Pathobiology of neuropathic pain

Activation of extracellular signal-regulated protein kinase in dorsal horn neurons in the rat neuropathic intermittent claudication model

Extracellular signal-regulated protein kinase (ERK) is a mitogen-activated protein kinase (MAPK) that mediates several cellular responses to mitogenic and differentiation signals, and activation of ERK in dorsal horn neurons by noxious stimulation is known to contribute to pain hypersensitivity. In order to elucidate the pathophysiological mechanisms of the cauda equina syndrome, secondary to spinal canal stenosis, we evaluated walking dysfunction triggered by forced exercise and activation of ERK in the dorsal horn using a rat model of neuropathic intermittent claudication. Rats in the lumbar canal stenosis (LCS) group showed a shorter running distance from 1 to 14 days after surgery. Two minutes after running on the treadmill apparatus, phosphorylation of ERK was induced in neurons in the superficial laminae in the LCS group but not in the sham group, whereas there was no change in the deeper laminae. Intrathecal administration of the MAPK kinase inhibitor, U0126, 30 min before running, clearly increased the running distance, whereas there was no significant change in the vehicle control group 3 days after surgery. In addition, a prostaglandin E1 analog, OP-1206 alpha-CD, administered orally, improved the walking dysfunction, and further, inhibited activation of ERK following running 7 days after surgery. These findings suggest that intermittent claudication triggered by forced walking might affect the phosphorylation of ERK in the superficial laminae, possibly via transient (partial) ischemia of the spinal cord. ERK activation in the dorsal horn neurons may be involved in the transient pain in the neuropathic intermittent claudication model.

Peri-sciatic administration of indomethacin early after nerve injury can attenuate the development of tactile allodynia in a rat model of L5 single spinal nerve injury

To clarify the role of cyclooxygenase in the peripheral nerve on the development of neuropathic pain, we investigated the effects of peri-sciatic administration of indomethacin on the development of allodynia in a model of L5 single spinal nerve injury. Peri-sciatic administration of indomethacin (1 mg/kg) was performed 3, 24, or 72 h after nerve injury (n=6/each). In rats with indomethacin 3 or 24 h after nerve injury, ipsi-lateral paw withdrawal thresholds 7-35 days after nerve injury were significantly higher compared with those in the control group (n=6: without peri-sciatic treatment) (P<0.05). However, such efficacy was no longer apparent when indomethacin was administered 72 h after nerve injury. These results suggest that peri-sciatic administration of indomethacin early (less than 24 h) after nerve injury can attenuate the development of allodynia.

Efficacy of transdermal nitroglycerin combined with etodolac for the treatment of chronic post-thoracotomy pain: an open-label prospective clinical trial

Chronic post-thoracotomy pain (CPP) is associated with surgical intercostal nerve injury. Like other forms of neuropathic pain, there is no ideal treatment. Nitroglycerin (NTG) has been found efficacious in acute pain, but has not been tested for chronic pain with neuropathic characteristics. The present study investigated the efficacy of NTG combined with the nonsteroidal anti-inflammatory drug etodolac for the treatment of CPP. Thirty of 129 patients who underwent thoracotomy within an 18-month period had moderate to severe pain that did not respond to etodolac. NTG, 5 mg/day, was added to the treatment. A significant reduction in VAS was observed on day 14 of treatment (from 66.7 +/- 11 to 42.1 +/- 5, P< 0.05). Similar changes were noted in breakthrough pain intensity and and sleep efficiency. The only side effect was mild headache, which was self-limited to the first few days of NTG administration. We conclude that NTG added to etodolac appears to be effective for the treatment of CPP, with minimal side effects. Further randomized blinded studies are required.

3-(4-Phenoxyphenyl)pyrazoles: A Novel Class of Sodium Channel Blockers

A series of 3-(4-phenoxyphenyl)-1H-pyrazoles were synthesized and characterized as potent state-dependent sodium channel blockers. A limited SAR study was carried out to delineate the chemical requirements for potency. The results indicate that the distal phenyl group is critical for activity but will tolerate lipophilic (+pi) electronegative (+sigma) substituents at the ortho and/or para position. Substitution at the pyrazole nitrogen with a H-bond donor improves potency. Compound 18 showed robust activity in the rat Chung neuropathy paradigm.

Peripheral nerve injury induces trans-synaptic modification of channels, receptors and signal pathways in rat dorsal spinal cord

Peripheral tissue injury-induced central sensitization may result from the altered biochemical properties of spinal dorsal horn. However, peripheral nerve injury-induced modification of genes in the dorsal horn remains largely unknown. Here we identified strong changes of 14 channels, 25 receptors and 42 signal transduction related molecules in Sprague-Dawley rat dorsal spinal cord 14 days after peripheral axotomy by cDNA microarray. Twenty-nine genes were further confirmed by semiquantitative RT-PCR, Northern blotting, in situ hybridization and immunohistochemistry. These regulated genes included Ca2+ channel alpha1E and alpha2/delta1 subunits, alpha subunits for Na+ channel 1 and 6, Na+ channel beta subunit, AMAP receptor GluR3 and 4, GABAA receptor alpha5 subunit, nicotinic acetylcholine receptor alpha5 and beta2 subunits, PKC alpha, betaI and delta isozymes, JNK1-3, ERK2-3, p38 MAPK and BatK and Lyn tyrosine-protein kinases, indicating that several signal transduction pathways were activated in dorsal spinal cord by peripheral nerve injury. These results demonstrate that peripheral nerve injury causes phenotypic changes in spinal dorsal horn. Increases in Ca2+ channel alpha2/delta1 subunit, GABAA receptor alpha5 subunit, Na+ channels and nicotinic acetylcholine receptors in both dorsal spinal cord and dorsal root ganglia indicate their potential roles in neuropathic pain control.

Induction of long-term potentiation of C fibre-evoked spinal field potentials requires recruitment of group I, but not group II/III metabotropic glutamate receptors

In superficial layers of the lumbar spinal dorsal horn, N-methyl-D-aspartate-dependent long-term potentiation (LTP) of C fibre-evoked field potentials, a synaptic model of central sensitisation and hyperalgesia, ensues the application of electrical high-frequency, high-intensity conditioning stimulation to the sciatic nerve. In order to investigate the putative involvement of the G protein-coupled metabotropic glutamate receptors (mGluRs) in the induction of this form of LTP, we applied a series of mGluR antagonists exhibiting distinct group-specific activity profiles to the spinal lumbar enlargement, prior to conditioning stimulation. The group I (mGluR1/5) and group II (mGluR2/3) mGluR antagonist (S)-alpha-methyl-4-carboxyphenylglycine or the selective mGluR1/5 antagonist (S)-4-carboxyphenylglycine consistently impaired the development of spinal LTP. However, potentiation occurred in the presence of the inactive enantiomer (R)-alpha-methyl-4-carboxyphenylglycine. LTP proved insensitive to the selective mGluR2/3 antagonists (2S)-alpha-ethylglutamic acid and LY341495, either spinally or intravenously delivered. LTP could also be induced in the presence of the selective group III (mGluR4/mGluR6-mGluR8) mGluR antagonist (RS)-alpha-methylserine-O-phosphate. However, none of the mGluR-active compounds alone noticeably altered the amplitudes of C fibre-evoked field potentials in the absence of conditioning stimulation. These findings suggest that the induction of LTP of C fibre-evoked field potentials in the spinal dorsal horn by high-frequency, high-intensity stimulation of afferent C fibres requires a group-specific mGluR recruitment, activation of mGluR1/5 but not that of mGluR4/6-8 and mGluR2/3 being a requisite step.

