Ethological profiling defines pain behaviors and parses analgesia from drug side effects in a mouse model of complex regional pain syndrome.

Abstract

Complex regional pain syndrome (CRPS) is a form of chronic postinjury pain affecting the extremities with contributions from the somatic and sympathetic nervous systems. The mouse tibial fracture-cast model was developed to enable preclinical study of CRPS mechanisms and guide condition-specific drug development. Given the inherent limitations of reflex pain measures in mice, we sought to holistically characterize pain-like behaviors in this model in neutral and aversive environments using quantitative behavior analysis with LabGym, a user-friendly automation tool that requires no special equipment or extensive computational resources. This study shows that tibial fracture-cast injury causes distinct changes in naturalistic and nocifensive behaviors in male and female mice in neutral and aversive environments, demonstrating reliable behavioral categorization using robust learning-based holistic assessment. As proof-of-concept for therapeutic testing, we leveraged this behavioral evaluation to characterize the peripheral vs central effects of targeting alpha-2 receptors (α2-AR) with dexmedetomidine (DEX), a selective α2-AR agonist with analgesic, sedative, and anxiolytic properties. We found that DEX reduced mechanical allodynia primarily through central α2-ARs. Dexmedetomidine also decreased motion metrics, grooming, and rearing in an open field and distinctly affected the quality and quantity of grooming in an aversive environment, and systemic α2-AR antagonists did not suppress this effect. Importantly, we also determined that the sedative effects of DEX were attenuated in injured compared with uninjured mice, consistent with known sympathetic nervous system activation in CRPS. Overall, this study highlights the use of automated behavioral testing to parse analgesia from sedation in a unique preclinical pain model.