Formalin-Induced Neuropathic Pain in Mice

Neuropathic pain is a chronic condition of the nervous system often associated with inflammatory diseases such as multiple sclerosis, diabetes, and chemotherapy-induced damage. Current therapies are ineffective, highlighting the need for improved preclinical models. Neuropathic pain in mice has been modeled using invasive procedures such as spinal nerve ligation (SNL) and chronic constriction injury (CCI), which require survival surgery. In contrast, this study investigates the use of subcutaneous formalin injection as a non-surgical, reproducible alternative for modeling chronic pain in mice. Formalin is known to produce a biphasic pain response and has been used to model nociceptive pain. Recent studies in rats suggest that higher concentrations of formalin can induce long-lasting inflammatory pain comparable to surgical models. This approach has not yet been validated in mice. Our study aims to test the efficacy of a formalin-induced chronic pain model in C57BL/6 mice, assessing both behavioral and molecular outcomes such as increased pro-inflammatory cytokines, glial activation, neuronal sensitization markers, and altered expression of pain-related ion channels. Behavioral tests will include the Von Frey filament test to assess mechanical sensitivity and the Hargreaves’ plantar test to assess thermal sensitivity, conducted 14 days post-injection. These tests measure pain thresholds and responses, with cut-offs to prevent injury. Mice are expected to exhibit hyperalgesia, shown by faster paw withdrawal to thermal stimuli and lower force thresholds in mechanical testing. This model offers a minimally invasive, cost-effective alternative to traditional neuropathic pain models, especially suited for undergraduate research settings where surgical training may be limited. The findings may provide a new platform for studying the molecular and cellular mechanisms of chronic pain and developing targeted therapeutics. Future directions include refining dosing strategies, examining genetic influences on pain sensitivity, and evaluating translational relevance for human pain disorders.