Association of prenatal maternal perceived stress with a sexually dimorphic measure of cognition in 4.5-month-old infants.

Maternal prenatal stress can adversely impact subsequent child neurodevelopment, but little is known about its effect on cognitive development in infancy. This analysis of 107 infants from a prospective birth cohort assessed whether prenatal stress disrupts sexually dimorphic performance typically observed on a physical reasoning task. Maternal stress was assessed at 8-14 and 33-37 gestational weeks using the Perceived Stress Scale. Stress was defined as: low (scores below the median at both times), medium (scores above the median at one of the two times), and high (scores above the median at both times). At 4.5 months infants saw videos of two events: one impossible and the other possible. In the impossible event a box was placed against a wall without support underneath. In the possible event the box was placed against the wall, supported by the floor. Looking time at each event was recorded via infrared eye-tracking. Previous literature has shown that, at 4.5 months of age, girls typically look significantly longer at the impossible than at the possible event, suggesting that they expect the unsupported box to fall and are surprised when it does not. Boys tend to look equally at the two events suggesting that they do not share this expectation. This sex difference was replicated in the current study. General linear models stratified by sex and adjusted for household income, maternal education, mother's age at birth, infant's age at exam, and order of event presentation revealed that girls whose mothers reported high perceived stress during pregnancy had shorter looking time differences between the impossible and possible events than girls whose mothers reported low perceived stress (ß = -7.1; 95% CI: -12.0, -2.2 s; p = 0.006). Similar to boys, girls in the highest stress category spent about the same amount of time looking at each event. For boys, there were no significant looking time differences by maternal stress level. This finding suggests prenatal stress is associated with a delay in the development of physical reasoning in girls.

Early exposure to bisphenol A alters neuron and glia number in the rat prefrontal cortex of adult males, but not females.

Previous work has shown that exposure to bisphenol A (BPA) during early development can alter sexual differentiation of the brain in rodents, although few studies have examined effects on areas of the brain associated with cognition. The current study examined if developmental BPA exposure alters the total number of neurons and glia in the medial prefrontal cortex (mPFC) in adulthood. Pregnant Long-Evans rats were orally exposed to 0, 4, 40, or 400-µg/kg BPA in corn oil throughout pregnancy. From postnatal days 1 to 9, pups were given daily oral doses of oil or BPA, at doses corresponding to those given during gestation. Brains were examined in adulthood, and the volume of layers 2/3 and layers 5/6 of the mPFC was parcellated. The density of neurons and glia in these layers was quantified stereologically with the optical disector, and density was multiplied by volume for each animal. Males exposed to 400-µg/kg BPA were found to have increased numbers of neurons and glia in layers 5/6. Although there were no significant effects of BPA in layers 2/3, the pattern of increased neuron number in males exposed to 400-µg/kg BPA was similar to that seen in layers 5/6. No effects of BPA were seen in females or in males exposed to the other doses of BPA. This study indicates that males are more susceptible to the long-lasting effects of BPA on anatomy of the mPFC, an area implicated in neurological disorders.

Tetrachlorodibenzo-p-dioxin exposure alters radial arm maze performance and hippocampal morphology in female AhR mice.

Perinatal exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has been reported to alter spatial learning in rats tested on a radial arm maze (RAM). TCDD is believed to exert most of its effects through binding to the aryl hydrocarbon receptor (AhR). To determine whether the AhR mediates TCDD-induced alterations in spatial learning, we tested male and female AhR-knockout (AhR-/-), heterozygous (AhR+/-) and wild-type (AhR+/+) mice on the RAM. AhR+/- male and female mice were time mated, and treated dams were dosed with 5 microg TCDD/kg body weight on day 13 of gestation. When offspring reached adulthood, male and female AhR+/+, AhR+/- and AhR-/- mice from TCDD-exposed and unexposed litters were tested on the eight-arm RAM. After testing, we examined hippocampal morphology as visualized by the Timm's silver sulfide stain. TCDD-exposed female AhR+/- mice made more errors than their respective controls on the RAM and exhibited a decrease in the size of the intra- and infrapyramidal mossy fiber (IIP-MF) field of the hippocampus. None of the other TCDD-exposed groups differed from their respective control groups with regard to maze performance or hippocampal morphology. The reduction of IIP-MF field indicates a possible morphological basis for the learning deficit that was observed in the female AhR+/- mice. It is hypothesized that the effect of TCDD exposure is AhR dependent and that TCDD may alter GABAergic activity in the hippocampus of female mice during development.

