TRPs and pain

Ligustilide covalently binds to Cys703 in the pre-S1 helix of TRPA1, blocking the opening of channel and relieving pain in rats with acute soft tissue injury

ETHNOPHARMACOLOGICAL RELEVANCE

The natural anodyne Ligustilide (Lig), derived from Angelica sinensis (Oliv.) Diels and Ligusticum chuanxiong Hort., has been traditionally employed for its analgesic properties in the treatment of dysmenorrhea and migraine, and rheumatoid arthritis pain. Despite the existing reports on the correlation between TRP channels and the analgesic effects of Lig, a comprehensive understanding of their underlying mechanisms of action remains elusive.

AIM OF THE STUDY

The objective of this study is to elucidate the mechanism of action of Lig on the analgesic target TRPA1 channel.

METHODS

The therapeutic effect of Lig was evaluated in a rat acute soft tissue injury model. The analgesic target was identified through competitive inhibition of TRP channel agonists at the animal level, followed by Fluo-4/Ca2+ imaging on live cells overexpressing TRP proteins. The potential target was verified through in-gel imaging, colocalization using a Lig-derived molecular probe, and a drug affinity response target stability assay. The binding site of Lig was identified through protein spectrometry and further analyzed using molecular docking, site-specific mutation, and multidisciplinary approaches.

RESULTS

The administration of Lig effectively ameliorated pain and attenuated oxidative stress and inflammatory responses in rats with soft tissue injuries. Moreover, the analgesic effects of Lig were specifically attributed to TRPA1. Mechanistic studies have revealed that Lig directly activates TRPA1 by interacting with the linker domain in the pre-S1 region of TRPA1. Through metabolic transformation, 6,7-epoxyligustilide (EM-Lig) forms a covalent bond with Cys703 of TRPA1 at high concentrations and prolonged exposure time. This irreversible binding prevents endogenous electrophilic products from entering the cysteine active center of ligand-binding pocket of TRPA1, thereby inhibiting Ca2+ influx through the channel opening and ultimately relieving pain.

CONCLUSIONS

Lig selectively modulates the TRPA1 channel in a bimodal manner via non-electrophilic/electrophilic metabolic conversion. The epoxidized metabolic intermediate EM-Lig exerts analgesic effects by irreversibly inhibiting the activation of TRPA1 on sensory neurons. These findings not only highlight the analgesic mechanism of Lig but also offer a novel nucleophilic attack site for the development of TRPA1 antagonists in the pre-S1 region.

Transient Receptor Potential Channels: Multiple Modulators of Peripheral Neuropathic Pain in Several Rodent Models

Neuropathic pain, a prevalent chronic condition in clinical settings, has attracted widespread societal attention. This condition is characterized by a persistent pain state accompanied by affective and cognitive disruptions, significantly impacting patients' quality of life. However, current clinical therapies fall short of addressing its complexity. Thus, exploring the underlying molecular mechanism of neuropathic pain and identifying potential targets for intervention is highly warranted. The transient receptor potential (TRP) receptors, a class of widely distributed channel proteins, in the nervous system, play a crucial role in sensory signaling, cellular calcium regulation, and developmental influences. TRP ion channels are also responsible for various sensory responses including heat, cold, pain, and stress. This review highlights recent advances in understanding TRPs in various rodent models of neuropathic pain, aiming to uncover potential therapeutic targets for clinical management.

Role of soluble epoxide hydrolase in pain and depression comorbidity

The coexistence of chronic pain and depression in clinical practice places a substantial social burden and profoundly impacts in patients. Although a clear correlation exists, the underlying mechanism of comorbidity between chronic pain and depression remains elusive. Research conducted in recent decades has uncovered that soluble epoxide hydrolase, a pivotal enzyme in the metabolism of polyunsaturated fatty acids, plays a crucial role in inflammation. Interestingly, this enzyme is intricately linked to the development of both pain and depression. With this understanding, this review aims to summarize the roles of soluble epoxide hydrolase in pain, depression, and their comorbidity. Simultaneously, we will also explore the underlying mechanisms, providing guidance for future research and drug development.

Downregulation of mesenteric afferent sensitivity following long-term systemic treatment of vincristine in mice

AIMS

Gastrointestinal paresthesia and dysmotility are common side effects of vincristine (VCR) chemotherapy, which have become one of the factors for dose reduction, therapy delay or discontinuation. However, the mechanism is not entirely clear, whether it is related to autonomic nerves injury remains unknown. Therefore, we aimed to study whether VCR-induced gastrointestinal toxicity is related to changes in mesenteric afferent activity.

METHODS

The effects of a single VCR stimulation and long-term systemic VCR treatment on mesenteric afferent activity were investigated by directly recording mesenteric afferent discharge in vitro.

RESULTS

Our results showed that a single VCR (0.001-1 μmol/L) stimulation obviously increased the spontaneous, chemically evoked and mechanically evoked discharge of jejunal and colonic mesenteric afferents. This kind of hypersensitivity of VCR could be blocked by capsazepine, a transient receptor potential vanilloid 1 (TRPV1) antagonist. For the mice treated with VCR (0.1 mg/kg/d, i.p.) for 14 days, the abdominal withdrawal reflex and writhing response scores were reduced. Meanwhile, the spontaneous discharge of colonic mesenteric afferents and the afferent response to VCR was downregulated, and the afferent sensitivity to chemical and mechanical stimulation was reduced. Moreover, the expression of TRPV1 in colon was decreased.

CONCLUSIONS

These results suggest that the direct stimulation by VCR increases the mesenteric afferent sensitivity by activating TRPV1, which may be the reason of VCR-induced abdominal pain; the long-term systemic treatment of VCR decreases mesenteric afferent sensitivity by reducing TRPV1, which may be the reason of VCR-induced constipation.

G Protein-Coupled Receptors and Ion Channels Involvement in Cisplatin-Induced Peripheral Neuropathy: A Review of Preclinical Studies

Cisplatin is a platinum-based chemotherapy drug widely used to treat various solid tumours. Although it is effective in anti-cancer therapy, many patients develop peripheral neuropathy during and after cisplatin treatment. Peripheral neuropathy results from lesions or diseases in the peripheral somatosensory nervous system and is a significant cause of debilitation and suffering in patients. In recent years, preclinical studies have been conducted to elucidate the mechanisms involved in chemotherapy-induced peripheral neuropathic pain, as well as to promote new therapeutic targets since current treatments are ineffective and are associated with adverse effects. G-protein coupled receptors and ion channels play a significant role in pain processing and may represent promising targets for improving the management of cisplatin-induced neuropathic pain. This review describes the role of G protein-coupled receptors and ion channels in cisplatin-induced pain, analysing preclinical experimental studies that investigated the role of each receptor subtype in the modulation of cisplatin-induced pain.

Targeting TRP channels: recent advances in structure, ligand binding, and molecular mechanisms

Transient receptor potential (TRP) channels are a large and diverse family of transmembrane ion channels that are widely expressed, have important physiological roles, and are associated with many human diseases. These proteins are actively pursued as promising drug targets, benefitting greatly from advances in structural and mechanistic studies of TRP channels. At the same time, the complex, polymodal activation and regulation of TRP channels have presented formidable challenges. In this short review, we summarize recent progresses toward understanding the structural basis of TRP channel function, as well as potential ligand binding sites that could be targeted for therapeutics. A particular focus is on the current understanding of the molecular mechanisms of TRP channel activation and regulation, where many fundamental questions remain unanswered. We believe that a deeper understanding of the functional mechanisms of TRP channels will be critical and likely transformative toward developing successful therapeutic strategies targeting these exciting proteins. This endeavor will require concerted efforts from computation, structural biology, medicinal chemistry, electrophysiology, pharmacology, drug safety and clinical studies.

Molecular mechanisms of pain in acute pancreatitis: recent basic research advances and therapeutic implications

Although severe abdominal pain is the main symptom of acute pancreatitis, its mechanisms are poorly understood. An emerging body of literature evidence indicates that neurogenic inflammation might play a major role in modulating the perception of pain from the pancreas. Neurogenic inflammation is the result of a crosstalk between injured pancreatic tissue and activated neurons, which leads to an auto-amplification loop between inflammation and pain during the progression of acute pancreatitis. In this review, we summarize recent findings on the role of neuropeptides, ion channels, and the endocannabinoid system in acute pancreatitis-related pain. We also highlight potential therapeutic strategies that could be applied for managing severe pain in this disease.

