to A1254 on the age of eye opening was unexpected, based on our previous work. Goldey et al. (1995b) and Goldey and Crofton (1998) demonstrated an accelerated age of eye opening following perinatal exposure to A1254. There may be a number of reasons for this discrepancy. First and foremost is dose, the previous work used a dose of 8 mg/kg/day, whereas the present work used only 6 mg/kg/day. This dose may be below that needed to induce an effect. An alternative explanation may involve a combination of the dose and the manufacturer's batch number for the A1254. Recent work by Frame and colleagues (Frame et al., 1996) has demonstrated considerable differences in congener composition for some commercial batches of Aroclor mixtures. The previous work used a batch of A1254 (Accustandard Lot #6024) that has been shown to contain higher concentrations of dibenzofurans and coplanar PCB congeners compared to the batch (Accustandard Lot#124–191) used in the present work (Burgin et al., 1999). The total aryl hydrocarbon toxic-equivalent (AH-TEQ) for batch #6024 is 10-fold higher compared to batch #124–191 (Burgin et al., 1999). As postulated previously (Goldey and Crofton, 1998), an Ah receptor-based mechanism may explain the early eye opening seen with developmental A1254 exposure. Early eye opening in TCDD-exposed rats has been postulated to result from Ah receptor activation and subsequent alterations in epidermal growth factor (EGF) signaling pathways (Madhukar et al., 1988). Activation of EGF receptors during development is well known to induce early eye opening (Aulerich et al., 1988; Cohen, 1962; Lakshmanan et al., 1985; Schlessinger et al., 1983). Thus, the batch with the higher AH-TEQ would be a more potent mixture for induction of early eye opening compared to the lot used in the present work.
Maternal A1254 exposure at 6 mg/kg/day did affect offspring growth. This effect was less pronounced and more transient compared to that previously reported from this lab (Goldey and Crofton, 1998; Goldey et al., 1995b). In addition, we found no increase in postnatal mortality, as was previously reported (Goldey and Crofton, 1998; Goldey et al., 1995b). This difference is most likely to be due to the lower dosage used (i.e., 6 mg/kg/day), as well as to the difference in lot number as discussed above.
The results of this cross-fostering experiment support some important principles for developmental studies. First, it underscores that the timing of exposure and the duration of effect are integrally linked. In the present study, measurement of T4 during the late postnatal period would underestimate or, even miss, the hypothyroxinemic effect of prenatal-only exposure to A1254. This is due to the transient nature of the effect. If the effect of exposure is permanent, then measurement can be taken at any time post-exposure. Admittedly, maternal exposure during only the prenatal period is not the same as a prenatal-only cohort in a cross-fostering study. In the former, the pups are weaned on the exposed dams, so there will be a postnatal exposure component for lipophilic xenobiotics. However, this exposure will not be of the same magnitude as pups reared with a dam that had been exposed perinatally. Second, caution is urged in interpreting results from studies where exposure was only prenatal. The kinetics of the compound(s) and the windows of vulnerability of the endpoint of concern must be taken into account prior to concluding negative findings.
The present findings may have relevance for predicting the effects of PCB exposure on human health, clearly indicating that the postnatal period is the vulnerable window of exposure for the ototoxicity of developmental exposure to A1254 in rats. This is consistent with the degree of exposure, the adverse impact on thyroxine, and the postnatal ontogeny of the cochlear structures that are damaged (Crofton et al., 2000). However, in comparing cochlear ontogeny between rats and humans, one major difference is readily apparent: cochlear tissue in humans develops almost entirely prenatally (cf., Sulik, 1995). This does not rule out effects of PCBs on human cochlear structure and/or function. However, dose-response data for hypothyroxinemia in the rat, gathered from previous work in this laboratory (Goldey et al., 1995a, 1995b; Goldey and Crofton, 1998), suggest that in the rat hypothyroxinemia must exceed approximately 60% before detectable auditory dysfunction occurs. A considerable degree of fetal hypothyroxinemia may need to occur before human cochlear development is adversely affected. Clearly, human epidemiological studies that track fetal thyroid hormone concentrations as well as auditory function are needed to definitively address this issue.
In summary, the present findings demonstrated that the critical period for the ototoxicity of developmental A1254 exposure is within the first few postnatal weeks in the rat. This effect is consistent with the greater magnitude of postnatal exposure via lactation, as well as the greater degree of postnatal hypothyroxinemia that occurs during a critical window of vulnerability for the developing rat cochlea.
