Increased nerve growth factor after rat plantar incision contributes to guarding behavior and heat hyperalgesia

Disruption of mitochondrial pyruvate oxidation in dorsal root ganglia drives persistent nociceptive sensitization and causes pervasive transcriptomic alterations

ABSTRACT

Metabolism is inextricably linked to every aspect of cellular function. In addition to energy production and biosynthesis, metabolism plays a crucial role in regulating signal transduction and gene expression. Altered metabolic states have been shown to maintain aberrant signaling and transcription, contributing to diseases like cancer, cardiovascular disease, and neurodegeneration. Metabolic gene polymorphisms and defects are also associated with chronic pain conditions, as are increased levels of nerve growth factor (NGF). However, the mechanisms by which NGF may modulate sensory neuron metabolism remain unclear. This study demonstrated that intraplantar NGF injection reprograms sensory neuron metabolism. Nerve growth factor suppressed mitochondrial pyruvate oxidation and enhanced lactate extrusion, requiring 24 hours to increase lactate dehydrogenase A and pyruvate dehydrogenase kinase 1 (PDHK1) expression. Inhibiting these metabolic enzymes reversed NGF-mediated effects. Remarkably, directly disrupting mitochondrial pyruvate oxidation induced severe, persistent allodynia, implicating this metabolic dysfunction in chronic pain. Nanopore long-read sequencing of poly(A) mRNA uncovered extensive transcriptomic changes upon metabolic disruption, including altered gene expression, splicing, and poly(A) tail lengths. By linking metabolic disturbance of dorsal root ganglia to transcriptome reprogramming, this study enhances our understanding of the mechanisms underlying persistent nociceptive sensitization. These findings imply that impaired mitochondrial pyruvate oxidation may drive chronic pain, possibly by impacting transcriptomic regulation. Exploring these metabolite-driven mechanisms further might reveal novel therapeutic targets for intractable pain.

Widespread latent hyperactivity of nociceptors outlasts enhanced avoidance behavior following incision injury

Nociceptors with somata in dorsal root ganglia (DRGs) exhibit an unusual readiness to switch from an electrically silent state to a hyperactive state of tonic, nonaccommodating, low-frequency, irregular discharge of action potentials (APs). Ongoing activity (OA) during this state is present in vivo in rats months after spinal cord injury (SCI), and has been causally linked to SCI pain. OA induced by various neuropathic conditions in rats, mice, and humans is retained in nociceptor somata after dissociation and culturing, providing a powerful tool for investigating its mechanisms and functions. An important question is whether similar nociceptor OA is induced by painful conditions other than neuropathy. The present study shows that probable nociceptors dissociated from DRGs of rats subjected to postsurgical pain (induced by plantar incision) exhibit OA. The OA was most apparent when the soma was artificially depolarized to a level within the normal range of membrane potentials where large, transient depolarizing spontaneous fluctuations (DSFs) can approach AP threshold. This latent hyperactivity persisted for at least 3 weeks, whereas behavioral indicators of affective pain - hindpaw guarding and increased avoidance of a noxious substrate in an operant conflict test - persisted for 1 week or less. An unexpected discovery was latent OA in neurons from thoracic DRGs that innervate dermatomes distant from the injured tissue. The most consistent electrophysiological alteration associated with OA was enhancement of DSFs. Potential in vivo functions of widespread, low-frequency nociceptor OA consistent with these and other findings are to amplify hyperalgesic priming and to drive anxiety-related hypervigilance.

Increases in local skin temperature correlate with spontaneous foot lifting and heat hyperalgesia in both incisional inflammatory models of pain

Background

This study investigated if a localized increase in skin temperature in rat models of incisional and inflammatory pain correlates with the intensity of spontaneous and evoked pain behaviors.

Methods

Anesthetized rats received either a 20-mm longitudinal incision made through the skin, fascia, and muscle of the plantar hind paw or an injection of complete Freund adjuvant into the plantar hind paw of anesthetized rats to induce local inflammation. Spontaneous and evoked pain behaviors were assessed, and changes in skin temperature were measured using a noncontact infrared thermometer.

Results

There were no differences in skin temperature between the ipsilateral and contralateral hind paw before the incision or inflammation. Skin temperature increased at 2 hours after hind paw plantar incision or 1 day after inflammation of the affected paw, which gradually returned to baseline by the first day and fourth days after treatment, respectively. The increase in skin temperature correlated with the intensity of spontaneous pain behaviors and heat but not with mechanical allodynia.

Conclusions

Our results suggest that a simple measurement of localized skin temperature using a noncontact infrared thermometer could measure the extent of spontaneous pain behaviors and heat hyperalgesia following plantar incision or inflammation in animals. In the absence of a reliable objective marker of pain, these results are encouraging. However, studies are warranted to validate our results using analgesics and pain-relieving interventions, such as nerve block on skin temperature changes.

STING-IFN-I pathway relieves incision induced acute postoperative pain via inhibiting the neuroinflammation in dorsal root ganglion of rats

BACKGROUND

The purpose of this study was to study the effect of STING-IFN-I pathway on incision induced postoperative pain in rats and its possible mechanisms.

METHODS

The pain thresholds were evaluated by measuring the mechanical withdrawal threshold and the thermal withdrawal latency. The satellite glial cell and macrophage of DRG were analyzed. The expression of STING, IFN-a, P-P65, iNOS, TNF-α, IL-1β and IL-6 in DRG was evaluated.

RESULTS

The activation of STING-IFN-I pathway can reduce the mechanical hyperalgesia, thermal hyperalgesia, down-regulate the expression of P-P65, iNOS, TNF-α, IL-1β and IL-6, and inhibit the activation of satellite glial cell and macrophage in DRG.

CONCLUSIONS

The activation of STING-IFN-I pathway can alleviate incision induced acute postoperative pain by inhibiting the activation of satellite glial cell and macrophage, which reducing the corresponding neuroinflammation in DRG.

The mechanisms and management of persistent postsurgical pain

An estimated 10%-50% of patients undergoing a surgical intervention will develop persistent postsurgical pain (PPP) lasting more than 3 months despite adequate acute pain management and the availability of minimally invasive procedures. The link between early and late pain outcomes for surgical procedures remains unclear-some patients improve while others develop persistent pain. The elective nature of a surgical procedure offers a unique opportunity for prophylactic or early intervention to prevent the development of PPP and improve our understanding of its associated risk factors, such as pre-operative anxiety and the duration of severe acute postoperative pain. Current perioperative pain management strategies often include opioids, but long-term consumption can lead to tolerance, addiction, opioid-induced hyperalgesia, and death. Pre-clinical models provide the opportunity to dissect mechanisms underpinning the transition from acute to chronic, or persistent, postsurgical pain. This review highlights putative mechanisms of PPP, including sensitisation of peripheral sensory neurons, neuroplasticity in the central nervous system and nociceptive signalling along the neuro-immune axis.

Sensitization of supra-threshold pain responses-Translational aspects and mechanisms

A substantial translational gap in pain research has been reflected by a mismatch of relevant primary pain assessment endpoints in preclinical vs. clinical trials. Since activity-dependent mechanisms may be neglected during reflexive tests, this may add as a confounding factor during preclinical pain assessment. In this perspective, we consider the evidence for a need for supra-threshold pain assessment in the pain research literature. In addition to that, we focus on previous results that may demonstrate an example mechanism, where the detection of neuron-glial interactions on pain seems to be substantially depending on the assessment of pain intensity beyond threshold levels.

Nerve Growth Factor (NGF) Inhibitors and Related Agents for Chronic Musculoskeletal Pain: A Comprehensive Review

Musculoskeletal pain such as osteoarthritis (OA) and low back pain (LBP) are very common and contribute to enormous burden and societal costs, despite dramatic therapeutic advances over recent decades. Novel approaches and targeted therapies are required to satisfy the urgent unmet medical need of musculoskeletal pain relief in both conditions. Nerve growth factor (NGF) inhibitors have utilized novel mechanisms different from conventional drugs, which have a variety of gastrointestinal, cardiac, or renal adverse effects. Several phase 2/3 studies have been accomplished for these drugs, such as tanezumab, fasinumab, and tyrosine receptor kinase A (TrkA) inhibitors. We searched the literature using the PubMed database and clinical trials using ClinicalTrials.gov to identify original papers, meta-analyses as well as ongoing clinical trials assessing the efficacy and safety profile of these drugs. In this narrative review, we briefly overview the disease burden of musculoskeletal pain, the role of NGF signaling and its receptors in the genesis of pain, and the mechanisms of action of inhibitors of NGF signaling and downstream pathways, and then discuss the efficacy and safety of each investigational drug in OA and LBP. Finally, we briefly review two serious adverse effects of NGF inhibitors, namely rapidly progressive OA and sympathetic system effects, and conclude with possible barriers and potential research directions to overcome these.

