Pro- and Anti-inflammatory Cytokine Gene Expression in Rat Sciatic Nerve Chronic Constriction Injury Model of Neuropathic Pain

Wallerian degeneration after crush injury of rat sciatic nerve increases endo- and epineurial tumor necrosis factor-alpha protein

The pro-inflammatory cytokine tumor necrosis factor-alpha (TNF) contributes to injury-induced peripheral nerve pathology and to the development of neuropathic pain. Here, we investigated whether TNF protein is altered at the site of crush injury of rat sciatic nerve using enzyme-linked immunoassay (ELISA) and immunohistochemistry (IHC). TNF protein levels determined by ELISA were low in nerve homogenates from naive rats. After crush injury, local TNF increased rapidly with a two-fold increase on day 0.5. TNF content remained elevated on day 3 and returned to baseline levels again by day 14 after crush. IHC revealed prominent TNF-immunoreactivity in many epineurial macrophages on days 0.5 to 3 after crush injury. These data indicate that TNF protein is early and transiently upregulated at the site of peripheral nerve trauma.

New and emerging pharmacological targets for neuropathic pain

Increasing knowledge of the molecular consequences of nerve injury and the availability of genome databases has greatly increased the range of potential targets for the pharmacological management of neuropathic pain. Controlling neuronal sensitization and the associated alterations in gene expression, protein modification, and neuronal excitability is the key to managing neuropathic pain. Control of neuronal sensitization can occur through inhibition of nerve injury-associated production of cytokines, activation of glial cells, modulation of potassium channel subtypes, mitogen-activated protein kinases, the ubiquitin-proteasome system, or the protection and amplification of spinal cord dorsal horn inhibitory systems. These new and already established targets promise unparalleled opportunities for the prevention, management, and resolution of persistent pain states following nerve injury.

Partial sciatic nerve transection causes redistribution of pain-related peptides and lowers withdrawal threshold

Complete nerve transection results in loss of sensation and paralysis of the involved extremity. Such injury drastically reduces content of the nociceptive peptides, substance P, and somatostatin in the dorsal horn of the spinal cord and dorsal root ganglia innervating the limb. Partial nerve injuries occur more commonly in clinical practice, however, and frequently result in the development of chronic neuropathic pain. To investigate mechanisms underlying this pathologic pain syndrome, rats were subjected to partial sciatic nerve transection. Withdrawal thresholds determined with Von Frey hairs dropped dramatically in the operated limb. On postoperative Day 4, thresholds had decreased from 15 g to less than 5 g on the operated side, whereas those on the contralateral (unoperated) side or those from sham-operated rats did not change. Sciatic hemisection had no effect on total content of either substance P or somatostatin in the dorsal spinal cord and lumbar dorsal root ganglia as measured by radioimmunoassay on postoperative Days 4, 7, or 14. However, when examined immunohistochemically, there was a marked redistribution of both peptides associated with partial transection. On the contralateral side or in sham-operated rats, both substance P and somatostatin were confined to the superficial laminae of the dorsal horn. By contrast, on the operated side, content of both peptides was reduced by more than half in the superficial laminae. There was a compensatory increase in content in the deeper laminae where nociceptive peptides are not usually found. Redistribution of substance P and somatostatin may be due to axonal sprouting, increased peptide expression by interneurons, or aberrant expression of nociceptive peptides by neurons normally mediating mechanical sensation. The presence of increased levels of nociceptive peptides in regions of the spinal cord that mediate innocuous sensation may underlie development of allodynia.

