Breaking barriers to novel analgesic drug development

Strategies for developing μ opioid receptor agonists with reduced adverse effects

Opioids are currently the most effective and widely used painkillers in the world. Unfortunately, the clinical use of opioid analgesics is limited by serious adverse effects. Many researchers have been working on designing and optimizing structures in search of novel μ opioid receptor(MOR) agonists with improved analgesic activity and reduced incidence of adverse effects. There are many strategies to develop MOR drugs, mainly focusing on new low efficacy agonists (potentially G protein biased agonists), MOR agonists acting on different Gα subtype, targeting opioid receptors in the periphery, acting on multiple opioid receptor, and targeting allosteric sites of opioid receptors, and others. This review summarizes the design methods, clinical applications, and structure-activity relationships of small-molecule agonists for MOR based on these different design strategies, providing ideas for the development of safer novel opioid ligands with therapeutic potential.

Cannabinoid CB2 receptors in primary sensory neurons are implicated in CB2 agonist-mediated suppression of paclitaxel-induced neuropathic nociception and sexually-dimorphic sparing of morphine tolerance

Cannabinoid CB2 agonists show therapeutic efficacy without unwanted CB1-mediated side effects. The G protein-biased CB2 receptor agonist LY2828360 attenuates the maintenance of chemotherapy-induced neuropathic nociception in male mice and blocks development of morphine tolerance in this model. However, the cell types involved in this phenomenon are unknown and whether this therapeutic profile is observed in female mice has never been investigated. We used conditional deletion of CB2 receptors to determine the cell population(s) mediating the anti-allodynic and morphine-sparing effects of CB2 agonists. Anti-allodynic effects of structurally distinct CB2 agonists (LY2828360 and AM1710) were present in paclitaxel-treated CB2f/f mice and in mice lacking CB2 receptors in CX3CR1 expressing microglia/macrophages (CX3CR1CRE/+; CB2f/f), but were absent in mice lacking CB2 receptors in peripheral sensory neurons (AdvillinCRE/+; CB2f/f). The morphine-sparing effect of LY28282360 occurred in a sexually-dimorphic manner, being present in male, but not female, mice. LY2828360 treatment (3 mg/kg per day i.p. x 12 days) blocked the development of morphine tolerance in male CB2f/f and CX3CR1CRE/+; CB2f/f mice with established paclitaxel-induced neuropathy but was absent in male (or female) AdvillinCRE/+; CB2f/f mice. Co-administration of morphine with a low dose of LY2828360 (0.1 mg/kg per day i.p. x 6 days) reversed morphine tolerance in paclitaxel-treated male CB2f/f mice, but not AdvillinCRE/+; CB2f/f mice of either sex. LY2828360 (3 mg/kg per day i.p. x 8 days) delayed, but did not prevent, the development of paclitaxel-induced mechanical or cold allodynia in either CB2f/f or CX3CR1CRE/+; CB2f/f mice of either sex. Our findings have potential clinical implications.

MP-13, a novel chimeric peptide of morphiceptin and pepcan-9, produces potent antinociception with limited side effects

Pharmacological investigations have substantiated the potential of bifunctional opioid/cannabinoid agonists in delivering potent analgesia while minimizing adverse reactions. Peptide modulators of cannabinoid receptors, known as pepcans, have been investigated before. In this study, we designed a series of chimeric peptides based on pepcans and morphiceptin (YPFP-NH2). Here, we combined injections of pepcans and morphiceptin to investigate the combination treatment of opioids and cannabis and compared the analgesic effect with chimeric compounds. Subsequently, we employed computational docking to screen the compounds against opioid and cannabinoid receptors, along with an acute pain model, to identify the most promising peptide. Among these peptides, MP-13, a morphiceptin and pepcan-9 (PVNFKLLSH) construct, exhibited superior supraspinal analgesic efficacy in the tail-flick test, with an ED50 value at 1.43 nmol/mouse, outperforming its parent peptides and other chimeric analogs. Additionally, MP-13 displayed potent analgesic activity mediated by mu-opioid receptor (MOR), delta-opioid receptor (DOR), and cannabinoid type 1 (CB1) receptor pathways. Furthermore, MP-13 did not induce psychological dependence and gastrointestinal motility inhibition at the effective analgesic doses, and it maintained non-tolerance-forming antinociception throughout a 7-day treatment regimen, with an unaltered count of microglial cells in the periaqueductal gray region, supporting this observation. Moreover, intracerebroventricular administration of MP-13 demonstrated dose-dependent antinociception in murine models of neuropathic, inflammatory, and visceral pain. Our findings provide promising insights for the development of opioid/cannabinoid peptide agonists, addressing a crucial gap in the field and holding significant potential for future research and development. PERSPECTIVE: This article offers insights into the combination treatment of pepcans with morphiceptin. Among the chimeric peptides, MP-13 exhibited potent analgesic effects in a series of preclinical pain models with a favorable side-effect profile.

A historical perspective and recent advances on the evolution of the relationship between acute and chronic pain and cardiovascular disease

The relationship between acute pain and the cardiovascular system was recognized approximately 50 years ago following the initial observation, along with several subsequent experimental studies, that hypertension can result in decreases in the perception of pain. These studies provided a strong impetus to study potential mechanisms to clarify commonalities between the regulatory pathways associated with pain and the cardiovascular system. Attention subsequently shifted to an emphasis on the impact of chronic pain on cardiovascular diseases and mortality with several large meta-analyses of longitudinal studies providing clear evidence that chronic widespread pain increases the risk for developing cardiovascular disease and is associated with excess morbidity and mortality. Cardiovascular associated mortality from myocardial infarction and stroke appears to be directly related to the duration and severity of chronic pain, a result often characterized as a 'dose-response' relationship. The availability and reproducibility of extensive large-scale observational and retrospective studies have emphasized the critical need for more research, including prospective studies, along with the need for the development of preclinical animal models, to better understand the relationship(s) and underlying mechanisms between chronic pain, associated comorbidities, and cardiovascular disease. Elucidation and a deeper understanding of these relationships, including a focus on the link between chronic pain, cardiovascular disease, and depression, could provide valuable information to guide the development of potential treatment interventions to aid in attenuating pain while preventing pain-associated cardiovascular disease, comorbidities, and mortality.

Strategies for Measuring Non-Evoked Pain in Preclinical Models of Neuropathic Pain: Systematic Review

The development of new analgesics for neuropathic pain treatment is crucial. The failure of promising drugs in clinical trials may be related to the over-reliance on reflex-based responses (evoked pain) in preclinical drug testing, which may not fully represent clinical neuropathic pain, characterized by spontaneous non-evoked pain (NEP). Hence, strategies for assessing NEP in preclinical studies emerged. This systematic review identified 443 articles evaluating NEP in neuropathic pain models (mainly traumatic nerve injuries in male rodents). An exponential growth in NEP evaluation was observed, which was assessed using 48 different tests classified in 12 NEP-related outcomes: anxiety, exploration/locomotion, paw lifting, depression, conditioned place preference, gait, autotomy, wellbeing, facial grooming, cognitive impairment, facial pain expressions and vocalizations. Although most of these outcomes showed clear limitations, our analysis suggests that conditioning-associated outcomes, pain-related comorbidities, and gait evaluation may be the most effective strategies. Moreover, a minimal part of the studies evaluated standard analgesics. The greater emphasis on evaluating NEP aligning with clinical pain symptoms may enhance analgesic drug development, improving clinical translation.

An orally available Cav2.2 calcium channel inhibitor for the treatment of neuropathic pain

BACKGROUND AND PURPOSE

Neuropathic pain affects up to 10% of the global population and is caused by an injury or a disease affecting the somatosensory, peripheral, or central nervous system. NP is characterized by chronic, severe and opioid-resistant properties. Therefore, its clinical management remains very challenging. The N-type voltage-gated calcium channel, Cav2.2, is a validated target for therapeutic intervention in chronic and neuropathic pain. The conotoxin ziconotide (Prialt®) is an FDA-approved drug that blocks Cav2.2 channel but needs to be administered intrathecally. Thus, although being principally efficient, the required application route is very much in disfavour.

EXPERIMENTAL APPROACH AND KEY RESULTS

Here, we describe an orally available drug candidate, RD2, which competes with ziconotide binding to Cav2.2 at nanomolar concentrations and inhibits Cav2.2 almost completely reversible. Other voltage-gated calcium channel subtypes, like Cav1.2 and Cav3.2, were affected by RD2 only at concentrations higher than 10 μM. Data from sciatic inflammatory neuritis rat model demonstrated the in vivo proof of concept, as low-dose RD2 (5 mg·kg-1) administered orally alleviated neuropathic pain compared with vehicle controls. High-dose RD2 (50 mg·kg-1) was necessary to reduce pain sensation in acute thermal response assessed by the tail flick test.

