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A recently identified novel peptide, orphanin FQ/nociceptin (OFQ/N), is an endogenous ligand for a unique member of the cloned opioid receptor family. Saturation studies in mouse brain membranes reveal curvilinear Scatchard plots with both a higher (K_D3.8 pM, B_max31.6 fmol/mg protein) and a lower (K_D896 pM, B_max233 fmol/mg protein) affinity site in brain tissue, compared to only a single site in transfected CHO cells (K_D36 pM). Competition studies confirm the high affinity of OFQ/N for this site, but shallow Hill slopes suggest heterogeneity. Traditional opioids have poor affinity for this receptor and OFQ/N and its fragments do not label traditional opioid receptors. In brain homogenates, both OFQ/N and OFQ/N(1-11) inhibit forskolin-stimulated camp accumulation with IC₅₀values of 1 nM or less, an action which is readily reversed by opioid antagonists. OFQ/N(1-7) shows little activity. Together, these studies …