KCNQ2/3/5 channels in dorsal root ganglion neurons can be therapeutic targets of neuropathic pain in diabetic 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.

Disease-modifying rdHSV-CA8* non-opioid analgesic gene therapy treats chronic osteoarthritis pain by activating Kv7 voltage-gated potassium channels

Chronic pain is common in our population, and most of these patients are inadequately treated, making the development of safer analgesics a high priority. Knee osteoarthritis (OA) is a primary cause of chronic pain and disability worldwide, and lower extremity OA is a major contributor to loss of quality-adjusted life-years. In this study we tested the hypothesis that a novel JDNI8 replication-defective herpes simplex-1 viral vector (rdHSV) incorporating a modified carbonic anhydrase-8 transgene (CA8*) produces analgesia and treats monoiodoacetate-induced (MIA) chronic knee pain due to OA. We observed transduction of lumbar DRG sensory neurons with these viral constructs (vHCA8*) (~40% of advillin-positive cells and ~ 50% of TrkA-positive cells colocalized with V5-positive cells) using the intra-articular (IA) knee joint (KJ) route of administration. vHCA8* inhibited chronic mechanical OA knee pain induced by MIA was dose- and time-dependent. Mechanical thresholds returned to Baseline by D17 after IA KJ vHCA8* treatment, and exceeded Baseline (analgesia) through D65, whereas negative controls failed to reach Baseline responses. Weight-bearing and automated voluntary wheel running were improved by vHCA8*, but not negative controls. Kv7 voltage-gated potassium channel-specific inhibitor XE-991 reversed vHCA8*-induced analgesia. Using IHC, IA KJ of vHCA8* activated DRG Kv7 channels via dephosphorylation, but negative controls failed to impact Kv7 channels. XE-991 stimulated Kv7.2-7.5 and Kv7.3 phosphorylation using western blotting of differentiated SH-SY5Y cells, which was inhibited by vHCA8* but not by negative controls. The observed prolonged dose-dependent therapeutic effects of IA KJ administration of vHCA8* on MIA-induced chronic KJ pain due to OA is consistent with the specific activation of Kv7 channels in small DRG sensory neurons. Together, these data demonstrate for the first-time local IA KJ administration of vHCA8* produces opioid-independent analgesia in this MIA-induced OA chronic pain model, supporting further therapeutic development.

Kv7 Channel Activators Flupirtine and ML213 Alleviate Neuropathic Pain Behavior in the Streptozotocin Rat Model of Diabetic Neuropathy

Background & Objective

Chronic peripheral neuropathic pain (PNP) is a debilitating condition that is associated with many types of injury/diseases, including diabetes mellitus. Patients with longstanding diabetes develop diabetic PNP (DPNP), which is resilient to currently available drugs. The underlying molecular mechanisms of DPNP are still illusive, but Kv7 channels that have been implicated in the pathogenesis of various types of chronic pain are likely to be involved. Indeed, using the streptozotocin (STZ) rat model of DPNP, we have previously shown that Kv7 activation with their non-selective activator retigabine attenuated neuropathic pain behavior suggesting that these channels are implicated in DPNP pathogenesis. Here, we evaluated, in the same STZ model, whether the more potent and more selective Kv7 channel openers flupirtine and ML213 attenuate STZ-induced pain hypersensitivity.

Methods

Male Sprague Dawley rats (250-300 g) were used. The STZ model involved a single injection of STZ (60 mg/kg, i.p.). Behavioral testing for mechanical and heat pain sensitivity was performed using a dynamic plantar aesthesiometer and Hargreaves analgesiometer, respectively.

Results

STZ rats exhibited behavioral signs of mechanical and heat hypersensitivity as indicated by significant decreases in the mean paw withdrawal threshold (PWT) and mean paw withdrawal latency (PWL), respectively, at 35 days post-STZ treatment. Single injections of flupirtine (10 mg/kg, i.p.) and ML213 (5 mg/kg, i.p.) to STZ rats (35-days after STZ treatment) caused significant increases in the mean PWT, but not PWL, indicating attenuation of mechanical, but not heat hypersensitivity. Both flupirtine and ML213 were as effective as the positive control gabapentin (10/kg, i.p.), and their anti-allodynic effects were prevented by the Kv7 channel-specific blocker XE991 (3 mg/kg, i.p.).

