Bisphenol A (BPA) is a major constituent of plastic products including epoxy resin containers, mobile phones, dental sealants, as well as medical and electronic equipment. The routes of entry of BPA to human body are through ingestion, inhalation and transdermal. BPA is one of endocrine disrupting chemicals and it can have an effect on brain and reproductive system. BPA has estrogenic and anti-androgenic effects on physiological and behavioral functions [1
]. BPA may exacerbate allergic diseases such as allergic dermatitis, rhinitis, and allergic asthma [3
]. Perinatal exposure to BPA not only enhanced lung inflammation in the presence of allergen [4
] but also induced aggressive and anxiety behaviors in adult rats [5
]; it also altered brain sexual differentiation of various brain regions in rodents [6
According to epidemiological studies, exposure to endocrine-disrupting chemicals can enhance underlying asthma or allergic diseases by modulating the immune responses [8
]. In a previous study, we demonstrated that exposure to toluene, a volatile organic compound, aggravated airway inflammatory responses in a mouse model of allergy by modulating the number of inflammatory cells and enhancing the plasma levels of nerve growth factor (NGF) [11
]. Furthermore, long-term exposure to toluene was shown to increase the mRNA expression levels of NGF and tropomyosin receptor kinase A (TrkA) in the lungs of ovalbumin (OVA)-immunized mice [12
]. Recent studies have indicated that early-life exposure to environmental chemicals may lead to lifestyle-related diseases such as obesity and diabetes mellitus and mental illnesses such as schizophrenia [13
] in later life. The increased risk of mental illnesses, such as autism, autism spectrum disorders, schizophrenia, Parkinson’s disease, and Alzheimer’s disease, observed in offspring exposed to chemicals during early life may be due to immune system disturbances or neuroinflammation in the brain [15
The prevalence of allergic diseases such as asthma, allergic rhinitis, and atopic dermatitis has increased sharply in many countries. Asthma is a chronic airway disease and is known to share common immune dysfunctions with mental disorders such as bipolar disease [17
]. A recent study indicated a key role of Th2-mediated inflammation in the association between asthma and bipolar disease in integrin β4-KO mice [19
]. Recently, we also showed that low-dose BPA exposure can aggravate allergic airway inflammation via induction of immune dysfunction and enhancement of Th2 responses [20
]. It was also shown in an adult female rat model that acute, low-dose BPA exposure can interfere with estradiol-dependent consolidation of memory and alter the dendritic spine density [21
Asthma not only affects the airways but also affects the brain. Anxiety and depression are more prevalent in asthmatic patients in comparison with healthy individuals [22
]. Patients with mild asthma also show mild cognitive deficit [25
], and brain MRI abnormalities have been observed in patients with mild to moderate asthma [26
]. A recent study demonstrated a marked reduction of the hippocampal volume in asthmatic subjects suggesting the existence of an association between asthma and brain regional structure that triggers cognitive deficit [27
]. Acute hypoxia, which can occur in asthma, as well as chronic obstructive pulmonary disease, sleep apnea, and heart failure can activate neuroimmune markers such as interleukin (IL)-1β in the brain and cause social behavioral impairment [28
]. In addition, an animal study reported increased plasma tumor necrosis factor (TNF)-α and IL-6 levels in mice exposed to 5% oxygen for one hour, and a human study also indicated increased plasma IL-6 levels at high altitudes [29
]. Moreover, the blood levels of inflammatory markers and oxidative stress markers are known to be increased in persons with sleep apnea [30
Exposure to endocrine-active metals, such as lead and mercury, during the brain developmental period can alter the hypothalamic-pituitary-adrenal (HPA) axis and neurobehaviors [31
]. In a previous study, we showed that high-dose phthalate di-(2-ethylhexyl) phthalate (DEHP) exposure during adolescence induced neuroinflammation via neuroimmune biomarkers in the hypothalami of allergic asthma mice [34
]. A recent study indicated that prenatal stress enhanced the neurotoxicity of lead and mercury by simultaneously affecting the HPA axis and central nervous system (CNS), including the hippocampus and the mesocorticolimbic system [35
Taken together, we hypothesize that asthma can cause brain hypoxia and elicit inflammatory responses via inducing inflammatory and oxidative stress markers; asthma also may induce cognitive impairment via modulation of the hippocampal memory function-related genes. However, the effects of BPA exposure on the cognitive function and neuroinflammatory responses in asthmatic subjects are largely unknown, as seen in Figure 1
. Therefore, we investigated the effects of BPA exposure on (1) the cognitive function using the novel object recognition ability test and the hippocampal NMDA receptor expressions, and (2) neuroinflammation using inflammatory biomarkers such as IL-1β, TNF-α, cyclooxygenase-2 (COX2), ionized calcium binding adapter molecule (Iba)1, and behavior-related genes, such as the estrogen receptor (ER)α and oxytocin receptor (oxtr), in the hypothalamus of a mouse model with allergic asthma (AA).
