Background & Aims

Multiple sclerosis (MS) is an inflammatory autoimmune disorder of the central nervous system (CNS)1. Over half of people living with MS experience chronic pain that is refractive to current treatments2. The pathophysiology of MS includes proinflammatory microglial activation that can mediate neuropathic pain in models of peripheral nerve injury3,4. Spinal microglia are activated not only by peripheral nerve injury, but also central neuron injury, and this activation contributes to spinal neuroinflammation and neuronal sensitization; however, the contribution of microglia to central neuropathic pain, especially in the context of neuroimmune disease like MS, is poorly understood. This study seeks to determine whether spinal microglia maintain multiple sclerosis-associated neuropathic pain (MSNP).

Methods

Adult male and female mice, some with tamoxifen-inducible CX3CR1-CreER::hM4Di-DREADD transgenes, underwent experimental autoimmune encephalomyelitis in the absence of pertussis toxin (EAE-nPTX) with the subcutaneous injection of MOG33-55 and complete Freund’s adjuvant (CFA). EAE-nPTX mice rapidly developed plantar hypersensitivity to mechanical and cold stimuli, and then they and their control groups (CFA without MOG35-55 or no injections) received either: 1) Global CNS depletion of microglia with a colony-stimulating factor 1 receptor (CSFR1) inhibitor, PLX3397 (600ppm in chow); 2) Local microglial depletion at the spinal cord or brain with injection of liposome encapsulated clodronate (LEC) by the intrathecal (50µL/10µL, i.t.) or intracerebroventricular route (50µg/10µL), respectively; or 3) in transgenic mice, reversible inhibition of microglia in spinal cord or throughout the CNS with clozapine N-oxide (CNO) injected i.t. (60µg/10µL) or intraperitoneal (3mg/kg), respectively.

Results

PLX3397 or i.t. LEC reduced IBA-1 immunoreactivity in the spinal cord by 50%-90%, confirming depletion of microglia. When tested 72-hr after intrathecal injection, LEC robustly attenuated mechanical, but not cold allodynia, in male and female EAE-nPTX mice but not control mice (P<0.01). In a separate cohort of mice expressing inhibitory DREADDS on microglia, spinal inhibition via i.t. CNO robustly attenuated mechanical, but not cold allodynia, at 30-90 minutes post-CNO (P<0.05). Surprisingly, neither global nor brain-directed microglial depletion influenced mechanical or cold hypersensitivity. Additionally, no treatment had lasting effects on EAE disease severity as measured by EAE clinical scores.

Conclusions

We conclude that spinal microglia maintain EAE induced mechanical hypersensitivity and represent a target for future pain treatments. Most drugs designed to target microglia, including those used in the above study, exert effects on peripheral macrophages as well. We do not yet fully understand the diversity of microglia within the CNS and how the disease changes these populations. We will need to conduct further studies to better identify specific subpopulations that represent targets for pain management. Our findings emphasize the complexity of site-specific immune contributions to pain in the MS population. The effects of spinal microglial inhibition appears to be masked by brain microglia or macrophage off-target effects. This adds to the challenges faced in pain management in multiple sclerosis and highlights the need for future investigations that focus on spinal microglial-specific targeting with analgesic drugs.

References

1.Khan, N., & Smith, M. T. (2014). Multiple sclerosis-induced neuropathic pain: Pharmacological management and pathophysiological insights from rodent EAE models. Inflammopharmacology, 22(1), 1–22. https://doi.org/10.1007/s10787-013-0195-3
2.Compston, A., & Coles, A. (208). Multiple sclerosis. The Lancet, 372(9648), 1502–1517.
https://doi.org/https://doi.org/10.1016/S0140-6736(08)61620-7
3.Iannitti, T., Kerr, B., & Taylor, B. (2014). Mechanisms and Pharmacology of Neuropathic Pain in Multiple Sclerosis. Curr Top Behav Neurosci, 20, 75–97. https://doi.org/doi:10.1007/7854_2014_288
4.Jack, C., Ruffini, F., Bar-Or, A., & Antel, J. P. (2005). Microglia and multiple sclerosis. Journal of Neuroscience Research, 81(3), 363–373. https://doi.org/10.1002/jnr.20482

Presenting Author

Sydney R Lamerand

Poster Authors

Sydney Lamerand

BSc

University of Pittsburgh

Lead Author

Anvita Anumolu

BS

University of Pittsburgh

Lead Author

Ronit Deshpande

BS

University of Pittsburgh

Lead Author

Ridhima Jain

University of Pittsburgh

Lead Author

Skyy Steber

University of Pittsburgh

Lead Author

Bradley Taylor

University of Pittsburgh

Lead Author

Topics

  • Specific Pain Conditions/Pain in Specific Populations: Neuropathic Pain - Central