Sulfasalazine blocks the development of tactile allodynia in diabetic rats

Peripheralized sepiapterin reductase inhibition as a safe analgesic therapy

The development of novel analgesics for chronic pain in the last 2 decades has proven virtually intractable, typically failing due to lack of efficacy and dose-limiting side effects. Identified through unbiased gene expression profiling experiments in rats and confirmed by human genome-wide association studies, the role of excessive tetrahydrobiopterin (BH4) in chronic pain has been validated by numerous clinical and preclinical studies. BH4 is an essential cofactor for aromatic amino acid hydroxylases, nitric oxide synthases, and alkylglycerol monooxygenase so a lack of BH4 leads to a range of symptoms in the periphery and central nervous system (CNS). An ideal therapeutic goal therefore would be to block excessive BH4 production, while preventing potential BH4 rundown. In this review, we make the case that sepiapterin reductase (SPR) inhibition restricted to the periphery (i.e., excluded from the spinal cord and brain), is an efficacious and safe target to alleviate chronic pain. First, we describe how different cell types that engage in BH4 overproduction and contribute to pain hypersensitivity, are themselves restricted to peripheral tissues and show their blockade is sufficient to alleviate pain. We discuss the likely safety profile of peripherally restricted SPR inhibition based on human genetic data, the biochemical alternate routes of BH4 production in various tissues and species, and the potential pitfalls to predictive translation when using rodents. Finally, we propose and discuss possible formulation and molecular strategies to achieve peripherally restricted, potent SPR inhibition to treat not only chronic pain but other conditions where excessive BH4 has been demonstrated to be pathological.

The Development of Mechanical Allodynia in Diabetic Rats Revealed by Single-Cell RNA-Seq

Mechanical allodynia (MA) is the main reason that patients with diabetic peripheral neuropathy (DPN) seek medical advice. It severely debilitates the quality of life. Investigating hyperglycemia-induced changes in neural transcription could provide fundamental insights into the complex pathogenesis of painful DPN (PDPN). Gene expression profiles of physiological dorsal root ganglia (DRG) have been studied. However, the transcriptomic changes in DRG neurons in PDPN remain largely unexplored. In this study, by single-cell RNA sequencing on dissociated rat DRG, we identified five physiological neuron types and a novel neuron type MAAC (Fxyd7⁺/Atp1b1⁺) in PDPN. The novel neuron type originated from peptidergic neuron cluster and was characterized by highly expressing genes related to neurofilament and cytoskeleton. Based on the inferred gene regulatory networks, we found that activated transcription factors Hobx7 and Larp1 in MAAC could enhance Atp1b1 expression. Moreover, we constructed the cellular communication network of MAAC and revealed its receptor-ligand pairs for transmitting signals with other cells. Our molecular investigation at single-cell resolution advances the understanding of the dynamic peripheral neuron changes and underlying molecular mechanisms during the development of PDPN.

Loss of POMC-mediated antinociception contributes to painful diabetic neuropathy

Painful neuropathy is a frequent complication in diabetes. Proopiomelanocortin (POMC) is an endogenous opioid precursor peptide, which plays a protective role against pain. Here, we report dysfunctional POMC-mediated antinociception in sensory neurons in diabetes. In streptozotocin-induced diabetic mice the Pomc promoter is repressed due to increased binding of NF-kB p50 subunit, leading to a loss in basal POMC level in peripheral nerves. Decreased POMC levels are also observed in peripheral nervous system tissue from diabetic patients. The antinociceptive pathway mediated by POMC is further impaired due to lysosomal degradation of μ-opioid receptor (MOR). Importantly, the neuropathic phenotype of the diabetic mice is rescued upon viral overexpression of POMC and MOR in the sensory ganglia. This study identifies an antinociceptive mechanism in the sensory ganglia that paves a way for a potential therapy for diabetic neuropathic pain.

Sulfasalazine alleviates neuropathic pain hypersensitivity in mice through inhibition of SGK-1 in the spinal cord

Diurnal variations in pain hypersensitivity are common in chronic pain disorders. Temporal exacerbation of neuropathic pain hypersensitivity is dependent on diurnal variations in glucocorticoid secretion from the adrenal glands. We previously demonstrated that spinal expression of serum- and glucocorticoid-inducible kinase-1 (SGK-1) is associated with glucocorticoid- induced exacerbation of pain hypersensitivity, but there are no available strategies to inhibit SGK-1 in the spinal cord. By screening a clinically approved drug library (more than 1,200 drugs), we found that sulfasalazine (SSZ) has inhibitory effects on SGK-1. SSZ is a prodrug composed of 5-aminosalicylic acid and sulfapyridine linked by NN bond, which is therapeutically effective for inflammatory bowel diseases. However, the NN bond in SSZ was necessary for its inhibitory action against SGK-1. Although intrathecal injection of SSZ to nerve-injured mice significantly alleviated mechanical pain hypersensitivity, no significant anti- neuropathic pain effects of SSZ were detected after oral administration due to its low bioavailability and limited spinal distribution, which were associated with efflux by the xenobiotic transporter breast cancer resistance protein (BCRP). Concomitant oral administration of SSZ with febuxostat (FBX), which is an approved drug to inhibit BCRP, improved the distribution of SSZ to the spinal cord. The concomitant oral administration with FBX also increased the anti-neuropathic pain effects of SSZ. Our study revealed a previously unrecognized pharmacological effect of SSZ to alleviate SGK-1-induced painful peripheral neuropathy, and concomitant oral administration of SSZ with FBX may also be a preventative option for diurnal exacerbation of neuropathic pain hypersensitivity.

