Targeting voltage-gated calcium channels: developments in peptide and small-molecule inhibitors for the treatment of neuropathic pain

Porous silicon and silica carriers for delivery of peptide therapeutics

Peptides have gained tremendous popularity as biological therapeutic agents in recent years due to their favourable specificity, diversity of targets, well-established screening methods, ease of production, and lower cost. However, their poor physiological and storage stability, pharmacokinetics, and fast clearance have limited their clinical translation. Novel nanocarrier-based strategies have shown promise in overcoming these issues. In this direction, porous silicon (pSi) and mesoporous silica nanoparticles (MSNs) have been widely explored as potential carriers for the delivery of peptide therapeutics. These materials possess several advantages, including large surface areas, tunable pore sizes, and adjustable pore architectures, which make them attractive carriers for peptide delivery systems. In this review, we cover pSi and MSNs as drug carriers focusing on their use in peptide delivery. The review provides a brief overview of their fabrication, surface modification, and interesting properties that make them ideal peptide drug carriers. The review provides a systematic account of various studies that have utilised these unique porous carriers for peptide delivery describing significant in vitro and in vivo results. We have also provided a critical comparison of the two carriers in terms of their physicochemical properties and short-term and long-term biocompatibility. Lastly, we have concluded the review with our opinion of this field and identified key areas for future research for clinical translation of pSi and MSN-based peptide therapeutic formulations.

Covalently Binding Adenosine A3 Receptor Agonist ICBM Irreversibly Reduces Voltage-Gated Ca2+ Currents in Dorsal Root Ganglion Neurons

Interest has been focused in recent years on the analgesic effects exerted by adenosine and its receptors, A1, A2A, A2B, and A3 adenosine receptor (AR) subtypes, in different in vivo models of chronic pain. In particular, it was demonstrated that selective A3AR agonists reduced pro-nociceptive N-type Ca2+ channels in dorsal root ganglion (DRG) neurons isolated from rats and, by this mechanism, inhibit post inflammatory visceral hypersensitivity. In the present study, we investigate the effect of a previously reported irreversibly binding A3AR agonist, ICBM, on Ca2+ currents (ICa) in rat DRG neurons. Present data demonstrate that ICBM, an isothiocyanate derivative designed for covalent binding to the receptor, concentration-dependently inhibits ICa. This effect is irreversible, since it persists after drug removal, differently from the prototypical A3AR agonist, Cl-IB-MECA. ICBM pre-exposure inhibits the effect of a subsequent Cl-IB-MECA application. Thus, covalent A3AR agonists such as ICBM may represent an innovative, beneficial, and longer-lasting strategy to achieve efficacious chronic pain control versus commonly used, reversible, A3AR agonists. However, the possible limitations of this drug and other covalent drugs may be, for example, a characteristic adverse effect profile, suggesting that more pre-clinical studies are needed.

Snake Venom: A Promising Source of Neurotoxins Targeting Voltage-Gated Potassium Channels

The venom derived from various sources of snakes represents a vast collection of predominantly protein-based toxins that exhibit a wide range of biological actions, including but not limited to inflammation, pain, cytotoxicity, cardiotoxicity, and neurotoxicity. The venom of a particular snake species is composed of several toxins, while the venoms of around 600 venomous snake species collectively encompass a substantial reservoir of pharmacologically intriguing compounds. Despite extensive research efforts, a significant portion of snake venoms remains uncharacterized. Recent findings have demonstrated the potential application of neurotoxins derived from snake venom in selectively targeting voltage-gated potassium channels (Kv). These neurotoxins include BPTI-Kunitz polypeptides, PLA2 neurotoxins, CRISPs, SVSPs, and various others. This study provides a comprehensive analysis of the existing literature on the significance of Kv channels in various tissues, highlighting their crucial role as proteins susceptible to modulation by diverse snake venoms. These toxins have demonstrated potential as valuable pharmacological resources and research tools for investigating the structural and functional characteristics of Kv channels.

Self-Assembly Nanostructure of Myristoylated ω-Conotoxin MVIIA Increases the Duration of Efficacy and Reduces Side Effects

Chronic pain is one of the most prevalent health problems worldwide. An alternative to suppress or alleviate chronic pain is the use of peptide drugs that block N-type Ca2+ channels (Cav2.2), such as ω-conotoxin MVIIA. Nevertheless, the narrow therapeutic window, severe neurological side effects and low stability associated with peptide MVIIA have restricted its widespread use. Fortunately, self-assembly endows the peptide with high stability and multiple functions, which can effectively control its release to prolong its duration of action. Inspired by this, MVIIA was modified with appropriate fatty acid chains to render it amphiphilic and easier to self-assemble. In this paper, an N-terminal myristoylated MVIIA (Myr-MVIIA, medium carbon chain length) was designed and prepared to undergo self-assembly. The present results indicated that Myr-MVIIA can self-assemble into micelles. Self-assembled micelles formed by Myr-MVIIA at higher concentrations than MVIIA can prolong the duration of the analgesic effect and significantly reduce or even eliminate the side effects of tremor and coordinated motor dysfunction in mice.

N-type calcium channel blockers: a new approach towards the treatment of chronic neuropathic pain

Neuropathic pain (NP) remains maltreated for a wide number of patients by the currently available treatments and little research has been done in finding new drugs for treating NP. Ziconotide (PrialtTM) had been developed as the new drug, which belongs to the class of ω-conotoxin MVIIA. It inhibits N-type calcium channels. Ziconotide is under the last phase of the clinical trial, a new non-narcotic drug for the management of NP. Synthetically it has shown the similarities with ω-conotoxin MVIIA, a constituent of poison found in fish hunting snails (Conus magus). Ziconotide acts by selectively blocking neural N-type voltage-sensitized Ca2+ channels (NVSCCs). Certain herbal drugs also have been studied but no clinical result is there and the study is only limited to preclinical data. This review emphasizes the N-type calcium channel inhibitors, and their mechanisms for blocking calcium channels with their remedial prospects for treating chronic NP.

An Update On Proficiency of Voltage-gated Ion Channel Blockers in the Treatment of Inflammation-associated Diseases

Inflammation is the body's mechanism to trigger the immune system, thereby preventing bacteria and viruses from manifesting their toxic effect. Inflammation plays a vital role in regulating inflammatory mediator levels to initiate the wound healing process depending on the nature of the stimuli. This process occurs due to chemical release from white blood cells by elevating blood flow to the site of action, leading to redness and increased body temperature. Currently, there are numerous Non-steroidal anti-inflammatory drugs (NSAIDs) available, but these drugs are reported with adverse effects such as gastric bleeding, progressive kidney damage, and increased risk of heart attacks when prolonged use. For such instances, alternative options need to be adopted. The introduction of voltage-gated ion channel blockers can be a substantial alternative to mask the side effects of these currently available drugs. Chronic inflammatory disorders such as rheumatoid and osteoarthritis, cancer and migraine, etc., can cause dreadful pain, which is often debilitating for the patient. The underlying mechanism for both acute and chronic inflammation involves various complex receptors, different types of cells, receptors, and proteins. The working of voltage-gated sodium and calcium channels is closely linked to both inflammatory and neuropathic pain. Certain drugs such as carbamazepine and gabapentin, which are ion channel blockers, have greater pharmacotherapeutic activity for sodium and calcium channel blockers for the treatment of chronic inflammatory pain states. This review intends to provide brief information on the mechanism of action, latest clinical trials, and applications of these blockers in treating inflammatory conditions.

