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

Androgen regulation of behavioral stress responses and the hypothalamic-pituitary-adrenal axis

Testosterone is a powerful steroid hormone that can impact the brain and behavior in various ways, including regulating behavioral and neuroendocrine (hypothalamic-pituitary-adrenal (HPA) axis) stress responses. Early in life androgens can act to alter development of brain regions associated with stress regulation, which ultimately impacts the display of stress responses later in life. Adult circulating androgens can also influence the expression of distinct genes and proteins that regulate stress responses. These changes in the brain are hypothesized to underlie the potent effects of androgens in regulating behaviors related to stress and stress-induced activation of the HPA axis. Androgens can induce alterations in these functions through direct binding to the androgen receptor (AR) or following conversion to estrogens and subsequent binding to estrogen receptors including estrogen receptor alpha (ERα), beta (ERβ), and G protein-coupled estrogen receptor 1 (GPER1). In this review, we focus on the role of androgens in regulating behavioral and neuroendocrine stress responses at different stages of the lifespan and the sex hormone receptors involved in regulating these effects. We also review the specific brain regions and cell phenotypes upon which androgens are proposed to act to regulate stress responses with an emphasis on hypothalamic and extended amygdala subregions. This knowledge of androgen effects on these neural systems is critical for understanding how sex hormones regulate stress responses.

Hyperexcitation of ovBNST CRF neurons during stress contributes to female-biased expression of anxiety-like avoidance behaviors

Corticotropin releasing factor (CRF) network in the oval nucleus of bed nuclei of the stria terminalis (ovBNST) is generally indicated in stress, but its role in female-biased susceptibility to anxiety is unknown. Here, we established a female-biased stress paradigm. We found that the CRF release in ovBNST during stress showed female-biased pattern, and ovBNST CRF neurons were more prone to be hyperexcited in female mice during stress in both in vitro and in vivo studies. Moreover, optogenetic modulation to exchange the activation pattern of ovBNST CRF neurons during stress between female and male mice could reverse their susceptibility to anxiety. Last, CRF receptor type 1 (CRFR1) mediated the CRF-induced excitation of ovBNST CRF neurons and showed female-biased expression. Specific knockdown of the CRFR1 level in ovBNST CRF neurons in female or overexpression that in male could reverse their susceptibility to anxiety. Therefore, we identify that CRFR1-mediated hyperexcitation of ovBNST CRF neurons in female mice encode the female-biased susceptibility to anxiety.

Inhibiting CRF Projections from the Central Amygdala to Lateral Hypothalamus and Amygdala Deletion of CRF Alters Binge-Like Ethanol Drinking in a Sex-Dependent Manner

Background

Binge alcohol drinking is a dangerous pattern of consumption that can contribute to the development of more severe alcohol use disorders (AUDs). Importantly, the rate and severity of AUDs has historically differed between men and women, suggesting that there may be sex differences in the central mechanisms that modulate alcohol (ethanol) consumption. Corticotropin releasing factor (CRF) is a centrally expressed neuropeptide that has been implicated in the modulation of binge-like ethanol intake, and emerging data highlight sex differences in central CRF systems.

Methods

In the present report we characterized CRF+ neurocircuitry arising from the central nucleus of the amygdala (CeA) and innervating the lateral hypothalamus (LH) in the modulation of binge-like ethanol intake in male and female mice.

Results

Using chemogenetic tools we found that silencing the CRF+ CeA to LH circuit significantly blunted binge-like ethanol intake in male, but not female, mice. Consistently, genetic deletion of CRF from neurons of the CeA blunted ethanol intake exclusively in male mice. Furthermore, pharmacological blockade of the CRF type-1 receptor (CRF1R) in the LH significantly reduced binge-like ethanol intake in male mice only, while CRF2R activation in the LH failed to alter ethanol intake in either sex. Finally, a history of binge-like ethanol drinking blunted CRF mRNA in the CeA regardless of sex.

Conclusions

These observations provide novel evidence that CRF+ CeA to LH neurocircuitry modulates binge-like ethanol intake in male, but not female mice, which may provide insight into the mechanisms that guide known sex differences in binge-like ethanol intake.

Perinatal exposure to bisphenol A or S: Effects on anxiety-related behaviors and serotonergic system

Bisphenols, synthetic organic compounds used in the production of plastics, are an extremely abundant class of Endocrine Disrupting Chemicals, i.e., exogenous chemicals or mixtures of chemicals that can interfere with any aspect of hormone action. Exposure to BPs can lead to a wide range of effects, and it is especially dangerous if it occurs during specific critical periods of life. Focusing on the perinatal exposure to BPA or its largely used substitute BPS, we investigated the effects on anxiety-related behaviors and the serotonergic system, which is highly involved in controlling these behaviors, in adult mice. We treated C57BL/6J dams orally with a dose of 4 μg/kg body weight/day (i.e., EFSA TDI) of BPA or BPS dissolved in corn oil or with vehicle alone, at the onset of mating and continued treatment until the offspring were weaned. Adult offspring of both sexes performed the elevated plus maze and the open field tests. Then, we analyzed the serotonergic system in dorsal (DR) and median (MnR) raphe nuclei by immunohistochemical techniques. Behavioral tests highlighted alterations in BPA- and BPS-treated mice, suggesting different effects of the bisphenols exposure on anxiety-related behavior in males (anxiolytic) and females (anxiogenic). The analysis of the serotonergic system highlighted a sex dimorphism in the DR only, with control females showing higher values of serotonin immunoreactivity (5-HT-ir) than control males. BPA-treated males displayed a significant increase of 5-HT-ir in all analyzed nuclei, whereas BPS-treated males showed an increase in ventral DR only. In females, both bisphenols-treated groups showed a significant increase of 5-HT-ir in dorsal DR compared to the controls, and BPA-treated females also showed a significant increase in MnR.These results provide evidence that exposure during the early phases of life to BPA or BPS alters anxiety and the raphe serotonergic neurons in a sex-dependent manner.

Distinct populations of corticotropin-releasing factor (CRF) neurons mediate divergent yet complementary defensive behaviors in response to a threat

Defensive behaviors in response to a threat are shared across the animal kingdom. Active (fleeing, sheltering) or passive (freezing, avoiding) defensive responses are adaptive and facilitate survival. Selecting appropriate defensive strategy depends on intensity, proximity, temporal threat threshold, and past experiences. Hypothalamic corticotropin-releasing factor (CRF) is a major driver of an acute stress response, whereas extrahypothalamic CRF mediates stress-related affective behaviors. In this review, we shift the focus from a monolithic role of CRF as an anxiogenic peptide to comprehensively dissecting contributions of distinct populations of CRF neurons in mediating defensive behaviors. Direct interrogation of CRF neurons of the central amygdala (CeA) or the bed nucleus of the stria terminalis (BNST) show they drive unconditioned defensive responses, such as vigilance and avoidance of open spaces. Although both populations also contribute to learned fear responses in familiar, threatening contexts, CeA-CRF neurons are particularly attuned to the ever-changing environment. Depending on threat intensities, they facilitate discrimination of salient stimuli predicting manageable threats, and prevent their generalization. Finally, hypothalamic CRF neurons mediate initial threat assessment and active defense such as escape to shelter. Overall, these three major populations of CRF neurons demonstrate divergent, yet complementary contributions to the versatile defense system: heightened vigilance, discriminating salient threats, and active escape, representing three legs of the defense tripod. Despite the 'CRF exhaustion' in the field of affective neuroscience, understanding contributions of specific CRF neurons during adaptive defensive behaviors is needed in order to understand the implications of their dysregulation in fear- and anxiety-related psychiatric disorders. This article is part of the Special Issue on "Fear, Anxiety and PTSD".