Inhibitory effects of linalool, an essential oil component of lavender, on nociceptive TRPA1 and voltage-gated Ca2+ channels in mouse sensory neurons

Linalool, an essential oil component of lavender is commonly used in fragrances. It is known that linalool has anxiolytic, sedative, and analgesic actions. However, the mechanism of its analgesic action has not yet been fully clarified. Pain signals elicited by the activation of nociceptors on peripheral neurons are transmitted to the central nervous system. In the present study, we investigated the effects of linalool on transient receptor potential (TRP) channels and voltage-gated channels, both of which are important for pain signaling via nociceptors in somatosensory neurons. For detection of channel activity, the intracellular Ca²⁺ concentration ([Ca²⁺]_i) was measured using a Ca²⁺-imaging system, and membrane currents were recorded using the whole-cell patch-clamp technique. Analgesic actions were also examined in vivo. In mouse sensory neurons linalool at concentrations that did not induce [Ca²⁺]_i increases did not affect [Ca²⁺]_i responses to capsaicin and acids, TRPV1 agonists, but suppressed those induced by allyl isothiocyanate (AITC) and carvacrol, TRPA1 agonists. Similar inhibitory effects of linalool were observed in cells that heterologously expressed TRPA1. Linalool attenuated the [Ca²⁺]_i increases induced by KCl and voltage-gated Ca²⁺ currents but only slightly suppressed voltage-gated Na^＋currents in mouse sensory neurons. Linalool diminished TRPA1-mediated nociceptive behaviors. The present data suggest that linalool exerts an analgesic action via the suppression of nociceptive TRPA1 and voltage-gated Ca²⁺ channels.

Multiple Chemical Sensitivity: It's time to catch up to the science

Multiple chemical sensitivity (MCS) is a complex medical condition associated with low dose chemical exposures. MCS is characterized by diverse features and common comorbidities, including fibromyalgia, cough hypersensitivity, asthma, and migraine, and stress/anxiety, with which the syndrome shares numerous neurobiological processes and altered functioning within diverse brain regions. Predictive factors linked to MCS comprise genetic influences, gene-environment interactions, oxidative stress, systemic inflammation, cell dysfunction, and psychosocial influences. The development of MCS may be attributed to the sensitization of transient receptor potential (TRP) receptors, notably TRPV1 and TRPA1. Capsaicin inhalation challenge studies demonstrated that TRPV1 sensitization is manifested in MCS, and functional brain imaging studies revealed that TRPV1 and TRPA1 agonists promote brain-region specific neuronal variations. Unfortunately, MCS has often been inappropriately viewed as stemming exclusively from psychological disturbances, which has fostered patients being stigmatized and ostracized, and often being denied accommodation for their disability. Evidence-based education is essential to provide appropriate support and advocacy. Greater recognition of receptor-mediated biological mechanisms should be incorporated in laws, and regulation of environmental exposures.

Evaluation of a polymer-coated nanoparticle cream formulation of resiniferatoxin for the treatment of painful diabetic peripheral neuropathy

ABSTRACT

Painful diabetic peripheral neuropathy (PDPN) is one of the major complications of diabetes. Currently, centrally acting drugs and topical analgesics are used for treating PDPN. These drugs have adverse effects; some are ineffective, and treatment with opioids is associated with use dependence and addiction. Recent research indicates that transient receptor potential vanilloid 1 (TRPV1) expressed in the peripheral sensory nerve terminals is an emerging target to treat pain associated with PDPN. Block of TRPV1 ion channel with specific antagonists, although effective as an analgesic, induced hyperthermia in clinical trials. However, TRPV1 agonists are useful to treat pain by virtue of their ability to cause Ca 2+ influx and subsequently leading to nerve terminal desensitization. Here, we report the effectiveness of an ultrapotent TRPV1 agonist, resiniferatoxin (RTX) nanoparticle, in a topical formulation (RTX-cream; RESINIZIN) that alleviates pain associated with DPN in animal models of diabetes. Resiniferatoxin causes nerve terminal depolarization block in the short term, which prevents pain during application and leading to nerve terminal desensitization/depletion in the long term resulting in long-lasting pain relief. Application of RTX cream to the hind limbs suppresses thermal hyperalgesia in streptozotocin-induced diabetic rats and mini pigs without any adverse effects as compared with capsaicin at therapeutic doses, which induces intense pain during application. Resiniferatoxin cream also decreases the expression of TRPV1 in the peripheral nerve endings and suppresses TRPV1-mediated calcitonin gene-related peptide release in the skin samples of diabetic rats and mini pigs. Our preclinical data confirm that RTX topical formulation is an effective treatment option for PDPN.

A TRPV4-dependent neuroimmune axis in the spinal cord promotes neuropathic pain

Microglia, resident macrophages of the CNS, are essential to brain development, homeostasis, and disease. Microglial activation and proliferation are hallmarks of many CNS diseases, including neuropathic pain. However, molecular mechanisms that govern the spinal neuroimmune axis in the setting of neuropathic pain remain incompletely understood. Here, we show that genetic ablation or pharmacological blockade of transient receptor potential vanilloid type 4 (TRPV4) markedly attenuated neuropathic pain-like behaviors in a mouse model of spared nerve injury. Mechanistically, microglia-expressed TRPV4 mediated microglial activation and proliferation and promoted functional and structural plasticity of excitatory spinal neurons through release of lipocalin-2. Our results suggest that microglial TRPV4 channels reside at the center of the neuroimmune axis in the spinal cord, which transforms peripheral nerve injury into central sensitization and neuropathic pain, thereby identifying TRPV4 as a potential new target for the treatment of chronic pain.

Changes in Acupuncture-Induced Specific Acupoint Neurotransmitters are Possibly Related to Their Physiological Functions in Rats

This study investigated changes in neurotransmitters induced by the application of electroacupuncture (EA) at Zusanli (ST36) and Neiguan (PC6). A total of 30 rats were divided into five groups: sham, ST (EA at bilateral ST36 and ST37), ScT (ST plus previous neurectomy of the bilateral sciatic nerves), ScS (sham plus previous neurectomy of the bilateral sciatic nerve), and PC (EA at bilateral PC6 and PC7). The P2X2 receptor expression was stronger in the sham group than in the ST and PC groups (both p < 0.05) but similar between the sham and ScT groups (p > 0.05). Dopamine levels in the extracellular fluid surrounding the acupoints were higher in the PC group than in the sham and ST groups during the postacupuncture period (both p < 0.05). Glutamate levels in the extracellular fluid surrounding the acupoints were higher in the ST group than in the sham group during the acupuncture period (p < 0.05) and higher in the ST group than in the sham and PC groups during the postacupuncture period (both p < 0.05). Serum adrenaline and noradrenaline levels were higher in the PC group than in the sham, ST, and ScT groups (all p < 0.05). Glutamate levels in the CSF were higher in the ST group than in the sham, ScS, and PC groups (all p < 0.05). GABA levels in the CSF were higher in the ST group than in the sham, ScT, and PC groups (all p < 0.05). EA at ST36 and ST37 and PC6 and PC7 exerted an analgesic effect, EA at PC6 and PC7 can enhance heart function, and EA at ST36 and ST37 modulates the cerebral cortex. However, the study needs an evaluation of direct pain behavior, heart function, and brain function in the future.

Rs11726196 Single-Nucleotide Polymorphism of the Transient Receptor Potential Canonical 3 (TRPC3) Gene Is Associated with Chronic Pain

Chronic pain is reportedly associated with the transient receptor potential canonical 3 (TRPC3) gene. The present study examined the genetic associations between the single-nucleotide polymorphisms (SNPs) of the TRPC3 gene and chronic pain. The genomic samples from 194 patients underwent linkage disequilibrium (LD) analyses of 29 SNPs within and around the vicinity of the TRPC3 gene. We examined the associations between the SNPs and the susceptibility to chronic pain by comparing the genotype distribution of 194 patients with 282 control subjects. All SNP genotype data were extracted from our previous whole-genome genotyping results. Twenty-nine SNPs were extracted, and a total of four LD blocks with 15 tag SNPs were observed within and around the TRPC3 gene. We further analyzed the associations between these tag SNPs and chronic pain. The rs11726196 SNP genotype distribution of patients was significantly different from the control subjects even after multiple-testing correction with the number of SNPs. The TT + TG genotype of rs11726196 is often carried by chronic pain patients, suggesting a causal role for the T allele. These results contribute to our understanding of the genetic risk factors for chronic pain.