Spinal and cortical spreading depression enhance spinal cord activity

Cortical spreading depression (CSD) has been suggested to underlie some neurological disorders such as migraine. Despite the intensity with which many investigators have studied SD in the brain, only a few studies have aimed to identify SD in the spinal cord. Here we described the main characteristic features of SD in the spinal cord induced by different methods including various spinal cord injury models and demonstrated that SD enhances the spinal cord activity following a transient suppressive period. These findings suggest that SD may play a role in the mechanisms of spinal neurogenic shock, spinal cord injury, and pain. Furthermore, we studied the effect of CSD on the neuronal activity of the spinal cord. CSD was induced via cortical pinprick injury or KCl injection in the somatosensory cortex. CSD did not propagate into the cervical spinal cord. However, intracellular recordings of the neurons in the dorsal horn of C2 segment, ipsilateral to the hemisphere in which CSD was evoked, showed a transient suppression of spontaneous burst discharges, followed by a significant enhancement of the neuronal activity. This indicates a link between a putative cause of the neurological symptoms and the subsequent pain of migraine.

Inflammation and hyperalgesia induced by nerve injury in the rat: a key role of mast cells

Inflammatory cells and their mediators are known to contribute to neuropathic pain following nerve injury. Mast cells play a key role in non-neural models of inflammation and we propose that mast cells and their mediators (in particular histamine) are important in the development of neuropathic pain. In rats, where the sciatic nerve was partially ligated, we showed that stabilisation of mast cells with sodium cromoglycate reduced the recruitment of neutrophils and monocytes to the injured nerve and suppressed the development of hyperalgesia. Treatment with histamine receptor antagonists suppressed the development of hyperalgesia following nerve injury and alleviated hyperalgesia once it was established. These results suggest that mast cell mediators such as histamine released within hours of nerve injury contribute to the recruitment of leukocytes and the development of hyperalgesia.

The role of uninjured nerve in spinal nerve ligated rats points to an improved animal model of neuropathic pain

L5 and L6 spinal nerve ligation (SNL) in rats leads to behavioral signs of neuropathic pain including mechanical allodynia. The purposes of this study were to investigate the role of the intact L4 spinal nerve in the development of mechanical allodynia following L5 and L6 SNL and, as a result, to develop a modified model of neuropathic pain. As a first set of experiments, in addition to tight ligation of the left L5 and L6 spinal nerves, the intact L4 spinal nerve was manipulated either (1) by gentle repeated stretching of the L4 spinal nerve immediately after L5 and L6 SNL or (2) by intermittent mechanical stimulation to the ipsilateral paw during the first week after SNL. Tactile sensitivity was measured by determining the foot withdrawal threshold before and after SNL. Mild irritation of L4 spinal nerve and application of mechanical stimuli to the ipsilateral paw significantly increased the development of mechanical allodynia after SNL. In a second set of experiments, SNL was produced by tightly ligating only the left L5 spinal nerve with or without a loop of 5-0 chromic gut placed loosely around the L4 spinal nerve. This additional L4 loop significantly increased long-lasting tactile sensitivity compared to L5 SNL alone. These results suggest that afferent activity of the intact L4 spinal nerve aids in the development of mechanical allodynia in the SNL model of neuropathic pain. The addition of a chromic gut loop around the intact L4 spinal nerve can augment the development of mechanical allodynia following SNL of L5. We propose this latter as a useful and practical animal model of neuropathic pain.

Coupling gene chip analyses and rat genetic variances in identifying potential target genes that may contribute to neuropathic allodynia development

Genetic factors and nerve injury-induced changes of gene expression in sensory neurons are potential contributors to tactile allodynia, a neuropathic pain state manifested as hypersensitivity to innocuous mechanical stimulation. To uncover genes relevant to neuropathic allodynia, we analyzed gene expression profiles in dorsal root ganglia (DRG) of spinal nerve-ligated Harlan and Holtzman Sprague Dawley rats, strains with different susceptibilities to neuropathic allodynia. Using Affymetrix gene chips, we identified genes showing differential basal-level expression in these strains without injury-induced regulation. Of more than 8000 genes analyzed, less than 180 genes in each strain were regulated after injury, and 19-22% of that was regulated in a strain-specific manner. Importantly, we identified functionally related genes that were co-regulated post injury in one or both strains. In situ hybridization and real-time PCR analyses of a subset of identified genes confirmed the patterns of the microarray data, and the former also demonstrated that injury-induced changes occurred, not only in neurons, but also in non-neuronal cells. Together, our studies provide a global view of injury plasticity in DRG of these rat stains and support a plasticity-based mechanism mediating variations in allodynia susceptibility, thus providing a source for further characterization of neuropathic pain-relevant genes and potential pathways.

Enzyme replacement therapy improves peripheral nerve and sweat function in Fabry disease

Fabry disease is an X-linked disorder caused by a deficiency of lysosomal alpha-galactosidase A resulting in accumulation of alpha-D-galatosyl conjugated glycosphingolipids. Clinical manifestations include a small-fiber neuropathy associated with debilitating pain and hypohidrosis. We report the effect of a 3-year open-label extension of a previously reported 6-month placebo-controlled enzyme replacement therapy (ERT) trial in which 26 hemizygous patients with Fabry disease received 0.2 mg/kg of alpha-galactosidase A every 2 weeks. The effect of ERT on neuropathic pain scores while off pain medications, quantitative sensory testing, quantitative sudomotor axon reflex test (QSART), and thermoregulatory sweat test (TST) is reported. In the patients who crossed-over from placebo to ERT (n = 10), mean pain-at-its-worst scores on a 0-10 scale decreased (from 6.9 to 4.5). There was a significant reduction in the threshold for cold and warm sensation in the foot. At the 3-year time-point, pre-ERT sweat excretion in 17 Fabry patients was 0.24 +/- 0.33 microl/mm(2) vs. 1.05 +/- 0.81 in concurrent controls (n = 38). Sweat function improved 24-72 h post-enzyme infusion (0.57 +/- 0.71 microl/mm(2)) and normalized in four anhidrotic patients. TST confirmed the QSART results. We conclude that prolonged ERT in Fabry disease leads to a modest but significant improvement in the clinical manifestations of the small-fiber neuropathy associated with this disorder. QSART may be useful to further optimize the dose and frequency of ERT.

