Aconitum-Derived Bulleyaconitine A Exhibits Antihypersensitivity Through Direct Stimulating Dynorphin A Expression in Spinal Microglia

Spinal neuron-glial crosstalk and ion channel dysregulation in diabetic neuropathic pain

Diabetic neuropathic pain (DNP) is one of the most prevalent complications of diabetes, characterized by a high global prevalence and a substantial affected population with limited effective therapeutic options. Although DNP is closely associated with hyperglycemia, an increasing body of research suggests that elevated blood glucose levels are not the sole inducers of DNP. The pathogenesis of DNP is intricate, involving the release of inflammatory mediators, alterations in synaptic plasticity, demyelination of nerve fibers, and ectopic impulse generation, yet the precise mechanisms remain to be elucidated. The spinal dorsal horn coordinates dynamic interactions between peripheral and central pain pathways, wherein dorsal horn neurons, microglia, and astrocytes synergize with Schwann cell-derived signals to process nociceptive information flow. Abnormally activated neurons can alter signal transduction by modifying the local microenvironment, compromising myelin integrity, and diminishing trophic support, leading to neuronal sensitization and an amplifying effect on peripheral pain signals, which in turn triggers neuropathic pain. Ion channels play a pivotal role in signal conduction, with the modulation of sodium, potassium, and calcium channels being particularly crucial for the regulation of pain signals. In light of the rising incidence of diabetes and the current scarcity of effective DNP treatments, a thorough investigation into the interactions between neurons and glial cells, especially the mechanisms of ion channel function in DNP, is imperative for identifying potential drug targets, developing novel therapeutic strategies, and thereby enhancing the prospects for DNP management.

In silico exploration of bioactive secondary metabolites with anesthetic effects on sodium channels Nav 1.7, 1.8, and 1.9 in painful human dental pulp

AIM

To investigate the efficacy of medicinal plant bioactive secondary metabolites as inhibitors of voltage-gated sodium channels (Nav1.7, Nav1.8, and Nav1.9) in managing painful states of dental pulps.

METHODOLOGY

Molecular docking, ADME prediction, toxicity profiling, and pharmacophore modeling were used to assess the binding affinities, pharmacokinetic properties, toxicological profiles, and active pharmacophores of the selected bioactive compounds.

RESULTS

Three compounds (Sepaconitine, Lappaconitine, and Ranaconitine) showed binding affinities (ΔG = -8.95 kcal/mol, -7.77 kcal/mol, and -7.44 kcal/mol, respectively) with all three Nav1.7, Nav1.8, and Nav1.9 sodium channels. The sepaconitine amine group formed hydrophobic interactions with key residues. The Lappaconitine benzene ring contributed to hydrophobic interactions and hydrogen bond acceptor interactions. The hydrophobic interactions of the ranaconitine amine group play a critical role with specific residues on Nav1.8 and Nav1.9.

CONCLUSION

The natural fusicoccane diterpenoid derivatives Sepaconitine, Lappaconitine, and Ranaconitine are potential lead compounds for the development of novel analgesics as selective antihyperalgesic drugs, which will provide a new dental pharmacological intervention for managing painful dental pulp conditions. Further experimental validation and clinical studies that confirm the efficacy and safety of these compounds will strengthen their applicability in dental practice.

Biologically active franchetine-type diterpenoid alkaloids: Isolation, synthesis, anti-inflammatory, agalgesic activities, and molecular docking

In this study, four franchetine-type diterpenoid alkaloids (1-4) were isolated from Aconitum sinoaxillare, and fourteen diverse franchetine analogs (5-18) were synthesized. Compounds 1, 2, 7 and 16 exhibited stronger inhibitory effects on NO production when compared to celecoxib. Among them, compound 1 had the best inhibitory effect on iNOS and COX-2 inflammatory proteins. The in vitro studies displayed that the anti-inflammatory effect of the most active compound 1 was ascribed to the inhibition of the TLR4-MyD88/NF-κB/MAPKs signalling pathway. Consequently, this led to a inhibition in the expression of inflammatory factors or mediators including NO, ROS, TNF-α, IL-6, IL-1β, iNOS, and COX-2. Additionally, compound 1 had low toxicity (LD50 > 20 mg/kg) in mice, and it had notable analgesic effects on acetic acid-induced visceral pain (ED50 = 2.15 ± 0.07 mg/kg). Moreover, compound 1 exhibited a distinct reduction in the NaV1.7 and NaV1.8 channel currents during both resting and half-inactivated states at 50 μM. The present study enriches the pharmacological activities of franchetine derivatives and provides valuable insights for the development of novel anti-inflammatory and analgesic agents.

Microglia in morphine tolerance: cellular and molecular mechanisms and therapeutic potential

Morphine has a crucial role in treating both moderate to severe pain and chronic pain. However, prolonged administration of morphine can lead to tolerance of analgesia, resulting in increased doses and poor treatment of pain. Many patients, such as those with terminal cancer, require high doses of morphine for long periods. Addressing morphine tolerance can help this group of patients to escape pain, and the mechanisms behind this need to be investigated. Microglia are the key cells involved in morphine tolerance and chronic morphine administration leads to microglia activation, which in turn leads to activation of internal microglia signalling pathways and protein transcription, ultimately leading to the release of inflammatory factors. Inhibiting the activation of microglia internal signalling pathways can reduce morphine tolerance. However, the exact mechanism of how morphine acts on microglia and ultimately leads to tolerance is unknown. This article discusses the mechanisms of morphine induced microglia activation, reviews the signalling pathways within microglia and the associated therapeutic targets and possible drugs, and provides possible directions for clinical prevention or retardation of morphine induced analgesic tolerance.

Advancements in Non-Addictive Analgesic Diterpenoid Alkaloid Lappaconitine: A Review

The perennial herb Aconitum sinomontanum Nakai (Ranunculaceae) has been utilized as a traditional oriental medicine in China for numerous years. The principal pharmacological constituent of A. sinomontanum, lappaconitine (LA), exhibits analgesic, anti-inflammatory, anti-tumor, anti-arrhythmic, and anti-epileptic activities. Due to its potent efficacy and non-addictive nature, LA is widely utilized in the management of cancer pain and postoperative analgesia. This review encompasses the research advancements pertaining to LA including extraction methods, separation techniques, pharmacological properties, chemical modifications, and clinical applications. Additionally, it offers insights into the potential applications and current challenges associated with LA to facilitate future research endeavors.

Screening tools to evaluate the neurotoxic potential of botanicals: building a strategy to assess safety

AREAS COVERED

This paper outlines the selection of NAMs, including in vitro assays using primary rat cortical neurons, zebrafish embryos, and Caenorhabditis elegans. These assays aim to assess neurotoxic endpoints such as neuronal activity and behavioral responses. Microelectrode array recordings of rat cortical neurons provide insights into the impact of botanical extracts on neuronal function, while the zebrafish embryos and C. elegans assays evaluate neurobehavioral responses. The paper also provides an account of the selection of botanical case studies based on expert judgment and existing neuroactivity/toxicity information. The proposed battery of assays will be tested with these case studies to evaluate their utility for neurotoxicity screening.

EXPERT OPINION

The complexity of botanicals necessitates the use of multiple NAMs for effective neurotoxicity screening. This paper discusses the evaluation of methodologies to develop a robust framework for evaluating botanical safety, including complex neuronal models and key neurodevelopmental process assays. It aims to establish a comprehensive screening framework.

Dexamethasone attenuates neuropathic pain through spinal microglial expression of dynorphin A via the cAMP/PKA/p38 MAPK/CREB signaling pathway

This study aimed to elucidate the opioid mechanisms underlying dexamethasone-induced pain antihypersensitive effects in neuropathic rats. Dexamethasone (subcutaneous and intrathecal) and membrane-impermeable Dex-BSA (intrathecal) administration dose-dependently inhibited mechanical allodynia and thermal hyperalgesia in neuropathic rats. Dexamethasone and Dex-BSA treatments increased expression of dynorphin A in the spinal cords and primary cultured microglia. Dexamethasone specifically enhanced dynorphin A expression in microglia but not astrocytes or neurons. Intrathecal injection of the microglial metabolic inhibitor minocycline blocked dexamethasone-stimulated spinal dynorphin A expression; intrathecal minocycline, the glucocorticoid receptor antagonist Dex-21-mesylate, dynorphin A antiserum, and κ-opioid receptor antagonist GNTI completely blocked dexamethasone-induced mechanical antiallodynia and thermal antihyperalgesia. Additionally, dexamethasone elevated spinal intracellular cAMP levels, leading to enhanced phosphorylation of PKA, p38 MAPK and CREB. The specific adenylate cyclase inhibitor DDA, PKA inhibitor H89, p38 MAPK inhibitor SB203580 and CREB inhibitor KG-501 completely blocked dexamethasone-induced anti-neuropathic pain and increased microglial dynorphin A exprression. In conclusion, this study reveal that dexamethasone mitigateds neuropathic pain through upregulation of dynorphin A in spinal microglia, likely involving the membrane glucocorticoid receptor/cAMP/PKA/p38 MAPK/CREB signaling pathway.

