Synergistic regulation of serotonin and opioid signaling contributes to pain insensitivity in Nav1.7 knockout mice

Safety, Tolerability, and Dose-Limiting Toxicity of Lacosamide in Patients With Painful Chronic Pancreatitis: Protocol for a Phase 1 Clinical Trial to Determine Safety and Identify Side Effects

BACKGROUND

Chronic abdominal pain is the hallmark symptom of chronic pancreatitis (CP), with 50% to 80% of patients seeking medical attention for pain control. Although several management options are available, outcomes are often disappointing, and opioids remain a mainstay of therapy. Opioid-induced hyperalgesia is a phenomenon resulting in dose escalation, which may occur partly because of the effects of opioids on voltage-gated sodium channels associated with pain. Preclinical observations demonstrate that the combination of an opioid and the antiseizure drug lacosamide diminishes opioid-induced hyperalgesia and improves pain control.

OBJECTIVE

In this phase 1 trial, we aim to determine the safety, tolerability, and dose-limiting toxicity of adding lacosamide to opioids for the treatment of painful CP and assess the feasibility of performance of a pilot study of adding lacosamide to opioid therapy in patients with CP. As an exploratory aim, we will assess the efficacy of adding lacosamide to opioid therapy in patients with painful CP.

METHODS

Using the Bayesian optimal interval design, we will conduct a dose-escalation trial of adding lacosamide to opioid therapy in patients with painful CP enrolled in cohorts of size 3. The initial dose will be 50 mg taken orally twice a day, followed by incremental increases to a maximum dose of 400 mg/day, with lacosamide administered for 7 days at each dose level. Adverse events will be documented according to Common Terminology Criteria for Adverse Events (version 5.0).

RESULTS

As of December 2023, we have currently enrolled 6 participants. The minimum number of participants to be enrolled is 12 with a maximum of 24. We expect to publish the results by March 2025.

CONCLUSIONS

This trial will test the feasibility of the study design and provide reassurance regarding the tolerability and safety of opioids in treating painful CP. It is anticipated that lacosamide will prove to be safe and well tolerated, supporting a subsequent phase 2 trial assessing the efficacy of lacosamide+opioid therapy in patients with painful CP, and that lacosamide combined with opiates will lower the opioid dose necessary for pain relief and improve the safety profile of opioid use in treating painful CP.

TRIAL REGISTRATION

Clinicaltrials.gov NCT05603702; https://clinicaltrials.gov/study/NCT05603702.

INTERNATIONAL REGISTERED REPORT IDENTIFIER (IRRID)

PRR1-10.2196/50513.

Intrathecal dopamine and serotonin enhance motor and nociceptive blockades of lidocaine in rats

The study examined the effect of intrathecal injection of dopamine (serotonin) and/or lidocaine. Intrathecal injections of dopamine (serotonin or epinephrine), lidocaine, or their combination were carried out in male Sprague Dawley rats. Neurobehavioral examinations (motor and nociceptive reactions) were performed before and after spinal injection. Intrathecal serotonin (1.5 μmol), dopamine (2.5 μmol), epinephrine (1:40000), and lidocaine (0.75 μmol) produced 29%, 33%, 29%, and 54% nociceptive blockade, whereas serotonin (1.5 μmol), dopamine (2.5 μmol), or epinephrine (1:40000) produced a longer duration of nociceptive blockade than lidocaine (0.75 μmol) (P < 0.05). Serotonin (1.5 μmol), dopamine (1.25 and 2.5 μmol), or epinephrine (1:40000 and 1:80000) prolonged the duration and increased the potency of spinal motor and nociceptive blockades of lidocaine (50% effective dose, ED50) (P < 0.05). The motor and nociceptive blockades caused by lidocaine (ED50) plus dopamine (2.5 μmol) or lidocaine (ED50) plus epinephrine (1:40000) were more outstanding than lidocaine (ED50) plus serotonin (0.75 μmol) (P < 0.05). Our study provides evidence that intrathecal dopamine or serotonin produces spinal nociceptive blockade dose-dependently. Dopamine and serotonin are less potent than lidocaine in inducing spinal nociceptive blockade. When mixed with lidocaine solution, dopamine or serotonin improves spinal motor and nociceptive blockades. The motor and nociceptive blockade caused by lidocaine (ED50) plus dopamine (2.5 μmol) is similar to that caused by lidocaine (ED50) plus epinephrine (1:40000).

Nav1.7 is essential for nociceptor action potentials in the mouse in a manner independent of endogenous opioids

Loss-of-function mutations in Nav1.7, a voltage-gated sodium channel, cause congenital insensitivity to pain (CIP) in humans, demonstrating that Nav1.7 is essential for the perception of pain. However, the mechanism by which loss of Nav1.7 results in insensitivity to pain is not entirely clear. It has been suggested that loss of Nav1.7 induces overexpression of enkephalin, an endogenous opioid receptor agonist, leading to opioid-dependent analgesia. Using behavioral pharmacology and single-cell RNA-seq analysis, we find that overexpression of enkephalin occurs only in cLTMR neurons, a subclass of sensory neurons involved in low-threshold touch detection, and that this overexpression does not play a role in the analgesia observed following genetic removal of Nav1.7. Furthermore, we demonstrate using laser speckle contrast imaging (LSCI) and in vivo electrophysiology that Nav1.7 function is required for the initiation of C-fiber action potentials (APs), which explains the observed insensitivity to pain following genetic removal or inhibition of Nav1.7.

Glycyrrhizic Acid Prevents Paclitaxel-Induced Neuropathy via Inhibition of OATP-Mediated Neuronal Uptake

Peripheral neuropathy is a common side effect of cancer treatment with paclitaxel. The mechanisms by which paclitaxel is transported into neurons, which are essential for preventing neuropathy, are not well understood. We studied the uptake mechanisms of paclitaxel into neurons using inhibitors for endocytosis, autophagy, organic anion-transporting polypeptide (OATP) drug transporters, and derivatives of paclitaxel. RT-qPCR was used to investigate the expression levels of OATPs in different neuronal tissues and cell lines. OATP transporters were pharmacologically inhibited or modulated by overexpression and CRISPR/Cas9-knock-out to investigate paclitaxel transport in neurons. Through these experiments, we identified OATP1A1 and OATP1B2 as the primary neuronal transporters for paclitaxel. In vitro inhibition of OATP1A1 and OAT1B2 by glycyrrhizic acid attenuated neurotoxicity, while paclitaxel's antineoplastic effects were sustained in cancer cell lines. In vivo, glycyrrhizic acid prevented paclitaxel-induced toxicity and improved behavioral and electrophysiological measures. This study indicates that a set of OATPs are involved in paclitaxel transport into neurons. The inhibition of OATP1A1 and OATP1B2 holds a promising strategy to prevent paclitaxel-induced peripheral neuropathy.

