title: Loss of vasomotor responsiveness to the mu-opioid receptor ligand endomorphin-1 in adjuvant monoarthritic rat knee joints.
abstract: Endomorphin-1 is a short-chain neuropeptide with a high affinity fo the mu-opioid receptor and has recently been localized in acutely inflamed knee joints where it was found to reduce inflammation. The present study examined the propensity of endomorphin-1 to modulate synovial blood flow in normal and adjuvant-inflamed at knee joints. Under deep urethane anesthesia, endomorphin-1 was topically applied to exposed normal and 1 wk adjuvant monoarthritic knee joints (0.1 ml bolus; 10(-12)-10(-9) mol). Relative changes in articular blood flow were measured by laser Doppler perfusion imaging and vascular resistances in response to the opioid were calculated. In normal knees, endomorphin-1 caused a dose-dependent increase in synovial vascular resistance and this effect was significantly inhibited by the specific mu-opioid receptor antagonist d-Phe-Cys-Tyr-d-Trp-O n-Thr-Pen-Th amide (CTOP) (P < 0.0001, 2-factor ANOVA, n = 5-7). One week after adjuvant inflammation, the hypoaemic effect of endomorphin-1 was completely abolished (P < 0.0001, 2-factor ANOVA, n = 5-7). Immunohistochemical analysis of normal and adjuvant-inflamed joints showed a ninefold increase in endomorphin-1 levels in the monoarthritic knee compared with normal control. Western blotting and immunohistochemistry revealed a moderate number of mu-opioid receptors in normal knees; however, mu-opioid receptors were almost undetectable in arthritic joints. These findings demonstrate that peripheral administration of endomorphin-1 reduces knee joint blood flow and this effect is not sustainable during advanced inflammation. The loss of this hypoaemic response appears to be due to down regulation of mu-opioid receptors as a consequence of endomorphin-1 accumulation within the arthritic joint.
authors.affiliation: Department of Physiology & Biophysics, University of Calgary, 3330 Hospital Drive NW, Calgary, Alberta, Canada T2N 4N1. mcdougaj@ucalgary.ca
authors.forename: Jason J
authors.initials: JJ
authors.lastname: McDougall
authors.forename: A Kursat
authors.initials: AK
authors.lastname: Barin
authors.forename: Chelsea M
authors.initials: CM
authors.lastname: McDougall
mesh_terms: D000818:Animals; D001169:Arthritis, Experimental / Q000473:pathology / Q000503:physiopathology*; D015153:Blotting, Western; D004305:Dose-Response Relationship, Drug; D007150:Immunohistochemistry; D007596:Joints / Q000098:blood supply / Q000473:pathology / Q000503:physiopathology*; D008297:Male; D009131:Muscle, Smooth, Vascular / Q000187:drug effects / Q000502:physiology*; D009842:Oligopeptides / Q000494:pharmacology*; D051381:Rats; D017208:Rats, Wistar; D017450:Receptors, Opioid, mu / Q000037:antagonists & inhibitors / Q000187:drug effects*; D012039:Regional Blood Flow / Q000502:physiology; D013004:Somatostatin / Q000031:analogs & derivatives* / Q000494:pharmacology; D014655:Vascular Resistance
chemical_list: D009842:Oligopeptides; D017450:Receptors, Opioid, mu; C105293:endomorphin 1; C049352:phenylalanyl-cyclo(cysteinyltyrosyl-tryptophyl-ornithyl-threonyl-penicillamine)threoninamide; D013004:Somatostatin