Taking into account that a PFOS inhibits
Taking into account that: (a) PFOS inhibits CRF gene expression as well as CRF and corticosterone secretion (Pereiro et al., 2014); (b) CRF1r mediates the stimulation of the pituitary ACTH secretion by this neuropeptide (Rivier et al., 2003); (c) corticosterone regulates its own synthesis by a negative feedback in the hypothalamus and pituitary gland through its receptors (Gr) (Herman and Cullinan, 1997; Bagosi et al., 2015); as well as (d) the bidirectional relationship between the HPA axis and several limbic IL-4, murine recombinant regions (hippocampus, prefrontal cortex and amygdala) (Smith and Vale, 2006), we have seen fit to evaluate the toxic effects of PFOS on the gene and protein expression of CRF1r and Gr at different levels of the HPA axis and in some limbic brain areas. More concretely after PFOS exposure, gene and protein expression of CRF1r will be determined in the hypothalamus, pituitary and adrenal glands as well as in the hippocampus. At the same time, mRNA and protein levels of Gr will be analysed in the hypothalamus, pituitary gland, prefrontal cortex, amygdala and hippocampus. The hypothalamic concentration of CRF and serum levels of ACTH and corticosterone will be also quantified.
Material and methods
Discussion The results obtained in this study showed an inhibitory effect of PFOS on the HPA axis activity, at the administered doses in adult male rats by diminishing CRF, ACTH and corticosterone secretion, which corroborates the effects observed in previous studies by our laboratory (Pereiro et al., 2014). However, according to the consulted literature, PFOS appears to affect the secretion of corticosterone depending on the animal species, sex, physiological conditions and administration guidelines (Ribes et al., 2010; Zheng et al., 2009, Zheng et al., 2011; Mortensen et al., 2011; Zhao et al., 2011a, Zhao et al., 2011b; Li et al., 2016; Goudarzi et al., 2017). Rats treated with PFOS exhibited a decrease in the CRF gene expression and secretion in the hypothalamus (Pereiro et al., 2014). However, in this same brain area, high doses of the xenobiotic (3.0 and 6.0 mg/kg/day) induced an increase in the mRNA levels of CRF1r. At the same time the protein expression of this same receptor decreased with the lower administered dose and increased with the dose of 1.0 mg/kg/day. This fact shows that PFOS differently affects the mechanisms involved in transcription and translation of the CRF1r in the hypothalamus. Moreover, CRF activates its own gene expression through an ultra-short positive feedback in the PVN (Konishi et al., 2003) and stimulates CRF1r gene expression in the hypothalamic CRF neurons (Konishi et al., 2003). Thus, in light of these self-regulation mechanisms, the high hypothalamic CRF1r mRNA levels induced by PFOS exposure are not due to the effects of this xenobiotic in the CRF secretion, showing a disruption on the CRF action in the hypothalamic CRF1r. At pituitary level, CRF stimulates the ACTH release through CRF1r (Van Pett et al., 2000; Bagosi et al., 2015). As evidenced, PFOS can inhibit the gene and protein expression of CRF1r at the pituitary gland, which could explain, at least in part, the inhibition that induces this toxic in the ACTH secretion. On the other hand, gene expression and affinity of pituitary CRF1r may decrease by elevated plasma CRF concentration (Wynn et al., 1988), so circulating CRF levels do not appear to mediate PFOS effects on CRF1r gene and protein expression in the pituitary gland. Adrenocortical steroids release by the adrenal gland is directly regulated by CRF (Nussdorfer, 1996) through its receptors, predominantly CRF1r (Müller et al., 2001). In the adrenal gland, PFOS at the dose of 0.5 mg/kg/day activates CRF1r gene expression while the doses of 3.0 and 6.0 mg/kg/day stimulate the protein synthesis of this receptor. These changes do not correspond to the inhibition induced by PFOS in corticosterone secretion, so adrenal CRF1r does not appear to mediate the effects of this contaminant on the release of this steroid hormone.