Acetic Acid-Induced Writhing

The acute visceral pain was induced by i.p. acetic acid injection refer to previous studies,¹⁷ which consisted of administering 10 mL/kg of 0.6% acetic acid into the abdominal cavity of animals. Before acetic acid injection, all rats were acclimatized to testing chamber for at least 30 min individually. Immediately after injection, the rats were returned to the testing chamber and spontaneous pain behaviors were observed for 60 min. The number of abdominal writhing episodes (contraction of the abdominal area with elongation of hind legs) were recorded for 60 min starting from 5 min after acetic acid injection and the latency from acetic acid injection to the first writhing were also recoded in each rat. In the present study, we performed acetic acid-induced writhing assay to evaluate the effect of ulinastatin on visceral pain because this assay has been widely used to determine the degree of anti-nociception of opioids and NSAIDs, and it has been suggested as a screening tool for assessing the analgesic or anti-inflammatory agents.^18–21 For evaluating the preventive effect of ulinastatin on acetic acid-induced acute visceral pain, ulinastatin (2000, 10,000 and 50,000 U/kg) or saline were i.p. administered 10 min prior to acetic acid injection. For evaluating the therapeutic effect of ulinastatin on acetic acid-induced acute visceral pain, ulinastatin (50,000 U/kg) or saline were i.p. administered 5 min post acetic acid injection.