Distribution of functional opioid receptors in human dorsal root ganglion neurons

Peripherally restricted cannabinoid and mu-opioid receptor agonists synergistically attenuate neuropathic mechanical hypersensitivity in mice

ABSTRACT

Many medications commonly used to treat neuropathic pain are associated with significant, dose-limiting adverse effects, including sedation, dizziness, and fatigue. These adverse effects are due to the activity of these medications within the central nervous system. The objective of this work was to investigate the interactions between peripherally restricted cannabinoid receptor and mu-opioid receptor (MOR) agonists on ongoing and evoked neuropathic pain behaviors in mouse models. RNAscope analysis of cannabinoid receptor type 1 (CB1R) and MOR mRNA demonstrated that the mRNA of both receptors is colocalized in both mouse and human dorsal root ganglion. Single-cell RNAseq of dorsal root ganglion from chronic constriction injury mice showed that the mRNA of both receptors (Cnr1 and Oprm1) is coexpressed across different neuron clusters. Myc-CB1R and FLAG-MOR were cotransfected into immortalized HEK-293T cells and were found to interact at a subcellular level. We also find that CB-13 (a peripherally restricted dual CB1R and cannabinoid receptor type 2 agonist) and DALDA (a peripherally restricted MOR agonist) both attenuate mechanical hypersensitivity in a murine model of neuropathic pain. Using isobolographic analysis, we demonstrate that when coadministered, these agents synergistically attenuate mechanical hypersensitivity. Importantly, combination dosing of these agents does not cause any detectable preferential behaviors or motor impairment. However, repeated dosing of these agents is associated with the development of tolerance to these drugs. Collectively, these findings suggest that leveraging synergistic pain inhibition between cannabinoid receptor and MOR agonists in peripheral sensory neurons may be worth examining in patients with neuropathic pain.

RNA isoform expression landscape of the human dorsal root ganglion generated from long-read sequencing

ABSTRACT

Splicing is a posttranscriptional RNA processing mechanism that enhances genomic complexity by creating multiple isoforms from the same gene. We aimed to characterize the isoforms expressed in the human peripheral nervous system, with the goal of creating a resource to identify novel isoforms of functionally relevant genes associated with somatosensation and nociception. We used long-read sequencing to document isoform expression in the human dorsal root ganglia from 3 organ donors and validated in silico by confirming expression in short-read sequencing from 3 independent organ donors. Nineteen thousand five hundred forty-seven isoforms of protein-coding genes were detected and validated. We identified 763 isoforms with at least one previously undescribed splice junction. Previously unannotated isoforms of multiple pain-associated genes, including ASIC3, MRGPRX1, and HNRNPK, were identified. In the novel isoforms of ASIC3, a region comprising approximately 35% of the 5'UTR was excised. By contrast, a novel splice junction was used in isoforms of MRGPRX1 to include an additional exon upstream of the start codon, consequently adding a region to the 5'UTR. Novel isoforms of HNRNPK were identified, which used previously unannotated splice sites to both excise exon 14 and include a sequence in the 3' end of exon 13. This novel insertion is predicted to introduce a tyrosine phosphorylation site potentially phosphorylated by SRC. We also independently confirm a recently reported DRG-specific splicing event in WNK1 that gives insight into how painless peripheral neuropathy occurs when this gene is mutated. Our findings give a clear overview of mRNA isoform diversity in the human dorsal root ganglia obtained using long-read sequencing.

Sickle cell disease iPSC-derived sensory neurons exhibit increased excitability and sensitization to patient plasma

ABSTRACT

Individuals living with sickle cell disease (SCD) experience severe recurrent acute and chronic pain. Challenges to gaining mechanistic insight into pathogenic SCD pain processes include differential gene expression and function of sensory neurons between humans and mice with SCD, and extremely limited availability of neuronal tissues from patients with SCD. Here, we used induced pluripotent stem cells (iPSCs), derived from patients with SCD, differentiated into sensory neurons (SCD iSNs) to begin to overcome these challenges. We characterize key gene expression and function of SCD iSNs to establish a model to investigate intrinsic and extrinsic factors that may contribute to SCD pain. Despite similarities in receptor gene expression, SCD iSNs show pronounced excitability using patch clamp electrophysiology. Furthermore, we find that plasma taken from patients with SCD during acute pain associated with a vaso-occlusive event increases the calcium responses to the nociceptive stimulus capsaicin in SCD iSNs compared with those treated with paired plasma from patients with SCD at steady state baseline or healthy control plasma samples. We identified high levels of the polyamine spermine in baseline and acute pain states of plasma from patients with SCD, which sensitizes SCD iSNs to subthreshold concentrations of capsaicin. Together, these data identify potential intrinsic mechanisms within SCD iSNs that may extend beyond a blood-based pathology.

Bradykinin receptor expression and bradykinin-mediated sensitization of human sensory neurons

ABSTRACT

Bradykinin is a peptide implicated in inflammatory pain in both humans and rodents. In rodent sensory neurons, activation of B1 and B2 bradykinin receptors induces neuronal hyperexcitability. Recent evidence suggests that human and rodent dorsal root ganglia (DRG), which contain the cell bodies of sensory neurons, differ in the expression and function of key GPCRs and ion channels; whether bradykinin receptor expression and function are conserved across species has not been studied in depth. In this study, we used human DRG tissue from organ donors to provide a detailed characterization of bradykinin receptor expression and bradykinin-induced changes in the excitability of human sensory neurons. We found that B2 and, to a lesser extent, B1 receptors are expressed by human DRG neurons and satellite glial cells. B2 receptors were enriched in the nociceptor subpopulation. Using patch-clamp electrophysiology, we found that acute bradykinin increases the excitability of human sensory neurons, whereas prolonged exposure to bradykinin decreases neuronal excitability in a subpopulation of human DRG neurons. Finally, our analyses suggest that donor's history of chronic pain and age may be predictors of higher B1 receptor expression in human DRG neurons. Together, these results indicate that acute bradykinin-induced hyperexcitability, first identified in rodents, is conserved in humans and provide further evidence supporting bradykinin signaling as a potential therapeutic target for treating pain in humans.