Roles of CaMKII, PKA, and PKC in the induction and maintenance of LTP of C-fiber-evoked field potentials in rat spinal dorsal horn

Long-term potentiation (LTP) of C-fiber-evoked field potentials in spinal dorsal horn may be relevant to hyperalgesia, an increased response to noxious stimulation. The mechanism underlying this form of synaptic plasticity is, however, still unclear. Considerable evidence has shown that calcium/calmodulin-dependent protein kinase II (CaMKII), protein kinase A (PKA), and protein kinase C (PKC) are important for LTP in hippocampus. In this study, the roles of these three protein kinases in the induction and maintenance of LTP of C-fiber-evoked field potentials were evaluated by application of specific inhibitors of CaMKII (KN-93 and AIP), PKA (Rp-CPT-cAMPS), and PKC (chelerythrine and Gö 6983) at the recording segments before and after LTP induction in urethane-anesthetized Sprague-Dawley rats. We found both KN-93 and AIP, when applied at 30 min prior to tetanic stimulation, completely blocked LTP induction. At 30 min after LTP induction, KN-93 and AIP reversed LTP completely, and at 60 min after LTP induction, they depressed spinal LTP in most rats tested. Three hours after LTP induction, however, KN-93 or AIP did not affect the spinal LTP. Rp-CPT-cAMPS, chelerythrine, and Gö 6983 blocked the spinal LTP when applied at 30 min before tetanic stimulation and reversed LTP completely at 15 min after LTP induction. In contrast, at 30 min after LTP induction, the drugs never affected the spinal LTP. These results suggest that activation of CaMKII, PKA, and PKC may be crucial for the induction and the early-phase but not for the late-phase maintenance of the spinal LTP.

Anti-allodynic and anti-oedematogenic properties of the extract and lignans from Phyllanthus amarus in models of persistent inflammatory and neuropathic pain

This study investigated the anti-allodynic and anti-oedematogenic effects of the hexanic extract, lignan-rich fraction and purified lignans from a plant used in the traditional medicine, Phyllanthus amarus, in the inflammatory and neuropathic models of nociception. The hexanic extract inhibited the allodynia and the oedema induced by the intraplantar injection of complete Freund's adjuvant (CFA). The inhibition observed was 76 +/- 7% (ipsilateral paw), 64 +/- 7% (contralateral paw), and 41 +/- 2% (oedema). Otherwise, the lignan-rich fraction or the pure lignans did not affect CFA-induced allodynia. Administered chronically, the lignan fraction reduced CFA-induced paw oedema (39 +/- 9%). When evaluated in the model of neuropathic pain caused by partial ligation of sciatic nerve, the hexanic extract inhibited the mechanical allodynia (77 +/- 7%), with a similar efficacy to the gabapentin (71 +/- 10%). The anti-allodynic effects of hexanic extract of P. amarus seem not to be associated with the impairment of motor co-ordination or with the development of tolerance. Finally, the treatment with hexanic extract inhibited the increase of myeloperoxidase activity, either following intraplantar injection of CFA or after sciatic nerve injury. It is concluded that, apart from its anti-inflammatory actions, which are probably linked to the presence of lignans, another as yet unidentified active principle(s) present in the hexanic extract of P. amarus produces pronounced anti-allodynia in two models of inflammatory and neuropathic pain. Considering that few drugs are currently available for the treatment of chronic pain, especially of the neuropathic type, the present results may have clinical relevance and open new possibilities for the development of new anti-allodynic drugs.

Phosphorylation of extracellular signal-regulated kinases 1/2 is predominantly enhanced in the microglia of the rat spinal cord following dorsal root transection

The present study was initiated to investigate the role of extracellular signal-regulated kinases (ERK) 1/2 signaling pathway in the early response of spinal cord and associated dorsal root ganglion (DRG) to rhizotomy by using Western blotting and immunohistochemical techniques in a rat model of L3 and L4 dorsal root transection. The results showed that there were a considerable amount of total and phosphorylated ERK 1/2 protein in both spinal cord and DRG in normal animals killed under pentobarbital anesthesia. The total ERK 1/2 distributed in both glia and neurons, while phosphorylated ERK 1/2 dominantly existed in the latter in the gray matter of spinal cord, as demonstrated with double immunofluorescent staining. Twenty-four and forty-eight hours after axotomy, the phosphorylation level of ERK 1/2 in the operation side of dorsal spinal cord was much higher than that in the contralateral side, while the total ERK 1/2 level seemed unchanged. The increased expression of Fos protein was also seen in the dorsal spinal cord at lesion side twelve and twenty-four hours after axotomy. Double fluorescent staining proved that the phosphorylated ERK 1/2 positive cells in the ipsilateral dorsal spinal cord after axotomy predominantly were microglia and small portion was oligodendrocytes, whereas the Fos expression was mainly in neurons. In normal DRG, most neurons, especially the medium and small-sized ones, and the satellite cells contained total ERK 1/2-like immunoreactivity, whereas only a small portion of neurons and satellite cells contained phosphorylated ERK 1/2. After unilateral dorsal rhizotomy, there were no detectable changes for the phosphorylation of ERK 1/2 in either neurons or satellite cells in DRG.Collectively, the present results suggest that both ERK and Fos signal pathways involve the cellular activation in the spinal cord following dorsal rhizotomy, with ERK mainly in microglia and Fos in neurons. The increase of phosphorylation of ERK 1/2 in microglia of spinal cord after rhizotomy implicates that ERK signaling pathway involves intracellular activity of microglia responding to the experimental injury.

Muscimol prevents long-lasting potentiation of dorsal horn field potentials in rats with chronic constriction injury exhibiting decreased levels of the GABA transporter GAT-1

The inhibitory activity of gamma-aminobutyric acid (GABA) is considered critical in setting the conditions for synaptic plasticity, and many studies support an important role of GABA in the suppression of nociceptive transmission in the dorsal horn. Consequently, any injury-induced modification of the GABA action has the potential to critically modify spinal synaptic plasticity. We have previously reported that chronic constriction injury of the sciatic nerve was accompanied by long-lasting potentiation of superficial spinal dorsal horn field potentials following high-frequency tetanus. In this study we examined whether the GABA-A receptor agonist muscimol would modify post-tetanic responses in rats with chronic constriction injury. In animals exhibiting maximal thermal hyperalgesia as one sign of neuropathic pain 7 days after loose ligation of the sciatic nerve, spinal application of muscimol (5, 10 or 20 microg) before the high-frequency (50 Hz) tetanus produced a long-lasting depression (rather than potentiation) of spinal dorsal horn field potentials. In separate but related Western immunoblot experiments, we also established that the chronic constriction injury was accompanied by significant decreases in the content of the GABA transporter GAT-1. These data demonstrated that GABA-A receptor agonists may effectively influence the expression of long-lasting synaptic plasticity in the spinal dorsal horn, and that an injury-induced loss in GABA transporter content may have contributed to a depletion of GABA from its terminals within the spinal dorsal horn. These data lent further support to the notion that the loss of GABA inhibition may have important consequences for the development of neuropathic pain.

