Protein kinase C in pain: involvement of multiple isoforms

Interaction between SGLT2 and the sympathetic nervous system in normal and various cardiovascular metabolic disease states

Sodium-glucose cotransporter 2 (SGLT2) inhibitors have been shown to suppress cardiovascular events and are widely used for treating diabetes, chronic heart failure and chronic kidney disease. Although the underlying mechanisms by which SGLT2 inhibitors suppress cardiovascular events are not entirely clear, several mechanisms have been proposed to explain the cardiorenal protective effects of SGLT2 inhibitors. One of these involves sympathoinhibition. In vitro, SGLT2 expression is upregulated by norepinephrine, and SGLT2 inhibitors have been shown to attenuate SGLT2 expression and normalize the diuretic response to volume expansion with isotonic saline in rats with heart failure. These findings suggest that inhibition of renal sympathetic nerve activity is the mechanism underlying the beneficial effects of SGLT2 inhibitors on heart failure. Increased resting afferent renal nerve activity has been observed in several disease models, including models of hypertension, heart failure, and kidney disease, and might induce augmented sympathetic outflow via the central nervous system. SGLT2 inhibitors may suppress afferent renal nerve activity via intrarenal environmental modifications such as renal tissue hypoxia, inflammation, oxidative stress, mitochondrial function, and congestion, thereby inhibiting sympathetic outflow to the peripheral organs, including the heart and kidneys. On the other hand, SGLT2 is also expressed in the brain, and electrophysiological techniques in rats have shown that SGLT2 inhibitors suppress the activities of the rostral ventrolateral medulla neurons which project to the sympathetic preganglionic nuclei of the spinal cord to control sympathetic outflow, suggesting decreased sympathetic nerve activities. This mini review focuses on the bidirectional interaction between SGLT2 and the sympathetic nervous system and introduces recent related findings from Hypertension Research and other journals.

Exploring the Role of RhoA/ROCK Signaling in Pain: A Narrative Review

Despite significant progress in understanding the mechanisms of pain and developing therapeutic agents, pain remains a challenging and unresolved clinical issue. The Ras homolog gene family member A (RhoA), a member of the small guanosine triphosphate hydrolases (GTPases) of the Ras homolog family, is involved in transmitting signals that regulate various cellular processes. RhoA exerts its effects through a range of downstream effectors, with Rho-associated kinase (ROCK) being the most extensively studied. Emerging evidence suggests that the RhoA/ROCK signaling pathway plays a crucial role in pain transmission and sensitization. Our work indicates that targeting the RhoA/ROCK signaling pathway may offer a promising therapeutic avenue for alleviating pain.

Established and emerging roles of protein kinases in regulating primary sensory neurons in injury-and inflammation-associated pain

INTRODUCTION

Recent seminal neuroscience research has significantly increased our knowledge on cellular and molecular responses of various cells in the pain pathway to peripheral nerve injuries and inflammatory processes. Transcriptomic and epigenetic analysis of primary sensory neurons (PSNs) in animal models of peripheral injuries revealed new insights into altered gene expression profiles and epigenetic modifications, which, via increasing spinal nociceptive input, lead to the development of pain. Among the various classes of molecules involved in driving differential gene expression, protein kinases, the enzymes that catalyze the phosphorylation of molecules, are emerging to control histone modification and chromatin remodeling needed for the alteration in transcriptional activity.

AREAS COVERED

Here, we focused on how protein kinases contribute to transcriptomic changes and pathways of induced reprogramming within PSNs upon peripheral nerve injury and inflammation. We conducted systematic literature search across multiple databases, including PubMed, NIH ClinicalTrials.gov portal and GEOData from 1980 to 2024 and compared protein kinase expression frequencies between publicly available RNA sequencing datasets of PSNs and investigated differences in protein kinase expression levels after peripheral nerve injury.

EXPERT OPINION

Novel findings support a new concept that protein kinases constitute regulatory hubs of reprogramming of PSNs, which offers novel analgesic approaches.

BDNF Signaling and Pain Modulation

Brain-derived neurotrophic factor (BDNF) is an important neuromodulator of nervous system functions and plays a key role in neuronal growth and survival, neurotransmission, and synaptic plasticity. The effects of BDNF are mainly mediated by the activation of tropomyosin receptor kinase B (TrkB), expressed in both the peripheral and central nervous system. BDNF has been implicated in several neuropsychiatric conditions such as schizophrenia and anxio-depressive disorders, as well as in pain states. This review summarizes the evidence for a critical role of BDNF throughout the pain system and describes contrasting findings of its pro- and anti-nociceptive effects. Different cellular sources of BDNF, its influence on neuroimmune signaling in pain conditions, and its effects in different cell types and regions are described. These and endogenous BDNF levels, downstream signaling mechanisms, route of administration, and approaches to manipulate BDNF functions could explain the bidirectional effects in pain plasticity and pain modulation. Finally, current knowledge gaps concerning BDNF signaling in pain are discussed, including sex- and pathway-specific differences.

Phosphorylation of distal C-terminal residues promotes TRPV4 channel activation in response to arachidonic acid

Transient receptor potential vanilloid 4 (TRPV4) is a Ca2+-permeable channel activated by diverse physical and chemical stimuli, including mechanical stress and endogenous lipid arachidonic acid (AA) and its metabolites. Phosphorylation of TRPV4 by protein kinase A (PKA) and protein kinase C (PKC) is a predominant mechanism for channel regulation, especially in the cytoplasmic domains due to their importance in protein assembly, and channelopathies. However, studies corresponding to phosphorylation sites for these kinases remain incomplete. We investigated the role of Ser-823 residue as a potential phosphorylation site in regulating TRPV4 activity and chemical agonist-induced channel activation. Using mass spectrometry, we identified a new phosphorylation site Ser-823 residue and confirmed the previously known phosphorylation site Ser-824 in the C-terminal tail. The low level of phosphorylation at Ser-823 was stimulated by PKC and to a lesser extent by PKA in human coronary artery endothelial cells (HCAECs) and human embryonic kidney 293 (HEK 293) cells. AA-induced TRPV4 activation was enhanced in the phosphomimetic S823E but was blunted in the S823A/S824A mutants, whereas the channel activation by the synthetic agonist GSK1016790A was unaffected. Further, TRPV4 activation by AA but not GSK1016790A was blunted or abolished by PKA inhibitor alone or in combination with PKC inhibitor, respectively. Using computational modeling, we refined a previously proposed structural model for TRPV4 regulation by Ser-823 and Ser-824 phosphorylation. Together, these results provide insight into how stimuli-specific TRPV4 activation is regulated by the phosphorylation of discrete residues (e.g., Ser-823 and Ser-824) in the C-terminal domains of the TRPV4 channel.

Taxane-Associated Acute Pain Syndrome: a Review of its Features and Management

OPINION STATEMENT

Taxane-associated acute pain syndrome (T-APS) is one of the most common adverse effects of taxane treatment and significantly reduces the quality of life and activities of daily living of patients. T-APS is recognized as myalgia and arthralgia, which generally appear 1-3 days after taxane administration and last for approximately 7 days, at a wide range of sites. Recently, T-APS has been suggested to be not only an acute symptom but also a chronic symptom associated with chemotherapy-induced peripheral neuropathy (CIPN). The reported incidence of T-APS varies among studies, possibly owing to differences in observation points, evaluation methods, taxane administration methods, concomitant medications, or patient factors. Several factors, such as high taxane dose, paclitaxel use, metastatic setting, breast cancer, younger age, and co-administration of pegfilgrastim, are associated with symptom development. Several findings regarding T-APS management, such as prophylaxis using corticosteroids, Shakuyaku-Kanzo-to, and non-steroidal anti-inflammatory drugs (NSAIDs), are present. Corticosteroids for several days after taxane administration dose-dependently prevents and attenuates T-APS although we should be cautious about its longer administration. Prophylactic administration of Shakuyaku-Kanzo-to, a herbal compound, may be useful, although prescriptions are only available in limited areas. Etoricoxib, a selective cyclooxygenase-2 inhibiting NSAID, also reduces the incidence and severity of T-APS. Additionally, its prophylactic administration decreases CIPN. In contrast, evidence of symptomatic medication is limited. Taxanes are key chemotherapeutic agents used in the treatment of several types of cancer; therefore, further assessment of mechanisms of action and treatment of T-APS is necessary.

