Effects of prenatal and postnatal exposure to a low dose of bisphenol A on behavior and memory in rats
Introduction
Exposure to gonadal steroid hormones during intrauterine and neonatal life plays a pivotal role in the development of brain circuits, and this differentiation persists, determining behavior patterns throughout life (Kubo et al., 2001, Patisaul and Polston, 2008). Estrogens cause proliferation and differentiation of nervous system cells and synaptic plasticity. Estrogens could act through genomic (nuclear-receptor) or non-genomic (membrane receptors) pathways. Estrogen receptors (ERs), ERα and ERβ, are estrogen-regulated, are expressed in various regions of the central nervous system (CNS) including the hippocampus, and have an important direct and indirect influence on higher cognitive functions (Prange-Kiel and Rune, 2006).
Endocrine disruptors (EDs) are synthetic or natural substances that are able to affect the action of endogenous hormones. EDs mimic or block endogenous hormones and may modify the synthesis, transport, metabolism, or elimination of natural hormones (Mlynarcıkova et al., 2005, Hanet et al., 2008). These chemicals influence the development and function of the CNS and the reproductive system (Kubo et al., 2001, Kubo et al., 2003, Fujimoto et al., 2007). Bisphenol A (BPA) is a well-known ED, and is recognized as having both estrogenic and anti-estrogenic effects, in addition to anti-androgenic and anti-thyroid effects (Zoeller et al., 2005, Fujimoto et al., 2007; Gonçalves et al., unpublished). About 3.0 million tons of BPA are produced annually, and it is used in the manufacture of polycarbonate, epoxy resins, polysulfone, polyester, and polyether ketones, and also as an antioxidant and flame retardant. BPA has been shown to leach from these materials, and in this way has become ubiquitous in the environment. Suspected of being hazardous to humans, BPA has been found in human urine (Calafat et al., 2005), milk, and plasma samples, and also in placental tissues (Schönfelder et al., 2002). Despite hepatic BPA metabolism (mainly glucurodation) be fast, BPA presence in human fluids indicates a constant exposure.
Low dose of BPA increases ERα mRNA and ERβ mRNA brain levels (see review in Vom Saal and Hughes, 2005). Therefore, BPA has been linked as a potential cause of alteration in sexual dimorphisms of the CNS and behavioral impairment in rats (Kubo et al., 2001).
For BPA, the lowest-observed-adverse-effect-level (LOAEL) was 50 mg/kg/day, and was used to calculate a reference dose of 50 μg/kg/day based on experiments conducted in the 1980s (IRIS, 1988). Exposure to a low dose (low compared to the reference dose) of BPA during the fetal and suckling periods in mice affected their exploratory emotional behavior, decreasing the normal sexual differences in behavior (Kubo et al., 2003). Rats exposed to a low dose of BPA showed behavioral defeminization in females and demasculinization in males (Kubo et al., 2001, Gioiosa et al., 2007).
Taking into consideration the widespread presence of BPA in the environment, and considering its weak estrogen activity and experimental limitations, some authors have affirmed (Vom Saal and Hughes, 2005) and others have doubted (Dekant and Völkel, 2008) the real risk of human exposure to BPA.
Concerning experimental condition, via of administration deserves some consideration: while the oral route is the main pathway in BPA exposure, parenteral administration avoids BPA first-pass in the liver (glucuronide conjugation). On the other hand, the toxicological core “the dose makes the poison” is confronted with hormesis mechanisms (i.e., low dose effect) or non-monotonic dose response curves (Vandenberg et al., 2009). Current human BPA exposure occurs in low dosage. Thus, low dose BPA effect in a single dose is interesting to observe, under adequate experimental conditions.
Independently of dosage, exposure period is crucial. Organisms exposed to EDs such as BPA during embryonic, fetal (free BPA increase for deconjugation β-glucuronidase-mediated in placental and fetal tissue) or neonatal (hepatic immaturity in glucuronidation capacity) life would present predisposition to dysfunctions observed in puberty or adulthood. Therefore, an important experimental approach is to study the effect of different BPA exposure times, at the same dosage, on memory and behavioral tasks.
This study evaluated the effects of a low dose of BPA (40 μg/kg/day, administered orally) on litters of female and male Wistar rats during the pregnancy and/or lactation periods, with respect to conditioning and spatial memory, locomotion, exploratory activity, and emotional state.
Section snippets
Materials and methods
All the procedures involving animal subjects were reviewed and approved by the Institutional Research Ethics Committee of the Universidade Federal do Rio Grande, Rio Grande, Rio Grande do Sul (CEPAS-Approval: 15/2008).
Female and male Wistar rats (Rattus norvegicus) at 16 weeks of age obtained from the Universidade Federal do Rio Grande breeding colony were housed in standard cages. After confirmed copulation, females were exposed to one of the following treatments: corn oil-treated, with BPA,
Statistical analysis
Data were expressed as mean ± SEM. When the assumptions of homogeneity and normality of variance were verified, one- or two-way ANOVA was carried out to determine the main effect of sex and treatment and possible sex × treatment interactions. One-way ANOVA was performed first between litters for each treatment and sex, to exclude litter effects. Once litter effect was excluded, statistical tests were performed. The ANOVA was followed by a Student–Newman–Keuls post hoc test. If the assumption of
Step-down inhibitory avoidance task (IAT)
The training session step-down latency among groups did not differ significantly in a Kruskal–Wallis analysis of variance (p > 0.05).
