Activation of voltage-gated KCNQ/Kv7 channels by anticonvulsant retigabine attenuates mechanical allodynia of inflammatory temporomandibular joint in rats

Selective inhibition of A-fiber-mediated excitatory transmission underlies the analgesic effects of KCNQ channel opening in the spinal dorsal horn

Neuronal voltage-gated KCNQ (Kv7) channels, expressed centrally and peripherally, mediate low-threshold and non-inactivating M-currents responsible for the control of tonic excitability of mammalian neurons. Pharmacological opening of KCNQ channels has been reported to generate analgesic effects in animal models of neuropathic pain. Here, we examined the possible involvement of central KCNQ channels in the analgesic effects of retigabine, a KCNQ channel opener. Behaviorally, intraperitoneally applied retigabine exerted analgesic effects on thermal and mechanical hypersensitivity in male mice developing neuropathic pain after partial sciatic nerve ligation, which was antagonized by the KCNQ channel blocker XE991 preadministered intraperitoneally and intrathecally. Intrathecally applied retigabine also exerted analgesic effects that were inhibited by intrathecally injected XE991. We then explored the synaptic mechanisms underlying the analgesic effects of retigabine in the spinal dorsal horn. Whole-cell recordings were made from dorsal horn neurons in spinal slices with attached dorsal roots from adult male mice developing neuropathic pain, and the effects of retigabine on miniature and afferent-evoked postsynaptic currents were examined. Retigabine reduced the amplitude of A-fiber-mediated EPSCs without affecting C-fiber-mediated excitatory synaptic transmission. A-fiber-mediated EPSCs remained unaltered by retigabine in the presence of XE991, consistently with the behavioral findings. The frequency and amplitude of mEPSCs were not affected by retigabine. Thus, opening of KCNQ channels in the central terminals of primary afferent A-fibers inhibits excitatory synaptic transmission in the spinal dorsal horn, most likely contributing to the analgesic effect of retigabine.

Glial KCNQ K+ channels control neuronal output by regulating GABA release from glia in C. elegans

KCNQs are voltage-gated K+ channels that control neuronal excitability and are mutated in epilepsy and autism spectrum disorder (ASD). KCNQs have been extensively studied in neurons, but their function in glia is unknown. Using voltage, calcium, and GABA imaging, optogenetics, and behavioral assays, we show here for the first time in Caenorhabditis elegans (C. elegans) that glial KCNQ channels control neuronal excitability by mediating GABA release from glia via regulation of the function of L-type voltage-gated Ca2+ channels. Further, we show that human KCNQ channels have the same role when expressed in nematode glia, underscoring conservation of function across species. Finally, we show that pathogenic loss-of-function and gain-of-function human KCNQ2 mutations alter glia-to-neuron GABA signaling in distinct ways and that the KCNQ channel opener retigabine exerts rescuing effects. This work identifies glial KCNQ channels as key regulators of neuronal excitability via control of GABA release from glia.

TET1 Participates in Complete Freund's Adjuvant-induced Trigeminal Inflammatory Pain by Regulating Kv7.2 in a Mouse Model

Trigeminal inflammatory pain is one of the most severe pain-related disorders in humans; however, the underlying mechanisms remain largely unknown. In this study, we investigated the possible contribution of interaction between ten-eleven translocation methylcytosine dioxygenase 1 (TET1) and the voltage-gated K+ channel Kv7.2 (encoded by Kcnq2) to orofacial inflammatory pain in mice. We found that complete Freund's adjuvant (CFA) injection reduced the expression of Kcnq2/Kv7.2 in the trigeminal ganglion (TG) and induced orofacial inflammatory pain. The involvement of Kv7.2 in CFA-induced orofacial pain was further confirmed by Kv7.2 knockdown or overexpression. Moreover, TET1 knockdown in Tet1flox/flox mice significantly reduced the expression of Kv7.2 and M currents in the TG and led to pain-like behaviors. Conversely, TET1 overexpression by lentivirus rescued the CFA-induced decreases of Kcnq2 and M currents and alleviated mechanical allodynia. Our data suggest that TET1 is implicated in CFA-induced trigeminal inflammatory pain by positively regulating Kv7.2 in TG neurons.

Enkephalin Rescues Temporomandibular Joint Pain-Related Behavior in Rats

Temporomandibular joint disorders include a variety of clinical syndromes that are difficult to manage if associated with debilitating severe jaw pain. Thus, seeking additional experimental therapies for temporomandibular joint pain reduction is warranted. Targeted enkephalin gene therapy approaches provide clear promise for pain control. The studies detailed here indicate significant analgesia and protection of joint tissue are provided after injection of an overexpression viral vector gene therapy near the joint. The viral vector gene therapy described provides overexpression of naturally occurring opioid peptides after its uptake by trigeminal nerve endings. The viral vectors act as independent "minipump" sources for the opioid peptide synthesis in the neuronal cytoplasm producing the intended biological function, reduction of pain, and tissue repair. The antinociceptive effects provided with this delivery method of opioid expression persist for over 4 weeks. This is coincident with the expected time frame for the duration of the transgene overproduction of the endogenous opioid peptide before its diminution due to dormancy of the virus. These experimental studies establish a basis for the use of replication-defective herpes simplex type 1-based gene therapy for severe chronic inflammatory temporomandibular joint destruction and pain. As innovative means of significantly reducing joint inflammation and preserving tissue architecture, gene therapies may extend their clinical usefulness for patients with temporomandibular joint disorders.

Behavioral and neuro-functional consequences of eliminating the KCNQ3 GABA binding site in mice

Voltage-gated potassium (Kv) channels formed by α subunits KCNQ2-5 are important in regulating neuronal excitability. We previously found that GABA directly binds to and activates channels containing KCNQ3, challenging the traditional understanding of inhibitory neurotransmission. To investigate the functional significance and behavioral role of this direct interaction, mice with a mutated KCNQ3 GABA binding site (Kcnq3-W266L) were generated and subjected to behavioral studies. Kcnq3-W266L mice exhibited distinctive behavioral phenotypes, of which reduced nociceptive and stress responses were profound and sex-specific. In female Kcnq3-W266L mice, the phenotype was shifted towards more nociceptive effects, while in male Kcnq3-W266L mice, it was shifted towards the stress response. In addition, female Kcnq3-W266L mice exhibited lower motor activity and reduced working spatial memory. The neuronal activity in the lateral habenula and visual cortex was altered in the female Kcnq3-W266L mice, suggesting that GABAergic activation of KCNQ3 in these regions may play a role in the regulation of the responses. Given the known overlap between the nociceptive and stress brain circuits, our data provide new insights into a sex-dependent role of KCNQ3 in regulating neural circuits involved in nociception and stress, via its GABA binding site. These findings identify new targets for effective treatments for neurological and psychiatric conditions such as pain and anxiety.

