Abstract
ObjectiveIntractable pain after peripheral nerve injury has become a major concern in the field of pain. Current evidence shows that routine medications or surgical treatment is associated with inconsistent results and different curative effects. Stable and effective treatment methods in clinical practice are also lacking. To date, there is no consensus on the pathophysiological mechanisms of pain. The present study investigates the potential regulatory role of regulatory T cells in the differentiation of macrophages on dorsal root ganglion (DRG) and explores the mechanism of nociceptive signals in the signal transfer station. The findings are expected to guide the prevention of various types of peripheral neuropathic pain.MethodsThirty-six male Sprague Dawley (SD) rats and 18 male Nude rats, of equal weight (250–300g), were used in this study. The rats were divided into 3 groups: SD rat sciatic nerve transection group (SNT group, n = 18), SD rat nerve transection experimental group (SNT/RAPA group, n = 18) and Nude rat nerve transection experimental group (SNT/NUDE group, n = 18). The behavior related to neuropathic pain of animals were comprehensively evaluated in all groups. Furthermore, we analyzed the degree of neuroma development, histology, gene, and protein expression, and compared their correlation with the ultrastructural changes of M1/M2 type differentiation of macrophages in DRG.ResultsSciatic nerve transection (SNT), induced the aggregation of several types of macrophages in lumbar DRG of SD rats leading to a higher ratio of M1/M2. Following the inhibition of the M1 type polarization of macrophages, axon outgrowth increased significantly. A significantly lower average autotomy score was reported in the SNT/NUDE group (*p < 0.05) and the SNT/RAPA group (@p < 0.05) as compared to that of the SNT group. The SNT/NUDE group showed no noticeable neuroma formation 30 days after the nerve transection. However, bulbous neuromas were observed in the nerve stumps of both the SNT control and SNT/RAPA groups. Immunofluorescence staining revealed a significant decrease in the proportion of M1/M2 macrophages in lumbar DRG of the SNT/NUDE group (**p < 0.001) and the SNT/RAPA group (@p < 0.05) compared to the SNT group. The expression of pain-related proteins was also decreased (@p < 0.05, *p < 0.05,**p < 0.001). Also, the expression of alpha-smooth muscle actin (α-SMA), neurofilament 200 (NF-200), and nerve growth factor low-affinity receptor p75 were significantly down-regulated in the nerve tissue (@p < 0.05, @@p < 0.001, **p < 0.001).ConclusionM1/M2 type differentiation of macrophages on DRG plays a significant role in the formation of traumatic painful neuroma after neurotomy. In combination with our previous study, the results of this study suggest that regulatory T cells reduce the ratio of M1/M2 macrophages and alleviate the pain of neuroma by regulating the polarization direction of macrophages on neuroma. These findings provide key insights into developing new strategies to manage painful neuroma.
Highlights
Despite the advancement of modern industrial civilization, the recent past has seen an unprecedented rise in trauma and amputation caused by natural and unnatural factors (Bowsher, 2002; Bonfiglioli et al, 2015)
Rat footprints revealed that rats in the SNT group exhibited a significantly larger gait area on the right front, right back, and left front, which is manifested as an increase in step length and step width as compared to normal rats without surgery
The results suggest that the regulatory effect of regulatory T cells on the M2-type polarization of macrophages holds great promise to prevent neuropathic pain associated with a traumatic neuroma
Summary
Despite the advancement of modern industrial civilization, the recent past has seen an unprecedented rise in trauma and amputation caused by natural and unnatural factors (Bowsher, 2002; Bonfiglioli et al, 2015). While limbs are faced with various soft tissue injuries, the peripheral nerves suffer different types and varying degrees of injury, among them, nerve crush injury and nerve severance injury (Noble et al, 1998). Peripheral neuropathic pain has the characteristics of refractory, repetitive, and intractable, which brings great physical and mental pain to patients (Zimmermann, 2001). Such pain response is often accompanied by symptoms of autonomic dysfunction associated with a severe impact on the psychological mood and quality of life of individuals. More and more factors that contribute to the pain perception in response to peripheral nerve injury have been discovered, no consensus has been reached on the precise pathophysiological mechanism of pain (Mackinnon, 1997). Drug therapy or surgical treatment yield unsatisfactory results with varying curative effects, which is why there is a need for more stable and effective treatment methods (Zeilhofer et al, 2011; Yao et al, 2017)
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