Anatomical specificity of acidic saline model of chronic pain and the role of glia
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Research into the mechanisms of hyperalgesia is ongoing with the goal of improving clinical management of chronic pain. One animal model of chronic musculoskeletal pain uses two injections of acidic saline into a lateral gastrocnemius muscle to induce a long-lasting bilateral decrease in paw withdrawal thresholds. This study tested whether the two injections need to occur in the same muscle. Male Sprague-Dawley rats were injected with acidic saline (pH 4.0) in either the lateral or medial head of gastrocnemius or the contralateral gastrocnemius (lateral head). All animals received a second injection in the ipsilateral gastrocnemius (lateral head). Mechanical withdrawal thresholds were reduced in all groups when tested 24 hours after the second injection. Animals in which the first muscle injection was substituted with a non-specific treatment (intraperitoneal injection of lipopolysaccharide) developed bilateral hyperalgesia after a single acidic saline injection. Thus, the mechanism of hyperalgesia in this model is not restricted to the injected tissues and may include central nervous system structures. Consistent with this, an inhibitor of glia cell activation (minocycline) blocked the development of bilateral hyperalgesia. These data indicate that the central nervous system may play a large role in mediating chronic musculoskeletal pain.