Global transcriptome analysis of rat dorsal root ganglia to identify molecular pathways involved in incisional pain

To develop non-opioid therapies for postoperative incisional pain, we must understand its underlying molecular mechanisms. In this study, we assessed global gene expression changes in dorsal root ganglia neurons in a model of incisional pain to identify pertinent molecular pathways. Male, Sprague-Dawley rats underwent infiltration of 1% capsaicin or vehicle into the plantar hind paw (n = 6-9/group) 30 min before plantar incision. Twenty-four hours after incision or sham (control) surgery, lumbar L4-L6 dorsal root ganglias were collected from rats pretreated with vehicle or capsaicin. RNA was isolated and sequenced by next generation sequencing. The genes were then annotated to functional networks using a knowledge-based database, Ingenuity Pathway Analysis. In rats pretreated with vehicle, plantar incision caused robust hyperalgesia, up-regulated 36 genes and downregulated 90 genes in dorsal root ganglias one day after plantar incision. Capsaicin pretreatment attenuated pain behaviors, caused localized denervation of the dermis and epidermis, and prevented the incision-induced changes in 99 of 126 genes. The pathway analyses showed altered gene networks related to increased pro-inflammatory and decreased anti-inflammatory responses in dorsal root ganglias. Insulin-like growth factor signaling was identified as one of the major gene networks involved in the development of incisional pain. Expression of insulin-like growth factor -2 and IGFBP6 in dorsal root ganglia were independently validated with quantitative real-time polymerase chain reaction. We discovered a distinct subset of dorsal root ganglia genes and three key signaling pathways that are altered 24 h after plantar incision but are unchanged when incision was made after capsaicin infiltration in the skin. Further exploration of molecular mechanisms of incisional pain may yield novel therapeutic targets.

c-Jun/p38MAPK/ASIC3 pathways specifically activated by nerve growth factor through TrkA are crucial for mechanical allodynia development

Mechanical allodynia is a cardinal sign of several inflammatory pain disorders where nerve growth factor, a prototypic neurotrophin, plays a crucial role by binding to TrkA receptors. Here, we took the advantage of our generated knock-in mouse model expressing a chimeric TrkA/TrkC receptor that seems to not specifically develop mechanical allodynia after inflammation, to identify the TrkA downstream pathways involved in this phenomenon. We confirmed and extended that disrupting TrkA-specific pathways leads to a specific deficit in mechanical hypersensitivity development after somatic (systemic nerve growth factor administration and paw incision) and, to a lesser extent, visceral injuries. Despite a deficit in thin, mainly peptidergic, fibre innervation in TrkAC mice, thermal hyperalgesia development was not different from WT mice. Inflammatory reaction (oedema, IL-6 content), pain behaviours after intraplantar capsaicin, as well as TRPV1 calcium imaging response of dorsal root ganglion neurons were similar between TrkAC and WT mice. This deficiency in mechanical allodynia development in TrkAC mice is likely due to the alteration of the expression of different TrkA transduction pathways (ie, Akt, p38 MAPK, and c-Jun) especially p38 MAPK, in the dorsal root ganglion cell bodies, ultimately leading to an alteration of at least, ASIC3 channel overexpression, known to participate in nociceptor mechanosensory function.

NGF-Induced Nav1.7 Upregulation Contributes to Chronic Post-surgical Pain by Activating SGK1-Dependent Nedd4-2 Phosphorylation

At present, chronic post-surgical pain (CPSP) is difficult to prevent and cure clinically because of our lack of understanding of its mechanisms. Surgical injury induces the upregulation of voltage-gated sodium channel Nav1.7 in dorsal root ganglion (DRG) neurons, suggesting that Nav1.7 is involved in the development of CPSP. However, the mechanism leading to persistent dysregulation of Nav1.7 is largely unknown. Given that nerve growth factor (NGF) induces a long-term increase in the neuronal hyperexcitability after injury, we hypothesized that NGF might cause the long-term dysregulation of Nav1.7. In this study, we aimed to investigate whether Nav1.7 regulation by NGF is involved in CPSP and thus contributes to the specific mechanisms involved in the development of CPSP. Using conditional nociceptor-specific Nav1.7 knockout mice, we confirmed the involvement of Nav1.7 in NGF-induced pain and identified its role in the maintenance of pain behavior during long-term observations (up to 14 days). Using western blot analyses and immunostaining, we showed that NGF could trigger the upregulation of Nav1.7 expression and thus support the development of CPSP in rats. Using pharmacological approaches, we showed that the increase of Nav1.7 might be partly regulated by an NGF/TrkA-SGK1-Nedd4-2-mediated pathway. Furthermore, reversing the upregulation of Nav1.7 in DRG could alleviate spinal sensitization. Our results suggest that the maintained upregulation of Nav1.7 triggered by NGF contributes to the development of CPSP. Attenuating the dysregulation of Nav1.7 in peripheral nociceptors may be a strategy to prevent the transition from acute post-surgical pain to CPSP.

Acid-sensing Ion Channel 3 Overexpression in Incisions Regulated by Nerve Growth Factor Participates in Postoperative Nociception in Rats

BACKGROUND

Acid-sensing ion channel 3 (ASIC3) upregulation has been reported in dorsal root ganglion neurons after incision and contributes to postoperative nociception. This study hypothesized that upregulation of ASIC3 in incised tissues is induced by nerve growth factor through the phosphoinositide 3-kinase/protein kinase B signaling pathway.

METHODS

A plantar incision model was established in adult male and female Sprague-Dawley rats. ASIC3 was inhibited by APETx2 treatment, small interfering RNA treatment, or ASIC3 knockout. Sciatic nerve ligation was performed to analyze ASIC3 transport. A nerve growth factor antibody and a phosphoinositide 3-kinase inhibitor were used to investigate the mechanism by which nerve growth factor regulates ASIC3 expression.

RESULTS

Acid-sensing ion channel 3 inhibition decreased incisional guarding and mechanical nociception. ASIC3 protein levels were increased in skin and muscle 4 h after incision (mean ± SD: 5.4 ± 3.2-fold in skin, n = 6, P = 0.001; 4.3 ± 2.2-fold in muscle, n = 6, P = 0.001). Sciatic nerve ligation revealed bidirectional ASIC3 transport. Nerve growth factor antibody treatment inhibited the expression of ASIC3 (mean ± SD: antibody 2.3 ± 0.8-fold vs. vehicle 4.9 ± 2.4-fold, n = 6, P = 0.036) and phosphorylated protein kinase B (mean ± SD: antibody 0.8 ± 0.3-fold vs. vehicle 1.8 ± 0.8-fold, n = 6, P = 0.010) in incised tissues. Intraplantar injection of nerve growth factor increased the expression of ASIC3 and phosphorylated protein kinase B. ASIC3 expression and incisional pain-related behaviors were inhibited by pretreatment with the phosphoinositide 3-kinase inhibitor LY294002.

CONCLUSIONS

Acid-sensing ion channel 3 overexpression in incisions contributes to postoperative guarding and mechanical nociception. Bidirectional transport of ASIC3 between incised tissues and dorsal root ganglion neurons occurs through the sciatic nerve. Nerve growth factor regulates ASIC3 expression after plantar incision through the phosphoinositide 3-kinase/protein kinase B signaling pathway.