Emergence of nociceptive functionality and opioid signaling in human induced pluripotent stem cell-derived sensory neurons

ABSTRACT

Induced pluripotent stem cells (iPSCs) have enabled the generation of various difficult-to-access cell types such as human nociceptors. A key challenge associated with human iPSC-derived nociceptors (hiPSCdNs) is their prolonged functional maturation. While numerous studies have addressed the expression of classic neuronal markers and ion channels in hiPSCdNs, the temporal development of key signaling cascades regulating nociceptor activity has remained largely unexplored. In this study, we used an immunocytochemical high-content imaging approach alongside electrophysiological staging to assess metabotropic and ionotropic signaling of large scale-generated hiPSCdNs across 70 days of in vitro differentiation. During this period, the resting membrane potential became more hyperpolarized, while rheobase, action potential peak amplitude, and membrane capacitance increased. After 70 days, hiPSCdNs exhibited robust physiological responses induced by GABA, pH shift, ATP, and capsaicin. Direct activation of protein kinase A type II (PKA-II) through adenylyl cyclase stimulation with forskolin resulted in PKA-II activation at all time points. Depolarization-induced activation of PKA-II emerged after 35 days of differentiation. However, effective inhibition of forskolin-induced PKA-II activation by opioid receptor agonists required 70 days of in vitro differentiation. Our results identify a pronounced time difference between early expression of functionally important ion channels and emergence of regulatory metabotropic sensitizing and desensitizing signaling only at advanced stages of in vitro cultivation, suggesting an independent regulation of ionotropic and metabotropic signaling. These data are relevant for devising future studies into the development and regulation of human nociceptor function and for defining time windows suitable for hiPSCdN-based drug discovery.

Na(V)1.7 Channel Blocker [Ala(5), Phe(6), Leu(26), Arg(28)]GpTx-1 Attenuates CFA-induced Inflammatory Hypersensitivity in Rats via Endogenous Enkephalin Mechanism

Venom-derived Na_V1.7 channel blockers have promising prospects in pain management. The 34-residue tarantula peptide GpTx-1 is a potent Na_V1.7 channel blocker. Its powerful analog [Ala⁵, Phe⁶, Leu²⁶, Arg²⁸]GpTx-1 (GpTx-1-71) displayed excellent Na_V1.7 selectivity and analgesic properties in mice. The current study aimed to elucidate the anti-hyperalgesic activities of GpTx-1-71 in inflammatory pain and reveal the underlying mechanisms. Our results demonstrated that intrathecal and intraplantar injections of GpTx-1-71 dose-dependently attenuated CFA-induced inflammatory hypersensitivity in rats. Moreover, GpTx-1-71-induced anti-hyperalgesia was significantly reduced by opioid receptor antagonists and the enkephalin antibody and diminished in proenkephalin (Penk) gene knockout animals. Consistently, GpTx-1-71 treatment increased the enkephalin level in the spinal dorsal horn and promoted the Penk transcription and enkephalin release in primary dorsal root ganglion (DRG) neurons, wherein sodium played a crucial role in these processes. Mass spectrometry analysis revealed that GpTx-1-71 mainly promoted the secretion of Met-enkephalin but not Leu-enkephalin from DRG neurons. In addition, the combination of subtherapeutic Met-enkephalin and GpTx-1-71 produced synergistic anti-hyperalgesia in CFA-induced inflammatory hypersensitivity. These findings suggest that the endogenous enkephalin pathway is essential for GpTx-1-71-induced spinal and peripheral analgesia in inflammatory pain. PERSPECTIVE: This article presents a possible pharmacological mechanism underlying Na_V1.7 blocker-induced analgesia in inflammatory pain, which helps us to better understand and develop venom-based painkillers for incurable pain.

Dose-dependent consequences of sub-chronic fentanyl exposure on neuron and glial co-cultures

Fentanyl is one of the most common opioid analgesics administered to patients undergoing surgery or for chronic pain management. While the side effects of chronic fentanyl abuse are recognized (e.g., addiction, tolerance, impairment of cognitive functions, and inhibit nociception, arousal, and respiration), it remains poorly understood what and how changes in brain activity from chronic fentanyl use influences the respective behavioral outcome. Here, we examined the functional and molecular changes to cortical neural network activity following sub-chronic exposure to two fentanyl concentrations, a low (0.01 μM) and high (10 μM) dose. Primary rat co-cultures, containing cortical neurons, astrocytes, and oligodendrocyte precursor cells, were seeded in wells on either a 6-well multi-electrode array (MEA, for electrophysiology) or a 96-well tissue culture plate (for serial endpoint bulk RNA sequencing analysis). Once networks matured (at 28 days in vitro), co-cultures were treated with 0.01 or 10 μM of fentanyl for 4 days and monitored daily. Only high dose exposure to fentanyl resulted in a decline in features of spiking and bursting activity as early as 30 min post-exposure and sustained for 4 days in cultures. Transcriptomic analysis of the complex cultures after 4 days of fentanyl exposure revealed that both the low and high dose induced gene expression changes involved in synaptic transmission, inflammation, and organization of the extracellular matrix. Collectively, the findings of this in vitro study suggest that while neuroadaptive changes to neural network activity at a systems level was detected only at the high dose of fentanyl, transcriptomic changes were also detected at the low dose conditions, suggesting that fentanyl rapidly elicits changes in plasticity.

Genetic pain loss disorders

Genetic pain loss includes congenital insensitivity to pain (CIP), hereditary sensory neuropathies and, if autonomic nerves are involved, hereditary sensory and autonomic neuropathy (HSAN). This heterogeneous group of disorders highlights the essential role of nociception in protecting against tissue damage. Patients with genetic pain loss have recurrent injuries, burns and poorly healing wounds as disease hallmarks. CIP and HSAN are caused by pathogenic genetic variants in >20 genes that lead to developmental defects, neurodegeneration or altered neuronal excitability of peripheral damage-sensing neurons. These genetic variants lead to hyperactivity of sodium channels, disturbed haem metabolism, altered clathrin-mediated transport and impaired gene regulatory mechanisms affecting epigenetic marks, long non-coding RNAs and repetitive elements. Therapies for pain loss disorders are mainly symptomatic but the first targeted therapies are being tested. Conversely, chronic pain remains one of the greatest unresolved medical challenges, and the genes and mechanisms associated with pain loss offer new targets for analgesics. Given the progress that has been made, the coming years are promising both in terms of targeted treatments for pain loss disorders and the development of innovative pain medicines based on knowledge of these genetic diseases.

Differential DNA methylation in Black and White individuals with chronic low back pain enrich different genomic pathways

Compared to Non-Hispanic Whites (NHWs), individuals who self-identify as Non-Hispanic Blacks (NHBs) in the United States experience more severe and disabling chronic low back pain (cLBP). We hypothesized that differences in DNA methylation (DNAm) play a role in racial disparities in cLBP.

