GTP cyclohydrolase and tetrahydrobiopterin regulate pain sensitivity and persistence

Central sensitization associated with low fetal hemoglobin levels in adults with sickle cell anemia

BACKGROUND AND AIMS

Pain is the hallmark of sickle cell anemia (SCA), presenting as recurrent acute events or chronic pain. Central sensitization, or enhanced excitability of the central nervous system, alters pain processing and contributes to the maintenance of chronic pain. Individuals with SCA demonstrate enhanced sensitivity to painful stimuli however central mechanisms of pain have not been fully explored. We hypothesized that adults with SCA would show evidence of central sensitization as observed in other diseases of chronic pain.

METHODS

We conducted a prospective study of static and dynamic quantitative sensory tests in 30 adults with SCA and 30 matched controls.

RESULTS

Static thermal testing using cold stimuli showed lower pain thresholds (p=0.04) and tolerance (p=0.04) in sickle cell subjects, but not for heat. However, SCA subjects reported higher pain ratings with random heat pulses (p<0.0001) and change in scores with temporal summation at the heat pain threshold (p=0.002). Similarly, with the use of pressure pain stimuli, sickle cell subjects reported higher pain ratings (p=0.04), but not higher pressure pain tolerance/thresholds or allodynia to light tactile stimuli. Temporal summation pain score changes using 2 pinprick probes (256 and 512mN) were significantly greater (p=0.004 and p=0.008) with sickle cell, and delayed recovery was associated with lower fetal hemoglobin (p=0.002 and 0.003).

CONCLUSIONS

Exaggerated temporal summation responses provide evidence of central sensitization in SCA.

IMPLICATIONS

The association with fetal hemoglobin suggests this known SCA modifier may have a therapeutic role in modulating central sensitization.

Overlapping Chronic Pain Conditions: Implications for Diagnosis and Classification

There is increasing recognition that many if not most common chronic pain conditions are heterogeneous with a high degree of overlap or coprevalence of other common pain conditions along with influences from biopsychosocial factors. At present, very little attention is given to the high degree of overlap of many common pain conditions when recruiting for clinical trials. As such, many if not most patients enrolled into clinical studies are not representative of most chronic pain patients. The failure to account for the heterogeneous and overlapping nature of most common pain conditions may result in treatment responses of small effect size when these treatments are administered to patients with chronic overlapping pain conditions (COPCs) represented in the general population. In this brief review we describe the concept of COPCs and the putative mechanisms underlying COPCs. Finally, we present a series of recommendations that will advance our understanding of COPCs.

PERSPECTIVE

This brief review describes the concept of COPCs. A mechanism-based heuristic model is presented and current knowledge and evidence for COPCs are presented. Finally, a set of recommendations is provided to advance our understanding of COPCs.

Breaking barriers to novel analgesic drug development

Acute and chronic pain complaints, although common, are generally poorly served by existing therapies. This unmet clinical need reflects a failure to develop novel classes of analgesics with superior efficacy, diminished adverse effects and a lower abuse liability than those currently available. Reasons for this include the heterogeneity of clinical pain conditions, the complexity and diversity of underlying pathophysiological mechanisms, and the unreliability of some preclinical pain models. However, recent advances in our understanding of the neurobiology of pain are beginning to offer opportunities for developing novel therapeutic strategies and revisiting existing targets, including modulating ion channels, enzymes and G-protein-coupled receptors.

Genetics of HIV-associated sensory neuropathy and related pain in Africans

Despite the use of safer antiretroviral medications, the rate of HIV-associated sensory neuropathy (HIV-SN), the most common neurological complication of HIV, remains high. This condition is often painful and has a negative effect on quality of life. Up to 90% of those with HIV-SN experience pain for which there is no effective analgesic treatment. Genetic factors are implicated, but there is a lack of a comprehensive body of research for African populations. This knowledge gap is even more pertinent as Africans are most affected by HIV. However, recent studies performed in Southern African populations have identified genes displaying potential as genetic markers for HIV-SN and HIV-SN-associated pain in Africans. Here, we review the published studies to describe current knowledge of genetic risk factors for this disease in Africa.

Low back pain in older adults: risk factors, management options and future directions

Low back pain (LBP) is one of the major disabling health conditions among older adults aged 60 years or older. While most causes of LBP among older adults are non-specific and self-limiting, seniors are prone to develop certain LBP pathologies and/or chronic LBP given their age-related physical and psychosocial changes. Unfortunately, no review has previously summarized/discussed various factors that may affect the effective LBP management among older adults. Accordingly, the objectives of the current narrative review were to comprehensively summarize common causes and risk factors (modifiable and non-modifiable) of developing severe/chronic LBP in older adults, to highlight specific issues in assessing and treating seniors with LBP, and to discuss future research directions. Existing evidence suggests that prevalence rates of severe and chronic LBP increase with older age. As compared to working-age adults, older adults are more likely to develop certain LBP pathologies (e.g., osteoporotic vertebral fractures, tumors, spinal infection, and lumbar spinal stenosis). Importantly, various age-related physical, psychological, and mental changes (e.g., spinal degeneration, comorbidities, physical inactivity, age-related changes in central pain processing, and dementia), as well as multiple risk factors (e.g., genetic, gender, and ethnicity), may affect the prognosis and management of LBP in older adults. Collectively, by understanding the impacts of various factors on the assessment and treatment of older adults with LBP, both clinicians and researchers can work toward the direction of more cost-effective and personalized LBP management for older people.

Genes associated with persistent lumbar radicular pain; a systematic review

Background

The aim of the present study was to provide an overview of the literature addressing the role of genetic factors and biomarkers predicting pain recovery in newly diagnosed lumbar radicular pain (LRP) patients.

Methods

The search was performed in Medline OVID, Embase, PsycInfo and Web of Science (2004 to 2015). Only prospective studies of patients with LRP addressing the role of genetic factors (genetic susceptibility) and pain biomarkers (proteins in serum) were included. Two independent reviewers extracted the data and assessed methodological quality.