Cell-type specific molecular architecture for mu opioid receptor function in pain and addiction circuits

Opioids are potent analgesics broadly used for pain management; however, they can produce dangerous side effects including addiction and respiratory depression. These harmful effects have led to an epidemic of opioid abuse and overdose deaths, creating an urgent need for the development of both safer pain medications and treatments for opioid use disorders. Both the analgesic and addictive properties of opioids are mediated by the mu opioid receptor (MOR), making resolution of the cell types and neural circuits responsible for each of the effects of opioids a critical research goal. Single-cell RNA sequencing (scRNA-seq) technology is enabling the identification of MOR-expressing cell types throughout the nervous system, creating new opportunities for mapping distinct opioid effects onto newly discovered cell types. Here, we describe molecularly defined MOR-expressing neuronal cell types throughout the peripheral and central nervous systems and their potential contributions to opioid analgesia and addiction.

RNA isoform expression landscape of the human dorsal root ganglion (DRG) generated from long read sequencing

Splicing is a post-transcriptional RNA processing mechanism that enhances genomic complexity by creating multiple isoforms from the same gene. Diversity in splicing in the mammalian nervous system is associated with neuronal development, synaptic function and plasticity, and is also associated with diseases of the nervous system ranging from neurodegeneration to chronic pain. We aimed to characterize the isoforms expressed in the human peripheral nervous system, with the goal of creating a resource to identify novel isoforms of functionally relevant genes associated with somatosensation and nociception. We used long read sequencing (LRS) to document isoform expression in the human dorsal root ganglia (hDRG) from 3 organ donors. Isoforms were validated in silico by confirming expression in hDRG short read sequencing (SRS) data from 3 independent organ donors. 19,547 isoforms of protein-coding genes were detected using LRS and validated with SRS and strict expression cutoffs. We identified 763 isoforms with at least one previously undescribed splice-junction. Previously unannotated isoforms of multiple pain-associated genes, including ASIC3, MRGPRX1 and HNRNPK were identified. In the novel isoforms of ASIC3, a region comprising ~35% of the 5'UTR was excised. In contrast, a novel splice-junction was utilized in isoforms of MRGPRX1 to include an additional exon upstream of the start-codon, consequently adding a region to the 5'UTR. Novel isoforms of HNRNPK were identified which utilized previously unannotated splice-sites to both excise exon 14 and include a sequence in the 5' end of exon 13. The insertion and deletion in the coding region was predicted to excise a serine-phosphorylation site favored by cdc2, and replace it with a tyrosine-phosphorylation site potentially phosphorylated by SRC. We also independently confirm a recently reported DRG-specific splicing event in WNK1 that gives insight into how painless peripheral neuropathy occurs when this gene is mutated. Our findings give a clear overview of mRNA isoform diversity in the hDRG obtained using LRS. Using this work as a foundation, an important next step will be to use LRS on hDRG tissues recovered from people with a history of chronic pain. This should enable identification of new drug targets and a better understanding of chronic pain that may involve aberrant splicing events.

Human OPRM1 and murine Oprm1 promoter driven viral constructs for genetic access to μ-opioidergic cell types

With concurrent global epidemics of chronic pain and opioid use disorders, there is a critical need to identify, target and manipulate specific cell populations expressing the mu-opioid receptor (MOR). However, available tools and transgenic models for gaining long-term genetic access to MOR+ neural cell types and circuits involved in modulating pain, analgesia and addiction across species are limited. To address this, we developed a catalog of MOR promoter (MORp) based constructs packaged into adeno-associated viral vectors that drive transgene expression in MOR+ cells. MORp constructs designed from promoter regions upstream of the mouse Oprm1 gene (mMORp) were validated for transduction efficiency and selectivity in endogenous MOR+ neurons in the brain, spinal cord, and periphery of mice, with additional studies revealing robust expression in rats, shrews, and human induced pluripotent stem cell (iPSC)-derived nociceptors. The use of mMORp for in vivo fiber photometry, behavioral chemogenetics, and intersectional genetic strategies is also demonstrated. Lastly, a human designed MORp (hMORp) efficiently transduced macaque cortical OPRM1+ cells. Together, our MORp toolkit provides researchers cell type specific genetic access to target and functionally manipulate mu-opioidergic neurons across a range of vertebrate species and translational models for pain, addiction, and neuropsychiatric disorders.