A novel tRNA-derived fragment tRF-3023b suppresses inflammation in RAW264.7 cells by targeting Cul4a through NF-κB signaling

The role of transfer RNA (tRNA)-derived fragment (tRF) in various diseases has been established. However, the effect of tRF-3023b on inflammation remains unclear. Inflammation was imitated in RAW264.7 cells by adding Lipopolysaccharide (LPS). Cells were first divided into control, LPS, and LPS + Bulleyaconitine A (BLA) groups. The contents of TNF-α, IL-6, and MCP-1 were quantified using ELISA. The levels of cyclooxygenase-2 (COX2), inducible nitric oxide synthase (iNOS), and the phosphorylation of nuclear factor-kappa B (NF-κB)-P65 (p-P65) were detected by Western blotting. RNA sequencing was utilized to find differentially expressed tRFs (DE-tRFs) among three groups. The levels of various tRFs were checked by quantitative real-time PCR (qRT-PCR). Cell cycle and apoptosis were checked by flow cytometry. Dluciferase reporter assay was applied to predict and confirm the interaction between tRF-3023b and Cullin 4A (Cul4a), subsequently RNA pull-down followed by mass spectrometry analysis were conducted. BLA treatment decreased the contents of TNF-α, IL-6, MCP-1, and the expression levels of COX2, iNOS, p-P65. We found 6 DE-tRFs in LPS + BLA group compared to LPS group, tRF-3023b was high expression in control and BLA groups, and the lowest in LPS group. Cul4a was a direct target of tRF-3023b. tRF-3023b mimic affected the cell cycle distribution, promoted cells apoptosis, and suppressed the TNF-α, IL-6, MCP-1, COX2, iNOS and p-P65. The suppression of Cul4a affected the cell cycle distribution, resulted in an increase of cell apoptosis while a decrease of TNF-α, IL-6, MCP-1, COX2, iNOS and p-P65. Furthermore, Cul4a overexpression reversed the effect of tRF-3023b mimic. Cul4a knockdown reversed the effect of tRF-3023b inhibitor. Our study positions tRF-3023b as a compelling candidate, through its interaction with Cul4a, the underlying mechanism on inflammation maybe related to NF-κB pathway. The study provides a basis for exploring new therapeutic strategies for inflammation.

Global Trends and Hotspots on Microglia Associated with Pain from 2002 to 2022: A Bibliometric Analysis

Background

Researchers have made significant progress in microglia associated with pain in recent years. However, more relevant bibliometric analyses are still needed on trends and directions in this field. The aim of this study is to provide a comprehensive perspective and to predict future directions of pain-related microglia research via bibliometric tools.

Methods

English articles and reviews related with pain and microglia were extracted from the Web of Science core collection (WosCC) database between 2002 to 2022. Bibliometric tools such as VOSviewer, CiteSpace, and Bibliometrix R package were used to analyze publication characteristics, countries, authors, institutions, journals, research hotspots, and trend topics.

Results

A total of 2761 articles were included in this analysis. Research on microglia associated with pain has increased significantly over the last two decades. China (n = 1020, 36.94%) and the United States (n = 751, 27.20%) contributed the most in terms of publications and citations, respectively. Kyushu University published the most articles in this field compared to other institutions, and Professor Inoue Kazuhide (n = 54) at this university made outstanding contributions in this field. Molecular Pain (n = 113) was the journal with the most publication, while Journal of Neuroscience had the highest number of citations. According to the authors keywords analysis, the research in this area can be summarized into 7 clusters such as "microglia activation pathways", "pain treatment research", "mental symptoms of chronic pain", and so on.

Conclusion

This study provides a comprehensive analysis of pain-related microglia research in the past two decades. We identified the countries, institutions, scholars, and journals with the highest number of publications and the most influence in the field, and the research trends identified in this paper may provide new insights for future research.

Neuropharmacological Potential of Diterpenoid Alkaloids

This study provides a narrative review of diterpenoid alkaloids (DAs), a family of extremely important natural products found predominantly in some species of Aconitum and Delphinium (Ranunculaceae). DAs have long been a focus of research attention due to their numerous intricate structures and diverse biological activities, especially in the central nervous system (CNS). These alkaloids originate through the amination reaction of tetra or pentacyclic diterpenoids, which are classified into three categories and 46 types based on the number of carbon atoms in the backbone structure and structural differences. The main chemical characteristics of DAs are their heterocyclic systems containing β-aminoethanol, methylamine, or ethylamine functionality. Although the role of tertiary nitrogen in ring A and the polycyclic complex structure are of great importance in drug-receptor affinity, in silico studies have emphasized the role of certain sidechains in C13, C14, and C8. DAs showed antiepileptic effects in preclinical studies mostly through Na⁺ channels. Aconitine (1) and 3-acetyl aconitine (2) can desensitize Na⁺ channels after persistent activation. Lappaconitine (3), N-deacetyllapaconitine (4), 6-benzoylheteratisine (5), and 1-benzoylnapelline (6) deactivate these channels. Methyllycaconitine (16), mainly found in Delphinium species, possesses an extreme affinity for the binding sites of α7 nicotinic acetylcholine receptors (nAChR) and contributes to a wide range of neurologic functions and the release of neurotransmitters. Several DAs such as bulleyaconitine A (17), (3), and mesaconitine (8) from Aconitum species have a drastic analgesic effect. Among them, compound 17 has been used in China for decades. Their effect is explained by increasing the release of dynorphin A, activating the inhibitory noradrenergic neurons in the β-adrenergic system, and preventing the transmission of pain messages by inactivating the Na⁺ channels that have been stressed. Acetylcholinesterase inhibitory, neuroprotective, antidepressant, and anxiolytic activities are other CNS effects that have been investigated for certain DAs. However, despite various CNS effects, recent advances in developing new drugs from DAs were insignificant due to their neurotoxicity.

Bullatine A has an antidepressant effect in chronic social defeat stress mice; Implication of microglial inflammasome

Inflammatory microglia and P2X7R are involved in the development of stress-induced depression. Endoplasmic reticulum (ER) stress and mitochondrial damage play an important role in depression and microglial activation. Bullatine A (BLA) has anti-inflammatory and anti-rheumatic effects, and can be used as a P2X7R antagonist. We found that Bullatine A can effectively inhibit the calcium overload of mitochondria and the increase of ER and mitochondrial colocalization caused by eATP (extracellular ATP) in BV2-cells. Bullatine A can also inhibit the activation of PERK-elF-2α unfolded protein response (UPR), lysosome production and the increase of NLRP3 inflammasome protein expression in BV2-cells Both intragastric administration and intra-hippocampal microinjection of Bullatine A can significantly improve the despair behavior but not anhedonia of Chronic chronic social defeat stress (CSDS) mice. Bullatine A may have a beneficial therapeutic effect in treating diseases related to stress stimulation, such as depression.

Ginsenoside Rb1, a principal effective ingredient of Panax notoginseng, produces pain antihypersensitivity by spinal microglial dynorphin A expression

Panax notoginseng (Chinese ginseng, Sanqi), one of the major ginseng species, has been traditionally used to alleviate different types of chronic pain. The raw P. notoginseng powder is commonly available in China as a non-prescription drug to treat various aliments including arthritic pain. However, strong scientific evidence is needed to illustrate its pain antihypersensitive effects, effective ingredients and mechanism of action. The oral P. notoginseng powder dose-dependently alleviated formalin-induced tonic hyperalgesia, and its total ginsenosides remarkably inhibited neuropathic pain hypersensitivity. Ginsenoside Rb1, the most abundant ginsenoside of P. notoginseng, dose-dependently produced neuropathic pain antihypersensitivity. Conversely, ginsenosides Rg1, Re and notoginseng R1, the other major saponins from P. notoginseng, failed to inhibit formalin-induced tonic pain or mechanical allodynia in neuropathic pain. Ginsenoside Rb1 metabolites ginsenosides Rg3, Compound-K and protopanaxadiol also had similar antineuropathic pain efficacy to ginsenoside Rb1. Additionally, intrathecal ginsenoside Rb1 specifically stimulated dynorphin A expression which was colocalized with microglia but not neurons or astrocytes in the spinal dorsal horn and primary cultured cells. Pretreatment with microglial metabolic inhibitor minocycline, dynorphin A antiserum and specific κ-opioid receptor antagonist GNTI completely blocked Rb1-induced mechanical antiallodynia in neuropathic pain. Furthermore, the specific glucocorticoid receptor (GR) antagonist Dex-21-mesylate (but not GPR30 estrogen receptor antagonist G15) also entirely attenuated ginsenoside Rb1-related antineuropathic pain effects. All these results, for the first time, show that P. notoginseng alleviates neuropathic pain and ginsenoside Rb1 is its principal effective ingredient. Furthermore, ginsenoside Rb1 inhibits neuropathic pain by stimulation of spinal microglial dynorphin A expression following GR activation.

