Normal sensitivity to acute pain, but increased inflammatory hyperalgesia in mice lacking the nociceptin precursor polypeptide or the nociceptin receptor.

Nociceptin/orphanin FQ (N/OFQ) is the endogenous agonist of the N/OFQ peptide receptor (NOP receptor). It is released from a larger precursor polypeptide, called prepro-nociceptin (ppN/OFQ) from which, in addition to N/OFQ, other biologically active neuropeptides may be derived. Increasing evidence indicates that exogenous application of N/OFQ to the central nervous system of mice and rats induces pro- and antinociceptive effects depending on the dose and site of administration. Much less is known about a potential contribution of endogenous N/OFQ to pain control. Here, we have used a genetic approach to address this topic. Mice deficient in either the NOP receptor (NOP-R-/- mice) or the N/OFQ precursor polypeptide (ppN/OFQ-/- mice) or both (double knockout mice) were compared with wild-type littermates in animal models of acute and tonic pain. Nociceptive responses to acute noxious heat of all three types of mutant mice were indistinguishable from those of wild-type mice. Accordingly, nociceptive behaviour was very similar in the early phase of the formalin test. However, NOP-R-/-, ppN/OFQ-/- and double knockout mice showed markedly stronger nociceptive responses during prolonged nociceptive stimulation in the second phase of the formalin test and significantly lower thermal pain thresholds in inflamed tissue after zymosan A injection. These results indicate that N/OFQ contributes significantly to endogenous pain control during prolonged nociceptive stimulation but does not affect acute pain sensitivity. Among the three types of mutant mice nociceptive behaviour was nearly identical, indicating that the lack of other potential ppN/OFQ products in the ppN/OFQ-/- mice was apparently without effect on the nociceptive phenotype.