The K(+) channel GIRK2 is both necessary and sufficient for peripheral opioid-mediated analgesia

Opioids-Induced Long QT Syndrome: A Challenge to Cardiac Health

The challenge posed by opioid overdose has become a significant concern for health systems due to the complexities associated with drug prohibition, widespread clinical use, and potential abuse. In response, healthcare professionals have primarily concentrated on mitigating the hallucinogenic and respiratory depressant consequences of opioid overdose to minimize associated risks. However, it is crucial to acknowledge that most opioids possess the capacity to prolong the QT interval, particularly in cases of overdose, thereby potentially resulting in severe ventricular arrhythmias and even sudden death if timely intervention is not implemented. Consequently, alongside addressing the typical adverse effects of opioids, it is imperative to consider their cardiotoxicity. To enhance comprehension of the correlation between opioids and arrhythmias, identify potential targets for prompt intervention, and mitigate the hazards associated with clinical utilization, an exploration of the interaction between drugs and ion channels, as well as their underlying mechanisms, becomes indispensable. This review primarily concentrates on elucidating the impact of opioid drugs on diverse ion channels, investigating recent advancements in this domain, and attaining a deeper understanding of the mechanisms underlying the prolongation of the QT interval by opioid drugs, along with potential interventions.

Novel Opioids in the Setting of Acute Postoperative Pain: A Narrative Review

Although traditional opioids such as morphine and oxycodone are commonly used in the management of acute postoperative pain, novel opioids may play a role as alternatives that provide potent pain relief while minimizing adverse effects. In this review, we discuss the mechanisms of action, findings from preclinical studies and clinical trials, and potential advantages of several novel opioids. The more established include oliceridine (biased ligand activity to activate analgesia and downregulate opioid-related adverse events), tapentadol (mu-opioid agonist and norepinephrine reuptake inhibitor), and cebranopadol (mu-opioid agonist with nociceptin opioid peptide activity)-all of which have demonstrated success in the clinical setting when compared to traditional opioids. On the other hand, dinalbuphine sebacate (DNS; semi-synthetic mu partial antagonist and kappa agonist), dual enkephalinase inhibitors (STR-324, PL37, and PL265), and endomorphin-1 analog (CYT-1010) have shown good efficacy in preclinical studies with future plans for clinical trials. Rather than relying solely on mu-opioid receptor agonism to relieve pain and risk opioid-related adverse events (ORAEs), novel opioids make use of alternative mechanisms of action to treat pain while maintaining a safer side-effect profile, such as lower incidence of nausea, vomiting, sedation, and respiratory depression as well as reduced abuse potential.

Regulatory lipid vicinal diols counteract the biological activity of epoxy fatty acids and can act as biomarkers and mechanisms for disease progression

Polyunsaturated fatty acids (PUFAs) are essential fatty acids required for human health and are obtained primarily from food or synthesized in the body by highly regulated processes. The metabolites of these lipids, formed largely through the action of cyclooxygenase, lipoxygenase, or cytochrome P450 (CYP450) enzymes, are responsible for multiple biological functions including inflammation, tissue repair, cell proliferation, blood vessel permeability, and immune cell behavior. The role of these regulatory lipids in disease has been well studied since their discovery as druggable targets; however, the metabolites generated downstream of these pathways have only recently gained attention for regulating biology. Specifically, the biological activity of lipid vicinal diols formed from the metabolism of CYP450-generated epoxy fatty acids (EpFA) by epoxide hydrolases were previously thought to have little biological activity but increasingly are recognized as promoting inflammation and brown fat adipogenesis, and exciting neurons through the regulation of ion channel activity at low concentrations. These metabolites also appear to balance the action of the EpFA precursor. For example, EpFA demonstrate the ability to resolve inflammation and reduce pain, while some lipid diols, through opposing mechanisms, promote inflammation and pain. This review describes recent studies that highlight the role of regulatory lipids, focusing on the balance between EpFA and their diol metabolites in promoting or resolving disease.

Modulation of G-protein activation, calcium currents and opioid receptor phosphorylation by the pH-dependent antinociceptive agonist NFEPP

N-(3-fluoro-1-phenethylpiperidine-4-yl)-N-phenyl propionamide is a newly-designed pain killer selectively activating G-protein-coupled mu-opioid receptors (MOR) in acidic injured tissues, and therefore devoid of central side effects which are typically elicited at normal pH values in healthy tissues. However, the neuronal mechanisms underlying NFEPP's antinociceptive effects were not examined in detail so far. Voltage-dependent Ca²⁺ channels (VDCCs) in nociceptive neurons play a major role in the generation and inhibition of pain. In this study, we focused on the effects of NFEPP on calcium currents in rat dorsal root ganglion (DRG) neurons. The inhibitory role of the G-protein subunits G_i/o and Gβγ on VDCCs was investigated using the blockers pertussis toxin and gallein, respectively. GTPγS binding, calcium signals and MOR phosphorylation were also investigated. All experiments were performed at acidic and normal pH values using NFEPP in comparison to the conventional opioid agonist fentanyl. At low pH, NFEPP produced more efficient G-protein activation in transfected HEK293 cells and significantly reduced VDCCs in depolarized DRG neurons. The latter effect was mediated by Gβγ subunits, and NFEPP-mediated MOR phosphorylation was pH-dependent. Fentanyl's responses were not affected by pH changes. Our data indicate that NFEPP-induced MOR signaling is more effective at low pH and that the inhibition of calcium channels in DRG neurons underlies NFEPP's antinociceptive actions.

Opioid Receptors Modulate Firing and Synaptic Transmission in the Paraventricular Nucleus of the Thalamus

The paraventricular nucleus of the thalamus (PVT) is involved in drug addiction-related behaviors, and morphine is a widely used opioid for the relief of severe pain. Morphine acts via opioid receptors, but the function of opioid receptors in the PVT has not been fully elucidated. Here, we used in vitro electrophysiology to study neuronal activity and synaptic transmission in the PVT of male and female mice. Activation of opioid receptors suppresses the firing and inhibitory synaptic transmission of PVT neurons in brain slices. On the other hand, the involvement of opioid modulation is reduced after chronic morphine exposure, probably because of desensitization and internalization of opioid receptors in the PVT. Overall, the opioid system is essential for the modulation of PVT activities.SIGNIFICANCE STATEMENT Opioid receptors modulate the activities and synaptic transmission in the PVT by suppressing the firing rate and inhibitory synaptic inputs. These modulations were largely diminished after chronic morphine exposure.

Alternative pain management via endocannabinoids in the time of the opioid epidemic: Peripheral neuromodulation and pharmacological interventions

The use of opioids in pain management is hampered by the emergence of analgesic tolerance, which leads to increased dosing and side effects, both of which have contributed to the opioid epidemic. One promising potential approach to limit opioid analgesic tolerance is activating the endocannabinoid system in the CNS, via activation of CB1 receptors in the descending pain inhibitory pathway. In this review, we first discuss preclinical and clinical evidence revealing the potential of pharmacological activation of CB1 receptors in modulating opioid tolerance, including activation by phytocannabinoids, synthetic CB1 receptor agonists, endocannabinoid degradation enzyme inhibitors, and recently discovered positive allosteric modulators of CB1 receptors. On the other hand, as non-pharmacological pain relief is advocated by the US-NIH to combat the opioid epidemic, we also discuss contributions of peripheral neuromodulation, involving the electrostimulation of peripheral nerves, in addressing chronic pain and opioid tolerance. The involvement of supraspinal endocannabinoid systems in peripheral neuromodulation-induced analgesia is also discussed. LINKED ARTICLES: This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

Sodium Channels and Local Anesthetics-Old Friends With New Perspectives

The long history of local anesthetics (LAs) starts out in the late 19th century when the content of coca plant leaves was discovered to alleviate pain. Soon after, cocaine was established and headed off to an infamous career as a substance causing addiction. Today, LAs and related substances-in modified form-are indispensable in our clinical everyday life for pain relief during and after minor and major surgery, and dental practices. In this review, we elucidate on the interaction of modern LAs with their main target, the voltage-gated sodium channel (Navs), in the light of the recently published channel structures. Knowledge of the 3D interaction sites of the drug with the protein will allow to mechanistically substantiate the comprehensive data available on LA gating modification. In the 1970s it was suggested that LAs can enter the channel pore from the lipid phase, which was quite prospective at that time. Today we know from cryo-electron microscopy structures and mutagenesis experiments, that indeed Navs have side fenestrations facing the membrane, which are likely the entrance for LAs to induce tonic block. In this review, we will focus on the effects of LA binding on fast inactivation and use-dependent inhibition in the light of the proposed new allosteric mechanism of fast inactivation. We will elaborate on subtype and species specificity and provide insights into modelling approaches that will help identify the exact molecular binding orientation, access pathways and pharmacokinetics. With this comprehensive overview, we will provide new perspectives in the use of the drug, both clinically and as a tool for basic ion channel research.

