PAR-2 agonists activate trigeminal nociceptors and induce functional competence in the delta opioid receptor

Activation of Protease-Activated Receptor-1 Causes Chronic Pain in Lupus-Prone Mice Via Suppressing Spinal Glial Glutamate Transporter Function and Enhancing Glutamatergic Synaptic Activity

Systemic lupus erythematosus (SLE) is an unpredictable autoimmune disease where the body's immune system mistakenly attacks healthy tissues in many parts of the body. Chronic pain is one of the most frequently reported symptoms among SLE patients. We previously reported that MRL lupus prone (MRL/lpr) mice develop hypersensitivity to mechanical and heat stimulation. In the present study, we found that the spinal protease-activated receptor-1(PAR1) plays an important role in the genesis of chronic pain in MRL/lpr mice. Female MRL/lpr mice with chronic pain had activation of astrocytes, over-expression of thrombin and PAR1, enhanced glutamatergic synaptic activity, as well as suppressed activity of adenosine monophosphate-activated protein kinase (AMPK) and glial glutamate transport function in the spinal cord. Intrathecal injection of either the PAR1 antagonist, or AMPK activator attenuated heat hyperalgesia and mechanical allodynia in MRL/lpr mice. Furthermore, we also identified that the enhanced glutamatergic synaptic activity and suppressed activity of glial glutamate transporters in the spinal dorsal horn of MRL/lpr mice are caused by activation of the PAR1 and suppression of AMPK signaling pathways. These findings suggest that targeting the PAR1 and AMPK signaling pathways in the spinal cord may be a useful approach for treating chronic pain caused by SLE. PERSPECTIVE: Our study provides evidence suggesting activation of PAR1 and suppression of AMPK in the spinal cord induces thermal hyperalgesia and mechanical allodynia in a lupus mouse model. Targeting signaling pathways regulating the PAR1 and AMPK could potentially provide a novel approach to the management of chronic pain caused by SLE.

Protease-activated receptors in health and disease

Proteases are signaling molecules that specifically control cellular functions by cleaving protease-activated receptors (PARs). The four known PARs are members of the large family of G protein-coupled receptors. These transmembrane receptors control most physiological and pathological processes and are the target of a large proportion of therapeutic drugs. Signaling proteases include enzymes from the circulation; from immune, inflammatory epithelial, and cancer cells; as well as from commensal and pathogenic bacteria. Advances in our understanding of the structure and function of PARs provide insights into how diverse proteases activate these receptors to regulate physiological and pathological processes in most tissues and organ systems. The realization that proteases and PARs are key mediators of disease, coupled with advances in understanding the atomic level structure of PARs and their mechanisms of signaling in subcellular microdomains, has spurred the development of antagonists, some of which have advanced to the clinic. Herein we review the discovery, structure, and function of this receptor system, highlight the contribution of PARs to homeostatic control, and discuss the potential of PAR antagonists for the treatment of major diseases.

14-3-3γ mediates the long-term inhibition of peripheral kappa opioid receptor antinociceptive signaling by norbinaltorphimine

Long-term inhibition of kappa opioid receptor (KOR) signaling in peripheral pain-sensing neurons is a potential obstacle for development of peripherally-restricted KOR agonists that produce analgesia. Such a long-term inhibitory mechanism is invoked from activation of c-Jun N-terminal kinase (JNK) that follows a single injection of the KOR antagonist norbinaltorphimine (norBNI). This effect requires protein synthesis of an unknown mediator in peripheral pain-sensing neurons. Using 2D difference gel electrophoresis with tandem mass spectrometry, we have identified that the scaffolding protein 14-3-3γ is upregulated in peripheral sensory neurons following activation of JNK with norBNI. Knockdown of 14-3-3γ by siRNA eliminates the long-term reduction in KOR-mediated cAMP signaling by norBNI in peripheral sensory neurons in culture. Similarly, knockdown of 14-3-3γ in the rat hind paw abolished the norBNI-mediated long-term reduction in peripheral KOR-mediated antinociception. Further, overexpression of 14-3-3γ in KOR expressing CHO cells prevented KOR-mediated inhibition of cAMP signaling. These long-term effects are selective for KOR as heterologous regulation of other receptor systems was not observed. These data suggest that 14-3-3γ is both necessary and sufficient for the long-term inhibition of KOR by norBNI in peripheral sensory neurons.

Age-related changes in peripheral nociceptor function

Pain and pain management in the elderly population is a significant social and medical problem. Pain sensation is a complex phenomenon that typically involves activation of peripheral pain-sensing neurons (nociceptors) which send signals to the spinal cord and brain that are interpreted as pain, an unpleasant sensory experience. In this work, young (4-5 months) and aged (26-27 months) Fischer 344 x Brown Norway (F344xBN) rats were examined for nociceptor sensitivity to activation by thermal (cold and heat) and mechanical stimulation following treatment with inflammatory mediators and activators of transient receptor potential (TRP) channels. Unlike other senses that decrease in sensitivity with age, sensitivity of hindpaw nociceptors to thermal and mechanical stimulation was not different between young and aged F344xBN rats. Intraplantar injection of bradykinin (BK) produced greater thermal and mechanical allodynia in aged versus young rats, whereas only mechanical allodynia was greater in aged rats following injection of prostaglandin E₂ (PGE₂). Intraplantar injection of TRP channel activators, capsaicin (TRPV1), mustard oil (TRPA1) and menthol (TRPM8) each resulted in greater mechanical allodynia in aged versus young rats and capsaicin-induced heat allodynia was also greater in aged rats. A treatment-induced allodynia that was greater in young rats was never observed. The anti-allodynic effects of intraplantar injection of kappa and delta opioid receptor agonists, salvinorin-A and D-Pen²,D-Pen⁵]enkephalin (DPDPE), respectively, were greater in aged than young rats, whereas mu opioid receptor agonists, [D-Ala2, N-MePhe4, Gly-ol]-enkephalin (DAMGO) and morphine, were not effective in aged rats. Consistent with these observations, in primary cultures of peripheral sensory neurons, inhibition of cAMP signaling in response to delta and kappa receptor agonists was greater in cultures derived from aged rats. By contrast, mu receptor agonists did not inhibit cAMP signaling in aged rats. Thus, age-related changes in nociceptors generally favor increased pain signaling in aged versus young rats, suggesting that changes in nociceptor sensitivity may play a role in the increased incidence of pain in the elderly population. These results also suggest that development of peripherally-restricted kappa or delta opioid receptor agonists may provide safer and effective pain relief for the elderly.

Delta opioid receptor regulation of calcitonin gene-related peptide dynamics in the trigeminal complex

ABSTRACT

Migraine is highly prevalent and is the sixth leading cause worldwide for years lost to disability. Therapeutic options specifically targeting migraine are limited, and delta opioid receptor (DOP) agonists were recently identified as a promising pharmacotherapy. The mechanisms by which DOPs regulate migraine are currently unclear. Calcitonin gene-related peptide (CGRP) has been identified as an endogenous migraine trigger and plays a critical role in migraine initiation and susceptibility. The aim of this study was to determine the behavioral effects of DOP agonists on the development of chronic migraine-associated pain and to investigate DOP coexpression with CGRP and CGRP receptor (CGRPR) in the trigeminal system. Chronic migraine-associated pain was induced in mice through repeated intermittent injection of the known human migraine trigger, nitroglycerin. Chronic nitroglycerin resulted in severe chronic cephalic allodynia which was prevented with cotreatment of the DOP-selective agonist, SNC80. In addition, a corresponding increase in CGRP expression in the trigeminal ganglia and trigeminal nucleus caudalis was observed after chronic nitroglycerin, an augmentation that was blocked by SNC80. Moreover, DOP was also upregulated in these head pain-processing regions following the chronic migraine model. Immunohistochemical analysis of the trigeminal ganglia revealed coexpression of DOP with CGRP as well as with a primary component of the CGRPR, RAMP1. In the trigeminal nucleus caudalis, DOP was not coexpressed with CGRP but was highly coexpressed with RAMP1 and calcitonin receptor-like receptor. These results suggest that DOP agonists inhibit migraine-associated pain by attenuating CGRP release and blocking pronociceptive signaling of the CGRPR.

Peripherally Acting Opioids in Orofacial Pain

The activation of opioid receptors by exogenous or endogenous opioids can produce significant analgesic effects in peripheral tissues. Numerous researchers have demonstrated the expression of peripheral opioid receptors (PORs) and endogenous opioid peptides (EOPs) in the orofacial region. Growing evidence has shown the involvement of PORs and immune cell-derived EOPs in the modulation of orofacial pain. In this review, we discuss the role of PORs and EOPs in orofacial pain and the possible cellular mechanisms involved. Furthermore, the potential development of therapeutic strategies for orofacial pain is also summarized.