Cognitive effects of endocrine-disrupting chemicals in animals.

A large number of chemical pollutants including phthalates, alkylphenolic compounds, polychlorinated biphenyls and polychlorinated dibenzodioxins, organochlorine pesticides, bisphenol A, and metals including lead, mercury, and cadmium have the ability to disrupt endocrine function in animals. Some of these same chemicals have been shown to alter cognitive function in animals and humans. Because hormonally mediated events play a central role in central nervous system development and function, a number of researchers have speculated that the changes in cognitive function are mediated by the endocrine-like actions of these chemicals. In this paper we review the evidence that cognitive effects of chemicals classified as environmental endocrine disruptors are mediated by changes in hormonal function. We begin by briefly reviewing the role of gonadal steroids, thyroid hormones, and glucocorticoids in brain development and brain function. We then review the endocrine changes and cognitive effects that have been reported for selected endocrine-disrupting chemicals, discuss the evidence for causal relationships between endocrine disruption and cognitive effects, and suggest directions for future research.

Impairments of memory and learning in older adults exposed to polychlorinated biphenyls via consumption of Great Lakes fish.

An association between in utero polychlorinated biphenyl (PCB) exposure and impaired childhood intellectual functioning has been reported, but the potential impact of PCB exposure during adulthood on intellectual functioning has received little attention. We assessed the impact of PCBs and other fish-borne contaminants on intellectual functioning in older adults. The subjects were 49- to 86-year-old Michigan residents recruited from an existing cohort. Fish eaters ate > 24 lb of sport-caught Lake Michigan fish per year and non-fish eaters ate < 6 lb of Lake Michigan fish per year. A battery of cognitive tests including tests of memory and learning, executive function, and visual-spatial function was administered to 180 subjects (101 fish eaters and 79 non-fish eaters). Blood samples were analyzed for PCBs and 10 other contaminants. We evaluated cognitive outcomes using multiple regression. PCBs and dichlorodiphenyl dichloroethene (DDE) were markedly elevated in fish eaters. After controlling for potential confounders PCB, but not DDE, exposure was associated with lower scores on several measures of memory and learning. These included the Weschler Memory Scale verbal delayed recall (p = 0.001), the semantic cluster ratio (p = 0.006), and list A, trial 1 (p = 0.037), from the California Verbal Learning Test. In contrast, executive and visual-spatial function were not impaired by exposure to either PCBs or DDE. In conclusion, PCB exposure during adulthood was associated with impairments in memory and learning, whereas executive and visual-spatial function were unaffected. These results are consistent with previous research showing an association between in utero PCB exposure and impairments of memory during infancy and childhood.

Spatial reversal learning in Aroclor 1254-exposed rats: sex-specific deficits in associative ability and inhibitory control.

Polychlorinated biphenyls (PCBs) are ubiquitous environmental contaminants that have been associated with cognitive deficits in children exposed in utero. Cognitive deficits due to PCB exposure have also been documented in animal models, but the underlying behavioral mechanisms responsible for those deficits remain to be elucidated. The current study examined the effects of gestational and lactational exposure to PCBs on spatial discrimination-reversal learning (spatial RL) in rats using standard two-lever operant testing chambers. Pregnant Long-Evans rats (10/dose) received either 0 or 6 mg/kg Aroclor 1254 (A1254) po in corn oil from gestational day 6 to postnatal day 21. One male and one female from each litter were tested on spatial RL beginning at 190-220 days of age. Animals were reinforced with a 45-mg food pellet for pressing the lever associated with the correct spatial location (either left or right). After reaching 85% correct performance for 2 consecutive days, the opposite spatial location was reinforced. Five of these position reversals were given. Male rats exposed to A1254 made significantly more total errors (121.6 +/- 12.5) on the first reversal than controls (90.7 +/- 5.8). In contrast, female rats exposed to A1254 exhibited deficits on the fourth and fifth reversals (23.6 +/- 4.2, 17.0 +/- 2.8 and 36.7 +/- 4.7, 26.8 +/- 2.5 for control and exposed animals, respectively). Response-pattern analyses in the A1254-exposed male and female rats revealed fundamental differences in the underlying behavioral mechanisms responsible for the deficits. A1254-exposed males exhibited an increased tendency to incorrectly respond to the previously correct stimulus (i.e., perseverate) following a reversal while A1254-exposed females exhibited impairments in their ability to make new associations with a reinforced spatial location (i.e., associative deficit). These data provide new insights into the underlying behavioral mechanisms that may be responsible for the spatial learning deficits observed in PCB-exposed rodents and monkeys.

