Possible role of serotonin and neuropeptide Y on the disruption of the reproductive axis activity by perfluorooctane sulfonate

Highlights

• PFOS increases serotonin concentration in hypothalamus and in the median eminence.

• PFOS induces a decrease of the hypothalamic neuropeptide Y (NPY) concentration.

• PFOS can inhibit reproductive axis through serotonin and NPY changes in adult rats.

Abstract

Perfluorooctane sulfonate (PFOS) is an endocrine disruptor, whose exposure can induce several alterations on the reproductive axis activity in males during adulthood. This study was undertaken to evaluate the possible role of serotonin and neuropeptide Y (NPY) on the disruption of the hypothalamic–pituitary–testicular (HPT) axis induced by PFOS in adult male rats. For that, adult male rats were orally treated with 0.5; 1.0; 3.0 and 6.0 mg of PFOS/kg/day for 28 days. After PFOS exposure, serotonin concentration increased in the anterior and mediobasal hypothalamus as well as in the median eminence. The metabolism of this amine (expressed as the ratio 5-hydroxyindolacetic acid (5-HIAA)/serotonin) was diminished except in the anterior hypothalamus, with the doses of 3.0 and 6.0 mg/kg/day, being this dose 0.5 mg/kg/day in the median eminence. In general terms, PFOS-treated rats presented a decrease of the hypothalamic concentration of the gonadotropin releasing hormone (GnRH) and NPY. A diminution of the serum levels of the luteinizing hormone (LH), testosterone and estradiol were also shown. These results suggest that both serotonin and NPY could be involved in the inhibition induced by PFOS on the reproductive axis activity in adult male rats.

Introduction

Perfluorooctane sulfonate (PFOS) is one of the most important fluorinated organic compounds, since it is the product of degradation of more than 50 of them. It is a widely distributed environmentally persistent organic pollutant (Stockholm Convention, 2009). This compound has been used during the past few decades, possessing extreme thermal, biological, and chemical stability as well as both hydrophobic and lipophobic characteristics. It is persistent in the environment as it is neither hydrolyzed, nor photolyzed or degraded by any of the environmental conditions tested (Organisation for Economic Co-operation and Development (OECD) 2002), leading in a widespread numerous environmental compartments such as water, sediments, soils, dust and also in biological organisms were they might accumulate (Sinclair et al., 2006, Zhao et al., 2011b). Currently and according to the European Directive 2013/39/EU, this xenobiotic has been declared a priority hazardous substance (European Comission, 2013). In addition, PFOS and perfluorooctanoic acid (PFOA) have been subject to restrictions in production and use in North America, Europe, and Japan (US EPA, 2005, EU, 2006), but these chemicals are still manufactured and somehow widely used in some Asian countries, particularly in China (Wang et al., 2014) and South Korea (Kim et al., 2012).

PFOS and PFOA are widely used as surfactants in several industry and consumer products, such as repellents in textile, furniture, carpets, in lubricants, surfactant, polymers, firefighting foams, as component of adhesives and paper coatings, in cosmetics, pesticides and food packaging, as well as in semiconductor production or in hydraulic fluids in the aviation industry. The general population is exposed to PFOS and PFOA via indoor dust, but mainly through the contamination of food chains, in particular foods of animal origin such as seafood (Rivière et al., 2014) and fish (EFSA, 2011). Despite the fact that most of the data concerns the marine fish species, several studies have also indicated that the concentration of perfluoroalkyl substances (PFAS) in the fish from fresh water can be higher than in marine species (Schuetze et al., 2010). In fish, PFOS was found to be the predominant PFAS in muscle and liver with a highest concentrations of 79 ng/g wet weight in muscle and 1500 ng/g wet weight in liver (Pan et al., 2014), being the total daily intake limits suggested by European Food Safety Authority for PFOS: 150 ng/kg body weight/day; and PFOA: 1500 ng/kg body weight/day. Dietary intakes of PFOS have been estimated to be 100–250 ng/day in the UK (UKFSA, 2006), USA (Schecter et al., 2010), and Canada (Tittlemier et al., 2007). Consequently, diet is the main exposure route of perfluoralkyl acids in humans only exposed to background levels (Domingo et al., 2012), contributing more than 60% of total lifetime exposure (Tittlemier et al., 2007).

Many studies have reported the occurrence of PFOA and PFOS in the blood of people in the general population (Olsen and Church, 2004). Both of them are detected in wildlife and in the human body (Hoffman et al., 2010), even in cord blood (Apelberg et al., 2007) and breast milk (Tan et al., 2012). PFOS is primarily concentrated in the liver, showing values of 26.6 ng/g wet weight in humans from Catalonia, Spain (Kärrman et al., 2010), and blood, whereas its brain levels gradually increase with time after exposure (Sato et al., 2009). Although the human-derived partition coefficient (Pk) values are more suitable than rat-based Pk values for the application in a physiologically-based pharmacokinetic (PBPK) model, a deeper knowledge on the distribution of these compounds in human body is needed in order to conduct studies on perfluoroalkyl substances (Fàbrega et al., 2014).

