In vivo intrathecal IL-1β quantification in rats: Monitoring the molecular signals of neuropathic pain

Background: Neuropathic pain, or pain after nerve injury, is a disorder with a significant reliance on the signalling of cytokines such as IL-1 beta. However, quantifying the cytokine release repeatedly over time in vivo is technically challenging. Aim: To evaluate if changes in IL-1 beta are correlated with the presentation of mechanical allodynia over time, by repeatedly quantifying intrathecal IL-1 beta concentrations following chronic constriction injury of the sciatic nerve in rats. Also, to establish any possible correlation between biochemical spinal marker expression and the in vivo quantification of IL-1 beta. Finally, to assess the expression of the mature IL-1 beta in lumbar spinal cord samples. Method: The Chronic Constriction Injury model (CCI) was used to initiate nerve injury in male Sprague Dawley rats and the generation of behavioural mechanical allodynia was quantified. Using an indwelling intrathecal catheter, a stainless steel (SS) wire biosensing device was repeatedly introduced to quantify intrathecal IL-1 beta concentrations at three timepoints of 0, 7, and 14 days post CCI. Fixed spinal cord samples (L4-L5), collected on day 14, were imaged for the expression of glial fibrillary acidic protein (GFAP, astrocytes) and ionized calcium binding adaptor molecule 1 (IBA1, microglia). Snap frozen spinal cord tissues (L4-L5) were also processed for western blot analysis. Results: Using the novel SS based biosensing device we established that CCI caused a significant increase in intrathecal IL-1 beta concentrations from day 0 to day 7 (p = 0.001) and to day 14 (p < 0.0001), while the sham group did not show any significant increase. We also further showed that the degree of mechanical allodynia correlated positively with the increase in the intrathecal concentration of IL-1 beta in the active CCI animals (p = 0.0007). While there was a significant increase in the ipsilateral GFAP expression in injured animals compared to sham animals (p = 0.03), we did not find any significant correlation between in vivo IL-1 beta concentration on days 7 and 14 and the area of dorsal horn GFAP or IBA1 positive structures on day 14. The result of western blot analysis of whole lumbar spinal cord revealed that there was no significant change (p = 0.7579) in IL-1 beta expression on day 14 in the CCI group compared to the sham group. Conclusion: For the first time we have established that the SS based immunosensing platform technology can repeatedly sample the intrathecal space for bioactive peptides, such as IL-1 beta. Using this novel approach, we have been able to establish the correlation of the intrathecal concentration of IL-1 beta with the extent of mechanical allodynia, providing a molecular biomarker of the degree of the exaggerated pain state.