Kv2 dysfunction after peripheral axotomy enhances sensory neuron responsiveness to sustained input

Mono-allelic KCNB2 variants lead to a neurodevelopmental syndrome caused by altered channel inactivation

Ion channels mediate voltage fluxes or action potentials that are central to the functioning of excitable cells such as neurons. The KCNB family of voltage-gated potassium channels (Kv) consists of two members (KCNB1 and KCNB2) encoded by KCNB1 and KCNB2, respectively. These channels are major contributors to delayed rectifier potassium currents arising from the neuronal soma which modulate overall excitability of neurons. In this study, we identified several mono-allelic pathogenic missense variants in KCNB2, in individuals with a neurodevelopmental syndrome with epilepsy and autism in some individuals. Recurrent dysmorphisms included a broad forehead, synophrys, and digital anomalies. Additionally, we selected three variants where genetic transmission has not been assessed, from two epilepsy studies, for inclusion in our experiments. We characterized channel properties of these variants by expressing them in oocytes of Xenopus laevis and conducting cut-open oocyte voltage clamp electrophysiology. Our datasets indicate no significant change in absolute conductance and conductance-voltage relationships of most disease variants as compared to wild type (WT), when expressed either alone or co-expressed with WT-KCNB2. However, variants c.1141A>G (p.Thr381Ala) and c.641C>T (p.Thr214Met) show complete abrogation of currents when expressed alone with the former exhibiting a left shift in activation midpoint when expressed alone or with WT-KCNB2. The variants we studied, nevertheless, show collective features of increased inactivation shifted to hyperpolarized potentials. We suggest that the effects of the variants on channel inactivation result in hyper-excitability of neurons, which contributes to disease manifestations.

Small fibre neuropathy in Fabry disease: a human-derived neuronal in vitro disease model and pilot data

Acral burning pain triggered by fever, thermal hyposensitivity and skin denervation are hallmarks of small fibre neuropathy in Fabry disease, a life-threatening X-linked lysosomal storage disorder. Variants in the gene encoding alpha-galactosidase A may lead to impaired enzyme activity with cellular accumulation of globotriaosylceramide. To study the underlying pathomechanism of Fabry-associated small fibre neuropathy, we generated a neuronal in vitro disease model using patient-derived induced pluripotent stem cells from three Fabry patients and one healthy control. We further generated an isogenic control line via gene editing. We subjected induced pluripotent stem cells to targeted peripheral neuronal differentiation and observed intra-lysosomal globotriaosylceramide accumulations in somas and neurites of Fabry sensory neurons using super-resolution microscopy. At functional level, patch-clamp analysis revealed a hyperpolarizing shift of voltage-gated sodium channel steady-state inactivation kinetics in isogenic control neurons compared with healthy control neurons (P < 0.001). Moreover, we demonstrate a drastic increase in Fabry sensory neuron calcium levels at 39°C mimicking clinical fever (P < 0.001). This pathophysiological phenotype was accompanied by thinning of neurite calibres in sensory neurons differentiated from induced pluripotent stem cells derived from Fabry patients compared with healthy control cells (P < 0.001). Linear-nonlinear cascade models fit to spiking responses revealed that Fabry cell lines exhibit altered single neuron encoding properties relative to control. We further observed mitochondrial aggregation at sphingolipid accumulations within Fabry sensory neurites utilizing a click chemistry approach together with mitochondrial dysmorphism compared with healthy control cells. We pioneer pilot insights into the cellular mechanisms contributing to pain, thermal hyposensitivity and denervation in Fabry small fibre neuropathy and pave the way for further mechanistic in vitro studies in Fabry disease and the development of novel treatment approaches.

Distinct cellular expression and subcellular localization of Kv2 voltage-gated K+ channel subtypes in dorsal root ganglion neurons conserved between mice and humans

The distinct organization of Kv2 voltage-gated potassium channels on and near the cell body of brain neurons enables their regulation of action potentials and specialized membrane contact sites. Somatosensory neurons have a pseudounipolar morphology and transmit action potentials from peripheral nerve endings through axons that bifurcate to the spinal cord and the cell body within ganglia including the dorsal root ganglia (DRG). Kv2 channels regulate action potentials in somatosensory neurons, yet little is known about where Kv2 channels are located. Here, we define the cellular and subcellular localization of the Kv2 paralogs, Kv2.1 and Kv2.2, in DRG somatosensory neurons with a panel of antibodies, cell markers, and genetically modified mice. We find that relative to spinal cord neurons, DRG neurons have similar levels of detectable Kv2.1 and higher levels of Kv2.2. In older mice, detectable Kv2.2 remains similar, while detectable Kv2.1 decreases. Both Kv2 subtypes adopt clustered subcellular patterns that are distinct from central neurons. Most DRG neurons co-express Kv2.1 and Kv2.2, although neuron subpopulations show preferential expression of Kv2.1 or Kv2.2. We find that Kv2 protein expression and subcellular localization are similar between mouse and human DRG neurons. We conclude that the organization of both Kv2 channels is consistent with physiological roles in the somata and stem axons of DRG neurons. The general prevalence of Kv2.2 in DRG as compared to central neurons and the enrichment of Kv2.2 relative to detectable Kv2.1 in older mice, proprioceptors, and axons suggest more widespread roles for Kv2.2 in DRG neurons.

Kv2 channels contribute to neuronal activity within the vagal afferent-nTS reflex arc

Diversity in the functional expression of ion channels contributes to the unique patterns of activity generated in visceral sensory A-type myelinated neurons versus C-type unmyelinated neurons in response to their natural stimuli. In the present study, Kv2 channels were identified as underlying a previously uncharacterized delayed rectifying potassium current expressed in both A- and C-type nodose ganglion neurons. Kv2.1 and 2.2 appear confined to the soma and initial segment of these sensory neurons; however, neither was identified in their central presynaptic terminals projecting onto relay neurons in the nucleus of the solitary tract (nTS). Kv2.1 and Kv2.2 were also not detected in the peripheral axons and sensory terminals in the aortic arch. Functionally, in nodose neuron somas, Kv2 currents exhibited frequency-dependent current inactivation and contributed to action potential repolarization in C-type neurons but not A-type neurons. Within the nTS, the block of Kv2 currents does not influence afferent presynaptic calcium influx or glutamate release in response to afferent activation, supporting our immunohistochemical observations. On the other hand, Kv2 channels contribute to membrane hyperpolarization and limit action potential discharge rate in second-order neurons. Together, these data demonstrate that Kv2 channels influence neuronal discharge within the vagal afferent-nTS circuit and indicate they may play a significant role in viscerosensory reflex function.NEW & NOTEWORTHY We demonstrate the expression and function of the voltage-gated delayed rectifier potassium channel Kv2 in vagal nodose neurons. Within sensory neurons, Kv2 channels limit the width of the broader C-type but not narrow A-type action potential. Within the nucleus of the solitary tract (nTS), the location of the vagal terminal field, Kv2 does not influence glutamate release. However, Kv2 limits the action potential discharge of nTS relay neurons. These data suggest a critical role for Kv2 in the vagal-nTS reflex arc.