Targeting Vascular endothelial growth factor A with soluble vascular endothelial growth factor receptor 1 ameliorates nerve injury-induced neuropathic pain

Neuropathic pain is a distressing medical condition with few effective treatments. The role of Vascular endothelial growth factor A (VEGFA) in inflammation pain has been confirmed in many researches. However, the mechanism of VEGFA affects neuropathic pain remains unclear. In this study, we demonstrated that VEGFA plays an important role in spare nerve injury (SNI)-induced neuropathic pain, which is mediated by enhanced expression and colocalized of VEGFA, p-AKT and TRPV1 in SNI-induced neuropathic pain model. Soluble VEGFR1 (sFlt1) not only relieved mechanical hyperalgesia and the expression of inflammatory markers, but ameliorated the expression of VEGFA, VEGFR2, p-AKT, and TRPV1 in spinal cord. However, these effects of sFlt1 can be blocked by rpVEGFA and by 740 Y-P. Therefore, our study indication that targeting VEGFA with sFlt1 reduces neuropathic pain development via the AKT/TRPV1 pathway in SNI-induced nerve injury. This study elucidates a new therapeutic target for neuropathic pain.

The neurobiology of pain and facial movements in rodents: Clinical applications and current research

One of the most controversial aspects of the use of animals in science is the production of pain. Pain is a central ethical concern. The activation of neural pathways involved in the pain response has physiological, endocrine, and behavioral consequences, that can affect both the health and welfare of the animals, as well as the validity of research. The strategy to prevent these consequences requires understanding of the nociception process, pain itself, and how assessment can be performed using validated, non-invasive methods. The study of facial expressions related to pain has undergone considerable study with the finding that certain movements of the facial muscles (called facial action units) are associated with the presence and intensity of pain. This review, focused on rodents, discusses the neurobiology of facial expressions, clinical applications, and current research designed to better understand pain and the nociceptive pathway as a strategy for implementing refinement in biomedical research.

Sex-related differences in oxaliplatin-induced changes in the expression of transient receptor potential channels and their contribution to cold hypersensitivity

Transient receptor potential (TRP) channels are involved in the development of oxaliplatin-induced neuropathic pain, a frequent and debilitating side effect of cancer therapy. Here we explored whether oxaliplatin-induced changes in the expression of TRP channels, as well as the development of pain-related behaviours, differed between male and female animals. Adult rats were injected with oxaliplatin or saline and mechanical and cold allodynia were evaluated using Von Frey and Choi Tests. The mRNA levels of TRPV1, TRPM8 and TRPA1 were assessed in lumbar ganglia and spinal cord by using real time RT-PCR. Oxaliplatin administration induced mechanical and cold hypersensitivity and allodynia in both sexes, with more severe responses to cold stimulation detected in females. Oxaliplatin also induced a significant increase in the expression of TRPV1, TRPM8 and TRPA1 in lumbar dorsal root ganglia. Interestingly, while TRPV1 and TRPA1 upregulation showed no sex difference, the increase in TRPM8 mRNA levels was more pronounced in female ganglia, correlating with the increased sensitivity to innocuous cold stimuli observed in females. TRPV1 and TRPM8 were also found to be upregulated in the spinal cord of animals of both sexes. Our results reveal previously undescribed changes in the expression of TRP channels occurring in peripheral ganglia and spinal cord of both male and female oxaliplatin-treated animals, with some of these changes exhibiting sex-related differences that could underlie the development of sex-specific patterns of pain-related behaviours.

TRP channels and monoterpenes: Past and current leads on analgesic properties

The activation of the transient receptor potential (TRP) channels expressed by sensory neurons is essential to the transduction of thermal and mechanical sensory information. In the setting of chronic inflammatory conditions, the activation of the melastatin family member 8 (TRPM8), the TRP vanilloid 1 (TRPV1), and the TRP ankyrin 1 (TRPA1) is correlated with pain hypersensitivity reactions. Monoterpenes, among which pulegone and menthol, a major class of phytocompounds present in essential oils of medicinal plants, are known modulators of those TRP channels activity. In the present review, we correlate the monoterpene content of plants with their historical therapeutic properties. We then describe how monoterpenes exert their anti-inflammatory and antihyperalgesia effects through modulation of TRP channels activity. Finally, we discuss the importance and the potential of characterizing new plant extracts and reassessing studied plant extracts for the development of ethnopharmacology-based innovative treatments for chronic pain. This review suggests that monoterpene solutions, based on composition from traditional healing herbs, offer an interesting avenue for the development of new phytotherapeutic treatments to alleviate chronic inflammatory pain conditions.

Plastic changes in nociceptive pathways contributing to persistent orofacial pain

BACKGROUND

Pain is a warning signal for the body defense mechanisms and is a critical sensation for supporting life. However, orofacial pain is not a vital sensation, but a disease. However, there are still many unclear points about the pathophysiological mechanism of orofacial pain. This situation makes it difficult for many clinicians to treat orofacial pain hypersensitivity.

HIGHLIGHT

Noxious information on the orofacial region received by trigeminal ganglion neurons is recognized as "orofacial pain" by being transmitted to the somatosensory cortex and limbic system via the spinal trigeminal nucleus and the thalamic sensory nuclei. Orofacial inflammation or trigeminal nerve injury causes neuropathic changes in various nociceptive signaling pathways, resulting in persistent orofacial pain. It is considered that persistent oral facial pain is triggered by plastic changes in nociceptive signaling pathways involving various cells such as satellite glial cells, astrocytes, microglia, and macrophages, as well as nociceptive neurons.

CONCLUSION

Recent studies have shown that hyperexcitability of nociceptive neurons in the nociceptive signaling pathways of the orofacial region caused by a variety of factors causes persistent orofacial pain. This review outlines the pathophysiology of orofacial pain along with the results of our study.

Transcriptional Alterations of Mouse Trigeminal Ganglion Neurons Following Orofacial Inflammation Revealed by Single-Cell Analysis

Orofacial inflammation leads to transcriptional alterations in trigeminal ganglion (TG) neurons. However, diverse alterations and regulatory mechanisms following orofacial inflammatory pain in different types of TG neurons remain unclear. Here, orofacial inflammation was induced by injection of complete Freund’s adjuvant (CFA) in mice. After 7 days, we performed single-cell RNA-sequencing on TG cells of mice from control and treatment groups. We identified primary sensory neurons, Schwann cells, satellite glial cells, oligodendrocyte-like cells, immune cells, fibroblasts, and endothelial cells in TG tissue. After principal component analysis and hierarchical clustering, we identified six TG neuronal subpopulations: peptidergic nociceptors (PEP1 and PEP2), non-peptidergic nociceptors (NP1 and NP2), C-fiber low-threshold mechanoreceptors (cLTMR) and myelinated neurons (Nefh-positive neurons, NF) based on annotated marker gene expression. We also performed differential gene expression analysis among TG neuronal subtypes, identifying several differential genes involved in the inflammatory response, neuronal excitability, neuroprotection, and metabolic processes. Notably, we identified several potential novel targets associated with pain modulation, including Arl6ip1, Gsk3b, Scn7a, and Zbtb20 in PEP1, Rgs7bp in PEP2, and Bhlha9 in cLTMR. The established protein–protein interaction network identified some hub genes, implying their critical involvement in regulating orofacial inflammatory pain. Our study revealed the heterogeneity of TG neurons and their diverse neuronal transcriptomic responses to orofacial inflammation, providing a basis for the development of therapeutic strategies for orofacial inflammatory pain.