The effects of prenatal bisphenol A exposure on brain volume of children and young mice

Bisphenol A (BPA) is a synthetic chemical used for the manufacturing of plastics, epoxy resin, and many personal care products. This ubiquitous endocrine disruptor is detectable in the urine of over 80% of North Americans. Although adverse neurodevelopmental outcomes have been observed in children with high gestational exposure to BPA, the effects of prenatal BPA on brain structure remain unclear. Here, using magnetic resonance imaging (MRI), we studied the associations of maternal BPA exposure with children's brain structure, as well as the impact of comparable BPA levels in a mouse model. Our human data showed that most maternal BPA exposure effects on brain volumes were small, with the largest effects observed in the opercular region of the inferior frontal gyrus (ρ = -0.2754), superior occipital gyrus (ρ = -0.2556), and postcentral gyrus (ρ = 0.2384). In mice, gestational exposure to an equivalent level of BPA (2.25 μg BPA/kg bw/day) induced structural alterations in brain regions including the superior olivary complex (SOC) and bed nucleus of stria terminalis (BNST) with larger effect sizes (1.07≤ Cohens d ≤ 1.53). Human (n = 87) and rodent (n = 8 each group) sample sizes, while small, are considered adequate to perform the primary endpoint analysis. Combined, these human and mouse data suggest that gestational exposure to low levels of BPA may have some impacts on the developing brain at the resolution of MRI.

Sex differences in pain-induced modulation of corticotropin-releasing hormone neurons in the dorsolateral part of the stria terminalis in mice

We earlier reported female-biased, sex-specific involvement of the dorsolateral bed nucleus of the stria terminalis (dl BST) in the formalin-induced pain response in rats. The present study investigated pain effects on mice behaviors. Because the dl BST is densely populated with corticotropin-releasing hormone (CRH) neurons, we examined sex differences in these parameters for the dl BST CRH neurons in male and female mice of a mouse line for which the CRH gene promoter (corticotropin-releasing factor [CRF]-Venus ΔNeo) controls the expression of the modified yellow fluorescent protein (Venus). Approximately 92% of Venus-positive cells in the dl BST were also CRH mRNA-positive, irrespective of sex. Therefore, the cells identified using Venus fluorescence were regarded as CRH neurons. A female-biased sex difference was observed in pain-induced behaviors during the interphase (5-15 min after formalin injection) but not during the later phase (phase 2, 15-60 min) in wild-type mice. In CRF-Venus ΔNeo mice, a female-biased difference was observed in either the earlier phase (phase 1, 0-5 min) or the interphase, but not in phase 2. Patch-clamp recordings taken using an acute BST slice obtained from a CRF-Venus ΔNeo mouse after formalin injection showed miniature excitatory postsynaptic currents (mEPSCs) and miniature inhibitory postsynaptic currents (mIPSCs). Remarkably, the mEPSCs frequency was higher in the Venus-expressing cells of formalin-injected female mice than in vehicle-treated female mice. Male mice showed no increase in mEPSC frequency by formalin injection. Formalin injection had no effect on mEPSC or mIPSC amplitudes in either sex. Pain-induced changes in mEPSC frequency in putative CRH neurons were phase-dependent. Results show that excitatory synaptic inputs to BST CRH neurons are temporally enhanced along with behavioral sex differences in pain response, suggesting that pain signals alter the BST CRH neurons excitability in a sex-dependent manner.

Corticotropin releasing factor and norepinephrine related circuitry changes in the bed nucleus of the stria terminalis in stress and alcohol and substance use disorders

Alcohol Use Disorder (AUD) affects around 14.5 million individuals in the United States, with Substance Use Disorder (SUD) affecting an additional 8.3 million individuals. Relapse is a major barrier to effective long-term treatment of this illness with stress often described as a key trigger for a person with AUD or SUD to relapse during a period of abstinence. Two signaling molecules, norepinephrine (NE) and corticotropin releasing factor (CRF), are released during the stress response, and also play important roles in reward behaviors and the addiction process. Within the addiction literature, one brain region in which there has been increasing research focus in recent years is the bed nucleus of the stria terminalis (BNST). The BNST is a limbic structure with numerous cytoarchitecturally and functionally different subregions that has been implicated in drug-seeking behaviors and stress responses. This review focuses on drug and stress-related neurocircuitry changes in the BNST, particularly within the CRF and NE systems, with an emphasis on differences and similarities between the major dorsal and ventral BNST subregions.

Sex differences in CRF1, CRF, and CRFBP expression in C57BL/6J mouse brain across the lifespan and in response to acute stress

Signaling pathways mediated by corticotropin-releasing factor and its receptor 1 (CRF1) play a central role in stress responses. Dysfunction of the CRF system has been associated with neuropsychiatric disorders. However, dynamic changes in the CRF system during brain development and aging are not well investigated. In this study, we characterized CRF1, CRF, and corticotropin-releasing factor binding protein (CRFBP) expression in different brain regions in both male and female C57BL/6J mice from 1 to 18 months of age under basal conditions as well as after an acute 2-hr-restraint stress. We found that CRF and CRF1 levels tended to increase in the hippocampus and hypothalamus, and to decrease in the prefrontal cortex with aging, especially at 18 months of age, whereas CRFBP expression followed an opposite direction in these brain areas. We also observed area-specific sex differences in the expression of these three proteins. For example, CRF expression was lower in females than in males in all the brain regions examined except the prefrontal cortex. After acute stress, CRF and CRF1 were up-regulated at 1, 6, and 12 months of age, and down-regulated at 18 months of age. Females showed more robust changes compared to males of the same age. CRFBP expression either decreased or remained unchanged in most of the brain areas following acute stress. Our findings suggest that brain CRF1, CRF, and CRFBP expression changes dynamically across the lifespan and under stress condition in a sex- and regional-specific manner. Sex differences in the CRF system in response to stress may contribute to the etiology of stress-related neuropsychiatric disorders.