Avulsion injury of the rat brachial plexus triggers hyperalgesia and allodynia in the hindpaws: a new model for the study of neuropathic pain

In the present study, we sought to characterise a behavioural model of persistent peripheral neuropathic pain produced by avulsion of the right brachial plexus in rats. In addition, we compared the effects of avulsion with those of ligation or crush injury of the brachial plexus. Avulsion and, to a lesser extent, ligation and crushing of brachial plexus caused a long-lasting (up to 90 days) and highly reproducible mechanical hyperalgesia, in both ipsilateral and contralateral hindpaws. However, the same injury did not produce thermal hyperalgesia. The avulsion and, to a lesser extent, ligation and crushing of the brachial plexus elicited a significant and long-lasting (up to 90 days) ipsilateral and contralateral cold and mechanical allodynia. Furthermore, the brachial plexus injury caused a significant decrease in functional activity of the forepaws as assessed in the grasping strength test, but did not alter the locomotor activity of the rats in the open field test in comparison with control or sham groups. Taken together these results show that avulsion of the brachial plexus in rat produces persistent mechanical and cold allodynia and mechanical hyperalgesia, and might represent a valuable method for understanding the mechanisms underlying the aetiology of neuropathic pain.

Animal and cellular models of chronic pain

Chronic pain, especially neuropathic pain and cancer pain, is often not adequately treated by currently available analgesics. Animal models provide pivotal systems for preclinical study of pain. This article reviews some of the most widely used or promising new models for chronic pain. Partial spinal ligation, chronic constriction injury, and L5/L6 spinal nerve ligation represent three of the best characterized rodent models of peripheral neuropathy. Recently, several mouse and rat bone cancer pain models have been reported. Primary or permanent cultures of sensory neurons have been established to study the molecular mechanism of pain, especially for neurotransmitter release and signal transduction. The emerging gene microarray, genomics and proteomics methods may be applied to throughly characterize these cells. Each model is uniquely created with distinct mechanisms, it is therefore essential to report and interpret results in the context of a specific model.

The challenge of chronic pain

Chronic pain is a complex problem with staggering negative health and economic consequences. The complexity of chronic pain is presented within Cervero and Laird's model that describes three phases of pain, including pain without tissue damage, pain with tissue damage and inflammation, and neuropathic pain. The increased afferent input in phases 2 and 3 of chronic pain produces marked changes in primary afferents, dorsal root ganglia, and spinal cord dorsal horn. These changes promote the symptoms of chronic pain, including spontaneous pain, hyperalgesia, and allodynia. Increased afferent input also evokes supraspinal input to the dorsal horn, including biphasic innervation from the ventromedial medulla and A7 catecholamine cell group, that promotes hyperalgesia and allodynia. More rostral brain structures, such as the lateral hypothalamus, amygdala, and hippocampus, may also play a role in chronic pain. Although much has been discovered about the multiple pathological mechanisms involved in chronic pain, further research is needed to fully comprehend these mechanisms.

Continuous administration of the 5-hydroxytryptamine1A agonist (3-Chloro-4-fluoro-phenyl)-[4-fluoro-4-[[(5-methyl-pyridin-2-ylmethyl) -amino]-methyl]piperidin-1-yl]-methadone (F 13640) attenuates allodynia-like behavior in a rat model of trigeminal neuropathic pain

(3-Chloro-4-fluoro-phenyl)-[4-fluoro-4-[[(5-methyl-pyridin-2-ylmethyl)-amino]-methyl]piperidin-1-yl]-methadone (F 13640) is a recently discovered high-efficacy 5-hydroxytryptamine (HT)1A receptor agonist that produces central analgesia through the neuroadaptive mechanisms of inverse tolerance and cooperation. In a rat model of trigeminal neuropathic pain, the chronic constriction injury of the infraorbital nerve causes allodynia-like behavior that develops within 2 weeks and remains stable thereafter. We report that early after surgery, during which time allodynia develops, the continuous 2-week infusion of 0.63 mg/day F 13640 inhibited the allodynia-like behavior, whereas 5 mg/day morphine showed no significant effect. When F 13640 infusion was initiated late after surgery, when allodynia was well established, it produced an antiallodynic effect that was apparent during the entire infusion period. In contrast, morphine infusion caused an initially marked antiallodynic effect to which tolerance developed within the 2-week infusion period. The GABA-B receptor agonist baclofen (1.06 mg/day) that has a recognized usefulness in the treatment of trigeminal neuralgia, demonstrated effectiveness in both conditions. The data are consistent with a theory of nociceptive signal transduction, as well as with previous data, in demonstrating the neuroadaptive mechanisms of inverse tolerance and cooperation. That is, in contrast with morphine, the antiallodynic effect induced by 5-HT1A receptor activation does not decay, but, if anything, grows with chronicity. Also, 5-HT1A receptor activation seemed to cooperate with nociceptive stimulation in, paradoxically, inducing an antiallodynic effect. The data presented here suggest that F 13640 may perhaps offer a lasting treatment of trigeminal neuralgia.

Anti-hyperalgesic and morphine-sparing actions of propentofylline following peripheral nerve injury in rats: mechanistic implications of spinal glia and proinflammatory cytokines

Injury to peripheral nerves often produces non-physiological, long-lasting spontaneous pain, hyperalgesia and allodynia that are refractory to standard treatment and often insensitive to opioids, such as morphine. Recent studies demonstrate spinal glial activation and increased proinflammatory cytokines in animal models of neuropathic pain. When these data are considered together, a unifying hypothesis emerges which implicates a role of central neuroimmune processes in the etiology of neuronal and behavioral hypersensitivity. The present investigation assessed the influence of propentofylline, a glial modulating and anti-inflammatory agent, on the development of L5 spinal nerve transection-induced hyperalgesia and associated enhancement of spinal neuroimmune responses using real-time reverse transcription-polymerase chain reaction, RNase protection assay, enzyme-linked immunosorbent assay, and immunocytochemistry in rats. The results show that chronic propentofylline treatment attenuated the development of hyperalgesia and restored the analgesic activity of acute morphine in neuropathic rats. These findings directly correlated with the ability of propentofylline to inhibit glial activation and enhanced spinal proinflammatory cytokines following peripheral nerve injury. These findings along with our earlier observations of an anti-allodynic activity of propentofylline using the identical animal model of mononeuropathy supports the concept that modulation of glial and neuroimmune activation may be potential therapeutic targets to treat or prevent neuropathic pain. Further, restoration of the analgesic activity of morphine by propentofylline treatment suggests that increased glial activity and proinflammatory cytokine responses may account for the decreased analgesic efficacy of morphine observed in the treatment of neuropathic pain.

Fatty acid amide hydrolase: an emerging therapeutic target in the endocannabinoid system

The medicinal properties of exogenous cannabinoids have been recognized for centuries and can largely be attributed to the activation in the nervous system of a single G-protein-coupled receptor, CB1. However, the beneficial properties of cannabinoids, which include relief of pain and spasticity, are counterbalanced by adverse effects such as cognitive and motor dysfunction. The recent discoveries of anandamide, a natural lipid ligand for CB1, and an enzyme, fatty acid amide hydrolase (FAAH), that terminates anandamide signaling have inspired pharmacological strategies to augment endogenous cannabinoid ('endocannabinoid') activity with FAAH inhibitors, which might exhibit superior selectivity in their elicited behavioral effects compared with direct CB1 agonists.