An atlas of neuropathic pain-associated molecular pathological characteristics in the mouse spinal cord

Peripheral nerve injury (PNI)-induced neuropathic pain (NP) is a severe disease with high prevalence in clinics. Gene reprogramming and tissue remodeling in the dorsal root ganglia (DRG) and spinal cord (SC) drive the development and maintenance of neuropathic pain (NP). However, our understanding of the NP-associated spatial molecular processing landscape of SC and the non-synaptic interactions between DRG neurons and SC cells remains limited. We here integrate spatial transcriptomics (ST) with single-nucleus RNA-sequencing (snRNA-seq) and bulk RNA-sequencing (bulk RNA-seq) to characterize regional pathological heterogeneity of the SC under NP conditions. First, the SC of NP mice manifests unique spatial atlases of genes, cell populations, cell-cell cross-talks, signaling pathways, and transcriptional regulatory networks compared to sham mice. We further report that injured DRG sensory neurons and the corresponding ventral horn of the SC show similar expression patterns after PNI. In addition, for the first time, we systematically exhibit "cross-talk omics" between the DRG neurons and SC dorsal horn neurons and glial cells, indicating an altered communication profile under NP conditions. Together, our findings decode the spatial and cellular heterogeneity of molecular pathological mechanisms underlying NP, providing a foundation for designing therapeutic targets for this disorder.

Transcriptome Analysis Suggests PKD3 Regulates Proliferative Glucose Metabolism, Calcium Homeostasis and Microtubule Dynamics After MEF Spontaneous Immortalization

Cell immortalization corresponds to a biologically relevant clinical feature that allows cells to acquire a high proliferative potential during carcinogenesis. In multiple cancer types, Protein Kinase D3 (PKD3) has often been reported as a dysregulated oncogenic kinase that promotes cell proliferation. Using mouse embryonic fibroblasts (MEFs), in a spontaneous immortalization model, PKD3 has been demonstrated as a critical regulator of cell proliferation after immortalization. However, the mechanisms by which PKD3 regulates proliferation in immortalized MEFs require further elucidation. Using a previously validated Prkd3-deficient MEF model, we performed a poly-A transcriptomic analysis to identify putative Prkd3-regulated biological processes and downstream targets in MEFs after spontaneous immortalization. To this end, differentially expressed genes (DEGs) were identified and further analyzed by gene ontology (GO) enrichment and protein-protein interaction (PPI) network analyses to identify potential hub genes. Our results suggest that Prkd3 modulates proliferation through the regulation of gene expression associated with glucose metabolism (Tnf, Ucp2, Pgam2, Angptl4), calcium homeostasis and transport (Calcr and P2rx7) and microtubule dynamics (Stmn2 and Map10). These candidate processes and associated genes represent potential mechanisms involved in Prkd3-induced proliferation in spontaneously immortalized cells as well as clinical targets in several cancer types.

Brain-Derived Neurotrophic Factor, Nociception, and Pain

This article examines the involvement of the brain-derived neurotrophic factor (BDNF) in the control of nociception and pain. BDNF, a neurotrophin known for its essential role in neuronal survival and plasticity, has garnered significant attention for its potential implications as a modulator of synaptic transmission. This comprehensive review aims to provide insights into the multifaceted interactions between BDNF and pain pathways, encompassing both physiological and pathological pain conditions. I delve into the molecular mechanisms underlying BDNF's involvement in pain processing and discuss potential therapeutic applications of BDNF and its mimetics in managing pain. Furthermore, I highlight recent advancements and challenges in translating BDNF-related research into clinical practice.

TRPV4 Activation and its Intracellular Modulation Mediated by Kinin Receptors Contribute to Painful Symptoms Induced by Anastrozole

Anastrozole, an aromatase inhibitor, induces painful musculoskeletal symptoms, which affect patients' quality of life and lead to therapy discontinuation. Efforts have been made to understand the mechanisms involved in these painful symptoms to manage them better. In this context, we explored the role of the Transient Receptor Potential Vanilloid 4 (TRPV4), a potential transducer of several nociceptive mechanisms, in anastrozole-induced musculoskeletal pain in mice. Besides, we evaluated the possible sensibilization of TRPV4 by signalling pathways downstream, PLC, PKC and PKCε from kinin B2 (B2R) and B1 (B1R) receptors activation in anastrozole-induced pain. Anastrozole caused mechanical allodynia and muscle strength loss in mice. HC067047, TRPV4 antagonist, reduced the anastrozole-induced mechanical allodynia and muscle strength loss. In animals previously treated with anastrozole, the local administration of sub-nociceptive doses of the TRPV4 (4α-PDD or hypotonic solution), B2R (Bradykinin) or B1R (DABk) agonists enhanced the anastrozole-induced pain behaviours. The sensitizing effects induced by local injection of the TRPV4, B2R and B1R agonists in animals previously treated with anastrozole were reduced by pre-treatment with TRPV4 antagonist. Furthermore, inhibition of PLC, PKC or PKCε attenuated the mechanical allodynia and muscle strength loss induced by TRPV4, B2R and B1R agonists. The generation of painful conditions caused by anastrozole depends on direct TRPV4 activation or indirect, e.g., PLC, PKC and PKCε pathways downstream from B2R and B1R activation. Thus, the TRPV4 channels act as sensors of extracellular and intracellular changes, making them potential therapeutic targets for alleviating pain related to aromatase inhibitors use, such as anastrozole.

CDDO regulates central and peripheral sensitization to attenuate post-herpetic neuralgia by targeting TRPV1/PKC-δ/p-Akt signals

Postherpetic neuralgia (PHN) is a notorious neuropathic pain featuring persistent profound mechanical hyperalgesia with significant negative impact on patients' life quality. CDDO can regulate inflammatory response and programmed cell death. Its derivative also protects neurons from damages by modulating microglia activities. As a consequence of central and peripheral sensitization, applying neural blocks may benefit to minimize the risk of PHN. This study aimed to explore whether CDDO could generate analgesic action in a PHN-rats' model. The behavioural test was determined by calibrated forceps testing. The number of apoptotic neurons and degree of glial cell reaction were assessed by immunofluorescence assay. Activation of PKC-δ and the phosphorylation of Akt were measured by western blots. CDDO improved PHN by decreasing TRPV1-positive nociceptive neurons, the apoptotic neurons, and reversed glial cell reaction in adult rats. It also suppressed the enhanced PKC-δ and p-Akt signalling in the sciatic nerve, dorsal root ganglia (DRG) and spinal dorsal horn. Our research is the promising report demonstrating the analgesic and neuroprotective action of CDDO in a PHN-rat's model by regulating central and peripheral sensitization targeting TRPV1, PKC-δ and p-Akt. It also is the first study to elucidate the role of oligodendrocyte in PHN.

An Update on Protein Kinases as Therapeutic Targets-Part I: Protein Kinase C Activation and Its Role in Cancer and Cardiovascular Diseases

Protein kinases are one of the most significant drug targets in the human proteome, historically harnessed for the treatment of cancer, cardiovascular disease, and a growing number of other conditions, including autoimmune and inflammatory processes. Since the approval of the first kinase inhibitors in the late 1990s and early 2000s, the field has grown exponentially, comprising 98 approved therapeutics to date, 37 of which were approved between 2016 and 2021. While many of these small-molecule protein kinase inhibitors that interact orthosterically with the protein kinase ATP binding pocket have been massively successful for oncological indications, their poor selectively for protein kinase isozymes have limited them due to toxicities in their application to other disease spaces. Thus, recent attention has turned to the use of alternative allosteric binding mechanisms and improved drug platforms such as modified peptides to design protein kinase modulators with enhanced selectivity and other pharmacological properties. Herein we review the role of different protein kinase C (PKC) isoforms in cancer and cardiovascular disease, with particular attention to PKC-family inhibitors. We discuss translational examples and carefully consider the advantages and limitations of each compound (Part I). We also discuss the recent advances in the field of protein kinase modulators, leverage molecular docking to model inhibitor-kinase interactions, and propose mechanisms of action that will aid in the design of next-generation protein kinase modulators (Part II).

Combination Drug Therapy for the Management of Chronic Neuropathic Pain

Chronic neuropathic pain (NP) is an increasingly prevalent disease and leading cause of disability which is challenging to treat. Several distinct classes of drugs are currently used for the treatment of chronic NP, but each drug targets only narrow components of the underlying pathophysiological mechanisms, bears limited efficacy, and comes with dose-limiting side effects. Multimodal therapies have been increasingly proposed as potential therapeutic approaches to target the multiple mechanisms underlying nociceptive transmission and modulation. However, while preclinical studies with combination therapies showed promise to improve efficacy over monotherapy, clinical trial data on their efficacy in specific populations are lacking and increased risk for adverse effects should be carefully considered. Drug-drug co-crystallization has emerged as an innovative pharmacological approach which can combine two or more different active pharmaceutical ingredients in a single crystal, optimizing pharmacokinetic and physicochemical characteristics of the native molecules, thus potentially capitalizing on the synergistic efficacy between classes of drugs while simplifying adherence and minimizing the risk of side effects by reducing the doses. In this work, we review the current pharmacological options for the treatment of chronic NP, focusing on combination therapies and their ongoing developing programs and highlighting the potential of co-crystals as novel approaches to chronic NP management.

