Vascular endothelial cells mediate mechanical stimulation-induced enhancement of endothelin hyperalgesia via activation of P2X2/3 receptors on nociceptors

Understanding Headaches Attributed to Cranial and/or Cervical Vascular Disorders: Insights and Challenges for Neurologists

In recent decades, cranial and cervical vascular disorders have become major global health concerns, significantly impacting patients, families, and societies. Headache is a prevalent symptom of these vascular diseases and can often be the initial, primary, or sole manifestation. The intricate relationship between headaches and cranial/cervical vascular disorders poses a diagnostic and therapeutic challenge, with the underlying mechanisms remaining largely elusive. Understanding this association is crucial for the early diagnosis, prevention, and intervention of such conditions. This review aims to provide a comprehensive overview of the clinical features and potential pathogenesis of headaches attributed to cranial and cervical vascular disorders and provide a reference for disease management and a basis for potential pathological mechanisms.

Profiling of plasma extracellular vesicles identifies proteins that strongly associate with patient's global assessment of disease activity in rheumatoid arthritis

Background

Rheumatoid arthritis (RA) is an autoimmune disease characterized by chronic synovial inflammation and cartilage/bone damage. Intercellular messengers such as IL-1 and TNF play a crucial role in the pathophysiology of RA but have limited diagnostic and prognostic values. Therefore, we assessed whether the protein content of the recently discovered extracellular vesicles (EVs), which have gained attention in the pathogenesis of RA, correlates with disease activity parameters in RA patients.

Methods

We identified and quantified proteins in plasma-derived EVs (pEVs), isolated by size exclusion chromatography from 17 RA patients by mass spectrophotometry (MS). Quantified protein levels were correlated with laboratory and clinical parameters and the patient's own global assessment of their disease activity (PGA-VAS). In a second MS run, the pEV proteins of nine other RA patients were quantified and compared to those from nine healthy controls (HC).

Results

No differences were observed in the concentration, size, and protein content of pEVs from RA patients. Proteomics revealed >95% overlapping proteins in RA-pEVs, compared to HC-pEVs (data are available via ProteomeXchange with identifier PXD046058). Remarkably, in both runs, the level of far more RA-pEV proteins correlated positively to PGA-VAS than to either clinical or laboratory parameters. Interestingly, all observed PGA-VAS positively correlated RA-pEV proteins were associated with the actin-cytoskeleton linker proteins, ezrin, and moesin.

Conclusion

Our observation suggests that PGA-VAS (loss of vitality) may have a different underlying pathological mechanism in RA, possibly related to enhanced muscle actin-cytoskeleton activity. Furthermore, our study contributes to the growing awareness and evidence that pEVs contain valuable biomarkers for diseases, with added value for RA patients.

Relationship of PIEZO1 and PIEZO2 vascular expression with diabetic neuropathy

Introduction: Diabetic distal symmetric polyneuropathy (DDSP) is the most prevalent form of diabetic peripheral neuropathy, and 25% of patients develop pain in their toes. DDSP is associated with increased cutaneous microvessel density (MVD), reduced skin blood flow, endothelial dysfunction, and impaired fluid filtration with vasodilation. The Piezo family of mechanosensitive channels is known to be involved in the control of vascular caliber by converting mechanical force into intracellular signals. Furthermore, Piezo2 is particularly involved in peripheral pain mechanisms of DDSP patients. To date, very little is known about the number, structure, and PIEZO expression in cutaneous blood vessels (BVs) of individuals with DDSP and their relation with pain and time span of diabetes. Methods and results: We studied microvessels using endothelial markers (CD34 and CD31) and smooth cell marker (α-SMA) by indirect immunohistochemical assay in sections of the glabrous skin of the toes from patients and controls. MVD was assessed through CD34 and CD31 immunoreaction. MVD determined by CD34 is higher in short-term DDSP patients (less than 15 years of evolution), regardless of pain. However, long-term DDSP patients only had increased BV density in the painful group for CD31. BVs of patients with DDSP showed structural disorganization and loss of shape. The BVs affected by painful DDSP underwent the most dramatic structural changes, showing rupture, leakage, and abundance of material that occluded the BV lumen. Moreover, BVs of DDSP patients displayed a Piezo1 slight immunoreaction, whereas painful DDSP patients showed an increase in Piezo2 immunoreaction. Discussion: These results suggest that alterations in the number, structure, and immunohistochemical profile of specific BVs can explain the vascular impairment associated with painful DDSP, as well as the temporal span of diabetes. Finally, this study points out a possible correlation between increased vascular Piezo2 immunostaining and pain and decreased vascular Piezo1 immunostaining and the development of vasodilation deficiency.

Role and therapeutic target of P2X2/3 receptors in visceral pain

Visceral pain (VP) is caused by internal organ disease. VP is involved in nerve conduction and related signaling molecules, but its specific pathogenesis has not yet been fully elucidated. Currently, there are no effective methods for treating VP. The role of P2X2/3 in VP has progressed. After visceral organs are subjected to noxious stimulation, cells release ATP, activate P2X2/3, enhance the sensitivity of peripheral receptors and the plasticity of neurons, enhance sensory information transmission, sensitize the central nervous system, and play an important role in the development of VP. However, antagonists possess the pharmacological effect of relieving pain. Therefore, in this review, we summarize the biological functions of P2X2/3 and discuss the intrinsic link between P2X2/3 and VP. Moreover, we focus on the pharmacological effects of P2X2/3 antagonists on VP therapy and provide a theoretical basis for its targeted therapy.

The role of the neuronal microenvironment in sensory function and pain pathophysiology

The high prevalence of pain and the at times low efficacy of current treatments represent a significant challenge to healthcare systems worldwide. Effective treatment strategies require consideration of the diverse pathophysiologies that underlie various pain conditions. Indeed, our understanding of the mechanisms contributing to aberrant sensory neuron function has advanced considerably. However, sensory neurons operate in a complex dynamic microenvironment that is controlled by multidirectional interactions of neurons with non-neuronal cells, including immune cells, neuronal accessory cells, fibroblasts, adipocytes, and keratinocytes. Each of these cells constitute and control the microenvironment in which neurons operate, inevitably influencing sensory function and the pathology of pain. This review highlights the importance of the neuronal microenvironment for sensory function and pain, focusing on cellular interactions in the skin, nerves, dorsal root ganglia, and spinal cord. We discuss the current understanding of the mechanisms by which neurons and non-neuronal cells communicate to promote or resolve pain, and how this knowledge could be used for the development of mechanism-based treatments.

Infrapatellar Fat Pad-Synovial Membrane Anatomo-Fuctional Unit: Microscopic Basis for Piezo1/2 Mechanosensors Involvement in Osteoarthritis Pain

The Infrapatellar Fat Pad (IFP) is a fibro-adipose tissue of the knee recently reconsidered as part of a single anatomo-functional unit (AFU) together with the synovial membrane (SM). Several evidence support the role of this unit in the mechanisms that trigger and perpetuate the onset and progression of osteoarthritis (OA) disease. Additionally, the contribution of IFP-SM AFU in OA-associated pain has also been supposed, but this assumption still needs to be fully elucidated. Within this context, the recent discovery of the mechanoceptive Piezo ion channels (i.e., Piezo1 and Piezo2) in mammals and consciousness on their role in mediating both mechanoceptive and inflammatory stimuli could shed some light on knee OA pain, as well as on the process leading from acute to chronic nociceptive responses. For this purpose, the IFP-SM AFUs of both healthy donors (non-OA IFP-SM AFUs, n = 10) and OA patients (OA IFP-SM AFUs, n = 10) were processed by histology and immunohistochemistry. After the attribution of a histopathological score to IFP-SM AFUs to confirm intrinsic differences between the two groups, the specimens were investigated for the expression and localization/distribution pattern of the mechanosensors Piezo1 and Piezo2. In addition, the presence of monocytes/macrophages (CD68), peripheral nerve endings (PGP9.5) and neoangiogenesis signs (YAP1) was evaluated for a broad tissue characterization. The study results lead to a better description of the IFP-SM AFU microscopic features in both healthy and pathological conditions, highlighting peculiar differences in the study cohort. Specifically, immunopositivity towards Piezo1/2, CD68 and YAP1 markers was detected at vessels level in the OA- IFP-SM AFUs compartments, differently from the non-OA-group. A correlation with pain was also inferred, paving the way for the identification of new and effective molecules in OA management.