Kv7-specific activators hyperpolarize resting membrane potential and modulate human iPSC-derived sensory neuron excitability

Chronic pain is highly prevalent and remains a significant unmet global medical need. As part of a search for modulatory genes that confer pain resilience, we have studied two family cohorts where one individual reported much less pain than other family members that share the same pathogenic gain-of-function Nav1.7 mutation that confers hyperexcitability on pain-signaling dorsal root ganglion (DRG) neurons. In each of these kindreds, the pain-resilient individual carried a gain-of-function variant in Kv7.2 or Kv7.3, two potassium channels that stabilize membrane potential and reduce excitability. Our observation in this molecular genetic study that these gain-of-function Kv7.2 and 7.3 variants reduce DRG neuron excitability suggests that agents that activate or open Kv7 channels should attenuate sensory neuron firing. In the present study, we assess the effects on sensory neuron excitability of three Kv7 modulators-retigabine (Kv7.2 thru Kv7.5 activator), ICA-110381 (Kv7.2/Kv7.3 specific activator), and as a comparator ML277 (Kv7.1 specific activator)-in a "human-pain-in-a-dish" model (human iPSC-derived sensory neurons, iPSC-SN). Multi-electrode-array (MEA) recordings demonstrated inhibition of firing with retigabine and ICA-110381 (but not with ML277), with the concentration-response curve indicating that retigabine can achieve a 50% reduction of firing with sub-micromolar concentrations. Current-clamp recording demonstrated that retigabine hyperpolarized iPSC-SN resting potential and increased threshold. This study implicates Kv7.2/Kv7.3 channels as effective modulators of sensory neuron excitability, and suggest that compounds that specifically target Kv7.2/Kv7.3 currents in sensory neurons, including human sensory neurons, might provide an effective approach toward pain relief.

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.

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.

KCNQ Channels in the Mesolimbic Reward Circuit Regulate Nociception in Chronic Pain in Mice

Mesocorticolimbic dopaminergic (DA) neurons have been implicated in regulating nociception in chronic pain, yet the mechanisms are barely understood. Here, we found that chronic constructive injury (CCI) in mice increased the firing activity and decreased the KCNQ channel-mediated M-currents in ventral tegmental area (VTA) DA neurons projecting to the nucleus accumbens (NAc). Chemogenetic inhibition of the VTA-to-NAc DA neurons alleviated CCI-induced thermal nociception. Opposite changes in the firing activity and M-currents were recorded in VTA DA neurons projecting to the medial prefrontal cortex (mPFC) but did not affect nociception. In addition, intra-VTA injection of retigabine, a KCNQ opener, while reversing the changes of the VTA-to-NAc DA neurons, alleviated CCI-induced nociception, and this was abolished by injecting exogenous BDNF into the NAc. Taken together, these findings highlight a vital role of KCNQ channel-mediated modulation of mesolimbic DA activity in regulating thermal nociception in the chronic pain state.

Pharmacological Activation of Neuronal Voltage-Gated Kv7/KCNQ/M-Channels for Potential Therapy of Epilepsy and Pain

Native M-current is a low-threshold, slowly activating potassium current that exerts an inhibitory control over neuronal excitability. The M-channel is primarily co-assembled by heterotetrameric Kv7.2/KCNQ2 and Kv7.3/KCNQ3 subunits that are specifically expressed in the brain and peripheral nociceptive and visceral sensory neurons in the spinal cord. Reduction of M-channel function leads to neuronal hyperexcitability that defines the fundamental mechanism of neurological disorders such as epilepsy and pain, indicating that pharmacological activation of Kv7/KCNQ/M-channels may serve the basis for the therapy. The well-known KCNQ opener retigabine (ezogabine or Potiga) was approved by FDA in 2011 as an anticonvulsant used for an adjunctive treatment of partial epilepsies. Unfortunately, retigabine was discontinued in 2017 due to its side effects of blue-colored appearance of the skin and eyes after prolonged intake. In addition, flupirtine, a structural derivative of retigabine and a centrally acting non-opioid analgesic, was also withdrawn in 2018 for liver toxicity. Fortunately, these side effects are compound-structures related and can be avoided. Thus, further identification and development of novel potent and selective Kv7 channel openers may lead to an effective therapy with improved safety window for anti-epilepsy and anti-nociception.

Nociceptive behavioural assessments in mouse models of temporomandibular joint disorders

Orofacial pain or tenderness is a primary symptom associated with temporomandibular joint (TMJ) disorders (TMDs). To understand the pathological mechanisms underlying TMDs, several mouse models have been developed, including mechanical stimulus-induced TMD and genetic mouse models. However, a lack of feasible approaches for assessing TMD-related nociceptive behaviours in the orofacial region of mice has hindered the in-depth study of TMD-associated mechanisms. This study aimed to explore modifications of three existing methods to analyse nociceptive behaviours using two TMD mouse models: (1) mechanical allodynia was tested using von Frey filaments in the mouse TMJ region by placing mice in specially designed chambers; (2) bite force was measured using the Economical Load and Force (ELF) system; and (3) spontaneous feeding behaviour tests, including eating duration and frequency, were analysed using the Laboratory Animal Behaviour Observation Registration and Analysis System (LABORAS). We successfully assessed changes in nociceptive behaviours in two TMD mouse models, a unilateral anterior crossbite (UAC)-induced TMD mouse model and a β-catenin conditional activation mouse model. We found that the UAC model and β-catenin conditional activation mouse model were significantly associated with signs of increased mechanical allodynia, lower bite force, and decreased spontaneous feeding behaviour, indicating manifestations of TMD. These behavioural changes were consistent with the cartilage degradation phenotype observed in these mouse models. Our studies have shown reliable methods to analyse nociceptive behaviours in mice and may indicate that these methods are valid to assess signs of TMD in mice.

Novel Analgesics with Peripheral Targets

A limited number of peripheral targets generate pain. Inflammatory mediators can sensitize these. The review addresses targets acting exclusively or predominantly on sensory neurons, mediators involved in inflammation targeting sensory neurons, and mediators involved in a more general inflammatory process, of which an analgesic effect secondary to an anti-inflammatory effect can be expected. Different approaches to address these systems are discussed, including scavenging proinflammatory mediators, applying anti-inflammatory mediators, and inhibiting proinflammatory or facilitating anti-inflammatory receptors. New approaches are contrasted to established ones; the current stage of progress is mentioned, in particular considering whether there is data from a molecular and cellular level, from animals, or from human trials, including an early stage after a market release. An overview of publication activity is presented, considering a IuPhar/BPS-curated list of targets with restriction to pain-related publications, which was also used to identify topics.