Bisphenol F Exposure in Adolescent Heterogeneous Stock Rats Affects Growth and Adiposity

Bisphenol F (BPF) is increasingly substituting bisphenol A in manufacturing polycarbonates and consumer products. The cardiometabolic effects of BPF in either humans or model organisms are not clear, and no studies to date have investigated the role of genetic background on susceptibility to BPF-induced cardiometabolic traits. The primary goal of this project was to determine if BPF exposure influences growth and adiposity in male N:NIH heterogeneous stock (HS) rats, a genetically heterogeneous population. Littermate pairs of male HS rats were randomly exposed to either vehicle (0.1% ethanol) or 1.125 µg/ml BPF in 0.1% ethanol for 5 weeks in drinking water starting at 3 weeks-of-age. Water consumption and body weight was measured weekly, body composition was determined using nuclear magnetic resonance, urine and feces were collected in metabolic cages, and blood and tissues were collected at the end of the study. BPF-exposed rats showed significantly increased body growth and abdominal adiposity, risk factors for cardiometabolic disease. Urine output was increased in BPF-exposed rats, driving a trend in increased creatinine clearance. We also report the first relationship between a bisphenol metabolizing enzyme and a bisphenol-induced phenotype. Preliminary heritability estimates of significant phenotypes suggest that BPF exposure may alter trait variation. These findings support BPF exposure as a cardiometabolic disease risk factor and indicate that the HS rat will be a useful model for dissecting gene by BPF interactions on metabolic health.

Prenatal and postnatal bisphenol A exposure inhibits postnatal neurogenesis in the hippocampal dentate gyrus

Bisphenol A (BPA), an endocrine disruptor with estrogenic effects, is widely used as a raw material for manufacturing polycarbonate plastic and epoxy resins. Prenatal and postnatal exposure to BPA affects brain morphogenesis. However, the effects of prenatal and postnatal BPA exposure on postnatal neurogenesis in mice are poorly understood. In this study, we developed a mouse model of prenatal and postnatal BPA exposure and analyzed its effects on hippocampal neurogenesis. The hippocampal dentate gyrus is vulnerable to chemical exposure, as neurogenesis continues in this region even after birth. Our results showed that in mice, prenatal and postnatal BPA exposure decreased the number of type-1, 2a, 2b, and 3 neural progenitor cells, as well as in granule cells, in the hippocampal dentate gyrus on postnatal days 16 and 70. The effect of prenatal and postnatal BPA exposure on neural progenitors were affected at all differentiation stages. In addition, prenatal and postnatal BPA exposure affects the maintenance of long-term memory on postnatal day 70. Our results suggest that neurodevelopmental toxicity due to prenatal and postnatal BPA exposure might affect postnatal morphogenesis and functional development of the hippocampal dentate gyrus.

Roles for androgens in mediating the sex differences of neuroendocrine and behavioral stress responses

Estradiol and testosterone are powerful steroid hormones that impact brain function in numerous ways. During development, these hormones can act to program the adult brain in a male or female direction. During adulthood, gonadal steroid hormones can activate or inhibit brain regions to modulate adult functions. Sex differences in behavioral and neuroendocrine (i.e., hypothalamic pituitary adrenal (HPA) axis) responses to stress arise as a result of these organizational and activational actions. The sex differences that are present in the HPA and behavioral responses to stress are particularly important considering their role in maintaining homeostasis. Furthermore, dysregulation of these systems can underlie the sex biases in risk for complex, stress-related diseases that are found in humans. Although many studies have explored the role of estrogen and estrogen receptors in mediating sex differences in stress-related behaviors and HPA function, much less consideration has been given to the role of androgens. While circulating androgens can act by binding and activating androgen receptors, they can also act by metabolism to estrogenic molecules to impact estrogen signaling in the brain and periphery. This review focuses on androgens as an important hormone for modulating the HPA axis and behaviors throughout life and for setting up sex differences in key stress regulatory systems that could impact risk for disease in adulthood. In particular, impacts of androgens on neuropeptide systems known to play key roles in HPA and behavioral responses to stress (corticotropin-releasing factor, vasopressin, and oxytocin) are discussed. A greater knowledge of androgen action in the brain is key to understanding the neurobiology of stress in both sexes.

Perinatal exposure to bisphenol A at the intersection of stress, anxiety, and depression

Endocrine-disrupting compounds (EDCs) are common contaminants in our environment that interfere with typical endocrine function. EDCs can act on steroid and nuclear receptors or alter hormone production. One particular EDC of critical concern is bisphenol A (BPA) due to its potential harm during the perinatal period of development. Previous studies suggest that perinatal exposure to BPA alters several neurotransmitter systems and disrupts behaviors associated with depression and anxiety in the rodent offspring later in life. Thus, dysregulation in neurotransmission may translate to behavioral phenotypes observed in mood and arousal. Many of the systems disrupted by BPA also overlap with the stress system, although little evidence exists on the effects of perinatal BPA exposure in relation to stress and behavior. The purpose of this review is to explore studies involved in perinatal BPA exposure and the stress response at neurochemical and behavioral endpoints. Although more research is needed, we suggest that perinatal BPA exposure is likely inducing variations in behavioral phenotypes that modulate their action through dysregulation of neurotransmitter systems sensitive to stress and endocrine disruption.

Biological intersection of sex, age, and environment in the corticotropin releasing factor (CRF) system and alcohol

The neuropeptide corticotropin-releasing factor (CRF) is critical in neural circuit function and behavior, particularly in the context of stress, anxiety, and addiction. Despite a wealth of preclinical evidence for the efficacy of CRF receptor 1 antagonists in reducing behavioral pathology associated with alcohol exposure, several clinical trials have had disappointing outcomes, possibly due to an underappreciation of the role of biological variables. Although he National Institutes of Health (NIH) now mandate the inclusion of sex as a biological variable in all clinical and preclinical research, the current state of knowledge in this area is based almost entirely on evidence from male subjects. Additionally, the influence of biological variables other than sex has received even less attention in the context of neuropeptide signaling. Age (particularly adolescent development) and housing conditions have been shown to affect CRF signaling and voluntary alcohol intake, and the interaction between these biological variables is particularly relevant to the role of the CRF system in the vulnerability or resilience to the development of alcohol use disorder (AUD). Going forward, it will be important to include careful consideration of biological variables in experimental design, reporting, and interpretation. As new research uncovers conditions in which sex, age, and environment play major roles in physiological and/or pathological processes, our understanding of the complex interaction between relevant biological variables and critical signaling pathways like the CRF system in the cellular and behavioral consequences of alcohol exposure will continue to expand ultimately improving the ability of preclinical research to translate to the clinic. This article is part of the special issue on Neuropeptides.

Sex-dependent effects of chronic variable stress on discrete corticotropin-releasing factor receptor 1 cell populations

Anxiety and depression are strikingly more prevalent in women compared with men. Dysregulation of corticotropin-releasing factor (CRF) binding to its cognate receptor (CRFR1) is thought to play a critical role in the etiology of these disorders. In the present study, we investigated whether there were sex differences in the effects of chronic variable stress (CVS) on CRFR1 cells using CRFR1-GFP reporter mice experiencing a 9-day CVS paradigm. Brains were collected from CVS and stress naïve female and male mice following exposure to the open field test. This CVS paradigm effectively increased anxiety-like behavior in female and male mice. In addition, we assessed changes in activation of CRFR1 cells (co-localization with c-Fos and phosphorylated CREB (pCREB)) in stress associated brain structures, including two sexually dimorphic CRFR1 cell groups in the anteroventral periventricular nucleus (AVPV/PeN; F>M) and paraventricular hypothalamus (PVN; M>F). CVS increased CRFR1-GFP cell number as well as the number of CRFR1/pCREB co-expressing cells in the female but not male AVPV/PeN. In the PVN, the number of CRFR1/pCREB co-expressing cells was overall greater in males regardless of treatment and CVS resulted in a male-specific reduction of CRFR1/c-Fos cells. In addition, CVS induced a female-specific reduction in CRFR1/c-Fos cells within the anteroventral bed nucleus of the stria terminalis and both sexes exhibited a reduction in CRFR1/c-Fos co-expressing cells following CVS within the ventral basolateral amygdala. Overall, these sex-specific effects of CVS on CRFR1 populations may have implications for sex differences in stress-induction of mood disorders.

