Ethological profiling of pain and analgesia in a mouse model of complex regional pain syndrome.

Complex regional pain syndrome (CRPS) is a form of chronic post-injury pain affecting the extremities. 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 characterize pain-like behaviors in this model more holistically. We evaluated spontaneous and evoked pain and naturalistic behaviors after tibial fracture-cast injury in male mice in neutral and aversive environments using LabGym. Here, we report a unique ethological signature of pain in injured mice characterized by reflexive allodynia, thermal hyperalgesia, frequent grooming and reduced rearing in neutral and aversive environments, and decreased paw withdrawal and increased paw licking in an aversive environment. As proof-of-concept, we also leveraged this holistic behavioral evaluation for drug screening by characterizing the peripheral versus central effects of targeting alpha-2 receptors (2-AR) in the tibial fracture-cast model. We evaluated the impact of systemic delivery of dexmedetomidine (DEX), a selective 2-AR agonist, with or without antagonists, on holistic behavioral metrics in injured male mice. We found that DEX reduced mechanical allodynia primarily via central 2-ARs. DEX 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 this effect was not suppressed by systemic 2-AR antagonists. Ultimately, this study holistically captures pain-related behaviors and provides a detailed characterization of the relative contributions of peripheral and central 2-ARs to alpha2-mediated analgesia in male mice after tibial fracture-cast injury.