Local effects of natural alkylamides from Acmella oleracea and synthetic isobutylalkyl amide on neuropathic and postoperative pain models in mice

Neuropathic and postoperative pain are clinical conditions that impair the patient's quality of life. The current pharmacotherapy of both painful states is ineffective and accompanied by several side effects. In order to develop new therapeutics targets, the secondary metabolites of plants have been extensively studied. Acmella oleracea ("jambu") is a native plant from the Amazon region and rich in alkylamides, bioactive compounds responsible for inducing anesthetic and chemesthetic sensations. We previously demonstrated that the intraplantar administration of an hexanic fraction (HF) rich in alkylamides from jambu and the synthetic isobutylalkyl amide (IBA) at 0.1 μg/20 μL can promote antinociceptive and anti-inflammatory effects. Thus, this study aimed to evaluate the local effect of HF and IBA (0.1 μg/20 μL) on neuropathic (partial sciatic nerve ligation, PSNL) and postoperative pain (plantar incision surgery, PIS) models in mice. Seven days after the PSNL, the mechanical (von Frey test) and cold (acetone-evoked evaporative cooling) allodynia, and digital gait parameters were analyzed. The intraplantar HF and IBA treatments attenuated the mechanical and cold allodynia as well as the static (max. Contact and print area) and dynamic (stand duration) parameters of digital gait analyses. On the day after PIS, the mechanical allodynia, heat hyperalgesia (hot plate, 52 ± 0.1°C), and spontaneous nociception scores were evaluated. Topical treatment with HF reduced the mechanical allodynia, heat hyperalgesia, and spontaneous nociception scores. In contrast, IBA treatment only partially reduced the mechanical allodynia. In summary, the local treatment with HF was effective on both neuropathic and postoperative pain, as opposed to IBA, which only had an effect on neuropathic pain.

Single dose and chronic oral administration of cannabigerol and cannabigerolic acid-rich hemp extract in fed and fasted dogs: Physiological effect and pharmacokinetic evaluation

The use of cannabinoids in veterinary medicine has been increasing exponentially recently and there is little information regarding the pharmacokinetics of cannabinoids except for cannabidiol (CBD) and tetrahydrocannabinol (THC), with even more sparse information related to their native acid forms found in cannabis. Cannabigerol (CBG) is the precursor molecule to cannabinoid formation in the cannabis plant which may have medicinal properties as well, yet there are no publications related to CBG or the native cannabigerolic acid (CBGA) in companion animal species. The aim of this study was to investigate similar dosing of CBG and CBGA from hemp plants that have been used for cannabidiol pharmacokinetic studies. Administration in the fed and fasted state was performed to better understand absorption and retention of these unique hemp-derived cannabinoids in dogs. Results suggest that when providing a hemp-derived CBG/CBGA formulation in equal quantities, CBGA is absorbed approximately 40-fold better than CBG regardless of being given to fed or fasted dogs. After twice daily dosing for two weeks at 2 mg/kg in the fasted and then fed state, no differences in the mean serum CBG (5 ng/ml) or CBGA (250 ng/ml) serum concentrations were observed between states. Importantly, physical examination, complete blood counts, and serum chemistry evaluations over the two weeks suggest no adverse events during this short-term dosing trial.

Drosophila as a Model to Study the Mechanism of Nociception

Nociception refers to the process of encoding and processing noxious stimuli, which allow animals to detect and avoid potentially harmful stimuli. Several types of stimuli can trigger nociceptive sensory transduction, including thermal, noxious chemicals, and harsh mechanical stimulation that depend on the corresponding nociceptors. In view of the high evolutionary conservation of the mechanisms that govern nociception from Drosophila melanogaster to mammals, investigation in the fruit fly Drosophila help us understand how the sensory nervous system works and what happen in nociception. Here, we present an overview of currently identified conserved genetics of nociception, the nociceptive sensory neurons responsible for detecting noxious stimuli, and various assays for evaluating different nociception. Finally, we cover development of anti-pain drug using fly model. These comparisons illustrate the value of using Drosophila as model for uncovering nociception mechanisms, which are essential for identifying new treatment goals and developing novel analgesics that are applicable to human health.

Ocular Surface Ion-Channels Are Closely Related to Dry Eye: Key Research Focus on Innovative Drugs for Dry Eye

Abundant ion-channels, including various perceptual receptors, chloride channels, purinergic receptor channels, and water channels that exist on the ocular surface, play an important role in the pathogenesis of dry eye. Channel-targeting activators or inhibitor compounds, which have shown positive effects in in vivo and in vitro experiments, have become the focus of the dry eye drug research and development, and individual compounds have been applied in clinical experimental treatment. This review summarized various types of ion-channels on the ocular surface related to dry eye, their basic functions, and spatial distribution, and discussed basic and clinical research results of various channel receptor regulatory compounds. Therefore, further elucidating the relationship between ion-channels and dry eye will warrant research of dry eye targeted drug therapy.

Reactive Oxygen Species Cause Exercise-Induced Angina in a Myocardial Ischaemia-Reperfusion Injury Model

Percutaneous coronary intervention (PCI) effectively treats obstructive coronary artery syndrome. However, 30–40% patients continue to have angina after a successful PCI, thereby reducing patient satisfaction. The mechanisms underlying persistent angina after revascularisation therapy are still poorly understood; hence, the treatment or guideline for post-PCI angina remains unestablished. Thus, this study aimed to investigate the mechanisms underlying effort angina in animals following myocardial ischaemia-reperfusion (I/R) injury. Phosphorylated extracellular signal-regulated kinase (p-ERK), a marker for painful stimulation-induced neuronal activation, was used for the investigation. After a forced treadmill exercise (FTE), the number of p-ERK-expressing neurons increased in the superficial dorsal horn of the I/R model animals. Moreover, FTE evoked hydrogen peroxide (H₂O₂) production in the I/R-injured heart, inducing angina through TRPA1 activation on cardiac sensory fibres. Notably, the treatment of a TEMPOL, a reactive oxygen species scavenger, or TRPA1^−/− mice successfully alleviated the FTE-induced p-ERK expression in the dorsal horn. The production of H₂O₂, a reactive oxygen species, through physical exercise contributes to angina development following I/R. Hence, our findings may be useful for understanding and treating angina following revascularisation therapy.

Vincristine-Induced Peripheral Neuropathy in Childhood Acute Lymphoblastic Leukemia: Genetic Variation as a Potential Risk Factor

Vincristine (VCR) is the first-line chemotherapeutic medication often co-administered with other drugs to treat childhood acute lymphoblastic leukemia. Dose-dependent neurotoxicity is the main factor restricting VCR’s clinical application. VCR-induced peripheral neuropathy (VIPN) sometimes results in dose reduction or omission, leading to clinical complications or affecting the patient’s quality of life. With regard to the genetic basis of drug responses, preemptive pharmacogenomic testing and simultaneous blood level monitoring could be helpful for the transformation of various findings into individualized therapies. In this review, we discussed the potential associations between genetic variants in genes contributing to the pharmacokinetics/pharmacodynamics of VCR and VIPN incidence and severity in patients with acute lymphoblastic leukemia. Of note, genetic variants in the CEP72 gene have great potential to be translated into clinical practice. Such a genetic biomarker may help clinicians diagnose VIPN earlier. Besides, genetic variants in other genes, such as CYP3A5, ABCB1, ABCC1, ABCC2, TTPA, ACTG1, CAPG, SYNE2, SLC5A7, COCH, and MRPL47, have been reported to be associated with the VIPN, but more evidence is needed to validate the findings in the future. In fact, a variety of complex factors jointly determine the VIPN. In implementing precision medicine, the combination of genetic, environmental, and personal variables, along with therapeutic drug monitoring, will allow for a better understanding of the mechanisms of VIPN, improving the effectiveness of VCR treatment, reducing adverse reactions, and improving patients’ quality of life.

TRP Channels as Molecular Targets to Relieve Cancer Pain

Transient receptor potential (TRP) channels are critical receptors in the transduction of nociceptive stimuli. The microenvironment of diverse types of cancer releases substances, including growth factors, neurotransmitters, and inflammatory mediators, which modulate the activity of TRPs through the regulation of intracellular signaling pathways. The modulation of TRP channels is associated with the peripheral sensitization observed in patients with cancer, which results in mild noxious sensory stimuli being perceived as hyperalgesia and allodynia. Secondary metabolites derived from plant extracts can induce the activation, blocking, and desensitization of TRP channels. Thus, these compounds could act as potential therapeutic agents, as their antinociceptive properties could be beneficial in relieving cancer-derived pain. In this review, we will summarize the role of TRPV1 and TRPA1 in pain associated with cancer and discuss molecules that have been reported to modulate these channels, focusing particularly on the mechanisms of channel activation associated with molecules released in the tumor microenvironment.