Research Trends and Hotspots of Neuromodulation in Neuropathic Pain: A Bibliometric Analysis

BACKGROUND

Neuropathic pain (NeuP), the result of a lesion or disease of the somatosensory nervous system, is tricky to cure clinically. Mounting researches reveal that neuromodulation can safely and effectively ameliorate NeuP. The number of publications associated with neuromodulation and NeuP increases with time. However, bibliometric analysis on the field is rare. The present study aims to analyze trends and topics in neuromodulation and NeuP research by using a bibliometric method.

METHODS

This study systematically collected the relevant publications on the Science Citation Index Expanded of Web of Science from January 1994 to January 17, 2023. CiteSpace software was used to draw and analyze corresponding visualization maps.

RESULTS

A total of 1404 publications were ultimately obtained under our specified inclusion criteria. The analysis showed that the focus of research on neuromodulation and NeuP had been developing steadily in recent years, with papers published in 58 countries/regions and 411 academic journals. The Journal of Neuromodulation and the author J.P. Lefaucheur published the most papers. The papers published in Harvard University and the United States contributed significantly. The cited keywords show that motor cortex stimulation, spinal cord stimulation, electrical stimulation, transcranial magnetic stimulation, and mechanism are the research hotspots in the field.

CONCLUSIONS

The bibliometric analysis showed that the number of publications on neuromodulation and NeuP are increasing rapidly, especially in the past 5 years. "Motor cortex stimulation," "electrical stimulation," "spinal cord stimulation," "transcranial magnetic stimulation" and "mechanism" catch the most attention among researchers in this field.

Ketoprofen, lysine and gabapentin co-crystal magnifies synergistic efficacy and tolerability of the constituent drugs: Pre-clinical evidences towards an innovative therapeutic approach for neuroinflammatory pain

Chronic pain is an enormous public health concern, and its treatment is still an unmet medical need. Starting from data highlighting the promising effects of some nonsteroidal anti-inflammatory drugs in combination with gabapentin in pain treatment, we sought to combine ketoprofen lysine salt (KLS) and gabapentin to obtain an effective multimodal therapeutic approach for chronic pain. Using relevant in vitro models, we first demonstrated that KLS and gabapentin have supra-additive effects in modulating key pathways in neuropathic pain and gastric mucosal damage. To leverage these supra-additive effects, we then chemically combined the two drugs via co-crystallization to yield a new compound, a ternary drug-drug co-crystal of ketoprofen, lysine and gabapentin (KLS-GABA co-crystal). Physicochemical, biodistribution and pharmacokinetic studies showed that within the co-crystal, ketoprofen reaches an increased gastrointestinal solubility and permeability, as well as a higher systemic exposure in vivo compared to KLS alone or in combination with gabapentin, while both the constituent drugs have increased central nervous system permeation. These unique characteristics led to striking, synergistic anti-nociceptive and anti-inflammatory effects of KLS-GABA co-crystal, as well as significantly reduced spinal neuroinflammation, in translational inflammatory and neuropathic pain rat models, suggesting that the synergistic therapeutic effects of the constituent drugs are further boosted by the co-crystallization. Notably, while strengthening the therapeutic effects of ketoprofen, KLS-GABA co-crystal showed remarkable gastrointestinal tolerability in both inflammatory and chronic neuropathic pain rat models. In conclusion, these results allow us to propose KLS-GABA co-crystal as a new drug candidate with high potential clinical benefit-to-risk ratio for chronic pain treatment.

Understanding the physiological role of NaV1.9: Challenges and opportunities for pain modulation

Voltage-activated Na⁺ (Na_V) channels are crucial contributors to rapid electrical signaling in the human body. As such, they are among the most targeted membrane proteins by clinical therapeutics and natural toxins. Several of the nine mammalian Na_V channel subtypes play a documented role in pain or other sensory processes such as itch, touch, and smell. While causal relationships between these subtypes and biological function have been extensively described, the physiological role of Na_V1.9 is less understood. Yet, mutations in Na_V1.9 can cause striking disease phenotypes related to sensory perception such as loss or gain of pain and chronic itch. Here, we explore our current knowledge of the mechanisms by which Na_V1.9 may contribute to pain and elaborate on the challenges associated with establishing links between experimental conditions and human disease. This review also discusses the lack of comprehensive insights into Na_V1.9-specific pharmacology, an unfortunate situation since modulatory compounds may have tremendous potential in the clinic to treat pain or as precision tools to examine the extent of Na_V1.9 participation in sensory perception processes.

Kinin B2 and B1 Receptors Activation Sensitize the TRPA1 Channel Contributing to Anastrozole-Induced Pain Symptoms

Aromatase inhibitors (AIs) cause symptoms of musculoskeletal pain, and some mechanisms have been proposed to explain them. However, signaling pathways downstream from kinin B2 (B2R) and B1 (B1R) receptor activation and their possible sensitizing of the Transient Receptor Potential Ankyrin 1 (TRPA1) remain unknown. The interaction between the kinin receptor and the TRPA1 channel in male C57BL/6 mice treated with anastrozole (an AI) was evaluated. PLC/PKC and PKA inhibitors were used to evaluate the signaling pathways downstream from B2R and B1R activation and their effect on TRPA1 sensitization. Anastrozole caused mechanical allodynia and muscle strength loss in mice. B2R (Bradykinin), B1R (DABk), or TRPA1 (AITC) agonists induced overt nociceptive behavior and enhanced and prolonged the painful parameters in anastrozole-treated mice. All painful symptoms were reduced by B2R (Icatibant), B1R (DALBk), or TRPA1 (A967079) antagonists. We observed the interaction between B2R, B1R, and the TRPA1 channel in anastrozole-induced musculoskeletal pain, which was dependent on the activation of the PLC/PKC and PKA signaling pathways. TRPA1 seems to be sensitized by mechanisms dependent on the activation of PLC/PKC, and PKA due to kinin receptors stimulation in anastrozole-treated animals. Thus, regulating this signaling pathway could contribute to alleviating AIs-related pain symptoms, patients’ adherence to therapy, and disease control.

Quercetin Ameliorates Neuropathic Pain after Brachial Plexus Avulsion via Suppressing Oxidative Damage through Inhibition of PKC/MAPK/ NOX Pathway

BACKGROUND

Brachial plexus avulsion (BPA) animally involves the separation of spinal nerve roots themselves and the correlative spinal cord segment, leading to formidable neuropathic pain of the upper limb.

METHODS

The right seventh cervical (C7) ventral and dorsal roots were avulsed to establish a neuropathic pain model in rats. After operation, rats were treated with quercetin (QCN) by intragastric administration for 1 week. The effects of QCN were evaluated using mechanical allodynia tests and biochemical assay kits.

RESULTS

QCN treatment significantly attenuated the avulsion-provoked mechanical allodynia, elevated the levels of catalase (CAT), superoxide dismutase (SOD) and glutathione peroxidase (GPx) and total antioxidant capacity (TAC) in the C7 spinal dorsal horn. In addition, QCN administration inhibited the activations of macrophages, microglia and astrocytes in the C6 dorsal root ganglion (DRG) and C6-8 spinal dorsal horn, as well as attenuated the release of purinergic 2X (P2X) receptors in C6 DRG. The molecular mechanism underlying the above alterations was found to be related to the suppression of the PKC/MAPK/NOX signal pathway. To further study the anti-oxidative effects of QCN, we applied QCN on the H2O2-induced BV-2 cells in vitro, and the results attested that QCN significantly ameliorated the H2O2-induced ROS production in BV-2 cells, inhibited the H2O2-induced activation of PKC/MAPK/NOX pathway.

CONCLUSION

Our study for the first time provided evidence that QCN was able to attenuate pain hypersensitivity following the C7 spinal root avulsion in rats, and the molecular mechanisms involve the reduction of both neuro-inflammatory infiltration and oxidative stress via suppression of P2X receptors and inhibition of the activation of PKC/MAPK/NOX pathway. The results indicate that QCN is a natural compound with great promise worthy of further development into a novel therapeutic method for the treatment of BPA-induced neuropathic pain.

