Expression of the nociceptin/orphanin FQ receptor in the intestinal mucosa of IBS patients.

Nociceptin/orphanin FQ (N/OFQ) and the N/OFQ peptide (NOP) receptor play important roles in regulating gastrointestinal function. To assess whether the NOP receptor is implicated in the etiopathogenesis of irritable bowel syndrome (IBS), we measured the levels of NOP receptor mRNA and protein in the jejunal and colonic tissues of healthy subjects and patients with diarrhea-predominant IBS (D-IBS) and constipation-predominant IBS (C-IBS). Mucosal biopsies were obtained from the jejunum and colon of patients diagnosed with D-IBS and C-IBS by the Rome III criteria and from healthy control subjects. The expression of NOP receptor mRNA was measured quantitatively using quantitative PCR (qPCR) and NOP protein expression was assayed immunohistochemically using a rabbit monoclonal antibody to OFQ. NOP receptor mRNA was detected in the jejunum and colon of healthy subjects and was more highly expressed in the jejunum than in the colon. Expression was lower in the jejunum and colon of patients with D-IBS; however, it was similar in patients with C-IBS and healthy subjects. The numbers of OFQ-positive cells in the jejunum and colon were similar among the three groups. The NOP receptor may be involved in the regulation of intestinal movement in healthy individuals. Its involvement in the pathophysiology of IBS may depend on whether the IBS is constipation- or diarrhea-predominant.