Probable gamma-aminobutyric acid involvement in bisphenol A effect at the hypothalamic level in adult male rats

Multi- and Transgenerational Effects of Environmental Toxicants on Mammalian Reproduction

Environmental toxicants (ETs) are an exogenous chemical group diffused in the environment that contaminate food, water, air and soil, and through the food chain, they bioaccumulate into the organisms. In mammals, the exposure to ETs can affect both male and female fertility and their reproductive health through complex alterations that impact both gametogeneses, among other processes. In humans, direct exposure to ETs concurs to the declining of fertility, and its transmission across generations has been recently proposed. However, multi- and transgenerational inheritances of ET reprotoxicity have only been demonstrated in animals. Here, we review recent studies performed on laboratory model animals investigating the effects of ETs, such as BPA, phthalates, pesticides and persistent contaminants, on the reproductive system transmitted through generations. This includes multigenerational effects, where exposure to the compounds cannot be excluded, and transgenerational effects in unexposed animals. Additionally, we report on epigenetic mechanisms, such as DNA methylation, histone tails and noncoding RNAs, which may play a mechanistic role in a nongenetic transmission of environmental information exposure through the germline across generations.

Early programming of reproductive health and fertility: novel neuroendocrine mechanisms and implications in reproductive medicine

BACKGROUND

According to the Developmental Origins of Health and Disease (DOHaD) hypothesis, environmental changes taking place during early maturational periods may alter normal development and predispose to the occurrence of diverse pathologies later in life. Indeed, adverse conditions during these critical developmental windows of high plasticity have been reported to alter the offspring developmental trajectory, causing permanent functional and structural perturbations that in the long term may enhance disease susceptibility. However, while solid evidence has documented that fluctuations in environmental factors, ranging from nutrient availability to chemicals, in early developmental stages (including the peri-conceptional period) have discernible programming effects that increase vulnerability to develop metabolic perturbations, the impact and eventual mechanisms involved, of such developmental alterations on the reproductive phenotype of offspring have received less attention.

OBJECTIVE AND RATIONALE

This review will summarize recent advances in basic and clinical research that support the concept of DOHaD in the context of the impact of nutritional and hormonal perturbations, occurring during the periconceptional, fetal and early postnatal stages, on different aspects of reproductive function in both sexes. Special emphasis will be given to the effects of early nutritional stress on the timing of puberty and adult gonadotropic function, and to address the underlying neuroendocrine pathways, with particular attention to involvement of the Kiss1 system in these reproductive perturbations. The implications of such phenomena in terms of reproductive medicine will also be considered.

SEARCH METHODS

A comprehensive MEDLINE search, using PubMed as main interface, of research articles and reviews, published mainly between 2006 and 2021, has been carried out. Search was implemented using multiple terms, focusing on clinical and preclinical data from DOHaD studies, addressing periconceptional, gestational and perinatal programming of reproduction. Selected studies addressing early programming of metabolic function have also been considered, when relevant.

OUTCOMES

A solid body of evidence, from clinical and preclinical studies, has documented the impact of nutritional and hormonal fluctuations during the periconceptional, prenatal and early postnatal periods on pubertal maturation, as well as adult gonadotropic function and fertility. Furthermore, exposure to environmental chemicals, such as bisphenol A, and maternal stress has been shown to negatively influence pubertal development and gonadotropic function in adulthood. The underlying neuroendocrine pathways and mechanisms involved have been also addressed, mainly by preclinical studies, which have identified an, as yet incomplete, array of molecular and neurohormonal effectors. These include, prominently, epigenetic regulatory mechanisms and the hypothalamic Kiss1 system, which likely contribute to the generation of reproductive alterations in conditions of early nutritional and/or metabolic stress. In addition to the Kiss1 system, other major hypothalamic regulators of GnRH neurosecretion, such as γ-aminobutyric acid and glutamate, may be targets of developmental programming.

WIDER IMPLICATIONS

This review addresses an underdeveloped area of reproductive biology and medicine that may help to improve our understanding of human reproductive disorders and stresses the importance, and eventual pathogenic impact, of early determinants of puberty, adult reproductive function and fertility.

A Putative Adverse Outcome Pathway Network for Disrupted Female Pubertal Onset to Improve Testing and Regulation of Endocrine Disrupting Chemicals

The average age for pubertal onset in girls has declined over recent decades. Epidemiological studies in humans and experimental studies in animals suggest a causal role for endocrine disrupting chemicals (EDCs) that are present in our environment. Of concern, current testing and screening regimens are inadequate in identifying EDCs that may affect pubertal maturation, not least because they do not consider early-life exposure. Also, the causal relationship between EDC exposure and pubertal timing is still a matter of debate. To address this issue, we have used current knowledge to elaborate a network of putative adverse outcome pathways (pAOPs) to identify how chemicals can affect pubertal onset. By using the AOP framework, we highlight current gaps in mechanistic understanding that need to be addressed and simultaneously point towards events causative of pubertal disturbance that could be exploited for alternative test methods. We propose 6 pAOPs that could explain the disruption of pubertal timing by interfering with the central hypothalamic trigger of puberty, GnRH neurons, and by so doing highlight specific modes of action that could be targeted for alternative test method development.

