美國杜克醫學中心科學家最新研究發現，人體暴露於雙酚A（BPA，一種出現在塑膠及樹脂的化學物質），其掌管神經細胞功能與中樞神經系統發展的基因，可能會受到抑制。
這份研究刊登在美國《國家科學研究院學報》（Proceedings of the National Academy of Sciences）上，研究主要是調查BPA對老鼠及人類的皮質神經元所產生的影響。
皮質神經元是腦中最大區域─大腦皮層中的細胞，大部分腦部複雜的活動，例如思考、感知、和其他的自主運動，都是透過這些神經元運作才能做到。
過去經由動物實驗發現，暴露在BPA環境下可能會導致行為問題、內分泌和生殖系統紊亂、肥胖、癌症、免疫系統疾病等健康問題。但這是第一次有研究指出，暴露在BPA中會抑制大腦和神經系統發展。
BPA是一種會模仿雌性激素的分子，它會影響體內的內分泌系統。在許多不同的人造產品中都有這種分子的存在，包括CD、紙鈔、牙齒填充物、阻燃劑、感熱影印紙、部分塑膠水瓶、和做為食物容器的金屬罐之襯裡等。
BPA可能會因容器受到高熱而釋放到其容器所盛裝的食品中，如奶瓶中的嬰兒牛奶。
一些研究指出，嬰兒和兒童最容易受到BPA影響，這樣的顧慮讓美國食品與藥物管理局於2012年7月下令禁止在奶瓶及兒童用杯子中使用BPA。
雖然研究顯示BPA會影響神經系統的發展，我們卻仍不清楚這是如何發生的。研究團隊在囓齒動物和人類的神經細胞上做實驗，以了解BPA所產生的影響和其如何干擾基因的運行。
在神經元發展初期，細胞中會有高濃度的氯化物，當神經元成熟時氯化物的濃度也就跟著下降，一種名為KCC2的運輸體蛋白會將氯離子趕出細胞外。
若細胞中氯化物的濃度持續升高，它會損害神經迴路、產生發展中的神經細胞功能，並取代它在腦中的位置。
若神經元暴露在微量BPA中，BPA會利用某種方法阻斷製造KCC2蛋白的基因，使細胞中的氯化物排出速度減緩。
另一種腦部正常運作所需的重要蛋白MECP2，也可能是和這種變化有關的關鍵因素。
研究人員發現，當暴露在BPA環境中，MECP2會變得更多，與KCC2基因的結合率也會變高，也可能因此阻斷KCC2基因的運行。這種反應會導致氯化物的排除速度變慢，因此這可能就是導致發展中的腦部發生病變的原因。
這項研究發現點出了一個疑問──BPA是否能有助於治療神經發育疾病，例如「雷特氏症」，一種只在女孩身上發生的嚴重自閉症。這種疾病是由製造MECP2的基因突變所導致的。
Liedtke表示，「這份研究讓我們了解了暴露於BPA會影響KCC2基因的運行，不過，我們也期待在未來將研究重點放在BPA對KCC2以外物質可能造成的影響。這個研究是『現在進行式』。」
Environmental exposure to bisphenol A, a chemical found in plastics and resins, may suppress a gene vital to nerve cell function and to the development of the central nervous system, finds new research led by scientists at Duke Medicine.
Published today in the journal "Proceedings of the National Academy of Sciences," the study investigated the effects of BPA on the cortical neurons of mice, rats and humans.
Cortical neurons are the cells of the brain's largest region, the cerebral cortex. Most of the complex activity of the brain enabling thought, perception, and voluntary movement is made possible by the activity of these neurons.
Research in animals has raised concerns that exposure to BPA may cause health problems such as behavioral issues, endocrine and reproductive disorders, obesity, cancer and immune system disorders. But this is one of the first studies indicating that brain and central nervous system development could be impaired by BPA exposure.
BPA is a molecule that mimics the hormone estrogen and interferes with the body's endocrine system. It is found in a wide variety of manufactured products, including CDs, paper money, dental fillings and flame retardants, thermal printer paper, some plastic water bottles and the lining of metal cans used to contain food.
BPA can be released from containers into the food they carry, such as infant formula in the case of baby bottles, if the containers are heated at high temperatures.
Some studies suggest that infants and young children may be the most vulnerable to the effects of BPA. This concern led the U.S. Food and Drug Administration to ban the use of the chemical in baby bottles and cups in July 2012.
While BPA has been shown to affect the developing nervous system, little is understood as to how this occurs. The research team developed a series of experiments in rodent and human nerve cells to learn how BPA induces changes that disrupt gene regulation.
During early development of neurons, high levels of chloride are present in the cells. These levels drop as neurons mature, due to a chloride transporter protein called KCC2, which churns chloride ions out of the cells.
If the level of chloride within neurons remains elevated, it can damage neural circuits and compromise a developing nerve cell's ability to migrate to its proper position in the brain.
Exposing neurons to minute amounts of BPA alters the chloride levels inside the cells by somehow shutting down the Kcc2 gene, which makes the KCC2 protein, thereby delaying the removal of chloride from neurons.
MECP2, another protein important for normal brain function, was found to be a possible factor behind this change.
When exposed to bisphenol A, MECP2 is more abundant and binds to the Kcc2 gene at a higher rate, which might help to shut it down, the researchers found. This could contribute to problems in the developing brain due to a delay in chloride being removed.
These findings raise the question of whether BPA could contribute to neurodevelopmental disorders such as Rett syndrome, a severe autism spectrum disorder found only in girls. This disorder is characterized by mutations in the gene that produces MECP2.
"Our findings improve our understanding of how environmental exposure to BPA can affect the regulation of the Kcc2 gene. However, we expect future studies to focus on what targets aside from Kcc2 are affected by BPA," Liedtke said. "This is a chapter in an ongoing story."
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