Opioid receptor modulation of norepinephrine secretion in the caudal dorsomedial medulla of rats: in vivo studies

Our knowledge of how opioids mfluence brainstem neurochemistry is derived principally from in vitro studies. Before we can fiiUy understand how opioids impact cardiovascular function, it is necessary to learn more about their role in vivo. Using microdialysis, male Sprague-Dawley rats were perfused with a microdialysis probe located adjacent to the nucleus of the solitary tract (nTS), with Ringer's, or Ringer's containing beta-endorphin or the selective opioid receptor agonists, [D-Ala^ N-Me-Phe^ Gly-ol]-Enkephalm(DAMGO), U-50488H. or [DPen23]-Enkephalin (non-pCL DPDPE). Microdialysate samples were collected and analyzed for norepinephrine (NE) by high performance liquid chromatography electrochemical (HPLC-EC) detection. DAMGO, a selective muopioid agonist, inhibited potassium (K^)-induced NE release in the nTS. DPDPE, a selective deltaopioid agonist and U-50488H, a kappa-agonist, did not have any affects on K^-induced NE release in the nTS. DAMGO effects on NE secretion were not observed in the presence of the mu/delta selective-opioid receptor antagonist, naloxone. These studies illustrate the ability of selective opioid receptor agonists to influence K"^ -evoked NE release and that the inhibitory effects are mediated through mu-opioid receptors in vivo.