Paeoniflorin alleviates CFA-induced inflammatory pain by inhibiting TRPV1 and succinate/SUCNR1-HIF-1α/NLPR3 pathway

BACKGROUND

Treatment of chronic inflammatory pain remains a major goal in the clinic. It is thus of prime importance to characterize inherent pathophysiological pathways to design new therapeutic strategies and analgesics for pain management. Paeoniflorin (PF), a monoterpenoid glycoside from Paeonia lactiflora Pallas plants, possesses promising anti-nociceptive property. However, therapeutic effect and underlying mechanism of action of PF on inflammatory pain have not yet been fully elucidated. In this study, we aim to investigate the analgesic effect further and clarify its mechanism of action of PF on complete freund's adjuvant (CFA)-evoked inflammatory pain.

METHODS

Twenty-four male mice were divided into 3 groups: sham, CFA, and CFA + PF groups (n = 8/group). Mice were treated with normal saline or PF (30 mg/kg) for 11 days. Footpad swelling (n = 8/group), mechanical (n = 8/group) and thermal hypersensitivity (n = 8/group) were measured to evaluate the analgesic effect of PF on CFA-injected mice. At the end of the animal experiment, blood and L4-L6 dorsal root ganglion neurons were collected to assess the therapeutic effect of PF on CFA-induced inflammatory pain. Next, hematoxylin and eosin, quantitative realtime PCR, ELISA, capsaicin and dimethyl succinate induced pain test (n = 8/group), motor coordination test (n = 8/group), tail flicking test (n = 8/group), pyruvate and succinate dehydrogenase assay (n = 6/group), immunohistochemical staining, were performed to clarify the action mechanism of PF on CFA-evoked inflammatory pain. Besides, the effect of PF on TRPV1 was evaluated by whole-cell patch clamp recording on primary neurons (n = 7). Finally, molecular docking further performed to evaluate the binding ability of PF to TRPV1.

RESULTS

PF significantly relieved inflammatory pain (P < 0.001) and paw edema (P < 0.001) on a complete Freund adjuvant (CFA)-induced peripheral inflammatory pain model. Furthermore, PF inhibited neutrophil infiltration (P < 0.01), IL-1β increase (P < 0.01), and pain-related peptide substance P release (P < 0.001). Intriguingly, CFA-induced succinate aggregation was notably reversed by PF via modulating pyruvate and SDH activity (P < 0.01). In addition, PF dampened the high expression of subsequent succinate receptor SUCNR1 (P < 0.01), HIF-1α (P < 0.05), as well as the activation of NLPR3 inflammasome (P < 0.05) and TRPV1 (P < 0.05). More importantly, both capsaicin and dimethyl succinate supplementation obviously counteracted the pain-relieving effect of PF and TRPV1 (P < 0.01 or P < 0.001).

CONCLUSION

Our findings suggest that PF can significantly relieve CFA-induced paw swelling, as well as mechanical and thermal hyperalgesia. PF alleviated inflammatory pain partly through inhibiting the activation of TRPV1 and succinate/SUCNR1-HIF-1α/NLPR3 pathway. Furthermore, we found that PF exerted its analgesic effect without affecting motor coordination and pain-related cold ion-channels. In summary, this study may provide valuable evidence for the potential application of PF as therapeutic strategy for inflammatory pain treatment.

Periodontal acidification contributes to tooth pain hypersensitivity during orthodontic tooth movement

Tooth movements associated with orthodontic treatment often cause tooth pain. However, the detailed mechanism remains unclear. Here, we examined the involvement of periodontal acidification caused by tooth movement in mechanical tooth pain hypersensitivity. Elastics were inserted between the first and second molars to move the teeth in Sprague-Dawley rats. Mechanical head-withdrawal reflex threshold to first molar stimulation and the pH of the gingival sulcus around the tooth were measured. The expression of acid-sensing ion channel 3 (ASIC3) in trigeminal ganglion neurons and phosphorylation of ASIC3 in the periodontal tissue were analyzed. The mechanical head-withdrawal reflex threshold to first molar stimulation and pH in the gingival sulcus decreased on day 1 after the elastic insertion. These decreases recovered to the sham level by buffering periodontal acidification. Periodontal inhibition of ASIC3 channel activity reversed the decreased mechanical head-withdrawal reflex threshold to first molar stimulation. On day 1 after elastic insertion, the tooth movement did not change the number of ASIC3 immunoreactive trigeminal ganglion neurons innervating the periodontal tissue but increased phosphorylated-ASIC3 levels in the periodontal tissue. Periodontal acidification induced by tooth movement causes phosphorylation of ASIC3, resulting in mechanical pain hypersensitivity in mechanically forced tooth.

Enterochromaffin Cells: Sentinels to Gut Microbiota in Hyperalgesia?

In recent years, increasing studies have been conducted on the mechanism of gut microbiota in neuropsychiatric diseases and non-neuropsychiatric diseases. The academic community has also recognized the existence of the microbiota-gut-brain axis. Chronic pain has always been an urgent difficulty for human beings, which often causes anxiety, depression, and other mental symptoms, seriously affecting people's quality of life. Hyperalgesia is one of the main adverse reactions of chronic pain. The mechanism of gut microbiota in hyperalgesia has been extensively studied, providing a new target for pain treatment. Enterochromaffin cells, as the chief sentinel for sensing gut microbiota and its metabolites, can play an important role in the interaction between the gut microbiota and hyperalgesia through paracrine or neural pathways. Therefore, this systematic review describes the role of gut microbiota in the pathological mechanism of hyperalgesia, learns about the role of enterochromaffin cell receptors and secretions in hyperalgesia, and provides a new strategy for pain treatment by targeting enterochromaffin cells through restoring disturbed gut microbiota or supplementing probiotics.

Molecular mechanism in trigeminal nerve and treatment methods related to orthodontic pain

BACKGROUND

Orthodontic treatment is the main treatment approach for malocclusion. Orthodontic pain is an inevitable undesirable adverse reaction during orthodontic treatment. It is reported orthodontic pain has become one of the most common reason that patients withdraw from orthodontic treatment. Therefore, understanding the underlying mechanism and finding treatment of orthodontic pain are in urgent need.

AIMS

This article aims to sort out the mechanisms and treatments of orthodontic pain, hoping to provide some ideas for future orthodontic pain relief.

MATERIALS

Tooth movement will cause local inflammation. Certain inflammatory factors and cytokines stimulating the trigeminal nerve and further generating pain perception, as well as drugs and molecular targeted therapy blocking nerve conduction pathways, will be reviewed in this article.

METHOD

We review and summaries current studies related to molecular mechanisms and treatment approaches in orthodontic pain control.

RESULTS

Orthodontics pain related influencing factors and molecular mechanisms has been introduced. Commonly used clinical methods in orthodontic pain control has been evaluated.

DISCUSSION

With the clarification of more molecular mechanisms, the direction of orthodontic pain treatment will shift to targeted drugs.

Diurnal Variation in Trigeminal Pain Sensitivity in Mice

Management of time and circadian disruption is an extremely important factor in basic research on pain and analgesia. Although pain is known to vary throughout the day, the mechanism underlying this circadian variation remains largely unknown. In this study, we hypothesized that the process of pain transmission to the central nervous system (after receiving nociceptive stimuli from outside the body) would show day-night differences. Ten-week-old male mice were kept under a strict 12/12-h light/dark cycle for at least 10 days. Formalin was then injected into the second branch region of the trigeminal nerve and the duration of pain-related behaviors (PRBs) was assessed. Immunohistochemical staining was then performed, and the c-Fos-immunopositive cells in the trigeminal spinal tract subnucleus caudalis (Sp5C) were counted. The results showed that the duration of PRBs was longer and the number of c-Fos immunopositive cells in the Sp5C was higher at nighttime than during the day. In addition, the trigeminal ganglia (TG) were extracted from the mice and examined by quantitative real-time PCR to evaluate the daytime and nighttime expression of nociceptive receptors. The results showed that the mRNA expression of transient receptor potential ankyrin 1 in the TG was significantly higher at night than during the day. These results suggest that pain in the trigeminal nerve region is more intense at nighttime, when rodents are active, than during the daytime, partly due to differences in nociceptor expression.