Monocyte Adhesion Assays for Detecting Endothelial Cell Activation in Vascular Inflammation and Atherosclerosis

Monocyte adhesion assay, a fluorescence-based method, enables the detection and quantification of monocyte adhesion to endothelial cell (EC) monolayers in vitro and measures EC activation. We describe in this chapter a monocyte adhesion assay based on two published papers from our laboratory that can be effectively used in studying the mechanisms of both pro- and anti-inflammatory cytokines in EC activation. Endothelial cell monolayers are cultured and treated with desired drug, cytokines, or other stimuli and incubated with fluorescently labeled monocytes.

Endothelin receptor type A is involved in the development of oxaliplatin-induced mechanical allodynia and cold allodynia acting through spinal and peripheral mechanisms in rats

Oxaliplatin, a platinum-based chemotherapeutic agent, frequently causes severe neuropathic pain typically encompassing cold allodynia and long-lasting mechanical allodynia. Endothelin has been shown to modulate nociceptive transmission in a variety of pain disorders. However, the action of endothelin varies greatly depending on many variables, including pain causes, receptor types (endothelin type A (ET_A) and B (ET_B) receptors) and organs (periphery and spinal cord). Therefore, in this study, we investigated the role of endothelin in a Sprague–Dawley rat model of oxaliplatin-induced neuropathic pain. Intraperitoneal administration of bosentan, a dual ET_A/ET_B receptor antagonist, effectively blocked the development or prevented the onset of both cold allodynia and mechanical allodynia. The preventive effects were exclusively mediated by ET_A receptor antagonism. Intrathecal administration of an ET_A receptor antagonist prevented development of long-lasting mechanical allodynia but not cold allodynia. In marked contrast, an intraplantar ET_A receptor antagonist had a suppressive effect on cold allodynia but only had a partial and transient effect on mechanical allodynia. In conclusion, ET_A receptor antagonism effectively prevented long-lasting mechanical allodynia through spinal and peripheral actions, while cold allodynia was prevented through peripheral actions.

Eliapixant is a selective P2X3 receptor antagonist for the treatment of disorders associated with hypersensitive nerve fibers

ATP-dependent P2X3 receptors play a crucial role in the sensitization of nerve fibers and pathological pain pathways. They are also involved in pathways triggering cough and may contribute to the pathophysiology of endometriosis and overactive bladder. However, despite the strong therapeutic rationale for targeting P2X3 receptors, preliminary antagonists have been hampered by off-target effects, including severe taste disturbances associated with blocking the P2X2/3 receptor heterotrimer. Here we present a P2X3 receptor antagonist, eliapixant (BAY 1817080), which is both highly potent and selective for P2X3 over other P2X subtypes in vitro, including P2X2/3. We show that eliapixant reduces inflammatory pain in relevant animal models. We also provide the first in vivo experimental evidence that P2X3 antagonism reduces neurogenic inflammation, a phenomenon hypothesised to contribute to several diseases, including endometriosis. To test whether eliapixant could help treat endometriosis, we confirmed P2X3 expression on nerve fibers innervating human endometriotic lesions. We then demonstrate that eliapixant reduces vaginal hyperalgesia in an animal model of endometriosis-associated dyspareunia, even beyond treatment cessation. Our findings indicate that P2X3 antagonism could alleviate pain, including non-menstrual pelvic pain, and modify the underlying disease pathophysiology in women with endometriosis. Eliapixant is currently under clinical development for the treatment of disorders associated with hypersensitive nerve fibers.

Fabry disease pain: patient and preclinical parallels

ABSTRACT

Severe neuropathic pain is a hallmark of Fabry disease, a genetic disorder caused by a deficiency in lysosomal α-galactosidase A. Pain experienced by these patients significantly impacts their quality of life and ability to perform everyday tasks. Patients with Fabry disease suffer from peripheral neuropathy, sensory abnormalities, acute pain crises, and lifelong ongoing pain. Although treatment of pain through medication and enzyme replacement therapy exists, pain persists in many of these patients. Some has been learned in the past decades regarding clinical manifestations of pain in Fabry disease and the pathological effects of α-galactosidase A insufficiency in neurons. Still, it is unclear how pain and sensory abnormalities arise in patients with Fabry disease and how these can be targeted with therapeutics. Our knowledge is limited in part due to the lack of adequate preclinical models to study the disease. This review will detail the types of pain, sensory abnormalities, influence of demographics on pain, and current strategies to treat pain experienced by patients with Fabry disease. In addition, we discuss the current knowledge of Fabry pain pathogenesis and which aspects of the disease preclinical models accurately recapitulate. Understanding the commonalities and divergences between humans and preclinical models can be used to further interrogate mechanisms causing the pain and sensory abnormalities as well as advance development of the next generation of therapeutics to treat pain in patients with Fabry disease.

Novel Compounds Targeting Neuropilin Receptor 1 with Potential To Interfere with SARS-CoV-2 Virus Entry

Neuropilin-1 (NRP-1) is a multifunctional transmembrane receptor for ligands that affect developmental axonal growth and angiogenesis. In addition to a role in cancer, NRP-1 is a reported entry point for several viruses, including severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causal agent of coronavirus disease 2019 (COVID-19). The furin cleavage product of SARS-CoV-2 Spike protein takes advantage of the vascular endothelial growth factor A (VEGF-A) binding site on NRP-1 which accommodates a polybasic stretch ending in a C-terminal arginine. This site has long been a focus of drug discovery efforts for cancer therapeutics. We recently showed that interruption of the VEGF-A/NRP-1 signaling pathway ameliorates neuropathic pain and hypothesize that interference of this pathway by SARS-CoV-2 Spike protein interferes with pain signaling. Here, we report confirmed hits from a small molecule and natural product screen of nearly 0.5 million compounds targeting the VEGF-A binding site on NRP-1. We identified nine chemical series with lead- or drug-like physicochemical properties. Using ELISA, we demonstrate that six compounds disrupt VEGF-A-NRP-1 binding more effectively than EG00229, a known NRP-1 inhibitor. Secondary validation in cells revealed that all tested compounds inhibited VEGF-A triggered VEGFR2 phosphorylation. Further, two compounds displayed robust inhibition of a recombinant vesicular stomatitis virus protein that utilizes the SARS-CoV-2 Spike for entry and fusion. These compounds represent a first step in a renewed effort to develop small molecule inhibitors of the VEGF-A/NRP-1 signaling for the treatment of neuropathic pain and cancer with the added potential of inhibiting SARS-CoV-2 virus entry.

Peripheral neuroimmune interactions: selected review and some clinical implications

Purpose

To provide a brief and focused review on peripheral neuroimmune interactions and their implications for some clinical disorders.

Methods

Narrative review of the literature including of English-language articles published between 1985 and 2021 using PubMed and MEDLINE.