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ACKNOWLEDGMENTS |
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We thank Dennis House for repeated and patient consultation on statistics, K. Rigsbee for excellent technical assistance, and G. Ward, E. B. Bailey, and J. Ali for engineering the behavioral test equipment.
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NOTES |
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This manuscript has been reviewed by the National Health and Environmental Effects Research Laboratory, U.S. Environmental Protection Agency, and approved for publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.
1 To whom correspondence should be addressed. Phone: (919) 541-2672. Fax: (919) 541-4849. E-mail: crofton.kevin{at}epa.gov. 
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Aulerich, R. J., Bursian, S. J., and Napolitano, A. C. (1988). Biological effects of epidermal growth factor and 2,3,7,8-tetrachlorodibenzo-p-dioxin on developmental parameters of neonatal mink. Arch. Environ. Contam. Toxicol. 17, 27–31.[Web of Science][Medline]
Barter, R. A., and Klaassen, C. D. (1992). UDP-glucuronosyltransferase inducers reduce thyroid hormone levels in rats by an extrathyroidal mechanism. Toxicol. Appl. Pharmacol. 113, 36–42.[Web of Science][Medline]
Barter, R A., and Klaassen, C. D. (1994). Reduction of thyroid hormone levels and alteration of thyroid function by four representative UDP-glucuronosyltransferase inducers in rats. Toxicol. Appl. Pharmacol. 128, 9–17.[Web of Science][Medline]
Bastomsky, C. H., and Murthy, P. V. (1976). Enhanced in vitro hepatic glucuronidation of thyroxine in rats following cutaneous application or ingestion of polychlorinated biphenyls. Can. J. Physiol. Pharmacol. 54, 23–26.[Web of Science][Medline]
Birnbaum, L. S. (1995). Developmental effects of dioxins and related endocrine disrupting chemicals. Toxicol. Lett. 82–83, 743–750.
Brouwer, A., Longnecker, M. P., Birnbaum, L. S., Cogliano, J., Kostyniak, P., Moore, J., Schantz, S., and Winneke, G. (1999). Characterization of potential endocrine-related health effects at low-dose levels of exposure to PCBs. Environ. Health Perspect. 107(Suppl. 4), 639–649.
Brouwer, A., Morse, D. C., Lans, M. G., Schuur, A. G., Murk, A. J., Klasson-Wehler, E., Bergman, A., and Visser, T. J. (1998). Interactions of persistent environmental organohalogens with the thyroid hormone system: Mechanisms and possible consequences for animal and human health. Toxicol. Ind. Health 14, 59–84.[Web of Science][Medline]
Burgin, D. E., Dilberto, J. J., Derr-Yellin, E. C., Birnbaum, L. S., and Kodavanti, P. R. S. (1999). Differential effects of Aroclor 1254 with two lot numbers: Hepatic enzyme induction and circulating thyroid hormone levels. Toxicologist 48, 277 (abstract).
Bush, B., Murphy, M. J., Connor, S., Snow, J., and Barnard, E. (1985). Improvements in glass capillary gas chromatographic polychlorinatedbiphenyl analysis. J. Chrom. Sci. 23, 509–515.[Web of Science][Medline]
Bush, B., Streeter, R. W., and Sloan, R. J. (1989). Polychlorinated biphenyl (PCB) congeners in striped bass (Morone saxatilis) from marine and estuarine waters of New York state determined by glass capillary gas chromatography. Arch. Environ. Comtam. Toxicol. 19, 49–61.
Chauhan, K. R., Kodavanti, P. R., and McKinney, J. D. (2000) Assessing the role of ortho-substitution on polychlorinated biphenyl binding to transthyretin, a thyroxine transport protein. Toxicol. Appl. Pharmacol. 162, 10–21.[Web of Science][Medline]
Cheek, A. O., Kow, K., Chen, J., and McLachlan, J. A. (1999) Potential mechanisms of thyroid disruption in humans: Interaction of organochlorine compounds with receptor, transthyretin, and thyroid-binding globulin. Environ. Health Perspect. 107, 273–278.[Web of Science][Medline]
Cohen, S. (1962). Isolation of a mouse submaxillary gland protein accelerating incisor eruption and eyelid opening in the newborn animal. J. Biol. Chem. 237, 1555–1562.
