Several studies have already shown
Several studies have already shown that mPR activation can significantly reduce intracellular cAMP levels in different kind of cells, such as GT1-7 neuronal cells , MDA-MB-468 breast cancer cells  and primary vascular endothelial cells . The experiments performed with PTXa and 8-Br-cAMP show that both Gi protein inactivation and cell-permeable cAMP administration can block the mPR-mediated effect on cell migration, strongly suggesting that it is dependent on activation of a Gi proteins and decreases in intracellular cAMP levels. Given that mPRδ and mPRε are coupled to Gs protein  and mPRγ is expressed at extremely low levels, the observed effect is likely due to activation of mPRα and/or mPRβ, which are coupled to Gi proteins , .
The observation that Schwann cells treated with the specific mPR agonist 02 did not show alterations in the mRNA expression levels of the main protein components of compact myelin, P0 and PMP22 , , is in agreement with previous findings that progestogen regulation of P0 and PMP22 are mediated by PR and the GABA-A receptor, respectively . However, mPR-mediated progestogen signaling may have a role in myelin-axon interactions, because MAG mRNA and protein levels were upregulated by 02. MAG is a glycoprotein mainly expressed on the adaxonal membrane and in other specialized structures in peripheral nervous system myelin, such as Schmidt-Lanterman incisures , . It is involved in myelin-axon interactions, having a role in the bi-directional signaling between myelin-forming glia and IAA-94 . Different studies with knockout models suggest that MAG is not essential for the myelination process , but it is purported to stabilize myelinated axons  and a recent report suggests that MAG protects neurons from excitotoxicity . The progestogen upregulation of MAG mRNA and protein levels does not seem to be specifically mPR mediated, because it was mimicked by R5020. Therefore, progestogens may regulate MAG level both through classical and non-classical mechanisms.
Conflict of interest
Introduction Steroid hormones such as estrogens and progestogens are widely used in many formulations of combined oral contraceptive pills and hormone replacement therapies and they may enter the aquatic environment via sewage treatment plant (STP) effluents (Liu et al., 2011). However, in contrast to the synthetic oestrogen, ethinylestradiol, little attention has been paid to the environmental fate and potential effects of progestogens on aquatic organisms. Some studies have reported the presence of progestogens (e.g. norethindrone, levonorgestrel and progesterone) at the ng/L range in STP effluents (Petrovic et al., 2002, Fernandez et al., 2007, Vulliet and Baugros, 2007, Pu et al., 2008, Viglino et al., 2008), surface and groundwater samples (Vulliet et al., 2008) and river sediments (López de Alda et al., 2002). Furthermore, levonorgestrel has been reported to bioconcentrate in the blood of rainbow trout exposed to sewage effluent at concentrations four times higher than those normally in the effluent (Fick et al., 2010). Progestogens include a broad range of steroids with progesterone-like actions, derived from different parent structures. So far, those used for contraception are structurally related to either testosterone (estranes and gonanes) or progesterone (pregnanes and 19-norpregnanes) (Schindler et al., 2003). Thus, in addition to binding to the progesterone receptor (PR), they have various hormonal activities: estrogenic, anti-mineralocorticoid, anti-androgenic and androgenic (Sitruk-Ware and Nath, 2010). These compounds are suspected to pose a risk to aquatic organisms, particularly to fish. Thus, Runnalls et al. (2013) reported masculinization of female fathead minnow after exposure to gestodene (1–100ng/L) and desogestrel (0.1–10μg/L). Environmental levels of some progestogens and antiprogestogens were shown to affect the expression of genes involved in hormonal pathways in zebrafish embryos (Zucchi et al., 2012). Levels of medroxyprogesterone acetate (MPA) found in wastewater treatment plant effluents elicited pheromonal responses in fish, altered behaviour and interfered with reproduction (Kolodziej et al., 2003). Moreover, the inhibitory effect of levonogestrel, drospirenone and norethindrone on fish reproduction at the low μg/L or ng/L range has been reported (Zeilinger et al., 2009, Paulos et al., 2010, Runnalls et al., 2013), although the modes of action of these compounds remain unknown.