The Utility of Peripherally Restricted Kappa-Opioid Receptor Agonists for Inhibiting Below-Level Pain After Spinal Cord Injury in Mice

Pain after spinal cord injury (SCI) can be difficult to treat. Drugs that target the opioid receptor (OR) outside the central nervous system (CNS) have gained increasing interest in pain control owing to their low risk of central side effects. Asimadoline and ICI-204448 are believed to be peripherally restricted KOR agonists withlimited access to the CNS. This study examined whether they can attenuate pain hypersensitivity in mice subjected to a contusive T10 SCI. Subcutaneous (s.c.) injection of asimadoline (5, 20 mg/kg) and ICI-204448 (1, 10 mg/kg) inhibited heat hypersensitivity at both doses, but only attenuated mechanical hypersensitivity at the high dose. However, the high-dose asimadoline adversely affected animals' exploratory performance in SCI mice and caused aversion, suggesting CNS drug penetration. In contrast, high-dose ICI-204448 did not impair exploration and remained effective in reducing both mechanical and heat hypersensitivities after SCI. Accordingly, we chose to examine the potential peripheral neuronal mechanism for ICI-204448-induced pain inhibition by conducting in vivo calcium imaging of dorsal root ganglion (DRG) in Pirt-GCaMP6s+/- mice. High-dose ICI-204448 (10 mg/kg, s.c.) attenuated the increased fluorescence intensity of lumbar DRG neurons activated by a noxious pinch (400 g) stimulation in SCI mice. In conclusion, systemic administration of ICI-204448 achieved SCI pain inhibition at doses that did not induce notable side effects and attenuated DRG neuronal excitability which may partly contribute to its pain inhibition. These findings suggest that peripherally restricted KOR agonists may be useful for treating SCI pain, but the therapeutic window must be carefully examined.

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.

Bradykinin receptor expression and bradykinin-mediated sensitization of human sensory neurons

Bradykinin is a peptide implicated in inflammatory pain in both humans and rodents. In rodent sensory neurons, activation of B1 and B2 bradykinin receptors induces neuronal hyperexcitability. Recent evidence suggests that human and rodent dorsal root ganglia (DRG), which contain the cell bodies of sensory neurons, differ in the expression and function of key GPCRs and ion channels; whether BK receptor expression and function are conserved across species has not been studied in depth. In this study, we used human DRG tissue from organ donors to provide a detailed characterization of bradykinin receptor expression and bradykinin-induced changes in the excitability of human sensory neurons. We found that B2 and, to a lesser extent, B1 receptors are expressed by human DRG neurons and satellite glial cells. B2 receptors were enriched in the nociceptor subpopulation. Using patch-clamp electrophysiology, we found that acute bradykinin increases the excitability of human sensory neurons, while prolonged exposure to bradykinin decreases neuronal excitability in a subpopulation of human DRG neurons. Finally, our analyses suggest that donor’s history of chronic pain and age may be predictors of higher B1 receptor expression in human DRG neurons. Together, these results indicate that acute BK-induced hyperexcitability, first identified in rodents, is conserved in humans and provide further evidence supporting BK signaling as a potential therapeutic target for treating pain in humans.

Sickle cell disease patient plasma sensitizes iPSC-derived sensory neurons from sickle cell disease patients

Individuals living with sickle cell disease (SCD) experience severe recurrent acute and chronic pain. In order to develop novel therapies, it is necessary to better understand the neurobiological mechanisms underlying SCD pain. There are many barriers to gaining mechanistic insight into pathogenic SCD pain processes, such as differential gene expression and function of sensory neurons between humans and mice with SCD, as well as the limited availability of patient samples. These can be overcome by utilizing SCD patient-derived induced pluripotent stem cells (iPSCs) differentiated into sensory neurons (SCD iSNs). Here, we characterize the key gene expression and function of SCD iSNs to establish a model for higher-throughput investigation of intrinsic and extrinsic factors that may contribute to increased SCD patient pain. Importantly, identified roles for C-C Motif Chemokine Ligand 2 (CCL2) and endothelin 1 (ET1) in SCD pain can be recapitulated in SCD iSNs. Further, we find that plasma taken from SCD patients during acute pain increases SCD iSN calcium response to the nociceptive stimulus capsaicin compared to those treated with paired SCD patient plasma at baseline or healthy control plasma samples. Together, these data provide the framework necessary to utilize iSNs as a powerful tool to investigate the neurobiology of SCD and identify potential intrinsic mechanisms of SCD pain which may extend beyond a blood-based pathology.

Design and development of novel, short, stable dynorphin-based opioid agonists for safer analgesic therapy

Kappa opioid receptors have exceptional potential as an analgesic target, seemingly devoid of many problematic Mu receptor side-effects. Kappa-selective, small molecule pharmaceutical agents have been developed, but centrally mediated side-effects limit clinical translation. We modify endogenous dynorphin peptides to improve drug-likeness and develop safer KOP receptor agonists for clinical use. Using rational, iterative design, we developed a series of potent, selective, and metabolically stable peptides from dynorphin 1-7. Peptides were assessed for in vitro cAMP-modulation against three opioid receptors, metabolic stability, KOP receptor selectivity, desensitisation and pERK-signalling capability. Lead peptides were evaluated for in vivo efficacy in a rat model of inflammatory nociception. A library of peptides was synthesised and assessed for pharmacological and metabolic stability. Promising peptide candidates showed low nanomolar KOP receptor selectivity in cAMP assay, and improved plasma and trypsin stability. Selected peptides showed bias towards cAMP signalling over pERK activity, also demonstrating reduced desensitisation. In vivo, two peptides showed significant opioid-like antinociception comparable to morphine and U50844H. These highly potent and metabolically stable peptides are promising opioid analgesic leads for clinical translation. Since they are somewhat biased peptide Kappa agonists they may lack many significant side-effects, such as tolerance, addiction, sedation, and euphoria/dysphoria, common to opioid analgesics.

