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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^-^1^0 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 moVL) and reactive blue 2 (a P"2"Y-purinoceptor antagonist; 3 x 10^-^5 moVL), but not pyridoxalphosphate-6-azophenyl-2',4' disulfonic acid (PPADS; a P"2"X-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 UP, whereas PPADS had no effect. Cumulative addition of OFQ (10^-^1^0 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.

Background & Aims: The pathophysiological role of neuronal nitric oxide synthase (nNOS) in colitis remains unknown. Methods: We investigated colonic transit, nonadrenergic, noncholinergic (NANC) relaxation, nNOS activity, and nNOS synthesis in the myenteric plexus in dextran sulfate sodium (DSS)-induced colitis in rats. Results: Oral administration of 5% DSS for 7 days induced predominant distal colitis and delayed colonic transit. In the proximal colon, carbachol-, sodium nitroprusside-, and electrical field stimulation (EFS)-induced responses were not different between control and DSS-treated rats. In the distal colon, EFS-evoked cholinergic contraction, NANC relaxation, and orphanin FQ-induced contraction were significantly impaired in DSS-treated rats compared with those in control rats, but carbachol- and sodium nitroprusside-induced responses remained intact in DSS-treated rats. The number of nNOS-immunopositive cells, catalytic activity of NOS, and nNOS synthesis in the colonic wall were significantly reduced in the distal colon of DSS-treated rats. In contrast, the number of PGP 9.5-immunopositive cells and PGP 9.5 synthesis in the colonic wall remained intact in the distal colon of DSS-treated rats. Conclusions: These results suggest that impaired NANC relaxation in the distal colon is associated with reduced activity and synthesis of nNOS in the myenteric plexus in DSS-induced colitis. GASTROENTEROLOGY 2000;118:714-723