Disk injury in rats produces persistent increases in pain-related neuropeptides in dorsal root ganglia and spinal cord glia but only transient increases in inflammatory mediators: pathomechanism of chronic diskogenic low back pain.

STUDY DESIGN: Immunohistological analysis in an injured intervertebral disk (IVD) model.

OBJECTIVE: To elucidate and compare in rats the behavior of the sensory nervous system and inflammatory mediators in experimentally injured IVDs.

SUMMARY OF BACKGROUND DATA: Multiple human and animal studies have verified the presence of sensory nerve fibers in IVDs or investigated the behavior of inflammatory mediators in injured IVDs, but no in vivo study to date has examined the relationship between the 2.

METHODS: Eight-week-old female rats were used. In the disk-injured group, L5/L6 disks were injured with a 24-gauge needle; simultaneously, the neurotracer Fluoro-gold was injected into the L5/L6 IVD. The L5/L6 IVD dorsal root ganglia (DRGs) from the L1 to L6 levels, and the spinal cord was resected at several time points after surgery. Nerve growth factor, tumor necrosis factor (TNF)-α and interleukin (IL)-6 production in the IVDs were quantified using enzyme-linked immunosorbent assay. DRGs were immunostained for calcitonin gene-related peptide, and spinal cord sections were immunostained for ionized calcium-binding adaptor molecule-1 and glial fibrillary acidic protein.

RESULTS: Nerve growth factor, and TNF-α levels (through 1 week) and IL-6 levels (through 4 days) were significantly higher in the disk-injured group than in the noninjured group (P < 0.05). However, starting at 2 weeks (nerve growth factor and TNF-α) or 1 week (IL-6), the differences in inflammatory mediator levels between the 2 groups no longer were significant. In contrast, the percentage of calcitonin gene-related peptide-immunoreactive neurons among Fluoro-gold-labeled DRG neurons, and the numbers of ionized calcium-binding adaptor molecule-1-immunoreactive microglia and glial fibrillary acidic protein-immunoreactive astrocytes in the spinal dorsal horn remained significantly higher in the injured group than in the noninjured group at all-time points (P < 0.05).

CONCLUSION: Disk injury in rats produces persistent increases in neuropeptides in DRGs and glia in the spinal cord, but only transient increases in inflammatory mediators in IVDs.