Results

The search identified 880 citations of which 15 fulfilled the inclusion criteria. Five genetic variants; i.e., OPRM1 rs1799971 G allele, COMT rs4680 G allele, MMP1 rs1799750 2G allele, IL1α rs1800587 T allele, IL1RN rs2234677 A allele, were associated with reduced recovery of LRP. Three biomarkers; i.e., TNFα, IL6 and IFNα, were associated with persistent LRP.

Conclusion

The present results indicate that several genetic factors and biomarkers may predict slow recovery in LRP. Still, there is a need for replication of the findings. A stricter use of nomenclature is also highly necessary.

Trial registration

The review is registered PROSPERO 20^th of November 2015. Registration number is CRD42015029125.

Redox-guided axonal regrowth requires cyclic GMP dependent protein kinase 1: Implication for neuropathic pain

Cyclic GMP-dependent protein kinase 1 (PKG1) mediates presynaptic nociceptive long-term potentiation (LTP) in the spinal cord and contributes to inflammatory pain in rodents but the present study revealed opposite effects in the context of neuropathic pain. We used a set of loss-of-function models for in vivo and in vitro studies to address this controversy: peripheral neuron specific deletion (SNS-PKG1^-/-), inducible deletion in subsets of neurons (SLICK-PKG1^-/-) and redox-dead PKG1 mutants. In contrast to inflammatory pain, SNS-PKG1^-/- mice developed stronger neuropathic hyperalgesia associated with an impairment of nerve regeneration, suggesting specific repair functions of PKG1. Although PKG1 accumulated at the site of injury, its activity was lost in the proximal nerve due to a reduction of oxidation-dependent dimerization, which was a consequence of mitochondrial damage in injured axons. In vitro, PKG1 deficiency or its redox-insensitivity resulted in enhanced outgrowth and reduction of growth cone collapse in response to redox signals, which presented as oxidative hotspots in growing cones. At the molecular level, PKG1 deficiency caused a depletion of phosphorylated cofilin, which is essential for growth cone collapse and guidance. Hence, redox-mediated guidance required PKG1 and consequently, its deficiency in vivo resulted in defective repair and enhanced neuropathic pain after nerve injury. PKG1-dependent repair functions will outweigh its signaling functions in spinal nociceptive LTP, so that inhibition of PKG1 is no option for neuropathic pain.

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Axonal injury leads mitochondrial damage.

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The loss of signaling ROS is associated with a reduction of redox-dependent autoactivation of PKG1.

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Loss of PKG1 impairs peripheral nerve regeneration and aggravates neuropathic pain in mice.

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Oxidative hot spots are generated in spiky growth cones and trigger growth cone collapse via PKG1.

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Malfunctioning of this redox-PKG1 guided growth cone collapse leads to aberrant outgrowth.

Progranulin promotes peripheral nerve regeneration and reinnervation: role of notch signaling

Background

Peripheral nerve injury is a frequent cause of lasting motor deficits and chronic pain. Although peripheral nerves are capable of regrowth they often fail to re-innervate target tissues.

Results

Using newly generated transgenic mice with inducible neuronal progranulin overexpression we show that progranulin accelerates axonal regrowth, restoration of neuromuscular synapses and recovery of sensory and motor functions after injury of the sciatic nerve. Oppositely, progranulin deficient mice have long-lasting deficits in motor function tests after nerve injury due to enhanced losses of motor neurons and stronger microglia activation in the ventral horn of the spinal cord. Deep proteome and gene ontology (GO) enrichment analysis revealed that the proteins upregulated in progranulin overexpressing mice were involved in ‘regulation of transcription’ and ‘response to insulin’ (GO terms). Transcription factor prediction pointed to activation of Notch signaling and indeed, co-immunoprecipitation studies revealed that progranulin bound to the extracellular domain of Notch receptors, and this was functionally associated with higher expression of Notch target genes in the dorsal root ganglia of transgenic mice with neuronal progranulin overexpression. Functionally, these transgenic mice recovered normal gait and running, which was not achieved by controls and was stronger impaired in progranulin deficient mice.

Conclusion

We infer that progranulin activates Notch signaling pathways, enhancing thereby the regenerative capacity of partially injured neurons, which leads to improved motor function recovery.

Graphical abstract

Electronic supplementary material

The online version of this article (doi:10.1186/s13024-016-0132-1) contains supplementary material, which is available to authorized users.

Effect of endocannabinoid degradation on pain: role of FAAH polymorphisms in experimental and postoperative pain in women treated for breast cancer

Fatty acid amide hydrolase (FAAH) metabolizes the endocannabinoid anandamide, which has an important role in nociception. We investigated the role of common FAAH single-nucleotide polymorphisms (SNPs) in experimentally induced and postoperative pain. One thousand women undergoing surgery for breast cancer participated in the study. They were tested for cold (n = 900) and heat pain (n = 1000) sensitivity. After surgery, their pain intensities and analgesic consumption were carefully registered. FAAH genotyping was performed using MassARRAY platform and genome-wide chip (n = 926). Association between 8 FAAH SNPs and 9 pain phenotypes was analyzed using linear regression models. The results showed that carrying 2 copies of a missense variant converting proline at position 129 to threonine (rs324420) resulted in significantly lower cold pain sensitivity and less need for postoperative analgesia. More specifically, rs324420 and another highly correlated SNP, rs1571138, associated significantly with cold pain intensity (corrected P value, 0.0014; recessive model). Patients homozygous for the minor allele (AA genotype) were less sensitive to cold pain (β = -1.48; 95% CI, -2.14 to -0.8). Two other SNPs (rs3766246 and rs4660928) showed nominal association with cold pain, and SNPs rs4141964, rs3766246, rs324420, and rs1571138 nominal association with oxycodone consumption. In conclusion, FAAH gene variation was shown to associate with cold pain sensitivity with P129T/rs324420 being the most likely causal variant as it is known to reduce the FAAH enzyme activity. The same variant showed nominal association with postoperative oxycodone consumption. Our conclusions are, however, limited by the lack of replication and the results should be replicated in an independent cohort.