Spinal Astrocyte-Neuron Lactate Shuttle Contributes to the Pituitary Adenylate Cyclase-Activating Polypeptide/PAC1 Receptor-Induced Nociceptive Behaviors in Mice

We have previously shown that spinal pituitary adenylate cyclase-activating polypeptide (PACAP)/PACAP type 1 (PAC1) receptor signaling triggered long-lasting nociceptive behaviors through astroglial activation in mice. Since astrocyte-neuron lactate shuttle (ANLS) could be essential for long-term synaptic facilitation, we aimed to elucidate a possible involvement of spinal ANLS in the development of the PACAP/PAC1 receptor-induced nociceptive behaviors. A single intrathecal administration of PACAP induced short-term spontaneous aversive behaviors, followed by long-lasting mechanical allodynia in mice. These nociceptive behaviors were inhibited by 1,4-dideoxy-1,4-imino-d-arabinitol (DAB), an inhibitor of glycogenolysis, and this inhibition was reversed by simultaneous L-lactate application. In the cultured spinal astrocytes, the PACAP-evoked glycogenolysis and L-lactate secretion were inhibited by DAB. In addition, a protein kinase C (PKC) inhibitor attenuated the PACAP-induced nociceptive behaviors as well as the PACAP-evoked glycogenolysis and L-lactate secretion. Finally, an inhibitor for the monocarboxylate transporters blocked the L-lactate secretion from the spinal astrocytes and inhibited the PACAP- and spinal nerve ligation-induced nociceptive behaviors. These results suggested that spinal PAC1 receptor-PKC-ANLS signaling contributed to the PACAP-induced nociceptive behaviors. This signaling system could be involved in the peripheral nerve injury-induced pain-like behaviors.

Potential mechanisms of acupuncture for neuropathic pain based on somatosensory system

Neuropathic pain, caused by a lesion or disease of the somatosensory system, is common and distressing. In view of the high human and economic burden, more effective treatment strategies were urgently needed. Acupuncture has been increasingly used as an adjuvant or complementary therapy for neuropathic pain. Although the therapeutic effects of acupuncture have been demonstrated in various high-quality randomized controlled trials, there is significant heterogeneity in the underlying mechanisms. This review aimed to summarize the potential mechanisms of acupuncture on neuropathic pain based on the somatosensory system, and guided for future both foundational and clinical studies. Here, we argued that acupuncture may have the potential to inhibit neuronal activity caused by neuropathic pain, through reducing the activation of pain-related ion channels and suppressing glial cells (including microglia and astrocytes) to release inflammatory cytokines, chemokines, amongst others. Meanwhile, acupuncture as a non-pharmacologic treatment, may have potential to activate descending pain control system via increasing the level of spinal or brain 5-hydroxytryptamine (5-HT), norepinephrine (NE), and opioid peptides. And the types of endogenously opioid peptides was influenced by electroacupuncture-frequency. The cumulative evidence demonstrated that acupuncture provided an alternative or adjunctive therapy for neuropathic pain.

Phenotypic drug screen uncovers the metabolic GCH1/BH4 pathway as key regulator of EGFR/KRAS-mediated neuropathic pain and lung cancer

Increased tetrahydrobiopterin (BH4) generated in injured sensory neurons contributes to increased pain sensitivity and its persistence. GTP cyclohydrolase 1 (GCH1) is the rate-limiting enzyme in the de novo BH4 synthetic pathway, and human single-nucleotide polymorphism studies, together with mouse genetic modeling, have demonstrated that decreased GCH1 leads to both reduced BH4 and pain. However, little is known about the regulation of Gch1 expression upon nerve injury and whether this could be modulated as an analgesic therapeutic intervention. We performed a phenotypic screen using about 1000 bioactive compounds, many of which are target-annotated FDA-approved drugs, for their effect on regulating Gch1 expression in rodent injured dorsal root ganglion neurons. From this approach, we uncovered relevant pathways that regulate Gch1 expression in sensory neurons. We report that EGFR/KRAS signaling triggers increased Gch1 expression and contributes to neuropathic pain; conversely, inhibiting EGFR suppressed GCH1 and BH4 and exerted analgesic effects, suggesting a molecular link between EGFR/KRAS and pain perception. We also show that GCH1/BH4 acts downstream of KRAS to drive lung cancer, identifying a potentially druggable pathway. Our screen shows that pharmacologic modulation of GCH1 expression and BH4 could be used to develop pharmacological treatments to alleviate pain and identified a critical role for EGFR-regulated GCH1/BH4 expression in neuropathic pain and cancer in rodents.

Dapagliflozin Attenuates Sympathetic and Pressor Responses to Stress in Young Prehypertensive Spontaneously Hypertensive Rats

BACKGROUND

SGLT2i (sodium-glucose cotransporter 2 inhibitor), a class of anti-diabetic medications, is shown to reduce blood pressure (BP) in hypertensive patients with type 2 diabetes. Mechanisms underlying this action are unknown but SGLT2i-induced sympathoinhibition is thought to play a role. Whether SGLT2i reduces BP and sympathetic nerve activity (SNA) in a nondiabetic prehypertension model is unknown.

METHODS

Accordingly, we assessed changes in conscious BP using radiotelemetry and alterations in mean arterial pressure and renal SNA during simulated exercise in nondiabetic spontaneously hypertensive rats during chronic administration of a diet containing dapagliflozin (0.5 mg/kg per day) versus a control diet.

RESULTS

We found that dapagliflozin had no effect on fasting blood glucose, insulin, or hemoglobin A_1C levels. However, dapagliflozin reduced BP in young (8-week old) spontaneously hypertensive rats as well as attenuated the age-related rise in BP in adult spontaneously hypertensive rat up to 17-weeks of age. The rises in mean arterial pressure and renal SNA during simulated exercise (exercise pressor reflex activation by hindlimb muscle contraction) were significantly reduced after 4 weeks of dapagliflozin (Δmean arterial pressure: 10±7 versus 25±14 mm Hg, Δrenal SNA: 31±17% versus 68±39%, P<0.05). Similarly, rises in mean arterial pressure and renal SNA during mechanoreflex stimulation by passive hindlimb stretching were also attenuated by dapagliflozin. Heart weight was significantly decreased in dapagliflozin compared with the control group.

CONCLUSIONS

These data demonstrate a novel role for SGLT2i in reducing resting BP as well as the activity of skeletal muscle reflexes, independent of glycemic control. Our study may have important clinical implications for preventing hypertension and hypertensive heart disease in young prehypertensive individuals.

Proteomic and Phosphoproteomic Changes of MAPK-Related Inflammatory Response in an Animal Model of Neuropathic Pain by Differential Target Multiplexed SCS and Low-Rate SCS

Introduction

Neuropathic pain initiates an interplay of pathways, involving MAP kinases and NFκB-signaling, leading to expression of immune response factors and activation and inactivation of proteins via phosphorylation. Neuropathic pain models demonstrated that spinal cord stimulation (SCS) may provide analgesia by modulating gene and protein expression in neuroinflammatory processes. A differential target multiplexed programming (DTMP) approach was more effective than conventional SCS treatments at modulating these. This work investigated the effect of DTMP and low rate SCS (LR-SCS) on proteins associated with MAP kinases and NFκB-signaling relevant to neuroinflammation.

Methods

Animals subjected to the spared nerve injury model (SNI) of neuropathic pain were treated continuously (48h) with either DTMP or LR-SCS. No-SNI and No-SCS groups were included as controls. Proteomics and phosphoproteomics of stimulated spinal cord tissues were performed via liquid chromatography/tandem mass spectrometry. Proteins were identified from mass spectra using bioinformatics. Expression levels and fold changes (No-SCS/No-SNI and SCS/No-SCS) were obtained from spectral intensities.

Results

Analyses identified 7192 proteins, with 1451 and 705 significantly changed by DTMP and LR-SCS, respectively. Eighty-one proteins, including MAP kinases, facilitating NFκB-signaling as part of inflammatory processes were identified. The pain model significantly increased expression levels of complement pathway-related proteins (LBP, NRG1, APP, CFH, C3, C5), which were significantly reversed by DTMP. Expression levels of other complement pathway-related proteins (HMGB1, S100A8, S100A9, CRP, C4) were decreased by DTMP, although not significantly affected by SNI. Other proteins (ORM1, APOE, NG2, CNTF) involved in NFκB-signaling were increased by SNI and decreased by DTMP. Expression levels of phosphorylated protein kinases involved in NFκB-signaling (including MAP kinases, PKC, MARK1) were affected by the pain model and reverse modulated by DTMP. LR-SCS modulated inflammatory-related proteins although to a lesser extent than DTMP.

Conclusion

Proteomic analyses support the profound effect of the DTMP approach on neuroinflammation via MAP kinases and NFκB-mediated signaling to alleviate neuropathic pain.

