Orphanin FQ, agonist of orphan opioid receptor ORL₁, stimulates feeding in rats

It has long been known that opioids, whether administered exogenously or released endogenously, play a role in feeding behavior, and that blockade of opioid receptors with various antagonists can inhibit feeding. Recently, the orphan opioid receptor, ORL₁, and its presumed ligand, orphanin FQ, have been identified in human and rodent brain, and show structural similarity to classical opioid receptors and peptides, respectively. While it has been shown that central administration of orphanin FQ induces hyperalgesia, the effects of this peptide on food intake have not been studied. In the present study, we report that central injection of orphanin FQ induces feeding in satiated rats and that this effect can be blocked by peripheral administration of the opioid antagonist naloxone.