Biopsychosocial Influence on Shoulder Pain: Influence of Genetic and Psychological Combinations on Twelve-Month Postoperative Pain and Disability Outcomes

OBJECTIVE

To identify novel combinations of genetic and psychological factors that predicted 12-month postoperative pain and disability outcomes following arthroscopic shoulder surgery.

METHODS

A prospective presurgical cohort (n = 150) was recruited to complete validated psychological questionnaires and have their DNA collected from saliva. DNA was genotyped for a priori selected genes involved with pain modulation (ADRB2, OPRM1, AVPR1A, GCH1, and KCNS1) and inflammation (IL1B, TNF/LTA, and IL6). The outcome measures of interest were the Brief Pain Inventory and Disabilities of the Arm, Shoulder, and Hand questionnaire. Followup for the cohort was at 3, 6, and 12 months postoperatively. After controlling for age, sex, race, and preoperative status, genetic and psychological factors were entered as main effects and interaction terms in separate general linear models for predicting postoperative pain and disability outcomes.

RESULTS

Seven interactions involving pain-modulatory genes were identified. Three provided strong statistical evidence for different outcomes, including KCNS1 and kinesiophobia for preoperative pain intensity, ADRB2 and depressive symptoms for postoperative course, and GCH1 and anxiety symptoms for 12-month pain-intensity outcome. Ten interactions involving inflammatory genes were identified. Three provided strong statistical evidence for the 12-month postoperative course outcome, including 2 different IL6 single-nucleotide polymorphism and pain catastrophizing, and IL6 and depressive symptoms.

CONCLUSION

The current study identified novel genetic and psychological interactions that can be used in future studies to further understand the development of persistent postoperative pain and investigate the effectiveness of tailored treatment.

Toward a Mechanism-Based Approach to Pain Diagnosis

The past few decades have witnessed a huge leap forward in our understanding of the mechanistic underpinnings of pain, in normal states where it helps protect from injury, and also in pathological states where pain evolves from a symptom reflecting tissue injury to become the disease itself. However, despite these scientific advances, chronic pain remains extremely challenging to manage clinically. Although the number of potential treatment targets has grown substantially and a strong case has been made for a mechanism-based and individualized approach to pain therapy, arguably clinicians are not much more advanced now than 20 years ago, in their capacity to either diagnose or effectively treat their patients. The gulf between pain research and pain management is as wide as ever. We are still currently unable to apply an evidence-based approach to chronic pain management that reflects mechanistic understanding, and instead, clinical practice remains an empirical and often unsatisfactory journey for patients, whose individual response to treatment cannot be predicted. In this article we take a common and difficult to treat pain condition, chronic low back pain, and use its presentation in clinical practice as a framework to highlight what is known about pathophysiological pain mechanisms and how we could potentially detect these to drive rational treatment choice. We discuss how present methods of assessment and management still fall well short, however, of any mechanism-based or precision medicine approach. Nevertheless, substantial improvements in chronic pain management could be possible if a more strategic and coordinated approach were to evolve, one designed to identify the specific mechanisms driving the presenting pain phenotype. We present an analysis of such an approach, highlighting the major problems in identifying mechanisms in patients, and develop a framework for a pain diagnostic ladder that may prove useful in the future, consisting of successive identification of 3 steps: pain state, pain mechanism, and molecular target. Such an approach could serve as the foundation for a new era of individualized/precision pain medicine. The Analgesic, Anesthetic, and Addiction Clinical Trial Translations, Innovations, Opportunities, and Networks (ACTTION)-American Pain Society (APS) Pain Taxonomy (AAPT) includes pain mechanisms as 1 of the 5 dimensions that need to be considered when making a diagnostic classification. The diagnostic ladder proposed in this article is consistent with and an extension of the AAPT.

PERSPECTIVE

We discuss how identifying the specific mechanisms that operate in the nervous system to produce chronic pain in individual patients could provide the basis for a targeted and rational precision medicine approach to controlling pain, using chronic low back pain as our example.

Fibromyalgia: A Critical and Comprehensive Review

Fibromyalgia is a disorder that is part of a spectrum of syndromes that lack precise classification. It is often considered as part of the global overview of functional somatic syndromes that are otherwise medically unexplained or part of a somatization disorder. Patients with fibromyalgia share symptoms with other functional somatic problems, including issues of myalgias, arthralgias, fatigue and sleep disturbances. Indeed, there is often diagnostic and classification overlap for the case definitions of a variety of somatization disorders. Fibromyalgia, however, is a critically important syndrome for physicians and scientists to be aware of. Patients should be taken very seriously and provided optimal care. Although inflammatory, infectious, and autoimmune disorders have all been ascribed to be etiological events in the development of fibromyalgia, there is very little data to support such a thesis. Many of these disorders are associated with depression and anxiety and may even be part of what has been sometimes called affected spectrum disorders. There is no evidence that physical trauma, i.e., automobile accidents, is associated with the development or exacerbation of fibromyalgia. Treatment should be placed on education, patient support, physical therapy, nutrition, and exercise, including the use of drugs that are approved for the treatment of fibromyalgia. Treatment should not include opiates and patients should not become poly pharmacies in which the treatment itself can lead to significant morbidities. Patients with fibromyalgia are living and not dying of this disorder and positive outlooks and family support are key elements in the management of patients.

Tetrahydrobiopterin Attenuates DSS-evoked Colitis in Mice by Rebalancing Redox and Lipid Signalling

BACKGROUND AND AIMS

Guanosine triphosphate cyclohydrolase [GCH1] governs the production of the enzyme cofactor tetrahydrobiopterin [BH4] which is essential for biogenic amine synthesis, lipid metabolism via alkylglycerol monooxygenase [AGMO], and redox coupling of nitric oxide synthases [NOSs]. Inflammation-evoked unequal regulation of GCH1 and NOS or AGMO may cause redox stress and lipid imbalances.

