Progesterone, but not estrogen, modulates the cAMP system mediated ir-β-endorphin secretion and POMC mRNA expression from rat hypothalamic cells in culture

Abstract

It is now evident that hypothalamic β-endorphin (βEP) modulates reproductive physiology at the central level by inhibiting the function of neurons producing gonadotropin-releasing hormone (GnRH). Increasing evidence suggests that gonadal steroids, which play an important role in the long-loop negative feedback on the hypothalamus-pituitary-gonadal axis, may exert its indirect inhibitory action through modulating the production and release of hypothalamic βEP. However, it remains unclear whether progesterone or estrogen alone or their combination is important to exert this effect. Employing long-term monolayer neonatal hypothalamic cell cultures, we reported here that whereas progesterone significantly enhanced forskolin-, N 6,2′- O-dibutyryladenosine-3′5′-cyclic monophosphate [(Bu) 2cAMP]-, 3-isobutyl-1-methylxanthine (IBMX)- or cholera toxin-stimulated immunoreactive (ir)-βEP release from cultures treated daily for 4 consecutive days, the steroid alone produced little effect. This potentiation of progesterone was time-related and dose-dependent with an EC 50 value of the steroid being approximately 25 nM; at this concentration the steroid increased ir-βEP secretion about 1.6 times ( P < 0.05) that induced by 5 μM forskolin alone. Similar effects were also observed for POMC mRNA levels in cultures subjected to 6 h of the above treatment regime. This potentiating effect appears specific as it can be mimicked by progestin, a progesterone receptor agonist and blocked by the progesterone receptor antagonist RU38486, but not RU28318, a mineralocorticoid receptor antagonist. Furthermore, β-estradiol alone failed to exert a significant effect on basal, forskolin-induced or on forskolin and progesterone co-stimulated βEP release or POMC mRNA levels in hypothalamic cell cultures. Hence, our present results substantiate findings from in vivo studies suggesting that progesterone may play an important role in augmenting hypothalamic βEP production and release to inhibit GnRH secretion. Our present findings are thus consistent with the notion that in the central nervous system, progesterone, but not estrogen, appears to be the major ovarian steroid that exerts an indirect long-loop negative feedback on the hypothalamus-pituitary-gonadal axis of the female rat.