Prevalence of Migraine and Neuropathic Pain in Rheumatic Diseases

To investigate the physiopathology of pain in chronic inflammatory rheumatic diseases (CIRDs), we assessed the prevalence of migraine and neuropathic pain in 499 patients with CIRDs. We studied 238 patients with rheumatoid arthritis, 188 with spondyloarthritis (SpA), 72 with psoriatic arthritis (PsA), and 1 unclassified. Migraine was diagnosed according to IHS migraine diagnostic criteria. Neuropathic pain was diagnosed when patients scored at least 3 on the DN4 questionnaire. Participants completed a validated self-assessment questionnaire. Migraine prevalence was 34% (165/484), and it was highest in PsA. Risk factors for migraine were a high level of anxiety, female sex, young age, and TNF-alpha inhibitor treatment (OR = 1.90 (1.13–3.25)). Besides, high disease activity was a risk factor in SpA. Blood CRP level was not significantly associated with migraine. Of 493 patients with CIRDs, 21.5% had chronic pain with neuropathic characteristics. Compared to the French general population, these patients had significantly higher prevalences of migraine (two-fold) and neuropathic pain (three-fold). This study showed that migraine and neuropathic pain frequently occurred in patients with rheumatic diseases. Therefore, upon reporting residual pain, these patients should be checked for the presence of migraine or neuropathic pain, despite adequate clinical control of rheumatic disease.

Sulfasalazine attenuates chronic constriction injury-induced neuroinflammation and mechanical hypersensitivity in rats

Neuropathic pain is a severe and chronic neurological disease caused by injury or disease of the somatosensory system. Currently, there are no effective treatments for neuropathic pain. Neuroinflammation, characterized by activation of spinal glial cells and increased production of pro-inflammatory cytokines (for example, IL-1β, TNF-α and IL-6), is a pathophysiological process closely related to neuropathic pain. The anti-inflammatory drug sulfasalazine (SFZ) is approved for inflammatory bowel disease and rheumatoid arthritis. Although the analgesic effect of SFZ has been reported in diabetic mice, its role in neuropathic pain caused by peripheral nerve injury has not been clarified. Here, we show that SFZ significantly alleviated mechanical hypersensitivity and attenuated neuroinflammatory response in neuropathic pain induced by chronic constriction injury (CCI) in rats. Additionally, SFZ inhibited the activation of astrocytes and abolished the CCI-induced increase of NF-κB in the spinal cord. Hence, our results show that SFZ is a potential treatment for neuropathic pain induced by peripheral nerve injury.

Antihyperalgesic Activity of Atomoxetine on Diabetes-Induced Neuropathic Pain: Contribution of Noradrenergic and Dopaminergic Systems

Atomoxetine is a selective noradrenaline reuptake inhibitor drug. Based on the knowledge that agents increasing monoamine levels in the central nervous system have therapeutic potential for neuropathic pain, it is planned to investigate the possible efficacy of atomoxetine on diabetes-induced hyperalgesia, in this study. Randall-Selitto (mechanical noxious stimuli) and Hargreaves (thermal noxious stimuli) tests were used to evaluate nociceptive perception of rats. Obtained data indicated that streptozotocin-induced diabetes causes significant decreases in the paw withdrawal threshold and paw withdrawal latency values of the animals, respectively. However, atomoxetine administered at 3 mg/kg/day for 7 and 14 days improved these diabetes-induced hyperalgesia responses. Furthermore, antihyperalgesic activity was antagonized with α-methyl-para-tyrosine methyl ester, phentolamine, propranolol, and sulpiride pre-treatments. The same effect was not reversed, however, by SCH 23390. These findings demonstrated, for the first time, that atomoxetine possesses significant antihyperalgesic activity on diabetes-induced neuropathic pain and this effect seems to be mediated by α- and β-adrenergic and D₂/D₃ dopaminergic receptors. Results of this present study seem to offer a new indication for an old drug; atomoxetine, but these preclinical data should first be confirmed with further well-designed clinical trials.

Combining Human and Rodent Genetics to Identify New Analgesics

Most attempts at rational development of new analgesics have failed, in part because chronic pain involves multiple processes that remain poorly understood. To improve translational success, one strategy is to select novel targets for which there is proof of clinical relevance, either genetically through heritable traits, or pharmacologically. Such an approach by definition yields targets with high clinical validity. The biology of these targets can be elucidated in animal models before returning to the patients with a refined therapeutic. For optimal treatment, having biomarkers of drug action available is also a plus. Here we describe a case study in rational drug design: the use of controlled inhibition of peripheral tetrahydrobiopterin (BH4) synthesis to reduce abnormal chronic pain states without altering nociceptive-protective pain. Initially identified in a population of patients with low back pain, the association between BH4 production and chronic pain has been confirmed in more than 12 independent cohorts, through a common haplotype (present in 25% of Caucasians) of the rate-limiting enzyme for BH4 synthesis, GTP cyclohydrolase 1 (GCH1). Genetic tools in mice have demonstrated that both injured sensory neurons and activated macrophages engage increased BH4 synthesis to cause chronic pain. GCH1 is an obligate enzyme for de novo BH4 production. Therefore, inhibiting GCH1 activity eliminates all BH4 production, affecting the synthesis of multiple neurotransmitters and signaling molecules and interfering with physiological function. In contrast, targeting the last enzyme of the BH4 synthesis pathway, sepiapterin reductase (SPR), allows reduction of pathological BH4 production without completely blocking physiological BH4 synthesis. Systemic SPR inhibition in mice has not revealed any safety concerns to date, and available genetic and pharmacologic data suggest similar responses in humans. Finally, because it is present in vivo only when SPR is inhibited, sepiapterin serves as a reliable biomarker of target engagement, allowing potential quantification of drug efficacy. The emerging development of therapeutics that target BH4 synthesis to treat chronic pain illustrates the power of combining human and mouse genetics: human genetic studies for clinical selection of relevant targets, coupled with causality studies in mice, allowing the rational engineering of new analgesics.

