Validity of acute and chronic tactile sensory testing after spinal cord injury in rats

Spinal cord injury (SCI) impairs sensory systems causing allodynia. Measuring the development of allodynia in rodent models of SCI is challenging due to spinal shock and marked motor impairments. Assessment of SCI-induced allodynia is not standardized across labs, making interpretation of results difficult. Therefore, we validated sensory threshold assessment after SCI and developed a novel assessment of allodynia prior to motor recovery in a rat SCI model. One hundred fifty-six Sprague–Dawley rats received T8 laminectomy or mild to moderate SCI using the OSU SCI device (0.3 mm to 1.3mm cord displacement). To determine tactile thresholds, von Frey hairs (VFH) were applied in Up–Down or ascending order to the dorsal or plantar hindpaw. The most efficient and valid procedures that maintain high sensitivity and specificity were identified. Ten Up–Down VFH applications yielded stable thresholds; reducing the risk of threshold decay and unnecessary exposure to painful stimuli. Importantly, distraction of SCI-rats with food revealed differential decay of thresholds than when distraction is not provided. The new test uses dorsal VFH stimulation and is independent of trunk or hindlimb control. Acute dorsal VFH thresholds collected before recovery of hindlimb weight support accurately predicted plantar VFH thresholds measured at late timepoints (χ² = 8.479; p < 0.05). Thus, standardized testing early after SCI using the dorsal VFH test or later using 10 stimuli in the Up–Down test produces valid measures of tactile sensation across many SCI severities. Early detection of allodynia in experimental SCI will allow identification of mechanisms responsible for pain development and determine targets for therapeutic interventions.

Research Highlights
► The Up–Down method is a valid, accurate and efficient method to assess tactile sensation following spinal cord injury.
► Distraction of rats with food stabilizes paw withdrawal thresholds.
► Acute testing of tactile sensation prior to the recovery of hindlimb weight support using the new dorsal von Frey Hair test can accurately predict the development of chronic plantar withdrawal thresholds.