Elsevier

Environmental Pollution

Volume 229, October 2017, Pages 40-48
Environmental Pollution

Neurotoxicity of low bisphenol A (BPA) exposure for young male mice: Implications for children exposed to environmental levels of BPA

https://doi.org/10.1016/j.envpol.2017.05.043Get rights and content

Highlights

  • Low dose BPA exposure could lead to DNA damage in brain cell.

  • Long-term BPA exposure could impair the learning and memory ability of male mice.

  • Non-monotonic dose dependant was observed in the CA3 region of the hippocampus.

Abstract

To investigate the neuron toxicities of low-dose exposure to bisphenol A (BPA) in children, mice were used as an animal model. We examined brain cell damage and the effects of learning and memory ability after BPA exposure in male mice (4 weeks of age) that were divided into four groups and chronically received different BPA treatments for 8 weeks. The comet assay and hippocampal neuron counting were used to detect the brain cell damage. The Y-maze test was applied to test alterations in learning and memory ability. Long term potentiation induction by BPA exposure was performed to study the potential mechanism of performance. The percentages of tail DNA, tail length and tail moment in brain cells increased with increasing BPA exposure concentrations. Significant differences in DNA damage were observed among the groups, including between the low-dose and control groups. In the Y-maze test, the other three groups qualified for the learned standard one day earlier than the high-exposed group. Furthermore, the ratio of qualified mice in the high-exposed group was always the lowest among the groups, indicating that high BPA treatment significantly altered the spatial memory performance of mice. Different BPA treatments exerted different effects on the neuron numbers of different regions in the hippocampus. In the CA1 region, the high-exposed group had a significant decrease in neuron numbers. A non-monotonic relationship was observed between the exposure concentrations and neuron quantity in the CA3 region. The hippocampal slices in the control and medium-exposed groups generated long-term potentiation after induction by theta burst stimulation, but the low-exposed group did not. A significant difference was observed between the control and low-exposed groups. In conclusion, chronic exposure to a low level of BPA had adverse effects on brain cells and altered the learning and memory ability of adolescent mice.

Introduction

Bisphenol A (BPA), a xenoestrogenic environmental endocrine disruptor, is an additive used to produce plastic products of polycabonate plastics and epoxy resin Additionally, it is also widely used in the inner coating of canned foods, thermal papers, dental sealants and many other domestic containers (Chouhan et al., 2014). The production of BPA was reported to be 15 billion pounds in 2013 (Grand View Research, 2014). Given its large volume, BPA is ubiquitous in the environment. In addition to the general detection of BPA in soil, water, dust, sediments and foods (Careghini et al., 2015), many biomonitoring studies showed that BPA could be detectable in more than 70% of urine samples from general populations in different countries and areas at a range of 0.1–2.4 μg/kg·bw (Chouhan et al., 2014, Battal et al., 2014). Increasing concerns have been raised about BPA exposure due to its negative effects on human health. Mounting epidemiologic studies have shown that BPA exposure is correlated with the risk of some diseases, including heart disease, obesity and diabetes (Melzer et al., 2010; Li et al., 2015, Lang et al., 2008).

Previous studies often focused on the reproductive toxicity of BPA exposure. However, recent animal studies have suggested that early exposure to BPA has a significant impact on brain cells because the embryonic and infant period is crucial for the development of the nervous system and brain morphology. For example, perinatal exposure to BPA was confirmed to affect sexual differentiation of brain development in early life (Mccaffrey et al., 2013, Rubin et al., 2006), alter adult sexual behavior (Boudalia et al., 2013, Decatanzaro et al., 2013), increase anxiety-like behavior, reduce exploratory behavior in a corticosterone-regulated way (Poimenova et al., 2010), and impair spatial and passive avoidance memory (Xu et al., 2010). The adverse effects of BPA exposure in the embryonic and infant periods on neural development are well acknowledged. However, BPA studies on the neurotoxic effects for adolescents have been limited.