Characterization and preclinical evaluation of a protease activated receptor 2 (PAR2) monoclonal antibody as a preventive therapy for migraine

AIM

Migraine pain is thought to result from activation of meningeal nociceptors that might involve dural mast cell degranulation and release of proteases and pronociceptive mediators. Tryptase, the most abundant dural mast cell protease, has been demonstrated to stimulate dural mast cells, as well as trigeminal nociceptors by activating the protease activated receptor 2. Mast cell or neuronal protease activated receptors 2 may therefore represent a novel target for migraine treatment. In this study, we characterized and evaluated a novel protease activated receptor 2 monoclonal antibody as a preventive anti-migraine pain therapy in preclinical models.

METHODS

Flow cytometry, immunocytochemistry, calcium imaging, Homogeneous Time Resolved Technology (HTRF) epitope competition assay and serum pharmacokinetic (PK) assay in rats were performed to confirm the activity, specificity and in vivo stability of PAR650097, a novel anti- protease activated receptor 2 monoclonal antibody. In vivo assessment was performed in female C57BL/6J mice by evaluation of PAR650097 in preventing cutaneous allodynia elicited by (a) supradural injection of the protease activated receptor 2 agonist, Ser-Leu-Ile-Gly-Arg-Leu-amide trifluoroacetate (SLIGRL), or calcitonin gene-related (CGRP) peptide, and (b) induction of latent sensitization by priming with three daily episodes of restraint stress followed by challenge with a subthreshold inhalational exposure to umbellulone (UMB), a transient receptor potential ankyrin 1 (TRPA1) agonist. PAR650097 was administered as a pretreatment prior to the first restraint stress, umbellulone exposure, SLIGRL or calcitonin gene-related peptide injection. Additionally, fremanezumab, a calcitonin gene-related peptide antibody was administered as pre-treatment prior to supradural administration of calcitonin gene-related peptide or SLIGRL.

RESULTS

In vitro, PAR650097 demonstrated rapid interaction with protease activated receptor 2, enabling it to fully inhibit protease-induced protease activated receptor 2 activation, in human and mouse cells, with high potency. Furthermore, PAR650097 was highly selective for protease activated receptor 2, demonstrating no affinity for protease activated receptor 1 protein and no functional effect on the activation of cellular protease activated receptor 1 with thrombin. In addition, PAR650097 had an acceptable PK profile, compatible with testing the effects of selective protease activated receptor 2 inhibition in vivo. In vivo, PAR650097 blocked cutaneous allodynia induced by either supradural SLIGRL or calcitonin gene-related peptide. Fremanezumab abolished cutaneous allodynia induced by supradural CGRP, and partially attenuated cutaneous allodynia induced by SLIGRL. Administration of PAR650097, before the first restraint stress episode, did not prevent the acute stress-induced cutaneous allodynia or restraint stress priming revealed by cutaneous allodynia induced by inhalational umbellulone. In contrast, PAR650097 prevented expression of cutaneous allodynia when given before the umbellulone challenge in restraint stress-primed animals.

CONCLUSION

PAR650097 specifically inhibits endogenously expressed protease activated receptor 2 in human and mouse cells with high potency. This antibody has an acceptable PK profile in rodents and effectively blocked SLIGR-induced cutaneous allodynia. PAR650097 additionally prevented cutaneous allodynia induced by supradural calcitonin gene-related peptide, indicating that the protease activated receptor 2 receptor is a downstream consequence of calcitonin gene-related peptide actions. Fremanezumab effectively blocked calcitonin gene-related peptide-induced cutaneous allodynia and only partially reduced cutaneous allodynia induced by a protease activated receptor 2 activator, suggesting both calcitonin gene-related peptide-dependent and -independent mechanisms in promoting migraine pain. While PAR650097 did not prevent stress-induced cutaneous allodynia or priming, it effectively prevented cutaneous allodynia induced by a TRPA1 agonist in animals with latent sensitization. Activation of protease activated receptor 2, therefore, contributes to both calcitonin gene-related peptide-dependent and -independent mechanisms in promoting migraine-like pain. Therapeutic targeting of protease activated receptor 2 receptors may represent an anti-migraine pain strategy with a potentially broad efficacy profile.

Quantitative proteomic analysis of human plasma using tandem mass tags to identify novel biomarkers for herpes zoster

Herpes zoster (HZ), commonly called shingles, it is a distinctive syndrome caused by reactivation of varicella zoster virus (VZV). A better understanding of the biological characteristics of HZ patients can help develop new targeted therapies to improve the prognosis. High-throughput proteomics technology can deeply study the molecular changes in the development and progression of HZ disease and integrate different levels of information, this is important to help make clinical decisions. Circulating blood contains a lot of biological information, we conducted a proteomics study of patient plasma, hoping to identify key proteins that could indicate the development of HZ. Compared to healthy human plasma, we found 44 differentially expressed proteins in the plasma of HZ patients, the main pathways involved in these molecules are MAPK signaling pathway, Neuroactive ligand-receptor interaction, Acute myeloid leukemia, Transcriptional misregulation in cancer. We found that 27 proteins have direct protein-protein interactions. Based on the comprehensive score, we identified six key molecules as candidate molecules for further study, and then validated another 80 plasma samples (40 HZ patient plasma and 40 healthy human plasma) using enzyme-linked immunosorbent assay (ELISA), immunoblot assay and receiver operating characteristic (ROC) curve analysis. Finally, we found that the expression levels of these three proteins (PLG, F2, VTN) were significantly lower than those of healthy controls (P < .05). To the best of our knowledge, we first used tandem mass tag (TMT) combined with liquid chromatography-mass spectrometry (LC-MS/MS) to screen for differentially expressed proteins in plasma between HZ patients and healthy individuals. It is preliminarily proved that the plasma protein expression profile of HZ patients is different from that of uninfected patients, it has also been found that these three altered key proteins may be used as biomarkers to test early HZ infection. This study reveals new insights into HZ that help to more accurately identify early HZ patients and to find new therapeutic targets. SIGNIFICANCE: Varicella-zoster virus (VZV; termed human alphaherpesvirus 3 by the International Committee on Taxonomy of Viruses) is a herpesvirus that is ubiquitous in humans and can cause chickenpox and herpes zoster (HZ). After the initial infection of varicella, the VZV goes into a dormant state in the sensory ganglia and cranial nerves. As age or immunosuppression increases, the cellular immunity to VZV decreases, and the virus reactivates and spreads along the sensory nerves to the skin, causing a unique prodromal pain followed by a rash. About one in five people around the world may be infected with VZV at some point in their lives. According to statistics, about one-third of infected people will develop HZ in their lifetime, and an estimated 1 million cases of herpes zoster occur in the United States each year. Herpes zoster can occur at any age and is usually less severe in children and young adults, but the greatest morbidity and mortality are observed in elderly and immunocompromised patients. 20% of patients with HZ have complications including vasculitis, increased risk of myocardial infarction, or postherpetic neuralgia, the overall mortality rate of patients with HZ in the United States is close to 5%. Considering the wide clinical severity and complications of this disease, there is a great need for biomarkers that contribute to early diagnosis, classification of risks, and prediction of outcomes, which will help elucidate the mechanisms underlying their clinical development. As a useful tool in biology, quantitative proteomics can repeatedly identify and accurately quantify proteins in a variety of biological samples. Proteomic analysis focuses on translational proteins, which play a direct role in most biological processes. Although a small number of proteins can be studied simultaneously with traditional methods, such as ELISA and Western blotting, typical proteomics studies can simultaneously analyze thousands of proteins for a more comprehensive identification. Proteomics has been successfully applied to human-based disease research, Analysis of exposed and unexposed subjects based on mass spectrometry (MS) has been found to reveal altered expression of proteins that can be identified as intermediate biomarkers of early disease effects. Tandem mass tags (TMTs) are chemical labels used for MS-based identification and quantification of biological molecules. TMTs play an important role in proteomic analysis in a variety of samples such as cells, tissues, and body fluids. The body fluids that are often detected clinically are blood, which are easy to obtain and contain abundant biological information related to physiological and pathological processes, we hope to develop protein biomarkers from these blood. Therefore, in order to better characterize the pathological process of HZ patients, we performed proteomic analysis of HZ patients and healthy human plasma using the TMT method. This comparison aims to identify specific processes in the development of HZ disease through protein profiling, which may help to improve our biological understanding of HZ.