Effect of Fentanyl as an Adjuvant to Brachial Plexus Block for Upper Extremity Surgeries: A Systematic Review and Meta-Analysis of RCTs

Objective. To assess if the addition of fentanyl to brachial plexus block has an impact on anesthetic outcomes and complication rates in patients undergoing upper extremity surgeries. Methods. We explore the PubMed, Embase, ScienceDirect, CENTRAL, and Google Scholar databases for all randomized controlled trials (RCTs) comparing adjuvant fentanyl with placebo/no drug for patients undergoing upper extremity surgery under brachial plexus block. Outcomes assessed were onset, duration of sensory and motor anesthesia, complications, and postoperative analgesia scores. Meta-analysis was conducted utilizing a random-effects model. The risk of bias was assessed using the Cochrane Collaboration’s risk of bias assessment tool 2. Certainty of evidence was assessed using GRADE. Subgroup analysis was conducted depending upon the approach of brachial plexus block and type of local anesthetic. Results. Twelve RCTs with 660 patients were included. Addition of fentanyl had no effect on onset of sensory anesthesia (11 studies; MD: 0.48; 95% CI: −1.81, 0.85; I2 = 96%; $p=0.48$ ) but significantly shortened onset of motor anesthesia (8 studies; MD: −2.36; 95% CI: −3.99, −0.74; I2 = 96%; $p=0.48$ ). Duration of sensory anesthesia (9 studies; MD: 82.81; 95% CI: 41.81, 123.81; I2 = 99%; $p<0.0001$ ) and motor anesthesia (7 studies; MD: 93.41; 95% CI: 42.35, 144.46; I2 = 99%; $p=0.0003$ ) was significantly increased with addition of fentanyl. The certainty of evidence-based on GRADE was deemed to be moderate for both onset and duration of anesthesia. The incidence of overall complications (nausea/vomiting and pruritis) was significantly higher in the fentanyl group (7 studies; OR: 2.14; 95% CI: 1.04, 4.40; I2 = 8%; $p=0.04$ ) but with low certainty of evidence. Conclusions. Adjuvant fentanyl with brachial plexus block improves the onset of motor anesthesia but not sensory anesthesia. The duration of both sensory and motor anesthesia is significantly prolonged with fentanyl by around 83–93 minutes. However, clinicians should be aware that complications such as nausea/vomiting and pruritis are increased twofold with the addition of the drug. Current evidence is limited risk of bias in the RCTs and high heterogeneity in the meta-analyses.

Peripheral ablation of type Ⅲ adenylyl cyclase induces hyperalgesia and eliminates KOR-mediated analgesia in mice

Ca2+/calmodulin-stimulated group Ⅰ adenylyl cyclase (AC) isoforms AC1 and AC8 have been involved in nociceptive processing and morphine responses. However, whether AC3, another member of group I ACs, is involved in nociceptive transmission and regulates opioid receptor signaling remain elusive. Here we report that conditional knockout of AC3 (AC3CKO) in L3 and L4 DRGs robustly facilitates the mouse nociceptive responses, decreases voltage-gated potassium (Kv) channel currents and increases neuronal excitability. Also, AC3CKO eliminates the analgesic effect of κ opioid receptor (KOR) agonist and its inhibition on Kv channel by classical Gαi/o signaling or nonclassical direct interaction of KOR and AC3 proteins. Interestingly, significantly upregulated AC1 level and cAMP concentration are detected in AC3 deficient DRGs. Inhibition of AC1 completely reversed cAMP upregulation, neuronal excitability enhancement and nociceptive behavioral hypersensitivity in AC3CKO mice. Our findings suggest a crucial role of peripheral AC3 in nociceptive modulation and KOR opioid analgesia.

Neuropharmacological basis for multimodal analgesia in chronic pain

Managing chronic pain remains a major unmet clinical challenge. Patients can be treated with a range of interventions, but pharmacotherapy is the most common. These include opioids, antidepressants, calcium channel modulators, sodium channel blockers, and nonsteroidal anti-inflammatory drugs. Many of these drugs target a particular mechanism; however, chronic pain in many diseases is multifactorial and induces plasticity throughout the sensory neuroaxis. Furthermore, comorbidities such as depression, anxiety, and sleep disturbances worsen quality of life. Given the complexity of mechanisms and symptoms in patients, it is unsurprising that many fail to achieve adequate pain relief from a single agent. The efforts to develop novel drug classes with better efficacy have not always proved successful; a multimodal or combination approach to analgesia is an important strategy in pain control. Many patients frequently take more than one medication, but high-quality evidence to support various combinations is often sparse. Ideally, combining drugs would produce synergistic action to maximize analgesia and reduce side effects, although sub-additive and additive analgesia is still advantageous if additive side-effects can be avoided. In this review, we discuss pain mechanisms, drug actions, and the rationale for mechanism-led treatment selection.Abbreviations: COX - cyclooxygenase, CGRP - calcitonin gene-related peptide, CPM - conditioned pain modulation, NGF - nerve growth factor, NNT - number needed to treat, NMDA - N-methyl-d-aspartate, NSAID - nonsteroidal anti-inflammatory drugs, TCA - tricyclic antidepressant, SNRI - serotonin-noradrenaline reuptake inhibitor, QST - quantitative sensory testing.

Analgesic α-conotoxins modulate native and recombinant GIRK1/2 channels via activation of GABAB receptors and reduce neuroexcitability

BACKGROUND AND PURPOSE

Activation of GIRK channels via G protein-coupled GABA_B receptors has been shown to attenuate nociceptive transmission. The analgesic α-conotoxin Vc1.1 activates GABA_B receptors resulting in inhibition of Ca_v 2.2 and Ca_v 2.3 channels in mammalian primary afferent neurons. Here, we investigated the effects of analgesic α-conotoxins on recombinant and native GIRK-mediated K⁺ currents and on neuronal excitability.

EXPERIMENTAL APPROACH

The effects of analgesic α-conotoxins, Vc1.1, RgIA, and PeIA, were investigated on inwardly-rectifying K⁺ currents in HEK293T cells recombinantly co-expressing either heteromeric human GIRK1/2 or homomeric GIRK2 subunits, with GABA_B receptors. The effects of α-conotoxin Vc1.1 and baclofen were studied on GIRK-mediated K⁺ currents and the passive and active electrical properties of adult mouse dorsal root ganglion neurons.

KEY RESULTS

Analgesic α-conotoxins Vc1.1, RgIA, and PeIA potentiate inwardly-rectifying K⁺ currents in HEK293T cells recombinantly expressing human GIRK1/2 channels and GABA_B receptors. GABA_B receptor-dependent GIRK channel potentiation by Vc1.1 and baclofen occurs via a pertussis toxin-sensitive G protein and is inhibited by the selective GABA_B receptor antagonist CGP 55845. In adult mouse dorsal root ganglion neurons, GABA_B receptor-dependent GIRK channel potentiation by Vc1.1 and baclofen hyperpolarizes the cell membrane potential and reduces excitability.

CONCLUSIONS AND IMPLICATIONS

This is the first report of GIRK channel potentiation via allosteric α-conotoxin Vc1.1-GABA_B receptor agonism, leading to decreased neuronal excitability. Such action potentially contributes to the analgesic effects of Vc1.1 and baclofen observed in vivo.

Peripherally Acting Opioids in Orofacial Pain

The activation of opioid receptors by exogenous or endogenous opioids can produce significant analgesic effects in peripheral tissues. Numerous researchers have demonstrated the expression of peripheral opioid receptors (PORs) and endogenous opioid peptides (EOPs) in the orofacial region. Growing evidence has shown the involvement of PORs and immune cell-derived EOPs in the modulation of orofacial pain. In this review, we discuss the role of PORs and EOPs in orofacial pain and the possible cellular mechanisms involved. Furthermore, the potential development of therapeutic strategies for orofacial pain is also summarized.

TRV130 partial agonism and capacity to induce anti-nociceptive tolerance revealed through reducing available μ-opioid receptor number

BACKGROUND AND PURPOSE

β-Arrestin2 recruitment to μ-receptors may contribute to the development of opioid side effects. This possibility led to the development of TRV130 and PZM21, opioids reportedly biased against β-arrestin2 recruitment in favour of G-protein signalling. However, low efficacy β-arrestin2 recruitment by TRV130 and PZM21 may simply reflect partial agonism overlooked due to overexpression of μ-receptors.