Promoted Interaction of Nuclear Factor-κB With Demethylated Purinergic P2X3 Receptor Gene Contributes to Neuropathic Pain in Rats With Diabetes

Painful diabetic neuropathy is a common complication of diabetes produced by mechanisms that as yet are incompletely defined. The aim of this study was to investigate the roles of nuclear factor-κB (NF-κB) in the regulation of purinergic receptor P2X ligand-gated ion channel 3 (P2X3R) plasticity in dorsal root ganglion (DRG) neurons of rats with painful diabetes. Here, we showed that hindpaw pain hypersensitivity in streptozocin-induced diabetic rats was attenuated by treatment with purinergic receptor antagonist suramin or A-317491. The expression and function of P2X3Rs was markedly enhanced in hindpaw-innervated DRG neurons in diabetic rats. The CpG (cytosine guanine dinucleotide) island in the p2x3r gene promoter region was significantly demethylated, and the expression of DNA methyltransferase 3b was remarkably downregulated in DRGs in diabetic rats. The binding ability of p65 (an active form of NF-κB) with the p2x3r gene promoter region and p65 expression were enhanced significantly in diabetes. The inhibition of p65 signaling using the NF-κB inhibitor pyrrolidine dithiocarbamate or recombinant lentiviral vectors designated as lentiviral vector-p65 small interfering RNA remarkably suppressed P2X3R activities and attenuated diabetic pain hypersensitivity. Insulin treatment significantly attenuated pain hypersensitivity and suppressed the expression of p65 and P2X3Rs. Our findings suggest that the p2x3r gene promoter DNA demethylation and enhanced interaction with p65 contributes to P2X3R sensitization and diabetic pain hypersensitivity.

Effect of 1.5% Topical Diclofenac on Clinical Neuropathic Pain

Background:

Neuropathic pain is a condition resulting from injury to the peripheral and/or central nervous system. Despite extensive research over the last several decades, neuropathic pain remains difficult to manage.

Methods:

The authors conducted a randomized, placebo-controlled, double-blinded, and crossover clinical trial to examine the effect of 1.5% topical diclofenac (TD) on neuropathic pain. The authors hypothesized that 1.5% TD would reduce the visual pain score and improve both quantitative sensory testing and functional status in subjects with neuropathic pain. The authors recruited subjects with postherpetic neuralgia and complex regional pain syndrome. The primary outcome was subject’s visual pain score.

Results:

Twenty-eight subjects completed the study (12 male and 16 female) with the mean age of 48.8 yr. After 2 weeks of topical application, subjects in 1.5% TD group showed lower overall visual pain score compared with placebo group (4.9 [1.9] vs. 5.6 [2.1], difference: 0.8; 95% CI, 0.1 to 1.3; P = 0.04) as well as decreased burning pain (2.9 [2.6] vs. 4.3 [2.8], difference, 1.4; 95% CI, 0.2 to 2.6; P = 0.01). There were no statistical differences in constant pain, shooting pain, or hypersensitivity over the painful area between the groups. This self-reported improvement of pain was corroborated by the decreased pain summation detected by quantitative sensory testing. There were no statistically significant changes in functional status in these subjects. There were no complications in both groups.

Conclusion:

The findings indicate that 1.5% TD may serve as an effective treatment option for patients with neuropathic pain from postherpetic neuralgia and complex regional pain syndrome.

Dietary fish oil inhibits mechanical allodynia and thermal hyperalgesia in diabetic rats by blocking nuclear factor-κB-mediated inflammatory pathways

One of the most common complications of early-onset diabetes mellitus is peripheral diabetic neuropathy, which is manifested either by loss of nociception or by allodynia and hyperalgesia. Diabetes mellitus is a common metabolic disease in human beings with characteristic symptoms of hyperglycemia, chronic inflammation and insulin resistance. Dietary fatty acids, especially polyunsaturated fatty acids, have been shown anti-inflammatory role in various experimental conditions. The present study investigated the effects of fish oil supplementation on the inflammation in the dorsal root ganglion (DRG) of streptozotocin (STZ)-induced diabetes rats. The effects of diabetes and fish oil treatment on the allodynia and hyperalgesia were also evaluated. Dietary fish oil effectively attenuated both allodynia and hyperalgesia induce by STZ injection. Along with the behavioral findings, DRG from fish oil-treated diabetic rats displayed a decrease in inflammatory cytokines and the expression of nuclear factor-κB (NF-κB) compared with untreated diabetic rats. Fish oil supplementation also increased the phosphorylation of AKT in DRG of diabetic rats. These results suggested that dietary fish oil-inhibited allodynia and hyperalgesia in diabetic rats may stem from its anti-inflammatory potential by regulating NF-κB and AKT. Fish oil might be useful as an adjuvant therapy for the prevention and treatment of diabetic complications.

The effects of capillary dysfunction on oxygen and glucose extraction in diabetic neuropathy

Diabetic neuropathy is associated with disturbances in endoneurial metabolism and microvascular morphology, but the roles of these factors in the aetiopathogenesis of diabetic neuropathy remain unclear. Changes in endoneurial capillary morphology and vascular reactivity apparently predate the development of diabetic neuropathy in humans, and in manifest neuropathy, reductions in nerve conduction velocity correlate with the level of endoneurial hypoxia. The idea that microvascular changes cause diabetic neuropathy is contradicted, however, by reports of elevated endoneurial blood flow in early experimental diabetes, and of unaffected blood flow when early histological signs of neuropathy first develop in humans. We recently showed that disturbances in capillary flow patterns, so-called capillary dysfunction, can reduce the amount of oxygen and glucose that can be extracted by the tissue for a given blood flow. In fact, tissue blood flow must be adjusted to ensure sufficient oxygen extraction as capillary dysfunction becomes more severe, thereby changing the normal relationship between tissue oxygenation and blood flow. This review examines the evidence of capillary dysfunction in diabetic neuropathy, and whether the observed relation between endoneurial blood flow and nerve function is consistent with increasingly disturbed capillary flow patterns. The analysis suggests testable relations between capillary dysfunction, tissue hypoxia, aldose reductase activity, oxidative stress, tissue inflammation and glucose clearance from blood. We discuss the implications of these predictions in relation to the prevention and management of diabetic complications in type 1 and type 2 diabetes, and suggest ways of testing these hypotheses in experimental and clinical settings.