CONCLUSIONS AND IMPLICATIONS

Taken together, these results demonstrate that RD2 has antiallodynic properties. RD2 is orally available, which is the most convenient application form for patients and caregivers. The surprising and novel result from standard receptor screens opens the room for further optimization into new promising drug candidates, which address an unmet medical need.

Using automated patch clamp electrophysiology platforms in ion channel drug discovery: an industry perspective

INTRODUCTION

Automated patch clamp (APC) is now well established as a mature technology for ion channel drug discovery in academia, biotech and pharma companies, and in contract research organizations (CRO), for a variety of applications including channelopathy research, compound screening, target validation and cardiac safety testing.

AREAS COVERED

Ion channels are an important class of drugged and approved drug targets. The authors present a review of the current state of ion channel drug discovery along with new and exciting developments in ion channel research involving APC. This includes topics such as native and iPSC-derived cells in ion channel drug discovery, channelopathy research, organellar and biologics in ion channel drug discovery.

EXPERT OPINION

It is our belief that APC will continue to play a critical role in ion channel drug discovery, not only in 'classical' hit screening, target validation and cardiac safety testing, but extending these applications to include high throughput organellar recordings and optogenetics. In this way, with advancements in APC capabilities and applications, together with high resolution cryo-EM structures, ion channel drug discovery will be re-invigorated, leading to a growing list of ion channel ligands in clinical development.

Spontaneous activity in peripheral sensory nerves: a systematic review

ABSTRACT

In the peripheral nervous system, spontaneous activity in sensory neurons is considered to be one of the 2 main drivers of chronic pain states, alongside neuronal sensitization. Despite this, the precise nature and timing of this spontaneous activity in neuropathic pain is not well-established. Here, we have performed a systematic search and data extraction of existing electrophysiological literature to shed light on which fibre types have been shown to maintain spontaneous activity and over what time frame. We examined both in vivo recordings of preclinical models of neuropathic pain, as well as microneurography recordings in humans. Our analyses reveal that there is broad agreement on the presence of spontaneous activity in neuropathic pain conditions, even months after injury or years after onset of neuropathic symptoms in humans. However, because of the highly specialised nature of the electrophysiological methods used to measure spontaneous activity, there is also a high degree of variability and uncertainty around these results. Specifically, there are very few directly controlled experiments, with less directly comparable data between human and animals. Given that spontaneous peripheral neuron activity is considered to be a key mechanistic feature of chronic pain conditions, it may be beneficial to conduct further experiments in this space.

Mimicking opioid analgesia in cortical pain circuits

The anterior cingulate cortex plays a pivotal role in the cognitive and affective aspects of pain perception. Both endogenous and exogenous opioid signaling within the cingulate mitigate cortical nociception, reducing pain unpleasantness. However, the specific functional and molecular identities of cells mediating opioid analgesia in the cingulate remain elusive. Given the complexity of pain as a sensory and emotional experience, and the richness of ethological pain-related behaviors, we developed a standardized, deep-learning platform for deconstructing the behavior dynamics associated with the affective component of pain in mice-LUPE (Light aUtomated Pain Evaluator). LUPE removes human bias in behavior quantification and accelerated analysis from weeks to hours, which we leveraged to discover that morphine altered attentional and motivational pain behaviors akin to affective analgesia in humans. Through activity-dependent genetics and single-nuclei RNA sequencing, we identified specific ensembles of nociceptive cingulate neuron-types expressing mu-opioid receptors. Tuning receptor expression in these cells bidirectionally modulated morphine analgesia. Moreover, we employed a synthetic opioid receptor promoter-driven approach for cell-type specific optical and chemical genetic viral therapies to mimic morphine's pain-relieving effects in the cingulate, without reinforcement. This approach offers a novel strategy for precision pain management by targeting a key nociceptive cortical circuit with on-demand, non-addictive, and effective analgesia.

12/15-Lipoxygenases mediate neuropathic-like pain hypersensitivity in female mice

It is estimated that chronic neuropathic pain conditions exhibit up to 10% prevalence in the general population, with increased incidence in females. However, nonsteroidal inflammatory drugs (NSAIDs) are ineffective, and currently indicated prescription treatments such as opioids, anticonvulsants, and antidepressants provide only limited therapeutic benefit. In the current work, we extended previous studies in male rats utilizing a paradigm of central Toll-like receptor 4 (TLR4)-dependent, NSAID-unresponsive neuropathic-like pain hypersensitivity to male and female C57BL/6N mice, uncovering an unexpected hyperalgesic phenotype in female mice following intrathecal (IT) LPS. In contrast to previous reports in female C57BL/6J mice, female C57BL/6N mice displayed tactile and cold allodynia, grip force deficits, and locomotor hyperactivity in response to IT LPS. Congruent with our previous observations in male rats, systemic inhibition of 12/15-Lipoxygenases (12/15-LOX) in female B6N mice with selective inhibitors - ML355 (targeting 12-LOX-p) and ML351 (targeting 15-LOX-1) - completely reversed allodynia and grip force deficits. We demonstrate here that 12/15-LOX enzymes also are expressed in mouse spinal cord and that 12/15-LOX metabolites produce tactile allodynia when administered spinally (IT) or peripherally (intraplantar in the paw, IPLT) in a hyperalgesic priming model, similar to others observations with the cyclooxygenase (COX) metabolite Prostaglandin E 2 (PGE 2 ). Surprisingly, we did not detect hyperalgesic priming following IT administration of LPS, indicating that this phenomenon likely requires peripheral activation of nociceptors. Collectively, these data suggest that 12/15-LOX enzymes contribute to neuropathic-like pain hypersensitivity in rodents, with potential translatability as druggable targets across sexes and species using multiple reflexive and non-reflexive outcome measures.

Design, Synthesis, and In Vivo Evaluation of a New Series of Indole-Chalcone Hybrids as Analgesic and Anti-Inflammatory Agents

Indole-chalcone hybrids have burst into prominence as potent weapons in the battle against pain and inflammation due to their unique features, allowing these ligands to form pivotal interactions with biological targets. In this context, the base-catalyzed Claisen-Schmidt condensation of 3',4'-(methylenedioxy)acetophenone with heteroaromatic aldehydes carrying an indole scaffold yielded new chalcones (1-7). The central and peripheral antinociceptive activities of all chalcones (compounds 1-7) at the dose of 10 mg/kg (i.p.) were evaluated by hot plate (supraspinal response), tail immersion (spinal response), and acetic acid-induced writhing tests in mice. The anti-inflammatory activities of compounds 1-7 were also investigated by means of a carrageenan-induced mouse paw edema model. The results revealed that compounds 1-7 extended the latency of response to thermal stimulus significantly in a hot-plate test similar to dipyrone (300 mg/kg; i.p.), the positive control drug. However, only compounds 2-7 were found to be significantly effective in the tail-immersion test. Compounds 1-7 also significantly showed analgesic effect by reducing the number of writhes and anti-inflammatory activity by inhibiting edema formation at different time intervals and levels. 1-(1,3-Benzodioxol-5-yl)-3-(1-methyl-1H-indol-2-yl)prop-2-en-1-one (4) drew attention by providing the highest efficacy results in both acute analgesia and inflammation models. Based on the in silico data acquired from the QikProp module, compound 4 was predicted to possess favorable oral bioavailability and drug-like properties. Taken together, it can be concluded that chalcones (1-7), especially compound 4, are outstanding candidates for further research to investigate their potential use in the management of pain and inflammation.