Neurotrophic factors as novel therapeutics for neuropathic pain

Neuropathic pain is a chronic condition that is caused by injury to the nervous system. Unlike acute pain, which is protective, neuropathic pain persists and serves no useful purpose, and severely affects quality of life. However, present therapies have modest efficacy in most patients, are palliative rather than curative, and their side effects represent significant limitations. Tremendous progress has been made over the past decade in our understanding of the biology of pain sensory neurons. The recent discovery that neurotrophic factors play an important role in neuropathic pain indicates that these pathways could serve as novel intervention points for therapy. Moreover, neurotrophic factors have the potential to address the underlying pathophysiology of neuropathic pain, thereby halting or reversing the disease process.

Tumor necrosis factor-alpha induces mechanical allodynia after spinal nerve ligation by activation of p38 MAPK in primary sensory neurons

Tumor necrosis factor-alpha (TNF) is implicated in the initiation of neuropathic pain. In vitro, TNF activates p38 mitogen-activated kinase. Accordingly, we investigated whether TNF activates the p38 cascade in vivo to trigger pain behavior after spinal nerve ligation (SNL). Treatment starting 2 d before SNL with the TNF antagonist etanercept (1 mg, i.p., every third day) attenuated mechanical allodynia. Treatment starting 1 or 7 d after SNL was ineffective. Similarly, intrathecal infusion of a p38 inhibitor (SB203580, 4 mg/d) was effective only if it was started before but not 7 d after SNL. For both treatments, the cessation of therapy resulted in increased allodynia. In separate experiments using Western blots and immunohistochemistry, ipsilateral lumbar spinal cord and L5 and L6 DRG were analyzed for total and phosphorylated p38 after SNL alone or SNL combined with etanercept pretreatment. In DRG, activated p38 was transiently elevated 5 hr after SNL and returned to baseline by 1 d after SNL. Phosphorylated p38 was localized in small TNF-positive DRG neurons. In spinal cord, p38 was activated between 5 hr and 3 d after SNL and returned to baseline within 5 d. In DRG, but not spinal cord, etanercept pretreatment blocked p38 activation. These data indicate that after SNL treatment, phosphorylated p38 levels in spinal cord and DRG are transiently elevated. In DRG, p38 activation is blocked by systemic TNF inhibition. Parallel inhibition of p38 activation and allodynia may represent a clinically relevant therapeutic window. These data suggest a sequential role for TNF and p38 in the induction of neuropathic pain.

Pathophysiology and assessment of neuropathic pain in Fabry disease

Severe neuropathic pain and hypohidrosis are important symptoms of Fabry disease, particularly in the first three decades of life. The pain is associated with a length-dependent small-fibre neuropathy that also causes a selective deficiency of cold perception. Cold exposure often accentuates the pain and worsens thermal perception. The hypohidrosis leads to poor exercise and heat tolerance. The mechanisms by which alpha-galactosidase A deficiency causes these physiological abnormalities are poorly understood. The stored glycolipid (globotriaosylceramide) may interfere with the function of cellular membrane proteins, such as ion channels, or may lead to cytotoxicity. The characteristic neuropathic pain can be symptomatically treated with various types of anticonvulsant drugs, such as carbamazepine. Improvement in neuropathic pain as a primary outcome measure has been useful in demonstrating that enzyme replacement therapy is effective in improving pain-related quality of life in Fabry disease.

CONCLUSIONS

The dysfunction of the peripheral nervous system is easily assessable and more readily reversible with specific therapy than the destructive processes that occur in organs such as the kidney. In future, therefore, it is likely that neuropathic pain, quantitative sensory testing and hypohidrosis will serve as clinical outcome measures for studies of specific and effective therapies for Fabry disease.

Protein synthesis inhibition blocks the late-phase LTP of C-fiber evoked field potentials in rat spinal dorsal horn

Previous studies have demonstrated that in the hippocampus the maintenance of long-term potentiation (LTP) requires de novo protein synthesis. To investigate the role of protein synthesis in the maintenance of LTP of C-fiber evoked field potentials in spinal dorsal horn, which may be relevant to hyperalgesia, protein synthesis inhibitor (either cycloheximide or anisomycin) was applied locally to the recording segments of spinal cord in anesthetized rats, 30 min prior to tetanic stimulation to the sciatic nerve. We found that both cycloheximide and anisomycin selectively inhibited late-phase maintenance of the spinal LTP but affected neither LTP induction nor baseline responses of C-fiber evoked field potentials. In the presence of cycloheximide, LTP of C-fiber evoked field potentials was 281.5 +/- 16.5% (n = 6) of baseline 1 h after tetanic stimulation and the potentiation significantly decreased to 235.5 +/- 18.5% at 145 min after tetanic stimulation (P < 0.05). Afterward, LTP of C-fiber evoked field potentials decreased continuously and at 270 min after tetanic stimulation reached 130.8 +/- 18.0%, which was no longer different from baseline (P > 0.05). Spinal application of anisomycin at 30 min before tetanic stimulation yielded similar results (n = 6). These results suggest that protein synthesis may be crucial for the late-phase maintenance of LTP of C-fiber evoked field potentials in spinal dorsal horn.

A subpopulation of rats show social and sleep-waking changes typical of chronic neuropathic pain following peripheral nerve injury

Neuropathic conditions for which treatment is sought, the so-called chronic pain syndrome, are characterized usually by complex behavioural disturbances as well as pain. In this study we evaluated whether social behavioural and sleep disruptions occurred after nerve injury. Before and after chronic constriction of the sciatic nerve, resident-intruder and sleep-wake cycles, as well as mechanical and thermal allodynia/hyperalgesia, were quantified. Sciatic nerve injury in all animals reduced withdrawal thresholds to tactile and thermal (cold) stimuli. Resident-intruder and sleep-waking behaviours were altered in some but not all animals. One group (30%, 'persistent change') had enduring reductions in dominant behaviour to an intruder and decreased slow-wave sleep and increased wakefulness during both light and dark cycles. Another group (25%, 'recovery') had a transient reduction in dominant behaviours and decreased slow-wave sleep and increased wakefulness during only the light cycle. In a third group (45%, 'no effect') resident-intruder and sleep-waking behaviours remained normal. Our finding that the degree of 'pain' as inferred from the allodynia/hyperalgesia was identical in all animals suggests that the alterations to resident-intruder and sleep-wake cycles were independent of the level of sensory disturbance. An absence of correlation between intensity of sensory disturbances and measures of disability (loss of sleep, familial/social problems) is also characteristic of human neuropathic pain. These data indicate that: (i) in a subpopulation of animals sciatic injury results in two of the major complex behavioural changes which are characteristic of neuropathic pain in humans; (ii) testing only for allodynia and hyperalgesia is not sufficient to detect this subpopulation.