Conclusion

The findings suggest that Kv7 channels are involved in the mechanisms of mechanical but not heat hypersensitivity associated with DPNP, and that their activation may prove to be effective in alleviating DPNP symptoms.

Painful diabetic neuropathy: The role of ion channels

Painful diabetic neuropathy (PDN) is a common chronic complication of diabetes that causes neuropathic pain and negatively affects the quality of life. The management of PDN is far from satisfactory. At present, interventions are primarily focused on symptomatic treatment. Ion channel disorders are a major cause of PDN, and a complete understanding of their roles and mechanisms may provide better options for the clinical treatment of PDN. Therefore, this review summarizes the important role of ion channels in PDN and the current drug development targeting these ion channels.

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.

Diabetes-induced electrophysiological alterations on neurosomes in ganglia of peripheral nervous system

Diabetes mellitus (DM) leads to medical complications, the epidemiologically most important of which is diabetic peripheral neuropathy (DPN). Electrophysiology is a major component of neural functioning and several studies have been undertaken to elucidate the neural electrophysiological alterations caused by DM and their mechanisms of action. Due to the importance of electrophysiology for neuronal function, the review of the studies dealing predominantly with electrophysiological parameters and mechanisms in the neuronal somata of peripheral neural ganglia of diabetic animals during the last 45 years is here undertaken. These studies, using predominantly techniques of electrophysiology, most frequently patch clamp for voltage clamp studies of transmembrane currents through ionic channels, have investigated the experimental DPN. They also have demonstrated that various cellular and molecular mechanisms of action of diabetic physiopathology at the level of biophysical electrical parameters are affected in DPN. Thus, they have demonstrated that several passive and active transmembrane voltage parameters, related to neuronal excitability and neuronal functions, are altered in diabetes. The majority of the studies agreed that DM produces depolarization of the resting membrane potential; alters excitability, increasing and decreasing it in dorsal root ganglia (DRG) and in nodose ganglion, respectively. They have tried to relate these changes to sensorial alterations of DPN. Concerning ionic currents, predominantly studied in DRG, the most frequent finding was increases in Na+, Ca2+, and TRPV1 cation current, and decreases in K+ current. This review concluded that additional studies are needed before an understanding of the hierarchized, time-dependent, and integrated picture of the contribution of neural electrophysiological alterations to the DPN could be reached. DM-induced electrophysiological neuronal alterations that so far have been demonstrated, most of them likely important, are either consistent with the DPN symptomatology or suggest important directions for improvement of the elucidation of DPN physiopathology, which the continuation seems to us very relevant.

Inflammation differentially controls transport of depolarizing Nav versus hyperpolarizing Kv channels to drive rat nociceptor activity

Inflammation causes pain by shifting the balance of ionic currents in nociceptors toward depolarization, leading to hyperexcitability. The ensemble of ion channels within the plasma membrane is regulated by processes including biogenesis, transport, and degradation. Thus, alterations in ion channel trafficking may influence excitability. Sodium channel NaV1.7 and potassium channel KV7.2 promote and oppose excitability in nociceptors, respectively. We used live-cell imaging to investigate mechanisms by which inflammatory mediators (IM) modulate the abundance of these channels at axonal surfaces through transcription, vesicular loading, axonal transport, exocytosis, and endocytosis. Inflammatory mediators induced a NaV1.7-dependent increase in activity in distal axons. Further, inflammation increased the abundance of NaV1.7, but not of KV7.2, at axonal surfaces by selectively increasing channel loading into anterograde transport vesicles and insertion at the membrane, without affecting retrograde transport. These results uncover a cell biological mechanism for inflammatory pain and suggest NaV1.7 trafficking as a potential therapeutic target.