GCH1 variants, tetrahydrobiopterin and their effects on pain sensitivity

Background A great proportion of the variation in pain experience and chronicity is caused by heritable factors. Within the last decades several candidate genes have been discovered either increasing or decreasing pain sensitivity or the risk of chronic pain in humans. One of the most studied genes is the GCH1 gene coding for the enzyme GTP cyclohydrolase 1 (GCH1). GCH1 catalyses the initial and rate-limiting step in the biosynthesis of tetrahydrobiopterin (BH4). The main function of BH4 is regulation of monoamine and nitric oxide biosynthesis, all involved in nociceptive signalling. Methods In this topical review we focus on the implication of the GCH1 gene and BH4 in painful conditions. We discuss experimental evidence from our group in relation to relevant research publications evaluating the BH4 pathway in pain. Studies assessing the role of GCH1 and BH4 in pain consist of human and animal studies, including DOPA-responsive dystonia (DRD) patients and hph-1 mice (a genetic mouse model of DRD) having mutations in the GCH1 gene as well as preclinical studies with the GCH1 inhibitor 2,4-diamino-6-hydroxypyrimidine (DAHP). The hypothesis is that genetic and pharmacological reduction of GCH1 would result in lower pain sensitivity. Results Previous studies have demonstrated that a particular "pain protective" GCH1 haplotype, found in 15% of the general human population, is linked to decreased pain sensitivity. We further support these findings in DRD patients, showing normal thresholds to mechanical and thermal stimuli, whereas a trend towards lower pain sensitivity is seen following chemical pain sensitisation. Consistent with these observations, non-injured hph-1 mice displayed normal mechano- and thermosensation compared to wild-type mice. After peripheral inflammation with Complete Freund' Adjuvant or sensitisation with capsaicin the mutant mice exhibited lower sensitivity to mechanical and heat stimuli. Moreover, hph-1 mice showed decreased nociception in the first phase of the formalin test. Several studies report analgesic effects of GCH1 inhibition with 90-270 mg/kg DAHP in rat models of inflammatory and neuropathic pain. However, we could not completely replicate these findings in mice. Fairly higher doses of DAHP (≥270 mg/kg) were needed to reduce inflammatory pain in mice, but the window between antinociception and toxic effects was small, since 400 mg/kg DAHP affected motor performance and general appearance. Also, the analgesic effects were marginal in mice compared to that observed in rats. Conclusions Variations in the GCH1 gene in both humans and mice appear to regulate pain sensitivity and pain behaviours, particularly after pain sensitisation, whereas pain sensitivity to phasic mechanical and thermal stimuli is normal. Moreover, pharmacological inhibition of GCH1 shows antinociceptive effects in preclinical pain studies, though our studies imply that GCH1 inhibition may have a small therapeutic index. Implications The implication of the GCH1 gene in pain may increase our understanding of the risk factors of chronic pain development and improve current pain therapy by personalised medicine. In addition, inhibition of GCH1 provides a potential target for analgesic drug development, though GCH1 inhibitors should possess local or partial effects to avoid serious side-effects to the central nervous system and cardiovascular system.

Marrow-derived cells regulate the development of early diabetic retinopathy and tactile allodynia in mice

The hypothesis that marrow-derived cells, and specifically proinflammatory proteins in those cells, play a critical role in the development of diabetes-induced retinopathy and tactile allodynia was investigated. Abnormalities characteristic of the early stages of retinopathy and allodynia were measured in chimeric mice lacking inducible nitric oxide synthase (iNOS) or poly(ADP-ribosyl) polymerase (PARP1) in only their marrow-derived cells. Diabetes-induced capillary degeneration, proinflammatory changes, and superoxide production in the retina and allodynia were inhibited in diabetic animals in which iNOS or PARP1 was deleted from bone marrow cells only. Of the various marrow cells, neutrophils (and monocytes) play a major role in retinopathy development, because retinal capillary degeneration likewise was significantly inhibited in diabetic mice lacking the receptor for granulocyte colony-stimulating factor in their marrow-derived cells. Immunodepletion of neutrophils or monocytes inhibited the endothelial death otherwise observed when coculturing leukocytes from wild-type diabetic animals with retinal endothelium. iNOS and PARP1 are known to play a role in inflammatory processes, and we conclude that proinflammatory processes within marrow-derived cells play a central role in the development of diabetes complications in the retina and nerve.

Normalization of NF-κB activity in dorsal root ganglia neurons cultured from diabetic rats reverses neuropathy-linked markers of cellular pathology

AIMS/HYPOTHESIS

Dorsal root ganglia (DRG) sensory neurons cultured from 3 to 5 month streptozotocin (STZ)-induced diabetic rats exhibit structural and biochemical changes seen in peripheral nerve fibers in vivo, including axonal swellings, oxidative damage, reduced axonal sprouting, and decreased NF-κB activity. NF-κB is a transcription factor required by DRG neurons for survival and plasticity, and regulates transcription of antioxidant proteins (e.g. MnSOD). We hypothesized that the diabetes-induced decrease in NF-κB activity in DRG contributes to pathological phenomena observed in cultured DRG neurons from diabetic rats.

METHODS

NF-κB localization was assessed in intact DRG and neuron cultures using immunostaining. NF-κB activity was manipulated in sensory neuron cultures derived from age-matched normal or 3-5 month STZ-diabetic rats using pharmacological means and lentiviral expression of shRNA. The impact of diabetes and altered NF-κB activity on neuronal phenotype involved analysis of neurite outgrowth, neurite morphology, oxidative stress (lipid peroxidation) and expression of MnSOD.

RESULTS

STZ-induced diabetes caused a significant decrease in nuclear localization of NF-κB subunits p50 and c-rel, but no change in p65 in intact DRG. Inhibition of NF-κB in normal neuron cultures significantly increased axonal swellings and oxidative stress, and reduced both neurite outgrowth and expression of MnSOD. These phenomena mimicked markers of pathology in cultured DRG neurons from diabetic rats. Enhancement of NF-κB activity in cultured diabetic DRG neurons ameliorated the sub-optimal neurite outgrowth and MnSOD levels triggered by diabetes. Exogenous insulin enhanced nuclear localization of p50 and c-rel but not p65 in diabetic neuronal cultures.

CONCLUSION/INTERPRETATION

The diabetes-induced decrease of nuclear localization of NF-κB subunits p50 and c-rel in DRG contributes to development of in vitro markers of peripheral neuropathy, possibly through impaired mitochondrial ROS scavenging by deficient MnSOD.

