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: (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.