Regulation of CRF mRNA in the Rat Extended Amygdala Following Chronic Cocaine: Sex Differences and Effect of Delta Opioid Receptor Agonism

BACKGROUND

Cocaine withdrawal activates stress systems. Females are more vulnerable to relapse to cocaine use and more sensitive to withdrawal-induced negative affect. Delta opioid receptors modulate anxiety-like behavior during cocaine withdrawal in rats. This study measured the time course of gene regulation of one of the main stress peptides, corticotropin-releasing factor (CRF), and its type 1 receptor in male and female rats as well as the ability of the delta opioid receptor agonist SNC80 to normalize cocaine withdrawal-induced changes in CRF mRNA.

METHODS

Rats were injected with cocaine or saline 3 times daily for 14 days. Brains were collected 30 minutes, 24 hours, 48 hours, 7 days, and 14 days following the last injection. The paraventricular nucleus of the hypothalamus, central amygdala, and bed nucleus of the stria terminalis were processed for quantitative reverse transcriptase PCR measurement of CRF and CRFR1 mRNA. Additional rats received SNC80 during early cocaine withdrawal, and CRF mRNA was measured in the central amygdala.

RESULTS

CRF mRNA was elevated in the central amygdala at 24 hours and the paraventricular nucleus at 48 hours of cocaine withdrawal in males and females. Significant sex differences in cocaine-induced CRF upregulation were found in the bed nucleus of the stria terminalis at 30 minutes and 24 hours. SNC80 administration attenuated the increase in CRF mRNA in the central amygdala of female rats only.

CONCLUSIONS

CRF mRNA regulation during cocaine withdrawal is sex, time, and brain region dependent. Administration of a delta opioid receptor agonist during early withdrawal may ameliorate stress-related negative affect in females by abrogating the induction of CRF mRNA.

A sexually dimorphic distribution of corticotropin-releasing factor receptor 1 in the paraventricular hypothalamus

Sex differences in neural structures are generally believed to underlie sex differences reported in anxiety, depression, and the hypothalamic-pituitary-adrenal axis, although the specific circuitry involved is largely unclear. Using a corticotropin-releasing factor receptor 1 (CRFR1) reporter mouse line, we report a sexually dimorphic distribution of CRFR1 expressing cells within the paraventricular hypothalamus (PVN; males > females). Relative to adult levels, PVN CRFR1-expressing cells are sparse and not sexually dimorphic at postnatal days 0, 4, or 21. This suggests that PVN cells might recruit CRFR1 during puberty or early adulthood in a sex-specific manner. The adult sex difference in PVN CRFR1 persists in old mice (20-24 months). Adult gonadectomy (6 weeks) resulted in a significant decrease in CRFR1-immunoreactive cells in the male but not female PVN. CRFR1 cells show moderate co-expression with estrogen receptor alpha (ERα) and high co-expression with androgen receptor, indicating potential mechanisms through which circulating gonadal hormones might regulate CRFR1 expression and function. Finally, we demonstrate that a psychological stressor, restraint stress, induces a sexually dimorphic pattern of neural activation in PVN CRFR1 cells (males >females) as assessed by co-localization with the transcription/neural activation marker phosphorylated CREB. Given the known role of CRFR1 in regulating stress-associated behaviors and hormonal responses, this CRFR1 PVN sex difference might contribute to sex differences in these functions.

Female-biased sexual dimorphism of corticotropin-releasing factor neurons in the bed nucleus of the stria terminalis

Background

The bed nucleus of the stria terminalis (BNST) contains the highest density of corticotropin-releasing factor (CRF)-producing neurons in the brain. CRF-immunoreactive neurons show a female-biased sexual dimorphism in the dorsolateral BNST in the rat. Since CRF neurons cannot be immunostained clearly with available CRF antibodies in the mouse, we used a mouse line, in which modified yellow fluorescent protein (Venus) was inserted to the CRF gene, and the Neo cassette was removed, to examine the morphological characteristics of CRF neurons in the dorsolateral BNST. Developmental changes of CRF neurons were examined from postnatal stages to adulthood. Gonadectomy (GDX) was carried out in adult male and female mice to examine the effects of sex steroids on the number of CRF neurons in the dorsolateral BNST.

Methods

The number of Venus-expressing neurons, stained by immunofluorescence, was compared between male and female mice over the course of development. GDX was carried out in adult mice. Immunohistochemistry, in combination with Nissl staining, was carried out, and the effects of sex or gonadal steroids were examined by estimating the number of Venus-expressing neurons, as well as the total number of neurons or glial cells, in each BNST subnucleus, using a stereological method.

Results

Most Venus-expressing neurons co-expressed Crf mRNA in the dorsolateral BNST. They constitute a group of neurons without calbindin immunoreactivity, which makes a contrast to the principal nucleus of the BNST that is characterized by calbindin immunostaining. In the dorsolateral BNST, the number of Venus-expressing neurons increased across developmental stages until adulthood. Sexual difference in the number of Venus-expressing neurons was not evident by postnatal day 5. In adulthood, however, there was a significant female predominance in the number of Venus expressing neurons in two subnuclei of the dorsolateral BNST, i.e., the oval nucleus of the BNST (ovBNST) and the anterolateral BNST (alBNST). The number of Venus-expressing neurons was smaller significantly in ovariectomized females compared with proestrous females in either ovBNST or alBNST, and greater significantly in orchiectomized males compared with gonadally intact males in ovBNST. The total number of neurons was also greater significantly in females than in males in ovBNST and alBNST, but it was not affected by GDX.

Conclusion

Venus-expressing CRF neurons showed female-biased sexual dimorphism in ovBNST and alBNST of the mouse. Expression of Venus in these subnuclei was controlled by gonadal steroids.