Pharmacological blockade of neurokinin1 receptor restricts morphine-induced tolerance and hyperalgesia in the rat

OBJECTIVES

The most notable adverse side effects of chronic morphine administration include tolerance and hyperalgesia. This study investigated the involvement of dorsal root ganglion (DRG) protein kinase Cɛ (PKCɛ) expression during chronic morphine administration and also considered the relationship between DRG PKCɛ expression and the substance P- neurokinin1 receptor (SP- NK1R) activity.

METHODS

Thirty-six animals were divided into six groups (n=6) in this study. In the morphine and sham groups, rats received 10 µg intrathecal (i.t.) morphine or saline for eight consecutive days, respectively. Behavioral tests were performed on days 1 and 8 before and after the first injections and then 48 h after the last injection (day 10). In the treatment groups, rats received NK1R antagonist (L-732,138, 25 µg) daily, either alone or 10 min before a morphine injection, Sham groups received DMSO alone or 10 min before a morphine injection. Animals were sacrificed on days 8 and 10, and DRG PKCɛ and SP expression were analyzed by western blot and immunohistochemistry techniques, respectively.

RESULTS

Behavioral tests indicated that tolerance developed following eight days of chronic morphine injection. Hyperalgesia was induced 48 h after the last morphine injection. Expression of SP and PKCɛ in DRG significantly increased in rats that developed morphine tolerance on day 8 and hyperalgesia on day 10, respectively. NK1R antagonist (L-732,138) not only blocked the development of hyperalgesia and the increase of PKCɛ expression but also alleviated morphine tolerance.

CONCLUSIONS

Our results provide evidence that DRG PKCɛ and SP-NK1R most likely participated in the generation of morphine tolerance and hyperalgesia. Pharmacological inhibition of SP-NK1R activity in the spinal cord suggests a role for NK1R and in restricting some side effects of chronic morphine. All experiments were performed by the National Institute of Health (NIH) Guidelines for the Care and Use of Laboratory Animals (NIH Publication No. 80-23, revised1996) and were approved by the Animal Ethics Committee of Shahid Beheshti University of Medical Sciences, Tehran, Iran (IR.SBMU.MSP.REC.1396.130).

The redundancy and diversity between two novel PKC isotypes that regulate learning in Caenorhabditis elegans

The nematode Caenorhabditis elegans learns the concentration of NaCl and moves toward the previously experienced concentration. In this behavior, the history of NaCl concentration change is reflected in the level of diacylglycerol and the activity of protein kinase C, PKC-1, in the gustatory sensory neuron ASER and determines the direction of migration. Here, through a genetic screen, we found that the activation of Gq protein compensates for the behavioral defect of the loss-of-function mutant of pkc-1 We found that Gq activation results in hyperproduction of diacylglycerol in ASER sensory neuron, which leads to recruitment of TPA-1, an nPKC isotype closely related to PKC-1. Unlike the pkc-1 mutants, loss of tpa-1 did not obviously affect migration directions in the conventional learning assay. This difference was suggested to be due to cooperative functions of the C1 and C2-like domains of the nPKC isotypes. Furthermore, we investigated how the compensatory capability of tpa-1 contributes to learning and found that learning was less robust in the context of cognitive decline or environmental perturbation in tpa-1 mutants. These results highlight how two nPKC isotypes contribute to the learning system.

C-X-C motif chemokine ligand 1 and its receptor C-X-C motif chemokine receptor 2 in trigeminal ganglion contribute to nerve injury-induced orofacial mechanical allodynia

BACKGROUND

Orofacial ectopic pain induced by trigeminal nerve injury is a serious complication of dental treatment. C-X-C motif chemokine ligand 1 (CXCL1) and its primary receptor C-X-C motif chemokine receptor 2 (CXCR2) contribute to the development and maintenance of neuropathic pain in the spinal nervous system, but their roles in trigeminal neuropathic sensation are still poorly understood.

OBJECTIVES

This study aimed to investigate the exact role of CXCL1 and CXCR2 in the regulation of orofacial ectopic mechanical allodynia and their potential downstream mechanisms in the trigeminal ganglion (TG).

METHODS

The head withdrawal threshold (HWT) of C57BL/6 mice was evaluated after inferior alveolar nerve (IAN) transection (IANX). Then, the distribution and expression of CXCL1 and CXCR2, and their potential downstream mechanisms in the TG were further measured using immunohistochemistry, real-time reverse transcription-quantitative polymerase chain reaction and Western blotting. Moreover, the effect of SB225002 (an inhibitor of CXCR2) on mechanical allodynia was examined. The data were analysed using the Student's t test and a analysis of variance (ANOVA).

RESULTS

IANX triggered persistent (>21 days) mechanical allodynia and upregulation of CXCL1 and CXCR2 in the TG. In addition, exogenous CXCL1 also lowered the HWT, which was alleviated by CXCR2 and protein kinase C (PKC) antagonists (p < .05). In addition, IANX increased the phosphorylated PKC (p-PKC) levels and decreased the expression of voltage-gated potassium channels (Kv), and these effects were reversed by inhibition of CXCR2 (p < .05).

CONCLUSION

Our results demonstrated that CXCR2 participated in orofacial ectopic mechanical allodynia via downregulation of Kv1.4 and Kv1.1 through the PKC signalling pathway. This mechanism may be a potential target in developing a treatment strategy for ectopic orofacial pain.

Effect of Acupuncture on Diabetic Neuropathy: A Narrative Review

Diabetic neuropathy, a major complication of diabetes mellitus, refers to a collection of clinically diverse disorders affecting the nervous system that may present with pain. Although the number of patients suffering from severe neuropathy is increasing, no optimal treatment method has been developed yet. Acupuncture is well known for its ability to reduce various kinds of pain, and a number of studies have also reported its effect on diabetes mellitus; however, its effect and underlying mechanism against diabetic neuropathy are not yet clearly understood. In this review, ten and five studies performed in humans and animals, respectively, were analyzed. All studies reported that acupuncture significantly relieved diabetic neuropathy. ST36, BL13, BL20, SP6, and SP9 were the most widely used acupoints. Five studies used electro-acupuncture, whereas other studies used manual acupuncture. Furthermore, the effect of acupuncture was shown to be mediated through the various molecules present in the peripheral nerves and spinal cord, such as P65, GPR78, and TRPV1. Five studies reported side effects, such as swelling, numbness, and nausea, but none were reported to be serious. Based on these results, we suggest that acupuncture should be considered as a treatment option for diabetic neuropathy.

The role of cyclin-dependent kinase 5 in neuropathic pain

The chronification of pain can be attributed to changes in membrane receptors and channels underlying neuronal plasticity and signal transduction largely within nociceptive neurons that initiate and maintain pathological pain states. These proteins are subject to dynamic modification by posttranslational modifications, creating a code that controls protein function in time and space. Phosphorylation is an important posttranslational modification that affects ∼30% of proteins in vivo. Increased phosphorylation of various nociceptive ion channels and of their modulators underlies sensitization of different pain states. Cyclin-dependent kinases are proline-directed serine/threonine kinases that impact various biological and cellular systems. Cyclin-dependent kinase 5 (Cdk5), one member of this kinase family, and its activators p35 and p39 are expressed in spinal nerves, dorsal root ganglia, and the dorsal horn of the spinal cord. In neuropathic pain conditions, expression and/or activity of Cdk5 is increased, implicating Cdk5 in nociception. Experimental evidence suggests that Cdk5 is regulated through its own phosphorylation, through increasing p35's interaction with Cdk5, and through cleavage of p35 into p25. This narrative review discusses the molecular mechanisms of Cdk5-mediated regulation of target proteins involved in neuropathic pain. We focus on Cdk5 substrates that have been linked to nociceptive pathways, including channels (eg, transient receptor potential cation channel and voltage-gated calcium channel), proteins involved in neurotransmitter release (eg, synaptophysin and collapsin response mediator protein 2), and receptors (eg, glutamate, purinergic, and opioid). By altering the phosphoregulatory "set point" of proteins involved in pain signaling, Cdk5 thus appears to be an attractive target for treating neuropathic pain conditions.