EDITOR’S PERSPECTIVE

A Thermal Place Preference Test for Discovery of Neuropathic Pain Drugs

Developing potent non-opioid pain medications is an integral part of the battle to conquer both chronic pain and the current opioid crisis. Although most screening approaches use in vitro surrogate targets, in vivo screening of analgesic candidates is a necessary preclinical step in drug discovery. Here, we report the design of a new automated behavioral testing apparatus based on the principle of a thermal place preference test (TPPT). This new design can detect, quantify, and differentiate behavioral responses to cold stimuli between sham and chronic constriction injury (CCI) rodents with up to 12 animals tested simultaneously. At an optimized temperature pair of 12.5 °C vs 30.0 °C (±0.5 °C), the TPPT design has captured the antinociceptive effects of morphine and pregabalin on CCI rats in individual 10 min tests. Moreover, it can differentiate analgesic effects by morphine or pregabalin from anxiolytic effects by diazepam. The results, along with the relatively low cost to construct the apparatus and moderately high throughput, make our TPPT design applicable for behavioral studies of chronic pain in rodents and for high-throughput in vivo screening of the next generation of pain medications.

AAAPT Diagnostic Criteria for Acute Pain Following Breast Surgery

Acute pain after breast surgery decreases the quality of life of cancer survivors. Previous studies using a variety of definitions and methods report prevalence rates between 10% and 80%, which suggests the need for a comprehensive framework that can be used to guide assessment of acute pain and pain-related outcomes after breast surgery. A multidisciplinary task force with clinical and research expertise performed a focused review and synthesis and applied the 5 dimensional framework of the AAAPT (Analgesic, Anesthetic, and Addiction Clinical Trial Translations, Innovations, Opportunities, and Networks [ACTTION], American Academy of Pain Medicine [AAPM], American Pain Society [APS] Pain Taxonomy) to acute pain after breast surgery. Application of the AAAPT taxonomy yielded the following: 1) Core Criteria: Location, timing, severity, and impact of breast surgery pain were defined; 2) Common Features: Character and expected trajectories were established in relevant surgical subgroups, and common pain assessment tools for acute breast surgery pain identified; 3) Modulating Factors: Biological, psychological, and social factors that modulate interindividual variability were delineated; 4) Impact/Functional Consequences: Domains of impact were outlined and defined; 5) Neurobiologic Mechanisms: Putative mechanisms were specified ranging from nerve injury, inflammation, peripheral and central sensitization, to affective and social processing of pain. PERSPECTIVE: The AAAPT provides a framework to define and guide improved assessment of acute pain after breast surgery, which will enhance generalizability of results across studies and facilitate meta-analyses and studies of interindividual variation, and underlying mechanism. It will allow researchers and clinicians to better compare between treatments, across institutions, and with other types of acute pain.

Indirect AMP-Activated Protein Kinase Activators Prevent Incision-Induced Hyperalgesia and Block Hyperalgesic Priming, Whereas Positive Allosteric Modulators Block Only Priming in Mice

AMP-activated protein kinase (AMPK) is a multifunctional kinase that negatively regulates the mechanistic target of rapamycin (mTOR) and mitogen-activated protein kinase (MAPK) signaling, two signaling pathways linked to pain promotion after injury, such as surgical incision. AMPK can be activated directly using positive allosteric modulators, as well as indirectly through the upregulation of upstream kinases, such as liver kinase B1 (LKB1), which is a mechanism of action of metformin. Metformin’s antihyperalgesic effects occur only in male mice, raising questions about how metformin regulates pain sensitivity. We used metformin and other structurally distinct AMPK activators narciclasine (NCLS), ZLN-024, and MK8722, to treat incision-induced mechanical hypersensitivity and hyperalgesic priming in male and female mice. Metformin was the only AMPK activator to have sex-specific effects. We also found that indirect AMPK activators metformin and NCLS were able to reduce mechanical hypersensitivity and block hyperalgesic priming, whereas direct AMPK activators ZLN-024 and MK8722 only blocked priming. Direct and indirect AMPK activators stimulated AMPK in dorsal root ganglion (DRG) neuron cultures to a similar degree; however, incision decreased phosphorylated AMPK (p-AMPK) in DRG. Because AMPK phosphorylation is required for kinase activity, we interpret our findings as evidence that indirect AMPK activators are more effective for treating pain hypersensitivity after incision because they can drive increased p-AMPK through upstream kinases like LKB1. These findings have important implications for the development of AMPK-targeting therapeutics for pain treatment.

Oxycodone regulates incision-induced activation of neurotrophic factors and receptors in an acute post-surgery pain rat model

Background

Oxycodone, which is one of the most commonly used opiates in postoperative pain management, has a different affinity for μ-opioid receptors (MOR), κ-opioid receptors (KOR), and δ-opioid receptors (DOR). Accumulating research has suggested that neurotrophins (NTs) are involved in opioid analgesia. In the current exploratory study, we aimed to investigate the underlying mechanisms of the analgesic effects of oxycodone on post-surgery pain in rats and to determine whether neurotrophic factors and receptors were involved in these effects.

Methods

Mechanical and thermal sensitivity tests were used to evaluate the validity of the postoperative pain rat model and to determine the analgesic effect of oxycodone. Quantitative PCR and Western blot analysis were used to detect the changes in the expression of three types of opioid receptors and NTs and their high-affinity receptors in the spinal cord after surgery and oxycodone administration.

Results

Oxycodone showed an analgesic effect on plantar incision (PI)-induced hyperalgesia, especially thermal hyperalgesia. We detected an obvious increase in MOR expression levels but insignificant changes in KOR and DOR levels in the spinal cord after PI. Moreover, we found that oxycodone was able to reverse the increased expression of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), tyrosine kinase receptor (TrK) A, and TrkB and the decreased expression of NT-3 and TrkC, after PI. Pretreatment with oxycodone also altered the expression of these mediators.

Conclusion

Based on the results, possible underlying mechanisms for the antinociceptive properties of oxycodone in acute postoperative pain include the activation of MOR downstream signaling and the regulation of NTs and receptor expression through attenuation of glial activation and fortification of antinociceptive mediators in the spinal cord. This study may provide new insights into the molecular mechanisms underlying the analgesic action of oxycodone.

Persistent Postsurgical Pain

The development of chronic pain is considered a major complication after surgery. Basic science research in animal models helps us understand the transition from acute to chronic pain by identifying the numerous molecular and cellular changes that occur in the peripheral and central nervous systems. It is now well recognized that inflammation and nerve injury lead to long-term synaptic plasticity that amplifies and also maintains pain signaling, a phenomenon referred to as pain sensitization. In the context of surgery in humans, pain sensitization is both responsible for an increase in postoperative pain via the expression of wound hyperalgesia and considered a critical factor for the development of persistent postsurgical pain. Using specific drugs that block the processes of pain sensitization reduces postoperative pain and prevents the development of persistent postoperative pain. This narrative review of the literature describes clinical investigations evaluating different preventative pharmacologic strategies that are routinely used by anesthesiologists in their daily clinical practices for preventing persistent postoperative pain. Nevertheless, further efforts are needed in both basic and clinical science research to identify preclinical models and novel therapeutics targets. There remains a need for more patient numbers in clinical research, for more reliable data, and for the development of the safest and the most effective strategies to limit the incidence of persistent postoperative pain.

Systemic Depletion of Nerve Growth Factor Inhibits Disease Progression in a Genetically Engineered Model of Pancreatic Ductal Adenocarcinoma

OBJECTIVES

In patients with pancreatic ductal adenocarcinoma (PDAC), increased expression of proinflammatory neurotrophic growth factors (eg, nerve growth factor [NGF]) correlates with a poorer prognosis, perineural invasion, and, with regard to NGF, pain severity. We hypothesized that NGF sequestration would reduce inflammation and disease in the KPC mouse model of PDAC.

METHODS

Following biweekly injections of NGF antibody or control immunoglobulin G, beginning at 4 or 8 weeks of age, inflammation and disease stage were assessed using histological, protein expression, and quantitative polymerase chain reaction analyses.

RESULTS

In the 8-week anti-NGF group, indicators of neurogenic inflammation in the dorsal root ganglia (substance P and calcitonin gene-related peptide) and spinal cord (glial fibrillary acidic protein) were significantly reduced. In the 4-week anti-NGF group, TRPA1 mRNA in dorsal root ganglia and spinal phosphorylated ERK protein were elevated, but glial fibrillary acidic protein expression was unaffected. In the 8-week anti-NGF group, there was a 40% reduction in the proportion of mice with microscopic perineural invasion, and no macrometastases were observed.