Results

Many studies on experimental models and in vitro indicate that there are close interactions between the neural and immune systems. Processes from sensory afferents and autonomic efferents co-localize with immune cells and interact at discrete anatomical sites forming neuroimmune units. These neuroimmune interactions are bidirectional and mediated by a wide range of soluble factors including neuropeptides, classical neurotransmitters, cytokines, and other molecules that mediate complex cross-talk among nerves and immune cells. Small-diameter sensory afferents express a wide range of receptors that respond directly to tissue damage or pathogen signals and to chemokines, cytokines, or other molecules released from immune cells. Reciprocally, immune cells respond to neurotransmitters released from nociceptive and autonomic fibers. Neuroimmune interactions operate both at peripheral tissues and at the level of the central nervous system. Both centrally and peripherally, glial cells have a major active role in this bidirectional communication.

Conclusions

Peripheral neuroimmune interactions are complex and importantly contribute to the pathophysiology of several disorders, including skin, respiratory, and intestinal inflammatory disorders typically associated with pain and altered barrier function. These interactions may be relevant for persistence of symptoms in disorders associated with intense immune activation.

Nonhormonal Treatment for Endometriosis Focusing on Redox Imbalance

The aim of this review is to investigate the oxidant/antioxidant status and its regulatory mechanisms in patients with endometriosis and to summarize the antioxidant therapy as an alternative to hormonal therapy for endometriosis. Each keyword alone or in combination was used to search from PubMed and Embase by applying the filters of the title and the publication years between January 2000 and March 2020. Endometriosis is a chronic inflammatory disease characterized by repeated episodes of hemorrhage. Methemoglobin in repeated hemorrhage produces large amounts of superoxide anion via the autoxidation of hemoglobin. Excessive free-radical production causes redox imbalance, leading to inadequate antioxidant defenses and damage to endometrial cells, but may contribute to endometrial cell growth and survival through activation of various signaling pathways. In addition, to overcome excessive oxidative stress, estradiol participates in the induction of antioxidants such as superoxide dismutase in mitochondria. Several antioxidants that suppress free radicals may be effective in endometriosis-related pain. We searched for 23 compounds and natural substances that could reduce the pain caused by superoxide/reactive oxygen species in basic research and animal models. Next, we built a list of 16 drugs that were suggested to be effective against endometriosis other than hormone therapy in preclinical studies and clinical trials. Of the 23 and 16 drugs, 4 overlapping drugs could be potential candidates for clinically reducing endometriosis-related pain caused by superoxide anion/reactive oxygen species. These drugs include polyphenols (resveratrol and polydatin), dopamine agonists (cabergoline), and statins (simvastatin). However, no randomized controlled trials have evaluated the efficacy of these drugs. In conclusion, this review summarizes the following 2 points: superoxide anion generation by methemoglobin is enhanced in endometriosis, resulting in redox imbalance; and some compounds and natural substances that can suppress free radicals may be effective in endometriosis-related pain. Further randomized clinical trials based on larger series are mandatory to confirm the promising role of antioxidants in the nonhormonal management of endometriosis.

SARS-CoV-2 spike protein co-opts VEGF-A/neuropilin-1 receptor signaling to induce analgesia

Global spread of severe acute respiratory syndrome coronavirus 2 continues unabated. Binding of severe acute respiratory syndrome coronavirus 2's spike protein to host angiotensin-converting enzyme 2 triggers viral entry, but other proteins may participate, including the neuropilin-1 receptor (NRP-1). Because both spike protein and vascular endothelial growth factor-A (VEGF-A)-a pronociceptive and angiogenic factor, bind NRP-1, we tested whether spike could block VEGF-A/NRP-1 signaling. VEGF-A-triggered sensory neuron firing was blocked by spike protein and NRP-1 inhibitor EG00229. Pronociceptive behaviors of VEGF-A were similarly blocked through suppression of spontaneous spinal synaptic activity and reduction of electrogenic currents in sensory neurons. Remarkably, preventing VEGF-A/NRP-1 signaling was antiallodynic in a neuropathic pain model. A "silencing" of pain through subversion of VEGF-A/NRP-1 signaling may underlie increased disease transmission in asymptomatic individuals.

Management of pain in colorectal cancer patients

In this review we focus on the pathophysiology of CRC-related pain and discuss currently applied pain management. Pain is a symptom reported by over 70 % of colorectal cancer (CRC) patients. It remains a feared and debilitating consequence of both cancer and cancer-related treatment. There are many options for pain management in CRC, consisting of intravenous, oral or topical medications. In order to address the full spectrum of pain, proper treatment should address the nociceptive, neuropathic and/or psychogenic pain component. Currently available methods do not bring pain relief to satisfying number of patients and, if used improperly, can cause a number of complications. Therefore, future treatments should focus primarily on alleviating pain, but also on reducing possible side effects. In this article we cover recent and promising pharmacological and non- pharmacological developments emerging in the field of CRC treatment.

Potentiation of P2X3 receptor mediated currents by endothelin-1 in rat dorsal root ganglion neurons

Endothelin-1 (ET-1), an endogenous vasoconstrictor, has been known as a pro-nociceptive agent involved in multitude of pain. ET-1 acts on endothelin receptors on vascular endothelial cells, sensitizes release of ATP, which then acts on P2X3 receptors on nociceptors and results in mechanical hyperalgesia. Both endothelin receptors and P2X3 receptors are present in primary sensory neuron, where it remains unclear whether there is an interaction between them. Herein, we reported that ET-1 potentiated the electrophysiological activity of P2X3 receptors in rat dorsal root ganglia (DRG) neurons. ET-1 concentration-dependently increased α,β-methylene-ATP (α,β-meATP)-evoked inward currents, which were mediated by P2X3 receptors. ET-1 shifted the α,β-meATP concentration-response curve upwards, with an increase of 34.38 ± 4.72% in the maximal current response to α,β-meATP in the presence of ET-1. ET-1 potentiation of α,β-meATP-evoked currents was voltage-independent. ET-1 potentiated P2X3 receptor-mediated currents through endothelin-A receptors (ET_AR), but not endothelin-B receptors (ET_BR). ET-1 potentiation was supressed by blockade of intracellular G-protein or protein kinase C (PKC) signaling. Moreover, there is a synergistic effect on mechanical allodynia induced by intraplantar injection of ET-1 and α,β-meATP in rats. Pharmacological blockade of P2X3 receptors also alleviated ET-1-induced mechanical allodynia. These results suggested that ET-1 sensitized P2X3 receptors in primary sensory neurons via an ET_AR and PKC signaling pathway. Our data provide evidence that cutaneous ET-1 induced mechanical allodynia not only by increasing the release of ATP from vascular endothelial cells, but also by sensitizing P2X3 receptors on nociceptive DRG neurons.

In silico identification and validation of inhibitors of the interaction between neuropilin receptor 1 and SARS-CoV-2 Spike protein

Neuropilin-1 (NRP-1) is a multifunctional transmembrane receptor for ligands that affect developmental axonal growth and angiogenesis. In addition to a role in cancer, NRP-1 is a reported entry point for several viruses, including severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causal agent of coronavirus disease 2019 (COVID-19). The furin cleavage product of SARS-CoV-2 Spike protein takes advantage of the vascular endothelial growth factor A (VEGF-A) binding site on NRP-1 which accommodates a polybasic stretch ending in a C-terminal arginine. This site has long been a focus of drug discovery efforts for cancer therapeutics. We recently showed that interruption of the VEGF-A/NRP-1 signaling pathway ameliorates neuropathic pain and hypothesize that interference of this pathway by SARS-CoV-2 spike protein interferes with pain signaling. Here, we report hits from a small molecule and natural product screen of nearly 0.5 million compounds targeting the VEGF-A binding site on NRP-1. We identified nine chemical series with lead- or drug-like physico-chemical properties. Using an ELISA, we demonstrate that six compounds disrupt VEGF-A-NRP-1 binding more effectively than EG00229, a known NRP-1 inhibitor. Secondary validation in cells revealed that almost all tested compounds inhibited VEGF-A triggered VEGFR2 phosphorylation. Two compounds displayed robust inhibition of a recombinant vesicular stomatitis virus protein that utilizes the SARS-CoV-2 Spike for entry and fusion. These compounds represent a first step in a renewed effort to develop small molecule inhibitors of the VEGF-A/NRP-1 signaling for the treatment of neuropathic pain and cancer with the added potential of inhibiting SARS-CoV-2 virus entry.