High-dose effects of BPA were investigated in both in vitro and in vivo experiments and confirmed to exert potential effects on the development of brain and behavior differentiation in infants, children and adolescence (Xu et al., 2011a, Xu et al., 2011b, Iwakura et al., 2010). However, low-dose effects of BPA exposure on human health have often been underestimated. “Low dose” was defined as human exposure in daily lives (i.e., the dose is equal to or lower than the level of human environmental exposure) (Melnick et al., 2002). Although environmental exposure is lower than the reference dose (RfD, 50 μg/kg·bw/day) given by the Environmental Protection Agency (U. S. EPA (United States Environmental Protection Agency), 2012, Rubin, 2011), it is unclear whether a low dose of BPA exposure is safe for children. Several studies reported an inability to detect any effects of BPA within the “low dose” range (Myers et al., 2009, Richter et al., 2006, Vandenberg et al., 2012). However, other researchers reported the opposite views because human exposure to BPA is long-term and results in chronic accumulation; furthermore, their experiments confirmed the relationship between the low dose exposure to BPA and adverse human health effects (vom Saal and Hughes, 2005, vom Saal and Welshons, 2006, Ziv-Gal et al., 2015).

To confirm and investigate the effects of BPA exposure on brain cells and the learning and memory ability of children/adolescents, particularly the effects of the low dose, young male mice aged 4 weeks were used as the model animals. The mice were divided into four groups that were given different levels of oral BPA treatments over 8 weeks to simulate the daily different exposure levels in humans. The comet assay and hippocampal neuron counting were performed to detect brain cell damage. The Y-maze test was conducted to test alterations in the memory performance and ability induced by BPA exposure. Long term potentiation (LTP) induction in the Schaffer-CA1 (cornu ammonis 1) of the hippocampus was used as the potential mechanism for evaluating performance.

Section snippets

Chemicals and instruments

BPA was purchased from Sigma-Aldrich (HPLC grade, St. Louis, MO, USA). Na2EDTA, NaCl, NaOH, Na2HPO4, NaH2PO4, DMSO, ethidium bromide, paraformaldehyde and pentobarbital sodium were purchased from Guangzhou Chemical Reagent Factory (Guangzhou, China). Agarose was purchased from Shanghai Biological Technology Co., LTD (Shanghai, China). Tris-base was purchased from Shanghai Juyuan Biotechnology Co. LTD (Shanghai, China). Triton-X 100 was purchased from Qiangsheng Chemical Co. LTD (Jiangsu,

Results

After a few weeks of oral exposure to BPA, the exposed mice were inclined to be anxious, excited, hyperactive and aggressive, especially the high-exposed group. Some mice had inflammation from biting or fighting each other. After solitary rearing for a period of time, they quickly recovered. The others remained in healthy condition with tight fur and normal coloration. The body weights of the mice steadily increased and no significant differences were observed between the groups.

Through

Discussion

In addition to reports that BPA has reproductive and genetic toxicity (Melnick et al., 2002, Ulutaş et al., 2011, Iso et al., 2006), in vivo experiments indicated that a low dose of BPA exposure in utero reduces the spine densities in the hippocampal CA1 of 14-month-old mice (Kimura et al., 2016) and perinatal exposure to BPA impairs spatial memory through upregulating the expression of synaptic proteins, Nrxn1 and Nlgn3, and increasing the dendritic spine density in the cerebral cortex and

Conclusion

In conclusion, our results showed that chronic BPA exposure in adolescent male mice increased DNA damage in brain cells, impaired learning and memory ability, changed the cell density in the hippocampus and inhibited induction of hippocampal long-term potentiation in Schaffer-CA1. The comet assay indicated that even a low dose exposure to BPA could result in DNA damage in brain cells. The effects of a non-monotonic, dose-independent trend were observed in neuron quantity counting experiments.

Declaration of conflict of interest

The authors declare no conflict of interest.

Acknowledgments

This research was supported by grants from National Natural Science Foundation of China (No. 81671946 and No. 21477041) and Science and Technology Plan Projects in Guangzhou City (201510010107).

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