Analgesic Action of Acetaminophen via Kv7 Channels

The mechanism of acetaminophen (APAP) analgesia is at least partially unknown. Previously, we showed that the APAP metabolite N-acetyl-p-benzoquinone imine (NAPQI) activated Kv7 channels in neurons in vitro, and this activation of Kv7 channels dampened neuronal firing. Here, the effect of the Kv7 channel blocker XE991 on APAP-induced analgesia was investigated in vivo. APAP had no effect on naive animals. Induction of inflammation with λ-carrageenan lowered mechanical and thermal thresholds. Systemic treatment with APAP reduced mechanical hyperalgesia, and co-application of XE991 reduced APAP's analgesic effect on mechanical pain. In a second experiment, the analgesic effect of systemic APAP was not antagonized by intrathecal XE991 application. Analysis of liver samples revealed APAP and glutathione-coupled APAP indicative of metabolization. However, there were no relevant levels of these metabolites in cerebrospinal fluid, suggesting no relevant APAP metabolite formation in the CNS. In summary, the results support an analgesic action of APAP by activating Kv7 channels at a peripheral site through formation of the metabolite NAPQI.

Combinations of classical and non-classical voltage dependent potassium channel openers suppress nociceptor discharge and reverse chronic pain signs in a rat model of Gulf War illness

In a companion paper we examined whether combinations of K_v7 channel openers (Retigabine and Diclofenac; RET, DIC) could be effective modifiers of deep tissue nociceptor activity; and whether such combinations could then be optimized for use as safe analgesics for pain-like signs that developed in a rat model of GWI (Gulf War Illness) pain. In the present report, we examined the combinations of Retigabine/Meclofenamate (RET/MEC) and Meclofenamate/Diclofenac (MEC/DIC). Voltage clamp experiments were performed on deep tissue nociceptors isolated from rat DRG (dorsal root ganglion). In voltage clamp studies, a stepped voltage protocol was applied (-55 to -40 mV; V_h=-60 mV; 1500 msec) and K_v7 evoked currents were subsequently isolated by Linopirdine subtraction. MEC greatly enhanced voltage dependent conductance and produced exceptional maximum sustained currents of 6.01 ± 0.26 pA/pF (EC₅₀: 62.2 ± 8.99 μM). Combinations of RET/MEC, and MEC/DIC substantially amplified resting currents at low concentrations. MEC/DIC also greatly improved voltage dependent conductance. In current clamp experiments, a cholinergic challenge test (Oxotremorine-M, 10 μM; OXO), associated with our GWI rat model, produced powerful action potential (AP) bursts (85 APs). Optimized combinations of RET/MEC (5 and 0.5 μM) and MEC/DIC (0.5 and 2.5 μM) significantly reduced AP discharges to 3 and 7 Aps, respectively. Treatment of pain-like ambulatory behavior in our rat model with a RET/MEC combination (5 and 0.5 mg/kg) successfully rescued ambulation deficits, but could not be fully separated from the effect of RET alone. Further development of this approach is recommended.

Derivation of nociceptive sensory neurons from hiPSCs with early patterning and temporally controlled NEUROG2 overexpression

Despite development of protocols to differentiate human pluripotent stem cells (hPSCs), those used to produce sensory neurons remain difficult to replicate and result in heterogenous populations. There is a growing clinical burden of chronic pain conditions, highlighting the need for relevant human cellular models. This study presents a hybrid differentiation method to produce nociceptive sensory neurons from hPSCs. Lines harboring an inducible NEUROG2 construct were patterned toward precursors with small molecules followed by NEUROG2 overexpression. Neurons expressed key markers, including BRN3A and ISL1, with single-cell RNA sequencing, revealing populations of nociceptors expressing SCN9A and TRP channels. Physiological profiling with multi-electrode arrays revealed that neurons responded to noxious stimuli, including capsaicin. Finally, we modeled pain-like states to identify genes and pathways involved in pain transduction. This study presents an optimized method to efficiently produce nociceptive sensory neurons and provides a tool to aid development of chronic pain research.

