Papers of the Week

This study evaluated a battery of pain-stimulated, pain-depressed, and pain-independent behaviors for preclinical pharmacological assessment of candidate analgesics in mice. Intraperitoneal injection of dilute lactic acid (IP acid) served as an acute visceral noxious stimulus to produce four pain-related behaviors in male and female ICR mice: stimulation of (1) stretching and (2) facial grimace, and depression of (3) rearing and (4) nesting. Additionally, nesting and locomotion in the absence of the noxious stimulus were used to assess pain-independent drug effects. These six behaviors were used to compare effects of two mechanistically distinct but clinically-effective positive controls (ketoprofen and oxycodone), and two negative controls that are not clinically approved as analgesics but produce either general motor depression (diazepam) or motor stimulation (amphetamine). We predicted that analgesics would alleviate all IP acid effects at doses that did not alter pain-independent behaviors, whereas negative controls would not. Consistent with this prediction, ketoprofen (0.1-32 mg/kg) produced the expected analgesic profile, whereas oxycodone (0.32-3.2 mg/kg) alleviated all IP acid effects except depression of rearing at doses lower than those that altered pain-independent behaviors. For the negative controls, diazepam (1-10 mg/kg) failed to block IP acid-induced depression of either rearing or nesting, and only decreased IP acid-stimulated behaviors at doses that also decreased pain-independent behaviors. Amphetamine (0.32-3.2 mg/kg) alleviated all IP acid effects, but only at doses that also stimulated locomotion. These results support utility of this model as a framework to evaluate candidate-analgesic effects in a battery of complementary pain-stimulated, pain-depressed, and pain-independent behavioral endpoints. Preclinical assays of pain and analgesia often yield false-positive effects with candidate analgesics. This study used two positive-control analgesics (ketoprofen, oxycodone) and two active negative controls (diazepam, amphetamine) to validate a strategy for distinguishing analgesics from non-analgesics by profiling drug effects in a battery of complementary pain-stimulated, pain-depressed, and pain-independent behaviors in male and female mice.