Cytokine inhibition and time-related influence of inflammatory stimuli on the hyperalgesia induced by the nucleus pulposus

Abstract

Introduction: The symptoms of lumbar disc herniation, such as low back pain and sciatica, have been associated with local release of cytokines following the inflammatory process induced by the contact of the nucleus pulposus (NP) with the spinal nerve.

Material and methods: Using an animal experimental model of intervertebral disc herniation and behavioral tests to evaluate mechanical (electronic von Frey test) and thermal (Hargreaves Plantar test) hyperalgesia in the hind paw of rats submitted to the surgical model, this study aimed to detect in normal intervertebral disc the cytokines known to be involved in the mechanisms of inflammatory hyperalgesia, to observe if previous exposure of the intervertebral disc tissue to specific antibodies could affect the pain behavior (mechanical and thermal hyperalgesia) induced by the NP, and to observe the influence of the time of contact of the NP with the fifth lumbar dorsal root ganglion (L5-DRG) in the mechanical and thermal hyperalgesia.

Results: The cytokines present at highest concentrations in the rat NP were TNF-α, IL-1β and CINC-1. Rats submitted to the disc herniation experimental model, in which a NP from the sacrococcygeal region is deposited over the right L5-DRG, showed increased mechanical and thermal hyperalgesia that lasted at least 7 weeks. When the autologous NP was treated with antibodies against the three cytokines found at highest concentrations in the NP (TNF-α, IL-1β and CINC-1), there was decrease in both mechanical and thermal hyperalgesia in different time points, suggesting that each cytokine may be important for the hyperalgesia in different steps of the inflammatory process. The surgical remotion of the NP from herniated rats 1 week after the implantation reduced the hyperalgesia to the level similar to the control group. This reduction in the hyperalgesia was also observed in the group that had the NP removed 3 weeks after the implantation, although the intensity of the hyperalgesia did not decreased totally. The removal of the NP after 5 weeks did not changed the hyperalgesia observed in the hind paw, which suggests that the longer the contact of the NP with the DRG, the greater is the possibility of development of chronic pain.

Conclusion: Together our results indicate that specific cytokines released during the inflammatory process induced by the herniated intervertebral disc play fundamental role in the development of the two modalities of hyperalgesia (mechanical and thermal) and that the maintenance of this inflammation may be the most important point for the chronification of the pain.

Fig. 1

Determination of inflammatory cytokines in the sacrococcygeal nucleus pulposus and adipose tissue of rats. Data are reported as the mean ± SEM of the material obtained from ten animals. The asterisk indicates a statistical difference (P < 0.05) between cytokine concentration in the nucleus pulposus and in adipose issue (Student’s t test for unpaired data)

Fig. 2

Intensity of mechanical (electronic von Frey) and thermal (Hargreaves) hyperalgesia induced by placement of a sponge soaked in cytokines (TNF-α, IL-1β and CINC-1—positive control group) or saline solution (negative control group) over the L5-DRG. The asterisk indicates a statistically significant difference (P < 0.005) between the control and the treated groups

Fig. 3

Intensity of mechanical (electronic von Frey) and thermal (Hargreaves) hyperalgesia induced by placement of a sponge soaked in anti-IL-1β, anti-TNF-α, anti-CINC-1 antibody or in non-immune serum and placed over the L5-DRG. The asterisk indicates a statistically significant difference (P < 0.005) between the group treated with IL-1β, anti-TNF-α and anti-CINC-1 antibody and the group treated with non-immune serum

Fig. 4

Intensity of mechanical and thermal hyperalgesia after surgical removal of the nucleus pulposus (NP) 1 week after the induction of hyperalgesia. The asterisk indicates a significant difference (P < 0.05) (MANOVA, followed by ANOVA with Bonferroni test) between group II (hernia group) and group III (NP removal group). The results are reported as the mean ± SEM of five animals

Fig. 5

Intensity of mechanical and thermal hyperalgesia after surgical removal of the nucleus pulposus (NP) 3 weeks after the induction of hyperalgesia. The asterisk indicates a significant difference (P < 0.05) (MANOVA, followed by ANOVA with Bonferroni test) between group II (hernia group) and group III (NP removal group). The results are reported as the mean ± SEM of five animals

Fig. 6

Intensity of mechanical and thermal hyperalgesia after the surgical removal of the nucleus pulposus (NP) 5 weeks after the induction of hyperalgesia. There was no significant difference (P < 0.05) (MANOVA, followed by ANOVA with Bonferroni test) between group II (hernia group) and group III (NP removal group). The results are reported as the mean ± SEM of five animals