Distinct cellular expression and subcellular localization of Kv2 voltage-gated K+ channel subtypes in dorsal root ganglion neurons conserved between mice and humans

The distinct organization of Kv2 voltage-gated potassium channels on and near the cell body of brain neurons enables their regulation of action potentials and specialized membrane contact sites. Somatosensory neurons have a pseudounipolar morphology and transmit action potentials from peripheral nerve endings through axons that bifurcate to the spinal cord and the cell body within ganglia including the dorsal root ganglia (DRG). Kv2 channels regulate action potentials in somatosensory neurons, yet little is known about where Kv2 channels are located. Here we define the cellular and subcellular localization of the Kv2 paralogs, Kv2.1 and Kv2.2, in DRG somatosensory neurons with a panel of antibodies, cell markers, and genetically modified mice. We find that relative to spinal cord neurons, DRG neurons have similar levels of detectable Kv2.1, and higher levels of Kv2.2. In older mice, detectable Kv2.2 remains similar while detectable Kv2.1 decreases. Both Kv2 subtypes adopt clustered subcellular patterns that are distinct from central neurons. Most DRG neurons co-express Kv2.1 and Kv2.2, although neuron subpopulations show preferential expression of Kv2.1 or Kv2.2. We find that Kv2 protein expression and subcellular localization is similar between mouse and human DRG neurons. We conclude that the organization of both Kv2 channels is consistent with physiological roles in the somata and stem axons of DRG neurons. The general prevalence of Kv2.2 in DRG as compared to central neurons and the enrichment of Kv2.2 relative to detectable Kv2.1, in older mice, proprioceptors, and axons suggest more widespread roles for Kv2.2 in DRG neurons.

A cholinergic circuit that relieves pain despite opioid tolerance

Chronic pain is a tremendous burden for afflicted individuals and society. Although opioids effectively relieve pain, significant adverse outcomes limit their utility and efficacy. To investigate alternate pain control mechanisms, we explored cholinergic signaling in the ventrolateral periaqueductal gray (vlPAG), a critical nexus for descending pain modulation. Biosensor assays revealed that pain states decreased acetylcholine release in vlPAG. Activation of cholinergic projections from the pedunculopontine tegmentum to vlPAG relieved pain, even in opioid-tolerant conditions, through ⍺7 nicotinic acetylcholine receptors (nAChRs). Activating ⍺7 nAChRs with agonists or stimulating endogenous acetylcholine inhibited vlPAG neuronal activity through Ca2+ and peroxisome proliferator-activated receptor α (PPAR⍺)-dependent signaling. In vivo 2-photon imaging revealed that chronic pain induces aberrant excitability of vlPAG neuronal ensembles and that ⍺7 nAChR-mediated inhibition of these cells relieves pain, even after opioid tolerance. Finally, pain relief through these cholinergic mechanisms was not associated with tolerance, reward, or withdrawal symptoms, highlighting its potential clinical relevance.

Spinal cord injury significantly alters the properties of reticulospinal neurons: delayed repolarization mediated by potassium channels

After rostral spinal cord injury (SCI) of lampreys, the descending axons of injured (axotomized) reticulospinal (RS) neurons regenerate and locomotor function gradually recovers. Our previous studies indicated that relative to uninjured lamprey RS neurons, injured RS neurons display several dramatic changes in their biophysical properties, called the "injury phenotype." In the present study, at the onset of applied depolarizing current pulses for membrane potentials below as well as above threshold for action potentials (APs), injured RS neurons displayed a transient depolarization consisting of an initial depolarizing component followed by a delayed repolarizing component. In contrast, for uninjured neurons the transient depolarization was mostly only evident at suprathreshold voltages when APs were blocked. For injured RS neurons, the delayed repolarizing component resisted depolarization to threshold and made these neurons less excitable than uninjured RS neurons. After block of voltage-gated sodium and calcium channels for injured RS neurons, the transient depolarization was still present. After a further block of voltage-gated potassium channels, the delayed repolarizing component was abolished or significantly reduced, with little or no effect on the initial depolarizing component. Voltage-clamp experiments indicated that the delayed repolarizing component was due to a noninactivating outward-rectifying potassium channel whose conductance (gK) was significantly larger for injured RS neurons compared to that for uninjured neurons. Thus, SCI results in an increase in gK and other changes in the biophysical properties of injured lamprey RS neurons that lead to a reduction in excitability, which is proposed to create an intracellular environment that supports axonal regeneration.NEW & NOTEWORTHY After spinal cord injury (SCI), lamprey reticulospinal (RS) neurons responded to subthreshold depolarizing current pulses with a transient depolarization, which included an initial depolarization that was due to passive channels followed by a delayed repolarization that was mediated by voltage-gated potassium channels. The conductance of these channels (gK) was significantly increased for RS neurons after SCI and contributed to a reduction in excitability, which is expected to provide supportive conditions for subsequent axonal regeneration.

ACTIVATION OF CYCLIN-DEPENDENT KINASE 5 BROADENS ACTION POTENTIALS IN HUMAN SENSORY NEURONS

Chronic pain is one of the most devastating and unpleasant conditions, associated with many pathological conditions. Tissue or nerve injuries induce comprehensive neurobiological plasticity in nociceptive neurons, which leads to chronic pain. Recent studies suggest that cyclin-dependent kinase 5 (CDK5) in primary afferents is a key neuronal kinase that modulates nociception through phosphorylation-dependent manner under pathological conditions. However, the impact of the CDK5 on nociceptor activity especially in human sensory neurons are not known. To determine the CDK5-mediated regulation of human dorsal root ganglia (hDRG) neuronal properties, we have performed the whole-cell patch clamp recordings in neurons dissociated from hDRG. CDK5 activation induced by overexpression of p35 depolarized the resting membrane potential and reduced the rheobase currents as compared to the uninfected neurons. CDK5 activation evidently changed the shape of the action potential (AP) by increasing AP rise time, AP fall time, and AP half width. The application of a prostaglandin E2 (PG) and bradykinin (BK) cocktail in uninfected hDRG neurons induced the depolarization of RMP and the reduction of rheobase currents along with increased AP rise time. However, PG and BK applications failed to induce any further significant changes in addition to the aforementioned changes of the membrane properties and AP parameters in the p35-overexpressing group. We conclude that CDK5 activation through the overexpression of p35 in dissociated hDRG neurons broadens AP in hDRG neurons and that CDK5 may play important roles in the modulation of AP properties in human primary afferents under pathological conditions, contributing to chronic pain.

Gene therapy for chronic pain: emerging opportunities in target-rich peripheral nociceptors

With sweeping advances in precision delivery systems and manipulation of the genomes and transcriptomes of various cell types, medical biotechnology offers unprecedented selectivity for and control of a wide variety of biological processes, forging new opportunities for therapeutic interventions. This perspective summarizes state-of-the-art gene therapies enabled by recent innovations, with an emphasis on the expanding universe of molecular targets that govern the activity and function of primary sensory neurons and which might be exploited to effectively treat chronic pain.

Activation of cyclin-dependent kinase 5 broadens action potentials in human sensory neurons

Chronic pain is one of the most devastating and unpleasant conditions, associated with many pathological states. Tissue or nerve injuries induce extensive neurobiological plasticity in nociceptive neurons, which leads to chronic pain. Recent studies suggest that cyclin-dependent kinase 5 (CDK5) in primary afferents is a key neuronal kinase that modulates nociception through phosphorylation under pathological conditions. However, the impact of the CDK5 on nociceptor activity especially in human sensory neurons is not known. To determine the CDK5-mediated regulation of human dorsal root ganglia (hDRG) neuronal properties, we have performed the whole-cell patch clamp recordings in neurons dissociated from hDRG. CDK5 activation induced by overexpression of p35 depolarized the resting membrane potential (RMP) and reduced the rheobase currents as compared to the control neurons. CDK5 activation changed the shape of the action potential (AP) by increasing AP -rise time, -fall time, and -half width. The application of a prostaglandin E2 (PG) and bradykinin (BK) cocktail in control hDRG neurons induced the depolarization of RMP and the reduction of rheobase currents along with increased AP rise time. However, PG and BK applications failed to induce any significant changes in the p35-overexpressing group. We conclude that, in dissociated hDRGs neurons, CDK5 activation through the overexpression of p35 broadens the AP and that CDK5 may play important roles in the modulation of AP properties in human primary afferents under the condition in which CDK5 is upregulated, contributing to chronic pain.

