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In this study, the authors evaluated the analgesic efficacy of tramadol (an opioid-like analgesic), carprofen (a nonsteroidal anti-inflammatory drug) and a combination of both drugs (multimodal therapy) in a rat laparotomy model. The authors randomly assigned rats to undergo either surgery (abdominal laparotomy with visceral manipulation and anesthesia) or anesthesia only. Rats in each group were treated with tramadol (12.5 mg per kg body weight), carprofen (5 mg per kg body weight), a combination of tramadol and carprofen (12.5 mg per kg body weight and 5 mg per kg body weight, respectively) or saline (anesthesia control group only; 5 mg per kg body weight). The authors administered analgesia 10 min before anesthesia, 4 h after surgery or (for the rats that received anesthesia only) anesthesia and 24 h after surgery or anesthesia. They measured locomotor activity, running wheel activity, feed and water consumption, body weight and fecal corticosterone concentration of each animal before and after surgery. Clinical observations were made after surgery or anesthesia to evaluate signs of pain and distress. The authors found that carprofen, tramadol and a combination of carprofen and tramadol were all acceptable analgesia regimens for a rat laparotomy model.

Tramadol is an opioid-like analgesic with relatively mild side effects. Because it is inexpensive and is not classified as a controlled substance by the US federal government, the authors wanted to evaluate its applicability as a practical and effective analgesic in male Sprague Dawley rats. They measured the efficacy of four dosages (4, 12.5, 25 or 50 mg tramadol per kg body weight) and three routes of administration (per os (p.o.) in a flavored gelatin cube, subcutaneous (s.c.) or intraperitoneal (i.p.)) using the hot-plate test and the tail-flick test, which were carried out 1 week apart. Rats that were dosed p.o. were given flavored gelatin cubes without tramadol on the 2 d before testing to help them become acclimated to the gelatin, in an effort to increase the likelihood that they would consume the gelatin on the testing day. Results from the hot-plate and tail-flick tests for rats that were given tramadol p.o. were similar before and after administration, regardless of tramadol dosage, suggesting that this route of administration was not effective. The s.c. route of administration was effective at dosages of 25 mg and 50 mg tramadol per kg body weight, although these dosages also resulted in sedation and skin lesions. The i.p. route of administration was also effective at dosages of 12.5 mg, 25 mg and 50 mg tramadol per kg body weight, though sedation was observed at dosages of 25 mg and 50 mg per kg body weight. Intraperitoneal administration of 12.5 mg tramadol per kg body weight had no notable side effects, and the authors plan to further study this dosage and route of administration in a rodent surgical model of pain.