CONCLUSIONS

Anti-NGF treatment beginning at 4 weeks may increase inflammation and negatively impact disease. Treatment starting at 8 weeks (after disease onset), however, reduces neural inflammation, neural invasion, and metastasis. These data indicate that NGF impacts PDAC progression and metastasis in a temporally dependent manner.

Inhibition of Poly(A)-binding protein with a synthetic RNA mimic reduces pain sensitization in mice

Nociceptors rely on cap-dependent translation to rapidly induce protein synthesis in response to pro-inflammatory signals. Comparatively little is known regarding the role of the regulatory factors bound to the 3' end of mRNA in nociceptor sensitization. Poly(A)-binding protein (PABP) stimulates translation initiation by bridging the Poly(A) tail to the eukaryotic initiation factor 4F complex associated with the mRNA cap. Here, we use unbiased assessment of PABP binding specificity to generate a chemically modified RNA-based competitive inhibitor of PABP. The resulting RNA mimic, which we designated as the Poly(A) SPOT-ON, is more stable than unmodified RNA and binds PABP with high affinity and selectivity in vitro. We show that injection of the Poly(A) SPOT-ON at the site of an injury can attenuate behavioral response to pain. Collectively, these results suggest that PABP is integral for nociceptive plasticity. The general strategy described here provides a broad new source of mechanism-based inhibitors for RNA-binding proteins and is applicable for in vivo studies.

Nerve growth factor induces facial heat hyperalgesia and plays a role in trigeminal neuropathic pain in rats

There is preclinical evidence that nerve growth factor (NGF) contributes toward inflammatory hyperalgesia in the orofacial region, but the mechanisms underlying its hyperalgesic effect as well as its role in trigeminal neuropathic pain require further investigation. This study investigated the ability of NGF to induce facial heat hyperalgesia and the involvement of tyrosine kinase receptor A, transient receptor potential vanilloid 1, and mast cells in NGF pronociceptive effects. In addition, the role of NGF in heat hyperalgesia in a model of trigeminal neuropathic pain was evaluated. NGF injection into the upper lip of naive rats induced long-lasting heat hyperalgesia. Pretreatment with an antibody anti-NGF, antagonists of tyrosine kinase receptor A, and transient receptor potential vanilloid 1 receptors or compound 48/80, to induce mast-cell degranulation, all attenuated NGF-induced hyperalgesia. In a rat model of trigeminal neuropathic pain, local treatment with anti-NGF significantly reduced heat hyperalgesia. In addition, increased NGF levels were detected in the ipsilateral infraorbital nerve branch at the time point that represents the peak of heat hyperalgesia. The results suggest that NGF is a prominent hyperalgesic mediator in the trigeminal system and it may represent a potential therapeutic target for the management of painful orofacial conditions, including trigeminal neuropathic pain.

Adenosine Monophosphate-activated Protein Kinase (AMPK) Activators For the Prevention, Treatment and Potential Reversal of Pathological Pain

Pathological pain is an enormous medical problem that places a significant burden on patients and can result from an injury that has long since healed or be due to an unidentifiable cause. Although treatments exist, they often either lack efficacy or have intolerable side effects. More importantly, they do not reverse the changes in the nervous system mediating pathological pain, and thus symptoms often return when therapies are discontinued. Consequently, novel therapies are urgently needed that have both improved efficacy and disease-modifying properties. Here we highlight an emerging target for novel pain therapies, adenosine monophosphate-activated protein kinase (AMPK). AMPK is capable of regulating a variety of cellular processes including protein translation, activity of other kinases, and mitochondrial metabolism, many of which are thought to contribute to pathological pain. Consistent with these properties, preclinical studies show positive, and in some cases disease-modifying effects of either pharmacological activation or genetic regulation of AMPK in models of nerve injury, chemotherapy-induced peripheral neuropathy (CIPN), postsurgical pain, inflammatory pain, and diabetic neuropathy. Given the AMPK-activating ability of metformin, a widely prescribed and well-tolerated drug, these preclinical studies provide a strong rationale for both retrospective and prospective human pain trials with this drug. They also argue for the development of novel AMPK activators, whether orthosteric, allosteric, or modulators of events upstream of the kinase. Together, this review will present the case for AMPK as a novel therapeutic target for pain and will discuss future challenges in the path toward development of AMPK-based pain therapeutics.

Pharmacological activation of AMPK inhibits incision-evoked mechanical hypersensitivity and the development of hyperalgesic priming in mice

New therapeutics to manage post-surgical pain are needed to mitigate the liabilities of opioid and other analgesics. Our previous work shows that key modulators of excitability in peripheral nociceptors, such as extracellular signal-regulated kinases (ERK) are inhibited by activation of adenosine monophosphate activated protein kinase (AMPK). We hypothesized that AMPK activation would attenuate acute incision-evoked mechanical hypersensitivity and the development of hyperalgesic priming caused by surgery in mice. Here we have used a variety of administration routes and combinations of AMPK activators to test this hypothesis. Topical administration of a resveratrol-based cream inhibited acute mechanical hypersensitivity evoked by incision and blocked the development of hyperalgesic priming. We also observed that systemic administration of metformin dose-dependently inhibited incision-evoked mechanical hypersensitivity and hyperalgesic priming. Interestingly, low doses of systemic metformin and local resveratrol that had no acute effect were able to mitigate development of hyperalgesic priming. Combined treatment with doses of systemic metformin and local resveratrol that were not effective on their own enhanced the acute efficacy of the individual AMPK activators for post-surgical mechanical pain alleviation and blocked the development of hyperalgesic priming. Finally, we used dorsal root ganglion (DRG) neurons in culture to show that resveratrol and metformin given in combination shift the concentration-response curve for AMPK activation to the left and increase the magnitude of AMPK activation. Therefore, we find that topical administration is an effective treatment route of administration and combining systemic and local treatments led to anti-nociceptive efficacy in acute mechanical hypersensitivity at doses that were not effective alone. Collectively our work demonstrates a specific effect of AMPK activators on post-surgical pain and points to novel therapeutic opportunities with potential immediate impact in the clinical setting.

Postoperative pain-from mechanisms to treatment

INTRODUCTION

Pain management after surgery continues to be suboptimal; there are several reasons including lack of translation of results from basic science studies and scientific clinical evidence into clinical praxis.

OBJECTIVES

This review presents and discusses basic science findings and scientific evidence generated within the last 2 decades in the field of acute postoperative pain.

METHODS

In the first part of the review, we give an overview about studies that have investigated the pathophysiology of postoperative pain by using rodent models of incisional pain up to July 2016. The second focus of the review lies on treatment recommendations based on guidelines and clinical evidence, eg, by using the fourth edition of the "Acute Pain Management: Scientific Evidence" of the Australian and New Zealand College of Anaesthetists and Faculty of Pain Medicine.

RESULTS

Preclinical studies in rodent models characterized responses of primary afferent nociceptors and dorsal horn neurons as one neural basis for pain behavior including resting pain, hyperalgesia, movement-evoked pain or anxiety- and depression-like behaviors after surgery. Furthermore, the role of certain receptors, mediators, and neurotransmitters involved in peripheral and central sensitization after incision were identified; many of these are very specific, relate to some modalities only, and are unique for incisional pain. Future treatment should focus on these targets to develop therapeutic agents that are effective for the treatment of postoperative pain as well as have few side effects. Furthermore, basic science findings translate well into results from clinical studies. Scientific evidence is able to point towards useful (and less useful) elements of multimodal analgesia able to reduce opioid consumption, improve pain management, and enhance recovery.

CONCLUSION

Understanding basic mechanisms of postoperative pain to identify effective treatment strategies may improve patients' outcome after surgery.