The Acute Antiallodynic Effect of Tolperisone in Rat Neuropathic Pain and Evaluation of Its Mechanism of Action

Current treatment approaches to manage neuropathic pain have a slow onset and their use is largely hampered by side-effects, thus there is a significant need for finding new medications. Tolperisone, a centrally acting muscle relaxant with a favorable side effect profile, has been reported to affect ion channels, which are targets for current first-line medications in neuropathic pain. Our aim was to explore its antinociceptive potency in rats developing neuropathic pain evoked by partial sciatic nerve ligation and the mechanisms involved. Acute oral tolperisone restores both the decreased paw pressure threshold and the elevated glutamate level in cerebrospinal fluid in neuropathic rats. These effects were comparable to those of pregabalin, a first-line medication in neuropathy. Tolperisone also inhibits release of glutamate from rat brain synaptosomes primarily by blockade of voltage-dependent sodium channels, although inhibition of calcium channels may also be involved at higher concentrations. However, pregabalin fails to affect glutamate release under our present conditions, indicating a different mechanism of action. These results lay the foundation of the avenue for repurposing tolperisone as an analgesic drug to relieve neuropathic pain.

Pathophysiological Responses to Conotoxin Modulation of Voltage-Gated Ion Currents

Voltage-gated ion channels are plasma membrane proteins that generate electrical signals following a change in the membrane voltage. Since they are involved in several physiological processes, their dysfunction may be responsible for a series of diseases and pain states particularly related to neuronal and muscular systems. It is well established for decades that bioactive peptides isolated from venoms of marine mollusks belonging to the Conus genus, collectively known as conotoxins, can target different types and isoforms of these channels exerting therapeutic effects and pain relief. For this reason, conotoxins are widely used for either therapeutic purposes or studies on ion channel mechanisms of action disclosure. In addition their positive property, however, conotoxins may generate pathological states through similar ion channel modulation. In this narrative review, we provide pieces of evidence on the pathophysiological impacts that different members of conotoxin families exert by targeting the three most important voltage-gated channels, such as sodium, calcium, and potassium, involved in cellular processes.

Rational Design of a Modality-Specific Inhibitor of TRPM8 Channel against Oxaliplatin-Induced Cold Allodynia

Platinum-based compounds in chemotherapy such as oxaliplatin often induce peripheral neuropathy and neuropathic pain such as cold allodynia in patients. Transient Receptor Potential Melastatin 8 (TRPM8) ion channel is a nociceptor critically involved in such pathological processes. Direct blockade of TRPM8 exhibits significant analgesic effects but also incurs severe side effects such as hypothermia. To selectively target TRPM8 channels against cold allodynia, a cyclic peptide DeC-1.2 is de novo designed with the optimized hot-spot centric approach. DeC-1.2 modality specifically inhibited the ligand activation of TRPM8 but not the cold activation as measured in single-channel patch clamp recordings. It is further demonstrated that DeC-1.2 abolishes cold allodynia in oxaliplatin treated mice without altering body temperature, indicating DeC-1.2 has the potential for further development as a novel analgesic against oxaliplatin-induced neuropathic pain.

A review of a new voltage-gated Ca2+ channel α2δ ligand, mirogabalin, for the treatment of peripheral neuropathic pain

INTRODUCTION

Neuropathic pain (NeP) is a chronic and refractory condition in many patients, and its treatment is a challenge for physicians. A new voltage-gated Ca²⁺ channel α₂δ ligand, mirogabalin, has a high specific binding affinity for the α₂δ subunit, with a slower dissociation rate for α₂δ-1 than α₂δ-2 compared to that of pregabalin. Mirogabalin was shown to be effective in NeP animal models, with a margin of safety between central nervous system side effects and the analgesic effect of the dose. It exerted a favorable analgesic effect, was well tolerated in patients with peripheral NeP (P-NeP), and was first approved in Japan in 2019 and subsequently in Korea and Taiwan in 2020.

AREAS COVERED

The purpose of this article is to review the pharmacological characteristics, pharmacokinetics, and efficacy and safety of mirogabalin for NeP based on the results of non-clinical and clinical studies.

EXPERT OPINION

Although there are several first-line therapies for NeP, insufficient efficacy and adverse drug reactions of NeP drugs often cause patient dissatisfaction. Mirogabalin was effective and well tolerated with a step-wise dose increase in clinical studies on P-NeP patients. Thus, mirogabalin is expected to be a useful treatment option for patients with P-NeP.

In silico Screening and Behavioral Validation of a Novel Peptide, LCGA-17, With Anxiolytic-Like Properties

The aim of the study was to develop better anxiolytics and antidepressants. We focused on GABA_A receptors and the α2δ auxiliary subunit of V-gated Ca²⁺ channels as putative targets because they are established as mediators of efficacious anxiolytics, antidepressants, and anticonvulsants. We further focused on short peptides as candidate ligands because of their high safety and tolerability profiles. We employed a structural bioinformatics approach to develop novel tetrapeptides with predicted affinity to GABA_A receptors and α2δ. In silico docking studies of one of these peptides, LCGA-17, showed a high binding score for both GABA_A receptors and α2δ, combined with anxiolytic-like properties in a Danio rerio behavioral screen. LCGA-17 showed anxiolytic-like effects in the novel tank test, the light–dark box, and the social preference test, with efficacy comparable to fluvoxamine and diazepam. In binding assays using rat brain membranes, [³H]-LCGA-17 was competed more effectively by gabapentinoid ligands of α2δ than ligands of GABA_A receptors, suggesting that α2δ represents a likely target for LCGA-17. [³H]-LCGA-17 binding to brain lysates was unaffected by competition with ligands for GABA_B, glutamate, dopamine, serotonin, and other receptors, suggesting specific interaction with α2δ. Dose-finding studies in mice using acute administration of LCGA-17 (i.p.) demonstrated anxiolytic-like effects in the open field test, elevated plus maze, and marble burying tests, as well as antidepressant-like properties in the forced swim test. The anxiolytic effects were effectively blocked by bicuculline. Therefore, LCGA-17 is a novel candidate anxiolytic and antidepressant that may act through α2δ, with possible synergism by GABA_A receptors.