Sympathetic modulation of tumor necrosis factor alpha-induced nociception in the presence of oral squamous cell carcinoma

ABSTRACT

Head and neck squamous cell carcinoma (HNSCC) causes more severe pain and psychological stress than other types of cancer. Despite clinical evidence linking pain, stress, and cancer progression, the underlying relationship between pain and sympathetic neurotransmission in oral cancer is unknown. We found that human HNSCC tumors and mouse tumor tissue are innervated by peripheral sympathetic and sensory nerves. Moreover, β-adrenergic 1 and 2 receptors (β-ARs) are overexpressed in human oral cancer cell lines, and norepinephrine treatment increased β-AR2 protein expression as well as cancer cell proliferation in vitro. We have recently demonstrated that inhibition of tumor necrosis factor alpha (TNFα) signaling reduces oral cancer-induced nociceptive behavior. Norepinephrine-treated cancer cell lines secrete more TNFα which, when applied to tongue-innervating trigeminal neurons, evoked a larger Ca 2+ transient; TNF-TNFR inhibitor blocked the increase in the evoked Ca 2+ transient. Using an orthotopic xenograft oral cancer model, we found that mice demonstrated significantly less orofacial cancer-induced nociceptive behavior during systemic β-adrenergic inhibitory treatment with propranolol. Furthermore, chemical sympathectomy using guanethidine led to a significant reduction in tumor size and nociceptive behavior. We infer from these results that sympathetic signaling modulates oral cancer pain through TNFα secretion and tumorigenesis. Further investigation of the role of neurocancer communication in cancer progression and pain is warranted.

Spinal NLRP3 inflammasome activation mediates IL-1β release and contributes to remifentanil-induced postoperative hyperalgesia by regulating NMDA receptor NR1 subunit phosphorylation and GLT-1 expression in rats

BACKGROUND

Trafficking and activation of N-methyl-D-aspartate (NMDA) receptors play an important role in initiating and maintaining postoperative remifentanil-induced hyperalgesia (RIH). Activation of the NOD-like receptor protein 3 (NLRP3) inflammasome has been linked to the development of inflammatory and neuropathic pain. We hypothesized that activation of NLRP3 inflammasome mediates IL-1β release and contributes to RIH in rats by increasing NMDA receptor NR1 (NR1) subunit phosphorylation and decreasing glutamate transporter-1 (GLT-1) expression.

METHODS

Acute exposure to remifentanil (1.2 μg/kg/min for 60 min) was used to establish RIH in rats. Thermal and mechanical hyperalgesia were tested at baseline (24 h before remifentanil infusion) and 2, 6, 24, and 48 h after remifentanil infusion. The levels of IL-1β, GLT-1, phosphorylated NR1 (phospho-NR1), and NLRP3 inflammasome activation indicators [NLRP3, Toll-like receptor 4 (TLR4), P2X purinoceptor 7 (P2X7R), and caspase-1] were measured after the last behavioral test. A selective IL-1β inhibitor (IL-1β inhibitor antagonist; IL-1ra) or three different selective NLRP3 inflammasome activation inhibitors [(+)-naloxone (a TLR4 inhibitor), A438079 (a P2X7R inhibitor), or ac-YVADcmk (a caspase-1 inhibitor)] were intrathecally administered immediately before remifentanil infusion into rats.

RESULTS

Remifentanil induced significant postoperative hyperalgesia, increased IL-1β and phospho-NR1 levels and activated the NLRP3 inflammasome by increasing TLR4, P2X7R, NLRP3, and caspase-1 expression, but it decreased GLT-1 expression in the L4-L6 spinal cord segments of rats, which was markedly improved by intrathecal administration of IL-1ra, (+)-naloxone, A438079, or ac-YVADcmk.

CONCLUSION

NLRP3 inflammasome activation mediates IL-1β release and contributes to RIH in rats by inducing NMDA receptor NR1 subunit phosphorylation and decreasing GLT-1 expression. Inhibiting the activation of the NLRP3 inflammasome may be an effective treatment for RIH.

Pharmacological Probes to Validate Biomarkers for Analgesic Drug Development

There is an urgent need for analgesics with improved efficacy, especially in neuropathic and other chronic pain conditions. Unfortunately, in recent decades, many candidate analgesics have failed in clinical phase II or III trials despite promising preclinical results. Translational assessment tools to verify engagement of pharmacological targets and actions on compartments of the nociceptive system are missing in both rodents and humans. Through the Innovative Medicines Initiative of the European Union and EFPIA, a consortium of researchers from academia and the pharmaceutical industry was established to identify and validate a set of functional biomarkers to assess drug-induced effects on nociceptive processing at peripheral, spinal and supraspinal levels using electrophysiological and functional neuroimaging techniques. Here, we report the results of a systematic literature search for pharmacological probes that allow for validation of these biomarkers. Of 26 candidate substances, only 7 met the inclusion criteria: evidence for nociceptive system modulation, tolerability, availability in oral form for human use and absence of active metabolites. Based on pharmacokinetic characteristics, three were selected for a set of crossover studies in rodents and healthy humans. All currently available probes act on more than one compartment of the nociceptive system. Once validated, biomarkers of nociceptive signal processing, combined with a pharmacometric modelling, will enable a more rational approach to selecting dose ranges and verifying target engagement. Combined with advances in classification of chronic pain conditions, these biomarkers are expected to accelerate analgesic drug development.

Voltage-gated calcium currents in human dorsal root ganglion neurons

ABSTRACT

Voltage-gated calcium channels in sensory neurons underlie processes ranging from neurotransmitter release to gene expression and remain a therapeutic target for the treatment of pain. Yet virtually all we know about voltage-gated calcium channels has been obtained through the study of rodent sensory neurons and heterologously expressed channels. To address this, high voltage-activated (HVA) Ca2+ currents in dissociated human and rat dorsal root ganglion neurons were characterized with whole-cell patch clamp techniques. The HVA currents from both species shared basic biophysical and pharmacological properties. However, HVA currents in human neurons differed from those in the rat in at least 3 potentially important ways: (1) Ca2+ current density was significantly smaller, (2) the proportion of nifedipine-sensitive currents was far greater, and (3) a subpopulation of human neurons displayed relatively large constitutive current inhibition. These results highlight the need to for the study of native proteins in their native environment before initiating costly clinical trials.