Colborn, T., vom Saal, F. S., and Soto, A. M. (1993). Developmental effects of endocrine-disrupting chemicals in wildlife and humans. Environ. Health. Perspect. 101, 378–384.[Web of Science][Medline]
Collins, T., Jr., and Capen, C. C. (1980). Fine structural lesions and hormonal alterations in thyroid glands of perinatal rats exposed in utero and by the milk to polychlorinated biphenyls. Am. J. Pathol. 99, 125–142.[Abstract]
Corey, D. A., Juarez de Ku, L. M., Bingman, V. P, and Meserve, L. A. (1996). Effects of exposure to polychlorinated biphenyl (PCB) from conception, on growth and development of endocrine, neurochemical, and cognitive measures in 60-day-old rats. Growth Dev. Aging 60, 131–143.[Web of Science][Medline]
Crofton, K. M. (1992). Reflex modification and the assessment of sensory dysfunction. In Neurotoxicology (H.Tilson and C. Mitchell, Eds.), pp. 181–211. Raven Press, Ltd., New York.
Crofton, K. M., Dean, K. F., Menache, M. G., and Janssen, R. (1990). Trimethyltin (TMT) effects on auditory function and cochlear pathology. Toxicol. Appl. Pharmacol. 105, 123–132.[Web of Science][Medline]
Crofton, K. M., Ding, D., Padich, R., Taylor, M., and Henderson, D. (2000) Hearing loss following developmental exposure to polychlorinated biphenyls: A cochlear site of action. Hear. Res. 144, 196–204.[Web of Science][Medline]
Crofton, K. M., and Rice, D. C. (1999). Perinatal exposure to PCB 126: Thyroid hormones and ototoxicity. Neurotoxicol. Teratol. 465, 299–301.
Darnerud, P. O., Morse, D., Klasson-Wehler, E., and Brouwer, A. (1996). Binding of a 3,3',4,4'-tetrachlorobiphenyl (CB-77) metabolite to fetal transthyretin and effects on fetal thyroid hormone levels in mice. Toxicology 106, 105–114.[Web of Science][Medline]
Deol, M. S. (1973). An experimental approach to the understanding and treatment of hereditary syndromes with congenital deafness and hypothyroidism. J. Med. Genet. 10, 235–242.
Deol, M. S. (1976). The role of thyroxine in the differentiation of the organ of Corti. Acta Otolaryngol. (Stockh) 81, 429–435.[Medline]
Fiolet, D. C. M., Cuijpers, C. E. J., and Lebret, E. (1997). Exposure to organic compounds and thyroid hormone plasma levels in human newborns. Organohalogen Compounds 34, 459–465.
Frame, G. M., Cochran, J. W., and Bøwadt, S. S. (1996). Complete PCB congener distributions for 17 Aroclor mixtures determined by 3 HRGC systems optimized for comprehensive, quantitative, congener-specific analysis. J. High Res. Chromat. 19, 657–668.
Giesy, J. P., and Kannan, K. (1998). Dioxin-like and non-dioxin-like toxic effects of polychlorinated biphenyls (PCBs): Implications for risk assessment. Crit. Rev. Toxicol. 28, 511–569.[Web of Science][Medline]
Goldey, E. S., and Crofton, K. M. (1998). Thyroxine replacement attenuates hypothyroxinemia, hearing loss, and motor deficits following developmental exposure to Aroclor 1254 in rats. Toxicol. Sci. 45, 94–105.
Goldey, E. S., Kehn, L. S., Lau, C., Rhenberg. G. L., and Crofton, K. M. (1995b). Developmental exposure to polychlorinated biphenyls (Aroclor 1254) reduces circulating thyroid hormone concentrations and causes hearing deficits in rats. Toxicol. Appl. Pharmacol. 135, 77–88.[Web of Science][Medline]
Goldey, E. S., Kehn, L. S., Rhenberg, G. L., and Crofton, K. M.. (1995a). Effects of developmental hypothyroidism on auditory and motor function in the rat. Toxicol. Appl. Pharmacol. 135, 67–76.[Web of Science][Medline]
Gray, L. E., Jr. (1998). Xenoendocrine disrupters: Laboratory studies on male reproductive effects. Toxicol. Lett. 102–103, 331–335.
