Nociceptin/orphanin FQ: role in nociceptive information processing

Recently, opioid receptor like1 (ORL1) receptor was identified. The ORL1 receptor is a G protein coupled receptor and the sequence of the ORL1 receptor is closely related to that of the opioid receptors. Nociceptin/orphanin FQ has been identified as a potent endogenous agonist of the ORL1 receptor and the sequence of nociceptin/orphanin FQ is closely related to that of dynorphin A. Nociceptin/orphanin FQis not active at the classical opioid receptors, such as mu, kappa and delta receptors. The distribution of prepronociceptin mRNA is distinct from that of the opioid peptide precursor. Mice lacking the ORL1 receptor showed no significant differences in nociceptive threshold compared with wild mice. The role of nociceptin/orphanin FQ on nociceptive transmission is unclear. Intracerebroventricular (i.c.v.) injection of nociceptin/orphanin FQ produced hyperalgesia and allodynia and antagonized morphine analgesia. On the other hand, intrathecal injection of low dose nociceptin/orphanin FQ produces allodynia, but high dose of nociceptin/orphanin FQ produces an analgesic effect. Although we do not fully understand the mechanisms that produce the difference between the effect of i.c.v. injection of nociceptin/orphanin FQ and that of intrathecal injection of nociceptin/orphanin FQ, we believe that spinal ORL1 receptor may be the next receptor which should be targeted by drugs designed for the treatment of pain.