A1254 Exposure Research Articles

TOXIC EFFECTS OF AROCLOR 1254 ON RAT HEART AND THYROID AND MODIFYING ROLE OF SELENIUM STATUS

Objective: Polychlorinated biphenyls (PCBs) were very widely used in industrial products in past. These chemicals were banned in the 1970s due to their toxic effects. PCBs can still affect human health, as they are persistent in the environment. Aroclor 1254 (A1254) is a commercial PCB congener which was used in electrical transformers, fluorescent lighting fixtures and old appliances such as televisions or refrigerators. In this study, we aimed to evaluate the toxic effects of A1254 on heart and thyroid in male Sprague-Dawley rats. In addition, the modifying role of selenium status was also evaluated.
 Material and Method: 8-week-old male Sprague-Dawley (SD) rats were used in the experiment. The animals were separated randomly into 6 groups (n=6) as control; selenium supplemented (SeS); selenium deficient (SeD); A1254 exposed (A); selenium supplemented A1254 exposed (ASeS) and selenium deficient A1254 exposed (ASeD). A1254 was applied by gavage during the last 15 days of feeding period. Heart and thyroid weights and relative weights, plasma thyroid hormone levels, as well as thyroid and heart tissue oxidative/antioxidative parameters were evaluated.
 Result and Discussion: Results showed that A1254 exposure and selenium deficiency caused oxidative stress on both heart and thyroid. Plasma fT3 and fT4 levels markedly changed in ASeD group. In conclusion, it can be stated that A1254 exposure can cause lead to oxidative/antioxidative imbalance in both thyroid and heart and can disrupt functioning of thyroid hormones. Selenium seems to have a modifying role in A1254 toxicity in both organs, the importance of which should be evaluated with further mechanistic experiments.

PDGFRα/β-PI3K-Akt pathway response to the interplay of mitochondrial dysfunction and DNA damage in Aroclor 1254-exposed porcine granulosa cells

Metabolic dysfunction and genomic instability are known to affect female fertility. Aroclor 1254 (A1254) is an endocrine disruptor that affects mitochondrial function following ingestion, inhalation, or dermal exposure. Numerous studies to date have addressed associations between A1254 toxicity and chronic neurological disorders, while A1254 exposure is little known to have a toxic effect on the female reproductive system. Furthermore, interactive mechanisms between metabolic dysfunction and the repair of DNA damage deserve further investigation. In this paper, an in vitro porcine primary granulosa cell (GC) culture model was used to investigate the mechanisms of exposure and effects of the exogenous chemical carcinogen A1254 on reproductive toxicology. High-throughput RNA sequencing obtained 2329 differentially expressed genes (DEGs) to be analyzed using COG classification, GO, and KEGG. When combined with immunofluorescence, Western blot analysis, and real-time RT-PCR analysis, this data showed that the mitochondrial-ROS-driven feed-forward loop increased phospho-PDGFRα/β, which stimulates apoptosis by suppressing the PI3K-Akt pathway. We also noticed that inhibition of the Akt-PDP1-PDK1 axis attenuated mitochondrial function. In contrast, following iPath analysis, partial metabolic pathways were enhanced. Importantly, we found that A1254 activated a DNA damage response, the major regulators of which belong to the PI3K-related protein kinases (PIKKs) and oncogenes, which led to the “Warburg effect”. It is not easy to restore the damage that A1254 causes to metabolism through dysregulation and the Warburg effect, owing to the fact that oncogenes can regulate cytoplasmic metabolism. Therefore, we suspect that the PDGFR-PI3K-Akt pathway may be a latent interaction between mitochondrial dysfunction and the response of DNA damage.

MS-275 inhibits aroclor 1254-induced SH-SY5Y neuronal cell toxicity by preventing the formation of the HDAC3/REST complex on the synapsin-1 promoter.