PURPOSE

To determine the relationship between DNAm levels and racial group differences in adults with cLBP. Our study's secondary purpose was to perform a race-stratified comparison of adults with cLBP and pain-free controls and identify functional genomic pathways enriched by annotated differentially methylated genes.

PATIENTS AND METHODS

We recruited 49 NHBs and 49 NHWs (49 cLBP and 49 pain-free controls, PFCs), analyzed DNAm from whole blood using reduced representation bisulfite sequencing, and identified enriched genomic pathways.

RESULTS

Among participants with cLBP, we identified 2873 differentially methylated loci (DML; methylation differences of at least 10% and p < 0.0001), many of which were annotated to genes of importance to pain pathology. These DMLs significantly enriched pathways to involved in nociception/pain processing (Dopamine-DARPP32 Feedback in cAMP signaling, GABA Receptor Signaling, Opioid Signaling) and neuronal differentiation (e.g., Calcium Signaling, Axon Guidance Signaling, and Endocannabinoid Neuronal Synapse). Our race stratified analyses of individuals with cLBP versus PFCs revealed 2356 DMLs in NHBs and 772 DMLs in NHWs with p < 0.0001 and > 10% methylation difference. Ingenuity Pathway Analysis revealed that many pathways of significance to pain such as Corticotropin Releasing Hormone Signaling, White Adipose Tissue Browning, and GABA Receptor Signaling pathways, were more significant in NHBs than NHWs.

CONCLUSION

Even though an individual's self-identified race is a social construct, not a biological variable, racism associated with that classification affects virtually every aspect of life, including disease risk. DNAm induced alterations in stress signaling pathways may explain worse pain outcomes in NHBs.

Selective targeting of NaV1.7 via inhibition of the CRMP2-Ubc9 interaction reduces pain in rodents

The voltage-gated sodium NaV1.7 channel, critical for sensing pain, has been actively targeted by drug developers; however, there are currently no effective and safe therapies targeting NaV1.7. Here, we tested whether a different approach, indirect NaV1.7 regulation, could have antinociceptive effects in preclinical models. We found that preventing addition of small ubiquitin-like modifier (SUMO) on the NaV1.7-interacting cytosolic collapsin response mediator protein 2 (CRMP2) blocked NaV1.7 functions and had antinociceptive effects in rodents. In silico targeting of the SUMOylation site in CRMP2 (Lys374) identified >200 hits, of which compound 194 exhibited selective in vitro and ex vivo NaV1.7 engagement. Orally administered 194 was not only antinociceptive in preclinical models of acute and chronic pain but also demonstrated synergy alongside other analgesics—without eliciting addiction, rewarding properties, or neurotoxicity. Analgesia conferred by 194 was opioid receptor dependent. Our results demonstrate that 194 is a first-in-class protein-protein inhibitor that capitalizes on CRMP2-NaV1.7 regulation to deliver safe analgesia in rodents.

Depolarization induces nociceptor sensitization by CaV1.2-mediated PKA-II activation

Depolarization drives neuronal plasticity. However, whether depolarization drives sensitization of peripheral nociceptive neurons remains elusive. By high-content screening (HCS) microscopy, we revealed that depolarization of cultured sensory neurons rapidly activates protein kinase A type II (PKA-II) in nociceptors by calcium influx through Ca_V1.2 channels. This effect was modulated by calpains but insensitive to inhibitors of cAMP formation, including opioids. In turn, PKA-II phosphorylated Ser1928 in the distal C terminus of Ca_V1.2, thereby increasing channel gating, whereas dephosphorylation of Ser1928 involved the phosphatase calcineurin. Patch-clamp and behavioral experiments confirmed that depolarization leads to calcium- and PKA-dependent sensitization of calcium currents ex vivo and local peripheral hyperalgesia in the skin in vivo. Our data suggest a local activity-driven feed-forward mechanism that selectively translates strong depolarization into further activity and thereby facilitates hypersensitivity of nociceptor terminals by a mechanism inaccessible to opioids.

Locus revealed: Painlessness via loss of NaV1.7 at central terminals of sensory neurons

How genetic loss of the sodium channel Na_V1.7 results in painlessness is puzzling. MacDonald et al. (2021) demonstrate that instead of impairing peripheral excitability, Na_V1.7 channels at central terminals of pain-sensing afferents play a pivotal role in the balance between pain and analgesia.

A central mechanism of analgesia in mice and humans lacking the sodium channel NaV1.7

Deletion of SCN9A encoding the voltage-gated sodium channel NaV1.7 in humans leads to profound pain insensitivity and anosmia. Conditional deletion of NaV1.7 in sensory neurons of mice also abolishes pain, suggesting that the locus of analgesia is the nociceptor. Here we demonstrate, using in vivo calcium imaging and extracellular recording, that NaV1.7 knockout mice have essentially normal nociceptor activity. However, synaptic transmission from nociceptor central terminals in the spinal cord is greatly reduced by an opioid-dependent mechanism. Analgesia is also reversed substantially by central but not peripheral application of opioid antagonists. In contrast, the lack of neurotransmitter release from olfactory sensory neurons is opioid independent. Male and female humans with NaV1.7-null mutations show naloxone-reversible analgesia. Thus, inhibition of neurotransmitter release is the principal mechanism of anosmia and analgesia in mouse and human Nav1.7-null mutants.

The physiological function of different voltage-gated sodium channels in pain

Evidence from human genetic pain disorders shows that voltage-gated sodium channel α-subtypes Nav1.7, Nav1.8 and Nav1.9 are important in the peripheral signalling of pain. Nav1.7 is of particular interest because individuals with Nav1.7 loss-of-function mutations are congenitally insensitive to acute and chronic pain, and there is considerable hope that phenocopying these effects with a pharmacological antagonist will produce a new class of analgesic drug. However, studies in these rare individuals do not reveal how and where voltage-gated sodium channels contribute to pain signalling, which is of critical importance for drug development. More than a decade of research utilizing rodent genetic models and pharmacological tools to study voltage-gated sodium channels in pain has begun to unravel the role of different subtypes. Here, we review the contribution of individual channel subtypes in three key physiological processes necessary for transmission of sensory information to the CNS: transduction of stimuli at peripheral nerve terminals, axonal transmission of action potentials and neurotransmitter release from central terminals. These data suggest that drugs seeking to recapitulate the analgesic effects of loss of function of Nav1.7 will need to be brain-penetrant - which most of those developed to date are not.