Gupta, B. N., McConnell, E. E., Goldstein, J. A., Harris, M. W., and Moore, J. A. (1983). Effects of a polybrominated biphenyl mixture in the rat and mouse. I. Six-month exposure. Toxicol. Appl. Pharmacol. 68, 1–18.[Web of Science][Medline]
Hebert, R., Langlois, J. M., and Dussault, J. H. (1985). Permanent defects in rat peripheral auditory function following perinatal hypothyroidism: Determination of a critical period. Develop. Brain Res. 355, 161–170.
Herr, D. W., Goldey, E. S., and Crofton, K. M. (1996). Developmental exposure to Arochlor 1254 produces low-frequency alterations in adult rat brainstem auditory evoked responses. Fundam. Appl. Toxicol. 33, 120–128.[Web of Science][Medline]
Ilsen, A., Briet, J. M., Koppe, J. G., Pluim, H. J., and Oosting, J. (1996). Signs of enhanced neuromotor maturation in children due to perinatal load with background levels of dioxins. Follow-up until age 2 years and 7 months. Chemosphere 33, 1317–1326.[Medline]
Koopman-Esseboom, C., Morse, D. C., Weisglas-Kuperus, N., Lutke-Schipholt, I. J., Van der Paauw, C. G., Tuinstra, L. G., Brouwer, A., and Sauer, P. J. (1994). Effects of dioxins and polychlorinated biphenyls on thyroid hormone status of pregnant women and their infants. Pediatr. Res. 36, 468–473.[Web of Science][Medline]
Lakshmanan, J., Perheentupa, J., Hoath, S. B., Kim, H., Gruters, A., Odell, C., and Fisher, D. A. (1985). Epidermal growth factor in mouse ocular tissue: effects of thyroxine and exogenous epidermal growth factor. Pediatr. Res. 19, 315–319.[Web of Science][Medline]
Langer, P., Tajtakova, M., Fodor, G., Kocan, A., Bohov, P., Michalek, J., and Kreze, A. (1998). Increased thyroid volume and prevalence of thyroid disorders in an area heavily polluted by polychlorinated biphenyls. Eur. J. Endocrinol. 139, 402–409.[Abstract]
Lans, M. C., Brouwer, A., Koppe, J. G., and van den Berg, M. (1990). Enzyme induction and alterations in thyroid hormone, and vitamin A and K levels by TCDD in neonatal and maternal rats. Chemosphere 20, 1129–1134.
Lans, M. C., Klasson-Wehler, E., Willemsen, M., Meussen, E., Safe, S., and Brouwer, A. (1993). Structure-dependent, competitive interaction of hydroxy-polychlorobiphenyls, -dibenzo-p-dioxins and -dibenzofurans with human transthyretin. Chem. Biol. Interact. 88, 7–21.[Web of Science][Medline]
Liu, M.-H., and Hansen, L. G. (1998). Consideration of enzyme and endocrine interactions in the risk assessment of PCBs. Rev. Toxicol. 1, 71–156.
