OBJECTIVE(S): There are many reports about the role of rostral ventromedial 
medulla (RVM) in modulating stress-induced analgesia (SIA). In the previous 
study we demonstrated that temporal inactivation of RVM by lidocaine potentiated 
stress-induced analgesia. In this study, we investigated the effect of permanent 
lesion of the RVM on SIA by using formalin test as a model of acute inflammatory 
pain.
MATERIALS AND METHODS: Three sets of experiments were conducted: (1) Application 
of stress protocol (2) Formalin injection after exposing the animals to the swim 
stress (3) Either the relevant vehicle or dopamine receptor 1 (D1) agonist 
R-SKF38393 was injected into the RVM to cause a lesion. For permanent lesion of 
RVM, R-SKF38393 was injected into the RVM. Forced swim stress in water was 
employed in adult male rats. Nociceptive responses were measured by formalin 
test (50µl injection of formalin 2% subcutaneously into hind paw) and pain 
related behaviors were monitored for 90 min.
RESULTS: In the unstressed rats, permanent lesion of the RVM by R-SKF38393 
decreased formalin-induced nociceptive behaviors in phase 1, while in stressed 
rats, injection of R-SKF38393 into the RVM potentiated swim stress-induced 
antinociception in phase 1 and interphase, phase 2A of formalin test. 
Furthermore, R-SKF38393 had pronociceptive effects in phase2B whereas injections 
of R-SKF38393 resulted in significant difference in nociceptive bahaviours in 
all phases of formalin test (P<0.05).
CONCLUSION: The result of the present study demonstrated that permanent 
inactivation of RVM can potentiate stress-induced analgesia in formalin test.