Cyclooxygenase inhibitors and thalidomide ameliorate vincristine-induced hyperalgesia in rats

In this study ibuprofen (50.0 mg/kg, i.p.), rofecoxib (10.0 mg/kg, i.p.) and thalidomide (50.0 mg/kg, oral) were shown to prevent vincristine-induced mechanical hyperalgesia. Sprague-Dawley rats were injected every other day with vincristine (0.1 mg/kg) over 13 days. The animals were cotreated daily with vehicle (saline), ibuprofen, rofecoxib or thalidomide throughout the period of vincristine treatment. Mechanical withdrawal threshold to punctuate and radiant heat stimuli were determined prior to and then on alternate days throughout the treatment period. Vincristine vehicle-treated animals developed marked mechanical hyperalgesia from day 5 of chemotherapy and this lasted until the end of the experiment. Thermal thresholds were not altered by the administration of vincristine vehicle. Animals in the vincristine vehicle group neither gained nor lost weight during the treatment period. All three active drugs showed an antihyperalgesic effect on the responses to mechanical stimulation of the hind paw that was significant from day 5 for ibuprofen and thalidomide and from day 7 for rofecoxib. Thermal thresholds increased after the administration of both the NSAIDs and thalidomide. Rofecoxib was the only drug to show any beneficial effect in protecting the animals from failure to gain body weight.

Wallerian degeneration after crush or chronic constriction injury of rodent sciatic nerve is associated with a depletion of endoneurial interleukin-10 protein

We used enzyme-linked immunoassay (ELISA), immunohistochemistry (IHC), and quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) to determine whether interleukin (IL)-10 protein is changed after unilateral crush or chronic constriction injury (CCI) of mouse or rat sciatic nerve and whether IL-10 protein and mRNA are differentially regulated. In the mouse sciatic nerve, IL-10 protein declined rapidly to 10-20% of baseline early after crush or CCI, while the IL10 mRNA was up-regulated with a maximum on Days 1 and 3. In the rat sciatic nerve, IL-10 protein was significantly reduced on Day 3 after CCI, and IL-10 mRNA was up-regulated in both models. These results suggest that changes of the local cytokine network during wallerian degeneration include an early deficiency of the antiinflammatory cytokine IL-10 despite up-regulation at the mRNA level.

Effect of etodolac, a COX-2 inhibitor, on neuropathic pain in a rat model

Etodolac, a cyclooxygenase-2 inhibitor, may alleviate nociceptive pain and inhibit the activation of osteoclasts. The aim of the present study was to determine whether etodolac can alleviate heat-evoked hyperalgesia and investigate its possible protective effects on osteoporosis induced by chronic constriction injury (CCI) in rats. A CCI to the sciatic nerve was performed, after which the rats received etodolac orally in a volume of 2 ml at 0, 1, and 10 mg/kg/day for 1 to 5 weeks following surgery (experiment 1); at 0 and 10 mg/kg/day for 1 day to 5 weeks following surgery (experiment 2); and at 0 mg/kg/day for 1 to 5 weeks, 10 mg/kg/day for 1 to 2 weeks after surgery, or 10 mg/kg/day for 1 to 3 weeks after surgery (experiment 3). Paw withdrawal latency after exposure to heat, bone mineral content (BMC) and bone mineral density (BMD) in the whole tibial bone, and the number of tartrate resistant acid phosphate (TRAP)-positive multinucleated osteoclasts were measured. Etodolac alleviated heat-evoked hyperalgesia in the CCI rats and the increase in number of TRAP-positive multinucleated osteoclasts on the CCI-side was abrogated, however, it did not inhibit the decrease of BMC and BMD on the CCI-side. Our results suggest that etodolac is useful for treatment of neuropathic pain.

Transgenic mice possessing increased numbers of nociceptors do not exhibit increased behavioral sensitivity in models of inflammatory and neuropathic pain

