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

Various gases for the treatment of neuropathic pain: mechanisms, current status, and future perspectives

In recent years, medical gas therapy has emerged as a promising approach for treating neuropathic pain. This review article aimed to investigate the therapeutic effects of medical gas therapy on neuropathic pain and its underlying mechanisms, thereby providing a theoretical foundation for clinical practice. A literature search was conducted using the Web of Science Core Collection database. Co-occurrence analysis of keywords revealed that terms including "neuropathic pain," "nitric oxide," "nitric oxide synthase," "pain," and "ozone" frequently appeared. Cluster analysis grouped these keywords into four primary categories: intervertebral disc disease and gas therapy, mechanisms of neuropathic pain and gas interventions, the role of nitric oxide in modulating neuropathic pain and gas therapy, and the effects of gas therapy on mental disorders in the context of neuropathic pain treatment. The analysis of highly cited literature in the field of medical gas therapy for neuropathic pain emphasizes the crucial roles of nitric oxide and nitric oxide synthase in nerve injury and pain. Various types of gas therapy, including oxygen-ozone therapy and nitric oxide-related therapies, show promise in treating pain following peripheral nerve injury. Oxidative stress and nitric oxide are crucial regulatory factors in the pain signaling associated with trigeminal neuralgia. Ozone therapy alleviates trigeminal pain by inhibiting inflammatory responses, reducing oxidative stress, and modulating neurotransmitter release. Novel nanomaterials, such as manganese oxide nanoparticles, have also demonstrated potential in scavenging free radicals and alleviating sciatic nerve pain. Ozone therapy has shown good clinical efficacy in treating lumbar disc herniation and sciatica, whereas both ozone therapy and hyperbaric oxygen therapy have demonstrated effectiveness and safety in managing postherpetic neuralgia. In conclusion, medical gas therapy for neuropathic pain primarily includes oxygen-ozone therapy, nitric oxide-related therapies, hydrogen sulfide-related therapies, and hyperbaric oxygen therapy. While these therapies exhibit efficacy in managing neuropathic pain, further research is necessary to elucidate their mechanisms of action and safety profiles. Although hyperbaric oxygen therapy and ozone therapy have already been implemented in clinical research, other types of gas therapy are still in the animal testing phase. Therefore, future studies should focus on conducting more multicenter, large-sample randomized controlled trials to accelerate clinical translation and provide more effective treatment options for patients suffering from neuropathic pain.

The Input-Output Relation of Primary Nociceptive Neurons is Determined by the Morphology of the Peripheral Nociceptive Terminals

The output from the peripheral terminals of primary nociceptive neurons, which detect and encode the information regarding noxious stimuli, is crucial in determining pain sensation. The nociceptive terminal endings are morphologically complex structures assembled from multiple branches of different geometry, which converge in a variety of forms to create the terminal tree. The output of a single terminal is defined by the properties of the transducer channels producing the generation potentials and voltage-gated channels, translating the generation potentials into action potential (AP) firing. However, in the majority of cases, noxious stimuli activate multiple terminals; thus, the output of the nociceptive neuron is defined by the integration and computation of the inputs of the individual terminals. Here, we used a computational model of nociceptive terminal tree to study how the architecture of the terminal tree affects the input-output relation of the primary nociceptive neurons. We show that the input-output properties of the nociceptive neurons depend on the length, the axial resistance (Ra), and location of individual terminals. Moreover, we show that activation of multiple terminals by a capsaicin-like current allows summation of the responses from individual terminals, thus leading to increased nociceptive output. Stimulation of the terminals in simulated models of inflammatory or neuropathic hyperexcitability led to a change in the temporal pattern of AP firing, emphasizing the role of temporal code in conveying key information about changes in nociceptive output in pathologic conditions, leading to pain hypersensitivity.SIGNIFICANCE STATEMENT Noxious stimuli are detected by terminal endings of primary nociceptive neurons, which are organized into morphologically complex terminal trees. The information from multiple terminals is integrated along the terminal tree, computing the neuronal output, which propagates toward the CNS, thus shaping the pain sensation. Here, we revealed that the structure of the nociceptive terminal tree determines the output of nociceptive neurons. We show that the integration of noxious information depends on the morphology of the terminal trees and how this integration and, consequently, the neuronal output change under pathologic conditions. Our findings help to predict how nociceptive neurons encode noxious stimuli and how this encoding changes in pathologic conditions, leading to pain.

An integrated review on new targets in the treatment of neuropathic pain

Neuropathic pain is a complex chronic pain state caused by the dysfunction of somatosensory nervous system, and it affects the millions of people worldwide. At present, there are very few medical treatments available for neuropathic pain management and the intolerable side effects of medications may further worsen the symptoms. Despite the presence of profound knowledge that delineates the pathophysiology and mechanisms leading to neuropathic pain, the unmet clinical needs demand more research in this field that would ultimately assist to ameliorate the pain conditions. Efforts are being made globally to explore and understand the basic molecular mechanisms responsible for somatosensory dysfunction in preclinical pain models. The present review highlights some of the novel molecular targets like D-amino acid oxidase, endoplasmic reticulum stress receptors, sigma receptors, hyperpolarization-activated cyclic nucleotide-gated cation channels, histone deacetylase, Wnt/β-catenin and Wnt/Ryk, ephrins and Eph receptor tyrosine kinase, Cdh-1 and mitochondrial ATPase that are implicated in the induction of neuropathic pain. Studies conducted on the different animal models and observed results have been summarized with an aim to facilitate the efforts made in the drug discovery. The diligent analysis and exploitation of these targets may help in the identification of some promising therapies that can better manage neuropathic pain and improve the health of patients.

Membrane cholesterol depletion as a trigger of Nav1.9 channel-mediated inflammatory pain

Cholesterol is a major lipid component of the mammalian plasma membrane. While much is known about its metabolism, its transport, and its role in atherosclerotic vascular disease, less is known about its role in neuronal pathophysiology. This study reveals an unexpected function of cholesterol in controlling pain transmission. We show that inflammation lowers cholesterol content in skin tissue and sensory DRG culture. Pharmacological depletion of cellular cholesterol entails sensitization of nociceptive neurons and promotes mechanical and thermal hyperalgesia through the activation of voltage-gated Nav1.9 channels. Inflammatory mediators enhance the production of reactive oxygen species and induce partitioning of Nav1.9 channels from cholesterol-rich lipid rafts to cholesterol-poor non-raft regions of the membrane. Low-cholesterol environment enhances voltage-dependent activation of Nav1.9 channels leading to enhanced neuronal excitability, whereas cholesterol replenishment reversed these effects. Consistently, we show that transcutaneous delivery of cholesterol alleviates hypersensitivity in animal models of acute and chronic inflammatory pain. In conclusion, our data establish that membrane cholesterol is a modulator of pain transmission and shed a new light on the relationship between cholesterol homeostasis, inflammation, and pain.

