CONICET | Buscador de Institutos y Recursos Humanos

In the pituitary, progesterone (P4) actions remain controversial because its effect depends on the hormonal milieu and the receptor types involved. Our previous results demonstrated that, besides the nuclear progestin receptors (nPRs), all membrane progestin receptors (mPRs) subtypes (α, β, γ, δ, ε) are also expressed in the anterior pituitary of female Sprague Dawley (SD) rats, being the mPRa the most abundant. In the present work, we observed by IF and Flow citometry that 65% of lactotrophs express mPRα. Focusing on the role of mPRα in the lactotroph physiology, we first performed a single-point competitive binding assay to demonstrate that P4 and the mPRα specific agonist Org OD 02-0 (02), significantly displaced [3H]-P4 binding in plasma membranes of GH3 cell line, whereas the nuclear PR agonist R5020 was ineffective. Next, we demonstrated, in ex vivo-incubated SD-female pituitaries, that P4 100nM and 02 100nM, but not R5020 10nM decreased PRL secretion in medium (measured by RIA) and increased PRL content in the pituitary. We also observed the inhibitory effect of P4 and 02 on PRL release in GH3 cells, and again, R5020 was ineffective. We next assayed putative second messengers involved. We found that both P4 and 02 induced a significant decrease in cAMP accumulation. The inhibition of 02 on PRL release was blocked by pre-treatment with pertussis toxin (PTX 2,5ug/ml). In addition, 15min-treatment of P4 and 02 induced a significant increase in ERK phosphorylation, while R5020 did not. Finally, since TGFβ1 is one of the main inhibitors of lactotroph function, we evaluated whether this system could be regulated by mPRα, mediating in part, the effects of P4 on PRL secretion. In fact, P4 and 02, but not R5020, increased active TGFβ1 levels, measured by ELISA, in ex vivo-incubated SD-female pituitaries, and also in in vitro GH3 cell-culture. All these results provide the first evidence of specific mPRα involvement in the control of lactotroph function.