Papers of the Week

Studies using rodent models of neuropathic pain employ sham surgery control procedures that cause deep tissue damage. Sham surgeries would thus be expected to induce potentially long-lasting postsurgical pain, but little evidence for such pain has been reported. Operant tests of voluntary behavior can reveal negative motivational and cognitive aspects of pain that may provide sensitive tools for detecting pain-related alterations. In a previously described operant mechanical conflict (MC) test involving lengthy familiarization and training, rodents freely choose to either escape from a brightly lit chamber by crossing sharp probes or refuse to cross. Here, we describe a brief (2-day) MC protocol that exploits rats' innate exploratory response to a novel environment in order to detect persistently enhanced pain-avoidance behavior after sham surgeries for two neural injury models: thoracic spinal cord injury (SCI) and chronic constriction injury (CCI) of the sciatic nerve. Pitting the combined motivations to avoid the bright light and to explore the novel device against pain from crossing noxious probes disclosed a conflicting, hyperalgesia-related reluctance to repeatedly cross the probes after injury. Rats receiving standard sham surgeries demonstrated enhanced pain-like avoidance behavior compared to naive controls, and this behavior was similar to that of corresponding CCI or SCI rats weeks or months after injury. In the case of sham surgery for SCI, video analysis of voluntary exploratory behavior directed at the probes revealed enhanced forepaw withdrawal responses. These findings have important implications for preclinical investigations into behavioral alterations and physiological mechanisms associated with postsurgical and neuropathic pain.