Quantitative Proteomics Analysis of VEGF-Responsive Endothelial Protein S-Nitrosylation Using Stable Isotope Labeling by Amino Acids in Cell Culture (SILAC) and LC-MS/MS

Adduction of a nitric oxide moiety (NO•) to cysteine(s), termed S-nitrosylation (SNO), is a novel mechanism for NO to regulate protein function directly. However, the endothelial SNO-protein network that is affected by endogenous and exogenous NO is obscure. This study was designed to develop a quantitative proteomics approach using stable isotope labeling by amino acids in cell culture for comparing vascular endothelial growth factor (VEGFA)- and NO donor-responsive endothelial nitroso-proteomes. Primary placental endothelial cells were labeled with "light" (L-(12)C6 (14)N4-Arg and L-(12)C6 (14)N2-Lys) or "heavy" (L-(13)C6 (15)N4-Arg and L-(13)C6 (15)N2-Lys) amino acids. The light cells were treated with an NO donor nitrosoglutathione (GSNO, 1 mM) or VEGFA (10 ng/ml) for 30 min, while the heavy cells received vehicle as control. Equal amounts of cellular proteins from the light (GSNO or VEGFA treated) and heavy cells were mixed for labeling SNO-proteins by the biotin switch technique and then trypsin digested. Biotinylated SNO-peptides were purified for identifying SNO-proteins by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Ratios of light to heavy SNO-peptides were calculated for determining the changes of the VEGFA- and GSNO-responsive endothelial nitroso-proteomes. A total of 387 light/heavy pairs of SNO-peptides were identified, corresponding to 213 SNO-proteins that include 125 common and 27 VEGFA- and 61 GSNO-responsive SNO-proteins. The specific SNO-cysteine(s) in each SNO-protein were simultaneously identified. Pathway analysis revealed that SNO-proteins are involved in various endothelial functions, including proliferation, motility, metabolism, and protein synthesis. We collectively conclude that endogenous NO on VEGFA stimulation and exogenous NO from GSNO affect common and different SNO-protein networks, implicating SNO as a critical mechanism for VEGFA stimulation of angiogenesis.

Sensory Neurodegeneration in Diabetes: Beyond Glucotoxicity

Diabetic polyneuropathy in humans is of gradual, sometimes insidious onset, and is more likely to occur if glucose control is poor. Arguments that the disorder arises chiefly from glucose toxicity however ignore the greater complexity of a unique neurodegenerative disorder. For example, sensory neurons regularly thrive in media with levels of glucose at or exceeding those of poorly controlled diabetic persons. Also, all of the linkages between hyperglycemia and neuropathy develop in the setting of altered insulin availability or sensitivity. Insulin itself is recognized as a potent growth, or trophic factor for adult sensory neurons. Low doses of insulin, insufficient to alter blood glucose levels, reverse features of diabetic neurodegeneration in animal models. Insulin resistance, as occurs in diabetic adipose tissue, liver, and muscle, also develops in sensory neurons, offering a mechanism for neurodegeneration in the setting of normal or elevated insulin levels. Other interventions that "shore up" sensory neurons prevent features of diabetic polyneuropathy from developing despite persistent hyperglycemia. More recently evidence has emerged that a series of subtle molecular changes in sensory neurons can be linked to neurodegeneration including epigenetic changes in the control of gene expression. Understanding the new complexity of sensory neuron degeneration may give rise to therapeutic strategies that have a higher chance of success in the clinical trial arena.

Rapid Management of Trigeminal Neuralgia and Comorbid Major Depressive Disorder With Duloxetine

Objective: To describe a case of a patient diagnosed with major depressive disorder whose trigeminal neuralgia was unexpectedly but rapidly and efficiently responsive to duloxetine. Case Summary: A 37-year-old woman was diagnosed with trigeminal neuralgia, and the initial treatment with carbamazepine 800 mg/d did not improve her pain. In the following 3 years, she was poorly responsive to the combination therapy with several medications, including carbamazepine, valproate, baclofen, diclofenac, and acetaminophen. The repeated gamma knife radiosurgery still did not relieve her symptoms. She developed clinically significant depressive symptoms, and a diagnosis of major depressive disorder was made. Duloxetine 30 mg/d was initiated for the management of depression, with the dose gradually increased to 60 mg/d. Unexpectedly, at the dose of 60 mg/d, the patient reported remarkable relief in her trigeminal neuralgia within the first week. Her depressed mood gradually improved in the following 3 weeks. At the 4-year follow-up, she was gradually tapered off her medications, and her depression and trigeminal neuralgia were well managed on duloxetine 60 mg/d and carbamazepine 600 mg/d. Discussion: The mechanisms may be related to duloxetine’s ability to modulate norepinephrine and serotonin and antagonize N-methyl-d-aspartate (NMDA) receptors. The ignition hypothesis is a proposed etiology of trigeminal neuralgia, in that any individual hyperexcitable neuron can spread its discharge quickly to activate the entire population of neurons. We suggest that duloxetine exerts desynchronizing effects through its NMDA antagonism, modulating the hyperexcitable state of the trigeminal afferents. Conclusions: Duloxetine may be an adjuvant in treatment-resistant trigeminal neuralgia.

Mechanisms of diabetic complications

It is increasingly apparent that not only is a cure for the current worldwide diabetes epidemic required, but also for its major complications, affecting both small and large blood vessels. These complications occur in the majority of individuals with both type 1 and type 2 diabetes. Among the most prevalent microvascular complications are kidney disease, blindness, and amputations, with current therapies only slowing disease progression. Impaired kidney function, exhibited as a reduced glomerular filtration rate, is also a major risk factor for macrovascular complications, such as heart attacks and strokes. There have been a large number of new therapies tested in clinical trials for diabetic complications, with, in general, rather disappointing results. Indeed, it remains to be fully defined as to which pathways in diabetic complications are essentially protective rather than pathological, in terms of their effects on the underlying disease process. Furthermore, seemingly independent pathways are also showing significant interactions with each other to exacerbate pathology. Interestingly, some of these pathways may not only play key roles in complications but also in the development of diabetes per se. This review aims to comprehensively discuss the well validated, as well as putative mechanisms involved in the development of diabetic complications. In addition, new fields of research, which warrant further investigation as potential therapeutic targets of the future, will be highlighted.

Sensory and signaling mechanisms of bradykinin, eicosanoids, platelet-activating factor, and nitric oxide in peripheral nociceptors

Peripheral mediators can contribute to the development and maintenance of inflammatory and neuropathic pain and its concomitants (hyperalgesia and allodynia) via two mechanisms. Activation or excitation by these substances of nociceptive nerve endings or fibers implicates generation of action potentials which then travel to the central nervous system and may induce pain sensation. Sensitization of nociceptors refers to their increased responsiveness to either thermal, mechanical, or chemical stimuli that may be translated to corresponding hyperalgesias. This review aims to give an account of the excitatory and sensitizing actions of inflammatory mediators including bradykinin, prostaglandins, thromboxanes, leukotrienes, platelet-activating factor, and nitric oxide on nociceptive primary afferent neurons. Manifestations, receptor molecules, and intracellular signaling mechanisms of the effects of these mediators are discussed in detail. With regard to signaling, most data reported have been obtained from transfected nonneuronal cells and somata of cultured sensory neurons as these structures are more accessible to direct study of sensory and signal transduction. The peripheral processes of sensory neurons, where painful stimuli actually affect the nociceptors in vivo, show marked differences with respect to biophysics, ultrastructure, and equipment with receptors and ion channels compared with cellular models. Therefore, an effort was made to highlight signaling mechanisms for which supporting data from molecular, cellular, and behavioral models are consistent with findings that reflect properties of peripheral nociceptive nerve endings. Identified molecular elements of these signaling pathways may serve as validated targets for development of novel types of analgesic drugs.