Prevention of Chronic Post-Thoracotomy Pain in Rats By Intrathecal Resolvin D1 and D2: Effectiveness of Perioperative and Delayed Drug Delivery

Thoracotomy results in a high frequency of chronic postoperative pain. Resolvins are endogenous molecules, synthesized and released by activated immune cells, effective against inflammatory and neuropathic pain. Different resolvins have differential actions on selective neuronal and glial receptors and enzymes. This article examines the ability of intrathecal resolvin D1 and resolvin D2 to reduce chronic post-thoracotomy pain in rats. Thoracotomy, involving intercostal incision and rib retraction, resulted in a decrease in the mechanical force threshold to induce nocifensive behavior, an enlargement of the pain-sensitive area, and an increase in the fraction of rats showing nocifensive behavior, all for at least 5 weeks. The qualitative nature of the behavioral responses to tactile stimulation changed dramatically after thoracotomy, including the appearance of vigorous behaviors, such as turning, shuddering, and squealing, all absent in naive rats. Intrathecal delivery of resolvin D1 (30 ng/30 μL), at surgery or 4 days later, halved the spread of the mechanosensitive area, lowered by 60% the percent of rats with tactile hypersensitivity, and reduced the drop in threshold for a nocifensive response, along with a reduction in the occurrence of vigorous nocifensive responses. Resolvin D2's actions on threshold changes were statistically the same. These findings suggest that intrathecal resolvins, delivered preoperatively or several days later, can prevent chronic postoperative hyperalgesia.

PERSPECTIVE

In studies of rats, the injection of the proresolving compounds of the resolvin-D series into spinal fluid, before or just after thoracotomy surgery, prevents the occurrence of acute and chronic pain. If these chemicals, which have shown no side-effects, were used in humans it might greatly reduce chronic postoperative pain.

The potent, indirect adenosine monophosphate- activated protein kinase activator R419 attenuates mitogen-activated protein kinase signaling, inhibits nociceptor excitability, and reduces pain hypersensitivity in mice

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There is a great need for new therapeutics for the treatment of pain. A possible avenue to development of such therapeutics is to interfere with signaling pathways engaged in peripheral nociceptors that cause these neurons to become hyperexcitable. There is strong evidence that mitogen-activated protein kinases and phosphoinositide 3-kinase (PI3K)/mechanistic target of rapamycin signaling pathways are key modulators of nociceptor excitability in vitro and in vivo. Activation of adenosine monophosphate-activated protein kinase (AMPK) can inhibit signaling in both of these pathways, and AMPK activators have been shown to inhibit nociceptor excitability and pain hypersensitivity in rodents. R419 is one of, if not the most potent AMPK activator described to date. We tested whether R419 activates AMPK in dorsal root ganglion (DRG) neurons and if this leads to decreased pain hypersensitivity in mice. We find that R419 activates AMPK in DRG neurons resulting in decreased mitogen-activated protein kinase signaling, decreased nascent protein synthesis, and enhanced P body formation. R419 attenuates nerve growth factor (NGF)-induced changes in excitability in DRG neurons and blocks NGF-induced mechanical pain amplification in vivo. Moreover, locally applied R419 attenuates pain hypersensitivity in a model of postsurgical pain and blocks the development of hyperalgesic priming in response to both NGF and incision. We conclude that R419 is a promising lead candidate compound for the development of potent and specific AMPK activation to inhibit pain hypersensitivity as a result of injury.

Cutaneous tissue damage induces long-lasting nociceptive sensitization and regulation of cellular stress- and nerve injury-associated genes in sensory neurons

Tissue damage is one of the major etiological factors in the emergence of chronic/persistent pain, although mechanisms remain enigmatic. Using incision of the back skin of adult rats as a model for tissue damage, we observed sensitization in a nociceptive reflex enduring to 28days post-incision (DPI). To determine if the enduring behavioral changes corresponded with a long-term impact of tissue damage on sensory neurons, we examined the temporal expression profile of injury-regulated genes and the electrophysiological properties of traced dorsal root ganglion (DRG) sensory neurons. The mRNA for the injury/stress-hub gene Activating Transcription Factor 3 (ATF3) was upregulated and peaked within 4 DPI, after which levels declined but remained significantly elevated out to 28 DPI, a time when the initial incision appears healed and tissue-inflammation largely resolved. Accordingly, stereological image analysis indicated that some neurons expressed ATF3 only transiently (mostly medium-large neurons), while in others it was sustained (mostly small neurons), suggesting cell-type-specific responses. In retrogradely-traced ATF3-expressing neurons, Calcium/calmodulin-dependent protein kinase type IV (CAMK4) protein levels and isolectin-B4 (IB4)-binding were suppressed whereas Growth Associated Protein-43 (GAP-43) and Neuropeptide Y (NPY) protein levels were enhanced. Electrophysiological recordings from DiI-traced sensory neurons 28 DPI showed a significant sensitization limited to ATF3-expressing neurons. Thus, ATF3 expression is revealed as a strong predictor of single cells displaying enduring pain-related electrophysiological properties. The cellular injury/stress response induced in sensory neurons by tissue damage and indicated by ATF3 expression is positioned to contribute to pain which can occur after tissue damage.

Targeting AMPK for the Alleviation of Pathological Pain

Chronic pain is a major clinical problem that is poorly treated with available therapeutics. Adenosine monophosphate-activated protein kinase (AMPK) has recently emerged as a novel target for the treatment of pain with the exciting potential for disease modification. AMPK activators inhibit signaling pathways that are known to promote changes in the function and phenotype of peripheral nociceptive neurons and promote chronic pain. AMPK activators also reduce the excitability of these cells suggesting that AMPK activators may be efficacious for the treatment of chronic pain disorders, like neuropathic pain, where changes in the excitability of nociceptors is thought to be an underlying cause. In agreement with this, AMPK activators have now been shown to alleviate pain in a broad variety of preclinical pain models indicating that this mechanism might be engaged for the treatment of many types of pain in the clinic. A key feature of the effect of AMPK activators in these models is that they can lead to a long-lasting reversal of pain hypersensitivity even long after treatment cessation, indicative of disease modification. Here, we review the evidence supporting AMPK as a novel pain target pointing out opportunities for further discovery that are likely to have an impact on drug discovery efforts centered around potent and specific allosteric activators of AMPK for chronic pain treatment.

Spontaneous pain-like behaviors are more sensitive to morphine and buprenorphine than mechanically evoked behaviors in a rat model of acute postoperative pain

BACKGROUND

Nonevoked spontaneous pain is most problematic for postoperative patients. Physicians assess this form of pain using the human visual analog scale or verbal numeric rating scale. Recent studies have proposed that spontaneous foot-lifting (SFL) behaviors are an expression of spontaneous pain in animals after spinal nerve injury or adjuvant-induced inflammation. In the current study, we characterize SFL behaviors in a rat model of acute postoperative pain, which includes comparisons with evoked behaviors to analgesic treatments.

METHODS

SFL was manually recorded over four 5-minute periods with 10-minute intervals between each testing session. Paw-withdrawal thresholds were subsequently measured with an electronic von Frey esthesiometer. To evaluate the effects of age, rats were tested in different age groups: 2, 7, and >26 months. The effects of buprenorphine and morphine were tested in a separate group of animals, which received intraperitoneal injections of saline, morphine (0.01, 0.1, 1, or 2 mg/kg), or buprenorphine (0.001, 0.01, or 0.1 mg/kg) before testing.

RESULTS

SFL behaviors peaked at 3 hours after incision and significantly recovered by the 3rd or 4th postoperative day (P < 0.0001). The presentation of these behaviors did not significantly vary with animal age (2, 7, and >26 months old; P = 0.30). SFL behaviors, with the exception of rapid SFL at 3 hours after incision, did not show significant correlation with paw-withdrawal threshold behaviors. The median effective dose of buprenorphine for reversal of mechanical hyperalgesia (0.0452 mg/kg; 95% CI, 0.0259-0.0787) was significantly larger than for reversing rapid (0.0027 mg/kg; 95% CI, 0.0009-0.0083; P < 0.0001) and prolonged (0.0004 mg/kg, 95% CI, 0.0000, 0.0035; P = 0.001) SFL at 3 hours after incision. Similarly, the median effective dose of morphine for reversal of mechanical hypersensitivity behaviors (2.901 mg/kg; 95% CI, 1.132-7.436) was larger than for SFL count (0.4044 mg/kg; 95% CI, 0.1048-1.561; P = 0.0103) and SFL duration (0.0309 mg/kg; 95% CI, 0.0095-0.0998; P < 0.0001) at 3 hours after incision.

