Acid increases inflammatory pain in rats: effect of local peripheral ASICs inhibitors

Neuromodulation of Acid-Sensitive Ion Channels (ASICs) and Anti-Inflammatory Potential by Lichenxanthone in Adult Zebrafish (Danio rerio): Experimental and Docking Studies

The xanthone lichenxanthone did not show toxic effects (LC50>1.0 mg/mL). lichenxanthone prevented nociceptive behavior induced by acidic saline, and its analgesic effect was blocked by amiloride, highlighting the involvement of neuromodulation of acid-sensitive ion channels (ASICs). In the analysis of anti-inflammatory activity, concentrations of 0.1 and 0.5 mg/mL of lichenxanthone reduced the edema induced by k-carrageenan 3.5 %, observed from the fourth hour of analysis. This effect was similar to that observed with ibuprofen (positive control). No leukocyte infiltrates were observed in lichenxanthone, suggesting that the compound acts in the acute inflammatory response. The results of the molecular docking study revealed that lichenxanthone exhibited better affinity energy when compared to the ibuprofen control against the two targets evaluated.

The role of acid-sensing ion channels in monosodium urate-induced gouty pain in mice

Acid-sensing ion channels (ASICs) in dorsal root ganglion (DRG) neurons play an important role in inflammatory pain. The objective of this study is to observe the regulatory role of ASICs in monosodium urate (MSU) crystal-induced gout pain and explore the basis for ASICs in DRG neurons as a target for gout pain treatment. The gout arthritis model was induced by injecting MSU crystals into the ankle joint of mice. The circumference of the ankle joint was used to evaluate the degree of swelling; the von Frey filaments were used to determine the withdrawal threshold of the paw. ASIC currents and action potentials (APs) were recorded by patch clamp technique in DRG neurons. The results displayed that injecting MSU crystals caused ankle edema and mechanical hyperalgesia of the paw, which was relieved after amiloride treatment. The ASIC currents in DRG neurons were increased to a peak on the second day after injecting MSU crystals, which were decreased after amiloride treatment. MSU treatment increased the current density of ASICs in different diameter DRG cells. MSU treatment does not change the characteristics of AP. The results suggest that ASICs in DRG neurons participate in MSU crystal-induced gout pain.

Hydrophobic Amines and Their Guanidine Analogues Modulate Activation and Desensitization of ASIC3

Acid-sensing ion channel 3 (ASIC3) is an important member of the acid-sensing ion channels family, which is widely expressed in the peripheral nervous system and contributes to pain sensation. ASICs are targeted by various drugs and toxins. However, mechanisms and structural determinants of ligands' action on ASIC3 are not completely understood. In the present work we studied ASIC3 modulation by a series of "hydrophobic monoamines" and their guanidine analogs, which were previously characterized to affect other ASIC channels via multiple mechanisms. Electrophysiological analysis of action via whole-cell patch clamp method was performed using rat ASIC3 expressed in Chinese hamster ovary (CHO) cells. We found that the compounds studied inhibited ASIC3 activation by inducing acidic shift of proton sensitivity and slowed channel desensitization, which was accompanied by a decrease of the equilibrium desensitization level. The total effect of the drugs on the sustained ASIC3-mediated currents was the sum of these opposite effects. It is demonstrated that drugs' action on activation and desensitization differed in their structural requirements, kinetics of action, and concentration and state dependencies. Taken together, these findings suggest that effects on activation and desensitization are independent and are likely mediated by drugs binding to distinct sites in ASIC3.

Acidic pH promotes oxidation-induced dissociation of C-reactive protein

BACKGROUND

Circulating levels of the systemic inflammation marker C-reactive protein (CRP) have been associated with increased risk and poor outcomes of many diseases, such as cardiovascular events and cancer. Accumulating evidence has indicated that the conformational rearrangement of human pentameric CRP (pCRP) to monomeric CRP (mCRP) is a prerequisite for participation in the pathogenesis. Therefore, determining the mechanism of the dissociation of pCRP into pro-inflammatory mCRP under physiological/pathological circumstances has been intriguing.

