Cloning, expression and functional role of a nociceptin/orphanin FQ receptor in the porcine gastrointestinal tract

The heptadecapeptide nociceptin/orphanin FQ is the cognate ligand for the opioid receptor-like orphanin FQ (OFQ) receptor, a member of the G protein-coupled receptor superfamily. The gastrointestinal tract is a major site of opioid action, and preliminary evidence suggests that an OFQ receptor may be expressed in rat small intestine. We addressed the hypothesis that this receptor is expressed in the gastrointestinal tract of the pig, a model for the human digestive system. A 1205-bp cDNA was isolated from porcine forebrain which contained the 370 amino acid open reading frame encoding the OFQ receptor. The receptor mRNA is likely to arise from a single gene, as determined by Southern blotting of porcine genomic DNA restriction digests using a porcine OFQ receptor cDNA probe. A semi-nested reverse transcriptase–polymerase chain reaction survey of receptor mRNA indicates that it is expressed in the porcine cerebral cortex and kidney, and along the length of the gastrointestinal tract. OFQ decreased initial contractile responses of porcine ileal smooth muscle strips to trains of electrical field stimulation with an IC 50 value of 1.3 nM; its effects were resistant to the opioid antagonist, naloxone. The peptide, at concentrations≥3 nM, also attenuated I sc elevations evoked by electrical transmural stimulation of mucosa–submucosa sheets from porcine ileum. The actions of OFQ appeared to differ from those previously reported for opioid receptor agonists in these tissue preparations. These results indicate that an OFQ receptor is expressed in the porcine intestine which modulates the neural control of intestinal smooth muscle contractility and mucosal transport.