Potassium channels as a potential therapeutic target for trigeminal neuropathic and inflammatory pain

Calcium-sensing receptor regulates Kv7 channels via Gi/o protein signalling and modulates excitability of human induced pluripotent stem cell-derived nociceptive-like neurons

BACKGROUND AND PURPOSE

Neuropathic pain, a debilitating condition with unmet medical needs, can be characterised as hyperexcitability of nociceptive neurons caused by dysfunction of ion channels. Voltage-gated potassium channels type 7 (Kv7), responsible for maintaining neuronal resting membrane potential and thus excitability, reside under tight control of G protein-coupled receptors (GPCRs). Calcium-sensing receptor (CaSR) is a GPCR that regulates the activity of numerous ion channels, but whether CaSR can control Kv7 channel function has been unexplored until now.

EXPERIMENTAL APPROACH

Experiments were conducted in recombinant cell models, mouse dorsal root ganglia (DRG) neurons and human induced pluripotent stem cell (hiPSC)-derived nociceptive-like neurons using patch-clamp electrophysiology and molecular biology techniques.

KEY RESULTS

Our results demonstrate that CaSR is expressed in recombinant cell models, hiPSC-derived nociceptive-like neurons and mouse DRG neurons, and its activation induced depolarisation via Kv7.2/7.3 channel inhibition. The CaSR-Kv7.2/7.3 channel crosslink was mediated via the Gi/o protein-adenylate cyclase-cyclicAMP-protein kinase A signalling cascade. Suppression of CaSR function demonstrated a potential to rescue hiPSC-derived nociceptive-like neurons from algogenic cocktail-induced hyperexcitability.

CONCLUSION AND IMPLICATIONS

This study demonstrates that the CaSR-Kv7.2/7.3 channel crosslink, via a Gi/o protein signalling pathway, effectively regulates neuronal excitability, providing a feasible pharmacological target for neuronal hyperexcitability management in neuropathic pain.

Ion Channel and Transporter Involvement in Chemotherapy-Induced Peripheral Neurotoxicity

The peripheral nervous system can encounter alterations due to exposure to some of the most commonly used anticancer drugs (platinum drugs, taxanes, vinca alkaloids, proteasome inhibitors, thalidomide), the so-called chemotherapy-induced peripheral neurotoxicity (CIPN). CIPN can be long-lasting or even permanent, and it is detrimental for the quality of life of cancer survivors, being associated with persistent disturbances such as sensory loss and neuropathic pain at limb extremities due to a mostly sensory axonal polyneuropathy/neuronopathy. In the state of the art, there is no efficacious preventive/curative treatment for this condition. Among the reasons for this unmet clinical and scientific need, there is an uncomplete knowledge of the pathogenetic mechanisms. Ion channels and transporters are pivotal elements in both the central and peripheral nervous system, and there is a growing body of literature suggesting that they might play a role in CIPN development. In this review, we first describe the biophysical properties of these targets and then report existing data for the involvement of ion channels and transporters in CIPN, thus paving the way for new approaches/druggable targets to cure and/or prevent CIPN.

Kv3.1 Interaction with UBR5 Is Required for Chronic Inflammatory Pain

Chronic inflammatory pain caused by neuronal hyperactivity is a common and refractory disease. Kv3.1, a member of the Kv3 family of voltage-dependent K+ channels, is a major determinant of the ability of neurons to generate high-frequency action potentials. However, little is known about its role in chronic inflammatory pain. Here, we show that although Kv3.1 mRNA expression was unchanged, Kv3.1 protein expression was decreased in the dorsal spinal horn of mice after plantar injection of complete Freund's adjuvant (CFA), a mouse model of inflammatory pain. Upregulating Kv3.1 expression alleviated CFA-induced mechanical allodynia and heat hyperalgesia, whereas downregulating Kv3.1 induced nociception-like behaviors. Additionally, we found that ubiquitin protein ligase E3 component n-recognin 5 (UBR5), a key factor in the initiation of chronic pain, binds directly to Kv3.1 to drive its ubiquitin degradation. Intrathecal injection of the peptide TP-CH-401, a Kv3.1 ubiquitination motif sequence, rescued the decrease in Kv3.1 expression and Kv currents through competitive binding to UBR5, and consequently attenuated mechanical and thermal hypersensitivity. These findings demonstrate a previously unrecognized pathway of Kv3.1 abrogation by UBR5 and indicate that Kv3.1 is critically involved in the regulation of nociceptive behavior. Kv3.1 is thus a promising new target for treating inflammatory pain.

Post-Traumatic Trigeminal Neuropathy: Neurobiology and Pathophysiology

Painful traumatic trigeminal neuropathy (PTTN) is a chronic neuropathic pain that may develop following injury to the trigeminal nerve. Etiologies include cranio-orofacial trauma that may result from dental, surgical, or anesthetic procedures or physical trauma, such as a motor vehicle accident. Following nerve injury, there are various mechanisms, including peripheral and central, as well as phenotypic changes and genetic predispositions that may contribute to the development of neuropathic pain. In this article, we review current literature pertaining to the cellular processes that occur following traumatic damage to the trigeminal nerve, also called cranial nerve V, that results in chronic neuropathic pain. We examine the neurobiology and pathophysiology based mostly on pre-clinical animal models of neuropathic/trigeminal pain.

Local Administration of the Phytochemical, Quercetin, Attenuates the Hyperexcitability of Rat Nociceptive Primary Sensory Neurons Following Inflammation Comparable to lidocaine

Although in vivo local injection of quercetin into the peripheral receptive field suppresses the excitability of rat nociceptive trigeminal ganglion (TG) neurons, under inflammatory conditions, the acute effects of quercetin in vivo, particularly on nociceptive TG neurons, remain to be determined. The aim of this study was to examine whether acute local administration of quercetin into inflamed tissue attenuates the excitability of nociceptive TG neurons in response to mechanical stimulation. The mechanical escape threshold was significantly lower in complete Freund's adjuvant (CFA)-inflamed rats compared to before CFA injection. Extracellular single-unit recordings were made from TG neurons of CFA-induced inflammation in anesthetized rats in response to orofacial mechanical stimulation. The mean firing frequency of TG neurons in response to both non-noxious and noxious mechanical stimuli was reversibly inhibited by quercetin in a dose-dependent manner (1-10 mM). The mean firing frequency of inflamed TG neurons in response to mechanical stimuli was reversibly inhibited by the local anesthetic, 1% lidocaine (37 mM). The mean magnitude of inhibition on TG neuronal discharge frequency with 1 mM quercetin was significantly greater than that of 1% lidocaine. These results suggest that local injection of quercetin into inflamed tissue suppresses the excitability of nociceptive primary sensory TG neurons. PERSPECTIVE: Local administration of the phytochemical, quercetin, into inflamed tissues is a more potent local analgesic than voltage-gated sodium channel blockers as it inhibits the generation of both generator potentials and action potentials in nociceptive primary nerve terminals. As such, it contributes to the area of complementary and alternative medicines.