Long-lasting analgesia via targeted in situ repression of NaV1.7 in mice

Current treatments for chronic pain rely largely on opioids despite their substantial side effects and risk of addiction. Genetic studies have identified in humans key targets pivotal to nociceptive processing. In particular, a hereditary loss-of-function mutation in NaV1.7, a sodium channel protein associated with signaling in nociceptive sensory afferents, leads to insensitivity to pain without other neurodevelopmental alterations. However, the high sequence and structural similarity between NaV subtypes has frustrated efforts to develop selective inhibitors. Here, we investigated targeted epigenetic repression of NaV1.7 in primary afferents via epigenome engineering approaches based on clustered regularly interspaced short palindromic repeats (CRISPR)-dCas9 and zinc finger proteins at the spinal level as a potential treatment for chronic pain. Toward this end, we first optimized the efficiency of NaV1.7 repression in vitro in Neuro2A cells and then, by the lumbar intrathecal route, delivered both epigenome engineering platforms via adeno-associated viruses (AAVs) to assess their effects in three mouse models of pain: carrageenan-induced inflammatory pain, paclitaxel-induced neuropathic pain, and BzATP-induced pain. Our results show effective repression of NaV1.7 in lumbar dorsal root ganglia, reduced thermal hyperalgesia in the inflammatory state, decreased tactile allodynia in the neuropathic state, and no changes in normal motor function in mice. We anticipate that this long-lasting analgesia via targeted in vivo epigenetic repression of NaV1.7 methodology we dub pain LATER, might have therapeutic potential in management of persistent pain states.

Serotonin enhances depolarizing spontaneous fluctuations, excitability, and ongoing activity in isolated rat DRG neurons via 5-HT4 receptors and cAMP-dependent mechanisms

Ongoing activity in nociceptors, a driver of spontaneous pain, can be generated in dorsal root ganglion neurons in the absence of sensory generator potentials if one or more of three neurophysiological alterations occur - prolonged depolarization of resting membrane potential (RMP), hyperpolarization of action potential (AP) threshold, and/or increased amplitude of depolarizing spontaneous fluctuations of membrane potential (DSFs) to bridge the gap between RMP and AP threshold. Previous work showed that acute, sustained exposure to serotonin (5-HT) hyperpolarized AP threshold and potentiated DSFs, leading to ongoing activity if a separate source of maintained depolarization was present. Cellular signaling pathways that increase DSF amplitude and promote ongoing activity acutely in nociceptors are not known for any neuromodulator. Here, isolated DRG neurons from male rats were used to define the pathway by which low concentrations of 5-HT enhance DSFs, hyperpolarize AP threshold, and promote ongoing activity. A selective 5-HT₄ receptor antagonist blocked these 5-HT-induced hyperexcitable effects, while a selective 5-HT₄ agonist mimicked the effects of 5-HT. Inhibition of cAMP effectors, protein kinase A (PKA) and exchange protein activated by cAMP (EPAC), attenuated 5-HT's hyperexcitable effects, but a blocker of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels had no significant effect. 5-HT₄-dependent PKA activation was specific to DRG neurons that bind isolectin B4 (a nonpeptidergic nociceptor marker). 5-HT's effects on AP threshold, DSFs, and ongoing activity were mimicked by a cAMP analog. Sustained exposure to 5-HT promotes ongoing activity in nonpeptidergic nociceptors through the G_s-coupled 5-HT₄ receptor and downstream cAMP signaling involving both PKA and EPAC.

Scorpion toxin MeuNaTxα-1 sensitizes primary nociceptors by selective modulation of voltage-gated sodium channels

Venoms are a rich source of highly specific toxins, which allow the identification of novel therapeutic targets. We have now applied high content screening (HCS) microscopy to identify toxins that modulate pain sensitization signaling in primary sensory neurons of rat and elucidated the underlying mechanism. A set of venoms and fractions thereof were analyzed for their ability to activate type II protein kinase A (PKA-II) and extracellular signal-regulated kinases (ERK1/2). We identified MeuNaTxα-1, a sodium channel-selective scorpion α-toxin from Mesobuthus eupeus, which affected both PKA-II and ERK1/2. Recombinant MeuNaTxα-1 showed identical activity to the native toxin on mammalian voltage-gated sodium channels expressed in Xenopus laevis oocytes and induced thermal hyperalgesia in adult mice. The effect of MeuNaTxα-1 on sensory neurons was dose-dependent and tetrodotoxin-sensitive. Application of inhibitors and toxin mutants with altered sodium channel selectivity demonstrated that signaling activation in sensory neurons depends on Na_V 1.2 isoform. Accordingly, the toxin was more potent in neurons from newborn rats, where Na_V 1.2 is expressed at a higher level. Our results demonstrate that HCS microscopy-based monitoring of intracellular signaling is a novel and powerful tool to identify and characterize venoms and their toxins affecting sensory neurons.

Oncostatin M induces hyperalgesic priming and amplifies signaling of cAMP to ERK by RapGEF2 and PKA

Hyperalgesic priming is characterized by enhanced nociceptor sensitization by pronociceptive mediators, prototypically PGE₂ . Priming has gained interest as a mechanism underlying the transition to chronic pain. Which stimuli induce priming and what cellular mechanisms are employed remains incompletely understood. In adult male rats, we present the cytokine Oncostatin M (OSM), a member of the IL-6 family, as an inducer of priming by a novel mechanism. We used a high content microscopy based approach to quantify the activation of endogenous PKA-II and ERK of thousands sensory neurons in culture. Incubation with OSM increased and prolonged ERK activation by agents that increase cAMP production such as PGE₂ , forskolin, and cAMP analogs. These changes were specific to IB4/CaMKIIα positive neurons, required protein translation, and increased cAMP-to-ERK signaling. In both, control and OSM-treated neurons, cAMP/ERK signaling involved RapGEF2 and PKA but not Epac. Similar enhancement of cAMP-to-ERK signaling could be induced by GDNF, which acts mostly on IB4/CaMKIIα-positive neurons, but not by NGF, which acts mostly on IB4/CaMKIIα-negative neurons. In vitro, OSM pretreatment rendered baseline TTX-R currents ERK-dependent and switched forskolin-increased currents from partial to full ERK-dependence in small/medium sized neurons. In summary, priming induced by OSM uses a novel mechanism to enhance and prolong coupling of cAMP/PKA to ERK1/2 signaling without changing the overall pathway structure.

Computer-aided Discovery of a New Nav1.7 Inhibitor for Treatment of Pain and Itch

BACKGROUND

Voltage-gated sodium channel Nav1.7 has been validated as a perspective target for selective inhibitors with analgesic and anti-itch activity. The objective of this study was to discover new candidate compounds with Nav1.7 inhibitor properties. The authors hypothesized that their approach would yield at least one new compound that inhibits sodium currents in vitro and exerts analgesic and anti-itch effects in mice.

METHODS

In silico structure-based similarity search of 1.5 million compounds followed by docking to the Nav1.7 voltage sensor of Domain 4 and molecular dynamics simulation was performed. Patch clamp experiments in Nav1.7-expressing human embryonic kidney 293 cells and in mouse and human dorsal root ganglion neurons were conducted to test sodium current inhibition. Formalin-induced inflammatory pain model, paclitaxel-induced neuropathic pain model, histamine-induced itch model, and mouse lymphoma model of chronic itch were used to confirm in vivo activity of the selected compound.