Cross-Talk of Toll-Like Receptor 5 and Mu-Opioid Receptor Attenuates Chronic Constriction Injury-Induced Mechanical Hyperalgesia through a Protein Kinase C Alpha-Dependent Signaling

Recently, Toll-like receptors (TLRs), a family of pattern recognition receptors, are reported as potential modulators for neuropathic pain; however, the desired mechanism is still unexplained. Here, we operated on the sciatic nerve to establish a pre-clinical rodent model of chronic constriction injury (CCI) in Sprague-Dawley rats, which were assigned into CCI and Decompression groups randomly. In Decompression group, the rats were performed with nerve decompression at post-operative week 4. Mechanical hyperalgesia and mechanical allodynia were obviously attenuated after a month. Toll-like receptor 5 (TLR5)-immunoreactive (ir) expression increased in dorsal horn, particularly in the inner part of lamina II. Additionally, substance P (SP) and isolectin B4 (IB4)-ir expressions, rather than calcitonin-gene-related peptide (CGRP)-ir expression, increased in their distinct laminae. Double immunofluorescence proved that increased TLR5-ir expression was co-expressed mainly with IB4-ir expression. Through an intrathecal administration with FLA-ST Ultrapure (a TLR5 agonist, purified flagellin from Salmonella Typhimurium, only the CCI-induced mechanical hyperalgesia was attenuated dose-dependently. Moreover, we confirmed that mu-opioid receptor (MOR) and phospho-protein kinase Cα (pPKCα)-ir expressions but not phospho-protein kinase A RII (pPKA RII)-ir expression, increased in lamina II, where they mostly co-expressed with IB4-ir expression. Go 6976, a potent protein kinase C inhibitor, effectively reversed the FLA-ST Ultrapure- or DAMGO-mediated attenuated trend towards mechanical hyperalgesia by an intrathecal administration in CCI rats. In summary, our current findings suggest that nerve decompression improves CCI-induced mechanical hyperalgesia that might be through the cross-talk of TLR5 and MOR in a PKCα-dependent manner, which opens a novel opportunity for the development of analgesic therapeutics in neuropathic pain.

Beyond Neuronal Heat Sensing: Diversity of TRPV1 Heat-Capsaicin Receptor-Channel Functions

Transient receptor potential vanilloid 1 (TRPV1) is a calcium-permeable ion channel best known for its ability to be gated by the pungent constituent of red chili pepper, capsaicin, and related chemicals from the group of vanilloids as well as by noxious heat. As such, it is mostly expressed in sensory neurons to act as a detector of painful stimuli produced by pungent chemicals and high temperatures. Its activation is also sensitized by the numerous endogenous inflammatory mediators and second messengers, making it an important determinant of nociceptive signaling. Except for such signaling, though, neuronal TRPV1 activation may influence various organ functions by promoting the release of bioactive neuropeptides from sensory fiber innervation organs. However, TRPV1 is also found outside the sensory nervous system in which its activation and function is not that straightforward. Thus, TRPV1 expression is detected in skeletal muscle; in some types of smooth muscle; in epithelial and immune cells; and in adipocytes, where it can be activated by the combination of dietary vanilloids, endovanilloids, and pro-inflammatory factors while the intracellular calcium signaling that this initiates can regulate processes as diverse as muscle constriction, cell differentiation, and carcinogenesis. The purpose of the present review is to provide a clear-cut distinction between neurogenic TRPV1 effects in various tissues consequent to its activation in sensory nerve endings and non-neurogenic TRPV1 effects due to its expression in cell types other than sensory neurons.

Spinal PAR2 Activation Contributes to Hypersensitivity Induced by Peripheral Inflammation in Rats

The mechanisms of inflammatory pain need to be identified in order to find new superior treatments. Protease-activated receptors 2 (PAR2) and transient receptor potential vanilloid 1 (TRPV1) are highly co-expressed in dorsal root ganglion neurons and implicated in pain development. Here, we examined the role of spinal PAR2 in hyperalgesia and the modulation of synaptic transmission in carrageenan-induced peripheral inflammation, using intrathecal (i.t.) treatment in the behavioral experiments and recordings of spontaneous, miniature and dorsal root stimulation-evoked excitatory postsynaptic currents (sEPSCs, mEPSCs and eEPSCs) in spinal cord slices. Intrathecal PAR2-activating peptide (AP) administration aggravated the carrageenan-induced thermal hyperalgesia, and this was prevented by a TRPV1 antagonist (SB 366791) and staurosporine i.t. pretreatment. Additionally, the frequency of the mEPSC and sEPSC and the amplitude of the eEPSC recorded from the superficial dorsal horn neurons were enhanced after acute PAR2 AP application, while prevented with SB 366791 or staurosporine pretreatment. PAR2 antagonist application reduced the thermal hyperalgesia and decreased the frequency of mEPSC and sEPSC and the amplitude of eEPSC. Our findings highlight the contribution of spinal PAR2 activation to carrageenan-induced hyperalgesia and the importance of dorsal horn PAR2 and TRPV1 receptor interactions in the modulation of nociceptive synaptic transmission.

Pain pathways and potential new targets for pain relief

Pain is an unpleasant sensory and emotional experience that affects a sizable percentage of people on a daily basis. Sensory neurons known as nociceptors built specifically to detect damaging stimuli can be found throughout the body. They transmit information about noxious stimuli from mechanical, thermal, and chemical sources to the central nervous system and higher brain centers via electrical signals. Nociceptors express various channels and receptors such as voltage-gated sodium and calcium channels, transient receptor potential channels, and opioid receptors that allow them to respond in a highly specific manner to noxious stimuli. Attenuating the pain response can be achieved by inhibiting or altering the expression of these pain targets. Achieving a deeper understanding of how these receptors can be affected at the molecular level can lead to the development of novel pain therapies. This review will discuss the mechanisms of pain, introduce the various receptors that are responsible for detecting pain, and future directions in pharmacological therapies.

Possible Participation of Ionotropic Glutamate Receptors and l-Arginine-Nitric Oxide-Cyclic Guanosine Monophosphate-ATP-Sensitive K+ Channel Pathway in the Antinociceptive Activity of Cardamonin in Acute Pain Animal Models

The perception of pain caused by inflammation serves as a warning sign to avoid further injury. The generation and transmission of pain impulses involves various pathways and receptors. Cardamonin isolated from Boesenbergia rotunda (L.) Mansf. has been reported to exert antinociceptive effects in thermal and mechanical pain models; however, the precise mechanism has yet to be examined. The present study investigated the possible mechanisms involved in the antinociceptive activity of cardamonin on protein kinase C, N-methyl-d-aspartate (NMDA) and non-NMDA glutamate receptors, l-arginine/cyclic guanosine monophosphate (cGMP) mechanism, as well as the ATP-sensitive potassium (K⁺) channel. Cardamonin was administered to the animals intra-peritoneally. Present findings showed that cardamonin significantly inhibited pain elicited by intraplantar injection of phorbol 12-myristate 13-acetate (PMA, a protein kinase C activator) with calculated mean ED₅₀ of 2.0 mg/kg (0.9–4.5 mg/kg). The study presented that pre-treatment with MK-801 (NMDA receptor antagonist) and NBQX (non-NMDA receptor antagonist) significantly modulates the antinociceptive activity of cardamonin at 3 mg/kg when tested with glutamate-induced paw licking test. Pre-treatment with l-arginine (a nitric oxide precursor), ODQ (selective inhibitor of soluble guanylyl cyclase) and glibenclamide (ATP-sensitive K⁺ channel inhibitor) significantly enhanced the antinociception produced by cardamonin. In conclusion, the present findings showed that the antinociceptive activity of cardamonin might involve the modulation of PKC activity, NMDA and non-NMDA glutamate receptors, l-arginine/nitric oxide/cGMP pathway and ATP-sensitive K⁺ channel.

Qualitative review on N-methyl-D-aspartate receptor expression in rat spinal cord during the postnatal development: Implications for central sensitization and pain

The N‐methyl‐D‐aspartate receptor (NMDAR) is an important mediator of central sensitization and nociception in the rat spinal dorsal horn. The NMDAR subunits and splice variants determine the properties of the receptor. Understanding the expression of NMDAR subunits in spinal cord during the neonatal development is important as it may have consequences for the process of central sensitization and nociception in later life. In this review, a systematic literature search was conducted using three databases: Medline, Embase, and PubMed. A quality assessment was performed on predetermined entities of bias. Thirteen articles were identified to be relevant. The results show that NMDAR subunits and splice variants are dynamically expressed during postnatal development in the spinal dorsal horn. During the first 2 weeks, the expression of less excitable GluN2A subunit and more sensitive GluN2B subunit increases while the expression of high excitable GluN2C subunit decreases. During the 2nd week of postnatal development GluN1 subunits with exon 21 spliced in but exon 22 spliced out are predominantly expressed, increasing phosphorylation, and transport to the membrane. The data suggest that in rats, the nociceptive system is most susceptible to central sensitization processes during the first two postnatal weeks. This may have important consequences for nociception and pain responses in later life. From this, we conclude that targeted therapy directed toward specific NMDAR subunits is a promising candidate for mechanism‐based treatment of pain in neonates.

