Summary.

Since its discovery in 1995, nociceptin (N/OFQ) and its N-terminal tridecapeptide sequence (N/OFQ (1-13)) were objects of intense investigation in order to ascertain their innumerous biological properties. Later, after chemical substitution of individual amino acids, the effects on nociception of the newly synthesized analogues were investigated. The aim of the present study was to investigate whether newly synthesized analogues of N/OFQ (1-13), where Lys at position 9 and/or 13 was substituted by ornitine (Orn), diaminobutanoic acid (Dab) or diaminopropanoic acid (Dap) had analgesic effects, and the involvement of the opioidergic system in these effects. The experiments were carried out on male Wistar rats. The changes in the mechanical nociceptive pain threshold of the rats were measured by paw pressure test. Nociceptin (1-13) and the newly synthesized analogues were administered intraperitoneally (i.p.). Our data showed that nociceptin (1-13), [Orn9]N/OFQ(1-13)NH2, [Orn13]N/OFQ(1-13)NH2, [Orn9,13]N/OFQ(1-13)NH2 and [Dab9] NO/FQ (1-13) exerted a naloxone-reversible analgesic effect in which the opioidergic system was involved.