Review of endocrine disruptors on male and female reproductive systems

Endocrine disruptors (EDs) interfere with different hormonal and metabolic processes and disrupt the development of organs and tissues, as well as the reproductive system. In toxicology research, various animal models have been utilized to compare and characterize the effects of EDs. We reviewed studies assessing the effect of ED exposure in humans, zebrafish, and mouse models and the adverse effects of EDs on male and female reproductive systems. This review outlines the distinctive morphological characteristics, as well as gene expression, factors, and mechanisms that are known to occur in response to EDs. In each animal model, disturbances in the reproductive system were associated with certain factors of apoptosis, the hypothalamic-pituitary-gonadal axis, estrogen receptor pathway-induced meiotic disruption, and steroidogenesis. The effects of bisphenol A, phthalate, and 17α-ethinylestradiol have been investigated in animal models, each providing supporting outcomes and elaborating the key regulators of male and female reproductive systems.

Cellular and molecular features of EDC exposure: consequences for the GnRH network

The onset of puberty and the female ovulatory cycle are important developmental milestones of the reproductive system. These processes are controlled by a tightly organized network of neurotransmitters and neuropeptides, as well as genetic, epigenetic and hormonal factors, which ultimately drive the pulsatile secretion of gonadotropin-releasing hormone. They also strongly depend on organizational processes that take place during fetal and early postnatal life. Therefore, exposure to environmental pollutants such as endocrine-disrupting chemicals (EDCs) during critical periods of development can result in altered brain development, delayed or advanced puberty and long-term reproductive consequences, such as impaired fertility. The gonads and peripheral organs are targets of EDCs, and research from the past few years suggests that the organization of the neuroendocrine control of reproduction is also sensitive to environmental cues and disruption. Among other mechanisms, EDCs interfere with the action of steroidal and non-steroidal receptors, and alter enzymatic, metabolic and epigenetic pathways during development. In this Review, we discuss the cellular and molecular consequences of perinatal exposure (mostly in rodents) to representative EDCs with a focus on the neuroendocrine control of reproduction, pubertal timing and the female ovulatory cycle.

Cord Blood Bisphenol A Levels and Reproductive and Thyroid Hormone Levels of Neonates: The Hokkaido Study on Environment and Children's Health

BACKGROUND

Bisphenol A (BPA) is widely used and BPA exposure is nearly ubiquitous in developed countries. While animal studies have indicated adverse health effects of prenatal BPA exposure including reproductive dysfunction and thyroid function disruption possibly in a sex-specific manner, findings from epidemiologic studies have not been enough to prove these adverse effects. Given very limited research on human, the aim of this study was to investigate associations between cord blood BPA levels and reproductive and thyroid hormone levels of neonates and whether associations differed by neonate sex.

METHODS

The study population included 514 participants of the Hokkaido study recruited from 2002 to 2005 at one hospital in Sapporo, Japan. The BPA level in cord blood was determined by ID-LC/MS/MS, and the limit of quantification was 0.040 ng/ml. We measured nine types of reproductive hormone levels in cord blood, and thyroid hormone levels were obtained from neonate mass screening test data. There were 283 subjects, who had both BPA and hormone levels measurements, included for the final analyses.

RESULTS

The geometric mean of cord blood BPA was 0.051 ng/ml. After adjustment, BPA level was negatively associated with prolactin (PRL) (β = -0.38). There was an interaction between infant sex and BPA levels on PRL; a weak negative association was found in boys (β = -0.12), whereas a weak positive association was found in girls (β = 0.14). BPA level showed weak positive association with testosterone, estradiol, and progesterone levels in boys. No association was found between BPA and thyroid hormone levels.

CONCLUSIONS

Our findings suggested that fetal BPA levels might be associated with changes in certain reproductive hormone levels of neonates in a sex-specific manner, though further investigations are necessary.