Endogenous opiates and behavior: 2020

This paper is the forty-third consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2020 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug abuse and alcohol (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

Agonists Specific for κ-Opioid Receptor Induces Apoptosis of HCC Cells Through Enhanced Endoplasmic Reticulum Stress

Cancer pain is an important factor affecting life quality of patients especially in the advanced stage and relieving pain is one of fundamental strategies for cancer treatment. Opioids such as morphine are the most widely used in clinics. However, they have been reported to be associated with the occurrence and development of several types of cancer. Thus, search for an opioid that has analgesic effect and can retard cancer progress simultaneously is critical for cancer management. In this study, we first examined the expression of μ and κ (MOR and KOR) in cell lines and tumor tissues of hepatocellular carcinoma (HCC), a malignant tumor with high mortality, and then compared the effects of opioid receptors-specific agonists on malignant phenotypes of HCC cells in vitro and tumor growth in an HCC xenograft mouse model. KOR and MOR were found to be highly expressed in HCC cell lines and HCC tissues. The KOR-specific agonist U50488h, oxycodone (agonist for both KOR and MOR) and the MOR-specific agonist morphine inhibited HCC cell proliferation, while only U50488h and oxycodone suppressed colony formation and migration of HCC cells. U50488h and oxycodone, but not morphine, induced HCC apoptosis. Further detection of PERK, GRP78 and CHOP revealed that PERK signaling was upregulated by treatment with U50488h, while treatment with the PERK inhibitor GSK2656157 partially reversed the promotion of apoptosis and inhibition of cell proliferation by U50488h, indicating that endoplasmic reticulum stress is associated with its suppressing effect on HCC malignant phenotypes. Similar to the in vitro results, HCC growth was significantly reduced by administration of U50488h and oxycodone, but not by morphine, in the HCC xenograft mouse model. PERK and caspase-3 in the HCC tissues were up-regulated by U50488h treatment as detected by immunohistochemistry and western blotting. Taken together, our results revealed that activation of KOR by U50488h inhibited malignant phenotypes of HCC both in vitro and in vivo, while activation of MOR by morphine did not have such effect. Because of their dual roles in the relief of pain and in the suppression of malignant phenotypes, opioids such as U50488h that act on KOR should be considered as the first choice for HCC management.

Spatial transcriptomics of dorsal root ganglia identifies molecular signatures of human nociceptors

Nociceptors are specialized sensory neurons that detect damaging or potentially damaging stimuli and are found in the dorsal root ganglia (DRG) and trigeminal ganglia. These neurons are critical for the generation of neuronal signals that ultimately create the perception of pain. Nociceptors are also primary targets for treating acute and chronic pain. Single-cell transcriptomics on mouse nociceptors has transformed our understanding of pain mechanisms. We sought to generate equivalent information for human nociceptors with the goal of identifying transcriptomic signatures of nociceptors, identifying species differences and potential drug targets. We used spatial transcriptomics to molecularly characterize transcriptomes of single DRG neurons from eight organ donors. We identified 12 clusters of human sensory neurons, 5 of which are C nociceptors, as well as 1 C low-threshold mechanoreceptors (LTMRs), 1 Aβ nociceptor, 2 Aδ, 2 Aβ, and 1 proprioceptor subtypes. By focusing on expression profiles for ion channels, G protein-coupled receptors (GPCRs), and other pharmacological targets, we provided a rich map of potential drug targets in the human DRG with direct comparison to mouse sensory neuron transcriptomes. We also compared human DRG neuronal subtypes to nonhuman primates showing conserved patterns of gene expression among many cell types but divergence among specific nociceptor subsets. Last, we identified sex differences in human DRG subpopulation transcriptomes, including a marked increase in calcitonin-related polypeptide alpha (CALCA) expression in female pruritogen receptor-enriched nociceptors. This comprehensive spatial characterization of human nociceptors might open the door to development of better treatments for acute and chronic pain disorders.

Spatial transcriptomics of dorsal root ganglia identifies molecular signatures of human nociceptors

Nociceptors are specialized sensory neurons that detect damaging or potentially damaging stimuli and are found in the dorsal root ganglia (DRG) and trigeminal ganglia. These neurons are critical for the generation of neuronal signals that ultimately create the perception of pain. Nociceptors are also primary targets for treating acute and chronic pain. Single-cell transcriptomics on mouse nociceptors has transformed our understanding of pain mechanisms. We sought to generate equivalent information for human nociceptors with the goal of identifying transcriptomic signatures of nociceptors, identifying species differences and potential drug targets. We used spatial transcriptomics to molecularly characterize transcriptomes of single DRG neurons from eight organ donors. We identified 12 clusters of human sensory neurons, 5 of which are C nociceptors, as well as 1 C low-threshold mechanoreceptors (LTMRs), 1 Aβ nociceptor, 2 Aδ, 2 Aβ, and 1 proprioceptor subtypes. By focusing on expression profiles for ion channels, G protein–coupled receptors (GPCRs), and other pharmacological targets, we provided a rich map of potential drug targets in the human DRG with direct comparison to mouse sensory neuron transcriptomes. We also compared human DRG neuronal subtypes to nonhuman primates showing conserved patterns of gene expression among many cell types but divergence among specific nociceptor subsets. Last, we identified sex differences in human DRG subpopulation transcriptomes, including a marked increase in calcitonin-related polypeptide alpha ( CALCA ) expression in female pruritogen receptor–enriched nociceptors. This comprehensive spatial characterization of human nociceptors might open the door to development of better treatments for acute and chronic pain disorders.