MiR215p alleviates trigeminal neuralgia in rats through downregulation of voltagegated potassium channel Kv1.1

OBJECTIVES

Trigeminal neuralgia (TN) is a common neuropathic pain. Voltage-gated potassium channel (Kv) has been confirmed to be involved in the occurrence and development of TN, but the specific mechanism is still unclear. MicroRNA may be involved in neuropathic pain by regulating the expression of Kv channels and neuronal excitability in trigeminal ganglion (TG). This study aims to explore the relationship between Kv1.1 and miR-21-5p in TG with a TN model, evaluate whether miR-21-5p has a regulatory effect on Kv1.1, and to provide a new target and experimental basis for the treatment of TN.

METHODS

A total of 48 SD rats were randomly divided into 6 groups: 1) a sham group (n=12), the rats were only sutured at the surgical incision without nerve ligation; 2) a sham+agomir NC group (n=6), the sham rats were microinjected with agomir NC through stereotactic brain injection in the surgical side of TG; 3) a sham+miR-21-5p agomir group (n=6), the sham rats were microinjected with miR-21-5p agomir via stereotactic brain injection in the surgical side of TG; 4) a TN group (n=12), a TN rat model was constructed using the chronic constriction injury of the distal infraorbital nerve (dIoN-CCI) method with chromium intestinal thread; 5) a TN+antagonist NC group (n=6), TN rats were microinjected with antagonist NC through stereotactic brain injection method in the surgical side of TG; 6) a TN+miR-21-5p antagonist group (n=6), TN rats were microinjected with miR-21-5p antagonist through stereotactic brain injection in the surgical side of TG. The change of mechanical pain threshold in rats of each group after surgery was detected. The expressions of Kv1.1 and miR-21-5p in the operative TG of rats were detected by Western blotting and real-time reverse transcription polymerase chain reaction. Dual luciferase reporter genes were used to determine whether there was a target relationship between Kv1.1 and miR-21-5p and whether miR-21-5p directly affected the 3'-UTR terminal of KCNA1. The effect of brain stereotaxic injection was evaluated by immunofluorescence assay, and then the analogue of miR-21-5p (agomir) and agomir NC were injected into the TG of rats in the sham group by brain stereotaxic apparatus to overexpress miR-21-5p. The miR-21-5p inhibitor (antagomir) and antagomir NC were injected into TG of rats in the TN group to inhibit the expression of miR-21-5p. The behavioral changes of rats before and after administration were observed, and the expression changes of miR-21-5p and Kv1.1 in TG of rats after intervention were detected.

RESULTS

Compared with the baseline pain threshold, the facial mechanical pain threshold of rats in the TN group was significantly decreased from the 5th to 15th day after the surgery (P<0.05), and the facial mechanical pain threshold of rats in the sham group was stable at the normal level, which proved that the dIoN-CCI model was successfully constructed. Compared with the sham group, the expression of Kv1.1 mRNA and protein in TG of the TN group was down-regulated (both P<0.05), and the expression of miR-21-5p was up-regulated (P<0.05). The results of dual luciferase report showed that the luciferase activity of rno-miR-21-5p mimics and KCNA1 WT transfected with 6 nmol/L or 20 nmol/L were significantly decreased compared with those transfected with mimic NC and wild-type KCNA1 WT, respectively (P<0.001). Compared with low dose rno-miR-21-5p mimics (6 nmol/L) co-transfection group, the relative activity of luciferase in the high dose rno-miR-21-5p mimics (20 nmol/L) cotransfection group was significantly decreased (P<0.001). The results of immunofluorescence showed that drugs were accurately injected into TG through stereotaxic brain. After the expression of miR-21-5p in the TN group, the mechanical pain threshold and the expression of Kv1.1 mRNA and protein in TG were increased. After overexpression of miR-21-5p in the sham group, the mechanical pain threshold and the expression of Kv1.1 mRNA and protein in TG were decreased.

CONCLUSIONS

Both Kv1.1 and miR-21-5p are involved in TN and miR-21-5p can regulate Kv1.1 expression by binding to the 3'-UTR of KCNA1.

Ion Channel Genes in Painful Neuropathies

Neuropathic pain (NP) is a typical symptom of peripheral nerve disorders, including painful neuropathy. The biological mechanisms that control ion channels are important for many cell activities and are also therapeutic targets. Disruption of the cellular mechanisms that govern ion channel activity can contribute to pain pathophysiology. The voltage-gated sodium channel (VGSC) is the most researched ion channel in terms of NP; however, VGSC impairment is detected in only <20% of painful neuropathy patients. Here, we discuss the potential role of the other peripheral ion channels involved in sensory signaling (transient receptor potential cation channels), neuronal excitation regulation (potassium channels), involuntary action potential generation (hyperpolarization-activated cyclic nucleotide-gated channels), thermal pain (anoctamins), pH modulation (acid sensing ion channels), and neurotransmitter release (calcium channels) related to pain and their prospective role as therapeutic targets for painful neuropathy.

Suppression of the Excitability of Rat Nociceptive Primary Sensory Neurons Following Local Administration of the Phytochemical, Quercetin

Although the modulatory effect of quercetin on voltage-gated Na, K, and Ca channels has been studied in vitro, the in vivo effect of quercetin on the excitability of nociceptive primary neurons remains to be determined. The aim of the present study was to examine whether acute local quercetin administration to rats attenuates the excitability of nociceptive trigeminal ganglion (TG) neurons in response to mechanical stimulation in vivo. Extracellular single unit recordings were made from TG neurons of anesthetized rats in response to orofacial non-noxious and noxious mechanical stimulation. The mean firing frequency of TG neurons in response to both non-noxious and noxious mechanical stimuli was dose-dependently inhibited by quercetin, and maximum inhibition of the discharge frequency of both non-noxious and noxious mechanical stimuli was seen within 10 min. The inhibitory effect of quercetin lasted for 15 minutes and was reversible. The mean magnitude of inhibition on TG neuronal discharge frequency with 10 mM quercetin was almost equal to that of the local anesthetic, 2% lidocaine. These results suggest that local injection of quercetin into the peripheral receptive field suppresses the excitability of nociceptive primary sensory neurons in the TG, possibly via inhibition of voltage-gated Na channels and opening voltage-gated K channels. PERSPECTIVE: Local administration of the phytochemical, quercetin, as a local anesthetic may provide relief from trigeminal nociceptive pain with smallest side effects, thus contributing to the area of complementary and alternative medicines.