Alleviating pain with delta opioid receptor agonists: evidence from experimental models

The use of opioids for the relief of pain and headache disorders has been studied for years. Nowadays, particularly because of its ability to produce analgesia in various pain models, delta opioid receptor (DOPr) emerges as a promising target for the development of new pain therapies. Indeed, their potential to avoid the unwanted effects commonly observed with clinically used opioids acting at the mu opioid receptor (MOPr) suggests that DOPr agonists could be a therapeutic option. In this review, we discuss the use of opioids in the management of pain in addition to describing the evidence of the analgesic potency of DOPr agonists in animal models.

Protease activated receptor 2 (PAR2) activation causes migraine-like pain behaviors in mice

BACKGROUND

Pain is the most debilitating symptom of migraine. The cause of migraine pain likely requires activation of meningeal nociceptors. Mast cell degranulation, with subsequent meningeal nociceptor activation, has been implicated in migraine pathophysiology. Degranulating mast cells release serine proteases that can cleave and activate protease activated receptors. The purpose of these studies was to investigate whether protease activated receptor 2 is a potential generator of nociceptive input from the meninges by using selective pharmacological agents and knockout mice.

METHODS

Ratiometric Ca++ imaging was performed on primary trigeminal and dural cell cultures after application of 2at-LIGRL-NH2, a specific protease activated receptor 2 agonist. Cutaneous hypersensitivity and facial grimace was measured in wild-type and protease activated receptor 2-/- mice after dural application of 2at-LIGRL-NH2 or compound 48-80, a mast cell degranulator. Behavioral experiments were also conducted in mice after dural application of 2at-LIGRL-NH2 (2AT) in the presence of either C391, a selective protease activated receptor 2 antagonist, or sumatriptan.

RESULTS

2at-LIGRL-NH2 evoked Ca2+ signaling in mouse trigeminal neurons, dural fibroblasts and in meningeal afferents. Dural application of 2at-LIGRL-NH2 or 48-80 caused dose-dependent grimace behavior and mechanical allodynia that were attenuated by either local or systemic application of C391 as well as in protease activated receptor 2-/- mice. Nociceptive behavior after dural injection of 2at-LIGRL-NH2 was also attenuated by sumatriptan.

CONCLUSIONS

Functional protease activated receptor 2 receptors are expressed on both dural afferents and fibroblasts and activation of dural protease activated receptor 2 produces migraine-like behavioral responses. Protease activated receptor 2 may link resident immune cells to meningeal nociceptor activation, driving migraine-like pain and implicating protease activated receptor 2 as a therapeutic target for migraine in humans.

TrpA1 activation in peripheral sensory neurons underlies the ionic basis of pain hypersensitivity in response to vinca alkaloids

Chemotherapy induced peripheral neuropathy (CIPN), a side effect of many anti-cancer drugs including the vinca alkaloids, is characterized by a severe pain syndrome that compromises treatment in many patients. Currently there are no effective treatments for this pain syndrome except for the reduction of anti-cancer drug dose. Existing data supports the model that the pain associated with CIPN is the result of anti-cancer drugs augmenting the function of the peripheral sensory nociceptors but the cellular mechanisms underlying the effects of anti-cancer drugs on sensory neuron function are not well described. Studies from animal models have suggested a number of disease etiologies including mitotoxicity, axonal degeneration, immune signaling, and reduced sensory innervations but these outcomes are the result of prolonged treatment paradigms and do not necessarily represent the early formative events associated with CIPN. Here we show that acute exposure to vinca alkaloids results in an immediate pain syndrome in both flies and mice. Furthermore, we demonstrate that exposure of isolated sensory neurons to vinca alkaloids results in the generation of an inward sodium current capable of depolarizing these neurons to threshold resulting in neuronal firing. These neuronal effects of vinca alkaloids require the transient receptor potential ankyrin-1 (TrpA1) channel, and the hypersensitization to painful stimuli in response to the acute exposure to vinca alkaloids is reduced in TrpA1 mutant flies and mice. These findings demonstrate the direct excitation of sensory neurons by CIPN-causing chemotherapy drugs, and identify TrpA1 as an important target during the pathogenesis of CIPN.

Regulation of δ Opioid Receptor-Mediated Signaling and Antinociception in Peripheral Sensory Neurons by Arachidonic Acid-Dependent 12/15-Lipoxygenase Metabolites

The function of δ opioid receptors (DOR) expressed by peripheral pain-sensing neurons (nociceptors) is regulated by both cyclooxygenase- and lipoxygenase (LOX)-dependent arachidonic acid (AA) metabolites. Whereas cyclooxygenase metabolites enhance responsiveness, LOX metabolites elicit a refractory, nonsignaling state of the DOR receptor system for antinociceptive signaling. In this study, using high-performance liquid chromatography-tandem mass spectrometry analyses, we have found that the 12-/15-LOX metabolites, 12-hydroxyeicosatetraenoic acid (HETE) and 15-HETE, were elevated after treatment of adult rat primary sensory neuron cultures with AA. Exogenously applied 12-HETE and 15-HETE, but not 5-HETE, completely prevented DOR and κ opioid receptor (KOR) agonist-mediated inhibition of prostaglandin E2 (PGE₂)-stimulated cAMP accumulation, but not inhibition, by the 5-HT₁ receptor agonist 5-carboxamidotryptamine in cultured peripheral sensory neurons and in Chinese hamster ovary (CHO) cells heterologously expressing DOR or KOR. Similarly, intraplantar injection of 12- or 15-HETE, either alone or in combination, prevented DOR agonist-mediated inhibition of PGE₂-evoked thermal allodynia. Further, both AA- and carrageenan-mediated induction of the nonresponsive state of the DOR system was blocked by an intraplantar coinjection of the 12-/15-LOX inhibitors baicalein and luteolin. In contrast to the regulation of cAMP signaling, pretreatment with 12- and 15-HETE had no effect on either DOR or KOR agonist- mediated activation of extracellular signal-regulated kinase in peripheral sensory neurons or CHO cells. These results suggest that the analgesic efficacy of peripherally restricted opioids for treatment of inflammatory pain may be enhanced by adjunct inhibition of LOX activity.

Molecular Pharmacology of δ-Opioid Receptors

Opioids are among the most effective analgesics available and are the first choice in the treatment of acute severe pain. However, partial efficacy, a tendency to produce tolerance, and a host of ill-tolerated side effects make clinically available opioids less effective in the management of chronic pain syndromes. Given that most therapeutic opioids produce their actions via µ-opioid receptors (MOPrs), other targets are constantly being explored, among which δ-opioid receptors (DOPrs) are being increasingly considered as promising alternatives. This review addresses DOPrs from the perspective of cellular and molecular determinants of their pharmacological diversity. Thus, DOPr ligands are examined in terms of structural and functional variety, DOPrs' capacity to engage a multiplicity of canonical and noncanonical G protein-dependent responses is surveyed, and evidence supporting ligand-specific signaling and regulation is analyzed. Pharmacological DOPr subtypes are examined in light of the ability of DOPr to organize into multimeric arrays and to adopt multiple active conformations as well as differences in ligand kinetics. Current knowledge on DOPr targeting to the membrane is examined as a means of understanding how these receptors are especially active in chronic pain management. Insight into cellular and molecular mechanisms of pharmacological diversity should guide the rational design of more effective, longer-lasting, and better-tolerated opioid analgesics for chronic pain management.

Identification of a signaling cascade that maintains constitutive δ-opioid receptor incompetence in peripheral sensory neurons

μ-Opioid receptor (MOR) agonists are often used to treat severe pain but can result in adverse side effects. To circumvent systemic side effects, targeting peripheral opioid receptors is an attractive alternative treatment for severe pain. Activation of the δ-opioid receptor (DOR) produces similar analgesia with reduced side effects. However, until primed by inflammation, peripheral DOR is analgesically incompetent, raising interest in the mechanism. We recently identified a novel role for G-protein-coupled receptor kinase 2 (GRK2) that renders DOR analgesically incompetent at the plasma membrane. However, the mechanism that maintains constitutive GRK2 association with DOR is unknown. Protein kinase A (PKA) phosphorylation of GRK2 at Ser-685 targets it to the plasma membrane. Protein kinase A-anchoring protein 79/150 (AKAP), residing at the plasma membrane in neurons, scaffolds PKA to target proteins to mediate downstream signal. Therefore, we sought to determine whether GRK2-mediated DOR desensitization is directed by PKA via AKAP scaffolding. Membrane fractions from cultured rat sensory neurons following AKAP siRNA transfection and from AKAP-knock-out mice had less PKA activity, GRK2 Ser-685 phosphorylation, and GRK2 plasma membrane targeting than controls. Site-directed mutagenesis revealed that GRK2 Ser-685 phosphorylation drives the association of GRK2 with plasma membrane-associated DOR. Moreover, overexpression studies with AKAP mutants indicated that impaired AKAP-mediated PKA scaffolding significantly reduces DOR-GRK2 association at the plasma membrane and consequently increases DOR activity in sensory neurons without a priming event. These findings suggest that AKAP scaffolds PKA to increase plasma membrane targeting and phosphorylation of GRK2 to maintain DOR analgesic incompetence in peripheral sensory neurons.