EXPERIMENTAL APPROACH

Efficacies and apparent potencies of DAMGO, morphine, PZM21 and TRV130 as stimulators of β-arrestin2 recruitment and inhibitors of cAMP accumulation were assessed in CHO cells stably expressing μ-receptors. Receptor availability was depleted through prior exposure of cells to the irreversible antagonist, β-FNA. We also examined whether μ-receptor availability influences TRV130 anti-nociception and/or tolerance using the tail withdrawal assay in wild-type C57BL/6 and μ+/- mice.

KEY RESULTS

Morphine, PZM21 and TRV130 were partial agonists in the β-arrestin2 recruitment assay. Only TRV130 exhibited partial agonism in the cAMP assay. Exposure to β-FNA to reduce μ-receptor availability further limited the efficacy of TRV130 and revealed morphine and PZM21 to be partial agonists. Despite having partial efficacy in vitro, TRV130 caused potent anti-nociception (ED50 : 0.33 mg·kg-1 ) in wild-type mice, without tolerance after daily administration for 10 days. TRV130 caused similar anti-nociception in μ+/- mice, with marked tolerance on day 4 of injections.

CONCLUSION AND IMPLICATIONS

Our findings emphasise the importance of receptor reserve when characterising μ-receptor agonists. Reduced receptor availability reveals that TRV130 is a partial agonist capable of tolerance, despite having limited efficacy for β-arrestin2 recruitment to the μ-receptor.

Central opioid receptors mediate morphine-induced itch and chronic itch via disinhibition

Opioids such as morphine are mainstay treatments for clinical pain conditions. Itch is a common side effect of opioids, particularly as a result of epidural or intrathecal administration. Recent progress has advanced our understanding of itch circuits in the spinal cord. However, the mechanisms underlying opioid-induced itch are not fully understood, although an interaction between µ-opioid receptor (MOR) and gastrin-releasing peptide receptor (GRPR) in spinal GRPR-expressing neurons has been implicated. In this study we investigated the cellular mechanisms of intrathecal opioid-induced itch by conditional deletion of MOR-encoding Oprm1 in distinct populations of interneurons and sensory neurons. We found that intrathecal injection of the MOR agonists morphine or DAMGO elicited dose-dependent scratching as well as licking and biting, but this pruritus was totally abolished in mice with a specific Oprm1 deletion in Vgat+ neurons [Oprm1-Vgat (Slc32a1)]. Loss of MOR in somatostatin+ interneurons and TRPV1+ sensory neurons did not affect morphine-induced itch but impaired morphine-induced antinociception. In situ hybridization revealed Oprm1 expression in 30% of inhibitory and 20% of excitatory interneurons in the spinal dorsal horn. Whole-cell recordings from spinal cord slices showed that DAMGO induced outward currents in 9 of 19 Vgat+ interneurons examined. Morphine also inhibited action potentials in Vgat+ interneurons. Furthermore, morphine suppressed evoked inhibitory postsynaptic currents in postsynaptic Vgat- excitatory neurons, suggesting a mechanism of disinhibition by MOR agonists. Notably, morphine-elicited itch was suppressed by intrathecal administration of NPY and abolished by spinal ablation of GRPR+ neurons with intrathecal injection of bombesin-saporin, whereas intrathecal GRP-induced itch response remained intact in mice lacking Oprm1-Vgat. Intrathecal bombesin-saporin treatment reduced the number of GRPR+ neurons by 97% in the lumber spinal cord and 91% in the cervical spinal cord, without changing the number of Oprm1+ neurons. Additionally, chronic itch from DNFB-induced allergic contact dermatitis was decreased by Oprm1-Vgat deletion. Finally, naloxone, but not peripherally restricted naloxone methiodide, inhibited chronic itch in the DNFB model and the CTCL model, indicating a contribution of central MOR signalling to chronic itch. Our findings demonstrate that intrathecal morphine elicits itch via acting on MOR on spinal inhibitory interneurons, leading to disinhibition of the spinal itch circuit. Our data also provide mechanistic insights into the current treatment of chronic itch with opioid receptor antagonist such as naloxone.

Morphine coordinates SST and PV interneurons in the prelimbic cortex to disinhibit pyramidal neurons and enhance reward

Opioids, such as morphine, are clinic analgesics which induce euphoria. Morphine exposure modifies the excitability and functional interactions between neurons, while the underlying cellular and molecular mechanisms, especially how morphine assembles heterogeneous interneurons (INs) in prelimbic cortex (PrL) to mediate disinhibition and reward, are not clear. Using approaches of optogenetics, electrophysiology, and cell type-specific RNA-seq, we show that morphine attenuates the inhibitory synaptic transmission from parvalbumin⁺ (PV)-INs onto pyramidal neurons in PrL via μ-opioid receptor (MOR) in PV-INs. Meanwhile, morphine enhances the inhibitory inputs from somatostatin⁺ (SST)-INs onto PV-INs, and thus disinhibits pyramidal neurons via δ-opioid receptor (DOR)-dependent Rac1 upregulation in SST-INs. We show that MOR in PV-INs is required for morphine-induced behavioral sensitization, while DOR as well as Rac1 activity in SST-INs is required for morphine-induced conditioned place preference and hyper-locomotion. These results reveal that SST- and PV-INs, functioning in PrL as a disinhibitory architecture, are coordinated by morphine via different opioid receptors to disinhibit pyramidal neurons and enhance reward.

A low pKa ligand inhibits cancer-associated pain in mice by activating peripheral mu-opioid receptors

The newly designed fentanyl derivative [( ±)-N-(3-fluoro-1-phenethylpiperidine-4-yl)-N-phenyl propionamide] (NFEPP) was recently shown to produce analgesia selectively via peripheral mu-opioid receptors (MOR) at acidic pH in rat inflamed tissues. Here, we examined the pH-dependency of NFEPP binding to brain MOR and its effects on bone cancer-induced pain in mice. The IC50 of NFEPP to displace bound [3H]-DAMGO was significantly higher compared to fentanyl at pH 7.4, but no differences were observed at pH 5.5 or 6.5. Intravenous NFEPP (30-100 nmol/kg) or fentanyl (17-30 nmol/kg) inhibited heat hyperalgesia in mice inoculated with B16-F10 melanoma cells. The peripherally-restricted opioid receptor antagonist naloxone-methiodide reversed the effect of NFEPP (100 nmol/kg), but not of fentanyl (30 nmol/kg). The antihyperalgesic effect of NFEPP was abolished by a selective MOR- (cyprodime), but not delta- (naltrindole) or kappa- (nor-binaltorphimine) receptor antagonists. Ten-fold higher doses of NFEPP than fentanyl induced maximal antinociception in mice without tumors, which was reversed by the non-restricted antagonist naloxone, but not by naloxone-methiodide. NFEPP also reduced heat hyperalgesia produced by fibrosarcoma- (NCTC 2472) or prostate cancer-derived (RM1) cells. These data demonstrate the increased affinity of NFEPP for murine MOR at low pH, and its ability to inhibit bone cancer-induced hyperalgesia through peripheral MOR. In mice, central opioid receptors may be activated by ten-fold higher doses of NFEPP.

Participation of the opioid receptor - nitric oxide - cGMP - K+ channel pathway in the peripheral antinociceptive effect of nalbuphine and buprenorphine in rats

The aim of this study was to examine if the peripheral antinociceptive effects of the opioid agonist/antagonist nalbuphine and buprenorphine involve the sequential participation of nitric oxide (NO) and cyclic guanosine monophosphate (cGMP) synthesis followed by K⁺ channel opening in the formalin test. Wistar rats (180-220 g) were injected in the dorsal surface of the right hind paw with formalin (1%). Rats received a subcutaneous (s.c.) injection into the dorsal surface of the paw of vehicles or increasing doses of nalbuphine (50-200 μg/paw) or buprenorphine (1-5 μg/paw) 20 min before formalin injection into the paw. Nalbuphine antinociception was reversed by the s.c. injection into the paw of the inhibitor of NO synthesis (N^G-nitro-l-arginine methyl ester (L-NAME)), by the inhibitor of guanylyl cyclase (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ)), by the K_ir6.1-2, ATP-sensitive K⁺ channel inhibitors (glibenclamide and glipizide), by the K_Ca2.1-3, small conductance Ca²⁺-activated K⁺ channel blocker (apamin), by the K_Ca1.1, large conductance Ca²⁺-activated K⁺ channel blocker (charybdotoxin), and by the K_V, voltage-dependent K⁺ channel inhibitors (4-aminopyridine (4-AP) and tetraethylammonium chloride (TEA)). The antinociceptive effect produced by buprenorphine was blocked by the s.c. injection of 4-AP and TEA but not by L-NAME, ODQ, glibenclamide, glipizide, apamin, or charybdotoxin. The present results provide evidence for differences in peripheral mechanisms of action between these opioid drugs.

