Abstract
Orexin A and B, are neuropeptides that are mainly produced in the hypothalamus. Orexins play important roles in food intake and in regulation of sleep and arousal. In the peripheral tissues, orexins also regulate various endocrine activities including the hypothalamic-pituitary-adrenal and -gonadal axes and growth hormone control. However, the effects of orexin on prolactin synthesis have not been clarified. Here, we focused on the role of bone morphogenetic protein (BMP)-4, which is involved in promoting PRL productivity and tumorigenesis of pituitary prolactinomas. We previously reported that BMP-4 induces prolactin secretion by rat lacto-somatotrope GH3 cells. In the present study, we studied the effects of orexin A and the interaction of orexin and BMP system on the regulatory role of prolactin synthesis by using rat lactotrope GH3 cells. Orexin type 1 receptor (OX1R), but not type 2 receptor (OX2R), was expressed in GH3 cells. Orexin A suppressed forskolin-induced, but not basal, prolactin mRNA expression without reducing cAMP levels. Of note, orexin A suppressed BMP-4-induced prolactin mRNA and cAMP synthesis. Interestingly, BMP-receptor signaling inhibitor reduced prolactin mRNA levels, which suggested that endogenous BMP action was to be involved in the activation of prolactin synthesis by GH3 cells. Orexin A suppressed Smad1/5/9 phosphorylation and Id-1 transcription induced by BMP-4, which was reversed by dual orexin-receptor antagonist, suggesting that the inhibitory effect of orexin A occurred via OX1R. Moreover, orexin A reduced BMP type I receptor, ALK-3 expression but increased inhibitory Smad6/7 expression, while BMP-4 treatment downregulated OX1R expression. These results indicated that orexin A has an inhibitory role in prolactin synthesis through suppression of endogenous BMP activity, suggesting that a new functional role of the interaction between orexin A and BMP-4 is modulation of prolactin levels in lactotrope cells.