The impact of neonatal bisphenol-A exposure on sexually dimorphic hypothalamic nuclei in the female rat

Abstract

Now under intense scrutiny, due to its endocrine disrupting properties, the potential threat the plastics component bisphenol-A (BPA) poses to human health remains unclear. Found in a multitude of polycarbonate plastics, food and beverage containers, and medical equipment, BPA is thought to bind to estrogen receptors (ERs), thereby interfering with estrogen-dependent processes. Our lab has previously shown that exposure to BPA (50mg/kg bw or 50μg/kg bw) during the neonatal critical period is associated with advancement of puberty, early reproductive senescence and ovarian malformations in female Long Evans rats. Here, using neural tissue obtained from the same animals, we explored the impact of neonatal BPA exposure on the development of sexually dimorphic hypothalamic regions critical for female reproductive physiology and behavior. Endpoints included quantification of oxytocin-immunoreactive neurons (OT-ir) in the paraventricular nucleus (PVN), serotonin (5-HT-ir) fiber density in the ventrolateral subdivision of the ventromedial nucleus (VMNvl) as well as ERα-ir neuron number in the medial preoptic area (MPOA), the VMNvl, and the arcuate nucleus (ARC). Both doses of BPA increased the number of OT-ir neurons within the PVN, but no significant effects were seen on 5-HT-ir fiber density or ERα-ir neuron number in any of the areas analyzed. In addition to hypothalamic development, we also assessed female sex behavior and body weight. No effect of BPA on sexual receptivity or proceptive behavior in females was observed. Females treated with BPA, however, weighed significantly more than control females by postnatal day 99. This effect of BPA on weight is critical because alterations in metabolism, are frequently associated with reproductive dysfunction. Collectively, the results of this and our prior study indicate that the impact of neonatal BPA exposure within the female rat hypothalamus is region specific and support the hypothesis that developmental BPA exposure may adversely affect reproductive development in females.

Figure 1

(A) Body weight on PND99 was significantly impacted by neonatal exposure with females exposed to EB or the higher dose of BPA weighing sitnificantly more than the vehicle (OIL) controls. (B) Effect of neonatal BPA exposure on female proceptive behaviors. There was no significant effect of exposure on proceptive hopping and darting behavior. Qualitatively, the EB exposed animals were not proceptive because none displayed this behavior. (*p ≤ 0.05; **p ≤ 0.001)

Figure 2

(A) Representative image depicting OT (white arrows) and OT/FOS double labeling (black arrows) within the PVN of a control (OIL) adult female rat. (B) Depiction of a co-labeled OT/FOS neuron observed at higher power. (C) Neonatal exposure to EB or BPA at either dose significantly increased the number of OT-ir neurons present within the adult PVN compared to OIL controls. The coexpression of OT-ir and FOS-ir was not significantly different between groups. (*p ≤ 0.05; **p ≤ 0.01; scale bar = 50 μm)

Figure 3

Representative confocal images depicting 5-HT fibers within the VMNvl of a (A) vehicle treated (OIL) female and an (B) EB exposed female. (C) Neonatal exposure to EB or PPT significantly increased 5-HT-ir fiber density within the adult female VMNvl. Although there was no significant effect of either BPA dose, there was a trend for increased fiber density in the 50μg BPA group (*p ≤ 0.05; **p ≤ 0.001;†p = 0.11; scale bar = 20μm).

Figure 4

Representative confocal images depicting ERα-ir labeling within neuronal nuclei within the MPOA of a (A) vehicle treated (OIL) female and an (B) EB exposed female. (C) Average ERα-ir neuronal number within the MPOA. Neonatal exposure to either EB or PPT significantly decreased the number of ERα-ir cells within the adult female MPOA while BPA had no significant effect. (*p ≤ 0.03, **p ≤ 0.001; scale bar = 40μm)