STUDY DESIGN: Using a retrograde tracing method and immunohistochemistry, we assessed the expression of activating transcription factor 3 (ATF3), a marker of nerve injury, and growth-associated protein 43 (GAP-43), a marker of axonal growth, in dorsal root ganglion (DRG) neurons innervating the lumbar intervertebral discs in rats. OBJECTIVES: To investigate ATF3 and GAP-43 expression in DRGs innervating the intervertebral discs after exposure of the nucleus pulposus to the outside of the anulus fibrosus. SUMMARY OF BACKGROUND DATA: Degeneration of lumbar intervertebral discs is considered as a cause of low back pain. We speculated that exposure of the nucleus pulposus to the outside of the anulus fibrosus may induce nerve injury and ingrowth into the disc. METHODS: A neurotracer, Fluoro-Gold (F-G), was applied to the ventral aspect of L5-L6 intervertebral discs in 20 rats. The rats were classified into 2 groups: an NP group whose disc was punctured to expose the nucleus pulposus (n = 10) and a sham-operated group whose anulus fibrosus surface was scratched superficially (n = 10). Ten days after surgery, bilateral L1-L5 DRGs were processed for staining of ATF3 and GAP-43. RESULTS: In the NP group, 13.9% +/- 2.9% of the F-G-labeled neurons innervating the discs were positive for ATF3, while 19.3% +/- 2.7% were positive for GAP-43. In contrast, in the sham-operated group, only 0.8% +/- 0.4% of the F-G-labeled neurons were positive for ATF3 while 7.4% +/- 1.7% were positive for GAP-43. The percentage of both ATF3-immunoreactive (IR) and GAP-43-IR neurons in the NP group was significantly higher than in the sham-operated group (P < 0.05). CONCLUSIONS: ATF3-IR and GAP-43-IR neurons were significantly increased in the NP group. These results suggested that exposure of the nucleus pulposus to the outside of the anulus fibrosus induced nerve injury and in growth into the discs. These findings may explain discogenic lower back pain in patients with lumbar disc degeneration.