Latency time in the test session showed no significant differences between sexes, in both STM (p = 0.239) and LTM (p = 0.625). The test results for STM (Fig. 1A) and LTM (Fig. 1B), expressed as mean and interquartile range (25/75), are shown for each sex and treatment. PRE-LAC-treated females and males showed impairment in conditioning memory as measured in the
Discussion
Memory formation is a process that involves many biochemical and molecular events in specific brain regions (Izquierdo and Medina, 1997). Soon after learning, memory is very susceptible to external and internal interferences (Gioiosa et al., 2007). We used the IAT test to evaluate the possible interference of BPA exposure during pregnancy and/or lactation with STM and LTM, in a paradigm of aversive learning. The aversive learning model is normally adaptive, and it contributes to remembering
Conflicts of interest
All authors declare they have no potential competing interest.
Acknowledgements
This study was supported by a grant, Proc. No. 0613848-FAPERGS (Fundação de Amparo à Pesquisa do Estado do Rio Grande do Sul), to the Physiological Sciences Post-graduate Program of the Universidade Federal do Rio Grande. D.M. Barros is a research fellow of the Brazilian agency CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico). C.R. Gonçalves received a graduate fellowship from the Brazilian agency CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior).
References (36)
- et al.
Estradiol valerate and tibolone: effects on memory
Pharmacol. Biochem. Behav.
(2006) - et al.
Molecular signaling pathways in the cerebral cortex are required for retrieval of one-trial inhibitory avoidance learning in rats
Behav. Brain Res.
(2000) - et al.
Simultaneous modulation of retrieval by dopaminergic D1, β noradrenergic, serotonergic-1A and cholinergic muscarinic receptors in cortical structures of the rat
Behav. Brain Res.
(2001) - et al.
Sex differences in learning processes of classical and operant conditioning
Physiol. Behav.
(2009) - et al.
Human exposure to bisphenol A by biomonitoring: methods, results and assessment of environmental exposures
Toxicol. Appl. Pharmacol.
(2008) - et al.
Applications of the Morris water maze in the study of learning and memory
Brain Res. Rev.
(2001) - et al.
Prenatal exposure to bisphenol A impairs sexual differentiation of exploratory behavior and increases depression-like behavior in rats
Brain Res.
(2006) - et al.
Environmental impacts on brain functions: low dose effects of bisphenol A during perinatal critical period
Int. Congr. Ser.
(2007) - et al.
Developmental exposure to low-dose estrogenic endocrine disruptors alters sex differences in exploration and emotional responses in mice
Horm. Behav.
(2007) - et al.
Effects of endocrine disruptors on genes associated with17β-estradiol metabolism and excretion
Steroids
(2008)
Memory formation: the sequence of biochemical events in the hippocampus and its connection to activity in other brain structures
Neurobiol. Learn. Mem.
Short- and long-term memory are differentially regulated by monoaminergic systems in the rat brain
Neurobiol. Learn. Mem.
Environment and endocrinology: the case of thyroidology
Ann. Endocrinol.
Exposure to bisphenol A during the fetal and suckling periods disrupts sexual differentiation of the locus coeruleus and of behavior in the rat
Neurosci. Lett.
Low dose effects of bisphenol A on sexual differentiation of the brain and behavior in rats
Neurosci. Res.
Effects of age and anxiety on learning and memory
Behav. Brain Res.
Pregnant rats show enhanced spatial memory, decreased anxiety, and altered levels of monoaminergic neurotransmitters
Brain Res.
Retrograde messengers, long-term potentiation and memory
Brain Res. Rev.
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2021, Neuroscience and Biobehavioral ReviewsCitation Excerpt :More specifically, following perinatal exposure to BPA, deficits in spatial learning and memory have been reported in male deer mice (Jašarević et al., 2011; 2013), male rats (Carr et al., 2003; Kuwahara et al., 2013; Liu et al., 2016; Xu et al., 2007; 2010a) and male mice (Kundakovic et al., 2015; Tian et al., 2010; Xu et al., 2010b). In contrast, females exposed to those same BPA doses exhibited either no change or a masculinized response (i.e., improved performance) in the learning task (Jašarević et al., 2011; 2013; Kundakovic et al., 2015) or were similarly affected by BPA exposure, suggesting a sex-independent effect of BPA on spatial learning (Gonçalves et al., 2010; Liu et al., 2016; Poimenova et al., 2010). Some studies report BPA effects to be more pronounced on females’ than on males’ learning and memory responses (in rats: Kubo et al., 2001; Hass et al., 2016; in mice: Matsuda et al., 2013; Jang et al., 2012).
Involvement of NMDAR/PSD-95/nNOS–NO–cGMP pathway in embryonic exposure to BPA induced learning and memory dysfunction of rats
2020, Environmental PollutionCitation Excerpt :The deficits in spatial memory ability were less sensitive to BPA, which a higher dose of embryonic BPA (5 mg/kg/day) could shorten the time staying of rats in the target area. Several investigations have shown that perinatal exposure to BPA (from gestation to lactation) can impair the learning and memory ability of rodents using MWM test (Chang et al., 2016; Gonçalves et al., 2010; Kumar and Thakur, 2014; Xu et al., 2010). Previous study conducted by Xu et al. (2014) have indicated that postnatal exposure to BPA can yield long-lasting behavioral changes in adulthood rats.