Characterization and gonadal hormone regulation of a sexually dimorphic corticotropin-releasing factor receptor 1 cell group

Corticotropin-releasing factor binds with high affinity to CRF receptor 1 (CRFR1) and is implicated in stress-related mood disorders such as anxiety and depression. Using a validated CRFR1-green fluorescent protein (GFP) reporter mouse, our laboratory recently discovered a nucleus of CRFR1 expressing cells that is prominent in the female rostral anteroventral periventricular nucleus (AVPV/PeN), but largely absent in males. This sex difference is present in the early postnatal period and remains dimorphic into adulthood. The present investigation sought to characterize the chemical composition and gonadal hormone regulation of these sexually dimorphic CRFR1 cells using immunohistochemical procedures. We report that CRFR1-GFP-ir cells within the female AVPV/PeN are largely distinct from other dimorphic cell populations (kisspeptin, tyrosine hydroxylase). However, CRFR1-GFP-ir cells within the AVPV/PeN highly co-express estrogen receptor alpha as well as glucocorticoid receptor. A single injection of testosterone propionate or estradiol benzoate on the day of birth completely eliminates the AVPV/PeN sex difference, whereas adult gonadectomy has no effect on CRFR1-GFP cell number. These results indicate that the AVPV/PeN CRFR1 is regulated by perinatal but not adult gonadal hormones. Finally, female AVPV/PeN CRFR1-GFP-ir cells are activated following an acute 30-min restraint stress, as assessed by co-localization of CRFR1-GFP cells with phosphorylated (p) CREB. CRFR1-GFP/pCREB cells were largely absent in the male AVPV/PeN. Together, these data indicate a stress and gonadal hormone responsive nucleus that is unique to females and may contribute to sex-specific stress responses.

Sex and β-Endorphin Influence the Effects of Ethanol on Limbic Gabra2 Expression in a Mouse Binge Drinking Model

Binge drinking is a widespread problem linked to increased risk for alcohol-related complications, including development of alcohol use disorders. In the last decade, binge drinking has increased significantly, specifically in women. Clinically, sexually dimorphic effects of alcohol are well-characterized, however, the underlying mechanisms for these dimorphisms in the physiological and behavioral effects of alcohol are poorly understood. Among its many effects, alcohol consumption reduces anxiety via the inhibitory neurotransmitter GABA, most likely acting upon receptors containing the α-2 subunit (Gabra2). Previous research from our laboratory indicates that female mice lacking the endogenous opioid peptide β-endorphin (βE) have an overactive stress axis and enhanced anxiety-like phenotype, coupled with increased binge-like alcohol consumption. Because βE works via GABA signaling to reduce anxiety, we sought to determine whether sexually dimorphic binge drinking behavior in βE deficient mice is coupled with differences in CNS Gabra2 expression. To test this hypothesis, we used βE knock-out mice in a "drinking in the dark" model where adult male and female C57BL/6J controls (βE +/+) and βE deficient (βE -/-; B6.129S2-Pomc^tm1Low/J) mice were provided with one bottle of 20% ethanol (EtOH) and one of water (EtOH drinkers) or two bottles of water (water drinkers) 3 h into the dark cycle for four consecutive days. Following a binge test on day 4, limbic tissue was collected and frozen for subsequent qRT-PCR analysis of Gabra2 mRNA expression. Water-drinking βE +/+ females expressed more Gabra2 in central nucleus of the amygdala and the bed nucleus of the stria terminalis than males, but this sex difference was absent in the βE -/- mice. Genotype alone had no effect on alcohol consumption or drug-induced increase in Gabra2 expression. In contrast, βE expression had bi-directional effects in females: in wildtypes, Gabra2 mRNA was reduced by binge EtOH consumption, while EtOH increased expression in βE -/- females to levels commensurate with drug-naïve βE +/+ females. These results support the contention that βE plays a role in sexually dimorphic binge-like EtOH consumption, perhaps through differential expression of GABA_A α2 subunits in limbic structures known to play key roles in the regulation of stress and anxiety.

Impairment of learning and memory performances induced by BPA: Evidences from the literature of a MoA mediated through an ED

Many rodent studies and a few non-human primate data report impairments of spatial and non-spatial memory induced by exposure to bisphenol A (BPA), which are associated with neural modifications, particularly in processes involved in synaptic plasticity. BPA-induced alterations involve disruption of the estrogenic pathway as established by reversal of BPA-induced effects with estrogenic receptor antagonist or by interference of BPA with administered estradiol in ovariectomized animals. Sex differences in hormonal impregnation during critical periods of development and their influence on maturation of learning and memory processes may explain the sexual dimorphism observed in BPA-induced effects in some studies. Altogether, these data highly support the plausibility that alteration of learning and memory and synaptic plasticity by BPA is essentially mediated by disturbance of the estrogenic pathways. As memory function in humans involves similar signaling pathways, this mode of action of BPA has the potential to alter human cognitive abilities.

CRF modulation of central monoaminergic function: Implications for sex differences in alcohol drinking and anxiety

Decades of research have described the importance of corticotropin-releasing factor (CRF) signaling in alcohol addiction, as well as in commonly co-expressed neuropsychiatric diseases, including anxiety and mood disorders. However, CRF signaling can also acutely regulate binge alcohol consumption, anxiety, and affect in non-dependent animals, possibly via modulation of central monoaminergic signaling. We hypothesize that basal CRF tone is particularly high in animals and humans with an inherent propensity for high anxiety and alcohol consumption, and thus these individuals are at increased risk for the development of alcohol use disorder and comorbid neuropsychiatric diseases. The current review focuses on extrahypothalamic CRF circuits, particularly those stemming from the bed nucleus of the stria terminalis (BNST), found to play a role in basal phenotypes, and examines whether the intrinsic hyperactivity of these circuits is sufficient to escalate the expression of these behaviors and steepen the trajectory of development of disease states. We focus our efforts on describing CRF modulation of biogenic amine neuron populations that have widespread projections to the forebrain to modulate behaviors, including alcohol and drug intake, stress reactivity, and anxiety. Further, we review the known sex differences and estradiol modulation of these neuron populations and CRF signaling at their synapses to address the question of whether females are more susceptible to the development of comorbid addiction and stress-related neuropsychiatric diseases because of hyperactive extrahypothalamic CRF circuits compared to males.

Mate choice, sexual selection, and endocrine-disrupting chemicals

Humans have disproportionately affected the habitat and survival of species through environmental contamination. Important among these anthropogenic influences is the proliferation of organic chemicals, some of which perturb hormone systems, the latter referred to as endocrine-disrupting chemicals (EDCs). EDCs are widespread in the environment and affect all levels of reproduction, including development of reproductive organs, hormone release and regulation through the life cycle, the development of secondary sexual characteristics, and the maturation and maintenance of adult physiology and behavior. However, what is not well-known is how the confluence of EDC actions on the manifestation of morphological and behavioral sexual traits influences mate choice, a process that requires the reciprocal evaluation of and/or acceptance of a sexual partner. Moreover, the outcomes of EDC-induced perturbations are likely to influence sexual selection; yet this has rarely been directly tested. Here, we provide background on the development and manifestation of sexual traits, reproductive competence, and the neurobiology of sexual behavior, and evidence for their perturbation by EDCs. Selection acts on individuals, with the consequences manifest in populations, and we discuss the implications for EDC contamination of these processes, and the future of species.