K+ Channels in Primary Afferents and Their Role in Nerve Injury-Induced Pain

Sensory abnormalities generated by nerve injury, peripheral neuropathy or disease are often expressed as neuropathic pain. This type of pain is frequently resistant to therapeutic intervention and may be intractable. Numerous studies have revealed the importance of enduring increases in primary afferent excitability and persistent spontaneous activity in the onset and maintenance of peripherally induced neuropathic pain. Some of this activity results from modulation, increased activity and /or expression of voltage-gated Na⁺ channels and hyperpolarization-activated cyclic nucleotide–gated (HCN) channels. K⁺ channels expressed in dorsal root ganglia (DRG) include delayed rectifiers (K_v1.1, 1.2), A-channels (K_v1.4, 3.3, 3.4, 4.1, 4.2, and 4.3), KCNQ or M-channels (K_v7.2, 7.3, 7.4, and 7.5), ATP-sensitive channels (K_IR6.2), Ca²⁺-activated K⁺ channels (K_Ca1.1, 2.1, 2.2, 2.3, and 3.1), Na⁺-activated K⁺ channels (K_Ca4.1 and 4.2) and two pore domain leak channels (K_2p; TWIK related channels). Function of all K⁺ channel types is reduced via a multiplicity of processes leading to altered expression and/or post-translational modification. This also increases excitability of DRG cell bodies and nociceptive free nerve endings, alters axonal conduction and increases neurotransmitter release from primary afferent terminals in the spinal dorsal horn. Correlation of these cellular changes with behavioral studies provides almost indisputable evidence for K⁺ channel dysfunction in the onset and maintenance of neuropathic pain. This idea is underlined by the observation that selective impairment of just one subtype of DRG K⁺ channel can produce signs of pain in vivo. Whilst it is established that various mediators, including cytokines and growth factors bring about injury-induced changes in DRG function and excitability, evidence presently available points to a seminal role for interleukin 1β (IL-1β) in control of K⁺ channel function. Despite the current state of knowledge, attempts to target K⁺ channels for therapeutic pain management have met with limited success. This situation may change with the advent of personalized medicine. Identification of specific sensory abnormalities and genetic profiling of individual patients may predict therapeutic benefit of K⁺ channel activators.

Antinociceptive effect of selective G protein-gated inwardly rectifying K+ channel agonist ML297 in the rat spinal cord

The G protein-gated inwardly rectifying K⁺ (GIRK) channels play important signaling roles in the central and peripheral nervous systems. However, the role of GIRK channel activation in pain signaling remains unknown mainly due to the lack of potent and selective GIRK channel activators until recently. The present study was designed to determine the effects and mechanisms of ML297, a selective GIRK1/2 activator, on nociception in the spinal cord by using behavioral studies and whole-cell patch-clamp recordings from substantia gelatinosa (SG) neurons. Rats were prepared for chronic lumber catheterization and intrathecal administration of ML297. The nociceptive flexion reflex was tested using an analgesy-meter, and the influence on motor performance was assessed using an accelerating rotarod. We also investigated pre- and post-synaptic actions of ML297 in spinal cord preparations by whole-cell patch-clamp recordings. Intrathecal administration of ML297 increased the mechanical nociceptive threshold without impairing motor function. In voltage-clamp mode of patch-clamp recordings, bath application of ML297 induced outward currents in a dose-dependent manner. The ML297-induced currents demonstrated specific equilibrium potential like other families of potassium channels. At high concentration, ML297 depressed miniature excitatory postsynaptic currents (mEPSCs) but not their amplitude. The ML297-induced outward currents and suppression of mEPSCs were not inhibited by naloxone, a μ-opioid receptor antagonist. These results demonstrated that intrathecal ML297 showed the antinociceptive effect, which was mediated through direct activation of pre- and post-synaptic GIRK channels. Selective GIRK channel activation is a promising strategy for the development of new agents against chronic pain and opioid tolerance.

Identification of Novel Targets of Knee Osteoarthritis Shared by Cartilage and Synovial Tissue

Arthritis is the leading cause of disability among adults, while osteoarthritis (OA) is the most common form of arthritis that results in cartilage loss. However, accumulating evidence suggests that the protective hyaline cartilage should not be the sole focus of OA treatment. Particularly, synovium also plays essential roles in OA’s initiation and progression and warrants serious consideration when battling against OA. Thus, biomarkers with similar OA-responsive expressions in cartilage and synovium should be the potential targets for OA treatment. On the other hand, molecules with a distinguished response during OA in cartilage and synovium should be ruled out as OA therapeutic(s) to avoid controversial effects in different tissues. Here, to pave the path for developing a new generation of OA therapeutics, two published transcriptome datasets of knee articular cartilage and synovium were analyzed in-depth. Genes with statistically significantly different expression in OA and healthy cartilage were compared with those in the synovium. Thirty-five genes with similar OA-responsive expression in both tissues were identified while recognizing three genes with opposite OA-responsive alteration trends in cartilage and synovium. These genes were clustered based on the currently available knowledge, and the potential impacts of these clusters in OA were explored.

Immune cell-mediated opioid analgesia

Pathological pain is regulated by a balance between pro-algesic and analgesic mechanisms. Interactions between opioid peptide-producing immune cells and peripheral sensory neurons expressing opioid receptors represent a powerful intrinsic pain control in animal models and in humans. Therefore, treatments based on general suppression of immune responses have been mostly unsuccessful. It is highly desirable to develop strategies that specifically promote neuro-immune communication mediated by opioids. Promising examples include vaccination-based recruitment of opioid-containing leukocytes to painful tissue and the local reprogramming of pro-algesic immune cells into analgesic cells producing and secreting high amounts of opioid peptides. Such approaches have the potential to inhibit pain at its origin and be devoid of central and systemic side effects of classical analgesics. In support of these concepts, in this article, we describe the functioning of peripheral opioid receptors, migration of opioid-producing immune cells to inflamed tissue, opioid peptide release, and the consequent pain relief. Conclusively, we provide clinical evidence and discuss therapeutic opportunities and challenges associated with immune cell-mediated peripheral opioid analgesia.

The role of peripheral opioid receptors in orofacial pain

Opioid receptors are widely distributed in the central and peripheral nervous systems and non-neuronal tissues. Numerous researchers have noted the pivotal role of peripheral opioid receptors (PORs) in analgesia. Accumulating evidence has shown the existence of PORs in the trigeminal nerve system, indicating that PORs may be involved in the modulation of orofacial pain. In this review, we summarise the recent evidence for the role of PORs in orofacial pain and discuss the possible cellular mechanisms.

Voltage modulates the effect of μ-receptor activation in a ligand-dependent manner

BACKGROUND AND PURPOSE

Various GPCRs have been described as being modulated in a voltage-dependent manner. Opioid analgesics act via activation of μ receptors in various neurons. As neurons are exposed to large changes in membrane potential, we were interested in studying the effects of depolarization on μ receptor signalling.

EXPERIMENTAL APPROACH

We investigated potential voltage sensitivity of μ receptors in heterologous expression systems (HEK293T cells) using electrophysiology in combination with Förster resonance energy transfer-based assays. Depolarization-induced changes in signalling were also tested in physiological rat tissue containing locus coeruleus neurons. We applied depolarization steps across the physiological range of membrane potentials.

KEY RESULTS

Studying μ receptor function and signalling in cells, we discovered that morphine-induced signalling was strongly dependent on the membrane potential (V_M ). This became apparent at the level of G-protein activation, G-protein coupled inwardly rectifying potassium channel (K_ir 3.X) currents and binding of GPCR kinases and arrestin3 to μ receptors by a robust increase in signalling upon membrane depolarization. The pronounced voltage sensitivity of morphine-induced μ receptor activation was also observed at the level of K_ir 3.X currents in rat locus coeruleus neurons. The efficacy of peptide ligands to activate μ receptors was not (Met-enkephalin) or only moderately ([D-Ala² , N-Me-Phe⁴ , Gly⁵ -ol]-enkephalin) enhanced upon depolarization. In contrast, depolarization reduced the ability of the analgesic fentanyl to activate μ receptors.