Development of KVO treatment strategies for chronic pain in a rat model of Gulf War Illness

We examined whether combinations of K_v7 channel openers could be effective modifiers of deep tissue nociceptor activity; and whether such combinations could then be optimized for use as safe analgesics for pain-like signs that developed in a rat model of GWI (Gulf War Illness) pain. Voltage clamp experiments were performed on subclassified nociceptors isolated from rat DRG (dorsal root ganglion). A stepped voltage protocol was applied (-55 to -40 mV; V_h = -60 mV; 1500 ms) and K_v7 evoked currents were subsequently isolated by linopirdine subtraction. Directly activated and voltage activated K⁺ currents were characterized in the presence and absence of Retigabine (5-100 μM) and/or Diclofenac (50-140 μM). Retigabine produced substantial voltage dependent effects and a maximal sustained current of 1.14 pA/pF ± 0.15 (ED₅₀: 62.7 ± 3.18 μM). Diclofenac produced weak voltage dependent effects but a similar maximum sustained current of 1.01 ± 0.26 pA/pF (ED₅₀: 93.2 ± 8.99 μM). Combinations of Retigabine and Diclofenac substantially amplified resting currents but had little effect on voltage dependence. Using a cholinergic challenge test (Oxotremorine, 10 μM) associated with our GWI rat model, combinations of Retigabine (5 uM) and Diclofenac (2.5, 20 and 50 μM) substantially reduced or totally abrogated action potential discharge to the cholinergic challenge. When combinations of Retigabine and Diclofenac were used to relieve pain-signs in our rat model of GWI, only those combinations associated with serious subacute side effects could relieve pain-like behaviors.

Scorpion Venom peptide, AGAP inhibits TRPV1 and potentiates the analgesic effect of lidocaine

The current study was designed to test the hypothesis that BmK AGAP (AGAP) potentiates the analgesic effect of lidocaine. The chronic constrictive injury was performed on 72 rats to induce a rapid onset and long-lasting pain. The rats were randomly assigned to one of six groups; Group A (n = 12) received an intrathecal administration of saline, Group B (n = 12) received an intrathecal injection of lidocaine, Group C (n = 12) received an intrathecal administration of AGAP, Group D, E, and F (n = 12 each) received an intrathecal administration of lidocaine 0.005 mg/ml + AGAP 25, 50, 100 μg/kg respectively. The von Frey filaments were used to assess mechanical allodynia. Nav1.7 and TRPV1 currents were recorded by the whole-cell aspiration patch-clamp technique, and KCNQ2/3 currents were recorded by the whole-cell drilling patch-clamp technique. The whole-cell aspiration patch-clamp technique showed that AGAP inhibited TRPV1and KCNQ2/3 currents and increased the analgesic effect of lidocaine. AGAP may have a synergistic effect with lidocaine which demonstrates a potential therapeutic approach for optimizing post-operative analgesia.

Posttranscriptional modulation of KCNQ2 gene expression by the miR-106b microRNA family

MicroRNAs (miRNAs) have recently emerged as important regulators of ion channel expression. We show here that select miR-106b family members repress the expression of the KCNQ2 K⁺ channel protein by binding to the 3'-untranslated region of KCNQ2 messenger RNA. During the first few weeks after birth, the expression of miR-106b family members rapidly decreases, whereas KCNQ2 protein level inversely increases. Overexpression of miR-106b mimics resulted in a reduction in KCNQ2 protein levels. Conversely, KCNQ2 levels were up-regulated in neurons transfected with antisense miRNA inhibitors. By constructing more specific and stable forms of miR-106b controlling systems, we further confirmed that overexpression of precursor-miR-106b-5p led to a decrease in KCNQ current density and an increase in firing frequency of hippocampal neurons, while tough decoy miR-106b-5p dramatically increased current density and decreased neuronal excitability. These results unmask a regulatory mechanism of KCNQ2 channel expression in early postnatal development and hint at a role for miR-106b up-regulation in the pathophysiology of epilepsy.