METHODS

The effects of oxidative and acidic stress occurring in inflammation on pCRP were examined by electrophoresis, electron microscopy, protein fluorescence, neoepitope expression and endothelial cell responses.

RESULTS

Reactive oxygen species (ROS) generated by the copper-hydrogen peroxide system could rapidly induce the dissociation of CRP at mild acidic pH within four hours, but not at physiological pH of 7.4. Meanwhile, mannitol, a ROS scavenger, could not protect against dissociation, which implied that local ROS from accessible histidine residues may be crucially beneficial to the formation of mCRP in a redox-balanced microenvironment. Furthermore, mCRP generated by ROS could be reduced by DTT, which indicated the exposure of functional motif aa35-47, and showed potent proinflammatory actions on endothelial cells, comparable to mCRP generated by urea.

CONCLUSION

dissociation of pCRP to mCRP could be rapidly induced by ROS from copper- hydrogen peroxide system in dependence on mildly acidic stress regardless of a redox-balanced microenvironment.

The orofacial antinociceptive effect of Kaempferol-3-O-rutinoside, isolated from the plant Ouratea fieldingiana, on adult zebrafish (Danio rerio)

The aim of this study was to evaluate the antinociceptive effect of Kaempferol-3-O-rutinoside (KR), isolated from the plant Ouratea fieldingiana, on the orofacial nociception and possible mechanisms of action. Adult zebrafish (Danio rerio) were tested as a behavioral model to study formalin, glutamate, capsaicin, cinnamaldehyde and acidic saline-induced orofacial nociception, using as parameter the number of times the fish crossed the lines between the quadrants of a glass Petri dish during a specific time. Morphine was used as positive control. The effect of KR was tested for modulation by opioid (naloxone), nitrergic (L-NAME), TRPV1 (ruthenium red), TRPA1 (camphor) or ASIC (amiloride) antagonists. The effect of KR on zebrafish locomotor behavior was evaluated with the open field test. KR did not alter the fish's locomotor system and significantly reduced the orofacial nociceptive behavior induced by all noxious agents compared to the control group. The antinociceptive effect of KR was similar to morphine. All antagonists inhibited the antinociceptive effect of KR. KR has pharmacological potential for the treatment of acute orofacial pain and this effect is modulated by the opioid and nitrergic systems as well as TRPV1, TRPA1 and ASIC channels. These results can lead to the development of a new natural product for the treatment of orofacial pain and confirm the popular use of O. fieldingiana leaf for pain relief.

Coordinate effects of P2X7 and extracellular acidification in microglial cells

Extracellular adenosine 5′-triphosphate (ATP) is a damage-associated molecular pattern and contributes to inflammation associated diseases including cancer. Extracellular acidosis is a novel danger signal in the inflammatory sites, where it can modulate inflammation, immunity and tumor growth. Extracellular acidification was shown to inhibit P2X7-mediated channel currents, while it remains unknown how acidification and P2X7 together affect cellular responses. Here, we treated BV-2 microglial cells with ATP in a short period (<15 min) or a sustained acidified condition. For short acidification we compared the actions of neutralized ATP and acidic ATP in a condition with pH buffering. For sustained acidification, we treated cells with neutralized ATP in acidic medium or acidic ATP in medium without pH buffering. In the short acidified condition, neutralized ATP induced higher responses than acidic ATP to increase intracellular calcium and reactive oxygen species, decrease intracellular potassium and induce cell death. In contrast, these cellular responses and mitochondrial fission caused by neutralized ATP were enhanced by pH 6.0 and pH 4.5 media. P2X7 activation can also rapidly block mitochondrial ATP turnover and respiration capacity, both of which were mimicked by nigericin and enhanced by acidity. Taken together P2X7-mediated ionic fluxes and reactive oxygen species production are attenuated under short acidification, while sustained acidification itself can induce mitochondrial toxicity which deteriorates the mitochondrial function under P2X7 activation.