Gestational and lactational exposure to TCDD or coplanar PCBs alters adult expression of saccharin preference behavior in female rats.

Previous studies have shown that maternal doses of 1 microg/kg or less of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in late gestation can demasculinize and feminize reproductive behavior in male rats. However, it was not known whether coplanar polychlorinated biphenyls (PCBs) had similar effects, or whether non-reproductive sexually dimorphic behaviors such as saccharin preference behavior were also altered. We determined the effects of TCDD or coplanar PCBs on saccharin consumption and saccharin preference in male and female rats. Sprague-Dawley rats were dosed with 3,3',4, 4'-tetrachlorobiphenyl (PCB 77; 2 or 8 mg/kg/day), 3,3',4,4', 5-pentachlorobiphenyl (PCB 126; 0.25 or 1.0 microg/kg/day), TCDD (0. 025 or 0.10 microg/kg/day), or corn oil vehicle on days 10-16 of gestation. Maternal exposure to TCDD or coplanar PCBs did not change saccharin consumption or saccharin preference in male rats. However, TCDD and coplanar PCB-exposed females showed decreased saccharin consumption and saccharin preference. The results indicate that saccharin consumption is masculinized in female rats exposed to TCDD or coplanar PCBs during perinatal development. This effect could be related to the anti-estrogenic actions of these chemicals.

Radial arm maze performance in rats following gestational and lactational exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD).

Recently, we reported that in utero and lactational exposure to 2,3, 7,8-tetrachlorodibenzo-p-dioxin (TCDD) resulted in a task-specific reduction of errors on the radial arm maze (RAM), without similar improvements on other spatial learning tasks including the Morris water maze. The effect was more pronounced in males than in females. This study further investigated the effects of in utero and lactational exposure to TCDD on RAM performance by testing male and female TCDD-exposed rats on either an eight-arm RAM with all arms baited or a 12-arm RAM with 8 of the 12 arms baited. If the rats have improved spatial learning or memory on the RAM, then they should be improved on both RAM tasks; whereas, if they are using adjacent arm selection or some other response strategy to solve the task, they should not show enhanced performance on the 12-arm RAM where not all the arms are rewarded. Time-mated Sprague-Dawley dams were gavaged with corn oil vehicle or one of two doses of TCDD in vehicle (0.1 or 0.2 microg/kg body weight) on gestational days 10 to 16. Litters were culled to eight on day 2 and weaned on day 21. Beginning on day 80, one male and female from each litter was tested on the eight-arm RAM with all arms baited. As in our previous studies, the 0.1-microg/kg TCDD-exposed male rats showed a significant decrease in the number of errors. However, the 0.2-microg/kg males did not differ from the controls. Neither group of TCDD-exposed females differed from the controls. None of the TCDD-exposed rats differed from the controls in adjacent arm selection behavior. An additional male and female from each litter were tested on the 12-arm RAM with only 8 of the 12 arms baited. In this task, neither TCDD group differed from the controls. These results suggest that the reduction of errors on the eight-arm RAM may be due to increased response patterning or use of intramaze cues rather than to improved spatial learning or memory. Also, the reduction in errors was only present at the lower dose of TCDD suggesting that the improvement in performance is only present at very low, nonovertly toxic doses of TCDD.