Endothelial nitric oxide synthase in dorsal root ganglia during chronic inflammatory nociception

Nitric oxide (NO) is a gaseous molecule implicated both in vascular tone and nociceptive transmission. The capillary blood supply to the dorsal root ganglia (DRG) is unique because it is highly permeable to several low and high molecular-weight compounds. This anatomical situation leads to a potential role of endothelial nitric oxide synthase (eNOS) in inflammatory nociception, which is not well established. Therefore, we examined the role of eNOS in DRG in a murine chronic inflammatory pain model induced by complete Freund's adjuvant using L-N(5)-(1-iminoethyl)ornithine (L-NIO), a potent inhibitor of eNOS activity. Pain state was examined using a behavioral test. The expression of eNOS, platelet endothelial cell adhesion molecule-1 (CD31) and vascular endothelial growth factor (VEGF) was examined by immunofluorescence. In control animals, CD31 was detected in vessels; VEGF was localized both in vessels and neurons while a weak eNOS immunopositivity was detected in both vessels and in neurons. Under inflammatory pain conditions, eNOS, CD31 and VEGF immunopositivity increased. Administration of L-NIO significantly attenuated thermal hyperalgesia by 24 h and decreased eNOS activity and CD31 immunopositivity by 7 days. VEGF was unaffected. Our results show that eNOS plays a nociceptive role in the early phases of inflammation while in the later phases it may be involved in neurotrophic support.

The three isoforms of nitric oxide synthase distinctively affect mouse nocifensive behavior

Nitric oxide synthases (NOSs) have been shown to modulate thermal hyperalgesia and mechanical hypersensitivity in inflammatory and neuropathic pain. However, little is known about the effect of NOSs on baseline function of sensory nerve fibers. Using genetic deficiency and pharmacologic inhibition of NOSs, we examined the impact of the three isoforms NOS1, NOS2, and NOS3 on baseline nocifensive behavior by measuring current vocalization threshold in response to electrical stimulation at 5, 250, 2000 Hz that preferentially stimulate C, Aδ, and Aβ fibers. In response to 5, 250 and 2000 Hz, NOS1-deficient animals had significantly higher current vocalization thresholds compared with wild-type. Genetic deficiency of NOS2 was associated with higher current vocalization thresholds in response to 5 Hz (C-fiber) stimulation. In contrast, NOS3-deficient animals had an overall weak trend toward lower current vocalization thresholds at 5 Hz and significantly lower current vocalization threshold compared with wild-type animals at 250 and 2000 Hz. Therefore, NOSs distinctively affect baseline mouse current vocalization threshold and appear to play a role on nocifensive response to electrical stimulation of sensory nerve fibers.

Reduced spinal microglial activation and neuropathic pain after nerve injury in mice lacking all three nitric oxide synthases

Background

Several studies have investigated the involvement of nitric oxide (NO) in acute and chronic pain using mice lacking a single NO synthase (NOS) gene among the three isoforms: neuronal (nNOS), inducible (iNOS) and endothelial (eNOS). However, the precise role of NOS/NO in pain states remains to be determined owing to the substantial compensatory interactions among the NOS isoforms. Therefore, in this study, we used mice lacking all three NOS genes (n/i/eNOS^-/-mice) and investigated the behavioral phenotypes in a series of acute and chronic pain assays.

Results

In a model of tissue injury-induced pain, evoked by intraplantar injection of formalin, both iNOS^-/-and n/i/eNOS^-/-mice exhibited attenuations of pain behaviors in the second phase compared with that in wild-type mice. In a model of neuropathic pain, nerve injury-induced behavioral and cellular responses (tactile allodynia, spinal microglial activation and Src-family kinase phosphorylation) were reduced in n/i/eNOS^-/-but not iNOS^-/-mice. Tactile allodynia after nerve injury was improved by acute pharmacological inhibition of all NOSs and nNOS. Furthermore, in MG-5 cells (a microglial cell-line), interferon-γ enhanced NOSs and Mac-1 mRNA expression, and the Mac-1 mRNA increase was suppressed by L-NAME co-treatment. Conversely, the NO donor, sodium nitroprusside, markedly increased mRNA expression of Mac-1, interleukin-6, toll-like receptor 4 and P2X4 receptor.

Conclusions

Our results provide evidence that the NOS/NO pathway contributes to behavioral pain responses evoked by tissue injury and nerve injury. In particular, nNOS may be important for spinal microglial activation and tactile allodynia after nerve injury.

The spinal cord expression of neuronal and inducible nitric oxide synthases and their contribution in the maintenance of neuropathic pain in mice

Background

Nitric oxide generated by neuronal (NOS1), inducible (NOS2) or endothelial (NOS3) nitric oxide synthases contributes to pain processing, but the exact role of NOS1 and NOS2 in the maintenance of chronic peripheral neuropathic pain as well as the possible compensatory changes in their expression in the spinal cord of wild type (WT) and NOS knockout (KO) mice at 21 days after total sciatic nerve ligation remains unknown.

Methodology/Principal Findings

The mechanical and thermal allodynia as well as thermal hyperalgesia induced by sciatic nerve injury was evaluated in WT, NOS1-KO and NOS2-KO mice from 1 to 21 days after surgery. The mRNA and protein levels of NOS1, NOS2 and NOS3 in the spinal cord of WT and KO mice, at 21 days after surgery, were also assessed. Sciatic nerve injury led to a neuropathic syndrome in WT mice, in contrast to the abolished mechanical allodynia and thermal hyperalgesia as well as the decreased or suppressed thermal allodynia observed in NOS1-KO and NOS2-KO animals, respectively. Sciatic nerve injury also increases the spinal cord expression of NOS1 and NOS2 isoforms, but not of NOS3, in WT and NOS1-KO mice respectively. Moreover, the presence of NOS2 is required to increase the spinal cord expression of NOS1 whereas an increased NOS1 expression might avoid the up-regulation of NOS2 in the spinal cord of nerve injured WT mice.

Conclusions/Significance

These data suggest that the increased spinal cord expression of NOS1, regulated by NOS2, might be responsible for the maintenance of chronic peripheral neuropathic pain in mice and propose these enzymes as interesting therapeutic targets for their treatment.

RhoA/Rho kinase pathway contributes to the pathogenesis of thermal hyperalgesia in diabetic mice

Diabetic neuropathy is one of the most common complications of diabetes and causes various problems in daily life. Several investigations have noted that many factors in the spinal cord are involved in the symptoms of painful diabetic neuropathy, and there are very few effective therapeutic regimens. In the present study, we sought to elucidate the role of the RhoA/Rho kinase (ROCK) pathway in thermal hyperalgesia in diabetic mice. The intracellular localization of RhoA and the expression of eNOS were measured by western blotting. Thermal hyperalgesia was assessed by the tail-flick test and mechanical allodynia was assessed by automated von Frey filament test in streptozotocin(STZ)-induced diabetic mice. The spinal cord of STZ-treated diabetic mice showed increased membrane-bound RhoA compared to non-diabetic control. Treatment with the RhoA inhibitor exoenzyme C3, Clostridium botulinum, and the ROCK inhibitor Y27632 attenuated thermal hyperalgesia and mechanical allodynia in diabetic mice. Moreover, daily treatment with simvastatin attenuated all of those changes in diabetic mice. The expression of eNOS and NO metabolite contents in the spinal cord was decreased in diabetic mice, and these changes were normalized by treatment with simvastatin. The present results show that HMG-CoA reductase inhibitors have an inhibitory effect on thermal hyperalgesia in diabetic mice, which is mediated by an increase in NO production through the inhibition of RhoA/ROCK pathways. These results suggest that ROCK inhibitors and HMG-CoA inhibitors may be attractive compounds to relieve the symptoms of painful diabetic neuropathies.

