End structure and neuropeptide immunoreactivity of axons in sciatic neuromas following nerve section in neonatal rats

Biology and pathophysiology of symptomatic neuromas

ABSTRACT

Neuromas are a substantial cause of morbidity and reduction in quality of life. This is not only caused by a disruption in motor and sensory function from the underlying nerve injury but also by the debilitating effects of neuropathic pain resulting from symptomatic neuromas. A wide range of surgical and therapeutic modalities have been introduced to mitigate this pain. Nevertheless, no single treatment option has been successful in completely resolving the associated constellation of symptoms. While certain novel surgical techniques have shown promising results in reducing neuroma-derived and phantom limb pain, their effectiveness and the exact mechanism behind their pain-relieving capacities have not yet been defined. Furthermore, surgery has inherent risks, may not be suitable for many patients, and may yet still fail to relieve pain. Therefore, there remains a great clinical need for additional therapeutic modalities to further improve treatment for patients with devastating injuries that lead to symptomatic neuromas. However, the molecular mechanisms and genetic contributions behind the regulatory programs that drive neuroma formation-as well as the resulting neuropathic pain-remain incompletely understood. Here, we review the histopathological features of symptomatic neuromas, our current understanding of the mechanisms that favor neuroma formation, and the putative contributory signals and regulatory programs that facilitate somatic pain, including neurotrophic factors, neuroinflammatory peptides, cytokines, along with transient receptor potential, and ionotropic channels that suggest possible approaches and innovations to identify novel clinical therapeutics.

Mechanisms and treatment of painful neuromas

A painful neuroma is a common complication of a peripheral nerve injury or amputation, and it can cause tremendous pain that is resistant to most analgesics. Furthermore, painful neuromas have a high postoperative recurrence rate. Painful neuromas are often accompanied by functional disorders, drastically reducing the patient’s quality of life. Several pathophysiological mechanisms have been proposed to explain this type of neuropathic pain, including peripheral and central sensitisation and the involvement of nerve growth factor, α-smooth muscle actin, the cannabinoid CB2 receptor and structural changes in neuroma fibres. Nevertheless, the mechanisms of neuroma-associated pain are not fully understood, contributing to the challenge of managing patients with painful neuromas. There are several effective treatment methods, although none are universally accepted. This review summarises the common mechanisms and treatments of painful neuromas, attempting to link the mechanisms and treatments. We hope to provide useful guidelines for choosing the appropriate treatment for the management of painful neuromas.

Novel expression pattern of neuropeptide Y immunoreactivity in the peripheral nervous system in a rat model of neuropathic pain

Background

Neuropeptide Y (NPY) has been implicated in the modulation of pain. Under normal conditions, NPY is found in interneurons in the dorsal horn of the spinal cord and in sympathetic postganglionic neurons but is absent from the cell bodies of sensory neurons. Following peripheral nerve injury NPY is dramatically upregulated in the sensory ganglia. How NPY expression is altered in the peripheral nervous system, distal to a site of nerve lesion, remains unknown. To address this question, NPY expression was investigated using immunohistochemistry at the level of the trigeminal ganglion, the mental nerve and in the skin of the lower lip in relation to markers of sensory and sympathetic fibers in a rat model of trigeminal neuropathic pain.

Results

At 2 and 6 weeks after chronic constriction injury (CCI) of the mental nerve, de novo expression of NPY was seen in the trigeminal ganglia, in axons in the mental nerve, and in fibers in the upper dermis of the skin. In lesioned animals, NPY immunoreactivity was expressed primarily by large diameter mental nerve sensory neurons retrogradely labelled with Fluorogold. Many axons transported this de novo NPY to the periphery as NPY-immunoreactive (IR) fibers were seen in the mental nerve both proximal and distal to the CCI. Some of these NPY-IR axons co-expressed Neurofilament 200 (NF200), a marker for myelinated sensory fibers, and occasionally colocalization was seen in their terminals in the skin. Peptidergic and non-peptidergic C fibers expressing calcitonin gene-related peptide (CGRP) or binding isolectin B4 (IB4), respectively, never expressed NPY. CCI caused a significant de novo sprouting of sympathetic fibers into the upper dermis of the skin, and most, but not all of these fibers, expressed NPY.

Conclusions

This is the first study to provide a comprehensive description of changes in NPY expression in the periphery after nerve injury. Novel expression of NPY in the skin comes mostly from sprouted sympathetic fibers. This information is fundamental in order to understand where endogenous NPY is expressed, and how it might be acting to modulate pain in the periphery.

Inflammatory cell accumulation in traumatic neuromas of the human lingual nerve

OBJECTIVE

To quantify the accumulation of inflammatory cells in traumatic neuromas of the human lingual nerve, and to establish any correlation with the patients' reported symptoms of dysaesthesia.