Effects of alkaloids on peripheral neuropathic pain: a review

Neuropathic pain is a debilitating pathological pain condition with a great therapeutic challenge in clinical practice. Currently used analgesics produce deleterious side effects. Therefore, it is necessary to investigate alternative medicines for neuropathic pain. Chinese herbal medicines have been widely used in treating intractable pain. Compelling evidence revealed that the bioactive alkaloids of Chinese herbal medicines stand out in developing novel drugs for neuropathic pain due to multiple targets and satisfactory efficacy. In this review, we summarize the recent progress in the research of analgesic effects of 20 alkaloids components for peripheral neuropathic pain and highlight the potential underlying molecular mechanisms. We also point out the opportunities and challenges of the current studies and shed light on further in-depth pharmacological and toxicological studies of these bioactive alkaloids. In conclusion, the alkaloids hold broad prospects and have the potentials to be novel drugs for treating neuropathic pain. This review provides a theoretical basis for further applying some alkaloids in clinical trials and developing new drugs of neuropathic pain.

Effect of PKC/NF-κB on the Regulation of P2X3 Receptor in Dorsal Root Ganglion in Rats with Sciatic Nerve Injury

Background

Protein kinase C (PKC), nuclear factor-kappa B p65 (NF-κB p65), and P2X₃ receptor (P2X₃R) play significant roles in the sensitization and transduction of nociceptive signals, which are considered as potential targets for the treatment of neuropathic pain. However, the mechanisms and relationships among them have not been clearly clarified.

Methods

80 rats were randomized and divided into 10 groups (n = 8). Sciatic chronic constriction injury (CCI) rats were intrathecally administered with bisindolylmaleimide I (GF109203X), a PKC-selective antagonist once a day, or pyrrolidine dithiocarbamate (PDTC), an NF-κB inhibitor twice a day. Sham-operated rats were intrathecally administered with saline. Thermal withdrawal latency (TWL) and mechanical withdrawal threshold (MWT) were evaluated in all the groups before CCI operation (baseline) and on the 1st, 3rd, 7th, 10th, and 14th day after CCI operation. Protein levels of p-PKCα, p-NF-κB p65, and P2X₃R were analyzed in the CCI ipsilateral L_4-6 dorsal root ganglions (DRGs).

Results

Intrathecal injection of GF109203X or PDTC alleviated the TWL and MWT in the following 2 weeks after CCI surgery. The protein levels of p-PKCα, p-NF-κB p65, and P2X₃R in the ipsilateral DRGs significantly increased after CCI operation, which could be partly reversed by intrathecal administration of GF109203X or PDTC.

Conclusion

The upregulation of p-PKCα, p-NF-κB p65, and P2X₃R expression in the DRGs of CCI rats was involved in the occurrence and development of neuropathic pain. Phosphorylated PKCα and phosphorylated NF-κB p65 regulated with each other. Phosphorylated NF-κB p65 and PKCα have a mutual regulation relationship with P2X₃R, respectively, while the specific regulatory mechanism needs further research.

Novel Analgesics with Peripheral Targets

A limited number of peripheral targets generate pain. Inflammatory mediators can sensitize these. The review addresses targets acting exclusively or predominantly on sensory neurons, mediators involved in inflammation targeting sensory neurons, and mediators involved in a more general inflammatory process, of which an analgesic effect secondary to an anti-inflammatory effect can be expected. Different approaches to address these systems are discussed, including scavenging proinflammatory mediators, applying anti-inflammatory mediators, and inhibiting proinflammatory or facilitating anti-inflammatory receptors. New approaches are contrasted to established ones; the current stage of progress is mentioned, in particular considering whether there is data from a molecular and cellular level, from animals, or from human trials, including an early stage after a market release. An overview of publication activity is presented, considering a IuPhar/BPS-curated list of targets with restriction to pain-related publications, which was also used to identify topics.

Negative Modulation of TRPM8 Channel Function by Protein Kinase C in Trigeminal Cold Thermoreceptor Neurons

TRPM8 is the main molecular entity responsible for cold sensing. This polymodal ion channel is activated by cold, cooling compounds such as menthol, voltage, and rises in osmolality. In corneal cold thermoreceptor neurons (CTNs), TRPM8 expression determines not only their sensitivity to cold, but also their role as neural detectors of ocular surface wetness. Several reports suggest that Protein Kinase C (PKC) activation impacts on TRPM8 function; however, the molecular bases of this functional modulation are still poorly understood. We explored PKC-dependent regulation of TRPM8 using Phorbol 12-Myristate 13-Acetate to activate this kinase. Consistently, recombinant TRPM8 channels, cultured trigeminal neurons, and free nerve endings of corneal CTNs revealed a robust reduction of TRPM8-dependent responses under PKC activation. In corneal CTNs, PKC activation decreased ongoing activity, a key parameter in the role of TRPM8-expressing neurons as humidity detectors, and also the maximal cold-evoked response, which were validated by mathematical modeling. Biophysical analysis indicated that PKC-dependent downregulation of TRPM8 is mainly due to a decreased maximal conductance value, and complementary noise analysis revealed a reduced number of functional channels at the cell surface, providing important clues to understanding the molecular mechanisms of how PKC activity modulates TRPM8 channels in CTNs.

Oral application of bulleyaconitine A attenuates morphine tolerance in neuropathic rats by inhibiting long-term potentiation at C-fiber synapses and protein kinase C gamma in spinal dorsal horn

Morphine is frequently used for the treatment of chronic pain, while long-term use of the drug leads to analgesic tolerance. At present, the prevention of the side effect remains a big challenge. Bulleyaconitine A, a diterpenoid alkaloid from Aconitum bulleyanum plants, has been used to treat chronic pain in China for more than 30 years. In the present study, we tested the effect of bulleyaconitine A on analgesic tolerance induced by morphine injections (10 mg/kg s.c., b.i.d.) in the lumbar 5 spinal nerve ligation model of neuropathic pain. We found that intragastrical application of bulleyaconitine A (0.4 mg/kg) 30 min before each morphine injection substantially inhibited the decrease in morphine’s inhibitory effect on mechanical allodynia and thermal hyperalgesia. Mechanistically, morphine injections further potentiated the lumbar 5 spinal nerve ligation induced long-term potentiation at C-fiber synapses in the spinal dorsal horn, a synaptic model of chronic pain. This effect was completely blocked by intragastrical bulleyaconitine A. It has been well established that activation of protein kinase C gamma and of glial cells in the spinal dorsal horn are critical for the development of opioid tolerance and neuropathic pain. We found that morphine injections exacerbated the upregulation of phospho-protein kinase C gamma (an active form of protein kinase C gamma), and the activation of microglia and astrocytes in the spinal dorsal horn induced by lumbar 5 spinal nerve ligation, and the effects were considerably prohibited by intragastrical bulleyaconitine A. Thus, spinal long-term potentiation at C-fiber synapses may underlie morphine tolerance. Oral administration of bulleyaconitine A may be a novel and simple approach for treating of opioid tolerance.

Methanol Extract of Dicranopteris linearis Leaves Attenuate Pain via the Modulation of Opioid/NO-Mediated Pathway

Dicranopteris linearis leaf has been reported to exert antinociceptive activity. The present study elucidates the possible mechanisms of antinociception modulated by the methanol extract of D. linearis leaves (MEDL) using various mouse models. The extract (25, 150, and 300 mg/kg) was administered orally to mice for 30 min priot to subjection to the acetic acid-induced writhing-, hot plate- or formalin-test to establish the antinociceptive profile of MEDL. The most effective dose was then used in the elucidation of possible mechanisms of action stage. The extract was also subjected to the phytochemical analyses. The results confirmed that MEDL exerted significant (p < 0.05) antinociceptive activity in those pain models as well as the capsaicin-, glutamate-, bradykinin- and phorbol 12-myristate 13-acetate (PMA)-induced paw licking model. Pretreatment with naloxone (a non-selective opioid antagonist) significantly (p < 0.05) reversed MEDL effect on thermal nociception. Only l-arginine (a nitric oxide (NO) donor) but not N(ω)-nitro-l-arginine methyl ester (l-NAME; a NO inhibitor) or 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; a specific soluble guanylyl cyclase inhibitor) significantly (p < 0.05) modified MEDL effect on the writhing test. Several polyphenolics and volatile antinociceptive compounds were detected in MEDL. In conclusion, MEDL exerted the opioid/NO-mediated antinociceptive activity, thus, justify D. linearis as a potential source for new analgesic agents development.