Late pregnancy is vulnerable period for exposure to BPA

Bisphenol A (BPA) is among the better-known endocrine disruptors. BPA is used in various food-contacting materials and is easily eluted into food; as a result, we are exposed to BPA on a daily basis. In adults, BPA is metabolized and eliminated rapidly from the body. However, numerous reports suggest that fetuses and young children are susceptible to BPA. One of the concerning adverse effects of BPA is disruption of behavior, especially anxiety-like behavior. In order to study the mechanism of influences on offspring, it is important to clarify the most vulnerable gestation period. We hypothesized that offspring in late pregnancy would be more susceptible to BPA, because late pregnancy is a critical time for functional brain development. In this study, C57BL/6 mouse fetuses were exposed prenatally by oral dosing of pregnant dams, once daily from gestational day 5.5 to 12.5 (early pregnancy) or 11.5 to 18.5 (late pregnancy), with BPA (0 or 10 mg/kg body weight). Following birth and weaning, the resulting pups were tested using an elevated plus maze at postnatal week 10. The behavior of the offspring was altered by prenatal BPA exposure during late pregnancy but not during early pregnancy. These results indicated that offspring are more vulnerable to exposure to BPA in late pregnancy.

Distribution of corticotropin-releasing factor receptor 1 in the developing mouse forebrain: A novel sex difference revealed in the rostral periventricular hypothalamus

Corticotropin-releasing factor (CRF) signaling through CRF receptor 1 (CRFR1) regulates autonomic, endocrine and behavioral responses to stress and has been implicated in the pathophysiology of several disorders including anxiety, depression, and addiction. Using a validated CRFR1 reporter mouse line (bacterial artificial chromosome identified by green fluorescence protein (BAC GFP-CRFR1)), we investigated the distribution of CRFR1 in the developing mouse forebrain. Distribution of CRFR1 was investigated at postnatal days (P) 0, 4, and 21 in male and female mice. CRFR1 increased with age in several regions including the medial amygdala, arcuate nucleus, paraventricular hypothalamus, medial septum, CA1 hippocampal area, and the lateral habenula. Regions showing decreased CRFR1 expression with increased age include the intermediate portion of the periventricular hypothalamic nucleus, and CA3 hippocampal area. We report a sexually dimorphic expression of CRFR1 within the rostral portion of the anteroventral periventricular nucleus of the hypothalamus (AVPV/PeN), a region known to regulate ovulation, reproductive and maternal behaviors. Females had a greater number of CRFR1-GFP-ir cells at all time points in the AVPV/PeN and CRFR1-GFP-ir was nearly absent in males by P21. Overall, alterations in CRFR1-GFP-ir distribution based on age and sex may contribute to observed age- and sex-dependent differences in stress regulation.

Long-term effects of adolescent exposure to bisphenol A on neuron and glia number in the rat prefrontal cortex: Differences between the sexes and cell type

Bisphenol A (BPA), an endocrine disruptor used in a variety of consumer products, has been found to alter the number of neurons in multiple brain areas in rats following exposure in perinatal development. Both the number of neurons and glia also change in the medial prefrontal cortex (mPFC) during adolescence, and this process is known to be influenced by gonadal hormones which could be altered by BPA. In the current study, we examined Long-Evans male and female rats that were administered BPA (0, 4, 40, or 400μg/kg/day) during adolescent development (postnatal days 27-46). In adulthood (postnatal day 150), the number of neurons and glia in the mPFC were stereologically assessed in methylene blue/azure II stained sections. There were no changes in the number of neurons, but there was a significant dose by sex interaction in number of glia in the mPFC. Pairwise comparisons between controls and each dose showed a significant increase in the number of glia between 0 and 40μg/kg/day in females, and a significant decrease in the number of glia between 0 and 4μg/kg/day in males. In order to determine the type of glial cells that were changing in these groups in response to adolescent BPA administration, adjacent sections were labelled with S100β (astrocytes) and IBA-1 (microglia) in the mPFC of the groups that differed. The number of microglia was significantly higher in females exposed to 40μg/kg/day than controls and lower in males exposed to 4μg/kg/day than controls. There were no significant effects of adolescent exposure to BPA on the number of astrocytes in male or females. Thus, adolescent exposure to BPA produced long-term alterations in the number of microglia in the mPFC of rats, the functional implications of which need to be explored.

Exposure to Bisphenol a and Gender Differences: From Rodents to Humans Evidences and Hypothesis about the Health Effects

Bisphenol A (BPA) interacts with the endocrine system and seems to produce different effects in relation to gender. The objective of the study was to clarify the possible health effects of exposure to BPA in relation to gender. A literature search was performed using three different search engines: Medline, PubMed and Scopus. Data on both animals and humans showed that BPA acts as a xenoestrogen and interacts with the androgens' metabolism, producing different outcomes: uterotropic effects, decreasing sperm production, stimulation of prolactin release. Gender difference plays a key role in understanding the real toxic effects, the BPA serum concentrations were, all the time, higher in male subjects, possibly due to the difference in androgen-related enzyme activity levels, compared with the healthly female subjects, to equal levels of exposure; while higher BPA levels in women have been associated with a variety of conditions including obesity, endometrial hyperplasia, recurrent miscarriages, and polycystic ovarian syndrome. The data collected are sufficiently robust to raise concerns about the potentially deleterious impact of BPA on humans, even with some methodological limitations; the different impact of BPA in men and in women is documented and of a certain interest. In toxicology it is necessary to assess effects in relation to gender differences, in order to set up prevention plans in the work environment targeting the specific risk.

Exposure to a low dose of bisphenol A impairs pituitary-ovarian axis in prepubertal rats: effects on early folliculogenesis

The research work studies the effect of providing a low dose of bisphenol A (BPA), on the reproductive axis of prepubertal female rats. Wistar mated rats were treated with either 0.1% ethanol or BPA in their drinking water until their offspring were weaned on the 21 day of birth. The estimated average dose of exposure to dams was approximately 3μg/kg/day. The pups were sacrificed at the 30th day of life. Body weight at the moment of the sacrifice was significantly higher in the group exposed to BPA; ovarian weight and its relative weight were not modified. LH and estradiol levels increased significantly, meanwhile FSH ones showed no significant changes. The number of primary, secondary and atretic follicles increased and antral ones was decreased. Our results demonstrated that early exposure to a low dose of BPA disrupts the normal function of the reproductive axis in prepubertal female rats.

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.

