abstract:The nociceptin receptor or NOP also known as the orphanin FQ receptor or kappa-type 3 opioid receptor is a protein that in humans is encoded by the OPRL1 (opioid receptor-like 1) gene. The nociceptin receptor is a G protein-coupled receptor whose natural ligand is known as nociceptin or orphanin FQ, a 17 amino acid neuropeptide.