A PTEN-Regulated Checkpoint Controls Surface Delivery of δ Opioid Receptors

The δ opioid receptor (δR) is a promising alternate target for pain management because δR agonists show decreased abuse potential compared with current opioid analgesics that target the μ opioid receptor. A critical limitation in developing δR as an analgesic target, however, is that δR agonists show relatively low efficacy in vivo, requiring the use of high doses that often cause adverse effects, such as convulsions. Here we tested whether intracellular retention of δR in sensory neurons contributes to this low δR agonist efficacy in vivo by limiting surface δR expression. Using direct visualization of δR trafficking and localization, we define a phosphatase and tensin homolog (PTEN)-regulated checkpoint that retains δR in the Golgi and decreases surface delivery in rat and mice sensory neurons. PTEN inhibition releases δR from this checkpoint and stimulates delivery of exogenous and endogenous δR to the neuronal surface both in vitro and in vivo PTEN inhibition in vivo increases the percentage of TG neurons expressing δR on the surface and allows efficient δR-mediated antihyperalgesia in mice. Together, we define a critical role for PTEN in regulating the surface delivery and bioavailability of the δR, explain the low efficacy of δR agonists in vivo, and provide evidence that active δR relocation is a viable strategy to increase δR antinociception.SIGNIFICANCE STATEMENT Opioid analgesics, such as morphine, which target the μ opioid receptor (μR), have been the mainstay of pain management, but their use is highly limited by adverse effects and their variable efficacy in chronic pain. Identifying alternate analgesic targets is therefore of great significance. Although the δ opioid receptor (δR) is an attractive option, a critical limiting factor in developing δR as a target has been the low efficacy of δR agonists. Why δR agonists show low efficacy is still under debate. This study provides mechanistic and functional data that intracellular localization of δR in neurons is a key factor that contributes to low agonist efficacy, and presents a proof of mechanism that relocating δR improves efficacy.

Delta Opioid Receptor Expression and Function in Primary Afferent Somatosensory Neurons

The functional diversity of primary afferent neurons of the dorsal root ganglia (DRG) generates a variety of qualitatively and quantitatively distinct somatosensory experiences, from shooting pain to pleasant touch. In recent years, the identification of dozens of genetic markers specifically expressed by subpopulations of DRG neurons has dramatically improved our understanding of this diversity and provided the tools to manipulate their activity and uncover their molecular identity and function. Opioid receptors have long been known to be expressed by discrete populations of DRG neurons, in which they regulate cell excitability and neurotransmitter release. We review recent insights into the identity of the DRG neurons that express the delta opioid receptor (DOR) and the ion channel mechanisms that DOR engages in these cells to regulate sensory input. We highlight recent findings derived from DORGFP reporter mice and from in situ hybridization and RNA sequencing studies in wild-type mice that revealed DOR presence in cutaneous mechanosensory afferents eliciting touch and implicated in tactile allodynia. Mechanistically, we describe how DOR modulates opening of voltage-gated calcium channels (VGCCs) to control glutamatergic neurotransmission between somatosensory neurons and postsynaptic neurons in the spinal cord dorsal horn. We additionally discuss other potential signaling mechanisms, including those involving potassium channels, which DOR may engage to fine tune somatosensation. We conclude by discussing how this knowledge may explain the analgesic properties of DOR agonists against mechanical pain and uncovers an unanticipated specialized function for DOR in cutaneous mechanosensation.

Involvement of the coatomer protein complex I in the intracellular traffic of the delta opioid receptor

The delta opioid receptor (DOPr) is known to be mainly expressed in intracellular compartments. It remains unknown why DOPr is barely exported to the cell surface, but it seems that a substantial proportion of the immature receptor is trapped within the endoplasmic reticulum (ER) and the Golgi network. In the present study, we performed LC-MS/MS analysis to identify putative protein partners involved in the retention of DOPr. Analysis of the proteins co-immunoprecipitating with Flag-DOPr in transfected HEK293 cells revealed the presence of numerous subunits of the coatomer protein complex I (COPI), a vesicle-coating complex involved in recycling resident proteins from the Golgi back to the ER. Further analysis of the amino acid sequence of DOPr identified multiple consensus di-lysine and di-arginine motifs within the intracellular segments of DOPr. Using cell-surface ELISA and GST pulldown assays, we showed that DOPr interacts with COPI through its intracellular loops 2 and 3 (ICL2 and ICL3, respectively) and that the mutation of the K¹⁶⁴AK¹⁶⁶ (ICL2) or K²⁵⁰EK²⁵² (ICL3) putative COPI binding sites increased the cell-surface expression of DOPr in transfected cells. Altogether, our results indicate that COPI is a binding partner of DOPr and provide a putative mechanism to explain why DOPr is highly retained inside the cells.

Long-Term Reduction of Kappa Opioid Receptor Function by the Biased Ligand, Norbinaltorphimine, Requires c-Jun N-Terminal Kinase Activity and New Protein Synthesis in Peripheral Sensory Neurons

A single administration of the κ opioid receptor (KOR) antagonist, norbinaltorphimine (norBNI), produces long-term reduction in KOR function in heterologous expression systems and brain that is mediated by activation of c-Jun N-terminal kinase (JNK). In this study, we examined the long-term effects of norBNI on adult rat peripheral sensory neurons in vivo and ex vivo. Following a single intraplantar (i.pl.) injection of norBNI into the hind paw, peripheral KOR-mediated antinociception in the ipsilateral, but not the contralateral, hindpaw was abolished for at least 9 days. By contrast, the antinociceptive response to mu and delta opioid receptor agonists was unaltered. The long-term inhibitory effect on antinociception produced by pretreatment with norBNI required occupancy of peripheral KOR and was completely blocked by i.pl. injection of the JNK inhibitor, SP600125. In cultures of peripheral sensory neurons, norBNI activated JNK for at least 30 minutes. Furthermore, norBNI blocked KOR-mediated inhibition of adenylyl cyclase activity measured 24 hours later in a JNK-dependent manner, but did not block activation of extracellular signal-regulated kinase (ERK). The long-term inhibitory effect of norBNI on KOR function in vivo and ex vivo was blocked by inhibitors of mRNA translation, cycloheximide and rapamycin. These data suggest that in peripheral sensory neurons norBNI is a KOR-biased ligand for activation of JNK signaling, resulting in long-term blockade of some (antinociception, inhibition of adenylyl cyclase activity), but not all (ERK), KOR signaling. Importantly, norBNI elicits de novo protein synthesis in sensory neuron terminals that produces selective long-term regulation of KOR.

GRK2 Constitutively Governs Peripheral Delta Opioid Receptor Activity

Opioids remain the standard for analgesic care; however, adverse effects of systemic treatments contraindicate long-term administration. While most clinical opioids target mu opioid receptors (MOR), those that target the delta class (DOR) also demonstrate analgesic efficacy. Furthermore, peripherally restrictive opioids represent an attractive direction for analgesia. However, opioid receptors including DOR are analgesically incompetent in the absence of inflammation. Here, we report that G protein-coupled receptor kinase 2 (GRK2) naively associates with plasma membrane DOR in peripheral sensory neurons to inhibit analgesic agonist efficacy. This interaction prevents optimal Gβ subunit association with the receptor, thereby reducing DOR activity. Importantly, bradykinin stimulates GRK2 movement away from DOR and onto Raf kinase inhibitory protein (RKIP). protein kinase C (PKC)-dependent RKIP phosphorylation induces GRK2 sequestration, restoring DOR functionality in sensory neurons. Together, these results expand the known function of GRK2, identifying a non-internalizing role to maintain peripheral DOR in an analgesically incompetent state.