Cannabinoid CB 2 receptors in primary sensory neurons are implicated in CB 2 agonist-mediated suppression of paclitaxel-induced neuropathic nociception and sexually-dimorphic sparing of morphine tolerance

Cannabinoid CB 2 agonists show therapeutic efficacy without the unwanted side effects commonly associated with direct activation of CB 1 receptors. The G protein-biased CB 2 receptor agonist LY2828360 attenuates the maintenance of chemotherapy-induced neuropathic nociception in male mice and blocks the development of morphine tolerance in this model. However, the specific cell types involved in this phenomenon have never been investigated and whether this therapeutic profile is observed in female mice remains poorly understood. We used conditional deletion of CB 2 receptors from specific cell populations to determine the population(s) mediating the anti-allodynic and morphine-sparing effects of CB 2 agonists. Anti-allodynic effects of structurally distinct CB 2 agonists (LY2828360 and AM1710) were present in paclitaxel-treated CB 2 f/f mice of either sex. The anti-allodynic effect of the CB 2 agonists were absent in conditional knockout (KO) mice lacking CB 2 receptors in peripheral sensory neurons (Advillin CRE/+ ; CB 2 f/f ) but preserved in mice lacking CB 2 receptors in CX3CR1 expressing microglia/macrophages (CX3CR1 CRE/+ ; CB 2 f/f ). The morphine-sparing effect of LY28282360 occurred in a sexually-dimorphic manner, being present in male mice but absent in female mice of any genotype. In mice with established paclitaxel-induced neuropathy, prior LY2828360 treatment (3 mg/kg per day i.p. x 12 days) blocked the subsequent development of morphine tolerance in male CB 2 f/f mice but was absent in male (or female) Advillin CRE/+ ; CB 2 f/f mice. LY2828360-induced sparing of morphine tolerance was preserved in male CX3CR1 CRE/+ ; CB 2 f/f mice, but this effect was not observed in female CX3CR1 CRE/+ ; CB 2 f/f mice. Similarly, co-administration of morphine with a low dose of LY2828360 (0.1 mg/kg per day i.p. x 6 days) reversed tolerance to the anti-allodynic efficacy of morphine in paclitaxel-treated male CB 2 f/f mice, but this effect was absent in female CB 2 f/f mice and Advillin CRE/+ ; CB 2 f/f mice of either sex. Additionally, LY2828360 (3 mg/kg per day i.p. x 8 days) delayed, but did not prevent, the development of paclitaxel-induced mechanical and cold allodynia in either CB 2 f/f or CX3CR1 CRE/+ ; CB 2 f/f mice of either sex. Our studies reveal that CB 2 receptors in primary sensory neurons are required for the anti-allodynic effects of CB 2 agonists in a mouse model of paclitaxel-induced neuropathic nociception. We also find that CB 2 agonists acting on primary sensory neurons produce a sexually-dimorphic sparing of morphine tolerance in males, but not female, paclitaxel-treated mice.

Digging deeper into pain: an ethological behavior assay correlating well-being in mice with human pain experience

ABSTRACT

The pressing need for safer, more efficacious analgesics is felt worldwide. Preclinical tests in animal models of painful conditions represent one of the earliest checkpoints novel therapeutics must negotiate before consideration for human use. Traditionally, the pain status of laboratory animals has been inferred from evoked nociceptive assays that measure their responses to noxious stimuli. The disconnect between how pain is tested in laboratory animals and how it is experienced by humans may in part explain the shortcomings of current pain medications and highlights a need for refinement. Here, we survey human patients with chronic pain who assert that everyday aspects of life, such as cleaning and leaving the house, are affected by their ongoing level of pain. Accordingly, we test the impact of painful conditions on an ethological behavior of mice, digging. Stable digging behavior was observed over time in naive mice of both sexes. By contrast, deficits in digging were seen after acute knee inflammation. The analgesia conferred by meloxicam and gabapentin was compared in the monosodium iodoacetate knee osteoarthritis model, with meloxicam more effectively ameliorating digging deficits, in line with human patients finding meloxicam more effective. Finally, in a visceral pain model, the decrease in digging behavior correlated with the extent of disease. Ultimately, we make a case for adopting ethological assays, such as digging, in studies of pain in laboratory animals, which we believe to be more representative of the human experience of pain and thus valuable in assessing clinical potential of novel analgesics in animals.

Spatial transcriptomics and single-nucleus RNA sequencing reveal a transcriptomic atlas of adult human spinal cord

Despite the recognized importance of the spinal cord in sensory processing, motor behaviors, and neural diseases, the underlying organization of neuronal clusters and their spatial location remain elusive. Recently, several studies have attempted to define the neuronal types and functional heterogeneity in the spinal cord using single-cell or single-nucleus RNA sequencing in animal models or developing humans. However, molecular evidence of cellular heterogeneity in the adult human spinal cord is limited. Here, we classified spinal cord neurons into 21 subclusters and determined their distribution from nine human donors using single-nucleus RNA sequencing and spatial transcriptomics. Moreover, we compared the human findings with previously published single-nucleus data of the adult mouse spinal cord, which revealed an overall similarity in the neuronal composition of the spinal cord between the two species while simultaneously highlighting some degree of heterogeneity. Additionally, we examined the sex differences in the spinal neuronal subclusters. Several genes, such as SCN10A and HCN1, showed sex differences in motor neurons. Finally, we classified human dorsal root ganglia (DRG) neurons using spatial transcriptomics and explored the putative interactions between DRG and spinal cord neuronal subclusters. In summary, these results illustrate the complexity and diversity of spinal neurons in humans and provide an important resource for future research to explore the molecular mechanisms underlying spinal cord physiology and diseases.

Optogenetic Activation of Peripheral Somatosensory Neurons in Transgenic Mice as a Neuropathic Pain Model for Assessing the Therapeutic Efficacy of Analgesics

Optogenetics is a novel biotechnology widely used to precisely manipulate a specific peripheral sensory neuron or neural circuit. However, the use of optogenetics to assess the therapeutic efficacy of analgesics is elusive. In this study, we generated a transgenic mouse stain in which all primary somatosensory neurons can be optogenetically activated to mimic neuronal hyperactivation in the neuropathic pain state for the assessment of analgesic effects of drugs. A transgenic mouse was generated using the advillin-Cre line mated with the Ai32 strain, in which channelrhodopsin-2 fused to enhanced yellow fluorescence protein (ChR2-EYFP) was conditionally expressed in all types of primary somatosensory neurons (advillincre/ChR2+/+). Immunofluorescence and transdermal photostimulation on the hindpaws were used to verify the transgenic mice. Optical stimulation to evoke pain-like paw withdrawal latency was used to assess the analgesic effects of a series of drugs. Injury- and pain-related molecular biomarkers were investigated with immunohistofluorescence. We found that the expression of ChR2-EYFP was observed in many primary afferents of paw skin and sciatic nerves and in primary sensory neurons and laminae I and II of the spinal dorsal horns in advillincre/ChR2+/+ mice. Transdermal blue light stimulation of the transgenic mouse hindpaw evoked nocifensive paw withdrawal behavior. Treatment with gabapentin, some channel blockers, and local anesthetics, but not opioids or COX-1/2 inhibitors, prolonged the paw withdrawal latency in the transgenic mice. The analgesic effect of gabapentin was also verified by the decreased expression of injury- and pain-related molecular biomarkers. These optogenetic mice provide a promising model for assessing the therapeutic efficacy of analgesics in neuropathic pain.

Potential analgesic effects of psychedelics on select chronic pain conditions: A survey study

BACKGROUND

Chronic pain is a major cause of suffering and disability and is often associated with psychiatric complications. Current treatments carry the risk of severe side effects and may lead to limited or no relief at all in a relevant portion of this patient population. Preliminary evidence suggests that classical psychedelics (e.g. LSD and psilocybin) may have analgesic effects in healthy volunteers, and in certain chronic pain conditions and observational studies reveal that they are used in naturalistic settings as a means to manage pain.

METHODS

In order to gain insight on the effectiveness of such compounds in chronic pain conditions, we set up a survey addressed to chronic pain patients inquiring about psychedelic use and the relief levels achieved with both conventional treatments, full psychedelic doses and microdoses. We analysed data related to five conditions selected based on diagnostic homogeneity within each of them: fibromyalgia, arthritis, migraine, tension-type headache and sciatica.

RESULTS

Except for sciatica, volunteers reported that psychedelics led to better pain relief compared to conventional medication in all examined conditions. More specifically, full doses performed better than conventional medication. Microdoses led to significantly better relief compared to conventional medication in migraines and achieved comparable relief in the remaining three categories. Implications for future research are discussed.

CONCLUSIONS

Full doses and microdoses may hold value in the treatment of some specific chronic pain conditions.

SIGNIFICANCE

Psychedelic substances are receiving increasing attention from the scientific literature because of evidence showing beneficial effects on several measures related to mental health in clinical samples and healthy volunteers samples. Previous evidence suggests that people suffering from chronic pain are using psychedelics to seek relief and the present paper presents the results of a survey study investigating their use and analgesic effects among individuals suffering from fibromyalgia, arthritis, migraine, tension-type headache and sciatica.

