INTRODUCTION: Even in patients without fractures, osteoporosis often induces low back pain (LBP). Inflammatory mediators secreted by osteoclasts, such as tumor necrosis factor‐α, increase expression of calcitonin gene‐related peptide in dorsal root ganglia (DRG) neurons innervating lumbar vertebrae of osteoporotic rats. These findings suggest that inflammatory pain is important in osteoporotic LBP without fractures. The objective of the present study was to evaluate the characteristics of osteoporotic pain without fractures.
METHODS: As a model of osteoporosis, 16‐week‐old female Sprague Dawley rats that had been ovariectomized (OVX) at 5 weeks were prepared (n=10). As a control group, sham‐operated rats without ovariectomy were used (n=10). The L3 was exposed and a neurotracer, Fluoro‐Gold (FG), was applied to detect DRG neuronal cells innervating L3 vertebrae. A week after FG‐application, bone mineral density (BMD) of femur was measured using peripheral quantitative computed tomography and bilateral L1 to L6 DRGs were resected. In FG‐labeled neurons, expression of activating transcription factor 3 (ATF3) and growth‐associated protein 43 (GAP43) was compared immunohistochemically between two groups.
RESULTS: The BMD of the OVX group was significantly decreased compared with controls (p<0.01). No FG‐labeled ATF3‐ or GAP43‐immunoreactive neurons were detected in the control group, and the proportions of FG‐labeled ATF3 or GAP43 immunoreactive neurons in the OVX group were significantly elevated (p<0.01; 0.6% and 0.5%).
DISCUSSION: The findings suggest that an osteoporotic state increases ATF3 and GAP43 expression in DRG neurons innervating L3 vertebrae. However, proportions of both were very low compared with total number of innervating cells. These markers may therefore indicate that osteoporotic LBP without fractures has both aspects of inflammatory and neuropathic pain, but is more strongly associated with inflammatory pain than neuropathic pain.