Polychlorinated biphenyl (PCB) exposure has been associated with neurodegenerative diseases, such as Parkinson's disease, amyotrophic lateral sclerosis, and dementia. Neuronal death elicited by the PCB mixture Aroclor 1254 (A1254) has been attributed to an increase in RE-1-silencing transcription factor (REST), which, in turn, correlates with a decrease in the synapsin-1 promoter gene. Although histone deacetylase (HDAC) inhibitors are known to be neuroprotective in several neurologic disorders, the core mechanisms governing this effect are not yet understood. Here, to examine how HDAC class I [N-(2-aminophenyl)-4-[N-(pyridin-3-yl-methoxycarbonyl)aminomethyl]-benzamide (MS-275)] and HDAC class II [3-[5-(3-(3-fluorophenyl)-3-oxopropen-1-yl)-1-methyl-1H-pyrrol-2-yl]-N-hydroxy-2-propenamide (MC-1568)] inhibitors prevent A1254-induced neuronal cell death, we exposed SH-SY5Y neuroblastoma cells to A1254. Exposure to A1254 (30.6 μM) for 24 and 48 hours resulted in a time-dependent cell death. Indeed, after 48 hours, MS-275, but not MC-1568, reverted A1254-induced cell death in a dose-dependent manner. Furthermore, A1254 significantly increased HDAC3, but not HDAC1 or HDAC2. Interestingly, REST physically interacted with HDAC3 after A1254 exposure. Chromatin immunoprecipitation assays revealed that MS-275 reverted the increased levels of HDAC3 binding and decreased acetylation of histone H3 within the synapsin-1 promoter region, thus reverting synapsin-1 mRNA reduction. Moreover, REST knockdown by small interfering RNA (siRNA) prevented HDAC3 from binding to the synapsin-1 promoter. Likewise, HDAC3 siRNA significantly reduced A1254-induced cell toxicity in SH-SY5Y cells and cortical neurons. Hence, this study demonstrates that inhibition of HDAC class I attenuates A1254-induced neuronal cell death by preventing HDAC3 binding and histone deacetylation within the synapsin-1 promoter region.

Gestational and lactational exposure to the polychlorinated biphenyl mixture Aroclor 1254 modulates retinoid homeostasis in rat offspring

Polychlorinated biphenyls (PCBs) induce a broad spectrum of biochemical and toxic effects in mammals including alterations of the vital retinoid (vitamin A) system. The aim of this study was to characterize alterations of tissue retinoid levels in rat offspring and their dams following gestational and lactational exposure to the PCB mixture Aroclor 1254 (A1254) and to assess the interrelationship of these changes with other established sensitive biochemical and toxicological endpoints. Sprague-Dawley rat dams were exposed orally to 0 or 15mg/kg body weight/day of A1254 from gestational day 1 to postnatal day (PND) 23. Livers, kidneys and serum were collected from the offspring on PNDs 35, 77 and 350. Tissue and serum retinoid levels, hepatic cytochrome P450 (CYP) enzymes and serum thyroid hormones were analyzed. A multivariate regression between A1254 treatment, hepatic retinoid levels, hepatic CYP enzymes activities, thyroid hormone levels and body/liver weights was performed using an orthogonal partial least-squares (PLS) analysis. The contribution of dioxin-like (DL) components of A1254 to the observed effects was also estimated using the toxic equivalency (TEQ) concept. In both male and female offspring short-term alterations in tissue retinoid levels occurred at PND35, i.e. decreased levels of hepatic retinol and retinoic acid (RA) metabolite 9-cis-4-oxo-13,14-dihydro-RA with concurrent increases in hepatic and renal all-trans-RA levels. Long-term changes consisted of decreased hepatic retinyl palmitate and increased renal retinol levels that were apparent until PND350. Retinoid system alterations were associated with altered CYP enzyme activities and serum thyroid hormone levels as well as body and liver weights in both offspring and dams. The estimated DL activity was within an order of magnitude of the theoretical TEQ for different endpoints, indicating significant involvement of DL congeners in the observed effects. This study shows that tissue retinoid levels are affected both short- and long-term by developmental A1254 exposure and are associated with alterations of other established endpoints of toxicological concern.