Design, synthesis, and biological evaluation of low-toxic lappaconitine derivatives as potential analgesics

The C₁₈-diterpenoid alkaloid lappaconitine (LA) is a non-addictive analgesic used in China. The toxicity (LD₅₀ = 11.7 mg/kg) limits its application. Two series of LA derivatives, including amides and sulfonamides (1-93), were designed and synthesized by modification on their C4 acetamidobenzoate side chains in this work. In vivo analgesic activity and toxicity of all derivatives were evaluated, and the structure-activity relationship was summarized. Six lead compounds (35, 36, 39, 49, 70, and 89) exhibited approximate analgesic activity to LA but with significantly reduced toxicity. The therapeutic index of these compounds is 14-30 times that of LA. In vivo metabolism study of the lead compounds 39, 49, 70, and 89 were conducted by UPLC-MS^E, indicating the reason for the low toxicity of the potential derivatives might be they are difficult to metabolize to toxic metabolite N-deacetyllappaconitine compared to LA. The effects of lead compounds on sodium channels and hERG channels were also studied by ion channel reader (ICR) which further revealed their analgesic and toxicity-attenuating mechanisms. Sodium channel assay revealed that the analgesic mechanism of these lead compounds was inhibiting the Na_v 1.7 channels. Taken together, compound 39 was provided as a new analgesic lead compound with significantly low toxicity and comparable activity to LA.

Injectable Erythrocyte Gel Loaded with Bulleyaconitine A for the Treatment of Rheumatoid Arthritis

Rheumatoid arthritis (RA) is a systemic autoimmune disease with clinical manifestations including joint cartilage, synovitis, and bone damage. Here we developed an injectable erythrocyte gel loaded with Bulleyaconitine A (BLA) for the treatment of RA and demonstrated its anti-inflammatory effects in vivo and in vitro. In vitro experiments showed that BLA could effectively down-regulate the expression of pro-inflammatory factor in activated macrophages through the nuclear factor-κB (NF-κB) pathway. In vivo experiments have shown that the injection of BLA@RBCs in the inflammatory joints of CIA mice increases the local concentration of BLA in a long time. Improved therapeutic outcomes and reduced toxicity of BLA are demonstrated in our work. Together, the developed BLA@RBCs drug delivery system provides an alternative strategy to treat RA joints and shows high potential in clinical RA treatment.

Microglial IL-10 and β-endorphin expression mediates gabapentinoids antineuropathic pain

Gabapentinoids are recommended first-line treatments for neuropathic pain. They are neuronal voltage-dependent calcium channel α2δ-1 subunit ligands and have been suggested to attenuate neuropathic pain via interaction with neuronal α2δ-1 subunit. However, the current study revealed their microglial mechanisms underlying antineuropathic pain. Intrathecal injection of gabapentin, pregabalin and mirogabalin rapidly inhibited mechanical allodynia and thermal hyperalgesia, with projected ED₅₀ values of 30.3, 6.2 and 1.5 µg (or 176.9, 38.9 and 7.2 nmol) and E_max values of 66%, 61% and 65% MPE respectively for mechanical allodynia. Intrathecal gabapentinoids stimulated spinal mRNA and protein expression of IL-10 and β-endorphin (but not dynorphin A) in neuropathic rats with the time point parallel to their inhibition of allodynia, which was observed in microglia but not astrocytes or neurons in spinal dorsal horns by using double immunofluorescence staining. Intrathecal gabapentin alleviated pain hypersensitivity in male/female neuropathic but not male sham rats, whereas it increased expression of spinal IL-10 and β-endorphin in male/female neuropathic and male sham rats. Treatment with gabapentin, pregabalin and mirogabalin specifically upregulated IL-10 and β-endorphin mRNA and protein expression in primary spinal microglial but not astrocytic or neuronal cells, with EC₅₀ values of 41.3, 11.5 and 2.5 µM and 34.7, 13.3 and 2.8 µM respectively. Pretreatment with intrathecal microglial metabolic inhibitor minocycline, IL-10 antibody, β-endorphin antiserum or μ-opioid receptor antagonist CTAP (but not κ- or δ-opioid receptor antagonists) suppressed spinal gabapentinoids-inhibited mechanical allodynia. Immunofluorescence staining exhibited specific α2δ-1 expression in neurons but not microglia or astrocytes in the spinal dorsal horns or cultured primary spinal cells. Thus the results illustrate that gabapentinoids alleviate neuropathic pain through stimulating expression of spinal microglial IL-10 and consequent β-endorphin.

A systematic review of pharmacological activities, toxicological mechanisms and pharmacokinetic studies on Aconitum alkaloids

The tubers and roots of Aconitum (Ranunculaceae) are widely used as heart medicine or analgesic agents for the treatment of coronary heart disease, chronic heart failure, rheumatoid arthritis and neuropathic pain since ancient times. As a type of natural products mainly extracted from Aconitum plants, Aconitum alkaloids have complex chemical structures and exert remarkable biological activity, which are mainly responsible for significant effects of Aconitum plants. The present review is to summarize the progress of the pharmacological, toxicological, and pharmacokinetic studies of Aconitum alkaloids, so as to provide evidence for better clinical application. Research data concerning pharmacological, toxicological and pharmacokinetic studies of Aconitum alkaloids were collected from different scientific databases (PubMed, CNKI, Google Scholar, Baidu Scholar, and Web of Science) using the phrase Aconitum alkaloids, as well as generic synonyms. Aconitum alkaloids are both bioactive compounds and toxic ingredients in Aconitum plants. They produce a wide range of pharmacological activities, including protecting the cardiovascular system, nervous system, and immune system and anti-cancer effects. Notably, Aconitum alkaloids also exert strong cardiac toxicity, neurotoxicity and liver toxicity, which are supported by clinical studies. Finally, pharmacokinetic studies indicated that cytochrome P450 proteins (CYPs) and efflux transporters (ETs) are closely related to the low bioavailability of Aconitum alkaloids and play an important role in their metabolism and detoxification in vivo.

Protopanaxadiol alleviates neuropathic pain by spinal microglial dynorphin A expression following glucocorticoid receptor activation

BACKGROUND AND PURPOSE

New remedies are required for the treatment of neuropathic pain due to insufficient efficacy of available therapies. This study provides a novel approach to develop painkillers for chronic pain treatment.

EXPERIMENTAL APPROACH

The rat formalin pain test and spinal nerve ligation model of neuropathic pain were used to evaluate antinociception of protopanaxadiol. Primary cell cultures, immunofluorescence staining, and gene and protein expression were also performed for mechanism studies.

KEY RESULTS

Gavage protopanaxadiol remarkably produces pain antihypersensitive effects in neuropathic pain, bone cancer pain and inflammatory pain, with efficacy comparable with gabapentin. Long-term PPD administration does not induce antihypersensitive tolerance, but prevents and reverses the development and expression of morphine analgesic tolerance. Oral protopanaxadiol specifically stimulates spinal expression of dynorphin A in microglia but not in astrocytes or neurons. Protopanaxadiol gavage-related pain antihypersensitivity is abolished by the intrathecal pretreatment with the microglial metabolic inhibitor minocycline, dynorphin antiserum or specific κ-opioid receptor antagonist GNTI. Intrathecal pretreatment with glucocorticoid receptor)antagonists RU486 and dexamethasone-21-mesylate, but not GPR-30 antagonist G15 or mineralocorticoid receptor antagonist eplerenone, completely attenuates protopanaxadiol-induced spinal dynorphin A expression and pain antihypersensitivity in neuropathic pain. Treatment with protopanaxadiol, the glucocorticoid receptor agonist dexamethasone and membrane-impermeable glucocorticoid receptor agonist dexamethasone-BSA in cultured microglia induces remarkable dynorphin A expression, which is totally blocked by pretreatment with dexamthasone-21-mesylate.

CONCLUSION AND IMPLICATIONS

All the results, for the first time, indicate that protopanaxadiol produces pain antihypersensitivity in neuropathic pain probably through spinal microglial dynorphin A expression after glucocorticoid receptor activation and hypothesize that microglial membrane glucocorticoid receptor/dynorphin A pathway is a potential target to discover and develop novel painkillers in chronic pain.

Kappa-opioid receptor-mediated thermal analgesia evoked by the intrathecal administration of the chemokine CCL1 in mice

BACKGROUND

The chemokine CC motif ligand 1 (CCL1) participates in immune cell recruitment and, as other chemokines, is also involved in nociceptive processing. In contrast with previous reports indicating its participation in allodynia and cold hypernociception when spinally administered, its ability to evoke heat thermal analgesia, mediated by circulating leukocytes and endocannabinoids, after systemic administration has recently been reported.

OBJECTIVES

Aiming to explore the role played by CCL1 on spinal nociception, we study here the effect of its intrathecal administration on thermal nociception in mice.

METHODS

Behavioral nociceptive assays, immunohistochemical experiments, white cell blood depletion procedures and qRT-PCR experiments were performed.

