Possible involvement of endogenous nociceptin/orphanin FQ in the pain-related behavioral responses induced by its own metabolite, nociceptin/orphanin FQ(14–17)

Nociceptin/orphanin FQ(14–17) (N/OFQ(14–17)) is one of the major fragments that are released from N/OFQ, an endogenous ligand for the opioid receptor like-1 (ORL-1) receptor by endopeptidase 24.11. In the present study, we determined the pharmacological profiles of N/OFQ(14–17) on pain-related behavioral responses in the mouse. Intrathecal (i.t.) administration of N/OFQ(14–17) (5–160 pmol) evoked pain-related behaviors, and these behavioral responses were reduced by i.t. co-administration of an ORL-1 receptor antagonist, [Nphe1]N/OFQ(1–13)NH2 (4 pmol). However, in the ligand-binding receptor assay, N/OFQ(14–17) had no affinity for the ORL-1 receptor. Furthermore, i.t. pretreatment with an antiserum against N/OFQ (1:50) diminished the N/OFQ(14–17)-induced pain-related behaviors, suggesting that endogenous N/OFQ is involved in their expression. Therefore, N/OFQ(14–17)-induced pain-related behaviors may be mediated through the release of endogenous N/OFQ in the mouse spinal cord.