Distinct effect of intracerebroventricular and intrathecal injections of nociceptin/orphanin FQ in the rat formalin test

Nociceptin/orphanin FQ (nociceptin/OFQ), a newly discovered heptadecapeptid e has been regarded as an endogenous ligand for orphan opioid receptor. The present study was designed to investigate the effect of nociceptin/OFQ on pain response and opioid analgesia in the rat formalin test. The results sh owed that intracerebroventricular injection of 1 mu g nociceptin/OFQ enhanc ed the pain response, and 0.1 or 0.5 mu g nociceptin/OFQ had no effect on f ormalin-induced pain. When 0.1 or 1 mu g nociceptin/OFQ were used together with mu-, delta-, or kappa-opioid receptor agonists, endomorphin-1, DSLET o r U50488H, respectively, it attenuated mu- and kappa- but not delta-recepto r mediated analgesia. On the other hand, intrathecal injection of nocicepti n/OFQ (0.1, 1 and 5 mu g) reduced the pain response in the formalin test. I n conclusion, nociceptin/OFQ potentiated formalin-induced pain response and antagonized opioid analgesia in the rat brain but inhibited pain response in the spinal cord. (C) 1999 Elsevier Science B.V. All rights reserved.