At least two classes of neciceptors can be distinguished based on their growth factor requirements: glial cell-line derived neurotrophic factor (GDNF)- and nerve growth factor (NGF)-dependent primary afferent neurons. Based on numerous anatomical and biochemical differences, GDNF- and NGF-dependent neurons have been proposed to be involved in the development of different types of persistent pain. To examine this hypothesis we used two lines of transgenic mice that contained a supernormal number of either NGF- or GDNF-dependent neurons (referred to as NGF-OE and GDNF-OE mice, respectively). These mice were tested in a model of inflammatory pain (induced by injection of complete Freund's adjuvant) and neuropathic pain (using a spinal nerve ligation protocol). Contrary to expectations, neither line of transgenic mice became more hyperalgesic following induction of persistent pain. In fact, NGF-OE mice recovered more rapidly and became hypoalgesic despite extensive paw swelling in the inflammatory pain model. In the neuropathic pain model, only wildtype mice became hyperalgesic. Real-time PCR analysis showed that the NGF-OE and GDNF-OE mice exhibited changes in neuronal-specific mRNAs in the dorsal root ganglia but not the spinal cord dorsal horn. These results indicate that increasing the number of nociceptors results in potent compensatory mechanisms that may begin with changes in the sensory neurons themselves.

The effects of endogenous interleukin-10 on gray matter damage and the development of pain behaviors following excitotoxic spinal cord injury in the mouse

Interleukin-10 (IL-10) has been utilized as a neuroprotective agent in experimental models of spinal cord injury because of its potent anti-inflammatory properties. Previous studies have delivered a single dose (5 microg) of IL-10 following experimental spinal cord injury in the rat, and demonstrated various degrees of neuroprotection. However, the role of endogenous production of IL-10 has not been considered. Therefore, the purpose of the current study was to establish the role of endogenous IL-10 and demonstrate the true potential of exogenous IL-10 administration through the use of IL-10((-/-)) mice. Using the quisqualic acid model of spinal cord injury, we examined the extent of gray matter damage and onset of injury-induced pain behaviors at various time points following injury in wild-type vs. IL-10((-/-)) mice. Additionally, IL-10 was reconstituted in IL-10 deficient mice by the intraperitoneal administration of 50 ng recombinant murine (rm) IL-10 30 min following quisqualic acid injection. Animals were observed daily following injury for the onset of pain-behaviors. At days 1, 7, and 14 following injection, lesion analysis revealed a greater extent of damage at early time points (1 day, 7 days) following injury in the IL-10((-/-)) animals as compared with wild-type animals. However, by 14 days post-experimental spinal cord injury, the extent of damage between the two groups was not significant. IL-10((-/-)) animals that received the single (50 ng) rmIL-10 injection following injury displayed gray matter damage patterns similar to wild-type animals. The pronounced early damage noted in the IL-10((-/-)) animals was associated with an approximately two-fold increase in peripheral neutrophils, an index of an innate immune response to injury, compared with wild-type mice. In addition, wild type and IL-10((-/-)) animals receiving rmIL-10 demonstrated a delay in the onset of injury-induced pain behaviors. However, by 14 days post-experimental spinal cord injury the overall incidence of pain behaviors was similar between all treatment groups. Therefore, the absence of IL-10 expression accelerates the kinetics of lesion expansion, the onset of pain behaviors, and the peripheral immune response to spinal cord injury. Endogenous IL-10 and low doses of exogenous IL-10 are neuroprotective at 1 and 7 days following injury. Therefore, the results of the current study suggest that low dose IL-10 administration acutely following spinal cord injury has potential as a therapeutic agent for limiting tissue loss following injury.

Peripheral norepinephrine exacerbates neuritis-induced hyperalgesia

Inflammation of a peripheral nerve (neuritis) causes mechanical and thermal hyperalgesia in the region in which the inflamed nerve innervates. We investigated whether peripherally applied norepinephrine (NE) would exacerbate mechanical hyperalgesia in rats with neuritis. After inflammation of the left L5 spinal nerve with complete Freund's adjuvant, the foot withdrawal thresholds to mechanical stimuli applied to the affected hind paw (mechanical thresholds) were decreased significantly, indicating the development of mechanical hyperalgesia. An intradermal injection of NE to the affected paw further aggravated mechanical hyperalgesia transiently (1-3 days) and then recovered to the pre-NE injection levels afterwards. This responsiveness to NE (adrenergic sensitivity) was observed not only while rats were showing inflammatory hyperalgesia but also after recovering from it. The effect of NE on mechanical hyperalgesia was mediated by both peripheral alpha(1)- and alpha(2)-adrenoceptors. Immunohistochemical study of the previously inflamed nerve showed that proinflammatory cytokine tumor necrosis factor immunoreactivity was significantly higher in the rats showing adrenergic sensitivity compared to rats without adrenergic sensitivity. The data thus suggest that peripheral NE, when released in an excessive amount from the sympathetic nervous system, might play an important role in the aggravation of pain in neuritis.

