A Novel Role for Lymphotactin (XCL1) Signaling in the Nervous System: XCL1 Acts via its Receptor XCR1 to Increase Trigeminal Neuronal Excitability.

Chemokines are known to have a role in the nervous system, influencing a range of processes including the development of chronic pain. To date there are very few studies describing the functions of the chemokine lymphotactin (XCL1) or its receptor (XCR1) in the nervous system. We investigated the role of the XCL1-XCR1 axis in nociceptive processing, using a combination of immunohistochemical, pharmacological and electrophysiological techniques. Expression of XCR1 in the rat mental nerve was elevated 3 days following chronic constriction injury (CCI), compared with 11 days post-CCI and sham controls. XCR1 co-existed with neuronal marker PGP9.5, leukocyte common antigen CD45 and Schwann cell marker S-100. In the trigeminal root and white matter of the brainstem, XCR1-positive cells co-expressed the oligodendrocyte marker Olig2. In trigeminal subnucleus caudalis (Vc), XCR1 immunoreactivity was present in the outer laminae and was colocalized with vesicular glutamate transporter 2 (VGlut2), but not calcitonin gene-related peptide (CGRP) or isolectin B4 (IB4). Incubation of brainstem slices with XCL1 induced activation of c-Fos, ERK and p38 in the superficial layers of Vc, and enhanced levels of intrinsic excitability. These effects were blocked by the XCR1 antagonist viral CC chemokine macrophage inhibitory protein-II (vMIP-II). This study has identified for the first time a role for XCL1-XCR1 in nociceptive processing, demonstrating upregulation of XCR1 at nerve injury sites and identifying XCL1 as a modulator of central excitability and signaling via XCR1 in Vc, a key area for modulation of orofacial pain, thus indicating XCR1 as a potential target for novel analgesics.

Correlation of Nav1.8 and Nav1.9 sodium channel expression with neuropathic pain in human subjects with lingual nerve neuromas.

BACKGROUND: Voltage-gated sodium channels Nav1.8 and Nav1.9 are expressed preferentially in small diameter sensory neurons, and are thought to play a role in the generation of ectopic activity in neuronal cell bodies and/or their axons following peripheral nerve injury. The expression of Nav1.8 and Nav1.9 has been quantified in human lingual nerves that have been previously injured inadvertently during lower third molar removal, and any correlation between the expression of these ion channels and the presence or absence of dysaesthesia investigated. RESULTS: Immunohistochemical processing and quantitative image analysis revealed that Nav1.8 and Nav1.9 were expressed in human lingual nerve neuromas from patients with or without symptoms of dysaesthesia. The level of Nav1.8 expression was significantly higher in patients reporting pain compared with no pain, and a significant positive correlation was observed between levels of Nav1.8 expression and VAS scores for the symptom of tingling. No significant differences were recorded in the level of expression of Nav1.9 between patients with or without pain. CONCLUSIONS: These results demonstrate that Nav1.8 and Nav1.9 are present in human lingual nerve neuromas, with significant correlations between the level of expression of Nav1.8 and symptoms of pain. These data provide further evidence that changes in expression of Nav1.8 are important in the development and/or maintenance of nerve injury-induced pain, and suggest that Nav1.8 may be a potential therapeutic target.

TRPA1 expression in human lingual nerve neuromas in patients with and without symptoms of dysaesthesia.

The TRPA1 receptor is a member of the ankyrin family and is found in both spinal and trigeminal neurones. There is evidence to suggest that this receptor may be a sensor of noxious thermal stimuli in normal animals. After nerve injury, TRPA1 shows increased expression in uninjured axons, and has been implicated in the development and maintenance of hyperalgesia. We examined expression of TRPA1 in lingual nerve neuromas and investigated any potential correlation with the presence or absence of symptoms of dysaesthesia. Thirteen neuroma-in-continuity specimens were obtained from patients undergoing repair of a lingual nerve that had previously been damaged during lower third molar removal. Visual analogue scales (VAS) were used to record the degree of pain, tingling and discomfort. Tissue was processed for indirect immunofluorescence and the percentage area of PGP 9.5-immunoreactive neuronal tissue also labelled for TRPA1 was quantified. No significant difference between levels of TRPA1 in neuromas from patients with or without symptoms of dysaesthesia and no relationship between TRPA1 expression and VAS scores for pain, tingling or discomfort were observed. TRPA1 expression and the time after initial injury that the specimen was obtained also showed no correlation. These data show that TRPA1 is expressed in lingual nerve neuromas, but, it appears that, at this site, TRPA1 does not play a principal role in the development of neuropathic pain.