CONCLUSION AND IMPLICATIONS

Our results indicate a strong ligand-dependent modulation of μ receptor activity by the membrane potential, suggesting preferential activity of morphine in neurons with high neuronal activity.

G Protein-Gated Inwardly Rectifying Potassium Channel Subunit 3 is Upregulated in Rat DRGs and Spinal Cord After Peripheral Nerve Injury

Background

G protein-gated inwardly rectifying potassium (GIRK) channels are involved in the regulation of neuronal excitability. Four GIRK subunits (GIRK1-4) are expressed in rat dorsal root ganglia (DRGs). Recently, we have characterized the expression of GIRK1 and −2, and both are downregulated in rat DRGs and spinal cord after a complete sciatic nerve transection (axotomy). Here, we aimed to study the neurochemical characteristics of GIRK3, and its regulation in rat DRGs and spinal cord induced by nerve injury.

Methods

A sciatic nerve axotomy was performed to study the influences of injury on GIRK3 expression in DRGs and spinal cord. A dorsal root rhizotomy and a sciatic nerve crush were employed to study the axonal transport of GIRK3 protein, respectively. Immunohistochemistry analysis was employed for investigating the neurochemical characteristics of GIRK3.

Results

In control DRGs, ~18% of neuron profiles (NPs) were GIRK3-positive (⁺), and ~41%, ~48% and ~45% of GIRK3⁺ NPs were CGRP⁺, IB4⁺ and NF200⁺, respectively. GIRK3-like immunoreactivity was observed in glabrous skin of hind paws and axons originating from DRG neurons. Fourteen days after axotomy, more than one-third of DRG NPs were GIRK3⁺, and among these ~51% and ~56% coexpressed galanin and neuropeptide Y, respectively. In control animals, a small group of interneurons found in the dorsal horn was GIRK3⁺. In addition, GIRK3⁺ processes could be observed in superficial laminae of spinal dorsal horn. After nerve injury, the intensity of GIRK3-like immunoreactivity in the superficial layers was increased. Evidence based on rhizotomy and sciatic nerve crush indicated both anterograde and retrograde transport of GIRK3.

Conclusion

Our study demonstrates that GIRK3 is expressed in sensory neurons and spinal cord. GIRK3 has both anterograde and retrograde axonal transport. GIRK3 expression can be regulated by peripheral nerve injury.

Differential Expression of Neuroinflammatory mRNAs in the Rat Sciatic Nerve Following Chronic Constriction Injury and Pain-Relieving Nanoemulsion NSAID Delivery to Infiltrating Macrophages

The neuroinflammatory response to peripheral nerve injury is associated with chronic pain and significant changes in the molecular expression profiles of mRNAs in neurons, glia and infiltrating immune cells. Chronic constriction injury (CCI) of the rat sciatic nerve provides an opportunity to mimic neuropathic injury and quantitatively assess behavior and differential gene expression in individual animals. Previously, we have shown that a single intravenous injection of nanoemulsion containing celecoxib (0.24 mg/kg) reduces inflammation of the sciatic nerve and relieves pain-like behavior for up to 6 days. Here, we use this targeted therapy to explore the impact on mRNA expression changes in both pain and pain-relieved states. Sciatic nerve tissue recovered from CCI animals is used to evaluate the mRNA expression profiles utilizing quantitative PCR. We observe mRNA changes consistent with the reduced recruitment of macrophages evident by a reduction in chemokine and cytokine expression. Furthermore, genes associated with adhesion of macrophages, as well as changes in the neuronal and glial mRNAs are observed. Moreover, genes associated with neuropathic pain including Maob, Grin2b/NMDAR2b, TrpV3, IL-6, Cacna1b/Ca_v2.2, Itgam/Cd11b, Scn9a/Na_v1.7, and Tac1 were all found to respond to the celecoxib loaded nanoemulsion during pain relief as compared to those animals that received drug-free vehicle. These results demonstrate that by targeting macrophage production of PGE₂ at the site of injury, pain relief includes partial reversal of the gene expression profiles associated with chronic pain.

Noncanonical Ion Channel Behaviour in Pain

Ion channels contribute fundamental properties to cell membranes. Although highly diverse in conductivity, structure, location, and function, many of them can be regulated by common mechanisms, such as voltage or (de-)phosphorylation. Primarily considering ion channels involved in the nociceptive system, this review covers more novel and less known features. Accordingly, we outline noncanonical operation of voltage-gated sodium, potassium, transient receptor potential (TRP), and hyperpolarization-activated cyclic nucleotide (HCN)-gated channels. Noncanonical features discussed include properties as a memory for prior voltage and chemical exposure, alternative ion conduction pathways, cluster formation, and silent subunits. Complementary to this main focus, the intention is also to transfer knowledge between fields, which become inevitably more separate due to their size.

[Pain inhibition by opioids-new concepts]

BACKGROUND

Opioids are the oldest and most potent drugs for the treatment of severe pain but they are burdened by detrimental side effects, such as respiratory depression, addiction potential, sedation, nausea and constipation. Their clinical application is undisputed in the treatment of acute (e.g. perioperative) and cancer pain but their long-term use in chronic pain has met increasing criticism and has contributed to the current "opioid crisis".

OBJECTIVES

This article reviews the pharmacological principles and new research strategies aiming at novel opioids with reduced side effects. The basic mechanisms underlying pain and opioid analgesia and other effects of opioids are outlined. To illustrate the clinical situation and medical problems, the plasticity of opioid receptors, intracellular signaling pathways, endogenous and exogenous opioid receptor ligands, central and peripheral sites of analgesic and side effects are discussed.

CONCLUSION

The epidemic of opioid misuse has shown that there is a lack of fundamental knowledge about the characteristics and management of chronic pain, that conflicts of interest and validity of models must be more intensively considered in the context of drug development and that novel analgesics with less addictive potential are urgently needed. Currently, the most promising perspectives appear to be augmenting endogenous opioid actions and the selective activation of peripheral opioid receptors.

Nociceptor Signalling through ion Channel Regulation via GPCRs

The prime task of nociceptors is the transformation of noxious stimuli into action potentials that are propagated along the neurites of nociceptive neurons from the periphery to the spinal cord. This function of nociceptors relies on the coordinated operation of a variety of ion channels. In this review, we summarize how members of nine different families of ion channels expressed in sensory neurons contribute to nociception. Furthermore, data on 35 different types of G protein coupled receptors are presented, activation of which controls the gating of the aforementioned ion channels. These receptors are not only targeted by more than 20 separate endogenous modulators, but can also be affected by pharmacotherapeutic agents. Thereby, this review provides information on how ion channel modulation via G protein coupled receptors in nociceptors can be exploited to provide improved analgesic therapy.

Analgesic Effects of the GIRK Activator, VU0466551, Alone and in Combination with Morphine in Acute and Persistent Pain Models

G protein-gated inwardly rectifying potassium (GIRK) channels are potassium-selective ion channels. As their name suggests, GIRK channels are effectors of G_i/o G protein-couple receptors whereby activation of these GPCRs leads to increased GIRK channel activity resulting in decreased cellular excitability. In this way, GIRK channels play diverse roles in physiology as effectors of G_i/o-coupled GPCRs: peacemaking in the heart rate, modulation of hormone secretion in endocrine tissues, as well as numerous CNS functions including learning, memory, and addiction/reward. Notably, GIRK channels are widely expressed along the spinothalamic tract and are positioned to play roles in both ascending and descending pain pathways. More notably, GIRK channel knockout and knock-down studies have found that GIRK channels play a major role in the action of opioid analgesics which act predominantly through G_i/o-coupled, opioid-activated GPCRs (e.g., μ-opioid receptors). Recent advances in GIRK channel pharmacology have led to the development of small molecules that directly and selectively activate GIRK channels. Based on research implicating the involvement of GIRK channels in pain pathways and as effectors of opioid analgesics, we conducted a study to determine whether direct pharmacological activation of GIRK channels could produce analgesic efficacy and/or augment the analgesic efficacy morphine, an opioid receptor agonist capable of activating μ-opioid receptors as well as other opioid receptor subtypes. In the present study, we demonstrate that the small-molecule GIRK activator, VU0466551, has analgesic effects when dosed alone or in combination with submaximally effective doses of morphine.