Roles of AMPK and Its Downstream Signals in Pain Regulation

Pain is an unpleasant sensory and emotional state that decreases quality of life. A metabolic sensor, adenosine monophosphate-activated protein kinase (AMPK), which is ubiquitously expressed in mammalian cells, has recently attracted interest as a new target of pain research. Abnormal AMPK expression and function in the peripheral and central nervous systems are associated with various types of pain. AMPK and its downstream kinases participate in the regulation of neuron excitability, neuroinflammation and axonal and myelin regeneration. Numerous AMPK activators have reduced pain behavior in animal models. The current understanding of pain has been deepened by AMPK research, but certain issues, such as the interactions of AMPK at each step of pain regulation, await further investigation. This review examines the roles of AMPK and its downstream kinases in neurons and non-neuronal cells, as well as their contribution to pain regulation.

Local Anesthetics Inhibit Transient Receptor Potential Vanilloid Subtype 3 Channel Function in Xenopus Oocytes

BACKGROUND

The transient receptor potential vanilloid subtype 3 (TRPV3) channel is activated by innocuous temperature and several chemical stimuli. It is proposed to be involved in pathological pain development and is therefore considered a potential target for treating pain. Local anesthetics have been used for patients with both acute and chronic pain. Although blockage of the voltage-gated sodium channel is the primary mechanism by which local anesthetics exert their effects, they cannot be explained by this mechanism alone, especially in pathologic states such as chronic pain. Indeed, the effects of local anesthetics on multiple targets involved in the pain pathway have been reported. It has also been suggested that modulating the function of transient receptor potential (TRP) channels (eg, TRPV1 and transient receptor potential ankyrin 1 [TRPA1]) is one of the mechanisms of action of local anesthetics. However, the effects of local anesthetics on TRPV3 have not been reported.

METHODS

We expressed TRPV3 in Xenopus oocytes and investigated the effects of local anesthetics on 2-aminoethoxydiphenyl borate (2APB)-induced currents using 2-electrode voltage-clamp techniques.

RESULTS

Clinically used local anesthetics inhibited the 2APB-activated currents from the TRPV3 channel in a concentration-dependent manner at pharmacologically relevant concentrations with half maximal inhibitory concentration (IC50) values of 2.5 (lidocaine), 1.4 (mepivacaine), 0.28 (ropivacaine), and 0.17 (bupivacaine) mmol/L, respectively. Conversely, these local anesthetics also directly induced currents at higher concentrations, although these currents were quite small compared to the 2APB-induced currents. We found that the inhibition of TRPV3 by lidocaine is noncompetitive and independent of intracellular signaling cascades. 2APB-induced TRPV3 currents were reduced by extracellular N-(2,6-dimethylphenylcarbamoylmethyl) triethylammonium bromide (QX-314) but not by intracellular QX-314 nor benzocaine. Moreover, lidocaine showed a use-dependent block in TRPV3 inhibition. Finally, QX-314 appeared to slightly permeate the activated TRPV3 channel pore based on examination of oocytes coexpressing TRPV3 and a sodium channel. These results suggest that local anesthetics could inhibit TRPV3 channel function by extracellular interactions of their charged forms with the channel pore.

CONCLUSIONS

Local anesthetics inhibited TRPV3 2APB-induced currents at pharmacologically relevant concentrations when TRPV3 was expressed in Xenopus oocytes. These effects seem to occur via an extracellular interaction between the charged form of the anesthetic with the TRPV3 channel pore. These results help to elucidate the mechanisms of action of local anesthetics.

Injectable Affinity and Remote Magnetothermal Effects of Bi-Based Alloy for Long-Term Bone Defect Repair and Analgesia

As alternatives, metallic/nonmetallic bone graft materials play significant roles in bone defect surgery to treat external trauma or bone disease. However, to date, there are rather limited long-term implantable materials owning to in situ molding incapability of metallics and poor mechanical property of nonmetallics. Here, Bi-based low melting point alloy, with unique properties of injectability, solid-liquid phase transition, mechanical capability, and biocompatibility, present obvious long-lasting bone affinity as the excellent artificial bone-substitute. It is particularly necessary to point out that the targeted injected Bi alloy remains in its original position for up to 210 days without moving, as well as, displays good osseointegration ability to resolve repeated revision trauma caused by losing bone repair material. Additionally, with outstanding electrical and thermal conductivity, an unconventional way using Bi alloy to realize very beneficial hyperthermia analgesia via non-invasive wireless energy delivery is first proposed, which avoids adverse effects on bone remodeling inflicted by traditional drugs. The significantly decreased expression of pain sensitizing factor, such as, interleukin-6, neuropeptide substance, and transient receptor potential vanilloid 1 reveals the potential mechanism of hyperthermia analgesia. The present findings suggest the combination therapy of Bi alloy in bone repair and analgesia, which owns far-reaching clinical application value.

Improving NKCC1 Function Increases the Excitability of DRG Neurons Exacerbating Pain Induced After TRPV1 Activation of Primary Sensory Neurons

Background Our aim was to investigate the effects of the protein expression and the function of sodium, potassium, and chloride co-transporter (NKCC1) in the dorsal root ganglion (DRG) after activation of transient receptor potential vanilloid 1 receptor (TRPV1) in capsaicin-induced acute inflammatory pain and the possible mechanism of action.

Methods Male Sprague–Dawley rats were randomly divided into control, capsaicin, and inhibitor groups. The expression and distribution of TRPV1 and NKCC1 in rat DRG were observed by immunofluorescence. Thermal radiation and acetone test were used to detect the pain threshold of heat and cold noxious stimulation in each group. The expressions of NKCC1 mRNA, NKCC1 protein, and p-NKCC1 in the DRG were detected by PCR and western blotting (WB). Patch clamp and chloride fluorescent probe were used to observe the changes of GABA activation current and intracellular chloride concentration. After intrathecal injection of protein kinase C (PKC) inhibitor (GF109203X) or MEK/extracellular signal-regulated kinase (ERK) inhibitor (U0126), the behavioral changes and the expression of NKCC1 and p-ERK protein in L_4–6 DRG were observed.

Result: TRPV1 and NKCC1 were co-expressed in the DRG. Compared with the control group, the immunofluorescence intensity of NKCC1 and p-NKCC1 in the capsaicin group was significantly higher, and the expression of NKCC1 in the nuclear membrane was significantly higher than that in the control group. The expression of NKCC1 mRNA and protein of NKCC1 and p-NKCC1 in the capsaicin group were higher than those in the control group. After capsaicin injection, GF109203X inhibited the protein expression of NKCC1 and p-ERK, while U0126 inhibited the protein expression of NKCC1. In the capsaicin group, paw withdrawal thermal latency (WTL) was decreased, while cold withdrawal latency (CWL) was prolonged. Bumetanide, GF109203X, or U0126 could reverse the effect. GABA activation current significantly increased in the DRG cells of the capsaicin group, which could be reversed by bumetanide. The concentration of chloride in the DRG cells of the capsaicin group increased, but decreased after bumetanide, GF109203X, and U0126 were administered.

Conclusion Activation of TRPV1 by exogenous agonists can increase the expression and function of NKCC1 protein in DRG, which is mediated by activation of PKC/p-ERK signaling pathway. These results suggest that DRG NKCC1 may participate in the inflammatory pain induced by TRPV1.

TRPV1 alleviates osteoarthritis by inhibiting M1 macrophage polarization via Ca2+/CaMKII/Nrf2 signaling pathway

Osteoarthritis (OA) is the major course of joint deterioration, in which M1 macrophage-driven synovitis exacerbates the pathological process. However, precise therapies for M1 macrophage to decrease synovitis and attenuate OA progression have been scarcely proposed. Transient receptor potential vanilloid 1 (TRPV1) is a cation channel that has been implicated in pain perception and inflammation. In this study, we investigated the role of TRPV1 in the M1 macrophage polarization and pathogenesis of OA. We demonstrated that TRPV1 expression and M1 macrophage infiltration were simultaneously increased in both human and rat OA synovium. More than 90% of the infiltrated M1 macrophages expressed TRPV1. In the rat OA model, intra-articular injection of capsaicin (CPS), a specific TRPV1 agonist, significantly attenuated OA phenotypes, including joint swelling, synovitis, cartilage damage, and osteophyte formation. CPS treatment markedly reduced M1 macrophage infiltration in the synovium. Further mechanistic analyses showed that TRPV1-evoked Ca2+ influx promoted the phosphorylation of calcium/calmodulin-dependent protein kinase II (CaMKII) and facilitated the nuclear localization of nuclear factor-erythroid 2-related factor 2 (Nrf2), which ultimately resulted in the inhibition of M1 macrophage polarization. Taken together, our findings establish that TRPV1 attenuates the progression of OA by inhibiting M1 macrophage polarization in synovium via the Ca2+/CaMKII/Nrf2 signaling pathway. These results highlight the effect of targeting TRPV1 for the development of a promising therapeutic strategy for OA.

