Effects of perinatal exposure to Bisphenols on anxiety-like behaviours and serotoninergic system

Bisphenol A (BPA) is an organic synthetic compound found in a huge variety of industrial products and consummatory goods. Due to its structural similarity with estrogens, BPA is listed among endocrine disruptors and widely studied for affecting human health. There can be identified several routes of exposure to BPA, including ingestion, inhalation, skin, and eye contact. It is also known to be transmitted from the mother to the offspring via the placenta and breast milk, and so to have effects on neurodevelopment, behavior, and metabolism. Because of these evidence on the effects of BPA on human health, many substitutes have been proposed to replace it. One of the most diffused is bisphenol S (BPS), an analogue of BPA, which shares its similarities with estrogens. The first studies on BPS describe it as similarly hormonal disrupting. In this study, we focused on the sexually dimorphic effects of perinatal exposure to either BPA or BPS on anxiety-related behaviour and, contextually, on one of the neural circuits that is mainly involved in the regulation of this behaviour: the serotoninergic system. To do so, we orally treated C57BL/6 dams with either BPA or BPS dissolved in corn oil or with vehicle, in a dose chosen for being the TDI dose for BPA (4µg/kg body weight/day), from mating until the weaning of the offspring. At PND90 the offspring (n= 10 ± 1 /group) was tested for the anxiety-related behavior through the Elevated Plus Maze (EPM) and the Open Field (OF) tests, and then they were sacrificed to perform immunohistochemical analysis (IHC) of serotonin (5-HT) (n= 4/group). The EPM shows a tendency of treated males to spend more time in the open arms with respect to the controls, and there is a significative decrease in the latency to first entry into the open arms, suggesting an increase in explorative behaviour and decrease in anxiety. In females, there is an opposite tendency, and a significative decrease of BPA-treated females time spent in open arms, suggesting an increase in anxiety. The OF shows a tendency of BPA- and BPS- treated males to spend more time in the center and less time in the border with respect to the controls; the opposite tendency is observed in females. These results seem to invert the sexual dimorphism in control males and females: the latter tend to be normally more explorative than males. A sexual dimorphism exists in serotonin neurons between male and female controls in dorsal raphe, females having more serotonergic neurons that males. IHC analysis partially confirm the data obtained in behavioural tests, especially the effects on BPA- treated males. There is a significative increase in serotoninergic neurons in both dorsal and median Raphe nuclei in BPA perinatally exposed male mice; also, BPS-treated males show an increase in the dorsal Raphe, mainly due to the ventral component, involved in coping with anxiety. The perinatal exposure to BPA and BPS on female offspring does not significantly affect the number of serotonin-positive cells, but there is a significative increase in the fractional area in median Raphe and the dorsal component of the dorsal raphe. Overall, this study suggests that perinatal exposure to both BPA and BPS have an anxiolytic effect on male mice and an anxiogenic effect on females, disrupting the sexually dimorphic anxiety-related behaviour and the dimorphism in serotonergic system.