New treatments for diabetic neuropathy: pathogenetically oriented treatment

Although there is clear evidence from experimental diabetic neuropathy (DN) models that the multiple pathways involved in neuronal degeneration cause overproduction of reactive oxygen species, oxidative stress, and cellular dysfunction, therapeutic approaches addressing these mechanisms have not yet provided a basis for a successful treatment of patients with DN. This review discusses the current knowledge on the pathomechanisms of unchecked reactive oxygen species accumulation, implications for specific treatment, and the need for carefully designed experimental studies and clinical trials closing the gap between promising results in experimental DN and its implementation into a pathogenetically oriented treatment.

Perioperative administration of the α2-adrenoceptor agonist clonidine at the site of nerve injury reduces the development of mechanical hypersensitivity and modulates local cytokine expression

The development of chronic pain after surgery is not rare. Nerve injury from complete or partial nerve section during surgery leads to macrophage recruitment and release of pro-inflammatory cytokines, leading in turn to sensitization. Macrophages also express alpha2-adrenoceptors, and we previously demonstrated a prolonged reduction in hypersensitivity following peri-neural injection of the alpha2-adrenoceptor agonist, clonidine, in rats with chronic nerve injury. The current study tested whether peri-neural clonidine at the time of injury could also prevent development of hypersensitivity. Rats underwent partial ligation of one sciatic nerve, and peri-neural saline, clonidine or a combination of clonidine and the alpha2A-adrenceptor-preferring antagonist, BRL44408, were administered before wound closure and, in some animals, also 24 and 48 h later. The single clonidine injection reduced hypersensitivity for only 5 h, whereas repeated injection for three days reduced hypersensitivity for 28 days. Peri-neural clonidine reduced the increase in tissue content of the proinflammatory cytokines IL-1beta and particularly TNFalpha in sciatic nerve, DRG and spinal cord while increasing concentrations of the anti-inflammatory cytokine TGF-beta1. Clonidine's effects on behavior and TNFalpha content were blocked by BRL44408. We conclude that peri-neural administration of clonidine at the site and time of injury reduces the degree of hypersensitivity in part by altering the balance of pro- and anti-inflammatory cytokines through activation of alpha2A-adrenoceptors. These results support testing of whether clonidine, as an adjuvant in continuous peripheral nerve blocks in settings of known major nerve injury, such as limb amputation, might prevent the development of chronic pain.

Identification of PINCH in Schwann cells and DRG neurons: shuttling and signaling after nerve injury

Particularly interesting new cysteine-histidine rich protein (PINCH) is a double zinc finger domain (LIM)-only adapter protein that functions to recruit the integrin-linked kinase (ILK) to sites of integrin activation. Genetic studies have shown that PINCH and ILK are required for integrin signaling. Since integrin activation is associated with Schwann cell migration, neurite outgrowth and regeneration, this study examined PINCH in the normal peripheral nervous system and after chronic constriction injury (CCI) in adult Sprague-Dawley rats. Immunohistochemistry identified PINCH immunoreactivity in cell bodies of dorsal root ganglia (DRG) neurons, axons, satellite cells, and Schwann cells. PINCH immunostaining was localized to the membrane of uninjured DRG cell bodies consistent with its localization at a site of integrin activation. In contrast, 5 days following CCI, PINCH immunostaining was diffuse throughout the DRG cell cytoplasm. Confocal microscopy of primary and transformed Schwann cells localized PINCH in cytoplasmic, perinuclear and nuclear areas. Examination of the PINCH sequence revealed a putative leucine-rich nuclear export signal (NES) and an overlapping basic nuclear localization signal (NLS). To demonstrate nuclear export of PINCH, rabbit anti-PINCH IgG was microinjected into Schwann cell nuclei and allowed to combine with PINCH contained within the nucleus. Immunofluorescence showed that the PINCH and anti-PINCH IgG complex rapidly translocated to the cytoplasm. Treatment with leptomycin B caused nuclear accumulation of PINCH, indicating that the CRM1 pathway mediates nuclear export of PINCH. ILK activity in Schwann cells was enhanced by platelet-derived growth factor (PDGF) and tumor necrosis factor alpha. PINCH immunoprecipitates from PDGF- and TNFalpha-stimulated Schwann cells contained several high-molecular-weight threonine-phosphorylated proteins. Taken together, these results indicate that PINCH is an abundant shuttling/signaling protein in Schwann cells and DRG neurons.

