Background: Discogenic low back pain has been shown to develop into chronic intractable pain due to an unknown pathogenesis. To study the mechanism of discogenic pain, we analyzed the serial expression of pain-related molecules in the dorsal root ganglia (DRG) and thalamus using a newly developed rat model of disc degeneration.
Methods: Ten microliters of complete Freund's adjuvant was injected into the L5-6 disc of male Sprague-Dawley rats for 10 minutes using a 26-gauge needle. Using a behavioral test, rats with significant pain were selected and subsequently serial gene expression of pain-related molecules in the DRG and the thalamus was analyzed by reverse transcriptase polymerase chain reaction.
Results: The expression of tumor necrosis factor-α and interleukin-1β significantly increased at 4 and 8 weeks in the DRG of rats with pain. Furthermore, interleukin-6 was significantly increased at 4 weeks in the DRG; however, these cytokines did not show a significant change in the thalamus. Calcitonin gene-related peptide and substance P were significantly increased in DRG at 4 and 8 weeks and in the thalamus at 2 and 4 weeks. The level of nerve growth factor-β did not significantly increase in the DRG or thalamus, whereas glial cell line-derived neurotropic factor (GDNF) was significantly increased at 2 weeks and was sustained through 8 weeks in both the DRG and thalamus.
Conclusions: The disc degeneration rat model described herein led to significant pain of a chronic nature. The gradual and persistent increase of GDNF in both the thalamus and DRG suggests that GDNF might be a key factor in the development of intractable, chronic discogenic pain.