Developmental Exposure to Polychlorinated Biphenyls Interferes with Experience-Dependent Dendritic Plasticity and Ryanodine Receptor Expression in Weanling Rats

BackgroundNeurodevelopmental disorders are associated with altered patterns of neuronal connectivity. A critical determinant of neuronal connectivity is the dendritic morphology of individual neurons, which is shaped by experience. The identification of environmental exposures that interfere with dendritic growth and plasticity may, therefore, provide insight into environmental risk factors for neurodevelopmental disorders.ObjectiveWe tested the hypothesis that polychlorinated biphenyls (PCBs) alter dendritic growth and/or plasticity by promoting the activity of ryanodine receptors (RyRs).Methods and ResultsThe Morris water maze was used to induce experience-dependent neural plasticity in weanling rats exposed to either vehicle or Aroclor 1254 (A1254) in the maternal diet throughout gestation and lactation. Developmental A1254 exposure promoted dendritic growth in cerebellar Purkinje cells and neocortical pyramidal neurons among untrained animals but attenuated or reversed experience-dependent dendritic growth among maze-trained littermates. These structural changes coincided with subtle deficits in spatial learning and memory, increased [3H]-ryanodine binding sites and RyR expression in the cerebellum of untrained animals, and inhibition of training-induced RyR upregulation. A congener with potent RyR activity, PCB95, but not a congener with negligible RyR activity, PCB66, promoted dendritic growth in primary cortical neuron cultures and this effect was blocked by pharmacologic antagonism of RyR activity.ConclusionsDevelopmental exposure to PCBs interferes with normal patterns of dendritic growth and plasticity, and these effects may be linked to changes in RyR expression and function. These findings identify PCBs as candidate environmental risk factors for neurodevelopmental disorders, especially in children with heritable deficits in calcium signaling.

Developmental Exposure to Polychlorinated Biphenyls Influences Stroke Outcome in Adult Rats

BackgroundThe “developmental origins of adult disease” hypothesis was originally derived from evidence linking low birth weight to cardiovascular diseases including stroke. Subsequently, it has been expanded to include developmental exposures to environmental contaminants as risk factors for adult onset disease.ObjectiveOur goal in this study was to test the hypothesis that developmental exposure to poly-chlorinated biphenyls (PCBs) alters stroke outcome in adults.MethodsWe exposed rats to the PCB mixture Aroclor 1254 (A1254) at 0.1 or 1 mg/kg/day in the maternal diet throughout gestation and lactation. Focal cerebral ischemia was induced at 6–8 weeks of age via middle cerebral artery occlusion, and infarct size was measured in the cerebral cortex and striatum at 22 hr of reperfusion. PCB congeners were quantified in brain tissue by gas chromatography with microelectron capture detection, and cortical and striatal expression of Bcl2 and Cyp2C11 were quantified by quantitative reverse transcriptase-polymerase chain reaction.ResultsDevelopmental exposure to A1254 significantly decreased striatal infarct in females and males at 0.1 and 1 mg/kg/day, respectively. Predominantly ortho-substituted PCB congeners were detected above background levels in brains of adult females and males exposed to A1254 at 1 but not 0.1 mg/kg/day. Effects of developmental A1254 exposure on Bcl2 and Cyp2C11 expression did not correlate with effects on infarct volume.ConclusionOur data provide proof of principle that developmental exposures to environmental contaminants influence the response of the adult brain to ischemic injury and thus represent potentially important determinants of stroke susceptibility.

Effects of aroclor 1254 on the expression of the KAP3 gene and reproductive function in rats