Local isoform-specific NOS inhibition: a promising approach to promote motor function recovery after nerve injury

Physical injury to a nerve is the most frequent cause of acquired peripheral neuropathy, which is responsible for loss of motor, sensory and/or autonomic functions. Injured axons in the peripheral nervous system maintain the capacity to regenerate in adult mammals. However, after nerve transection, stumps of damaged nerves must be surgically joined to guide regenerating axons into the distal nerve stump. Even so, severe functional limitations persist after restorative surgery. Therefore, the identification of molecules that regulate degenerative and regenerative processes is indispensable in developing therapeutic tools to accelerate and improve functional recovery. Here, I consider the role of nitric oxide (NO) synthesized by the three major isoforms of NO synthases (NOS) in motor neuropathy. Neuronal NOS (nNOS) seems to be the primary source of NO that is detrimental to the survival of injured motoneurons. Endothelial NOS (eNOS) appears to be the major source of NO that interferes with axonal regrowth, at least soon after injury. Finally, NO derived from inducible NOS (iNOS) or nNOS is critical to the process of lipid breakdown for Wallerian degeneration and thereby benefits axonal regrowth. Specific inhibitors of these isoforms can be used to protect injured neurons from degeneration and promote axonal regeneration. A cautious proposal for the treatment of acquired motor neuropathy using therapeutic tools that locally interfere with eNOS/nNOS activities seems to merit consideration.

Norepinephrine-induced nociception and vasoconstrictor hypersensitivity in rats with chronic post-ischemia pain

Painful hypersensitivity to norepinephrine (NE) has been reported in various chronic pain conditions that exhibit sympathetically-maintained pain (SMP), particularly CRPS-I and II. We investigated the parallels between the nociceptive and vascular sensitivity to NE in rats with chronic post-ischemia pain (CPIP), an animal model of CRPS-I induced by hind paw ischemia-reperfusion injury. Intradermal injections of NE to the affected hind paw induced dose-dependent nociceptive behaviours in CPIP rats, but not sham animals. These behaviours were blocked by alpha(1)- and alpha(2)-adrenergic receptor antagonists, or a nitric oxide (NO) donor. Using laser Doppler flowmetry, we detected vasoconstrictor hypersensitivity in the ipsilateral CPIP hind paw, as compared to responses in sham animals or the contralateral hind paw. The vasoconstrictor hypersensitivity was also attenuated by adrenergic antagonists. Intradermal injection of [Arg(8)] vasopressin (AVP) or the endothelial NO synthase (eNOS) inhibitor, L-NIO, to the affected paw also induced nociceptive behaviours in CPIP rats, but not sham rats. These results suggest CPIP rats display abnormal nociceptive responses to adrenergic and non-adrenergic vasoconstrictive agents. Furthermore, the nociceptive responses to NE in CPIP rats are paralleled by enhanced vasoconstrictive responses to NE, and are relieved by alpha-adrenergic antagonists or a vasodilator. We conclude that persistent tissue ischemia and hypersensitivity to sympathetic vasoconstriction are important mechanisms for pain in CPIP rats, and that either reducing vasoconstriction or enhancing vasodilatation may be effective methods of relieving the pain of CRPS-I.

Endothelial nitric oxide synthase overexpression by neuronal cells in neurodegeneration: a link between inflammation and neuroprotection

The roles of neuronal and inducible nitric oxide synthases in neurones have been extensively investigated; by contrast, the biological significance of endothelial nitric oxide synthase (eNOS) overexpression that occurs in several pathological conditions has not yet been studied. We have started addressing this issue in a cell model of neurodegeneration, i.e. human SKNBE neuroblastoma cells transfected with a mutant form of alsin, a protein causing an early-onset type of amyotrophic lateral sclerosis, ALS2. We found that eNOS, which is endogenously expressed by these cells, was activated by tumour necrosis factor-alpha (TNF-alpha), a proinflammatory cytokine that plays important roles in ALS2 and several neurodegenerative diseases. The TNF-alpha-dependent eNOS activation occurred through generation, by sphingosine-kinase-1, of sphingosine-1-phosphate, stimulation of its membrane receptors and activation of Akt, as determined using small interference RNA and dominant negative constructs specific for the enzymes and receptors. eNOS activation by TNF-alpha conferred cytoprotection from excitotoxicity and neurotoxic cues such as reactive oxygen species, endoplasmic reticulum stress, DNA damage, and mutated alsin itself. Our results suggest that overexpression of eNOS by neurones is a broad-range protective mechanism activated during damage and establish a link of pathophysiological relevance between this enzyme and inflammation accompanying neurodegenerative diseases. These findings also question the concept that high NO output in the presence of oxidative stress leads always to peroxynitrite formation contributing to neurodegeneration.

Changes in mRNA for CAPON and Dexras1 in adult rat following sciatic nerve transection

Peripheral nerve transection has been implicated to cause a production of neuronal nitric oxide synthase (nNOS), which may influence a range of post-axotomy processes necessary for neuronal survival and nerve regeneration. Carboxy-terminal post synaptic density protein/Drosophila disc large tumor suppressor/zonula occuldens-1 protein (PDZ) ligand of neuronal nitric oxide synthase (CAPON), as an adaptor, interacts with nNOS via the PDZ domain helping regulate nNOS activity at postsynaptic sites in neurons. And Dexras1, a small G protein mediating multiple signal transductions, has been reported to form a complex with CAPON and nNOS. A role for the physiologic linkage by CAPON of nNOS to Dexras1 has suggested that NO-mediated activation of Dexras1 is markedly enhanced by CAPON. We investigated the changes in mRNA for CAPON, Dexras1 and nNOS in the sciatic nerve, dorsal root ganglia and lumbar spinal cord of adult rat following sciatic axotomy by TaqMan quantitative real-time PCR and in situ hybridization combined with immunofluorescence. Signals of mRNA for CAPON and Dexras1 were initially expressed in these neural tissues mentioned, transiently increased at certain time periods after sciatic axotomy and finally recovered to the basal level. It was also found that nNOS mRNA underwent a similar change pattern during this process. These results suggest that CAPON as well as Dexras1 may be involved in the different pathological conditions including nerve regeneration, neuron loss or survival and even pain process, possibly via regulating the nNOS activity or through the downstream targets of Dexras1.

Effect of peripheral axotomy on gene expression of NIDD in rat neural tissues

Peripheral nerve lesion-induced production of neuronal nitric oxide synthase (nNOS) was implicated to influence a range of postaxotomy processes necessary for neuronal survival and nerve regeneration (Zochodne et al., Neuroscience, 91:1515-1527, 1999; Keilhoff et al., Journal of Chemical Neuroanatomy, 24:181-187, 2002, Nitric Oxide, 10:101-111, 2004). Protein-protein interactions represent an important mechanism in the control of NOS spatial distribution or activity (Alderton et al., Biochemical Journal, 357:593-615, 2001; Dedio et al., FASEB Journal, 15:79-89, 2001; Zimmermann et al., Proceedings of the National Academy of Sciences, 99:17167-17172, 2002). As one of the nNOS-binding proteins, nNOS-interacting DHHC domain-containing protein with dendritic mRNA (NIDD) has recently been identified to increase nNOS enzyme activity by targeting nNOS to the synaptic plasma membrane in a postsynaptic density protein 95/discs-large/zona occlusens-1 domain dependent manner (Saitoh et al., Journal of Biological Chemistry, 279:29461-29468, 2004). In this paper, we established a rat model with peripheral axotomy to investigate the gene expression patterns of NIDD in neural tissues using TaqMan quantitative real-time polymerase chain reaction and in situ hybridization combined with immunofluorescence. It revealed that NIDD mRNA was upregulated after sciatic nerve transection with the similar expressing styles as that of the nNOS in the injured nerves, corresponding dorsal root ganglia, and lumbar spinal cord. These findings imply that NIDD may be involved in the different pathological conditions including nerve regeneration, neuron loss or survival, and even pain process, possibly via regulating the enzyme nNOS activity.

