Background & Aims
The formation of neuroma following peripheral nerve injury is considered as a primary onset of a painful condition. The accumulation of sodium channels in the cell bodies and the terminal neuroma following nerve injury generates ectopic neuronal firings, which contributes to central sensitization leading to the development of chronic pain. However, the underlying mechanisms of neuroma-related pain are not fully understood. In the present study, we investigated the correlation between the structural characteristics and molecular mechanisms within the neuroma and the severity of neuroma pain. We examined the role of voltage-gated sodium (Nav) channels in neuroma pain by analyzing the alteration of Nav channels (Nav1.7 and Nav1.8) expression in the peripherally according to pain severity.
Methods
To develop an animal model of neuroma pain, the tibial nerve was completely ligated and transected in rats. Behavioral tests were quantified by using paw withdrawal threshold to mechanical stimulation, latency to noxious heat stimulation and duration of spontaneous foot lifting. Immunohistochemistry to quantify the calcitonin gene-related peptide (CGRP) expression was performed in the L4-6 segments of the spinal cord. Two months after the nerve injury, the diameter of the neuroma was measured and correlations between neuroma size and the pain severity were analyzed using Spearman’s rank correlation test. Immunofluorescence was performed to observe the distribution of sensory fibers and demyelinated axons in the neuroma. In vivo dorsal root single-unit recordings at the L4-6 levels were performed to record ongoing spontaneous activity (SA). The expression of axonal sodium channels (Nav1.7 and Nav1.8) in the neuroma and dorsal root ganglions (DRGs) were measured through western blot.
Results
Following nerve injury, approximately 20% of rats did not show pain like behavior (painless group); however, the formation of neuroma was all present in the painful and painless groups. The painless group showed lower expression of CGRP in the spinal dorsal horn compared to the painful group. There was no significant association between the size of neuroma and pain severity. The proportion of sensory fibers and degree of demyelination also showed any significant correlation between pain severity. However, in vivo dorsal root single-unit recordings, decreased SA was shown in the painless group when it compared to the painful group. In addition, the greater density of Nav channels (Nav1.7 and Nav1.8) was observed in the painful neuroma and DRGs than in the painless group at both 7 and 28 days after nerve injury.
Conclusions
The present data demonstrate that Nav1.7 and Nav1.8 are accumulated in the neuroma and DRGs following nerve injury, with a significant correlation between the Nav channel expression and the pain severity in both acute and chronic phases. In addition, the structural features of the neuroma such as size and demyelination were not related to pain. These findings provide further evidence that changes in the expression of Nav1.7 and Nav1.8 are important in the development and maintenance of neuroma pain. Our results suggest that Nav1.7 and Nav1.8 could be a possible mechanism for neuroma pain and may be a potential therapeutic target depending on the pain severity.
References
1. Neumeister, Michael W., and James N. Winters. “Neuroma.” Clinics in plastic surgery 47.2 (2020): 279-283.
2. Buch, Nina Stockfleth, et al. “Neuromas and postamputation pain.” Pain 161.1 (2020): 147-155.
3. Hameed, Shaila. “Nav1. 7 and Nav1. 8: Role in the pathophysiology of pain.” Molecular pain 15 (2019): 1744806919858801.
4. Persson, Anna-Karin, et al. “Nav1. 7 accumulates and co-localizes with phosphorylated ERK1/2 within transected axons in early experimental neuromas.” Experimental neurology 230.2 (2011): 273-279.
5. Roza, Carolina, et al. “The tetrodotoxin?resistant Na+ channel Nav1. 8 is essential for the expression of spontaneous activity in damaged sensory axons of mice.” The Journal of physiology 550.3 (2003): 921-926.
6. Devor, Marshall. “Ectopic discharge in A? afferents as a source of neuropathic pain.” Experimental brain research 196 (2009): 115-128.
7. Dorsi, Michael J., et al. “The tibial neuroma transposition (TNT) model of neuroma pain and hyperalgesia.” Pain 134.3 (2008): 320-334.