The Mechanisms Underlying Morroniside And Infant Suppressing Neuropathic Pain: The Spinal Interleukin-10/?-endorphin Pathway

Neuropathic pain is a kind of disease with high morbidity and complex mechanisms.7%-10% of world population suffer from neuropathic pain while its treatment is poorly developed.Wound,brain and spinal cord lesion and other diseases,such as diabetes and cancer,could lead to neuropathic pain.To develop effective neuropathic pain drugs with little adverse effect has always been a cutting-edge field of neuroscience research.Our lab has previously revealed that glucagon like peptide-1 receptor(GLP-1R)agonist exenatide stimulated spinal cord microglia interleukin-10(IL-10)expression,regulating ?-endorphin secretion and thus produce anti-allodynia effect.This mechanism was first discovered by our lab and was named as spinal cord microglial GLP-1R/IL-10/?-endorphin pathway.In this study,we mainly discussed the role of this pathway both in drug administration and in spontaneous antinociception.We first aimed to investigate the analgesic effect of GLP-1R agonist morroniside and its stimulation on microglial GLP-1R/IL-10/?-endorphin pathway.Results showed that morroniside attenuated rat mechanical hypersensitivity induced by spinal nerve ligation(SNL)and this effect was completely reversed by intrathecal injection of microglia inhibitor minocycline and microglia depletor clodronate liposome.Intrathecal injection of IL-10 neutralizing antibody could not only block the analgesic effect of morroniside,but also reverse its elevation on spinal cord ?-endorphin precursor gene proopiomelanocor(POMC)mRNA expression and ?-endorphin secretion.While intrathecal injection was able to block morroniside analgesia but not morroniside induced IL-10 mRNA and protein expression increase.These data indicate that morroniside could stimulate spinal cord microglia GLP-1R and increase IL-10 expression afterwards.The increased IL-10 then regulated ?-endorphin expression,thus producing anti-allodynia effect.The stimulation effect of morroniside on GLP-1R/IL-10/?-endorphin pathway was also verified in primary cultured microglia.Furthermore,we investigated the role of spinal IL-10/?-endorphin pathway in the suppression of early life neuropathic pain.As was reported,spared nerve injury(SNI)could evoke neuropathic pain like behavior in adult rats(postnatal day 33)instead of infant rats(postnatal day 10),which was constitutively suppressed by antiinflammation neuroimmune regulation.In this study,we confirmed that infant rats subjected to spared nerve injury(SNI)developed no mechanical allodynia.RT-PCR results demonstrated that the spinal cord IL-10 and POMC expression was increased simultaneously.Intrathecal injection of both IL-10 antibody and ?-endorphin antibody evoked mechanical hypersensitivity in infant rats.IL-10 antibody could block the POMC expression increase in infant rats while ?-endorphin antibody had no effect on IL-10 elevation.These data demonstrated that the suppression of infant neuropathic pain was mainly induced by spinal cord IL-10/?-endorphin pathway.This study demonstrated the mechanisms of morroniside analgesia effect and worked as a further evidence of spinal microglial GLP-1R/IL-10/?-endorphin pathway.More importantly,we displayed the involvement of spinal IL-10/?-endorphin in early life pain suppression,providing novel understandings in both neuropathic pain development and this signaling pathway.