Background & Aims

Sympathetically maintained pain (SMP) is that type of pain that results from increased activity in the sympathetic nervous system. Despite being prevalent in a substantial number of individuals with chronic nerve injury pain, the factors determining its occurrence or not remain unidentified.
In classical preclinical models of nerve injury, established through permanent transection or ligation of parts of the sciatic nerve, a sympathetic involvement in pain has been observed and characterized by both sympathetic nerve sprouting and the release of norepinephrine (NE) in the affected dorsal root ganglia (DRG).
Our previous research indicated that a transient partial sciatic nerve crush injury can lead to long-term hypersensitivity even after nerve regeneration. In this study, we explored whether sympathetic activity is also evident in the DRG following this crush injury model, which differs from other nerve transection or constriction models.

Methods

All surgical procedures were performed on 6 to 10-week-old C57BL/6J mice using aseptic techniques. The partial crush injury (PCI) was executed as reported, with the sciatic nerve crushed using ultra-fine hemostats equipped with a 30 ?m-thick custom spacer between the crush surfaces. As a positive control for sympathetically maintained pain, a L5 spinal nerve transection (SpNT) model was employed. Pain-related behavior was assessed by a von Frey assay or an automated hind paw behavior analysis. Sympathetic nerve sprouting in the DRG was identified using Tyrosine Hydroxylase (TH) staining on days 7, 15, and 30, focusing on TH-fibers innervating NeuN-positive neurons. NE concentration in the DRG was measured by high-sensitive ELISA. A chemical sympathectomy was induced by an intraperitoneal 6-OHDA injection, and its efficacy confirmed by decreased levels of NE in the spleen.

Results

Both PCI and L5 SpNT models induced mechanical hypersensitivity lasting up to 30 days, but with different onsets. L5 SpNT injury resulted in TH-fiber innervation in the L5 DRG, consistent with previous reports, while PCI groups showed no TH-fiber sprouting in the affected DRG tissue at any timepoints. Correspondingly, NE concentration in the injured DRG increased following SpNT but not after PCI. Chemical sympathectomy reduced hypersensitivity development in L5 SpNT models but had no effect on PCI models.
As the two nerve injury models are distinguished by both location and the type of injury, we further explored TH-fiber innervation after a spinal nerve crush (SpNC) and sciatic nerve transection (ScNT). TH-fiber sprouting was observed in ScNT injury but not after SpNC, which matched positively with occurrence of spontaneous pain behaviors.

Conclusions

We identified that the sciatic nerve PCI model, which produces chronic neuropathic hypersensitivity, lacks the characteristics and features of SMP. Additionally, our findings highlight transection injury as a major catalyst for sympathetic nerve sprouting in the DRG. These results indicate that PCI-induced hypersensitivity operates through sympathetically independent mechanisms, warranting further investigation of the precise mechanisms responsible.

References

McLachlan EM, Jänig W, Devor M, Michaelis M. Peripheral nerve injury triggers noradrenergic sprouting within dorsal root ganglia. Nature. 1993;363(6429):543-546. doi:10.1038/363543a0

Kim HW, Shim SW, Zhao AM, et al. Long-term tactile hypersensitivity after nerve crush injury in mice is characterized by the persistence of intact sensory axons. Pain. 2023;164(10):2327-2342. doi:10.1097/j.pain.0000000000002937

Chung K, Lee BH, Yoon YW, Chung JM. Sympathetic sprouting in the dorsal root ganglia of the injured peripheral nerve in a rat neuropathic pain model. J Comp Neurol. 1996;376(2):241-252. doi:10.1002/(SICI)1096-9861(19961209)376:2<241::AID-CNE6>3.0.CO;2-3

Ramer MS, Thompson SWN, McMahon SB. Causes and consequences of sympathetic basket formation in dorsal root ganglia. Pain. 1999;Suppl 6:S111-S120. doi:10.1016/S0304-3959(99)00144-X

Presenting Author

Hyoung Woo Kim

Poster Authors

Hyoung Woo Kim

PhD

Boston Children's Hospital

Lead Author

Sang Wook Shim

Lead Author

Yoon Kyung Lee

Lead Author

Omer Barkai

PhD

Lead Author

Biyao Zhang

BS

Lead Author

Xiangsunze Zeng

PhD

Lead Author

Kihwan Lee

PhD

Lead Author

Clifford Woolf

MB

Boston Childrens Hospital

Lead Author

Seog Bae Oh

Seoul National University

Lead Author

Topics

  • Models: Chronic Pain - Neuropathic