The kappa-opioid receptor agonist MR-2034 stimulates the rat hypothalamic-pituitary-adrenal axis: Studies in vivo and in vitro

There is increasing evidence that opiates not only have analgesic properties, but also regulate mechanisms activated during the stress response, such as the hypothalamic-pituitary-adrenal (HPA) axis. Indeed, opioid-containing neurons innervate the paraventricular nucleus and the median eminence, thus modulating inputs to ACTH-controlling neurons. In addition, dynorphin (the endogenous ligand of the kappa-opioid receptor)-like peptides have been found co-localized with corticotrophin-releasing hormone (CRH) and are believed to be co-secreted with it in the hypophyseal portal circulation to modulate ACTH release. In this study, we evaluated the effects of the selective κ-opioid receptor agonist MR-2034 [(-)-N-(2-tetrahydrofurfuryl)-normetazocine] on the HPA axis in vivo and in vitro. MR-2034 was given intravenously to catheterized, freely moving, male Sprague-Dawley rats and serial blood samples were collected for ACTH and corticosterone (B) measurements. We evaluated also the site of MR-2034 action on the HPA axis in vivo, after the administration of or-helical CRH_9-41, a CRH receptor antagonist, on hypothalamic CRH, pituitary ACTH, and B release in vitro. MR-2034 increased plasma ACTH and B levels in a dose-related fashion and this effect was antagonized by the selective κ-opioid receptor antagonist MR-1452. In the presence of α-helical GRH_9-41, the responses of plasma ACTH and B to MR-2034 were blunted significantly, suggesting that this compound activates the HPA axis through a CRH-dependent mechanism. Accordingly, MR-2034 stimulated hypothalamic CRH release in vitro in a concentration-dependent fashion and this effect was antagonized dose-dependently by MR-1452. However, the stimulatory effect of MR-2034 on plasma ACTH and B in vivo was not completely abolished by α-helical CRH_9-41, suggesting that an additional, CRH-independent, mechanism was involved. Indeed, MR-2034 was able to stimulate basal ACTH output in a dose-dependent manner and this effect was antagonized by MR-1452 in vitro. On the other hand, MR-2034 did not have any effect on B release from adrenocortical cells or adrenal quarters in vitro. These results show that the benzomorphan MR-2034 stimulates the HPA axis in the rat by acting at the hypothalamic and the pituitary level. We hypothesize that endogenous κ-opioid peptides not only act at the pituitary level to increase ACTH output, but may also act at the hypothalamic level to increase CRH release through an autocrine and/or ultrashort positive feedback mechanism.