Effects of Buprenorphine, Meloxicam, and Flunixin Meglumine as Postoperative Analgesia in Mice

C57BL/6NCrl male mice (n = 60; age, 6 to 7 wk) underwent partial hepatectomy or no surgery and were given 1 of 3 analgesics pre- and postoperatively. Food and water consumption, body weight, running wheel activity, locomotor activity, and serum corticosterone concentrations were measured before and after surgery. Mice that were surgically manipulated weighed significantly less on days 1 through 3 after surgery than did mice not manipulated surgically. On the day of surgery, the surgery groups consumed significantly less feed (–1.5 ± 0.35 g) than did nonsurgery groups. There were no differences in water consumption on any day between surgery and nonsurgery groups or among the 3 analgesic groups. For running wheel activity, significant decreases in the surgery groups were seen at day 1 after surgery compared with baseline. Surgery groups that received buprenorphine and meloxicam returned to baseline activity levels on day 2 after surgery. Open-field testing revealed no significant differences in locomotor activity in any groups; however, posttreatment locomotor activity in the buprenorphine nonsurgery group was increased compared with baseline, and posttreatment locomotor activity in the flunixin meglumine surgery group was decreased compared with baseline. Serum corticosterone concentrations were within normal limits regardless of treatment in all groups. Comparison of the overall results indicated that meloxicam and buprenorphine, at the dose given, appear to be suitable postoperative analgesics for partial hepatectomy in mice. Flunixin meglumine at the given dosage (2.5 mg/kg) may not provide adequate analgesia for partial hepatectomy.Abbreviation: NSAID, nonsteroidal antiinflammatory drugLaboratory animals commonly undergo procedures that may cause pain and distress. The Guide for the Care and Use of Laboratory Animals states, “An integral component of veterinary medical care is prevention or alleviation of pain associated with procedural and surgical protocols. Pain is a stressor and, if not relieved, can lead to unacceptable levels of stress and distress in animals.”²⁰ Such deleterious effects, while adversely affecting an animal''s wellbeing, may also confound research data and impede research study outcome. As a result, prevention or recognition and minimization or alleviation of pain and distress are integral components of an animal care and use program. In rats, unrelieved pain and distress after a surgical procedure has been shown to affect locomotor activity, body weight, and food and water intake.^12,31,32 Although mice comprise the majority of research animals, few studies have evaluated analgesia effectiveness in this species. Studies in mice have shown that the lack of postoperative analgesia can affect locomotor activity¹⁵ and food and water intake.¹⁹Locomotor activity testing involving an open field has been used as a method to assess exploratory and locomotor activity in mice.^{17,21,37,38,41,46,49,55} The assessment of unconditioned locomotor behavior in rodents has become one of the most widely used behavioral paradigms to determine the effects of various experimental manipulations ranging from genetic changes to pharmacologic challenges.^41,46 Levels of locomotor activity are measured by using a variety of methods based on the frequency of photocell beam breaks or distance traveled as assessed by video tracking.Running wheel activity has been used in research for a variety of experiments.^8,30,48 Running wheels have been used to study the hypothalamic–pituitary–adrenocortical axis, circadian rhythms, and social-stress-affected sleep in mice.^28,53 This parameter has also been used to monitor voluntary activity levels of mice by measuring distance traveled.^8,48Commonly used analgesics include buprenorphine hydrochloride, flunixin meglumine, and meloxicam. Buprenorphine hydrochloride is used in rodent surgical models^{13,15,22,26,32,45} primarily due to its longer duration of action over other opioid drugs. It is a partial µ opiate agonist and has a maximum biologic effect achieved regardless of the maximal dose given (ceiling effect).^7,34 The duration of effect in most species is 6 to 12 h.⁴⁴ The duration of buprenorphine in mice has been documented as 3 to 5 h.¹⁴ As a controlled substance, buprenorphine is regulated by the Drug Enforcement Administration, which requires strict storage and record keeping.Flunixin meglumine and meloxicam are nonsteroidal antiinflammatory drugs (NSAID) that reduce inflammatory pain by the inhibition of prostaglandin synthesis.^10,42 Because NSAID are not controlled substances, federal licensure and other restrictive measures are not necessary. Although the drugs are considered safe, NSAID-associated gastrointestinal bleeding is listed as a side effect reported with chronic use.⁴² A recent study by our laboratory found that flunixin meglumine did not interfere with embryo implantation when used in mice undergoing embryo transfer.¹⁸ We wanted to further investigate the properties of flunixin meglumine as an analgesic agent in a different mouse surgical model. Because meloxicam is a long-acting NSAID requiring once-daily dosing, we were interested in evaluating the effects of meloxicam in a mouse surgical model.Evaluations of physiologic parameters and results of behavioral tests are often used to assess pain in rodents; however, there are very few reports on assessing pain relief in mice. Various parameters have been used to monitor rodents during the postoperative period, including ethographic scoring,⁶ body weight changes, corticosterone concentrations, and food and water consumption.^{3,12,16,31,32} Our laboratory previously evaluated whether isoflurane anesthesia would alter the body weight, voluntary exercise or open-field locomotor activity of mice administered either flunixin, meloxicam, or buprenorphine. We found that isoflurane anesthesia had no effect on these parameters with any of the 3 analgesics.The purpose of the current study was to evaluate the effects of buprenorphine hydrochloride, flunixin meglumine, and meloxicam in mice after partial hepatectomy. We hypothesized that there would be no differences in physiologic parameters in mice with the use of the 3 analgesics. Partial hepatectomy is a surgical procedure performed frequently in our facility for research studies, and we were interested in investigating the effect of the 3 analgesics in this animal model. These mice were used specifically for this study to refine the analgesic protocol for this surgical procedure. Voluntary running-wheel activity, open-field locomotor activity, and serum corticosterone concentrations were used in conjunction with body weight and food and water consumption as objective parameters for assessment of analgesic effectiveness.