Effects of chronic triclosan exposure on social behaviors in adult mice

Highlights

• Chronic TCS exposure reduced social dominance in adult mice.

• Chronic TCS exposure impaired memory formation in female mice.

• Chronic TCS exposure altered the composition of gut microbiota in female mice.

• Chronic TCS exposure induced ultrastructural damage to hippocampal neurons and synapses in adult mice.

Abstract

Triclosan (TCS), a newly identified environmental endocrine disruptor (EED) in household products, has been reported to have toxic effects on animals and humans. The effects of TCS exposure on individual social behaviors and the potential underlying mechanisms are still unknown. This study investigated the behavioral effects of 42-day exposure to TCS (0, 50, 100 mg/kg) in drinking water using the open field test (OFT), social dominance test (SDT), social interaction test (SIT), and novel object recognition task (NOR). Using 16S rRNA sequencing analysis and transmission electron microscopy (TEM), we observed the effects of TCS exposure on the gut microbiota and ultrastructure of hippocampal neurons and synapses. Behavioral results showed that chronic TCS exposure reduced the social dominance of male and female mice. TCS exposure also reduced social interaction in male mice and impaired memory formation in female mice. Analysis of the gut microbiota showed that TCS exposure increased the relative abundance of the Proteobacteria and Actinobacteria phyla in female mice. Ultrastructural analysis revealed that TCS exposure induced ultrastructural damage to hippocampal neurons and synapses. These findings suggest that TCS exposure may affect social behaviors, which may be caused by altered gut microbiota and impaired plasticity of hippocampal neurons and synapses.

Introduction

A class of compounds that can interfere with normal biological systems is known as environmental endocrine disruptors (EEDs). EEDs exist in the environment and can disrupt the stability of the internal environment of the body by interfering with the function of endocrine hormones, thus affecting biological or human reproductive, nervous, and immune system functions. Triclosan (TCS), a new EED, is widely used as a broad-spectrum antibacterial agent in textiles, toys, medical devices, and many personal care products, such as toothpastes, soaps, mouthwashes and cosmetics (Ruszkiewicz et al., 2017, Tabari et al., 2019). TCS is similar to bisphenol A (BPA) in structure and is estrogenic and highly lipophilic. TCS is absorbed into the body circulation primarily via the dermal, oral mucosal, and gastrointestinal routes (Tabari et al., 2019). TCS has been detected in blood, urine, breast milk, amniotic fluid, liver, adipose, and brain tissue (Ruszkiewicz et al., 2017, Tabari et al., 2019, Sandborgh-Englund et al., 2006, Dann and Hontela, 2011, Chen et al., 2012, Dayan, 2007, Karzi et al., 2021) (Table S1). Several studies have shown that TCS is associated with human health issues. TCS has been reported to interfere with the metabolism of thyroid hormones in pregnancy, infancy, and childhood (Braun et al., 2018). TCS has reproductive toxicity and can affect the growth and development of a fetus through breast milk and the placenta (Trivedi et al., 2020, Tran et al., 2020, Bai et al., 2020). TCS can also disrupt liver integrity and function and induce hepatic tumorigenesis in mice (Yueh et al., 2014). In addition, in zebrafish, TCS altered lipid synthesis and metabolism and reduced the expression of β-oxidation transcripts in the embryonic liver (Ho et al., 2016). Moreover, TCS has a negative impact on the immune system (Anderson et al., 2016).

Recently, the effects of TCS on the brain have also received increasingly critical attention (Table S2). Arias-Cavieres and colleagues reported that direct hippocampal injection of TCS (5 pmol/side) impaired spatial memory and synaptic plasticity in male rats (Arias-Cavieres et al., 2018). Tabari and colleagues reported that TCS gavage at 4000 mg/kg/day induced anxiety-like behavior in mice (Tabari et al., 2019). In addition, Hao and colleagues reported that TCS (50 mg/kg) gavage during pregnancy increased the risk of autism in offspring mice (Hao et al., 2019). In a series of experiments with zebrafish, Ling et al. reported that TCS caused neurotoxicity in zebrafish by affecting the epigenetic effects of miR-219-related pathways (Ling et al., 2020). Pullaguri et al. reported that TCS impaired motor neurons in zebrafish larvae by suppressing the acetylcholinesterase and synapsin 2a genes (Pullaguri et al., 2021). Moreover, Wang et al. reported that TCS produced neurotoxicity in zebrafish through the RNA-binding protein CELF2 (Wang et al., 2021). Although the studies mentioned above indicated the potential effects of TCS on brain development and brain functions, scientific limitations still exist. The TCS exposure dose of 4000 mg/kg/day in Tabari’s study was too high (Tabari et al., 2019) because it was close to the median lethal dose (4350 mg/kg orally in mice) (Fang et al., 2010). Arias-Cavieres's study on the effects of TCS on memory and synaptic plasticity was considered important, although intranuclear administration can cause severe and sudden toxicity in the brain (Arias-Cavieres et al., 2018).

The gut microbiota has been considered to be involved in the regulation of autism- and cognition-related behavioral changes (Sherwin et al., 2019, Fattorusso et al., 2019). As an antimicrobial agent, the effect of TCS on the gut microbiota has been reported. Mei-Fei Yueh’s study found that TCS caused changes in the gut microbiota in high-fat diet-fed animal models, which may not be generalizable to normal mice (Yueh et al., 2020); Yue Ma’s study found that perinatal TCS exposure led to changes in the gut microbiota of offspring but did not explore behavioral changes (Ma et al., 2020). Therefore, relevant research on the role of gut microbiota in the influence of TCS on normal individual social behavior is lacking.

Social dominance is a significant social behavior of community creatures. Building and maintaining social hierarchies are essential for social stability and individual health. Meanwhile, the social dominance phenotypes of individuals are malleable and influenced by both genetic and environmental factors (Fitchett et al., 2005, Langley et al., 2018, Barnard and Luo, 2002). Different social hierarchies have different sensitivities to stress (Larrieu et al., 2017), and social dominance may affect the reaction to chronic stress differently for male and female mice (Karamihalev et al., 2020). Our recent studies indicated that postweaning exposure to microbiota-derived metabolites (trimethylamine N-oxide and sodium butyrate) could significantly decrease the social dominance in mice during adulthood, which was accompanied by hippocampal metabolic adaptation (Zhao et al., 2020, Luo et al., 2021). Notably, Kim B’s group reported that EEDs alter social behaviors and indirectly influence social hierarchies via changes in body weight (Kim et al., 2015). To date, the effects of TCS exposure on social dominance and the potential underlying mechanism are still elusive.

The present study was conducted to investigate the potential effects of chronic oral intake of low-dose TCS in drinking water on the social dominance of mice and to explore the potential mechanisms by focusing on the gut microbiota. In addition, the hippocampus is one of the key brain areas regulating various social behaviors (Alboni et al., 2020, Yuan et al., 2020, Qi et al., 2021). Neuroplasticity has been linked to a variety of social behavior disorders (Boschen et al., 2014, Nuytens et al., 2013, Wei et al., 2020). Therefore, in the present study, we also focused on the neuroplasticity of the hippocampus.