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.

A Novel Bifunctional μOR Agonist and σ1R Antagonist with Potent Analgesic Responses and Reduced Adverse Effects

Bifunctional ligands possessing both μOR agonism and σ1R antagonism have shown promise in producing strong analgesic effects with reduced opioid-related side effects. However, the μOR agonism activity of most dual ligands diminishes compared with classical opioids, raising concern about their effectiveness in managing nociceptive pain. In this study, a new class of dual μOR agonist/σ1R antagonist was reported. Through structure-activity relationship analyses, we identified the optimal compound, 4x, which displayed picomolar μOR agonism activity (EC50: 0.6 ± 0.2 nM) and good σ1R inhibitory activity (Ki: 363.7 ± 5.6 nM) with excellent selectivity. Compound 4x exhibited robust analgesic effects in various pain models, with significantly reduced side effects. Importantly, compound 4x also possessed good safety profiles and no abnormalities were observed in biological parameters even under a high dosage. Our findings suggest that 4x may be a promising lead compound for developing safer opioids and warrants further in-depth studies.

Targeting sensory neuron GPCRs for peripheral neuropathic pain

Despite the high prevalence of peripheral neuropathic pain (NP) conditions and significant progress in understanding its underlying mechanisms, the management of peripheral NP remains inadequate. Existing pharmacotherapies for NP act primarily on the central nervous system (CNS) and are often associated with CNS-related adverse effects, limiting their clinical effectiveness. Mounting preclinical evidence indicates that reducing the heightened activity in primary sensory neurons by targeting G-protein-coupled receptors (GPCRs), without activating these receptors in the CNS, relieves pain without central adverse effects. In this review, we focus on recent advancements in GPCR-mediated peripheral pain relief and discuss strategies to advance the development of more effective and safer therapies for peripheral NP by shifting from traditional CNS modulatory approaches toward selective targeting of GPCRs on primary sensory neurons.

Angiotensin II type 2 receptor signalling as a pain target: Bench, bedside and back-translation

Translating promising preclinical pain relief data for novel molecules from drug discovery to positive clinical trial outcomes is challenging. The angiotensin II type 2 (AT2) receptor is a clinically-validated target based upon positive proof-of-concept clinical trial data in patients with post-herpetic neuralgia. This trial was conducted because AT2 receptor antagonists evoked pain relief in rodent models of neuropathic pain. EMA401 was selected as the drug candidate based upon its suitable preclinical toxicity and safety profile and good pharmacokinetics. Herein, we provide an overview of the discovery, preclinical and clinical development of EMA401, for the alleviation of peripheral neuropathic pain.

The Analgesia Effect of Aucubin on CFA-Induced Inflammatory Pain by Inhibiting Glial Cells Activation-Mediated Inflammatory Response via Activating Mitophagy

BACKGROUND

Inflammatory pain, characterized by sustained nociceptive hypersensitivity, represents one of the most prevalent conditions in both daily life and clinical settings. Aucubin, a natural plant iridoid glycoside, possesses potent biological effects, encompassing anti-inflammatory, antioxidant, and neuroprotective properties. However, its impact on inflammatory pain remains unclear. The aim of this study is to investigate the therapeutic effects and underlying mechanism of aucubin in addressing inflammatory pain induced by complete Freund's adjuvant (CFA).

METHODS

The CFA-induced inflammatory pain model was employed to assess whether aucubin exerts analgesic effects and its potential mechanisms. Behavioral tests evaluated mechanical and thermal hyperalgesia as well as anxiety-like behaviors in mice. The activation of spinal glial cells and the expression of pro-inflammatory cytokines were examined to evaluate neuroinflammation. Additionally, RNA sequencing was utilized for the identification of differentially expressed genes (DEGs). Molecular biology experiments were conducted to determine the levels of the PINK1 gene and autophagy-related genes, along with PINK1 distribution in neural cells. Furthermore, mitophagy induced by carbonyl cyanide m-chlorophenylhydrazone (CCCP) was employed to examine the roles of PINK1 and mitophagy in pain processing.

RESULTS

Aucubin significantly ameliorated pain and anxiety-like behaviors induced by CFA in mice and reduced spinal inflammation. RNA sequencing indicated PINK1 as a pivotal gene, and aucubin treatment led to a significant downregulation of PINK1 expression. Further GO and KEGG analyses suggested the involvement of mitochondrial function in the therapeutic regulation of aucubin. Western blotting revealed that aucubin markedly decreased PINK1, Parkin, and p62 levels while increasing LC3B expression. Immunofluorescence showed the predominant co-localization of PINK1 with neuronal cells. Moreover, CCCP-induced mitophagy alleviated mechanical and thermal hyperalgesia caused by CFA and reversed CFA-induced mitochondrial dysfunction.

CONCLUSIONS

In summary, our data suggest that aucubin effectively alleviates CFA-induced inflammatory pain, potentially through triggering the PINK1 pathway, promoting mitophagy, and suppressing inflammation. These results provide a novel theoretical foundation for addressing the treatment of inflammatory pain.

Oxycodone: A Current Perspective on Its Pharmacology, Abuse, and Pharmacotherapeutic Developments

Oxycodone, a semisynthetic derivative of naturally occurring thebaine, an opioid alkaloid, has been available for more than 100 years. Although thebaine cannot be used therapeutically due to the occurrence of convulsions at higher doses, it has been converted to a number of other widely used compounds that include naloxone, naltrexone, buprenorphine, and oxycodone. Despite the early identification of oxycodone, it was not until the 1990s that clinical studies began to explore its analgesic efficacy. These studies were followed by the pursuit of several preclinical studies to examine the analgesic effects and abuse liability of oxycodone in laboratory animals and the subjective effects in human volunteers. For a number of years oxycodone was at the forefront of the opioid crisis, playing a significant role in contributing to opioid misuse and abuse, with suggestions that it led to transitioning to other opioids. Several concerns were expressed as early as the 1940s that oxycodone had significant abuse potential similar to heroin and morphine. Both animal and human abuse liability studies have confirmed, and in some cases amplified, these early warnings. Despite sharing a similar structure with morphine and pharmacological actions also mediated by the μ-opioid receptor, there are several differences in the pharmacology and neurobiology of oxycodone. The data that have emerged from the many efforts to analyze the pharmacological and molecular mechanism of oxycodone have generated considerable insight into its many actions, reviewed here, which, in turn, have provided new information on opioid receptor pharmacology. SIGNIFICANCE STATEMENT: Oxycodone, a μ-opioid receptor agonist, was synthesized in 1916 and introduced into clinical use in Germany in 1917. It has been studied extensively as a therapeutic analgesic for acute and chronic neuropathic pain as an alternative to morphine. Oxycodone emerged as a drug with widespread abuse. This article brings together an integrated, detailed review of the pharmacology of oxycodone, preclinical and clinical studies of pain and abuse, and recent advances to identify potential opioid analgesics without abuse liability.

Neuropathic pain; what we know and what we should do about it

Neuropathic pain can result from injury to, or disease of the nervous system. It is notoriously difficult to treat. Peripheral nerve injury promotes Schwann cell activation and invasion of immunocompetent cells into the site of injury, spinal cord and higher sensory structures such as thalamus and cingulate and sensory cortices. Various cytokines, chemokines, growth factors, monoamines and neuropeptides effect two-way signalling between neurons, glia and immune cells. This promotes sustained hyperexcitability and spontaneous activity in primary afferents that is crucial for onset and persistence of pain as well as misprocessing of sensory information in the spinal cord and supraspinal structures. Much of the current understanding of pain aetiology and identification of drug targets derives from studies of the consequences of peripheral nerve injury in rodent models. Although a vast amount of information has been forthcoming, the translation of this information into the clinical arena has been minimal. Few, if any, major therapeutic approaches have appeared since the mid 1990's. This may reflect failure to recognise differences in pain processing in males vs. females, differences in cellular responses to different types of injury and differences in pain processing in humans vs. animals. Basic science and clinical approaches which seek to bridge this knowledge gap include better assessment of pain in animal models, use of pain models which better emulate human disease, and stratification of human pain phenotypes according to quantitative assessment of signs and symptoms of disease. This can lead to more personalized and effective treatments for individual patients. Significance statement: There is an urgent need to find new treatments for neuropathic pain. Although classical animal models have revealed essential features of pain aetiology such as peripheral and central sensitization and some of the molecular and cellular mechanisms involved, they do not adequately model the multiplicity of disease states or injuries that may bring forth neuropathic pain in the clinic. This review seeks to integrate information from the multiplicity of disciplines that seek to understand neuropathic pain; including immunology, cell biology, electrophysiology and biophysics, anatomy, cell biology, neurology, molecular biology, pharmacology and behavioral science. Beyond this, it underlines ongoing refinements in basic science and clinical practice that will engender improved approaches to pain management.