Peripheral and spinal TRPA1 channels contribute to formalin-induced long-lasting mechanical hypersensitivity

Background

Transient receptor potential ankyrin 1 (TRPA1) is a non-selective cation channel expressed by a subset of nociceptive neurons that acts as a multimodal receptor. Its activity contributes to modulate nociceptive transmission in acute inflammatory pain. However, the role of this channel in chronic pain has been less studied. The purpose of this study was to investigate the local peripheral and spinal participation of TRPA1 channels in formalin-induced long-lasting hypersensitivity.

Materials and methods

Formalin (1%)-induced chronic hypersensitivity was determined by the application of von Frey filaments to ipsilateral and contralateral paws and through pharmacological testing using a selective TRPA1 blocker (A-967079). TRPA1 expression in the dorsal root ganglion (DRG) and spinal cord was analyzed by Western blotting.

Results

Formalin (1%) injection produced acute flinching behavior (1 h) as well as secondary allodynia and hyperalgesia (12 days). Local peripheral pretreatment (10 min before) or posttreatment (6 days later) with A-967079 (1–100 µM) partially prevented and reversed, respectively, in a dose-dependent manner, long-lasting secondary mechanical allodynia and hyperalgesia evoked by 1% formalin. Likewise, intrathecal pretreatment or posttreatment with A-967079 partially prevented and reversed, respectively, formalin-induced long-lasting hypersensitivity. A-967079 (100 µM) completely abolished the pro-nociceptive effect of formalin (adjusted to pH 7.4). Finally, formalin injection increased TRPA1 protein expression in the DRG and spinal cord.

Conclusion

Results indicate that TRPA1 expressed in the DRG and spinal cord plays a relevant role in formalin-induced long-lasting secondary nociceptive hypersensitivity.

Acid-induced experimental knee pain and hyperalgesia in healthy humans

Inflammation and the related acidity in peri-articular structures may be involved in pain generation and hyperalgesia in knee osteoarthritis. This study investigated pain and associated hyperalgesia provoked by infusion of acidic saline into the infrapatellar fat pad. Twenty-eight subjects participated in two sessions in which acidic saline (AS, pH 5) or neutral saline (NS, pH 7.4) were infused into the infrapatellar fat pad for 15 min. Pain intensity, pain area, mechanical and thermal sensitivity, and maximal voluntary knee extension force were recorded. Repeated infusions were performed in 14 subjects. Infusion of AS caused significantly higher pain intensity, larger pain areas, induced hyperalgesia around the infused knee, and reduced extension force. No significant pain facilitation or spreading of hyperalgesia was found after repeated infusions as compared with single infusions. Acidic saline infused into the infrapatellar fat pad provoked pain and localized mechanical hyperalgesia. Thus, this acid-induced pain model may mimic the early-stage responses to tissue injury of knee osteoarthritis.

Acid-induced experimental muscle pain and hyperalgesia with single and repeated infusion in human forearm

Background and purpose

Acid has long been thought to play an important role in the pain process. Animal study showed that repeated acid stimulation induced central sensitization. The purpose of the study is to investigate muscle pain and hyperalgesia evoked byintramuscular infusion of saline at different pH levels, and to compare the effect of a single versus repeated acid infusions.

Methods

Twenty healthy subjects received infusions of buffered saline (pH 5.0, 6.0, and 7.4) into the brachioradialis muscle in a randomized order. Twelve of the subjects received repeated infusions. The subjects rated the pain intensity on visual analogue scale (VAS). Thermal pain sensitivity, and pressure pain threshold (PPT) were assessed in both arm before, during, immediately after, one hour after, and one day after the infusion. A McGill Pain Questionnaire and pain mapping were completed after each infusion.

Results

The pH 5 solution caused significantly higher pain and larger areas than pH 6.0 or 7.4. The local PPTs were significantly decreased (hyperalgesia) during and immediately after infusion of all three solutions. No significant differences were detected between the first and second infusion.

Conclusions

The intensity of acid-induced muscle pain is pH-dependent. All three solutions induced pressure hyperalgesia at the infusion site. Repeated infusions did not induce increased pain or prolonged hyperalgesia as compared with a single injection. Human intramuscular acidic saline infusion could not produce chronic pain model.

