µ-opioid receptor, β-endorphin, and cannabinoid receptor-2 are increased in the colonic mucosa of irritable bowel syndrome patients.

Background And Aims: The gut immune, cannabinoid, and opioid systems constitute an integrated network contributing to visceral sensation and pain modulation. We aimed to assess the expression of the µ-opioid receptor (MOR), its ligand β-endorphin (β-END), and cannabinoid receptor-2 (CB ) in patients with irritable bowel syndrome (IBS) and asymptomatic controls (AC) and their correlation with sex and symptom perception.

Methods: Mucosal biopsies were obtained from the left colon of 31 IBS patients (45% women) with predominant constipation (IBS-C, 9) or diarrhea (IBS-D, 10) or with mixed bowel habits (IBS-M, 12) and 32 AC (44% women) and processed for qRT-PCR, Western blotting, and immunohistochemistry.

Key Results: µ-opioid receptor and CB mRNA and protein expression and β-END protein levels were increased in patients with IBS compared to AC (all Ps=0.021). A significant sex by IBS interaction was found in relation to CB mRNA expression (P = .003) with women showing a markedly higher expression to men (P = .035). In contrast, in AC, men had higher expression than women (P = .033). β-END, MOR, and CB immunoreactivities (IR) were localized to CD4+T cells including EMR-1+ eosinophils and CD31+ T cells but not to mast cells.

Conclusions: The increased expression of MOR, β-END, and CB in the mucosa of IBS patients, where they are localized to immune cells, suggests that opioid and cannabinoid systems play an immune-related compensatory role in visceral pain in IBS patients. Further work is necessary to support this hypothesis.