Gestational-lactational exposure to Aroclor 1254 impairs radial-arm maze performance in male rats.

Developmental exposure to polychlorinated biphenyls (PCBs) has been associated with cognitive deficits in children. The current study assessed effects of gestational and lactational exposure to a commercial PCB mixture, Aroclor 1254 (A1254), on spatial learning and memory in rats, using the radial-arm maze (RAM). Pregnant Long-Evans females (10/dose group) were exposed to 0 or 6-mg/kg/day A1254 (po in corn oil) from gestation day (GD) 6 to weaning at postnatal day (PND) 21. After they reached adulthood, 1 male and 1 female from each litter were tested on a working/reference memory task using a 12-arm RAM. Eight of the 12 arms were baited, with the pattern of baited arms remaining the same on every trial for each rat. Compared to control males, the A1254-exposed males made significantly more working memory errors (2.15 +/- 0.13 and 3.20 +/- 0.18 errors +/- SEM for control and A1254 males, respectively) and reference memory errors (3.17 +/- 0.10 and 4.13+/-0.14 errors +/- SEM for control and A1254 males, respectively) on the RAM. In contrast, A1254-exposed females were not impaired relative to control females on the RAM. Drug challenges with dizocilpine (MK-801) and scopolamine did not differentially affect working or reference memory of control and exposed rats. These data suggest that perinatal exposure to A1254 may cause sex-specific deficits in spatial learning and memory, and that NMDA-mediated and muscarinic neurotransmission, as assessed with the drug challenges, were not markedly impaired in the A1254-exposed animals.

PCB congener profile in the serum of humans consuming Great Lakes fish.

The State of Michigan has a long history of research into human exposure to environmental contaminants through consumption of recreationally caught fish. A large cohort of Lake Michigan residents who eat fish (fish-eaters) and those who do not eat fish (nonfish-eaters) established in 1980 served as the basis for the congener-specific polychlorinated biphenyl (PCB) exposure evaluation reported here. In this paper we present the serum PCB congener profile for a subset of this cohort who were over 50 years of age. Serum samples were collected in 1993-1995 and were evaluated by a dual column capillary column gas chromatography procedure capable of detecting over 90 PCB congeners. This evaluation demonstrated significant PCB exposure in the fish-eaters (mean serum PCB of 14.26 ppb; n = 101). This elevated exposure allowed the establishment of a detailed profile of the PCB congeners found in humans exposed by this route. Twenty-two congeners of varying concentrations were the most prevalent and constituted over 95% of the total PCB present in most subjects. Four congeners, 138/163 (2,2',3,4,4',5-PCB/2,3,3',4', 5,6-PCB), 180 (2,2',3,4,4',5,5'-PCB), and 153 (2,2',4,4',5,5'-PCB), accounted for 55-64% of the total PCB load. Other congeners, some of toxicologic significance, were also detected by this analytical protocol. Nonfish-eaters had lower total serum PCB levels (mean = 4. 56; n = 78), but the same general pattern of PCB congeners was present. It was demonstrated that careful selection of a subset of prevalent PCB congeners could provide a cost-effective assessment of exposure without losing critical scientific information.

Learning and memory in rats gestationally and lactationally exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD).

Recently we reported that in utero and lactational exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or coplanar polychlorinated biphenyls (PCBs) resulted in a reduction of errors on a radial arm maze (RAM) working memory task. The effect was more pronounced in males than in females. In this study, we further investigated the effects of in utero and lactational exposure to TCDD on learning and memory by testing male and female TCDD-exposed rats on three different spatial learning and memory tasks: the RAM, the Morris water maze (MWM), and spatial discrimination-reversal learning (RL), as well as on a nonspatial learning task, visual RL. Time-mated Sprague-Dawley rats were gavaged with either TCDD (0.1 microg/kg/day) or corn oil vehicle on gestation days 10-16. Litters were culled to eight on day 2 and weaned on day 21. Beginning on day 80, one male and one female from each litter were tested on the same RAM working memory task used in the previous study. Again, the TCDD-exposed male rats displayed a pronounced decrease in errors relative to control males. Following the RAM testing, the same animals were tested on the MWM, but no differences between the exposed and control rats were observed. Another male and female from each litter were tested on spatial RL on a T-maze. There were no differences between the exposed and control rats on this task. Following spatial RL, the same rats were tested on visual RL on the same maze. The exposed animals did not differ from controls on original learning, but took more trials to reach criterion on the first and second reversals. These results demonstrate a reliable, but task-specific, facilitation of spatial learning and memory in male rats exposed to TCDD during gestation and lactation. In contrast, both male and female TCDD-exposed rats showed a deficit in learning on the visual RL task. This pattern is consistent with that seen in earlier monkey studies. Perinatally TCDD-exposed monkeys were facilitated on certain spatial tasks, but impaired on visual RL tasks.