Identification of MEK1 as a novel target for the treatment of neuropathic pain

(1) In the present study we have attempted to identify changes in gene expression which are associated with neuropathic pain using subtractive suppression hybridization analysis of the lumbar spinal cord of animals suffering streptozocin induced diabetic neuropathy. (2) Using this approach, we found a significant up-regulation of several key components of the extracellular signal-regulated kinase (ERK) cascade. These findings were confirmed by Western blot analysis, which demonstrated that the levels of active ERK1 and 2 correlated with the onset of streptozocin-induced hyperalgesia. (3) Intrathecal administration of the selective MAPK/ERK-kinase (MEK) inhibitor PD 198306 dose-dependently (1-30 micro g) blocked static allodynia in both the streptozocin and the chronic constriction injury (CCI) models of neuropathic pain. (4) The antihyperalgesic effects of PD 198306, in both the streptozocin and CCI models of neuropathic pain, correlated with a reduction in the elevated levels of active ERK1 and 2 in lumbar spinal cord. (5) Intraplantar administration of PD 198306 had no effect in either model of hyperalgesia, indicating that changes in the activation of ERKs and the effect of MEK inhibition are localized to the central nervous system. (6) In summary, we have demonstrated for the first time that the development of neuropathic pain is associated with an increase in the activity of the MAPK/ERK-kinase cascade within the spinal cord and that enzymes in this pathway represent potential targets for the treatment of this condition.

Anti-allodynic effect of NW-1029, a novel Na(+) channel blocker, in experimental animal models of inflammatory and neuropathic pain

NW-1029, a benzylamino propanamide derivative, was selected among several molecules of this chemical class on the basis of its affinity for the [(3)H]batracotoxin ligand displacement of the Na(+) channel complex and also on the basis of its voltage and use-dependent inhibitory action on the Na(+) currents of the rat DRG (dorsal root ganglia) sensory neuron. This study evaluated the analgesic activity of NW-1029 in animal models of inflammatory and neuropathic pain (formalin test in mice, complete Freund's adjuvant and chronic constriction injury in rats) as well as in acute pain test (hot-plate and tail-flick in rats). Orally administered NW-1029 dose-dependently reduced cumulative licking time in the early and late phase of the formalin test (ED(50)=10.1 mg/kg in the late phase). In the CFA model, NW-1029 reversed mechanical allodynia (von Frey test) after both i.p. and p.o. administration (ED(50)=0.57 and 0.53 mg/kg), respectively. Similarly, NW-1029 reversed mechanical allodynia in the CCI model after both i.p. and p.o. administration yielding an ED(50) of 0.89 and 0.67 mg/kg, respectively. No effects were observed in the hot-plate and tail-flick tests up to 30 mg/kg p.o. The compound orally administered (0.1-10 mg/kg) was well tolerated, without signs of neurological impairment up to high doses (ED(50)=470 and 245 mg/kg in rat and mice Rotarod test, respectively). These results indicate that NW-1029 has anti-nociceptive properties in models of inflammatory and neuropathic pain.

Spinal glia and proinflammatory cytokines mediate mirror-image neuropathic pain in rats

Mirror-image allodynia is a mysterious phenomenon that occurs in association with many clinical pain syndromes. Allodynia refers to pain in response to light touch/pressure stimuli, which normally are perceived as innocuous. Mirror-image allodynia arises from the healthy body region contralateral to the actual site of trauma/inflammation. Virtually nothing is known about the mechanisms underlying such pain. A recently developed animal model of inflammatory neuropathy reliably produces mirror-image allodynia, thus allowing this pain phenomenon to be analyzed. In this sciatic inflammatory neuropathy (SIN) model, decreased response threshold to tactile stimuli (mechanical allodynia) develops in rats after microinjection of immune activators around one healthy sciatic nerve at mid-thigh level. Low level immune activation produces unilateral allodynia ipsilateral to the site of sciatic inflammation; more intense immune activation produces bilateral (ipsilateral + mirror image) allodynia. The present studies demonstrate that both ipsilateral and mirror-image SIN-induced allodynias are (1) reversed by intrathecal (peri-spinal) delivery of fluorocitrate, a glial metabolic inhibitor; (2) prevented and reversed by intrathecal CNI-1493, an inhibitor of p38 mitogen-activated kinases implicated in proinflammatory cytokine production and signaling; and (3) prevented or reversed by intrathecal proinflammatory cytokine antagonists specific for interleukin-1, tumor necrosis factor, or interleukin-6. Reversal of ipsilateral and mirror-image allodynias was rapid and complete even when SIN was maintained constantly for 2 weeks before proinflammatory cytokine antagonist administration. These results provide the first evidence that ipsilateral and mirror-image inflammatory neuropathy pain are created both acutely and chronically through glial and proinflammatory cytokine actions.

Serotonergic signaling inhibits hyperalgesia induced by spinal cord damage

Although dysesthesia is one of the most serious problems in patients with spinal cord injury, most of them being unresponsive to conventional treatments. In this study, we established a rat thoracic spinal cord mild-compression model that revealed thermal hyperalgesia in the hind limb. The thoracic spinal cord was compressed gently, using a 20 g weight for 20 min. The withdrawal latency of the thermal stimulation of the bilateral hind-limb was monitored using Hargreaves' Plantar test apparatus. In this model, thermal-hyperalgesia was observed for 1 week after the injury. The spinal cord injury-induced thermal-hyperalgesia was mimicked by the intrathecal application of metergoline, a non-selective 5-HT antagonist, 1-(2-methoxyphenyl)-4-[4-(2-phthalimido) butyl]-piperazine hydrobromide (NAN190), a selective 5-HT1 antagonist, and 3-tropanyl-3,5-dichlorobenzoate (MDL72222), a selective 5-HT3 antagonist. Intraperitoneal application of fluvoxamine maleate, a selective serotonin reuptake inhibitor, reduced the intensity of hyperalgesia induced by spinal cord injury. The inhibitory effect of fluvoxamine maleate on thermal hyperalgesia was prevented by the application of the aforementioned nonselective or selective 5-HT receptor antagonists. Intrathecal application of fluvoxamine maleate and selective 5-HT receptor agonists, i.e., 8-hydroxy-2-(di-n-proplyamino)-tetralin hydrobromide (8-OH-DPAT: 5HT-1 selective) and 2-methyl-5-hydroxytryptamine maleate (2-m-5-HT: 5HT-3 selective), inhibited the spinal cord injury-induced hyperalgesia. These results suggest that the change in the descending serotonergic signal plays an important role in hyperalgesia after the spinal cord injury, and that the application of selective serotonin reuptake inhibitors will be one of the candidates for new therapeutic methods against post-spinal cord injury dysesthesia.