RESULTS

The intrathecal administration of CCL1 (0.3-30 ng) produced analgesia as measured by the unilateral hot plate test. This effect peaked 1 h after injection, was prevented by the CCR8 antagonist R243 and was accompanied by a reduction of c-Fos expression in spinal neurons. Whereas blood leukocyte depletion did not modify it, analgesia was abolished by the microglial inhibitor minocycline, but not the astroglial inhibitor aminoadipate. Furthermore, antinociception remained unmodified by the coadministration of cannabinoid type 1 or 2 receptors antagonists. However, it was reversed by naloxone but not by selective blockade of mu- or delta-opioid receptors. The inhibitory effect induced by the selective kappa-opioid receptor antagonist, nor-binaltorphimine, and by an anti-dynorphin A 1-17 antibody indicates that analgesia evoked by spinal CCL1 is mediated by endogenous dynorphins acting on kappa-opioid receptors.

CONCLUSIONS

Endogenous dynorphin and microglia behave as key players in heat thermal analgesia evoked by spinal CCL1 in mice.

Bulleyaconitine A Inhibits Morphine-Induced Withdrawal Symptoms, Conditioned Place Preference, and Locomotor Sensitization Via Microglial Dynorphin A Expression

Bulleyaconitine A (BAA), a C19-diterpenoid alkaloid, has been prescribed as a nonnarcotic analgesic to treat chronic pain over four decades in China. The present study investigated its inhibition in morphine-induced withdrawal symptoms, conditioned place preference (CPP) and locomotor sensitization, and then explored the underlying mechanisms of actions. Multiple daily injections of morphine but not BAA up to 300 μg/kg/day into mice evoked naloxone-induced withdrawal symptoms (i.e., shakes, jumps, genital licks, fecal excretion and body weight loss), CPP expression, and locomotor sensitization. Single subcutaneous BAA injection (30–300 μg/kg) dose-dependently and completely attenuated morphine-induced withdrawal symptoms, with ED₅₀ values of 74.4 and 105.8 μg/kg in shakes and body weight loss, respectively. Subcutaneous BAA (300 μg/kg) also totally alleviated morphine-induced CPP acquisition and expression and locomotor sensitization. Furthermore, subcutaneous BAA injection also specifically stimulated dynorphin A expression in microglia but not astrocytes or neurons in nucleus accumbens (NAc) and hippocampal, measured for gene and protein expression and double immunofluorescence staining. In addition, subcutaneous BAA-inhibited morphine-induced withdrawal symptoms and CPP expression were totally blocked by the microglial metabolic inhibitor minocycline, dynorphin A antiserum, or specific KOR antagonist GNTI, given intracerebroventricularly. These results, for the first time, illustrate that BAA attenuates morphine-induced withdrawal symptoms, CPP expression, and locomotor sensitization by stimulation of microglial dynorphin A expression in the brain, suggesting that BAA may be a potential candidate for treatment of opioids-induced physical dependence and addiction.

Use of medicinal plants for headache, and their potential implication in medication-overuse headache: Evidence from a population-based study in Nepal

Background

In Nepal, traditional treatment using medicinal plants is popular. Whereas medication-overuse headache is, by definition, caused by excessive use of acute headache medication, we hypothesized that medicinal plants, being pharmacologically active, were as likely a cause.

Methods

We used data from a cross-sectional, nationwide population-based study, which enquired into headache and use of medicinal plants and allopathic medications. We searched the literature for pharmacodynamic actions of the medicinal plants.

Results

Of 2100 participants, 1794 (85.4%) reported headache in the preceding year; 161 (7.7%) reported headache on ≥15 days/month, of whom 28 (17.4%) had used medicinal plants and 117 (72.7%) allopathic medication(s). Of 46 with probable medication-overuse headache, 87.0% (40/46) were using allopathic medication(s) and 13.0% (6/46) medicinal plants, a ratio of 6.7:1, higher than the overall ratio among those with headache of 4.9:1 (912/185). Of 60 plant species identified, 49 were pharmacodynamically active on the central nervous system, with various effects of likely relevance in medication-overuse headache causation.

Conclusions

MPs are potentially a cause of medication-overuse headache, and not to be seen as innocent in this regard. Numbers presumptively affected in Nepal are low but not negligible. This pioneering project provides a starting point for further research to provide needed guidance on use of medicinal plants for headache.

Aconitine disrupts serotonin neurotransmission via 5-hydroxytryptamine receptor in zebrafish embryo

Medicinal plants of the genus Aconitum are one of the most commonly used herbs in traditional medicine in East Asia to treat conditions related to the heart, pain, or inflammation. However, these herbs are also dangerous as accidental poisoning due to misuse is a recurring issue. These plants contain a number of diester-diterpenoid alkaloid compounds and aconitine is the most abundant and active one. This study investigated neurotoxicity of aconitine to zebrafish embryos in early development in relation to serotonin regulation. Experimental results showed that aconitine exposure (1, 10, and 100 μM) increased frequency of coiling behavior in zebrafish embryos in a dose-dependent manner and this effect can be triggered by either exposure to 5-hydroxytryptamine 1A (5-HT1A) receptor agonist (±)-8-hydroxy-2-(dipropylamino)tetralin (8-OH-DPAT) or overexpression of serotonin receptor 5-htr1ab. At the same time, coiling behavior caused by aconitine exposure could be rescued by co-exposure to 5-HT1A receptor antagonist WAY-100635 Maleate (WAY100635) and knockdown of 5-htr1ab using morpholino. Exposure to aconitine also significantly increased serotonin receptor 5-htr1ab and 5-htr1bd gene expression at 24 h post fertilization (hpf), but decreased their expression and protein expression of the serotonin receptor at 96 hpf with the high dose. These results suggest that neurotoxicity caused by aconitine is mediated through the 5-HT receptor.

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.

Synthesis and Evaluation of a Series of New Bulleyaconitine A Derivatives as Analgesics

As a nonaddictive analgesic widely used in clinics, the LD₅₀ of bulleyaconitine A is just only 0.92 mg/kg, which exhibits obvious toxicity. Therefore, 31 new non-natural C₁₉-diterpenoid alkaloids (2a-w, 2'a-e, 3, 4a, and 4b) were designed and synthesized from bulleyaconitine A to develop nonaddictive analgesics with low toxicity. The chemical structures were characterized by ¹H NMR, ¹³C NMR, and high-resolution mass spectrometry (HRMS) spectra. The analgesic activities were evaluated by a hot plate test in mice. At the dosage of 10 mg/kg, six compounds (2d, 2j, 2k, 2m, 2t, 2w) exhibited good analgesic activities (increased pain threshold >100%) with a long duration. Among them, 2w showed the best analgesic activity and the longest duration. Its pain threshold reached 166.35% in 15 min, peaked at 30 min (182.35%), and remained 82.59% even at 60 min.

Bulleyaconitine A Inhibits Visceral Nociception and Spinal Synaptic Plasticity through Stimulation of Microglial Release of Dynorphin A

Background

Visceral pain is one of the most common types of pain and particularly in the abdomen is associated with gastrointestinal diseases. Bulleyaconitine A (BAA), isolated from Aconitum bulleyanum, is prescribed in China to treat chronic pain. The present study is aimed at evaluating the mechanisms underlying BAA visceral antinociception.

Methods

The rat model of chronic visceral hypersensitivity was set up by colonic perfusion of 2,4,6-trinitrobenzene sulfonic acid (TNBS) on postnatal day 10 with coapplication of heterotypic intermittent chronic stress (HeICS).

Results

The rat model of chronic visceral hypersensitivity exhibited remarkable abdominal withdrawal responses and mechanical hyperalgesia in hind paws, which were dose-dependently attenuated by single subcutaneous of administration of BAA (30 and 90 μg/kg). Pretreatment with the microglial inhibitor minocycline, dynorphin A antiserum, and κ-opioid receptor antagonist totally blocked BAA-induced visceral antinociception and mechanical antihyperalgesia. Spontaneous excitatory postsynaptic currents (sEPSCs) in spinal dorsal horn lamina II neurons were recorded by using whole-cell patch clamp. Its frequency (but not amplitude) from TNBS-treated rats was remarkably higher than that from naïve rats. BAA (1 μM) significantly reduced the frequency of sEPSCs from TNBS-treated rats but not naïve rats. BAA-inhibited spinal synaptic plasticity was blocked by minocycline, the dynorphin A antiserum, and κ-opioid receptor antagonist. Dynorphin A also inhibited spinal synaptic plasticity in a κ-opioid receptor-dependent manner.

Conclusions

These results suggest that BAA produces visceral antinociception by stimulating spinal microglial release of dynorphin A, which activates presynaptic κ-opioid receptors in afferent neurons and inhibits spinal synaptic plasticity, highlighting a novel interaction mode between microglia and neurons.