RESULTS

After in silico screening, nine compounds were selected for experimental assessment in vitro. Of those, four compounds inhibited sodium currents in Nav1.7-expressing human embryonic kidney 293 cells by 29% or greater (P < 0.05). Compound 9 (3-(1-benzyl-1H-indol-3-yl)-3-(3-phenoxyphenyl)-N-(2-(pyrrolidin-1-yl)ethyl)propanamide, referred to as DA-0218) reduced sodium current by 80% with a 50% inhibition concentration of 0.74 μM (95% CI, 0.35 to 1.56 μM), but had no effects on Nav1.5-expressing human embryonic kidney 293 cells. In mouse and human dorsal root ganglion neurons, DA-0218 reduced sodium currents by 17% (95% CI, 6 to 28%) and 22% (95% CI, 9 to 35%), respectively. The inhibition was greatly potentiated in paclitaxel-treated mouse neurons. Intraperitoneal and intrathecal administration of the compound reduced formalin-induced phase II inflammatory pain behavior in mice by 76% (95% CI, 48 to 100%) and 80% (95% CI, 68 to 92%), respectively. Intrathecal administration of DA-0218 produced acute reduction in paclitaxel-induced mechanical allodynia, and inhibited histamine-induced acute itch and lymphoma-induced chronic itch.

CONCLUSIONS

This study's computer-aided drug discovery approach yielded a new Nav1.7 inhibitor that shows analgesic and anti-pruritic activity in mouse models.

Depolarization-Dependent C-Raf Signaling Promotes Hyperexcitability and Reduces Opioid Sensitivity of Isolated Nociceptors after Spinal Cord Injury

Chronic pain caused by spinal cord injury (SCI) is notoriously resistant to treatment, particularly by opioids. After SCI, DRG neurons show hyperactivity and chronic depolarization of resting membrane potential (RMP) that is maintained by cAMP signaling through PKA and EPAC. Importantly, SCI also reduces the negative regulation by Gαi of adenylyl cyclase and its production of cAMP, independent of alterations in G protein-coupled receptors and/or G proteins. Opioid reduction of pain depends on coupling of opioid receptors to Gαi/o family members. Combining high-content imaging and cluster analysis, we show that in male rats SCI decreases opioid responsiveness in vitro within a specific subset of small-diameter nociceptors that bind isolectin B4. This SCI effect is mimicked in nociceptors from naive animals by a modest 5 min depolarization of RMP (15 mm K⁺; -45 mV), reducing inhibition of cAMP signaling by μ-opioid receptor agonists DAMGO and morphine. Disinhibition and activation of C-Raf by depolarization-dependent phosphorylation are central to these effects. Expression of an activated C-Raf reduces sensitivity of adenylyl cyclase to opioids in nonexcitable HEK293 cells, whereas inhibition of C-Raf or treatment with the hyperpolarizing drug retigabine restores opioid responsiveness and blocks spontaneous activity of nociceptors after SCI. Inhibition of ERK downstream of C-Raf also blocks SCI-induced hyperexcitability and depolarization, without direct effects on opioid responsiveness. Thus, depolarization-dependent C-Raf and downstream ERK activity maintain a depolarized RMP and nociceptor hyperactivity after SCI, providing a self-reinforcing mechanism to persistently promote nociceptor hyperexcitability and limit the therapeutic effectiveness of opioids.SIGNIFICANCE STATEMENT Chronic pain induced by spinal cord injury (SCI) is often permanent and debilitating, and usually refractory to treatment with analgesics, including opioids. SCI-induced pain in a rat model has been shown to depend on persistent hyperactivity in primary nociceptors (injury-detecting sensory neurons), associated with a decrease in the sensitivity of adenylyl cyclase production of cAMP to inhibitory Gαi proteins in DRGs. This study shows that SCI and one consequence of SCI (chronic depolarization of resting membrane potential) decrease sensitivity to opioid-mediated inhibition of cAMP and promote hyperactivity of nociceptors by enhancing C-Raf activity. ERK activation downstream of C-Raf is necessary for maintaining ongoing depolarization and hyperactivity, demonstrating an unexpected positive feedback loop to persistently promote pain.

Antiallodynic effects of the selective NaV1.7 inhibitor Pn3a in a mouse model of acute postsurgical pain: evidence for analgesic synergy with opioids and baclofen

Pain is the leading cause of disability in the developed world but remains a poorly treated condition. Specifically, postsurgical pain continues to be a frequent and undermanaged condition. Here, we investigate the analgesic potential of pharmacological NaV1.7 inhibition in a mouse model of acute postsurgical pain, based on incision of the plantar skin and underlying muscle of the hind paw. We demonstrate that local and systemic treatment with the selective NaV1.7 inhibitor μ-theraphotoxin-Pn3a is effectively antiallodynic in this model and completely reverses mechanical hypersensitivity in the absence of motor adverse effects. In addition, the selective NaV1.7 inhibitors ProTx-II and PF-04856264 as well as the clinical candidate CNV1014802 also reduced mechanical allodynia. Interestingly, co-administration of the opioid receptor antagonist naloxone completely reversed analgesic effects of Pn3a, indicating an involvement of endogenous opioids in the analgesic activity of Pn3a. In addition, we found superadditive antinociceptive effects of subtherapeutic Pn3a doses not only with the opioid oxycodone but also with the GABAB receptor agonist baclofen. Transcriptomic analysis of gene expression changes in dorsal root ganglia of mice after surgery did not reveal any changes in mRNA expression of endogenous opioids or opioid receptors; however, several genes involved in pain, including Runx1 (Runt related transcription factor 1), Cacna1a (CaV2.1), and Cacna1b (CaV2.2), were downregulated. In summary, these findings suggest that pain after surgery can be successfully treated with NaV1.7 inhibitors alone or in combination with baclofen or opioids, which may present a novel and safe treatment strategy for this frequent and poorly managed condition.

Complementary roles of murine NaV1.7, NaV1.8 and NaV1.9 in acute itch signalling

Acute pruritus occurs in various disorders. Despite severe repercussions on quality of life treatment options remain limited. Voltage-gated sodium channels (Na_V) are indispensable for transformation and propagation of sensory signals implicating them as drug targets. Here, Na_V1.7, 1.8 and 1.9 were compared for their contribution to itch by analysing Na_V-specific knockout mice. Acute pruritus was induced by a comprehensive panel of pruritogens (C48/80, endothelin, 5-HT, chloroquine, histamine, lysophosphatidic acid, trypsin, SLIGRL, β-alanine, BAM8-22), and scratching was assessed using a magnet-based recording technology. We report an unexpected stimulus-dependent diversity in Na_V channel-mediated itch signalling. Na_V1.7^−/− showed substantial scratch reduction mainly towards strong pruritogens. Na_V1.8^−/− impaired histamine and 5-HT-induced scratching while Na_V1.9 was involved in itch signalling towards 5-HT, C48/80 and SLIGRL. Furthermore, similar microfluorimetric calcium responses of sensory neurons and expression of itch-related TRP channels suggest no change in sensory transduction but in action potential transformation and conduction. The cumulative sum of scratching over all pruritogens confirmed a leading role of Na_V1.7 and indicated an overall contribution of Na_V1.9. Beside the proposed general role of Na_V1.7 and 1.9 in itch signalling, scrutiny of time courses suggested Na_V1.8 to sustain prolonged itching. Therefore, Na_V1.7 and 1.9 may represent targets in pruritus therapy.