Longnecker, M. P., Gladen, B. C., Patterson, D. G., Jr, and Rogan,W. J. (2000). Polychlorinated biphenyl (PCB) exposure in relation to thyroid hormone levels in neonates. Epidemiology 11, 249–254.[Web of Science][Medline]
Lucier, G. W., Sonawane, B. R., McDaniel, O. S., and Hook, G. E. (1975). Postnatal stimulation of hepatic microsomal enzymes following administration of TCDD to pregnant rats. Chem. Biol. Interact. 11, 15–26.[Web of Science][Medline]
Madhukar, B. V., Ebner, K., Matsumura, F., Bombick, D. W., Brewster, D. W., and Kawamoto,T. (1988). 2,3,7,8-Tetrachlorodibenzo-p-dioxin causes an increase in protein kinases associated with epidermal growth factor receptor in the hepatic plasma membrane. J. Biochem. Toxicol. 3, 261–277.[Web of Science][Medline]
Masuda, Y., Kagawa, R., Tokudome, S., and Kuratsune, M. (1978). Transfer of polychlorinated biphenyls to the foetuses and offspring of mice. Food Cosmet. Toxicol. 16, 33–37.[Web of Science][Medline]
McCormack, K. M., Melrose, P., Rickert, D. E., Dent, J. G., Gibson, J. E., and Hook, J. B. (1979). Concomitant dietary exposure to polychlorinated biphenyls and polybrominated biphenyls: Tissue distribution and arylhydrocarbon hydroxylase activity in lactating rats. Toxicol. Appl. Pharmacol. 47, 95–104.[Web of Science][Medline]
Meserve, L. A., Murray, B. A., and Landis, J. A. (1992). Influence of maternal ingestion of Aroclor 1254 (PCB) or FireMaster BP 6 (PBB) on unstimulated and stimulated corticosterone levels in young rats. Bull. Environ. Contam. Toxicol. 48, 715–720.[Web of Science][Medline]
Meza, G., Acuna, D., and Escobar, C. (1996). Development of vestibular and auditory function: Effects of hypothyroidism and thyroxine replacement therapy on nystagmus and auditory-evoked potentials in the pigmented rat. Int. J. Dev. Neurosci. 14, 515–522.[Web of Science][Medline]
Morse, D. C., Groen, D., Veerman, M., Van Amerongen, C. J., Koëter, H. B. W. M., Smits van Prooije, A. E., Visser, T. J., Koeman, J. H., and Brouwer, A. (1993). Interference of polychlorinated biphenyls in hepatic and brain thyroid hormone metabolism in fetal and neonatal rats. Toxicol. Appl. Pharmacol. 122, 27–33.[Web of Science][Medline]
Morse, D. C., Wehler, E. K., Wesseling, W., Koeman, J. H., and Brouwer, A. (1996). Alterations in rat brain thyroid hormone status following pre- and postnatal exposure to polychlorinated biphenyls (Aroclor 1254). Toxicol. Appl. Pharmacol. 136, 269–279.[Web of Science][Medline]
Müller, M. (1991a) Developmental changes of frequency representation in the rat cochlea. Hear. Res. 56, 1–7.[Web of Science][Medline]
Müller, M. (1991b) Frequency representation in the rat cochlea. Hear. Res. 51, 247–254.[Web of Science][Medline]
Nagayama, J., Okamura, K., Iida, T., Hirakawa, H., Matsueda, T., Tsuji, H., Hasegawa, M., Sato, K., Ma, H.-Y., Yanagawa, T., Igarashi, H., Fukushige, J., and Watanabe, T. (1998). Postnatal exposure to chlorinated dioxins and related chemicals on thyroid hormone status in Japanese breast-fed infants. Chemosphere 37, 1789–1793.[Medline]
Ness, D. K., Schantz, S. L., Moshtaghian, J., and Hansen, L. G. (1993). Effects of perinatal exposure to specific PCB congeners on thyroid hormone concentrations and thyroid histology in the rat. Toxicol. Lett. 68, 311–323.[Web of Science][Medline]
Pluim, H. J., de Vijlder, J. J. M., and Olie, K., Kok, J. H., Vulsma, T., van Tijn, D. A., Van der Slikke, J. W., and Koppe, J. G. (1993). Effects of pre- and postnatal exposure to chlorinated dioxins and furans on human neonatal thyroid hormone concentrations. Environ. Health Perspect. 101, 504–508.[Web of Science][Medline]
Puel, J.-L., and Uziel, A. (1987). Correlative development of cochlear action potential sensitivity, latency, and frequency selectivity. Brain Res. 37, 179–188.
Rosiak, K. L., Seo, B. W., Chu, I., and Francis, B. M. (1997). Effects of maternal exposure to chlorinated diphenyl ethers on thyroid hormone concentrations in maternal and juvenile rats. J. Environ. Sci. Health B 32, 377–393.[Medline]
Rubel, E. W. (1978). Ontogeny of structure and function in the vertebrate auditory system. In Handbook of Sensory Physiology, (M. Jacobson, Ed.), vol. 9, pp. 135–237. Springer-Verlag, Berlin.
Rubel, E. W. (1984). Ontogeny of auditory system function. Annu. Rev. Physiol. 46, 213–229.[Web of Science][Medline]
Ruppert, P. H., Dean, K. F., and Reiter, L. W. (1984). Trimethyltin disrupts acoustic startle responding in adult rats. Toxicol. Lett. 22, 33–38.[Web of Science][Medline]
Safe, S. H. (1994). Polychlorinated biphenyls (PCBs): Environmental impact, biochemical and toxic responses, and implications for risk assessment. Crit. Rev. Toxicol. 24, 87–149.[Web of Science][Medline]
Safe, S., Wang, F., Porter, W., Duan, R., and McDougal, A. (1998). Ah receptor agonists as endocrine disruptors: Antiestrogenic activity and mechanisms. Toxicol. Lett. 102–103, 343–347.