Cell-Based Reporter Release Assay to Determine the Activity of Calcium-Dependent Neurotoxins and Neuroactive Pharmaceuticals

The suitability of a newly developed cell-based functional assay was tested for the detection of the activity of a range of neurotoxins and neuroactive pharmaceuticals which act by stimulation or inhibition of calcium-dependent neurotransmitter release. In this functional assay, a reporter enzyme is released concomitantly with the neurotransmitter from neurosecretory vesicles. The current study showed that the release of a luciferase from a differentiated human neuroblastoma-based reporter cell line (SIMA-hPOMC1-26-GLuc cells) can be stimulated by a carbachol-mediated activation of the Gq-coupled muscarinic-acetylcholine receptor and by the Ca²⁺-channel forming spider toxin α-latrotoxin. Carbachol-stimulated luciferase release was completely inhibited by the muscarinic acetylcholine receptor antagonist atropine and α-latrotoxin-mediated release by the Ca²⁺-chelator EGTA, demonstrating the specificity of luciferase-release stimulation. SIMA-hPOMC1-26-GLuc cells express mainly L- and N-type and to a lesser extent T-type VGCC on the mRNA and protein level. In accordance with the expression profile a depolarization-stimulated luciferase release by a high K⁺-buffer was effectively and dose-dependently inhibited by L-type VGCC inhibitors and to a lesser extent by N-type and T-type inhibitors. P/Q- and R-type inhibitors did not affect the K⁺-stimulated luciferase release. In summary, the newly established cell-based assay may represent a versatile tool to analyze the biological efficiency of a range of neurotoxins and neuroactive pharmaceuticals which mediate their activity by the modulation of calcium-dependent neurotransmitter release.

From Animal Poisons and Venoms to Medicines: Achievements, Challenges and Perspectives in Drug Discovery

Animal poisons and venoms are comprised of different classes of molecules displaying wide-ranging pharmacological activities. This review aims to provide an in-depth view of toxin-based compounds from terrestrial and marine organisms used as diagnostic tools, experimental molecules to validate postulated therapeutic targets, drug libraries, prototypes for the design of drugs, cosmeceuticals, and therapeutic agents. However, making these molecules applicable requires extensive preclinical trials, with some applications also demanding clinical trials, in order to validate their molecular target, mechanism of action, effective dose, potential adverse effects, as well as other fundamental parameters. Here we go through the pitfalls for a toxin-based potential therapeutic drug to become eligible for clinical trials and marketing. The manuscript also presents an overview of the current picture for several molecules from different animal venoms and poisons (such as those from amphibians, cone snails, hymenopterans, scorpions, sea anemones, snakes, spiders, tetraodontiformes, bats, and shrews) that have been used in clinical trials. Advances and perspectives on the therapeutic potential of molecules from other underexploited animals, such as caterpillars and ticks, are also reported. The challenges faced during the lengthy and costly preclinical and clinical studies and how to overcome these hindrances are also discussed for that drug candidates going to the bedside. It covers most of the drugs developed using toxins, the molecules that have failed and those that are currently in clinical trials. The article presents a detailed overview of toxins that have been used as therapeutic agents, including their discovery, formulation, dosage, indications, main adverse effects, and pregnancy and breastfeeding prescription warnings. Toxins in diagnosis, as well as cosmeceuticals and atypical therapies (bee venom and leech therapies) are also reported. The level of cumulative and detailed information provided in this review may help pharmacists, physicians, biotechnologists, pharmacologists, and scientists interested in toxinology, drug discovery, and development of toxin-based products.

Conotoxins: Chemistry and Biology

The venom of the marine predatory cone snails (genus Conus) has evolved for prey capture and defense, providing the basis for survival and rapid diversification of the now estimated 750+ species. A typical Conus venom contains hundreds to thousands of bioactive peptides known as conotoxins. These mostly disulfide-rich and well-structured peptides act on a wide range of targets such as ion channels, G protein-coupled receptors, transporters, and enzymes. Conotoxins are of interest to neuroscientists as well as drug developers due to their exquisite potency and selectivity, not just against prey but also mammalian targets, thereby providing a rich source of molecular probes and therapeutic leads. The rise of integrated venomics has accelerated conotoxin discovery with now well over 10,000 conotoxin sequences published. However, their structural and pharmacological characterization lags considerably behind. In this review, we highlight the diversity of new conotoxins uncovered since 2014, their three-dimensional structures and folds, novel chemical approaches to their syntheses, and their value as pharmacological tools to unravel complex biology. Additionally, we discuss challenges and future directions for the field.

A Phase I, Randomized, Double-Blind, Laser-Evoked Potential Study to Evaluate the Analgesic/Antihyperalgesic Effect of ASP9226, a State-Dependent N-Type Voltage-Gated Calcium Channel Inhibitor, in Healthy Male Subjects

Objective

Evaluate the analgesic/antihyperalgesic effects of ASP9226, a state-dependent N-type voltage-gated calcium channel inhibitor, in healthy male subjects.

Design

Randomized, double-blind, double-dummy, placebo- and active comparator–controlled crossover study.

Setting

HPR Dr. Schaffler GmbH, Munich, Germany.

Subject

Healthy male subjects aged 18–55 years.

Methods

Twenty-four eligible subjects were randomly assigned to one of four treatment sequences and received single doses of ASP9226 (30 mg or 50 mg), pregabalin (150 mg), or placebo during four treatment periods. Laser-evoked potentials (LEP) and postlaser pain visual analog scales (VAS) on capsaicin-treated skin were assessed during main assessment days (the first day of each study period). Primary and secondary end points were the differences in LEP N2-P2 peak-to-peak (PtP) amplitudes and VAS score, respectively, in all subjects.

Results

Overall, treatment with pregabalin resulted in a significantly lower LEP N2-P2 PtP amplitude vs placebo (–3.30 μV, P < 0.0001). There were no clinically relevant differences in N2-P2 PtP amplitudes between placebo and either ASP9226 dose (–0.31 μV and –0.27 μV). Furthermore, subjects reported significantly lower VAS pain scores with pregabalin vs placebo (–9.90%, P < 0.0001) in contrast to ASP9226 30 mg (–2.1%) and ASP9226 50 mg (1.2%) vs placebo. Subgroup analysis of LEP and VAS pain in participants with positive prestudy capsaicin response (n = 13) were in keeping with results in all subjects.

Conclusions

ASP9226 was well tolerated; however, there was no improvement in LEP and VAS pain scores with ASP9226 at either dose vs placebo.

AAV-encoded CaV2.2 peptide aptamer CBD3A6K for primary sensory neuron-targeted treatment of established neuropathic pain

Transmission of pain signals from primary sensory neurons to secondary neurons of the central nervous system is critically dependent on presynaptic voltage-gated calcium channels. Calcium channel-binding domain 3 (CBD3), derived from the collapsin response mediator protein 2 (CRMP2), is a peptide aptamer that is effective in blocking N-type voltage-gated calcium channel (Ca_V2.2) activity. We previously reported that recombinant adeno-associated virus (AAV)-mediated restricted expression of CBD3 affixed to enhanced green fluorescent protein (EGFP) in primary sensory neurons prevents the development of cutaneous mechanical hypersensitivity in a rat neuropathic pain model. In this study, we tested whether this strategy is effective in treating established pain. We constructed AAV6-EGFP-CBD3A6K (AAV6-CBD3A6K) expressing a fluorescent CBD3A6K (replacing A to K at position 6 of CBD3 peptide), which is an optimized variant of the parental CBD3 peptide that is a more potent blocker of Ca_V2.2. Delivery of AAV6-CBD3A6K into lumbar (L) 4 and 5 dorsal root ganglia (DRG) of rats 2 weeks following tibial nerve injury (TNI) induced transgene expression in neurons of these DRG and their axonal projections, accompanied by attenuation of pain behavior. We additionally observed that the increased Ca_V2.2α1b immunoreactivity in the ipsilateral spinal cord dorsal horn and DRG following TNI was significantly normalized by AAV6-CBD3A6K treatment. Finally, the increased neuronal activity in the ipsilateral dorsal horn that developed after TNI was reduced by AAV6-CBD3A6K treatment. Collectively, these results indicate that DRG-restricted AAV6 delivery of CBD3A6K is an effective analgesic molecular strategy for the treatment of established neuropathic pain.

