Spinal glial activation and cytokine expression after lumbar root injury in the rat

Study Design. This study was designed to examine the behaviorial immunohist ochemical changes of spinal glial cells and spinal Interleukin (IL)-1 beta expression after various nerve root injuries used as models of lumbar radic ulopathy. Objectives. In order to better understand the role of central inflammation in the pathophysiologic mechanisms that give rise to pain associated with l umbar radiculopathy, this research studied the relationship between pain-re lated behavior associated with spinal glial activation and IL-1 beta expres sion generated by three types of nerve root injury: loose ligation with chr omic gut, loose ligation with silk, and tight ligation with silk. Summary of Background Data. An animal model of lumbar radiculopathy origina lly described by Kawakami and Weinstein involved loose ligation of unilater al L4-L6 nerve roots with chromic gut. Characterization and establishment o f such an animal model of low back pain enables further investigation of th e nature of the pathophysiologic mechanisms associated with lumbar radiculo pathy in humans. Methods. Seventy-three rats were divided into four treatment groups. Chromi c group (n = 25): The L5 nerve roots (dorsal and ventral) were exposed by h emilaminectomy and loosely ligated with chromic gut. Tight silk group (n = 18): The exposed L5 nerve roots were tightly ligated extradurally with 5-0 silk suture. Loose silk group (n = 15): two loose ligatures of 5-0 silk wer e placed around the exposed L5 nerve roots. Sham group (n = 15): the rats w ere subjected to laminectomy alone for exposing nerve roots. Following surg ery, thermal hyperalgesia and mechanical allodynia was assessed time-depend ently up to 42 days post operatively. At 1, 3, 7, 14, and 42 days postopera tively, the rats in each group were perfused with fixative. The L5 spinal c ord segments was harvested and cryosectioned for glial and cytokine immunoh istochemistry. Results, In the chromic and the tight silk group, an immediate and sustaine d mechanical allodynia was observed in the ipsilateral hind paw up to 35 da ys postoperatively. The loose silk group also showed an immediate mechanica l allodynia that subsided by 14 days postoperatively. Sham-treated animals exhibited mild mechanical allodynia for the initial 7 days after the surger y. Thermal-hyperalgesia was evident in the three primary treatment groups, but not in the sham-treated rats. OX-42 expression was elevated in the gray matter of the L5 spinal section by 3 days in the chromic, the tight silk, and the loose silk groups as compared to the sham group. Astrocytic activat ion increased over time in all groups except the sham group. There was no d irect correlation between degree of microglial response and severity of pai n behaviors. In contrast, astrocytic activation demonstrated a direct relat ionship with the elevation of mechanical allodynia for the first 7 days. In addition, spinal IL-1 beta protein expression was increased bilaterally in the superficial layer of the dorsal horn and cell nuclei of the ventral ho rns in the ligature treated groups as compared with the sham group. Conclusion. Direct mechanical and/or chemical injury to lumbar roots in the rat gives rise to pain behavior suggestive of lumbar radiculopathy. The fi nding that glial activation and enhanced IL-1 beta expression are observed in the spinal cord after root injury supports a central, neuroimmune compon ent in the generation of lumbar radiculopathy, A further understanding of t he immunologic consequences of root injury may lead to further development and the novel use of selective cytokine-inflammatory inhibitors for the tre atment of low back pain associated with radiculopathy.