SARS-CoV-2 Spike protein co-opts VEGF-A/Neuropilin-1 receptor signaling to induce analgesia

Global spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) continues unabated. Binding of SARS-CoV-2's Spike protein to host angiotensin converting enzyme 2 triggers viral entry, but other proteins may participate, including neuropilin-1 receptor (NRP-1). As both Spike protein and vascular endothelial growth factor-A (VEGF-A) - a pro-nociceptive and angiogenic factor, bind NRP-1, we tested if Spike could block VEGF-A/NRP-1 signaling. VEGF-A-triggered sensory neuronal firing was blocked by Spike protein and NRP-1 inhibitor EG00229. Pro-nociceptive behaviors of VEGF-A were similarly blocked via suppression of spontaneous spinal synaptic activity and reduction of electrogenic currents in sensory neurons. Remarkably, preventing VEGF-A/NRP-1 signaling was antiallodynic in a neuropathic pain model. A 'silencing' of pain via subversion of VEGF-A/NRP-1 signaling may underlie increased disease transmission in asymptomatic individuals.

Vascular Contributions to Migraine: Time to Revisit?

Migraine is one of the most prevalent and disabling neurovascular disorders worldwide. However, despite the increase in awareness and research, the understanding of migraine pathophysiology and treatment options remain limited. For centuries, migraine was considered to be a vascular disorder. In fact, the throbbing, pulsating quality of the headache is thought to be caused by mechanical changes in vessels. Moreover, the most successful migraine treatments act on the vasculature and induction of migraine can be accomplished with vasoactive agents. However, over the past 20 years, the emphasis has shifted to the neural imbalances associated with migraine, and vascular changes have generally been viewed as an epiphenomenon that is neither sufficient nor necessary to induce migraine. With the clinical success of peripherally-acting antibodies that target calcitonin gene-related peptide (CGRP) and its receptor for preventing migraine, this neurocentric view warrants a critical re-evaluation. This review will highlight the likely importance of the vasculature in migraine.

Current understanding of meningeal and cerebral vascular function underlying migraine headache

BACKGROUND

The exact mechanisms underlying the onset of a migraine attack are not completely understood. It is, however, now well accepted that the onset of the excruciating throbbing headache of migraine is mediated by the activation and increased mechanosensitivity (i.e. sensitization) of trigeminal nociceptive afferents that innervate the cranial meninges and their related large blood vessels.

OBJECTIVES

To provide a critical summary of current understanding of the role that the cranial meninges, their associated vasculature, and immune cells play in meningeal nociception and the ensuing migraine headache.

METHODS

We discuss the anatomy of the cranial meninges, their associated vasculature, innervation and immune cell population. We then debate the meningeal neurogenic inflammation hypothesis of migraine and its putative contribution to migraine pain. Finally, we provide insights into potential sources of meningeal inflammation and nociception beyond neurogenic inflammation, and their potential contribution to migraine headache.

Endothelin-1 Elicits TRP-Mediated Pain in an Acid-Induced Oral Ulcer Model

Oral ulcer is the most common oral disease and leads to pain during meals and speaking, reducing the quality of life of patients. Recent evidence using animal models suggests that oral ulcers induce cyclooxygenase-dependent spontaneous pain and cyclooxygenase-independent mechanical allodynia. Endothelin-1 is upregulated in oral mucosal inflammation, although it has not been shown to induce pain in oral ulcers. In the present study, we investigated the involvement of endothelin-1 signaling with oral ulcer-induced pain using our proprietary assay system in conscious rats. Endothelin-1 was significantly upregulated in oral ulcers experimentally induced by topical acetic acid treatment, while endothelin-1 production was suppressed by antibacterial pretreatment. Spontaneous nociceptive behavior in oral ulcer model rats was inhibited by swab applications of BQ-788 (ET_B receptor antagonist), ONO-8711 (prostanoid receptor EP₁ antagonist), and HC-030031 (TRPA1 antagonist). Prostaglandin E₂ production in the ulcers was suppressed by BQ-788. Mechanical allodynia in the model was inhibited not only by BQ-788 and HC-030031 but also by BQ-123 (ET_A receptor antagonist), SB-366791 (TRPV1 antagonist), and RN-1734 (TRPV4 antagonist). In naive rats, submucosal injection of endothelin-1 caused mechanical allodynia that was sensitive to HC-030031 and SB-366791 but not to RN-1734. These results suggest that endothelin-1 production following oral bacterial invasion via ulcerative regions elicits TRPA1-mediated spontaneous pain. This pain likely occurs through an indirect route that involves ET_B receptor-accelerated prostanoid production. Endothelin-1 elicits directly TRPA1- and TRPV1-mediated mechanical allodynia via both ET_A and ET_B receptors on nociceptive fibers. The TRPV4-mediated allodynia component seems to be independent of endothelin signaling. These findings highlight the potential of endothelin signaling blockers as effective analgesic approaches for oral ulcer patients.

Low-intensity pulsed ultrasound promotes endothelial cell-mediated osteogenesis in a conditioned medium coculture system with osteoblasts

Angiogenesis plays an important role during bone regeneration. Low-intensity pulsed ultrasound (LIPUS) has been proven to accelerate the process of bone fracture healing. However, the mechanism of the effect of LIPUS on bone regeneration is still unclear. In the present study, we used human umbilical vein endothelial cell (HUVEC) and human osteosarcoma cell (MG-63) to investigate the effect of LIPUS stimulation in an endothelial cell-osteoblast coculture system. At the same time, we used transwell and in vitro angiogenesis assay to observe how LIPUS affects endothelial cells. The results demonstrated that LIPUS could significantly increase the migratory ability and promote tube formation in angiogenesis of HUVECs. Furthermore, LIPUS could significantly elevate the expression of osteogenesis-related genes on osteoblasts such as Runt-related transcription factor 2, alkaline phosphatase, Osteorix, and Cyclin-D1, indicating the pro-osteogenesis effect of LIPUS in our coculture system. In conclusion, endothelial cell is involved in LIPUS-accelerated bone regeneration, the positive effect of LIPUS may be transferred via endothelial cells surrounding fracture healing site.

P2X3 receptor involvement in endometriosis pain via ERK signaling pathway

The purinergic receptor P2X ligand-gated ion channel 3 (P2X3) is crucially involved in peripheral nociceptive processes of somatic and visceral pain. Endometriosis pain is considered as a kind of inflammatory and neuropathic pain. However, whether P2X3 is involved in endometriosis pain has not been reported up to date. Here, we aimed to determine whether P2X3 expression in endometriotic lesions is involved in endometriosis pain, which is regulated by inflammatory mediators through extracellular regulated protein kinases (ERK) signalling pathway. We found that P2X3 expressions in endometriosis endometrium and endometriotic lesions were both significantly higher as compared with control endometrium (P<0.05), and both positively correlated with pain (P<0.05). The expression levels of phosphorylated –ERK (p-ERK), phosphorylated-cAMP-response element binding protein (p-CREB), and P2X3 in endometriotic stromal cells (ESCs) were all significantly increased in comparison to the initial levels after treated with interleukin (IL)-1β (P<0.05) or adenosine triphosphate (ATP) (P<0.05), respectively, and did not increase after the ESCs were pre-treated with ERK1/2 inhibitor. Additionally, P2X3 and calcitonin gene related peptide (CGRP) were co-expressed in endometriotic lesions. These obtained results suggest that P2X3 might be involved in endometriosis pain signal transduction via ERK signal pathway.