CONCLUSIONS

The present study demonstrates that a hindpaw plantar incision induces SFL behaviors in rats and that these behaviors have higher bioassay sensitivity to analgesic interventions with morphine and buprenorphine compared with mechanically evoked behaviors.

Role of somatostatin and somatostatin receptor type 2 in postincisional nociception in rats

Somatostatin (SST) and the somatostatin receptor type 2 (sstr2) are expressed in the superficial part (Laminae I-III) of the dorsal horn of the spinal cord. Since the neurons in these laminae also receive nociceptive sensation from the periphery, it was hypothesized that both SST and sstr2 could be involved in the modulation of nociceptive transmission. To the best of knowledge, there are no studies on the involvement of SST and sstr2 in hind paw incision model in rats, which mimics postoperative pain in humans. Sprague-Dawley rats were subjected to hind paw incision under isoflurane anaesthesia and the resulting mechanical allodynia and thermal hyperalgesia were evaluated for 5 days. In another set of animals, the spinal cord was isolated at specified time intervals after incision and examined for SST and sstr2 expression using immunohistochemistry and immunoblotting procedures. Finally, nociceptive parameters were again evaluated in incised rats, which had received SST (400 µg/kg i.p. three times per day). Blood glucose level and locomotor activity were determined after SST treatment. Both allodynia and hyperalgesia were highest immediately after incision. Spinal SST expression increased at 2 h. A further increase was noted on day 3. Expression of sstr2 increased initially but decreased at day 1. These changes could be due to exocytosis of SST and internalization of the ligand-receptor complex. SST injection significantly attenuated mechanical allodynia but not thermal hyperalgesia. Significant change in blood glucose level or locomotor activity was absent. SST appears to contribute to postincisional pain. This finding could be of clinical relevance.

Translational control of chronic pain

Pain is a crucial physiological response to injury and pathologies. The development and maintenance of pain requires the expression of novel genes. The expression of such genes occurs in highly regulated and orchestrated manner where protein translation provides an exquisite temporal and spatial fidelity within the axons and dendrites of neurons. Signaling pathways that regulate local translation are activated by cytokines, neurotrophic factors, or neurotransmitters, which are released either due to tissue damage or neuronal activity. In recent years, the ERK and mTOR pathways have been demonstrated to be central in regulating local translation in neurons of both the peripheral and central nervous systems in diverse models of chronic pain. The ERK and mTOR pathways converge onto the cap-dependent translational machinery that regulates genes essential for the development of nociceptive sensitization. Moreover, inhibition of these pathways has proved to be effective in normalizing the biochemical changes and the associated pain in various preclinical models.

TRPV1 antagonist attenuates postoperative hypersensitivity by central and peripheral mechanisms

BACKGROUND

Acute postoperative pain is one of the frequent reasons for pain treatment. However, the exact mechanisms of its development are still not completely clear. Transient receptor potential vanilloid 1 (TRPV1) receptors are involved in nociceptive signaling in various hypersensitive states. Here we have investigated the contribution of TRPV1 receptors expressed on cutaneous peripheral nociceptive fibers and in the spinal cord on the development and maintenance of hypersensitivity to thermal and mechanical stimuli following surgical incision. A rat plantar incision model was used to test paw withdrawal responses to thermal and mechanical stimuli. The effect of the TRPV1 receptor antagonist SB366791 was investigated 1) by intrathecal injection 15 min before incision and 2) intradermal injection before (30 min) and immediately after the surgery. Vehicle-injected rats and naïve animals treated identically were used as controls.

RESULTS

Plantar incision induced mechanical allodynia and hyperalgesia and thermal hyperalgesia. A single intrathecal administration of SB366791 significantly reduced postincisional thermal hyperalgesia and also attenuated mechanical allodynia, while mechanical hyperalgesia remained unaffected. Local intradermal SB366791 treatment reduced thermal hyperalgesia and mechanical allodynia without affecting mechanical hyperalgesia.

CONCLUSIONS

Our experiments suggest that both peripheral and spinal cord TRPV1 receptors are involved in increased cutaneous sensitivity following surgical incision. The analgesic effect of the TRPV1 receptor antagonist was especially evident in the reduction of thermal hyperalgesia. The activation of TRPV1 receptors represents an important mechanism in the development of postoperative hypersensitivity.

Nerve growth factor mediates a switch in intracellular signaling for PGE2-induced sensitization of sensory neurons from protein kinase A to Epac

We examined whether nerve growth factor (NGF), an inflammatory mediator that contributes to chronic hypersensitivity, alters the intracellular signaling that mediates the sensitizing actions of PGE2 from activation of protein kinase A (PKA) to exchange proteins directly activated by cAMP (Epacs). When isolated sensory neurons are grown in the absence of added NGF, but not in cultures grown with 30 ng/ml NGF, inhibiting protein kinase A (PKA) activity blocks the ability of PGE2 to augment capsaicin-evoked release of the neuropeptide CGRP and to increase the number of action potentials (APs) evoked by a ramp of current. Growing sensory neurons in culture in the presence of increasing concentrations of NGF increases the expression of Epac2, but not Epac1. An intradermal injection of complete Freund's adjuvant into the rat hindpaw also increases the expression of Epac2, but not Epac1 in the dorsal root ganglia and spinal cord: an effect blocked by intraplantar administration of NGF antibodies. Treating cultures grown in the presence of 30 ng/ml NGF with Epac1siRNA significantly reduced the expression of Epac1, but not Epac2, and did not block the ability of PGE2 to augment capsaicin-evoked release of CGRP from sensory neurons. Exposing neuronal cultures grown in NGF to Epac2siRNAreduced the expression of Epac2, but not Epac1 and prevented the PGE2-induced augmentation of capsaicin and potassium-evoked CGRP release in sensory neurons and the PGE2-induced increase in the number of APs generated by a ramp of current. In neurons grown with no added NGF, Epac siRNAs did not attenuate PGE2-induced sensitization. These results demonstrate that NGF, through increasing Epac2 expression, alters the signaling cascade that mediates PGE2-induced sensitization of sensory neurons, thus providing a novel mechanism for maintaining PGE2-induced hypersensitivity during inflammation.

Direct intrawound administration of dimethylsulphoxide relieves acute pain in rats

Wounds associated with injuries such as burns can produce moderate to severe pain. Besides causing distress to the patient, unrelieved pain could delay healing owing to stress-related problems. Thus, pain needs to be treated as early as possible after injury. It was hypothesised that local treatment of wounds with appropriate analgesic drugs could attenuate pain. HOE 140, a bradykinin receptor antagonist, reduced acute inflammatory pain in rats after intrawound administration. In this study, the analgesic effect of dimethylsulphoxide (DMSO) was investigated in a similar hind-paw incision model in rats. An extremely small quantity (10 µl) of 100% DMSO was administered into the incision site just before closure of the wound. It persistently attenuated guarding behaviour in rats over a period of 3 days without affecting thermal hyperalgesia or allodynia. Accumulated evidence indicates that guarding is equivalent to pain at rest in humans. The possible mechanisms of the analgesic effect could be inhibition of C group of peripheral nerve fibres or even free radical scavenging. Healing of the wound was found to be normal at the end of the study period. In conclusion, DMSO could be useful in the treatment of acute pain resulting from tissue injuries such as burns.

The neurobiology of skeletal pain

Disorders of the skeleton are one of the most common causes of chronic pain and long-term physical disability in the world. Chronic skeletal pain is caused by a remarkably diverse group of conditions including trauma-induced fracture, osteoarthritis, osteoporosis, low back pain, orthopedic procedures, celiac disease, sickle cell disease and bone cancer. While these disorders are diverse, what they share in common is that when chronic skeletal pain occurs in these disorders, there are currently few therapies that can fully control the pain without significant unwanted side effects. In this review we focus on recent advances in our knowledge concerning the unique population of primary afferent sensory nerve fibers that innervate the skeleton, the nociceptive and neuropathic mechanisms that are involved in driving skeletal pain, and the neurochemical and structural changes that can occur in sensory and sympathetic nerve fibers and the CNS in chronic skeletal pain. We also discuss therapies targeting nerve growth factor or sclerostin for treating skeletal pain. These therapies have provided unique insight into the factors that drive skeletal pain and the structural decline that occurs in the aging skeleton. We conclude by discussing how these advances have changed our understanding and potentially the therapeutic options for treating and/or preventing chronic pain in the injured, diseased and aged skeleton.