Ciliary neurotrophic factor activates NF-κB to enhance mitochondrial bioenergetics and prevent neuropathy in sensory neurons of streptozotocin-induced diabetic rodents

Diabetes causes mitochondrial dysfunction in sensory neurons that may contribute to peripheral neuropathy. Ciliary neurotrophic factor (CNTF) promotes sensory neuron survival and axon regeneration and prevents axonal dwindling, nerve conduction deficits and thermal hypoalgesia in diabetic rats. In this study, we tested the hypothesis that CNTF protects sensory neuron function during diabetes through normalization of impaired mitochondrial bioenergetics. In addition, we investigated whether the NF-κB signal transduction pathway was mobilized by CNTF. Neurite outgrowth of sensory neurons derived from streptozotocin (STZ)-induced diabetic rats was reduced compared to neurons from control rats and exposure to CNTF for 24 h enhanced neurite outgrowth. CNTF also activated NF-κB, as assessed by Western blotting for the NF-κB p50 subunit and reporter assays for NF-κB promoter activity. Conversely, blockade of NF-κB signaling using SN50 peptide inhibited CNTF-mediated neurite outgrowth. Studies in mice with STZ-induced diabetes demonstrated that systemic therapy with CNTF prevented functional indices of peripheral neuropathy along with deficiencies in dorsal root ganglion (DRG) NF-κB p50 expression and DNA binding activity. DRG neurons derived from STZ-diabetic mice also exhibited deficiencies in maximal oxygen consumption rate and associated spare respiratory capacity that were corrected by exposure to CNTF for 24 h in an NF-κB-dependent manner. We propose that the ability of CNTF to enhance axon regeneration and protect peripheral nerve from structural and functional indices of diabetic peripheral neuropathy is associated with targeting of mitochondrial function, in part via NF-κB activation, and improvement of cellular bioenergetics.

Analgesia by inhibiting tetrahydrobiopterin synthesis

Physiological control of the co-factor tetrahydrobiopterin (BH4) is tight in normal circumstances but levels increase pathologically in the injured somatosensory system. BH4 is an essential co-factor in the production of serotonin, dopamine, epinephrine, norepinephrine and nitric oxide. Excess BH4 levels cause pain, likely through excess production of one or more of these neurotransmitters or signaling molecules. The rate limiting step for BH4 production is GTP Cyclohydrolase 1 (GCH1). A human GCH1 gene haplotype exists that leads to less GCH1 transcription, translation, and therefore enzyme activity, following cellular stress. Carriers of this haplotype produce less BH4 and therefore feel less pain, especially following nerve injury where BH4 production is pathologically augmented. Sulfasalazine (SSZ) an FDA approved anti-inflammatory agent of unknown mechanism of action, has recently been shown to be a sepiapterin reductase (SPR) inhibitor. SPR is part of the BH4 synthesis cascade and is also upregulated by nerve injury. Inhibiting SPR will reduce BH4 levels and therefore should act as an analgesic. We propose SSZ as a novel anti-neuropathic pain medicine.

Beneficial effects of a novel RAGE inhibitor on early diabetic retinopathy and tactile allodynia

PURPOSE

The receptor for advanced glycation end products (RAGE) has been implicated in the pathogenesis of numerous complications of diabetes. We assessed the effect of a novel RAGE fusion protein inhibitor on retinal histopathology and nerve function, and on retinal inflammation and oxidative stress.

METHODS

C57BL/6J mice were made diabetic with streptozotocin, and some were given a RAGE fusion protein (10, 100, or 300 µg per mouse 3 times per week). Mice were sacrificed at 2 months and 10 months into the study to assess retinal vascular histopathology, accumulation of albumin in the neural retina, cell loss in the ganglion cell layer, and biochemical and physiologic abnormalities in the retina. Tactile allodynia (light touch) was measured on a paw of each animal at 2 months.

RESULTS

Leukostasis, expression of the intercellular adhesion molecule-1 (ICAM-1), accumulation of albumin in the neural retina, and nitration of retinal proteins were significantly increased in the retinas of mice diabetic for 2 months. The number of degenerate retinal capillaries was significantly increased in mice diabetic for 10 months, compared to the nondiabetic controls. Diabetes also enhanced sensitivity of peripheral nerves to tactile allodynia. All three doses of the RAGE fusion protein inhibited capillary degeneration, accumulation of albumin in the neural retina, nitration of retinal proteins, and tactile allodynia, demonstrating that biologically meaningful levels of the drug reached the retina. RAGE inhibition did tend to inhibit diabetes-induced retinal leukostasis and ICAM-1 expression (previously postulated to be important in the pathogenesis of retinopathy), but these effects were not statistically significant for the use of the lower doses of the drug that normalized the vascular histopathology.

CONCLUSIONS

Inhibition of RAGE blocked the development of important lesions of diabetic retinopathy, but these beneficial effects seemed not to be mediated via leukostasis. RAGE inhibition also blocked the development of sensory allodynia in diabetes. RAGE is an important therapeutic target to inhibit the development of vascular and neural complications of diabetes.

Beneficial effects of PKF275-055, a novel, selective, orally bioavailable, long-acting dipeptidyl peptidase IV inhibitor in streptozotocin-induced diabetic peripheral neuropathy

1-[(2-adamantyl)amino]acetyl-2-cyano-(S)-pyrrolidine, monohydrochloride (PKF275-055), a vildagliptin analog, is a novel, selective, potent, orally bioavailable, and long-acting dipeptidyl peptidase IV inhibitor. We studied the effect of PKF275-055 administration on the prevention, protection, and treatment of diabetic neuropathy in the streptozotocin-induced diabetic rat. PKF275-055 improved body and muscle weight. Oral glucose tolerance tests showed a marked improvement in glucose metabolism under all treatment schedules. When tested in prevention and protection experiments, PKF275-055 completely averted the decrease of Na⁺/K⁺-ATPase activity and partially counteracted the nerve conduction velocity (NCV) deficit observed in untreated diabetic rats but had no effects on abnormal mechanical and thermal sensitivity. When used in a therapeutic setting, PKF275-055 induced a significant correction in the alteration in Na⁺,K⁺-ATPase activity and NCV present in untreated diabetics. Diabetic rats developed mechanical hyperalgesia within 2 weeks after streptozotocin injection and exhibited significantly longer thermal response latencies. It is noteworthy that PKF275-055 treatment restored mechanical sensitivity thresholds by approximately 50% (p < 0.01) and progressively improved the alteration in thermal responsiveness. In conclusion, PKF275-055 showed an anabolic effect, improved oral glucose tolerance, and counteracted the alterations in Na⁺,K⁺-ATPase activity, NCV, and nociceptive thresholds in diabetic rats. The present data support a potential therapeutic effect of PKF275-055 in the treatment of rodent diabetic neuropathy.

