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Contribution of small diameter non-peptidergic primary afferent neurons to central neuropathic pain in a new, more clinically relevant mouse model of multiple sclerosis

Nguyen KL, Lamerand SR, Deshpande RP, Taylor BK (2021) Contribution of small diameter non-peptidergic primary afferent neurons to central neuropathic pain in a new, more clinically relevant mouse model of multiple sclerosis. Neuroscience 2021 Abstracts P377/07. Society for Neuroscience, Virtual.

Summary: Over 50% of multiple sclerosis (MS) patients suffer from neuropathic pain (MSNP). Current treatments give inadequate relief due to incomplete understanding of underlying mechanisms. Recent electrophysiological recordings of primary afferent neurons (PAN) in the dorsal root ganglion (DRG) following experimental autoimmune encephalomyelitis (EAE), a mouse model of MS, revealed increased afterhyperpolarization in small-diameter fibers. These data form the premise of our goal to understand the contribution of small-diameter (peptidergic or non-peptidergic) PANs to MSNP. Arguably the most common method to induce EAE is administration of myelin oligodendrocyte glycoprotein (MOG) to generate an autoimmune response targeting the myelin sheath. A MOG fragment is typically given with two adjuvants: complete Freund’s adjuvant (CFA) to boost immunogenicity and pertussis toxin (PTX) to breakdown the blood-brain barrier and facilitate CNS immune cell infiltration. However, PTX can disrupt G-protein coupled receptors, cause pain, and alter autoimmune response gene expression. In 10-week-old C57BL/6 mice, we conducted the first rigorous comparison of a classic PTX EAE model with the novel non-PTX (nPTX) EAE model. We found that both PTX and nPTX EAE mouse models showed the same degree of: 1) motor deficits; 2) plantar hindpaw mechanical and cold hypersensitivity (except cold hypersensitivity resolved more quickly after PTX EAE than nPTX EAE); and 3) lumbar spinal cord demyelination. Unlike most rodent models of MS including PTX EAE, the nPTX EAE group exhibited somatosensory cortex demyelination, a core feature of MS in human patients and cold hypersensitivity. We suggest nPTX EAE to be the most clinically relevant rodent model available to study not only MSNP, but MS in general. To evaluate the contribution of peptidergic and non-peptidergic neurons to MSNP, we induced nPTX EAE. After 12 days we administered capsaicin (10µg/mouse, i.t.) or IB4-saporin (1.5µg/mouse, i.t.) to primarily ablate peptidergic or nonpeptidergic C-fibers, respectively. Ablation efficacy was successfully confirmed with dramatic loss in DRG of TRPV1/CGRP immunoreactivity (peptidergic C-fibers) following capsaicin, and IB4 immunoreactivity (nonpeptidergic C-fibers) following IB4-saporin. IB4-saporin, but not capsaicin, partially reduced mechanical hypersensitivity and reversed cold hypersensitivity within 9 days. These data suggest nonpeptidergic but not peptidergic C-fibers contribute to MSNP. Our next studies will use genetic knockout, chemogenetic, and optogenetic strategies using MrgprdCreER mice to modulate the activity of nonpeptidergic C-fibers.

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