Effects of lidocaine injections into the rostral ventromedial medulla on nociceptive behviours in hot-plate and formalin tests in rats

 Introduction: Many studies are being run to understand the mechanisms of pain modulation. Rostral ventromedial medulla (RVM) contributes to pain control by its attenuating and augmentation. In this study, we investigated the role of RVM in chronic and acute pain while RVM inactivated temporarily by lidocaine.  Materials and Methods: White rats (weighing 250-200 g) were used in this study. The animals were divided into three groups: control, sham and test group. One week after surgery, saline or lidocaine was injected into the RVM and the hot plate (set temperature 52± 0.1 °) and formalin test (injection of 50 µL of formalin 2% to the right hindpaw) were performed and nociceptive responses were recorded. Results: The intra-RVM injection of lidocaine reduced pain in the first phase of formalin test, but it had no effect on the interphase or second phase. Also the lidocaine injection caused analgesia in the hot plate test, in all four time period (60, 30, 15 and 90 minutes) after injection.  Conclusion: Our results showed that injection of lidocaine into the RVM could affect acute pain model, but it didn’t show any effect on phase 2 formalin as chronic pain. So, it is likely that the RVM has a substantial role on acute pain through pain transferring or processing.