Paclitaxel Inhibits KCNQ Channels in Primary Sensory Neurons to Initiate the Development of Painful Peripheral Neuropathy

Cancer patients undergoing paclitaxel infusion usually experience peripheral nerve degeneration and serious neuropathic pain termed paclitaxel-induced peripheral neuropathy (PIPN). However, alterations in the dose or treatment schedule for paclitaxel do not eliminate PIPN, and no therapies are available for PIPN, despite numerous studies to uncover the mechanisms underlying the development/maintenance of this condition. Therefore, we aimed to uncover a novel mechanism underlying the pathogenesis of PIPN. Clinical studies suggest that acute over excitation of primary sensory neurons is linked to the pathogenesis of PIPN. We found that paclitaxel-induced acute hyperexcitability of primary sensory neurons results from the paclitaxel-induced inhibition of KCNQ potassium channels (mainly KCNQ2), found abundantly in sensory neurons and axons. We found that repeated application of XE-991, a specific KCNQ channel blocker, induced PIPN-like alterations in rats, including mechanical hypersensitivity and degeneration of peripheral nerves, as detected by both morphological and behavioral assays. In contrast, genetic deletion of KCNQ2 from peripheral sensory neurons in mice significantly attenuated the development of paclitaxel-induced peripheral sensory fiber degeneration and chronic pain. These findings may lead to a better understanding of the causes of PIPN and provide an impetus for developing new classes of KCNQ activators for its therapeutic treatment.

SUMOylation of Kir7.1 participates in neuropathic pain through regulating its membrane expression in spinal cord neurons

Aims

Potassium (K⁺) channels have been demonstrated to play a prominent involvement in nociceptive processing. Kir7.1, the newest members of the Kir channel family, has not been extensively studied in the CNS, and its function remains largely unknown. The present study investigated the role of spinal Kir7.1 in the development of pathological pain.

Methods and Results

Neuropathic pain was induced by spared nerve injury (SNI). The mechanical sensitivity was assessed by von Frey test. Immunofluorescence staining assay revealed that Kir7.1 was predominantly expressed in spinal neurons but not astrocytes or microglia in normal rats. Western blot results showed that SNI markedly decreased the total and membrane expression of Kir7.1 in the spinal dorsal horn accompanied by mechanical hypersensitivity. Blocking Kir7.1 with the specific antagonist ML418 or knockdown kir7.1 by siRNA led to mechanical allodynia. Co‐IP results showed that the spinal kir7.1 channels were decorated by SUMO‐1 but not SUMO‐2/3, and Kir7.1 SUMOylation was upregulated following SNI. Moreover, inhibited SUMOylation by GA (E1 inhibitor) or 2‐D08 (UBC9 inhibitor) can increase the spinal surface Kir7.1 expression.

Conclusion

SUMOylation of the Kir7.1 in the spinal cord might contribute to the development of SNI‐induced mechanical allodynia by decreasing the Kir7.1 surface expression in rats.

Cdyl Deficiency Brakes Neuronal Excitability and Nociception through Promoting Kcnb1 Transcription in Peripheral Sensory Neurons

Epigenetic modifications are involved in the onset, development, and maintenance of pain; however, the precise epigenetic mechanism underlying pain regulation remains elusive. Here it is reported that the epigenetic factor chromodomain Y-like (CDYL) is crucial for pain processing. Selective knockout of CDYL in sensory neurons results in decreased neuronal excitability and nociception. Moreover, CDYL facilitates histone 3 lysine 27 trimethylation (H3K27me3) deposition at the Kcnb1 intron region thus silencing voltage-gated potassium channel (Kv ) subfamily member Kv 2.1 transcription. Loss function of CDYL enhances total Kv and Kv 2.1 current density in dorsal root ganglia and knockdown of Kv 2.1 reverses the pain-related phenotypes of Cdyl deficiency mice. Furthermore, focal administration of a novel potent CDYL antagonist blunts nociception and attenuates neuropathic pain. These findings reveal that CDYL is a critical regulator of pain sensation and shed light on the development of novel analgesics targeting epigenetic mechanisms.

Peripheral Voltage-Gated Cation Channels in Neuropathic Pain and Their Potential as Therapeutic Targets

The persistence of increased excitability and spontaneous activity in injured peripheral neurons is imperative for the development and persistence of many forms of neuropathic pain. This aberrant activity involves increased activity and/or expression of voltage-gated Na⁺ and Ca²⁺ channels and hyperpolarization activated cyclic nucleotide gated (HCN) channels as well as decreased function of K⁺ channels. Because they display limited central side effects, peripherally restricted Na⁺ and Ca²⁺ channel blockers and K⁺ channel activators offer potential therapeutic approaches to pain management. This review outlines the current status and future therapeutic promise of peripherally acting channel modulators. Selective blockers of Na_v1.3, Na_v1.7, Na_v1.8, Ca_v3.2, and HCN2 and activators of K_v7.2 abrogate signs of neuropathic pain in animal models. Unfortunately, their performance in the clinic has been disappointing; some substances fail to meet therapeutic end points whereas others produce dose-limiting side effects. Despite this, peripheral voltage-gated cation channels retain their promise as therapeutic targets. The way forward may include (i) further structural refinement of K⁺ channel activators such as retigabine and ASP0819 to improve selectivity and limit toxicity; use or modification of Na⁺ channel blockers such as vixotrigine, PF-05089771, A803467, PF-01247324, VX-150 or arachnid toxins such as Tap1a; the use of Ca²⁺ channel blockers such as TTA-P2, TTA-A2, Z 944, ACT709478, and CNCB-2; (ii) improving methods for assessing “pain” as opposed to nociception in rodent models; (iii) recognizing sex differences in pain etiology; (iv) tailoring of therapeutic approaches to meet the symptoms and etiology of pain in individual patients via quantitative sensory testing and other personalized medicine approaches; (v) targeting genetic and biochemical mechanisms controlling channel expression using anti-NGF antibodies such as tanezumab or re-purposed drugs such as vorinostat, a histone methyltransferase inhibitor used in the management of T-cell lymphoma, or cercosporamide a MNK 1/2 inhibitor used in treatment of rheumatoid arthritis; (vi) combination therapy using drugs that are selective for different channel types or regulatory processes; (vii) directing preclinical validation work toward the use of human or human-derived tissue samples; and (viii) application of molecular biological approaches such as clustered regularly interspaced short palindromic repeats (CRISPR) technology.

Multitarget nociceptor sensitization by a promiscuous peptide from the venom of the King Baboon spider

The King Baboon spider, Pelinobius muticus, is a burrowing African tarantula. Its impressive size and appealing coloration are tempered by reports describing severe localized pain, swelling, itchiness, and muscle cramping after accidental envenomation. Hyperalgesia is the most prominent symptom after bites from P. muticus, but the molecular basis by which the venom induces pain is unknown. Proteotranscriptomic analysis of P. muticus venom uncovered a cysteine-rich peptide, δ/κ-theraphotoxin-Pm1a (δ/κ-TRTX-Pm1a), that elicited nocifensive behavior when injected into mice. In small dorsal root ganglion neurons, synthetic δ/κ-TRTX-Pm1a (sPm1a) induced hyperexcitability by enhancing tetrodotoxin-resistant sodium currents, impairing repolarization and lowering the threshold of action potential firing, consistent with the severe pain associated with envenomation. The molecular mechanism of nociceptor sensitization by sPm1a involves multimodal actions over several ion channel targets, including Na_V1.8, K_V2.1, and tetrodotoxin-sensitive Na_V channels. The promiscuous targeting of peptides like δ/κ-TRTX-Pm1a may be an evolutionary adaptation in pain-inducing defensive venoms.