Inflammatory hypersensitivity in a rat model of trigeminal neuropathic pain

Thermal and mechanical stimuli have been used to monitor the development of neuropathic pain following an experimental injury to a branch of the trigeminal nerve. However, the response to inflammatory challenge has not been evaluated in a model of orofacial neuropathic pain. The purpose of this study was to determine whether chronic constriction of the infraorbital nerve (IoN) enhances nociceptive responses elicited by the formalin test. The characteristic biphasic response (primarily directed grooming) to formalin injected subcutaneously in the right vibrissae pad was observed in sham-injury rats. Twenty-one days after IoN constriction, formalin injection provoked an immediate response that involved both directed grooming and other abnormal behaviors, e.g. flinching, trismus and shielding of the affected region. As with sham-injury rats, this was followed by a quiescent period and then a second phase of nocifensive behaviors. The total time recorded for all pain-related behaviors was significantly greater in rats with constrictive injuries (P<0.001), due primarily to the exhibition of novel pain-related responses. Histological examination (qualitative) revealed that chronic constriction resulted in a ligature-induced neuroma, as well as a partial denervation of the affected sensory field. Thus, an intense inflammatory hypersensitivity in a rat model of orofacial neuropathic pain develops in association with partial denervation and with an ongoing perineural inflammatory response and neuroma formation.

The glycine/NMDA receptor antagonist (+)-HA966 enhances the peripheral effect of morphine in neuropathic rats

Systemic opioid dosing until adequate analgesia in neuropathic pain may involve intolerable and untreatable side effects. Peripheral opioid receptor mechanisms may participate in the antinociceptive effect of systemic morphine. We evaluated the effect of peripherally injected morphine alone, and the ability of the functional antagonist at the glycine site of the N-methyl-D-aspartate (NMDA) receptor complex (+)-HA966 to modulate the antinociceptive effect of peripheral morphine in a rat model of neuropathic pain. Mononeuropathy was induced by placing four ligatures around the common sciatic nerve. Experiments were performed 2 weeks after the nerve ligature, when the pain-related behavior reached a stable maximum. Rats received injections of either subcutaneous (+)-HA966 (2.5mg/kg) or saline administered 20 min before morphine (50-150 microg injected into the nerve-injured hindpaw). The antinociceptive effect was tested against mechanical (vocalization threshold to hindpaw pressure) or thermal (struggle latency to hindpaw immersion into a 46 degrees C hot water bath) stimuli. In both tests, morphine alone (100-150 microg) produced antinociception. Pretreatment with (+)-HA966 did not potentiate the analgesic effectiveness of the two highest doses of morphine, but it did produce analgesia when combined with a low dose of morphine (50 microg), which did not produce analgesia by itself. These effects were reversed by intraplantar naloxone methiodide (50 microg injected into the nerve-injured hindpaw) indicating a peripherally opioid-mediated mechanism of action. The present studies suggested that combined administration of glycine/NMDA receptor antagonists, and peripherally acting morphine may be an interesting approach in the treatment of neuropathic pain.