Altered gene expression of NIDD in dorsal root ganglia and spinal cord of rats with neuropathic or inflammatory pain

Nitric oxide and nitric oxide synthases are key players in synaptic plasticity events in spinal cord (SC), which underlies the chronic pain states. To date, little is known about the molecular mechanisms regulating the activity of nitric oxide synthases in nociceptive systems. The present study was aimed at the determination of the gene expression of nNOS-interacting DHHC domain-containing protein with dendritic mRNA (NIDD), a recently identified protein regulating nNOS enzyme activity, in rat SC and dorsal root ganglia (DRG) and studying its regulation in states of nociceptive hypersensitivity in a rat model of neuropathic or inflammatory pain. It was found that NIDD mRNA was predominantly expressed in nociceptive primary neurons and in neurons of the spinal dorsal horn (DH) and the number of NIDD-positive neurons in the corresponding DRG or SC increased significantly following induction of chronic hyperalgesia. Meanwhile, remarkable changes of nNOS were detected under such pain conditions. Our data suggest a potential role for NIDD in the maintenance of thermal pain hypersensitivity possibly via regulating the nNOS activity.

Agmatine induces antihyperalgesic effects in diabetic rats and a superadditive interaction with R(-)-3-(2-carboxypiperazine-4-yl)-propyl-1-phosphonic acid, a N-methyl-D-aspartate-receptor antagonist

Agmatine, an endogenous cationic amine resulting from the decarboxylation of L-arginine, produces antihyperalgesic and antiallodynic effects in animal models of chronic neuropathic and inflammatory pain. We examined the effect of agmatine on tactile and thermal allodynia and on mechanical hyperalgesia in streptozocin-induced diabetic rats. To determine its mechanism of action and the potential interest of some of its combinations, the antihyperalgesic effect of agmatine was challenged with alpha(2)-adrenergic imidazoline and opioid-receptor antagonists, and its interaction with the opioid-receptor agonist morphine, the competitive N-methyl-D-aspartate receptor antagonist D-CPP [R(-)-3-(2-carboxypiperazine-4-yl)-propyl-1-phosphonic acid], and the nitric-oxide synthase inhibitor L-NAME (L-N(G)-nitro-L-arginine methyl ester) were examined. When intrathecally (i.t.) injected (4.4 to 438 nmol/rat), agmatine was ineffective in normal rats but suppressed tactile allodynia (von Frey hair test), thermal allodynia (tail immersion test), and mechanical hyperalgesia (paw-pressure test) in diabetic rats. This spinal antihyperalgesic effect was suppressed by idazoxan (40 micromol/rat i.t.) but not by yohimbine (40 micromol/rat i.t.) or naloxone (0.69 micromol/rat i.v.). In diabetic rats, an isobolographic analysis showed that combinations of i.t. agmatine with i.v. L-NAME or with i.t. morphine resulted in an additive antihyperalgesic effect, whereas the agmatine/D-CPP i.t. combination was superadditive. In summary, the present findings reveal that spinal agmatine produces antiallodynic and antihyperalgesic effects in diabetic neuropathic pain involving, at least for its antihyperalgesic effect, the imidazoline receptors. Moreover, agmatine combined with D-CPP produces an antinociceptive synergy in experimental neuropathy, opening opportunities in the development of new strategies for pain therapy.

NO pain: potential roles of nitric oxide in neuropathic pain

The disabling human syndrome of "neuropathic pain" is an intractable complication of peripheral nerve injury or degeneration. A complex interaction between injured peripheral axons, sensory neurons and central nervous system signaling is thought to account for it. In this brief review, we present evidence that the free radical signaling molecule, nitric oxide (NO) may act at several levels of the nervous system during the development of experimental neuropathic pain. For example, NO may directly influence injured axons in the periphery, may indirectly influence pain by its role in the process of Wallerian degeneration, and may signal in the dorsal horn of the spinal cord. While it is premature to argue for therapeutic approaches that alter NO actions, it may be an important player in the cascade of events that generate neuropathic pain.

Differences in inflammatory pain in nNOS-, iNOS- and eNOS-deficient mice

To assess the relative importance of the isoforms of nitric oxide synthase (NOS) in inflammatory pain, we directly compared pain behaviour and paw thickness after intraplantar injection of complete Freund's adjuvant (CFA) in wild-type (WT) mice and in mice lacking either inducible (iNOS), endothelial (eNOS) or neuronal NOS (nNOS). In mice deficient for nNOS, thermal hyperalgesia was reduced by approximately 50% compared to wild type mice at 4 and 8h after CFA injection, and mechanical hypersensitivity was absent. The only change in pain behaviour in iNOS and eNOS deficient mice compared to WT mice was a more rapid recovery from thermal hyperalgesia. A compensatory up-regulation of nNOS in dorsal root ganglia (DRG) and spinal cords of iNOS and eNOS knockout mice was excluded using RT-PCR. However, an increase of iNOS gene expression was found in spinal cords of eNOS and nNOS deficient mice. To study the downstream effects of nNOS deficiency on DRG neurones, we assessed their immunoreactivity for calcitonin gene-related peptide (CGRP) and cytokines. We found a significant reduction in the CFA induced increase in CGRP immunoreactive neurones as well as in CGRP gene expression in nNOS deficient mice, whereas the percentage of cells immunopositive for tumour necrosis factor-alpha (TNF-alpha) and interleukin-1beta (IL-1beta) was unchanged. These results support the proposed role of nNOS in sensitization of DRG neurones, and might indicate that CGRP is involved in this process.

Establishment of a rodent model of HIV-associated sensory neuropathy

Human immunodeficiency virus (HIV)-associated sensory neuropathy (SN) is the most common neurological complication of HIV infection in the current highly active antiretroviral therapy era. The painful sensory neuropathy is associated with the use of dideoxynucleoside antiretrovirals, and its development limits the choice of antiretroviral drugs in affected patients. There are presently no effective therapies for HIV-SN, and moreover there has been no robust animal model of HIV-SN in which candidate therapeutic agents can be tested. In this paper, we show that we have established a rodent model of HIV-SN by oral administration of a dideoxynucleoside drug, didanosine, to transgenic mice expressing the HIV coat protein gp120 under a GFAP promoter. The neuropathy in these rodents is characterized by distal degeneration of unmyelinated sensory axons, similar to the "dying back" pattern of C-fiber loss seen in patients with HIV-SN. This model will be useful in examining mechanisms of distal axonal degeneration and testing potential neuroprotective compounds that may prevent development of the sensory neuropathy.

Possible sources and sites of action of the nitric oxide involved in synaptic plasticity at spinal lamina I projection neurons

The synaptic long-term potentiation between primary afferent C-fibers and spinal lamina I projection neurons is a cellular model for hyperalgesia [Ikeda H, Heinke B, Ruscheweyh R, Sandkühler J (2003) Synaptic plasticity in spinal lamina I projection neurons that mediate hyperalgesia. Science 299:1237-1240]. In lamina I neurons with a projection to the periaqueductal gray, this long-term potentiation is dependent on nitric oxide. In the present study, we used immunohistochemistry to detect possible sources and sites of action of the nitric oxide necessary for the long-term potentiation at lamina I spino-periaqueductal gray neurons in rats. None of the three isoforms of the nitric oxide synthase was expressed in a significant number of lamina I spino-periaqueductal gray neurons or primary afferent C-fibers (as evaluated by staining of their cell bodies in the dorsal root ganglia). However, endothelial and inducible nitric oxide synthase were found throughout the spinal cord vasculature and neuronal nitric oxide synthase was present in a number of neurons in laminae II and III. The nitric oxide target soluble guanylyl cyclase was detected in most lamina I spino-periaqueductal gray neurons and in approximately 12% of the dorsal root ganglion neurons, all of them nociceptive as evaluated by coexpression of substance P. Synthesis of cyclic 3',5'-guanosine monophosphate upon stimulation by a nitric oxide donor confirmed the presence of active guanylyl cyclase in at least a portion of the spino-periaqueductal gray neuronal cell bodies. We therefore propose that nitric oxide generated in neighboring neurons or blood vessels acts on the spino-periaqueductal gray neuron and/or the primary afferent C-fiber to enable long-term potentiation. Lamina I spino-parabrachial neurons were stained for comparison and yielded similar results.