Tissue injury and related mediators of pain exacerbation

Tissue injury and inflammation result in release of various mediators that promote ongoing pain or pain hypersensitivity against mechanical, thermal and chemical stimuli. Pro-nociceptive mediators activate primary afferent neurons directly or indirectly to enhance nociceptive signal transmission to the central nervous system. Excitation of primary afferents by peripherally originating mediators, so-called “peripheral sensitization”, is a hallmark of tissue injury-related pain. Many kinds of pro-nociceptive mediators, including ATP, glutamate, kinins, cytokines and tropic factors, synthesized at the damaged tissue, contribute to the development of peripheral sensitization. In the present review we will discuss the molecular mechanisms of peripheral sensitization following tissue injury.

Modulation of neurotrophin signaling by monoclonal antibodies

The neurotrophin family is comprised of the structurally related secreted proteins nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophine-4 (NT-4). They bind and activate the tyrosine kinase receptors Trk A, B, and C in a ligand-specific manner and additionally bind a shared p75NTR receptor. The neurotrophins were originally defined by their ability to support the survival and maturation of embryonic neurons. However, they also control important physiological functions of the adult nervous system including learning and memory, sensation, and energy homeostasis. For example, NGF/trkA signaling is critical for normal and pathological sensation of pain. Likewise, the BDNF/trkB pathway controls feeding and metabolism, and its dysfunction leads to severe obesity. Antibodies can modulate neurotrophin signaling. Thus, NGF blocking agents can attenuate pain in several animal models, and a recombinant humanized NGF blocking antibody (Tanezumab) has shown promising results in human clinical trials for osteoarthritic pain. On the other hand trkB agonist antibodies can modulate food intake and body weight in rodents and nonhuman primates. The power of monoclonal antibodies to modulate neurotrophin signaling promises to turn the rich biological insights into novel human medicines.

Temporal mismatch between pain behaviour, skin Nerve Growth factor and intra-epidermal nerve fibre density in trigeminal neuropathic pain

BACKGROUND

The neurotrophin Nerve Growth factor (NGF) is known to influence the phenotype of mature nociceptors, for example by altering synthesis of neuropeptides, and changes in NGF levels have been implicated in the pathophysiology of chronic pain conditions such as neuropathic pain. We have tested the hypothesis that after partial nerve injury, NGF accumulates within the skin and causes 'pro-nociceptive' phenotypic changes in the remaining population of sensory nerve fibres, which could underpin the development of neuropathic pain.

RESULTS

Eleven days after chronic constriction injury of the rat mental nerve the intra-epidermal nerve fibre density of the chin skin from had reduced from 11.6 ± 4.9 fibres/mm to 1.0 ± 0.4 fibres/mm; this slowly recovered to 2.4 ± 2.0 fibres/mm on day 14 and 4.0 ± 0.8 fibres/mm on day 21. Cold hyperalgesia in the ipsilateral lower lip was detectable 11 days after chronic constriction injury, although at this time skin [NGF] did not differ between sides. At 14 days post-injury, there was a significantly greater [NGF] ipsilaterally compared to contralaterally (ipsilateral = 111 ± 23 pg/mg, contralateral = 69 ± 13 pg/mg), but there was no behavioural evidence of neuropathic pain at this time-point. By 21 days post-injury, skin [NGF] was elevated bilaterally and there was a significant increase in the proportion of TrkA-positive (the high-affinity NGF receptor) intra-epidermal nerve fibres that were immunolabelled for the neuropeptide Calcitonin Gene-related peptide.

CONCLUSIONS

The temporal mismatch in behaviour, skin [NGF] and phenotypic changes in sensory nerve fibres indicate that increased [NGF] does not cause hyperalgesia after partial mental nerve injury, although it may contribute to the altered neurochemistry of cutaneous nerve fibres.

The Qualitative Hyperalgesia Profile: A New Metric to Assess Chronic Post-Thoracotomy Pain

Thoracotomy often results in chronic pain, characterized by resting pain and elevated mechano-sensitivity. This paper defines complex behavioral responses to tactile stimulation in rats after thoracotomy, shown to be reversibly relieved by systemic morphine, in order to develop a novel qualitative "pain" score. A deep incision and 1 hour of rib retraction in male Sprague-Dawley rats resulted in reduced threshold and a change in the locus of greatest tactile (von Frey filament) sensitivity, from the lower back to a more rostral location around the wound site, and extending bilaterally. The fraction of rats showing nocifensive responses to mild stimulation (10 gm) increased after thoracotomy (from a pre-operative value of 0/10 to 8/10 at 10 days post-op), and the average threshold decreased correspondingly, from 15 gm to ∼4 gm. The nature of the nocifensive responses to tactile stimulation, composed pre-operatively only of no response (Grade 0) or brief contractions of the local subcutaneous muscles (Grade I), changed markedly after thoracotomy, with the appearance of new behaviors including a brisk lateral "escape" movement and/or a 180° rotation of the trunk (both included as Grade II), and whole body shuddering, and scratching and squealing (Grade III). Systemic morphine (2.5 mg/kg, i.p.) transiently raised the threshold for response and reduced the frequency of Grade II and III responses, supporting the interpretation that these represent pain. The findings support the development of a Qualitative Hyperalgesic Profile to assess the complex behavior that indicates a central integration of hyperalgesia.

AMPK: An emerging target for modification of injury-induced pain plasticity

Chronic pain is a critical medical problem afflicting hundreds of millions of people worldwide with costly effects on society and health care systems. Novel therapeutic avenues for the treatment of pain are needed that are directly targeted to the molecular mechanisms that promote and maintain chronic pain states. Recent evidence suggests that peripheral pain plasticity is promoted and potentially maintained via changes in translation control that are mediated by mTORC1 and MAPK pathways. While these pathways can be targeted individually, stimulating the AMPK pathway with direct or indirect activators achieves inhibition of these pathways via engagement of a single kinase. Here we review the form, function and pharmacology of AMPK with special attention to its emerging role as a potential target for pain therapeutics. We present the existing evidence supporting a role of AMPK activation in alleviating symptoms of peripheral nerve injury- and incision-induced pain plasticity and the blockade of the development of chronic pain following surgery. We argue that these preclinical findings support a strong rationale for clinical trials of currently available AMPK activators and further development of novel pharmacological strategies for more potent and efficacious manipulation of AMPK in the clinical setting. Finally, we posit that AMPK represents a unique opportunity for drug development in the kinase area for pain because it is pharmacologically manipulated via activation rather than inhibition potentially offering a wider therapeutic window with interesting additional pharmacological opportunities. Altogether, the physiology, pharmacology and therapeutic opportunities surrounding AMPK make it an attractive target for novel intervention for chronic pain and its prevention.

A modified Hargreaves' method for assessing threshold temperatures for heat nociception

This study describes a modified Hargreaves' method for assessing paw withdrawal threshold temperatures for heat (PWT-H) nociception in the hind paws of rats. This method utilises radiant heat to maintain controlled lamp temperatures (CLTs) on a glass floor beneath the rat hind paw. An ascending series of CLTs were applied for 10s, with 5-10min intervals between applications, until characteristic withdrawal behaviour was observed or a cutoff CLT was reached. The average plantar epicutaneous temperatures measured from anaesthetised rats corresponding to CLTs and withdrawal latencies were used for determining PWT-H. The mean PWT-H in 2-month-old (mo) naïve Sprague-Dawley rats (n=38) was 47.6±0.2°C, which is comparable to the noxious threshold temperature for human glabrous skin (46.5±0.5°C). The PWT-H was consistent between trials and daily assessments over four consecutive days. No significant differences were observed between the PWT-H in 2-, 6- to 8-, and >24-mo F344 rats, but the PWT-H in 1-mo rats was significantly reduced. Three hours following plantar incision, the PWT-H decreased to 37.5±0.2°C, consistent with previous observations of C-fibre afferents from incised glabrous skin firing at 36.7±3.6°C. Parallel testing, using the current method and an electronic von Frey device, illustrated similar degrees of incision-induced hyperalgesia, gradual improvements in hyperalgesia, and reversals induced through morphine and gabapentin. In conclusion, the present method facilitates a comparison of PWT-H using electrophysiological and human psychophysical studies involving thermosensation, and as a behavioural assay identical to von Frey testing, this method also measures the threshold for nociception.