Enhancing KCNQ Channel Activity Improves Neurobehavioral Recovery after Spinal Cord Injury

Spinal cord injury (SCI) usually leads to acute neuronal death and delayed secondary degeneration, resulting in sensory dysfunction, paralysis, and chronic pain. Excessive excitation is one of the critical factors leading to secondary neural damage initiated by various insults. KCNQ/Kv7 channels are highly expressed in spinal neurons and axons and play an important role in controlling their excitability. Enhancing KCNQ channel activity by using its specific opener retigabine could thus be a plausible treatment strategy to reduce the pathology after SCI. We produced contusive SCI at T10 in adult male rats, which then received 10 consecutive days' treatment with retigabine or vehicle starting 3 hours or 3 days after contusion. Two different concentrations and two different delivery methods were applied. Delivery of retigabine via Alzet osmotic pumps, but not intraperitoneal injections 3 hours after contusion, promoted recovery of locomotor function. Remarkably, retigabine delivery in both methods significantly attenuated the development of mechanical stimuli-induced hyperreflexia and spontaneous pain; however, no significant difference in the thermal threshold was observed. Although retigabine delivered 3 days after contusion significantly attenuated the development of mechanical hypersensitivity and spontaneous pain, the locomotor function is not improved by the delayed treatments. Finally, we found that early application of retigabine attenuates the inflammatory activity in the spinal cord and increases the survival of white matter after SCI. Our results suggest that decreasing neuronal excitability by targeting KCNQ/Kv7 channels at acute stage aids the recovery of locomotor function and attenuates the development of neuropathic pain after SCI. SIGNIFICANCE STATEMENT: Several pharmacological interventions have been proposed for spinal cord injury (SCI) treatment, but none have been shown to be both effective and safe in clinical trials. Necrotic neuronal death and chronic pain are often the cost of pathological neural excitation after SCI. We show that early, brief application of retigabine could aid locomotor and sensory neurobehavioral recovery after SCI, supporting the use of this drug in the clinic to promote motor and sensory function in patients with SCI.

Activation of KCNQ Channels Prevents Paclitaxel-Induced Peripheral Neuropathy and Associated Neuropathic Pain

Paclitaxel-induced peripheral neuropathy (PIPN) and associated neuropathic pain are the most common and serious adverse effects experienced by cancer patients receiving paclitaxel treatment. These effects adversely impact daily activities and consequently the quality of life, sometimes forcing the suspension of treatment and negatively influencing survival. Patients are usually at high risk of developing PIPN if paclitaxel induces acute pain, which strongly suggests that an acute increase in the excitability of nociceptors underlies the chronic alterations of PIPN. KCNQ/Kv7 channels are widely expressed in the primary sensory neurons to modulate their excitability. In the present study, we show that targeting KCNQ/Kv7 channels at an early stage is an effective strategy to attenuate the development of PIPN. We found that paclitaxel did not decrease the expression level of KCNQ/Kv7 channels in the primary sensory neurons as detected by quantitative reverse-transcription polymerase chain reaction (qRT-PCR) and Western blotting. However, retigabine, which is a specific KCNQ/Kv7 channel opener, attenuated significantly the development of PIPN, as shown by both morphologic and behavioral evidence. We also observed that retigabine had no obvious effect on the chemosensitivity of breast cancer cells to paclitaxel. Although retigabine has been approved by the FDA as an anticonvulsant, our study suggests that this drug can be repurposed to attenuate the development of PIPN. PERSPECTIVE: Paclitaxel-induced peripheral neuropathy and associated neuropathic pain are severe and resistant to intervention. The results of our study demonstrated that retigabine (a clinically available medicine) can be used to attenuate the development of paclitaxel-induced peripheral neuropathy.

Pore- and voltage sensor-targeted KCNQ openers have distinct state-dependent actions

Ion channels encoded by KCNQ2-5 generate a prominent K⁺ conductance in the central nervous system, referred to as the M current, which is controlled by membrane voltage and PIP2. The KCNQ2-5 voltage-gated potassium channels are targeted by a variety of activating compounds that cause negative shifts in the voltage dependence of activation. The underlying pharmacology of these effects is of growing interest because of possible clinical applications. Recent studies have revealed multiple binding sites and mechanisms of action of KCNQ activators. For example, retigabine targets the pore domain, but several compounds have been shown to influence the voltage-sensing domain. An important unexplored feature of these compounds is the influence of channel gating on drug binding or effects. In the present study, we compare the state-dependent actions of retigabine and ICA-069673 (ICA73, a voltage sensor-targeted activator). We assess drug binding to preopen states by applying drugs to homomeric KCNQ2 channels at different holding voltages, demonstrating little or no association of ICA73 with resting states. Using rapid solution switching, we also demonstrate that the rate of onset of ICA73 correlates with the voltage dependence of channel activation. Retigabine actions differ significantly, with prominent drug effects seen at very negative holding voltages and distinct voltage dependences of drug binding versus channel activation. Using similar approaches, we investigate the mechanistic basis for attenuation of ICA73 actions by the voltage-sensing domain mutation KCNQ2[A181P]. Our findings demonstrate different state-dependent actions of pore- versus voltage sensor-targeted KCNQ channel activators, which highlight that subtypes of this drug class operate with distinct mechanisms.

Kv7 channels a potential therapeutic target in fibromyalgia: A hypothesis

Fibromyalgia is characterized by the primary symptoms of persistent diffuse pain, fatigue, sleep disturbance and cognitive dysfunction. Persistent pain conditions, such as fibromyalgia, are often refractory to current available therapies. An involvement of K⁺ channels in the pathophysiology of fibromyalgia is emerging and supported by drug treatments for this condition exhibiting action at these molecular processes. K⁺ channels constitute potential novel target candidates for pain therapy offering peripheral and/or central actions. The Kv7 channel activators, flupirtine and retigabine, have exhibited pharmacological profiles compatible to the requirements needed for use as a therapeutic approach to fibromyalgia. Clinical trials to address the multidimensional challenges of fibromyalgia with flupirtine and retigabine will provide important insight to the role of K⁺ channels in this condition.

M-type K+ channels in peripheral nociceptive pathways

Pathological pain is a hyperexcitability disorder. Since the excitability of a neuron is set and controlled by a complement of ion channels it expresses, in order to understand and treat pain, we need to develop a mechanistic insight into the key ion channels controlling excitability within the mammalian pain pathways and how these ion channels are regulated and modulated in various physiological and pathophysiological settings. In this review, we will discuss the emerging data on the expression in pain pathways, functional role and modulation of a family of voltage-gated K+ channels called 'M channels' (KCNQ, Kv 7). M channels are increasingly recognized as important players in controlling pain signalling, especially within the peripheral somatosensory system. We will also discuss the therapeutic potential of M channels as analgesic drug targets.

LINKED ARTICLES

This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc/.

The signaling lipid sphingosine 1-phosphate regulates mechanical pain

Somatosensory neurons mediate responses to diverse mechanical stimuli, from innocuous touch to noxious pain. While recent studies have identified distinct populations of A mechanonociceptors (AMs) that are required for mechanical pain, the molecular underpinnings of mechanonociception remain unknown. Here, we show that the bioactive lipid sphingosine 1-phosphate (S1P) and S1P Receptor 3 (S1PR3) are critical regulators of acute mechanonociception. Genetic or pharmacological ablation of S1PR3, or blockade of S1P production, significantly impaired the behavioral response to noxious mechanical stimuli, with no effect on responses to innocuous touch or thermal stimuli. These effects are mediated by fast-conducting A mechanonociceptors, which displayed a significant decrease in mechanosensitivity in S1PR3 mutant mice. We show that S1PR3 signaling tunes mechanonociceptor excitability via modulation of KCNQ2/3 channels. Our findings define a new role for S1PR3 in regulating neuronal excitability and establish the importance of S1P/S1PR3 signaling in the setting of mechanical pain thresholds.

