Objective:To observe and investigate the effect and mechanism of intrathecal injecting miR-125b mimic on regulating microglial and its-mediated NOD-like receptor protein 3 (NLRP3) inflammasome activation after spinal cord ischemia reperfusion injury (SCIRI) in rats.Methods:120 Sprague-Dawley (SD) rats were randomly divided into four groups: sham group (sham group, intrathecal injection of normal saline), injury group (IR group, intrathecal injection of normal saline), mimic group (IR+ intrathecal injection of miR-125b mimic) and control group (IR+ intrathecal injection of miR-125b control). In sham group, the aortic arch was exposed without clamping. In other groups, the aortic arch was clamped for 14 minutes, and then the artery clamp was removed to prepare SCIRI model. Rats in each group were given intrathecal injection for 3 consecutive days before operation, once a day. Immunofluorescence was used to observe the morphology of microglia and count the number of activated cells 48 hours after injury; double immunofluorescence was used to observe the distribution of Iba-1 and NLRP3 in cells. The protein expression of Iba-1, NLRP3 and cleaved caspase-1 in spinal cord were determined by Western blot, and the inflammatory factor IL-1 β was determined by enzyme linked immunosorbent assay (ELISA).Results:The activated microglias exhibited the amoeba morphology and increased Iba-1 fluorescence signal in immunofluorescence. The microglias in spinal dorsal horn of Sham group were all branched-like morphology and absent of the increased Iba-1 signal at 48 h post-IR. And the microglias in IR and control groups was all amoeba-like morphology with obviously increased number and intensity of Iba-1 signal, whereas in mimic group, the microglias were mixed morphology with the less increased number and intensity of Iba-1 signal ( P<0.05). And NLRP3 signals were mainly distributed in microglias with increased Iba-1 signal. Meantime, compared with Sham group, the protein expression of Iba-1, NLRP3, cleaved caspase-1 and IL-1β were significantly increased in IR group ( P<0.05). Compared with IR group, the above-mentioned protein expression were decreased in mimic group ( P<0.05). There were no statistical differences between IR and control groups in the above-mentioned protein levels ( P>0.05). Conclusions:Intrathecal injection of miR-125b mimic inhibits the SCIRI-induced microglial activation and then reduces NLRP3 inflammasome expression in spinal cords, thereby exerting an effective anti-inflammatory effects.