Impact of Low Dose Oral Exposure to Bisphenol A (BPA) on the Neonatal Rat Hypothalamic and Hippocampal Transcriptome: A CLARITY-BPA Consortium Study

Bisphenol A (BPA) is an endocrine disrupting, high volume production chemical found in a variety of products. Evidence of prenatal exposure has raised concerns that developmental BPA may disrupt sex-specific brain organization and, consequently, induce lasting changes on neurophysiology and behavior. We and others have shown that exposure to BPA at doses below the no-observed-adverse-effect level can disrupt the sex-specific expression of estrogen-responsive genes in the neonatal rat brain including estrogen receptors (ERs). The present studies, conducted as part of the Consortium Linking Academic and Regulatory Insights of BPA Toxicity program, expanded this work by examining the hippocampal and hypothalamic transcriptome on postnatal day 1 with the hypothesis that genes sensitive to estrogen and/or sexually dimorphic in expression would be altered by prenatal BPA exposure. NCTR Sprague-Dawley dams were gavaged from gestational day 6 until parturition with BPA (0-, 2.5-, 25-, 250-, 2500-, or 25 000-μg/kg body weight [bw]/d). Ethinyl estradiol was used as a reference estrogen (0.05- or 0.5-μg/kg bw/d). Postnatal day 1 brains were microdissected and gene expression was assessed with RNA-sequencing (0-, 2.5-, and 2500-μg/kg bw BPA groups only) and/or quantitative real-time PCR (all exposure groups). BPA-related transcriptional changes were mainly confined to the hypothalamus. Consistent with prior observations, BPA induced sex-specific effects on hypothalamic ERα and ERβ (Esr1 and Esr2) expression and hippocampal and hypothalamic oxytocin (Oxt) expression. These data demonstrate prenatal BPA exposure, even at doses below the current no-observed-adverse-effect level, can alter gene expression in the developing brain.

The toxicological effects of bisphenol A and octylphenol on the reproductive system of prepubertal male rats

The aim of the present study was to assess and compare the individual adverse effects of bisphenol A (BPA) and octylphenol (OP) on the reproductive system of prepubertal male rats. Rats were exposed to BPA and OP at doses of 125 and 250 mg/kg/day, by gavage, for 90 days. At the end of the study, the testes, epididymis, prostate gland, and seminal vesicle were removed and examined histopathologically. Also, 3-β-hydroxysteroid dehydrogenase expressions were analyzed and serum testosterone and luteinizing hormone (LH) levels were measured. Sperm head count of caput epididymis was performed using a hemocytometer. Seminiferous and epididymal round tubules were evaluated for tubule diameter, lumen diameter, and height of tubule epithelium. There were significant increases in relative testes weights in BPA125, OP125, and OP250 groups compared with the control. Atrophic tubules, pyknotic tubules, combined tubules, congestion, vacuolization of Sertoli cell, cell debris in the lumen, tubules without sperm, and degeneration of tubules were noted in the tissue specimens obtained from the treatment groups compared with the control group. Sperm head counts were decreased in all treatment groups except for the low-dose BPA group. Testosterone (T) levels decreased in the BPA and high-dose OP treatment groups. LH levels increased in BPA treatment groups and the low-dose OP treatment group and decreased in the high-dose OP group. Epithelial height of high-dose BPA and OP treatment groups increased compared with the control group. Furthermore tubular height of low-dose BPA and high-dose OP groups increased with respect to control levels. In the OP250 treatment group, thyroxine hormone level was increased compared to other groups. Also, in the OP125 treatment group, triiodothyronine hormone level was increased compared with other groups.

The results of this study showed that BPA and OP affect the steroidogenic enzyme expression and T production in Leydig cells. In conclusion, BPA and OP have adverse effects on the male reproductive system of prepubertal rats.

Delayed Neuroendocrine Sexual Maturation in Female Rats After a Very Low Dose of Bisphenol A Through Altered GABAergic Neurotransmission and Opposing Effects of a High Dose

Rat sexual maturation is preceded by a reduction of the interpulse interval (IPI) of GnRH neurosecretion. This work aims at studying disruption of that neuroendocrine event in females after early exposure to a very low dose of bisphenol A (BPA), a ubiquitous endocrine disrupting chemical. Female rats were exposed to vehicle or BPA 25 ng/kg·d, 25 μg/kg·d, or 5 mg/kg·d from postnatal day (PND)1 to PND5 or PND15. Exposure to 25 ng/kg·d of BPA for 5 or 15 days was followed by a delay in developmental reduction of GnRH IPI studied ex vivo on PND20. After 15 days of exposure to that low dose of BPA, vaginal opening tended to be delayed. In contrast, exposure to BPA 5 mg/kg·d for 15 days resulted in a premature reduction in GnRH IPI and a trend toward early vaginal opening. RNA sequencing analysis on PND20 indicated that exposure to BPA resulted in opposing dose effects on the mRNA expression of hypothalamic genes involved in gamma aminobutyric acid A (GABAA) neurotransmission. The study of GnRH secretion in vitro in the presence of GABAA receptor agonist/antagonist confirmed an increased or a reduced GABAergic tone after in vivo exposure to the very low or the high dose of BPA, respectively. Overall, we show for the first time that neonatal exposure to BPA leads to opposing dose-dependent effects on the neuroendocrine control of puberty in the female rat. A very low and environmentally relevant dose of BPA delays neuroendocrine maturation related to puberty through increased inhibitory GABAergic neurotransmission.