The present study investigated the effects of aroclor 1254 (A1254) on the expression of the kinesin superfamily associated protein 3 (KAP3) gene in F1 rat brain during brain sexual differentiation and puberty. In addition, the effects of A1254 on reproductive function were examined. The KAP3 gene is involved in the neurogenesis and synaptogenesis of sexual differentiation in rats and also during puberty. In the present study, pregnant Sprague-Dawley rats each received a daily dose of A1254 (0, 10, 50 mg kg(-1)) dissolved in 1.0 mL corn oil by gavage, from gestational Day (GD) 8 to postnatal Day (PD) 21. The mRNA levels of the KAP3 gene in hypothalamic tissues were analysed by northern blot hybridisation during the critical periods of brain sexual differentiation (GD18 and PD5) and puberty (PD28). Variables affecting reproduction in F1 female rats, such as vaginal opening (VO), vaginal oestrus (VE) and oestrous cyclicity, were recorded. Depending on the sex and A1254 exposure (control or 50 mg kg(-1) day(-1)), F1 rats were divided into three mating groups, namely control male-control female, control male-A1254-treated female and A1254-treated male-control female. During the critical periods of brain sexual differentiation (GD18, PD5) and puberty (PD28), KAP3 mRNA levels were significantly reduced in A1254-treated fetal and pubertal rat brains relative to those of control groups. In A1254-treated F1 female rats, VO and VE were delayed, the percentage of irregular oestrous cycles was increased and the duration of the oestrous cycle was extended in a dose-dependent manner compared with control groups. Treatment with a high dose of A1254 significantly impaired the reproductive function of both male and female F1 rats, including mating and pregnancy indices and the number of live fetuses. These data suggest that A1254 disrupts transcriptional regulation of the KAP3 gene in fetal and pubertal rat brains and that these effects may be related to A1254-induced abnormal brain sexual differentiation and lowered reproductive function in F1 rats.

Heme-Oxygenase-1 Promotes Polychlorinated Biphenyl Mixture Aroclor 1254-Induced Oxidative Stress and Dopaminergic Cell Injury

Dopaminergic (DAergic) systems have been identified as putative targets for polycholorinated biphenyl (PCB) actions. However, the precise mechanisms leading to neurotoxicity are unresolved. Reactive oxygen species (ROS) were recently shown to mediate injury in DAergic MN9D cells following exposure to Aroclor 1254 (A1254), a commercial PCB mixture. The oxidative stress response in DAergic cells included a persistent expression of heme oxygenase-1 (HO-1). This study tested the hypothesis that a sustained PCB-induced HO-1 response leads to abnormally high Fe levels, which generates ROS production and mediates death in the MN9D DAergic cell model. Accordingly, results indicated that A1254 augmented intracellular Fe levels in MN9D cells after 24 h. Fe chelation by desferoxamine or pharmacologic inhibition of HO activity with tin-protoporphyrin reduced Fe accumulation, ROS production, and cytotoxicity following A1254 exposure. HO-1 over-expression predisposed MN9D DAergic cells to enhanced ROS production and cell death in response to PCBs. Conversely, antisense inhibition of HO-1 expression prevented PCB-induced ROS production and cell death. These observations suggest that enhanced HO-1 catalytic activity and subsequent liberation of Fe participate in neurotoxic DAergic cell injury caused by A1254 exposure in vitro.

Perinatal exposure to polychlorinated biphenyls differentially affects cerebellar development and motor functions in male and female rat neonates

Perinatal exposure to polychlorinated biphenyls (PCBs) interacts with genetics and impacts the course of the central nervous system (CNS) development in both humans and animals. To test the hypothesis that the neurobehavioral impairments, and specifically motor dysfunctions following perinatal PCB exposure in rats are associated with changes in a specific brain region, the cerebellum, we compared neurodevelopment, motor behavior, cerebellar structure, and protein expression in rat neonates exposed to the PCB mixture Aroclor 1254 (A1254, 10.0 mg/kg/day) from gestational day 11 until postnatal day (P) 21 with that of controls. Body mass of PCB-exposed pups was not affected at birth, but was significantly lower than that of controls between birth and weaning; by P21 the difference was greater in females than in males. A1254 exposure delayed ear unfolding and impaired performance on the following behavioral tests: (1) righting response on P3-P6; (2) negative geotaxis on P5-P7; (3) startle response on P10-P12; and (4) a rotorod on P12, with PCB-male pups more severely affected than female. Changes in the behavior of PCB pups were associated with changes in cerebellar structure and protein expression. Cerebellar mass was more reduced in PCB-male than PCB-female pups. Analysis of selected cerebellar proteins revealed an increase in GFAP expression, greater in male than in female, and a decrease in L1 expression in both sexes. These results suggest that PCB exposure affects behavior and cerebellar development differently in male and female rat neonates, with greater effects in males. Further studies of neonatal PCB exposure will establish whether the environmental pollutants can contribute to the sex-related preponderance of certain neuropsychiatric disorders.

