Pubertal development and reproductive functions of Crl:CD BR Sprague-Dawley rats exposed to bisphenol A during prenatal and postnatal development

Building a scientific framework for studying hormonal effects on behavior and on the development of the sexually dimorphic nervous system

There has been increasing concern that low-dose exposure to hormonally active chemicals disrupts sexual differentiation of the brain and peripheral nervous system. There also has been active drug development research on the therapeutic potential of hormone therapy on behaviors. These different research goals have in common the need to develop reliable animal models to study the effect of hormones on brain function and behaviors that are predictive of effects in humans. This paper summarizes presentations given at the June 2007 11th International Neurotoxicology Association (INA-11) meeting, which addressed these issues. Using a few examples from the bisphenol A neurobehavioral literature for illustrative purposes, Dr. Abby Li discussed some of the methodological issues that should be considered in designing developmental neurobehavioral animal studies so they can be useful for human health risk assessment. Dr. Earl Gray provided an overview of research on the role of androgens and estrogens in the development of the brain and peripheral nervous system and behavior. Based on this scientific foundation, Dr. Gray proposed a rational framework for the study of the effects of developmental exposures to chemicals on the organization of the sexually dimorphic nervous system, including specific recommendations for experimental design and statistical analyses that can increase the utility of the research for regulatory decision-making. Dr. Michael Baum and by Dr. Feng Liu presented basic research on the hormonal mechanisms underlying sexual preference and estrogenic effects of cognition, respectively. These behaviors are among those studied in adult animals following in utero exposure to hormonally active chemicals, to evaluate their potential effects on sexual differentiation of the brain. Understanding of the hormonal mechanisms of these behaviors, and of relevance to humans, is needed to develop biologically plausible hypotheses regarding the potential effects of hormonally active chemicals in humans.

Derivation of a bisphenol A oral reference dose (RfD) and drinking-water equivalent concentration

Human exposure to bisphenol A (BPA) is due to that found in the diet, and BPA and its metabolites were detected at parts per billion (or less) concentrations in human urine, milk, saliva, serum, plasma, ovarian follicular fluid, and amniotic fluid. Adverse health effects in mice and rats may be induced after parenteral injection or after massive oral doses. Controlled ingestion trials in healthy adult volunteers with 5 mg d16-BPA were unable to detect parent BPA in plasma despite exquisitely sensitive (limit of detection = 6 nM) methods, but by 96 h 100% of the administered dose was recovered in urine as the glucuronide. The extensive BPA glucuronidation following ingestion is not seen after parenteral injection; only the parent BPA binds plasma proteins and estrogen receptors (ER). The hypothesis that BPA dose-response may be described by a J- or U-shape curve was not supported by toxicogenomic data collected in fetal rat testes and epididymes (after repeated parenteral exposure at 2-400,000 microg/kg-d), where a clear monotonic dose-response both in the numbers of genes and magnitude of individual gene expression was evident. There is no clear indication from available data that the BPA doses normally consumed by humans pose an increased risk for immunologic or neurologic disease. There is no evidence that BPA poses a genotoxic or carcinogenic risk and clinical evaluations of 205 men and women with high-performance liquid chromatography (HPLC)-verified serum or urinary BPA conjugates showed (1) no objective signs, (2) no changes in reproductive hormones or clinical chemistry parameters, and (3) no alterations in the number of children or sons:daughters ratio. Results of benchmark dose (BMD10 and BMDL10) calculations and no-observed-adverse-effect level (NOAEL) inspections of all available and reproducible rodent studies with oral BPA found BMD and NOAEL values all greater than the 5 mg/kg-d NOAELs from mouse and rat multigeneration reproduction toxicity studies. While allometric and physiologically based pharmacokinetic (PBPK) models were constructed for interspecies scaling of BPA and its interaction with ER, multigeneration feeding studies with BPA at doses spanning 5 orders of magnitude failed to identify signs of developmental toxicity or adverse changes in reproductive tract tissues; the 5-mg/kg-d NOAELs identified for systemic toxicity in rats and mice were less than the oral NOAELs for reproductive toxicity. Thus, it is the generalized systemic toxicity of ingested BPA rather than reproductive, immunologic, neurobehavioral, or genotoxic hazard that represents the point of departure. Using U.S. Environmental Protection Agency (EPA) uncertainty factor guidance and application of a threefold database uncertainty factor (to account for the fact that the carcinogenic potential of transplacental BPA exposure has yet to be fully defined and comprehensive neurobehavioral and immunotoxicologic evaluations of BPA by relevant routes and at relevant doses have yet to be completed) to the administered dose NOAEL results in an oral RfD of 0.016 mg/kg-d. Assuming the 70-kg adult consumes 2 L of water each day and adopting the default 20% U.S. EPA drinking water relative source contribution yields a 100 microg/L BPA total allowable concentration (TAC).