Current concepts in neuroendocrine disruption

In the last few years, it has become clear that a wide variety of environmental contaminants have specific effects on neuroendocrine systems in fish, amphibians, birds and mammals. While it is beyond the scope of this review to provide a comprehensive examination of all of these neuroendocrine disruptors, we will focus on select representative examples. Organochlorine pesticides bioaccumulate in neuroendocrine areas of the brain that directly regulate GnRH neurons, thereby altering the expression of genes downstream of GnRH signaling. Organochlorine pesticides can also agonize or antagonize hormone receptors, adversely affecting crosstalk between neurotransmitter systems. The impacts of polychlorinated biphenyls are varied and in many cases subtle. This is particularly true for neuroedocrine and behavioral effects of exposure. These effects impact sexual differentiation of the hypothalamic-pituitary-gonadal axis, and other neuroendocrine systems regulating the thyroid, metabolic, and stress axes and their physiological responses. Weakly estrogenic and anti-androgenic pollutants such as bisphenol A, phthalates, phytochemicals, and the fungicide vinclozolin can lead to severe and widespread neuroendocrine disruptions in discrete brain regions, including the hippocampus, amygdala, and hypothalamus, resulting in behavioral changes in a wide range of species. Behavioral features that have been shown to be affected by one or more these chemicals include cognitive deficits, heightened anxiety or anxiety-like, sociosexual, locomotor, and appetitive behaviors. Neuroactive pharmaceuticals are now widely detected in aquatic environments and water supplies through the release of wastewater treatment plant effluents. The antidepressant fluoxetine is one such pharmaceutical neuroendocrine disruptor. Fluoxetine is a selective serotonin reuptake inhibitor that can affect multiple neuroendocrine pathways and behavioral circuits, including disruptive effects on reproduction and feeding in fish. There is growing evidence for the association between environmental contaminant exposures and diseases with strong neuroendocrine components, for example decreased fecundity, neurodegeneration, and cardiac disease. It is critical to consider the timing of exposures of neuroendocrine disruptors because embryonic stages of central nervous system development are exquisitely sensitive to adverse effects. There is also evidence for epigenetic and transgenerational neuroendocrine disrupting effects of some pollutants. We must now consider the impacts of neuroendocrine disruptors on reproduction, development, growth and behaviors, and the population consequences for evolutionary change in an increasingly contaminated world. This review examines the evidence to date that various so-called neuroendocrine disruptors can induce such effects often at environmentally-relevant concentrations.

Perinatal exposure to low-dose bisphenol A affects the neuroendocrine stress response in rats

Bisphenol A (BPA) is an estrogen-mimicking endocrine disruptor. Early-life exposures to low doses of BPA exert long-lasting effects on animals' reproductive and brain physiology. However, little is known about the effects of BPA on the stress-response system. Given the interaction of sex and stress hormones, we examined the effect of a low perinatal BPA exposure on the function of the hypothalamic-pituitary-adrenal (HPA) axis at rest and upon application of acute stress. Throughout pregnancy and lactation rats received daily 40 μg BPA/kg body weight orally via cornflakes. We studied the effect of this low but chronic exposure to BPA in the male and female offspring at puberty. BPA exposure led to abnormal adrenal histology including reduced zona reticularis especially in male offspring, hyperplasia of zona fasciculata in both sexes, and increased adrenal weight in female offspring. BPA-treated females had increased basal corticosterone and reduced hypothalamic glucocorticoid receptors (GR) levels. Stressed BPA-exposed females exhibited anxiety-like behavioral coping, a less rigorous corticosterone response, and did not downregulate GR in the hypothalamus, compared with control females. BPA-exposed males exhibited a heightened corticosterone stress response compared with females; they also displayed increased pro-opiomelanocortin mRNA levels and retained the prestress levels of pituitary corticotropin-releasing hormone-receptor 1, compared with control males. We found that perinatal chronic exposure to a low dose of BPA perturbs the basal and stress-induced activity of the HPA axis in a sexually dimorphic manner at adolescence. Exposure to BPA might contribute to increased susceptibility to stress-related disorders in later life.

Sex differences in activated corticotropin-releasing factor neurons within stress-related neurocircuitry and hypothalamic-pituitary-adrenocortical axis hormones following restraint in rats

Women may be more vulnerable to certain stress-related psychiatric illnesses than men due to differences in hypothalamic-pituitary-adrenocortical (HPA) axis function. To investigate potential sex differences in forebrain regions associated with HPA axis activation in rats, these experiments utilized acute exposure to a psychological stressor. Male and female rats in various stages of the estrous cycle were exposed to 30min of restraint, producing a robust HPA axis hormonal response in all animals, the magnitude of which was significantly higher in female rats. Although both male and female animals displayed equivalent c-fos expression in many brain regions known to be involved in the detection of threatening stimuli, three regions had significantly higher expression in females: the paraventricular nucleus of the hypothalamus (PVN), the anteroventral division of the bed nucleus of the stria terminalis (BSTav), and the medial preoptic area (MPOA). Dual fluorescence in situ hybridization analysis of neurons containing c-fos and corticotropin-releasing factor (CRF) mRNA in these regions revealed significantly more c-fos and CRF single-labeled neurons, as well as significantly more double-labeled neurons in females. Surprisingly, there was no effect of the estrous cycle on any measure analyzed, and an additional experiment revealed no demonstrable effect of estradiol replacement following ovariectomy on HPA axis hormone induction following stress. Taken together, these data suggest sex differences in HPA axis activation in response to perceived threat may be influenced by specific populations of CRF neurons in key stress-related brain regions, the BSTav, MPOA, and PVN, which may be independent of circulating sex steroids.

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.

Hormones and endocrine-disrupting chemicals: low-dose effects and nonmonotonic dose responses

For decades, studies of endocrine-disrupting chemicals (EDCs) have challenged traditional concepts in toxicology, in particular the dogma of "the dose makes the poison," because EDCs can have effects at low doses that are not predicted by effects at higher doses. Here, we review two major concepts in EDC studies: low dose and nonmonotonicity. Low-dose effects were defined by the National Toxicology Program as those that occur in the range of human exposures or effects observed at doses below those used for traditional toxicological studies. We review the mechanistic data for low-dose effects and use a weight-of-evidence approach to analyze five examples from the EDC literature. Additionally, we explore nonmonotonic dose-response curves, defined as a nonlinear relationship between dose and effect where the slope of the curve changes sign somewhere within the range of doses examined. We provide a detailed discussion of the mechanisms responsible for generating these phenomena, plus hundreds of examples from the cell culture, animal, and epidemiology literature. We illustrate that nonmonotonic responses and low-dose effects are remarkably common in studies of natural hormones and EDCs. Whether low doses of EDCs influence certain human disorders is no longer conjecture, because epidemiological studies show that environmental exposures to EDCs are associated with human diseases and disabilities. We conclude that when nonmonotonic dose-response curves occur, the effects of low doses cannot be predicted by the effects observed at high doses. Thus, fundamental changes in chemical testing and safety determination are needed to protect human health.

Epigenetic perspective on the developmental effects of bisphenol A

Bisphenol A (BPA) is an estrogenic environmental toxin widely used in the production of plastics and ubiquitous human exposure to this chemical has been proposed to be a potential risk to public health. Animal studies suggest that in utero and early postnatal exposure to this compound may produce a broad range of adverse effects, including impaired brain development, sexual differentiation, behavior, and immune function, which could extend to future generations. Molecular mechanisms that underlie the long-lasting effects of BPA continue to be elucidated, and likely involve disruption of epigenetic programming of gene expression during development. Several studies have provided evidence that maternal exposure to BPA results in postnatal changes in DNA methylation status and altered expression of specific genes in offspring. However, further studies are needed to extend these initial findings to other genes in different tissues, and to examine the correlations between BPA-induced epigenetic alterations, changes in gene expression, and various phenotypic outcomes. It will be also important to explore whether the epigenetic effects of BPA are related to its estrogenic activity, and to determine which downstream effector proteins could mediate changes in DNA methylation. In this review, we will highlight research indicating a consequence of prenatal BPA exposure for brain, behavior, and immune outcomes and discuss evidence for the role of epigenetic pathways in shaping these developmental effects. Based on this evidence, we will suggest future directions in the study of BPA-induced epigenetic effects and discuss the transgenerational implications of exposure to endocrine disrupting chemicals.