Acute Influences of Bisphenol A Exposure on Hypothalamic Release of Gonadotropin-Releasing Hormone and Kisspeptin in Female Rhesus Monkeys

Bisphenol A (BPA) is an industrial compound with pervasive distribution in the environments of industrialized countries. The U.S. Centers for Disease Control recently found that greater than 90% of Americans carry detectable levels of BPA, raising concern over the direct influences of this compound on human physiology. Epidemiologic evidence links elevated BPA serum concentrations to human reproductive dysfunction, although controlled studies on the acute effect of BPA exposure on reproductive function are limited, particularly in primates. We evaluated the effect of direct BPA exposure on female primate hypothalamic peptide release. Specifically, using a microdialysis method, we examined the effects of BPA (0.1, 1, and 10nM) directly infused to the stalk-median eminence on the release of GnRH and kisspeptin (KP) in mid to late pubertal ovarian intact female rhesus monkeys. We found that the highest level of BPA exposure (10nM) suppressed both GnRH and KP release, whereas BPA at lower concentrations (0.1 and 1nM) had no apparent effects. In addition, we measured BPA in plasma and hypothalamic dialysates after an iv bolus injection of BPA (100 μg/kg). We found a relatively stable distribution of BPA between the blood and brain (plasma:brain ≅ 5:1) persists across a wide range of blood BPA concentrations (1-620 ng/mL). Findings of this study suggest that persistent, high-level exposures to BPA could impair female reproductive function by directly influencing hypothalamic neuroendocrine function.

Serum bisphenol-A concentration and sex hormone levels in men

OBJECTIVE

To evaluate the association between serum bisphenol-A (BPA) concentration and sex hormone levels in men.

DESIGN

Cross-sectional study.

SETTING

Not applicable.

PATIENT(S)

A total of 290 men with or without BPA exposure in the workplace.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Serum sex hormone levels.

RESULT(S)

After adjustment for potential confounders using linear regression, increasing serum BPA concentration was statistically significantly associated with [1] decreased androstenedione levels, [2] decreased free testosterone levels, [3] decreased free androgen index, and [4] increased sex hormone-binding globulin levels. Comparison of hormone levels between workers exposed and unexposed to BPA showed similar associations.

CONCLUSION(S)

Exposure to a high BPA level may impact sex hormone levels in men.

In vitro evaluation of gene expression changes for gonadotropin-releasing hormone 1, brain-derived neurotrophic factor and neurotrophic tyrosine kinase, receptor, type 2, in response to bisphenol A treatment

We evaluated the effects of bisphenol A (BPA) on embryonic mouse hypothalamic cells. Real-time reverse transcription polymerase chain reaction (RT-PCR) indicated that gonadotropin-releasing hormone 1 (Gnrh1) expression in 0.02-20 μM BPA-treated cells did not differ from that in control cells but decreased significantly in 200 μMBPAtreated cells. The mRNA level for brain-derived neurotrophic factor (Bdnf), which participates in GNRH1 secretory system development, decreased significantly in 200 μM BPA-treated cells, but that for neurotrophic tyrosine kinase, receptor, type 2 (Ntrk2), did not change. This indicates that Gnrh1 gene expression in mice fetuses is not affected by exposure to <20 μM BPA and that the adverse effects of BPA on the BDNF-NTRK2 neurotrophin system are induced by decrease in the mRNA level of the ligand, not of its receptor.

Gene expression of epigenetic regulatory factors related to primary silencing mechanism is less susceptible to lower doses of bisphenol A in embryonic hypothalamic cells

DNA methyltransferases (DNMTs) are associated with epigenetic regulation of gene expression, and methyl-CpG binding protein 2 (MECP2) acts as a long-range regulator of methylated genes. We evaluated the effects of bisphenol A (BPA) on embryonic mouse hypothalamic cells, with particular emphasis on the gene expression of Dnmts (Dnmt1, Dnmt3a, and Dnmt3b) and Mecp2 isoforms. In a dose-dependent (0.02-200 µM BPA) 3-hr experiment, real-time reverse transcription polymerase chain reaction revealed that gene expression of both Dnmts and Mecp2_e2 was affected at 200 µM and that of Mecp2_e1 was affected at > 20 µM. These results suggest that gene expression of Dnmts and Mecp2 are less susceptible to lower doses of BPA in developing hypothalamic cells. However, as BPA concentration increases, this agent has the potential to alter gene expression of key players that provide stability and flexibility of epigenetic gene regulation, which could disrupt the normal development of hypothalamic functions.