ABSTRACT
OBJECTIVE: The purposes of this study were the development and assessment of an animal model of persistent muscle pain induced by repeated intramuscular injections of low pH buffered saline. METHOD: Buffered saline at pH 4, 6 or 7.2 were injected twice, 3 days apart, into the gastrocnemius muscles of thirty Sprague-Dawley rats. To quantify hyperalgesia, paw withdrawal response to von Frey filament (mechanical hyperalgesia) and acetone (cold hyperalgesia) were measured for the 4 weeks after injection. Also the locomotor performance with inclined plane board and treadmill, and electrophysiologic study were evaluated. RESULTS: Mechanical hyperalgesia that was produced by the single injection of saline lasted less than 24 hours. The repeated injection of acidic saline produced mechanical hyperalgesia for more than 4 weeks at injected side and 3 weeks at contralateral side. Cold hyperalgesia lasted for 1 week after injection bilaterally. The locomotor performance and nerve conduction studies were not changed after injection. CONCLUSION: We have developed and characterized a model of muscular pain that was long lasting and widespread. This might prove useful for studying pain of muscular origin that may be similar to the chronic pain syndromes observed clinically.