Abstract
The objective of this study was to establish an effective post-operative analgesic regimen for Sprague-Dawley (SD) and Dark Agouti (DA) rats. Buprenorphine (0.01 or 0.05 mg/kg), a partial µ opioid agonist, was administered subcutaneously immediately on completion of a standardized surgical procedure, involving anaesthesia, laparotomy and visceral manipulation. Two of the four treatment groups and the saline control group received a second injection 9 h later. Behavioural observations by three independent observers provided no information in assessing pain in this model. All rats lost weight, consumed less food and water after surgery. On the first day, both SD and DA rats receiving buprenorphine lost less weight than untreated control groups. Using weight loss as an efficacy criterion, low-dose buprenorphine, given once or twice, provided effective analgesia in SD rats. A higher single dose provided no additional benefit and a second dose was detrimental, reducing body weight and food intake. In DA rats, the high dose, given twice, appeared to be more effective than the lower dose. All DA cage cohorts consumed <10% pre-operative food despite buprenorphine treatment, suggesting a higher dosage may be necessary. However, all SD and 80% DA rats who received no buprenorphine gained body weight on the second day, whereas most of the buprenorphine-treated rats continued to lose weight for another 2 days, despite increased food consumption by both strains. Buprenorphine may adversely affect intestinal function over a number of days due to its enterohepatic circulation; this effect may be more severe in DA rats. Adverse metabolic effects of buprenorphine and other opioids may preclude their use in the future if it can be shown that non-steroidal anti-inflammatory drugs (NSAIDs) provide equally effective analgesia.