Convallatoxin enhance the ligand-induced mu-opioid receptor endocytosis and attenuate morphine antinociceptive tolerance in mice

Morphine is a unique opioid analgesic that activates the mu-opioid receptor (MOR) without efficiently promoting its endocytosis that may underlie side effects. Our objective was to discover a novel enhancer of ligand-induced MOR endocytosis and determine its effects on analgesia, tolerance and dependence. We used high-throughput screening to identify convallatoxin as an enhancer of ligand-induced MOR endocytosis with high potency and efficacy. Treatment of cells with convallatoxin enhanced morphine-induced MOR endocytosis through an adaptor protein 2 (AP2)/clathrin-dependent mechanism, attenuated morphine-induced phosphorylation of MOR, and diminished desensitization of membrane hyperpolarization. Furthermore, co-treatment with chronic convallatoxin reduced morphine tolerance in animal models of acute thermal pain and chronic inflammatory pain. Acute convallatoxin administration reversed morphine tolerance and dependence in morphine-tolerant mice. These findings suggest convallatoxin are potentially therapeutic for morphine side effects and open a new avenue to study MOR trafficking.

Analgesic effects of a novel pH-dependent μ-opioid receptor agonist in models of neuropathic and abdominal pain

Supplemental Digital Content is Available in the Text.

The newly designed, pH-dependent opioid agonist NFEPP induced analgesia exclusively through peripheral opioid receptors in models of neuropathic and abdominal pain.

Recently, (±)-N-(3-fluoro-1-phenethylpiperidine-4-yl)-N-phenyl propionamide (NFEPP), a newly designed μ-opioid receptor (MOR) agonist with a low pKa, has been shown to produce injury-restricted analgesia in models of inflammatory and postoperative pain, without exhibiting typical opioid side effects. Here, we investigated MOR binding of NFEPP in brain and dorsal root ganglia, pH in injured tissues, and the analgesic efficacy of NFEPP compared with fentanyl in a chronic constriction injury model of neuropathic pain, and in the acetic acid–induced abdominal writhing assay in rats. Binding experiments revealed significantly lower affinity of NFEPP compared with fentanyl at pH 7.4. In vivo, pH significantly dropped both at injured nerves after chronic constriction injury and in the abdominal cavity after acetic acid administration. Intravenous NFEPP as well as fentanyl dose-dependently diminished neuropathy-induced mechanical and heat hypersensitivity, and acetic acid–induced abdominal constrictions. In both models, NFEPP-induced analgesia was fully reversed by naloxone methiodide, a peripherally restricted opioid receptor antagonist, injected at the nerve injury site or into the abdominal cavity. Our results indicate that NFEPP exerts peripheral opioid receptor–mediated analgesia exclusively in damaged tissue in models of neuropathic and abdominal pain.

Candidate gene analyses for acute pain and morphine analgesia after pediatric day surgery: African American versus European Caucasian ancestry and dose prediction limits

Acute pain and opioid analgesia demonstrate inter-individual variability and polygenic influence. In 241 children of African American and 277 of European Caucasian ancestry, we sought to replicate select candidate gene associations with morphine dose and postoperative pain and then to estimate dose prediction limits. Twenty-seven single-nucleotide polymorphisms (SNPs) from nine genes (ABCB1, ARRB2, COMT, DRD2, KCNJ6, MC1R, OPRD1, OPRM1, and UGT2B7) met selection criteria and were analyzed along with TAOK3. Few associations replicated: morphine dose (mcg/kg) in African American children and ABCB1 rs1045642 (A allele, β = -9.30, 95% CI: -17.25 to -1.35, p = 0.02) and OPRM1 rs1799971 (G allele, β = 23.19, 95% CI: 3.27-43.11, p = 0.02); KCNJ6 rs2211843 and high pain in African American subjects (T allele, OR 2.08, 95% CI: 1.17-3.71, p = 0.01) and in congruent European Caucasian pain phenotypes; and COMT rs740603 for high pain in European Caucasian subjects (A allele, OR: 0.69, 95% CI: 0.48-0.99, p = 0.046). With age, body mass index, and physical status as covariates, simple top SNP candidate gene models could explain theoretical maximums of 24.2% (European Caucasian) and 14.6% (African American) of morphine dose variances.

[Pain inhibition by opioids-new concepts]

BACKGROUND

Opioids are the oldest and most potent drugs for the treatment of severe pain but they are burdened by detrimental side effects, such as respiratory depression, addiction potential, sedation, nausea and constipation. Their clinical application is undisputed in the treatment of acute (e.g. perioperative) and cancer pain but their long-term use in chronic pain has met increasing criticism and has contributed to the current "opioid crisis".

OBJECTIVES

This article reviews the pharmacological principles and new research strategies aiming at novel opioids with reduced side effects. The basic mechanisms underlying pain and opioid analgesia and other effects of opioids are outlined. To illustrate the clinical situation and medical problems, the plasticity of opioid receptors, intracellular signaling pathways, endogenous and exogenous opioid receptor ligands, central and peripheral sites of analgesic and side effects are discussed.

CONCLUSION

The epidemic of opioid misuse has shown that there is a lack of fundamental knowledge about the characteristics and management of chronic pain, that conflicts of interest and validity of models must be more intensively considered in the context of drug development and that novel analgesics with less addictive potential are urgently needed. Currently, the most promising perspectives appear to be augmenting endogenous opioid actions and the selective activation of peripheral opioid receptors.

Mu-Opioid Receptor Agonist Induces Kir3 Currents in Mouse Peripheral Sensory Neurons - Effects of Nerve Injury

Neuropathic pain often arises from damage to peripheral nerves and is difficult to treat. Activation of opioid receptors in peripheral sensory neurons is devoid of respiratory depression, sedation, nausea, and addiction mediated in the brain, and ameliorates neuropathic pain in animal models. Mechanisms of peripheral opioid analgesia have therefore gained interest, but the role of G protein-coupled inwardly rectifying potassium (Kir3) channels, important regulators of neuronal excitability, remains unclear. Whereas functional Kir3 channels have been detected in dorsal root ganglion (DRG) neurons in rats, some studies question their contribution to opioid analgesia in inflammatory pain models in mice. However, neuropathic pain can be diminished by activation of peripheral opioid receptors in mouse models. Therefore, here we investigated effects of the selective μ-opioid receptor (MOR) agonist [D-Ala², N-Me-Phe⁴, Gly⁵-ol]-enkephalin (DAMGO) on potassium conductance in DRG neurons upon a chronic constriction injury (CCI) of the sciatic nerve in mice. For verification, we also tested human embryonic kidney (HEK) 293 cells transfected with MOR and Kir3.2. Using patch clamp, we recorded currents at -80 mV and applied voltage ramps in high extracellular potassium concentrations, which are a highly sensitive measures of Kir3 channel activity. We found a significantly higher rate of HEK cells responding with potassium channel blocker barium-sensitive inward current (233 ± 51 pA) to DAMGO application in transfected than in untransfected group, which confirms successful recordings of inward currents through Kir3.2 channels. Interestingly, DAMGO induced similar inward currents (178 ± 36-207 ± 56 pA) in 15-20% of recorded DRG neurons from naïve mice and in 4-27% of DRG neurons from mice exposed to CCI, measured in voltage clamp or voltage ramp modes. DAMGO-induced currents in naïve and CCI groups were reversed by barium and a more selective Kir3 channel blocker tertiapin-Q. These data indicate the coupling of Kir3 channels with MOR in mouse peripheral sensory neuron cell bodies, which was unchanged after CCI. A comparative analysis of opioid-induced potassium conductance at the axonal injury site and peripheral terminals of DRG neurons could clarify the role of Kir3 channel-MOR interactions in peripheral nerve injury and opioid analgesia.

Nociceptin/Orphanin-FQ Inhibits Gonadotropin-Releasing Hormone Neurons via G-Protein-Gated Inwardly Rectifying Potassium Channels

The pulsatile release of gonadotropin-releasing hormone (GnRH) is a key feature of the hypothalamic-pituitary-gonadal axis. Kisspeptin neurons in the arcuate nucleus (ARC) trigger GnRH neuronal activity, but how GnRH neurons return to baseline electrical activity is unknown. Nociceptin/orphanin-FQ (OFQ) is an inhibitory neuromodulator. ARC proopiomelanocortin (POMC) neurons, known to receive inputs from ARC kisspeptin neurons, contact GnRH neurons and coexpress OFQ in the rat. In the present study, the effect of OFQ(1-13) on GnRH neurons was determined in the mouse. We identified transcripts for the OFQ receptor [opioid receptor like 1 (ORL1)] in GnRH neurons, and, using two-model systems (explants and slices), we found that OFQ exerted a potent inhibition on GnRH neurons, with or without excitatory inputs. We confirmed that the inhibition was mediated by ORL1 via Gi/o-protein coupling. The inhibition, occurring independently of levels of intracellular cyclic adenosine monophosphate, was sensitive to inwardly rectifying potassium channels. The only specific blocker of Gi/o-protein-coupled inwardly rectifying potassium (GIRK) channels, tertiapin-Q (TPNQ), was ineffective in the inhibition of OFQ. Two GIRK activators, one sharing the binding site of TPNQ and one active only on GIRK1-containing GIRK channels, failed to trigger an inhibition. In contrast, protein kinase C phosphorylation activation, known to inhibit GIRK2-mediated currents, prevented the OFQ inhibition. These results indicate a specific combination of GIRK subunits, GIRK2/3 in GnRH neurons. In vivo, double-labeled OFQ/POMC fibers were found in the vicinity of GnRH neurons, and OFQ fibers apposed GnRH neurons. Together, this study brings to light a potent neuromodulator of GnRH neurons.

