The Role of N-Methyl-D-Aspartate Receptors in the Release of Adrenocorticotropin by Dynorphin A _1–13

Abstract

We previously reported that dynorphin A_1–13 evokes a significant increase in plasma adrenocorticotropin (ACTH) after intravenous administration in the ovine fetus. This response was not sensitive to naloxone and was regulated differently from the response to U50,488H, a selective ĸ-opioid agonist. NMDA appears to play a role in many of the nonopioid actions of dynorphin. We therefore hypothesized that dynorphin A_1–13 may release ACTH via N-methyl-D-aspartate (NMDA) receptors. To test this hypothesis, we have compared the ACTH response to dynorphin A_1–13 and NMDA in the chronically-instrumented ovine fetus. Our data show that both dynorphin A_1–13 (0.5 mg/kg) and NMDA (4 mg/kg) induced a significant release of immunoreactive ACTH in the late-term ovine fetus. The ACTH response to NMDA was of a smaller magnitude, but of longer duration, when compared to dynorphin A_1–13. The response to both dynorphin A_1–13 and NMDA was significantly attenuated by pretreatment with the noncompetitive NMDA antagonist, MK-801, but was not affected by antagonists of corticotropin-releasing hormone and arginine vasopressin. Finally, the ACTH response to both dynorphin A_1–13 and NMDA were inhibited by dexamethasone. The results of this study indicate a role for NMDA receptors in the action of dynorphin A_1–13, and suggest that NMDA may act directly at the level of the pituitary to release ACTH without the involvement of hypothalamic secretagogues.