Ovariectomy uncovers purinergic receptor activation of endothelin-dependent natriuresis

We recently reported that natriuresis produced by renal medullary salt loading is dependent on endothelin (ET)-1 and purinergic (P2) receptors in male rats. Because sex differences in ET-1 and P2 signaling have been reported, we decided to test whether ovarian sex hormones regulate renal medullary ET-1 and P2-dependent natriuresis. The effect of medullary NaCl loading on Na⁺ excretion was determined in intact and ovariectomized (OVX) female Sprague-Dawley rats with and without ET-1 or P2 receptor antagonism. Isosmotic saline (284 mosmol/kgH₂O) was infused in the renal medullary interstitium of anesthetized rats during a baseline urine collection period, followed by isosmotic or hyperosmotic saline (1,800 mosmol/kgH₂O) infusion. Medullary NaCl loading significantly enhanced Na⁺ excretion in intact and OVX female rats. ET_A+B or P2 receptor blockade did not attenuate the natriuretic effect of medullary NaCl loading in intact females, whereas ET_A+B or P2 receptor blockade attenuated the natriuretic response to NaCl loading in OVX rats. Activation of medullary P2Y₂ and P2Y₄ receptors by UTP infusion had no significant effect in intact females but enhanced Na⁺ excretion in OVX rats. Combined ET_A+B receptor blockade significantly inhibited the natriuretic response to UTP observed in OVX rats. These data demonstrate that medullary NaCl loading induces ET-1 and P2-independent natriuresis in intact females. In OVX, activation of medullary P2 receptors promotes ET-dependent natriuresis, suggesting that ovarian hormones may regulate the interplay between the renal ET-1 and P2 signaling systems to facilitate Na⁺ excretion.

Interplay between renal endothelin and purinergic signaling systems

Alterations in extracellular fluid volume regulation and sodium balance may result in the development and maintenance of salt-dependent hypertension, a major risk factor for cardiovascular disease. Numerous pathways contribute to the regulation of sodium excretion and blood pressure, including endothelin and purinergic signaling. Increasing evidence suggests a link between purinergic receptor activation and endothelin production within the renal collecting duct as a means of promoting natriuresis. A better understanding of the relationship between these two systems, especially in regard to sodium homeostasis, will fill a significant knowledge gap and may provide novel antihypertensive treatment options. Therefore, this review focuses on the cross talk between endothelin and purinergic signaling as it relates to the renal regulation of sodium and blood pressure homeostasis.

Differential regulation of oxidative stress and cytokine production by endothelin ETA and ETB receptors in superoxide anion-induced inflammation and pain in mice

The present study investigated whether endothelin-1 acts via ET_A or ET_B receptors to mediate superoxide anion-induced pain and inflammation. Mice were treated with clazosentan (ET_A receptor antagonist) or BQ-788 (ET_B receptor antagonist) prior to stimulation with the superoxide anion donor, KO₂. Intraplantar treatment with 30 nmol of clazosentan or BQ-788 reduced mechanical hyperalgesia (47% and 42%), thermal hyperalgesia (68% and 76%), oedema (50% and 30%); myeloperoxidase activity (64% and 32%), and overt-pain like behaviours, such as paw flinching (42% and 42%) and paw licking (38% and 62%), respectively. Similarly, intraperitoneal treatment with 30 nmol of clazosentan or BQ-788 reduced leukocyte recruitment to the peritoneal cavity (58% and 32%) and abdominal writhing (81% and 77%), respectively. Additionally, intraplantar treatment with clazosentan or BQ-788 decreased spinal (45% and 41%) and peripheral (47% and 47%) superoxide anion production as well as spinal (47% and 47%) and peripheral (33% and 54%) lipid peroxidation, respectively. Intraplantar treatment with clazosentan, but not BQ-788, reduced spinal (71%) and peripheral (51%) interleukin-1 beta as well as spinal (59%) and peripheral (50%) tumor necrosis factor-alpha production. Therefore, the present study unveils the differential mechanisms by which ET-1, acting on ET_A or ET_B receptors, regulates superoxide anion-induced inflammation and pain.

Emerging factors in the progression of cancer-related pain

Most cancer patients experience severe pain during their disease course, and the management of cancer pain is a major challenge for patients and the healthcare team. Many diverse translational models of cancer pain in recent years have improved our understanding of cancer-related pain. Cancer and associated cells in the cancer microenvironment may release various peripheral mediators, including ATP, formaldehyde, protons, proteases, endothelin, bradykinin, TNF and NGF, that result in the activation and/or sensitization of peripheral and central neurons, that contribute to the clinical manifestations of cancer-related pain. Identification of these mediators and the peripheral and central mechanisms by which they contribute to cancer-related pain may provide novel therapeutic targets to alleviate cancer patient suffering.

Activation of purinergic receptors (P2) in the renal medulla promotes endothelin-dependent natriuresis in male rats

Renal endothelin-1 (ET-1) and purinergic signaling systems regulate Na(+) reabsorption in the renal medulla. A link between the renal ET-1 and purinergic systems was demonstrated in vitro, however, the in vivo interaction between these systems has not been defined. To test whether renal medullary activation of purinergic (P2) receptors promotes ET-dependent natriuresis, we determined the effect of increased medullary NaCl loading on Na(+) excretion and inner medullary ET-1 mRNA expression in anesthetized adult male Sprague-Dawley rats in the presence and absence of purinergic receptor antagonism. Isosmotic saline (NaCl; 284 mosmol/kgH2O) was infused into the medullary interstitium (500 μl/h) during a 30-min baseline urine collection period, followed by isosmotic or hyperosmotic saline (1,800 mosmol/kgH2O) for two further 30-min urine collection periods. Na(+) excretion was significantly increased during intramedullary infusion of hyperosmotic saline. Compared with isosmotic saline, hyperosmotic saline infused into the renal medulla caused significant increases in inner medullary ET-1 mRNA expression. Renal intramedullary infusion of the P2 receptor antagonist suramin inhibited the increase in Na(+) excretion and inner medullary ET-1 mRNA expression induced by NaCl loading in the renal medulla. Activation of medullary P2Y2/4 receptors by infusion of UTP increased urinary Na(+) excretion. Combined ETA and ETB receptor blockade abolished the natriuretic response to intramedullary infusion of UTP. These data demonstrate that activation of medullary P2 receptors promotes ET-dependent natriuresis in male rats, suggesting that the renal ET-1 and purinergic signaling systems interact to efficiently facilitate excretion of a NaCl load.

The Neurobiology of Cancer Pain

Oral cancers are often severely painful and clinically difficult to manage. Few researchers have investigated the neurobiologic factors responsible for cancer pain; however, the study of oral cancer pain might inform us about the fundamental biology of cancer. The purpose of the present report was to summarize the clinical challenges inherent in oral cancer pain management, oral cancer pain mechanisms and mediators, and the convergence of the investigation of carcinogenesis and pain.