Inhibitory Gi/O-coupled receptors in somatosensory neurons: Potential therapeutic targets for novel analgesics

Primary sensory neurons in the dorsal root ganglia and trigeminal ganglia are responsible for sensing mechanical and thermal stimuli, as well as detecting tissue damage. These neurons express ion channels that respond to thermal, mechanical, or chemical cues, conduct action potentials, and mediate transmitter release. These neurons also express a large number of G-protein coupled receptors, which are major transducers for extracellular signaling molecules, and their activation usually modulates the primary transduction pathways. Receptors that couple to phospholipase C via heterotrimeric G_q/11 proteins and those that activate adenylate cyclase via G_s are considered excitatory; they positively regulate somatosensory transduction and they play roles in inflammatory sensitization and pain, and in some cases also in inducing itch. On the other hand, receptors that couple to G_i/o proteins, such as opioid or GABA_B receptors, are generally inhibitory. Their activation counteracts the effect of G_s-stimulation by inhibiting adenylate cyclase, as well as exerts effects on ion channels, usually resulting in decreased excitability. This review will summarize knowledge on G_i-coupled receptors in sensory neurons, focusing on their roles in ion channel regulation and discuss their potential as targets for analgesic and antipruritic medications.

MBD1 Contributes to the Genesis of Acute Pain and Neuropathic Pain by Epigenetic Silencing of Oprm1 and Kcna2 Genes in Primary Sensory Neurons

The transmission of normal sensory and/or acute noxious information requires intact expression of pain-associated genes within the pain pathways of nervous system. Expressional changes of these genes after peripheral nerve injury are also critical for neuropathic pain induction and maintenance. Methyl-CpG-binding domain protein 1 (MBD1), an epigenetic repressor, regulates gene transcriptional activity. We report here that MBD1 in the primary sensory neurons of DRG is critical for the genesis of acute pain and neuropathic pain as DRG MBD1-deficient mice exhibit the reduced responses to acute mechanical, heat, cold, and capsaicin stimuli and the blunted nerve injury-induced pain hypersensitivities. Furthermore, DRG overexpression of MBD1 leads to spontaneous pain and evoked pain hypersensitivities in the WT mice and restores acute pain sensitivities in the MBD1-deficient mice. Mechanistically, MDB1 represses Oprm1 and Kcna2 gene expression by recruiting DNA methyltransferase DNMT3a into these two gene promoters in the DRG neurons. DRG MBD1 is likely a key player under the conditions of acute pain and neuropathic pain.SIGNIFICANCE STATEMENT In the present study, we revealed that the mice with deficiency of methyl-CpG-binding domain protein 1 (MBD1), an epigenetic repressor, in the DRG displayed the reduced responses to acute noxious stimuli and the blunted neuropathic pain. We also showed that DRG overexpression of MBD1 produced the hypersensitivities to noxious stimuli in the WT mice and rescued acute pain sensitivities in the MBD1-deficient mice. We have also provided the evidence that MDB1 represses Oprm1 and Kcna2 gene expression by recruiting DNA methyltransferase DNMT3a into these two gene promoters in the DRG neurons. DRG MBD1 may participate in the genesis of acute pain and neuropathic pain likely through regulating DNMT3a-controlled Oprm1 and Kcna2 gene expression in the DRG neurons.

New concepts in opioid analgesia

INTRODUCTION

Opioids are the oldest and most potent drugs for the treatment of severe pain, but they are burdened by detrimental side effects such as respiratory depression, addiction, sedation, nausea, and constipation. Their clinical application is undisputed in acute (e.g. perioperative) and cancer pain, but their long-term use in chronic pain has met increasing scrutiny and has contributed to the current 'opioid crisis.'

AREAS COVERED

This article reviews pharmacological principles and research strategies aiming at novel opioids with reduced side effects. Basic mechanisms underlying pain, opioid analgesia, and other opioid actions are outlined. To illustrate the clinical situation and medical needs, plasticity of opioid receptors, intracellular signaling pathways, endogenous and exogenous opioid receptor ligands, central and peripheral sites of analgesic, and side effects are discussed.

EXPERT OPINION

The epidemic of opioid misuse has taught us that there is a lack of fundamental knowledge about the characteristics and management of chronic pain, that conflicts of interest and validity of models must be considered in the context of drug development, and that novel analgesics with less abuse liability are badly needed. Currently, the most promising perspectives appear to be augmenting endogenous opioid actions and selectively targeting pathological conformations of peripheral opioid receptors.

New opioid receptor modulators and agonists

There has been significant research to develop an ideal synthetic opioid. Opioids with variable properties possessing efficacy and with reduced side effects have been synthesized when compared to previously used agents. An opioid modulator is a drug that can produce both agonistic and antagonistic effects by binding to different opioid receptors and therefore cannot be classified as one or the other alone. These compounds can differ in their structures while still possessing opioid-mediated actions. This review will discuss TRV130 receptor modulators and other novel opioid receptor modulators, including Mitragyna "Kratom," Ignavine, Salvinorin-A, DPI-289, UFP-505, LP1, SKF-10,047, Cebranopadol, Naltrexone-14-O-sulfate, and Naloxegol. In summary, the structural elucidation of opioid receptors, allosteric modulation of opioid receptors, new opioid modulators and agonists, the employment of optogenetics, optopharmacology, and next-generation sequencing of opioid receptor genes and related functionality should create exciting new avenues for research and therapeutic development to treat conditions including pain, opioid abuse, and addiction.

Opioid receptor signaling, analgesic and side effects induced by a computationally designed pH-dependent agonist

Novel pain killers without adverse effects are urgently needed. Opioids induce central and intestinal side effects such as respiratory depression, sedation, addiction, and constipation. We have recently shown that a newly designed agonist with a reduced acid dissociation constant (pK_a) abolished pain by selectively activating peripheral μ-opioid receptors (MOR) in inflamed (acidic) tissues without eliciting side effects. Here, we extended this concept in that pK_a reduction to 7.22 was achieved by placing a fluorine atom at the ethylidene bridge in the parental molecule fentanyl. The new compound (FF3) showed pH-sensitive MOR affinity, [³⁵S]-GTPγS binding, and G protein dissociation by fluorescence resonance energy transfer. It produced injury-restricted analgesia in rat models of inflammatory, postoperative, abdominal, and neuropathic pain. At high dosages, FF3 induced sedation, motor disturbance, reward, constipation, and respiratory depression. These results support our hypothesis that a ligand’s pK_a should be close to the pH of injured tissue to obtain analgesia without side effects.

A nontoxic pain killer designed by modeling of pathological receptor conformations

Indiscriminate activation of opioid receptors provides pain relief but also severe central and intestinal side effects. We hypothesized that exploiting pathological (rather than physiological) conformation dynamics of opioid receptor-ligand interactions might yield ligands without adverse actions. By computer simulations at low pH, a hallmark of injured tissue, we designed an agonist that, because of its low acid dissociation constant, selectively activates peripheral μ-opioid receptors at the source of pain generation. Unlike the conventional opioid fentanyl, this agonist showed pH-sensitive binding, heterotrimeric guanine nucleotide-binding protein (G protein) subunit dissociation by fluorescence resonance energy transfer, and adenosine 3',5'-monophosphate inhibition in vitro. It produced injury-restricted analgesia in rats with different types of inflammatory pain without exhibiting respiratory depression, sedation, constipation, or addiction potential.

