The Nociceptin/Orphanin Fq Receptor System As A Target To Alleviate Cancer-Induced Bone Pain In Rats: Model Validation And Pharmacological Evaluation

Background and Purpose Cancer-induced bone pain remains inadequately controlled, and current standard of care analgesics is accompanied by several side effects. Nociceptin/orphanin FQ peptide (NOP) receptor agonists have demonstrated broad analgesic properties in rodent neuropathic and inflammatory pain models. Here, we investigate the analgesic potential of NOP receptor activation in a rodent cancer-induced bone pain model. Experimental Approach Model validation by intratibial inoculation in male Sprague Dawley rats was performed with varying MRMT-1/Luc2 cell quantities (0.5-1.5 x 10(6)center dot ml(-1)) and a behavioural battery (>14 days post-surgery) including evoked and non-evoked readouts: paw pressure test, cold plate, von Frey, open field, and weight distribution. Anti-allodynic potential of the endogenous NOP receptor ligand nociceptin (i.t.) and NOP receptor agonist Ro65-6570 ( i.p.) was tested using von Frey filaments, followed by a combination experiment with Ro65-6570 and the NOP receptor antagonist J-113397 (i.p.). Plasma cytokine levels and NOP receptor gene expression in dorsal root ganglion (DRG, L4-L6) and bone marrow were examined. Key Results Inoculation with 1.5 x 10(6)center dot ml(-1) of MRMT-1/Luc2 cells resulted in a robust and progressive pain-related phenotype. Nociceptin and Ro65-6570 treatment inhibited cancer-induced mechanical allodynia. J-113397 selectively antagonized the effect of Ro65-6570. MRMT-1/Luc2-bearing animals demonstrated elevated plasma cytokine levels of IL-4, IL-5, IL-6 and IL-10 plus unaltered NOP-r gene expression in DRG and reduced expression in bone marrow. Conclusion and Implications Nociceptin and Ro65-6570 selectively and dose-dependently reversed cancer-induced bone pain-like behaviour. The NOP receptor system may be a potential target for cancer-induced bone pain treatment.