Protein Kinase C Controls the Excitability of Cortical Pyramidal Neurons by Regulating Kv2.2 Channel Activity

The family of voltage-gated potassium Kv2 channels consists of the Kv2.1 and Kv2.2 subtypes. Kv2.1 is constitutively highly phosphorylated in neurons and its function relies on its phosphorylation state. Whether the function of Kv2.2 is also dependent on its phosphorylation state remains unknown. Here, we investigated whether Kv2.2 channels can be phosphorylated by protein kinase C (PKC) and examined the effects of PKC-induced phosphorylation on their activity and function. Activation of PKC inhibited Kv2.2 currents and altered their steady-state activation in HEK293 cells. Point mutations and specific antibodies against phosphorylated S481 or S488 demonstrated the importance of these residues for the PKC-dependent modulation of Kv2.2. In layer II pyramidal neurons in cortical slices, activation of PKC similarly regulated native Kv2.2 channels and simultaneously reduced the frequency of action potentials. In conclusion, this study provides the first evidence to our knowledge that PKC-induced phosphorylation of the Kv2.2 channel controls the excitability of cortical pyramidal neurons.

Neuropathic pain modeling: Focus on synaptic and ion channel mechanisms

Animal models of pain consist of modeling a pain-like state and measuring the consequent behavior. The first animal models of neuropathic pain (NP) were developed in rodents with a total lesion of the sciatic nerve. Later, other models targeting central or peripheral branches of nerves were developed to identify novel mechanisms that contribute to persistent pain conditions in NP. Objective assessment of pain in these different animal models represents a significant challenge for pre-clinical research. Multiple behavioral approaches are used to investigate and to validate pain phenotypes including withdrawal reflex to evoked stimuli, vocalizations, spontaneous pain, but also emotional and affective behaviors. Furthermore, animal models were very useful in investigating the mechanisms of NP. This review will focus on a detailed description of rodent models of NP and provide an overview of the assessment of the sensory and emotional components of pain. A detailed inventory will be made to examine spinal mechanisms involved in NP-induced hyperexcitability and underlying the current pharmacological approaches used in clinics with the possibility to present new avenues for future treatment. The success of pre-clinical studies in this area of research depends on the choice of the relevant model and the appropriate test based on the objectives of the study.

Human Labor Pain Is Influenced by the Voltage-Gated Potassium Channel KV6.4 Subunit

By studying healthy women who do not request analgesia during their first delivery, we investigate genetic effects on labor pain. Such women have normal sensory and psychometric test results, except for significantly higher cuff pressure pain. We find an excess of heterozygotes carrying the rare allele of SNP rs140124801 in KCNG4. The rare variant K_V6.4-Met419 has a dominant-negative effect and cannot modulate the voltage dependence of K_V2.1 inactivation because it fails to traffic to the plasma membrane. In vivo, Kcng4 (K_V6.4) expression occurs in 40% of retrograde-labeled mouse uterine sensory neurons, all of which express K_V2.1, and over 90% express the nociceptor genes Trpv1 and Scn10a. In neurons overexpressing K_V6.4-Met419, the voltage dependence of inactivation for K_V2.1 is more depolarized compared with neurons overexpressing K_V6.4. Finally, K_V6.4-Met419-overexpressing neurons have a higher action potential threshold. We conclude that K_V6.4 can influence human labor pain by modulating the excitability of uterine nociceptors.

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KCNG4 variant highly prevalent in women requiring no analgesia in childbirth

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KCNG4 variant encodes K_V6.4Met-419; K_V6.4 is a silent subunit modifying K_V activity

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K_V6.4Met-419 is retained in the cytoplasm and acts in a dominant-negative manner

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K_V6.4Met-419 overexpression results in hypoexcitable sensory neurons

Blocking the Spinal Fbxo3/CARM1/K+ Channel Epigenetic Silencing Pathway as a Strategy for Neuropathic Pain Relief

Many epigenetic regulators are involved in pain-associated spinal plasticity. Coactivator-associated arginine methyltransferase 1 (CARM1), an epigenetic regulator of histone arginine methylation, is a highly interesting target in neuroplasticity. However, its potential contribution to spinal plasticity-associated neuropathic pain development remains poorly explored. Here, we report that nerve injury decreased the expression of spinal CARM1 and induced allodynia. Moreover, decreasing spinal CARM1 expression by Fbxo3-mediated CARM1 ubiquitination promoted H3R17me2 decrement at the K+ channel promoter, thereby causing K+ channel epigenetic silencing and the development of neuropathic pain. Remarkably, in naïve rats, decreasing spinal CARM1 using CARM1 siRNA or a CARM1 inhibitor resulted in similar epigenetic signaling and allodynia. Furthermore, intrathecal administration of BC-1215 (a novel Fbxo3 inhibitor) prevented CARM1 ubiquitination to block K+ channel gene silencing and ameliorate allodynia after nerve injury. Collectively, the results reveal that this newly identified spinal Fbxo3-CARM1-K+ channel gene functional axis promotes neuropathic pain. These findings provide essential insights that will aid in the development of more efficient and specific therapies against neuropathic pain.

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

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

Single-cell transcriptomic analysis of somatosensory neurons uncovers temporal development of neuropathic pain

Peripheral nerve injury could lead to chronic neuropathic pain. Understanding transcriptional changes induced by nerve injury could provide fundamental insights into the complex pathogenesis of neuropathic pain. Gene expression profiles of dorsal root ganglia (DRG) in neuropathic pain condition have been studied. However, little is known about transcriptomic changes in individual DRG neurons after peripheral nerve injury. Here we performed single-cell RNA sequencing on dissociated mouse DRG cells after spared nerve injury (SNI). In addition to DRG neuron types that are found under physiological conditions, we identified three SNI-induced neuronal clusters (SNIICs) characterized by the expression of Atf3/Gfra3/Gal (SNIIC1), Atf3/Mrgprd (SNIIC2) and Atf3/S100b/Gal (SNIIC3). These SNIICs originated from Cldn9⁺/Gal⁺, Mrgprd⁺ and Trappc3l⁺ DRG neurons, respectively. Interestingly, SNIIC2 switched to SNIIC1 by increasing Gal and reducing Mrgprd expression 2 days after nerve injury. Inferring the gene regulatory networks after nerve injury, we revealed that activated transcription factors Atf3 and Egr1 in SNIICs could enhance Gal expression while activated Cpeb1 in SNIIC2 might suppress Mrgprd expression within 2 days after SNI. Furthermore, we mined the transcriptomic changes in the development of neuropathic pain to identify potential analgesic targets. We revealed that cardiotrophin-like cytokine factor 1, which activates astrocytes in the dorsal horn of spinal cord, was upregulated in SNIIC1 neurons and contributed to SNI-induced mechanical allodynia. Therefore, our results provide a new landscape to understand the dynamic course of neuron type changes and their underlying molecular mechanisms during the development of neuropathic pain.

