Effect of transient inactivation of rostral ventromedial medulla on swim stress induced analgesia in formalin test in rats

Introduction: Despite significant progress in understanding pain control mechanism, there are numerous questions about central nervous mechanisms underlying stress-induced analgesia. The rostral ventromedial medulla (RVM) in the brainstem integrates a variety of functions, including pain modulation and pain perception. In the present study, we investigated the effect of temporary inactivation of RVM on stress-induced analgesia. Methods: This study was performed using adult male Wistar rats (200-250 gr). Swim stress was induced using a cylinder filled with water (50 cm height, 20±1°C) in which the rats were kept for 3 min. For induction of pain, 50 μL of 2% formalin was injected subcutaneously in the hind paw. For temporary inactivation of RVM, 0.5 μL of 2% lidocaine was injected into RVM. Results: Injection of lidocaine into RVM, before inducing swim stress, potentiated the anti-nociceptive effects of swim stress in phase 1 and phase 2A. In phase 2B swim stresses increased nociceptive scores of formalin test so administration of lidocaine into RVM inhibited the effect of swim stress. Conclusion: The result of this study demonstrated that temporal inactivation of RVM by lidocaine potentiated stress-induced analgesia.