Pain modulation effect on motor cortex after optogenetic stimulation in shPKCγ knockdown dorsal root ganglion-compressed Sprague-Dawley rat model

Neuropathic pain can be generated by chronic compression of dorsal root ganglion (CCD). Stimulation of primary motor cortex can disrupt the nociceptive sensory signal at dorsal root ganglion level and reduce pain behaviors. But the mechanism behind it is still implicit. Protein kinase C gamma is known as an essential enzyme for the development of neuropathic pain, and specific inhibitor of protein kinase C gamma can disrupt the sensory signal and reduce pain behaviors. Optogenetic stimulation has been emerged as a new and promising conducive method for refractory neuropathic pain. The aim of this study was to provide evidence whether optical stimulation of primary motor cortex can modulate chronic neuropathic pain in CCD rat model. Animals were randomly divided into CCD group, sham group, and control group. Dorsal root ganglion-compressed neuropathic pain model was established in animals, and knocking down of protein kinase C gamma was also accomplished. Pain behavioral scores were significantly improved in the short hairpin Protein Kinase C gamma knockdown CCD animals during optic stimulation. Ventral posterolateral thalamic firing inhibition was also observed during light stimulation on motor cortex in CCD animal. We assessed alteration of pain behaviors in pre-light off, stimulation-light on, and post-light off state. In vivo extracellular recording of the ventral posterolateral thalamus, viral expression in the primary motor cortex, and protein kinase C gamma expression in dorsal root ganglion were investigated. So, optical cortico-thalamic inhibition by motor cortex stimulation can improve neuropathic pain behaviors in CCD animal, and knocking down of protein kinase C gamma plays a conducive role in the process. This study provides feasibility for in vivo optogenetic stimulation on primary motor cortex of dorsal root ganglion-initiated neuropathic pain.

Developing Modern Pain Therapies

Chronic pain afflicts as much as 50% of the population at any given time but our methods to address pain remain limited, ineffective and addictive. In order to develop new therapies an understanding of the mechanisms of painful sensitization is essential. We discuss here recent progress in the understanding of mechanisms underlying pain, and how these mechanisms are being targeted to produce modern, specific therapies for pain. Finally, we make recommendations for the next generation of targeted, effective, and safe pain therapies.

Spinal and afferent PKC signaling mechanisms that mediate chronic pain in sickle cell disease

Pain is the most characteristic feature of sickle cell disease (SCD). Patients with SCD live with unpredictable, recurrent episodes of acute painful crisis, as well as chronic unremitting pain throughout their lifetime. While most of the research and medical efforts have focused on treating vaso-occlusion crisis and acute pain, chronic pain remains a significant challenge faced by patients and physicians. Emerging evidence from human and animal studies has suggested the presence of a neuropathic component in SCD pain. New knowledge on the neurobiology of chronic pain in SCD has significant implications in unraveling the underlying mechanisms. This review focuses on the recent advances on the role of protein kinase C or PKC in promoting and maintaining chronic pain conditions. With a highlight of a specific PKC isoform, PKCδ, we aim to propose PKC as an essential regulator of chronic pain in SCD, which may ultimately lead to innovative therapeutic strategies for treating this devastating life-long problem in patients with SCD.

Influence of intrathecal injection with dexmedetomidine on the behavioral ability and analgesic effects on rats with neuropathic pain and expression of protein kinase C in the spinal dorsal horn

This study aimed to investigate the influence of intrathecal injection with dexmedetomidine on the behavioral ability and analgesic effects of rat models with chronic neuropathic pain. It also discusses the role that the expression of protein kinase C (PKC) in the spinal dorsal horn plays in the analgesic mechanism of dexmedetomidine. A total of 35 Sprague Dawley rats were selected. Five rats were randomly chosen as the blank control group and the rest were prepared as models with chronic constriction injury, which were divided into the model and the observation group (n=15, each group). The observation group was injected with 10 ml dexmedetomidine intrathecally, while the model group was injected with an equivalent amount of saline. The behavioral abilities were evaluated by cumulative and motor function score. In addition, mechanical withdrawal threshold (MWT) and thermal withdrawal latency (TWL) were applied for the detection of pain threshold. Immunohistochemistry staining method was used to test the positive expression of PKC, and western blot analysis was used for the detection of PKC quantitative expression levels. After initiation, the instant cumulative and motor function score of the model and observation groups increased, while the values of MWT and TWL decreased. After initiation, the positive expression of PKC in the spinal dorsal horn of the model and observation groups rose dramatically, and the quantitative expression level was also on the rise. The positive and quantitative expression levels in the observation group reduced gradually over time. Thus, the intrathecal injection of dexmedetomidine may improve the behavioral ability of rats with chronic neuralgia and reduce the degree of pain, which may be associated with the inhibition of the expression of PKC in the spinal dorsal horn.

Suppressing PKC-dependent membrane P2X3 receptor upregulation in dorsal root ganglia mediated electroacupuncture analgesia in rat painful diabetic neuropathy

Painful diabetic neuropathy (PDN) is a common and troublesome diabetes complication. Protein kinase C (PKC)-mediated dorsal root ganglia (DRG) P2X3 receptor upregulation is one important mechanism underlying PDN. Accumulating evidence demonstrated that electroacupuncture (EA) at low frequency could effectively attenuate neuropathic pain. Our previous study showed that 2-Hz EA could relieve pain well in PDN. The study aimed to investigate whether 2-Hz EA relieves pain in PDN through suppressing PKC-mediated DRG P2X3 receptor upregulation. A 7-week feeding of high-fat and high-sugar diet plus a single injection of streptozotocin (STZ) in a dose of 35 mg/kg after a 5-week feeding of the diet successfully induced type 2 PDN in rats as revealed by the elevated body weight, fasting blood glucose, fasting insulin and insulin resistance, and the reduced paw withdrawal threshold (PWT), as well as the destructive ultrastructural change of sciatic nerve. DRG plasma membrane P2X3 receptor level and DRG PKC expression were elevated. Two-hertz EA failed to improve peripheral neuropathy; however, it reduced PWT, DRG plasma membrane P2X3 receptor level, and DRG PKC expression in PDN rats. Intraperitoneal administration of P2X3 receptor agonist αβ-meATP or PKC activator phorbol 12-myristate 13-acetate (PMA) blocked 2-Hz EA analgesia. Furthermore, PMA administration increased DRG plasma membrane P2X3 receptor level in PDN rats subject to 2-Hz EA treatment. These findings together indicated that the analgesic effect of EA in PDN is mediated by suppressing PKC-dependent membrane P2X3 upregulation in DRG. EA at low frequency is a valuable approach for PDN control.

Antinociceptive Activity of Methanolic Extract of Clinacanthus nutans Leaves: Possible Mechanisms of Action Involved

Methanolic extract of Clinacanthus nutans Lindau leaves (MECN) has been proven to possess antinociceptive activity that works via the opioid and NO-dependent/cGMP-independent pathways. In the present study, we aimed to further determine the possible mechanisms of antinociception of MECN using various nociceptive assays. The antinociceptive activity of MECN was (i) tested against capsaicin-, glutamate-, phorbol 12-myristate 13-acetate-, bradykinin-induced nociception model; (ii) prechallenged against selective antagonist of opioid receptor subtypes (β-funaltrexamine, naltrindole, and nor-binaltorphimine); (iii) prechallenged against antagonist of nonopioid systems, namely, α₂-noradrenergic (yohimbine), β-adrenergic (pindolol), adenosinergic (caffeine), dopaminergic (haloperidol), and cholinergic (atropine) receptors; (iv) prechallenged with inhibitors of various potassium channels (glibenclamide, apamin, charybdotoxin, and tetraethylammonium chloride). The results demonstrated that the orally administered MECN (100, 250, and 500 mg/kg) significantly (p < 0.05) reversed the nociceptive effect of all models in a dose-dependent manner. Moreover, the antinociceptive activity of 500 mg/kg MECN was significantly (p < 0.05) inhibited by (i) antagonists of μ-, δ-, and κ-opioid receptors; (ii) antagonists of α₂-noradrenergic, β-adrenergic, adenosinergic, dopaminergic, and cholinergic receptors; and (iii) blockers of different K⁺ channels (voltage-activated-, Ca²⁺-activated, and ATP-sensitive-K⁺ channels, resp.). In conclusion, MECN-induced antinociception involves modulation of protein kinase C-, bradykinin-, TRVP1 receptors-, and glutamatergic-signaling pathways; opioidergic, α₂-noradrenergic, β-adrenergic, adenosinergic, dopaminergic, and cholinergic receptors; and nonopioidergic receptors as well as the opening of various K⁺ channels. The antinociceptive activity could be associated with the presence of several flavonoid-based bioactive compounds and their synergistic action with nonvolatile bioactive compounds.