Mechanisms of interaction of endocrine-disrupting chemicals with glutamate-evoked secretion of gonadotropin-releasing hormone

In previous studies, we detected a dichlorodiphenyltrichloroethane (DDT) derivative in the serum of children with sexual precocity after migration from developing countries. Recently, we reported that DDT stimulated pulsatile gonadotropin-releasing hormone (GnRH) secretion and sexual maturation in the female rat. The aim of this study was to delineate the mechanisms of interaction of endocrine-disrupting chemicals including DDT with GnRH secretion evoked by glutamate in vitro. Using hypothalamic explants obtained from 15-day-old female rats, estradiol (E2) and DDT caused a concentration-related increase in glutamate-evoked GnRH release while p,p'-dichlorodiphenyldichloroethene and methoxychlor had no effect. The effective DDT concentrations in vitro were consistent with the serum concentrations measured in vivo 5 days after exposure of immature rats to 10 mg/kg/day of o,p'-DDT. Bisphenol A induced some stimulatory effect, whereas no change was observed with 4-nonylphenol. The o,p'-DDT effects in vitro were prevented partially by a selective antagonist of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) subtype of glutamate receptors. A complete prevention of o,p'-DDT effects was caused by an estrogen receptor (ER) antagonist as well as an aryl hydrocarbon receptor (AHR) antagonist and inhibitors of protein kinases A and C and mitogen-activated kinases. While an intermittent incubation with E2 caused no change in amplification of the glutamate-evoked GnRH release for 4 h, continuous incubation with E2 or o,p'-DDT caused an increase of this amplification after 3.5 h of incubation. In summary, DDT amplifies the glutamate-evoked GnRH secretion in vitro through rapid and slow effects involving ER, AHR, and AMPA receptor mediation.

Influence of endocrine active compounds on the developing rodent brain

Changes in the volumes of sexually dimorphic brain nuclei are often used as a biomarker for developmental disruption by endocrine-active compounds (EACs). However, these gross, morphological analyses do not reliably predict disruption of cell phenotype or neuronal function. Therefore, an experimental approach that simultaneously assesses anatomical, physiological and behavioral endpoints is required when developing risk assessment models for EAC exposure. Using this more comprehensive approach we have demonstrated that the disruption of nuclear volume does not necessarily coincide with disruption of cellular phenotype or neuroendocrine function in two sexually dimorphic brain nuclei: the anteroventral periventricular nucleus of the hypothalamus (AVPV) and the sexually dimorphic nucleus of the preoptic area (SDN). These results demonstrate that nuclear volume is likely not an appropriate biomarker for EAC exposure. We further demonstrated that neonatal exposure to the EACs genistein (GEN) and Bisphenol-A (BPA) can affect sexually dimorphic brain morphology and neuronal phenotypes in adulthood with regional and cellular specificity suggesting that effects observed in one brain region may not be predictive of effects within neighboring regions. Finally, developmental EAC exposure has been shown to affect a variety of sexually dimorphic behaviors including reproductive behavior. These effects are likely to have a broad impact as maladaptive behavior could translate to decreased fitness of entire populations. Collectively, these findings emphasize the need to employ a comprehensive approach that addresses anatomical, functional and behavioral endpoints when evaluating the potential effects of EAC exposure.

In vivo effects of bisphenol A in laboratory rodent studies

Concern is mounting regarding the human health and environmental effects of bisphenol A (BPA), a high-production-volume chemical used in synthesis of plastics. We have reviewed the growing literature on effects of low doses of BPA, below 50 mg/(kg day), in laboratory exposures with mammalian model organisms. Many, but not all, effects of BPA are similar to effects seen in response to the model estrogens diethylstilbestrol and ethinylestradiol. For most effects, the potency of BPA is approximately 10-1000-fold less than that of diethylstilbestrol or ethinylestradiol. Based on our review of the literature, a consensus was reached regarding our level of confidence that particular outcomes occur in response to low dose BPA exposure. We are confident that adult exposure to BPA affects the male reproductive tract, and that long lasting, organizational effects in response to developmental exposure to BPA occur in the brain, the male reproductive system, and metabolic processes. We consider it likely, but requiring further confirmation, that adult exposure to BPA affects the brain, the female reproductive system, and the immune system, and that developmental effects occur in the female reproductive system.

Differential disruption of nuclear volume and neuronal phenotype in the preoptic area by neonatal exposure to genistein and bisphenol-A

Changes in the volumes of sexually dimorphic brain nuclei are often used as a biomarker for developmental disruption by endocrine-active compounds (EACs). However, these gross, morphological analyses do not reliably predict disruption of cell phenotype or neuronal function. In the present experiments, we used a more comprehensive approach to assess whether postnatal exposure to the EACs genistein (GEN) or bisphenol-A (BIS) affected the development of two sexually dimorphic brain regions in male rats: the anteroventral periventricular nucleus of the hypothalamus (AVPV) and the sexually dimorphic nucleus of the preoptic area (SDN). In addition to nuclear volumes, we also measured the number of immunopositive calbindin neurons in the SDN and the activational patterns of gonadotropin-releasing hormone (GnRH) neurons, a neuronal population that is functionally linked to the AVPV. In rats, exposure of the neonatal male brain to endogenous estrogen, aromatized from testicular testosterone, is essential for the proper sexual differentiation of these endpoints. Thus, we hypothesized that exposure to BIS and GEN during this critical period could disrupt brain sexual differentiation. Animals were given four subcutaneous injections of sesame oil (control), 250 microg GEN, or 250 microg BIS at 12 h intervals over postnatal days (PND) 1 and 2, gonadectomized on PND 85, and treated sequentially with estrogen and progesterone to stimulate Fos expression in GnRH neurons, a marker for their activation. A cohort of age-matched ovariectomized (OVX) females that were given the same hormone treatment in adulthood served as a positive control group. SDN volume was unchanged by treatment, but the number of calbindin neurons in the SDN was significantly increased by both BIS and GEN. GEN, but not BIS, demasculinized male AVPV volume, but patterns of GnRH neuronal activation were not affected by either compound. These results suggest that acute exposure to EACs during a critical developmental period can independently alter nuclear volumes of sexually dimorphic nuclei and their phenotypic profiles in a region specific manner.