Activation of KCNQ Channels Suppresses Spontaneous Activity in Dorsal Root Ganglion Neurons and Reduces Chronic Pain after Spinal Cord Injury

A majority of people who have sustained spinal cord injury (SCI) experience chronic pain after injury, and this pain is highly resistant to available treatments. Contusive SCI in rats at T10 results in hyperexcitability of primary sensory neurons, which contributes to chronic pain. KCNQ channels are widely expressed in nociceptive dorsal root ganglion (DRG) neurons, are important for controlling their excitability, and their activation has proven effective in reducing pain in peripheral nerve injury and inflammation models. The possibility that activators of KCNQ channels could be useful for treating SCI-induced chronic pain is strongly supported by the following findings. First, SCI, unlike peripheral nerve injury, failed to decrease the functional or biochemical expression of KCNQ channels in DRG as revealed by electrophysiology, real-time quantitative polymerase chain reaction, and Western blot; therefore, these channels remain available for pharmacological targeting of SCI pain. Second, treatment with retigabine, a specific KCNQ channel opener, profoundly decreased spontaneous activity in primary sensory neurons of SCI animals both in vitro and in vivo without changing the peripheral mechanical threshold. Third, retigabine reversed SCI-induced reflex hypersensitivity, adding to our previous demonstration that retigabine supports the conditioning of place preference after SCI (an operant measure of spontaneous pain). In contrast to SCI animals, naïve animals showed no effects of retigabine on reflex sensitivity or conditioned place preference by pairing with retigabine, indicating that a dose that blocks chronic pain-related behavior has no effect on normal pain sensitivity or motivational state. These results encourage the further exploration of U.S. Food and Drug Administration-approved KCNQ activators for treating SCI pain, as well as efforts to develop a new generation of KCNQ activators that lack central side effects.

Kv7 channels in the nucleus accumbens are altered by chronic drinking and are targets for reducing alcohol consumption

Alcohol use disorders (AUDs) are a major public health issue and produce enormous societal and economic burdens. Current Food and Drug Administration (FDA)-approved pharmacotherapies for treating AUDs suffer from deleterious side effects and are only effective in a subset of individuals. It is therefore essential to find improved medications for the management of AUDs. Emerging evidence suggests that anticonvulsants are a promising class of drugs for treating individuals with AUDs. In these studies, we used integrative functional genomics to demonstrate that genes that encode Kv7 channels (i.e. Kcnq2/3) are related to alcohol (ethanol) consumption, preference and acceptance in rodents. We then tested the ability of the FDA-approved anticonvulsant retigabine, a Kv7 channel opener, to reduce voluntary ethanol consumption of Wistar rats in a two-bottle choice intermittent alcohol access paradigm. Systemic administration and microinjections of retigabine into the nucleus accumbens significantly reduced alcohol drinking, and retigabine was more effective at reducing intake in high- versus low-drinking populations of Wistar rats. Prolonged voluntary drinking increased the sensitivity to the proconvulsant effects of pharmacological blockade of Kv7 channels and altered surface trafficking and SUMOylation patterns of Kv7.2 channels in the nucleus accumbens. These data implicate Kcnq2/3 in the regulation of ethanol drinking and demonstrate that long-term drinking produces neuroadaptations in Kv7 channels. In addition, these results have identified retigabine as a potential pharmacotherapy for treating AUDs and Kv7 channels as a novel therapeutic target for reducing heavy drinking.

Hydroxymethylation of microRNA-365-3p Regulates Nociceptive Behaviors via Kcnh2

DNA 5-hydroxylmethylcytosine (5hmC) catalyzed by ten-eleven translocation methylcytosine dioxygenase (TET) occurs abundantly in neurons of mammals. However, the in vivo causal link between TET dysregulation and nociceptive modulation has not been established. Here, we found that spinal TET1 and TET3 were significantly increased in the model of formalin-induced acute inflammatory pain, which was accompanied with the augment of genome-wide 5hmC content in spinal cord. Knockdown of spinal TET1 or TET3 alleviated the formalin-induced nociceptive behavior and overexpression of spinal TET1 or TET3 in naive mice produced pain-like behavior as evidenced by decreased thermal pain threshold. Furthermore, we found that TET1 or TET3 regulated the nociceptive behavior by targeting microRNA-365-3p (miR-365-3p). Formalin increased 5hmC in the miR-365-3p promoter, which was inhibited by knockdown of TET1 or TET3 and mimicked by overexpression of TET1 or TET3 in naive mice. Nociceptive behavior induced by formalin or overexpression of spinal TET1 or TET3 could be prevented by downregulation of miR-365-3p, and mimicked by overexpression of spinal miR-365-3p. Finally, we demonstrated that a potassium channel, voltage-gated eag-related subfamily H member 2 (Kcnh2), validated as a target of miR-365-3p, played a critical role in nociceptive modulation by spinal TET or miR-365-3p. Together, we concluded that TET-mediated hydroxymethylation of miR-365-3p regulates nociceptive behavior via Kcnh2.

SIGNIFICANCE STATEMENT

Mounting evidence indicates that epigenetic modifications in the nociceptive pathway contribute to pain processes and analgesia response. Here, we found that the increase of 5hmC content mediated by TET1 or TET3 in miR-365-3p promoter in the spinal cord is involved in nociceptive modulation through targeting a potassium channel, Kcnh2. Our study reveals a new epigenetic mechanism underlying nociceptive information processing, which may be a novel target for development of antinociceptive drugs.

Activation of neuronal Kv7/KCNQ/M-channels by the opener QO58-lysine and its anti-nociceptive effects on inflammatory pain in rodents

AIM

The aim of this study was to examine the activation of neuronal Kv7/KCNQ channels by a novel modified Kv7 opener QO58-lysine and to test the anti-nociceptive effects of QO58-lysine on inflammatory pain in rodent models.

METHODS

Assays including whole-cell patch clamp recordings, HPLC, and in vivo pain behavioral evaluations were employed.