Motor function in aging Great Lakes fisheaters.

Exposure to contaminants in Great Lakes fish has been linked to impaired neuropsychological functioning in children, but neurological function of exposed adults has not been evaluated. This report describes a cross-sectional analysis of the effects of PCB/DDE exposure from contaminated fish on fine motor function in older adults. The subjects were 50-90-year-old Michigan residents who were members of a previously established study cohort. Fisheaters ate 24 lbs or more of sport-caught Lake Michigan fish/year at the time they were originally recruited in 1980-1982. Age- and sex-matched non-fisheaters ate 6 or fewer lbs/year. Outcome measures were scores on the Static Motor Steadiness Test (SMST) and Grooved Pegboard Test (GPT). PCB/DDE exposure was determined through serum analyses performed at the time of recruitment into the present study in 1993-1995. Because of the high correlation between serum PCB and DDE levels in this sample (Spearman r=0.64, P<0.0001), the effects of the two contaminants were assessed jointly using a single derived exposure variable=Low=both PCB and DDE at or below the medians of their respective distributions, intermediate=PCB and/or DDE in the third quartile, and high=PCB and/or DDE in the upper quartile. In unadjusted analyses, high exposure to PCBs/DDE was associated with significantly poorer performance on the GPT (P=0.03). However, in the multiple regression model, age and gender emerged as the most significant factors affecting GPT scores, and exposure to PCB/DDE was not significant. Performance on the SMST was not related to PCB/DDE exposure in initial unadjusted analyses, but performance with the dominant hand was marginally (P=0.052) associated with exposure in the final model. Scores on the SMST improved slightly as PCB/DDE exposure increased. A similar trend was not observed for the nondominant hand (P=0.46). These findings suggest that PCB/DDE exposure from Great Lakes fish has not significantly impaired hand steadiness or visual-motor coordination in this sample of older adults.

Ortho-substituted 2,2',3,5',6-pentachlorobiphenyl (PCB 95) alters rat hippocampal ryanodine receptors and neuroplasticity in vitro: evidence for altered hippocampal function.

The effects of PCBs on hippocampal function were studied in vitro, by radioligand-receptor binding analysis and electrophysiological measurements of the hippocampal slice preparation. [3H]Ryanodine, a conformation-sensitive probe for ryanodine receptors, was employed to determine how PCBs influence specific high-affinity occupancy to receptors found in microsomes isolated from rat hippocampus. PCB 95 (2,2',3,5'6-pentachlorobiphenyl) exhibited a dose-dependent enhancement of [3H]ryanodine receptor occupancy with an EC50 of 12 microM. In contrast, PCB 66 (2,3'4,4'-tetrachlorobiphenyl) showed no activity toward ryanodine receptors, up to its solubility limit (> or = 200 microM. Population spike (PS) and excitatory postsynaptic potential (EPSP) responses were recorded from striatum pyramidale of the CA1 region, which were generated from single pulse orthodromic stimulation of Schaffer collateral/commissural (SC/C) fibers at striatum radiatum of the hippocampal slice preparation. After the introduction of PCB 95 to the perfusion medium, PCB 95 depressed PS amplitude, especially at high stimulus intensities. Significant reductions in PS and EPSP maxima were seen, even after induction of long term potentiation, a model of neuroplasticity. However, these actions were not observed with PCB 66 which lacks ryanodine receptor activity, implicating a ryanodine receptor-mediated mechanism in the general depression of pyramidal cell excitability seen with PCB 95. Taken together, these results reveal a novel, arylhydrocarbon (Ah) receptor-independent, mechanism by which PCB 95 alters neuronal Ca2+ signaling and neuroplasticity in adult brain.

