Inhibition of glial activation by minocycline modifies the effects of botulinum neurotoxin in a model of neuropathic pain.

Background and aims: Botulinum neurotoxin serotype A (BoNT/A) shows long-lasting antinociceptive action in neuropathic pain models and its clinical application in pain therapy is continuously increasing. Our study aimed to examine the BoNT/A-induced effects in rats subjected to chronic constriction injury (CCI) to the sciatic nerve, after attenuation of glia activation by minocycline. Method: The experiments were carried out according to IASP recommendations and local Bioethics Committee. The CCI of the sciatic nerve was performed in Wistar rats. Minocycline was injected intraperitoneally and BoNT/A intraplantarly. Behavioral studies consisted of the allodynia/hyperalgesia measurement, biochemical studies comprised the RT-PCR analysis in the spinal cords and DRG. Results: A single intraplantar injection of BoNT/A induced long-lasting analgesic effect in neuropathic rats and diminished the injury-induced ipsilateral upregulation of C1q (microglial marker) and SNAP-25 mRNA in the spinal cord, independently from glia silencing by minocycline. On the contrary, it up-regulated C1q, GFAP and prodynorphin expression in DRG, previously decreased by minocycline. Conclusions: We have shown that BoNT/A-induced analgesia is dependent on glia activity and that, in case of earlier minocycline administration, BoNT/A-induced biochemical changes are modified. BoNT/A increases the expression of prodynorphin and of both glia markers, effects that may be important for the side effects caused by this neurotoxin. Acknowledgments: This research was supported by statutory funds from the Department of Pain Pharmacology and by a grant Harmonia 5 2013/10/M/NZ4/00261) both from the Ministry of Science and Higher Education. The scientific collaboration between PAN and CNR (Italy) is greatly acknowledged.