Mechanisms mediating nitroglycerin-induced delayed-onset hyperalgesia in the rat

Nitroglycerin (glycerol trinitrate, GTN) induces headache in migraineurs, an effect that has been used both diagnostically and in the study of the pathophysiology of this neurovascular pain syndrome. An important feature of this headache is a delay from the administration of GTN to headache onset that, because of GTN's very rapid metabolism, cannot be due to its pharmacokinetic profile. It has recently been suggested that activation of perivascular mast cells, which has been implicated in the pathophysiology of migraine, may contribute to this delay. We reported that hyperalgesia induced by intradermal GTN has a delay to onset of ∼ 30 min in male and ∼ 45 min in female rats. This hyperalgesia was greater in females, was prevented by pretreatment with the anti-migraine drug, sumatriptan, as well as by chronic pretreatment with the mast cell degranulator, compound 48/80. The acute administration of GTN and compound 48/80 both induced hyperalgesia that was prevented by pretreatment with octoxynol-9, which attenuates endothelial function, suggesting that GTN and mast cell-mediated hyperalgesia are endothelial cell-dependent. Furthermore, A-317491, a P2X3 antagonist, which inhibits endothelial cell-dependent hyperalgesia, also prevents GTN and mast cell-mediated hyperalgesia. We conclude that delayed-onset mechanical hyperalgesia induced by GTN is mediated by activation of mast cells, which in turn release mediators that stimulate endothelial cells to release ATP, to act on P2X3, a ligand-gated ion channel, in perivascular nociceptors. A role of the mast and endothelial cell in GTN-induced hyperalgesia suggests potential novel risk factors and targets for the treatment of migraine.

Purinergic Mechanisms and Pain

There is a brief introductory summary of purinergic signaling involving ATP storage, release, and ectoenzymatic breakdown, and the current classification of receptor subtypes for purines and pyrimidines. The review then describes purinergic mechanosensory transduction involved in visceral, cutaneous, and musculoskeletal nociception and on the roles played by receptor subtypes in neuropathic and inflammatory pain. Multiple purinoceptor subtypes are involved in pain pathways both as an initiator and modulator. Activation of homomeric P2X3 receptors contributes to acute nociception and activation of heteromeric P2X2/3 receptors appears to modulate longer-lasting nociceptive sensitivity associated with nerve injury or chronic inflammation. In neuropathic pain activation of P2X4, P2X7, and P2Y12 receptors on microglia may serve to maintain nociceptive sensitivity through complex neural-glial cell interactions and antagonists to these receptors reduce neuropathic pain. Potential therapeutic approaches involving purinergic mechanisms will be discussed.

Contribution of Piezo2 to endothelium-dependent pain

Background

We evaluated the role of a mechanically-gated ion channel, Piezo2, in mechanical stimulation-induced enhancement of hyperalgesia produced by the pronociceptive vasoactive mediator endothelin-1, an innocuous mechanical stimulus-induced enhancement of hyperalgesia that is vascular endothelial cell dependent. We also evaluated its role in a preclinical model of a vascular endothelial cell dependent painful peripheral neuropathy.

Results

The local administration of oligodeoxynucleotides antisense to Piezo2 mRNA, at the site of nociceptive testing in the rat’s hind paw, but not intrathecally at the central terminal of the nociceptor, prevented innocuous stimulus-induced enhancement of hyperalgesia produced by endothelin-1 (100 ng). The mechanical hyperalgesia induced by oxaliplatin (2 mg/kg. i.v.), which was inhibited by impairing endothelial cell function, was similarly attenuated by local injection of the Piezo2 antisense. Polymerase chain reaction analysis demonstrated for the first time the presence of Piezo2 mRNA in endothelial cells.

Conclusions

These results support the hypothesis that Piezo2 is a mechano-transducer in the endothelial cell where it contributes to stimulus-dependent hyperalgesia, and a model of chemotherapy-induced painful peripheral neuropathy.

Nociceptor beta II, delta, and epsilon isoforms of PKC differentially mediate paclitaxel-induced spontaneous and evoked pain

As one of the most effective and frequently used chemotherapeutic agents, paclitaxel produces peripheral neuropathy (paclitaxel-induced peripheral neuropathy or PIPN) that negatively affects chemotherapy and persists after cancer therapy. The mechanisms underlying this dose-limiting side effect remain to be fully elucidated. This study aimed to investigate the role of nociceptor protein kinase C (PKC) isoforms in PIPN. Employing multiple complementary approaches, we have identified a subset of PKC isoforms, namely βII, δ, and ϵ, were activated by paclitaxel in the isolated primary afferent sensory neurons. Persistent activation of PKCβII, PKCδ, and PKCϵ was also observed in the dorsal root ganglion neurons after chronic treatment with paclitaxel in a mouse model of PIPN. Isoform-selective inhibitors of PKCβII, PKCδ, and PKCϵ given intrathecally dose-dependently attenuated paclitaxel-induced mechanical allodynia and heat hyperalgesia. Surprisingly, spinal inhibition of PKCβII and PKCδ, but not PKCϵ, blocked the spontaneous pain induced by paclitaxel. These data suggest that a subset of nociceptor PKC isoforms differentially contribute to spontaneous and evoked pain in PIPN, although it is not clear whether PKCϵ in other regions regulates spontaneous pain in PIPN. The findings can potentially offer new selective targets for pharmacological intervention of PIPN.

Algodystrophy (CRPS) in minor orthopedic surgery

Algodystrophy or Chronic Regional Pain Syndrome (CRPS) is a painful disorder that develops especially at upper or lower extremities of the limbs after a fracture. This syndrome is probably due to bone microvascular changes with subsequent sympathetic nervous system involvement. The pain that characterizes CRPS is spontaneous, disproportionate to the traumatic event and is associated with hyperalgesia, and a variety of autonomic and trophic disorders. This condition has a variable incidence up to 37% of the cases, increasing along with the severity of the fracture. CRPS has a higher chance of developing in women, in older individuals, in smokers, and in patients with reduced bone strength. Early diagnosis is associated with remission in 80-90% of cases. Since the typical onset of the disease is insidious over 2 weeks after surgery, a diagnostic and therapeutic delay may occur. These are the major causes of a high percentage of chronic and disabling complications leading to impaired functional outcomes. In the acute or subacute phase, infusion of bisphosphonates has proven to be the first-choice of treatment with a high percentage of remissions. Moreover, it has been suggested the utility of vitamin C in prevention of CRPS. Furthermore, in the chronic phase electroanalgesia seems to provide promising results.

Homocysteine-induced attenuation of vascular endothelium-dependent hyperalgesia in the rat

We have recently demonstrated a role of the vascular endothelium in peripheral pain mechanism by disrupting endothelial cell function using intravascular administration of octoxynol-9, a non-selective membrane active agent. As an independent test of the role of endothelial cells in pain mechanisms, we evaluated the effect of homocysteine, an agent that damages endothelial cell function. Mechanical stimulus-induced enhancement of endothelin-1 hyperalgesia in the gastrocnemius muscle of the rat was first prevented then enhanced by intravenous administration of homocysteine, but was only inhibited by its precursor, methionine. Both homocysteine and methionine significantly attenuated mechanical hyperalgesia in two models of ergonomic muscle pain, induced by exposure to vibration, and by eccentric exercise, and cutaneous mechanical hyperalgesia in an ischemia-reperfusion injury model of Complex Regional Pain Syndrome type I, all previously shown responsive to octoxynol-9. This study provides independent support for a role of the endothelial cell in pain syndromes thought to have a vascular basis, and suggests that substances that are endothelial cell toxins can enhance vascular pain.

