Permanent lesion in rostral ventromedial medulla potentiates swim stress-induced analgesia in formalin test

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: (۱) Application of stress protocol (۲) Formalin injection after exposing the animals to the swim stress (۳) Either the relevant vehicle or dopamine receptor ۱ (D۱) agonist R-SKF۳۸۳۹۳ was injected into the RVM to cause a lesion. For permanent lesion of RVM, R-SKF۳۸۳۹۳ was injected into the RVM. Forced swim stress in water was employed in adult male rats. Nociceptive responses were measured by formalin test (۵۰µl injection of formalin ۲% subcutaneously into hind paw) and pain related behaviors were monitored for ۹۰ min. Results: In the unstressed rats, permanent lesion of the RVM by R-SKF۳۸۳۹۳ decreased formalin-induced nociceptive behaviors in phase ۱, while in stressed rats, injection of R-SKF۳۸۳۹۳ into the RVM potentiated swim stress-induced antinociception in phase ۱ and interphase, phase ۲A of formalin test. Furthermore, R-SKF۳۸۳۹۳ had pronociceptive effects in phase۲B whereas injections of R-SKF۳۸۳۹۳ resulted in significant difference in nociceptive bahaviours in all phases of formalin test (P<۰.۰۵). Conclusion: The result of the present study demonstrated that permanent inactivation of RVM can potentiate stress-induced analgesia in formalin test.

Keywords:

Analgesia , Formalin test , Inactivation , Lesion , Rostral ventromedial medulla , Swim stress