Implications

The acid-induced pain model may reflect the early stage responses to tissue injury of clinical conditions. Repeated intramuscular acidic saline injection model of prolonged hyperalgesia in rodents could not be translated into a human for modelling chronic musculoskeletal pain.

TRPA1 and TRPV1 Antagonists Do Not Inhibit Human Acidosis-Induced Pain

Acidosis occurs in a variety of pathophysiological and painful conditions where it is thought to excite or contribute to excitation of nociceptive neurons. Despite potential clinical relevance the principal receptor for sensing acidosis is unclear, but several receptors have been proposed. We investigated the contribution of the acid-sensing ion channels, transient receptor potential vanilloid type 1 (TRPV1) and transient receptor potential ankyrin type 1 (TRPA1) to peripheral pain signaling. We first established a human pain model using intraepidermal injection of the TRPA1 agonist carvacrol. This resulted in concentration-dependent pain sensations, which were reduced by experimental TRPA1 antagonist A-967079. Capsaicin-induced pain was reduced by the TRPV1 inhibitor BCTC. Amiloride was used to block acid-sensing ion channels. Testing these antagonists in a double-blind and randomized experiment, we probed the contribution of the respective channels to experimental acidosis-induced pain in 15 healthy human subjects. A continuous intraepidermal injection of pH 4.3 was used to counter the buffering capacity of tissue and generate a prolonged painful stimulation. In this model, addition of A-967079, BCTC or amiloride did not reduce the reported pain. In conclusion, target-validated antagonists, applied locally in human skin, have excluded the main hypothesized targets and the mechanism of the human acidosis-induced pain remains unclear.

PERSPECTIVE

An acidic milieu is a trigger of pain in many clinical conditions. The aim of this study was to identify the contribution of the currently hypothesized sensors of acid-induced pain in humans. Surprisingly, inhibition of these receptors did not alter acidosis-induced pain.

Participation of peripheral TRPV1, TRPV4, TRPA1 and ASIC in a magnesium sulfate-induced local pain model in rat

We previously showed that magnesium sulfate (MS) has systemic antinociceptive and local peripheral pronociceptive effects. The role of transient receptor potential (TRP) channels and acid-sensing ion channels (ASICs) in the mechanism of action of MS has not been investigated in detail. The aim of this study was to explore the participation of TRP channels in the pronociceptive action of MS in rats after its intraplantar injection. The paw withdrawal threshold (PWT) to mechanical stimuli was measured by the electronic von Frey test. Drugs that were tested were either co-administered with an isotonic pH-unadjusted or pH-adjusted solution of MS intraplantarily, or to the contralateral paw to exclude systemic effects. We found that the subcutaneous administration of both pH-adjusted (7.4) and pH-unadjusted (about 6.0) isotonic (6.2% w/v in water) solutions of MS induce the pain at the injection site. The pH-unadjusted MS solution-induced mechanical hyperalgesia decreased in a dose-dependent manner as a consequence of co-injection of capsazepine, a selective TRPV1 antagonist (20, 100 and 500pmol/paw), RN-1734, a selective TRPV4 antagonist (1.55, 3.1 and 6.2μmol/paw), HC-030031, a selective TRPA1 antagonist (5.6, 28.1 and 140nmol/paw), and amiloride hydrochloride, a non-selective ASIC inhibitor (0.83, 2.5 and 7.55μmol/paw). In pH-adjusted MS-induced hyperalgesia, the highest doses of TRPV1, TRPV4 and TRPA1 antagonists displayed effects that were, respectively, either similar, less pronounced or delayed in comparison to the effect induced by administration of the pH-unadjusted MS solution; the ASIC antagonist did not have any effect. These results suggest that the MS-induced local peripheral mechanical hyperalgesia is mediated via modulation of the activity of peripheral TRPV1, TRPV4, TRPA1 and ASICs. Specific local inhibition of TRP channels represents a novel approach to treating local injection-related pain.