Role of nociceptor estrogen receptor GPR30 in a rat model of endometriosis pain

Endometriosis, the most common cause of chronic pelvic pain, is an estrogen-dependent disease in which classic estrogen receptors (ERα, ERβ) play an important role. Although recent evidence suggests that the novel G protein-coupled estrogen receptor (GPR30) also plays a key role in the progression of endometriosis, whether it is also involved in endometriosis pain is still unknown. Here we tested the hypothesis that GPR30 expressed by nociceptors contributes to endometriosis pain. Intramuscular injection of the GPR30 agonists raloxifene or 17β-estradiol produced a fast-onset, persistent, mechanical hyperalgesia at the site of the injection. Intrathecal antisense (AS) oligodeoxynucleotides (ODN), but not mismatch (MM) ODN, targeting mRNA for GPR30 markedly inhibited its protein expression in nociceptors and attenuated the mechanical hyperalgesia induced by local raloxifene or 17β-estradiol. Pretreatment with the GPR30 antagonist G-36 also inhibited the hyperalgesia induced by raloxifene or 17β-estradiol in naive control rats. Surgical implant of autologous uterine tissue onto the gastrocnemius muscle, which induces endometriosis-like lesions, produced local mechanical hyperalgesia. Intrathecal AS, but not MM, ODN targeting GPR30 mRNA reversibly inhibited the mechanical hyperalgesia at the site of endometriotic lesions. Finally, intralesional injection of the GPR30 antagonist G-36 also inhibited the mechanical hyperalgesia at the site of ectopic uterine tissue. We conclude that local GPR30 agonists produce persistent mechanical hyperalgesia in naive female rats, whereas local GPR30 antagonists inhibit mechanical hyperalgesia in a model of endometriosis pain. Thus, GPR30 expressed by nociceptors innervating ectopic uterine lesions might play a major role in endometriosis pain.

Elevated peritoneal expression and estrogen regulation of nociceptive ion channels in endometriosis

CONTEXT

Ovarian suppression is a common treatment for endometriosis-associated pelvic pain. Its exact mechanism of action is poorly understood, although it is assumed to reflect reduced production/action of estrogens.

OBJECTIVE

The objective of the study was to measure the expression of mRNAs encoded by nociceptive genes in the peritoneum of women with chronic pelvic pain (CPP) with or without endometriosis and to investigate whether estrogens alter nociceptive gene expression in human sensory neurons.

DESIGN

The study was performed using human tissue analysis and cell culture.

SETTING

The study was conducted at a university research institute.

PATIENTS

Peritoneal biopsies were obtained from women with CPP and endometriosis (n = 12), CPP and no endometriosis (n = 10), and no pain or endometriosis (n = 5). Endometriosis lesions were obtained from women with endometriosis (n = 18).

MAIN OUTCOME MEASURES

mRNAs encoding ion channels (P2RX3, SCN9A, SCN11A, TRPA1, TRPV1) and the neurotransmitter TAC1 were measured in human tissue samples and in human embryonic stem cell-derived sensory neurons treated with estrogens.

RESULTS

TRPV1, TRPA1, and SCN11A mRNAs were significantly higher in the peritoneum from women with endometriosis (P < .001, P < .01). TRPV1, SCN9A, and TAC1 were elevated in endometriosis lesions (P < .05). P2RX3 mRNA was increased in the peritoneum of women with CPP, with and without endometriosis (P < .05). Incubation of sensory neurons with 17β-estradiol increased TRPV1 mRNA (P < .01). The estrogen receptor-β-selective agonist 2,3-bis(4-hydroxy-phenyl)-propionitrile increased concentrations of TRPV1, P2RX3, SCN9A, and TAC1 mRNAs.

CONCLUSIONS

Estrogen-dependent expression of TRPV1 in sensory neurons may explain why ovarian suppression can reduce endometriosis-associated pain. Strategies directly targeting ion channels may offer an alternative option for the management of CPP.

Regulation of alternative VEGF-A mRNA splicing is a therapeutic target for analgesia

Vascular endothelial growth factor-A (VEGF-A) is best known as a key regulator of the formation of new blood vessels. Neutralization of VEGF-A with anti-VEGF therapy e.g. bevacizumab, can be painful, and this is hypothesized to result from a loss of VEGF-A-mediated neuroprotection. The multiple vegf-a gene products consist of two alternatively spliced families, typified by VEGF-A₁₆₅a and VEGF-A₁₆₅b (both contain 165 amino acids), both of which are neuroprotective. Under pathological conditions, such as in inflammation and cancer, the pro-angiogenic VEGF-A₁₆₅a is upregulated and predominates over the VEGF-A₁₆₅b isoform.

We show here that in rats and mice VEGF-A₁₆₅a and VEGF-A₁₆₅b have opposing effects on pain, and that blocking the proximal splicing event – leading to the preferential expression of VEGF-A₁₆₅b over VEGF₁₆₅a – prevents pain in vivo. VEGF-A₁₆₅a sensitizes peripheral nociceptive neurons through actions on VEGFR2 and a TRPV1-dependent mechanism, thus enhancing nociceptive signaling. VEGF-A₁₆₅b blocks the effect of VEGF-A₁₆₅a.

After nerve injury, the endogenous balance of VEGF-A isoforms switches to greater expression of VEGF-A_xxxa compared to VEGF-A_xxxb, through an SRPK1-dependent pre-mRNA splicing mechanism. Pharmacological inhibition of SRPK1 after traumatic nerve injury selectively reduced VEGF-A_xxxa expression and reversed associated neuropathic pain. Exogenous VEGF-A₁₆₅b also ameliorated neuropathic pain.

We conclude that the relative levels of alternatively spliced VEGF-A isoforms are critical for pain modulation under both normal conditions and in sensory neuropathy. Altering VEGF-A_xxxa/VEGF-A_xxxb balance by targeting alternative RNA splicing may be a new analgesic strategy.

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The different vegf-a splice variants, VEGF-A₁₆₅a and VEGF-A₁₆₅b have pro- and anti-nociceptive actions respectively.

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Pro-nociceptive actions of VEGF-A₁₆₅a are dependent on TRPV1.

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Alternative pre-mRNA splicing underpins peripheral sensitization by VEGF-A isoforms in normal and neuropathic animals.

ATP release mechanisms of endothelial cell-mediated stimulus-dependent hyperalgesia

Endothelin-1 (ET-1) acts on endothelial cells to enhance mechanical stimulation-induced release of adenosine triphosphate (ATP), which in turn can act on sensory neurons innervating blood vessels to contribute to vascular pain, a phenomenon we have referred to as stimulus-dependent hyperalgesia (SDH). In the present study, we evaluated the role of the major classes of ATP release mechanisms to SDH: vesicular exocytosis, plasma membrane-associated ATP synthase, ATP-binding cassette transporters, and ion channels. Inhibitors of vesicular exocytosis (ie, monensin, brefeldin A, and bafilomycin), plasma membrane-associated ATPase (ie, oligomycin and pigment epithelium-derived factor peptide 34-mer), and connexin ion channels (carbenoxolone and flufenamic acid) but not ATP-binding cassette transporter (ie, dipyridamole, nicardipine, or CFTRinh-172) attenuated SDH. This study reports a role of ATP in SDH and suggests novel targets for the treatment of vascular pain syndromes.

PERSPECTIVE

ET-1 acts on endothelial cells to produce mechanical stimulation-induced hyperalgesia. Inhibitors of 3 different ATP release mechanisms attenuated this SDH. This study provides support for a role of ATP in SDH and suggests novel targets for the treatment of vascular pain syndromes.