Abolition of sex-dependent effects of prenatal exposure to diethylstilbestrol on emotional behavior in estrogen receptor-α knockout mice

To investigate the contribution of estrogen receptor-alpha in the effects of prenatal exposure to diethylstilbestrol on emotionality, estrogen receptor-alpha knockout heterozygous pregnant mice were orally given 0.1 microg/animal of diethylstilbestrol from gestational day 11 to 17. Emotional behavior of the offspring was assessed at 5 weeks in light-dark transition tests. Time spent in the light area was significantly decreased (i.e. decrease of emotionality) by diethylstilbestrol exposure in wild-type female mice, whereas in wild-type male mice this measurement tended to be increased (i.e. increase of emotionality) by diethylstilbestrol treatment. These sex-dependent effects of diethylstilbestrol were completely abolished in estrogen receptor-alpha knockout mice. These results suggest that the sex-dependent effects of diethylstilbestrol on emotionality are mainly produced by its action on estrogen receptor-alpha.

Female sexual maturation and reproduction after prepubertal exposure to estrogens and endocrine disrupting chemicals: a review of rodent and human data

Natural hormones and some synthetic chemicals spread into our surrounding environment share the capacity to interact with hormone action and metabolism. Exposure to such compounds can cause a variety of developmental and reproductive detrimental abnormalities in wildlife species and, potentially, in human. Many experimental and epidemiological data have reported that exposure of the developing fetus or neonate to environmentally relevant concentrations of some among these endocrine disrupters induces morphological, biochemical and/or physiological disorders in brain and reproductive organs, by interfering with the hormone actions. The impact of such exposures on the hypothalamic-pituitary-gonadal axis and subsequent sexual maturation is the subject of the present review. We will highlight epidemiological human studies and the effects of early exposure during gestational, perinatal or postnatal life in female rodents.

Effects of perinatal exposure to bisphenol A on brain neurotransmitters in female rat offspring

Pregnant Sprague-Dawley (CD IGS) rats were orally administered doses of bisphenol A (BPA) at 4, 40, and 400 mg/kg, from gestation days 6 to postnatal day 20. Neurotransmitters such as dopamine (DA) and serotonin (5HT) were extracted from the brains of dams and female offspring, and measured using liquid chromatography. BPA at 400 mg/kg was toxic and dosed rats died. At 3 wk after birth, brain levels of 3,4-dihydroxyphenylacetic acid (DOPAC, a DA metabolite), homovanillic acid (HVA, a DA metabolite), 5HT, 5-hydroxyindoleacetic acid (5HIAA, a 5HT metabolite) in female offspring were increased and the HVA/DA ratio was high in some brain areas of BPA-treated groups as compared with controls. At the age of 6 wk, levels of choline (Ch) in BPA-treated groups at 4 and 40 mg/kg were higher than control in all of eight brain areas. No changes were observed in acetylcholine (ACh) contents. In 9-wk-old offspring, changes in monoamines and metabolites were scattered and not great. At 3 wk after delivery, levels of 5HIAA in some brain areas of dams treated with BPA were higher than in control dams. Dose dependent increases in HVA and the HVA/DA ratio of the occipital cortex, and in the HVA/DA ratio of the frontal cortex were observed. The turnover of DA and 5HT was accelerated in 3-wk-old offspring and dams. BPA possesses very weak estrogenic activity. Changes in cerebral neurotransmitters observed in offspring and dams in this study may have been related to the estrogenic activity of BPA. However, further investigation is needed to examine the contribution of hormonal activity to such neurotransmitter changes.

Effects of in utero and lactational exposure to bisphenol A on thyroid status in F1 rat offspring

Bisphenol A (BPA), a xenoestrogen, has been reported to mimic the actions of estrogen or to affect the endocrine glands in vivo and in vitro. In this study, we examined whether in utero and lactational exposure to BPA alters thyroid status in rat F1 offspring. Dams were orally administered various doses of BPA (0, 4 or 40 mg/kg body weight per day) from gestation day (GD) 6 through postnatal day (PND) 20. The BPA and control groups did not differ significantly with respect to plasma thyroxine (T4) concentration. The thyroid glands from the BPA groups had normal T4 responses to exogenous thyroid-stimulating hormone in vivo. These results suggest that in utero and lactational exposure (indirect exposure) to BPA (4-40 mg/kg/day, GD 6 - PND 20) does not affect thyroid functions in the F1 generation of male and female rats.

