Mechanisms of non-steroid anti-inflammatory drugs action on ASICs expressed in hippocampal interneurons

Stress-mediated aggregation of disease-associated proteins in amyloid bodies

The formation of protein aggregates is a hallmark of many neurodegenerative diseases and systemic amyloidoses. These disorders are associated with the fibrillation of a variety of proteins/peptides, which ultimately leads to cell toxicity and tissue damage. Understanding how amyloid aggregation occurs and developing compounds that impair this process is a major challenge in the health science community. Here, we demonstrate that pathogenic proteins associated with Alzheimer's disease, diabetes, AL/AA amyloidosis, and amyotrophic lateral sclerosis can aggregate within stress-inducible physiological amyloid-based structures, termed amyloid bodies (A-bodies). Using a limited collection of small molecule inhibitors, we found that diclofenac could repress amyloid aggregation of the β-amyloid (1-42) in a cellular setting, despite having no effect in the classic Thioflavin T (ThT) in vitro fibrillation assay. Mapping the mechanism of the diclofenac-mediated repression indicated that dysregulation of cyclooxygenases and the prostaglandin synthesis pathway was potentially responsible for this effect. Together, this work suggests that the A-body machinery may be linked to a subset of pathological amyloidosis, and highlights the utility of this model system in the identification of new small molecules that could treat these debilitating diseases.

Physiologically relevant acid-sensing ion channel (ASIC) 2a/3 heteromers have a 1:2 stoichiometry

Acid-sensing ion channels (ASICs) sense extracellular protons and are involved in synaptic transmission and pain sensation. ASIC1a and ASIC3 are the ASIC subunits with the highest proton sensitivity. ASIC2a in contrast has low proton sensitivity but increases the variability of ASICs by forming heteromers with ASIC1a or ASIC3. ASICs are trimers and for the ASIC1a/2a heteromer it has been shown that subunits randomly assemble with a flexible 1:2/2:1 stoichiometry. Both heteromers have almost identical proton sensitivity intermediate between ASIC1a and ASIC2a. Here, we investigated the stoichiometry of the ASIC2a/3 heteromer. Using electrophysiology, we extensively characterized, first, cells expressing ASIC2a and ASIC3 at different ratios, second, concatemeric channels with a fixed subunit stoichiometry, and, third, channels containing loss-of-functions mutations in specific subunits. Our results conclusively show that only ASIC2a/3 heteromers with a 1:2 stoichiometry had a proton-sensitivity intermediate between ASIC2a and ASIC3. In contrast, the proton sensitivity of ASIC2a/3 heteromers with a 2:1 stoichiometry was strongly acid-shifted by more than one pH unit, which suggests that they are not physiologically relevant. Together, our results reveal that the proton sensitivity of the two ASIC2a/3 heteromers is clearly different and that ASIC3 and ASIC1a make remarkably different contributions to heteromers with ASIC2a.

Distribution of acid-sensing ion channel subunits in human sensory neurons contrasts with that in rodents

Acid-sensing ion channels (ASICs) play a critical role in nociception in human sensory neurons. Four genes (ASIC1, ASIC2, ASIC3, and ASIC4) encoding multiple subunits through alternative splicing have been identified in humans. Real time-PCR experiments showed strong expression of three subunits ASIC1, ASIC2, and ASIC3 in human dorsal root ganglia; however, their detailed expression pattern in different neuronal populations has not been investigated yet. In the current study, using an in situ hybridization approach (RNAscope), we examined the presence of ASIC1, ASIC2, and ASIC3 mRNA in three subpopulations of human dorsal root ganglia neurons. Our results revealed that ASIC1 and ASIC3 were present in the vast majority of dorsal root ganglia neurons, while ASIC2 was only expressed in less than half of dorsal root ganglia neurons. The distribution pattern of the three ASIC subunits was the same across the three populations of dorsal root ganglia neurons examined, including neurons expressing the REarranged during Transfection (RET) receptor tyrosine kinase, calcitonin gene-related peptide, and a subpopulation of nociceptors expressing Transient Receptor Potential Cation Channel Subfamily V Member 1. These results strongly contrast the expression pattern of Asics in mice since our previous study demonstrated differential distribution of Asics among the various subpopulation of dorsal root ganglia neurons. Given the distinct acid-sensitivity and activity dynamics among different ASIC channels, the expression differences between human and rodents should be taken under consideration when evaluating the translational potential and efficiency of drugs targeting ASICs in rodent studies.

Development and Challenges of Diclofenac-Based Novel Therapeutics: Targeting Cancer and Complex Diseases

Simple Summary

Diclofenac is a widely used drug for its anti-inflammatory and pain alleviating properties. This review summarizes the current understanding about the drug diclofenac. The potential applications of diclofenac beyond its well-known anti-inflammatory properties for other diseases such as cancer are discussed, along with existing limitations.

Abstract

Diclofenac is a highly prescribed non-steroidal anti-inflammatory drug (NSAID) that relieves inflammation, pain, fever, and aches, used at different doses depending on clinical conditions. This drug inhibits cyclooxygenase-1 and cyclooxygenase-2 enzymes, which are responsible for the generation of prostaglandin synthesis. To improve current diclofenac-based therapies, we require new molecular systematic therapeutic approaches to reduce complex multifactorial effects. However, the critical challenge that appears with diclofenac and other drugs of the same class is their side effects, such as signs of stomach injuries, kidney problems, cardiovascular issues, hepatic issues, and diarrhea. In this article, we discuss why defining diclofenac-based mechanisms, pharmacological features, and its medicinal properties are needed to direct future drug development against neurodegeneration and imperfect ageing and to improve cancer therapy. In addition, we describe various advance molecular mechanisms and fundamental aspects linked with diclofenac which can strengthen and enable the better designing of new derivatives of diclofenac to overcome critical challenges and improve their applications.