Plant-derived natural products targeting ion channels for pain

Chronic pain affects approximately one-fifth of people worldwide and reduces quality of life and in some cases, working ability. Ion channels expressed along nociceptive pathways affect neuronal excitability and as a result modulate pain experience. Several ion channels have been identified and investigated as potential targets for new medicines for the treatment of a variety of human diseases, including chronic pain. Voltage-gated channels Na⁺ and Ca²⁺ channels, K⁺ channels, transient receptor potential channels (TRP), purinergic (P2X) channels and acid-sensing ion channels (ASICs) are some examples of ion channels exhibiting altered function or expression in different chronic pain states. Pharmacological approaches are being developed to mitigate dysregulation of these channels as potential treatment options. Since natural compounds of plant origin exert promising biological and pharmacological properties and are believed to possess less adverse effects compared to synthetic drugs, they have been widely studied as treatments for chronic pain for their ability to alter the functional activity of ion channels. A literature review was conducted using Medline, Google Scholar and PubMed, resulted in listing 79 natural compounds/extracts that are reported to interact with ion channels as part of their analgesic mechanism of action. Most in vitro studies utilized electrophysiological techniques to study the effect of natural compounds on ion channels using primary cultures of dorsal root ganglia (DRG) neurons. In vivo studies concentrated on different pain models and were conducted mainly in mice and rats. Proceeding into clinical trials will require further study to develop new, potent and specific ion channel modulators of plant origin.

Blockade of Cholecystokinin Type 2 Receptors Prevents the Onset of Vincristine-Induced Neuropathy in Mice

Vincristine (VCR) is responsible for the onset of the VCR-induced peripheral neuropathy (VIPN), associated with neuropathic pain. Several reports have strongly linked the cholecystokinin type 2 receptor (CCK2R) to nociceptive modulation. Thus, our aim was to evaluate the effect of CCK2R blockade on the onset of VIPN, as well as its interaction on VCR anticancer efficacy. VCR was administrated in mice for 8 days (100 µg/kg/d, i.p.). Transcriptomic analysis of the dorsal root ganglia (DRG) was performed at day 7 in VCR and control mice. Proglumide (30 mg/kg/d), a CCK1R and CCK2R antagonist, and Ly225910 (1 mg/kg/d), a selective CCK2R antagonist, were administrated one day before and during VCR treatment. Tactile sensitivity was assessed during treatments. Immunofluorescence and morphological analyses were performed on the skin, DRG and sciatic nerve at day 7. The cytotoxicity of VCR in combination with proglumide/Ly225910 was evaluated in human cancer cell lines. Cck2r was highly upregulated in the DRG of VCR mice. Proglumide accelerated the recovery of normal sensitivity, while Ly225910 totally prevented the onset of allodynia and nerve injuries induced by VCR. Proglumide or Ly225910 in combination with VCR did not affect the cytotoxicity of VCR. Targeting CCK2R could therefore be an effective strategy to prevent the onset of VIPN.

Activation of TREK1 Channel in the Anterior Cingulate Cortex Improves Neuropathic Pain in a Rat Model

Objective. To explore the biological function and mechanism of TREK1 in neuropathic pain. Thirty-two healthy rats and rats with sciatic nerve chronic press-fitting model (chronic constriction injury of the sciatic nerve, CCI) were selected. Western blot, immunofluorescence staining, and patch clamp technique were performed to explore the biological functions of TREK1. The expression of TREK1 was decreased in the CCI model. The TREK1 channel current in the CCI model was decreased. After local administration of TREK1 channel activator in the anterior cingulate cortex area, the pain behavior of CCI rats and the expression of TREK1 protein were reversed. The expression of TREK1 was downregulated in the ACC area of CCI rats and the current of TREK1 was decreased, which played an important role in the regulation of neuropathic pain.

The effects of certain TRP channels and voltage-gated KCNQ/Kv7 channel opener retigabine on calcitonin gene-related peptide release in the trigeminovascular system

BACKGROUND

Calcitonin gene-related peptide release in trigeminovascular system is a pivotal component of neurogenic inflammation underlying migraine pathophysiology. Transient receptor potential channels and voltage-gated KCNQ/Kv7 potassium channels expressed throughout trigeminovascular system are important targets for modulation of calcitonin gene-related peptide release. We investigated the effects of certain transient receptor potential (TRP) channels the vanilloid 1 and 4 (TRPV1 and TRPV4), the ankyrin 1 (TRPA1), and metastatin type 8 (TRPM8), and voltage-gated potassium channel (Kv7) opener retigabine on calcitonin gene-related peptide release from peripheral (dura mater and trigeminal ganglion) and central (trigeminal nucleus caudalis) trigeminal components of rats.

METHODS

The experiments were carried out using well-established in-vitro preparations (hemiskull, trigeminal ganglion and trigeminal nucleus caudalis) from male Wistar rats. Agonists and antagonists of TRPV1, TRPV4, TRPA1 and TRPM8 channels, and also retigabine were tested on the in-vitro release of calcitonin gene-related peptide. Calcitonin gene-related peptide concentrations were measured using enzyme-linked immunosorbent assay.

RESULTS

Agonists of these transient receptor potential channels induced calcitonin gene-related peptide release from hemiskull, trigeminal ganglion and trigeminal nucleus caudalis, respectively. The transient receptor potential channels-induced calcitonin gene-related peptide releases were blocked by their specific antagonists and reduced by retigabine. Retigabine also decreased basal calcitonin gene-related peptide releases in all preparations.

CONCLUSION

Our findings suggest that favorable antagonists of these transient receptor potential channels, or Kv7 channel opener retigabine may be effective in migraine therapy by inhibiting neurogenic inflammation that requires calcitonin gene-related peptide release.

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.

(-)-Naringenin 4',7-dimethyl Ether Isolated from Nardostachys jatamansi Relieves Pain through Inhibition of Multiple Channels

(−)-Naringenin 4′,7-dimethyl ether ((−)-NRG-DM) was isolated for the first time by our lab from Nardostachys jatamansi DC, a traditional medicinal plant frequently used to attenuate pain in Asia. As a natural derivative of analgesic, the current study was designed to test the potential analgesic activity of (−)-NRG-DM and its implicated mechanism. The analgesic activity of (−)-NRG-DM was assessed in a formalin-induced mouse inflammatory pain model and mustard oil-induced mouse colorectal pain model, in which the mice were intraperitoneally administrated with vehicle or (−)-NRG-DM (30 or 50 mg/kg) (n = 10 for each group). Our data showed that (−)-NRG-DM can dose dependently (30~50 mg/kg) relieve the pain behaviors. Notably, (−)-NRG-DM did not affect motor coordination in mice evaluated by the rotarod test, in which the animals were intraperitoneally injected with vehicle or (−)-NRG-DM (100, 200, or 400 mg/kg) (n = 10 for each group). In acutely isolated mouse dorsal root ganglion neurons, (−)-NRG-DM (1~30 μM) potently dampened the stimulated firing, reduced the action potential threshold and amplitude. In addition, the neuronal delayed rectifier potassium currents (I_K) and voltage-gated sodium currents (I_Na) were significantly suppressed. Consistently, (−)-NRG-DM dramatically inhibited heterologously expressed Kv2.1 and Nav1.8 channels which represent the major components of the endogenous I_K and I_Na. A pharmacokinetic study revealed the plasma concentration of (−)-NRG-DM is around 7 µM, which was higher than the effective concentrations for the I_K and I_Na. Taken together, our study showed that (−)-NRG-DM is a potential analgesic candidate with inhibition of multiple neuronal channels (mediating I_K and I_Na).