METHODS

The present study assessed potential therapeutic effects of rebalancing these systems with BH4 in experimental colitis in mice.

RESULTS

Oral treatment with BH4 as a suspension of crushed tablets attenuated colitis, whereas inhibition of its production had opposite effects: aggravated weight loss, epithelial haemorrhages and ulcers, neutrophil infiltrates, production of reactive oxygen species, and unfavourable profile changes of endocannabinoids, ceramides, and lysophosphatidic acids. Conversely, oral BH4 normalised biopterin, reduced in vivo activity of oxidases and peroxidases in the inflamed gut, favoured nitric oxide over hydrogen peroxide, and maintained normal levels of lipid signalling molecules. BH4 favoured thereby resident CD3+CD8+ and regulatory CD3+CD25+ intraepithelial T cells that are important for epithelial integrity.

CONCLUSIONS

BH4 protected against colitis in mice via two major pathways: [i] by reduction of oxidative stress; and [ii] by re-orchestration of alkyl- and acylglycerolipid signalling via AGMO. Oral treatment with BH4 is a safe approved supplementary therapy for genetic BH4 deficiency and did not excessively increase systemic BH4 levels. Therefore, one may consider repurposing of oral BH4 as an adjunctive treatment for colitis.

Genetic susceptibility to postherniotomy pain. The influence of polymorphisms in the Mu opioid receptor, TNF-α, GRIK3, GCH1, BDNF and CACNA2D2 genes

BACKGROUND AND AIMS

Despite improvements in surgical technique, 5%-8% of patients undergoing herniorrhaphy still suffer from clinically relevant persistent postherniotomy pain. This is a problem at both individual and society levels. The aim of this study was to determine whether or not a single nucleotide polymorphism in a specific gene contributes to the development of persistent pain after surgery.

METHODS

One hundred individuals with persistent postherniotomy pain, along with 100 without pain matched for age, gender and type of surgery were identified in a previous cohort study on patients operated for groin hernia. All patients underwent a thorough sensory examination and blood samples were collected. DNA was extracted and analysed for single nucleotide polymorphism in the Mu opioid receptor, TNF-α, GRIK3, GCH1, BDNF and CACNA2D2 genes.

RESULTS

Patients with neuropathic pain were found to have a homozygous single nucleotide polymorph in the TNF-α gene significantly more often than pain-free patients (P=0.036, one-tailed test).

CONCLUSIONS

SNP in the TNF-α gene has a significant impact on the risk for developing PPSP.

IMPLICATIONS

The result suggests the involvement of genetic variance in the development of pain and this requires further investigation.

Pain Mechanisms and Centralized Pain in Temporomandibular Disorders

Until recently, most clinicians and scientists believed that the experience of pain is perceptually proportional to the amount of incoming peripheral nociceptive drive due to injury or inflammation in the area perceived to be painful. However, many cases of chronic pain have defied this logic, leaving clinicians perplexed as to how patients are experiencing pain with no obvious signs of injury in the periphery. Conversely, there are patients who have a peripheral injury and/or inflammation but little or no pain. What makes some individuals experience intense pain with minimal peripheral nociceptive stimulation and others experience minimal pain with serious injury? It is increasingly well accepted in the scientific community that pain can be generated and maintained or, through other mechanisms, suppressed by changes in the central nervous system, creating a complete mismatch between peripheral nociceptive drive and perceived pain. In fact, there is no known chronic pain condition where the observed extent of peripheral damage reproducibly engenders the same level of pain across individuals. Temporomandibular disorders (TMDs) are no exception. This review focuses on the idea that TMD patients range on a continuum-from those whose pain is generated peripherally to those whose pain is centralized (i.e., generated, exacerbated, and/or maintained by central nervous system mechanisms). This article uses other centralized chronic pain conditions as a guide, and it suggests that the mechanistic variability in TMD pain etiology has prevented us from adequately treating many individuals who are diagnosed with the condition. As the field moves forward, it will be imperative to understand each person's pain from its own mechanistic standpoint, which will enable clinicians to deliver personalized medicine to TMD patients and eventually provide relief in even the most recalcitrant cases.

Research design considerations for chronic pain prevention clinical trials: IMMPACT recommendations

Although certain risk factors can identify individuals who are most likely to develop chronic pain, few interventions to prevent chronic pain have been identified. To facilitate the identification of preventive interventions, an IMMPACT meeting was convened to discuss research design considerations for clinical trials investigating the prevention of chronic pain. We present general design considerations for prevention trials in populations that are at relatively high risk for developing chronic pain. Specific design considerations included subject identification, timing and duration of treatment, outcomes, timing of assessment, and adjusting for risk factors in the analyses. We provide a detailed examination of 4 models of chronic pain prevention (ie, chronic postsurgical pain, postherpetic neuralgia, chronic low back pain, and painful chemotherapy-induced peripheral neuropathy). The issues discussed can, in many instances, be extrapolated to other chronic pain conditions. These examples were selected because they are representative models of primary and secondary prevention, reflect persistent pain resulting from multiple insults (ie, surgery, viral infection, injury, and toxic or noxious element exposure), and are chronically painful conditions that are treated with a range of interventions. Improvements in the design of chronic pain prevention trials could improve assay sensitivity and thus accelerate the identification of efficacious interventions. Such interventions would have the potential to reduce the prevalence of chronic pain in the population. Additionally, standardization of outcomes in prevention clinical trials will facilitate meta-analyses and systematic reviews and improve detection of preventive strategies emerging from clinical trials.