Long-term effects of developmental exposure to 2,2',3,5',6-pentachlorobiphenyl (PCB 95) on locomotor activity, spatial learning and memory and brain ryanodine binding.

There is mounting evidence that perinatal exposure to ortho-substituted PCB congeners causes neurobehavioral and neurochemical alterations. The molecular mechanism for these effects is not understood, but certain ortho-substituted PCBs have been found to interact specifically with ryanodine-sensitive Ca2+ channels in vitro. These channels are widely expressed in brain and are thought to be responsible for Ca(2+)-induced Ca2+ release. Thus, the ryanodine receptor may represent a selective molecular target through which ortho-substituted PCBs disrupt calcium signaling in neurons, and produce neurochemical and neurobehavioral alterations. Of the PCBs evaluated, 2,2',3,5',6-pentachlorobiphenyl (PCB 95) exhibits the highest potency and efficacy towards the ryanodine receptor in vitro. Therefore, we conducted an in vivo study to investigate the effects of developmental exposure to PCB 95 on neurobehavioral functional and regional brain ryanodine binding. Time-mated Sprague-Dawley rats were dosed with PCB 95 (8 or 32 mg/kg/day) or corn oil vehicle via gavage on gestation days 10-16. One male and one female from each litter were evaluated for neurobehavioral effects. Locomotor activity was evaluated in an automated open field at 35 and 100 days of age. Spatial learning and memory was assessed using an eight arm radial maze working memory task at 60 days of age and a T-maze delayed spatial alternation task at 140 days of age. The animals were then euthanized and [3H] ryanodine binding was assayed in homogenates of cerebral cortex, hippocampus and cerebellum. Rats exposed to PCB 95 showed normal levels of activity as juveniles, but were hypoactive in adulthood. They also showed a faster acquisition of the working memory task on the radial arm maze, but did not differ from controls on the T-maze delayed spatial alteration task. Region-specific changes in ryanodine binding to Ca2+ channels were also observed, with decreased binding in the hippocampus, increased binding in the cerebral cortex and a biphasic effect in the cerebellum. How these changes in ryanodine receptor function are related to the alterations in behavior will be a challenging problem to elucidate.

Neuropsychological assessment of an aging population of Great Lakes fisheaters.

Because of the decline in central nervous system function that occurs with age, older people may be at greater risk of neurological dysfunction following exposure to neurotoxic contaminants in the environment. This study was designed to assess the neuropsychological functioning of a group of 50-90-year-old fisheaters exposed to polychlorinated biphenyls (PCBs) through Great Lakes fish consumption, and a group of age- and sex-matched nonfisheaters selected from the Michigan Department of Public Health's established cohort of fisheaters and nonfisheaters. A neuropsychological assessment battery, demographic interview, and fish consumption questionnaire were developed and piloted on similarly aged men and women in the Lansing and Detroit, Michigan, areas. The assessment battery included tests of motor function, memory and learning, executive functions, and visual-spatial functions, and took approximately two hours to administer. Most of the tests included in the battery have been shown to be sensitive to subtle, age-related declines in cognitive and motor function. The demographic questionnaire included questions on a number of important control variables that could influence the neuropsychological end points that were assessed in the study. These included demographic background, alcohol consumption, tobacco use, prescription and nonprescription drug use, medical history (including psychiatric illnesses), employment history, and activity level. The fish consumption questionnaire asked about historical and current consumption of specific fish species from each of the Great Lakes and its tributaries and was based on the fish consumption advisories published in the 1992 Michigan Fishing Guide. The questionnaire also asked about consumption of wild game, fish preparation and cooking methods, serving size, and changes in fish consumption patterns over time. After each subject completed the neuropsychological assessment, demographic interview, and fish consumption questionnaire, a blood sample was collected for analysis of PCBs, dichloro diphenyl dichloroethene (DDE), and ten other contaminants frequently detected in Great Lakes fish. Subject recruitment for the study began in July 1993 and was completed in November 1995. The data will be analyzed in two steps: first, to assess differences in confounding variables between fisheaters and nonfisheaters; and secondly, to determine the independent effects of Great Lakes fish consumption, as well as serum PCB and DDE levels, on cognitive and motor function after controlling for all identified covariates. Three indices of PCB exposure-total PCBs, total ortho-substituted PCBs and total coplanar PCBs-will be assessed. These studies should shed light on three questions: 1) Does consumption of contaminated fish from the Great Lakes exacerbate or accelerate the normal age-related decline in cognitive and motor function? 2) Do serum PCB or DDE concentrations predict the degree of behavioral dysfunction? and 3) If PCB exposure is related to behavioral outcomes, which class of PCB congeners, ortho-substituted or coplanar, are responsible for the cognitive and motor deficits?