Tumor necrosis factor-α elevates neurite outgrowth through an NF-κB-dependent pathway in cultured adult sensory neurons: Diminished expression in diabetes may contribute to sensory neuropathy

The presence of a proinflammatory environment in the sensory neuron axis in diabetes was tested by measuring levels of proinflammatory cytokines in lumbar dorsal root ganglia (DRG) and peripheral nerve from age matched control and streptozotocin (STZ)-induced diabetic rats. The levels of tumor necrosis factor-α (TNFα) and other cytokines were diminished in lumbar DRG from diabetic animals. Consequently, we tested the hypothesis that TNFα modulated axonal plasticity in adult sensory neurons and posited that impairments in this signal transduction pathway may underlie degeneration in diabetic sensory neuropathy. Cultured adult rat sensory neurons were grown under defined conditions and TNFα caused a dose-dependent 2-fold (P<0.05) elevation in neurite outgrowth. Neurons derived from 3 to 5month STZ-induced diabetic rats exhibited significantly reduced levels of neurite outgrowth in response to TNFα. TNFα enhanced NF-κB activity as assessed using Western blotting and plasmid reporter technology. Blockade of TNFα-induction of NF-κB activation caused inhibition of neurite outgrowth in cultured neurons. Immunofluorescent staining for NF-κB subunit p50 within neuronal nuclei revealed that medium to large diameter neurons were most susceptible to NF-κB inhibition and was associated with decreased neurite outgrowth. The results demonstrating reduced cytokine expression in DRG confirm that diabetic sensory neuropathy does not involve a neuroinflammatory component at this stage of the disease in experimental animal models. In addition, it is hypothesized that reduced TNFα expression in the DRG and possibly associated deficits in anterograde transport may contribute to impaired collatoral sprouting and regeneration in target tissue in type 1 diabetes.

JSH-23 targets nuclear factor-kappa B and reverses various deficits in experimental diabetic neuropathy: effect on neuroinflammation and antioxidant defence

AIM

Nuclear factor-kappa B (NF-κB) being reported to play an important role in the pathogenesis of diabetic neuropathy is believed to be a central mechanism involved in the genesis and promulgation of inflammatory insult. Here we have targeted the nuclear translocation of NF-κB using JSH-23 to elucidate its role in diabetic neuropathy.

METHODS

JSH-23 (1 and 3 mg/kg) was administered for 2 weeks in diabetic rats, after 6 weeks of diabetes induction using streptozotocin (55 mg/kg) as diabetogenic agent. Functional (motor nerve conduction velocity and blood flow), behavioural (mechanical hyperalgesia), biochemical [malondialdehyde, glutathione, tumour necrosis factor-α (TNF-α) and interleukin-6 (IL-6) levels] and NF-κB translocation studies (western blot technique) were then undertaken.

RESULTS

JSH-23 treatment significantly reversed the nerve conduction and nerve blood flow deficits seen in diabetic animals. Reduction in mechanical pain threshold was also partially corrected by the treatment. Protein expression studies showed that nuclear translocation of p65/p50 subunit was inhibited by JSH-23 treatment in the sciatic nerve. The treatment also lowered the elevated IL-6, TNF-α, cyclo-oxygenase (COX-2) and inducible nitric oxide synthase (iNOS) levels/expression, indicating reduction in the inflammatory damage of the sciatic nerve. Apart from these effects, JSH-23 also increased Nrf2 and hemeoxygenase-1 (HO-1) levels which could imply its potential in increasing the strength of antioxidant defence.

CONCLUSION

We observed that NF-κB inhibition partially reversed functional, behavioural and biochemical deficits with JSH-23 treatment. This study substantiates the role of NF-κB activation in the aetiology of diabetic neuropathy and protection afforded by inhibition of NF-κB by JSH-23, which can be attributed to its effect on neuroinflammation and oxidative stress.

Pharmacological treatment of diabetic neuropathic pain

Neuropathic pain continues to be a difficult and challenging clinical issue to deal with effectively. Painful diabetic polyneuropathy is a complex pain condition that occurs with reasonable frequency in the population and it may be extremely difficult for clinicians to provide patients with effective analgesia. Chronic neuropathic pain may occur in approximately one of every four diabetic patients. The pain may be described as burning or a deep-seated ache with sporadic paroxysms of lancinating painful exacerbations. The pain is often constant, moderate to severe in intensity, usually primarily involves the feet and generally tends to worsen at night. Treatment may be multimodal but largely involves pharmacological approaches. Pharmacological therapeutic options include antidepressants (tricyclic antidepressants, serotonin-norepinephrine reuptake inhibitors), α2δ ligands and topical (5%) lidocaine patch. Other agents may be different antiepileptic drugs (carbamazepine, lamotrigine, topiramate), topical capsaicin, tramadol and other opioids. Progress continues with respect to understanding various mechanisms that may contribute to painful diabetic neuropathy. Agents that may hold some promise include neurotrophic factors, growth factors, immunomodulators, gene therapy and poly (adenosine diphosphate-ribose) polymerase inhibitors. It is hoped that in the future clinicians will be able to assess patient pathophysiology, which may help them to match optimal therapeutic agents to target individual patient aberrant mechanisms.