Long-term daily vibration exposure alters current perception threshold (CPT) sensitivity and myelinated axons in a rat-tail model of vibration-induced injury

Repeated exposure to hand-transmitted vibration through the use of powered hand tools may result in pain and progressive reductions in tactile sensitivity. The goal of the present study was to use an established animal model of vibration-induced injury to characterize changes in sensory nerve function and cellular mechanisms associated with these alterations. Sensory nerve function was assessed weekly using the current perception threshold test and tail-flick analgesia test in male Sprague-Dawley rats exposed to 28 d of tail vibration. After 28 d of exposure, Aβ fiber sensitivity was reduced. This reduction in sensitivity was partly attributed to structural disruption of myelin. In addition, the decrease in sensitivity was also associated with a reduction in myelin basic protein and 2',3'- cyclic nucleotide phosphodiasterase (CNPase) staining in tail nerves, and an increase in circulating calcitonin gene-related peptide (CGRP) concentrations. Changes in Aβ fiber sensitivity and CGRP concentrations may serve as early markers of vibration-induced injury in peripheral nerves. It is conceivable that these markers may be utilized to monitor sensorineural alterations in workers exposed to vibration to potentially prevent additional injury.

Depressed mood, anxiety, and the use of labor analgesia

Relatively little is known about mental health and labor pain. The aim of this study was to assess if self-rated antenatal depressed mood and anxiety are associated with pain-related behaviors and self-reported labor pain. We also wanted to replicate our previous finding of altered labor pain behavior in carriers of a specific guanosine triphosphate cyclohydrolase 1 gene (GCH1) haplotype. Ninety-nine women in gestational weeks 37 to 40 filled out questionnaires on depression and anxiety symptoms and later rated their labor pain by use of visual analog scales. Each subject was also genotyped for GCH1. Following adjustment for relevant confounders, women who arrived early to the delivery unit (cervical dilation <5 cm) had a significantly higher antenatal Montgomery-Åsberg Depression Rating Scale (MADRS-S) score, p < 0.05, than late arrivers (cervical dilation >5 cm). Women with increased Spielberger State-Trait Anxiety Inventory (STAI-T) scores reported higher self-rated pain prior to labor analgesia, p < 0.05, than women with low STAI-T scores. No association between the GCH1 pain-protective haplotype and cervical dilation was found, but a previously demonstrated association with increased use of second-line analgesia was confirmed. Depressed mood during pregnancy is associated with early arrival to the delivery department, whereas antenatal anxiety is associated with increased self-rated pain prior to labor analgesia.

The Acute to Chronic Pain Transition: Can Chronic Pain Be Prevented?

Chronic postsurgical pain (CPSP) is a distressing disease process that can lead to long-term disability, reduced quality of life, and increased health care spending. Although the exact mechanism of development of CPSP is unknown, nerve injury and inflammation may lead to peripheral and central sensitization. Given the complexity of the disease process, no novel treatment has been identified. The preoperative use of multimodal analgesia has been shown to decrease acute postoperative pain, but it has no proven efficacy in preventing development of CPSP.

Nucleotide Variants of the BH4 Biosynthesis Pathway Gene GCH1 and the Risk of Orofacial Clefts

A deficiency of GTP cyclohydrolase, encoded by the GCH1 gene, results in two neurological diseases: hyperphenylalaninaemia type HPABH4B and DOPA-responsive dystonia. Genes involved in neurotransmitter metabolism and motor systems may contribute to palatogenesis. The purpose of the study was to analyse polymorphic variants of the GCH1 gene as risk factors for non-syndromic cleft lip with or without cleft palate (NSCL/P). Genotyping of nine polymorphisms was conducted in a group of 281 NSCL/P patients and 574 controls. The GCH1 variant rs17128077 was associated with a 1.7-fold higher risk for NSCL/P (95 %CI = 1.224-2.325; p = 0.001). We also found a significant correlation between the rs8004018 and rs17128050 variants and an increased risk of oral clefts (p trend = 0.003 and 0.004, respectively). The best evidence of the global haplotype association was observed for rs17128050 and rs8004018 (p corr = 0.0152). This study demonstrates that the risk of NSCL/P is associated with variants of the GCH1 gene related to BH4 metabolism and provides some evidence of the relationships between morphological/functional shifts in the central nervous system and orofacial clefts.

Redoxins in peripheral neurons after sciatic nerve injury

Peripheral nerve injury causes redox stress in injured neurons by upregulations of pro-oxidative enzymes, but most neurons survive suggesting an activation of endogenous defense against the imbalance. As potential candidates we assessed thioredoxin-fold proteins, called redoxins, which maintain redox homeostasis by reduction of hydrogen peroxide or protein dithiol-disulfide exchange. Using a histologic approach, we show that the peroxiredoxins (Prdx1-6), the glutaredoxins (Glrx1, 2, 3 and 5), thioredoxin (Txn1 and 2) and their reductases (Txnrd1 and 2) are expressed in neurons, glial and/or vascular cells of the dorsal root ganglia (DRGs) and in the spinal cord. They show distinct cellular and subcellular locations in agreement with the GO terms for "cellular component". The expression and localization of Glrx, Txn and Txnrd proteins was not affected by sciatic nerve injury but peroxiredoxins were upregulated in the DRGs, Prdx1 and Prdx6 mainly in non-neuronal cells and Prdx4 and Prdx5 in DRG neurons, the latter associated with an increase of respective mRNAs and protein accumulation in peripheral and/or central fibers. The upregulation of Prdx4 and Prdx5 in DRG neurons was reduced in mice with a cre-loxP mediated deficiency of hypoxia inducible factor 1 alpha (HIF1α) in these neurons. The results identify Prdx4 and Prdx5 as endogenous HIF1α-dependent, transcriptionally regulated defenders of nerve injury evoked redox stress that may be important for neuronal survival and regeneration.