RESULTS

QO58-lysine caused instant activation of Kv7.2/7.3 currents, and increasing the dose of QO58-lysine resulted in a dose-dependent activation of Kv7.2/Kv7.3 currents with an EC50 of 1.2±0.2 μmol/L. QO58-lysine caused a leftward shift of the voltage-dependent activation of Kv7.2/Kv7.3 to a hyperpolarized potential at V1/2=-54.4±2.5 mV from V1/2=-26.0±0.6 mV. The half-life in plasma (t1/2) was derived as 2.9, 2.7, and 3.0 h for doses of 12.5, 25, and 50 mg/kg, respectively. The absolute bioavailabilities for the three doses (12.5, 25, and 50 mg/kg) of QO58-lysine (po) were determined as 13.7%, 24.3%, and 39.3%, respectively. QO58-lysine caused a concentration-dependent reduction in the licking times during phase II pain induced by the injection of formalin into the mouse hindpaw. In the Complete Freund's adjuvant (CFA)-induced inflammatory pain model in rats, oral or intraperitoneal administration of QO58-lysine resulted in a dose-dependent increase in the paw withdrawal threshold, and the anti-nociceptive effect on mechanical allodynia could be reversed by the channel-specific blocker XE991 (3 mg/kg).

CONCLUSION

Taken together, our findings show that a modified QO58 compound (QO58-lysine) can specifically activate Kv7.2/7.3/M-channels. Oral or intraperitoneal administration of QO58-lysine, which has improved bioavailability and a half-life of approximately 3 h in plasma, can reverse inflammatory pain in rodent animal models.

Activation of peripheral KCNQ channels relieves gout pain

Supplemental Digital Content is Available in the Text.

Combining electrophysiology and in vivo pain models, the concept that activation of peripheral KCNQ channels relieves the gout pain is demonstrated.

Intense inflammatory pain caused by urate crystals in joints and other tissues is a major symptom of gout. Among therapy drugs that lower urate, benzbromarone (BBR), an inhibitor of urate transporters, is widely used because it is well tolerated and highly effective. We demonstrate that BBR is also an activator of voltage-gated KCNQ potassium channels. In cultured recombinant cells, BBR exhibited significant potentiation effects on KCNQ channels comparable to previously reported classical activators. In native dorsal root ganglion neurons, BBR effectively overcame the suppression of KCNQ currents, and the resultant neuronal hyperexcitability caused by inflammatory mediators, such as bradykinin (BK). Benzbromarone consistently attenuates BK-, formalin-, or monosodium urate–induced inflammatory pain in rat and mouse models. Notably, the analgesic effects of BBR are largely mediated through peripheral and not through central KCNQ channels, an observation supported both by pharmacokinetic studies and in vivo experiments. Moreover, multiple residues in the superficial part of the voltage sensing domain of KCNQ channels were identified critical for the potentiation activity of BBR by a molecular determinant investigation. Our data indicate that activation of peripheral KCNQ channels mediates the pain relief effects of BBR, potentially providing a new strategy for the development of more effective therapies for gout.

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.

Adiponectin-Mediated Analgesia and Anti-Inflammatory Effects in Rat

The adipose tissue-derived protein, adiponectin, has significant anti-inflammatory properties in a variety of disease conditions. Recent evidence that adiponectin and its receptors (AdipoR1 and AdipoR2) are expressed in central nervous system, suggests that it may also have a central modulatory role in pain and inflammation. This study set out to investigate the effects of exogenously applied recombinant adiponectin (via intrathecal and intraplantar routes; 10–5000 ng) on the development of peripheral inflammation (paw oedema) and pain hypersensitivity in the rat carrageenan model of inflammation. Expression of adiponectin, AdipoR1 and AdipoR2 mRNA and protein was characterised in dorsal spinal cord using real-time polymerase chain reaction (PCR) and Western blotting. AdipoR1 and AdipoR2 mRNA and protein were found to be constitutively expressed in dorsal spinal cord, but no change in mRNA expression levels was detected in response to carrageenan-induced inflammation. Adiponectin mRNA, but not protein, was detected in dorsal spinal cord, although levels were very low. Intrathecal administration of adiponectin, both pre- and 3 hours post-carrageenan, significantly attenuated thermal hyperalgesia and mechanical hypersensitivity. Intrathecal administration of adiponectin post-carrageenan also reduced peripheral inflammation. Intraplantar administration of adiponectin pre-carrageenan dose-dependently reduced thermal hyperalgesia but had no effect on mechanical hypersensitivity and peripheral inflammation. These results show that adiponectin functions both peripherally and centrally at the spinal cord level, likely through activation of AdipoRs to modulate pain and peripheral inflammation. These data suggest that adiponectin receptors may be a novel therapeutic target for pain modulation.

Thalamic Kv 7 channels: pharmacological properties and activity control during noxious signal processing

BACKGROUND AND PURPOSE

The existence of functional K(v)7 channels in thalamocortical (TC) relay neurons and the effects of the K(+)-current termed M-current (I(M)) on thalamic signal processing have long been debated. Immunocytochemical evidence suggests their presence in this brain region. Therefore, we aimed to verify their existence, pharmacological properties and function in regulating activity in neurons of the ventrobasal thalamus (VB).

EXPERIMENTAL APPROACH

Characterization of K(v)7 channels was performed by combining in vitro, in vivo and in silico techniques with a pharmacological approach. Retigabine (30 μM) and XE991 (20 μM), a specific K(v)7 channel enhancer and blocker, respectively, were applied in acute brain slices during electrophysiological recordings. The effects of intrathalamic injection of retigabine (3 mM, 300 nL) and/or XE991 (2 mM, 300 nL) were investigated in freely moving animals during hot-plate tests by recording behaviour and neuronal activity.

KEY RESULTS

K(v)7.2 and K(v)7.3 subunits were found to be abundantly expressed in TC neurons of mouse VB. A slow K(+)-current with properties of IM was activated by retigabine and inhibited by XE991. K(v)7 channel activation evoked membrane hyperpolarization, a reduction in tonic action potential firing, and increased burst firing in vitro and in computational models. Single-unit recordings and pharmacological intervention demonstrated a specific burst-firing increase upon I(M) activation in vivo. A K(v)7 channel-mediated increase in pain threshold was associated with fewer VB units responding to noxious stimuli, and increased burst firing in responsive neurons.

CONCLUSIONS AND IMPLICATIONS

K(v)7 channel enhancement alters somatosensory activity and may reflect an anti-nociceptive mechanism during acute pain processing.

A novel operant-based behavioral assay of mechanical allodynia in the orofacial region of rats

BACKGROUND

Detecting behaviors related to orofacial pain in rodent models often relies on subjective investigator grades or methods that place the animal in a stressful environment. In this study, an operant-based behavioral assay is presented for the assessment of orofacial tactile sensitivity in the rat.

NEW METHODS

In the testing chamber, rats are provided access to a sweetened condensed milk bottle; however, a 360° array of stainless steel wire loops impedes access. To receive the reward, an animal must engage the wires across the orofacial region. Contact with the bottle triggers a motor, requiring the animal to accept increasing pressure on the face during the test. To evaluate this approach, tolerated bottle distance was measured for 10 hairless Sprague Dawley rats at baseline and 30 min after application of capsaicin cream (0.1%) to the face. The experiment was repeated to evaluate the ability of morphine to reverse this effect.