Screening the role of pronociceptive molecules in a rodent model of endometriosis pain

Chronic pain is a major symptom in patients with endometriosis, a common gynecologic condition affecting women in their reproductive years. Although many proalgesic substances are produced by endometriosis lesions, experimental evidence supporting their relative roles is still lacking. Furthermore, it is unclear whether these proalgesic agents directly activate nociceptors to induce endometriosis pain. To determine their relative contribution to pain associated with endometriosis, we evaluated the intrathecal administration of oligodeoxynucleotides (ODNs) antisense to messenger RNA for receptors for 3 pronociceptive mediators known to be produced by the ectopic endometrium. Two weeks after the implant of autologous uterine tissue onto the gastrocnemius muscle, local mechanical hyperalgesia was observed in operated rats. Intrathecal antisense ODN targeting messenger RNA for the interleukin 6 receptor-signaling complex subunit glycoprotein 130 and the nerve growth factor tyrosine kinase receptor A, but not their mismatch ODNs, reversibly attenuated mechanical hyperalgesia at the implant site. In contrast, intrathecal antisense ODN targeting the tumor necrosis factor receptor 1, at a dose that markedly inhibited intramuscularly injected tumor necrosis factor alpha, had only a small antihyperalgesic effect in this model. These results indicate the relative contribution of pronociceptive mediators produced by ectopic endometrial tissue to endometriosis pain. The experimental approach presented here provides a novel method to evaluate for the differential contribution of mediators produced by other painful lesions as well as endometriosis lesions as targets for novel treatment of pain syndromes.

PERSPECTIVE

This article presents evidence for the relative contribution of proalgesic mediators to primary hyperalgesia displayed by rats submitted to a model of endometriosis pain. This approach can be used to identify potential targets for the treatment of endometriosis pain.

Cold hypersensitivity increases with age in mice with sickle cell disease

Sickle cell disease (SCD) is associated with acute vaso-occlusive crises that trigger painful episodes and frequently involves ongoing, chronic pain. In addition, both humans and mice with SCD experience heightened cold sensitivity. However, studies have not addressed the mechanism(s) underlying the cold sensitization or its progression with age. Here we measured thermotaxis behavior in young and aged mice with severe SCD. Sickle mice had a marked increase in cold sensitivity measured by a cold preference test. Furthermore, cold hypersensitivity worsened with advanced age. We assessed whether enhanced peripheral input contributes to the chronic cold pain behavior by recording from C fibers, many of which are cold sensitive, in skin-nerve preparations. We observed that C fibers from sickle mice displayed a shift to warmer (more sensitive) cold detection thresholds. To address mechanisms underlying the cold sensitization in primary afferent neurons, we quantified mRNA expression levels for ion channels thought to be involved in cold detection. These included the transient receptor potential melastatin 8 (Trpm8) and transient receptor potential ankyrin 1 (Trpa1) channels, as well as the 2-pore domain potassium channels, TREK-1 (Kcnk2), TREK-2 (Kcnk10), and TRAAK (Kcnk4). Surprisingly, transcript expression levels of all of these channels were comparable between sickle and control mice. We further examined transcript expression of 83 additional pain-related genes, and found increased mRNA levels for endothelin 1 and tachykinin receptor 1. These factors may contribute to hypersensitivity in sickle mice at both the afferent and behavioral levels.

Does the antihyperalgesic disruptor of endothelial cells, octoxynol-9, alter nociceptor function?

The vasoactive mediator, endothelin-1, elicits a novel form of hyperalgesia, stimulation-dependent hyperalgesia. Acting on its cognate receptor on the vascular endothelial cell, endothelin-1 produces a state in which mechanical stimulation now elicits release of pronociceptive mediators from endothelium that, in turn, acts at receptors on sensory neurons. The only evidence that octoxynol-9, a surface-active agent that attenuates both endothelial cell function and stimulus-dependent hyperalgesia, does not affect nociceptors is indirect (i.e., octoxynol-9 treatment did not affect behavioral nociceptive threshold or hyperalgesia induced by agents that act directly on nociceptors). To help address the question of whether the attenuation of stimulation-dependent hyperalgesia by octoxynol-9 treatment is due to alteration of nociceptor function, we used in vivo single-fiber electrophysiological recordings. Consistent with our previous behavioral observations, we observed no significant effect of octoxynol-9 on mechanical threshold in nociceptors, their response to sustained suprathreshold mechanical stimulation, conduction velocity, and change in mechanical threshold in response to the direct-acting hyperalgesic agent, PGE2. Although octoxynol-9 did not produce a biologically meaningful change in parameters of nociceptor function, we cannot exclude the possibility of a type II error. However, our data provide preliminary evidence of no effect of octoxynol-9 on nociceptors and are consistent with the suggestion that the primary action of octoxynol-9 in our studies is due to its action on the endothelium.

The neurobiology of cancer pain

The global burden of cancer pain is enormous and opioids, despite their side effects, remain the primary therapeutic approach. The cause of cancer pain is unknown. Mechanisms driving cancer pain differ from those mechanisms responsible for inflammatory and neuropathic pain. The prevailing hypothesis put forward to explain cancer pain posits that cancers generate and secrete mediators which sensitize and activate primary afferent nociceptors in the cancer microenvironment. Moreover, cancers induce neurochemical reorganization of the spinal cord, which contributes to spontaneous activity and enhanced responsiveness. The purpose of this review, which covers clinical and preclinical studies, is to highlight those peripheral and central mechanisms responsible for cancer pain. The challenges facing neuroscientists and clinicians studying and ultimately treating cancer pain are discussed.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

Mechanism of pain generation for endometriosis-associated pelvic pain

PURPOSE

Endometriosis-associated pelvic pain appears due to persistent nociceptive stimulation, but the precise mechanisms remain poorly understood.

METHODS

A search was conducted to screen and select articles from PubMed.

MAIN RESULTS

Neurotrophins (NTs), a family of neuronal growth factors, are overexpressed in endometriosis and encompass NGF, BDNF and NT-3 and NT-4/5. NT receptors, TrkA and p75NTR, and NT receptor-interacting proteins, MAGE and NDN, were also expressed. NTs and their receptors play a role in the development and maintenance of neural tissues in non-neuronal cell types such as endometriosis. Nerve fibers contain unmyelinated sensory C, myelinated sensory Adelta and adrenergic nerve fibers that innervate abnormal cell growths. An increased release of proinflammatory cytokines from endometriotic lesions is responsible for the excessive sensory innervation and development of chronic pelvic pain.

CONCLUSIONS

The preponderance of the inflammatory milieu and subsequent hyperinnervation might be involved in the pathophysiology of pain generation in women with endometriosis.

Expression of purinergic P2X receptor subtypes 1, 2, 3 and 7 in equine laminitis

Tissue sensitisation and chronic pain have been described in chronic-active laminitis in the horse, making treatment of such cases difficult. Purinergic P2X receptors are linked to chronic pain and inflammation. The aim of this study was to examine the expression of purinergic P2X receptor subtypes 1, 2, 3 and 7 in the hoof, palmar digital vessels and nerve, dorsal root ganglia and spinal cord in horses with chronic-active laminitis (n=5) compared to non-laminitic horses (n=5). Immunohistochemical analysis was performed on tissue sections using antibodies against P2X receptor subtypes 1-3 and 7. In horses with laminitis, there was a reduction in the thickness of the tunica media layer of the palmar digital vein as a proportion of the whole vessel diameter (0.48±0.05) compared to the non-laminitic group (0.57±0.04; P=0.02). P2X receptor subtype 3 was expressed in the smooth muscle layer (tunica media) of the palmar digital artery of horses with laminitis, but was absent in horses without laminitis. There was strong expression of P2X receptor subtype 7 in the proliferating, partially keratinised, epidermal cells of the secondary epidermal lamellae in the hooves of horses with laminitis, but no immunopositivity in horses without laminitis.