Down-regulation of GAT-1 mRNA expression in the microdissected hypothalamic medial preoptic area of rat offspring exposed maternally to ethinylestradiol

Steroid hormones are powerful regulators of gene transcription in the brain and have the potential to permanently alter the structure and function of the developing brain. Steroid-mediated altered gene expression may thus be responsible for the molecular cascade for sexual differentiation. In this study, to assess effects of maternal exposure to ethinylestradiol (EE) on brain sexual differentiation of offspring, region-specific mRNA expression of two estrogen-responsive genes, gamma-aminobutyric acid transporter type 1 (GAT-1) and anti-apoptotic bcl-xL was measured in the medial preoptic area (MPOA), including sexually dimorphic nucleus (SDN), at the late stage of brain sexual differentiation in rats. Pregnant Sprague-Dawley animals were fed diets containing EE at concentrations of 0, 0.02, 0.1, and 0.5 ppm from day 15 of pregnancy to day 9 after delivery. In another group, neonates were directly injected with estradiol benzoate (EB: 10 microg/pup, sc) on postnatal day (PND) 2. The MPOA on PND 9 was microdissected from methacarn-fixed paraffin-embedded brain sections to measure mRNA levels by competitive RT-PCR, followed by plate hybridization. EE-exposure decreased GAT-1 expression dose-dependently from 0.02 ppm in females and at 0.5 ppm in males, while EB-treatment caused reduction only in females. EE-exposure did not alter Bcl-xL levels. At week 11, EE-exposed females exhibited a similar spectrum of histopathological changes in endocrine-linked organs as with EB, evident from 0.1 ppm, while in males EE-exposure did not cause histopathological alteration despite clear change with EB-treatment. Measurement of SDN-POA dimensions at week 11 revealed volume reduction in males exposed to 0.5 ppm EE or EB. The results suggest that GAT-1 expression in the developing MPOA is a sensitive measure for the level of disruption of brain sexual differentiation due to maternal dietary exposure to estrogens, despite definite reproductive abnormalities may not be detectable in males with this exposure protocol.

Maternal exposure to low doses of bisphenol a has no effects on development of female reproductive tract and uterine carcinogenesis in Donryu rats

Effects of maternal exposure to low doses of bisphenol A (BPA), including those comparable with human exposure levels, on growth and development of the female reproductive system and uterine carcinogenesis in Donryu rats were investigated. Dams were administered BPA (0, 0.006 and 6 mg/kg/day) daily by gavage from gestation day 2 up to the day before weaning (postnatal day 21 at offspring). The serum levels of BPA were significantly elevated in the dams receiving 6 mg/kg/day, however, BPA levels in the milk of dams, and those in the serum and liver of offspring were similar between control and treated groups. The treatment did not exert any influences on uterine development including weight, gland genesis and estrogen receptor alpha expression, vaginal opening and gonadotropin secretion in the female offspring up to puberty. After maturation, no effects were evident with regard to estrous cyclicity in female offspring treated with BPA. In addition, the treatment had no effects on age-related morphological changes of the reproductive and endocrine organs and uterine carcinogenesis until 15 months of age. The results demonstrate that maternal exposure to BPA at levels comparable to human exposure did not have any effects on the female reproductive system of offspring in rats. In addition, BPA was also found in the serum, milk and liver of control dams and pups, and low levels of BPA were detected in drinking water and pellet diet. The present study showed that the experimental animals were also exposed to environmental BPA in the animal room.

Behavioral alterations in response to fear-provoking stimuli and tranylcypromine induced by perinatal exposure to bisphenol A and nonylphenol in male rats

The purpose of this study was to examine whether perinatal exposure to two major environmental endocrine-disrupting chemicals, bisphenol A (BPA; 0.1 mg/kg/day orally) and nonylphenol [NP; 0.1 mg/kg/day (low dose) and 10 mg/kg/day (high dose) orally] daily from gestational day 3 to postnatal day 20 (transplacental and lactational exposures) would lead to behavioral alterations in the male offspring of F344 rats. Neither BPA nor NP exposure affected behavioral characteristics in an open-field test (8 weeks of age), in a measurement of spontaneous motor activity (12 weeks of age), or in an elevated plus-maze test (14 weeks of age). A passive avoidance test (13 weeks of age) showed that both BPA- and NP-treated offspring tended to delay entry into a dark compartment. An active avoidance test at 15 weeks of age revealed that BPA-treated offspring showed significantly fewer avoidance responses and low-dose NP-treated offspring exhibited slightly fewer avoidance responses. Furthermore, BPA-treated offspring significantly increased the number of failures to avoid electrical unconditioned stimuli within 5-sec electrical shock presentation compared with the control offspring. In a monoamine-disruption test using 5 mg/kg (intraperitoneal) tranylcypromine (Tcy), a monoamine oxidase inhibitor, both BPA-treated and low-dose NP-treated offspring at 22-24 weeks of age failed to show a significant increment in locomotion in response to Tcy, whereas control and high-dose NP-treated offspring significantly increased locomotion behavior after Tcy injection. In addition, when only saline was injected during a monoamine-disruption test, low-dose NP-treated offspring showed frequent rearing compared with the control offspring. The present results indicate that perinatal low-dose BPA or NP exposure irreversibly influenced the reception of fear-provoking stimuli (e.g., electrical shock), as well as monoaminergic neural pathways. Key words: behavior, bisphenol A, fear, learning, monoamine, nonylphenol.

In utero effects of chemicals on reproductive tissues in females

Chemicals found in the environment as industrial byproducts or pollutants as well as those that are prescribed or part of our daily lives can have multiple effects on the human body. The manner in which we are exposed, and the levels we are exposed to are significant contributing factors. Adults have the bodily defense mechanisms in place to combat exposures to adverse toxicants and general pollution at a variety of levels. However, developing organisms may not have adequate defense mechanisms, and toxicants can have a significant effect on their health and development. In this review, we take particular note of the toxicities of chemicals on the developing female reproductive system as a result of in utero exposure. Environmental and prescribed chemicals such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), diethylstilbestrol, and genistein, as well as others, will be reviewed for their in utero toxicity in the neuroendocrine system, the ovary, oviduct, placenta, uterus, vagina, cervix, and mammary gland.