Neopterin as a potential cytoprotective brain molecule

Neopterin, a byproduct of the tetrahydrobiopterin de novo pathway, is found in increased levels in cerebrospinal fluid and plasma and significantly increases upon damage, infection or during immune system activation. The production of this compound seems almost restricted to the monocyte/macrophage linage cells, in response to interferon-γ stimulation. However, it is unclear whether and which nervous cells are able to synthesize neopterin, respond to any stressor applied extracellularly, or even the role of the compound in the central nervous system. Here we propose a potential cytoprotective role of neopterin in the brain, and show evidence that cultured rat astrocytes are responsive to the molecule; the pterin elicited increased hemeoxygenase-1 cellular content and decreased oxidative stress induced by mitochondrial dysfunction. Further studies are needed to clarify neopterin's cytoprotective effects in the central nervous system, and its potential role in different neuroinflammatory diseases.

Association of genetic and psychological factors with persistent pain after cosmetic thoracic surgery

The genetic control of pain has been repeatedly demonstrated in human association studies. In the present study, we assessed the relative contribution of 16 single nucleotide polymorphisms in pain-related genes, such as cathechol-O-methyl transferase gene (COMT), fatty acid amino hydrolase gene (FAAH), transient receptor potential cation channel, subfamily V, member 1 gene (TRPV1), and δ-opioid receptor gene (OPRD1), for postsurgical pain chronification. Ninety preoperatively pain-free male patients were assigned to good or poor outcome groups according to their intensity or disability score assessed at 1 week, 3 months, 6 months, and 1 year after funnel chest correction. The genetic effects were compared with those of two psychological predictors, the attentional bias toward positive words (dot-probe task) and the self-reported pain vigilance (Pain Vigilance and Awareness Questionnaire [PVAQ]), which were already shown to be the best predictors for pain intensity and disability at 6 months after surgery in the same sample, respectively. Cox regression analyses revealed no significant effects of any of the genetic predictors up to the end point of survival time at 1 year after surgery. Adding the genetics to the prediction by the attentional bias to positive words for pain intensity and the PVAQ for pain disability, again no significant additional explanation could be gained by the genetic predictors. In contrast, the preoperative PVAQ score was also, in the present enlarged sample, a meaningful predictor for lasting pain disability after surgery. Effect size measures suggested some genetic variables, for example, the polymorphism rs1800587G>A in the interleukin 1 alpha gene (IL1A) and the COMT haplotype rs4646312T>C/rs165722T>C/rs6269A>G/rs4633T>C/rs4818C>G/rs4680A>G, as possible relevant modulators of long-term postsurgical pain outcome. A comparison between pathophysiologically different predictor groups appears to be helpful in identifying clinically relevant predictors of chronic pain.

Neuropathic pain phenotyping by international consensus (NeuroPPIC) for genetic studies: a NeuPSIG systematic review, Delphi survey, and expert panel recommendations

For genetic research to contribute more fully to furthering our knowledge of neuropathic pain, we require an agreed, valid, and feasible approach to phenotyping, to allow collaboration and replication in samples of sufficient size. Results from genetic studies on neuropathic pain have been inconsistent and have met with replication difficulties, in part because of differences in phenotypes used for case ascertainment. Because there is no consensus on the nature of these phenotypes, nor on the methods of collecting them, this study aimed to provide guidelines on collecting and reporting phenotypes in cases and controls for genetic studies. Consensus was achieved through a staged approach: (1) systematic literature review to identify all neuropathic pain phenotypes used in previous genetic studies; (2) Delphi survey to identify the most useful neuropathic pain phenotypes and their validity and feasibility; and (3) meeting of experts to reach consensus on the optimal phenotype(s) to be collected from patients with neuropathic pain for genetic studies. A basic "entry level" set of phenotypes was identified for any genetic study of neuropathic pain. This set identifies cases of "possible" neuropathic pain, and controls, and includes: (1) a validated symptom-based questionnaire to determine whether any pain is likely to be neuropathic; (2) body chart or checklist to identify whether the area of pain distribution is neuroanatomically logical; and (3) details of pain history (intensity, duration, any formal diagnosis). This NeuroPPIC "entry level" set of phenotypes can be expanded by more extensive and specific measures, as determined by scientific requirements and resource availability.

Human pain and genetics: some basics

Human pain causes untold misery and suffering, with major impact on functioning and resources. Recent advances in genetics have revealed that subtle changes in DNA could partly explain the variation in individual differences in pain. Various genes encoding for receptors are now known to play a major role in the sensitivity, perception and expression of pain. The fields of epigenetics and proteomics hold promises in the way pain could be treated and managed in future.

[Pharmacological aspects of pain research in Germany]

In spite of several approved analgesics, the therapy of pain still constitutes a challenge due to the fact that the drugs do not exert sufficient efficacy or are associated with severe side effects. Therefore, the development of new and improved painkillers is still of great importance. A number of highly qualified scientists in Germany are investigating signal transduction pathways in pain, effectivity of new drugs and the so far incompletely investigated mechanisms of well-known analgesics in preclinical and clinical studies. The highlights of pharmacological pain research in Germany are summarized in this article.

Nonmotor Symptoms in Dopa-Responsive Dystonia

Background

Dopa-responsive dystonia (DRD) is a rare inherited dystonia, caused by an autosomal dominantly inherited defect in the gene GCH1 that encodes guanosine triphosphate cyclohydrolase 1. It catalyzes the first and rate-limiting enzyme in the biosynthesis of tetrahydrobiopterin, which is the essential co-factor for aromatic amino acid hydroxylases. Mutation results in the typical scenario of a young-onset lower-limb dystonia with diurnal fluctuations, concurrent or subsequent development of parkinsonism and excellent response to levodopa. Given the myriad functions of tetrahydrobiopterin, it is reasonable that other systems, apart from motor, would also be impaired. So far, non-motor symptoms have been overlooked and very few and often contrasting data are currently available on the matter.

Methods

Here by searching the Medline database for publications between 1971 to March 2015, we render an in-depth analysis of all published data on non-motor symptoms in DRD.