NS383 Selectively Inhibits Acid-Sensing Ion Channels Containing 1a and 3 Subunits to Reverse Inflammatory and Neuropathic Hyperalgesia in Rats

AIMS

Here, we investigate the pharmacology of NS383, a novel small molecule inhibitor of acid-sensing ion channels (ASICs).

METHODS

ASIC inhibition by NS383 was characterized in patch-clamp electrophysiological studies. Analgesic properties were evaluated in four rat behavioral models of pain.

RESULTS

NS383 inhibited H(+)-activated currents recorded from rat homomeric ASIC1a, ASIC3, and heteromeric ASIC1a+3 with IC50 values ranging from 0.61 to 2.2 μM. However, NS383 was completely inactive at homomeric ASIC2a. Heteromeric receptors containing AISC2a, such as ASIC1a+2a and ASIC2a+3, were only partially inhibited, presumably as a result of stoichiometry-dependent binding. NS383 (10-60 mg/kg, i.p.), amiloride (50-200 mg/kg, i.p.), acetaminophen (100-400 mg/kg, i.p.), and morphine (3-10 mg/kg, i.p.) all dose-dependently attenuated nocifensive behaviors in the rat formalin test, reversed pathological inflammatory hyperalgesia in complete Freund's adjuvant-inflamed rats, and reversed mechanical hypersensitivity in the chronic constriction injury model of neuropathic pain. However, in contrast to acetaminophen and morphine, motor function was unaffected by NS383 at doses at least 8-fold greater than those that were effective in pain models, whilst analgesic doses of amiloride were deemed to be toxic.

CONCLUSIONS

NS383 is a potent and uniquely selective inhibitor of rat ASICs containing 1a and/or 3 subunits. It is well tolerated and capable of reversing pathological painlike behaviors, presumably via peripheral actions, but possibly also via actions within central pain circuits.

Sensory nerves contribute to cutaneous vasodilator response to cathodal stimulation in healthy rats

Cutaneous current-induced vasodilation (CIV) in response to galvanic current application is an integrative model of neurovascular interaction that relies on capsaicin-sensitive fiber activation. The upstream and downstream mechanisms related to the activation of the capsaicin-sensitive fibers involved in CIV are not elucidated. In particular, the activation of cutaneous transient receptor potential vanilloid type-1 (TRPV1) channels and/or acid-sensing ion channels (ASIC) (activators mechanisms) and the release of calcitonin gene-related peptide (CGRP) and substance P (SP) (effector mechanisms) have been tested. To assess cathodal CIV, we measured cutaneous blood flow using laser Doppler flowmetry for 20min following cathodal current application (240s, 100μA) on the skin of the thigh in anesthetized healthy rats for 20min. CIV was studied in rats treated with capsazepine and amiloride to inhibit TRPV1 and ASIC channels, respectively; CGRP8-37 and SR140333 to antagonize CGRP and neurokinin-1 (NK1) receptors, respectively; compared to their respective controls. Cathodal CIV was attenuated by capsazepine (12±2% vs 54±6%, P<0.001), amiloride (19±8% vs 61±6%, P<0.01), CGRP8-37 (15±6% vs 61±6%, P<0.001) and SR140333 (9±5% vs 54±6%, P<0.001) without changing local acidification. This is the first integrative study performed in healthy rats showing that cutaneous vasodilation in response to cathodal stimulation is initiated by activation of cutaneous TRPV1 and ASIC channels likely through local acidification. The involvement of CGRP and NK1 receptors suggests that cathodal CIV is the result of CGRP and SP released through activated capsaicin-sensitive fibers. Therefore cathodal CIV could be a valuable method to assess sensory neurovascular function in the skin, which would be particularly relevant to evaluate the presence of small nerve fiber disorders and the effectiveness of treatments.