Mechanisms of μ-opioid receptor inhibition of NMDA receptor-induced substance P release in the rat spinal cord

The interaction between NMDA receptors and μ-opioid receptors in primary afferent terminals was studied by using NMDA to induce substance P release, measured as neurokinin 1 receptor internalization. In rat spinal cord slices, the μ-opioid receptor agonists morphine, DAMGO and endomorphin-2 inhibited NMDA-induced substance P release, whereas the antagonist CTAP right-shifted the concentration response of DAMGO. In vivo, substance P release induced by intrathecal NMDA after priming with BDNF was inhibited by DAMGO. ω-Conotoxins MVIIC and GVIA inhibited about half of the NMDA-induced substance P release, showing that it was partially mediated by the opening of voltage-gated calcium (Cav) channels. In contrast, DAMGO or ω-conotoxins did not inhibit capsaicin-induced substance P release. In cultured DRG neurons, DAMGO but not ω-conotoxin inhibited NMDA-induced increases in intracellular calcium, indicating that μ-opioid receptors can inhibit NMDA receptor function by mechanisms other than inactivation of Cav channels. Moreover, DAMGO decreased the ω-conotoxin-insensitive component of the substance P release. Potent inhibition by ifenprodil showed that these NMDA receptors have the NR2B subunit. Activators of adenylyl cyclase and protein kinase A (PKA) induced substance P release and this was decreased by the NMDA receptor blocker MK-801 and by DAMGO. Conversely, inhibitors of adenylyl cyclase and PKA, but not of protein kinase C, decreased NMDA-induced substance P release. Hence, these NMDA receptors are positively modulated by the adenylyl cyclase-PKA pathway, which is inhibited by μ-opioid receptors. In conclusion, μ-opioid receptors inhibit NMDA receptor-induced substance P release through Cav channel inactivation and adenylyl cyclase inhibition.

Using an engineered glutamate-gated chloride channel to silence sensory neurons and treat neuropathic pain at the source

See Basbaum (doi:10.1093/brain/awx227) for a scientific commentary on this article. Peripheral neuropathic pain arises as a consequence of injury to sensory neurons; the development of ectopic activity in these neurons is thought to be critical for the induction and maintenance of such pain. Local anaesthetics and anti-epileptic drugs can suppress hyperexcitability; however, these drugs are complicated by unwanted effects on motor, central nervous system and cardiac function, and alternative more selective treatments to suppress hyperexcitability are therefore required. Here we show that a glutamate-gated chloride channel modified to be activated by low doses of ivermectin (but not glutamate) is highly effective in silencing sensory neurons and reversing neuropathic pain-related hypersensitivity. Activation of the glutamate-gated chloride channel expressed in either rodent or human induced pluripotent stem cell-derived sensory neurons in vitro potently inhibited their response to both electrical and algogenic stimuli. We have shown that silencing is achieved both at nerve terminals and the soma and is independent of membrane hyperpolarization and instead likely mediated by lowering of the membrane resistance. Using intrathecal adeno-associated virus serotype 9-based delivery, the glutamate-gated chloride channel was successfully targeted to mouse sensory neurons in vivo, resulting in high level and long-lasting expression of the channel selectively in sensory neurons. This enabled reproducible and reversible modulation of thermal and mechanical pain thresholds in vivo; analgesia was observed for 3 days after a single systemic dose of ivermectin. We did not observe any motor or proprioceptive deficits and noted no reduction in cutaneous afferent innervation or upregulation of the injury marker ATF3 following prolonged glutamate-gated chloride channel expression. Established mechanical and cold pain-related hypersensitivity generated by the spared nerve injury model of neuropathic pain was reversed by ivermectin treatment. The efficacy of ivermectin in ameliorating behavioural hypersensitivity was mirrored at the cellular level by a cessation of ectopic activity in sensory neurons. These findings demonstrate the importance of aberrant afferent input in the maintenance of neuropathic pain and the potential for targeted chemogenetic silencing as a new treatment modality in neuropathic pain.

Anti-nociceptive action of peripheral mu-opioid receptors by G-beta-gamma protein-mediated inhibition of TRPM3 channels

Opioids, agonists of µ-opioid receptors (µORs), are the strongest pain killers clinically available. Their action includes a strong central component, which also causes important adverse effects. However, µORs are also found on the peripheral endings of nociceptors and their activation there produces meaningful analgesia. The cellular mechanisms downstream of peripheral µORs are not well understood. Here, we show in neurons of murine dorsal root ganglia that pro-nociceptive TRPM3 channels, present in the peripheral parts of nociceptors, are strongly inhibited by µOR activation, much more than other TRP channels in the same compartment, like TRPV1 and TRPA1. Inhibition of TRPM3 channels occurs via a short signaling cascade involving Gβγ proteins, which form a complex with TRPM3. Accordingly, activation of peripheral µORs in vivo strongly attenuates TRPM3-dependent pain. Our data establish TRPM3 inhibition as important consequence of peripheral µOR activation indicating that pharmacologically antagonizing TRPM3 may be a useful analgesic strategy.

There are very few treatments available for people suffering from strong or long-lasting pain. Currently, substances called opioids – which include the well-known drug morphine – are the strongest painkillers. However, these drugs also cause harmful side effects, which makes them less useful.

Like all drugs, opioids mediate their effects by interacting with molecules in the body. In the case of opioids, these interacting molecules belong to a group of receptor proteins called G-protein coupled receptors (or GPCRs for short). These opioid receptors are widely distributed in the nerve cells and brain regions that detect and transmit pain signals. It was poorly understood how activation of opioid receptors reduces the activity of pain-sensing nerve cells, however several lines of evidence had suggested that a protein called TRPM3 might be involved.

TRPM3 is a channel protein that allows sodium and calcium ions to enter into nerve cells by forming pores in cell membranes, and mice that lack this protein are less sensitive to certain kinds of pain. Dembla, Behrendt et al. now show that activating opioid receptors on nerve cells from mice, with morphine and a similar substance, rapidly reduces the flow of calcium ions through TRPM3 channels. Further experiments confirmed that activating opioid receptors in a mouse’s paw also reduced the pain caused when TRPM3 proteins are activated.

GPCRs interact with a group of small proteins called G-proteins that, when activated by the receptor, split into two subunits. Based on studies with human kidney cells, Dembla, Behrendt et al. found the so-called G-beta-gamma subunit then carries the signal from the opioid receptor to TRPM3. Two independent studies by Quallo et al. and Badheka, Yudin et al. also report similar findings.

These new findings show that drugs already used in the treatment of pain can indirectly alter how TRPM3 works in a dramatic way. These results might help scientists to find drugs that work in a more direct way to dial down the activity of TRPM3 and to combat pain with fewer side effects. Though first it will be important to confirm these new findings in human nerve cells.

Gi-DREADD Expression in Peripheral Nerves Produces Ligand-Dependent Analgesia, as well as Ligand-Independent Functional Changes in Sensory Neurons

Designer receptors exclusively activated by designer drugs (DREADDs) are an advanced experimental tool that could potentially provide a novel approach to pain management. In particular, expression of an inhibitory (Gi-coupled) DREADD in nociceptors might enable ligand-dependent analgesia. To test this possibility, TRPV1-cre mice were used to restrict expression of Gi-DREADDs to predominantly C-fibers. Whereas baseline heat thresholds in both male and female mice expressing Gi-DREADD were normal, 1 mg/kg clozapine-N-oxide (CNO) produced a significant 3 h increase in heat threshold that returned to baseline by 5 h after injection. Consistent with these behavioral results, CNO decreased action potential firing in isolated sensory neurons from Gi-DREADD mice. Unexpectedly, however, the expression of Gi-DREADD in sensory neurons caused significant changes in voltage-gated Ca²⁺ and Na⁺ currents in the absence of CNO, as well as an increase in Na⁺ channel (NaV1.7) expression. Furthermore, CNO-independent excitatory and inhibitory second-messenger signaling was also altered in these mice, which was associated with a decrease in the analgesic effect of endogenous inhibitory G-protein-coupled receptor activation. These results highlight the potential of this exciting technology, but also its limitations, and that it is essential to identify the underlying mechanisms for any observed behavioral phenotypes.

SIGNIFICANCE STATEMENT

DREADD technology is a powerful tool enabling manipulation of activity and/or transmitter release from targeted cell populations. The purpose of this study was to determine whether inhibitory DREADDs in nociceptive afferents could be used to produce analgesia, and if so, how. DREADD activation produced a ligand-dependent analgesia to heat in vivo and a decrease in neuronal firing at the single-cell level. However, we observed that expression of Gi-DREADD also causes ligand-independent changes in ion channel activity and second-messenger signaling. These findings highlight both the potential and the limitations of this exciting technology as well as the necessity to identify the mechanisms underlying any observed phenotype.