Peripheral mechanisms of peripheral neuropathic pain

Peripheral neuropathic pain (PNP), neuropathic pain that arises from a damage or disease affecting the peripheral nervous system, is associated with an extremely large disease burden, and there is an increasing and urgent need for new therapies for treating this disorder. In this review we have highlighted therapeutic targets that may be translated into disease modifying therapies for PNP associated with peripheral neuropathy. We have also discussed how genetic studies and novel technologies, such as optogenetics, chemogenetics and single-cell RNA-sequencing, have been increasingly successful in revealing novel mechanisms underlying PNP. Additionally, consideration of the role of non-neuronal cells and communication between the skin and sensory afferents is presented to highlight the potential use of drug treatment that could be applied topically, bypassing drug side effects. We conclude by discussing the current difficulties to the development of effective new therapies and, most importantly, how we might improve the translation of targets for peripheral neuropathic pain identified from studies in animal models to the clinic.

A systematic review and meta-analysis of thigmotactic behaviour in the open field test in rodent models associated with persistent pain

Thigmotaxis is an innate predator avoidance behaviour of rodents. To gain insight into how injury and disease models, and analgesic drug treatments affect thigmotaxis, we performed a systematic review and meta-analysis of studies that assessed thigmotaxis in the open field test. Systematic searches were conducted of 3 databases in October 2020, March and August 2022. Study design characteristics and experimental data were extracted and analysed using a random-effects meta-analysis. We also assessed the correlation between thigmotaxis and stimulus-evoked limb withdrawal. This review included the meta-analysis of 165 studies We report thigmotaxis was increased in injury and disease models associated with persistent pain and this increase was attenuated by analgesic drug treatments in both rat and mouse experiments. Its usefulness, however, may be limited in certain injury and disease models because our analysis suggested that thigmotaxis may be associated with the locomotor function. We also conducted subgroup analyses and meta-regression, but our findings on sources of heterogeneity are inconclusive because analyses were limited by insufficient available data. It was difficult to assess internal validity because reporting of methodological quality measures was poor, therefore, the studies have an unclear risk of bias. The correlation between time in the centre (type of a thigmotactic metric) and types of stimulus-evoked limb withdrawal was inconsistent. Therefore, stimulus-evoked and ethologically relevant behavioural paradigms should be viewed as two separate entities as they are conceptually and methodologically different from each other.

What's New in Neuropathy?

Neuropathic pain presents diagnostic and treatment challenges. Despite recent advances in our understanding of the diagnosis and treatment of neuropathy, much remains to be elucidated. Familiar with neuropathy is the paradox that aberrant nerve signaling causes both sensory loss and pain. Voltage-gated sodium channels play an important role in neuronal electrogenesis and communication among neurons, and their dysregulation leads to hyperexcitability and pain. While numerous validated diagnostic assessment tools are available for neuropathy, patients often experience a diagnostic delay about the cause of their neuropathy. New research is defining more specific types of neuropathy beyond peripheral and central forms. The prevalence of pain varies by type of neuropathy, with chronic idiopathic axonal polyneuropathy associated with the highest proportion of patients experiencing pain. In the majority of types, it exceeds 50%. Gluten neuropathy, a form of peripheral neuropathy, is a new diagnostic consideration. It may require electrochemical conductance testing of hands and feet to test for sudomotor dysfunction. Among those with serologically confirmed gluten sensitivity or celiac disease, gluten neuropathy is a common neurological manifestation and may be addressed at least partially by a gluten-free diet. In Greece, a new neuropathic pain registry was created in 2014 in order to help gather data from real-world neuropathic pain patients. While still in its earliest phase, this registry has already produced demographic and treatment data that suggest suboptimal prescribing and less than recommended use of interventional procedures. Awareness campaigns are underway to encourage more Greek pain clinics to participate in this important registry.

Nociceptin Receptor-Related Agonists as Safe and Non-addictive Analgesics

As clinical use of currently available opioid analgesics is often impeded by dose-limiting adverse effects, such as abuse liability and respiratory depression, new approaches have been pursued to develop safe, effective, and non-addictive pain medications. After the identification of the nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor more than 25 years ago, NOP receptor-related agonists have emerged as a promising target for developing novel and effective opioids that modulate the analgesic and addictive properties of mu-opioid peptide (MOP) receptor agonists. In this review, we highlight the effects of the NOP receptor-related agonists compared with those of MOP receptor agonists in experimental rodent and more translational non-human primate (NHP) models and the development status of key NOP receptor-related agonists as potential safe and non-addictive analgesics. Several lines of evidence demonstrated that peptidic and non-peptidic NOP receptor agonists produce potent analgesic effects by intrathecal delivery in NHPs. Moreover, mixed NOP/MOP receptor partial agonists (e.g., BU08028, BU10038, and AT-121) display potent analgesic effects when administered intrathecally or systemically, without eliciting adverse effects, such as respiratory depression, itch behavior, and signs of abuse liability. More importantly, cebranopadol, a mixed NOP/opioid receptor agonist with full efficacy at NOP and MOP receptors, produces robust analgesic efficacy with reduced adverse effects, conferring promising outcomes in clinical studies. A balanced coactivation of NOP and MOP receptors is a strategy that warrants further exploration and refinement for the development of novel analgesics with a safer and effective profile.

Long-term optical imaging of the spinal cord in awake, behaving animals

Advances in optical imaging approaches and fluorescent biosensors have enabled an understanding of the spatiotemporal and long-term neural dynamics in the brain of awake animals. However, methodological difficulties and the persistence of post-laminectomy fibrosis have greatly limited similar advances in the spinal cord. To overcome these technical obstacles, we combined in vivo application of fluoropolymer membranes that inhibit fibrosis; a redesigned, cost-effective implantable spinal imaging chamber; and improved motion correction methods that together permit imaging of the spinal cord in awake, behaving mice, for months to over a year. We also demonstrate a robust ability to monitor axons, identify a spinal cord somatotopic map, conduct Ca2+ imaging of neural dynamics in behaving animals responding to pain-provoking stimuli, and observe persistent microglial changes after nerve injury. The ability to couple neural activity and behavior at the spinal cord level will drive insights not previously possible at a key location for somatosensory transmission to the brain.

Network pharmacological analysis to reveal the mechanism governing the effect of Qin Xi Tong on osteoarthritis and rheumatoid arthritis

INTRODUCTION

Qin Xi Tong (QXT), produced by water extracts of Caulis Sinomenii, is clinically effective in the therapy of rheumatoid arthritis (RA). It is also a complementary agent for osteoarthritis (OA). This study aimed to screen the candidate targets and identify the potential mechanisms of QXT against RA and OA.

METHOD

The active ingredients contained in QXT were queried from the TCMSP database. Their predicted targets were obtained through web-based databases, including TCMSP, BATMAN-TCM, CTD, and PharmMapper. The OA and RA targets were collected from the Genecards database and the GSE55235 dataset. Based on the DAVID database, GO and KEGG enrichment analyses of disease-drug common targets predicted potential signaling pathways for QXT. In addition, core targets were identified by mapping component-target-disease interaction networks with Cytoscape 3.9.1 and STRING. The Swissdock and Pymol tools further validate the predicted results.

RESULTS

A total of 161 genes were put forward as potential targets for treating RA and OA. These genes might be involved in joint inflammation, including the IL-17 signaling pathway, MAPK signaling pathway, and TNF signaling pathway. They also regulated the progression of joint injuries, such as apoptosis, Th17 cell differentiation, and osteoclast differentiation. In addition, we identified 12 core targets of QXT. Molecular docking results showed that QXT has a high affinity with these core targets.

CONCLUSIONS

This study reveals the mechanism governing the effect of QXT on RA and OA, predicts the direct target, and provides new ideas for clinical treatment. Key Points • Our study reveals the underlying mechanism of QXT in the treatment of RA and OA. • Further research into the effects of compounds in QXT alone would be of interest.

