Objective Quantification of Limb Position in Mouse Spared Nerve Injury Reveals New Motor Phenotypes

Abstract

Difficulties translating basic animal research into novel pharmacotherapies for neuropathic pain can, in part, be attributed to a lack of predictive efficacy in preclinical pain models. Most animal models of neuropathic pain rely heavily on targeted nerve injuries quantified using reflexive measures in response to an applied noxious stimulus. However, data from these approaches fail to capture spontaneous and ongoing pain – a critical component of neuropathic pain in human patients. Furthermore, the common spared nerve injury (SNI) model damages the tibial and common peroneal nerves that should result in motor phenotypes not typically captured in preclinical literature. To overcome these limitations, we employed deep learning-based markerless pose estimation software (DeepLabCut) to quantify spontaneous limb position in C57BL/6J mice during a one-minute tail suspension following either SNI or sham surgery. Using this granular detail, we were able to characterize previously unidentified injury-induced motor behaviors that persist up to six weeks following SNI. In particular, we found a decrease in the median distance between body midline and injured limb position in the SNI group compared to sham controls. We also found some evidence for a decrease in toe movement relative to the heel movement. We repeated these analyses in subsequent cohorts of SNI mice to determine whether these outputs could be reversed by multiple analgesics with diverse mechanistic actions and thus far do not see evidence that these behaviors are reversed by analgesics. Furthermore, we do not see these phenotypes in the Complete Freund's Adjuvant model of inflammatory pain. Together these results suggest SNI causes previously unobserved motor phenotypes unrelated to altered sensation that are likely underappreciated while interpreting preclinical pain research. Grant support from R01 NS117899-01 F31 NS124301-01 Rita Allen Foundation.