Antispasmodic Drug Drofenine as an Inhibitor of Kv2.1 Channel Ameliorates Peripheral Neuropathy in Diabetic Mice

Diabetic peripheral neuropathy (DPN) is a common diabetic complication and has yet no efficient medication. Here, we report that antispasmodic drug drofenine (Dfe) blocks Kv2.1 and ameliorates DPN-like pathology in diabetic mice. The underlying mechanisms are investigated against the DPN mice with in vivo Kv2.1 knockdown through adeno associated virus AAV9-Kv2.1-RNAi. Streptozotocin (STZ) induced type 1 or db/db type 2 diabetic mice with DPN exhibited a high level of Kv2.1 protein in dorsal root ganglion (DRG) tissue and a suppressed neurite outgrowth in DRG neuron. Dfe promoted neurite outgrowth by inhibiting Kv2.1 channel and/or Kv2.1 mRNA and protein expression level. Moreover, it suppressed inflammation by repressing IκBα/NF-κB signaling, inhibited apoptosis by regulating Kv2.1-mediated Bcl-2 family proteins and Caspase-3 and ameliorated mitochondrial dysfunction through Kv2.1/CaMKKβ/AMPK/PGC1α pathway. Our work supports that Kv2.1 inhibition is a promisingly therapeutic strategy for DPN and highlights the potential of Dfe in treating this disease.

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Antispasmodic drug drofenine (Dfe) ameliorates DPN-like pathology in diabetic mice

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Dfe inhibits Kv2.1 channel and/or Kv2.1 mRNA and protein expression level

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Dfe represses inflammation, apoptosis, and mitochondrial dysfunction in DPN mice

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Kv2.1 inhibition is a therapeutic tactic and Dfe shows therapeutic potential for DPN

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 (Kv1.1, 1.2), A-channels (Kv1.4, 3.3, 3.4, 4.1, 4.2, and 4.3), KCNQ or M-channels (Kv7.2, 7.3, 7.4, and 7.5), ATP-sensitive channels (KIR6.2), Ca2+-activated K+ channels (KCa1.1, 2.1, 2.2, 2.3, and 3.1), Na+-activated K+ channels (KCa4.1 and 4.2) and two pore domain leak channels (K2p; 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.

Intercellular communication and ion channels in neuropathic pain chronicization

BACKGROUND

Neuropathic pain is caused by primary lesion or dysfunction of either peripheral or central nervous system. Due to its complex pathogenesis, often related to a number of comorbidities, such as cancer, neurodegenerative and neurovascular diseases, neuropathic pain still represents an unmet clinical need, lacking long-term effective treatment and complex case-by-case approach.

AIM AND METHODS

We analyzed the recent literature on the role of selective voltage-sensitive sodium, calcium and potassium permeable channels and non-selective gap junctions (GJs) and hemichannels (HCs) in establishing and maintaining chronic neuropathic conditions. We finally focussed our review on the role of extracellular microenvironment modifications induced by resident glial cells and on the recent advances in cell-to-cell and cell-to-extracellular environment communication in chronic neuropathies.

CONCLUSION

In this review, we provide an update on the current knowledge of neuropathy chronicization processes with a focus on both neuronal and glial ion channels, as well as on channel-mediated intercellular communication.

In vitro affinity optimization of an anti-BDNF monoclonal antibody translates to improved potency in targeting chronic pain states in vivo

The role of brain-derived neurotrophic factor (BDNF) signaling in chronic pain has been well documented. Given the important central role of BDNF in long term plasticity and memory, we sought to engineer a high affinity, peripherally-restricted monoclonal antibody against BDNF to modulate pain. BDNF shares 100% sequence homology across human and rodents; thus, we selected chickens as an alternative immune host for initial antibody generation. Here, we describe the affinity optimization of complementarity-determining region-grafted, chicken-derived R3bH01, an anti-BDNF antibody specifically blocking the TrkB receptor interaction. Antibody optimization led to the identification of B30, which has a > 300-fold improvement in affinity based on BIAcore, an 800-fold improvement in potency in a cell-based pERK assay and demonstrates exquisite selectivity over related neurotrophins. Affinity improvements measured in vitro translated to in vivo pharmacological activity, with B30 demonstrating a 30-fold improvement in potency over parental R3bH01 in a peripheral nerve injury model. We further demonstrate that peripheral BDNF plays a role in maintaining the plasticity of sensory neurons following nerve damage, with B30 reversing neuron hyperexcitability associated with heat and mechanical stimuli in a dose-dependent fashion. In summary, our data demonstrate that effective sequestration of BDNF via a high affinity neutralizing antibody has potential utility in modulating the pathophysiological mechanisms that drive chronic pain states.

Global Transcriptomic Profile of Dorsal Root Ganglion and Physiological Correlates of Cisplatin-Induced Peripheral Neuropathy

BACKGROUND

Multiple cell signaling pathways are implicated in the development, progression, and persistence of cisplatin-induced peripheral neuropathy. Although advances have been made in terms of understanding specific neurotoxic mechanisms, there are few predictive factors identified that can help inform the clinician approach to symptom prevention or management.

OBJECTIVE

We investigate the differential sensitivity to cisplatin-induced peripheral neuropathy and examine the contribution of dorsal root ganglion (DRG) transcriptional profiles across two inbred strains of mice.

METHODS

Cisplatin (4 mg/kg intraperitoneal or vehicle control) was administered twice a week for 4 weeks to adult female C57BL/6J and A/J mice-the C57BL/6J strain of mice characterized by a robust mechanical allodynia and the A/J with a mild largely resistant allodynia phenotype. Peripheral nerve conduction velocities (NCVs), electrophysiological evaluation of wide dynamic range (WDR) neurons, morphological examination of DRG neurons, and microarray analysis of spinal cord tissues were compared across the 4 weeks.

RESULTS

The A/J strain presents with an early, mild nocifensive response to cisplatin with reduced neuronal activity in WDR neurons and small changes in cross-sectional nucleus size in DRG neurons at 4 weeks. The more nocifensive-sensitive C57BL/6J strain presents with no early changes in WDR neuron responsiveness; however, there were significant changes in DRG size. Both strains demonstrate a drop in NCV after 4 weeks of treatment, with the greatest reduction present in the A/J strain. Transcriptome data implicate neuroimmune modulation in the differential response to cisplatin in the DRGs of A/J and C57BL/6J mice.

DISCUSSION

Nocifensive responses in both strains implicate involvement of small myelinated and unmyelinated fibers in neurotoxic cisplatin response, whereas reductions in NCV reflect involvement of the largest myelinated fibers in the peripheral nerves. Microarray data analysis identifies neuropathy-relevant gene sets with differential activation of pathways, suggesting a role for antigen presentation in the differential neurotoxic response to cisplatin across strains. Further research is indicated to determine the relative contributions of each of these potential pathological mechanisms to both the neurotoxic response to cisplatin and to the potential for targeted therapy.

Transcriptional profiling of intramembranous and endochondral ossification after fracture in mice

Bone fracture repair represents an important clinical challenge with nearly 1 million non-union fractures occurring annually in the U.S. Gene expression differs between non-union and healthy repair, suggesting there is a pattern of gene expression that is indicative of optimal repair. Despite this, the gene expression profile of fracture repair remains incompletely understood. In this work, we used RNA-seq of two well-established murine fracture models to describe gene expression of intramembranous and endochondral bone formation. We used top differentially expressed genes, enriched gene ontology terms and pathways, callus cellular phenotyping, and histology to describe and contrast these bone formation processes across time. Intramembranous repair, as modeled by ulnar stress fracture, and endochondral repair, as modeled by femur full fracture, exhibited vastly different transcriptional profiles throughout repair. Stress fracture healing had enriched differentially expressed genes associated with bone repair and osteoblasts, highlighting the strong osteogenic repair process of this model. Interestingly, the PI3K-Akt signaling pathway was one of only a few pathways uniquely enriched in stress fracture repair. Full fracture repair involved a higher level of inflammatory and immune cell related genes than did stress fracture repair. Full fracture repair also differed from stress fracture in a robust downregulation of ion channel genes following injury, the role of which in fracture repair is unclear. This study offers a broad description of gene expression in intramembranous and endochondral ossification across several time points throughout repair and suggests several potentially intriguing genes, pathways, and cells whose role in fracture repair requires further study.