Nerve growth factor alters microtubule targeting agent-induced neurotransmitter release but not MTA-induced neurite retraction in sensory neurons

Peripheral neuropathy is a dose-limiting side effect of anticancer treatment with the microtubule-targeted agents (MTAs), paclitaxel and epothilone B (EpoB); however, the mechanisms by which the MTAs alter neuronal function and morphology are unknown. We previously demonstrated that paclitaxel alters neuronal sensitivity, in vitro, in the presence of nerve growth factor (NGF). Evidence in the literature suggests that NGF may modulate the neurotoxic effects of paclitaxel. Here, we examine whether NGF modulates changes in neuronal sensitivity and morphology induced by paclitaxel and EpoB. Neuronal sensitivity was assessed using the stimulated release of calcitonin gene-related peptide (CGRP), whereas morphology of established neurites was evaluated using a high content screening system. Dorsal root ganglion cultures, maintained in the absence or presence of NGF, were treated from day 7 to day 12 in culture with paclitaxel (300nM) or EpoB (30nM). Following treatment, the release of CGRP was stimulated using capsaicin or high extracellular potassium. In the presence of NGF, EpoB mimicked the effects of paclitaxel: capsaicin-stimulated release was attenuated, potassium-stimulated release was slightly enhanced and the total peptide content was unchanged. In the absence of NGF, both paclitaxel and EpoB decreased capsaicin- and potassium-stimulated release and the total peptide content, suggesting that NGF may reverse MTA-induced hyposensitivity. Paclitaxel and EpoB both decreased neurite length and branching, and this attenuation was unaffected by NGF in the growth media. These differential effects of NGF on neuronal sensitivity and morphology suggest that neurite retraction is not a causative factor to alter neuronal sensitivity.

New insights into protease-activated receptor 4 signaling pathways in the pathogenesis of inflammation and neuropathic pain: a literature review

Pain is an unpleasant sensory and emotional experience that is commonly associated with actual or potential tissue damage. Despite decades of pain research, many patients continue to suffer from chronic pain that is refractory to current treatments. Accumulating evidence has indicated an important role of protease-activated receptor 4 (PAR4) in the pathogenesis of inflammation and neuropathic pain. Here we reviewed PAR4 expression and activation via intracellular signaling pathways and the role of PAR4 signaling pathways in the development and maintenance of pain. Understanding PAR4 and its corresponding signaling pathways will provide insight to further explore the molecular basis of pain, which will also help to identify new targets for pharmacological intervention for pain relief.

TMPRSS2, a novel membrane-anchored mediator in cancer pain

More than half of all cancer patients have significant pain during the course of their disease. The strategic localization of TMPRSS2, a membrane-bound serine protease, on the cancer cell surface may allow it to mediate signal transduction between the cancer cell and its extracellular environment. We show that TMPRSS2 expression is not only dramatically increased in the primary cancers of patients but TMPRSS2 immunopositivity is also directly correlated with cancer pain severity in these patients. TMPRSS2 induced proteolytic activity, activated trigeminal neurons, and produced marked mechanical hyperalgesia when administered into the hind paw of wild-type mice but not PAR2-deficient mice. Coculture of human cancer cells with murine trigeminal neurons demonstrated colocalization of TMPRSS2 with PAR2. These results point to a novel role for a cell membrane-anchored mediator in cancer pain, as well as pain in general.

Dual regulation of δ-opioid receptor function by arachidonic acid metabolites in rat peripheral sensory neurons

The regulation of opioid receptor system function in peripheral sensory neurons is not well understood. Opioid agonist efficacy to inhibit nociceptor function and to promote antinociception is generally weak under basal conditions and frequently no response occurs. However, in response to a cyclooxygenase-dependent metabolite of arachidonic acid (AA) after exposure to inflammatory mediators, such as bradykinin (BK) or exogenous AA, peripheral opioid receptor systems become much more responsive to opioid agonists. In this study, we examined the time course for the induction and maintenance of functional competence of the δ-opioid receptor (DOR) system in adult rat nociceptors in culture and in vivo. We found that the responsive state of DOR after pretreatment with BK or exogenous AA is transient (30-60 minutes) and persists for 15-30 minutes after a 15-minute exposure of nociceptors to BK or AA. Interestingly, whereas functional competence of the DOR system could be reinduced with a second application of BK 60 minutes after the first, responsiveness of the DOR system could not be reinduced after an initial exposure to AA. This nonresponsive state of DOR after exogenous AA was mediated by a lipoxygenase (LOX)-dependent metabolite of AA. Intraplantar carrageenan also produced transient DOR functional competence and responsiveness was also reinduced by inhibition of LOX. Thus, the DOR system expressed by peripheral sensory neurons is under dual regulation by cyclooxygenase- and LOX-dependent metabolites of AA.

Endogenous regulation of inflammatory pain by T-cell-derived opioids: when friend turns to foe

Painful sensation is a hallmark of microbe-induced inflammation. This inflammatory pain is downregulated a few days after infection by opioids locally released by effector T lymphocytes generated in response to microbe-derived antigens. This review focuses on the endogenous regulation of inflammatory pain associated with adaptive T-cell response and puts in perspective the clinical consequences of the opioid-mediated analgesic activity of colitogenic T lymphocytes in inflammatory bowel disease.

Activation of estrogen receptor α enhances bradykinin signaling in peripheral sensory neurons of female rats

Numerous studies have demonstrated that females have a higher risk of experiencing several pain disorders with either greater frequency or severity than males. Although the mechanisms that underlie this sex disparity remain unclear, several studies have shown an important role for sex steroids, such as estrogen, in the modulation of nociception. Receptors for estrogen are present in primary afferent neurons in the trigeminal and dorsal root ganglia, and brief exposure to estrogen increases responses to the inflammatory mediator bradykinin (BK). However, the mechanism for estrogen-mediated enhancement of BK signaling is not fully understood. The aim of the present study was to evaluate the relative contributions of estrogen receptor α (ERα), ERβ, and G protein-coupled estrogen receptor 1 (GPER) to the enhanced signaling of the inflammatory mediator BK by 17β-estradiol (17β-E2) in primary sensory neurons from female rats in culture (ex vivo) and in behavioral assays of nociception in vivo. The effects of 17β-E2 on BK responses were mimicked by ERα-selective agonists and blocked by ERα-selective antagonists and by small interfering RNA knockdown of ERα. The data indicate that ERα is required for 17β-E2-mediated enhancement of BK signaling in peripheral sensory neurons in female rats.

Mechanisms of transient signaling via short and long prolactin receptor isoforms in female and male sensory neurons

Prolactin (PRL) regulates activity of nociceptors and causes hyperalgesia in pain conditions. PRL enhances nociceptive responses by rapidly modulating channels in nociceptors. The molecular mechanisms underlying PRL-induced transient signaling in neurons are not well understood. Here we use a variety of cell biology and pharmacological approaches to show that PRL transiently enhanced capsaicin-evoked responses involve protein kinase C ε (PKCε) or phosphatidylinositol 3-kinase (PI3K) pathways in female rat trigeminal (TG) neurons. We next reconstituted PRL-induced signaling in a heterologous expression system and TG neurons from PRL receptor (PRLR)-null mutant mice by expressing rat PRLR-long isoform (PRLR-L), PRLR-short isoform (PRLR-S), or a mix of both. Results show that PRLR-S, but not PRLR-L, is capable of mediating PRL-induced transient enhancement of capsaicin responses in both male and female TG neurons. However, co-expression of PRLR-L with PRLR-S (1:1 ratio) leads to the inhibition of the transient PRL actions. Co-expression of PRLR-L deletion mutants with PRLR-S indicated that the cytoplasmic site adjacent to the trans-membrane domain of PRLR-L was responsible for inhibitory effects of PRLR-L. Furthermore, in situ hybridization and immunohistochemistry data indicate that in normal conditions, PRLR-L is expressed mainly in glia with little expression in rat sensory neurons (3-5%) and human nerves. The predominant PRLR form in TG neurons/nerves from rats and humans is PRLR-S. Altogether, PRL-induced transient signaling in sensory neurons is governed by PI3K or PKCε, mediated via the PRLR-S isoform, and transient effects mediated by PRLR-S are inhibited by presence of PRLR-L in these cells.

Protease-activated receptor 2 signalling pathways: a role in pain processing

INTRODUCTION

Pain is a complex biological phenomenon that includes intricate neurophysiological, behavioural, psychosocial and affective components. Despite decades of pain research, many patients continue suffering from chronic pain that may be refractory to current medical regimens. Accumulating evidence has indicated an important role of protease-activated receptor 2 (PAR2) in the pathogenesis of pain, including inflammation, neuropathic and cancer pain.

AREAS COVERED

In this review, the role of the PAR2 signalling pathway in pain processes, basic mechanism of PAR2 activation and expression of PAR2 in the nervous system is covered. Furthermore, intracellular signalling pathways that are activated by PAR2 are also described.

EXPERT OPINION

The role of PAR2 in pain processing is becoming increasingly clear, and although causal implication remains to be established, PAR2 activation has been observed in several disease model systems. Since PAR2 is activated after nerve injury as well as by trypsin and related serine proteases, and PAR2 plays an important role in pain development and maintenance, exploring PAR2 and its corresponding signalling pathways will provide unfathomable knowledge in understanding the molecular basis of pain. This will also help to identify new targets for pharmacological intervention; however, in the context of potential PAR2-directed therapies, several aspects should be clarified.