A Novel Progress: Glial Cells and Inflammatory Pain

Inflammatory pain is the perception of noxious stimuli that occurs during inflammation or an immune response. Glial cells are widespread in the central and peripheral nervous systems, supporting and guiding the migration of neurons, participating in the immune response, forming the myelin sheath and blood-brain barrier, and maintaining the concentration of potassium ions outside nerve cells. Recent studies have shown that glial cells have a significant connection with the production and development of inflammatory pain. This article reviews the relationship, mechanisms, therapeutic targets between five types of glial cells and inflammatory pain, and the medicine composition that can effectively inhibit inflammatory pain. It expands the study on the mechanism of glial cells regulating pain and provides new ideas for the therapy of inflammatory pain.

Role of voltage-gated sodium channels in axonal signal propagation of trigeminal ganglion neurons after infraorbital nerve entrapment

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Infraorbital nerve entrapment (IoNE) induces mechanical allodynia and enhances signal propagation in primary afferent A- and C-fibers.

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IoNE increases sensitivity of A- and C-fibers to conduction block by tetrodotoxin (TTX) and selective voltage-gated sodium channel 1.8 (NaV1.8) inhibitor, A-803467.

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IoNE increases signal propagation in vibrissal pad Ad -, but not Aβ-fibers, and their sensitivity to conduction block by the selective NaV1.8 inhibitor.

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IoNE increases membrane excitability of dissociated small and medium sized trigeminal neurons.

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IoNE increases nerve, but not ganglion, levels of NaV1.3, NaV1.7, and NaV1.8 mRNAs, and NaV1.8 protein.

Chronic pain arising from peripheral nerve injuries represents a significant clinical challenge because even the most efficacious anticonvulsant drug treatments are limited by their side effects profile. We investigated pain behavior, changes in axonal signal conduction and excitability of trigeminal neurons, and expression of voltage-gated sodium channels (NaVs) in the infraorbital nerve and trigeminal ganglion (TG) after infraorbital nerve entrapment (IoNE). Compared to Sham, IoNE rats had increased A- and C-fiber compound action potentials (CAPs) and Aδ component of A-CAP area from fibers innervating the vibrissal pad. After IoNE, A- and C-fiber CAPs were more sensitive to blockade by tetrodotoxin (TTX), and those fibers that were TTX-resistant were more sensitive to blockade by the NaV1.8 selective blocker, A-803467. Although NaV1.7 blocker, ICA-121431 alone, did not affect Aδ-fiber signal propagation, cumulative application with A-803467 and 4,9-anhydro-TTX significantly reduced the Aδ-fiber CAP in IoNE rats. In patch clamp recordings from small- and medium-sized TG neurons, IoNE resulted in reduced action potential (AP) depolarizing current threshold, hyperpolarized AP voltage threshold, increased AP duration, and a more depolarized membrane potential. While the transcripts of most NaVs were reduced in the ipsilateral TG after IoNE, NaV1.3, NaV1.7, and NaV1.8 mRNAs, and NaV1.8 protein, were significantly increased in the nerve. Altogether, our data suggest that axonal redistribution of NaV1.8, and to a lesser extent NaV1.3, and NaV1.7 contributes to enhanced nociceptive signal propagation in peripheral nerve after IoNE.

Vincristine-Induced Peripheral Neuropathy in Childhood Acute Lymphoblastic Leukemia: Genetic Variation as a Potential Risk Factor

Vincristine (VCR) is the first-line chemotherapeutic medication often co-administered with other drugs to treat childhood acute lymphoblastic leukemia. Dose-dependent neurotoxicity is the main factor restricting VCR’s clinical application. VCR-induced peripheral neuropathy (VIPN) sometimes results in dose reduction or omission, leading to clinical complications or affecting the patient’s quality of life. With regard to the genetic basis of drug responses, preemptive pharmacogenomic testing and simultaneous blood level monitoring could be helpful for the transformation of various findings into individualized therapies. In this review, we discussed the potential associations between genetic variants in genes contributing to the pharmacokinetics/pharmacodynamics of VCR and VIPN incidence and severity in patients with acute lymphoblastic leukemia. Of note, genetic variants in the CEP72 gene have great potential to be translated into clinical practice. Such a genetic biomarker may help clinicians diagnose VIPN earlier. Besides, genetic variants in other genes, such as CYP3A5, ABCB1, ABCC1, ABCC2, TTPA, ACTG1, CAPG, SYNE2, SLC5A7, COCH, and MRPL47, have been reported to be associated with the VIPN, but more evidence is needed to validate the findings in the future. In fact, a variety of complex factors jointly determine the VIPN. In implementing precision medicine, the combination of genetic, environmental, and personal variables, along with therapeutic drug monitoring, will allow for a better understanding of the mechanisms of VIPN, improving the effectiveness of VCR treatment, reducing adverse reactions, and improving patients’ quality of life.

C-X-C motif chemokine ligand 1 and its receptor C-X-C motif chemokine receptor 2 in trigeminal ganglion contribute to nerve injury-induced orofacial mechanical allodynia

BACKGROUND

Orofacial ectopic pain induced by trigeminal nerve injury is a serious complication of dental treatment. C-X-C motif chemokine ligand 1 (CXCL1) and its primary receptor C-X-C motif chemokine receptor 2 (CXCR2) contribute to the development and maintenance of neuropathic pain in the spinal nervous system, but their roles in trigeminal neuropathic sensation are still poorly understood.

OBJECTIVES

This study aimed to investigate the exact role of CXCL1 and CXCR2 in the regulation of orofacial ectopic mechanical allodynia and their potential downstream mechanisms in the trigeminal ganglion (TG).

METHODS

The head withdrawal threshold (HWT) of C57BL/6 mice was evaluated after inferior alveolar nerve (IAN) transection (IANX). Then, the distribution and expression of CXCL1 and CXCR2, and their potential downstream mechanisms in the TG were further measured using immunohistochemistry, real-time reverse transcription-quantitative polymerase chain reaction and Western blotting. Moreover, the effect of SB225002 (an inhibitor of CXCR2) on mechanical allodynia was examined. The data were analysed using the Student's t test and a analysis of variance (ANOVA).

RESULTS

IANX triggered persistent (>21 days) mechanical allodynia and upregulation of CXCL1 and CXCR2 in the TG. In addition, exogenous CXCL1 also lowered the HWT, which was alleviated by CXCR2 and protein kinase C (PKC) antagonists (p < .05). In addition, IANX increased the phosphorylated PKC (p-PKC) levels and decreased the expression of voltage-gated potassium channels (Kv), and these effects were reversed by inhibition of CXCR2 (p < .05).

CONCLUSION

Our results demonstrated that CXCR2 participated in orofacial ectopic mechanical allodynia via downregulation of Kv1.4 and Kv1.1 through the PKC signalling pathway. This mechanism may be a potential target in developing a treatment strategy for ectopic orofacial pain.