Opioid resistance in chronic daily headache: a synthesis of ideas from the bench and bedside

Chronic daily headache is a complex pain disorder that encompasses many diagnostic and therapeutic challenges. Our understanding of the pathophysiologic processes of intermittent migraine has improved over the past decade, but the biologic basis of chronic daily headache remains obscure. Some of the more common issues confounding management of patients who experience chronic daily headache are medication overuse, psychiatric comorbidity, refractoriness to pharmacologic treatments, and disability. Long-acting opioid analgesics would appear to provide a viable treatment option for those patients who remain refractory to other treatment interventions; however, clinical experience often does not support this belief. Current concepts of the pathophysiologic basis of chronic pain and associated neural plasticity may elucidate a biologic basis for the general inefficacy of opioids in the management of chronic daily headache. This article explores the models of pathophysiology for migraine and chronic daily headache, the concept of chronic daily headache as a neuropathic pain syndrome, neural plasticity in the context of neuropathic pain states, the physiologic basis for opioid tolerance and opioid-induced hyperalgesia, and how each of these conditions interact to provoke the general lack of opioid efficacy often observed in the management of chronic daily headache.

The anticonvulsant retigabine attenuates nociceptive behaviours in rat models of persistent and neuropathic pain

We have tested for anti-nociceptive effects of the anticonvulsant KCNQ channel opener, N-(2-amino-4-(4-fluorobenzylamino)-phenyl)carbamic acid ethyl ester (retigabine), in rat models of experimental pain. In the chronic constriction injury and spared nerve models of neuropathic pain, injection of retigabine (5 and 20 mg/kg, p.o.) significantly attenuated (P<0.05) mechanical hypersensitivity in response to pin prick stimulation of the injured hindpaw. In contrast, retigabine had no effect on mechanical hypersensitivity to von Frey stimulation of the injured hindpaw in either model. Cold sensitivity in response to ethyl chloride was only attenuated (P<0.05) in the chronic constriction injury model. In the formalin test, retigabine (20 mg/kg, p.o.) attenuated flinching behaviour in the second phase compared with vehicle (P<0.05), and this effect was completely reversed by the KCNQ channel blocker 10,10-bis(4-pyridinylmethyl)-9(10H)-anthracenone (XE-991; 3 mg/kg, i.p.). Neither retigabine nor XE-991 administration affected the latency to respond to noxious thermal stimulation of the tail in control animals. These results suggest that retigabine may prove to be effective in the treatment of neuropathic pain.

A comparison of the antinociceptive effects of voltage-activated Na+ channel blockers in two rat models of neuropathic pain

The pain-relieving effects of various voltage-activated Na(+) channel blockers have been evaluated in two rat models of neuropathic pain; the photochemically induced nerve injury model (Gazelius) and spared nerve injury model. Lidocaine (up to 40 mg/kg, i.p.) and lamotrigine (up to 60 mg/kg, i.p.) had no effect on mechanical or cold allodynia in either model. However, lamotrigine (10, 30 and 60 mg/kg) significantly attenuated mechanical hyperalgesia in the spared nerve injury model, while mexiletine (25 and 37.5 mg/kg, i.p.) attenuated mechanical allodynia in the Gazelius model. Tocainide (50, 75 and 100 mg/kg, i.p.) significantly reduced all types of pain behaviour measured. The present results show that these voltage-activated Na(+) channel blockers have broadly similar antinociceptive effects in these two models of neuropathic pain. They also show that these drugs can have markedly different effects on distinct neuropathic pain-related behaviours within models.

The enzymatic inactivation of the fatty acid amide class of signaling lipids

The fatty acid amide (FAA) class of signaling lipids modulates a number of neurobehavioral processes in mammals, including pain, sleep, feeding, and locomotor activity. Representative FAAs include the endogenous cannabinoid anandamide and the sleep-inducing lipid oleamide. Despite activating several neuroreceptor systems in vitro, most FAAs produce only weak and transient behavioral effects in vivo, presumably due to their expeditious catabolism. This review focuses on one enzyme, fatty acid amide hydrolase (FAAH) that appears to play a major role in regulating the amplitude and duration of FAA signals in vivo. In particular, we will highlight a series of recent papers that have investigated the physiological functions of the mouse and human FAAH enzymes. Collectively, these studies promote FAAH as a central component of FAA signaling pathways, especially those mediated by the endocannabinoid anandamide, and suggest that this enzyme may represent an attractive pharmaceutical target for the treatment of pain and related neurophysiological disorders.

Chronic neuropathic pain is accompanied by global changes in gene expression and shares pathobiology with neurodegenerative diseases

Neuropathic pain is induced by injury or disease of the nervous system. Studies aimed at understanding the molecular pathophysiology of neuropathic pain have so far focused on a few known molecules and signaling pathways in neurons. However, the pathophysiology of neuropathic pain appears to be very complex and remains poorly understood. A global understanding of the molecular mechanisms involved in neuropathic pain is needed for a better understanding of the pathophysiology and treatment of neuropathic pain. Towards this end, we examined global gene expression changes as well as the pathobiology at the cellular level in a spinal nerve ligation neuropathic pain model using DNA microarray, quantitative real-time PCR and immunohistochemistry. We found that the behavioral hypersensitivity that is manifested in the persistent pain state is accompanied by previously undescribed changes in gene expression. In the DRG, we found regulation of: (1) immediate early genes; (2) genes such as ion channels and signaling molecules that contribute to the excitability of neurons; and (3) genes that are indicative of secondary events such as neuroinflammation. In addition, we studied gene regulation in both injured and uninjured DRG by quantitative PCR, and observed differential gene regulation in these two populations of DRGs. Furthermore, we demonstrated unexpected co-regulation of many genes, especially the activation of neuroinflammation markers in both the PNS and CNS. The results of our study provide a new picture of the molecular mechanisms that underlie the complexity of neuropathic pain and suggest that chronic pain shares common pathobiology with progressive neurodegenerative disease.

Altered synaptophysin expression in the rat spinal cord after chronic constriction injury of sciatic nerve

Injury to the peripheral nervous system can lead to spontaneous pain, hyperalgesia and allodynia. Previous studies have shown sprouting of Abeta-fibres into lamina II of the spinal cord dorsal horn after nerve injury and the formation of new synapses by these sprouts. Synaptophysin is a presynaptic vesicle protein, useful in the identification of synaptogenesis. Here we investigated whether synaptogenesis as measured by the expression of synaptophysin protein correlates with symptoms of neuropathic pain in rats with a chronic constriction injury (CCI) of the sciatic nerve. We used immunohistochemistry, Western immunoblotting and densitometry to study the distribution of synaptophysin and to quantify relative protein. Synaptophysin was increased in the ipsilateral dorsal horn with a peak level on day 14 and returned to baseline on day 21 post-CCI. Synaptophysin levels temporally correlated with thermal hyperalgesia but not with tactile allodynia. Our results indicate that thermal hyperalgesia in CCI significantly correlates with synaptogenesis within the superficial layers of the dorsal horn.

Physiology of chronic spinal pain syndromes: from animal models to biomechanics

STUDY DESIGN

The literature and current research related to spinal pain mechanisms were reviewed, as well as animal models related to its study.

OBJECTIVES

To provide a pragmatic discussion of spinal pain that both reviews relevant research and coherently synthesizes the existing body of literature related to pain, nociception, animal modeling, and injury biomechanics.