Bradykinin controls pool size of sensory neurons expressing functional δ-opioid receptors

Analgesics targeting the δ-opioid receptor (DOR) may lead to fewer side effects than conventional opioid drugs, which mainly act on μ-opioid receptors (MOR), because of the less abundant expression of DOR in the CNS compared with MOR. Analgesic potential of DOR agonists increases after inflammation, an effect that may be mediated by DOR expressed in the peripheral sensory fibers. However, the expression of functional DOR at the plasma membrane of sensory neurons is controversial. Here we have used patch-clamp recordings and total internal reflection fluorescence microscopy to study the functional expression of DOR in sensory neurons from rat trigeminal (TG) and dorsal root ganglia (DRG). Real-time total internal reflection fluorescence microscopy revealed that treatment of TG and DRG cultures with the inflammatory mediator bradykinin (BK) caused robust trafficking of heterologously expressed GFP-tagged DOR to the plasma membrane. By contrast, treatment of neurons with the DOR agonist [d-Ala(2), d-Leu(5)]-enkephalin (DADLE) caused a decrease in the membrane abundance of DOR, suggesting internalization of the receptor after agonist binding. Patch-clamp experiments revealed that DADLE inhibited voltage-gated Ca(2+) channels (VGCCs) in 23% of small-diameter TG neurons. Pretreatment with BK resulted in more than twice as many DADLE responsive neurons (54%) but did not affect the efficacy of VGCC inhibition by DADLE. Our data suggest that inflammatory mediator-induced membrane insertion of DOR into the plasma membrane of peripheral sensory neurons may underlie increased DOR analgesia in inflamed tissue. Furthermore, the majority of BK-responsive TG neurons may have a potential to become responsive to DOR ligands in inflammatory conditions.

Disruption of δ-opioid receptor phosphorylation at threonine 161 attenuates morphine tolerance in rats with CFA-induced inflammatory hypersensitivity

OBJECTIVE

Our previous study identified Threonine 161 (Thr-161), located in the second intracellular loop of the δ-opioid receptor (DOR), as the only consensus phosphorylation site for cyclin-dependent kinase 5 (Cdk5). The aim of this study was to assess the function of DOR phosphorylation by Cdk5 in complete Freund's adjuvant (CFA)-induced inflammatory pain and morphine tolerance.

METHODS

Dorsal root ganglion (DRG) neurons of rats with CFA-induced inflammatory pain were acutely dissociated and the biotinylation method was used to explore the membrane localization of phosphorylated DOR at Thr-161 (pThr-161-DOR), and paw withdrawal latency was measured after intrathecal delivery of drugs or Tat-peptide, using a radiant heat stimulator in rats with CFA-induced inflammatory pain.

RESULTS

Both the total amount and the surface localization of pThr-161-DOR were significantly enhanced in the ipsilateral DRG following CFA injection. Intrathecal delivery of the engineered Tat fusion-interefering peptide corresponding to the second intracellular loop of DOR (Tat-DOR-2L) increased inflammatory hypersensitivity, and inhibited DOR- but not µ-opioid receptor-mediated spinal analgesia in CFA-treated rats. However, intrathecal delivery of Tat-DOR-2L postponed morphine antinociceptive tolerance in rats with CFA-induced inflammatory pain.

CONCLUSION

Phosphorylation of DOR at Thr-161 by Cdk5 attenuates hypersensitivity and potentiates morphine tolerance in rats with CFA-induced inflammatory pain, while disruption of the phosphorylation of DOR at Thr-161 attenuates morphine tolerance.

Different symptoms of neuropathic pain can be induced by different degrees of compressive force on the C7 dorsal root of rats

BACKGROUND CONTEXT

Neuropathic pain after nerve injuries is characterized by positive and negative sensory symptoms and signs. The extent of sensory fiber loss after nerve injuries has been demonstrated to correlate with symptoms of neuropathic pain by quantitative sensory testing and confirmed by biopsies of small nerve fibers. However, the relationship between the pathologic changes of large nerves on injuries and resulting pain symptoms remains unclear.

PURPOSE

To investigate the relationship between the extent of dorsal root injury and resulting symptoms of neuropathic pain.

STUDY DESIGN

Nerve injury and assessment of the following pain-related behaviors and neuropathologic changes.

METHODS

A total of 24 adult male Sprague-Dawley rats weighing 250 to 300 g were randomly divided into three groups (n=8 each): sham group operated on but without nerve compression, 70 gf group, and 180 gf group; a compression force of 70 or 180 g was applied to the right C7 dorsal root, separately. Threshold thermal and mechanical pains were measured before surgery (baseline) and on the first, third, fifth, and seventh day after surgery. On the seventh day after surgery, all rats were killed, and the structural alterations of nerve fibers within the compressed areas were examined.

RESULTS

A compression force of 70 g resulted in hyperalgesia, whereas a compression force of 180 g induced hypoalgesia in the ipsilateral forepaw in response to both mechanical and thermal stimulations within 7 days after injury. Light microscopy and electron microscopy revealed a mild to moderate sensory fiber loss after 70-gf compression and a more severe sensory fiber loss after 180-gf compression.

CONCLUSIONS

Transient injuries on sensory fibers can produce either positive or negative symptoms of neuropathic pain, and the different extent of sensory fiber loss after different degrees of injuries might account for the varied resulting symptoms of neuropathic pain.

The cytochrome P450 inhibitor, ketoconazole, inhibits oxidized linoleic acid metabolite-mediated peripheral inflammatory pain

Background

Oxidized linoleic acid metabolites (OLAMs) are a class of endogenous agonists to the transient receptor potential V1 (TRPV1) receptor. Although TRPV1 mediates inflammatory heat hyperalgesia, it is not known if the OLAMs contribute to the peripheral activation of this receptor during tissue inflammation. In the present study, we evaluated whether the OLAM system is activated during inflammation and whether cytochrome P450 enzymes mediate OLAM contributions to heat hyperalgesia using the complete Freund’s adjuvant (CFA) model of inflammation.

Results

Our results demonstrate that the intraplantar (ipl) injection of anti-OLAM antibodies significantly reversed CFA-induced heat hyperalgesia. Moreover, application of lipid extracts from inflamed rat skin to cultured sensory neurons triggered a significant release of iCGRP that is blocked by co-treatment with I-RTX, a TRPV1 antagonist. To determine the role of CYP enzymes in mediating OLAM effects, we used a broad spectrum CYP inhibitor, ketoconazole. Pretreatment with ketoconazole inhibited the release of TRPV1 agonists in lipid extracts from inflamed skin and significantly reversed CFA-induced heat hyperalgesia by a peripheral mechanism of action. Moreover, the ipl injection of linoleic acid to rats 24 hr after CFA evoked spontaneous nocifensive behaviors that were significantly reduced by capsazepine, by knockout of the TRPV1 gene, or by pretreatment with either anti-OLAM antibodies or ketoconazole.

Conclusions

Taken together, our data suggests that OLAMs contribute to inflammatory nociception in the periphery and that cytochrome P450 enzymes play a crucial role in mediating OLAM contributions to inflammatory heat hyperalgesia.

Plasticity of cytochrome P450 isozyme expression in rat trigeminal ganglia neurons during inflammation

Recently, specific oxidized linoleic acid metabolites (OLAMs) have been identified as transient receptor potential vanilloid 1 (TRPV1) channel agonists that contribute to inflammatory and heat hyperalgesia mechanisms, yet the specific mechanism responsible for OLAM synthesis in sensory neurons is unknown. Here, we use molecular, anatomical, calcium imaging, and perforated patch electrophysiology methods to demonstrate the specific involvement of cytochrome P450 enzymes (CYPs) in the oxidation of linoleic acid leading to neuronal activation and show that this is enhanced under inflammatory conditions. Additional studies evaluated CYP expressions in the native rat trigeminal ganglia (TG) tissue and cultures as well as changes in their expression pattern following the induction of peripheral inflammation. Fourteen of 20 candidate transcripts were detected in native TG, and 7 of these displayed altered expression under cultured conditions. Moreover, complete Freund's adjuvant-induced inflammation of vibrissal pad selectively increased expression of CYP3A23/3A1 and CYP2J4 transcripts in TG. In situ hybridization studies demonstrated broad expression pattern of CYP3A23/3A1 and CYP2J4 within TG neurons. Anatomical studies characterized the expression of CYP3A1 and the CYP2J families within TG sensory neurons, including those with TRPV1, with about half of all TRPV1-positive neurons showing more prominent CYP3A1 and CYP2J expression. Together, these findings show that CYP enzymes play a primary role in mediating linoleic acid-evoked activation of sensory neurons and furthermore, implicate the involvement of specific CYPs as contributing to the formation of OLAMs that act as TRPV1 agonists within this subpopulation of nociceptors.