Transcriptomics Analysis of Porcine Caudal Dorsal Root Ganglia in Tail Amputated Pigs Shows Long-Term Effects on Many Pain-Associated Genes

Tail amputation by tail docking or as an extreme consequence of tail biting in commercial pig production potentially has serious implications for animal welfare. Tail amputation causes peripheral nerve injury that might be associated with lasting chronic pain. The aim of this study was to investigate the short- and long-term effects of tail amputation in pigs on caudal DRG gene expression at different stages of development, particularly in relation to genes associated with nociception and pain. Microarrays were used to analyse whole DRG transcriptomes from tail amputated and sham-treated pigs 1, 8, and 16 weeks following tail treatment at either 3 or 63 days of age (8 pigs/treatment/age/time after treatment; n = 96). Tail amputation induced marked changes in gene expression (up and down) compared to sham-treated intact controls for all treatment ages and time points after tail treatment. Sustained changes in gene expression in tail amputated pigs were still evident 4 months after tail injury. Gene correlation network analysis revealed two co-expression clusters associated with amputation: Cluster A (759 down-regulated) and Cluster B (273 up-regulated) genes. Gene ontology (GO) enrichment analysis identified 124 genes in Cluster A and 61 genes in Cluster B associated with both “inflammatory pain” and “neuropathic pain.” In Cluster A, gene family members of ion channels e.g., voltage-gated potassium channels (VGPC) and receptors e.g., GABA receptors, were significantly down-regulated compared to shams, both of which are linked to increased peripheral nerve excitability after axotomy. Up-regulated gene families in Cluster B were linked to transcriptional regulation, inflammation, tissue remodeling, and regulatory neuropeptide activity. These findings, demonstrate that tail amputation causes sustained transcriptomic expression changes in caudal DRG cells involved in inflammatory and neuropathic pain pathways.

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.

Electrical stimulation of the superior sagittal sinus suppresses A-type K+ currents and increases P/Q- and T-type Ca2+ currents in rat trigeminal ganglion neurons

BACKGROUND

Migraine is a debilitating neurological disorder involving abnormal trigeminovascular activation and sensitization. However, the underlying cellular and molecular mechanisms remain unclear.

METHODS

A rat model of conscious migraine was established through the electrical stimulation (ES) of the dural mater surrounding the superior sagittal sinus. Using patch clamp recording, immunofluorescent labelling, enzyme-linked immunosorbent assays and western blot analysis, we studied the effects of ES on sensory neuronal excitability and elucidated the underlying mechanisms mediated by voltage-gated ion channels.

RESULTS

The calcitonin gene-related peptide (CGRP) level in the jugular vein blood and the number of CGRP-positive neurons in the trigeminal ganglia (TGs) were significantly increased in rats with ES-induced migraine. The application of ES increased actional potential firing in both small-sized IB₄-negative (IB₄^-) and IB₄⁺ TG neurons. No significant changes in voltage-gated Na⁺ currents were observed in the ES-treated groups. ES robustly suppressed the transient outward K⁺ current (I_A) in both types of TG neurons, while the delayed rectifier K⁺ current remained unchanged. Immunoblot analysis revealed that the protein expression of Kv4.3 was significantly decreased in the ES-treated groups, while Kv1.4 remained unaffected. Interestingly, ES increased the P/Q-type and T-type Ca²⁺ currents in small-sized IB₄^- TG neurons, while there were no significant changes in the IB₄⁺ subpopulation of neurons.

CONCLUSION

These results suggest that ES decreases the I_A in small-sized TG neurons and increases P/Q- and T-type Ca²⁺ currents in the IB₄^- subpopulation of TG neurons, which might contribute to neuronal hyperexcitability in a rat model of ES-induced migraine.

Inhibition of Kv2.1 Potassium Channels by MiDCA1, A Pre-Synaptically Active PLA2-Type Toxin from Micrurus dumerilii carinicauda Coral Snake Venom

MiDCA1, a phospholipase A2 (PLA2) neurotoxin isolated from Micrurus dumerilii carinicauda coral snake venom, inhibited a major component of voltage-activated potassium (Kv) currents (41 ± 3% inhibition with 1 μM toxin) in mouse cultured dorsal root ganglion (DRG) neurons. In addition, the selective Kv2.1 channel blocker guangxitoxin (GxTx-1E) and MiDCA1 competitively inhibited the outward potassium current in DRG neurons. MiDCA1 (1 µM) reversibly inhibited the Kv2.1 current by 55 ± 8.9% in a Xenopus oocyte heterologous system. The toxin showed selectivity for Kv2.1 channels over all the other Kv channels tested in this study. We propose that Kv2.1 channel blockade by MiDCA1 underlies the toxin's action on acetylcholine release at mammalian neuromuscular junctions.

Antisense oligonucleotides selectively suppress target RNA in nociceptive neurons of the pain system and can ameliorate mechanical pain

There is an urgent need for better treatments for chronic pain, which affects more than 1 billion people worldwide. Antisense oligonucleotides (ASOs) have proven successful in treating children with spinal muscular atrophy, a severe infantile neurological disorder, and several ASOs are currently being tested in clinical trials for various neurological disorders. Here, we characterize the pharmacodynamic activity of ASOs in spinal cord and dorsal root ganglia (DRG), key tissues for pain signaling. We demonstrate that activity of ASOs lasts up to 2 months after a single intrathecal bolus dose. Interestingly, comparison of subcutaneous, intracerebroventricular, and intrathecal administration shows that DRGs are targetable by systemic and central delivery of ASOs, while target reduction in the spinal cord is achieved only after direct central delivery. Upon detailed characterization of ASO activity in individual cell populations in DRG, we observe robust target suppression in all neuronal populations, thereby establishing that ASOs are effective in the cell populations involved in pain propagation. Furthermore, we confirm that ASOs are selective and do not modulate basal pain sensation. We also demonstrate that ASOs targeting the sodium channel Nav1.7 induce sustained analgesia up to 4 weeks. Taken together, our findings support the idea that ASOs possess the required pharmacodynamic properties, along with a long duration of action beneficial for treating pain.

Mice lacking Kcns1 in peripheral neurons show increased basal and neuropathic pain sensitivity

Voltage-gated potassium (Kv) channels are increasingly recognised as key regulators of nociceptive excitability. Kcns1 is one of the first potassium channels to be associated with neuronal hyperexcitability and mechanical sensitivity in the rat, as well as pain intensity and risk of developing chronic pain in humans. Here, we show that in mice, Kcns1 is predominantly expressed in the cell body and axons of myelinated sensory neurons positive for neurofilament-200, including Aδ-fiber nociceptors and low-threshold Aβ mechanoreceptors. In the spinal cord, Kcns1 was detected in laminae III to V of the dorsal horn where most sensory A fibers terminate, as well as large motoneurons of the ventral horn. To investigate Kcns1 function specifically in the periphery, we generated transgenic mice in which the gene is deleted in all sensory neurons but retained in the central nervous system. Kcns1 ablation resulted in a modest increase in basal mechanical pain, with no change in thermal pain processing. After neuropathic injury, Kcns1 KO mice exhibited exaggerated mechanical pain responses and hypersensitivity to both noxious and innocuous cold, consistent with increased A-fiber activity. Interestingly, Kcns1 deletion also improved locomotor performance in the rotarod test, indicative of augmented proprioceptive signalling. Our results suggest that restoring Kcns1 function in the periphery may be of some use in ameliorating mechanical and cold pain in chronic states.