SUMMARY OF BACKGROUND DATA

A detailed body of literature suggests that spinal pain mechanisms are quite complicated and involve a host of different processes (e.g., genetics, gender, neurophysiology, and biomechanics) that may contribute to clinical manifestations and symptoms.

METHODS

Both a review of the literature and a presentation of current and ongoing laboratory research are presented. Specific findings from the authors' laboratory using a rodent model of lumbar radiculopathy are presented to elucidate the role of local nerve root biomechanics in initiating and maintaining behavioral symptoms of nociception and pain.

RESULTS

For an understanding of chronic pain, a bidirectional-translational approach that incorporates cross-disciplinary methods such as in vivo biomechanical techniques is required. A conceptual model of chronic spine pain is proposed that details the dynamic and integrated roles of injury, biomechanics, and nociceptive physiology.

CONCLUSIONS

Areas of continued research are highlighted that may help guide the management of painful spine symptoms and syndromes.

Anti-allodynic action of the tormentic acid, a triterpene isolated from plant, against neuropathic and inflammatory persistent pain in mice

Experiments were designed to address whether the pentacyclic triterpene tormentic acid isolated from the stem bark of the plant Vochysia divergens exerts oral anti-allodynic properties in two models of chronic pain in mice: neuropathic pain caused by partial ligation of the sciatic nerve and inflammatory pain produced by intraplantar injection of Complete Freund's Adjuvant. Oral administration of tormentic acid (30 mg/kg) twice a day for several consecutive days produced time-dependent and pronounced anti-allodynia effect in both ispsilateral and contralateral paws after plantar injection of Complete Freund's Adjuvant. The inhibition observed was 82+/-9% and 100+/-11%, respectively. Interestingly, tormentic acid did not inhibit paw oedema formation following Complete Freund's Adjuvant plantar injection. Tormentic acid (30 mg/kg, p.o.) and gabapentin (70 mg/kg, p.o.), given twice a day, inhibited markedly the neuropathic allodynia induced by partial ligation of the sciatic nerve, with inhibition of 91+/-19% and 71+/-16%, respectively. The anti-allodynic action of tormentic acid was not associated with impairment of the motor activity of the animals. Together, the present results indicate that tormentic acid or its derivatives might be of potential interest in the development of new clinically relevant drugs for the management of persistent neuropathic and inflammatory allodynia.

Surgical technique to reduce scar discomfort after carpal tunnel surgery

A total of 379 patients (416 hands) with clinically diagnosed and electromyographically confirmed carpal tunnel syndrome were enrolled in a prospective study to determine the influence of a modified open decompression technique on postoperative scar discomfort. The new technique used in 184 patients (200 hands) is presented. Special attention was focused on identification and preservation of macroscopically detectable subcutaneous nerves. After using this method, which permits complete visualization of the entire transverse carpal ligament, the incidence of postoperative scar discomfort was 2.5%. This was significantly lower compared with the group of 195 patients (216 hands) treated by standard open decompression technique, without preservation of subcutaneous nerves. Primary results regarding relieving symptoms were comparable in both groups. Five anatomic variations of subcutaneous innervation, at the site of the incision in the line with the radial border of the ring finger, are described. The etiology of scar discomfort is discussed.

Effects of peripheral nerve injury on functional spinal VR1 receptors

VR1 receptors, present on Adelta- and C-fibres and post-synaptic sites within the spinal cord dorsal horn, is an integrator of noxious stimuli. Here, the contribution of spinal VR1 receptors to spinal nociceptive processing in nerve injured (selective spinal nerve ligated SNL) and sham anaesthetised rats was studied. Spinal capsazepine (0.5-30 microM), a competitive VR1 antagonist, reduced noxious evoked responses of spinal neurones to a greater extent in sham operated rats, compared to SNL rats. Significant differences between the effect of spinal capsazepine on the non-potentiated component of the C-fibre evoked response of SNL and sham operated rats are reported (p< 0.01, two-way ANOVA). Our data suggest there is a functional plasticity of the spinal VR1 receptor following nerve injury.

Brain plasticity and microglia: is transsynaptic glial activation in the thalamus after limb denervation linked to cortical plasticity and central sensitisation?

Microglia are a subset of tissue-macrophages that are ubiquitously distributed throughout the entire CNS. In health, they remain largely dormant until activated by a pathological stimulus. The availability of more sensitive detection techniques has allowed the early measurement of the cell responses of microglia in areas with few signs of active pathology. Subtle neuronal injury can induce microglial activation in retrograde and anterograde projection areas remote from the primary lesion focus. There is also evidence that in cases of long-standing abnormal neuronal activity, such as in patients after limb amputation with chronic pain and phantom sensations, glial activation may occur transsynaptically in the thalamus. Such neuronally driven glial responses may be related to the emergence central sensitisation in chronic pain states or plasticity phenomena in the cerebral cortex. It is suggested, that such persistent low-level microglial activation is not adequately described by the traditional concept of phagocyte-mediated tissue damage that largely evolved from studies of acute brain lesion models or acute human brain pathology. Due to the presence of signal molecules that can act on neurons and microglia alike, the communication between neurons and microglia is likely to be bi-directional. Persistent subtle microglial activity may modulate basal synaptic transmission and thus neuronal functioning either directly or through the interaction with astrocytes. The activation of microglia leads to the emergence of microstructural as well as functional compartments in which neurokines, interleukins and other signalling molecules introduce a qualitatively different, more open mode of cell-cell communication that is normally absent from the healthy adult brain. This 'neo-compartmentalisation', however, occurs along predictable neuronal pathways within which these glial changes are themselves under the modulatory influence of neurons or other glial cells and are subject to the evolving state of the pathology. Depending on the disease state, yet relatively independent of the specific disease cause, fluctuations in the modulatory influence by non-neuronal cells may form the cellular basis for the variability of brain plasticity phenomena, i.e. the plasticity of plasticity.

Reflex sympathetic dystrophy

Reflex sympathetic dystrophy (RSD) is composed of five major features: pain, swelling, autonomic dysregulation, movement disorders, and atrophy and dystrophy. RSD is caused by an injury to a specific nerve or the C- and A-delta fibers that innervate the involved tissue. It is a progressive illness that spreads with time and may encompass the entire body. There is no psychological disposition to the problem, but all patients are severely depressed because of the constant pain, lack of sleep, and complete disruption of their lifestyle. The continuing pain is usually secondary to the process of central sensitization. The autonomic dysregulation has a major central nervous system component. Atrophy and dystrophy are partly due to loss of nutritive blood supply to the affected tissues. The movement disorder is partly due to deficiency of GABAergic mechanisms; the tremor is an exaggeration of the normal physiologic tremor. Treatment consists of decreasing the afferent pain, maintaining barrage from the underlying defect, and blocking the sympathetic component of the process. New developments include the use of neurotrophic factors to reverse the phenotypic changes that occur in the dorsal horn and the use of pharmacologic agents to block the activity-dependent NMDA channels that appear to be instrumental in maintaining central sensitization.