Delta opioid receptor analgesia: recent contributions from pharmacology and molecular approaches

Delta opioid receptors represent a promising target for the development of novel analgesics. A number of tools have been developed recently that have significantly improved our knowledge of δ receptor function in pain control. These include several novel δ agonists with potent analgesic properties, and genetic mouse models with targeted mutations in the δ opioid receptor gene. Also, recent findings have further documented the regulation of δ receptor function at cellular level, which impacts on the pain-reducing activity of the receptor. These regulatory mechanisms occur at transcriptional and post-translational levels, along agonist-induced receptor activation, signaling and trafficking, or in interaction with other receptors and neuromodulatory systems. All these tools for in-vivo research, and proposed mechanisms at molecular level, have tremendously increased our understanding of δ receptor physiology, and contribute to designing innovative strategies for the treatment of chronic pain and other diseases such as mood disorders.

Allosteric interactions between δ and κ opioid receptors in peripheral sensory neurons

The peripheral δ opioid receptor (DOR) is an attractive target for analgesic drug development. There is evidence that DOR can form heteromers with the κ-opioid receptor (KOR). As drug targets, heteromeric receptors offer an additional level of selectivity and, because of allosteric interactions between protomers, functionality. Here we report that selective KOR antagonists differentially altered the potency and/or efficacy of DOR agonists in primary cultures of adult rat peripheral sensory neurons and in a rat behavioral model of thermal allodynia. In vitro, the KOR antagonist nor-binaltorphimine (nor-BNI) enhanced the potency of [D-Pen(2,5)]-enkephalin (DPDPE), decreased the potency of [D-Ala(2),D-Leu(5)]-enkephalin (DADLE), and decreased the potency and efficacy of 4-[(R)-[(2S,5R)-4-allyl-2,5-dimethylpiperazin-1-yl](3-methoxyphenyl)methyl]-N,N-diethylbenzamide (SNC80) to inhibit prostaglandin E(2) (PGE(2))-stimulated adenylyl cyclase activity. In vivo, nor-BNI enhanced the effect of DPDPE and decreased the effect of SNC80 to inhibit PGE(2)-stimulated thermal allodynia. In contrast to nor-BNI, the KOR antagonist 5'-guanidinonaltrindole (5'-GNTI) reduced the response of DPDPE both in cultured neurons and in vivo. Evidence for DOR-KOR heteromers in peripheral sensory neurons included coimmunoprecipitation of DOR with KOR, a DOR-KOR heteromer selective antibody augmented the antinociceptive effect of DPDPE in vivo, and the DOR-KOR heteromer agonist 6'-GNTI inhibited adenylyl cyclase activity in vitro as well as PGE(2)-stimulated thermal allodynia in vivo. Taken together, these data suggest that DOR-KOR heteromers exist in rat primary sensory neurons and that KOR antagonists can act as modulators of DOR agonist responses most likely through allosteric interactions between the protomers of the DOR-KOR heteromer.

A-kinase anchoring protein 150 mediates transient receptor potential family V type 1 sensitivity to phosphatidylinositol-4,5-bisphosphate

A-kinase anchoring protein 150 (AKAP150) is a scaffolding protein that controls protein kinase A- and C-mediated phosphorylation of the transient receptor potential family V type 1 (TRPV1), dictating receptor response to nociceptive stimuli. The phospholipid phosphatidylinositol-4,5-bisphosphate (PIP(2)) anchors AKAP150 to the plasma membrane in naive conditions and also affects TRPV1 activity. In the present study, we sought to determine whether the effects of PIP(2) on TRPV1 are mediated through AKAP150. In trigeminal neurons and CHO cells, the manipulation of cellular PIP(2) led to significant changes in the association of AKAP150 and TRPV1. Following PIP(2) degradation, increased TRPV1:AKAP150 coimmunoprecipitation was observed, resulting in increased receptor response to capsaicin treatment. Phospholipase C activation in neurons isolated from AKAP150(-/-) animals indicated that PIP(2)-mediated inhibition of TRPV1 in the whole-cell environment requires expression of the scaffolding protein. Furthermore, the addition of PIP(2) to neurons isolated from AKAP150 wild-type mice reduced PKA sensitization of TRPV1 compared with isolated neurons from AKAP150(-/-) mice. These findings suggest that PIP(2) degradation increases AKAP150 association with TRPV1 in the whole-cell environment, leading to sensitization of the receptor to nociceptive stimuli.

Activation of peripheral delta-opioid receptors leads to anti-hyperalgesic responses in the masseter muscle of male and female rats

In this project, we examined peripheral δ-opioid receptor (DOR)-mediated anti-hyperalgesic responses in the context of an acute orofacial muscle pain condition in both male and female rats. We also investigated whether the ATP-sensitive K+ channel (KATP), a downstream target of OR signaling, contributes to DOR-mediated anti-hyperalgesic responses. Local pretreatment of the masseter with a DOR agonist, [D-Pen², D-Pen⁶]-enkephalin (DPDPE), dose-dependently attenuated capsaicin-induced mechanical hypersensitivity in both male and female rats. However, there were sex differences in the potency of local DPDPE in that a 10-fold higher dose of DPDPE was required in female rats to produce the level of anti-hyperalgesia achieved in male rats. The sex differences in the DPDPE effect may not be fully explained by DOR expression level since there was no significant sex difference in DOR mRNA levels in trigeminal ganglia (TG). Finally, pretreatment of the masseter with the KATP antagonist, glibenclamide, significantly blocked the effects of DPDPE in male rats suggesting that the peripheral DOR effect is mediated by the KATP. These studies revealed novel information about sex differences with regards to peripherally localized DOR-mediated anti-hyperalgesia under an orofacial muscle pain condition.

Cannabinoid receptor antagonists AM251 and AM630 activate TRPA1 in sensory neurons

Cannabinoid receptor antagonists have been utilized extensively in vivo as well as in vitro, but their selectivity has not been fully examined. We investigated activation of sensory neurons by two cannabinoid antagonists - AM251 and AM630. AM251 and AM630 activated trigeminal (TG) sensory neurons in a concentration-dependent fashion (threshold 1 μM). AM251 and AM630 responses are mediated by the TRPA1 channel in a majority (90-95%) of small-to-medium TG sensory neurons. AM630 (1-100 μM), but not AM251, was a significantly more potent agonist in cells co-expressing both TRPA1 and TRPV1 channels. We next evaluated AM630 and AM251 effects on TRPV1- and TRPA1-mediated responses in TG neurons. Capsaicin (CAP) effects were inhibited by pre-treatment with AM630, but not AM251. Mustard oil (MO) and WIN55,212-2 (WIN) TRPA1 mediated responses were also inhibited by pre-treatment with AM630, but not AM251 (25 uM each). Co-treatment of neurons with WIN and either AM630 or AM251 had opposite effects: AM630 sensitized WIN responses, whereas AM251 inhibited WIN responses. WIN-induced inhibition of CAP responses in sensory neurons was reversed by AM630 pre-treatment and AM251 co-treatment (25 μM each), as these conditions inhibit WIN responses. Hindpaw injections of AM630 and AM251 did not produce nocifensive behaviors. However, both compounds modulated CAP-induced thermal hyperalgesia in wild-type mice and rats, but not TRPA1 null-mutant mice. AMs also partially regulate WIN inhibition of CAP-induced thermal hyperalgesia in a TRPA1-dependent fashion. In summary, these findings demonstrate alternative targets for the cannabinoid antagonists, AM251 and AM630, in peripheral antihyperalgesia which involve certain TRP channels.