DESIGN

Using fluorescence immunohistochemistry, the extent of any chronic inflammatory infiltrate was quantified in human lingual neuroma specimens removed from 24 patients at the time of microsurgical nerve repair. A pan-leucocyte marker (CD45) and a specific macrophage marker (CD68) were used, and comparisons made between neuromas-in-continuity (NICs) and nerve-end neuromas (NENs) in patients with or without symptoms of dysaesthesia.

RESULTS

CD68 and CD45 labelling was significantly associated with areas of viable nerve tissue in neuromas and the CD68 labelling was significantly higher in NICs than NENs. CD68 labelling density tended to decrease with increasing time after the initial nerve injury, but this correlation was only significant for labelling associated with viable nerve tissue in NENs. No significant difference was found between the level of CD68 or CD45 labelling in patients with or without symptoms of dysaesthesia.

CONCLUSION

This study has demonstrated the presence of inflammatory cells within traumatic neuromas of the human lingual nerve. These cells were found to be closely associated with regions of viable nerve tissue, but there was no correlation with the patients' clinical symptoms.

Close apposition and exposure of non-myelinated axons in traumatic neuromas of the human lingual nerve

Peripheral nerve injury is sometimes followed by the development of persistent painful sensory disorders, such as dysaesthesia. The aetiology of these disorders is not clear, but abnormal behaviour of damaged axons at the injury site is likely to be involved. In this study, we quantified some ultrastructural characteristics that may be related to the development of abnormal spontaneous activity, sympathetic interactions, and fibre-to-fibre crosstalk. Using electron microscopy, we have determined the frequency and extent of axonal exposure and close apposition among non-myelinated axons from 34 traumatic neuromas of the human lingual nerve. These specimens were removed at the time of microsurgical nerve repair, and the presence or absence of symptoms of dysaesthesia was determined pre-operatively. Comparisons were also made with eight normal control lingual nerve specimens obtained from patients undergoing organ donor retrieval. More non-myelinated axons showed signs of axonal exposure in traumatic neuromas (26%) than in controls (5%), and exposure was higher in nerve-end neuromas (31%) than in neuromas-in-continuity (22%). In addition, the proportion of the non-myelinated axolemma that was exposed was significantly higher in neuromas (32%) than in controls (21%). The frequency of close apposition between neighbouring non-myelinated axons was also higher in neuromas (11%) than in controls (0.35%). The majority of axons showing signs of exposure or close apposition had diameters <1 microm. These ultrastructural changes may account for some of the altered electrophysiological properties of axons within neuromas. However, no significant correlations were found between these ultrastructural characteristics and the patients' reported symptoms of dysaesthesia.

The microenvironment of injured and regenerating peripheral nerves

Local events in the milieu of injured peripheral nerve trunks may have an important influence on the likelihood of regenerative success or the development of neuropathic pain. Injury-related changes in the microcirculation of this milieu have provided some evidence that axonal endbulbs, structures that form at the proximal end of transected axons, dump peptides and other molecules into the injury milieu where they may exert local actions, including those on microvessels. During a later phase of nerve repair, macrophage influx and pancellular proliferative events appear to develop in a coordinated fashion. Nitric oxide is probably an important and prominent player in the injured nerve trunk, both at early and later stages of the repair process. A better understanding of the injured peripheral nerve microenvironment may allow therapeutic approaches that can enhance regeneration and diminish pain.

Transient action of the endothelial constitutive nitric oxide synthase (ecNOS) mediates the development of thermal hypersensitivity following peripheral nerve injury

Neuropathic pain is a disabling feature of peripheral nerve injury. Following injury, local inflammation and the release of mediators may contribute to ectopic mechanosensitivity of the nerve-trunk and pain hypersensitivity. In the present study we investigated whether nitric oxide (NO) action and local nitric oxide synthase (NOS) expression play a role in pain hypersensitivity and A fibre-mediated ectopic hyperexcitability following a chronic constriction injury to a rat sciatic nerve. Using immunohistochemical methods we provide evidence for a unique endothelial constitutive nitric oxide synthase (ecNOS) immunoreactivity localized in early axonal endbulb-like structures of injured peripheral nerve axons. Moreover, we show that following nerve injury there is increased ecNOS-mRNA expression within the lumbar sympathetic ganglia, and that axoplasmic transport in sympathetic and other axons rather than local non-neural synthesis accounts for its accumulation in nerve fibres. We also demonstrate here that local inhibition of NOS action with the broad-spectrum inhibitor NG-nitro-L-arginine-methyl ester (L-NAME), but not more specific inhibitors of other NOS isoforms, has stereospecific, dose- and time-dependent analgesic effects that were reversed by local administration of L-arginine, the natural precursor of NO. In further work, using a teased fibre preparation, we show that administration of L-NAME, but not D-NAME, to the injury site also blocks ectopic mechanosensitivity of injured A-fibres. Our results indicate that an early and transient local ecNOS expression within early axonal endbulb-like structures, some arising from sympathetic axons, plays a critical role in the development of neuropathic pain.