Rescue and regeneration of injured peripheral nerve axons by intrathecal insulin

Insulin peptide, acting through tyrosine kinase receptor pathways, contributes to nerve development or repair. In this work, we examined the direction, impact and repertoire of insulin signaling in vivo during peripheral nerve regeneration in rats. First, we demonstrated that insulin receptor is expressed on lumbar dorsal root ganglia neuronal perikarya using immunohistochemistry. Immunoblots and polymerase chain reactions confirmed the presence of both alpha and beta insulin receptor subunits in dorsal root ganglia. In vivo and in vitro assessment of dorsal root ganglion neurons showed preferential localization of insulin receptor to perikaryal sites. In vivo, intrathecal delivery of fluorescein isothiocyanate-labeled insulin identified localization around dorsal root ganglia neurons. The direction and impact of potential insulin signaling was evaluated by concurrently delivering insulin or carrier over a 2 week period using mini-osmotic pumps, either intrathecally, near nerve, or with both deliveries, following a selective sural nerve crush injury. Only intrathecal insulin increased the number and maturity of regenerating sensory sural nerve axons distal to the crush site. As well, only intrathecal insulin rescued retrograde loss of sural axons after crush. In a separate experiment, insulin also rescued retrograde loss and atrophy of deep peroneal, largely motor, axons post-injury. Intrathecal insulin increased the expression of calcitonin-gene-related peptide in regenerating sprouts, increased the number of visualized regenerating fiber clusters, and reduced downregulation of calcitonin-gene-related peptide in dorsal root ganglia neurons. Insulin delivered intrathecally does not appear to influence expression of insulin-like growth factor-1 at dorsal root ganglion neurons or near peripheral nerve injury, but was associated with upregulation of insulin receptor alpha subunit in dorsal root ganglia. Intrathecal insulin delivery was associated with greater recovery of thermal sensation and longer distances to stimulus response with the pinch test following sural nerve crush. Insulin signaling at neuron perikarya can drive distal sensory axon regrowth, rescue retrograde alterations of axons and alter axon peptide expression. Moreover, such actions are associated with upregulation of its own receptor.

Upregulation of chondroitin 6-sulphotransferase-1 facilitates Schwann cell migration during axonal growth

Cell migration is central to development and post-traumatic regeneration. The differential increase in 6-sulphated chondroitins during axonal growth in both crushed sciatic nerves and brain development suggests that chondroitin 6-sulphotransferase-1 (C6ST-1) is a key enzyme that mediates cell migration in the process. We have cloned the cDNA of the C6ST-1 gene (C6st1) (GenBank accession number AF178689) from crushed sciatic nerves of adult rats and produced ribonucleotide probes accordingly to track signs of 6-sulphated chondroitins at the site of injury. We found C6st1 mRNA expression in Schwann cells emigrating from explants of both sciatic nerve segments and embryonic dorsal root ganglia. Immunocytochemistry indicated pericellular 6-sulphated chondroitin products around C6ST-1-expressing frontier cells. Motility analysis of frontier cells in cultures subjected to staged treatment with chondroitinase ABC indicated that freshly produced 6-sulphated chondroitin moieties facilitated Schwann cell motility, unlike restrictions resulting from proteoglycan interaction with matrix components. Sciatic nerve crush provided further evidence of in vivo upregulation of the C6ST-1 gene in mobile Schwann cells that guided axonal regrowth 1-14 days post crush; downregulation then accompanied declining mobility of Schwann cells as they engaged in the myelination of re-growing axons. These findings are the first to identify upregulated C6st1 gene expression correlating with the motility of Schwann cells that guide growing axons through both developmental and injured environments.

Immune and inflammatory mechanisms in neuropathic pain

Tissue damage, inflammation or injury of the nervous system may result in chronic neuropathic pain characterised by increased sensitivity to painful stimuli (hyperalgesia), the perception of innocuous stimuli as painful (allodynia) and spontaneous pain. Neuropathic pain has been described in about 1% of the US population, is often severely debilitating and largely resistant to treatment. Animal models of peripheral neuropathic pain are now available in which the mechanisms underlying hyperalgesia and allodynia due to nerve injury or nerve inflammation can be analysed. Recently, it has become clear that inflammatory and immune mechanisms both in the periphery and the central nervous system play an important role in neuropathic pain. Infiltration of inflammatory cells, as well as activation of resident immune cells in response to nervous system damage, leads to subsequent production and secretion of various inflammatory mediators. These mediators promote neuroimmune activation and can sensitise primary afferent neurones and contribute to pain hypersensitivity. Inflammatory cells such as mast cells, neutrophils, macrophages and T lymphocytes have all been implicated, as have immune-like glial cells such as microglia and astrocytes. In addition, the immune response plays an important role in demyelinating neuropathies such as multiple sclerosis (MS), in which pain is a common symptom, and an animal model of MS-related pain has recently been demonstrated. Here, we will briefly review some of the milestones in research that have led to an increased awareness of the contribution of immune and inflammatory systems to neuropathic pain and then review in more detail the role of immune cells and inflammatory mediators.

Nitric oxide synthases II and III and vascular endothelial growth factor are up-regulated in sciatic nerve after spiral cuff electrode implantation

Nerve cuff electrodes, commonly used in functional electrical stimulation systems, induce local morphological changes that can affect nerve functionality. Nitric oxide (NO) and vascular endothelial growth factor (VEGF) have both neural and vascular effects. We investigated the time-dependent regulation of nitric oxide synthases (NOS) and of VEGF after implantation of spiral cuff electrode around rat sciatic nerve. NOSIII as well as VEGF were up-regulated in both epineurial and endoneurial compartments in cuff-implanted animals along with microvascular changes. Our results suggest that VEGF and NO are implied in morphological and functional alterations occurring in the early time after cuff implantation.

Nitric oxide synthase (NOS)-interacting protein interacts with neuronal NOS and regulates its distribution and activity

Mechanisms governing the activity of neuronal nitric oxide synthase (nNOS), the major source of nitric oxide (NO) in the nervous system, are not completely understood. We report here a protein-protein interaction between nNOS and NOSIP (nitric oxide synthase-interacting protein) in rat brain in vivo. NOSIP and nNOS are concentrated in neuronal synapses and demonstrate significant colocalization in various regions of the central and peripheral nervous systems. NOSIP produces a significant reduction in nNOS activity in a neuroepithelioma cell line stably expressing nNOS. Furthermore, overexpression of NOSIP in cultured primary neurons reduces the availability of nNOS in terminal dendrites. These results thus suggest that the interaction between NOSIP and nNOS is functionally involved in endogenous mechanisms regulating NO synthesis. Furthermore, we found that the subcellular distribution and expression levels of NOSIP are dynamically regulated by neuronal activity in vitro as well as in vivo, suggesting that NOSIP may contribute to a mechanism via which neuronal activity regulates the synaptic availability and activity of nNOS.