Innovations and advances in modelling and measuring pain in animals

Best practices in preclinical algesiometry (pain behaviour testing) have shifted over the past decade as a result of technological advancements, the continued dearth of translational progress and the emphasis that funding institutions and journals have placed on rigour and reproducibility. Here we describe the changing trends in research methods by analysing the methods reported in preclinical pain publications from the past 40 years, with a focus on the last 5 years. We also discuss how the status quo may be hampering translational success. This discussion is centred on four fundamental decisions that apply to every pain behaviour experiment: choice of subject (model organism), choice of assay (pain-inducing injury), laboratory environment and choice of outcome measures. Finally, we discuss how human tissues, which are increasingly accessible, can be used to validate the translatability of targets and mechanisms identified in animal pain models.

Synthetic opioids: a review and clinical update

The term 'opioids' refers to both the natural compounds ('opiates') which are extracted from the opium poppy plant (Papaver somniferum) and their semi-synthetic and synthetic derivatives. They all possess relatively similar biochemical profiles and interact with the opioid receptors within the human body to produce a wide range of physiological effects. They have historically been used for medicinal purposes, their analgesic and sedative effects, and in the management of chronic and severe pain. They have also been used for non-medicinal and recreational purposes to produce feelings of relaxation, euphoria and well-being. Over the last decade, the emergence of an illegal market in new synthetic opioids has become a major global public health issue, associated with a substantial increase in unintentional overdoses and drug-related deaths. Synthetic opioids include fentanyl, its analogues and emerging non-fentanyl opioids. Their popularity relates to changes in criminal markets, pricing, potency, availability compared to classic opioids, ease of transport and use, rapid effect and lack of detection by conventional testing technologies. This article expands on our previous review on new psychoactive substances. We now provide a more in-depth review on synthetic opioids and explore the current challenges faced by people who use drugs, healthcare professionals, and global public health systems.

Pleiotropic Effects of Kappa Opioid Receptor-Related Ligands in Non-human Primates

The kappa opioid receptor (KOR)-related ligands have been demonstrated in preclinical studies for several therapeutic potentials. This chapter highlights (1) how non-human primates (NHP) studies facilitate the research and development of ligands targeting the KOR, (2) effects of the endogenous opioid peptide, dynorphin A-(1-17), and its analogs in NHP, and (3) pleiotropic effects and therapeutic applications of KOR-related ligands. In particular, synthetic ligands targeting the KOR have been extensively studied in NHP in three therapeutic areas, i.e., the treatment for itch, pain, and substance use disorders. As the KORs are widely expressed in the peripheral and central nervous systems, pleiotropic effects of KOR-related ligands, such as discriminative stimulus effects, neuroendocrine effects (e.g., prolactin release and stimulation of hypothalamic-pituitary-adrenal axis), and diuresis, in NHP are discussed. Centrally acting KOR agonists are known to produce adverse effects including dysphoria, hallucination, and sedation. Nonetheless, with strategic advances in medicinal chemistry, three classes of KOR-related agonists, i.e., peripherally restricted KOR agonists, mixed KOR/mu opioid receptor partial agonists, and G protein-biased KOR agonists, warrant additional NHP studies to improve our understanding of their functional efficacy, selectivity, and tolerability. Pharmacological studies in NHP which carry high translational significance will facilitate future development of KOR-based medications.

Shedding Light on the Pharmacological Interactions between μ-Opioid Analgesics and Angiotensin Receptor Modulators: A New Option for Treating Chronic Pain

The current protocols for neuropathic pain management include µ-opioid receptor (MOR) analgesics alongside other drugs; however, there is debate on the effectiveness of opioids. Nevertheless, dose escalation is required to maintain their analgesia, which, in turn, contributes to a further increase in opioid side effects. Finding novel approaches to effectively control chronic pain, particularly neuropathic pain, is a great challenge clinically. Literature data related to pain transmission reveal that angiotensin and its receptors (the AT1R, AT2R, and MAS receptors) could affect the nociception both in the periphery and CNS. The MOR and angiotensin receptors or drugs interacting with these receptors have been independently investigated in relation to analgesia. However, the interaction between the MOR and angiotensin receptors has not been excessively studied in chronic pain, particularly neuropathy. This review aims to shed light on existing literature information in relation to the analgesic action of AT1R and AT2R or MASR ligands in neuropathic pain conditions. Finally, based on literature data, we can hypothesize that combining MOR agonists with AT1R or AT2R antagonists might improve analgesia.

Opioid Addiction and Opioid Receptor Dimerization: Structural Modeling of the OPRD1 and OPRM1 Heterodimer and Its Signaling Pathways

Opioid addiction is a complex phenomenon with genetic, social, and other components. Due to such complexity, it is difficult to interpret the outcome of clinical studies, and thus, mutations found in individuals with these addictions are still not indisputably classified as opioid addiction-causing variants. Here, we computationally investigated two such mutations, A6V and N40D, found in the mu opioid receptor gene OPRM1. The mutations are located in the extracellular domain of the corresponding protein, which is important to the hetero-dimerization of OPRM1 with the delta opioid receptor protein (OPRD1). The hetero-dimerization of OPRD1-OPRM1 affects the signaling pathways activated by opioids and natural peptides and, thus, could be considered a factor contributing to addiction. In this study, we built four 3D structures of molecular pathways, including the G-protein signaling pathway and the β-arrestin signaling pathway of the heterodimer of OPRD1-OPRM1. We also analyzed the effect of mutations of A6V and N40D on the stability of individual OPRM1/OPRD1 molecules and the OPRD1-OPRM1 heterodimer with the goal of inferring their plausible linkage with opioid addiction. It was found that both mutations slightly destabilize OPRM1/OPRD1 monomers and weaken their association. Since hetero-dimerization is a key step for signaling processes, it is anticipated that both mutations may be causing increased addiction risk.

