Assessment of an Orofacial Operant Pain Assay as a Preclinical Tool for Evaluating Analgesic Efficacy in Rodents

A model system capable of providing clinically relevant analgesic doses with minimal trauma has been elusive in laboratory animal medicine. Our laboratory has developed an orofacial operant pain system that effectively discriminates between nonnoxious and noxious thermal stimuli in rats and mice. Male and female rats (Crl:SD) and mice (Crl:SKR-HR^hr) were trained to perform a task (placing their face through an opening and having their cheeks stay in contact with thermodes) to receive a reward (a solution of sweetened condensed milk). Currently accepted doses of buprenorphine were tested by using a crossover design. Pain was induced in both species by sensitizing the depilated skin over both cheeks with capsaicin cream or by creating a surgical incision (rats only) and then allowing the animals to contact a temperature-regulated thermode while obtaining a reward. Optimal antinociceptive doses included 0.05 and 0.1 mg/kg in male mice but only 0.05 mg/kg in female mice. In rats, optimal antinociceptive doses included 0.03 and 0.05 mg/kg for male rats but only 0.03 mg/kg for female rats. The 2 pain-induction models in rats (capsaicin cream and surgical incision) did not differ. Our orofacial operant pain assay can determine clinically relevant analgesic doses for rodents in a preclinical assay. The automated, investigator-independent nature of the assay, in conjunction with its high sensitivity, makes this method an improvement over traditional noninvasive methods, providing better data for developing optimal analgesic recommendations for rats and mice.Abbreviation: TRVP1, transient receptor vanilloid receptor potential 1The relief of postsurgical pain in rats and mice has been a major topic of discussion during the past decades. However, the availability of a model system capable of assessing clinically relevant analgesic doses in the context of minimal trauma has been elusive in laboratory animal medicine. Traditionally, the methods for assessing analgesic efficacy in rodents have included thermal (for example, hot-plate test, tail-flick test) and mechanical (for example, von Frey filament) assays. Albeit simple to perform and nontraumatic when performed correctly, these assays may not represent the best way to assess analgesic clinical efficacy, given that they rely on either reflex-derived outcomes (for example, tail-flick assay) or unlearned, brainstem-mediated responses (for example, grooming).^14,15Postoperative behavior-based pain assessment techniques, including the recently described facial grimace scale, have been used as a more comprehensive method for assessing clinical analgesic efficacy in rats and mice.^20-22 The basic premise in these assays consists of quantifying the relative magnitude of behavioral alterations after producing an injury, typically a laparotomy. Although these models have improved the assessment accuracy for recommended analgesic dosages, they can be time-consuming to perform.⁹ In addition, these methods typically include a vehicle- or untreated control group, which introduces the ethical dilemma of uncontrolled pain in regard to some animals.The lack of clinical relevancy of the noninvasive methods on the one hand and the complexity of the behavior-based pain models on the other has limited the number of clinically effective analgesics or analgesic combinations that can be used effectively in rodents to minimize postoperative pain. As a consequence, several commonly available, centrally acting analgesics, such as tramadol, and multimodal analgesic combinations frequently used in human and other species remain underused in rodents. For this reason, there is still a need for a preclinical, minimally invasive pain model system that evaluates pain throughout the entire neuraxis, thus increasing the probability of providing clinically relevant analgesic doses.Our laboratory has developed an orofacial operant pain system that effectively discriminates between nonnoxious and noxious thermal stimuli in rats and mice.^1,15,16 The system uses a reward-conflict paradigm in which animals choose between obtaining a reward in the presence of a thermal nociceptive stimulus and avoiding both the stimulus and the reward. The objective of the current study was to evaluate the analgesic effects of currently recommended doses of buprenorphine in our novel orofacial operant pain system. Our hypothesis was that this operant assay would reliably predict an analgesic effect of buprenorphine within the currently published dose range and would clarify optimal doses for male and female rats and mice.