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 L 5-L6 facet joint were studied using the retrograde neurotransport method an d the immunohistochemistry of calcitonin gene-related peptide in an inflamm atory model of rats. Objectives. To determine by inflammatory stimulation the changes in calcito nin gene-related peptide-immunoreactive dorsal root ganglion neurons innerv ating 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-im munoreactive dorsal root ganglion neurons innervating the L5-L6 facet joint has been confirmed, but the changes in the number and distribution of thes e 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 d ays after application, complete Freund's adjuvant (50 mug Mycobacterium but yricum 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 profi les of fluorogold-labeled, calcitonin gene-related peptide-immunoreactive n eurons in the bilateral dorsal root ganglia of both groups were evaluated. Results. Fluorogold-labeled neurons were distributed throughout the ipsilat eral dorsal root ganglia from L1-L5 in both groups. Of the fluorogold-label ed neurons, the ratios of the calcitonin gene-related peptide-immunoreactiv e L1, L2, L3, L4, and L5 dorsal root ganglion neurons, respectively, were 1 7%, 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 p eptide-immunoreactive L1 and L2 dorsal root ganglion neurons labeled by flu orogold were significantly higher in the inflammatory group than in the con trol group (P < 0.05). The mean cross-sectional area of fluorogold-labeled, calcitonin gene-related peptide-immunoreactive cells from L1-L5 dorsal roo t ganglia increased from 621 +/- 64 mum(2) to 893 +/- 63 mum(2) in the infl ammatory group (P < 0.01). Conclusions. The ratio of fluorogold-labeled, calcitonin gene-related pepti de-immunoreactive neurons was significantly higher in the L1 and L2 dorsal rot ga ng lia 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 g anglion increased. Associated with the inflammation in the facet joints, th e change in calcitonin gene-related peptide-immunoreactive neuron distribut ion and the phenotypic switch to large neurons may Com plicate the mechanis m of facet joint inflammatory pain.