SAS Institute, Inc. (1989). SAS/STAT User's Guide, Vol. 2, Ver. 6, 4th ed. SAS Institute, Cary, NC.
Sauer, P. J. J., Huisman, M., Koopman-Esseboom, C., Morse, D. C., Smits-van Prooije, A. E., van de Berg, K. J., Tuinstra, L. G., van der Paauw, C. G., Boersma, E. R., and Weisglas-Kuperus, N., et al. (1994). Effects of polychlorinated biphenyls (PCBs) and dioxins on growth and development. Hum. Exp. Toxicol. 13, 900–906.[Web of Science][Medline]
Schlessinger, J., Schreiber, A. B., Levi, A., Lax, I., Libermann, T., and Yarden, Y. (1983). Regulation of cell proliferation by epidermal growth factor. CRC Crit. Rev. Biochem. 14, 93–111.[Web of Science][Medline]
Schulz, D. E., Gert, P., and Duinker, J. C. (1989). Complete characterization of polychlorinated biphyeny congeners in commmercial aroclor and clophen mixtures by multidimensional gas chromatography-electron capture detection. Environ. Sci. Technol. 23, 852–859.
Schuur, A. G., Legger, F. F., van Meeteren, M. E., Moonen, M. J., van Leeuwen-Bol, I., Bergman, A., Visser, T. J., and Brouwer, A. (1998). In vitro inhibition of thyroid hormone sulfation by hydroxylated metabolites of halogenated aromatic hydrocarbons. Chem. Res. Toxicol. 11, 1075–1081.[Web of Science][Medline]
Seo, B.-W., Li, M.-H., Hansen, L. G., Moore, R. W., Petersen, R. E., and Schantz, S. L. (1995). Effects of gestational and lactational exposure to coplanar polychlorinated biphenyl (PCB) congeners or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on thyroid hormone concentrations in weanling rats. Toxicol. Lett. 78, 253–262.[Web of Science][Medline]
Sinjari, T., Darnerud, P. O. (1998). Hydroxylated polychlorinated biphenyls: Placental transfer and effects on thyroxine in the fetal mouse. Xenobiotica 28, 21–30.[Web of Science][Medline]
Sulik, K. K. (1995). Embryology of the ear. In Hereditary Hearing Loss and Its Syndromes (R. J. Gorlin, H. V. Toriello, and M. M. Cohen, Eds.), pp.23–42. Oxford University Press, New York.
Uziel, A. (1986). Periods of sensitivity to thyroid hormone during the development of the organ of Corti. Acta Otolaryngol. 429(Suppl.), 23–27.
Uziel, A., Gabrion, J., Ohresser, M., Legrand, C. (1981). Effects of hypothyroidism on the structural development of the organ of Corti in the rat. Acta Otolaryngol. 92, 469–480.[Medline]
Uziel, A., Legrand, C., and Rabie, A. (1985a). Corrective effects of thyroxine on cochlear abnormalities induced by congenital hypothyroidism in the rat: I. Morphological study. Develop. Brain Res. 19, 111–122.
Uziel, A., Legrand, C., Rabie, A. (1985b). Corrective effects of thyroxine on cochlear abnormalities induced by congenital hypothyroidism in the rat. II. Electrophysiological study. Develop. Brain Res. 19, 123–127.
Uziel, A., Rabie, A., and Marot, M. (1980). The effect of hypothyroidism on the onset of cochlear potentials in developing rats. Brain Res. 182, 172–175.[Web of Science][Medline]
Visser, T. J., Kaptein, E., van Toor, H., van Raaij, J. A., van den Berg, K. J., Joe, C. T., van Engelen, J. G., and Brouwer, A. (1993). Glucuronidation of thyroid hormone in rat liver: Effects of in vivo treatment with microsomal enzyme inducers and in vitro assay conditions. Endocrinology 133, 2177–2186.