Increased nitric oxide levels and iNOS over-expression in oral squamous cell carcinoma

Inducible nitric oxide synthase (iNOS) is responsible for generating high levels of nitric oxide (NO) in tissues. Increased iNOS expression has been demonstrated in a number of carcinomas including head and neck squamous cell carcinoma (SCC). However, iNOS levels have not been evaluated specifically in oral cavity SCC, or in the precancerous lesions that progress to oral SCC. Also, NO levels have not been measured in oral precancerous or cancerous tissues. We therefore measured iNOS mRNA, iNOS protein and NO in oral SCC, oral dysplasias and normal oral epithelium. We used RT-PCR to quantify and compare iNOS mRNA levels in these oral tissue specimens. We found that iNOS mRNA was overexpressed in 41% of oral SCC but in only 8% of dysplasia specimens (P = 0.003). Immunohistochemistry was used to evaluate iNOS protein levels in oral SCC, oral dysplasias and normal oral epithelium. A significantly higher percentage of oral SCC specimens showed the highest level of iNOS staining relative to the oral dysplasias and normal oral epithelial samples (95% of oral SCC, 50% of dysplasias, and only 0% of normal epithelial controls, P < 0.0001). The positive staining for iNOS was limited to the SCC cells. Production of NO from iNOS was quantified using HPLC and found to be significantly higher in oral SCC (1.45 +/- 0.56 microg/ml) than normal epithelial controls (0.43 +/- 0.26 microg/ml) (P = 0.0013). We conclude that iNOS mRNA levels and NO production are significantly increased, in oral SCC compared to oral dysplasias and normal epithelial controls. These findings suggest that increased iNOS expression and the generation of high NO levels might have a role in oral SCC development.

nNOS expression following inferior alveolar nerve injury in the ferret

Damage to the inferior alveolar nerve (IAN) may result in permanent painful dysaesthesia, and there is compelling evidence to suggest that ectopic activity from the injury site plays a crucial role in the initiation of this disorder. The aim of this study was to determine whether neuronal nitric oxide synthase (nNOS), a regulator of neuronal excitability, could be involved in the development of the abnormal activity. In seven ferrets, the left IAN was exposed and a retrograde tracer, fluorogold, was applied to the nerve for the identification of cell bodies in the trigeminal ganglion with axons in the IAN. In four animals, the nerve was sectioned distal to the injection site, and three served as controls. After 3 days, the animals were perfused with fixative, and the left and right IANs and trigeminal ganglia were processed using indirect immunofluorescence for nNOS. Image analysis was used to quantify the percentage area of staining (PAS) at the injury site. In the ganglia, counts were made of positively labelled cells in the fluorogold population. At the injury site, PAS was significantly greater in injured nerves than in either contralateral or control nerves, and contralateral PAS was elevated compared to control. In the ganglia, the proportion of nNOS-labelled cells was significantly reduced following injury. These results indicate a possible translocation of the nNOS protein from the cell body to the site of nerve injury, where it accumulates. Thus, nNOS could play a role in the development of ectopic activity at a site of trigeminal nerve injury.

Modulation of Dural Nociceptor Mechanosensitivity by the Nitric Oxide-Cyclic GMP Signaling Cascade

The role of the nitric oxide (NO)-cGMP signaling cascade in modulation of peripheral nociception is controversial. Although behavioral studies have suggested both pro- and anti-nociceptive effects, little is known about the direct action of this signaling cascade on primary afferent nociceptive neurons that mediate these behaviors. Here, using single-unit recordings, we examined the direct effect of NO-cGMP signaling on spontaneous activity and mechanical responses of nociceptive afferents that innervate the dura mater. We found that the NO donor sodium nitroprusside (SNP), when applied topically to the neuronal receptive field, induced both sensitization and inhibition of the mechanical responses, albeit in different populations of neurons, which could be distinguished based on their baseline mechanical thresholds. SNP, however, did not change the level of spontaneous activity. Administration of the cGMP analogue 8-pCPT-cGMP mimicked only the inhibitory effect. When SNP was co-applied with either an inhibitor of guanylyl cyclase or a cGMP blocker, sensitization never occurred, and the inhibitory effect of SNP could also be blocked. Our findings suggest that NO can either increase or decrease the mechanical responsiveness of nociceptors and that its action might depend, in part, on the baseline level of neuronal excitability. Our results also implicate cGMP in mediating the inhibitory effect of NO.

Oxidized galectin-1 stimulates macrophages to promote axonal regeneration in peripheral nerves after axotomy

Various neurotrophic factors that promote axonal regeneration have been investigated in vivo, but the signals that prompt neurons to send out processes in peripheral nerves after axotomy are not well understood. Previously, we have shown oxidized galectin-1 (GAL-1/Ox) promotes initial axonal growth after axotomy in peripheral nerves. However, the mechanism by which GAL-1/Ox promotes axonal regeneration remains unclear and is the subject of the present study. To identify possible target cells of GAL-1/Ox, a fluorescently labeled recombinant human GAL-1/Ox (rhGAL-1/Ox) was incubated with DRG neurons, Schwann cells, and intraperitoneal macrophages from adult rats. Only the cell surfaces of intraperitoneal macrophages bound the rhGAL-1/Ox, suggesting that these cells possess a receptor for GAL-1/Ox. Experiments examining tyrosine phosphorylation revealed that rhGAL-1/Ox stimulated changes in signal transduction pathways in these macrophages. These changes caused macrophages to secrete an axonal growth-promoting factor. This was demonstrated when conditioned media of macrophages stimulated with rhGAL-1/Ox in 48 hr culture strongly enhanced axonal regeneration from transected-nerve sites of DRG explants. Furthermore, activated macrophage-conditioned media also improved Schwann cell migration from the transected-nerve sites. From these results, we propose that axonal regeneration occurs in axotomized peripheral nerves as a result of cytosolic reduced galectin-1 being released from Schwann cells and injured axons, which then becomes oxidized in the extracellular space. Oxidized galectin-1 then stimulates macrophages to secrete a factor that promotes axonal growth and Schwann cell migration, thus enhancing peripheral nerve regeneration.

Heightened pain and delayed regeneration of galanin axons in diabetic mice

Galanin peptide in primary sensory neurons may confer analgesia following injury. Its presence in regenerative axon sprouts where pain may be initiated has not been examined. We examined very early outgrowth of peptidergic axon sprouts after sciatic nerve crush in mice with experimental streptozotocin-induced diabetes. Diabetic mice had a retarded wave of outgrowing galanin axons, but those expressing calcitonin gene-related peptide grew normally. Diabetic mice also developed early, then persistent excessive autotomy behaviour, an index of pain behaviour in complete nerve lesions. Diabetes is associated with variations in the early outgrowth of peptide-containing axons. A relative delay in galanin axon outgrowth could contribute to heightened neuropathic pain in diabetes.

Pathophysiology of pain

Pain is a major symptom of many different diseases. Modern pain research has uncovered important neuronal mechanisms that are underlying clinically relevant pain states, and research goes on to define different types of pains on the basis of their neuronal and molecular mechanisms. This review will briefly outline neuronal mechanisms of pathophysiological nociceptive pain resulting from inflammation and injury, and neuropathic pain resulting from nerve damage. Pain is the sensation that is specifically evoked by potential or actual noxious (i.e. tissue damaging) stimuli or by tissue injury. Pain research has not only explored the neuronal and molecular basis of the "pain system" of the healthy subject but has also provided insights into the function and plasticity of the "pain system" during clinically relevant pains such as post-injury pain, inflammatory pain, postoperative pain, cancer pain and neuropathic pain. This review will briefly describe the "pain system" and then address neuronal mechanisms that are involved in clinical pain states.