Neurotropin inhibits neuronal activity through potentiation of sustained Kv currents in primary cultured DRG neurons

Neurotropin (NTP) is a Japanese analgesic agent for treating neuropathic pain; however, its method of action remains unclear. This study examined the effects of NTP on the activity of small dorsal root ganglion (DRG) neurons using whole-cell patch clamp recordings. After 3 days of treatment, NTP decreased current injection-induced firing activity of cultured DRG neurons by raising the current threshold for action potential generation. Additionally, NTP increased the sustained component of voltage-gated potassium (K_v) channel currents without affecting other K⁺ currents. These results suggest that NTP inhibits the firing activity of DRG neurons through augmentation of sustained K_v current.

A novel intrinsic analgesic mechanism: the enhancement of the conduction failure along polymodal nociceptive C-fibers

Supplemental Digital Content is Available in the Text.

Conduction failure represents a prime target for modulating pain signals along C-fibers and might provide a new promising strategy to modulating pain with little side effects.

Although conduction failure has been observed in nociceptive C-fibers, little is known regarding its significance or therapeutic potential. In a previous study, we demonstrated that C-fiber conduction failure, which is regarded as an intrinsic self-inhibition mechanism, was reduced in circumstances of painful diabetic neuropathy. In this study, we extend this finding in the complete Freund's adjuvant model of inflammatory pain and validate that the degree of conduction failure decreased and led to a greater amount of pain signals conveyed to the central nervous system. In complete Freund's adjuvant–injected animals, conduction failure occurred in a C-fiber-selective, activity-dependent manner and was associated with an increase in the rising slope of the C-fiber after-hyperpolarization potential. To target conduction failure in a therapeutic modality, we used ZD7288, an antagonist of hyperpolarization-activated, cyclic nucleotide–modulated channels which are activated by hyperpolarization and play a pivotal role in both inflammatory and neuropathic pain. ZD7288 promoted conduction failure by suppressing I_h as a mechanism to reduce the rising slope of the after-hyperpolarization potential. Moreover, perineuronal injection of ZD7288 inhibited abnormal mechanical allodynia and thermal hyperalgesia without affecting motor function or heart rate. Our data highlight the analgesic potential of local ZD7288 application and identify conduction failure as a novel target for analgesic therapeutic development.

Breaking barriers to novel analgesic drug development

Acute and chronic pain complaints, although common, are generally poorly served by existing therapies. This unmet clinical need reflects a failure to develop novel classes of analgesics with superior efficacy, diminished adverse effects and a lower abuse liability than those currently available. Reasons for this include the heterogeneity of clinical pain conditions, the complexity and diversity of underlying pathophysiological mechanisms, and the unreliability of some preclinical pain models. However, recent advances in our understanding of the neurobiology of pain are beginning to offer opportunities for developing novel therapeutic strategies and revisiting existing targets, including modulating ion channels, enzymes and G-protein-coupled receptors.

The pharmacology of voltage-gated sodium channel activators

Toxins and venom components that target voltage-gated sodium (Na_V) channels have evolved numerous times due to the importance of this class of ion channels in the normal physiological function of peripheral and central neurons as well as cardiac and skeletal muscle. Na_V channel activators in particular have been isolated from the venom of spiders, wasps, snakes, scorpions, cone snails and sea anemone and are also produced by plants, bacteria and algae. These compounds have provided key insight into the molecular structure, function and pathophysiological roles of Na_V channels and are important tools due to their at times exquisite subtype-selectivity. We review the pharmacology of Na_V channel activators with particular emphasis on mammalian isoforms and discuss putative applications for these compounds. This article is part of the Special Issue entitled 'Venom-derived Peptides as Pharmacological Tools.'

Centipede venom peptide SsmTX-I with two intramolecular disulfide bonds shows analgesic activities in animal models

Pain is a major symptom of many diseases and results in enormous pressures on human body or society. Currently, clinically used analgesic drugs, including opioids and nonsteroidal anti-inflammatory drugs, have adverse reactions, and thus, the development of new types of analgesic drug candidates is urgently needed. Animal venom peptides have proven to have potential as new types of analgesic medicine. In this research, we describe the isolation and characterization of an analgesic peptide from the crude venom of centipede, Scolopendra subspinipes mutilans. The amino acid sequence of this peptide was identical with SsmTX-I that was previously reported as a specific Kv2.1 ion channel blocker. Our results revealed that SsmTX-I was produced by posttranslational processing of a 73-residue prepropeptide. The intramolecular disulfide bridge motifs of SsmTX-I was Cys1-Cys3 and Cys2-Cys4. Functional assay revealed that SsmTX-I showed potential analgesic activities in formalin-induced paw licking, thermal pain, and acetic acid-induced abdominal writhing mice models. Our research provides the first report of cDNA sequences, disulfide motif, successful synthesis, and analgesic potential of SsmTX-I for the development of pain-killing drugs. It indicates that centipede peptide toxins could be a treasure trove for the search of novel analgesic drug candidates. Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.

HCN2 ion channels: basic science opens up possibilities for therapeutic intervention in neuropathic pain

Nociception — the ability to detect painful stimuli — is an invaluable sense that warns against present or imminent damage. In patients with chronic pain, however, this warning signal persists in the absence of any genuine threat and affects all aspects of everyday life. Neuropathic pain, a form of chronic pain caused by damage to sensory nerves themselves, is dishearteningly refractory to drugs that may work in other types of pain and is a major unmet medical need begging for novel analgesics. Hyperpolarisation-activated cyclic nucleotide (HCN)-modulated ion channels are best known for their fundamental pacemaker role in the heart; here, we review data demonstrating that the HCN2 isoform acts in an analogous way as a ‘pacemaker for pain’, in that its activity in nociceptive neurons is critical for the maintenance of electrical activity and for the sensation of chronic pain in pathological pain states. Pharmacological block or genetic deletion of HCN2 in sensory neurons provides robust pain relief in a variety of animal models of inflammatory and neuropathic pain, without any effect on normal sensation of acute pain. We discuss the implications of these findings for our understanding of neuropathic pain pathogenesis, and we outline possible future opportunities for the development of efficacious and safe pharmacotherapies in a range of chronic pain syndromes.

Exposure to Gulf War Illness chemicals induces functional muscarinic receptor maladaptations in muscle nociceptors

Chronic pain is a component of the multisymptom disease known as Gulf War Illness (GWI). There is evidence that pain symptoms could have been a consequence of prolonged and/or excessive exposure to anticholinesterases and other GW chemicals. We previously reported that rats exposed, for 8 weeks, to a mixture of anticholinesterases (pyridostigmine bromide, chlorpyrifos) and a Nav (voltage activated Na(+) channel) deactivation-inhibiting pyrethroid, permethrin, exhibited a behavior pattern that was consistent with a delayed myalgia. This myalgia-like behavior was accompanied by persistent changes to Kv (voltage activated K(+)) channel physiology in muscle nociceptors (Kv7, KDR). In the present study, we examined how exposure to the above agents altered the reactivity of Kv channels to a muscarinic receptor (mAChR) agonist (oxotremorine-M). Comparisons between muscle nociceptors harvested from vehicle and GW chemical-exposed rats revealed that mAChR suppression of Kv7 activity was enhanced in exposed rats. Yet in these same muscle nociceptors, a Stromatoxin-insensitive component of the KDR (voltage activated delayed rectifier K(+) channel) exhibited decreased sensitivity to activation of mAChR. We have previously shown that a unique mAChR-induced depolarization and burst discharge (MDBD) was exaggerated in muscle nociceptors of rats exposed to GW chemicals. We now provide evidence that both muscle and vascular nociceptors of naïve rats exhibit MDBD. Examination of the molecular basis of the MDBD in naïve animals revealed that while the mAChR depolarization was independent of Kv7, the action potential burst was modulated by Kv7 status. mAChR depolarizations were shown to be dependent, in part, on TRPA1. We argue that dysfunction of the MDBD could be a functional convergence point for maladapted ion channels and receptors consequent to exposure to GW chemicals.