In addition, some epidemiologic studies reported developmental deficiencies such as decrease in the birth weight of neonates (Washino et al., 2009) or infertility in mature women (Fei et al., 2009). PFASs are also known to easily accumulate in humans by combining with proteins (Giesy and Kannan, 2002, ASTDR, 2009). Moreover, evidence that PFOA and PFOS can cause endocrine disruption and developmental toxicity has been reported (Steenland et al., 2010). It is a neurotoxic agent (Long et al., 2013), that can disrupt the neuroendocrine system activity, fact evidenced in mammals (Zhao et al., 2011a, Pereiro et al., 2014). It is also an animal carcinogen (Wang et al., 2014), besides, carcinogenicity to human is also suggested by epidemiological studies for workers employed in PFCs manufacturer (Alexander et al., 2003). Toxicological studies suggest that PFOS is correlated with multiple pathologies such as hepatotoxicity, carcinogenicity, immunotoxicity as well as hormonal effects or on lipid metabolism and reproductive and developmental effects (EFSA, 2008). On the other hand, serum concentration of these compounds can be related to several biomarkers of the liver function such as alanine transferase, bilirubin, gamma-glutamyl transferase and uric acid (Gleason et al., 2015).

It is well known that PFOS exerts its toxicity at testicular level (Qiu et al., 2013, Zhao et al., 2014). This fact has been reported in several animals species such as rats (Jensen and Leffers, 2008), Xenopus laevis (Lou et al., 2013) and in human beings (Zhao et al., 2014). More concretely, in adult rats, it is observed Leydig cell hyperplasia, and eventually Leydig cell adenomas, as well as decreased testosterone production (Shi et al., 2007, Zhao et al., 2010). X. laevis exposed to PFOS showed a degeneration of spermatogonia in testes (Lou et al., 2013). In male mice, it was also found that PFOS significantly increased vacuolization in Sertoli cells in seminiferous tubules and blood–testis barrier ultrastructural disassembly, which subsequently increased its permeability and testicular PFOS levels (Qiu et al., 2013). These observations suggest that PFOS might also induce several alterations in other levels of the hypothalamic–pituitary–testicular (HPT) axis. The regulation of this physiological axis activity has been well established (Schlatt and Ehmcke, 2014). Serotonin exerts direct postsynaptic actions on gonadotropin-releasing hormone (GnRH) neurons to regulate their electrical excitability (Bhattarai et al., 2014). Most GnRH neurons exhibit a biphasic response to serotonin consisting of an immediate serotonin1A-dependent inhibition and a slower serotonin2A-mediated excitation (Bhattarai et al., 2014). This catecholamine inhibits LH secretion (McCann et al., 1998), and has a direct regulatory effect on steroidogenesis in testis (Csaba et al., 1998). Estradiol is synthetized from testosterone by the aromatase cytochrome P450 action (Lephart, 1996) and this steroid hormone inhibits LH secretion, perhaps through serotonin2 receptors (Johnson and Kitts, 1988). Furthermore, NPY is also involved in the regulation of GnRH and LH secretion (Crown et al., 2007), by modulating the gene expression of kisspeptinin the hypothalamus (Luque et al., 2007, Castellano et al., 2009).

On the other hand, reactive oxygen species (ROS) and oxidative stress induced by PFOS exposure has been reported in both in vivo and in vitro studies in different species and tissues (Qian et al., 2010, Wang et al., 2013). This chemical induces apoptosis (Liu et al., 2007) and modifies the permeability of the endothelial barrier through ROS (Qian et al., 2010). As a result, oxidative stress could be involved in the PFOS toxicity in the reproductive axis, lead to functional alterations of the reproductive axis at different levels. In fact, nitric oxide (NO) is a signaling molecule involved in the regulation of several physiological systems, including the endocrine system, and, of course, pituitary secretion activity (Pinilla et al., 1998, Gądek-Michalska et al., 2013).

Its synthesis is catalyzed by three different isoforms of nitric oxide synthase (NOS): a constitutive neuronal form (NOS1), an inducible form (NOS2) and a constitutive endothelial form (NOS3). NOS have been localized in the hypothalamus (Gao et al., 2014) and in the testis (Pérez et al., 2013), being regulated its gene expression by estrogens (Ceccatelli et al., 1993).

Heme oxygenase-1 (HO-1) represents a defensive mechanism against free radicals, being its gene expression induced by oxidative stress processes (Wagener et al., 2003). HO-1 seems to be a protective factor with potent anti-inflammatory, anti-oxidant, and anti-proliferative effects (Li et al., 2014).

In view of the facts above mentioned, the present study was undertaken to evaluate the possible role of serotonin and neuropeptide Y (NPY) on the disruption of reproductive axis activity exerted by PFOS exposure in adult male rats, by: (1) analyzing serotonin concentration and its metabolism in the anterior and mediobasal hypothalamus and in the median eminence; (2) quantifying hypothalamic concentration of NPY; (3) measuring hypothalamic concentration of GnRH and serum levels of LH, testosterone and estradiol; (4) determining NOS1 gene expression in hypothalamus; and finally (5) by analyzing in testis several parameters of cellular signaling adaptation to oxidative stress, and more concretely, the gene expression of HO-1, NOS1 and NOS2 as well as catalase concentration. All parameters evaluated in the present study are shown in Fig. 1.