Endogenous opiates and behavior: 2017

This paper is the fortieth consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2017 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).

Sensory neuron-derived NaV1.7 contributes to dorsal horn neuron excitability

Expression of the voltage-gated sodium channel Na_V1.7 in sensory neurons is required for pain sensation. We examined the role of Na_V1.7 in the dorsal horn of the spinal cord using an epitope-tagged Na_V1.7 knock-in mouse. Immuno-electron microscopy showed the presence of Na_V1.7 in dendrites of superficial dorsal horn neurons, despite the absence of mRNA. Rhizotomy of L5 afferent nerves lowered the levels of Na_V1.7 in the dorsal horn. Peripheral nervous system-specific Na_V1.7 null mutant mice showed central deficits, with lamina II dorsal horn tonic firing neurons more than halved and single spiking neurons more than doubled. Na_V1.7 blocker PF05089771 diminished excitability in dorsal horn neurons but had no effect on Na_V1.7 null mutant mice. These data demonstrate an unsuspected functional role of primary afferent neuron-generated Na_V1.7 in dorsal horn neurons and an expression pattern that would not be predicted by transcriptomic analysis.

Antibodies and venom peptides: new modalities for ion channels

Ion channels play fundamental roles in both excitable and non-excitable tissues and therefore constitute attractive drug targets for myriad neurological, cardiovascular and metabolic diseases as well as for cancer and immunomodulation. However, achieving selectivity for specific ion channel subtypes with small-molecule drugs has been challenging, and there currently is a growing trend to target ion channels with biologics. One approach is to improve the pharmacokinetics of existing or novel venom-derived peptides. In parallel, after initial studies with polyclonal antibodies demonstrated the technical feasibility of inhibiting channel function with antibodies, multiple preclinical programmes are now using the full spectrum of available technologies to generate conventional monoclonal and engineered antibodies or nanobodies against extracellular loops of ion channels. After a summary of the current state of ion channel drug discovery, this Review discusses recent developments using the purinergic receptor channel P2X purinoceptor 7 (P2X7), the voltage-gated potassium channel KV1.3 and the voltage-gated sodium channel NaV1.7 as examples of targeting ion channels with biologics.

Transcriptional Regulation of Voltage-Gated Sodium Channels Contributes to GM-CSF-Induced Pain

Granulocyte-macrophage colony-stimulating factor (GM-CSF) induces the production of granulocyte and macrophage populations from the hematopoietic progenitor cells; it is one of the most common growth factors in the blood. GM-CSF is also involved in bone cancer pain development by regulating tumor-nerve interactions, remodeling of peripheral nerves, and sensitization of damage-sensing (nociceptive) nerves. However, the precise mechanism for GM-CSF-dependent pain is unclear. In this study, we found that GM-CSF is highly expressed in human malignant osteosarcoma. Female Sprague Dawley rats implanted with bone cancer cells develop mechanical and thermal hyperalgesia, but antagonizing GM-CSF in these animals significantly reduced such hypersensitivity. The voltage-gated Na⁺ channels Nav1.7, Nav1.8, and Nav1.9 were found to be selectively upregulated in rat DRG neurons treated with GM-CSF, which resulted in enhanced excitability. GM-CSF activated the Janus kinase 2 (Jak2)-signal transducer and activator of transcription protein 3 (Stat3) signaling pathway, which promoted the transcription of Nav1.7-1.9 in DRG neurons. Accordingly, targeted knocking down of either Nav1.7-1.9 or Jak2/Stat3 in DRG neurons in vivo alleviated the hyperalgesia in male Sprague Dawley rats. Our findings describe a novel bone cancer pain mechanism and provide a new insight into the physiological and pathological functions of GM-CSF.SIGNIFICANCE STATEMENT It has been reported that granulocyte-macrophage colony-stimulating factor (GM-CSF) plays a key role in bone cancer pain, yet the underlying mechanisms involved in the GM-CSF-mediated signaling pathway in nociceptors is not fully understood. Here, we showed that GM-CSF promotes bone cancer-associated pain by enhancing the excitability of DRG neurons via the Janus kinase 2 (Jak2)-signal transducer and activator of transcription protein 3 (Stat3)-mediated upregulation of expression of nociceptor-specific voltage-gated sodium channels. Our study provides a detailed understanding of the roles that sodium channels and the Jak2/Stat3 pathway play in the GM-CSF-mediated bone cancer pain; our data also highlight the therapeutic potential of targeting GM-CSF.

The Role of Voltage-Gated Sodium Channels in Pain Signaling

Acute pain signaling has a key protective role and is highly evolutionarily conserved. Chronic pain, however, is maladaptive, occurring as a consequence of injury and disease, and is associated with sensitization of the somatosensory nervous system. Primary sensory neurons are involved in both of these processes, and the recent advances in understanding sensory transduction and human genetics are the focus of this review. Voltage-gated sodium channels (VGSCs) are important determinants of sensory neuron excitability: they are essential for the initial transduction of sensory stimuli, the electrogenesis of the action potential, and neurotransmitter release from sensory neuron terminals. Nav1.1, Nav1.6, Nav1.7, Nav1.8, and Nav1.9 are all expressed by adult sensory neurons. The biophysical characteristics of these channels, as well as their unique expression patterns within subtypes of sensory neurons, define their functional role in pain signaling. Changes in the expression of VGSCs, as well as posttranslational modifications, contribute to the sensitization of sensory neurons in chronic pain states. Furthermore, gene variants in Nav1.7, Nav1.8, and Nav1.9 have now been linked to human Mendelian pain disorders and more recently to common pain disorders such as small-fiber neuropathy. Chronic pain affects one in five of the general population. Given the poor efficacy of current analgesics, the selective expression of particular VGSCs in sensory neurons makes these attractive targets for drug discovery. The increasing availability of gene sequencing, combined with structural modeling and electrophysiological analysis of gene variants, also provides the opportunity to better target existing therapies in a personalized manner.