Vodicnik, M. J., and Lech, J. J. (1980). The transfer of 2,4,5,2',4',5'-hexachlorobiphenyl to fetuses and nursing offspring: I. Disposition in pregnant and lactating mice and accumulation in young. Toxicol. Appl. Pharmacol. 54, 293–300.[Web of Science][Medline]
Volp, R. P. (1988). Glucuronidation in 4-chlorobiphenyl metabolism by rat and human liver microsomes. Drug Metab. Dispos. 16, 650–652.[Web of Science][Medline]
Young, J. F., Branham, W. S., Sheehan, D. M., Baker, M. E., Wosilait, W. D., and Luecke, R. H. (1997). Physiological "constants" for PBPK models for pregnancy. J. Toxicol. Environ. Health 52, 385–401.[Web of Science][Medline]
Young, J. S., and Fechter, L. D. (1983). Reflex inhibition procedures for animal audiometry: A technique for assessing ototoxicity. J. Acoust. Soc. Amer. 73, 1686–1693.[Web of Science][Medline]
Zhou, T., Taylor, M. M., DeVito, M. J., and Crofton, K. M. (2000). Thyroid hormone disruptive effects of brominated diphenyl ethers following developmental exposure. Toxicologist 54, 260–261 (abstract).
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J. C. Bemis and R. F. Seegal PCB-Induced Inhibition of the Vesicular Monoamine Transporter Predicts Reductions in Synaptosomal Dopamine Content Toxicol. Sci., August 1, 2004; 80(2): 288 - 295. [Abstract] [Full Text] [PDF]
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 T Mankame, R Hokanson, R Chowdhary, and D Busbee Altered gene expression in human cells induced by the agricultural chemical Enable Toxicology and Industrial Health, July 1, 2004; 20(6-10): 89 - 102. [Abstract] [PDF]
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C. S. Roegge, V. C. Wang, B. E. Powers, A. Y. Klintsova, S. Villareal, W. T. Greenough, and S. L. Schantz Motor Impairment in Rats Exposed to PCBs and Methylmercury during Early Development Toxicol. Sci., February 1, 2004; 77(2): 315 - 324. [Abstract] [Full Text] [PDF]
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R. E. Lasky, J. J. Widholm, K. M. Crofton, and S. L. Schantz Perinatal Exposure to Aroclor 1254 Impairs Distortion Product Otoacoustic Emissions (DPOAEs) in Rats Toxicol. Sci., August 1, 2002; 68(2): 458 - 464. [Abstract] [Full Text] [PDF]
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P. J. Bushnell, V. C. Moser, R. C. MacPhail, W. M. Oshiro, E. C. Derr-Yellin, P. M. Phillips, and P. R. S. Kodavanti Neurobehavioral Assessments of Rats Perinatally Exposed to a Commercial Mixture of Polychlorinated Biphenyls Toxicol. Sci., July 1, 2002; 68(1): 109 - 120. [Abstract] [Full Text] [PDF]
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T. Zhou, M. M. Taylor, M. J. DeVito, and K. M. Crofton Developmental Exposure to Brominated Diphenyl Ethers Results in Thyroid Hormone Disruption Toxicol. Sci., March 1, 2002; 66(1): 105 - 116. [Abstract] [Full Text] [PDF]
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 J. R. Inglefield, W. R. Mundy, and T. J. Shafer Inositol 1,4,5-Triphosphate Receptor-Sensitive Ca2+ Release, Store-Operated Ca2+ Entry, and cAMP Responsive Element Binding Protein Phosphorylation in Developing Cortical Cells following Exposure to Polychlorinated Biphenyls J. Pharmacol. Exp. Ther., April 12, 2001; 297(2): 762 - 773. [Abstract] [Full Text]
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M. E. Gilbert, W. R. Mundy, and K. M. Crofton Spatial Learning and Long-Term Potentiation in the Dentate Gyrus of the Hippocampus in Animals Developmentally Exposed to Aroclor 1254 Toxicol. Sci., September 1, 2000; 57(1): 102 - 111. [Abstract] [Full Text] [PDF]
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C. S. Roegge, B.-W. Seo, K. M. Crofton, and S. L. Schantz Gestational-Lactational Exposure to Aroclor 1254 Impairs Radial-Arm Maze Performance in Male Rats Toxicol. Sci., September 1, 2000; 57(1): 121 - 130. [Abstract] [Full Text] [PDF]
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