Effect of ketamine on voltage-gated potassium channels in rat primary sensory cortex pyramidal neurons

Ketamine is an intravenous anesthetic commonly used in clinical, which has sedative and analgesic effects. Potassium channels exert many physiological functions in excitable cells. Therefore, potassium channels may be one of the targets of ketamine. In this study, we used patch clamp to study the effects of ketamine on voltage-gated potassium channels in primary sensory cortex (S1) neurons. We recorded the outward potassium currents (IA) and delayed rectifier potassium currents (IK) separately. We found that ketamine both concentration-dependently inhibited IA currents and IK currents in S1 neurons. Ketamine (100 and 300 μM) induced a concentration-dependent hyperpolarizing shift in V1/2, without affecting the slope factor (κ) or inactivation of IA. Ketamine induced a concentration-dependent hyperpolarizing shift in V1/2 of IK, without affecting its κ. Ketamine (100 and 300 μM) did not alter the steady-state activation or its κ. Hence, ketamine inhibits IA and IK in a concentration-dependent manner in S1 pyramidal neurons. The inactivation of IA does not appear to be involved in the inhibitory effect of ketamine on IA. Ketamine inhibits IK mainly by speeding up the inactivation of IK in S1 pyramidal neurons.

Overexpressed Na V 1.7 Channels Confer Hyperexcitability to in vitro Trigeminal Sensory Neurons of Ca V 2.1 Mutant Hemiplegic Migraine Mice

Trigeminal sensory neurons of transgenic knock-in (KI) mice expressing the R192Q missense mutation in the α1A subunit of neuronal voltage-gated Ca_V2.1 Ca²⁺ channels, which leads to familial hemiplegic migraine type 1 (FHM1) in patients, exhibit a hyperexcitability phenotype. Here, we show that the expression of Na_V1.7 channels, linked to pain states, is upregulated in KI primary cultures of trigeminal ganglia (TG), as shown by increased expression of its α1 subunit. In the majority of TG neurons, Na_V1.7 channels are co-expressed with ATP-gated P2X3 receptors (P2X3R), which are important nociceptive sensors. Reversing the trigeminal phenotype with selective Ca_V2.1 channel inhibitor ω-agatoxin IVA inhibited Na_V1.7 overexpression. Functionally, KI neurons revealed a TTX-sensitive inward current of larger amplitude that was partially inhibited by selective Na_V1.7 blocker Tp1a. Under current-clamp condition, Tp1a raised the spike threshold of both wild-type (WT) and KI neurons with decreased firing rate in KI cells. Na_V1.7 activator OD1 accelerated firing in WT and KI neurons, a phenomenon blocked by Tp1a. Enhanced expression and function of Na_V1.7 channels in KI TG neurons resulted in higher excitability and facilitated nociceptive signaling. Co-expression of Na_V1.7 channels and P2X3Rs in TGs may explain how hypersensitivity to local stimuli can be relevant to migraine.

Microvascular decompression for typical trigeminal neuralgia: Personal experience with intraoperative neuromonitoring with level-specific-CE-Chirp® brainstem auditory evoked potentials in preventing possible hearing loss

Background:

Permanent hearing loss after posterior fossa microvascular decompression (MVD) for typical trigeminal neuralgia (TTN) is one of the possible complications of this procedure. Intraoperative brainstem auditory evoked potentials (BAEPs) are used for monitoring the function of cochlear nerve during cerebellopontine angle (CPA) microsurgery. Level-specific (LS)-CE-Chirp® BAEPs are the most recent evolution of classical click BAEP, performed both in clinical studies and during intraoperative neuromonitoring (IONM) of acoustic pathways during several neurosurgical procedures.

Methods:

Since February 2016, we routinely use LS-CE-Chirp® BAEPs for monitoring the function of cochlear nerve during CPA surgery, including MVD for trigeminal neuralgia. From September 2011 to December 2018, 71 MVDs for TTN were performed in our department, 47 without IONM of acoustic pathways (Group A), and, from February 2016, 24 with LS-CE-Chirp BAEP (Group B).

Results:

Two patients of Group A developed a permanent ipsilateral anacusia after MVD. In Group B, we did not observe any permanent acoustic deficit after surgery. In one case of Group B, during arachnoid dissection, intraoperative LS-CE-Chirp BAEP showed a temporary lag of V wave, resolved in 5 min after application of intracisternal diluted papaverine (0.3% solution without excipients).

Conclusion:

MVD is widely considered a definitive surgical procedure in the management of TTN. Even though posterior fossa MVD is a safe procedure, serious complications might occur. In particular, the use of IONM of acoustic pathways during MVD for TTN might contribute to prevention of postoperative hearing loss.

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

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

Molecular mechanisms of painful traumatic trigeminal neuropathy-Evidence from animal research and clinical correlates

Painful traumatic trigeminal neuropathy (PTTN) may occur following major craniofacial or oral trauma, or may be subsequent to relatively minor dental interventions. Following injury, pain may originate from a peripheral nerve, a ganglion, or from the central nervous system. In this review, we focus on molecular mechanisms of pain resulting from injury to the peripheral branch of the trigeminal nerve. This syndrome has been termed painful traumatic trigeminal neuropathy (PTTN) by the International Headache Society and replaces previous terms including atypical odontalgia, deafferentation pain, traumatic neuropathy and phantom toothache. We emphasize the scientific evidence supporting the events purported to lead to PTTN by reviewing the pathophysiology of PTTN based on relevant animal models. Additionally, we briefly overview clinical correlates and pathophysiological manifestations of PTTN.

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.

Characteristics of voltage-gated potassium currents in monosodium urate induced gouty arthritis in mice

OBJECTIVE

To evaluate the role of K⁺ channels in pain following gouty arthritis.

METHODS

The model of acute gouty arthritis was induced by monosodium urate (MSU) in mice. The swelling degree was determined by measuring the circumference of the ankle joint. Mechanical hyperalgesia was detected by von Frey filaments. Two types of K⁺ currents, A-type currents (I_A) and delayed rectifier currents (I_K), were recorded in dorsal root ganglion (DRG) neurons using patch-clamp techniques.

RESULTS

The swelling degree reached its maximum at 10 h and the minimum pain threshold was maintained between 8 and 48 h after MSU treatment in mice. The amplitudes of I_A and I_K in DRG neurons were moderately increased on day 1 after MSU treatment, and then, they were gradually decreased with times and reached their minimums on day 4 (for I_A) or 5 (for I_K). Compared with control group, the activation curve of I_A was significantly shifted to more positive potential and the recovery time of I_A from inactivation was markedly prolonged, but inactivation and frequency dependence of I_A appeared unaffected in MSU-treated group. Additionally, no change was observed in the activation curve of I_K after MSU treatment. The excitability was significantly higher in the MSU group than in the control group.

CONCLUSIONS

MSU-induced gout pain may be related to the hyperexcitability of DRG neurons elicited by decreasing K⁺ currents.