Toxic and hormonal effects of polychlorinated biphenyls on cultured testicular germ cells of embryonic chickens

The toxic and hormonal effects of polychlorinated biphenyls (PCBs) on testicular germ cell development were revealed with a germ-Sertoli cell co-culture model from embryonic chickens. Testicular cells were dispersed from 18-day-old embryo and exposed to Aroclor 1254 (A1254) alone and combined with α-tocopherol, flutamide and tamoxifen for discretion of the toxic and hormonal actions of A1254. Cell damage was evaluated by determinations of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT) reduction and lactate dehydrogenase (LDH) leakage. Results showed that 10 μg/ml A1254 induced condensed nuclear and vacuolated cytoplasm; cell exfoliated and broke into pieces as a sign of cell degeneration after treatment for 6 h. The morphological cytotoxicity was confirmed by MTT reduction and LDH leakage assays. α-Tocopherol attenuated the toxic effect of A1254. After culture for 48 h, A1254 (0.1–1 μg/ml) manifested obvious hormonal effect to induce germ cell proliferation, while 10 μg/ml A1254 displayed obvious toxic and hormonal effects on germ and somatic cells. Blocking of either androgen receptor by flutamide or estrogen receptor by tamoxifen inhibited hormonal effect of A1254 on germ cell proliferation and increased the cytotoxicity. The above results indicated that A1254 exposure imposed both toxic and hormonal effects on embryonic testicular germ cell proliferation, which may cause reproductive disorder and even infertility at adulthood.

Exposure of pig oocytes to PCBs during in vitro maturation: effects on developmental competence, cytoplasmic remodelling and communications with cumulus cells.

Polychlorinated biphenyls (PCBs) are one of the most persistent and widespread groups of endocrine disrupting compounds in the ecosystem. These substances are present in sewage sludge that is spread in increasing amounts on arable land and pasture as fertilizer, and are ingested by farm animals with food and drinking water. This study investigated the effect of different PCB concentrations on pig oocyte in vitro maturation and developmental competence as well as examined the possible mechanisms involved. A concentration ranging from 0 to 1 mg/mL of Aroclor 1254 (A1254), a pool of more than 60 PCB congeners, was added to the maturation medium, as its composition is considered environmentally relevant. A1254 had no effect on maturation of pig oocytes and on the number of oocytes that cleaved following parthenogenetic activation at any of the doses tested. By contrast, a significant decrease in the number of zygotes that developed to blastocyst stage became evident at a concentration of 10 ng/mL. The number of blastocysts obtained decreased significantly, and in a dose response manner with higher concentrations. Exposure to PCBs altered mitochondria relocation during maturation and this was associated with the lack of a cytoplasmic microtubule network. No effect on mitochondria activity was observed. A1254 exposure also perturbed gap-junction mediated communications between oocytes and cumulus cells. In conclusion, PCB exposure of pig oocytes during in vitro maturation significantly decreased oocyte developmental competence, altered both their cytoplasmic remodelling and the communication with the somatic compartment. These data indicated that accumulation of PCBs in the pig organism may have a detrimental effect on the reproductive efficiency in this species.

Biological effects of gestational and lactational PCB exposure in neonatal and juvenile C57BL/6 mice.