Roles of AMPK and Its Downstream Signals in Pain Regulation

Pain is an unpleasant sensory and emotional state that decreases quality of life. A metabolic sensor, adenosine monophosphate-activated protein kinase (AMPK), which is ubiquitously expressed in mammalian cells, has recently attracted interest as a new target of pain research. Abnormal AMPK expression and function in the peripheral and central nervous systems are associated with various types of pain. AMPK and its downstream kinases participate in the regulation of neuron excitability, neuroinflammation and axonal and myelin regeneration. Numerous AMPK activators have reduced pain behavior in animal models. The current understanding of pain has been deepened by AMPK research, but certain issues, such as the interactions of AMPK at each step of pain regulation, await further investigation. This review examines the roles of AMPK and its downstream kinases in neurons and non-neuronal cells, as well as their contribution to pain regulation.

Nerve Excitability and Neuropathic Pain is Reduced by BET Protein Inhibition After Spared Nerve Injury

Neuropathic pain is a common disability produced by enhanced neuronal excitability after nervous system injury. The pathophysiological changes that underlie the generation and maintenance of neuropathic pain require modifications of transcriptional programs. In particular, there is an induction of pro-inflammatory neuromodulators levels, and changes in the expression of ion channels and other factors intervening in the determination of the membrane potential in neuronal cells. We have previously found that inhibition of the BET proteins epigenetic readers reduced neuroinflammation after spinal cord injury. Within the present study we aimed to determine if BET protein inhibition may also affect neuroinflammation after a peripheral nerve injury, and if this would beneficially alter neuronal excitability and neuropathic pain. For this purpose, C57BL/6 female mice underwent spared nerve injury (SNI), and were treated with the BET inhibitor JQ1, or vehicle. Electrophysiological and algesimetry tests were performed on these mice. We also determined the effects of JQ1 treatment after injury on neuroinflammation, and the expression of neuronal components important for the maintenance of axon membrane potential. We found that treatment with JQ1 affected neuronal excitability and mechanical hyperalgesia after SNI in mice. BET protein inhibition regulated cytokine expression and reduced microglial reactivity after injury. In addition, JQ1 treatment altered the expression of SCN3A, SCN9A, KCNA1, KCNQ2, KCNQ3, HCN1 and HCN2 ion channels, as well as the expression of the Na⁺/K⁺ ATPase pump subunits. In conclusion, both, alteration of inflammation, and neuronal transcription, could be the responsible epigenetic mechanisms for the reduction of excitability and hyperalgesia observed after BET inhibition. Inhibition of BET proteins is a promising therapy for reducing neuropathic pain after neural injury.

Physiology and Therapeutic Potential of SK, H, and M Medium AfterHyperPolarization Ion Channels

SK, HCN, and M channels are medium afterhyperpolarization (mAHP)-mediating ion channels. The three channels co-express in various brain regions, and their collective action strongly influences cellular excitability. However, significant diversity exists in the expression of channel isoforms in distinct brain regions and various subcellular compartments, which contributes to an equally diverse set of specific neuronal functions. The current review emphasizes the collective behavior of the three classes of mAHP channels and discusses how these channels function together although they play specialized roles. We discuss the biophysical properties of these channels, signaling pathways that influence the activity of the three mAHP channels, various chemical modulators that alter channel activity and their therapeutic potential in treating various neurological anomalies. Additionally, we discuss the role of mAHP channels in the pathophysiology of various neurological diseases and how their modulation can alleviate some of the symptoms.