Synaptic signals mediated by protons and acid-sensing ion channels

Extracellular pH changes may constitute significant signals for neuronal communication. During synaptic transmission, changes in pH in the synaptic cleft take place. Its role in the regulation of presynaptic Ca²⁺ currents through multivesicular release in ribbon-type synapses is a proven phenomenon. In recent years, protons have been recognized as neurotransmitters that participate in neuronal communication in synapses of several regions of the CNS such as amygdala, nucleus accumbens, and brainstem. Protons are released by nerve stimulation and activate postsynaptic acid-sensing ion channels (ASICs). Several types of ASIC channels are expressed in the peripheral and central nervous system. The influx of Ca²⁺ through some subtypes of ASICs, as a result of synaptic transmission, agrees with the participation of ASICs in synaptic plasticity. Pharmacological and genetical inhibition of ASIC1a results in alterations in learning, memory, and phenomena like fear and cocaine-seeking behavior. The recognition of endogenous molecules, such as arachidonic acid, cytokines, histamine, spermine, lactate, and neuropeptides, capable of inhibiting or potentiating ASICs suggests the existence of mechanisms of synaptic modulation that have not yet been fully identified and that could be tuned by new emerging pharmacological compounds with potential therapeutic benefits.

Identification of Isoform 2 Acid-Sensing Ion Channel Inhibitors as Tool Compounds for Target Validation Studies in CNS

Acid-sensing ion channels (ASICs) are a family of ion channels permeable to cations and largely responsible for the onset of acid-evoked ion currents both in neurons and in different types of cancer cells, thus representing a potential target for drug discovery. Owing to the limited attention ASIC2 has received so far, an exploratory program was initiated to identify ASIC2 inhibitors using diminazene, a known pan-ASIC inhibitor, as a chemical starting point for structural elaboration. The performed exploration enabled the identification of a novel series of ASIC2 inhibitors. In particular, compound 2u is a brain penetrant ASIC2 inhibitor endowed with an optimal pharmacokinetic profile. This compound may represent a useful tool to validate in animal models in vivo the role of ASIC2 in different neurodegenerative central nervous system pathologies.

Modulation of Proton-Gated Channels by Antidepressants

The chemical structures of some antidepressants are similar to those of recently described amine-containing ligands of acid-sensing ion channels (ASICs). ASICs are expressed in brain neurons and participate in numerous CNS functions. As such, they can be related to antidepressant action or side effects. We therefore studied the actions of a series of antidepressants on recombinant ASIC1a and ASIC2a and on native ASICs in rat brain neurons. Most of the tested compounds prevented steady-state ASIC1a desensitization evoked by conditioning acidification to pH 7.1. Amitriptyline also potentiated ASIC1a responses evoked by pH drops from 7.4 to 6.5. We conclude that amitriptyline has a twofold effect: it shifts activation to less acidic values while also shifting steady-state desensitization to more acidic values. Chlorpromazine, desipramine, amitriptyline, fluoxetine, and atomoxetine potentiated ASIC2a response. Tianeptine caused strong inhibition of ASIC2a. Both potentiation and inhibition of ASIC2a were accompanied by the slowdown of desensitization, suggesting distinct mechanisms of action on activation and desensitization. In experiments on native heteromeric ASICs, tianeptine and amitriptyline demonstrated the same modes of action as on ASIC2a although with reduced potency.

The Role of Toxins in the Pursuit for Novel Analgesics

Chronic pain is a major medical issue which reduces the quality of life of millions and inflicts a significant burden on health authorities worldwide. Currently, management of chronic pain includes first-line pharmacological therapies that are inadequately effective, as in just a portion of patients pain relief is obtained. Furthermore, most analgesics in use produce severe or intolerable adverse effects that impose dose restrictions and reduce compliance. As the majority of analgesic agents act on the central nervous system (CNS), it is possible that blocking pain at its source by targeting nociceptors would prove more efficient with minimal CNS-related side effects. The development of such analgesics requires the identification of appropriate molecular targets and thorough understanding of their structural and functional features. To this end, plant and animal toxins can be employed as they affect ion channels with high potency and selectivity. Moreover, elucidation of the toxin-bound ion channel structure could generate pharmacophores for rational drug design while favorable safety and analgesic profiles could highlight toxins as leads or even as valuable therapeutic compounds themselves. Here, we discuss the use of plant and animal toxins in the characterization of peripherally expressed ion channels which are implicated in pain.

Inhibition on acid-sensing ion channels and analgesic activities of flavonoids isolated from dragon's blood resin

Acid-sensing ion channel (ASIC) serves important roles in the transmission of nociceptive information. To confirm the analgesic mechanism of dragon's blood resin, patch-clamp technique, in vivo animal experiments, and immunohistochemical staining were used to observe the effects of the three flavonoids (loureirin B, cochinchinemin A, and cochinchinemin B) isolated from dragon's blood resin on ASIC. Results showed that the three flavonoids exerted various inhibitory effects on ASIC currents in rat dorsal root ganglion (DRG) neurons. The combination of the three flavonoids with total concentration of 6.5 μM could decrease (53.8 ± 4.3%) of the peak amplitude and (45.8 ± 4.5%) of the sustained portion of ASIC currents. The combination of the three flavonoids was fully efficacious on complete Freud's adjuvant (CFA)-induced inflammatory thermal hyperalgesia at a dose of 6.5 mM similar with amiloride at 10 mM. The analgesic effects of the combination could be weakened by an ASIC activator 2-guanidine-4-methylquinazoline. CFA-induced hyperalgesia was accompanied by c-Fos up-regulation in DRG neurons, and the combination rescued thermal hyperalgesia through down-regulation of c-Fos and ASIC3 expression in CFA-induced inflammation. These collective results suggested that the flavonoids isolated from dragon's blood resin could be considered as the chemical compounds that exert analgesic effects on inflammatory thermal pain due to action on ASIC.

Multiple modes of action of hydrophobic amines and their guanidine analogues on ASIC1a

Hydrophobic monoamines containing only a hydrophobic/aromatic moiety and protonated amino group are a recently described class of acid-sensing ion channel (ASIC) modulators. Intensive studies have revealed a number of active compounds including endogenous amines and pharmacological agents and shown that these compounds potentiate and inhibit ASICs depending on their specific structure and on subunit composition of the target channel. The action of monoamines also depends on the application protocol, membrane voltage, conditioning and activating pH, suggesting complex mechanism(s) of the ligand-receptor interaction. Without understanding of these mechanisms analysis of structure-function relationships and predictive search for new potent and selective drugs are hardly possible. To this end, we investigated the modes of action for a representative series of amine and guanidine derivatives of adamantane and phenylcyclohexyl. The study was performed on transfected Chinese hamster ovary (CHO) cells and rat hippocampal interneurons using whole-cell patch clamp recording. We found that complex picture of monoamine action can be rationalized assuming four modes of action: (1) voltage-dependent pore block, (2) acidic shift of activation, (3) alkaline shift of activation and (4) acidic shift of steady-state desensitization. Structure-activity relationships are discussed in the light of this framework. The experiments on native heteromeric ASICs have shown that some of these mechanisms are shared between them and recombinant ASIC1a, implying that our results could also be relevant for amine action in physiological and pathological conditions.