Results

Depression and subtle sleep quality impairment have been reported among the different cohorts, while current data do not support any alterations of the cardiologic and autonomic systems. However, there is debate about the occurrence of sleep-related movement disorders and cognitive function. Non-motor symptoms are instead frequently reported among the clinical spectrum of other neurotransmitter disorders which may sometimes mimic DRD phenotype, ie, DRD plus diseases.

Conclusions

Further studies in larger and treatment-naïve cohorts are needed to better elucidate the extend of non-motor symptoms in DRD and also to consider treatment for these.

Reduction of Neuropathic and Inflammatory Pain through Inhibition of the Tetrahydrobiopterin Pathway

Human genetic studies have revealed an association between GTP cyclohydrolase 1 polymorphisms, which decrease tetrahydrobiopterin (BH4) levels, and reduced pain in patients. We now show that excessive BH4 is produced in mice by both axotomized sensory neurons and macrophages infiltrating damaged nerves and inflamed tissue. Constitutive BH4 overproduction in sensory neurons increases pain sensitivity, whereas blocking BH4 production only in these cells reduces nerve injury-induced hypersensitivity without affecting nociceptive pain. To minimize risk of side effects, we targeted sepiapterin reductase (SPR), whose blockade allows minimal BH4 production through the BH4 salvage pathways. Using a structure-based design, we developed a potent SPR inhibitor and show that it reduces pain hypersensitivity effectively with a concomitant decrease in BH4 levels in target tissues, acting both on sensory neurons and macrophages, with no development of tolerance or adverse effects. Finally, we demonstrate that sepiapterin accumulation is a sensitive biomarker for SPR inhibition in vivo.

The search for novel analgesics: targets and mechanisms

The management of the pain state is of great therapeutic relevance to virtually every medical specialty. Failure to manage its expression has deleterious consequence to the well-being of the organism. An understanding of the complex biology of the mechanisms underlying the processing of nociceptive information provides an important pathway towards development of novel and robust therapeutics. Importantly, preclinical models have been of considerable use in determining the linkage between mechanism and the associated behaviorally defined pain state. This review seeks to provide an overview of current thinking targeting pain biology, the use of preclinical models and the development of novel pain therapeutics. Issues pertinent to the strengths and weaknesses of current development strategies for analgesics are considered.

Diagnosing and treating chronic musculoskeletal pain based on the underlying mechanism(s)

Until recently, most clinicians considered chronic pain to be typically due to ongoing peripheral nociceptive input (i.e., damage or inflammation) in the region of the body where the individual is experiencing pain. Clinicians are generally aware of a few types of pain (e.g., headache and phantom limb pain) where chronic pain is not due to such causes, but most do not realize there is not a single chronic pain state where any radiographic, surgical, or pathological description of peripheral nociceptive damage has been reproducibly shown to be related to the presence or severity of pain. The primary reason for this appears to be that both the peripheral and central nervous systems play a critical role in determining which nociceptive input being detected by sensory nerves in the peripheral tissues will lead to the perception of pain in humans. This manuscript reviews some of the latest findings regarding the neural processing of pain, with a special focus on how clinicians can use information gleaned from the history and physical examination to assess which mechanisms are most likely to be responsible for pain in a given individual, and tailors therapy appropriately. A critical construct is that, within any specific diagnostic category (e.g., fibromyalgia (FM), osteoarthritis (OA), and chronic low back pain (CLBP) are specifically reviewed), individual patients may have markedly different peripheral/nociceptive and neural contributions to their pain. Thus, just as low back pain has long been acknowledged to have multiple potential mechanisms, so also is this true of all chronic pain states, wherein some individuals will have pain primarily due to peripheral nociceptive input, whereas in others peripheral (e.g., peripheral sensitization) or central nervous system factors ("central sensitization" or "centralization" of pain via augmented pain processing in spinal and brain) may be playing an equally or even more prominent role in their pain and other symptoms.

Genetic and Clinical Factors Associated with Chronic Postsurgical Pain after Hernia Repair, Hysterectomy, and Thoracotomy

Background:

Chronic postsurgical pain (CPSP) has been linked to many surgical settings. The authors aimed to analyze functional genetic polymorphisms and clinical factors that might identify CPSP risk after inguinal hernia repair, hysterectomy, and thoracotomy.

Methods:

This prospective multicenter cohort study enrolled 2,929 patients scheduled for inguinal hernia repair, hysterectomy (vaginal or abdominal), or thoracotomy. The main outcome was the incidence of CPSP confirmed by physical examination 4 months after surgery. The secondary outcome was CPSP incidences at 12 and 24 months. The authors also tested the associations between CPSP and 90 genetic markers plus a series of clinical factors and built a CPSP risk model.

Results:

Within a median of 4.4 months, CPSP had developed in 527 patients (18.0%), in 13.6% after hernia repair, 11.8% after vaginal hysterectomy, 25.1% after abdominal hysterectomy, and 37.6% after thoracotomy. CPSP persisted after a median of 14.6 months and 26.3 months in 6.2% and 4.1%, respectively, after hernia repair, 4.1% and 2.2% after vaginal hysterectomy, 9.9% and 6.7% after abdominal hysterectomy, and 19.1% and 13.2% after thoracotomy. No significant genetic differences between cases and controls were identified. The risk model included six clinical predictors: (1) surgical procedure, (2) age, (3) physical health (Short Form Health Survey-12), (4) mental health (Short Form Health Survey-12), (5) preoperative pain in the surgical field, and (6) preoperative pain in another area. Discrimination was moderate (c-statistic, 0.731; 95% CI, 0.705 to 0.755).

Conclusions:

Until unequivocal genetic predictors of CPSP are understood, the authors encourage systematic use of clinical factors for predicting and managing CPSP risk.

Evidence of a role for GTP cyclohydrolase-1 in visceral pain

BACKGROUND

The enzyme guanosine triphosphate-cyclohydrolase-1 (GCH-1) is a rate limiting step in the de novo synthesis of tetrahydrobiopterin (BH4) a co-factor in monoamine synthesis and nitric oxide production. GCH-1 is strongly implicated in chronic pain based on data generated using the selective GCH-1 inhibitor 2,4-diamino-6-hydroxypyrimidine (DAHP), and studies which have identified a pain protective GCH-1 haplotype associated with lower BH4 production and reduced pain.

