THE ORL(1) receptor, an orphan receptor whose human(1) and murine(2-8) complementary DNAs have recently been characterized, structurally resembles opioid receptors and is negatively coupled with adenylate cyclase(1). ORL(1) transcripts are particularly abundant in the central nervous system. Here we report the isolation, on the basis of its ability to inhibit the cyclase in a stable recombinant CHO(ORL(1)(+)) cell line, of a neuropeptide that resembles dynorphin A(9) and whose amino acid sequence is Phe-Gly-Gly-Phe-Thr-Gly-Ala-Arg-Lys-Ser-Ala-Arg-Lys-Leu-Ala-Asn- Gln. A rat-brain cDNA encodes the peptide flanked by Lys-Arg proteolytic cleavage motifs. The synthetic heptadecapeptide potently inhibits adenylate cyclase in CHO(ORL(1)(+)) cells in culture and induces hyperalgesia when administered intracerebroventricularly to mice. Taken together, these data indicate that the newly discovered heptadecapeptide is an endogenous agonist of the ORL(1) receptor and that it may be endowed with pro-nociceptive properties.