Exposure to bisphenol A during gestation and lactation causes loss of sex difference in corticotropin-releasing hormone-immunoreactive neurons in the bed nucleus of the stria terminalis of rats

It has been suspected that endocrine disrupters induce abnormal differentiation and development of reproductive organs. In the present study, we examined whether exposure to bisphenol A (BPA), a known endocrine disrupter, during gestation and lactation affects sex difference in the number of corticotropin-releasing hormone-immunoreactive neurons (CRH neurons) in the preoptic area (POA) and the bed nucleus of the stria terminalis (BST). For that purpose, pregnant female Wistar rats (n=8-11 per treatment group) were treated with either 0.1% ethanol (control group) or 10 mg/l BPA (BPA group) dissolved in their drinking water until their offspring were weaned. In the control group, we confirmed a previous report that the POA of female rats contained significantly more CRH neurons than that of male rats (p<0.05). This significant sex difference was also evident in the BPA group, indicating that BPA exposure used in the present study had no effect on the sex difference in CRH neurons in the POA. We also found in the control group that the BST of female rats contained significantly more CRH neurons (p<0.05) than that of male rats. However, this significant sex difference was not observed in the BPA group (p>0.05), suggesting that BPA exposure affected the sex difference in CRH neurons in the BST. Since there was no statistically significant difference in the number of CRH neurons between the control and the BPA group, irrespective of the sex, the results suggested that a loss of sex difference in CRH neurons was due to both an increase in CRH neurons in male rats and a decrease in CRH neurons in female rats. The present study indicates that there is a significant sex difference in the number of CRH neurons in the BST as well as in the POA and that exposure to BPA during gestation and lactation causes a loss of this sex difference in the rat BST, but not in the POA. We suggest that CRH neurons in the BST are more susceptible to endocrine disrupters than those in the POA, irrespective of the sex.

Low-dose perinatal diethylstilbestrol exposure affected behaviors and hypothalamic estrogen receptor-α-positive cells in the mouse

To estimate the potential risk of perinatal exposure to estrogenic endocrine disrupters, pregnant female mice received daily oral administration of diethylstilbestrol (DES; either 0.3 or 3 microg/kg body weight) dissolved in corn oil from gestation days 11 to 17 and from postnatal days 2 to 6. Multiple behaviors that are sexually dimorphic were examined, and the numbers of estrogen receptor-alpha and tyrosine hydroxylase-immunoreactive (ER-IR and TH-IR) cells in some brain loci related to these behaviors were investigated. Perinatal exposure to DES caused significantly enhanced open-field activity in both males and females and significantly poorer passive avoidance performance in males. In addition, a significant increase in the number of ER-IR cells in the ventromedial hypothalamic nucleus (VMH) was demonstrated for the first time. The DES-induced increases in the sexual and aggressive behaviors, although statistically nonsignificant, and the increase in the number of ER-IR cells did not agree with those obtained in previous studies using high-dose DES, which suggests that DES may have a different effect on these endpoints depending on the dose used. The relationship between the increase in ER-IR cells and behavioral changes should be further examined.

Effects of perinatal exposure to bisphenol A on the behavior of offspring in F344 rats

The objective of this investigation is to evaluate whether perinatal maternal exposure to bisphenol A (BPA) at 4, 40, and 400 mg/kg per day affects the behavior of offspring in F344 rats. Perinatal BPA exposure inhibited the body weight increases of male and female offspring in a dose-dependent manner, which continued after weaning. Spontaneous activity analyses revealed that BPA elongated immobile time during the dark phase in female offspring. At 4 weeks of age, male offspring exposed to BPA at 40 and 400 mg/kg per day performed avoidance responses significantly higher in the shuttlebox avoidance test. At 8 weeks of age, however, male offspring only at 4 mg/kg per day showed significantly lower responses. In the open-field behavior test at 8 weeks of age, male offspring exposed to BPA only at 4 mg/kg per day showed a higher percent of grooming than the control male offspring. In conclusion, perinatal exposure to BPA caused the behavioral alterations in the offspring.