A Review of Chronic Musculoskeletal Pain: Central and Peripheral Effects of Diclofenac

Diclofenac is widely used to manage chronic inflammatory and degenerative joint diseases such as osteoarthritis (OA), rheumatoid arthritis (RA), ankylosing spondylitis, and extra-articular rheumatism. Its various mechanisms of action make it particularly effective in treating nociceptive pain, but it is also an alternative for treating spinal and chronic central pain. Osteoarthritis and rheumatoid arthritis are the most frequently encountered arthritic conditions in adults. The management of nociceptive pain requires a sequential hierarchical approach, with the initial NSAID treatment being characterized by the replacement of one drug with another, or complete discontinuation usually because of insufficient pain control. OA- and RA-related pain is complex and multifactorial, and due to physiological interactions between the signaling of the central and peripheral nervous systems. The mechanisms of action of diclofenac make it particularly effective in treating both nociceptive pain and chronic central pain. This review underlines the mechanisms of diclofenac involved in chronic and acute joint pain, the most relevant adverse events.

Molecular basis of inhibition of acid sensing ion channel 1A by diminazene

Acid-sensing ion channels (ASICs) are trimeric proton-gated cation permeable ion channels expressed primarily in neurons. Here we employed site-directed mutagenesis and electrophysiology to investigate the mechanism of inhibition of ASIC1a by diminazene. This compound inhibits mouse ASIC1a with a half-maximal inhibitory concentration (IC50) of 2.4 μM. At first, we examined whether neutralizing mutations of Glu79 and Glu416 alter diminazene block. These residues form a hexagonal array in the lower palm domain that was previously shown to contribute to pore opening in response to extracellular acidification. Significantly, single Gln substitutions at positions 79 and 416 in ASIC1a reduced diminazene apparent affinity by 6-7 fold. This result suggests that diminazene inhibits ASIC1a in part by limiting conformational rearrangement in the lower palm domain. Because diminazene is charged at physiological pHs, we assessed whether it inhibits ASIC1a by blocking the ion channel pore. Consistent with the notion that diminazene binds to a site within the membrane electric field, diminazene block showed a strong dependence with the membrane potential. Moreover, a Gly to Ala mutation at position 438, in the ion conduction pathway of ASIC1a, increased diminazene IC50 by one order of magnitude and eliminated the voltage dependence of block. Taken together, our results indicate that the inhibition of ASIC1a by diminazene involves both allosteric modulation and blocking of ion flow through the conduction pathway. Our findings provide a foundation for the development of more selective and potent ASIC pore blockers.

Targeted Acid-Sensing Ion Channel Therapies for Migraine

Acid-sensing ion channels (ASICs) are a family of ion channels, consisting of four members; ASIC1 to 4. These channels are sensitive to changes in pH and are expressed throughout the central and peripheral nervous systems-including brain, spinal cord, and sensory ganglia. They have been implicated in a number of neurological conditions such as stroke and cerebral ischemia, traumatic brain injury, and epilepsy, and more recently in migraine. Their expression within areas of interest in the brain in migraine, such as the hypothalamus and PAG, their demonstrated involvement in preclinical models of meningeal afferent signaling, and their role in cortical spreading depression (the electrophysiological correlate of migraine aura), has enhanced research interest into these channels as potential therapeutic targets in migraine. Migraine is a disorder with a paucity of both acute and preventive therapies available, in which at best 50% of patients respond to available medications, and these medications often have intolerable side effects. There is therefore a great need for therapeutic development for this disabling condition. This review will summarize the understanding of the structure and CNS expression of ASICs, the mechanisms for their potential role in nociception, recent work in migraine, and areas for future research and drug development.

Inhibition of acid-sensing ion channels by diminazene and APETx2 evoke partial and highly variable antihyperalgesia in a rat model of inflammatory pain

BACKGROUND AND PURPOSE

Acid-sensing ion channels (ASICs) are primary acid sensors in mammals, with the ASIC1b and ASIC3 subtypes being involved in peripheral nociception. The antiprotozoal drug diminazene is a moderately potent ASIC inhibitor, but its analgesic activity has not been assessed.

EXPERIMENTAL APPROACH

We determined the ASIC subtype selectivity of diminazene and the mechanism by which it inhibits ASICs using voltage-clamp electrophysiology of Xenopus oocytes expressing ASICs 1-3. Its peripheral analgesic activity was then assessed relative to APETx2, an ASIC3 inhibitor, and morphine, in a Freund's complete adjuvant (FCA)-induced rat model of inflammatory pain.

KEY RESULTS

Diminazene inhibited homomeric rat ASICs with IC₅₀ values of ~200-800 nM, via an open channel and subtype-dependent mechanism. In rats with FCA-induced inflammatory pain in one hindpaw, diminazene and APETx2 evoked more potent peripheral antihyperalgesia than morphine, but the effect was partial for APETx2. APETx2 potentiated rat ASIC1b at concentrations 30-fold to 100-fold higher than the concentration inhibiting ASIC3, which may have implications for its use in in vivo experiments.

CONCLUSIONS AND IMPLICATIONS

Diminazene and APETx2 are moderately potent ASIC inhibitors, both inducing peripheral antihyperalgesia in a rat model of chronic inflammatory pain. APETx2 has a more complex ASIC pharmacology, which must be considered when it is used as a supposedly selective ASIC3 inhibitor in vivo. Our use of outbred rats revealed responders and non-responders when ASIC inhibition was used to alleviate inflammatory pain, which is aligned with the concept of number-needed-to-treat in human clinical studies.