This study investigated the effects of exposure to BPA in susceptible groups, such as subjects with allergic airway inflammation, and examined the effects of co-exposure to BPA and an allergen (OVA) in adolescence. In regard to AA mouse models, we have already reported that airway inflammation and increased serum levels of OVA-specific antibodies were observed in the same OVA-immunized mouse models [20
]. The major findings of this study were the appearance of impaired novel object recognition ability in the male allergic asthmatic mice following high-dose BPA exposure, accompanied by downregulation of the NMDA receptor subunit NR2B in the hippocampus. The mRNA expressions of inflammatory markers, such as IL-1, TNF-α, COX2, and microglia marker Iba1, were not different in the hypothalami of the allergic asthmatic mice exposed to BPA. Moreover, the expression levels of ERα and oxtr were not different between the control and BPA-exposed groups, regardless of the status of OVA immunization. To the best of our knowledge, this is the first study to show that intratracheal instillation of high-dose BPA alone could not affect memory function in normal condition and could affect in individuals with underlying asthma or allergy.
In humans, a common pathway of exposure to BPA is via dietary ingestion. However, other routes of exposure, such as inhalation, cannot be excluded because BPA has been identified in common house dust and has been found in the atmosphere [20
]. Three doses of BPA such as BPA-L, BPA-M and BPA-H- were used in that study which were equivalent to 0.25, 5, and 100 times the estimated peak exposure to BPA from the atmosphere in Japan (0.0003 μg/kg/day) [36
]. We selected the intratracheal instillation route to detect the effects of specific airway exposure to BPA. Human studies have indicated the existence of an association between BPA exposure and behavioral problems such as anxiety and depression in a gender-dependent manner in children [44
]. In animal studies, BPA-induced working memory deficits in the adult male monkey have been reported [47
]. BPA can enter the human body via ingestion, inhalation, and transdermal pathways. We examined the effects of intratracheal instillation of BPA on the novel object recognition ability in male mice and found that male allergic asthmatic mice exposed to high-dose BPA showed appearance of impaired novel object recognition ability.
The nervous and immune systems communicate with each other, and neuroimmune interactions have a role in the pathophysiology of neurodegenerative diseases. Immune responses in the CNS involve not only the activation of the resident cells such as microglia and astrocytes but also infiltration of circulating immune cells, such as monocytes, neutrophils, and T cells. Both activated resident cells and infiltrating cells in the CNS act as regulators of immunity and modulators of the neuronal and glial functions [48
]. Interactions among neurons, immune cells, and neurotrophins are potentially responsible for the regulation and control of neuroimmune crosstalk. Little was known regarding the effects of BPA exposure on the neuroimmune biomarker expression levels in the hypothalamus of allergic asthmatic mice. Therefore, to evaluate the neuroimmune interactions in animals exposed to BPA, we examined potential neuroimmune biomarker levels, such as the expression levels of proinflammatory cytokines and an oxidative stress marker, as well as the levels of microglia markers in allergic asthmatic mice.
Juvenile age is the last developmental stage of the central nervous system [49
] and there are limited studies for this age group. In the present study, we developed an AA mouse model using OVA immunization and investigated the effects of exposure to BPA on the memory function and expression levels of neuroimmune biomarkers in the hypothalami of juvenile mice. The hippocampus plays a partial role in recognition memory [50
]. Our previous studies also showed the involvement of the hippocampal NMDA receptor in novel object recognition memory [38
]. Thus, the effects of BPA exposure on novel object recognition memory were examined in AA mouse models and it was found that the BPA-H-exposed mice showed poor discrimination between familiar and novel objects. Second, to confirm these effects were due to BPA or OVA or co-exposure, the object recognition test was performed in mice exposed to vehicle, BPA alone, OVA alone, and OVA + BPA-H. OVA alone and OVA + BPA-H-exposed mice showed poor discrimination between novel and familial objects. Our findings suggest that a high dose of BPA impaired memory function in allergic asthmatic individuals; however, it did not affect memory function in normal individuals.
