Hypocretin/orexin suppresses corticotroph responsiveness in vitro

The hypocretin/orexins (Hcrts/ORXs) are peptides produced in neurons in the lateral hypothalamic area that project to neuroendocrine centers in the hy pothalamus. Hcrt/ORX receptors are present in the hypothalamus and anterior pituitary gland. We examined the possibility that the Hcrts/ORXs, which we have demonstrated previously to act in the brain to stimulate sympathetic function, could alter stress hormone secretion by a direct pituitary action . In vitro studies revealed a dose-related inhibitory effect of the Hcrts/O RXs on corticotropin-releasing hormone-stimulated ACTH secretion that appea red to be mediated via the orexin-1 receptor and to be expressed at doses ( threshold dose 1 nM orexin A) similar to the affinity constant for the rece ptor. The effect was not due to abrogation of the cAMP response of the cort icotroph to corticotropin-releasing hormone and was not pertussis toxin sen sitive, suggesting a non-G(i)-mediated mechanism. Instead, a G(q)-mediated signaling mechanism was indicated by the ability of protein kinase C blocka de with calphostin C to reverse the inhibitory action of orexin A. Orexin A and orexin B did not significantly alter basal ACTH secretion in vitro and did not alter basal or releasing factor-stimulated secretion of luteinizin g hormone, prolactin, thyroid-stimulating hormone or growth hormone from ce lls harvested from male or random-cycle female donors. Our data suggest a d irect, pituitary action of the Hcrts/ ORXs to modulate the endocrine respon se to stress and identify the potential cellular mechanism of a unique biol ogical action of the peptides in the anterior pituitary gland.