A yeast-based screen reveals that sulfasalazine inhibits tetrahydrobiopterin biosynthesis

We introduce an approach for detection of drug-protein interactions that combines a new yeast three-hybrid screening for identification of interactions with affinity chromatography for their unambiguous validation. We applied the methodology to the profiling of clinically approved drugs, resulting in the identification of previously known and unknown drug-protein interactions. In particular, we were able to identify off-targets for erlotinib and atorvastatin, as well as an enzyme target for the anti-inflammatory drug sulfasalazine. We demonstrate that sulfasalazine and its metabolites, sulfapyridine and mesalamine, are inhibitors of the enzyme catalyzing the final step in the biosynthesis of the cofactor tetrahydrobiopterin. The interference with tetrahydrobiopterin metabolism provides an explanation for some of the beneficial and deleterious properties of sulfasalazine and furthermore suggests new and improved therapies for the drug. This work thus establishes a powerful approach for drug profiling and provides new insights in the mechanism of action of clinically approved drugs.

A yeast-based screen reveals that sulfasalazine inhibits tetrahydrobiopterin biosynthesis

We introduce an approach for detection of drug-protein interactions that combines a new yeast three-hybrid screening for identification of interactions with affinity chromatography for their unambiguous validation. We applied the methodology to the profiling of clinically approved drugs, resulting in the identification of previously known and unknown drug-protein interactions. In particular, we were able to identify off-targets for erlotinib and atorvastatin, as well as an enzyme target for the anti-inflammatory drug sulfasalazine. We demonstrate that sulfasalazine and its metabolites, sulfapyridine and mesalamine, are inhibitors of the enzyme catalyzing the final step in the biosynthesis of the cofactor tetrahydrobiopterin. The interference with tetrahydrobiopterin metabolism provides an explanation for some of the beneficial and deleterious properties of sulfasalazine and furthermore suggests new and improved therapies for the drug. This work thus establishes a powerful approach for drug profiling and provides new insights in the mechanism of action of clinically approved drugs.

Transcriptional repression of the M channel subunit Kv7.2 in chronic nerve injury

Neuropathic pain is a severe health problem for which there is a lack of effective therapy. A frequent underlying condition of neuropathic pain is a sustained overexcitability of pain-sensing (nociceptive) sensory fibres. Therefore, the identification of mechanisms for such abnormal neuronal excitability is of utmost importance for understanding neuropathic pain. Despite much effort, an inclusive model explaining peripheral overexcitability is missing. We investigated transcriptional regulation of the Kcnq2 gene, which encodes the Kv7.2 subunit of membrane potential-stabilizing M channel, in peripheral sensory neurons in a model of neuropathic pain—partial sciatic nerve ligation (PSNL). We show that Kcnq2 is the major Kcnq gene transcript in dorsal root ganglion (DRG); immunostaining and patch-clamp recordings from acute ganglionic slices verified functional expression of Kv7.2 in small-diameter nociceptive DRG neurons. Neuropathic injury induced substantial downregulation of Kv7.2 expression. Levels of repressor element 1–silencing transcription factor (REST), which is known to suppress Kcnq2 expression, were upregulated in response to neuropathic injury identifying the likely mechanism of Kcnq2 regulation. Behavioural experiments demonstrated that neuropathic hyperalgesia following PSNL developed faster than the downregulation of Kcnq2 expression could be detected, suggesting that this transcriptional mechanism may contribute to the maintenance rather than the initiation of neuropathic pain. Importantly, the decrease in the peripheral M channel abundance could be functionally compensated by peripherally applied M channel opener flupirtine, which alleviated neuropathic hyperalgesia. Our work suggests a novel mechanism for neuropathic overexcitability and brings focus on M channels and REST as peripheral targets for the treatment of neuropathic pain.

Neuropathic injury induces transcriptional downregulation of the Kcnq2 potassium channel gene by the transcriptional suppressor repressor element 1–silencing transcription factor; this mechanism contributes to peripheral sensitization of the afferent fibres.

NF-κB subunits are differentially distributed in cells of lumbar dorsal root ganglia in naïve and diabetic rats

Previous studies demonstrated that nuclear factor κB (NF-κB) activation is decreased in dorsal root ganglia (DRG) of rats having streptozotocin (STZ)-induced diabetes. DRG contain cell bodies of neurons that convey sensory signals from the periphery. To determine the relationship between diabetes-induced neuropathy and NF-κB expression in DRG, behavioral, immunohistochemical, and biochemical studies were performed on naïve and 3-month diabetic rats. Behavioral studies confirmed that many diabetic rats develop tactile allodynia, or increased sensitivity to light touch, in the hind paws. Immunohistochemical studies on lumbar DRG that receive input from the affected regions revealed that p50 and p65, frequent NF-κB subunit partners, are differentially localized. Intense p65 immunostaining was detected in the cytoplasm of small- and medium-sized neurons as well as in satellite cells. In contrast, p50 was localized in the cytoplasm of virtually all neurons. In many cases, prominent staining was also present in nuclei, a location consistent with transcription factor activation. Immunohistochemical and biochemical studies found that the nuclear to cytoplasmic ratio of p50 expression was significantly reduced in diabetic rats compared to that in naïve animals. Our findings raise the possibility that changes in NF-κB activation in a subset of DRG neurons participates in mediating diabetes-induced sensory neuropathy.

Comparison of three strains of diabetic rats with respect to the rate at which retinopathy and tactile allodynia develop

PURPOSE

We compared three rat strains to determine if different strains develop early-stage diabetic retinopathy or sensory neuropathy at different rates.

METHODS

Sprague Dawley, Lewis, and Wistar rats were made diabetic with streptozotocin. Diabetic and nondiabetic animals had retinal vascular pathology measured at eight months of diabetes. The number of cells in the retinal ganglion cell layer (GCL), retinal function (using electroretinography [ERG]), and retinal levels of inducible nitric oxide synthase (iNOS), cyclooxygenase2 (COX2), and vascular endothelial growth factor (VEGF) were measured at four months of diabetes. Tactile allodynia was assessed in hind paws at two months of diabetes.

