Effects of chronic exposure to bisphenol-S on social behaviors in adult zebrafish: Disruption of the neuropeptide signaling pathways in the brain

Highlights

• Exposure to bisphenol S and estradiol impaired group preferences in zebrafish.

• High concentration of bisphenol S and estradiol impaired shoaling behavior in zebrafish.

• BPS and estradiol did not influence the locomotor activity and number of excursions.

• BPS and estradiol altered the expression of the neuropeptide genes.

• Impaired social behavior was associated with dysfunction of neuropeptide system.

Abstract

Bisphenol S (BPS), considered to be a safe alternative to Bisphenol A, is increasingly used in a wide variety of consumer and industrial products. However, mounting evidence suggests that BPS can act as a xenoestrogen targeting a wide range of neuro-endocrine functions in animals. At present, very little is known about the impacts of BPS on social behaviors and/or the potential underlying mechanisms. To this end, we exposed adult male and female zebrafish to environmentally relevant concentrations of BPS (0 (control), 1, 10, and 30 μg/L), as well as to 17β-estradiol (E2; 1 μg/L; as positive control) for 75 days. Subsequently, alterations in social behaviors were evaluated by measuring shoal cohesion, group preferences, and locomotor activity. Furthermore, to elucidate the possible molecular mechanism underlying the neuro-behavioral effects of BPS, we also quantified the changes in the mRNA abundance of arginine vasotocin (AVT), isotocin (IT), and their corresponding receptors in the zebrafish brain. The results showed that E2 and BPS (30 μg/L) decreased shoal cohesion in both males and females. Moreover, a marked decline in group preferences was observed in all treatment groups, while locomotor activity remained unaffected. Alterations in the social behaviors were associated with sex-specific changes in the mRNA expression of genes involved in IT and AVT signaling. Taken together, the results of this study suggest that chronic exposure to BPS can impair zebrafish social behaviors via disruption of isotocinergic and vasotocinergic neuro-endocrine systems.

Introduction

Bisphenol-S (BPS, bis-(4-hydroxyphenyl)-sulfone) is one of the main alternatives of BPA widely used for manufacturing a variety of products including epoxy glues, electroplating solvent, dyestuffs, canned foodstuffs, thermal receipt papers, luggage tags and other products marked as “BPA-free” (Liao & Kannan, 2013; Liao, Liu & Kannan, 2012a). BPS is known to have lower estrogenic activity (Chen, Ike & Fujita, 2002) and higher resistance to physical degradation (i.e. greater stability against temperature, UV, and biological degradation) compared to BPA (Ozaki et al., 2004). However, the increased usage of BPS has led to the contamination of different ecosystems at concentrations similar to or even greater than BPA (Chen et al., 2016). Detectable levels of BPS have been repeatedly reported in surface waters, sediment, and sewage sludge ranging from 0.02 to 65.60 μg/L (Huang et al., 2018; Kienhuis & Geerdink, 2000; Song et al., 2014; Yamazaki et al., 2015; Jin & Zhu, 2016). As a result, daily exposure of humans and animals to BPS has become inevitable (Chen et al., 2016; Liao et al., 2012b). For instance, urine samples collected from the United States and multiple Asian countries revealed that BPS was present in 81% of individuals with concentrations of up to 21 ng/ml (Liao et al., 2012b). Likewise, Wan et al recently showed that BPS was detected in 97.5% of human serum samples (n = 240) in 4 different cities in China along the Yangtze River that the highest detected concentration of BPS was 169 ng/ml (Wan et al., 2018). These observations are of concern because a growing body of evidence demonstrated that BPS can interact with estrogen receptors and thyroid hormone receptor (Catanese & Vandenberg, 2017; Zhang, Zhou & Yang, 2017). In line with this, BPS-induced reproductive abnormality, cytotoxicity, genotoxicity, and teratogenicity have been reported in various animals (Castro et al., 2015; Zhang et al., 2018; Naderi, Wong & Gholami, 2014). BPS, such as BPA, has structural homology with 17β-estradiol (E2) that can act as an estrogenic and anti-androgenic compound by binding to estrogen receptors (ERs); therefore, BPS may not necessarily be a safer alternative to BPA and can actually cause adverse effects in organisms (Rosenmai et al., 2014).

