Activities of mixed NOP and μ-opioid receptor ligands

Background and purpose: Compounds that activate both NOP and mu-opioid receptors might be useful as analgesics and drug abuse medications. Studies were carried out to better understand the biological activity of such compounds. Experimental approach: Binding affinities were determined on membranes from cells transfected with NOP and opioid receptors. Functional activity was determined by [S-35] GTP gamma S binding on cell membranes and using the mouse vas deferens preparation in vitro and the tail flick antinociception assay in vivo. Key results: Compounds ranged in affinity from SR14150, 20-fold selective for NOP receptors, to buprenorphine, 50-fold selective for mu-opioid receptors. In the [S-35] GTP gamma S assay, SR compounds ranged from full agonist to antagonist at NOP receptors and most were partial agonists at mu-opioid receptors. Buprenorphine was a low efficacy partial agonist at mu-opioid receptors, but did not stimulate [S-35] GTP gamma S binding through NOP. In the mouse vas deferens, each compound, except for SR16430, inhibited electrically induced contractions. In each case, except for N/OFQ itself, the inhibition was due to mu-opioid receptor activation, as determined by equivalent results in NOP receptor knockout tissues. SR14150 showed antinociceptive activity in the tail flick test, which was reversed by the opioid antagonist naloxone. Conclusions and implications: Compounds that bind to both mu-opioid and NOP receptors have antinociceptive activity but the relative contribution of each receptor is unclear. These experiments help characterize compounds that bind to both receptors, to better understand the mechanism behind their biological activities, and identify new pharmacological tools to characterize NOP and opioid receptors.