Activation of the RAS/B-RAF-MEK-ERK pathway in satellite glial cells contributes to substance p-mediated orofacial pain

The cross talk between trigeminal ganglion (TG) neurons and satellite glial cells (SGCs) is crucial for the regulation of inflammatory orofacial pain. Substance P (SP) plays an important role by activating neurokinin (NK)-I receptors in this cross talk. The activation of extracellular signal-regulated kinase (ERK) 1/2, protein kinase A (PKA) and protein kinase C (PKC) in neurons and SGCs of peripheral ganglions by peripheral inflammation is associated with inflammatory hypersensitivity. This study tested the hypothesis that SP evoked SP-NK-I receptor positive feedback via the Renin-Angiotensin System/B-Protein Kinase A-Rapidly Accelerates Fibrosarcoma-MEK-Extracellular Signal-Regulated Kinase (RAS/PKA-RAF-MEK-ERK) pathway, which is involved in pain hypersensitivity. Inflammatory models were induced in vivo by injecting Complete Freund's adjuvant (CFA) into the whisker pad of rats. SP was administrated to SGCs in vitro for investigating, whether SP regulates the expression of NK-I receptor in the SGC nucleus. The effects of RAS-RAF-MEK, PKA and PKC pathways in this process were measured by co-incubating SGCs with respective Raf, PKA, PKC and MEK inhibitors in vitro and by pre-injecting these inhibitors into the TG in vivo. SP significantly upregulated NK-I receptor, p-ERK1/2, Ras, B-Raf, PKA and PKC in SGCs under inflammatory conditions. In addition, L703,606 (NK-I receptor antagonist), U0126 (MEK inhibitor), Sorafenib (Raf inhibitor) and H892HCL (PKA inhibitor) but not chelerythrine chloride (PKC inhibitor) significantly decreased NK-I mRNA and protein levels induced by SP. The allodynia-related behavior evoked by CFA was inhibited by pre-injection of L703,606, U0126, Sorafenib and H892HCL into the TG. Overall, SP upregulates NK-I receptor in TG SGCs via PKA/RAS-RAF-MEK-ERK pathway activation, contributing to a positive feedback of SP-NK-I receptor in inflammatory orofacial pain.

[Effect of cinobufagin on transient outward potassium current in dorsal root ganglion cells of rats with cancer-induced bone pain]

OBJECTIVE

To observe the effect of cinobufagin on transient outward potassium current (I_A) in rat dorsal root ganglion cells of cancer-induced bone pain (CIBP) and explore the possible analgesic mechanism of cinobufagin.

METHODS

Whole cell patch clamp technique was used to examine the effect of cionbufagin on I_A in acutely isolated dorsal root ganglion (DRG) cells from normal SD rats and rats with bone cancer pain.

RESULTS

The DRG cells from rats with CIBP showed obviously decreased I_A current density, an activation curve shift to the right, and an inactivation curve shift to the left. Cinobufagin treatment significantly increased the I_A current density and reversed the changes in the activation and inactivation curves in the DRG cells.

CONCLUSIONS

I_A current is decreased in DRG neurons from rats with CIBP. Cinobufagin can regulate the activation and inactivation of I_A current in the DRG cells, which may be related to its analgesic mechanism.

Effect of lutein on the acute inflammation-induced c-Fos expression of rat trigeminal spinal nucleus caudalis and C1 dorsal horn neurons

Although lutein is known to inhibit chronic inflammation, its effect on acute inflammation-induced nociceptive processing in the trigeminal system remains to be determined. The aim of the present study was to investigate whether pretreatment with lutein attenuates acute inflammation-induced sensitization of nociceptive processing in rat spinal trigeminal nucleus caudalis (SpVc) and upper cervical (C1) dorsal horn neurons, via c-Fos immunoreactivity. Mustard oil, a transient receptor potential ankyrin-1 channel agonist, was injected into the whisker pads to induce inflammation. Pretreatment of rats with lutein resulted in significant decreases in the inflammation-induced mean times of face grooming and the thickness of inflammation-induced edema in whisker pads relative to those features in inflamed rats (i.e., rats with no lutein pretreatment). In both the ipsilateral superficial and deep laminae of the SpVc and C1 dorsal horn, there were significantly larger numbers of c-Fos-positive neurons in inflamed rats than in naïve rats, and lutein pretreatment significantly decreased that number relative to inflamed rats. These results suggest that systemic administration of lutein attenuates acute inflammation-induced nocifensive behavior and augmented nociceptive processing of SpVc and C1 neurons that send stimulus localization and intensity information to higher pain centers. These findings support lutein as a potential therapeutic agent for use as an alternative, complementary medicine to attenuate, or even prevent, acute inflammatory pain.

Noncanonical Ion Channel Behaviour in Pain

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

Spinal glial cell line-derived neurotrophic factor infusion reverses reduction of Kv4.1-mediated A-type potassium currents of injured myelinated primary afferent neurons in a neuropathic pain model

High frequency spontaneous activity in injured primary afferents has been proposed as a pathological mechanism of neuropathic pain following nerve injury. Although spinal infusion of glial cell line-derived neurotrophic factor reduces the activity of injured myelinated A-fiber neurons after fifth lumbar (L5) spinal nerve ligation in rats, the implicated molecular mechanism remains undetermined. The fast-inactivating transient A-type potassium current (I_A) is an important determinant of neuronal excitability, and five voltage-gated potassium channel (Kv) alpha-subunits, Kv1.4, Kv3.4, Kv4.1, Kv4.2, and Kv4.3, display I_A in heterologous expression systems. Here, we examined the effect of spinal glial cell line-derived neurotrophic factor infusion on I_A and the expression of these five Kv mRNAs in injured A-fiber neurons using the in vitro patch clamp technique and in situ hybridization histochemistry. Glial cell line-derived neurotrophic factor infusion reversed axotomy-induced reduction of the rheobase, elongation of first spike duration, and depolarization of the resting membrane potential. L5 spinal nerve ligation significantly reduced the current density of I_A and glial cell line-derived neurotrophic factor treatment reversed the reduction. Among the examined Kv mRNAs, only the change in Kv4.1-expression was parallel with the change in I_A after spinal nerve ligation and glial cell line-derived neurotrophic factor treatment. These findings suggest that glial cell line-derived neurotrophic factor should reduce the hyperexcitability of injured A-fiber primary afferents by I_A recurrence. Among the five I_A-related Kv channels, Kv4.1 should be a key channel, which account for this I_A recurrence.

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.

Medicinal Chemistry of Potassium Channel Modulators: An Update of Recent Progress (2011-2017)

BACKGROUND

Potassium (K+) channels participate in many physiological processes, cardiac function, cell proliferation, neuronal signaling, muscle contractility, immune function, hormone secretion, osmotic pressure, changes in gene expression, and are involved in critical biological functions, and in a variety of diseases. Potassium channels represent a large family of tetrameric membrane proteins. Potassium channels activation reduces excitability, whereas channel inhibition increases excitability.

OBJECTIVE

Small molecule K+ channel activators and inhibitors interact with voltage-gated, inward rectifying, and two-pore tandem potassium channels. Due to their involvement in biological functions, and in a variety of diseases, small molecules as potassium channel modulators have received great scientific attention.