RESULTS

The application of capsaicin cream reduced tolerated bottle distance measures relative to baseline (p<0.05). As long as morphine did not cause reduced participation due to sedation, subcutaneous morphine dosing reduced the effects of capsaicin (p<0.001). Comparison with existing method: For behavioral tests, experimenters often make subjective decisions of an animal's response. Operant methods can reduce these effects by measuring an animal's selection in a reward-conflict decision. Herein, a method to measure orofacial sensitivity is presented using an operant system.

CONCLUSIONS

This operant device allows for consistent measurement of heightened tactile sensitivity in the orofacial regions of the rat.

A continuous spinal cord stimulation model attenuates pain-related behavior in vivo following induction of a peripheral nerve injury

OBJECTIVES

Models that simulate clinical conditions are needed to gain an understanding of the mechanism involved during spinal cord stimulation (SCS) treatment of chronic neuropathic pain. An animal model has been developed for continuous SCS in which animals that have been injured to develop neuropathic pain behavior were allowed to carry on with regular daily activities while being stimulated for 72 hours.

MATERIAL AND METHODS

Sprague-Dawley rats were randomized into each of six different groups (N = 10-13). Three groups included animals in which the spared nerve injury (SNI) was induced. Animals in two of these groups were implanted with a four-contact electrode in the epidural space. Animals in one of these groups received stimulation for 72 hours continuously. Three corresponding sham groups (no SNI) were included. Mechanical and cold-thermal allodynia were evaluated using von Frey filaments and acetone drops, respectively. Mean withdrawal thresholds were compared. Statistical significance was established using one-way ANOVAs followed by Holm-Sidak post hoc analysis.

RESULTS

Continuous SCS attenuates mechanical allodynia in animals with neuropathic pain behavior. Mechanical withdrawal threshold increases significantly in SNI animals after 24 and 72 hours stimulation vs. SNI no stimulation (p = 0.007 and p < 0.001, respectively). SCS for 24 and 72 hours provides significant increase in mechanical withdrawal thresholds relative to values before stimulation (p = 0.001 and p < 0.001, respectively). Stimulation did not provide recovery to baseline values. SCS did not seem to attenuate cold-thermal allodynia.

CONCLUSION

A continuous SCS model has been developed. Animals with neuropathic pain behavior that were continuously stimulated showed significant increase in withdrawal thresholds proportional to stimulation time.

Development and pharmacological characterization of a model of sleep disruption-induced hypersensitivity in the rat

BACKGROUND

Sleep disturbance is a commonly reported co-morbidity in chronic pain patients, and conversely, disruption of sleep can cause acute and long-lasting hypersensitivity to painful stimuli. The underlying mechanisms of sleep disruption-induced pain hypersensitivity are poorly understood. Confounding factors of previous studies have been the sleep disruption protocols, such as the 'pedestal over water' or 'inverted flower pot' methods, that can cause large stress responses and therefore may significantly affect pain outcome measures.

METHODS

Sleep disruption was induced by placing rats for 8 h in a slowly rotating cylindrical cage causing arousal via the righting reflex. Mechanical (Von Frey filaments) and thermal (Hargreaves) nociceptive thresholds were assessed, and plasma corticosterone levels were measured (mass spectroscopy). Sleep disruption-induced hypersensitivity was pharmacologically characterized with drugs relevant for pain treatment, including gabapentin (30 mg/kg and 50 mg/kg), Ica-6p (Kv7.2/7.3 potassium channel opener; 10 mg/kg), ibuprofen (30 mg/kg and 100 mg/kg) and amitriptyline (10 mg/kg).

RESULTS

Eight hours of sleep disruption caused robust mechanical and heat hypersensitivity in the absence of a measurable change in plasma corticosterone levels. Gabapentin had no effect on reduced nociceptive thresholds. Ibuprofen attenuated mechanical thresholds, while Ica-6p and amitriptyline attenuated only reduced thermal nociceptive thresholds.

CONCLUSIONS

These results show that acute and low-stress sleep disruption causes mechanical and heat hypersensitivity in rats. Mechanical and heat hypersensitivity exhibited differential sensitivity to pharmacological agents, thus suggesting dissociable mechanisms for those two modalities. Ultimately, this model could help identify underlying mechanisms linking sleep disruption and hypersensitivity.

Possible involvement of nitric oxide modulatory mechanism in the protective effect of retigabine against spinal nerve ligation-induced neuropathic pain

Decreasing the hyperexcitability of neurons through opening of voltage-gated potassium (Kv7) channels has been suggested as one of the protective mechanisms in the effective management of neuropathic pain. Reactive oxygen/nitrogen species are well implicated in the pathophysiology of neuropathic pain. Further, M current generated by opening of voltage-gated potassium channels (Kv7) has been modulated by reactive oxygen/nitrogen species. The present study has been designed to elucidate the nitric oxide modulatory mechanism in the protective effect of retigabine against spinal nerve ligation-induced neuropathic pain in rats. Ligation of L5/L6 spinal nerves resulted in alterations in various behavioral (as evident from marked increase in thermal and mechanical hyperalgesia, and allodynia) and biochemical (raised lipid peroxidation, nitrite, and depletion of GSH, SOD, and catalase) cascades as compared to sham treatment. Administration of retigabine (10 mg/kg) for 28 days attenuated these behavioral and biochemical cascades as compared to control rats. Further, L-arginine (100 mg/kg) pretreatment with retigabine (5 mg/kg) significantly reversed the protective effect of retigabine in spinal nerve-ligated rats. However, L-NAME (10 mg/kg) pretreatment with retigabine (5 mg/kg) significantly potentiated their protective effects which were significant as compared to their effect per se, respectively. The present study highlights the possible involvement of nitric oxide modulatory mechanism in the protective effect of retigabine against L5/L6 spinal nerve ligation-induced behavioral and biochemical alterations in rats.

Retigabine calms seizure-induced behavior following status epilepticus

In adult rats, intraperitoneal injection of kainate (KA) results in sustained status epilepticus and persistent behavioral comorbidities such as hyperexcitability, anxiety, and altered response to environmental cues. Intrahippocampal KA also results in sustained status epilepticus and continuous high frequency oscillations in the electroencephalograph (EEG), although subsequent behavioral side effects are unknown. We hypothesized that retigabine, a recently discovered anticonvulsant and potent positive modulator of Kv7 channels, may attenuate seizure-induced behavioral abnormalities. Status epilepticus was induced by administration of KA either intraperitoneally (15 mg/kg) or by single intrahippocampal injection (1.0 μg/0.5 μL). After 24 h, half of systemically KA-treated animals that reached stage 6 seizures were injected once daily with retigabine (5 mg/kg) for 14 continuous days. All groups underwent three behavioral tests--capture and handling, open field, and elevated plus maze--24 h following the last retigabine treatment and were sacrificed at 25-28 days. In the capture and handling test, systemic KA treatment resulted in frisky behavior and resistance to capture with wild attempts to escape during the 1st, 2nd, and 3rd weeks of the observation period. In contrast, these behaviors were attenuated in KA+retigabine-treated animals. In the open-field test, KA-treated animals spent more time in the center zone, but KA+retigabine-treated rats had greater overall activity compared with those having vehicle, KA, or retigabine-only treatment. In the elevated plus maze, KA+retigabine-treated animals traveled greater distances in open and closed arms (proximal and distal) compared with controls, also signifying anxiety reduction. Retigabine-only-treated rats traveled more in the open proximal arms compared with controls, indicating increased hyperlocomotion in normotensive rats. Although treatment with KA+retigabine resulted in anxiolytic-like effects in all three behavioral tasks compared with vehicle, this group did not significantly differ from systemically KA-treated rats in most measurements in open-field and elevated plus maze tasks, suggesting that retigabine may also cause hyperlocomotion unrelated to anxiety level. Despite that intrahippocampal KA-treated rats displayed comparable seizure behavior, epileptiform activity, and hippocampal injury, their behavior resembled the controls, suggesting that molecular and subsequent cellular changes are also partially responsible for anxiolytic-like effects and that these results are likely independent of the hippocampus.

