Colchicine treatment of the sciatic nerve reduces neurogenic extravasation, but does not affect nociceptive thresholds or collateral sprouting in neuropathic or normal rats

Pain in cancer survivors

Cancer and its treatment exert a heavy psychological and physical toll. Of the myriad symptoms which result, pain is common, encountered in between 30% and 60% of cancer survivors. Pain in cancer survivors is a major and growing problem, impeding the recovery and rehabilitation of patients who have beaten cancer and negatively impacting on cancer patients' quality of life, work prospects and mental health. Persistent pain in cancer survivors remains challenging to treat successfully. Pain can arise both due to the underlying disease and the various treatments the patient has been subjected to. Chemotherapy causes painful chemotherapy-induced peripheral neuropathy (CIPN), radiotherapy can produce late effect radiation toxicity and surgery may lead to the development of persistent post-surgical pain syndromes. This review explores a selection of the common causes of persistent pain in cancer survivors, detailing our current understanding of the pathophysiology and outlining both the clinical manifestations of individual pain states and the treatment options available.

Induction of monocyte chemoattractant protein-1 (MCP-1) and its receptor CCR2 in primary sensory neurons contributes to paclitaxel-induced peripheral neuropathy

The use of paclitaxel (Taxol), a microtubule stabilizer, for cancer treatment is often limited by its associated peripheral neuropathy (chemotherapy-induced peripheral neuropathy [CIPN]), which predominantly results in sensory dysfunction, including chronic pain. Here we show that paclitaxel CIPN was associated with induction of chemokine monocyte chemoattractant protein-1 (MCP-1) and its cognate receptor CCR2 in primary sensory neurons of dorsal root ganglia. Immunostaining revealed that MCP-1 was mainly expressed in small nociceptive neurons whereas CCR2 was expressed in large and medium-sized myelinated neurons. Direct application of MCP-1 consistently induced intracellular calcium increases in dorsal root ganglia large and medium-sized neurons but not in small neurons mainly dissociated from paclitaxel-treated but not vehicle-treated animals. Paclitaxel also induced increased expression of MCP-1 in spinal astrocytes, but no CCR2 signal was detected in the spinal cord. Local blockade of MCP-1/CCR2 signaling by anti-MCP-1 antibody or CCR2 antisense oligodeoxynucleotides significantly attenuated paclitaxel CIPN phenotypes including mechanical hypersensitivity and loss of intraepidermal nerve fibers in hindpaw glabrous skin. These results suggest that activation of paracrine MCP-1/CCR2 signaling between dorsal root ganglion neurons plays a critical role in the development of paclitaxel CIPN, and targeting MCP-1/CCR2 signaling could be a novel therapeutic approach.

PERSPECTIVE

CIPN is a severe side effect accompanying paclitaxel chemotherapy and lacks effective treatments. The current study suggests that blocking MCP-1/CCR2 signaling could be a new therapeutic strategy to prevent or reverse paclitaxel CIPN. This preclinical evidence encourages future clinical evaluation of this strategy.

Epigenetic regulation of opioid-induced hyperalgesia, dependence, and tolerance in mice

Repeated administration of opioids such as morphine induces persistent behavioral changes including opioid-induced hyperalgesia (OIH), tolerance, and physical dependence. In the current work we explored how the balance of histone acetyltransferase (HAT) versus histone deacetylase (HDAC) might regulate these morphine-induced changes. Nociceptive thresholds, analgesia, and physical dependence were assessed during and for a period of several weeks after morphine exposure. To probe the roles of histone acetylation, the HAT inhibitor curcumin or a selective HDAC inhibitor suberoylanilide hydroxamic acid (SAHA) was administered daily to groups of animals. Histone acetylation in spinal cord was assessed by Western blot and immunohistochemistry. Concurrent administration of curcumin with morphine for 4 days significantly reduced development of opioid-induced mechanical allodynia, thermal hyperalgesia, tolerance, and physical dependence. Conversely, the HDAC inhibitor SAHA enhanced these responses. Interestingly, SAHA treatment after the termination of opioid administration sustained these behavioral changes for at least 4 weeks. Histone H3 acetylation in the dorsal horn of the spinal cord was increased after chronic morphine treatment, but H4 acetylation was unchanged. Moreover, we observed a decrease in HDAC activity in the spinal cords of morphine-treated mice while overall HAT activity was unchanged, suggesting a shift toward a state of enhanced histone acetylation.