Mining the Nav1.7 interactome: Opportunities for chronic pain therapeutics

The peripherally expressed voltage-gated sodium Na_V1.7 (gene SCN9A) channel boosts small stimuli to initiate firing of pain-signaling dorsal root ganglia (DRG) neurons and facilitates neurotransmitter release at the first synapse within the spinal cord. Mutations in SCN9A produce distinct human pain syndromes. Widely acknowledged as a "gatekeeper" of pain, Na_V1.7 has been the focus of intense investigation but, to date, no Na_V1.7-selective drugs have reached the clinic. Elegant crystallographic studies have demonstrated the potential of designing highly potent and selective Na_V1.7 compounds but their therapeutic value remains untested. Transcriptional silencing of Na_V1.7 by a naturally expressed antisense transcript has been reported in rodents and humans but whether this represents a viable opportunity for designing Na_V1.7 therapeutics is currently unknown. The demonstration that loss of Na_V1.7 function is associated with upregulation of endogenous opioids and potentiation of mu- and delta-opioid receptor activities, suggests that targeting only Na_V1.7 may be insufficient for analgesia. However, the link between opioid-dependent analgesic mechanisms and function of sodium channels and intracellular sodium-dependent signaling remains controversial. Thus, additional new targets - regulators, modulators - are needed. In this context, we mine the literature for the known interactome of Na_V1.7 with a focus on protein interactors that affect the channel's trafficking or link it to opioid signaling. As a case study, we present antinociceptive evidence of allosteric regulation of Na_V1.7 by the cytosolic collapsin response mediator protein 2 (CRMP2). Throughout discussions of these possible new targets, we offer thoughts on the therapeutic implications of modulating Na_V1.7 function in chronic pain.

High-Content Imaging of Immunofluorescently Labeled TRPV1-Positive Sensory Neurons

Studying TRP channel expressing nociceptors requires the identification of the respective subpopulations as well as the quantification of dynamic cellular events. However, the heterogeneity of sensory neurons and associated nonneuronal cells demands the analysis of large numbers of cells to reflect the distribution of entire populations. Here we report a detailed workflow how to apply high-content screening (HCS) microscopy to signaling events in TRPV1-positive neurons as well as an approach to use the selective elimination of TRPV1 positive cells from dissociated rat sensory ganglia as base for transcriptomic analysis of TRPV1-positive cells and/or as control for TRPV1 antibody specificity.

Alterations of Expression of the Serotonin 5-HT4 Receptor in Brain Disorders

The serotonin 4 receptor, 5-HT₄R, represents one of seven different serotonin receptor families and is implicated in a variety of physiological functions and their pathophysiological variants, such as mood and depression or anxiety, food intake and obesity or anorexia, or memory and memory loss in Alzheimer's disease. Its central nervous system expression pattern in the forebrain, in particular in caudate putamen, the hippocampus and to lesser extent in the cortex, predispose it for a role in executive function and reward-related actions. In rodents, regional overexpression or knockdown in the prefrontal cortex or the nucleus accumbens of 5-HT₄R was shown to impact mood and depression-like phenotypes, food intake and hypophagia; however, whether expression changes are causally involved in the etiology of such disorders is not clear. In this context, more data are emerging, especially based on PET technology and the use of ligand tracers that demonstrate altered 5-HT₄R expression in brain disorders in humans, confirming data stemming from post-mortem tissue and preclinical animal models. In this review, we would like to present the current knowledge of 5-HT₄R expression in brain regions relevant to mood/depression, reward and executive function with a focus on 5-HT₄R expression changes in brain disorders or caused by drug treatment, at both the transcript and protein levels.

Insensitivity to Pain upon Adult-Onset Deletion of Nav1.7 or Its Blockade with Selective Inhibitors

Strong human genetic evidence points to an essential contribution of the voltage-gated sodium channel Nav1.7 to pain sensation: loss of Nav1.7 function leads to congenital insensitivity to pain, whereas gain-of-function mutations in the SCN9A gene that encodes Nav1.7 cause painful neuropathies, such as inherited erythromelalgia, a syndrome characterized by episodic spontaneous pain. Selective Nav1.7 channel blockers thus hold promise as potential painkillers with improved safety and reduced unwanted side effects compared with existing therapeutics. To determine the maximum effect of a theoretically perfectly selective Nav1.7 inhibitor, we generated a tamoxifen-inducible KO mouse model enabling genetic deletion of Nav1.7 from adult mice. Electrophysiological recordings of sensory neurons from these mice following tamoxifen injection demonstrated the loss of Nav1.7 channel current and the resulting decrease in neuronal excitability of small-diameter neurons. We found that behavioral responses to most, but surprisingly not all, modalities of noxious stimulus are abolished following adult deletion of Nav1.7, pointing toward indications where Nav1.7 blockade should be efficacious. Furthermore, we demonstrate that isoform-selective acylsulfonamide Nav1.7 inhibitors show robust analgesic and antinociceptive activity acutely after a single dose in mouse pain models shown to be Nav1.7-dependent. All experiments were done with both male and female mice. Collectively, these data expand the depth of knowledge surrounding Nav1.7 biology as it relates to pain, and provide preclinical proof of efficacy that lays a clear path toward translation for the therapeutic use of Nav1.7-selective inhibitors in humans.SIGNIFICANCE STATEMENT Loss-of-function mutations in the sodium channel Nav1.7 cause congenital insensitivity to pain in humans, making Nav1.7 a top target for novel pain drugs. Targeting Nav1.7 selectively has been challenging, however, in part due to uncertainties in which rodent pain models are dependent on Nav1.7. We have developed and characterized an adult-onset Nav1.7 KO mouse model that allows us to determine the expected effects of a theoretically perfect Nav1.7 blocker. Importantly, many commonly used pain models, such as mechanical allodynia after nerve injury, appear to not be dependent on Nav1.7 in the adult. By defining which models are Nav1.7 dependent, we demonstrate that selective Nav1.7 inhibitors can approximate the effects of genetic loss of function, which previously has not been directly established.

Substance P and pain chronicity

Substance P (SP) is a highly conserved member of the tachykinin peptide family that is widely expressed throughout the animal kingdom. The numerous members of the tachykinin peptide family are involved in a multitude of neuronal signaling pathways, mediating sensations and emotional responses (Steinhoff et al. in Physiol Rev 94:265–301, 2014). In contrast to receptors for classical transmitters, such as glutamate (Parsons et al. in Handb Exp Pharmacol 249–303, 2005), only a minority of neurons in certain brain areas express neurokinin receptors (NKRs) (Mantyh in J Clin Psychiatry 63:6–10, 2002). SP is also expressed by a variety of non-neuronal cell types such as microglia, as well as immune cells (Mashaghi et al. in Cell Mol Life Sci 73:4249–4264, 2016). SP is an 11-amino acid neuropeptide that preferentially activates the neurokinin-1 receptor (NK1R). It transmits nociceptive signals via primary afferent fibers to spinal and brainstem second-order neurons (Cao et al. in Nature 392:390–394, 1998). Compounds that inhibit SP’s action are being investigated as potential drugs to relieve pain. More recently, SP and NKR have gained attention for their role in complex psychiatric processes. It is a key goal in the field of pain research to understand mechanisms involved in the transition between acute pain and chronic pain. The influence of emotional and cognitive inputs and feedbacks from different brain areas makes pain not only a perception but an experience (Zieglgänsberger et al. in CNS Spectr 10:298–308, 2005; Trenkwaldner et al. Sleep Med 31:78–85, 2017). This review focuses on functional neuronal plasticity in spinal dorsal horn neurons as a major relay for nociceptive information.

