Orphanin FQNociceptin

Recently, an opioid-related peptide has been identified termed orphanin FQ or nociceptin (OFQ/N). OFQ/N is the endogenous ligand for the fourth member of the opioid receptor gene family, ORL-1. Although it has some similarities to the traditional opioid peptides, it also has many significant differences. Like dynorphin A, OFQ/N is a heptadecapeptide (Table I) and its first four amino acids (Phe-Gly-Gly-Phe-) are similar to those of the traditional opioid peptides (Tyr-Gly-Gly-Phe-). However, OFQ/N has very poor affinity for the traditional opioid receptors, whereas the opioid pep-tides show no appreciable affinity for the ORL-1 receptor. Functionally, OFQ/N is also quite distinct from the opioid peptides. Although high doses of OFQ/N can elicit analgesia, low doses given suprasp-inally functionally reverse the opioid analgesia. The precursor for OFQ/N has been cloned, and it too contains additional putative neuropeptides, including nocistatin and OFQ2 (Fig. 2D).

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