We have recently reported that injury to a lumbar root in a rat model of
radiculopathy produces spinal glial activation associated with elevated proinflammatory
cytokines. Based on our hypothesis that central neuroinflammatory processes may manifest clinically as radicular
pain, we undertook pharmacological intervention using the
immunosuppressive agent methotrexate (MTX). The L5 lumbar spinal root (central to the dorsal root ganglia) was exposed unilaterally and loosely constricted with chromic gut. In the prevention (phase I) study, MTX was administered intrathecally (1 mg/kg) and around the spinal root (1 mg/kg) at surgery and at days 2 and 4 postsurgery (group A). Saline injection was employed for the control group (group B).
Sham operated animals were administered MTX to determine the potential for behavioral/neural side effects (group C). In the existing
pain paradigm (phase II) study, the experiment was extended to day 14 with three additional groups. The same dose and method of delivery of MTX or saline was administered as in phase I in the first week on days 0, 2, and 4 and in the second week on days 7, 9, and 11 postsurgery. To measure the effects of MTX on existing behaviors saline was administered in the first week and MTX during the second (group D; Saline:MTX). The control group received saline during both weeks (group E; Saline:Saline). To examine the possible recurrence of radicular
pain after MTX termination, MTX was given in the first week and saline in the second (group
F; MTX:Saline). Gait disturbance and
mechanical allodynia (using von Frey filaments) were assessed up to day 7 in the prevention study (Phase I) and day 14 in the existing
pain paradigm (Phase II). The L5 spinal cord segments were harvested for assessment of immunohistochemical glial activation using the
antibodies OX-42 (microglial marker) and
glial fibrillary acidic protein (GFAP: astrocytic marker) and for the presence of Major Histocompatibility Complex (MHC) Class II expression. Group C (Sham+MTX) did not demonstrate any evidence of gait disturbance or
mechanical allodynia after MTX administration. The rats in group B (Surgery+Saline) demonstrated
mechanical allodynia from one day postsurgery to the time of euthanization. When
allodynia was assessed using the 12 g von Frey filament, the MTX treated rats in group A showed significantly decreased
mechanical allodynia as compared to the saline treated rats (group B) (repeated measured ANOVA, P<0.0001). In the phase II study, the rats in group D (Saline:MTX) and E (Saline:Saline) showed robust
allodynia in the first week after the surgery. In the second week,
mechanical allodynia significantly decreased in group D, while
mechanical allodynia continued in the saline treated group (repeated measured ANOVA, P=0.0121).
Allodynia was significantly attenuated in group
F (MTX: Saline) as compared to the response in groups D and E at day 7 (one-way ANOVA, P<0.0001) and remained significantly lower as compared to group E up to day 11 postsurgery (one-way ANOVA, P9=0. 0013: P11=0.0048). OX-42 and GFAP expression were elevated in the gray matter of the L5 spinal section in all groups that underwent the root
ligature with chromic gut (Groups A, B, D-F). There were no significant differences in glial activation between the groups. However, spinal expression of MHC II was markedly reduced in the MTX treated group as compared with the saline treated group. The exact mechanism of action of MTX in attenuating
mechanical allodynia has not yet been elucidated. The present results indicate that MTX administration may offer a new treatment modality for radicular
pain with or without
disc herniation as well as directing new research into the development of novel
immunomodulators for the treatment of chronic neuropathic and radicular
pain.