Glucocorticoids potentiate the adenylyl cyclase—cAMP system mediated immunoreactive β-endorphin production and secretion from hypothalamic neurons in culture

Abstract

Beta-endorphin(βEP) 1−31, a potent opioid peptide of proopiomelanocortin (POMC) derivatives, is produced and released from neurons at arcuate nuclei of the rat hypothalamus. Although dexamethasone (DM) suppresses the production and secretion of POMC related peptides from rat pituitary corticotrophs, the effect of glucocorticoids on the function of hypothalamic βEP neurons remains unclear. Employing long term monolayer cultures of neonatal rat hypothalamic cells, we report here that 4 day treatment with 10 μM of forskolin increased ir-βEP levels in cell content and culture media by approximately1.7 ( P < 0.05) and 4.1 times ( P < 0.01) above vehicle treated control cultures (mean ±S.E.M., 47.3 ± 2.6 pg/well 40.4 ± 3.0 pg/well; n = 3) respectively. Although 4 day treatment with DM alone had little effect on the release and the cell content of ir-βEP, it significantly enhanced forskolin-induced elevation of ir-βEP levels in cell content and in culture media. The effect of DM was dose-related and time-dependent, with an EC 50 of about 1 nM; at this concentration DM enhanced ir-βEP secretion about2.1 times ( P < 0.01) above that induced by 10 μM of forskolin alone. Furthermore, the potentiating effect of DM was specifically suppressed by 100 nM of RU38486 ( P < 0.01), a glucocorticoid receptor antagonist, but not by an equivalent dose of RU28318, a mineralocorticoid receptor antagonist. In addition, Northern blot analysis showed that forskolin (10 μM) increased the abundance of POMC mRNA 1.4 fold above that of vehicle treated control cultures. Whereas by itself, DM (10 nM) had little effect on the level of POMC mRNA, it enhanced forskolin-stimulated increase of the abundance of POMC mRNA approximately 2.6 times. Moreover, DM also augmented1.6 times ( P < 0.05) forskolin-induced but not 3-isobutyl-1-methylxanthine (IBMX)-induced increase of cAMP production (5.5 ± 0.4 pmol/well; mean ± S.E.M., n = 3) in the cultures. Taken together, our findings suggest that in contrast to the inhibitory effect on pituitary corticotrophs, glucocorticoids enhance the production and secretion of βEP from rat hypothalamic neurons by facilitating the stimulatory effect mediated, in part, through the adenylyl cyclase-cAMP system.