Spinal upregulation of the nitric oxide synthase-interacting protein NOSIP in a rat model of inflammatory pain

Nitric oxide and nitric oxide synthases are key players in synaptic plasticity events in the spinal cord, which underly the development of chronic pain states. To date, little is known about the molecular mechanisms regulating the activity of nitric oxide synthases in nociceptive systems. The present study was aimed at the immunohistochemical determination of the expression of a nitric oxide synthase-interacting protein (NOSIP) in the rat spinal cord and dorsal root ganglia and studying its regulation in states of nociceptive hypersensitivity in a rat model of post-inflammatory pain. NOSIP is predominantly expressed in nociceptive primary neurons and in neurons of the spinal dorsal horn and the number of NOSIP-positive spinal neurons increases significantly following induction of unilateral intraplantar injection of complete Freund's adjuvant. Thus, NOSIP may modulate nitric oxide homeostasis in physiological and pathological pain conditions.

Mu opioid receptors and analgesia at the site of a peripheral nerve injury

Opioid ligands may exert antinociception through receptors expressed on peripheral afferent axons. Whether local opioid receptors might attenuate neuropathic pain is uncertain. In this work, we examined the function and expression of local mu opioid receptors (MORs) associated with the chronic constriction injury (CCI) model of sciatic neuropathic pain in rats. Low-dose morphine or its carrier were percutaneously superfused over the CCI site with the injector blinded to the identity of the injectate. Morphine, but not its carrier, and not equimolar systemic doses of morphine reversed thermal hyperalgesia in a dose-related, naloxone-sensitive fashion. Moreover, analgesia was conferred at both 48 hours and 14 days after CCI, times associated with very different stages of nerve repair. Equimolar local DAGO ([D-Ala2, N-Me-Phe4, Gly5-(ol)] enkephalin), a selective MOR ligand, provided similar analgesia. Local morphine also attenuated mechanical allodynia. MOR protein was expressed in axonal endbulbs of Cajal just proximal to the injury site, in aberrantly regenerating small axons in the epineurial sheath around the CCI site and in residual small axons distal to the CCI lesion. Sensory neurons ipsilateral to CCI had an increase in the proportion of neurons expressing MOR. We suggest that local MOR expressed in axons may be exploited to modulate some forms of neuropathic pain.

The kappa-opioid agonist (+/-)-bremazocine elicits peripheral antinociception by activation of the L-arginine/nitric oxide/cyclic GMP pathway

In view of the scarce information about the analgesic mechanism of kappa-opioid receptor agonists, the objective of the present study was to determine whether nitric oxide (NO) is involved in the peripheral antinociception of bremazocine, a kappa-opioid receptor agonist. Three drugs all interfering with the L-arginine/NO/cyclic GMP pathway were tested using the rat paw model of carrageenan-induced (250 microg) hyperalgesia: (a) N(G)-nitro-L-arginine (a nonselective NO-synthase inhibitor), (b) methylene blue (a guanylate cyclase inhibitor), and (c) zaprinast (a cyclic GMP phosphodiesterase inhibitor). Intraplantar administration of bremazocine (20, 40 and 50 microg) caused a dose-dependent peripheral antihyperalgesia against carrageenan-induced hyperalgesia. The possibility of the higher dose of bremazocine (50 microg) having central or systemic effect was excluded since administration of the drug into the left paw did not elicit antinociception in the contralateral paw. However, when the dose of bremazocine was increased to 100 microg, a significant increase in the nociceptive threshold was observed, as measured in the hyperalgesic contralateral paw. Peripheral antihyperalgesia induced by bremazocine (50 microg) was significantly reduced in a dose-dependent manner when N(G)-nitro-L-arginine (6, 9, 12 and 25 microg) or methylene blue (250, 375 and 500 microg) was injected before. Previous treatment with 50 microg of zaprinast (which had no effect when administered alone) potentiated the antihyperalgesic effect of bremazocine (20 microg). Our data suggest that bremazocine elicits peripheral antinociception by activation of the L-arginine/NO/cyclic GMP pathway and that nitric oxide is an intermediary in this mechanism, forming cyclic GMP.

Human neural stem cells express extra-neural markers

Neural stem cells can be derived from the adult/embryonic nervous system as well as from more primitive embryonic stem cells but, because of the lack of specific markers, only their differentiated progeny can be characterized. We here report the presence of several endothelial and hematopoietic receptors (at protein and mRNA level) on the surface of embryonic human neural stem cells, which are partially maintained during differentiation. This suggests that neural stem cells have a greater potential than previously thought, which involves the ability to respond to different and so far unconsidered environmental signals and may be responsible for the recently discovered process of stem cell-fate conversion.

Do denervated peripheral nerve trunks become ischemic? The impact of chronic denervation on vasa nervorum

The long-term relationship between the peripheral nerve trunk and its vascular supply, the vasa nervorum, has not been considered in the context of denervation and regeneration. While the microvessels of peripheral nerve are not thought to influence Wallerian degeneration itself, in this work we explored how vasa nervorum respond to denervation of the nerve trunk. Our hypotheses were that the presence of axons had a significant impact on the vasa nervorum and that the absence of reinnervation might eventually lead to an unfavorable ischemic regenerative microenvironment. We studied rat sciatic nerve trunks for up to 6 months following transection and either prevented regeneration or allowed it to proceed. Vasa nervorum were studied in several ways: (i) measurements of local endoneurial blood flow using microelectrode hydrogen clearance polarography; (ii) measurements of erythrocyte flux (flow) in the extrinsic nerve plexus using laser Doppler flowmetry; (iii) India ink perfusion of microvessels in unfixed nerve; (iv) mRNA expression of vascular endothelial growth factor (VEGF) using reverse transcription polymerase chain reaction. Early after injury, there were rises in endoneurial and extrinsic flow, microvessel numbers, and VEGF mRNA expression. Angiogenesis was apparently confined to the epineurial and perineurial compartments. Later, however, there were substantial declines in flow observed in long-term (6-month) denervated sciatic nerve trunks associated with declines in the caliber of new microvessels. Reinnervated sciatic nerves had restored endoneurial blood flow. The findings confirm important relationships between axon presence and local blood flow. Angiogenesis is a feature of the injured peripheral nerve, but long term denervated nerve trunks have declines of flow despite retaining new microvessels.

Delayed peripheral nerve degeneration, regeneration, and pain in mice lacking inducible nitric oxide synthase

Inducible nitric oxide synthase (iNOS) may be a critical factor in the repair of injured tissues. In mice lacking iNOS we observed abnormalities in how the peripheral nerve responds to each of 3 fundamental types of injury: chronic constriction partial nerve injury (a model of neuropathic pain), nerve crush, and nerve transection. In each type of injury, mice lacking iNOS had evidence of a regenerative delay, preceded by slowing of myelinated fiber Wallerian degeneration (WD). In wild-type mice, iNOS immunoreactivity and the presence and upregulation of its mRNA were demonstrated distal to injury, but neither was observed in the knockout mice. Slowed WD was suggested by the abnormal persistence of apparent myelinated fiber profiles distal to the injury zones in mice lacking iNOS compared to wild-type controls. In mice lacking iNOS there were fewer regenerating myelinated fibers, smaller caliber regenerating fibers, and slowed reinnervation of muscle endplates distal to the injury zone. Slowed degeneration was also associated with normal initiation but delayed expression of neuropathic pain. Our findings highlight important relationships among nitric oxide, WD, neuropathic pain, and axon regeneration.