PERSPECTIVE

The current study indicates that epigenetic mechanisms play a crucial role in opioid-induced long-lasting neuroplasticity. These results provide new sight into understanding the mechanisms of opioid-induced neuroplasticity and suggest new strategies to limit opioid abuse potential and increase the value of these drugs as analgesics.

Chemotherapy-induced neuropathic pain

PURPOSE OF REVIEW

To discuss the importance, clinical features, possible pathology and treatments of chemotherapy-induced neuropathic pain. Newer biological agents such as bortezomib will be considered in greater detail.

RECENT FINDINGS

Chemotherapy-induced peripheral neuropathy (CIPN) is a frequent complication of common anticancer therapies. It may lead to treatment compromise, significantly adds to the symptom burden and interferes with quality of life of cancer survivors. Recent investigations have identified processes involved in CIPN which may give some insight for the development of novel treatments. CIPN induced by different anticancer therapies may be heterogeneous and present as distinct neuropathic pains. Recent work has focussed on the newer anticancer drugs such as bortezomib. Contemporaneous studies have failed to find good evidence for the use of several common antineuropathic agents and further research is required.

SUMMARY

Painful CIPN remains under recognized and undertreated. It is an important cause of pain during cancer treatment and is a common pain in the cancer survivor. Difficulties in assessment and limitations in treatment contribute to management problems. Improvements in education (patient and clinician), assessment and treatment would potentially reduce the often debilitating effects of painful CIPN.

Effects of removal of dietary polyunsaturated fatty acids on plasma extravasation and mechanical allodynia in a trigeminal neuropathic pain model

Background

Neuropathic pain (NP) is partially mediated by neuroinflammatory mechanisms, and also modulates local neurogenic inflammation. Dietary lipids, in particular the total amount and relative proportions of polyunsaturated fatty acids (PUFAs) of the ω-3 and ω-6 families, have been reported to modify the threshold for thermal and mechanical allodynia in the partial sciatic nerve ligation model of NP in rats. The effects of dietary lipids on other popular NP models, such as the chronic constriction injury (CCI), have not yet been examined. It is also unknown whether dietary PUFAs exert any effect on the capsaicin (CAP)-induced neurogenic inflammation under control or NP conditions. In this study we investigated these interrelated phenomena in the trigeminal territory, which has been much less explored, and for which not all data derived from limb nerves can be directly applied.

Results

We studied the effects of a CCI of the infraorbital nerve (IoN) on the development of mechanical allodynia and CAP-induced plasma extravasation in rats fed either a regular diet (RD), or a modified diet (MD) with much lower total content and ω-3:ω-6 ratio of PUFAs. In rats kept on MD, mechanical allodynia following CCI-IoN was more pronounced and developed earlier. Extravasation was substantially increased in naive rats fed MD, and displayed differential diet-depending changes one and four weeks after CCI-IoN. When compared with basal levels (in naive and/or sham cases), the net effect of CCI-IoN on ipsilateral extravasation was a reduction in the MD group, but an increase in the RD group, effectively neutralizing the original intergroup differences.

Conclusion

In summary, PUFA intake reduces CAP-induced neurogenic plasma extravasation in the trigeminal territory, and their removal significantly alters the mechanical allodynia and the plasma extravasation that result from a unilateral CCI-IoN. It is likely that this "protective" effect of dietary lipids is temporary. Also, the presence of contralateral effects of CCI-IoN precludes using the contralateral side as control.