Transient receptor potential ankyrin 1 channel: An evolutionarily tuned thermosensor

The discovery of the role of the transient receptor potential ankyrin 1 (TRPA1) channel as a polymodal detector of cold and pain-producing stimuli almost two decades ago catalyzed the consequent identification of various vertebrate and invertebrate orthologues. In different species, the role of TRPA1 has been implicated in numerous physiological functions, indicating that the molecular structure of the channel exhibits evolutionary flexibility. Until very recently, information about the critical elements of the temperature-sensing molecular machinery of thermosensitive ion channels such as TRPA1 had lagged far behind information obtained from mutational and functional analysis. Current developments in single-particle cryo-electron microscopy are revealing precisely how the thermosensitive channels operate, how they might be targeted with drugs, and at which sites they can be critically regulated by membrane lipids. This means that it is now possible to resolve a huge number of very important pharmacological, biophysical and physiological questions in a way we have never had before. In this review, we aim at providing some of the recent knowledge on the molecular mechanisms underlying the temperature sensitivity of TRPA1. We also demonstrate how the search for differences in temperature and chemical sensitivity between human and mouse TRPA1 orthologues can be a useful approach to identifying important domains with a key role in channel activation.

Increasing Efficiency of Repetitive Electroacupuncture on Purine- and Acid-Induced Pain During a Three-Week Treatment Schedule

Acupuncture (AP) is an important constituent of the therapeutic repertoire of traditional Chinese medicine and has been widely used to alleviate chronic painful conditions all over the world. We studied in rats the efficiency of electroacupuncture (EAP) applied to the Zusanli acupoint (ST36) as an analgesic treatment over a 3-week period of time on purine (α,β-methylene ATP, dibenzoyl-ATP)- and acid (pH 6.0 medium)-induced pain in the rat paw. The two ATP derivatives stimulated P2X3 and P2X7 receptors, respectively, while the slightly acidic medium stimulated the “acid-sensitive ion channel 3” (ASIC3). It was found that the P2X7 receptor and ASIC-mediated pain was counteracted by EAP with greater efficiency at the end than at the beginning of the treatment schedule, while the P2X3 receptor–mediated pain was not. Our findings have important clinical and theoretical consequences, among others, because they are difficult to reconcile with the assumption that AP is primarily due to the release of peripheral and central opioid peptides causing the well-known tolerance to their effects. In consequence, AP is a convenient therapeutic instrument to treat subacute and chronic pain.

Complete Freund's adjuvant-induced decrement of pruriceptor-mediated suppression of itch

Peripheral inflammation is always accompanied by a noxious sensation, either pain or itch, providing a protective warning for the occurrence of pathological changes; however, the mechanisms determining whether pain, itch, or both will be elicited under certain inflammatory statuses are still far from clear. Complete Freund's adjuvant (CFA) contains heat killed and dried Mycobacterium tuberculosis widely used to induce inflammatory pain models, but how CFA treatment affects itch sensation and the possible mechanisms are still unclear. In this study, using itch behavior testing and calcium imaging, we showed that both the behaviors and calcium responses associated with Transient Receptor Potential Vanilloid 1 (TRPV1)-mediated histamine-dependent itch and Transient Receptor Potential Ankyrin 1 (TRPA1)-mediated histamine-independent itch were significantly suppressed by CFA treatment. Furthermore, to explore the possible cellular mechanisms, high-throughput single-cell RNA sequencing and real-time PCR were used to detect CFA-induced changes of itch-related genes in dorsal root ganglion (DRG) neurons. Our results revealed that although both nociceptive Trpv1+ and Trpa1+ DRG neurons were increased after CFA treatment, most known pruriceptors, including Hrh1+, Mrgpra3+, Mrgprd+, Htr3a+, Htr1f+, IL31ra+, Osmr+, and Lpar3+ DRG neurons, were significantly decreased, which may explain that CFA treatment caused itch suppression. This study indicated that itch sensation was affected after CFA treatment, although negatively, and comprehensive but not specific suppression of different pruriceptors was observed after CFA treatment, suggesting that a unified adaptive change of increased pain and decreased itch will occur simultaneously under CFA-induced inflammatory conditions.

Topical Treatments and Their Molecular/Cellular Mechanisms in Patients with Peripheral Neuropathic Pain-Narrative Review

Neuropathic pain in humans results from an injury or disease of the somatosensory nervous system at the peripheral or central level. Despite the considerable progress in pain management methods made to date, peripheral neuropathic pain significantly impacts patients' quality of life, as pharmacological and non-pharmacological methods often fail or induce side effects. Topical treatments are gaining popularity in the management of peripheral neuropathic pain, due to excellent safety profiles and preferences. Moreover, topical treatments applied locally may target the underlying mechanisms of peripheral sensitization and pain. Recent studies showed that peripheral sensitization results from interactions between neuronal and non-neuronal cells, with numerous signaling molecules and molecular/cellular targets involved. This narrative review discusses the molecular/cellular mechanisms of drugs available in topical formulations utilized in clinical practice and their effectiveness in clinical studies in patients with peripheral neuropathic pain. We searched PubMed for papers published from 1 January 1995 to 30 November 2020. The key search phrases for identifying potentially relevant articles were "topical AND pain", "topical AND neuropathic", "topical AND treatment", "topical AND mechanism", "peripheral neuropathic", and "mechanism". The result of our search was 23 randomized controlled trials (RCT), 9 open-label studies, 16 retrospective studies, 20 case (series) reports, 8 systematic reviews, 66 narrative reviews, and 140 experimental studies. The data from preclinical studies revealed that active compounds of topical treatments exert multiple mechanisms of action, directly or indirectly modulating ion channels, receptors, proteins, and enzymes expressed by neuronal and non-neuronal cells, and thus contributing to antinociception. However, which mechanisms and the extent to which the mechanisms contribute to pain relief observed in humans remain unclear. The evidence from RCTs and reviews supports 5% lidocaine patches, 8% capsaicin patches, and botulinum toxin A injections as effective treatments in patients with peripheral neuropathic pain. In turn, single RCTs support evidence of doxepin, funapide, diclofenac, baclofen, clonidine, loperamide, and cannabidiol in neuropathic pain states. Topical administration of phenytoin, ambroxol, and prazosin is supported by observational clinical studies. For topical amitriptyline, menthol, and gabapentin, evidence comes from case reports and case series. For topical ketamine and baclofen, data supporting their effectiveness are provided by both single RCTs and case series. The discussed data from clinical studies and observations support the usefulness of topical treatments in neuropathic pain management. This review may help clinicians in making decisions regarding whether and which topical treatment may be a beneficial option, particularly in frail patients not tolerating systemic pharmacotherapy.

Cold pain sensitivity is associated with single-nucleotide polymorphisms of PAR2/F2RL1 and TRPM8

Pain sensitivity differs individually, but the mechanisms and genetic factors that underlie these differences are not fully understood. To investigate genetic factors that are involved in sensing cold pain, we applied a cold-induced pain test and evaluated protease-activated receptor 2 (PAR2/F2RL1) and transient receptor potential melastatin 8 (TRPM8), which are related to pain. We statistically investigated the associations between genetic polymorphisms and cold pain sensitivity in 461 healthy patients who were scheduled to undergo cosmetic orthognathic surgery for mandibular prognathism. We found an association between cold pain sensitivity and the rs2243057 polymorphism of the PAR2 gene. We also found a significant association between cold pain sensitivity and the rs12992084 polymorphism of the TRPM8 gene. Carriers of the minor A allele of the rs2243057 polymorphism of PAR2 and minor C allele of the rs12992084 polymorphism of TRPM8 exhibited a longer latency to pain perception in the cold-induced pain test, reflecting a decrease in cold pain sensitivity. These results suggest that genetic polymorphisms of both PAR2 and TRPM8 are involved in individual differences in cold pain sensitivity.

