Orphanin FQ causes contractions via inhibiting purinergic pathway in the rat colon

Background & Aims: We have previously shown that orphanin FQ (OFQ) preferentially stimulates muscle contraction in the rat colon. However, the mechanism of action of OFQ remains unclear. Methods: We studied the effects of OFQ on muscle contractions and inhibitory junction potentials (IJPs) in rat colon. The site of action of OFQ was also investigated by in situ hybridization of OFQ receptors. Results: OFQ (10(-10) to 10(-6) mol/L) caused circular muscle contractions that were blocked by tetrodotoxin (10(-7) mol/L), suggesting the contractions were nerve mediated. Suramin (a nonselective P-2-purinoceptor antagonist; 10(-4) mol/L) and reactive blue 2 (a P-2Y-purinoceptor antagonist; 3 x 10(-5) mol/L), but not pyridoxalphosphate-6-azophenyl-2',4' disulfonic acid (PPADS; a P-2X-purinoceptor antagonist; 3 x 10(-5) mol/L), abolished OFQ-induced colonic contractions. Focal stimulation of interganglionic fiber tracts evoked biphasic IJPs in colonic circular muscle cells. Suramin and reactive blue 2 inhibited the peak amplitude of the IJP, whereas PPADS had no effect. Cumulative addition of OFQ (10(-10) to 10(-6) mol/L) significantly inhibited the IJPs. In situ hybridization revealed that OFQ receptor messenger RNA was expressed in the colonic myenteric plexus but not in the smooth muscle cells, suggesting that the site of action of OFQ is neuronal. Conclusions: These results suggest that OFQ causes muscle contractions by inhibiting purinergic inhibitory motorneurons in the rat colon.