Effects of tramadol on behavioural indicators of colic pain in a rat model of ureteral calculosis

This study investigated the effect of prolonged administration of tramadol vs. placebo on behavioural indicators of ureteral pain and referred lumbar muscle hyperalgesia in a rat model of artificial ureteral calculosis. Four groups of 10 rats each (female, Sprague‐Dawley) were treated twice a day, for 4 days, with i.p. injections of tramadol 1.25 mg/kg, 2.5 mg/kg, 5 mg/kg or saline, respectively. The first injection was delivered 45 min before laparotomy (under pentobarbital anaesthesia) for formation of the stone in the upper left ureter via injection of dental cement. All rats were video‐taped 24 h non‐stop from the immediate postoperative period until the 4th day for recording of behavioural ureteral crises indicative of colic pain. Lumbar muscle sensitivity was tested daily over the same period by verifying presence or absence of vocalization upon pinching of the parietal layers at L1 level, bilaterally, at a constant predefined pressure value with calibrated forceps. Tramadol significantly reduced number and global duration (ANOVA, P  < 0.008 and P  < 0.004) of ureteral crises with respect to saline and the effect was dose‐dependent (linear regression analysis between doses and parameters of crises, P  < 0.003 and P  < 0.002). The drug also significantly reduced the incidence of referred muscle hyperalgesia (ANOVA, P  < 0.0001). It is concluded that tramadol is highly effective in controlling pain phenomena from urinary stones and can represent a valid therapeutic approach in patients with urinary colics.