Bulleyaconitine A inhibits the lung inflammation and airway remodeling through restoring Th1/Th2 balance in asthmatic model mice

The current study aimed to study the effects of Bulleyaconitine A (BLA) on asthma. Asthmatic mice model was established by ovalbumin (OVA) stimulation, and the model mice were treated by BLA. After BLA treatment, the changes in lung and airway resistances, total and differential leukocytes in the bronchoalveolar lavage fluid (BALF) were detected, and the changes in lung inflammation and airway remodeling were observed. Moreover, the secretion of IgE, Th1/Th2-type and IL-17A cytokines in BALF and serum of the asthmatic mice were determined. The resuts showed that BLA attenuated OVA-induced lung and airway resistances, inhibited the inflammatory cell recruitment in BALF and the inflammation and airway remodeling of the asthmatic mice. In addition, BLA suppressed the secretion of IgE, Th2-type cytokines, and IL-17A, but enhanced secretions of Th1-type cytokines in BALF and serum. The current study discovered that BLA inhibited the lung inflammation and airway remodeling via restoring the Th1/Th2 balance in asthmatic mice.

Bulleyaconitine A Exerts Antianxiety and Antivisceral Hypersensitivity Effects

Visceral pain is one of the leading causes for abdominal pain in gastroenterological diseases and is still hard to treat effectively. Bulleyaconitine A (BAA) is an aconitine analog and has been used for the treatment of pain. Our previous work suggested that BAA exerted analgesic effects on neuropathic pain through stimulating the expression of dynorphin A in spinal microglia. Here, we investigated the inhibitory effect of BAA on visceral pain and examined whether the expression of dynorphin A in spinal microglia was responsible for its effects. We found that BAA produced significant antivisceral pain effect induced by acetic acid through stimulating dynorphin A expression in spinal microglia. In addition, anxiety and chronic visceral pain are highly prevalent comorbid conditions in clinical research, which is still a problem to be solved. We also aimed to evaluate the effects of BAA on anxiety. A comorbidity model with characteristics of both chronic visceral pain and anxiety was developed by colorectal injection of 2,4,6-trinitrobenzene sulfonic acid and the induction of heterotypic intermittent chronic stress protocol. In comorbid animals, BAA exerted great antianxiety effects. Meanwhile, the antianxiety mechanism of BAA was different with the antivisceral pain mechanism of BAA. In conclusion, our study demonstrated, for the first time, that BAA exerted marked antivisceral pain and antianxiety effects, which expands the analgesic spectrum and clinical application of BAA. Furthermore, it also it provides a better guidance for the clinical use of BAA.

Opioid Receptors in Immune and Glial Cells-Implications for Pain Control

Opioid receptors comprise μ (MOP), δ (DOP), κ (KOP), and nociceptin/orphanin FQ (NOP) receptors. Opioids are agonists of MOP, DOP, and KOP receptors, whereas nociceptin/orphanin FQ (N/OFQ) is an agonist of NOP receptors. Activation of all four opioid receptors in neurons can induce analgesia in animal models, but the most clinically relevant are MOP receptor agonists (e.g., morphine, fentanyl). Opioids can also affect the function of immune cells, and their actions in relation to immunosuppression and infections have been widely discussed. Here, we analyze the expression and the role of opioid receptors in peripheral immune cells and glia in the modulation of pain. All four opioid receptors have been identified at the mRNA and protein levels in immune cells (lymphocytes, granulocytes, monocytes, macrophages) in humans, rhesus monkeys, rats or mice. Activation of leukocyte MOP, DOP, and KOP receptors was recently reported to attenuate pain after nerve injury in mice. This involved intracellular Ca²⁺-regulated release of opioid peptides from immune cells, which subsequently activated MOP, DOP, and KOP receptors on peripheral neurons. There is no evidence of pain modulation by leukocyte NOP receptors. More good quality studies are needed to verify the presence of DOP, KOP, and NOP receptors in native glia. Although still questioned, MOP receptors might be expressed in brain or spinal cord microglia and astrocytes in humans, mice, and rats. Morphine acting at spinal cord microglia is often reported to induce hyperalgesia in rodents. However, most studies used animals without pathological pain and/or unconventional paradigms (e.g., high or ultra-low doses, pain assessment after abrupt discontinuation of chronic morphine treatment). Therefore, the opioid-induced hyperalgesia can be viewed in the context of dependence/withdrawal rather than pain management, in line with clinical reports. There is convincing evidence of analgesic effects mediated by immune cell-derived opioid peptides in animal models and in humans. Together, MOP, DOP, and KOP receptors, and opioid peptides in immune cells can ameliorate pathological pain. The relevance of NOP receptors and N/OFQ in leukocytes, and of all opioid receptors, opioid peptides and N/OFQ in native glia for pain control is yet to be clarified.

Isotalatizidine, a C19-diterpenoid alkaloid, attenuates chronic neuropathic pain through stimulating ERK/CREB signaling pathway-mediated microglial dynorphin A expression

Background

Isotalatizidine is a representative C₁₉-diterpenoid alkaloid extracted from the lateral roots of Aconitum carmichaelii, which has been widely used to treat various diseases on account of its analgesic, anti-inflammatory, anti-rheumatic, and immunosuppressive properties. The aim of this study was to evaluate the analgesic effect of isotalatizidine and its underlying mechanisms against neuropathic pain.

Methods

A chronic constrictive injury (CCI)-induced model of neuropathic pain was established in mice, and the limb withdrawal was evaluated by the Von Frey filament test following isotalatizidine or placebo administration. The signaling pathways in primary or immortalized microglia cells treated with isotalatizidine were analyzed by Western blotting and immunofluorescence.

Results

Intrathecal injection of isotalatizidine attenuated the CCI-induced mechanical allodynia in a dose-dependent manner. At the molecular level, isotalatizidine selectively increased the phosphorylation of p38 and ERK1/2, in addition to activating the transcription factor CREB and increasing dynorphin A production in cultured primary microglia. However, the downstream effects of isotalatizidine were abrogated by the selective ERK1/2 inhibitor U0126-EtOH or CREB inhibitor of KG-501, but not by the p38 inhibitor SB203580. The results also were confirmed in in vivo experiments.

Conclusion

Taken together, isotalatizidine specifically activates the ERK1/2 pathway and subsequently CREB, which triggers dynorphin A release in the microglia, eventually leading to its anti-nociceptive action.

Acoapetaludines A-K, C20 and C19-diterpenoid alkaloids from the whole plants of Aconitum apetalum (Huth) B.Fedtsch

A C₂₀-diterpenoid alkaloid with an unprecedented carbon skeleton, acoapetaludine A, together with ten undescribed aconitine-type C₁₉-diterpenoid alkaloids, acoapetaludines B-K, were isolated from the whole plants of Aconitum apetalum (Huth) B. Fedtsch. (Ranunculaceae). The structures were elucidated based on a comprehensive spectroscopic data analysis. The absolute configuration of acoapetaludine A was determined by quantum ECD calculation. Acoapetaludines D and E exhibited weak anti-Helicobacter pylori activity at a minimum inhibitory concentration (MIC) of 100 and 50 μg/mL, respectively.

Activation of GPR40 produces mechanical antiallodynia via the spinal glial interleukin-10/β-endorphin pathway

Background

The G protein-coupled receptor 40 (GPR40), broadly expressed in various tissues such as the spinal cord, exerts multiple physiological functions including pain regulation. This study aimed to elucidate the mechanisms underlying GPR40 activation-induced antinociception in neuropathic pain, particularly related to the spinal glial expression of IL-10 and subsequent β-endorphin.

Methods

Spinal nerve ligation-induced neuropathic pain model was used in this study. β-Endorphin and IL-10 levels were measured in the spinal cord and cultured primary microglia, astrocytes, and neurons. Double immunofluorescence staining of β-endorphin with glial and neuronal cellular biomarkers was also detected in the spinal cord and cultured primary microglia, astrocytes, and neurons.

Results

GPR40 was expressed on microglia, astrocytes, and neurons in the spinal cords and upregulated by spinal nerve ligation. Intrathecal injection of the GPR40 agonist GW9508 dose-dependently attenuated mechanical allodynia and thermal hyperalgesia in neuropathic rats, with E_max values of 80% and 100% MPE and ED₅₀ values of 6.7 and 5.4 μg, respectively. Its mechanical antiallodynia was blocked by the selective GPR40 antagonist GW1100 but not GPR120 antagonist AH7614. Intrathecal GW9508 significantly enhanced IL-10 and β-endorphin immunostaining in spinal microglia and astrocytes but not in neurons. GW9508 also markedly stimulated gene and protein expression of IL-10 and β-endorphin in cultured primary spinal microglia and astrocytes but not in neurons, originated from 1-day-old neonatal rats. The IL-10 antibody inhibited GW9508-stimulated gene expression of the β-endorphin precursor proopiomelanocortin (POMC) but not IL-10, whereas the β-endorphin antibody did not affect GW9508-stimulated IL-10 or POMC gene expression. GW9508 increased phosphorylation of mitogen-activated protein kinases (MAPKs) including p38, extracellular signal-regulated kinase (ERK), and c-Jun N-terminal kinase (JNK), and its stimulatory effects on IL-10 and POMC expression were blocked by each MAPK isoform inhibitor. Spinal GW9508-induced mechanical antiallodynia was completely blocked by intrathecal minocycline, IL-10 neutralizing antibody, β-endorphin antiserum, and μ-opioid receptor-preferred antagonist naloxone.

