Loss of Enrichment Induces Widespread Muscle Mechanical Hypersensitivity and Enhances Pain Related Behaviors after Ischemia with Reperfusion Injury via Macrophage Dependent Mechanisms

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      The development of chronic musculoskeletal pain is frequently associated with the presence of high levels of psychological stress. Multiple animal studies have shown that a variety of stress paradigms are effective at inducing deep tissue hyperalgesia and enhanced behavioral responses to subthreshold stimuli or injury. Nevertheless, the mechanisms behind stress-induced hyperalgesia are not clearly understood. Many of the current paradigms induce stress in rodents using physical stimuli that are known to induce changes in sensory function, such as repeated cold exposure or electrical shock. Non-invasive strategies to induce stress in animal models may uncover new mechanisms that drive stress related hyperalgesia. To address this issue, our lab has adapted a stress paradigm that utilizes environmental enhancement and subsequent loss of enrichment (LOE) to induce stress in mice, paired with fore limb ischemia/reperfusion injury (I/R) as a model of muscle hypersensitivity. After LOE, mice developed prolonged immobility time in a forced swimming test and presented with increased weight gain, suggesting increased stress/anxiety-like behaviors. At baseline (1 week after LOE), the mice also developed a bilateral decrease in mechanical withdrawal thresholds to plantar muscle squeezing, indicating stress related hypersensitivity. Immunohistochemical analysis of the plantar muscles showed increased macrophage infiltration one week after LOE. Mice exposed to LOE and I/R injury displayed a prolonged recovery time, compared to normal housing controls. Systemic ablation of macrophages only during the stress period facilitated the recovery after I/R. Our results suggests that LOE can significantly enhance the development of muscle pain after I/R. Together with our immunocytochemical data, results suggest that macrophages could play a role in the development of pain after LOE. These findings highlight the importance of stress in the development of long-lasting pain and may shed light in potential mechanisms behind chronic widespread muscle pain conditions like fibromyalgia. Grant Support from Anesthesia Innovation and Pilot (AIP) Program at Cincinnati Children's Hospital (LFQ) R01NS113965 and R01NS105715 (MPJ).
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