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.

Potentiation and Block of ASIC1a by Memantine

Acid-sensing ion channels (ASICs) are modulated by various classes of ligands, including the recently described hydrophobic monoamines, which inhibit and potentiate ASICs in a subunit-specific manner. In particular, memantine inhibits ASIC1a and potentiates ASIC2a homomers. The aim of the present work was to characterize action mechanism of memantine on recombinant ASIC1a expressed in CHO (Chinese hamster ovary) cells. We have demonstrated that effect of memantine on ASIC1a strongly depends on membrane voltage, conditioning pH value and application protocol. When applied simultaneously with activating acidification at hyperpolarized voltages, memantine caused the strongest inhibition. Surprisingly, application of memantine between ASIC1a activations at zero voltage caused significant potentiation. Analysis of the data suggests that memantine produces two separate effects, voltage-dependent open-channel block and shift of steady-state desensitization curve to more acidic values. Putative binding sites are discussed based on the computer docking of memantine to the acidic pocket and the pore region.

Diminazene Is a Slow Pore Blocker of Acid-Sensing Ion Channel 1a (ASIC1a)

Acid-sensing ion channels (ASICs) are neuronal receptors for extracellular protons. They contribute to the excitatory postsynaptic current and to the detection of painful acidosis. Moreover, they are activated during peripheral inflammation and acidosis associated with various neuronal disorders, such as stroke and neuroinflammation, rendering them interesting drug targets. Diminazene aceturate is a small-molecule inhibitor of ASICs with a reported apparent affinity in the low micromolar range, making it an interesting lead compound. It was reported that diminazene accelerates desensitization of ASICs, which was, however, not explained mechanistically. Furthermore, a binding site in a groove of the extracellular domain was proposed but not experimentally verified. In this study, we revisited the mechanism of inhibition by diminazene and its binding site on ASIC1a, the ASIC subunit with the greatest importance in the central nervous system. We show that diminazene slowly blocks ASIC1a, leading to the apparent acceleration of desensitization and underestimating its potency; we show that diminazene indeed has a submicromolar potency at ASIC1a (IC₅₀ 0.3 μM). Moreover, we show that the inhibition is voltage-dependent and competes with that by amiloride, a pore blocker of ASICs. Finally, we identify by molecular docking a binding site in the ion pore that we confirm by site-directed mutagenesis. In summary, our results show that diminazene blocks ASIC1a by a slow open-channel block and suggest that diminazene is an interesting lead compound for high-affinity blockers of ASICs.

Molecular Basis for Allosteric Inhibition of Acid-Sensing Ion Channel 1a by Ibuprofen

A growing body of evidence links certain aspects of nonsteroidal anti-inflammatory drug (NSAID) pharmacology with acid-sensing ion channels (ASICs), a small family of excitatory neurotransmitter receptors implicated in pain and neuroinflammation. The molecular basis of NSAID inhibition of ASICs has remained unknown, hindering the exploration of this line of therapy. Here, we characterized the mechanism of inhibition, explored the molecular determinants of sensitivity, and sought to establish informative structure-activity relationships, using electrophysiology, site-directed mutagenesis, and voltage-clamp fluorometry. Our results show that ibuprofen is an allosteric inhibitor of ASIC1a, which binds to a crucial site in the agonist transduction pathway and causes conformational changes that oppose channel activation. Ibuprofen inhibits several ASIC subtypes, but certain ibuprofen derivatives show some selectivity for ASIC1a over ASIC2a and vice versa. These results thus define the NSAID/ASIC interaction and pave the way for small-molecule drug design targeting pain and inflammation.

Acid-Sensing Ion Channels as Potential Therapeutic Targets in Neurodegeneration and Neuroinflammation

Acid-sensing ion channels (ASICs) are a family of proton-sensing channels that are voltage insensitive, cation selective (mostly permeable to Na⁺), and nonspecifically blocked by amiloride. Derived from 5 genes (ACCN1-5), 7 subunits have been identified, 1a, 1b, 2a, 2b, 3, 4, and 5, that are widely expressed in the peripheral and central nervous system as well as other tissues. Over the years, different studies have shown that activation of these channels is linked to various physiological and pathological processes, such as memory, learning, fear, anxiety, ischemia, and multiple sclerosis to name a few, so their potential as therapeutic targets is increasing. This review focuses on recent advances that have helped us to better understand the role played by ASICs in different pathologies related to neurodegenerative diseases, inflammatory processes, and pain.

Evaluation of DNA damage and physiological responses in Nile tilapia, Oreochromis niloticus (Linnaeus, 1758) exposed to sub-lethal diclofenac (DCF)

The frequent bioaccumulation of pharmaceuticals in the aquatic ecosystem has raised a concern about their possible ecotoxicological consequences. DNA damage, haematological changes and activities of oxidative stress enzymes in Nile tilapia, Oreochromis niloticus in response to diclofenac (DCF) exposure were investigated for up to 60 days at the concentrations of 0.17, 0.34 and 0.68mgL^-1 in the fish liver. Evaluation of genotoxic effects of the drug in the liver, using single-cell gel electrophoresis, showed DNA damage on exposure at the concentrations of 0.34 and 0.68mgL^-1 after day 30. Compared with the control, there was a reduction in haemoglobin and red blood cell counts with a significant increase (p<0.05) in white blood cell counts, mean corpuscular volume and mean corpuscular haemoglobin level after day 30 at 0.34 and 0.68mgL^-1. The levels of pack cell volume, red cell distribution width and mean corpuscular haemoglobin concentration were not significant (p>0.05) between the exposed group and the control. The indices of hepatic oxidative stress biomarkers, including lipid peroxidation and carbonyl protein, showed elevated level, depicting a positive correlation with both time and concentration. More so, activity of catalase was inhibited while reduced glutathione level decreased in the liver tissue. There was increase in the activities of superoxide dismutase, glutathione peroxidase and glutathione-S-transferase after 30 days at 0.34mgL^-1. Further, activity of Na⁺-K⁺-ATPase in the tissue was significantly inhibited (p<0.05) at the end of 60 days. Prolonged exposure to diclofenac at sub-lethal concentration can cause both DNA and oxidative damages in O. niloticus, suggesting the use of oxidative stress biomarkers as early warning signals in environmental monitoring of residual pharmaceutical and assessment.