To date, significant strides have been made towards understanding the effects of bisphenolic compounds, such as BPS, on sex steroid hormones and reproductive parameters in animals. In this regard, Chen et al. (2016) provided an excellent overview of how bisphenol analogs affect different aspects of the endocrine system in animals. Growing evidence shows that bisphenols and BPA, in particular, can interfere with neuroendocrine processes involved in animal cognition and behavior leading to a broad spectrum of neurobehavioral abnormalities (Saili et al., 2012; Weber et al., 2015; Wolstenholme et al., 2012). However, there is a dearth of literature on the effects of BPS on the central nerve system (CNS) and its consequences for animal behaviors. Across vertebrate taxa, different stress hormones, sex hormones, and neuropeptides play a critical role in the regulation and plasticity of social behaviors. Arginine vasopressin (AVP) and oxytocin (OT) are families of neuropeptides that are primarily secreted into the blood (i.e. peripheral systemic effects of neuropeptides) from magnocellular neuronsitr (MCNs) located inside the supraoptic (SON) and paraventricular nuclei (PVN) (Lemos, 2012). These neuropeptides are involved in regulating various physiological processes such as smooth muscle contraction and homeostasis of blood pressure (Balment et al., 2006; Gruber, 2014). The central release of these neuropeptides from the parvocellular neurons to limbic areas plays a fundamental role in mediating neurobehavioral responses in both males and females (Neumann & Landgraf, 2012; Gainer & Wray, 1992), which include aggression, sexual behavior, social behaviors, pair bonding, and parental care (Lema, Sanders & Walti, 2015; Minerbo et al., 1994; Churchland & Winkielman, 2012; Heinrichs & Domes, 2008; Cho et al., 1999). These two neuropeptide families can induce independent and/or opposite effects on male and female social behaviors. For example, the anxiolytic and antidepressive effects of OT is of special relevance for female behavior whereas the anxiogenic effects of AVP primarily modulate male behaviors (Neumann & Landgraf, 2012; Campbell, 2010). Several receptors mediate the effects of these neuropeptides on the CNS of mammals. The expression and activity of these receptors have also been correlated with various fundamental behavioral functions (Hasunuma et al., 2013; Insel, 2010).

Teleost fishes provide valuable model systems to investigate links between the neuropeptides and behaviors. In fish, the nonapeptides vasotocin (AVT) and isotocin (IT) are the homologs of AVP and OT, respectively. These peptides and their receptors (e.g., V1a1, V1a2, and V2 type for AVT, and ITr for IT) are involved in the regulation of non-reproductive social behaviors such as group preferences (Goodson et al., 2009), shoaling behaviors (Lindeyer et al., 2015), and social approaches (Thompson & Walton, 2004). These behaviors are critical for mating, foraging efficiency, and antipredator defense in fish (Krause & Ruxton, 2008). Hence, the impairment of these behaviors may lead to decreased fitness which may ultimately reduce the survival of individuals and populations. Although BPA has been reported to impair group preferences in zebrafish (Danio rerio) (Wang et al., 2013), it remains unknown whether exposure to BPS has any effects on social behaviors such as group preferences and shoaling behaviors in fish. In this context, it would be also important to investigate whether the behavioral effects of BPS are mediated by the perturbation of the AVT/IT pathways in the brain.

To address these unresolved questions, zebrafish were selected as a model organism in the present study, as it is one of the most social vertebrates among commonly used laboratory species (Buske & Gerlai, 2011). Moreover, zebrafish are a popular model organism for assessing chemical toxicity, neurological diseases, and behavioral disorders (Gerlai, 2010; de Esch et al., 2012; Guo, 2004; Hill et al., 2005). It has been suggested that zebrafish increase their preference for conspecifics with ontogeny (Buske & Gerlai, 2011; Gerlai, 2014), hence we used 9 months old adult zebrafish in the present study. Groups of zebrafish were exposed chronically to environmentally relevant concentrations of BPS and 17 β-estradiol (E2) as a positive control since E2 is the most predominant form of estrogen that potentially acts as a homeostatic modulator of brain tissue. Following exposure, shoal cohesion, group preferences, and locomotor activities were assessed in both males and females. In addition, to gain insights into the underlying mechanisms by which BPS may affect these behaviors, we also evaluated the expression of genes involved in the AVT/IT neuropeptide systems in the zebrafish brain.