Regulation of κ-opioid receptor signaling in peripheral sensory neurons in vitro and in vivo

There is considerable interest in understanding the regulation of peripheral opioid receptors to avoid central nervous system side effects associated with systemically administered opioid analgesics. Here, we investigated the regulation of the κ-opioid receptor (KOR) on rat primary sensory neurons in vitro and in a rat model of thermal allodynia. Under basal conditions, application of the KOR agonist trans-(1S,2S)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeneacetamide hydrochloride hydrate (U50488) did not inhibit adenylyl cyclase (AC) activity nor release of calcitonin gene-related peptide (CGRP) in vitro and did not inhibit thermal allodynia in vivo. However, after 15-min pretreatment with bradykinin (BK), U50488 became capable of inhibiting AC activity, CGRP release, and thermal allodynia. Inhibition of AC by 5-hydroxytryptamine 1 or neuropeptide Y(1) receptor agonists and stimulation of extracellular signal-regulated kinase activity by U50488 did not require BK pretreatment. The effect of U50488 in BK-primed tissue was blocked by the KOR antagonist nor-binaltorphimine both in vitro and in vivo. The effect of BK in vitro was blocked by either indomethacin or bisindolylmaleimide, suggesting that an arachidonic acid (AA) metabolite and protein kinase C (PKC) activation mediate BK-induced regulation of the KOR system. Furthermore, the effect of U50488 in BK-treated tissue was blocked by a soluble integrin-blocking peptide (GRGDSP), but not the inactive reverse sequence peptide (GDGRSP), suggesting that, in addition to AA and PKC, RGD-binding integrins participate in the regulation of KOR signaling in response to U50488. Understanding the mechanisms by which peripheral KOR agonist efficacy is regulated may lead to improved pharmacotherapy for the treatment of pain with reduced adverse effects.

Co-administration of δ- and μ-opioid receptor agonists promotes peripheral opioid receptor function

Enhancement of peripheral opioid analgesia following tissue injury or inflammation in animal models is well-documented, but clinical results of peripheral opioid therapy remain inconsistent. Previous studies in the central nervous system have shown that co-administration of μ- and δ-opioid receptor agonists can enhance analgesic outcomes; however, less is known about the functional consequences of opioid receptor interactions in the periphery. The present study examines the effects of intraplantar injection of the μ- and δ-opioid receptor agonists, morphine and deltorphin, alone and in combination on behavioral tests of nociception in naïve rats and on potassium-evoked release of CGRP from sciatic nerves of naïve rats. Neither drug alone affected nociceptive behaviors or CGRP release. Two separate measures of mechanical nociceptive sensitivity remained unchanged after co-administration of the two drugs. In contrast, when deltorphin was co-injected with morphine, dose-dependent and peripherally restricted increases in paw withdrawal latencies to radiant heat were observed. Similarly, concentration-dependent inhibition of CGRP release was observed when deltorphin and morphine were administered in sequence prior to potassium stimulation. However, no inhibition was observed when morphine was administered prior to deltorphin. All combined opioid effects were blocked by co-application of antagonists. Deltorphin exposure also enhanced the in vivo and in vitro effects of another μ-opioid receptor agonist, DAMGO. Together, these results suggest that under normal conditions, δ-opioid receptor agonists enhance the effect of μ-opioid receptor agonists in the periphery, and local co-administration of δ- and μ-opioid receptor agonists may improve results of peripheral opioid therapy for the treatment of pain.

Cannabinoid receptor CB1-immunoreactive nerve fibres in painful and non-painful human tooth pulp

The cannabinoid receptor CB1 is involved in modulation of neuronal hypersensitivity and pain. The aim of this study was to evaluate CB1 receptor levels for the first time in dental pain. A total of 19 patients due for molar extraction were divided into two groups, those with existing dental pain (n=9), and those with no history of pain (n=10). Immunohistochemistry and computer image analysis was used to evaluate CB1-positive nerve fibres in tooth pulp, with neurofilament-immunostaining as a structural nerve marker. CB1-immunoreactive nerve fibres were scattered throughout the tooth pulp and often seen in nerve bundles, but the fibres did not penetrate the subodontoblastic layer. There was no statistically significant change in the CB1 nerve fibre percentage area in the painful group compared to the non-painful group (p=0.146); the neurofilament fibres were significantly reduced in the painful group compared to the controls (p=0.028), but there was no difference in the ratio of CB1 to neurofilaments between the two groups. Thus, CB1 expression is maintained by nerve fibres in painful human dental pulp, and peripherally-restricted CB1 agonists currently in development may advance the treatment of dental pain.

17beta-estradiol rapidly enhances bradykinin signaling in primary sensory neurons in vitro and in vivo

Many studies have demonstrated that premenopausal women are at increased risk for various pain disorders. Pain-sensing neurons, termed "nociceptors," in the trigeminal ganglia (TG) and dorsal root ganglia (DRG) express receptors for inflammatory mediators and noxious physical stimuli and transmit signals for central processing of pain sensation. Estrogen receptors (ERs) are also expressed on nociceptors in the TG and DRG, and there is ample literature to suggest that activation of ERs can influence pain mechanisms. However, the mechanism for ER modulation of nociceptor activity is incompletely understood. The aim of this study was to characterize the effect of 17β-estradiol (17β-E(2)) on signaling of the inflammatory mediator bradykinin (BK) in primary cultures of rat sensory neurons and a behavioral model of thermal allodynia in rats. Here, we show that exposure to 17β-E(2) rapidly (within 15 min) enhanced responses to BK in vitro and in vivo. The 17β-E(2)-mediated enhancement of BK signaling was not blocked by the transcription inhibitor anisomycin and was mediated by a membrane-associated ER. The effect of 17β-E(2) to enhance BK responses required activation of β1-containing, RGD-binding integrins. These data show that 17β-E(2) rapidly enhances inflammatory mediator responses both in vitro and in vivo and suggest that 17β-E(2) acting at primary sensory pain neurons may participate in regulating the sensitivity of women to painful stimuli.

PAR-2 regulates dental pulp inflammation associated with caries

Protease-activated receptors (PARs) are G-protein-coupled receptors that are activated enzymatically by proteolysis of an N-terminal domain. The cleavage and activation of PARs by serine proteases represent a novel mechanism by which such enzymes could influence the host inflammatory response. The aim of this study was to determine whether PAR-2 expression and activation were increased in dental caries. Using immunohistochemistry, we showed PAR-2 to be localized to pulp cells subjacent to caries lesions, but minimally expressed by healthy pulp tissue. Trypsin and the PAR-2 agonist (PAR2-AP) activated PAR-2 in an in vitro functional assay. Endogenous molecules present in pulp cell lysates from carious teeth specifically activated PAR-2, but those from healthy teeth failed to do so. The activation of PAR-2 in vitro was shown to increase the expression of the pro-inflammatory mediator cyclo-oxygenase-2 (COX-2), providing a mechanism whereby PAR-2 could modulate pulpal inflammation.

Heat generates oxidized linoleic acid metabolites that activate TRPV1 and produce pain in rodents

The transient receptor potential vanilloid 1 (TRPV1) channel is the principal detector of noxious heat in the peripheral nervous system. TRPV1 is expressed in many nociceptors and is involved in heat-induced hyperalgesia and thermoregulation. The precise mechanism or mechanisms mediating the thermal sensitivity of TRPV1 are unknown. Here, we have shown that the oxidized linoleic acid metabolites 9- and 13-hydroxyoctadecadienoic acid (9- and 13-HODE) are formed in mouse and rat skin biopsies by exposure to noxious heat. 9- and 13-HODE and their metabolites, 9- and 13-oxoODE, activated TRPV1 and therefore constitute a family of endogenous TRPV1 agonists. Moreover, blocking these substances substantially decreased the heat sensitivity of TRPV1 in rats and mice and reduced nociception. Collectively, our results indicate that HODEs contribute to the heat sensitivity of TRPV1 in rodents. Because oxidized linoleic acid metabolites are released during cell injury, these findings suggest a mechanism for integrating the hyperalgesic and proinflammatory roles of TRPV1 and linoleic acid metabolites and may provide the foundation for investigating new classes of analgesic drugs.

Delta and kappa opioid receptors as suitable drug targets for pain

Similar to mu opioid receptors, kappa and delta opioid receptors reside in the periphery, the dorsal root ganglion, the spinal cord, and in supraspinal regions associated with pain modulation. Both delta and kappa opioid agonists have been shown to activate pain inhibitory pathways in the central nervous system. Yet, currently there are only a few pharmacologic agents that target kappa receptors, and none that target delta receptors. Spurred by the need for an efficacious analgesic without the unwanted side effects associated with the typical clinical profile of mu opioid agonists, new research has provided insight into why the development of effective kappa and delta opioid receptor agonists has remained elusive thus far, and importantly, how these obstacles may be overcome. For example, for delta opioid agonists to be effective, a state of inflammation may be required as this induces delta opioid receptors to migrate to the surface of neuronal cells and thereby become accessible to delta opioid agonists. Studies have shown that delta opioid agonists can provide relief of inflammatory pain and malignant bone pain. Meanwhile, peripherally restricted kappa opioid agonists have been developed to target kappa opioid receptors located on visceral and somatic afferent nerves for relief of inflammatory, visceral, and neuropathic chronic pain. The recently shown efficacy of these analgesics combined with a possible lower abuse potential and side effect burden than mu opioid receptor agonists makes delta and peripherally restricted kappa opioid receptor agonists promising targets for treating pain.