Pharmacotherapy in Generalized Anxiety Disorder: Novel Experimental Medicine Models and Emerging Drug Targets

Many pharmacological and psychological approaches have been found efficacious in patients with generalized anxiety disorder (GAD), but many treatment-seeking patients will not respond and others will relapse despite continuing with interventions that initially had beneficial effects. Other patients will respond but then stop treatment early because of untoward effects such as sexual dysfunction, drowsiness, and weight gain. There is much scope for the development of novel approaches that could have greater overall effectiveness or acceptability than currently available interventions or that have particular effectiveness in specific clinical subgroups. 'Experimental medicine' studies in healthy volunteers model disease states and represent a proof-of-concept approach for the development of novel therapeutic interventions: they determine whether to proceed to pivotal efficacy studies and so can reduce delays in translating innovations into clinical practice. Investigations in healthy volunteers challenged with the inhalation of air 'enriched' with 7.5% carbon dioxide (CO₂) indicate this technique provides a validated and robust experimental medicine model, mirroring the subjective, autonomic, and cognitive features of GAD. The anxiety response during CO₂ challenge probably involves both central noradrenergic neurotransmission and effects on acid-base sensitive receptors and so may stimulate development of novel agents targeted at central chemosensors. Increasing awareness of the potential role of altered cytokine balance in anxiety and the interplay of cytokines with monoaminergic mechanisms may also encourage the investigation of novel agents with modulating effects on immunological profiles. Although seemingly disparate, these two approaches to treatment development may pivot on a shared mechanism in exerting anxiolytic-like effects through pharmacological effects on acid-sensing ion channels.

Acid-Sensing Ion Channel Pharmacology, Past, Present, and Future …

pH is one of the most strictly controlled parameters in mammalian physiology. An extracellular pH of ~7.4 is crucial for normal physiological processes, and perturbations to this have profound effects on cell function. Acidic microenvironments occur in many physiological and pathological conditions, including inflammation, bone remodeling, ischemia, trauma, and intense synaptic activity. Cells exposed to these conditions respond in different ways, from tumor cells that thrive to neurons that are either suppressed or hyperactivated, often fatally. Acid-sensing ion channels (ASICs) are primary pH sensors in mammals and are expressed widely in neuronal and nonneuronal cells. There are six main subtypes of ASICs in rodents that can form homo- or heteromeric channels resulting in many potential combinations. ASICs are present and activated under all of the conditions mentioned earlier, suggesting that they play an important role in how cells respond to acidosis. Compared to many other ion channel families, ASICs were relatively recently discovered-1997-and there is a substantial lack of potent, subtype-selective ligands that can be used to elucidate their structural and functional properties. In this chapter I cover the history of ASIC channel pharmacology, which began before the proteins were even identified, and describe the current arsenal of tools available, their limitations, and take a glance into the future to predict from where new tools are likely to emerge.

Computer modeling of ischemic stroke

The occlusion of a blood vessel in the brain causes an ischemic stroke. Current treatment relies restoration of blood flow within 3 hours. Substantial research has focused on neuroprotection to spare compromised neural tissue and extend the treatment time window. Despite success with animal models and extensive associated clinical testing, there are still no therapies of this kind. Ischemic stroke is fundamentally a multiscale phenomenon where a cascade of changes triggered by loss of blood flow involves processes at spatial scales from molecular to centimeters with damage occurring in milliseconds to days and recovery into years. Multiscale computational modeling is a technique to assist understanding of the many agents involved in these multitudinous interacting pathways to provide clues for in silico development of multi-target polypharmacy drug cocktails.

Transient Receptor Potential Channels in Microglia: Roles in Physiology and Disease

Microglia modulate the nervous system cellular environment and induce neuroprotective and neurotoxic effects. Various molecules are involved in these processes, including families of ion channels expressed in microglial cells, such as transient receptor potential (TRP) channels. TRP channels comprise a family of non-selective cation channels that can be activated by mechanical, thermal, and chemical stimuli, and which contribute to the regulation of intracellular calcium concentrations. TRP channels have been shown to be involved in cellular processes such as osmotic regulation, cytokine production, proliferation, activation, cell death, and oxidative stress responses. Given the significance of these processes in microglial activity, studies of TRP channels in microglia have focused on determining their roles in both neuroprotective and neurotoxic processes. TRP channel activity has been proposed to play an important function in neurodegenerative diseases, ischemia, inflammatory responses, and neuropathic pain. Modulation of TRP channel activity may thus be considered as a potential therapeutic strategy for the treatment of various diseases associated with alterations of the central nervous system (CNS). In this review, we describe the expression of different subfamilies of TRP channels in microglia, focusing on their physiological and pathophysiological roles, and consider their potential use as therapeutic targets in CNS diseases.

Acid-sensing ion channels and their modulators

According to a modern look acid-sensing ion channels (ASICs) are one of the most important receptors that perceive pH change in the body. ASICs represent proton-gated Na+-selective channels, which are expressed in neurons of the central and peripheral nervous system. These channels are attracting attention of researchers around the world, as they are involved in various physiological processes in the body. Drop of pH may occur in tissues in norm (e.g. the accumulation of lactic acid, the release of protons upon ATP hydrolysis) and pathology (inflammation, ischemic stroke, tissue damage and seizure). These processes are accompanied by unpleasant pain sensations, which may be short-lived or can lead to chronic inflammatory diseases. Modulators of ASIC channels activity are potential candidates for new effective analgesic and neuroprotection drugs. This review summarizes available information about structure, function, and physiological role of ASIC channels. In addition a description of all known ligands of these channels and their practical relevance is provided.

Acid-sensing ion channels: potential therapeutic targets for neurologic diseases

Maintaining the physiological pH of interstitial fluid is crucial for normal cellular functions. In disease states, tissue acidosis is a common pathologic change causing abnormal activation of acid-sensing ion channels (ASICs), which according to cumulative evidence, significantly contributes to inflammation, mitochondrial dysfunction, and other pathologic mechanisms (i.e., pain, stroke, and psychiatric conditions). Thus, it has become increasingly clear that ASICs are critical in the progression of neurologic diseases. This review is focused on the importance of ASICs as potential therapeutic targets in combating neurologic diseases.