Conclusions

Our results illustrate that GPR40 activation produces antinociception via the spinal glial IL-10/β-endorphin antinociceptive pathway.

Electronic supplementary material

The online version of this article (10.1186/s12974-019-1457-9) contains supplementary material, which is available to authorized users.

Bulleyaconitine A Effectively Relieves Allergic Lung Inflammation in a Murine Asthmatic Model

Background

Bulleyaconitine A (BLA) has been widely used as analgesic against chronic inflammatory pain in China. However, its potential therapeutic role in asthma remains unclear. The purpose of this study was to investigate the effect of BLA on airway inflammation in mice with allergic asthma.

Material/Methods

Specific-pathogen-free (SPF) female Balb/c mice were randomly divided into the following 6 groups: (1) Control group (NC), (2) Asthma group (AS), (3) BLA-L group, (4) BLA-M group, (5) BLA-H group, and (6) Dexamethasone group. An asthma mouse model was established by administration of ovalbumin (OVA) and mice were sacrificed within 24 h after the last challenge. Enzyme-linked immunosorbent assay (ELISA) method was used to determine the relative expression levels of IgE and IgG in mouse serum. In addition, bronchoalveolar lavage fluid (BALF) was collected and IL-4, TNF-α, and MCP-1 levels were determined by ELISA. Furthermore, eosinophils, lymphocytes, and macrophages in BALF were classified and analyzed, and inflammatory cell infiltration in the airways of mice was determined by hematoxylin-eosin (HE) staining. The expression of NF-κB1 and PKC-δ in mouse lung tissue was determined by Western blot analysis.

Results

The levels of serum IgE and IgG in BLA- or Dex- treated mice were significantly reduced compared to those in the asthma (AS) group (P<0.01), whereas the levels of cytokines IL-4, TNF-α, and MCP-1 were significantly decreased (P<0.01). HE-staining showed that BLA significantly reduced inflammatory cell infiltration and mucus secretion in lung tissue. Moreover, BLA inhibited the expression of NF-κB1 and PKC-δ via the NF-κB signaling pathway in the lung.

Conclusions

Our data show that BLA activates PKC-δ/NF-κB to reduce airway inflammation in allergic asthma mice.

Gelsemine and koumine, principal active ingredients of Gelsemium, exhibit mechanical antiallodynia via spinal glycine receptor activation-induced allopregnanolone biosynthesis

Gelsemine, the principal active alkaloid from Gelsemium sempervirens Ait., and koumine, the most dominant alkaloids from Gelsemium elegans Benth., produced antinociception in a variety of rodent models of painful hypersensitivity. The present study explored the molecular mechanisms underlying gelsemine- and koumine-induced mechanical antiallodynia in neuropathic pain. The radioligand binding and displacement assays indicated that gelsemine and koumine, like glycine, were reversible and orthosteric agonists of glycine receptors with full efficacy and probably acted on same binding site as the glycine receptor antagonist strychnine. Treatment with gelsemine, koumine and glycine in primary cultures of spinal neurons (but not microglia or astrocytes) concentration dependently increased 3α-hydroxysteroid oxidoreductase (3α-HSOR) mRNA expression, which was inhibited by pretreatment with strychnine but not the glial inhibitor minocycline. Intrathecal injection of gelsemine, koumine and glycine stimulated 3α-HSOR mRNA expression in the spinal cords of neuropathic rats and produced mechanical antiallodynia. Their spinal mechanical antiallodynia was completely blocked by strychnine, the selective 3α-HSOR inhibitor medroxyprogesterone acetate (MPA), 3α-HSOR gene silencer siRNA/3α-HSOR and specific GABA_A receptor antagonist isoallopregnanolone, but not minocycline. All the results taken together uncovered that gelsemine and koumine are orthosteric agonists of glycine receptors, and produce mechanical antiallodynia through neuronal glycine receptor/3α-HSOR/allopregnanolone/GABA_A receptor pathway.

Mechanisms for therapeutic effect of bulleyaconitine A on chronic pain

Bulleyaconitine A, a diterpenoid alkaloid isolated from Aconitum bulleyanum plants, has been used for the treatment of chronic pain in China since 1985. Clinical studies show that the oral administration of bulleyaconitine A is effective for treating different kinds of chronic pain, including back pain, joint pain, and neuropathic pain with minimal side effect in human patients. The experimental studies have revealed that bulleyaconitine A at therapeutic doses potently inhibits the peripheral sensitization and central sensitization that underlie chronic pain and has no effect on acute pain. Bulleyaconitine A preferably blocks tetrodotoxin-sensitive voltage-gated sodium channels in dorsal root ganglion neurons by inhibition of protein kinase C, and the effect is around 600 times more potent in neuropathic animals than in naïve ones. Bulleyaconitine A at 5 nM inhibits the hypersensitivity of dorsal root ganglion neurons in neuropathic rats but has no effect on excitability of dorsal root ganglion neurons in sham group. Bulleyaconitine A inhibits long-term potentiation at C-fiber synapses in spinal dorsal horn, a synaptic model of pathological pain, preferably in neuropathic pain rats over naïve rats. The following mechanisms may underlie the selective effect of bulleyaconitine A on chronic pain. (1) In neuropathic conditions, protein kinase C and voltage-gated sodium channels in dorsal root ganglion neurons are upregulated, which enhances bulleyaconitine A's effect. (2) Bulleyaconitine A use-dependently blocks voltage-gated sodium channels and therefore inhibits the ectopic discharges that are important for neuropathic pain. (3) Bulleyaconitine A is shown to inhibit neuropathic pain by the modulation of spinal microglia, which are involved in the chronic pain but not in acute (nociceptive) pain. Moreover, bulleyaconitine A facilitates the anesthetic effect of morphine and inhibits morphine tolerance in rats. Together, bulleyaconitine A is able to inhibit chronic pain by targeting at multiple molecules. Further clinical and experimental studies are needed for evaluating the efficacy of bulleyaconitine A in different forms of chronic pain in patients and for exploring the underlying mechanisms.

Bulleyaconitine A attenuates hyperexcitability of dorsal root ganglion neurons induced by spared nerve injury: The role of preferably blocking Nav1.7 and Nav1.3 channels

Background

Oral administration of Bulleyaconitine A, an extracted diterpenoid alkaloid from Aconitum bulleyanum plants, is effective for treating chronic pain in rats and in human patients, but the underlying mechanisms are poorly understood.

Results

As the hyperexcitability of dorsal root ganglion neurons resulting from the upregulation of voltage-gated sodium (Nav) channels has been proved critical for development of chronic pain, we tested the effects of Bulleyaconitine A on Nav channels in rat spared nerve injury model of neuropathic pain. We found that Bulleyaconitine A at 5 nM increased the threshold of action potentials and reduced the firing rate of dorsal root ganglion neurons in spared nerve injury rats but not in sham rats. Bulleyaconitine A preferably blocked tetrodotoxin-sensitive Nav channels over tetrodotoxin-resistant ones in dorsal root ganglion neurons of spared nerve injury rats. Bulleyaconitine A was more potent for blocking Nav1.3 and Nav1.7 than Nav1.8 in cell lines. The half maximal inhibitory concentration (IC₅₀) values for resting Nav1.3, Nav1.7, and Nav1.8 were 995.6 ± 139.1 nM, 125.7 ± 18.6 nM, and 151.2 ± 15.4 μM, respectively, which were much higher than those for inactivated Nav1.3 (20.3 ± 3.4 pM), Nav1.7 (132.9 ± 25.5 pM), and Nav1.8 (18.0 ± 2.5 μM). The most profound use-dependent blocking effect of Bulleyaconitine A was observed on Nav1.7, less on Nav1.3, and least on Nav1.8 at IC₅₀ concentrations. Bulleyaconitine A facilitated the inactivation of Nav channels in each subtype.

Conclusions

Preferably blocking tetrodotoxin-sensitive Nav1.7 and Nav1.3 in dorsal root ganglion neurons may contribute to Bulleyaconitine A’s antineuropathic pain effect.

Spinal interleukin-10 produces antinociception in neuropathy through microglial β-endorphin expression, separated from antineuroinflammation

Interleukin 10 (IL-10) is antinociceptive in various animal models of pain without induction of tolerance, and its mechanism of action was generally believed to be mediated by inhibition of neuroinflammation. Here we reported that intrathecal IL-10 injection dose dependently attenuated mechanical allodynia and thermal hyperalgesiain male and female neuropathic rats, with ED₅₀ values of 40.8 ng and 24 ng, and E_max values of 61.5% MPE and 100% MPE in male rats. Treatment with IL-10 specifically increased expression of the β-endorphin (but not prodynorphin) gene and protein in primary cultures of spinal microglia but not in astrocytes or neurons. Intrathecal injection of IL-10 stimulated β-endorphin expression from microglia but not neurons or astrocytes in both contralateral and ipsilateral spinal cords of neuropathic rats. However, intrathecal injection of the β-endorphin neutralizing antibody, opioid receptor antagonist naloxone, or μ-opioid receptor antagonist CTAP completely blocked spinal IL-10-induced mechanical antiallodynia, while the microglial inhibitor minocycline and specific microglia depletor reversed spinal IL-10-induced β-endorphin overexpression and mechanical antiallodynia. IL-10 treatment increased spinal microglial STAT3 phosphorylation, and the STAT3 inhibitor NSC74859 completely reversed IL-10-increased spinal expression of β-endorphin and neuroinflammatory cytokines and mechanical antiallodynia. Silence of the Bcl3 and Socs3 genes nearly fully reversed IL-10-induced suppression of neuroinflammatory cytokines (but not expression of β-endorphin), although it had no effect on mechanical allodynia. In contrast, disruption of the POMC gene completely blocked IL-10-stimulated β-endorphin expression and mechanical antiallodynia, but had no effect on IL-10 inhibited expression of neuroinflammatory cytokines. Thus this study revealed that IL-10 produced antinociception through spinal microglial β-endorphin expression, but not inhibition of neuroinflammation.

