KAPPA-OPIOID, BUT NOT MU-OPIOID OR DELTA-OPIOID, ATTENUATE RESPONSES TO DISTENSION OF AFFERENT-FIBERS INNERVATING THE RAT COLON

Background & Aims: Discomfort and pain are the principal conscious sen sations that arise from the viscera, and both are increased in frequen cy and intensity in patients with a functional bower disorder, Viscera l receptors, perhaps sensitized, may contribute to these altered sensa tions, The aim of this study was to evaluate the effects of opioid rec eptor-selective agonists on afferent fibers innervating the colon. Met hods: Mechanosensitive pelvic nerve afferent fibers were recorded from the decentralized S-1 dorsal root in anesthetized rats. The effects o f opioid agonists, given intra-arterially, were studied based on the f iber's responses to noxious colorectal distention (CRD) (80 mm Hg, 30 seconds), Results: A total of 115 distention-sensitive fibers innervat ing the colon were studied, including 32 that were studied after colon ic inflammation with 2.5% acetic acid, Neither mu- (morphine and fenta nyl) nor delta- ([D-Pen(2),D-Pen(5)]enkephalin- and SNC-80) opioid rec eptor agonists affected responses to CRD, In contrast, kappa- (U-50,48 8 and fedotozine) opioid receptor agonists dose-dependently attenuated responses to CRD, Acetic acid sensitized about half of the fibers stu died, but neither the potency nor the efficacy of U-50,488 or FDZ were changed after colonic inflammation, Conclusions: These results sugges t a role for peripheral kappa-opioid receptors in the modulation of vi sceral nociception.