The contribution of Kv2.2-mediated currents decreases during the postnatal development of mouse dorsal root ganglion neurons

Delayed rectifier voltage-gated K(+)(Kv) channels play an important role in the regulation of the electrophysiological properties of neurons. In mouse dorsal root ganglion (DRG) neurons, a large fraction of the delayed rectifier current is carried by both homotetrameric Kv2 channels and heterotetrameric channels consisting of Kv2 and silent Kv (KvS) subunits (i.e., Kv5-Kv6 and Kv8-Kv9). However, little is known about the contribution of Kv2-mediated currents during the postnatal development ofDRGneurons. Here, we report that the Stromatoxin-1 (ScTx)-sensitive fraction of the total outward K(+)current (IK) from mouseDRGneurons gradually decreased (~13%,P < 0.05) during the first month of postnatal development. Because ScTx inhibits both Kv2.1- and Kv2.2-mediated currents, this gradual decrease may reflect a decrease in currents containing either subunit. However, the fraction of Kv2.1 antibody-sensitive current that only reflects the Kv2.1-mediated currents remained constant during that same period. These results suggested that the fractional contribution of Kv2.2-mediated currents relative toIKdecreased with postnatal age. SemiquantitativeRT-PCRanalysis indicated that this decrease can be attributed to developmental changes in Kv2.2 expression as themRNAlevels of the Kv2.2 subunit decreased gradually between 1 and 4 weeks of age. In addition, we observed age-dependent fluctuations in themRNAlevels of the Kv6.3, Kv8.1, Kv9.1, and Kv9.3 subunits. These results support an important role of both Kv2 and KvS subunits in the postnatal maturation ofDRGneurons.

Emerging potassium channel targets for the treatment of pain

PURPOSE OF REVIEW

Poor management of chronic pain remains a significant cause of misery with huge socioeconomic costs. Accumulating research in potassium (K+) channel physiology has uncovered several promising leads for the development of novel analgesics.

RECENT FINDINGS

We now recognize that certain K+ channel subunits are directly gated to pain-relevant stimuli (Kv1.1, K2P) whereas others are specifically modulated by inflammatory processes (Kv7, BKCA, K2P). Genetic analyses illustrate that K+ channel gene variation can predict pain sensitivity (KCNS1, GIRKs), risk for persistent pain (KCNS1, GIRKs, TRESK) and analgesic effectiveness (GIRK2). Importantly, preclinical studies confirm that K+ channel dysfunction can be a pain trigger in traumatic neuropathies (Kv9.1/Kv2.1, Kv7, Kv1.2) and migraine (TRESK). Finally, emerging data suggest that even pain in diabetes, bone cancer and autoimmune neuropathies may have K+ channel dysfunction constituents.

SUMMARY

There is a long-sought need for superior pharmacotherapy of pain syndromes. Although universal enhancement of K+ channel function in the periphery can decrease nociceptive excitability irrespective of the underlying cause, a more refined targeting of subunits with dominant nociceptive roles could yield highly efficacious treatments with fewer side-effects. The ongoing characterization of molecular interactions linking K+ channel dysfunction to pain is instrumental for identifying candidates with the most therapeutic potential.

THE INHIBITORY EFFECT OF PACLITAXEL ON (Kv2.1) K+ CURRENT IN H9c2 CELLS

Using the whole-cell voltage clamp technique, we investigated the effect of paclitaxel, an anticancer agent which promotes microtubule formation, on K(+) current in H9c2 cells originated from rat embryonic cardiac myocytes. Paclitaxel inhibited Kv2.1 voltage-dependent K(+) current (IKur) with ultra-rapidly activating and slowly inactivating kinetics in a concentration-dependent manner. The inhibitory effect of paclitaxel on IKur was time-dependent and more marked at 200 ms after the onset than at the beginning of the depolarizing pulse. The IC50 value of paclitaxel was 1.1 µM at 200 ms. The time-dependent inhibition suggests that paclitaxel might be an open channel blocker of Kv2.1. This inhibition of Kv2.1 may be involved in the adverse effects of paclitaxel on cardiac and neuronal cells.

The role of slow and persistent TTX-resistant sodium currents in acute tumor necrosis factor-α-mediated increase in nociceptors excitability

Tetrodotoxin-resistant (TTX-r) sodium channels are key players in determining the input-output properties of peripheral nociceptive neurons. Changes in gating kinetics or in expression levels of these channels by proinflammatory mediators are likely to cause the hyperexcitability of nociceptive neurons and pain hypersensitivity observed during inflammation. Proinflammatory mediator, tumor necrosis factor-α (TNF-α), is secreted during inflammation and is associated with the early onset, as well as long-lasting, inflammation-mediated increase in excitability of peripheral nociceptive neurons. Here we studied the underlying mechanisms of the rapid component of TNF-α-mediated nociceptive hyperexcitability and acute pain hypersensitivity. We showed that TNF-α leads to rapid onset, cyclooxygenase-independent pain hypersensitivity in adult rats. Furthermore, TNF-α rapidly and substantially increases nociceptive excitability in vitro, by decreasing action potential threshold, increasing neuronal gain and decreasing accommodation. We extended on previous studies entailing p38 MAPK-dependent increase in TTX-r sodium currents by showing that TNF-α via p38 MAPK leads to increased availability of TTX-r sodium channels by partial relief of voltage dependence of their slow inactivation, thereby contributing to increase in neuronal gain. Moreover, we showed that TNF-α also in a p38 MAPK-dependent manner increases persistent TTX-r current by shifting the voltage dependence of activation to a hyperpolarized direction, thus producing an increase in inward current at functionally critical subthreshold voltages. Our results suggest that rapid modulation of the gating of TTX-r sodium channels plays a major role in the mediated nociceptive hyperexcitability of TNF-α during acute inflammation and may lead to development of effective treatments for inflammatory pain, without modulating the inflammation-induced healing processes.

Opening paths to novel analgesics: the role of potassium channels in chronic pain

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Potassium (K⁺) channels are crucial determinants of neuronal excitability.

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Nerve injury or inflammation alters K⁺ channel activity in neurons of the pain pathway.

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These changes can render neurons hyperexcitable and cause chronic pain.

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Therapies targeting K⁺ channels may provide improved pain relief in these states.

Chronic pain is associated with abnormal excitability of the somatosensory system and remains poorly treated in the clinic. Potassium (K⁺) channels are crucial determinants of neuronal activity throughout the nervous system. Opening of these channels facilitates a hyperpolarizing K⁺ efflux across the plasma membrane that counteracts inward ion conductance and therefore limits neuronal excitability. Accumulating research has highlighted a prominent involvement of K⁺ channels in nociceptive processing, particularly in determining peripheral hyperexcitability. We review salient findings from expression, pharmacological, and genetic studies that have untangled a hitherto undervalued contribution of K⁺ channels in maladaptive pain signaling. These emerging data provide a framework to explain enigmatic pain syndromes and to design novel pharmacological treatments for these debilitating states.