Developmental neurotoxicity of PCBs in humans: what do we know and where do we go from here?

The potential neurotoxicity of PCBs was first recognized in 1968 when a number of Japanese people became ill after ingesting rice oil that was contaminated with PCBs during the manufacturing process (Yusho). Later a similar exposure occurred in Taiwan (YuCheng). Children born to Taiwanese mothers who consumed PCB-contaminated rice oil were followed and a number of developmental abnormalities, including lower body weight and height, higher activity levels, greater incidence of behavior problems, and lower IQ scores, were observed. However, interpretation of these findings is complicated by the fact that there did not appear to be any relationship between available indices of exposure and severity of effects, and by the fact that the PCBs to which the Taiwanese were exposed contained unusually high concentrations of dibenzofurans, which are many times more toxic than PCBs, and may have been responsible for some or all of the observed effects. Since the Yusho and YuCheng episodes, several studies have been initiated to study the neurobehavioral effects of exposure to the lower levels of PCBs present in the environment. The two studies published to date have yielded conflicting results. Jacobson, Jacobson, and colleagues reported that in utero PCB exposure was associated with decreased birth weight and head circumference, shorter gestation, and several adverse outcomes on the Brazelton Neonatal Assessment Battery. Later they reported that the body weight deficits associated with prenatal PCB exposure were still present at 5 months and 4 years of age. Deficits in memory function were observed at 7 months and 4 years. Rogan, Gladen, and colleagues did not find any evidence of decreased birth weight or head circumference. Nor did they find any evidence of deficits in memory function. However, they did observe some similar effects on the Brazelton Neonatal Assessment Battery. They also observed a small delay in psychomotor development in the most highly PCB-exposed children, but the effect did not persist beyond 2 years of age. A number of methodological concerns have been raised about the Jacobson study, including issues related to exposure assessment, sample selection, and control of potential confounding variables. However, it is not clear that these shortcomings can explain the discrepancies between their findings and those of Rogan and Gladen. Other possible explanations include differences in exposure levels or PCB congener patterns between the two cohorts, differences in sociodemographic variables between the two cohorts, or other problems inherent in trying to detect subtle neuropsychological deficits at exposure levels that are near the threshold for effects. Hopefully, several new studies that are currently underway will help to resolve the uncertainties regarding the risks of perinatal PCB exposure that have been created by the conflicting results of these early studies.

Effects of gestational and lactational exposure to TCDD or coplanar PCBs on spatial learning.

Recently we reported that in utero and lactational exposure to specific ortho-substituted polychlorinated biphenyl (PCB) congeners resulted in a learning deficit on a delayed spatial alternation (DSA) task in female rats. In this study, spatial learning and memory was assessed following in utero and lactational exposure to coplanar PCBs or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Time-mated Sprague-Dawley rats were dosed with PCB 77 (3,3',4,4'-tetrachlorobiphenyl), 2 or 8 mg/kg/day; PCB 126 (3,3',4,4',5-pentachlorobiphenyl), 0.25 or 1.0 micrograms/kg/day; TCDD, 0.025 or 0.1 micrograms/kg/day; or corn oil vehicle via gavage on gestation days 10-16. Litters were culled to eight on day 2 and weaned on day 21. Beginning on day 80, one male and one female from each litter were tested on an eight-arm radial maze working memory task. The TCDD-exposed rats displayed pronounced decreases in errors relative to controls. PCB 77- and PCB 126-exposed rats showed similar, but less pronounced, decreases in errors. The same animals were later tested on a T-maze DSA task, but no differences among groups were observed. In conclusion, perinatal exposure to low doses of TCDD or structurally related coplanar PCBs appeared to facilitate acquisition of a working memory task on the radial arm maze. This effect was very different from that previously observed in rats exposed to ortho-substituted PCB congeners. The rats exposed to ortho-substituted PCBs did not differ from controls on the radial arm maze and were impaired on the T-maze DSA task. Together these findings suggest that coplanar and ortho-substituted PCBs may have different mechanisms of action on the CNS.