Adenosine A3 receptor activation inhibits pronociceptive N-type Ca2+ currents and cell excitability in dorsal root ganglion neurons

Recently, studies have focused on the antihyperalgesic activity of the A3 adenosine receptor (A3AR) in several chronic pain models, but the cellular and molecular basis of this effect is still unknown. Here, we investigated the expression and functional effects of A3AR on the excitability of small- to medium-sized, capsaicin-sensitive, dorsal root ganglion (DRG) neurons isolated from 3- to 4-week-old rats. Real-time quantitative polymerase chain reaction experiments and immunofluorescence analysis revealed A3AR expression in DRG neurons. Patch-clamp experiments demonstrated that 2 distinct A3AR agonists, Cl-IB-MECA and the highly selective MRS5980, inhibited Ca-activated K (KCa) currents evoked by a voltage-ramp protocol. This effect was dependent on a reduction in Ca influx via N-type voltage-dependent Ca channels, as Cl-IB-MECA-induced inhibition was sensitive to the N-type blocker PD173212 but not to the L-type blocker, lacidipine. The endogenous agonist adenosine also reduced N-type Ca currents, and its effect was inhibited by 56% in the presence of A3AR antagonist MRS1523, demonstrating that the majority of adenosine's effect is mediated by this receptor subtype. Current-clamp recordings demonstrated that neuronal firing of rat DRG neurons was also significantly reduced by A3AR activation in a MRS1523-sensitive but PD173212-insensitive manner. Intracellular Ca measurements confirmed the inhibitory role of A3AR on DRG neuronal firing. We conclude that pain-relieving effects observed on A3AR activation could be mediated through N-type Ca channel block and action potential inhibition as independent mechanisms in isolated rat DRG neurons. These findings support A3AR-based therapy as a viable approach to alleviate pain in different pathologies.

Rationally Designed α-Conotoxin Analogues Maintained Analgesia Activity and Weakened Side Effects

A lack of specificity is restricting the further application of conotoxin from Conus bullatus (BuIA). In this study, an analogue library of BuIA was established and virtual screening was used, which identified high α7 nicotinic acetylcholine receptor (nAChR)-selectivity analogues. The analogues were synthesized and tested for their affinity to functional human α7 nAChR and for the regulation of intracellular calcium ion capacity in neurons. Immunofluorescence, flow cytometry, and patch clamp results showed that the analogues maintained their capacity for calcium regulation. The results of the hot-plate model and paclitaxel-induced peripheral neuropathy model indicated that, when compared with natural BuIA, the analgesia activities of the analogues in different models were maintained. To analyze the adverse effects and toxicity of BuIA and its analogues, the tail suspension test, forced swimming test, and open field test were used. The results showed that the safety and toxicity of the analogues were significantly better than BuIA. The analogues of BuIA with an appropriate and rational mutation showed high selectivity and maintained the regulation of Ca²⁺ capacity in neurons and activities of analgesia, whereas the analogues demonstrated that the adverse effects of natural α-conotoxins could be reduced.

Mechanisms for therapeutic effect of bulleyaconitine A on chronic pain

Bulleyaconitine A, a diterpenoid alkaloid isolated from Aconitum bulleyanum plants, has been used for the treatment of chronic pain in China since 1985. Clinical studies show that the oral administration of bulleyaconitine A is effective for treating different kinds of chronic pain, including back pain, joint pain, and neuropathic pain with minimal side effect in human patients. The experimental studies have revealed that bulleyaconitine A at therapeutic doses potently inhibits the peripheral sensitization and central sensitization that underlie chronic pain and has no effect on acute pain. Bulleyaconitine A preferably blocks tetrodotoxin-sensitive voltage-gated sodium channels in dorsal root ganglion neurons by inhibition of protein kinase C, and the effect is around 600 times more potent in neuropathic animals than in naïve ones. Bulleyaconitine A at 5 nM inhibits the hypersensitivity of dorsal root ganglion neurons in neuropathic rats but has no effect on excitability of dorsal root ganglion neurons in sham group. Bulleyaconitine A inhibits long-term potentiation at C-fiber synapses in spinal dorsal horn, a synaptic model of pathological pain, preferably in neuropathic pain rats over naïve rats. The following mechanisms may underlie the selective effect of bulleyaconitine A on chronic pain. (1) In neuropathic conditions, protein kinase C and voltage-gated sodium channels in dorsal root ganglion neurons are upregulated, which enhances bulleyaconitine A's effect. (2) Bulleyaconitine A use-dependently blocks voltage-gated sodium channels and therefore inhibits the ectopic discharges that are important for neuropathic pain. (3) Bulleyaconitine A is shown to inhibit neuropathic pain by the modulation of spinal microglia, which are involved in the chronic pain but not in acute (nociceptive) pain. Moreover, bulleyaconitine A facilitates the anesthetic effect of morphine and inhibits morphine tolerance in rats. Together, bulleyaconitine A is able to inhibit chronic pain by targeting at multiple molecules. Further clinical and experimental studies are needed for evaluating the efficacy of bulleyaconitine A in different forms of chronic pain in patients and for exploring the underlying mechanisms.

Four novel interaction partners demonstrate diverse modulatory effects on voltage-gated CaV2.2 Ca2+ channels

Voltage-gated Ca²⁺ channels are embedded in a network of protein interactions that are fundamental for channel function and modulation. Different strategies such as high-resolution quantitative MS analyses and yeast-two hybrid screens have been used to uncover these Ca²⁺ channel nanodomains. We applied the yeast split-ubiquitin system with its specific advantages to search for interaction partners of the Ca_V2.2 Ca²⁺ channel and identified four proteins: reticulon 1 (RTN1), member 1 of solute carrier family 38 (SLC38), prostaglandin D2 synthase (PTGDS) and transmembrane protein 223 (TMEM223). Interactions were verified using the yeast split-ubiquitin system and narrowed down to Ca_V2.2 domain IV. Colocalization studies using fluorescent constructs demonstrated defined regions of subcellular localization. Detailed electrophysiological studies revealed that coexpression of RTN1 modulated Ca_V2.2 channels only to a minor extent. SLC38 accelerated the cumulative current inactivation during a high-frequency train of brief depolarizing pulses. As neurons expressing Ca_V2.2 channels were exposed to high-frequency bursts under physiological conditions, observed regulation may have a negative modulatory effect on transmitter release. Coexpression of PTGDS significantly lowered the average current density and slowed the kinetics of cumulative current inactivation. Since the latter effect was not significant, it may only partly compensate the first one under physiological conditions. Expression of TMEM223 lowered the average current density, accelerated the kinetics of cumulative current inactivation and slowed the kinetics of recovery from inactivation. Therefore, TMEM223 and, to a lesser extent, PTGDS, may negatively modulate Ca²⁺ entry required for transmitter release and/or for dendritic plasticity under physiological conditions.