An overview on transient receptor potential channels superfamily

The transient receptor potential (TRP) channel superfamily is comprised of a large group of cation-permeable channels, which display an extraordinary diversity of roles in sensory signaling and are involved in plethora of animal behaviors. These channels are activated through a wide variety of mechanisms and participate in virtually every sensory modality. Modulating TRP channel activity provides an important way to regulate membrane excitability and intracellular calcium levels. This is reflected by the fact that small molecule compounds modulating different TRPs have all entered clinical trials for a variety of diseases. The role of TRPs will be further elucidated in complex diseases of the nervous, intestinal, renal, urogenital, respiratory, and cardiovascular systems in diverse therapeutic areas including pain and itch, headache, pulmonary function, oncology, neurology, visceral organs, and genetic diseases. This review focuses on recent developments in the TRP ion channel-related area and highlights evidence supporting TRP channels as promising targets for new analgesic drugs for therapeutic intervention. This review presents a variety of: (1) phylogeny aspects of TRP channels; (2) some structural and functional characteristics of TRPs; (3) a general view and short characteristics of main seven subfamilies of TRP channels; (4) the evidence for consider TRP channels as therapeutic and analgesic targets; and finally (5) further perspectives of TRP channels research.

Role of peripheral and central sensitization in the anti-hyperalgesic effect of hecogenin acetate, an acetylated sapogenin, complexed with β-cyclodextrin: Involvement of NFκB and p38 MAPK pathways

Neuropathic pain develops due to injury to the somatosensory system, affecting the patient's quality of life. In view of the ineffectiveness of the current pharmacotherapy, substances obtained from natural products (NPs) are a promising alternative. One NP that has been discussed in the literature is hecogenin acetate (HA), a steroidal sapogenin with anti-inflammatory and antinociceptive activity. However, HA has low water solubility, which affects its bioavailability. Thus, the objective of this study was to evaluate the anti-hyperalgesic activity of pure and complexed hecogenin acetate (HA/βCD) in an animal model of chronic neuropathic and inflammatory pain. The inclusion complex was prepared at a molar ratio of 1:2 (HA:βCD) by the lyophilization method. For the induction of chronic inflammatory pain, the mice received an intraplantar injection of CFA (complete Freund's adjuvant), and were evaluated for mechanical hyperalgesia and for the levels of myeloperoxidase (MPO) in the skin of the paw after eight days of treatment. HA and HA/βCD reduced mechanical hyperalgesia in relation to the vehicle group until the fourth and fifth hours, respectively, in the acute evaluation, with a superior effect of the complexed form over the pure form in the second and third hour after treatment (p < 0.001). In the chronic evaluation, HA and HA/βCD reduced hyperalgesia in relation to the vehicle in the eight days of treatment (p < 0.001). Both pure (p < 0.01) and complexed (p < 0.001) forms reduced myeloperoxidase activity in the skin of the animals' paw. Groups of animals subjected to the same pharmacological protocol were submitted to the partial sciatic nerve ligation (PSNL) model and evaluated for mechanical and thermal hyperalgesia, and cold allodynia. HA and HA/βCD reduced mechanical hyperalgesia until the fourth and sixth hours, respectively, and both reduced hyperalgesia in relation to the vehicle in the chronic evaluation (p < 0.001). HA and HA/βCD also reduced thermal hyperalgesia and cold allodynia (p < 0.05 and p < 0.001, respectively). The analysis of the spinal cord of these animals showed a decrease in the levels of the pro-inflammatory cytokines TNF-α, IL-1β and IL-6 and a reduction in the phosphorylation of NFκB and p38MAPK, as well as a decrease in microglioses compared to the vehicle group. In addition, HA/βCD reduced the nociception induced by intraplantar injection of agonist TRPA1 (p < 0.01) and TRPM8 (p < 0.05). Treatment for eight days with HA and HA/βCD showed no signs of gastric or liver damage. HA and HA/βCD were, therefore, shown to have antinociceptive effects in chronic pain models. Based on our exploration of the mechanisms of the action of HA, these effects are likely to be related to inhibited leukocyte migration, interaction with the TRPA1 and TRPM8 receptors, reduced pro-inflammatory cytokines levels, microglial expression and suppression of NF-κB p65 and p38 MAPK pathway signaling. Therefore, HA/βCD has great potential for use in the treatment of chronic pain.

Peripheral receptors and neuromediators involved in the antihyperalgesic effects of acupuncture: a state-of-the-art review

The present study aims to describe state-of-the-art of preclinical studies that have investigated peripheral receptors and neuromediators involved in the antihyperalgesic effects of acupuncture. The PubMed, Scopus, and Web of Science databases were searched using the integrative review method. Preclinical articles that involved the study of peripheral receptors and neuromediators on the pain control effects of acupuncture in rats or mice were selected using a predefined search strategy. From this search, 456 articles were found, and 29 of them met the inclusion criteria of the study. The selected articles addressed the following peripheral receptors: opioid (n = 9), adenosine (n = 5), cannabinoid (n = 5), transient receptor potential vanilloid (TRPV) (n = 3), histamine (n = 2), adrenergic (n = 1), muscarinic (n = 1), corticotrophin-releasing factor (CRF) (n = 2), IL-1 (n = 1), and endothelin (n = 1) receptors. The peripheral neuromediators correlated with the peripheral pain control effect were as follows: opioid peptides (n = 4), adenosine (n = 3), histamine (n = 1), substance P (n = 1) calcitonin gene-related peptide (CGRP) (n = 1), anandamide (n = 1), nitric oxide (n = 1), and norepinephrine (n = 1). This review summarizes the methods used to investigate the peripheral effects of acupuncture and discusses the main findings on each family of receptors and neuromediators. Ten families of peripheral receptors and 8 types of neuromediators were correlated with the antihyperalgesic effects of acupuncture in preclinical studies. Considering the benefits of a better understanding of the role of peripheral receptors and neuromediators in the context pain management, the findings of the present study highlight the importance of deepening the exploration of the peripheral mechanisms of acupuncture.

Peripheral Mechanisms of Neuropathic Pain-the Role of Neuronal and Non-Neuronal Interactions and Their Implications for Topical Treatment of Neuropathic Pain

Neuropathic pain in humans arises as a consequence of injury or disease of somatosensory nervous system at peripheral or central level. Peripheral neuropathic pain is more common than central neuropathic pain, and is supposed to result from peripheral mechanisms, following nerve injury. The animal models of neuropathic pain show extensive functional and structural changes occurring in neuronal and non-neuronal cells in response to peripheral nerve injury. These pathological changes following damage lead to peripheral sensitization development, and subsequently to central sensitization initiation with spinal and supraspinal mechanism involved. The aim of this narrative review paper is to discuss the mechanisms engaged in peripheral neuropathic pain generation and maintenance, with special focus on the role of glial, immune, and epithelial cells in peripheral nociception. Based on the preclinical and clinical studies, interactions between neuronal and non-neuronal cells have been described, pointing out at the molecular/cellular underlying mechanisms of neuropathic pain, which might be potentially targeted by topical treatments in clinical practice. The modulation of the complex neuro-immuno-cutaneous interactions in the periphery represents a strategy for the development of new topical analgesics and their utilization in clinical settings.

Resolvins: Potent Pain Inhibiting Lipid Mediators via Transient Receptor Potential Regulation

Chronic pain is a serious condition that occurs in the peripheral nervous system (PNS) and the central nervous system (CNS). It is caused by inflammation or nerve damage that induces the release of inflammatory mediators from immune cells and/or protein kinase activation in neuronal cells. Both nervous systems are closely linked; therefore, inflammation or nerve damage in the PNS can affect the CNS (central sensitization). In this process, nociceptive transient receptor potential (TRP) channel activation and expression are increased. As a result, nociceptive neurons are activated, and pain signals to the brain are amplified and prolonged. In other words, suppressing the onset of pain signals in the PNS can suppress pain signals to the CNS. Resolvins, endogenous lipid mediators generated during the resolution phase of acute inflammation, inhibit nociceptive TRP ion channels and alleviate chronic pain. This paper summarizes the effect of resolvins in chronic pain control and discusses future scientific perspectives. Further study on the effect of resolvins on neuropathic pain will expand the scope of pain research.