Uncertainties for endocrine disrupters: our view on progress

The hypothesis that hormonally active compounds in the environment--endocrine disrupters--are having a significant impact on human and ecological health has captured the public's attention like no other toxicity concern since the publication of Rachel Carson's Silent Spring 1962. In the early 1990s, Theo Colborn and others began to synthesize information about the potential impacts of endocrine-mediated toxicity in the scientific literature (Colborn and Clement, 1992) and the popular press (Colborn et al., 1997). Recognizing the possibility of an emerging health threat, the U.S. Environmental Protection Agency (EPA) convened two international workshops in 1995 (Ankley et al., 1997; Kavlock et al., 1996) that identified research needs relative to future risk assessments for endocrine-disrupting chemicals (EDCs). These workshops identified effects on reproductive, neurological, and immunological function, as well as carcinogenesis as the major endpoints of concern and made a number of recommendations for research. Subsequently, the EPA developed a research strategy to begin addressing the recommendations (EPA, 1998a), and the federal government as a whole, working through the White House's Committee on the Environment and Natural Resources, increased funding levels and coordinated research programs to fill the major data gaps (Reiter et al., 1998). In parallel with these research efforts that were attempting to define the scope and nature of the endocrine disruptor hypothesis, the U.S. Congress added provisions to the Food Quality Protection Act (FQPA) and the Safe Drinking Water Act of 1996 to require the testing of food-use pesticides and drinking water contaminants, respectively, for estrogenicity and other hormonal activity. These bills were enacted into law, giving the EPA the mandate to implement them. The EPA, with the help of an external advisory committee, the Endocrine Disruptor Screening and Testing Advisory Committee (EDSTAC), determined that other hormonal activity should include androgens and compounds that affect thyroid function, and expanded the mandate to include all chemicals under EPA's jurisdiction, potentially including the 70,000 chemicals regulated under the Toxic Substances Control Act (Endocrine Disruptor Screening and Testing Advisory Committee [EDSTAC], 1998). EDSTAC recommended an extensive process of prioritization, screening, and testing of chemicals for endocrine-disrupting activity, including a screening battery that involves a combination of at least eight in vitro and in vivo assays spanning a number of taxa (EDSTAC, 1998). What started out as a hypothesis has become one of the biggest testing programs conceived in the history of toxicology and the only one that has ever been based on mechanism of action as its premise. As we pass the 10th anniversary of the emergence of the endocrine disruptor hypothesis, it is useful to look back on the progress that has been made in answering the nine questions posed as data gaps in the EPA's research strategy (EPA, 1998a)--not only to see what we have learned, but also to examine whether the questions are still appropriate for the goal, what gaps remain, and what directions should be emphasized in the future.

Lack of carcinogenic risk in the prostate with transplacental and lactational exposure to bisphenol A in rats

The current study was designed to examine the modulating effects of bisphenol A (BPA) on prostate cancer risk in male offspring exposed transplacentally and lactationally. BPA was administered to F344 female rats by gavage at 0, 0.05, 7.5, 30, 120 mg/kg/day during pregnancy and lactation periods. When F1 males reached 5 weeks old, they were given 10 subcutaneous injections of 3,2'-dimethyl-4-aminobiphenyl (DMAB) or corn oil vehicle and rats were then sacrificed under ether anesthesia at week 60. There were no observable effects on the accessory sex organ weights of male offspring. Transplacental and lactational exposure to BPA did not affect the incidences of preneoplastic and neoplastic lesions in the accessory sex organs (prostate and seminal vesicle) of F1 rats and did not induce any proliferating lesions without DMAB. Our data suggest that maternal exposure to BPA during the period of pregnancy and lactation does not affect the risk of prostate carcinogenesis in male offspring.

Testicular toxicity of dietarily or parenterally administered bisphenol A in rats and mice

Male Crj:Wistar rats, HsdHot:Holtzman SD rats, Crj:CD-1(ICR) mice and C57BL/6CrSlc mice were administered bisphenol A (BPA) in the diet at a level of 0 (control) and 0.25% for 8 weeks. Daily BPA intake was about 200 and 400 mg/kg for rats and mice, respectively. No conspicuous signs of general or reproductive toxicity were observed after administration in any strain of these animals. Serum testosterone concentrations were not decreased in BPA-fed rats and mice. Successive subcutaneous administration of BPA at a dose of 200 mg/kg/day for 4 weeks significantly decreased the testis, epididymis, prostate and seminal vesicle weights, and the testicular daily sperm production in Jcl:Wistar rats. Successive intraperitoneal administration of BPA at a dose of 20 mg/kg/day for 4 weeks decreased the prostate and seminal vesicle weights but not the testis or epididymis weights. An intraperitoneal dose of 2 mg BPA/kg/day did not cause any toxicity. These results indicate that dietarily administered BPA is less toxic to most strains of rats and mice, and the maximum non-toxic dose and/or minimum toxic dose may be about 200 mg/kg/day. Subcutaneous or intraperitoneal BPA is much more toxic on male reproductive and sex accessory organs than dietary.

Low dose effects of bisphenol A on sexual differentiation of the brain and behavior in rats

There is an endocrinological concern that environmental endocrine disrupters (EEDs) may influence sexual differentiation. Bisphenol A (BPA), one of EEDs, is released from polycarbonate plastics, and has been detected in the human umbilical cord. In this study, we examined the effect of BPA on the sexual differentiation of open-field behavior and the sexually dimorphic nuclei in the brain in the offspring of rats exposed to BPA during the fetal and suckling periods at a dosage below the human tolerable daily intake (TDI) level. In the control group, females were more active in the open field and had a larger locus coeruleus (LC) volume than males. BPA abolished and inverted the sex differences of the open-field behavior and the LC volume, respectively, without affecting the reproductive system. We also compared the effects of estrogenic compounds, diethylstilbestrol (DES) and resveratrol (RVT), to that of BPA because of their structural similarities. DES affected the open-field behavior, LC volume and reproductive system, while RVT affected the LC volume and the reproductive system. These results suggest that the brain is highly sensitive to BPA at a dosage below TDI and that the disrupting effects of BPA on sexual differentiation may vary from those of RVT and DES.