Natural Peptides in Drug Discovery Targeting Acetylcholinesterase

Acetylcholinesterase-inhibitory peptide has gained much importance since it can inhibit acetylcholinesterase (AChE) and increase the availability of acetylcholine in cholinergic synapses, enhancing cholinergic transmission in pharmacological treatment of Alzheimer’s disease (AD). Natural peptides have received considerable attention as biologically important substances as a source of AChE inhibitors. These natural peptides have high potential pharmaceutical and medicinal values due to their bioactivities as neuroprotective and neurodegenerative treatment activities. These peptides have attracted great interest in the pharmaceutical industries, in order to design potential peptides for use in the prophylactic and therapy purposes. Some natural peptides and their derivatives have high commercial values and have succeeded in reaching the pharmaceutical market. A large number of peptides are already in preclinical and clinical pipelines for treatment of various diseases. This review highlights the recent researches on the various natural peptides and future prospects for AD management.

Injury-induced maladaptation and dysregulation of calcium channel α2 δ subunit proteins and its contribution to neuropathic pain development

Voltage-gated calcium channels (VGCCs) play important roles in physiological functions including the modulation of neurotransmitter release, neuronal network activities, intracellular signalling pathways and gene expression. Some pathological conditions, including nerve injuries, can cause the dysregulation of VGCCs and their subunits. This in turn can lead to a functional maladaptation of VGCCs and their subunits, which can contribute to the development of disorders such as pain sensations. This review has summarized recent findings related to maladaptive changes in the dysregulated VGCC α2 δ1 subunit (Cav α2 δ1 ) with a focus on exploring the mechanisms underlying the contribution of Cav α2 δ1 to pain signal transduction. At least under neuropathic pain conditions, the dysregulated Cav α2 δ1 can modulate VGCC functions as well as other plasticity changes. The latter includes abnormal excitatory synaptogenesis resulting from its interactions with injury-induced extracellular matrix glycoprotein molecule thrombospondins, which is independent of the VGCC functions. Blocking Cav α2 δ1 with gabapentinoids can reverse neuropathic pain significantly with relatively mild side effects, but only in a small population of neuropathic pain patients due to reasons yet to be explored. There are emerging data suggesting that early preventive treatment with gabapentinoids can prevent aberrant excitatory synapse formation and the development of chronic pain. If these findings are confirmed clinically, this could be an attractive approach for neuropathic pain management.

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.

Cisplatin alters the function and expression of N-type voltage-gated calcium channels in the absence of morphological damage of sensory neurons

Platinum-based chemotherapeutic agents, such as cisplatin, are still frequently used for treating various types of cancer. Besides its high effectiveness, cisplatin has several serious side effects. One of the most common side effects is dorsal root ganglion (DRG) neurotoxicity. However, the mechanisms underlying this neurotoxicity are still unclear and controversially discussed. Cisplatin-mediated modulation of voltage-gated calcium channels (VGCCs) in the DRG neurons has been shown to alter intracellular calcium homeostasis, a process critical for the induction of neurotoxicity. Using the whole-cell patch-clamp technique, immunostaining, behavioural experiments and electron microscopy (EM) of rat DRGs, we here demonstrate that cisplatin-induced neurotoxicity is due to functional alteration of VGCC, but not due to morphological damage. In vitro application of cisplatin (0.5 µM) increased N-type VGCC currents (I_Ca(V)) in small DRG neurons. Repetitive in vivo administration of cisplatin (1.5 mg/kg, cumulative 12 mg/kg) increased the protein level of N-type VGCC over 26 days, with the protein level being increased for at least 14 days after the final cisplatin administration. Behavioural studies revealed that N-type VGCCs are crucial for inducing symptoms of cisplatin-related neuropathic pain, such as thermal and mechanical hyperalgesia. EM and histology showed no evidence of any structural damage, apoptosis or necrosis in DRG cells after cisplatin exposure for 26 days. Furthermore, no nuclear DNA damage in sensory neurons was observed. Here, we provide evidence for a mainly functionally driven induction of neuropathic pain by cisplatin.

Homology-guided mutational analysis reveals the functional requirements for antinociceptive specificity of collapsin response mediator protein 2-derived peptides

BACKGROUND AND PURPOSE

N-type voltage-gated calcium (Ca_v 2.2) channels are critical determinants of increased neuronal excitability and neurotransmission accompanying persistent neuropathic pain. Although Ca_v 2.2 channel antagonists are recommended as first-line treatment for neuropathic pain, calcium-current blocking gabapentinoids inadequately alleviate chronic pain symptoms and often exhibit numerous side effects. Collapsin response mediator protein 2 (CRMP2) targets Ca_v 2.2 channels to the sensory neuron membrane and allosterically modulates their function. A 15-amino-acid peptide (CBD3), derived from CRMP2, disrupts the functional protein-protein interaction between CRMP2 and Ca_v 2.2 channels to inhibit calcium influx, transmitter release and acute, inflammatory and neuropathic pain. Here, we have mapped the minimal domain of CBD3 necessary for its antinociceptive potential.

EXPERIMENTAL APPROACH

Truncated as well as homology-guided mutant versions of CBD3 were generated and assessed using depolarization-evoked calcium influx in rat dorsal root ganglion neurons, binding between CRMP2 and Ca_v 2.2 channels, whole-cell voltage clamp electrophysiology and behavioural effects in two models of experimental pain: post-surgical pain and HIV-induced sensory neuropathy induced by the viral glycoprotein 120.

KEY RESULTS

The first six amino acids within CBD3 accounted for all in vitro activity and antinociception. Spinal administration of a prototypical peptide (TAT-CBD3-L5M) reversed pain behaviours. Homology-guided mutational analyses of these six amino acids identified at least two residues, Ala1 and Arg4, as being critical for antinociception in two pain models.

CONCLUSIONS AND IMPLICATIONS

These results identify an antinociceptive scaffold core in CBD3 that can be used for development of low MW mimetics of CBD3.

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.

