TONIC ENDOGENOUS OPIOID INHIBITION OF VISCERAL NOXIOUS INFORMATION INRABBITS

Background & Aims: A tonic intrinsic spinal inhibitory system on spina l motor reflexes in rabbits has been shown earlier. The aim of this st udy was to examine the effects of different opioid antagonists against visceral noxious stimulation in awake rabbits. Methods: The opioid re ceptor antagonists examined were naloxone (nonselective), MR2266 (kapp a), and naltrindole (delta). The effects on the visceromotor response thresholds induced by colorectal distention in rabbits were determined after intrathecal and intramuscular administration of the antagonists . Results: Intrathecal naloxone resulted in a dose-dependent decrease of visceromotor response thresholds. The selective antagonists MR2266 and naltrindole had no significant effects. In the presence of MR2266, intrathecal naloxone reduced thresholds to the same degree as when gi ven alone. Analysis of the data from all rabbits showed a statisticall y significant reduction in visceromotor response thresholds after intr athecal naloxone compared with intramuscular administration. Conclusio ns: In rabbits, tonic active intrinsic spinal and supraspinal endogeno us nous opioids modulate visceral noxious information. This inhibition is exerted at the mu opioid receptor.