Ligand of the opioid receptor-like receptor OPRL1. It may act as a transmitter in the brain by modulating nociceptive and locomotor behavior. May be involved in neuronal differentiation and development. Blocks nociceptin action in pain transmission by inhibiting nociceptin-induced hyperalgesia and allodynia. Has potent analgesic activity. Specific enzymatic cleavages at paired basic residues probably yield other active peptides besides nociceptin. The N-terminal domain contains 6 conserved cysteines thought to be involved in disulfide bonding and/or processing. Belongs to the opioid neuropeptide precursor family.