METHODS

To investigate the role for GCH-1 in visceral pain we examined the effects of DAHP on pain behaviors elicited by colorectal injection of mustard oil in rats, and the pain protective GCH-1 haplotype in healthy volunteers characterized by esophageal pain sensitivity before and after acid injury, and assessed using depression and anxiety questionnaires.

KEY RESULTS

In rodents pretreatment with DAHP produced a substantial dose related inhibition of pain behaviors from 10 to 180 mg/kg i.p. (p < 0.01 to 0.001). In healthy volunteers, no association was seen between the pain protective GCH-1 haplotype and the development of hypersensitivity following injury. However, a substantial increase in baseline pain thresholds was seen between first and second visits (26.6 ± 6.2 mA) in subjects who sensitized to esophageal injury and possessed the pain protective GCH-1 haplotype compared with all other groups (p < 0.05). Furthermore the same subjects who sensitized to acid and possessed the haplotype, also had significantly lower depression scores (p < 0.05).

CONCLUSIONS & INFERENCES

The data generated indicate that GCH-1 plays a role in visceral pain processing that requires more detailed investigation.

The serine protease inhibitor SerpinA3N attenuates neuropathic pain by inhibiting T cell-derived leukocyte elastase

Neuropathic pain is a major, intractable clinical problem and its pathophysiology is not well understood. Although recent gene expression profiling studies have enabled the identification of novel targets for pain therapy, classical study designs provide unclear results owing to the differential expression of hundreds of genes across sham and nerve-injured groups, which can be difficult to validate, particularly with respect to the specificity of pain modulation. To circumvent this, we used two outbred lines of rats, which are genetically similar except for being genetically segregated as a result of selective breeding for differences in neuropathic pain hypersensitivity. SerpinA3N, a serine protease inhibitor, was upregulated in the dorsal root ganglia (DRG) after nerve injury, which was further validated for its mouse homolog. Mice lacking SerpinA3N developed more neuropathic mechanical allodynia than wild-type (WT) mice, and exogenous delivery of SerpinA3N attenuated mechanical allodynia in WT mice. T lymphocytes infiltrate the DRG after nerve injury and release leukocyte elastase (LE), which was inhibited by SerpinA3N derived from DRG neurons. Genetic loss of LE or exogenous application of a LE inhibitor (Sivelastat) in WT mice attenuated neuropathic mechanical allodynia. Overall, we reveal a novel and clinically relevant role for a member of the serpin superfamily and a leukocyte elastase and crosstalk between neurons and T cells in the modulation of neuropathic pain.

Translational pain research: Lessons from genetics and genomics

Pharmacological, surgical, psychological, and alternative medicine approaches for the treatment of chronic pain, including neuropathic pain, provide only partial relief for most patients, with the efficacy of existing medications often blunted by dose-limiting side effects arising from drug actions on cells outside the pain-signaling axis. The development of more effective treatments for pain--particularly chronic pain states such as neuropathic pain--has been hampered by lack of predictive animal models and biomarkers, variation in pain characteristics between patients or on a day-to-day basis for single patients, patient stratification on the basis of symptoms rather than mechanism, and a high rate of placebo responses. We discuss genetic and genomic approaches to translational pain research. We review examples of the identification and validation of human pain targets through rodent genome-wide association studies (GWAS) and global mRNA expression studies, functional screening in flies and mice, human GWAS and whole-exome sequencing studies, and the targeted candidate gene approach. These and other emerging genetic and genomic strategies are likely to facilitate the development of new, more effective pain therapeutics.

Lost but making progress--Where will new analgesic drugs come from?

There is a critical need for effective new pharmacotherapies for pain. The paucity of new drugs successfully reaching the clinic calls for a reassessment of current analgesic drug discovery approaches. Many points early in the discovery process present significant hurdles, making it critical to exploit advances in pain neurobiology to increase the probability of success. In this review, we highlight approaches that are being pursued vigorously by the pain community for drug discovery, including innovative preclinical pain models, insights from genetics, mechanistic phenotyping of pain patients, development of biomarkers, and emerging insights into chronic pain as a disorder of both the periphery and the brain. Collaborative efforts between pharmaceutical, academic, and public entities to advance research in these areas promise to de-risk potential targets, stimulate investment, and speed evaluation and development of better pain therapies.

Notch signaling activation is critical to the development of neuropathic pain

Background

Nerve injury-induced neuropathic pain is a major health problem worldwide. Notch signaling is a highly conserved pathway in evolution, which has an important role in synaptic plasticity and inflammation in central nervous system. The present study was designed to investigate the potential role of notch signaling in the development of neuropathic pain.

Methods

The neuropathic pain was induced by spared nerve injury (SNI) in rats. The activation of notch signaling in the lumbar spinal dorsal horn was measured. DAPT, an inhibitor of notch signaling, was intrathecally (i.t.) administered before SNI or after appearance of pain sensitivity. Moreover, Jagged-1 (JAG-1) peptide, a ligand of notch signaling, was i.t. administered to normal rats. The mechanical allodynia was assessed by von Frey test.

Results

Here, we found that DAPT administered 0.5 h before SNI operation could significantly prevent the decrease of mechanical paw withdrawal threshold (PWT) for more than 4 weeks (P < 0.05 vs. SNI group). DAPT administered after appearance of pain sensitivity could also significantly reverse the decrease of mechanical PWT in a dose-dependent manner (P < 0.05). In addition, administration of Jagged-1 (JAG-1) peptide significantly decreased the mechanical PWT of normal rats in a dose-dependent manner (P < 0.05).

Conclusions

Therefore, notch signaling activation might contribute to the development of neuropathic pain. This study might provide a new therapeutic target for neuropathic pain.