Lappaconitine, a C18-diterpenoid alkaloid, exhibits antihypersensitivity in chronic pain through stimulation of spinal dynorphin A expression

Lappaconitine is a representative C18-diterpenoid alkaloid extracted from Aconitum sinomontanum Nakai and has been prescribed as a pain relief medicine in China for more than 30 years. This study evaluated its antihypersensitivity activity in the rat models of neuropathic and cancer pains and explored its underlying mechanisms. Subcutaneous injection of cumulative doses of lappaconitine produced dose-dependent mechanical antiallodynia and thermal antihyperalgesia in spinal nerve ligation-induced neuropathic rats. The cumulative dose-response analysis exhibited their E_max values of 53.3 and 58.3% MPE, and ED₅₀ values of 1.1 and 1.6 mg/kg. Single intrathecal lappaconitine dose in neuropathy also dose- and time-dependently blocked mechanical allodynia, with an E_max of 66.1% MPE and an ED₅₀ of 0.8 μg. Its multiple twice-daily intrathecal administration over 7 days did not induce mechanical antiallodynic tolerance. Subcutaneous cumulative doses of lappaconitine also produced dose-dependent blockade of mechanical allodynia in the rat bone cancer pain model induced by tibia implantation of cancer cells, with the E_max of 57.9% MPE and ED₅₀ of 2.0 mg/kg. Furthermore, lappaconitine treatment stimulated spinal dynorphin A expression in neuropathic rats, and in primary cultures of microglia but not neurons or astrocytes. Intrathecal pretreatment with the specific microglia depletor liposome-encapsulated clodronate, dynorphin A antibody, and κ-opioid receptor antagonist GNTI totally suppressed intrathecal and subcutaneous lappaconitine-induced mechanical antiallodynia. This study suggests that lappaconitine exhibits antinociception through directly stimulating spinal microglial dynorphin A expression. Graphical Abstract ᅟ.

Liposome-encapsulated clodronate specifically depletes spinal microglia and reduces initial neuropathic pain

Liposome-encapsulated clodronate (LEC) is a specific depletor of macrophages. Our study characterized the LEC depletory effects, given intrathecally, on spinal microglia and assessed its effects on initiation and maintenance of neuropathic pain. Measured by using the MTT assay, LEC treatment specifically inhibited cell viability of cultured primary microglia, but not astrocytes or neurons, from neonatal rats, with an IC₅₀ of 43 μg/mL. In spinal nerve ligation-induced neuropathic rats, pretreatment (1 day but not 5 days earlier) with intrathecal LEC specifically depleted microglia (but not astrocytes or neurons) in both contralateral and ipsilateral dorsal horns by the same degree (63% vs. 71%). Intrathecal injection of LEC reversibly blocked the antinociceptive effects of the GLP-1 receptor agonist exenatide and dynorphin A stimulator bulleyaconitine, which have been claimed to be mediated by spinal microglia, whereas it failed to alter morphine- or the glycine receptor agonist gelsemine-induced mechanical antiallodynia which was mediated via the neuronal mechanisms. Furthermore, intrathecal LEC injection significantly attenuated initial (one day after nerve injury) but not existing (2 weeks after nerve injury) mechanical allodynia. Our study demonstrated that LEC, given intrathecally, is a specific spinal microglial inhibitor and significantly reduces initiation but not maintenance of neuropathic pain, highlighting an opposite role of spinal microglia in different stages of neuropathic pain.

Endogenous Opiates and Behavior: 2016

This paper is the thirty-ninth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2016 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia, stress and social status, tolerance and dependence, learning and memory, eating and drinking, drug abuse and alcohol, sexual activity and hormones, pregnancy, development and endocrinology, mental illness and mood, seizures and neurologic disorders, electrical-related activity and neurophysiology, general activity and locomotion, gastrointestinal, renal and hepatic functions, cardiovascular responses, respiration and thermoregulation, and immunological responses.

Optimization and In Vivo Profiling of a Refined Rat Model of Walker 256 Breast Cancer Cell-Induced Bone Pain Using Behavioral, Radiological, Histological, Immunohistochemical and Pharmacological Methods

In the majority of patients with advanced breast cancer, there is metastatic spread to bones resulting in pain. Clinically available drug treatments for alleviation of breast cancer-induced bone pain (BCIBP) often produce inadequate pain relief due to dose-limiting side-effects. A major impediment to the discovery of novel well-tolerated analgesic agents for the relief of pain due to bony metastases is the fact that most cancer-induced bone pain models in rodents relied on the systemic injection of cancer cells, causing widespread formation of cancer metastases and poor general animal health. Herein, we have established an optimized, clinically relevant Wistar Han female rat model of breast cancer induced bone pain which was characterized using behavioral assessments, radiology, histology, immunohistochemistry and pharmacological methods. In this model that is based on unilateral intra-tibial injection (ITI) of Walker 256 carcinoma cells, animals maintained good health for at least 66 days post-ITI. The temporal development of hindpaw hypersensitivity depended on the initial number of Walker 256 cells inoculated in the tibiae. Hindpaw hypersensitivity resolved after approximately 25 days, in the continued presence of bone tumors as evidenced by ex vivo histology, micro-computed tomography scans and immunohistochemical assessments of tibiae. A possible role for the endogenous opioid system as an internal factor mediating the self-resolving nature of BCIBP was identified based upon the observation that naloxone, a non-selective opioid antagonist, caused the re-emergence of hindpaw hypersensitivity. Bolus dose injections of morphine, gabapentin, amitriptyline and meloxicam all alleviated hindpaw hypersensitivity in a dose-dependent manner. This is a first systematic pharmacological profiling of this model by testing standard analgesic drugs from four important diverse classes, which are used to treat cancer induced bone pain in the clinical setting. Our refined rat model more closely mimics the pathophysiology of this condition in humans and hence is well-suited for probing the mechanisms underpinning breast cancer induced bone pain. In addition, the model may be suitable for efficacy profiling of new molecules from drug discovery programs with potential to be developed as novel agents for alleviation of intractable pain associated with disseminated breast cancer induced bony metastases.

Cynandione A attenuates neuropathic pain through p38β MAPK-mediated spinal microglial expression of β-endorphin

Cynanchi Wilfordii Radix (baishouwu), a medicinal herb, has been widely used in Asia to treat a variety of diseases or illnesses. Cynandione A isolated from C. Wilfordii is the principle acetophenone and exhibits neuroprotective and anti-inflammatory activities. This study aims to evaluate the antihypersensitivity activities of cynandione A in neuropathy and explored its mechanisms of action. Intrathecal injection of cynandione A dose-dependently attenuated spinal nerve ligation-induced mechanical allodynia and thermal hyperalgesia, with maximal possible effects of 57% and 59%, ED₅₀s of 14.9μg and 6.5μg, respectively. Intrathecal injection of cynandione A significantly increased β-endorphin levels in spinal cords of neuropathic rats and its treatment concentration-dependently induced β-endorphin expression in cultured primary microglia (but not in neurons or astrocytes), with EC₅₀s of 38.8 and 20.0μM, respectively. Cynandione A also non-selectively upregulated phosphorylation of mitogen-activated protein kinases (MAPKs), including p38, extracellular signal regulated kinase (ERK1/2), and extracellular signal regulated kinase (JNK) in primary microglial culture; however, cynandione A-stimulated β-endorphin expression was completely inhibited by the specific p38 activation inhibitor SB203580, but not by the ERK1/2 or JNK activation inhibitors. Knockdown of spinal p38β but not p38α using siRNA also completely blocked cynandione A-induced β-endorphin expression in cultured microglial cells. Furthermore, cynandione A-induced antiallodynia in neuropathy was totally inhibited by the microglial inhibitor minocycline, SB203580, anti-β-endorphin antibody, and μ-opioid receptor antagonist CTAP (but not the κ- or δ-opioid receptor antagonist). These results suggest that cynandione A attenuates neuropathic pain through upregulation of spinal microglial expression β-endorphin via p38β MAPK activation.