In contrast, no difference in the effects on microglia activation was observed between the vehicle and test compound exposure groups. The hypothalamus is characterized as the principal brain region for neuroendocrine activity: it sends signals to and receives information from endocrine glands to maintain body homeostatic functions. Moreover, the hypothalamus also regulates body processes such as metabolism, immunity reactions, aggression, emotions, learning, and memory [51
]. Accordingly, we selected the hypothalamus to detect the roles of neuroimmune biomarkers in BPA-induced neurotoxicity in AA mouse models.
Cytokines are the principal biomarkers of neuroinflammation in neuropathology and neurodegenerative processes [53
]. Previously, we demonstrated that proinflammatory cytokine mRNA levels were upregulated in the brains of mice exposed to environmental pollutants, such as carbon black nanoparticles, nanoparticle-rich diesel exhaust, and volatile organic compound toluene [55
]. COX is required for the conversion of arachidonic acid to prostaglandins, which are involved in the inflammatory responses in the brain. Exposure to BPA and allergen from 5 to 11 weeks old may induce chronic stress. Chronic stressors disrupt cytokine homeostasis through the activation of the HPA axis, sympathetic adrenal medullary axis, and vagal fibers, promoting the secretions of glucocorticoids, catecholamines, and acetylcholine to inhibit proinflammatory cytokine secretion [60
]. However, no significant changes of mRNA expression levels of inflammatory markers in the hypothalamus were observed in this study.
Microglia are the major immune cells resident in the brain [61
] and the principal source of brain immune mediators. Activated microglia generally release several cytotoxic substances, including free radicals, excitatory amino acids, arachidonic acid derivatives, and cytokines. We detected major microglia activation using the microglia marker Iba1 but no significant difference in the Iba1 mRNA level in the hypothalamus between the BPA-exposed mice, regardless of the status of OVA immunization and the control mice.
Estrogen receptors show a sex-specific distribution in the rat hypothalamus [62
]. BPA exerts potential neurotoxic effects on the neuronal morphology and brain functions by binding ERs in a different manner. In the present study, we examined the effects of juvenile BPA exposure on the expression of ERα, which has been implicated in synaptic plasticity and in the pathophysiology of anxiety behaviors [63
]. We found no difference in the ERα expression level between the BPA- exposed mice that either received or did not receive OVA immunization. This indicates that juvenile BPA exposure may not affect the hypothalamic ER in the regulation of cognitive functions. Brain oxytocin plays a role in sociosexual behaviors [64
] and anxiolytic effects [66
]. Disrupted ontogeny of the oxytocin signaling pathways may underlie juvenile affective behavior. Prenatal BPA exposure resulted in significantly lower whole brain levels of oxytocin in mice just prior to birth when compared with controls [67
]. In this study, BPA-M-exposed mice showed significantly increased ortr mRNA expression level when compared with vehicle-exposed mice. We did not know exactly the cause of this effect, but it was suggested that the dose-specific effect of BPA alone exposure may affect the oxytocin receptor expression in the hypothalamus.
From our findings, co-exposure to allergen plus high-dose BPA from the juvenile period to adulthood may affect novel object recognition ability, accompanied by alterations in memory function-related gene expressions in the hippocampus. In the present study, significant responses were observed in the animals exposed to high-dose BPA. Previously, we have shown that intratracheal administration of high-dose DEHP during adolescence induced neuroinflammation by modulating the neuroimmune biomarker expression levels in the hypothalamus in AA mouse models [34
]. One possibility is that a very high level of BPA exposure is needed for the expression levels of the neuroimmune biomarkers to be affected. Another possibility is that the duration of exposure, route of administration, and exposure schedule also influence the dose–response effects. The limitations of our study were the small sample size of animals and lack of behavioral assessment in BPA-L, BPA-M-exposed groups. In a future study, we will increase the number of animals and perform behavioral tests for different doses of BPA-exposed mice.