Up-regulation of p55 TNF alpha-receptor in dorsal root ganglia neurons following lumbar facet joint injury in rats

Abstract

The rat L5/6 facet joint is multisegmentally innervated from the L1 to L6 dorsal root ganglia (DRG). Tumor necrosis factor (TNF) is a known mediator of inflammation. It has been reported that satellite cells are activated, produce TNF and surround DRG neurons innervating L5/6 facet joints after facet injury. In the current study, changes in TNF receptor (p55) expression in DRG neurons innervating the L5/6 facet joint following facet joint injury were investigated in rats using a retrograde neurotransport method followed by immunohistochemistry. Twenty rats were used for this study. Two crystals of Fluorogold (FG; neurotracer) were applied into the L5/6 facet joint. Seven days after surgery, the dorsal portion of the capsule was cut in the injured group (injured group n = 10). No injury was performed in the non-injured group (n = 10). Fourteen days after the first application of FG, bilateral DRGs from T13 to L6 levels were resected and sectioned. They were subsequently processed for p55 immunohistochemistry. The number of FG labeled neurons and number of FG labeled p55-immunoreactive (IR) neurons were counted. FG labeled DRG neurons innervating the L5/6 facet joint were distributed from ipsilateral L1 to L6 levels. Of FG labeled neurons, the ratio of DRG neurons immunoreactive for p55 in the injured group (50%) was significantly higher than that in the non-injured group (13%). The ratio of p55-IR neurons of FG labeled DRG neurons was significantly higher in total L1 and L2 DRGs than that in total L3, 4, 5 and 6 DRGs in the injured group (L1 and 2 DRG, 67%; L3, 4, 5 and 6 DRG, 37%, percentages of the total number of p55-IR neurons at L1 and L2 level or L3-6 level/the total number of FG-labeled neurons at L1 and L2 level or L3-6 level). These data suggest that up-regulation of p55 in DRG neurons may be involved in the sensory transmission from facet joint injury. Regulation of p55 in DRG neurons innervating the facet joint was different between upper DRG innervated via the paravertebral sympathetic trunks and lower DRG innervated via other direct routes.

Fig. 1

Distribution of the average number of FG-labeled DRG neurons innervating L5/6 facet joints. These neurons were observed from L1 to L6 DRG. Error bars represent the SEM. The numbers of FG labeled neurons in L2 DRGs in both groups were significantly higher than those of L1, L3, L4, L5, and L6 DRGs in both groups (P < 0.05). a Non-injured group, b injured group

Fig. 2

Fluorescent photomicrographs show FG labeled DRG neurons, and p55-IR. a and c show the FG labeled neurons innervating the L5/6 facet joint. b, d Labeled cells in green show p55-IR neurons. a and b are the same section in the non-injured group. c, d The same section in the facet joint injury group. In the non-injured group, FG labeled neurons did not express p55 receptor. In the facet joint injury group, most FG labeled cells expressed the p55 receptor (arrows). Scale bar = 100 μm

Fig. 3

a Distribution of the average number of FG-labeled and p55-IR cells at each level in both non-injured (blue) and facet joint injury (pink) groups. The number of FG-labeled and p55-IR cells in the facet joint injury group was significantly higher than that in the non-injured group (P < 0.05). b The ratios of FG-labeled and p55-IR cells at each level in the facet joint injury group. The ratio of FG-labeled and p55-IR cells in L1 and L2 DRG was significantly higher than those in L3, L4 and L5 DRG (P < 0.05)