Comparison of the antinociceptive and antirewarding profiles of novel bifunctional nociceptin receptor/mu-opioid receptor ligands: implications for therapeutic applications

The nociceptin receptor (NOPr), a member of the Opioid Receptor family, is a target for the treatment of pain and drug abuse. Nociceptin/orphanin FQ (N/OFQ), the endogenous peptide for NOPr, not only modulates opioid antinociception, but also blocks the rewarding effects of several abused drugs, such as morphine, cocaine, and amphetamine. We hypothesized that NOPr agonists, with bifunctional activity at the mu-opioid receptor (MOPr), may function as nonaddicting analgesics or as drug abuse medications. Bifunctional small-molecule NOPr agonists possessing different selectivities and efficacies at MOPr were evaluated in an acute thermal antinociception assay, and for their ability to induce conditioned place preference (CPP) and their effect on morphine-induced CPP. 1-(1-Cyclooctylpiperidin-4-yl)-indolin-2-one) (SR14150), a high-affinity NOPr partial agonist, with low MOPr affinity and efficacy, produced analgesia that was naloxone-reversible. SR14150 did not induce CPP alone, nor did it attenuate morphine-induced CPP. 3-Ethyl-1-(1-(4-isopropylcyclohexyl)piperidin-4-yl)-indolin-2-one (SR16507), which has high affinity for both NOPr and MOPr, full agonist activity at NOPr, and partial agonist activity at MOPr, was also a potent analgesic and produced CPP alone, but also modestly attenuated morphine CPP. 1-(1-(2,3,3a,4,5,6-hexahydro-1H-phenalen-1-yl)piperidinl-4-yl)-indolin-2-one (SR16835), a NOPr full agonist and low-affinity MOPr partial agonist, was not antinociceptive, did not produce CPP alone, but attenuated morphine CPP. Our results suggest that NOPr full-agonist activity is required to modulate opioid-induced reward, whereas a bifunctional NOPr/MOPr partial agonist profile may be suitable as a nonaddicting analgesic. The opioid-modulating effects of the NOPr ligands may be used effectively to produce better medications for treatment of drug abuse and pain.