Participation of pro- and anti-nociceptive interleukins in botulinum toxin A-induced analgesia in a rat model of neuropathic pain

Botulinum neurotoxin serotype A (BoNT/A) shows antinociceptive properties, and its clinical applications in pain therapy are continuously increasing. BoNT/A specifically cleaves SNAP-25, which results in the formation of a non-functional SNARE complex, thereby potently inhibiting the release of neurotransmitters and neuropeptides, including those involved in nociception. The aim of the present study was to determine the effects of BoNT/A (300 pg/paw) on pain-related behavior and the levels of glial markers and interleukins in the spinal cord and dorsal root ganglia (DRG) after chronic constriction injury (CCI) to the sciatic nerve in rats. Glial activity was also examined after repeated intraperitoneal injection of minocycline combined with a single BoNT/A injection. Our results show that a single intraplantar BoNT/A injection did not influence motor function but strongly diminished pain-related behaviors in naïve and CCI-exposed rats. Additionally, microglial inhibition using minocycline enhanced the analgesic effects of BoNT/A. Western blotting results suggested that CCI induces the upregulation of the pronociceptive proteins IL-18, IL-6 and IL-1β in the ipsilateral lumbar spinal cord and DRG, but no changes in the levels of the antinociceptive proteins IL-18BP, IL-1RA and IL-10 were observed. Interestingly, BoNT/A injection suppressed the CCI-induced upregulation of IL-18 and IL-1β in the spinal cord and/or DRG and increased the levels of IL-10 and IL-1RA in the DRG. In summary, our results suggest that BoNT/A significantly attenuates pain-related behavior and microglial activation and restores the neuroimmune balance in a CCI model by decreasing the levels of pronociceptive factors (IL-1β and IL-18) and increasing the levels of antinociceptive factors (IL-10 and IL-1RA) in the spinal cord and DRG.