Increase of anteroventral periventricular kisspeptin neurons and generation of E2-induced LH-surge system in male rats exposed perinatally to environmental dose of bisphenol-A

Perinatal exposure to environmental levels of bisphenol-A (BPA) impairs sexually dimorphic behaviors in rodents. Kisspeptin neurons in anteroventral periventricular nucleus (AVPV), which plays an important role in the activation of GnRH neurons and the initiation of LH-surge, have been suggested to be sexual dimorphism in rats. This study focused on exploring the influence of a perinatal exposure to an environmental dose of BPA on the development and maturation of male AVPV kisspeptin neurons and hypothalamus-pituitary-gonadal axis. Female rats were injected sc with 2 μg BPA/kg·d from gestation d 10 through lactation d 7. Anatomical and functional changes in AVPV kisspeptin neurons and hypothalamus-pituitary-gonadal axis were examined in prepubertal, pubertal, and adult male rats exposed perinatally to BPA (BPA-rats). Here, we show that in postnatal d (PND)30/50/90 BPA-rats, the number of AVPV kisspeptin-immunoreactive cells was persistently increased in comparison with age-matched control male rats. The number of GnRH-immunoreactive cells in PND30 BPA-rats declined approximately 40% compared with control male rats, whereas that in PND50/90 BPA-rats was increased in a G protein-coupled receptor 54-dependent manner. Estradiol could induce a stable LH-surge in PND90 BPA-rats and control female rats, which was sensitive to the G protein-coupled receptor 54 inhibitor. In PND30/50 BPA-rats, plasma level of LH was higher, but the level of testosterone was lower than control male rats. These findings provide evidence that perinatal exposure to an environmental dose of BPA causes a sustained increase in AVPV kisspeptin neurons in male rats, leading to the generation of estradiol-induced LH-surge system.

Bisphenol A: developmental toxicity from early prenatal exposure

Bisphenol A (BPA) exposure has been documented in pregnant women, but consequences for development are not yet widely studied in human populations. This review presents research on the consequences for offspring of BPA exposure during pregnancy. Extensive work in laboratory rodents has evaluated survival and growth of the conceptus, interference with embryonic programs of development, morphological sex differentiation, sex differentiation of the brain and behavior, immune responsiveness, and mechanism of action. Sensitive measures include RAR, aryl hydrocarbon receptor, and Hox A10 gene expression, anogenital distance, sex differentiation of affective and exploratory behavior, and immune hyperresponsiveness. Many BPA effects are reported at low doses (10-50 µg/kg d range) by the oral route of administration. At high doses (>500,000 µg/kg d) fetal viability is compromised. Much of the work has centered around the implications of the estrogenic actions of this agent. Some work related to thyroid mechanism of action has also been explored. BPA research has actively integrated current knowledge of developmental biology, concepts of endocrine disruption, and toxicological research to provide a basis for human health risk assessment.

Gestational exposure to bisphenol A and cross-fostering affect behaviors in juvenile mice

Bisphenol-A (BPA) is a component of polycarbonate resins, and, lately, concern has been raised about its potential negative effects on human health. BPA is an estrogen analog and, in addition, it can act as a DNA hypomethylator. We examined the effects of gestational exposure to BPA on several behaviors in C57BL/6J mice. Because BPA affects maternal care, which, may have long-lasting effects on offspring behavior, we tested mice raised by either biological or fostered dams. Both diet and dam affected behavior in juvenile mice in a social novelty task and the elevated plus maze (EPM). In a social novelty task, the amount of time spent interacting with an adult male was affected by sex and gestational diet, but only in juveniles raised by a foster dam. Control females spent less time sniffing a novel adult than did control males or females exposed to BPA during gestation. In the EPM, juveniles reared by foster dams and exposed to BPA during gestation spent less time in the distal half of the open arm as compared with juveniles gestated on a control diet. Adult offspring raised by their biological dams showed the same response pattern; gestational BPA increased anxiety as compared with control diet. Our results show that prenatal BPA exposure affects social behavior and anxiety in the EPM. Moreover, some facet(s) of the infant-maternal interaction may modify these effects.

Alterations in male infant behaviors towards its mother by prenatal exposure to bisphenol A in cynomolgus monkeys (Macaca fascicularis) during early suckling period

Bisphenol A (BPA) is an environmental chemical with physiological potencies that cause adverse effects, even at environmentally relevant exposures, on the basis of a number of studies in experimental rodents. Thus, there is an increasing concern about environmental exposure of humans to BPA. In the present study, we used experimentally controlled cynomolgus monkeys (Macaca fascicularis) to assess the influence of prenatal exposure to BPA (10 microg/(kg day)) via subcutaneously implanted pumps and examined social behaviors between infants and their mothers during the suckling period. Mother-infant interactions in cynomolgus monkeys had behavioral sexual dimorphism associated with sex of infant from early suckling period. Prenatal exposure to BPA altered the behaviors of male infants significantly; BPA-exposed male infants behaved as female infants. And it also affected some of female infant behaviors. Consequently, gestational BPA exposure altered some behaviors of their mothers, mainly in male-nursing mothers. These results suggest that BPA exposure affects behavioral sexual differentiation in male monkeys, which promotes the understanding of risk of BPA exposure in human.

Low-dose effects of bisphenol A: a serious threat to human health?

The author tried to review and summarize low-dose effects of endocrine disrupting chemicals (EDCs) through an extensive literature survey of toxicological studies with bisphenol A (BPA), taking BPA as an example for which many studies were published. Data on low-dose effects with BPA, especially on neurobehavioral effects after fetal or early postnatal exposures, suggested that there would be new aspects to be considered. Specific mention for future tasks was made. Firstly, toxicity tests should be designed with more elaboration to ensure a sufficient number of animals with careful handling of litters to allow adequate statistical analysis and appropriate selection of dosages to obtain insight in dose-response relationship. Secondly, precise measurement of plasma levels in both humans and rodents and construction of relevant physiologically-based pharmacokinetic models would help obtain quantitative estimates of intake and target-organ exposure relationship. Thirdly, biological backgrounds, particularly differences and similarities in endocrinological, neurological and immunological aspects among species, should be revisited. Fourthly, mechanistic deliberations on the possibilities of epigenetic mechanism and examinations of putative neurobehavioral effects or a presumptive link of miscarriage with BPA exposures are requested. Finally, general public concerns must be addressed in a thoughtful way so that a simple precautionary approach is not pursued, but uncertainties of the new toxicological aspects should be carefully explained. Further researches and internationally concerted efforts on elucidating risk of low-dose effects by integrating knowledge will contribute to setting new directions in toxicology and improving chemical risk assessments.