Evaluation of developmental toxicity and teratogenicity of diclofenac using Xenopus embryos

Diclofenac is a non-steroidal anti-inflammatory drug (NSAID) with analgesic and anti-pyretic properties. This compound is therefore used to treat pain, inflammatory disorders, and dysmenorrhea. Due to its multimodal mechanism of action and ability to penetrate placenta, diclofenac is known to have undesirable side effects including teratogenicity. However, limited data exist on its teratogenicity, and a detailed investigation regarding harmful effects of this drug during embryogenesis is warranted. Here, we analyzed the developmental toxic effects of diclofenac using Xenopus embryos according to the Frog Embryo Teratogenesis Assay-Xenopus (FETAX) protocol. Diclofenac treatment exerted a teratogenic effect on Xenopus embryos with a teratogenic index (TI) value of 2.64 TI; if this value is higher than 1.2, the cut-off value indicative of toxicity. In particular, mortality of embryos treated with diclofenac increased in a concentration-dependent manner and a broad spectrum of malformations such as shortening and kinking of the axis, abdominal bulging, and prominent blister formation, was observed. The shape and length of internal organs also differed compared to the control group embryos and show developmental retardation on histological label. However, the expression of major tissue-specific markers did not change when analyzed by reverse transcription-polymerase chain reaction (RT-PCR). In conclusion, diclofenac treatment can promote teratogenicity that results in morphological anomalies, but not disrupt the developmental tissue arrangement during Xenopus embryogenesis.

Monoamine NMDA receptor channel blockers inhibit and potentiate native and recombinant proton-gated ion channels

Acid-sensing ion channels (ASICs) are widely distributed in the peripheral and central nervous system. Although they are involved in many physiological functions, the actual processes that activate ASICs remain unclear. This is particularly true for brain ASICs, which produce only a transient response to a fast drop in pH and cannot mediate sustained current. Therefore, the search for ASIC inhibitors and, especially, potentiators/activators is important. We report that NMDA receptor channel blockers with a comparatively simple structure (9-aminoacridine, memantine, IEM-2117 and IEM-1921) potentiate and/or inhibit ASICs in submillimolar concentrations. The experiments were performed using the patch clamp technique on native ASICs from rat hippocampal interneurons and recombinant ASICs of different subunit compositions expressed in CHO cells. Native ASICs were potentiated by IEM-1921 and IEM-2117, and inhibited by memantine and 9-aminoacridine. Homomeric ASIC1a were inhibited by memantine, IEM-2117 and 9-aminoacridine while IEM-1921 was ineffective. In contrast, homomeric ASIC2a were potentiated by IEM-2117, memantine and IEM-1921, whereas 9-aminoacridine was inactive. The compounds caused a complex effect on ASIC3. 9-aminoacridine and IEM-1921 potentiated the steady-state response of ASIC3 and inhibited the peak component. IEM-2117 not only potentiated ASIC3-mediated currents caused by acidification but also evoked steady-state currents at neutral pH. Our results demonstrate that, depending on the subunit composition, ASICs can be activated or inhibited by simple compounds that possess only amino group and aromatic/hydrophobic moieties. This opens up the possibility to search for new ASIC modulators among a number of endogenous ligands.

Expression of acid-sensing ion channels in nucleus pulposus cells of the human intervertebral disk is regulated by non-steroid anti-inflammatory drugs

Non-steroid anti-inflammatory drugs (NSAIDs) are generally used in the treatment of inflammation and pain through cyclooxygenase (COX) inhibition. Mounting evidence has indicated additional COX-independent targets for NSAIDs including acid-sensing ion channels (ASICs) 1a and 3. However, detailed function and mechanism of ASICs still remain largely elusive. In this study, the impact of NSAIDs on ASICs in nucleus pulposus cells of the human intervertebral disk was investigated. Nucleus pulposus cells were isolated and cultured from protruded disk tissues of 40 patients. It was shown that ASIC1a and ASIC3 were expressed and functional in these cells by analyzing proton-gated currents after ASIC inhibition. We further investigated the neuroprotective capacity of ibuprofen (a COX inhibitor), psalmotoxin-1 (PcTX1, a tarantula toxin specific for homomeric ASIC1a), and amiloride (a classic inhibitor of the epithelial sodium channel ENaC/DEG family to which ASICs belong). PcTX1-containing venom has been shown to be comparable with amiloride in its neuroprotective features in rodent models of ischemia. Taken together, our data showed that amiloride, PcTX1, and ibuprofen decreased ASIC protein expression and thereby exerted protective effects from ASIC inhibition-mediated cell damage.

Electrophysiological and metabolic effects of CHF5074 in the hippocampus: protection against in vitro ischemia

CHF5074 is a non-steroidal anti-inflammatory derivative holding disease-modifying potential for the treatment of Alzheimer's disease. The aim of the present study was to characterize the electrophysiological and metabolic profile of CHF5074 in the hippocampus. Electrophysiological recordings show that CHF5074 inhibits in a dose-dependent manner the current-evoked repetitive firing discharge in CA1 pyramidal neurons. This result is paralleled by a dose-dependent reduction of field excitatory post-synaptic potentials with no effect on the paired-pulse ratio. The effects of CHF5074 were not mediated by AMPA or NMDA receptors, since the inward currents induced by local applications of AMPA and NMDA remained constant in the presence of this compound. We also suggest a possible activity of CHF5074 on ASIC1a receptor since ASIC1a-mediated current, evoked by application of a pH 5.5 solution, is reduced by pretreatment with this compound. Moreover, we demonstrate that CHF5074 treatment is able to counteract in hippocampal slices the OGD-induced increase in alanine, lactate and acetate levels. Finally, CHF5074 significantly reduced the apoptosis in hippocampal neurons exposed to OGD, as revealed by cleaved-caspase-3 immunoreactivity and TUNEL staining. Overall, the present work identifies novel mechanisms for CHF5074 in reducing metabolic acidosis, rendering this compound potentially useful also in conditions of brain ischemia.