Analgesia linked to Nav1.7 loss of function requires µ- and δ-opioid receptors

Background: Functional deletion of the Scn9a (sodium voltage-gated channel alpha subunit 9) gene encoding sodium channel Nav1.7 makes humans and mice pain-free. Opioid signalling contributes to this analgesic state. We have used pharmacological and genetic approaches to identify the opioid receptors involved in this form of analgesia. We also examined the regulation of proenkephalin expression by the transcription factor Nfat5 that binds upstream of the Penk gene. Methods: We used specific µ-, δ- and κ-opioid receptor antagonists alone or in combination to examine which opioid receptors were necessary for Nav1.7 loss-associated analgesia in mouse behavioural assays of thermal pain. We also used µ- and δ-opioid receptor null mutant mice alone and in combination in behavioural assays to examine the role of these receptors in Nav1.7 knockouts pain free phenotype. Finally, we examined the levels of Penk mRNA in Nfat5-null mutant mice, as this transcription factor binds to consensus sequences upstream of the Penk gene. Results: The pharmacological block or deletion of both µ- and δ-opioid receptors was required to abolish Nav1.7-null opioid-related analgesia. κ-opioid receptor antagonists were without effect. Enkephalins encoded by the Penk gene are upregulated in Nav1.7 nulls. Deleting Nfat5, a transcription factor with binding motifs upstream of Penk, induces the same level of enkephalin mRNA expression as found in Nav1 .7 nulls, but without consequent analgesia. These data confirm that a combination of events linked to Scn9a gene loss is required for analgesia. Higher levels of endogenous enkephalins, potentiated opioid receptors, diminished electrical excitability and loss of neurotransmitter release together contribute to the analgesic phenotype found in Nav1.7-null mouse and human mutants. Conclusions: These observations help explain the failure of Nav1.7 channel blockers alone to produce analgesia and suggest new routes for analgesic drug development.

PKA-RII subunit phosphorylation precedes activation by cAMP and regulates activity termination

Activity of endogenous protein kinase A (PKA) could never be analyzed directly in the cellular environment. Isensee et al. used antibodies to quantify conformational changes leading to an open conformation of endogenous PKA-II holoenzymes, which allowed them to analyze and model its activation cycle in primary sensory neurons.

Type II isoforms of cyclic adenosine monophosphate (cAMP)–dependent protein kinase A (PKA-II) contain a phosphorylatable epitope within the inhibitory domain of RII subunits (pRII) with still unclear function. In vitro, RII phosphorylation occurs in the absence of cAMP, whereas staining of cells with pRII-specific antibodies revealed a cAMP-dependent pattern. In sensory neurons, we found that increased pRII immunoreactivity reflects increased accessibility of the already phosphorylated RII epitope during cAMP-induced opening of the tetrameric RII₂:C₂ holoenzyme. Accordingly, induction of pRII by cAMP was sensitive to novel inhibitors of dissociation, whereas blocking catalytic activity was ineffective. Also in vitro, cAMP increased the binding of pRII antibodies to RII₂:C₂ holoenzymes. Identification of an antibody specific for the glycine-rich loop of catalytic subunits facing the pRII-epitope confirmed activity-dependent binding with similar kinetics, proving that the reassociation is rapid and precisely controlled. Mechanistic modeling further supported that RII phosphorylation precedes cAMP binding and controls the inactivation by modulating the reassociation involving the coordinated action of phosphodiesterases and phosphatases.

Modulation of sodium channels as pharmacological tool for pain therapy-highlights and gaps

Voltage-gated sodium channels are crucially involved in the transduction and transmission of nociceptive signals and pathological pain states. In the past decades, a lot of effort has been spent examining and characterizing biophysical properties of the different sodium channels and their role in signaling pathways. Several gains of function mutations of the sodium channels Nav1.7, Nav1.8, and Nav1.9 are associated with pain disorders. Due to their critical role in nociceptive pathways voltage-gated sodium channels are regarded interesting targets for pharmacological pain treatment. However we still need to fill the gap that exists in the translation of efficacy in preclinical in vitro experiments and in models of pain into the clinic. This review summarizes biological and electrophysiological properties of voltage-gated sodium channels and aims to discuss limitations and promising pharmacological strategies in sodium channel research in the context of pain therapy.

Mapping protein interactions of sodium channel NaV1.7 using epitope-tagged gene-targeted mice

The voltage-gated sodium channel NaV1.7 plays a critical role in pain pathways. We generated an epitope-tagged NaV1.7 mouse that showed normal pain behaviours to identify channel-interacting proteins. Analysis of NaV1.7 complexes affinity-purified under native conditions by mass spectrometry revealed 267 proteins associated with Nav1.7 in vivo The sodium channel β3 (Scn3b), rather than the β1 subunit, complexes with Nav1.7, and we demonstrate an interaction between collapsing-response mediator protein (Crmp2) and Nav1.7, through which the analgesic drug lacosamide regulates Nav1.7 current density. Novel NaV1.7 protein interactors including membrane-trafficking protein synaptotagmin-2 (Syt2), L-type amino acid transporter 1 (Lat1) and transmembrane P24-trafficking protein 10 (Tmed10) together with Scn3b and Crmp2 were validated by co-immunoprecipitation (Co-IP) from sensory neuron extract. Nav1.7, known to regulate opioid receptor efficacy, interacts with the G protein-regulated inducer of neurite outgrowth (Gprin1), an opioid receptor-binding protein, demonstrating a physical and functional link between Nav1.7 and opioid signalling. Further information on physiological interactions provided with this normal epitope-tagged mouse should provide useful insights into the many functions now associated with the NaV1.7 channel.

The Genetics of Pain: Implications for Therapeutics

Pain is an increasing clinical challenge affecting about half the population, with a substantial number of people suffering daily intense pain. Such suffering can be linked to the dramatic rise in opioid use and associated deaths in the United States. There is a pressing need for new analgesics with limited side effects. Here, we summarize what we know about the genetics of pain and implications for drug development. We make the case that chronic pain is not one but a set of disease states, with peripheral drive a key element in most. We argue that understanding redundancy and plasticity, hallmarks of the nervous system, is critical in developing analgesic drug strategies. We describe the exploitation of monogenic pain syndromes and genetic association studies to define analgesic targets, as well as issues associated with animal models of pain. We appraise present-day screening technologies and describe recent approaches to pain treatment that hold promise.