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. When administered intracerebroventricularly, nociceptin induces hyperalgesia and decreases locomotor activity. Blocks nociceptin action in pain transmission by inhibiting nociceptin-induced hyperalgesia and allodynia. Has potent analgesic activity. Brain and spinal cord. Low levels in kidney and spleen. In embryonic brain, first detected at day 14 and in postnatal brain, levels increase in day 1 and day 18. Levels decrease significantly in adults. 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.