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.

Perinatal exposure to polychlorinated biphenyl (PCB) mixtures or to certain ortho-substituted PCB congeners dramatically reduces circulating thyroxine (T4) concentrations. It is not clear whether perinatal exposure to coplanar PCBs or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has a similar effect. In this study, time-mated Sprague-Dawley rats were dosed with 2 or 8 mg/kg/day PCB 77 (3,3',4,4'-tetrachlorobiphenyl), 0.25 or 1.00 micrograms/kg/day PCB 126 (3,3',4,4',5-pentachlorobiphenyl), 0.025 or 0.10 microgram/kg/day TCDD, or corn oil vehicle orally on gestation days 10-16. At weaning, plasma total T4 concentrations in PCB 77 and TCDD high-dose female pups were significantly depressed, but the changes were modest (84.4 and 79.6% of control, respectively). T4 concentrations in PCB 126 high-dose females and all high-dose males were also depressed slightly, but the changes were not statistically significant. UDP-Glucuronosyl transferase (UDP-GT) activity towards 4-nitrophenol was increased in all high-dose groups. Thus, the modest decreases in T4 could be due in part to increased T4 glucuronidation by UDP-GT. Triiodothyronine (T3) and thyroid stimulating hormone (TSH) concentrations were unchanged in all groups. In contrast to the minor changes in thyroid hormone status, liver microsomal ethoxyresorufin-O-deethylase (EROD) was markedly induced in all exposure groups and thymus weights were depressed in the high-dose groups. Because doses of coplanar PCBs or TCDD that caused marked induction of EROD activity had only minor effects on T4, we conclude that changes in thyroid hormone status at weaning are not among the more sensitive effects of perinatal exposure to these compounds.

Spatial learning deficits in adult rats exposed to ortho-substituted PCB congeners during gestation and lactation.

Spatial learning and memory was assessed in rats following gestational and lactational exposure to specific ortho-substituted PCBs. Time-mated Sprague-Dawley rats were exposed to PCB 28 (2,4,4'-trichlorobiphenyl), 8 or 32 mg/kg/day, PCB 118 (2,3',4,4',5-pentachlorobiphenyl), 4 or 16 mg/kg/day, PCB 153 (2,2',4,4',5,5'-hexachlorobiphenyl), 16 or 64 mg/kg/day, or corn oil vehicle via gavage on Gestation Days 10-16. Litters were culled to eight on Day 2 and weaned on Day 21. Beginning on Day 90, one male and one female from each litter were tested on a working/reference memory task on an eight-arm maze. For each rat, the same four arms were baited throughout training. Animals were tested Monday-Friday, for seven consecutive weeks. No differences in working or reference memory errors were observed. The same animals were later tested on a T-maze delayed spatial alternation task. On each trial, the reinforcer was placed in the arm opposite that chosen by the rat on the previous trial. Intertrial delays of 15, 25, or 40 sec appeared in counterbalanced order. Rats were tested Monday-Friday for three consecutive weeks. The higher doses of all three congeners resulted in slower acquisition by female rats. Males were not affected. PCB-exposed females were impaired at all delays and were not differentially more impaired at longer delays, suggesting a learning or attentional deficit, rather than a mnemonic deficit. These findings demonstrate that perinatal exposure to ortho-substituted PCBs can result in long-lasting deficits in learning and suggest that the effects of PCBs on learning may be sex specific.