THE EFFECT OF INTRATHECAL OPIOID-RECEPTOR AGONISTS ON VISCERAL NOXIOUS-STIMULATION IN RABBITS

Background & Aims: Conflicting results have been published concerning the effects of different opioid-receptor agonists against visceral nox ious stimulation, The introduction of colorectal distention facilitate s research in this field, The aim of this study was to examine intrath ecally administered opioid agonists against colorectal distention in c onscious rabbits, Methods: Rabbits were equipped with a subcutaneous i ntrathecal injection system, Colorectal distention was induced by infl ation of a balloon inserted into the descending colon. The test parame ter was the pressure eliciting a characteristic visceromotor response, Examinations were performed before and after administration of the fo llowing drugs: morphine, U50488H, [D-Pen(2), D-Pen(5)]enkephalin (DPDP E), naloxone, MR2266, naltrindole, saline, and acidified saline, Resul ts: The visceromotor response to colorectal distention was inhibited i n a dose-dependent fashion by intrathecal opioids acting as agonists a t all three types of opioid receptors. Morphine was antagonized more e ffectively by intrathecal than intramuscular naloxone, U50488H and DPD PE were equally antagonized by the specific antagonists MR2266 and nal trindole, Electrical thresholds in the lumbar region were increased, a lthough they remained unaltered in the cervical region after administr ation of all three agonists, Conclusions: Intrathecal administration o f different opioid agonists produces a dose-dependent spinal effect, T he rank order of potencies in this model is DPDPE > U50488H > morphine > saline = 0.