The purpose of this study was to obtain a better understanding of polychlorinated biphenyl (PCB) immunotoxicity in the developing mouse. Adult female mice were dosed with three subcutaneous injections per week of 50 mg/kg Aroclor 1242 (A1242), Aroclor 1254 (A1254), or corn oil for 2 weeks and then mated with nondosed males. First-litter pups were sacrificed at 7 or 28 days of age. At both ages, the tissue concentration of PCB was significantly higher in both the A1242 and A1254 pups than in oil-treated controls. Seven-day-old pups exposed to A1242 or A1254 had significantly decreased splenic IL-2 production. Alterations in the percentages of T cell subsets compared to controls were observed in A1242-exposed pups; an increased spleen somatic index was noted only in A1254-exposed pups. Twenty-eight-day-old pups exposed to A1254 demonstrated a significant decrease in thymus somatic index, an increase in liver somatic index, a 25% decrease in total circulating T(4), and decreased B cell percentages relative to their controls. Alteration in the percentages of CD3(int) T cells was observed in A1254-exposed 28-day-old pups. A significant increase in 7-ethoxyresorufin- O-deethylase (EROD) and 7-benzoxyresorufin-O-dearylase (BROD) activity was measured at both ages in A1254-exposed pups and in A1242-exposed 28-day-old pups. These data confirm that during gestation and lactation A1242 and A1254 are transferred from dams to pups and that such exposure results in immune-related effects in neonatal (7-day-old) and juvenile (28-day-old) mice. Furthermore, A1254 exposure produces more frequent and pronounced effects than exposure to A1242.

Perinatal exposure to Aroclor 1254 impairs distortion product otoacoustic emissions (DPOAEs) in rats.

Polychlorinated biphenyls (PCBs) are ubiquitous environmental contaminants that are a potential health hazard to human and wildlife populations. Low-frequency auditory impairments have previously been documented in Aroclor 1254 (A 1254)-exposed rats, including elevated behavioral auditory thresholds and decreased amplitude and prolonged latency auditory evoked brain stem responses (ABRs). Furthermore, outer hair-cell loss on the basilar membrane of the cochlea has been documented, demonstrating that the cochlea is a target organ for PCB ototoxicity. The current experiment assessed the effects of A1254 on cochlear function by measuring distortion product otoacoustic emissions (DPOAEs). ABRs were measured to determine the effects of A1254 on the central nervous system auditory pathways. Pregnant Long-Evans rats received either 0 or 6 mg/kg A1254 (po) in corn oil from gestation day 6 to lactational day 21. The auditory function of male and female offspring was assessed at approximately 18 months of age. The rats were anesthetized and a probe-unit, consisting of 2 insert earphones and a microphone, was positioned in the ear canal. DPOAE amplitudes were reduced and thresholds increased in the A1254-exposed rats. The deficits were most pronounced at the lowest frequencies tested (2.1-3.2 kHz), but deficits were also observed at higher frequencies (3.7-8.6 kHz). Males and females were equally affected at the lower frequencies, but females were more impaired at the higher frequencies. In contrast, ABR latencies and amplitudes were not altered by A1254 exposure. These findings provide the first functional evidence supporting a cochlear site of damage in PCB-induced hearing loss.

Schedule-controlled behavior in rats exposed perinatally to the PCB mixture Aroclor 1254

Exposure to polychlorinated biphenyls (PCBs) has been shown to detrimentally affect learning and memory in children as well as schedule-controlled behavior in experimental animals. The objective of the present series of experiments was to extend research into the effects of PCBs on behavior maintained under both short (30 s) and long (5 min) fixed-interval (FI) schedules as well as an FI 3-min with reinforcement omission. Long–Evans rats were exposed to 0 or 6 mg/kg/day Aroclor 1254 (A1254) via oral gavage from Gestation Day 6 (GD 6) through Postnatal Day 21 (PND 21). At approximately PND 90, acquisition and steady-state performance were assessed under a series of FI reinforcement schedules consisting of FI 30-s, FI 5-min, and FI 3-min with 33% of the scheduled reinforcers omitted. Performance measures included index of curvature (IOC), response rate, and postreinforcement pause (PRP). There were no effects of A1254 on the acquisition of behavior under the FI 30-s schedule. Subsequently, there was an initial decrease in response rate and IOC and an increase in PRP following the transition from FI 30-s to the FI 5-min; there were, however, no treatment-related effects on any measure. During the reinforcement-omission procedure, there was an increase in the rate of responding and a decrease in IOC and PRP following omission intervals irrespective of treatment. These data are inconsistent with previous findings and suggest that perinatal A1254 exposure in the rat does not disrupt temporally organized behavior.

PCBs, thyroid hormones, and ototoxicity in rats: cross-fostering experiments demonstrate the impact of postnatal lactation exposure.

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 (T(4)) 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 T(4) 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.