The absence of neuromas in beaks of adult hens after conservative trimming at hatch

OBJECTIVE

To determine the effects of the amount of break removed and cauterisation time on neuroma formation in hens.

DESIGN

A pathology study with controls.

ANIMALS

Twenty domestic fowl were beak-trimmed. Three non-beak-trimmed domestic fowl were used as controls.

PROCEDURE

Beaks of two age groups with two levels of beak removal and either 2 s or 4 s cauterisation, were investigated macroscopically and microscopically for deformities.

RESULTS

Scattered trauma-associated neuromas were present in the beaks of pullets 10 weeks after moderate trimming at hatch. Neuromas were not present in beaks of adult hens that had been similarly trimmed. Sensory corpuscles were present 10 and 70 weeks after moderate trimming, though fewer in number than in intact control hens. In contrast, trauma-associated neuromas persisted in beaks of 70-week-old hens that had been severely trimmed at hatch. A range of deformities that were absent in moderately trimmed hens, were observed in hens with severely trimmed beaks. Receptors were not seen in severely trimmed beaks.

CONCLUSION

Beak-trimming at hatch induces the formation of neuromas, regardless of the amount of tissue removed. There is a critical amount of beak tissue that can be removed, beyond which trauma-associated neuromas will not resolve, but will persist in mature hens.

Distribution and axonal relations of macrophages in a neuroma

After axotomy in the peripheral nervous system, most axons regrow and re-establish contact with their targets. Depending on the type of lesion, a varying number of nerve fibers fail to regenerate and terminate far from the target, forming a neuroma. Sensory axons trapped in a neuroma show abnormal sensitivity to various stimuli, and often fire spontaneously. In this study we have examined the distribution and axonal relations of macrophages in rat sciatic neuromas three days to one year after cutting and ligating the nerve. ED1-immunoreactive macrophages migrated into the neuroma in large numbers within the two first weeks after the injury. Most cells were at that time located 0.5-1 mm proximal to the ligature. From three weeks on, a majority of the ED1-immunoreactive cells contained numerous large vacuoles filled with myelin fragments. At sites of focal demyelination, macrophages often had direct contact with axonal membranes. At later survival stages (three months to one year) ED1-immunoreactive cells were seen not only in the area just proximal to the ligature, but also several millimeters proximal to this. Macrophages persisted in considerable numbers in the neuroma for at least one year. These data suggest that neuroma macrophages may participate in the genesis of electrophysiological abnormalities thought to underly chronic pain after neuroma formation, possibly by creating demyelinated axonal regions susceptible to external stimuli from e.g. neighboring nerve fibers, by releasing substances which influence regeneration and remodelling of axonal growth cones, or by direct actions on the denuded axonal membranes.

Signs of neuropathic pain depend on signals from injured nerve fibers in a rat model

The present study was undertaken to determine the role of injured fibers in the development of neuropathic pain using our earlier established rat model. Our model was produced by placing tight ligatures to the L5 or both the L5 and L6 spinal nerves on one side in the rat. These rats showed long-lasting behavioral signs of mechanical allodynia and heat hyperalgesia. Using the uniqueness of our model, 3 specific questions are being asked concerning the initiation and maintenance of behavioral signs for neuropathic pain. The results of behavioral tests performed after various surgical manipulations suggest that: (1) peripheral nerve injury itself is the critical factor for the development of behavioral signs of neuropathic pain; (2) signs of neuropathic pain appear only when injury occurs at a part of the peripheral nerve distal to the dorsal root ganglion; and (3) signals (either electrical or chemical) entering the spinal cord from the injured fibers or the dorsal root ganglion cells play a critical role for both initiation and maintenance of the neuropathic pain state.

NPY-like immunoreactivity in sensory nerve fibers in rat sciatic neuroma

Neuropeptide Y (NPY)-like immunoreactivity was examined by fluorescence immunohistochemistry in rat sciatic neuromas, L5 dorsal root ganglias (DRGs) and L5 dorsal roots 1-3 weeks after chronic nerve injury. Anterograde tracing demonstrated that a large number of NPY-positive neuroma fibers were sensory. These fibers were mostly large diameter axons, in line with the finding that a majority of NPY-immunoreactive neurons in the DRG were medium- to large-sized neurons which showed immunoreactivity to the neurofilament antibody RT 97. In dorsal roots NPY immunoreactivity was strong after sciatic neuroma formation. Dorsal rhizotomy and ligation, on the other hand, did not induce NPY immunoreactivity at any of the sites examined.