Cisplatin-induced neuropathic pain is mediated by upregulation of N-type voltage-gated calcium channels in dorsal root ganglion neurons

Cisplatin is important in the treatment of various types of cancer. Although it is highly effective, it also has severe side effects, with neurotoxicity in dorsal root ganglion (DRG) neurons being one of the most common. The key mechanisms of neurotoxicity are still controversially discussed; however, disturbances of the calcium homeostasis in DRG neurons have been suggested to mediate cisplatin neurotoxicity. By using the whole-cell patch-clamp technique, immunostaining and behavioral experiments with Sprague-Dawley rats, we examined the influence of short- and long-term exposure to cisplatin on voltage-gated calcium channel (VGCC) currents (I_Ca(V)) in small DRG neurons. In vitro exposure to cisplatin reduced I_Ca(V) in a concentration-dependent manner (0.01-50μM; 13.8-77.3%; IC₅₀ 5.07μM). Subtype-specific measurements of VGCCs showed differential effects on I_Ca(V). While the I_Ca(V) of P/Q-, L- and T-type VGCCs were reduced, I_Ca(V) of N-type VGCCs were increased by 30.3% during depolarization to 0mV. Exposure of DRG neurons to cisplatin (0.5 or 5μM) for 24-48h in vitro significantly increased a CaMK II-mediated I_Ca(V) current density. Immunostaining and western blot analysis revealed an increase of N-type VGCC protein level in DRG neurons 24h after cisplatin exposure. Cisplatin-mediated activation of caspase-3 was prevented by inhibition of N-type VGCCs using Ɯ-conotoxin MVIIA. Behavioral experiments showed that Ɯ-conotoxin MVIIA treatment prevented neuropathic syndromes in vivo by inhibiting upregulation of the N-type protein level. Here we show evidence for the first time for a crucial role of N-type VGCC in the genesis of cisplatin-induced polyneuropathy.

Recent progress in designing inhibitors that target the drug-resistant M2 proton channels from the influenza A viruses

Influenza viruses are the causative agents for seasonal influenza, which results in thousands of deaths and millions of hospitalizations each year. Moreover, sporadic transmission of avian or swan influenza viruses to humans often leads to an influenza pandemic, as there is no preimmunity in the human body to fight against such novel strains. The metastable genome of the influenza viruses, coupled with the reassortment of different strains from a wide range of host origins, leads to the continuous evolution of the influenza virus diversity. Such characteristics of influenza viruses present a grand challenge in devising therapeutic strategies to combat influenza virus infection. This review summarizes recent progress in designing small molecule inhibitors that target the drug-resistant influenza A virus M2 proton channels and highlights the contribution of mechanistic studies of proton conductance to drug discovery. The lessons learned throughout the course of M2 drug discovery might provide insights for designing inhibitors that target other therapeutically important ion channels.

Involvement of ion channels and transporters in carcinoma angiogenesis and metastasis

Angiogenesis is a finely tuned process, which is the result of the equilibrium between pro- and antiangiogenic factors. In solid tumor angiogenesis, the balance is highly in favor of the production of new, but poorly functional blood vessels, initially intended to provide growing tumors with nutrients and oxygen. Among the numerous proteins involved in tumor development, several types of ion channels are overexpressed in tumor cells, as well as in stromal and endothelial cells. Ion channels thus actively participate in the different hallmarks of cancer, especially in tumor angiogenesis and metastasis. Indeed, from their strategic localization in the plasma membrane, ion channels are key operators of cell signaling, as they sense and respond to environmental changes. This review aims to decipher how ion channels of different families are intricately involved in the fundamental angiogenesis and metastasis hallmarks, which lead from a nascent tumor to systemic dissemination. An overview of the possible use of ion channels as therapeutic targets will also be given, showing that ion channel inhibitors or specific antibodies may provide effective tools, in the near future, in the treatment of carcinomas.

Simultaneous Determination of Sufentanil and Ziconotide in Combination for Intrathecal Analgesia by UPLC-UV

Intrathecal analgesia has increased over the past two decades based on high level proof of efficacy in patients with cancer. Morphine and Ziconotide remains the reference. Polyanalgesic Consensus Conference IT treatment algorithm recommends as the second line therapy opioids/ziconotide combination. Sufentanil and ziconotide combination can be used. The implantable pumps development helped to improve the comfort of the patient. The refills were prepared under a laminar airflow hood under strictly aseptic conditions, by the hospital pharmacist. In order to secure the process, a new analytical method by simple liquid chromatography ultraviolet spectrometry method was developed for the simultaneous quantification of two analgesic drugs (sufentanil, ziconotide). The method was validated according to the recommendation of the US Food and Drug Administration (FDA). The method was linear between 0.1 to 4 μg/mL for ziconotide and 3.125 to 50 µg/mL for sufentanil. This routine quality control analysis secures the production process.

Bioactive Mimetics of Conotoxins and other Venom Peptides

Ziconotide (Prialt^®), a synthetic version of the peptide ω-conotoxin MVIIA found in the venom of a fish-hunting marine cone snail Conus magnus, is one of very few drugs effective in the treatment of intractable chronic pain. However, its intrathecal mode of delivery and narrow therapeutic window cause complications for patients. This review will summarize progress in the development of small molecule, non-peptidic mimics of Conotoxins and a small number of other venom peptides. This will include a description of how some of the initially designed mimics have been modified to improve their drug-like properties.

3-Benzamides and 3,4,5-trimethoxyphenyl amines as calcium channel blockers

T- and N-type calcium channels have known for relating to therapy of neuropathic pain which is chronic, debilitating pain state. Neuropathic pain is caused by damage of the somatosensory system. It may be associated with abnormal sensations and pain produced by normally non-painful stimuli (allodynia). Neuropathic pain is very difficult to treat, and only some 40-60% of patients achieve partial relief. For a neuropathic pain therapy, anticonvulsant like Lamotrigine, Carbamazepine and a topical anesthetic such as Lidocaine are used. We synthesized 15 novel amine derivatives and evaluated their activities against T-type and N-type calcium channels by whole-cell patch clamp recording on HEK293 cells. Among the tested compounds, compound 10 showed good inhibitory activity for both T-type and N-type calcium channels with the IC50 value of 1.9 μM and 4.3 μM, respectively. Compound 10 also showed good analgesic activity on rat spinal cord injury model.

Therapeutic conotoxins: a US patent literature survey

INTRODUCTION

Conotoxins are a large family of bioactive peptides derived from cone snail venom. They target specific classes of ion channels and other membrane proteins and may have therapeutic value, primarily in the management of pain.

AREAS COVERED

The authors surveyed the US patent literature covering conotoxins, and their potential therapeutic applications. They describe the various subclasses of conotoxins that are the subject of current patent applications and their therapeutic indications. Limitations that may preclude broader application of these molecules are discussed and strategies for overcoming these limitations are presented.

EXPERT OPINION

Despite more than 25 years of intense global conotoxin research, only one molecule has successfully reached the market. Several other conotoxin-derived candidates failed in clinical trials, indicating that 'from the bench into the clinic' translation has been more difficult than originally anticipated. Nevertheless, we are optimistic that the potent activities of these molecules and the potential for improving their biopharmaceutical properties may lead to next-generation drug candidates with favorable pharmacological properties.

Inhibition of N-type calcium channels by fluorophenoxyanilide derivatives

A set of fluorophenoxyanilides, designed to be simplified analogues of previously reported ω-conotoxin GVIA mimetics, were prepared and tested for N-type calcium channel inhibition in a SH-SY5Y neuroblastoma FLIPR assay. N-type or Ca_v2.2 channel is a validated target for the treatment of refractory chronic pain. Despite being significantly less complex than the originally designed mimetics, up to a seven-fold improvement in activity was observed.