Activation of peripheral KCNQ channels attenuates inflammatory pain

Background

Refractory chronic pain dramatically reduces the quality of life of patients. Existing drugs cannot fully achieve effective chronic pain control because of their lower efficacy and/or accompanying side effects. Voltage-gated potassium channels (KCNQ) openers have demonstrated their analgesic effect in preclinical and clinical studies, and are thus considered to be a potential therapeutic target as analgesics. However, these drugs exhibit a narrow therapeutic window due to their imposed central nerve system (CNS) side effects. To clarify the analgesic effect by peripheral KCNQ channel activation, we investigated whether the analgesic effect of the KCNQ channel opener, retigabine, is inhibited by intracerebroventricular (i.c.v.) administration of the KCNQ channel blocker, 10, 10-bis (4-Pyridinylmethyl)-9(10H) -anthracenone dihydrochloride (XE-991) in rats.

Results

Oral administration (p.o.) of retigabine showed an anticonvulsant effect on maximal electronic seizures and an analgesic effect on complete Freund’s adjuvant-induced thermal hyperalgesia. However, impaired motor coordination and reduced exploratory behavior were also observed at the analgesic doses of retigabine. Administration (i.c.v.) of XE-991 reversed the retigabine-induced anticonvulsant effect, impaired motor coordination and reduced exploratory behavior but not the analgesic effect. Moreover, intraplantar administration of retigabine or an additional KCNQ channel opener, N-(6-Chloro-pyridin-3-yl)-3,4-difluoro-benzamide (ICA-27243), inhibited formalin-induced nociceptive behavior.

Conclusions

Our findings suggest that the peripheral sensory neuron is the main target for KCNQ channel openers to induce analgesia. Therefore, peripheral KCNQ channel openers that do not penetrate the CNS may be suitable analgesic drugs as they would prevent CNS side effects.

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.

Developing Effective and Efficient care pathways in chronic Pain: DEEP study protocol

BACKGROUND

Pain affecting the face or mouth and lasting longer than three months ("chronic orofacial pain", COFP) is relatively common in the UK. This study aims to describe and model current care pathways for COFP patients, identify areas where current pathways could be modified, and model whether these changes would improve outcomes for patients and use resources more efficiently.

METHODS/DESIGN

The study takes a prospective operations research approach. A cohort of primary and secondary care COFP patients (n = 240) will be recruited at differing stages of their care in order to follow and analyse their journey through care. The cohort will be followed for two years with data collected at baseline 6, 12, 18, and 24 months on: 1) experiences of the care pathway and its impacts; 2) quality of life; 3) pain; 4) use of health services and costs incurred; 5) illness perceptions. Qualitative in-depth interviews will be used to collect data on patient experiences from a purposive sub-sample of the total cohort (n = 30) at baseline, 12 and 24 months. Four separate appraisal groups (public, patient, clincian, service manager/commissioning) will then be given data from the pathway analysis and asked to determine their priority areas for change. The proposals from appraisal groups will inform an economic modelling exercise. Findings from the economic modelling will be presented as incremental costs, Quality Adjusted Life Years (QALYs), and the incremental cost per QALY gained. At the end of the modelling a series of recommendations for service change will be available for implementation or further trial if necessary.

DISCUSSION

The recent white paper on health and the report from the NHS Forum identified chronic conditions as priority areas and whilst technology can improve outcomes, so can simple, appropriate and well-defined clinical care pathways. Understanding the opportunity cost related to care pathways benefits the wider NHS. This research develops a method to help design efficient systems built around one condition (COFP), but the principles should be applicable to a wide range of other chronic and long-term conditions.

TRPV1 channel-mediated bilateral allodynia induced by unilateral masseter muscle inflammation in rats

Pain in masticatory muscles is among the most prominent symptoms of temperomandibular disorders (TMDs) that have diverse and complex etiology. A common complaint of TMD is that unilateral pain of craniofacial muscle can cause a widespread of bilateral pain sensation, although the underlying mechanism remains unknown. To investigate whether unilateral inflammation of masseter muscle can cause a bilateral allodynia, we generated masseter muscle inflammation induced by unilateral injection of complete Freund’s adjuvant (CFA) in rats, and measured the bilateral head withdrawal threshold at different time points using a von Frey anesthesiometer. After behavioral assessment, both right and left trigeminal ganglia (TRG) were dissected and examined for histopathology and transient receptor potential vanilloid 1 (TRPV1) mRNA expression using quantitative real-time PCR analysis. A significant increase in TRPV1 mRNA expression occurred in TRG ipsilateral to CFA injected masseter muscle, whereas no significant alteration in TRPV1 occurred in the contralateral TRG. Interestingly, central injection of TRPV1 antagonist 5-iodoresiniferatoxin into the hippocampus significantly attenuated the head withdrawal response of both CFA injected and non-CFA injected contralateral masseter muscle. Our findings show that unilateral inflammation of masseter muscle is capable of inducing bilateral allodynia in rats. Upregulation of TRPV1 at the TRG level is due to nociception caused by inflammation, whereas contralateral nocifensive behavior in masticatory muscle nociception is likely mediated by central TRPV1, pointing to the involvement of altered information processing in higher centers.

Potassium channels in peripheral pain pathways: expression, function and therapeutic potential

Electrical excitation of peripheral somatosensory nerves is a first step in generation of most pain signals in mammalian nervous system. Such excitation is controlled by an intricate set of ion channels that are coordinated to produce a degree of excitation that is proportional to the strength of the external stimulation. However, in many disease states this coordination is disrupted resulting in deregulated peripheral excitability which, in turn, may underpin pathological pain states (i.e. migraine, neuralgia, neuropathic and inflammatory pains). One of the major groups of ion channels that are essential for controlling neuronal excitability is potassium channel family and, hereby, the focus of this review is on the K+ channels in peripheral pain pathways. The aim of the review is threefold. First, we will discuss current evidence for the expression and functional role of various K+ channels in peripheral nociceptive fibres. Second, we will consider a hypothesis suggesting that reduced functional activity of K+ channels within peripheral nociceptive pathways is a general feature of many types of pain. Third, we will evaluate the perspectives of pharmacological enhancement of K+ channels in nociceptive pathways as a strategy for new analgesic drug design.

