Phenotypic inflammation switch in rats shown by calcitonin gene-related peptide immunoreactive dorsal root ganglion neurons innervating the lumbar facet joints.

STUDY DESIGN: The changes in dorsal root ganglion neurons innervating the L5-L6 facet joint were studied using the retrograde neurotransport method and the immunohistochemistry of calcitonin gene-related peptide in an inflammatory model of rats.

OBJECTIVES: To determine by inflammatory stimulation the changes in calcitonin gene-related peptide-immunoreactive dorsal root ganglion neurons innervating the L5-L6 facet.

SUMMARY OF BACKGROUND DATA: The rat L5-L6 facet joint is innervated from L1-L5 dorsal root ganglia. The presence of calcitonin gene-related peptide-immunoreactive dorsal root ganglion neurons innervating the L5-L6 facet joint has been confirmed, but the changes in the number and distribution of these neurons caused by inflammation have not been studied.

METHODS: Retrograde transport of fluorogold was used in 20 rats: 10 in the control group and 10 in the inflammatory group. Using the dorsal approach, fluorogold crystals were injected into the left L5-L6 facet joint. Then 5 days after application, complete Freund's adjuvant (50 microg Mycobacterium butyricum in oil saline emulsion) was injected into the same L5-L6 facet joint (inflammatory group). Of the total fluorogold-labeled dorsal root ganglion neurons from T13-L6, the number and cross-sectional area of the cell profiles of fluorogold-labeled, calcitonin gene-related peptide-immunoreactive neurons in the bilateral dorsal root ganglia of both groups were evaluated.

RESULTS: Fluorogold-labeled neurons were distributed throughout the ipsilateral dorsal root ganglia from L1-L5 in both groups. Of the fluorogold-labeled neurons, the ratios of the calcitonin gene-related peptide-immunoreactive L1, L2, L3, L4, and L5 dorsal root ganglion neurons, respectively, were 17%, 24%, 44%, 56%, and 50% in the control group and 50%, 39%, 51%, 61%, and 56% in the inflammatory group. The ratios of the calcitonin gene-related peptide-immunoreactive L1 and L2 dorsal root ganglion neurons labeled by fluorogold were significantly higher in the inflammatory group than in the control group (P < 0.05). The mean cross-sectional area of fluorogold-labeled, calcitonin gene-related peptide-immunoreactive cells from L1-L5 dorsal root ganglia increased from 621 +/- 64 microm2 to 893 +/- 63 microm2 in the inflammatory group (P < 0.01).

CONCLUSIONS: The ratio of fluorogold-labeled, calcitonin gene-related peptide-immunoreactive neurons was significantly higher in the L1 and L2 dorsal root ganglia of the inflammatory group than in those of the control group, and the average cross-sectional area of the cells from L1-L5 dorsal root ganglion increased. Associated with the inflammation in the facet joints, the change in calcitonin gene-related peptide-immunoreactive neuron distribution and the phenotypic switch to large neurons may complicate the mechanism of facet joint inflammatory pain.