Effects of arginine 10 to lysine substitution on ω-conotoxin CVIE and CVIF block of Cav2.2 channels

BACKGROUND AND PURPOSE

ω-Conotoxins CVIE and CVIF (CVIE&F) selectively inhibit Cav2.2 channels and are lead molecules in the development of novel analgesics. At physiological membrane potentials, CVIE&F block of Cav2.2 channels is weakly reversible. To improve reversibility, we designed and synthesized arginine CVIE&F analogues in which arginine was substituted for lysine at position 10 ([R10K]CVIE&F), and investigated their serum stability and pharmacological actions on voltage-gated calcium channels (VGCCs).

EXPERIMENTAL APPROACH

Changes in peptide structure due to R10K substitution were assessed by NMR. Peptide stability in human serum was analysed by reversed-phase HPLC and MS over a 24 h period. Two-electrode voltage-clamp and whole-cell patch clamp techniques were used to study [R10K]CVIE&F effects on VGCC currents in Xenopus oocytes and rat dorsal root ganglion neurons respectively.

KEY RESULTS

R10K substitution did not change the conserved ω-conotoxin backbone conformations of CVIE&F nor the ω-conotoxin selectivity for recombinant or native Cav2.2 channels, although the inhibitory potency of [R10K]CVIF was better than that of CVIF. At -80 mV, the R10K chemical modification significantly affected ω-conotoxin-channel interaction, resulting in faster onset kinetics than those of CVIE&F. Heterologous and native Cav2.2 channels recovered better from [R10K]CVIE&F block than CVIE&F. In human serum, the ω-conotoxin half-lives were 6-10 h. CVIE&F and [R10K]CVIE&F were more stable than CVID.

CONCLUSIONS AND IMPLICATIONS

R10K substitution in CVIE&F significantly alters the kinetics of ω-conotoxin action and improves reversibility without diminishing conotoxin potency and specificity for the Cav2.2 channel and without diminishing the serum stability. These results may help generate ω-conotoxins with optimized kinetic profiles for target binding.

Spinal blockage of P/Q- or N-type voltage-gated calcium channels modulates functional and symptomatic changes related to haemorrhagic cystitis in mice

BACKGROUND AND PURPOSE

Spinal voltage-gated calcium channels (VGCCs) are pivotal regulators of painful and inflammatory alterations, representing attractive therapeutic targets. We examined the effects of epidural administration of the P/Q- and N-type VGCC blockers Tx3-3 and Phα1β, respectively, isolated from the spider Phoneutria nigriventer, on symptomatic, inflammatory and functional changes allied to mouse cyclophosphamide (CPA)-induced haemorrhagic cystitis (HC). The effects of P. nigriventer-derived toxins were compared with those displayed by MVIIC and MVIIA, extracted from the cone snail Conus magus.

EXPERIMENTAL APPROACH

HC was induced by a single i.p. injection of CPA (300 mg·kg(-1) ). Dose- and time-related effects of spinally administered P/Q and N-type VGCC blockers were assessed on nociceptive behaviour and macroscopic inflammation elicited by CPA. The effects of toxins were also evaluated on cell migration, cytokine production, oxidative stress, functional cystometry alterations and TRPV1, TRPA1 and NK1 receptor mRNA expression.

KEY RESULTS

The spinal blockage of P/Q-type VGCC by Tx3-3 and MVIIC or N-type VGCC by Phα1β attenuated nociceptive and inflammatory events associated with HC, including bladder oxidative stress and cytokine production. CPA produced a slight increase in bladder TRPV1 and TRPA1 mRNA expression, which was reversed by all the toxins tested. Noteworthy, Phα1β strongly prevented bladder neutrophil migration, besides HC-related functional alterations, and its effects were potentiated by co-injecting the selective NK1 receptor antagonist CP-96345.

CONCLUSIONS AND IMPLICATIONS

Our results shed new light on the role of spinal P/Q and N-type VGCC in bladder dysfunctions, pointing out Phα1β as a promising alternative for treating complications associated with CPA-induced HC.

Transient receptor potential ion channels in primary sensory neurons as targets for novel analgesics

The last decade has witnessed an explosion in novel findings relating to the molecules involved in mediating the sensation of pain in humans. Transient receptor potential (TRP) ion channels emerged as the greatest group of molecules involved in the transduction of various physical stimuli into neuronal signals in primary sensory neurons, as well as, in the development of pain. Here, we review the role of TRP ion channels in primary sensory neurons in the development of pain associated with peripheral pathologies and possible strategies to translate preclinical data into the development of effective new analgesics. Based on available evidence, we argue that nociception-related TRP channels on primary sensory neurons provide highly valuable targets for the development of novel analgesics and that, in order to reduce possible undesirable side effects, novel analgesics should prevent the translocation from the cytoplasm to the cell membrane and the sensitization of the channels rather than blocking the channel pore or binding sites for exogenous or endogenous activators.

Sodium hydrosulfide relieves neuropathic pain in chronic constriction injured rats

Aberrant neuronal activity in injured peripheral nerves is believed to be an important factor in the development of neuropathic pain (NPP). Channel protein pCREB of that activity has been shown to mitigate the onset of associated molecular events in the nervous system, and sodium hydrosulfide (NaHS) could inhibit the expression of pCREB. However, whether NaHS could relieve the pain, it needs further experimental research. Furthermore, the clinical potential that NaHS was used to relieve pain was limited so it would be required. To address these issues, the rats of sciatic nerve chronic constriction injury (CCI) were given intraperitoneal injection of NaHS containing hydrogen sulfide (H₂S). The experimental results showed that NaHS inhibited the reduction of paw withdrawal thermal latency (PWTL), mechanical withdrawal threshold (MWT), and the level of pCREB in CCI rats in a dose-dependent manner and they were greatly decreased in NaHS_M group (P < 0.05). NaHS alleviates chronic neuropathic pain by inhibiting expression of pCREB in the spinal cord of Sprague-Dawley rats.

Inhibition of A-type potassium current by the peptide toxin SNX-482

SNX-482, a peptide toxin isolated from tarantula venom, has become widely used as an inhibitor of Cav2.3 voltage-gated calcium channels. Unexpectedly, we found that SNX-482 dramatically reduced the A-type potassium current in acutely dissociated dopamine neurons from mouse substantia nigra pars compacta. The inhibition persisted when calcium was replaced by cobalt, showing that it was not secondary to a reduction of calcium influx. Currents from cloned Kv4.3 channels expressed in HEK-293 cells were inhibited by SNX-482 with an IC50 of <3 nM, revealing substantially greater potency than for SNX-482 inhibition of Cav2.3 channels (IC50 20-60 nM). At sub-saturating concentrations, SNX-482 produced a depolarizing shift in the voltage dependence of activation of Kv4.3 channels and slowed activation kinetics. Similar effects were seen on gating of cloned Kv4.2 channels, but the inhibition was less pronounced and required higher toxin concentrations. These results reveal SNX-482 as the most potent inhibitor of Kv4.3 channels yet identified. Because of the effects on both Kv4.3 and Kv4.2 channels, caution is needed when interpreting the effects of SNX-482 on cells and circuits where these channels are present.

