Objective: Our previous work has shown that motor nerve injury by a lumber 5 ventral root transection(L5-VRT)leads to the overexpression of interleukin-6(IL-6)in the bilateral spinal cord and intrathecal administration of IL-6 neutralizing antibody delays the induction of mechanical allodynia in bilateral hindpaws.However,what factors contribute to the spinal IL-6 secretion following L5-VRT are still not very clear yet.Methods: We accessed the role of calpain-2 and IL-6 in the spinal cord with immunohistochemistry and western blot analysis early after L5-VRT.Drug administration was performed to evaluate the effect of calpian-2 on spinal IL-6 secretion.Results: Here we evidence for the first time that both IL-6 and calpain-2(CALP2,a calcium-dependent protease)are increased in ipsilateral lumbar spinal cord as early as 0.5 h after L5-VRT.IL-6 but not CALP2 is also increased on contralateral side at the same time point.The increased IL-6 is colocalized with CALP2-immunoreactivity neurons in both the spinal dorsal horn and the ventral horn.Intrathecal pretreatment with adenovirus vector-mediated CALP2 knockdown partially suppresses the VRT-induced spinal IL-6 on ipsilateral side and reduces mechanical hyperalgesia.Without any nerve injury,addition of recombinant rat CALP2(rr-CALP2)onto the surface of the L5 nerve root directly leads to the up-regulation of IL-6 protein level in the bilateral spinal cord at post-drug day 3.Furthermore,the elevated IL-6 is also obtained in the activated spinal microglia cells,combined with no co-localization between CALP2 and Iba1(a microglia marker),implicating another potential factor of microglia in IL-6 secretion early after VRT surgery.Conclusion: The increased CALP2 and activated microglia cells in the spinal cord might play the causative role in spinal IL-6 secretion early after motor nerve injury.The agents blocking CALP2 and/or microglia actions might be a therapy of neuropathic pain.