Disruption of axoplasmic transport induces mechanical sensitivity in intact rat C-fibre nociceptor axons

Peripheral nerve inflammation can cause axons conducting through the inflamed site to become mechanically sensitive. Axonal mechanical sensitivity (AMS) of intact axons may explain symptoms in a diverse number of conditions characterized by radiating pain evoked by movements of the affected nerve. Because nerve inflammation also disrupts axoplasmic transport, we hypothesized that the disruption of axoplasmic transport by nerve inflammation could cause the cellular components responsible for mechanical transduction to accumulate and become inserted at the inflamed site, causing AMS. This was tested by examining AMS in C-fibre nociceptors following the application of axoplasmic transport blockers (colchicine and vinblastine) to the sciatic nerve. Both 10 mm colchicine and 0.1 mm vinblastine caused AMS to develop in 30.6% and 33.3% of intact axons, respectively (P < 0.05 compared to sham treatment). Since high doses of colchicine (> 50 mm) can damage axons, and inflammation is involved in the removal of axonal debris, experiments were performed to assess conduction across the treatment site as well as signs of inflammation. Results indicated minimal axonal loss (95% of A- and C-fibres conducting), consistent with the normal microscopic appearance of the colchicine treatment site and absence of ED1-positive (recruited) macrophages. In a separate series of experiments, the block of axoplasmic transport proximal to a localized neuritis significantly reduced inflammation-induced AMS (15.6% compared to 55.6%; P < 0.05), further supporting that the components necessary for AMS are moved by anterograde transport. In summary, nerve inflammation that causes the disruption of axoplasmic transport in patients with painful conditions may result in the accumulation and insertion of mechanosensitive elements at the inflamed site.

Taxol-induced sensory disturbance is characterized by preferential impairment of myelinated fiber function in cancer patients

Taxol produces neuropathic pain with three distinct zones of involvement in the extremities. Most distally is an area of on-going pain and proximal to this is a zone of sensory disturbance but not overt pain. These two areas were confined in all but one case to the glabrous skin of the hands and/or feet. More proximal is an area not recognized by the patients as involved with pain or sensory disturbance yet wherein quantitative sensory tests nevertheless reveal altered sensibility. Impairment of perception to light touch, normally conveyed by myelinated fibers, was dramatically altered in all three areas, being approximately 50-fold greater than normal in areas of pain and sensory disturbance as well as in areas of skin perceived by the patients as not affected. Impairment of perception to sharpness, normally conveyed by small myelinated fibers, was most pronounced in areas of on-going pain, intermediate in areas of sensory disturbance and near baseline in more proximal skin of chemotherapy patients. In contrast to mechanical sensibility, thermal thresholds for warm and heat pain detection were normal throughout. Finally, chemotherapy patients showed paradoxical burning pain to skin cooling that was most pronounced in proximal areas of skin thought to be unaffected by the patients, intermediate in the border zone of altered sensibility and least pronounced in areas of on-going pain. These data suggest that taxol produces a neuropathy characterized by pronounced impairment of function in A-beta myelinated fibers, intermediate impairment of A-delta myelinated fibers, and a relative sparing of C-fibers.

Anterograde transport of tumor necrosis factor-alpha in the intact and injured rat sciatic nerve

Tumor necrosis factor-alpha (TNF) appears as a key player at both central and peripheral terminals in early degenerative pathology and pain behavior after peripheral nerve injury. Recent studies suggest that TNF may be axonally transported and thereby contribute to these central and peripheral actions. To characterize this transport, we used a double ligation (DL) procedure that distinguishes between anterograde and retrograde flow to visualize the axonal transport of endogenous TNF compared with the neurotrophin nerve growth factor (NGF) and to the neuropeptide calcitonin gene-related peptide (CGRP). In the intact nerve, TNF and CGRP immunoreactivity predominantly accumulated proximal to the DL (anterograde transport), whereas NGF displayed exclusive retrograde transport. At 20 hr after chronic constrictive injury (CCI), the anterograde transport of TNF and CGRP to the nerve injury site was dramatically increased. The results were corroborated by the analysis of axonal transport of exogenously applied 125I-TNF and 125I-NGF. After intraneural injection, 125I-TNF accumulated proximally to a DL, suggesting anterograde transport. In the unligated nerve, 125I-TNF was specifically transported anterogradely to the innervated muscle but not to skin. After CCI, 125I-TNF accumulated proximally to the peripheral nerve injury site, and endogenous TNF was exclusively increased in medium-sized and large dorsal root ganglion (DRG) neurons, suggesting that DRG neurons are a major contributing source of increased TNF traffic in the injured sciatic nerve. Our results suggest that anterograde transport of TNF plays a major role in the early neuronal response to peripheral nerve injury at sites distal to the cell body.

