Low dose effects of bisphenol A on sexual differentiation of the brain and behavior in rats

Abstract

There is an endocrinological concern that environmental endocrine disrupters (EEDs) may influence sexual differentiation. Bisphenol A (BPA), one of EEDs, is released from polycarbonate plastics, and has been detected in the human umbilical cord. In this study, we examined the effect of BPA on the sexual differentiation of open-field behavior and the sexually dimorphic nuclei in the brain in the offspring of rats exposed to BPA during the fetal and suckling periods at a dosage below the human tolerable daily intake (TDI) level. In the control group, females were more active in the open field and had a larger locus coeruleus (LC) volume than males. BPA abolished and inverted the sex differences of the open-field behavior and the LC volume, respectively, without affecting the reproductive system. We also compared the effects of estrogenic compounds, diethylstilbestrol (DES) and resveratrol (RVT), to that of BPA because of their structural similarities. DES affected the open-field behavior, LC volume and reproductive system, while RVT affected the LC volume and the reproductive system. These results suggest that the brain is highly sensitive to BPA at a dosage below TDI and that the disrupting effects of BPA on sexual differentiation may vary from those of RVT and DES.

Introduction

A great endocrinological concern has recently arisen that certain industrial chemical substances, which are released into our surrounding environment, may influence the central nervous system (CNS) as well as the reproductive system in many animals (Colborn et al., 1993). These chemicals are called ‘environmental endocrine disrupters (EEDs)’. Bisphenol A (BPA) is one of the most famous EEDs and it is released from polycarbonate plastics, epoxy resins of food cans etc. (Krishnan et al., 1993, Society of the Plastics Industry, 1996). Human fetuses have also been reported to be contaminated with BPA (Sakurai and Mori, 2000, Brock et al., 2001). BPA binds to both the estrogen receptor α (ERα) and β (ERβ), and BPA induces a proliferation of estrogen receptor expressing MCF-7 cells in an estrogen-dependent manner (Krishnan et al., 1993, Sohoni and Sumpter, 1998, Hiroi et al., 1999). In addition, BPA has been found to increase both the weight of the uterus and the release of prolactin into the blood in female rodents, similar to estrogen (Steinmetz et al., 1997, Ashby and Tinwell, 1998, Steinmetz et al., 1998). These results indicate that BPA has a common property with estrogenic substances.

The gonadal steroids differentiate neurons and then create the sexual dimorphisms of the brain structure and behavior (MacLusky and Naftolin, 1981, Kelly et al., 1999). Up to now, there is little information about the effect of BPA on the CNS, especially regarding sexual differentiation of the brain and behavior. In a previous study, we found that the BPA exposure during the fetal and suckling periods at a dosage (1.5 mg/kg per day) below the no-observed-adverse-effect level (NOAEL) disrupted the sex difference in open-field behavior and moreover the gender difference of the size of the locus coeruleus (LC), but BPA did not affect the reproductive function or the size of the sexually dimorphic nucleus of the preoptic area (SDN-POA; Kubo et al., 2001). The NOAEL of BPA is determined as 50 mg/kg per day by various in vivo toxicity studies including a reproductive toxicity test. On the basis of the NOAEL, the resolution limitation is calculated as 2.5 mg/l (ppm), and the tolerable daily intake (TDI), which is the maximum acceptable dose in humans, is also calculated as 50 μg/kg per day (from the web site: www: http://www.bisphenol-a.org/health/exposure/consumer/research.html).

For the risk assessment of BPA, it is important to confirm that BPA at or below the TDI does not cause any adverse effects. The main purpose of the present study is to elucidate whether the exposure of very low dose of BPA during the fetal and suckling periods also affects non-reproductive sexual differentiation of both the open-field behavior and the LC structure, and the reproductive function including sexual behavior. Two concentrations of BPA were set at lower levels than that used in our previous study, and the daily intake of BPA was under the TDI in the lower BPA group. An additional purpose of the study is to compare these effects of BPA to those of other estrogenic compounds. In this study, we used two estrogenic chemicals showing structural similarity to BPA. One is trans-resveratrol (RVT) which is a phytoestrogen found in grapes, red wine, peanuts, and other fruits. RVT has an estrogenic potential (Gehm et al., 1997), however, there is little information about the effect of RVT on sexual differentiation of the CNS. The other is diethylstilbestrol (DES) which is a synthetic estrogen, and the estrogenic potential of DES is similar to that of estradiol. DES is often used as an estrogenic control to study the effect of EEDs, because the effects of DES have already been reported regarding sexual differentiation of both the CNS and the reproductive system (MacLusky and Naftolin, 1981, Döhler et al., 1984, Csaba et al., 1986, Tarttelin and Gorski, 1988, Sharpe et al., 1995, Ashby et al., 1997, Vancutsem and Roessler, 1997, Kwon et al., 2000, Atanassova et al., 2000).