Cellular models of pain: New technologies and their potential to progress preclinical research

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Human sensory neurons can reduce the translational gap in analgesic development.

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Access to dorsal root ganglion (hDRG) neurons is increasing.

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Diverse sensory neuron subtypes can now be generated via stem cell technology.

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Advances of these technologies will improve our understanding of human nociception.

In vitro models fill a vital niche in preclinical pain research, allowing detailed study of molecular pathways, and in the case of humanised systems, providing a translational bridge between in vivo animal models and human patients. Significant advances in cellular technology available to basic pain researchers have occurred in the last decade, including developing protocols to differentiate sensory neuron-like cells from stem cells and greater access to human dorsal root ganglion tissue. In this review, we discuss the use of both models in preclinical pain research: What can a human sensory neuron in a dish tell us that rodent in vivo models cannot? How similar are these models to their endogenous counterparts, and how should we judge them? What limitations do we need to consider? How can we leverage cell models to improve translational success? In vitro human sensory neuron models equip pain researchers with a valuable tool to investigate human nociception. With continual development, consideration for their advantages and limitations, and effective integration with other experimental strategies, they could become a driving force for the pain field's advancement.

Studying human nociceptors: from fundamentals to clinic

Chronic pain affects one in five of the general population and is the third most important cause of disability-adjusted life-years globally. Unfortunately, treatment remains inadequate due to poor efficacy and tolerability. There has been a failure in translating promising preclinical drug targets into clinic use. This reflects challenges across the whole drug development pathway, from preclinical models to trial design. Nociceptors remain an attractive therapeutic target: their sensitization makes an important contribution to many chronic pain states, they are located outside the blood-brain barrier, and they are relatively specific. The past decade has seen significant advances in the techniques available to study human nociceptors, including: the use of corneal confocal microscopy and biopsy samples to observe nociceptor morphology, the culture of human nociceptors (either from surgical or post-mortem tissue or using human induced pluripotent stem cell derived nociceptors), the application of high throughput technologies such as transcriptomics, the in vitro and in vivo electrophysiological characterization through microneurography, and the correlation with pain percepts provided by quantitative sensory testing. Genome editing in human induced pluripotent stem cell-derived nociceptors enables the interrogation of the causal role of genes in the regulation of nociceptor function. Both human and rodent nociceptors are more heterogeneous at a molecular level than previously appreciated, and while we find that there are broad similarities between human and rodent nociceptors there are also important differences involving ion channel function, expression, and cellular excitability. These technological advances have emphasized the maladaptive plastic changes occurring in human nociceptors following injury that contribute to chronic pain. Studying human nociceptors has revealed new therapeutic targets for the suppression of chronic pain and enhanced repair. Cellular models of human nociceptors have enabled the screening of small molecule and gene therapy approaches on nociceptor function, and in some cases have enabled correlation with clinical outcomes. Undoubtedly, challenges remain. Many of these techniques are difficult to implement at scale, current induced pluripotent stem cell differentiation protocols do not generate the full diversity of nociceptor populations, and we still have a relatively poor understanding of inter-individual variation in nociceptors due to factors such as age, sex, or ethnicity. We hope our ability to directly investigate human nociceptors will not only aid our understanding of the fundamental neurobiology underlying acute and chronic pain but also help bridge the translational gap.

Quantitative differences in neuronal subpopulations between mouse and human dorsal root ganglia demonstrated with RNAscope in situ hybridization

Next-generation transcriptomics in combination with imaging-based approaches have emerged as powerful tools for the characterization of dorsal root ganglion (DRG) neuronal subpopulations. The mouse DRG has been well characterized by many independently conducted studies with convergent findings, but few studies have directly compared expression of population markers between mouse and human. This is important because of our increasing reliance on the mouse as a preclinical model for translational studies. Although calcitonin gene-related peptide (CGRP) and P2X purinergic ion channel type 3 receptor (P2X3R) have been used to define peptidergic and nonpeptidergic nociceptor subpopulations, respectively, in mouse DRG, these populations may be different in other species. To directly test this, as well as a host of other markers, we used multiplex RNAscope in situ hybridization to elucidate the distribution of a multitude of unique and classic neuronal mRNAs in peptidergic (CGRP-expressing) and nonpeptidergic (P2X3R-expressing) nociceptor subpopulations in mouse and human DRG. We found a large overlapping CGRP and P2X3R neuronal subpopulation in human, lumbar DRG that was not present in mouse. We also found differential expression in a variety of mRNAs for transient receptor potential channels, cholinergic receptors, potassium channels, sodium channels, and other markers/targets. These data offer insights into the spatial and functional organization of neuronal cell subpopulations in the rodent and human DRG and support the idea that sensory system organizational principles are likely different between both species.