FXYD2 antisense oligonucleotide provides an efficient approach for long-lasting relief of chronic peripheral pain

Chronic pain, whether of inflammatory or neuropathic origin, affects about 18% of the population of developed countries, and most current treatments are only moderately effective and/or cause serious side effects. Therefore, the development of novel therapeutic approaches still represents a major challenge. The Na,K-ATPase modulator FXYD2 is critically required for the maintenance of neuropathic pain in rodents. Here, we set up a therapeutic protocol based on the use of chemically modified antisense oligonucleotides (ASOs) to inhibit FXYD2 expression and treat chronic pain. We identified an ASO targeting a 20-nucleotide stretch in the FXYD2 mRNA that is evolutionarily conserved between rats and humans and is a potent inhibitor of FXYD2 expression. We used this sequence to synthesize lipid-modified forms of ASO (FXYD2-LASO) to facilitate their entry into dorsal root ganglia neurons. We established that intrathecal or intravenous injections of FXYD2-LASO in rat models of neuropathic or inflammatory pain led to a virtually complete alleviation of their pain symptoms, without causing obvious side effects. Remarkably, by using 2'-O-2-methoxyethyl chemical stabilization of the ASO (FXYD2-LASO-Gapmer), we could significantly prolong the therapeutic action of a single treatment up to 10 days. This study establishes FXYD2-LASO-Gapmer administration as a promising and efficient therapeutic strategy for long-lasting relief of chronic pain conditions in human patients.

Scalable generation of sensory neurons from human pluripotent stem cells

Development of new non-addictive analgesics requires advanced strategies to differentiate human pluripotent stem cells (hPSCs) into relevant cell types. Following principles of developmental biology and translational applicability, here we developed an efficient stepwise differentiation method for peptidergic and non-peptidergic nociceptors. By modulating specific cell signaling pathways, hPSCs were first converted into SOX10+ neural crest, followed by differentiation into sensory neurons. Detailed characterization, including ultrastructural analysis, confirmed that the hPSC-derived nociceptors displayed cellular and molecular features comparable to native dorsal root ganglion (DRG) neurons, and expressed high-threshold primary sensory neuron markers, transcription factors, neuropeptides, and over 150 ion channels and receptors relevant for pain research and axonal growth/regeneration studies (e.g., TRPV1, NAV1.7, NAV1.8, TAC1, CALCA, GAP43, DPYSL2, NMNAT2). Moreover, after confirming robust functional activities and differential response to noxious stimuli and specific drugs, a robotic cell culture system was employed to produce large quantities of human sensory neurons, which can be used to develop nociceptor-selective analgesics.

Inhibition of the Human Neuronal Sodium Channel Nav1.9 by Arachidonyl-2-Chloroethylamide, An Analogue of Anandamide in a hNav1.9/rNav1.4 Chimera, An Experimental and in Silico Study

Cannabinoids regulate analgesia, which has aroused much interest in identifying new pharmacological therapies in the management of refractory pain. Voltage-gated Na+ channels (Na_vs) play an important role in inflammatory and neuropathic pain. In particular, Na_v1.9 is involved in nociception and the understanding of its pharmacology has lagged behind because it is difficult to express in heterologous systems. Here, we utilized the chimeric channel hNa_v1.9_C4, that comprises the extracellular and transmembrane domains of hNa_v1.9, co-expressed with the ß1 subunit on CHO-K1 cells to characterize the electrophysiological effects of ACEA, a synthetic surrogate of the endogenous cannabinoid anandamide. ACEA induced a tonic block, decelerated the fast inactivation, markedly shifted steady-state inactivation in the hyperpolarized direction, decreasing the window current and showed use-dependent block, with a high affinity for the inactivated state (k_i = 0.84 µM). Thus, we argue that ACEA possess a local anaesthetic-like profile. To provide a mechanistic understanding of its mode of action at the molecular level, we combined induced fit docking with Monte Carlo simulations and electrostatic complementarity. In agreement with the experimental evidence, our computer simulations revealed that ACEA binds Tyr1599 of the local anaesthetics binding site of the hNa_v1.9, contacting residues that bind cannabinol (CBD) in the Na_vMs channel. ACEA adopted a conformation remarkably similar to the crystallographic conformation of anandamide on a non-homologous protein, obstructing the Na+ permeation pathway below the selectivity filter to occupy a highly conserved binding pocket at the intracellular side. These results describe a mechanism of action, possibly involved in cannabinoid analgesia.

The core of maintaining neuropathic pain: Crosstalk between glial cells and neurons (neural cell crosstalk at spinal cord)

BACKGROUND

Neuropathic pain (NP) caused by the injury or dysfunction of the nervous system is a chronic pain state accompanied by hyperalgesia, and the available clinical treatment is relatively scarce. Hyperalgesia mediated by pro-inflammatory factors and chemokines plays an important role in the occurrence and maintenance of NP.

DATA TREATMENT

Therefore, we conducted a systematic literature review of experimental NP (PubMed Medline), in order to find the mechanism of inducing central sensitization and explore the intervention methods of hyperalgesia caused by real or simulated injury.

RESULT

In this review, we sorted out the activation pathways of microglia, astrocytes and neurons, and the process of crosstalk among them. It was found that in NP, the microglia P2X4 receptor is the key target, which can activate the mitogen-activated protein kinase pathway inward and then activate astrocytes and outwardly activate neuronal tropomyosin receptor kinase B receptor to activate neurons. At the same time, activated neurons continue to maintain the activation of astrocytes and microglia through chemokines on CXCL13/CXCR5 and CX3CL1/CX3CR1. This crosstalk process is the key to maintaining NP.

CONCLUSION

We summarize the further research on crosstalk among neurons, microglia, and astrocytes in the central nervous system, elaborate the ways and connections of relevant crosstalk, and find potential crosstalk targets, which provides a reference for drug development and preclinical research.

Kinins and their B1 and B2 receptors as potential therapeutic targets for pain relief

Kinins are endogenous peptides that belong to the kallikrein-kinin system, which has been extensively studied for over a century. Their essential role in multiple physiological and pathological processes is demonstrated by activating two transmembrane G-protein-coupled receptors, the kinin B₁ and B₂ receptors. The attention is mainly given to the pathological role of kinins in pain transduction mechanisms. In the past years, a wide range of preclinical studies has amounted to the literature reinforcing the need for an updated review about the participation of kinins and their receptors in pain disorders. Here, we performed an extensive literature search since 2004, describing the historical progress and the current understanding of the kinin receptors' participation and its potential therapeutic in several acute and chronic painful conditions. These include inflammatory (mainly arthritis), neuropathic (caused by different aetiologies, such as cancer, multiple sclerosis, antineoplastic toxicity and diabetes) and nociplastic (mainly fibromyalgia) pain. Moreover, we highlighted the pharmacological actions and possible clinical applications of the kinin B₁ and B₂ receptor antagonists, kallikrein inhibitors or kallikrein-kinin system signalling pathways-target molecules in these different painful conditions. Notably, recent findings sought to elucidate mechanisms for guiding new and better drug design targeting kinin B₁ and B₂ receptors to treat a disease diversity. Since the kinin B₂ receptor antagonist, Icatibant, is clinically used and well-tolerated by patients with hereditary angioedema gives us hope kinin receptors antagonists could be more robustly tested for a possible clinical application in the treatment of pathological pains, which present limited pharmacology management.

Solving an Old Puzzle: Elucidation and Evaluation of the Binding Mode of Salvinorin A at the Kappa Opioid Receptor

The natural product Salvinorin A (SalA) was the first nitrogen-lacking agonist discovered for the opioid receptors and exhibits high selectivity for the kappa opioid receptor (KOR) turning SalA into a promising analgesic to overcome the current opioid crisis. Since SalA's suffers from poor pharmacokinetic properties, particularly the absence of gastrointestinal bioavailability, fast metabolic inactivation, and subsequent short duration of action, the rational design of new tailored analogs with improved clinical usability is highly desired. Despite being known for decades, the binding mode of SalA within the KOR remains elusive as several conflicting binding modes of SalA were proposed hindering the rational design of new analgesics. In this study, we rationally determined the binding mode of SalA to the active state KOR by in silico experiments (docking, molecular dynamics simulations, dynophores) in the context of all available mutagenesis studies and structure-activity relationship (SAR) data. To the best of our knowledge, this is the first comprehensive evaluation of SalA's binding mode since the determination of the active state KOR crystal structure. SalA binds above the morphinan binding site with its furan pointing toward the intracellular core while the C2-acetoxy group is oriented toward the extracellular loop 2 (ECL2). SalA is solely stabilized within the binding pocket by hydrogen bonds (C210^ECL2, Y312^7.35, Y313^7.36) and hydrophobic contacts (V118^2.63, I139^3.33, I294^6.55, I316^7.39). With the disruption of this interaction pattern or the establishment of additional interactions within the binding site, we were able to rationalize the experimental data for selected analogs. We surmise the C2-substituent interactions as important for SalA and its analogs to be experimentally active, albeit with moderate frequency within MD simulations of SalA. We further identified the non-conserved residues 2.63, 7.35, and 7.36 responsible for the KOR subtype selectivity of SalA. We are confident that the elucidation of the SalA binding mode will promote the understanding of KOR activation and facilitate the development of novel analgesics that are urgently needed.