METHODS

In this review, we have compiled the literature, patents and patent applications (2011 to 2017) related to different chemical classes of potassium channel openers and blockers as therapeutic agents for the treatment of various diseases. Many different chemical classes of selective small molecule have emerged as potassium channel modulators over the past years.

CONCLUSION

This review discussed the current understanding of medicinal chemistry research in the field of potassium channel modulators to update the key advances in this field.

Cholecystokinin type B receptor-mediated inhibition of A-type K+ channels enhances sensory neuronal excitability through the phosphatidylinositol 3-kinase and c-Src-dependent JNK pathway

Background

Cholecystokinin (CCK) is implicated in the regulation of nociceptive sensitivity of primary afferent neurons. Nevertheless, the underlying cellular and molecular mechanisms remain unknown.

Methods

Using patch clamp recording, western blot analysis, immunofluorescent labelling, enzyme-linked immunosorbent assays, adenovirus-mediated shRNA knockdown and animal behaviour tests, we studied the effects of CCK-8 on the sensory neuronal excitability and peripheral pain sensitivity mediated by A-type K⁺ channels.

Results

CCK-8 reversibly and concentration-dependently decreased A-type K⁺ channel (I_A) in small-sized dorsal root ganglion (DRG) neurons through the activation of CCK type B receptor (CCK-BR), while the sustained delayed rectifier K⁺ current was unaffected. The intracellular subunit of CCK-BR coimmunoprecipitated with Gα_o. Blocking G-protein signaling with pertussis toxin or by the intracellular application of anti-G_β antibody reversed the inhibitory effects of CCK-8. Antagonism of phosphatidylinositol 3-kinase (PI3K) but not of its common downstream target Akts abolished the CCK-BR-mediated I_A response. CCK-8 application significantly activated JNK mitogen-activated protein kinase. Antagonism of either JNK or c-Src prevented the CCK-BR-mediated I_A decrease, whereas c-Src inhibition attenuated the CCK-8-induced p-JNK activation. Application of CCK-8 enhanced the action potential firing rate of DRG neurons and elicited mechanical and thermal pain hypersensitivity in mice. These effects were mediated by CCK-BR and were occluded by I_A blockade.

Conclusion

Our findings indicate that CCK-8 attenuated I_A through CCK-BR that is coupled to the G_βγ-dependent PI3K and c-Src-mediated JNK pathways, thereby enhancing the sensory neuronal excitability in DRG neurons and peripheral pain sensitivity in mice.

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.

Melanocortin type 4 receptor-mediated inhibition of A-type K+ current enhances sensory neuronal excitability and mechanical pain sensitivity in rats

α-Melanocyte-stimulating hormone (α-MSH) has been shown to be involved in nociception, but the underlying molecular mechanisms remain largely unknown. In this study, we report that α-MSH suppresses the transient outward A-type K⁺ current (I _A) in trigeminal ganglion (TG) neurons and thereby modulates neuronal excitability and peripheral pain sensitivity in rats. Exposing small-diameter TG neurons to α-MSH concentration-dependently decreased I _A This α-MSH-induced I _A decrease was dependent on the melanocortin type 4 receptor (MC4R) and associated with a hyperpolarizing shift in the voltage dependence of A-type K⁺ channel inactivation. Chemical inhibition of phosphatidylinositol 3-kinase (PI3K) with wortmannin or of class I PI3Ks with the selective inhibitor CH5132799 prevented the MC4R-mediated I _A response. Blocking G_i/o-protein signaling with pertussis toxin or by dialysis of TG neurons with the G_βγ-blocking synthetic peptide QEHA abolished the α-MSH-mediated decrease in I _A Further, α-MSH increased the expression levels of phospho-p38 mitogen-activated protein kinase, and pharmacological or genetic inhibition of p38α abrogated the α-MSH-induced I _A response. Additionally, α-MSH significantly increased the action potential firing rate of TG neurons and increased the sensitivity of rats to mechanical stimuli applied to the buccal pad area, and both effects were abrogated by I _A blockade. Taken together, our findings suggest that α-MSH suppresses I _A by activating MC4R, which is coupled sequentially to the G_βγ complex of the G_i/o-protein and downstream class I PI3K-dependent p38α signaling, thereby increasing TG neuronal excitability and mechanical pain sensitivity in rats.

Verapamil Inhibits TRESK (K2P18.1) Current in Trigeminal Ganglion Neurons Independently of the Blockade of Ca2+ Influx

Tandem pore domain weak inward rectifier potassium channel (TWIK)-related spinal cord K⁺ (TRESK; K_2P18.1) channel is the only member of the two-pore domain K⁺ (K_2P) channel family that is activated by an increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) and linked to migraines. This study was performed to identify the effect of verapamil, which is an L-type Ca²⁺ channel blocker and a prophylaxis for migraines, on the TRESK channel in trigeminal ganglion (TG) neurons, as well as in a heterologous system. Single-channel and whole-cell currents were recorded in TG neurons and HEK-293 cells transfected with mTRESK using patch-clamping techniques. In TG neurons, changes in [Ca²⁺]_i were measured using the fluo-3-AM Ca²⁺ indicator. Verapamil, nifedipine, and NiCl₂ inhibited the whole-cell currents in HEK-293 cells overexpressing mTRESK with IC₅₀ values of 5.2, 54.3, and >100 μM, respectively. The inhibitory effect of verapamil on TRESK channel was also observed in excised patches. In TG neurons, verapamil (10 μM) inhibited TRESK channel activity by approximately 76%. The TRESK channel activity was not dependent on the presence of extracellular Ca²⁺. In addition, the inhibitory effect of verapamil on the TRESK channel remained despite the absence of extracellular Ca²⁺. These findings show that verapamil inhibits the TRESK current independently of the blockade of Ca²⁺ influx in TG neurons. Verapamil will be able to exert its pharmacological effects by modulating TRESK, as well as Ca²⁺ influx, in TG neurons in vitro. We suggest that verapamil could be used as an inhibitor for identifying TRESK channel in TG neurons.

Atom-by-atom tuning of the electrostatic potassium-channel modulator dehydroabietic acid

Dehydroabietic acid was recently shown to open voltage-gated potassium channels. Silverå Ejneby et al. show that its effect peaks when the carboxyl-group charge and hydrophobic anchor are separated by three atoms and use this rule to design molecules that open the human K_v7.2/7.3 potassium channel.

Dehydroabietic acid (DHAA) is a naturally occurring component of pine resin that was recently shown to open voltage-gated potassium (K_V) channels. The hydrophobic part of DHAA anchors the compound near the channel’s positively charged voltage sensor in a pocket between the channel and the lipid membrane. The negatively charged carboxyl group exerts an electrostatic effect on the channel’s voltage sensor, leading to the channel opening. In this study, we show that the channel-opening effect increases as the length of the carboxyl-group stalk is extended until a critical length of three atoms is reached. Longer stalks render the compounds noneffective. This critical distance is consistent with a simple electrostatic model in which the charge location depends on the stalk length. By combining an effective anchor with the optimal stalk length, we create a compound that opens the human K_V7.2/7.3 (M type) potassium channel at a concentration of 1 µM. These results suggest that a stalk between the anchor and the effector group is a powerful way of increasing the potency of a channel-opening drug.