A ligand-receptor interactome platform for discovery of pain mechanisms and therapeutic targets

In the peripheral nervous system, ligand-receptor interactions between cells and neurons shape sensory experience, including pain. We set out to identify the potential interactions between sensory neurons and peripheral cell types implicated in disease-associated pain. Using mouse and human RNA sequencing datasets and computational analysis, we created interactome maps between dorsal root ganglion (DRG) sensory neurons and an array of normal cell types, as well as colitis-associated glial cells, rheumatoid arthritis-associated synovial macrophages, and pancreatic tumor tissue. These maps revealed a common correlation between the abundance of heparin-binding EGF-like growth factor (HBEGF) in peripheral cells with that of its receptor EGFR (a member of the ErbB family of receptors) in DRG neurons. Subsequently, we confirmed that increased abundance of HBEGF enhanced nociception in mice, likely acting on DRG neurons through ErbB family receptors. Collectively, these interactomes highlight ligand-receptor interactions that may lead to treatments for disease-associated pain and, furthermore, reflect the complexity of cell-to-neuron signaling in chronic pain states.

Skin-Nerve Co-Culture Systems for Disease Modeling and Drug Discovery

Prominent clinical problems related to the skin-nerve interface include barrier dysfunction and erythema, but it is the symptoms of pain and itch that most often lead patients to seek medical treatment. Tissue-engineered innervated skin models provide an excellent solution for studying the mechanisms underlying neurocutaneous disorders for drug screening, and cutaneous device development. Innervated skin substitutes provide solutions beyond traditional monolayer cultures and have advantages that make them preferable to in vivo animal studies for certain applications, such as measuring somatosensory transduction. The tissue-engineered innervated skin models replicate the complex stratified epidermis that provides barrier function in native skin, a feature that is lacking in monolayer co-cultures, while allowing for a level of detail in measurement of nerve morphology and function that cannot be achieved in animal models. In this review, the advantages and disadvantages of different cell sources and scaffold materials will be discussed and a presentation of the current state of the field is reviewed. Impact statement A review of the current state of innervated skin substitutes and the considerations that need to be addressed when developing these models. Tissue-engineered skin substitutes are customizable and provide barrier function allowing for screening of topical drugs and for studying nerve function.

GCaMP as an indirect measure of electrical activity in rat trigeminal ganglion neurons

While debate continues over whether somatosensory information is transmitted via labeled line, population coding, frequency coding, or some combination therein, researchers have begun to address this question at the level of the primary afferent by using optical approaches that enable the assessment of neural activity in hundreds to even thousands of neurons simultaneously. However, with limited availability of tools to optically assess electrical activity in large populations of neurons, researchers have turned to genetically encoded Ca²⁺ indicators (GECIs) including GCaMP to enable the detection of increases in cytosolic Ca²⁺ concentrations as a correlate for neuronal activity. One of the most widely used GECIs is GCaMP6, which is available in three different versions tuned for sensitivity (GCaMP6s), speed (GCaMP6f), or a balance of the two (GCaMP6m). In order to determine if these issues were unique to GCaMP6 itself, or if they were inherent to more than one generation of GCaMP, we also characterized jGCaMP7. In the present study, we sought to determine the utility of the three GCaMP6 isoforms to detect changes in activity in primary afferents at frequencies ranging from 0.1-30 Hz. Given the heterogeneity of sensory neurons, we also compared the performance of each GCaMP6 isoform in subpopulations of neurons defined by properties used to identify putative nociceptive afferents: cell body size, isolectin B4 (IB4) binding, and capsaicin sensitivity. Finally, we compared results generated with GCaMP6 with that generated from neurons expressing the next generation of GCaMP, jGCaMP7s and jGCaMP7f. A viral approach, with AAV9-CAG-GCaMP6s/m/f, was used to drive GECI expression in acutely dissociated rat trigeminal ganglion (TG) neurons, and neural activity was driven by electrical field stimulation. Infection efficiency with the AAV serotype was high >95 %, and the impact of GCaMP6 expression in TG neurons over the period of study (<10 days) on the regulation of intracellular Ca²⁺, as assessed with fura-2, was minimal. Having confirmed that the field stimulation evoked Ca²⁺ transients were dependent on Ca²⁺ influx secondary to the activation of action potentials and voltage-gated Ca²⁺ channels, we also confirmed that the signal-to-noise ratio for each of the isoforms was excellent, enabling detection of a single spike in>90% of neurons. However, the utility of the GCaMP6 isoforms to enable an assessment of the firing frequency let alone changes in firing frequency of each neuron was relatively limited and isoform specific: GCaMP6s and 6m had the lowest resolution, enabling detection of spikes at 3 Hz in 15% and 32% of neurons respectively, but it was possible to resolve discrete single spikes up to 10 Hz in 36% of GCaMP6f neurons. Unfortunately, using other parameters of the Ca²⁺ transient, such as magnitude of the transient or the rate of rise, did not improve the range over which these indicators could be used to assess changes in spike number or firing frequency. Furthermore, in the presence of ongoing neural activity, it was even more difficult to detect a change in firing frequency. The frequency response relationship for the increase in Ca²⁺ was highly heterogeneous among sensory neurons and was influenced by both the GCaMP6 isoform used to assess it, the timing between the delivery of stimulation trains (inter-burst interval), and afferent subpopulation. Notably, the same deficiencies were observed with jGCaMP7s and 7f in resolving the degree of activity as were present for the GCaMP6 isoforms. Together, these data suggest that while both GCaMP6 and jGCaMP7 are potentially useful tools in sensory neurons to determine the presence or absence of neural activity, the ability to discriminate changes in firing frequency ≥ 3 Hz is extremely limited. As a result, GECIs should probably not be used in sensory neurons to assess changes in activity within or between subpopulations of neurons.