Inhibition of NKCC1 in spinal dorsal horn and dorsal root ganglion results in alleviation of neuropathic pain in rats with spinal cord contusion

Previous studies have confirmed the relationship between chloride homeostasis and pain. However, the role of sodium potassium chloride co-transporter isoform 1 (NKCC1) in dorsal horn and dorsal root ganglion neurons (DRGs) in spinal cord injury (SCI)-induced neuropathic pain (NP) remains inconclusive. Therefore, we aimed to explore whether suppression of NKCC1 in the spinal cord and DRGs alleviate the NP of adult rats with thoracic spinal cord contusion. Thirty adult female Sprague-Dawley rats (8 week-old, weighing 250-280 g) were randomly divided into three groups with ten animals in each group (sham, SCI, and bumetanide groups). The paw withdrawal mechanical threshold and paw withdrawal thermal latency were recorded before injury (baseline) and on post-injury days 14, 21, 28, and 35. At the end of experiment, western blotting (WB) analysis, quantitative real-time Polymerase Chain Reaction (PCR) and immunofluorescence were performed to quantify NKCC1 expression. Our results revealed that NKCC1 protein expression in the spinal cord and DRGs was significantly up-regulated in rats with SCI. Intraperitoneal treatment of bumetanide (an NKCC1 inhibitor) reversed the expression of NKCC1 in the dorsal horn and DRGs and ameliorated mechanical ectopic pain and thermal hypersensitivities in the SCI rats. Our study demonstrated the occurrence of NKCC1 overexpression in the spinal cord and DRGs in a rodent model of NP and indicated that changes in the peripheral nervous system also play a major role in promoting pain sensitization after SCI.

Sophoridine alleviates hyperalgesia and anxiety-like behavior in an inflammatory pain mouse model induced by complete freund's adjuvant

Chronic pain, along with comorbid psychiatric disorders, is a common problem worldwide. A growing number of studies have focused on non-opioid-based medicines, and billions of funds have been put into digging new analgesic mechanisms. Peripheral inflammation is one of the critical causes of chronic pain, and drugs with anti-inflammatory effects usually alleviate pain hypersensitivity. Sophoridine (SRI), one of the most abundant alkaloids in Chinese herbs, has been proved to exert antitumor, antivirus and anti-inflammation effects. Here, we evaluated the analgesic effect of SRI in an inflammatory pain mouse model induced by complete Freund's adjuvant (CFA) injection. SRI treatment significantly decreased pro-inflammatory factors release after LPS stimuli in microglia. Three days of SRI treatment relieved CFA-induced mechanical hypersensitivity and anxiety-like behavior, and recovered abnormal neuroplasticity in the anterior cingulate cortex of mice. Therefore, SRI may be a candidate compound for the treatment of chronic inflammatory pain and may serve as a structural basis for the development of new drugs.

A Comprehensive Survey of Prospective Structure-Based Virtual Screening for Early Drug Discovery in the Past Fifteen Years

Structure-based virtual screening (SBVS), also known as molecular docking, has been increasingly applied to discover small-molecule ligands based on the protein structures in the early stage of drug discovery. In this review, we comprehensively surveyed the prospective applications of molecular docking judged by solid experimental validations in the literature over the past fifteen years. Herein, we systematically analyzed the novelty of the targets and the docking hits, practical protocols of docking screening, and the following experimental validations. Among the 419 case studies we reviewed, most virtual screenings were carried out on widely studied targets, and only 22% were on less-explored new targets. Regarding docking software, GLIDE is the most popular one used in molecular docking, while the DOCK 3 series showed a strong capacity for large-scale virtual screening. Besides, the majority of identified hits are promising in structural novelty and one-quarter of the hits showed better potency than 1 μM, indicating that the primary advantage of SBVS is to discover new chemotypes rather than highly potent compounds. Furthermore, in most studies, only in vitro bioassays were carried out to validate the docking hits, which might limit the further characterization and development of the identified active compounds. Finally, several successful stories of SBVS with extensive experimental validations have been highlighted, which provide unique insights into future SBVS drug discovery campaigns.

Synthesis and Characterization of an Analgesic Potential Conotoxin Lv32.1

In our work of screening analgesic peptides from the conotoxin libraries of diverse Conus species, we decoded a peptide sequence from Conus lividus and named it Lv32.1 (LvXXXIIA). The folding conditions of linear Lv32.1 on buffer, oxidizing agent, concentration of GSH/GSSG and reaction time were optimized for a maximum yield of (34.94 ± 0.96)%, providing an efficient solution for the synthesis of Lv32.1. Its disulfide connectivity was identified to be 1-3, 2-6, 4-5, which was first reported for the conotoxins with cysteine framework XXXII and different from the common connectivities established for conotoxins with six cysteines. The analgesic effect of Lv32.1 was determined by a hot plate test in mice. An evident increase in the pain threshold with time illustrated that Lv32.1 exhibited analgesic potency. The effects on Na_v1.8 channel and α9α10 nAChR were detected, but weak inhibition was observed. In this work, we highlight the efficient synthesis, novel disulfide linkage and analgesic potential of Lv32.1, which laid a positive foundation for further development of conotoxin Lv32.1 as an analgesic candidate.

Analgesic potential of macrodoses and microdoses of classical psychedelics in chronic pain sufferers: a population survey

Although several studies and reports have shown the potential analgesic use of serotonergic psychedelics in cancer pain, phantom limb pain and cluster headache, evidence supporting their use for chronic pain is still limited. The past years have seen a considerable renewal of interest toward the therapeutic use of these compounds for mood disorders, resulting in a marked increase in the number of people turning to psychedelics in an attempt to self-medicate a health condition or improve their wellbeing. In western countries particularly, this population of users overlaps substantially with chronic pain sufferers, representing a unique opportunity to evaluate the effects these compounds have on pain and wellbeing. Here, we report results from an online survey conducted between August 2020 and July 2021 in a population of 250 chronic pain sufferers who had experience with psychedelics, either in microdoses (small sub-hallucinogenic doses), macrodoses (hallucinogenic doses), or both. Macrodoses, while less often used for analgesic purposes than microdoses, were reported to induce a higher level of pain relief than both microdoses and conventional pain medications (including opioids and cannabis). Although the effects were weaker and potentially more prone to expectation bias than with macrodoses, our results also suggested some benefits of psychedelics in microdoses for pain management. The reported analgesic effect appeared unrelated to mood improvements associated with psychedelic use, or the advocacy of psychedelic use. Taken together, our findings indicate interesting potential analgesic applications for psychedelics that warrant further clinical research.

Identifying SM-miRNA associations based on layer attention graph convolutional network and matrix decomposition

The accurate prediction of potential associations between microRNAs (miRNAs) and small molecule (SM) drugs can enhance our knowledge of how SM cures endogenous miRNA-related diseases. Given that traditional methods for predicting SM-miRNA associations are time-consuming and arduous, a number of computational models have been proposed to anticipate the potential SM-miRNA associations. However, several of these strategies failed to eliminate noise from the known SM-miRNA association information or failed to prioritize the most significant known SM-miRNA associations. Therefore, we proposed a model of Graph Convolutional Network with Layer Attention mechanism for SM-MiRNA Association prediction (GCNLASMMA). Firstly, we obtained the new SM-miRNA associations by matrix decomposition. The new SM-miRNA associations, as well as the integrated SM similarity and miRNA similarity were subsequently incorporated into a heterogeneous network. Finally, a graph convolutional network with an attention mechanism was used to compute the reconstructed SM-miRNA association matrix. Furthermore, four types of cross validations and two types of case studies were performed to assess the performance of GCNLASMMA. In cross validation, global Leave-One-Out Cross Validation (LOOCV), miRNA-fixed LOOCV, SM-fixed LOOCV and 5-fold cross-validation achieved excellent performance. Numerous hypothesized associations in case studies were confirmed by experimental literatures. All of these results confirmed that GCNLASMMA is a trustworthy association inference method.

