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, on both somatic and visceral pain after visceral stress conditions. METHODS: Twenty six male rats were examined. Tail-flick (TF) and colorectal distension (CD) tests were used to determine somatic and visceral antinociceptive effects, respectively. Measurements were performed in rats under immediate post-stress conditions (group ST; n = 14) and in rats nor under stress conditions (group NST; n = 12). In the stressed group, the same device, CD, for visceral antinociceptive effects was used for visceral stress and was applied with an intracolonic pressure of 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 % maximum possible effect (%MPE) in either TF or CD tests in the unstressed 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 test did not alter even after the CD-induced stress condition. CONCLUSION: These results suggest that ONO-9902 may have analgesic effects on visceral pain but not on somatic pain under immediate post-stress conditions.