From foe to friend: using animal toxins to investigate ion channel function

Ion channels are vital contributors to cellular communication in a wide range of organisms, a distinct feature that renders this ubiquitous family of membrane-spanning proteins a prime target for toxins found in animal venom. For many years, the unique properties of these naturally occurring molecules have enabled researchers to probe the structural and functional features of ion channels and to define their physiological roles in normal and diseased tissues. To illustrate their considerable impact on the ion channel field, this review will highlight fundamental insights into toxin-channel interactions and recently developed toxin screening methods and practical applications of engineered toxins.

N-type calcium current, Cav2.2, is enhanced in small-diameter sensory neurons isolated from Nf1+/- mice

Major aspects of neuronal function are regulated by Ca(2+) including neurotransmitter release, excitability, developmental plasticity, and gene expression. We reported previously that sensory neurons isolated from a mouse model with a heterozygous mutation of the Nf1 gene (Nf1+/-) exhibited both greater excitability and evoked release of neuropeptides compared to wildtype mice. Furthermore, augmented voltage-dependent sodium currents but not potassium currents contribute to the enhanced excitability. To determine the mechanisms giving rise to the enhanced release of substance P and calcitonin gene-related peptide in the Nf1+/- sensory neurons, the potential differences in the total voltage-dependent calcium current (ICa) as well as the contributions of individual Ca(2+) channel subtypes were assessed. Whole-cell patch-clamp recordings from small-diameter capsaicin-sensitive sensory neurons demonstrated that the average peak ICa densities were not different between the two genotypes. However, by using selective blockers of channel subtypes, the current density of N-type (Cav2.2) ICa was significantly larger in Nf1+/- neurons compared to wildtype neurons. In contrast, there were no significant differences in L-, P/Q- and R-type currents between the two genotypes. Quantitative real-time polymerase chain reaction measurements made from the isolated but intact dorsal root ganglia indicated that N-type (Cav2.2) and P/Q-type (Cav2.1) Ca(2+) channels exhibited the highest mRNA expression levels although there were no significant differences in the levels of mRNA expression between the genotypes. These results suggest that the augmented N-type (Cav2.2) ICa observed in the Nf1+/- sensory neurons does not result from genomic differences but may reflect post-translational or some other non-genomic modifications. Thus, our results demonstrate that sensory neurons from Nf1+/- mice, exhibit increased N-type ICa and likely account for the increased release of substance P and calcitonin gene-related peptide that occurs in Nf1+/- sensory neurons.

Isolation, synthesis and characterization of ω-TRTX-Cc1a, a novel tarantula venom peptide that selectively targets L-type Cav channels

Spider venoms are replete with peptidic ion channel modulators, often with novel subtype selectivity, making them a rich source of pharmacological tools and drug leads. In a search for subtype-selective blockers of voltage-gated calcium (CaV) channels, we isolated and characterized a novel 39-residue peptide, ω-TRTX-Cc1a (Cc1a), from the venom of the tarantula Citharischius crawshayi (now Pelinobius muticus). Cc1a is 67% identical to the spider toxin ω-TRTX-Hg1a, an inhibitor of CaV2.3 channels. We assembled Cc1a using a combination of Boc solid-phase peptide synthesis and native chemical ligation. Oxidative folding yielded two stable, slowly interconverting isomers. Cc1a preferentially inhibited Ba(2+) currents (IBa) mediated by L-type (CaV1.2 and CaV1.3) CaV channels heterologously expressed in Xenopus oocytes, with half-maximal inhibitory concentration (IC50) values of 825nM and 2.24μM, respectively. In rat dorsal root ganglion neurons, Cc1a inhibited IBa mediated by high voltage-activated CaV channels but did not affect low voltage-activated T-type CaV channels. Cc1a exhibited weak activity at NaV1.5 and NaV1.7 voltage-gated sodium (NaV) channels stably expressed in mammalian HEK or CHO cells, respectively. Experiments with modified Cc1a peptides, truncated at the N-terminus (ΔG1-E5) or C-terminus (ΔW35-V39), demonstrated that the N- and C-termini are important for voltage-gated ion channel modulation. We conclude that Cc1a represents a novel pharmacological tool for probing the structure and function of L-type CaV channels.

Icilin reduces voltage-gated calcium channel currents in naïve and injured DRG neurons in the rat spinal nerve ligation model

Recently, the transient receptor potential (TRP) channels TRPM8 and TRPA1 have been identified as molecular sensors for cold, and it has been suggested that they play a crucial role in allodynia by modulating voltage-gated calcium channel currents (ICa(V)). The aim of this study was to analyze the modulation of ICa(V) by the TRPM8-agonist icilin in vitro and to investigate the analgesic effect of icilin in a neuropathic pain model in vivo. Whole cell patch-clamp recordings were performed on isolated naïve and injured rat dorsal root ganglia (DRG) neurons, and the analgesic efficacy of icilin applied topically to the paws or intrathecally was tested in rats after spinal nerve ligation (SNL). ICa(V) (depolarization from -80 to 0mV) in naïve DRG neurons was reduced dose dependently (0.002-200µM) by icilin (18-80%). Subtype isolation of calcium channels show a marked reduction of L-type channel currents compared to N-type channel currents. The effects of icilin on ICa(V) were not significantly different in non-injured and SNL-injured DRG neurons. In vivo, neither topical (10-200µM) nor intrathecal application of icilin (0.1nM to 1µM) affected tactile allodynia or thermal hyperalgesia after SNL, but it increases cold allodynia 6h after application. We conclude that the icilin-induced modulation of ICa(V) in DRG neurons is unlikely to mediate analgesic effects or contribute directly to the pathogenesis of cold allodynia in the rat SNL model, but it is a potential mechanism for the analgesic effects of icilin in other pain models.

Management of cancer pain: 1. Wider implications of orthodox analgesics

In this review, the first of two parts, we first provide an overview of the orthodox analgesics used commonly against cancer pain. Then, we examine in more detail the emerging evidence for the potential impact of analgesic use on cancer risk and disease progression. Increasing findings suggest that long-term use of nonsteroidal anti-inflammatory drugs, particularly aspirin, may reduce cancer occurrence. However, acetaminophen may raise the risk of some hematological malignancies. Drugs acting upon receptors of gamma-aminobutyric acid (GABA) and GABA “mimetics” (eg, gabapentin) appear generally safe for cancer patients, but there is some evidence of potential carcinogenicity. Some barbiturates appear to slightly raise cancer risks and can affect cancer cell behavior in vitro. For cannabis, studies suggest an increased risk of squamous cell carcinoma of the tongue, larynx, and possibly lung. Morphine may stimulate human microvascular endothelial cell proliferation and angiogenesis; it is not clear whether this might cause harm or produce benefit. The opioid, fentanyl, may promote growth in some tumor cell lines. Opium itself is an emerging risk factor for gastric adenocarcinoma and possibly cancers of the esophagus, bladder, larynx, and lung. It is concluded that analgesics currently prescribed for cancer pain can significantly affect the cancer process itself. More futuristically, several ion channels are being targeted with novel analgesics, but many of these are also involved in primary and/or secondary tumorigenesis. Further studies are needed to elucidate possible cellular and molecular effects of orthodox analgesics and their possible long-term impact, both positive and negative, and thus enable the best possible clinical gain for cancer patients.