Effect Of Immune Inhibitor MTX On The Chronic Neuropathic Pain By Intrathecal Injection

Objective: To investigate the effect and the likely mechanism of immune inhibitor methotrexate(MTX) on the chronic neuropathic pain by intrathecal injection, and to provide a new treatment to the chronic pain.Methods:1. Experiment groups: 32 SD rats were divided into 4 groups. The volume of saline was 100μl/kg each time. Methotrexate was reconstituted to 5mg/ml in saline, the volume of MTX was 0.5mg/kg. Group A(n=5, sham operation + saline treatment); Group B(n=9, root injury + saline treatment); Group C(n=9, root injury ,MTX:saline): MTX was administered at days 3,5,and 7 postsurgery. Saline was administered at days 9,11,and 13 postsurgery. Group D(n=9, root injury ,saline:MTX): Saline was administered at days 3,5,and 7 postsurgery. MTX was administered at days 9,11,and 13 postsurgery.2. Established model of chronic neuropathic pain: . established the CCI model which was made by Bennet-Xie(1988), and was regarded most similar to human's chronic neuropathic pain.3. Administrated method: intrathecally injected by passing a PE-10 catheter which was inserted into L₄,5 spinal subarachnoid space at days 3,5,7,9,11, and 13 postsurgery.4. Behavioral -tests of threshold: tested the threshold of rats who responded to heat noxious stimuli at the day before operation , days 4,6,8,10,12 and 14 postsurgery.5. Immunohistochemistry: Animals were fixed by transcardiac perfusion at days 14 postsurgery after threshold test. Immunohistochemistry was taken to investigate the expression of the astrocytic GFAP and TNF-a.Results:1. Behavioral tests of threshold: The threshold of group B(saline treatment) decreased after CCI operation(P<0.01). The hyperalgesia appeared on the 4th day. The threshold of group C(MTX:saline) decreased not significantly compared with the sham group(P>0.05). The threshold of group D(saline:MTX) was significantly attenuated at the first week after operation(P<0.01), and recovered slowly at the second week(P<0.05).2. Immunohistochemistry:(1) GFAP staining: Spinal astrocytic activity was elevated in rats of the group B, group C and group D except the sham group(P<0.05).(2) TNF-oexpression: TNF-aexpression was not elevated in the sham group. It was intense response in group B(P<0.05), moderate in group D(P<0.05), and mild in group C(P<0.05).Conclusion:As an immune inhibitor, MTX has the action to prevent and treat the chronic neuropathic pain. The likely mechanism is to inhibit thecytokine which secreted by actived glial to attain the analgesia effect.