RESULTS

Diabetes of eight months' duration resulted in a significant increase in retinal degenerate capillaries and pericyte ghosts in Lewis and Wistar rats, but not in Sprague Dawley rats. A significant loss of cells in the GCL occurred only in diabetic Lewis rats, whereas Wistar and Sprague Dawley rats showed little change. Diabetes-induced iNOS and VEGF were statistically significant in all strains. Cyclooxygenase 2 (COX2) was significantly elevated in the Sprague Dawley and Wistar strains. Lewis rats showed a similar trend, however, the results were not statistically significant. All strains tended to show diabetes-induced impairment of dark-adapted b-wave amplitude, but only Sprague Dawley and Lewis strains had a significant reduction in latency. All strains showed significant tactile allodynia in peripheral nerves.

CONCLUSIONS

At the durations studied, Lewis rats showed accelerated loss of both retinal capillaries and ganglion cells in diabetes, whereas diabetic Wistar rats showed degeneration of the capillaries without significant neurodegeneration, and Sprague Dawley rats showed neither lesion. Identification of strains that develop retinal lesions at different rates should be of value in investigating the pathogenesis of retinopathy.

Effects of p38 MAPK inhibition on early stages of diabetic retinopathy and sensory nerve function

Purpose. p38 mitogen-activated protein kinase (MAPK) is known to play a regulatory role in inflammatory processes in disease. Inflammation has been linked also to the development of diabetic retinopathy in rodents. This study was conducted to evaluate the effect of a p38 MAPK inhibitor on the development of early stages of diabetic retinopathy in rats. Methods. Streptozotocin-diabetic rats were assigned to two groups-treated with the p38 MAPK inhibitor PHA666859 (Pfizer, New York, NY) and untreated-and compared with age-matched nondiabetic control animals. Results. At 2 months of diabetes, insulin-deficient diabetic control rats exhibited significant increases in retinal superoxide, nitric oxide (NO), cyclooxygenase (COX)-2, and leukostasis within retinal microvessels. All these abnormalities were significantly inhibited by the p38 MAPK inhibitor (25 mg/kgBW/d). At 10 months of diabetes, significant increases in the number of degenerate (acellular) capillaries and pericyte ghosts were measured in control diabetic rats versus those in nondiabetic control animals, and pharmacologic inhibition of p38 MAPK significantly inhibited all these abnormalities (all P < 0.05). This therapy also had beneficial effects outside the eye in diabetes, as evidenced by the inhibition of a diabetes-induced hypersensitivity of peripheral nerves to light touch (tactile allodynia). Conclusions. p38 MAPK plays an important role in diabetes-induced inflammation in the retina, and inhibition of p38 MAPK offers a novel therapeutic approach to inhibiting the development of early stages of diabetic retinopathy and other complications of diabetes.

The anti-inflammatory and antinociceptive effects of NF-kappaB inhibitory guanidine derivative ME10092

The guanidine compound ME10092 (1-(3,4-dimethoxy-2-chlorobenzylideneamino)-guanidine) is known to possess anti-radical and anti-ischemic activity but its molecular targets have not been identified. This study investigated whether ME10092 regulates the nuclear factor kappa B (NF-kappaB)-mediated signal transduction in vivo. The effect of ME10092 treatment (1-100 pmol/mouse) on nuclear translocation of NF-kappaB, activation of expression of inflammatory mediators and production of nitric oxide were measured in the lipopolysaccharide (LPS)-induced brain inflammation model in mice in vivo. The antinociceptive activity of ME10092 was tested in the formalin-induced paw licking test. ME10092 dose-dependently inhibited LPS-induced nuclear translocation of NF-kappaB, transcription of tumour necrosis factor-alpha (TNF-alpha), interleukin-1beta (IL-1beta), inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2). Electron paramagnetic resonance measurements showed that ME10092 inhibited the LPS-induced increase in nitric oxide content in mouse brain tissue in a dose-dependent manner. In the formalin-induced paw licking test, ME10092 (at the dose of 3mg/kg, p.o. twice daily for eight days) significantly reduced nociceptive response. In conclusion, above results indicate that ME10092 inhibits NF-kappaB activation and suppresses the up-regulation of inflammatory mediators in experimental models in vivo.

Diabetic painful and insensate neuropathy: pathogenesis and potential treatments

Advanced peripheral diabetic neuropathy (PDN) is associated with elevated vibration and thermal perception thresholds that progress to sensory loss and degeneration of all fiber types in peripheral nerve. A considerable proportion of diabetic patients also describe abnormal sensations such as paresthesias, allodynia, hyperalgesia, and spontaneous pain. One or several manifestations of abnormal sensation and pain are described in all the diabetic rat and mouse models studied so far (i.e., streptozotocin-diabetic rats and mice, type 1 insulinopenic BB/Wor and type 2 hyperinsulinemic diabetic BBZDR/Wor rats, Zucker diabetic fatty rats, and nonobese diabetic, Akita, leptin- and leptin-receptor-deficient, and high-fat diet-fed mice). Such manifestations are 1) thermal hyperalgesia, an equivalent of a clinical phenomenon described in early PDN; 2) thermal hypoalgesia, typically present in advanced PDN; 3) mechanical hyperalgesia, an equivalent of pain on pressure in early PDN; 4) mechanical hypoalgesia, an equivalent to the loss of sensitivity to mechanical noxious stimuli in advanced PDN; 5) tactile allodynia, a painful perception of a light touch; and 5) formalin-induced hyperalgesia. Rats with short-term diabetes develop painful neuropathy, whereas those with longer-term diabetes and diabetic mice typically display manifestations of both painful and insensate neuropathy, or insensate neuropathy only. Animal studies using pharmacological and genetic approaches revealed important roles of increased aldose reductase, protein kinase C, and poly(ADP-ribose) polymerase activities, advanced glycation end-products and their receptors, oxidative-nitrosative stress, growth factor imbalances, and C-peptide deficiency in both painful and insensate neuropathy. This review describes recent achievements in studying the pathogenesis of diabetic neuropathic pain and sensory disorders in diabetic animal models and developing potential pathogenetic treatments.