Transient nerve root compression load and duration differentially mediate behavioral sensitivity and associated spinal astrocyte activation and mGLuR5 expression.

Injury to the cervical nerve roots is a common source of neck pain. Animal models of nerve root compression have previously established the role of compression magnitude and duration in nerve root-mediated pain and spinal inflammation; yet, the response of the spinal glutamatergic system to transient nerve root compression and its relationship to compression mechanics have not been studied. The glutamate receptor, mGluR5, has a central role in pain, and its expression by neurons and astrocytes in the spinal cord may be pivotal for neuronal-glial signaling. This study quantified spinal GFAP and mGluR5 expression following nerve root compressions of different magnitudes and durations in the rat. Compression to the C7 nerve root was applied for a duration that was either above (10 min) or below (3 min) the critical duration for mediating afferent discharge rates during compression. To also test for the effect of the magnitude of the compression load, either a 10 gf or a 60 gf was applied to the nerve root for each duration. Mechanical allodynia was assessed, and the C7 spinal cord was harvested on day 7 for immunofluorescent analysis. Double labeling was used to localize the expression of mGluR5 on astrocytes (GFAP) and neurons (MAP2). Seven days after injury, 10 min of compression produced significantly greater behavioral sensitivity (P<0.001) and spinal GFAP expression (P=0.002) than 3 min of compression, regardless of the compression magnitude. Nerve root compression at 60 gf produced a significant increase (P<0.001) in spinal mGluR5 for both of the durations studied. There was no difference in the distribution of mGluR5 between astrocytes and neurons following nerve root compression of any type. The glutamatergic and glial systems are differentially modulated by the mechanics of nerve root compression despite the known contribution of glia to pain through glutamatergic signaling.