Translational strategies for neuroprotection in ischemic stroke--focusing on acid-sensing ion channel 1a

Ischemic stroke contributes to the majority of brain injuries and remains to be a leading cause of death and long-term disability. Despite the devastating pathology and high incidence of disease, there remain only few treatment options (TPA and endovascular procedures), which may be hampered by time-dependent administration among a variety of other factors. Promising research of glutamate receptor antagonists has been unsuccessful in clinical trial. But, the mechanism by which glutamate receptors initiate injury by excessive calcium overload has spurred investigation of new and potentially successful candidates for stroke therapy. Acid-sensing ion channels (ASICs) may contribute to poor stroke prognosis due to localized drop in brain pH, resulting in excessive calcium overload, independent of glutamate activation. Accumulating studies targeting ASICs have underscored the importance of understanding inhibition, regulation, desensitization, and trafficking of this channel and its role in disease. This review will discuss potential directions in translational ASIC research for future stroke therapies.

Aquaporin-4 inhibition mediates piroxicam-induced neuroprotection against focal cerebral ischemia/reperfusion injury in rodents

BACKGROUND AND PURPOSE

Aquaporin-4(AQP4) is an abundant water channel protein in brain that regulates water transport to maintain homeostasis. Cerebral edema resulting from AQP4 over expression is considered to be one of the major determinants for progressive neuronal insult during cerebral ischemia. Although, both upregulation and downregulation of AQP4 expression is associated with brain pathology, over expression of AQP4 is one of the chief contributors of water imbalance in brain during ischemic pathology. We have found that Piroxicam binds to AQP4 with optimal binding energy value. Thus, we hypothesized that Piroxicam is neuroprotective in the rodent cerebral ischemic model by mitigating cerebral edema via AQP4 regulation.

METHODS

Rats were treated with Piroxicam OR placebo at 30 min prior, 2 h post and 4 h post 60 minutes of MCAO followed by 24 hour reperfusion. Rats were evaluated for neurological deficits and motor function just before sacrifice. Brains were harvested for infarct size estimation, water content measurement, biochemical analysis, RT-PCR and western blot experiments.

RESULTS

Piroxicam pretreatment thirty minutes prior to ischemia and four hour post reperfusion afforded neuroprotection as evident through significant reduction in cerebral infarct volume, improvement in motor behavior, neurological deficit and reduction in brain edema. Furthermore, ischemia induced surge in levels of nitrite and malondialdehyde were also found to be significantly reduced in ischemic brain regions in treated animals. This neuroprotection was found to be associated with inhibition of acid mediated rise in intracellular calcium levels and also downregulated AQP4 expression.

CONCLUSIONS

Findings of the present study provide significant evidence that Piroxicam acts as a potent AQP4 regulator and renders neuroprotection in focal cerebral ischemia. Piroxicam could be clinically exploited for the treatment of brain stroke along with other anti-stroke therapeutics in future.

Extracellular acidosis modulates the endocytosis and maturation of macrophages

Extracellular acidosis is involved in various pathological situations of central nervous system and the effects are largely mediated by acid sensing ion channels (ASICs). However, it remains unclear whether extracellular acidosis affects immune cells. Macrophages are immune cells that play important role in immune reactions. In this study we investigated the impact of extracellular acidosis on the function of bone marrow derived macrophages (BMMs). The results showed that extracellular acidosis upregulated the endocytosis, surface molecular expression and interleukin-10 secretion of BMMs, in which the expression of ASIC1 and ASIC3 was detected. Notably, extracellular acidosis stimulated endocytosis and upregulation of surface molecules expression in BMMs could be abolished by amiloride, a blocker of ASICs, and nonsteroid anti-inflammatory drugs. Our findings provide new insight into the role of extracellular acidosis in the regulation of immune function and suggest ASICs as new targets for the modulation of immune response.

Aspirin-induced microscopic surface changes stimulate thrombopoiesis in rat megakaryocytes

During the process of thrombopoiesis, invaginations of the plasma membrane occur in megakaryocytes. Since acetylsalicylic acid (aspirin), the most commonly used anti-inflammatory and antiplatelet drug, interacts with the lipid bilayers of the plasma membranes, this drug would affect the process of thrombopoiesis. In the present study, employing a standard patch-clamp whole-cell recording technique, we examined the effects of aspirin on delayed rectifier K(+)-channel (Kv1.3) currents and the membrane capacitance in megakaryocytes. Using confocal imaging of di-8-butyl-amino-naphthyl-ethylene-pyridinium-propyl-sulfonate (di-8-ANEPPS) staining, we also monitored the membrane invaginations in megakaryocytes. Aspirin suppressed both the peak and the pulse-end currents with a significant increase in the membrane capacitance. Massive di-8-ANEPPS staining after treatment with aspirin demonstrated the impaired membrane micro-architecture of megakaryocytes. This study demonstrated for the first time that aspirin induces microscopic surface changes in megakaryocytes. Such surface changes were thought to stimulate thrombopoiesis in megakaryocytes as detected by the increase in the membrane invaginations.

Regulation of TRPC6 Channels by Non-Steroidal Anti-Inflammatory Drugs

Family focal segmental glomerulosclerosis (FSGS) is characterized by sclerosis and hyalinosis of particular loops of glomeruli and is one of the causes of the nephrotic syndrome. Certain mutations in the structure of TRPC6 channels are the genetic impetus for FSGS development resulting in podocytes functional abnormalities and various nephropathies. We have recently demonstrated that non-steroid anti-inflammatory drugs (NSAID) ibuprofen and diclofenac decrease the activity of endogenous TRPC like cal cium channels in the podocytes of the freshly isolated rat glomeruli. It has also been shown that TRPC6 chan nels are expressed in the podocytes. In the current study we have functionally reconstituted TRPC6 channels in mammalian cells to investigate the effects of diclofenac on the activity of wild type TRPC6 channel and TRPC6^P112Q channel containing a mutation in the N-terminus that was described in FSGS patients. Intracellular calcium level measurements in transfected cells revealed a more intensive carbachol induced increase of calcium concentration in HEK 293 cells expressing TRPC6^P112Q versus the cells expressing wild-type TRPC6. We also performed patch-clamp experiments to study TRPC6 channels reconstituted in Chinese hamster ovary (CHO) cell line and found that application of diclofenac (500 μM) acutely reduced single channel activity. Preincubation with diclofenac (100 μM) also decreased the whole cell current in CHO cells overexpressing TRPC6^P112Q. Therefore, our previously published data on the effects of NSAID on TRPC-like channels in the isolated rat glomeruli, along with this current investigation on the cultured overexpressed mammalian cells, allows hypothesizing that TRPC6 channels may be a target for NSAID that can be impor tant in the treatment of FSGS.