Comprehensive RNA-Seq expression analysis of sensory ganglia with a focus on ion channels and GPCRs in Trigeminal ganglia

The specific functions of sensory systems depend on the tissue-specific expression of genes that code for molecular sensor proteins that are necessary for stimulus detection and membrane signaling. Using the Next Generation Sequencing technique (RNA-Seq), we analyzed the complete transcriptome of the trigeminal ganglia (TG) and dorsal root ganglia (DRG) of adult mice. Focusing on genes with an expression level higher than 1 FPKM (fragments per kilobase of transcript per million mapped reads), we detected the expression of 12984 genes in the TG and 13195 in the DRG. To analyze the specific gene expression patterns of the peripheral neuronal tissues, we compared their gene expression profiles with that of the liver, brain, olfactory epithelium, and skeletal muscle. The transcriptome data of the TG and DRG were scanned for virtually all known G-protein-coupled receptors (GPCRs) as well as for ion channels. The expression profile was ranked with regard to the level and specificity for the TG. In total, we detected 106 non-olfactory GPCRs and 33 ion channels that had not been previously described as expressed in the TG. To validate the RNA-Seq data, in situ hybridization experiments were performed for several of the newly detected transcripts. To identify differences in expression profiles between the sensory ganglia, the RNA-Seq data of the TG and DRG were compared. Among the differentially expressed genes (> 1 FPKM), 65 and 117 were expressed at least 10-fold higher in the TG and DRG, respectively. Our transcriptome analysis allows a comprehensive overview of all ion channels and G protein-coupled receptors that are expressed in trigeminal ganglia and provides additional approaches for the investigation of trigeminal sensing as well as for the physiological and pathophysiological mechanisms of pain.

Axonal voltage-gated ion channels as pharmacological targets for pain

Upon peripheral nerve injury (caused by trauma or disease process) axons of the dorsal root ganglion (DRG) somatosensory neurons have the ability to sprout and regrow/remyelinate to reinnervate distant target tissue or form a tangled scar mass called a neuroma. This regenerative response can become maladaptive leading to a persistent and debilitating pain state referred to as chronic pain corresponding to the clinical description of neuropathic/chronic inflammatory pain. There is little agreement to what causes peripheral chronic pain other than hyperactivity of the nociceptive DRG neurons which ultimately depends on the function of voltage-gated ion channels. This review focuses on the pharmacological modulators of voltage-gated ion channels known to be present on axonal membrane which represents by far the largest surface of DRG neurons. Blockers of voltage-gated Na(+) channels, openers of voltage-gated K(+) channels and blockers of hyperpolarization-activated cyclic nucleotide-gated channels that were found to reduce neuronal activity were also found to be effective in neuropathic and inflammatory pain states. The isoforms of these channels present on nociceptive axons have limited specificity. The rationale for considering axonal voltage-gated ion channels as targets for pain treatment comes from the accumulating evidence that chronic pain states are associated with a dysregulation of these channels that could alter their specificity and make them more susceptible to pharmacological modulation. This drives the need for further development of subtype-specific voltage-gated ion channels modulators, as well as clinically available neurophysiological techniques for monitoring axonal ion channel function in peripheral nerves.

Evaluating dermal myelinated nerve fibers in skin biopsy

Although there has been extensive research on small, unmyelinated fibers in the skin, little research has investigated dermal myelinated fibers in comparison. Glabrous, nonhairy skin contains mechanoreceptors that afford a vantage point for observation of myelinated fibers that have previously been seen only with invasively obtained nerve biopsies. This review discusses current morphometric and molecular expression data of normative and pathogenic glabrous skin obtained by various processing and analysis methods for cutaneous myelinated fibers. Recent publications have shed light on the role of glabrous skin biopsy in identifying signs of peripheral neuropathy and as a potential biomarker of distal myelin and mechanoreceptor integrity. The clinical relevance of a better understanding of the role of dermal myelinated nerve terminations in peripheral neuropathy will be addressed in light of recent publications in the growing field of skin biopsy.

Visceral hyperalgesia induced by forebrain-specific suppression of native Kv7/KCNQ/M-current in mice

BACKGROUND

Dysfunction of brain-gut interaction is thought to underlie visceral hypersensitivity which causes unexplained abdominal pain syndromes. However, the mechanism by which alteration of brain function in the brain-gut axis influences the perception of visceral pain remains largely elusive. In this study we investigated whether altered brain activity can generate visceral hyperalgesia.

RESULTS

Using a forebrain specific αCaMKII promoter, we established a line of transgenic (Tg) mice expressing a dominant-negative pore mutant of the Kv7.2/KCNQ2 channel which suppresses native KCNQ/M-current and enhances forebrain neuronal excitability. Brain slice recording of hippocampal pyramidal neurons from these Tg mice confirmed the presence of hyperexcitable properties with increased firing. Behavioral evaluation of Tg mice exhibited increased sensitivity to visceral pain induced by intraperitoneal (i.p.) injection of either acetic acid or magnesium sulfate, and intracolon capsaicin stimulation, but not cutaneous sensation for thermal or inflammatory pain. Immunohistological staining showed increased c-Fos expression in the somatosensory SII cortex and insular cortex of Tg mice that were injected intraperitoneally with acetic acid. To mimic the effect of cortical hyperexcitability on visceral hyperalgesia, we injected KCNQ/M channel blocker XE991 into the lateral ventricle of wild type (WT) mice. Intracerebroventricular injection of XE991 resulted in increased writhes of WT mice induced by acetic acid, and this effect was reversed by co-injection of the channel opener retigabine.

CONCLUSIONS

Our findings provide evidence that forebrain hyperexcitability confers visceral hyperalgesia, and suppression of central hyperexcitability by activation of KCNQ/M-channel function may provide a therapeutic potential for treatment of abdominal pain syndromes.

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

Previous studies in several different trigeminal nerve injury/inflammation models indicated that the hyperexcitability of primary afferent neurons contributes to the pain pathway underlying mechanical allodynia. Although multiple types of voltage-gated ion channels are associated with neuronal hyperexcitability, voltage-gated K⁺ channels (Kv) are one of the important physiological regulators of membrane potentials in excitable tissues, including nociceptive sensory neurons. Since the opening of K⁺ channels leads to hyperpolarization of cell membrane and a consequent decrease in cell excitability, several Kv channels have been proposed as potential target candidates for pain therapy. In this review, we focus on common changes measured in the Kv channels of several different trigeminal neuropathic/inflammatory pain animal models, particularly the relationship between changes in Kv channels and the excitability of trigeminal ganglion (TRG) neurons. We also discuss the potential of Kv channel openers as therapeutic agents for trigeminal neuropathic/inflammatory pain, such as mechanical allodynia.