Exposure to the environmental estrogen bisphenol A differentially modulated estrogen receptor-alpha and -beta immunoreactivity and mRNA in male mouse testis

We examined the effects of bisphenol A (0.5 microg/ml or 50 microg/ml) in the drinking water on estrogen receptor (ER) alpha and beta proteins and mRNA in the testis of young mice following 8-weeks of oral administration of bisphenol A utilizing immunohistochemistry and semiquantitative reverse transcription polymerase chain reaction amplification (RT-PCR). ER beta was clearly localized in the nuclei of spermatogonia and/or spermatocytes. ER beta immunopositive cell numbers per testis section were significantly decreased in the 50 microg/ml bisphenol A-treated group compared with control and the 0.5 microg/ml bisphenol A-treated group. The number of ER alpha positive cells in the testis was significantly lower than ER beta positive cells in control group. ER alpha immunopositive cell numbers per testis section were markedly increased in the 50 microg/ml bisphenol A-treated group compared with the control and the 0.5 microg/ml bisphenol A-treated group. ER beta mRNA expression was significantly decreased in the 50 microg/ml bisphenol A-treated group compared with the control and the 0.5 microg/ml bisphenol A-treated group. In contrast, ER alpha mRNA expression was markedly increased in the 50 microg/ml bisphenol A-treated group compared with the control and the 0.5 microg/ml bisphenol A-treated group. The existence of ER alpha and beta in the testis suggests that estrogens directly affect germ cells during testicular development and spermatogenesis, and differential modulation of ER alpha and beta in the testis could be involved in the effects of bisphenol A.

Lack of significant alteration in the prostate or testis of F344 rat offspring after transplacental and lactational exposure to bisphenol A

Bisphenol A (BPA), a compound of great concern as an estrogenic xenobiotic, was assessed for its ability to cause alteration in the accessory sex organs and spermatogenesis in male offspring exposed preneonatally and neonatally. In a series of experiments focusing on rat sensitivity to gestational and lactational exposure to BPA, we investigated its effects on gestation period and reproductive organs in male offspring. In the first instance, BPA was administered to F344 female rats by gavage at 0, 7.5, 120 mg/kg/day during pregnancy and lactation period. There were no observable adverse effects in pregnant rats and the treatment did not induce any morphological abnormalities in the accessory sex organs of male offspring. However, lowered numbers of sperm in the testis were found with a dose of 120 mg/kg/day. In the second study, the same protocol with a higher number of male offspring was applied, but no reduction in the sperm count was apparent. We conclude that transplacental and lactational exposure to BPA dose not exert any adverse effects on morphogenesis of rat accessory sex organs or spermatogenesis.

Maternal-fetal disposition of bisphenol a in pregnant Sprague-Dawley rats

This study describes the maternal-fetal disposition of bisphenol A and its distribution into the placenta and amniotic fluid after iv injection (2 mg/kg) to pregnant Sprague-Dawley rats. Bisphenol A was distributed extensively to the placenta and fetus, with their respective AUC values 4.4- and 2.2-fold greater than AUC for the maternal serum. In contrast, the distribution of bisphenol A into the amniotic fluid was low, with the mean amniotic fluid-to-maternal serum AUC ratio of 0.2. The decay curves of bisphenol A in the placenta, fetus, and amniotic fluid paralleled that of the maternal serum during the terminal elimination phase. A five-compartment open model consisting of the maternal central, maternal peripheral, placental, fetal, and amniotic fluid compartments was used to describe the disposition of bisphenol A in pregnant rats, with the elimination occurring from the maternal central and fetal compartments. Based on this model, bisphenol A delivered to the placenta was transferred primarily to the fetus [kpf/(kpf + kpc + kpa) = 65.4 %], with the remaining fraction transported to the maternal central (33.2%) and amniotic fluid (1.4%) compartments. Bisphenol A was eliminated from the amniotic fluid by the fetal (63.9%) and placental (36.1%) routes. On the other hand, bisphenol A was eliminated from the fetus primarily by the placental route back to mother [kfp/(kfp + kfa + kfo) = 100%], with the amniotic route playing an insignificant role in fetal elimination. The percent contribution of the fetal elimination to the total elimination in the maternal-fetal unit was 0.0% [CLfoAUCfetus/(CLcoAUCmaternal serum + CLfoAUCfetus)]. The pharmacokinetic model used in this study provides insights into the routes of elimination of bisphenol A in the maternal-fetal rat upon maternal administration.

Low-dose bisphenol A does not affect reproductive organs in estrogen-sensitive C57BL/6N mice exposed at the sexually mature, juvenile, or embryonic stage

Bisphenol A (BPA) is used on a large scale in the manufacture of polycarbonate plastics. BPA has been shown to bind weakly to both estrogen receptor (ER) alpha and ER beta. The objective of this study was to evaluate the effects of low-dose BPA on male sexual development after exposure at various stages of development. Mice of the estrogen-sensitive strain C57BL/6N were exposed to BPA orally at doses of 2, 20, or 200 microg/kg at various stages, i.e. adulthood, the immature stage just after weaning, or the embryonic/fetal stage, to evaluate the effects of low-dose BPA on male reproductive organs. Body weight changes, weights of reproductive organs (testes, epididymides, seminal vesicles), cauda epididymal sperm density, and histology of reproductive organs including the ventral prostate were not affected by exposure to BPA at any dose examined. The results of this study indicate that exposure of estrogen-sensitive C57BL/6N mice to low-dose BPA did not reduce sperm density or disrupt development of the male reproductive organs.

