Exercise induced hypoalgesia profile in rats is associated with IL-10 and IL-1 β levels and pain severity following nerve injury

Background Pain may undergo modulation in the central nervous system prior to reaching the primary somatosensory cortex and being perceived as pain. Faulty pain modulation mechanisms have been linked to various chronic pain conditions. Cytokines such as IL-10 and IL-1beta, are known to be involved in initiation and maintenance of neuropathic pain. In this study, we investigated the association between pain modulation profile, pain intensity and cytokines (IL-10 and IL-1beta) levels in a rat model of neuropathic pain. Methods Exercise-Induced Hypoalgesia (EIH) was assessed by evaluating the percentage of responses to a train of 60g mechanical stimuli before and after 180 seconds of exercise on a rotating rod. The differences in the response rates before and after the exercise were used to divide the rats into low and high EIH responders. Rats from low and high EIH groups underwent constriction injury of the left sciatic nerve. Pain behavior (allodynia and hyperalgesia) were assessed by measuring responses to mechanical and thermal stimuli applied to the plantar surface of the foot. Serum, sciatic nerve and the related Dorsal Root Ganglia (DRG) levels of IL-10 and IL-1beta were determined by ELISA. The DRG mRNA levels of IL-10 and IL-1beta measured with PCR. A comparison between the low and high EIH rats of all measured parameters was made. Results The low EIH rats developed significantly more severe allodynia and hyperalgesia in the affected paw and allodynia in the contralateral paw compared to the high EIH rats, 7 days following the injury. The low EIH rats had higher IL-1beta protein levels in serum prior to and following injury, higher affected and contralateral sciatic nerve IL-1beta levels following injury and higher IL-1beta levels in the contralateral DRG (protein and mRNA) following injury when compared to high EIH rats. The high EIH rats had higher affected sciatic nerve IL-10 levels following nerve injury and higher IL-10 levels of both protein and mRNA in the affected and contralateral DRG at baseline and following injury. Conclusion EIH profile was found to be predictive of pain behavior following nerve injury, low EIH rats developed more severe allodynia and hyperalgesia. IL-1beta may be associated with painful neuropathy developed in rats with low EIH while the anti-inflammatory cytokine IL-10 may have a protective role, inhibiting the development of painful.