Development of a PET radioligand for α2δ-1 subunit of calcium channels for imaging neuropathic pain

Neuropathic pain affects 7-10% of the adult population. Being able to accurately monitor biological changes underlying neuropathic pain will improve our understanding of neuropathic pain mechanisms and facilitate the development of novel therapeutics. Positron emission tomography (PET) is a noninvasive molecular imaging technique that can provide quantitative information of biochemical changes at the whole-body level by using radiolabeled ligands. One important biological change underlying the development of neuropathic pain is the overexpression of α2δ-1 subunit of voltage-dependent calcium channels (the target of gabapentin). Thus, we hypothesized that a radiolabeled form of gabapentin may allow imaging changes in α2δ-1 for monitoring the underlying pathophysiology of neuropathic pain. Here, we report the development of two 18F-labeled derivatives of gabapentin (trans-4-[18F]fluorogabapentin and cis-4-[18F]fluorogabapentin) and their evaluation in healthy rats and a rat model of neuropathic pain (spinal nerve ligation model). Both isomers were found to selectively bind to the α2δ-1 receptor with trans-4-[18F]fluorogabapentin having higher affinity. Both tracers displayed around 1.5- to 2-fold increased uptake in injured nerves over the contralateral uninjured nerves when measured by gamma counting ex vivo. Although the small size of the nerves and the signal from surrounding muscle prevented visualizing these changes using PET, this work demonstrates that fluorinated derivatives of gabapentin retain binding to α2δ-1 and that their radiolabeled forms can be used to detect pathological changes in vitro and ex vivo. Furthermore, this work confirms that α2δ-1 is a promising target for imaging specific features of neuropathic pain.

Antinociceptive efficacy of Clerodendrum petasites S. Moore, a Thai medicinal plant, and its CNS safety profiles

Background

Clerodendrum petasites, an herbal plant in Thailand, has been used for many years in folk medicine. However, scientific evidence regarding CNS safety pharmacology and antinociceptive activity of C. petasites (CP) has not yet been well characterized.

Purpose

The present study aimed to assess the CNS safety pharmacology and antinociceptive and antiinflammatory effects of CP extract.

Methods

The effect of CP extract on CNS safety pharmacology was assessed using LABORAS automated home cage monitoring and rotarod test. Its pharmacological activity was evaluated both in-vitro, and in-vivo using hot-plate, acetic acid-induced writhing, formalin, and carrageenan-induced paw edema models.

Results and conclusion

CP extract significantly improved thermal and chemical nociceptive behaviors and acute inflammatory pain at all doses: 300, 600, and 1200 mg/kg, p.o. The antiinflammatory effect of CP extract in inflammatory pain models was comparable to the effect of positive control: indomethacin 10 mg/kg at all dose levels tested. Further, the CP extract at 600 mg/kg dose significantly inhibited 82.3% of carrageenan-induced total edema. In-vitro, CP extract at 12.5, 25, and 50 μg/mL concentrations significantly reduced the expression of LPS-induced nitric oxide, IL-6, and TNF-α expression in both RAW 264.7 macrophage and BV-2 microglial cell lines. In addition, CP extract did not show any potential effects on the CNS, indicated by no significant effects on motor coordination, spontaneous locomotor activity, general behaviors, and well-being compared to vehicle-treated mice (p > 0.05). Overall, the present study evidences the potential antinociceptive, antiinflammatory efficacies of CP extract with a favorable CNS safety profile.

Characterization of the Dahl salt-sensitive rat as a rodent model of inherited, widespread, persistent pain

Animal models are essential for studying the pathophysiology of chronic pain disorders and as screening tools for new therapies. However, most models available do not reproduce key characteristics of clinical persistent pain. This has limited their ability to accurately predict which new medicines will be clinically effective. Here, we characterize the Dahl salt-sensitive (SS) rat strain as the first rodent model of inherited widespread hyperalgesia. We show that this strain exhibits physiological phenotypes known to contribute to chronic pain, such as neuroinflammation, defective endogenous pain modulation, dysfunctional hypothalamic-pituitary-adrenal axis, increased oxidative stress and immune cell activation. When compared with Sprague Dawley and Brown Norway rats, SS rats have lower nociceptive thresholds due to increased inflammatory mediator concentrations, lower corticosterone levels, and high oxidative stress. Treatment with dexamethasone, the reactive oxygen species scavenger tempol, or the glial inhibitor minocycline attenuated the pain sensitivity in SS rats without affecting the other strains while indomethacin and gabapentin provided less robust pain relief. Moreover, SS rats presented impaired diffuse noxious inhibitory controls and an exacerbated response to the proalgesic mediator PGE₂, features of generalized pain conditions. These data establish this strain as a novel model of spontaneous, widespread hyperalgesia that can be used to identify biomarkers for chronic pain diagnosis and treatment.

Systematic review and meta-analysis of studies in which burrowing behaviour was assessed in rodent models of disease-associated persistent pain

ABSTRACT

Burrowing behaviour is used to assess pain-associated behaviour in laboratory rodents. To gain insight into how models of disease-associated persistent pain and analgesics affect burrowing behaviour, we performed a systematic review and meta-analysis of studies that assessed burrowing behaviour. A systematic search in March 2020 and update in September 2020 was conducted in 4 databases. Study design characteristics and experimental data were extracted, followed by a random-effects meta-analysis. We explored the association between burrowing and monofilament-induced limb withdrawal. Dose response relationship was investigated for some analgesics. Forty-five studies were included in the meta-analysis, in which 16 model types and 14 drug classes were used. Most experiments used rat (79%) and male (72%) animals. Somatic inflammation and trauma-induced neuropathy models were associated with reduced burrowing behaviour. Analgesics (nonsteroidal anti-inflammatory drug and gabapentinoids) attenuated burrowing deficits in these models. Reporting of measures to reduce risk of bias was unclear except for randomisation which was high. There was not a correlation ( R2 = 0.1421) between burrowing and monofilament-induced limb withdrawal. Opioids, gabapentin, and naproxen showed reduced burrowing behaviour at high doses, whereas ibuprofen and celecoxib showed opposite trend. The findings indicate that burrowing could be used to assess pain-associated behaviour. We support the use of a portfolio of composite measures including spontaneous and stimulus-evoked tests. The information collected here could help in designing experiments involving burrowing assessment in models of disease-associated pain.

Structure-based discovery of nonopioid analgesics acting through the α2A-adrenergic receptor

Because nonopioid analgesics are much sought after, we computationally docked more than 301 million virtual molecules against a validated pain target, the α2A-adrenergic receptor (α2AAR), seeking new α2AAR agonists chemotypes that lack the sedation conferred by known α2AAR drugs, such as dexmedetomidine. We identified 17 ligands with potencies as low as 12 nanomolar, many with partial agonism and preferential Gi and Go signaling. Experimental structures of α2AAR complexed with two of these agonists confirmed the docking predictions and templated further optimization. Several compounds, including the initial docking hit '9087 [mean effective concentration (EC50) of 52 nanomolar] and two analogs, '7075 and PS75 (EC50 4.1 and 4.8 nanomolar), exerted on-target analgesic activity in multiple in vivo pain models without sedation. These newly discovered agonists are interesting as therapeutic leads that lack the liabilities of opioids and the sedation of dexmedetomidine.

Neuropathic ocular surface pain: Emerging drug targets and therapeutic implications

INTRODUCTION

Dysfunction at various levels of the somatosensory system can lead to ocular surface pain with a neuropathic component. Compared to nociceptive pain (due to noxious stimuli at the ocular surface), neuropathic pain tends to be chronic and refractory to therapies, making it an important source of morbidity in the population. An understanding of the options available for neuropathic ocular surface pain, including new and emerging therapies, is thus an important topic.

AREAS COVERED

This review will examine studies focusing on ocular surface pain, emphasizing those examining patients with a neuropathic component. Attention will be placed toward recent (after 2017) studies that have examined new and emerging therapies for neuropathic ocular surface pain.

EXPERT OPINION

Several therapies have been studied thus far, and continued research is needed to identify which individuals would benefit from specific therapies. Gaps in our understanding exist, especially with availability of in-clinic diagnostics for neuropathic pain. A focus on improving diagnostic capabilities and researching gene-modulating therapies could help us to provide more specific mechanism-based therapies for patients. In the meantime, continuing to uncover new modalities and examining which are likely to work depending on pain phenotype remains an important short-term goal.