Rat two-generation reproductive toxicity study of bisphenol A

This study was conducted to determine the low-dose effects of bisphenol A (BPA) in a rat two-generation reproduction study. Groups of 25 male and 25 female Crj: CD (SD) IGS rats were given BPA at 0.2, 2, 20, or 200 microg/kg/day by gastric intubation throughout the study beginning at the onset of a 10- and 2-week premating period, in F0 males and females, respectively, and continuing through the mating, gestation, and lactation periods, for two generations. There were adult (F0, F1, F2) and postnatal day (PND) 22 (F1, F2) necropsies: the oldest F2 males and females being killed at postnatal weeks 7 and 14, respectively. No compound-related clinical signs or effects on body weight or food consumption were observed in any generation. There were no compound-related changes in surface righting reflex, negative geotaxis reflex, mid-air righting reflex, pinna detachment, incisor eruption, eye opening, testes descent, preputial separation, or vaginal opening in F1 and F2 generations, or behavior in the open field or water filled multiple T-maze in the F1 generation. No test compound-related changes in estrous cyclicity, copulation index, fertility index, number of implantations, gestation length, litter size, pup weight, pup sex ratio, pup viability, or other functional reproductive measures were noted in any generation. A few significant changes in the anogenital distance (AGD) per cube root of body weight ratio were found at 0.2 and 20 microg/kg in F1 males, at 2, 20, and 200 microg/kg in F1 females, and at 20 and 200 microg/kg in F2 females. However, the changes in the AGD were consistently small (within 5% of control values), and no continuous changes in the AGD or AGD/cube root of body weight ratio were detected. There were no compound-related changes in epididymal sperm counts or motility in F0 and F1 males. No compound-related necropsy findings or effects on organ weight including the reproductive organs were found in any generation. Histopathologic examinations revealed no evidence of compound-related changes in any organs including the reproductive organs of both sexes. The data indicate that oral doses of BPA of between 0.2 and 200 microg/kg over 2 generations did not cause significant compound-related changes in reproductive or developmental parameters in rats.

Exposure to bisphenol A during the fetal and suckling periods disrupts sexual differentiation of the locus coeruleus and of behavior in the rat

This study tested the effect of exposure to bisphenol A (BPA) early in life on the sexual differentiation in the brain and behavior in Wistar rats. We administered BPA only to mother rats during pregnancy and lactation at a dosage of approximately 1.5 mg/kg per day far less than the no-observed-adverse-effect level (NOAEL; 50 mg/kg per day). Control female offspring showed a higher activity, a lower avoidance memory, and larger locus coeruleus than the male controls, while the BPA-exposed group did not show any sexual dimorphism. BPA did not affect the reproductive organs or sex hormones. Our results suggest that the current methods to determine the NOAEL of artificial industrial chemicals may not be sufficient to detect a disruption of the sexual differentiation in the brain.

Metabolism and disposition of bisphenol A in female rats

Bisphenol A (BPA), which is used in the manufacture of polycarbonates, elicits weak estrogenic activity in in vitro and in vivo test systems. The objectives of this study were to compare the patterns of disposition of radioactivity in adult female F-344 and CD rats after oral administration of (14)C BPA (100 mg/kg), to isolate the glucuronide of BPA and to assess its estrogenic activity in vitro, and to evaluate the transfer of radioactivity to pups from lactating dams administered (14)C BPA. Over 6 days, F-344 rats excreted more radioactivity in urine than CD rats. The major metabolite in urine was identified as bisphenol A glucuronide (BPA gluc) by incubation with beta-glucuronidase and (1)H and (13)C NMR spectroscopy. In lactating CD rats administered (14)C BPA (100 mg/kg) by gavage, only a small fraction of the label was found in milk, with 0.95 +/- 0.66, 0.63 +/- 0.13, and 0.26 +/- 0.10 microg equiv/ml (mean +/- SD) from dams collected 1, 8, and 26 h after dosing, respectively. Radioactivity in pup carcasses indicated exposure in the range of microgram equivalents per kilogram; those values ranged from 44.3 +/- 24.4 for pups separated from their lactating dams at 2 h to 78.4 +/- 10.9 at 24 h. BPA gluc was the prominent metabolite in milk and plasma. In test systems for activation of in vitro estrogen receptors alpha and beta, BPA gluc did not show appreciable efficacy at concentrations up to 0.03 mM, indicating that metabolism via glucuronidation is a detoxication reaction.

Effect of different sampling designs on outcome of endocrine disruptor studies

In this article, we demonstrate how sampling strategy can influence the outcome of endocrine disruptor studies. In a study of the weak xenoestrogen bisphenol A (BPA), possible treatment-related effects on ventral prostate (VP) fresh weight were found in rat offspring at 6 months of age when only one or two male pups were randomly selected from each litter. In subsequent BPA and di-n-butyl phthalate studies, large intralitter variability of this specific end point was apparent when the VP weights from entire litter complements were examined. We modeled the effects of sampling 1, 2, or 3 pups from each litter using the full-litter complement data. When one pup was randomly selected, a substantial percentage of incorrect conclusions about the presence or absence of treatment effects occurred. These statistical modeling analyses raise significant concern about the selection of one pup per litter for highly variable end points.