Chemical modulation of Kv7 potassium channels

The rising interest in Kv7 modulators originates from their ability to evoke fundamental electrophysiological perturbations in a tissue-specific manner. A large number of therapeutic applications are, in part, based on the clinical experience with two broad-spectrum Kv7 agonists, flupirtine and retigabine. Since precise molecular structures of human Kv7 channel subtypes in closed and open states have only very recently started to emerge, computational studies have traditionally been used to analyze binding modes and direct the development of more potent and selective Kv7 modulators with improved safety profiles. Herein, the synthetic and medicinal chemistry of small molecule modulators and the representative biological properties are summarized. Furthermore, new therapeutic applications supported by in vitro and in vivo assay data are suggested.

Diabetic neuropathy and neuropathic pain: a (con)fusion of pathogenic mechanisms?

Neuropathy is a common complication of long-term diabetes that impairs quality of life by producing pain, sensory loss and limb amputation. The presence of neuropathy in both insulin-deficient (type 1) and insulin resistant (type 2) diabetes along with the slowing of progression of neuropathy by improved glycemic control in type 1 diabetes has caused the majority of preclinical and clinical investigations to focus on hyperglycemia as the initiating pathogenic lesion. Studies in animal models of diabetes have identified multiple plausible mechanisms of glucotoxicity to the nervous system including post-translational modification of proteins by glucose and increased glucose metabolism by aldose reductase, glycolysis and other catabolic pathways. However, it is becoming increasingly apparent that factors not necessarily downstream of hyperglycemia can also contribute to the incidence, progression and severity of neuropathy and neuropathic pain. For example, peripheral nerve contains insulin receptors that transduce the neurotrophic and neurosupportive properties of insulin, independent of systemic glucose regulation, while the detection of neuropathy and neuropathic pain in patients with metabolic syndrome and failure of improved glycemic control to protect against neuropathy in cohorts of type 2 diabetic patients has placed a focus on the pathogenic role of dyslipidemia. This review provides an overview of current understanding of potential initiating lesions for diabetic neuropathy and the multiple downstream mechanisms identified in cell and animal models of diabetes that may contribute to the pathogenesis of diabetic neuropathy and neuropathic pain.

Changes in expression of Kv7.5 and Kv7.2 channels in dorsal root ganglion neurons in the streptozotocin rat model of painful diabetic neuropathy

Diabetic peripheral neuropathic pain (DPNP), the most debilitating complication of diabetes mellitus, is resistant to current therapy. The pathogenesis of DPNP is still elusive, but several mechanisms have been proposed including abnormal hyperexcitability of dorsal root ganglion (DRG) neurons. The underlying molecular mechanisms of such aberrant hyperexcitability are incompletely understood. Using the streptozotocin (STZ) rat model of DPNP, we have recently provided evidence implicating neuronal K_v7 channels that normally exert a powerful stabilizing influence on neuronal excitability, in the abnormal hyperexcitability of DRG neurons and in pain hypersensitivity associated with DPNP. In the present immunohistochemical study, we sought to determine whether K_v7.2 and/or K_v7.5 channel expression is altered in DRG neurons in STZ rats. We found 35 days post-STZ: (1) a significant decrease in K_v7.5-immunoreactivity in small (<30 μm) DRG neurons (both IB4 positive and IB4 negative) and medium-sized (30-40 μm) neurons, and (2) a significant increase in K_v7.2-immunoreactivity in small (<30 μm) neurons, and a non-significant increase in medium/large neurons. The decrease in K_v7.5 channel expression in small and medium-sized DRG neurons in STZ rats is likely to contribute to the mechanisms of hyperexcitability of these neurons and thereby to the resulting pain hypersensitivity associated with DPNP. The upregulation of K_v7.2 subunit in small DRG neurons may be an activity dependent compensatory mechanism to limit STZ-induced hyperexcitability of DRG neurons and the associated pain hypersensitivity. The findings support the notion that K_v7 channels may represent a novel target for DPNP treatment.

