ANTINOCICEPTIVE EFFECTS OF ONO-9902, AN ENKEPHALINASE INHIBITOR, AFTER VISCERAL STRESS CONDITION IN RATS

Purpose: The present study was designed to examine the antinociceptive effects of orally administered ONO-9902, an enkephalinase inhibitor o n both somatic and visceral pain after visceral stress conditions. Met hods: Twenty six male rats were examined. Tail-Slick (TF) and colorect al distension (CD) tests were used to determine somatic and visceral a ntinociceptive effects, respectively. Measurements were performed in r ats under immediate post-stress conditions (group ST; n = 14) and in r ats nor under stress conditions (group NST; n = 12). In the stressed g roup, the same device, CD, for visceral antinociceptive effects was us ed for visceral stress and;was applied with an intracolonic pressure o f 60 mmHg for 20 min after drug administration. The TF latency and CD threshold were measured before and at 30, 40, 50, 60 and 90 min after administration of ONO-9902 300 mg . kg(-1) or distilled water. Results : Orally administered ONO-9902 did not produce any changes in the % ma ximum possible effect (%MPE) in either TF or CD tests in the unstresse d group. In the stressed group, %MPE in the CD test increased 18%, and 31% at 30 and 40 min, respectively, after oral administration of ONO- 9902 compared with the control group (P < 0.05). However, %MPE to TF t est did not alter even after the CD-induced stress condition. Conclusi on: These results suggest that ONO-9902 may have analgesic effects on visceral pain but not on somatic pain under immediate post-stress cond itions.