The effect of microinjection of dimethyl sulfoxide into the rostral ventromedial medulla on swim stress-induced analgesia

Background: Dimethyl sulfoxide (DMSO) is an important solvent for compounds that used in pain research. Rostral ventromedial medulla (RVM) plays an important role in modulating nociception and stress-induced analgesia (SIA). Objective: The aim of this study was to investigate the effect of DMSO administration into the RVM on SIA by using formalin test. Methods: This experimental study was conducted on 27 Wistar male rats (200±30 gr) were randomly assigned to control, stress and stress+DMSO groups. Animals were placed in a water reservoir (20±1°C) for 3 minutes to induce forced swimming stress. Stereotaxic surgery was performed to microinjection of DMSO (0.5μl, 100%) into RVM. The pain behavior score was evaluated by subcutaneous injection of formalin 2% in the dorsal plantar region of hid paw. Findings: The pain score of phase 1, interphase and phase 2 of formalin test in swim stress group decreased significantly in comparison to control group (P<0.001, P< 0.05, P<0.001) respectively. In addition, the pain score of three phase of formalin test after DMSO injection in swim stress group decreased significantly in comparison to control and stress group (P<0.001, P<0.05) respectively. Conclusion: Also microinjections of DMSO into the RVM potentiate the swim stress analgesia. According to the analgesic effects of dimethyl sulfoxide, as well as its ability to potentiate stress-induced analgesia, DMSO should be used with caution as a solvent in pain studies. Conclusion: Force swim stress induces analgesia in, and microinjections of DMSO into the RVM potentiate the swim stress analgesia. According to the analgesic effects of DMSO, as well as its ability to potentiate stress-induced analgesia, it should be used with caution as solvent in pain studies.