Molecular signaling underlying bulleyaconitine A (BAA)-induced microglial expression of prodynorphin

Bulleyaconitine (BAA) has been shown to possess antinociceptive activities by stimulation of dynorphin A release from spinal microglia. This study investigated its underlying signal transduction mechanisms. The data showed that (1) BAA treatment induced phosphorylation of CREB (rather than NF-κB) and prodynorphin expression in cultured primary microglia, and antiallodynia in neuropathy, which were totally inhibited by the CREB inhibitor KG-501; (2) BAA upregulated phosphorylation of p38 (but not ERK or JNK), and the p38 inhibitor SB203580 (but not ERK or JNK inhibitor) and p38β gene silencer siRNA/p38β (but not siRNA/p38α) completely blocked BAA-induced p38 phosphorylation and/or prodynorphin expression, and antiallodynia; (3) BAA stimulated cAMP production and PKA phosphorylation, and the adenylate cyclase inhibitor DDA and PKA inhibitor H-89 entirely antagonized BAA-induced prodynorphin expression and antiallodynia; (4) The Gs-protein inhibitor NF449 completely inhibited BAA-increased cAMP level, prodynorphin expression and antiallodynia, whereas the antagonists of noradrenergic, corticotrophin-releasing factor, A1 adenosine, formyl peptide, D1/D2 dopamine, and glucagon like-peptide-1 receptors failed to block BAA-induced antiallodynia. The data indicate that BAA-induced microglial expression of prodynorphin is mediated by activation of the cAMP-PKA-p38β-CREB signaling pathway, suggesting that its possible target is a Gs-protein-coupled receptor - "aconitine receptor", although the chemical identity is not illustrated.

Concurrent bullatine A enhances morphine antinociception and inhibits morphine antinociceptive tolerance by indirect activation of spinal κ-opioid receptors

ETHNOPHARMACOLOGICAL RELEVANCE

Bullatine A, a C₂₀-diterpenoid alkaloid and one of the major effective ingredients in Aconiti brachypodi Radix (Xue-shang-yi-zhi-hao), can block pain hypersensitivity in a variety of rodent models through expression of spinal microglial dynorphin A.

AIM OF THE STUDY

To assess the interaction between bullatine A and morphine on antinociception in acute nociception and pain hypersensitivity states, with the exogenous synthetic dynorphin A as a comparison MATERIALS AND METHODS: Spinal nerve ligation-induced neuropathic rats and naïve mice were used for assessing the acute and chronic interactions of bullatine A/dynorphin A with morphine.

RESULTS

Single subcutaneous injection of bullatine A or dynorphin A(1-17) did not either alter formalin- and thermally (hot-plate and water immersion tests)-induced acute nociception or potentiate morphine antinociception in naïve mice. In contrast, bullatine A dose-dependently inhibited formalin-induced tonic pain with the efficacy of 54% inhibition and the half-effective dose of 0.9mg/kg. Concurrent bullatine A additively enhanced morphine antinociception. In neuropathic rats, the antinociceptive effects of multiple bidaily intrathecal injections of bullatine A and dynorphin A remained consistent over 13 days, whereas morphine produced progressive and complete tolerance to antinociception, which was completely inhibited by concurrent bullatine A and dynorphin A. A single intrathecal injection of bullatine A and dynorphin A immediately reversed established morphine tolerance in neuropathic rats, although the blockade was a less degree in the thermally induced mouse acute nociceptive tests. The inhibitory effects of bullatine A and dynorphin A on morphine tolerance were immediately and completely attenuated by intrathecal dynorphin A antibody and/or selective κ-opioid receptor antagonist GNTI.

CONCLUSION

These results suggest that bullatine A produces antinociception without induction of tolerance and inhibits morphine antinociceptive tolerance, and provide pharmacological basis for concurrent bullatine A and morphine treatment for chronic pain and morphine analgesic tolerance.

Ester Hydrolysis Differentially Reduces Aconitine-Induced Anti-hypersensitivity and Acute Neurotoxicity: Involvement of Spinal Microglial Dynorphin Expression and Implications for Aconitum Processing

Aconitines, including bulleyaconitine A, probably the most bioactive and abundant alkaloids in Aconitum plant, are a group of diester C19-diterpenoid alkaloids with one acetylester group attached to C8 of the diterpenoid skeleton and one benzoylester group to C14. Hydrolysis of both groups is involved in the processing of Aconitum, a traditional Chinese medicinal approach. We recently demonstrated that bulleyaconitine A produced anti-hypersensitivity, which was mediated by stimulation of spinal microglial dynorphin A expression. This study aimed to elucidate whether the acetylester and benzoylester groups are involved in aconitine-induced dynorphin A expression, anti-hypersensitivity, neurotoxicity in neuropathic rats. Intrathecal administration of aconitine and benzoylaconine (but not aconine) attenuated mechanical allodynia and heat hyperalgesia, with normalized ED₅₀ values of 35 pmol and 3.6 nmol, respectively. Aconitine and benzoylaconine anti-allodynia was completely blocked by the microglial inhibitor, dynorphin A antiserum, and κ-opioid receptor antagonist. Aconitine and benzoylaconine, but not aconine, stimulated dynorphin A expression in cultured primary spinal microglia, with EC₅₀ values of 32 nM and 3 μM, respectively. Intrathecal aconitine, benzoylaconine and aconine induced flaccid paralysis and death, with normalized TD₅₀ values of 0.5 nmol, 0.2 μmol, and 1.6 μmol, respectively. The TD₅₀/ED₅₀ ratios of aconitine and benzolyaconine were 14:1 and 56:1. Our results suggest that both the C8-acetyl and C14-benzoyl groups are essential for aconitine to stimulate spinal microglial dynorphin A expression and subsequent anti-hypersensitivity, which can be separated from neurotoxicity, because both benzoylaconine and aconine differentially produced anti-hypersensitivity and neurotoxicity due to their different stimulatory ability on dynorphin A expression. Our results support the scientific rationale for Aconitum processing, but caution should be taken to avoid overprocessing and excess hydrolysis of benzolyaconine to aconine.

The Walker 256 Breast Cancer Cell- Induced Bone Pain Model in Rats

The majority of patients with terminal breast cancer show signs of bone metastasis, the most common cause of pain in cancer. Clinically available drug treatment options for the relief of cancer-associated bone pain are limited due to either inadequate pain relief and/or dose-limiting side-effects. One of the major hurdles in understanding the mechanism by which breast cancer causes pain after metastasis to the bones is the lack of suitable preclinical models. Until the late twentieth century, all animal models of cancer induced bone pain involved systemic injection of cancer cells into animals, which caused severe deterioration of animal health due to widespread metastasis. In this mini-review we have discussed details of a recently developed and highly efficient preclinical model of breast cancer induced bone pain: Walker 256 cancer cell- induced bone pain in rats. The model involves direct localized injection of cancer cells into a single tibia in rats, which avoids widespread metastasis of cancer cells and hence animals maintain good health throughout the experimental period. This model closely mimics the human pathophysiology of breast cancer induced bone pain and has great potential to aid in the process of drug discovery for treating this intractable pain condition.

Bullatine A stimulates spinal microglial dynorphin A expression to produce anti-hypersensitivity in a variety of rat pain models

Background

Aconiti brachypodi Radix (Xue-shang-yi-zhi-hao) has been prescribed to manage chronic pain, arthritis, and traumatic injuries. Bullatine A, a C₂₀-diterpenoid alkaloid, is one of its principle effective compounds. This study aimed to investigate the anti-hypersensitivity of bullatine A in a variety of rat pain models and explore its mechanisms of action.

Methods

Rat neuropathic pain, inflammatory pain, diabetic neuropathic pain, and bone cancer pain models were used. Dynorphin A and pro-inflammatory cytokines were measured in the spinal cord and cultured primary microglia. Double immunofluorescence staining of dynorphin A and glial and neuronal cellular markers was also measured in the spinal cord.

Results

Subcutaneous and intrathecal injection of bullatine A dose-dependently attenuated spinal nerve ligation-, complete Freud’s adjuvant-, diabetes-, and bone cancer-induced mechanical allodynia and thermal hyperalgesia, with the efficacies of 45–70 % inhibition, and half-effective doses of 0.9–1.9 mg/kg for subcutaneous injection. However, bullatine A was not effective in blocking acute nociceptive response in the normal condition. Bullatine A specifically stimulated dynorphin A expression in microglia in the spinal cord in vivo and cultured primary microglia in vitro; the stimulatory effects were completely inhibited by the microglial inhibitor minocycline. In contrast, bullatine A did not have an inhibitory effect on peripheral nerve injury- or lipopolysaccharide-induced pro-inflammatory cytokine expression. The spinal anti-allodynic effects of bullatine A were entirely blocked by intrathecal injection of minocycline, the specific dynorphin A antiserum, and the selective k-opioid receptor antagonist.

Conclusions

We, for the first time, demonstrate that bullatine A specifically attenuates pain hypersensitivity, regardless of the pain models employed. The results also suggest that stimulation of spinal microglial dynorphin A expression mediates bullatine A anti-nociception in pain hypersensitivity conditions.