Preprotachykinin-A gene disruption attenuates nociceptive sensitivity after opioid administration and incision by peripheral and spinal mechanisms in mice

The preprotachykinin A gene (ppt-A) codes for Substance P (SP), supports nociceptive sensitization, and modulates inflammatory responses after incision. Repeated opioid use produces paradoxical pain sensitization-termed opioid-induced hyperalgesia (OIH) -which can exacerbate pain after incision. Here the contribution of SP to peri-incisional nociceptive sensitization and nociceptive mediator production after opioid treatment was examined utilizing ppt-A knockout (-/-) mice and the neurokinin (NK1) receptor antagonist LY303870. Less mechanical allodynia was observed in ppt-A(-/-) mice compared to wild types (wt) after morphine treatment both before and after incision. Moreover, LY303870 administered with morphine reduced incisional hyperalgesia in wt mice. Incision after saline or escalating morphine treatment upregulated skin IL-1β, IL-6, G-CSF and MIP-1α levels in ppt-A(-/-) and wt mice similarly. However, chronic morphine treatment greatly exacerbated increases in skin nerve growth factor levels after incision, an effect entirely dependent upon intact SP signaling. Additionally, SP dependent upregulation of prodynorphin, NMDA1 and NK1 receptor expression in spinal cord was seen after morphine treatment and incision. A similar pattern was seen for 5-HT3 receptor expression in tissue from dorsal root ganglia. Therefore, SP may work at both central and peripheral sites to enhance nociceptive sensitization after morphine treatment and incision.

PERSPECTIVE

These studies show that SP signaling modulates enhanced nerve growth factor production and changes in neuronal gene expression seen after incision in mice previously exposed to morphine.

Activation of p38 mitogen-activated protein kinase in the dorsal root ganglion contributes to pain hypersensitivity after plantar incision

BACKGROUND

The phosphorylation of p38 mitogen-activated protein kinase (MAPK) in the dorsal root ganglion (DRG) promotes primary afferent sensitization. The role of p38MAPK signaling in the DRG in the pathogenesis of plantar incision hyperalgesia has not been investigated.

RESULTS

Levels of phosphorylated p38MAPK (p-p38MAPK) obviously increased in the DRG after plantar incision. Unmyelinated and myelinated DRG neurons that express p-p38MAPK contained small to medium cell bodies, suggesting that p-p38MAPK expression is induced in neurons with C- and Aδ-fibers. The p-p38MAPK inhibitors FR167653 or SB203580 inhibited incision-induced mechanical hypersensitivity and spontaneous pain behavior. The systemic administration of tumor necrosis factor-α (TNF-α) inhibitor prevented subsequent incision-induced activation of p38MAPK in the DRG and alleviated mechanical hypersensitivity after the incision.

CONCLUSIONS

p38MAPK signaling in the DRG plays a crucial role in the development of primary afferent sensitization and pain behavior caused by plantar incision.

Neutrophils recruited by CXCR1/2 signalling mediate post-incisional pain

BACKGROUND

Neutrophil recruitment mediated by the CXCL1/KC chemokine and its receptors CXCR1/CXCR2 plays a critical role in inflammatory diseases. Recently, neutrophil migration and activation triggered by CXCL1-CXCR1/2 signalling was implicated in inflammatory nociception; however, their role in post-surgical pain has not been elucidated. In this study, we addressed the function of neutrophils in the genesis of post-incisional pain in an experimental model of post-surgical pain.

METHODS

Mechanical hyperalgesia was determined with an electronic von Frey test in a mouse hindpaw incisional model. Neutrophil accumulation and the level of CXCL1/KC in the plantar tissue were determined by myeloperoxidase activity assay and enzyme-linked immunosorbent assay, respectively.

RESULTS

An incision in the mouse hindpaw produces long-lasting mechanical hyperalgesia that persists for at least 72 h after surgery. Following surgery, there was an increase in both neutrophil accumulation and the CXCL1/KC level in the incised paws. The depletion of the mouse neutrophils by vinblastine sulphate or anti-neutrophil antibody treatments reduced the mechanical hyperalgesia after paw incision. Furthermore, the treatment of mice with ladarixin, an orally acting CXCR1/2 antagonist, also reduced both the mechanical hyperalgesia and the infiltration of neutrophils in the incised paws.

CONCLUSION

In conclusion, it appears that after surgical processes, neutrophils are recruited by CXCL1-CXCR1/2 signalling and participate in the cascade of events, leading to mechanical hyperalgesia. These results suggest that blocking neutrophil migration through the inhibition of CXCL1-CXCR1/2 signalling might be a target to control post-surgical pain.

Characterization of nerve growth factor-induced mechanical and thermal hypersensitivity in rats

BACKGROUND

Injection of nerve growth factor (NGF) produces mechanical and thermal hypersensitivity in rodents and humans. Treatment with sequestering antibodies demonstrates the importance of NGF in various pain states, with efficacy seen in a number of animal pain models and in painful human conditions. However, these phenomena have not been evaluated in the context of using NGF-induced hypersensitivities as a model of pain.

METHODS

NGF-induced behaviours were characterized using von Frey filament, pinprick and thermal endpoints and then pharmacologically evaluated with known reference agents.

RESULTS

Intraplantar NGF injection produced a dose-dependent increase in thermal sensitivity that lasted through 24 h post-injection and an immediate long-lasting (2 week) increase in mechanical sensitivity at the injection site, with no effects detected at secondary sites. NGF-induced mechanical sensitivity was pharmacologically characterized at 4 h and 1 week post-NGF injection. The nonsteroidal anti-inflammatory drugs (NSAIDs), celecoxib and diclofenac, were minimally effective against both thermal and mechanical endpoints. Gabapenitn and duloxetine were only moderately effective against thermal and mechanical hypersensitivity. Morphine was effective against thermal and mechanical endpoints at every time point examined. Treatment with the transient receptor potential vanilloid 1 (TRPV1) antagonist A-784168 partially attenuated NGF-induced thermal and mechanical sensitivity at all time points examined.

CONCLUSIONS

The results reported here suggest that effects of NGF on thermal and mechanical sensitivity in rats are similar to those reported in human and are partially driven by TRPV1. The rat NGF model may serve as a potential translational model for exploring the effects of novel analgesic agents.

Neural mobilization reverses behavioral and cellular changes that characterize neuropathic pain in rats

BACKGROUND

The neural mobilization technique is a noninvasive method that has proved clinically effective in reducing pain sensitivity and consequently in improving quality of life after neuropathic pain. The present study examined the effects of neural mobilization (NM) on pain sensitivity induced by chronic constriction injury (CCI) in rats. The CCI was performed on adult male rats, submitted thereafter to 10 sessions of NM, each other day, starting 14 days after the CCI injury. Over the treatment period, animals were evaluated for nociception using behavioral tests, such as tests for allodynia and thermal and mechanical hyperalgesia. At the end of the sessions, the dorsal root ganglion (DRG) and spinal cord were analyzed using immunohistochemistry and Western blot assays for neural growth factor (NGF) and glial fibrillary acidic protein (GFAP).

RESULTS

The NM treatment induced an early reduction (from the second session) of the hyperalgesia and allodynia in CCI-injured rats, which persisted until the end of the treatment. On the other hand, only after the 4th session we observed a blockade of thermal sensitivity. Regarding cellular changes, we observed a decrease of GFAP and NGF expression after NM in the ipsilateral DRG (68% and 111%, respectively) and the decrease of only GFAP expression after NM in the lumbar spinal cord (L3-L6) (108%).

CONCLUSIONS

These data provide evidence that NM treatment reverses pain symptoms in CCI-injured rats and suggest the involvement of glial cells and NGF in such an effect.