Drug management of visceral pain: concepts from basic research

Visceral pain is experienced by 40% of the population, and 28% of cancer patients suffer from pain arising from intra- abdominal metastasis or from treatment. Neuroanatomy of visceral nociception and neurotransmitters, receptors, and ion channels that modulate visceral pain are qualitatively or quantitatively different from those that modulate somatic and neuropathic pain. Visceral pain should be recognized as distinct pain phenotype. TRPV1, Na 1.8, and ASIC3 ion channels and peripheral kappa opioid receptors are important mediators of visceral pain. Mu agonists, gabapentinoids, and GABAB agonists reduce pain by binding to central receptors and channels. Combinations of analgesics and adjuvants in animal models have supra-additive antinociception and should be considered in clinical trials. This paper will discuss the neuroanatomy, receptors, ion channels, and neurotransmitters important to visceral pain and provide a basic science rationale for analgesic trials and management.

Insights into the effects of diclofenac and other non-steroidal anti-inflammatory agents on ion channels

OBJECTIVES

Diclofenac and other non-steroidal anti-inflammatory drugs (NSAIDs) are widely used in the treatment of inflammation and pain. Most effects of NSAIDs are attributed to the inhibition of cyclooxygenases (COX). However, many NSAIDs may have other effects not related to COX, including the modulation of various ion channels. The clinical implications of the effects on channels are not fully understood. This review outlines the effects of NSAIDs, with special attention to diclofenac, on ion channels and highlights the possible underlying mechanisms.

KEY FINDINGS

NSAIDs have effects on channels such as inhibition, activation or changes in expression patterns. The channels affected include voltage-gated Na(+) , Ca(2+) , or K(+) channels, ligand-gated K(+) channels, transient receptor potential and other cation channels as well as chloride channels in several types of cells. The mechanisms of drug actions not related to COX inhibition may involve drug-channel interactions, interference with the generation of second messengers, changes in channel expression, or synergistic/antagonist interactions with other channel modulators.

SUMMARY

The effects on ion channels may account for novel therapeutic actions of NSAIDs or for adverse effects. Among the NSAIDs, diclofenac may serve as a template for developing new channel modulators and as a tool for investigating the actions of other drugs.

High doses of salicylate and aspirin are inhibitory on acid-sensing ion channels and protective against acidosis-induced neuronal injury in the rat cortical neuron

Aspirin and its main metabolite salicylate are widely used to relieve pain, treat inflammatory diseases, and prevent ischemic stroke. Multiple pathways are responsible for the therapeutic actions exerted by these drugs. One of the pathways is targeting neuronal receptors/ion channels in the central nervous system. Correspondingly, increasing evidence has implicated acid-sensing ion channels (ASICs) in the processes of the diseases that are medicated by aspirin and salicylate. We therefore employed whole-cell patch-clamp recordings to examine the effects of salicylate as well as aspirin on ASICs in cultured cortical neurons of the rat. We recorded rapid and reversible inhibition of ASIC current by millimolar concentrations of aspirin and salicylate and found that salicylate reduced acidosis-induced membrane depolarization. These data suggest that ASICs in the cortex are molecular targets of high doses of aspirin and salicylate. In addition, the results from lactate dehydrogenase release measurement showed that high doses of aspirin and salicylate protected the cortical neuron from acidosis-induced neuronal injury. These findings may contribute to a better understanding of the therapeutic mechanisms of aspirin and salicylate actions in the brain and provide new evidence on aspirin and salicylate used as neuroprotective agents in the treatment of ischemic stroke.

Acid-sensing ion channels contribute to the effect of acidosis on the function of dendritic cells

As an H(+)-gated subgroup of the degenerin/epithelial Na(+) channel family, acid-sensing ion channels (ASICs) were reported to be involved in various physiological and pathological processes in neurons. However, little is known about the role of ASICs in the function of dendritic cells (DCs). In this study, we investigated the expression of ASICs in mouse bone marrow-derived DCs and their possible role in the function of DCs. We found that ASIC1, ASIC2, and ASIC3 are expressed in DCs at the mRNA and protein levels, and extracellular acid can evoke ASIC-like currents in DCs. We also demonstrated that acidosis upregulated the expression of CD11c, MHC class II, CD80, and CD86 and enhanced the Ag-presenting ability of DCs via ASICs. Moreover, the effect of acidosis on DCs can be abolished by the nonsteroidal anti-inflammatory drugs ibuprofen and diclofenac. These results suggest that ASICs are involved in the acidosis-mediated effect on DC function.

Acid-sensing ion channels (ASICs): pharmacology and implication in pain

Tissue acidosis is a common feature of many painful conditions. Protons are indeed among the first factors released by injured tissues, inducing a local pH fall that depolarizes peripheral free terminals of nociceptors and leads to pain. ASICs are excitatory cation channels directly gated by extracellular protons that are expressed in the nervous system. In sensory neurons, they act as "chemo-electrical" transducers and are involved in somatic and visceral nociception. Two highly specific inhibitory peptides isolated from animal venoms have considerably helped in the understanding of the physiological roles of these channels in pain. At the peripheral level, ASIC3 is important for inflammatory pain. Its expression and its activity are potentiated by several pain mediators present in the "inflammatory soup" that sensitize nociceptors. ASICs have also been involved in some aspects of mechanosensation and mechanonociception, notably in the gastrointestinal tract, but the underlying mechanisms remain to be determined. At the central level, ASIC1a is largely expressed in spinal cord neurons where it has been proposed to participate in the processing of noxious stimuli and in central sensitization. Blocking ASIC1a in the spinal cord also produces a potent analgesia in a broad range of pain conditions through activation of the opiate system. Targeting ASIC channels at different levels of the nervous system could therefore be an interesting strategy for the relief of pain.