Immunohistochemical analysis of the purinoceptor P2X7 in human lingual nerve neuromas.

AIMS: Recent evidence suggests that the purinoceptor P2X7 may be involved in the development of dysesthesia following nerve injury, therefore, the aim of the present study was to investigate whether a correlation exists between the level of P2X7 receptor expression in damaged human lingual nerves and the severity of the patients' symptoms. METHODS: Neuroma-in-continuity specimens were obtained from patients undergoing surgical repair of the damaged lingual nerve. Specimens were categorized preoperatively according to the presence or absence of dysesthesia, and visual analog scales scores were used to record the degree of pain, tingling, and discomfort. Indirect immunofluorescence using antibodies raised against S-100 (a Schwann cell marker) and P2X7 was employed to quantify the percentage area of S-100 positive cells that also expressed P2X7. RESULTS: P2X7 was found to be expressed in Schwann cells of lingual nerve neuromas. No significant difference was found between the level of P2X7 expression in patients with or without symptoms of dysesthesia, and no relationship was observed between P2X7 expression and VAS scores for pain, tingling, or discomfort. No correlation was found between P2X7 expression and the time between initial injury and nerve repair. CONCLUSION: These data show that P2X7 is expressed in human lingual nerve neuromas from patients with and without dysesthesia. It therefore appears that the level of P2X7 expression at the injury site may not be linked to the maintenance of neuropathic pain after lingual nerve injury.

Peripheral mechanisms for the initiation of pain following trigeminal nerve injury.

Injury to a branch of the trigeminal nerve may lead to the development of chronic pain in the affected area. The etiology of this condition is not clear, but there is strong evidence to suggest that spontaneous and mechanically induced neural discharge from the injury site plays a crucial role. In laboratory studies, we have characterized this discharge following injury to the inferior alveolar or lingual nerves and have shown a temporal association with the accumulation of neuropeptides in the damaged axons. Substance P, calcitonin gene-related peptide, and vasoactive intestinal polypeptide were all found to be capable of increasing the discharge when applied systemically, and enkephalin caused a decrease. There were also changes in the expression of specific sodium channels and nitric oxide synthase, both at the injury site and in the trigeminal ganglion. Studies on lingual nerve neuromas taken from patients undergoing nerve repair also revealed accumulation of peptides, as well as inflammatory and structural changes, but the presence of these features did not correlate directly with the reported symptoms. The application of corticosteroids to an experimental injury site decreased the mechanically induced discharge, and the anticonvulsant carbamazepine reduced the spontaneous discharge in some axons. Some of the responses that result from damage to a branch of the trigeminal nerve appear to differ from those that follow damage to other peripheral nerves. These differences will need to be taken into account when developing new therapeutic approaches for the management of injury-induced trigeminal pain.

Neuropeptide expression following constriction or section of the inferior alveolar nerve in the ferret.

Some of the sensory abnormalities that follow peripheral nerve injury may result from the development of ectopic discharge from the damaged axons. Previous studies in our laboratory have shown that, following tight ligation of the inferior alveolar nerve (IAN), there is a close association between the time-course of this neural activity and the accumulation of neuropeptides at the injury site. In this study we investigated whether the type of injury has any effect on the time-course or level of neuropeptide expression. In 36 adult ferrets, the IAN was either loosely constricted or sectioned, and the animals left to recover for 3 days, 3 weeks, or 3 months. The tissue was processed using indirect immunofluorescence and image analysis was used to quantify levels of substance P, calcitonin gene-related peptide, vasoactive intestinal polypeptide, enkephalin, galanin, and neuropeptide Y. Immunoreactivity to all of the neuropeptides was present within the injured nerve 3 days after both types of injury, and decreased to lower levels by 3 weeks and 3 months. Comparisons between the levels of neuropeptide immunoreactivity in each group revealed that the pattern of accumulation was similar following loose constriction or section, and also similar to that found in our previous study on tight ligation. For each injury the time-course of neuropeptide expression was similar to that of the spontaneous activity we had previously recorded. These data support the suggestion that neuropeptide accumulation may be linked to the development of ectopic neural activity but indicate that the type of injury has little effect on the extent of expression.