EXCITABILITY PROPERTIES OF TRIGEMINAL GANGLION NEURONS

The firing properties of small neurons (with diameters of soma less than 25 µm) were investigated using patch-clamp technique in whole-cell configuration in primary culture of trigeminal ganglia (TG) of postnatal rats. TG neurons were divided into three groups according to their firing responses to long-lasting depolarizing pulses: adaptive neurons (AN) characterized by a strongly adaptive responses; tonic neurons (TN) characterized by a multiple tonic firing; neurons with a delay before initiation of AP generation, namely, NDG. AN, TN and NDG also differed in AP electrophysiological and pharmacological characteristics. TN was distinguished by responses to hyperpolarization and the greatest value of input resistance. TN, AN and NDG were characterized by different active properties (amplitude of action potential and afterhyperpolarization, reobase, threshold). Each group of neurons was characterized by heterogeneity of AP duration and of frequency properties for TN. The application of tetrodotoxin (TTX) (250 nM) resulted in full or partial inhibition of AP generation and some neurons had TTX – insensitive firing responses. Neurons that were not affected by TTX had markedly longer AP. TTX had no effect on electrical activity of some AN and NDG. Based on sensitivity to TTX and their electrophysiological properties, AN and NDG seem to be C-fiber nococeptors.

Endothelin-1 Decreases Excitability of the Dorsal Root Ganglion Neurons via ETB Receptor

Endothelin-1 (ET-1) has been demonstrated to be a pro-nociceptive as well as an anti-nociceptive agent. However, underlying molecular mechanisms for these pain modulatory actions remain unclear. In the present study, we evaluated the ability of ET-1 to alter the nociceptor excitability using a patch clamp technique in acutely dissociated rat dorsal root ganglion (DRG) neurons. ET-1 produced an increase in threshold current to evoke an action potential (I _threshold) and hyperpolarization of resting membrane potential (RMP) indicating decreased excitability of DRG neurons. I _threshold increased from 0.25 ± 0.08 to 0.33 ± 0.07 nA and hyperpolarized RMP from -57.51 ± 1.70 to -67.41 ± 2.92 mV by ET-1 (100 nM). The hyperpolarizing effect of ET-1 appears to be orchestrated via modulation of membrane conductances, namely voltage-gated sodium current (I _Na) and outward transient potassium current (I _KT). ET-1, 30 and 100 nM, decreased the peak I _Na by 41.3 ± 6.8 and 74 ± 15.2%, respectively. Additionally, ET-1 (100 nM) significantly potentiated the transient component (I _KT) of the potassium currents. ET-1-induced effects were largely attenuated by BQ-788, a selective ET_BR blocker. However, a selective ET_AR blocker BQ-123 did not alter the effects of ET-1. A selective ET_BR agonist, IRL-1620, mimicked the effect of ET-1 on I _Na in a concentration-dependent manner (IC₅₀ 159.5 ± 92.6 μM). In conclusion, our results demonstrate that ET-1 hyperpolarizes nociceptors by blocking I _Na and potentiating I _KT through selective activation of ET_BR, which may represent one of the underlying mechanisms for reported anti-nociceptive effects of ET-1.

Afferent Fiber Remodeling in the Somatosensory Thalamus of Mice as a Neural Basis of Somatotopic Reorganization in the Brain and Ectopic Mechanical Hypersensitivity after Peripheral Sensory Nerve Injury

Plastic changes in the CNS in response to peripheral sensory nerve injury are a series of complex processes, ranging from local circuit remodeling to somatotopic reorganization. However, the link between circuit remodeling and somatotopic reorganization remains unclear. We have previously reported that transection of the primary whisker sensory nerve causes the abnormal rewiring of lemniscal fibers (sensory afferents) on a neuron in the mouse whisker sensory thalamus (V2 VPM). In the present study, using transgenic mice whose lemniscal fibers originate from the whisker sensory principle trigeminal nucleus (PrV2) are specifically labeled, we identified that the transection induced retraction of PrV2-originating lemniscal fibers and invasion of those not originating from PrV2 in the V2 VPM. This anatomical remodeling with somatotopic reorganization was highly correlated with the rewiring of lemniscal fibers. Origins of the non-PrV2-origin lemniscal fibers in the V2 VPM included the mandibular subregion of trigeminal nuclei and the dorsal column nuclei (DCNs), which normally represent body parts other than whiskers. The transection also resulted in ectopic receptive fields of V2 VPM neurons and extraterritorial pain behavior on the uninjured mandibular region of the face. The anatomical remodeling, emergence of ectopic receptive fields, and extraterritorial pain behavior all concomitantly developed within a week and lasted more than three months after the transection. Our findings, thus, indicate a strong linkage between these plastic changes after peripheral sensory nerve injury, which may provide a neural circuit basis underlying large-scale reorganization of somatotopic representation and abnormal ectopic sensations.

Neuronal plasticity of trigeminal ganglia in mice following nerve injury

Background

Nerve injury may induce neuropathic pain. In studying the mechanisms of orofacial neuropathic pain, attention has been paid to the plastic changes that occur in the trigeminal ganglia (TGs) and nucleus in response to an injury of the trigeminal nerve branches. Previous studies have explored the impact of sciatic nerve injury on dorsal root ganglia (DRGs) and it has shown dramatic changes in the expression of multiple biomarkers. In large, the changes in biomarker expression in TGs after trigeminal nerve injury are similar to that in DRGs after sciatic nerve injury. However, important differences exist. Therefore, there is a need to study the plasticity of biomarkers in TGs after nerve injury in the context of the development of neuropathic pain-like behaviors.

Aim

The aim of this study was to investigate the plasticity of biomarkers associated with chronic persistent pain in TGs after trigeminal nerve injury.

Materials and methods

To mimic the chronic nature of the disorder, we used an intraoral procedure to access the infraorbital nerve (ION) and induced a nerve injury in mice. Immunohistochemistry and quantification were used for revealing the expression level of each biomarker in TGs after nerve injury.

Results

Two weeks after partial ION injury, immunohistochemistry results showed strongly upregulated expressions of activating transcription factor 3 and neuropeptide Y (NPY) in the ipsilateral TGs. Microglial cells were also activated after nerve injury. In regard to positive neuronal profile counting, however, no significant difference in expression was observed in galanin, substance P, calcitonin gene-related peptide, neuronal nitric oxide synthase, phosphorylated AKT, or P2X3 in ipsilateral TGs when compared to contralateral TGs.

Conclusion

In this study, the expression and regulation of biomarkers in TGs have been observed in response to trigeminal nerve injury. Our results suggest that NPY and Iba1 might play crucial roles in the pathogenesis of orofacial neuropathic pain following this type of injury. Further investigations on the relevance of these changes may help to target suitable treatment possibilities for trigeminal neuralgia.