Limited role of macrophages in generation of nerve injury-induced mechanical allodynia

The purpose of this study was to investigate the role of peripheral macrophages in the generation of mechanical allodynia utilizing a modification of the Chung rat model of neuropathy. Three distinct methods were used: (1) systemic and perineural macrophage inhibition utilizing CNI-1493; (2) depletion of the peripheral macrophage population by liposome-encapsulated clodronate; and (3) perineural administration of activated or inactivated bone marrow-derived macrophages (BMDM) in sham-surgery rats. Mechanical allodynia was tested on days 1, 3, 5, 7, and 10 post-intervention or surgery using von Frey monofilaments. In order to assess the role of spinal glia following these interventions, microglial (CNS macrophages) and astrocytic activation was assessed using immunohistochemistry. CNI-1493 did not attenuate mechanical allodynia, or spinal glial expression as compared to the saline control group. Similarly, the clodronate depletion of peripheral macrophages prior to nerve injury did not have any effect on the resultant mechanical allodynia or spinal glial activation. Perineural administration of activated or inactivated BMDM did not evoke mechanical allodynia in sham surgery rats. Of interest, we observed an ipsilateral, dorsal horn increase in microglial expression following perineural administration of activated macrophages. In summary, these data suggest a limited role of activated macrophages in the onset of mechanical allodynia in an animal model of neuropathy.

The alpha(2A) adrenoceptor and the sympathetic postganglionic neuron contribute to the development of neuropathic heat hyperalgesia in mice

We have addressed the role of the sympathetic nervous system in the development and maintenance of neuropathic pain. Using a new neuropathic mouse model, we examined the development of hyperalgesia in transgenic mice lacking functional alpha(2A) adrenoceptors and in sympathectomized wild-type mice, to determine if sympathetic-sensory coupling generates hyperalgesia. The development of neuropathic heat hyperalgesia required the presence of both the alpha(2A) adrenoceptor and the sympathetic postganglionic neuron (SPGN), but the development of mechanical hyperalgesia did not require either the alpha(2A) adrenoceptor or the SPGN, indicating different mechanisms of sensitization. These results suggest that the development of neuropathic heat hyperalgesia, but not mechanical hyperalgesia, requires sympathetic-sensory coupling in the peripheral nervous system. Nerve injury enhanced the analgesic efficacy of the alpha(2) adrenoceptor agonist dexmedetomidine, and paradoxically also induced an analgesic response to alpha(2) adrenoceptor antagonists. The alpha(2) agonist-evoked analgesia to mechanical stimuli was mediated by activating central alpha(2A) adrenoceptors, possibly at the spinal level. The peripherally restricted alpha(2) antagonist L659,066 evoked analgesia for heat, but not for mechanical stimuli, findings which support the hypothesis that the peripheral alpha(2) adrenoceptor plays a role in both the development and the maintenance of neuropathic heat hyperalgesia. The alpha(2) antagonist-evoked analgesia for heat stimuli was mediated by blocking peripheral and probably central alpha(2) adrenoceptors, while the analgesia for mechanical stimuli was mediated by blocking central alpha(2A) adrenoceptors. Intradermal injections with an alpha(2) agonist or antagonist had no effect on nociceptive thresholds, indicating that sympathetic-sensory coupling at the level of the cutaneous nociceptor did not contribute to the maintenance of neuropathic hyperalgesia.

Effect of colchicine on neuropeptide Y expression in rat dorsal root ganglia and spinal cord

Neuropeptide Y (NPY) expression in the spinal cord and dorsal root ganglia (DRG) was examined after application of colchicine, an axonal transport blocker, on the intact sciatic nerve or prior to axotomy or chronic constriction injury (CCI). Rats that underwent topical application of colchicine on the sciatic nerve showed decreased responsiveness to heat stimulation, ipsilaterally. CCI-induced hyperalgesia was prevented by prior application of colchicine. However, colchicine did not block axotomy-induced NPY increase when applied proximally to the injury. In fact, colchicine induced the expression of NPY in the DRG and spinal cord in an identical manner to axotomy. The present data indicates that the increase in NPY observed after nerve injury could be initiated by the suppression of retrograde transport of factors, possibly neurotrophins, rather than by the production of an active factor at the site of injury.