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 kappa-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 alpha-helical CRH9-41, 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 kappa-opioid receptor antagonist MR-1452. In the presence of alpha-helical CRH9-41, 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 alpha-helical CRH9-41, 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 kappa-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.

Hypothalamic involvement in the activation of the pituitary-adrenocortical axis by nerve growth factor.

Intravenous injection of nerve growth factor (NGF) into rats produces a dose-dependent (from 0.1 to 5 nmol/kg) increase in circulating concentrations of adrenocorticotropin (ACTH) and corticosterone. We have investigated whether this effect is produced through a direct action on a component of the hypothalamo-pituitary-adrenocortical axis. NGF (50 and 500 nM), added to the incubation medium of in vitro isolated pituitary segments or dispersed adrenal cells, did not modify either basal and stimulated release of biologically active or immunoreactive ACTH or release of corticosterone, respectively. The presence of NGF in the incubation medium of in vitro isolated hypothalami produced a dose-dependent (from 150 to 600 nM) increase of both release and content of some material with corticotropin-releasing bioactivity. The nature of this corticotropin-releasing bioactivity was determined directly by radioimmunoassays. Results have indicated that NGF induced an increase of both release and content of hypothalamic arginine-vasopressin (AVP), while no changes were observed in the release and content of hypothalamic corticotropin-releasing hormone (CRH). These results suggest that adrenocortical stimulation by NGF in vivo could be mediated by the release of hypothalamic AVP rather than CRH. The finding that in vivo NGF stimulatory effect was not abolished by the specific CRH antagonist alpha-helical CRH(9-41), while it was accompanied by an increase in circulating AVP levels, supports this interpretation. However, the fact that the hypothalamus is stimulated in vitro by NGF concentrations higher than those expected to reach this structure after systemic injection of active doses raises the possibility that other brain areas such as the hippocampus participate in NGF-induced adrenocortical activation.