Diabetes mellitus alters electrophysiological properties in neurons of superior cervical ganglion of rats

Diabetic neuropathy is the most prevalent complication associated with diabetes mellitus (DM). The superior cervical ganglion (SCG) is an important sympathetic component of the autonomic nervous system. We investigated the changes in cellular electrophysiological properties and on Na⁺K⁺-ATPase activity of SCG neurons of rats with DM induced by streptozotocin (STZ). Three types of action potentials (AP) firing pattern were observed in response to a long (1 s) depolarizing pulse. Whilst some neurons fired a single AP (single firing phasic, SFP), others fired few APs (multiple firing phasic, MFP). A third type fired APs during more than 80% of the stimulus duration (tonic-like, TL). The occurrence of SFP, MFP and TL was 84.5, 13.8, and 1.7%, respectively. SFP and MFP differed significantly in their membrane input resistance (R_in). At the end of the 4th week of its time course, DM differently affected most types of neurons: DM induced depolarization of resting membrane potential (RMP), decreased AP amplitude in SFP, and decreased R_in in MFP. DM decreased spike after-hyperpolarization amplitude in MFP and the duration in SFP. Based on the RMP depolarization, we investigated the Na⁺K⁺-ATPase action and observed that DM caused a significant decrease in Na⁺K⁺-ATPase activity of SCG. In conclusion, we have demonstrated that DM affects several parameters of SCG physiology in a manner likely to have pathophysiological relevance.

Downregulation of glucose-6-phosphate dehydrogenase contributes to diabetic neuropathic pain through upregulation of toll-like receptor 4 in rats

Background and aim

Diabetic neuropathic pain is a refractory and disabling complication of diabetes mellitus. The pathogenesis of the diabetic neuropathic pain is still unclear, and treatment is insufficient. The aim of this study is to investigate the roles of glucose-6-phosphate dehydrogenase (G6PD) and toll-like receptor 4 (TLR4) in neuropathic pain in rats with diabetes.

Methods

Type 1 diabetes model was induced by intraperitoneal injection of streptozotocin (STZ, 75 mg/kg) in adult female Sprague-Dawley rats. Paw withdrawal threshold and paw withdrawal latency of rats were measured by von Frey filaments and thermal radiation, respectively. The expressions of G6PD and TLR4 in L4-L6 dorsal root ganglions (DRGs) were measured by western blotting and quantitative real-time polymerase chain reaction analysis. Fluorescent immunohistochemistry was employed to detect expressions of G6PD and TLR4 and co-location of G6PD with TLR4.

Results

The mRNA and protein expression levels of G6PD in DRGs were significantly decreased in diabetic rats when compared with age-matched control rats. Upregulation of G6PD by intrathecal injection of G6PD overexpression adenovirus markedly attenuated hindpaw pain hypersensitivity of diabetic rats. The mRNA and protein expression levels of TLR4 in DRGs of diabetic rats were significantly increased when compared with control rats. Intrathecal injection of TLR4-selective inhibitor CLI-095 attenuated diabetic pain in dose- and time-dependent manners. Furthermore, G6PD and TLR4 were co-localized in DRG neurons. Intrathecal injection of G6PD overexpression adenovirus greatly reduced TLR4 expression, while intrathecal injection of CLI-095 had no significant effect on G6PD expression in diabetic rats.

Conclusions

Our results suggest that decrease in G6PD expression was involved in diabetic peripheral neuropathic pain, which was most likely through upregulation of TLR4 expression in the DRGs of rats.

Nociceptor Signalling through ion Channel Regulation via GPCRs

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

How to replace the lost keys? Strategies toward safer KV7 channel openers

The highly structurally similar drugs flupirtine and retigabine have been regarded as safe and effective for many years but lately they turned out to exert intolerable side effects. While the twin molecules share the mode of action, both stabilize the open state of voltage-gated potassium channels, the form and severity of adverse effects is different. The analgesic flupirtine caused drug-induced liver injury in rare but fatal cases, whereas prolonged use of the antiepileptic retigabine led to blue tissue discoloration. Because the adverse effects seem unrelated to the mode of action, it is likely, that both drugs that occupied important therapeutic niches, could be replaced. Reasons for the clinically relevant toxicity will be clarified and future substitutes for these drugs presented in this review.