Intraplantar injection of tetrahydrobiopterin induces nociception in mice

Tetrahydrobiopterin (BH4) is implicated in the development and maintenance of chronic pain. After injury/inflammation, the biosynthesis of BH4 is markedly increased in sensory neurons, and the pharmacological and genetic inhibition of BH4 shows analgesic effects in pre-clinical animal pain models. Intrathecal injections of BH4 have been shown to induce and enhance pain-like behaviours in rats, suggesting that under chronic pain conditions BH4 may act by facilitating central sensitisation. So far it is unknown whether BH4 acts on peripheral sites of the somatosensory system or whether BH4 per se provokes nociceptive pain behaviours. The purpose of this study was therefore to investigate the acute nociceptive effects of intraplantar injection of BH4. BH4 was found to induce dose-dependent licking/biting of the paw lasting 5 min, which was not observed following an injection of biopterin (inactive BH4 metabolite). Paw swelling, measured as paw thickness and weight, was not observed after BH4 injection. To explore possible mechanisms of action of BH4, the effect of local pre-treatment with indomethacin, Nω-nitro-L-arginine methyl ester, Nω-nitro-L-arginine, capsazepine and ruthenium red was tested. Morphine served as a positive control. Intraplantar pre-injection of morphine dose-dependently inhibited BH4-induced nociception, while none of the other compounds showed any statistical significant antinociception. These results suggest that BH4 exhibits nociceptive properties at peripheral sites of the somatosensory system, proposing an as yet unexplored involvement of BH4 in peripheral nociceptive processes. However, this appears not to be mediated through nitric oxide and prostaglandin release or by activation of the transient receptor potential vanilloid 1.

[Acid-Sensing Ion Channels (ASICs) in pain]

The discovery of new drug targets represents a real opportunity for developing fresh strategies against pain. Ion channels are interesting targets because they are directly involved in the detection and the transmission of noxious stimuli by sensory fibres of the peripheral nervous system and by neurons of the spinal cord. Acid-Sensing Ion Channels (ASICs) have emerged as important players in the pain pathway. They are neuronal, voltage-independent depolarizing sodium channels activated by extracellular protons. The ASIC family comprises several subunits that need to associate into homo- or hetero-trimers to form a functional channel. The ASIC1 and ASIC3 isoforms are particularly important in sensory neurons, whereas ASIC1a, alone or in association with ASIC2, is essential in the central nervous system. The potent analgesic effects associated with their inhibition in animals (which can be comparable to those of morphine) and data suggesting a role in human pain illustrate the therapeutic potential of these channels.

Gastrodin inhibits the activity of acid-sensing ion channels in rat primary sensory neurons

Acid-sensing ion channels (ASICs), a family of proton-gated cation channels, are believed to mediate pain caused by extracellular acidification. Gastrodin is a main bioactive constituent of the traditional herbal Gastrodia elata Blume, which has been widely used in Oriental countries for centuries. As an analgesic, gastrodin has been used clinically to treat pain such as migraine and headache. However, the mechanisms underlying analgesic action of gastrodin are still poorly understood. Here, we have found that gastrodin inhibited the activity of native ASICs in rat dorsal root ganglion (DRG) neurons. Gastrodin dose-dependently inhibited proton-gated currents mediated by ASICs. Gastrodin shifted the proton concentration-response curve downwards, with a decrease of 36.92 ± 6.23% in the maximum current response but with no significant change in the pH0.5 value. Moreover, gastrodin altered acid-evoked membrane excitability of rat DRG neurons and caused a significant decrease in the amplitude of the depolarization and the number of action potentials induced by acid stimuli. Finally, peripheral applied gastrodin relieved pain evoked by intraplantar injection of acetic acid in rats. Our results indicate that gastrodin can inhibit the activity of ASICs in the primary sensory neurons, which provided a novel mechanism underlying analgesic action of gastrodin.

Calcium-permeable ion channels in pain signaling

The detection and processing of painful stimuli in afferent sensory neurons is critically dependent on a wide range of different types of voltage- and ligand-gated ion channels, including sodium, calcium, and TRP channels, to name a few. The functions of these channels include the detection of mechanical and chemical insults, the generation of action potentials and regulation of neuronal firing patterns, the initiation of neurotransmitter release at dorsal horn synapses, and the ensuing activation of spinal cord neurons that project to pain centers in the brain. Long-term changes in ion channel expression and function are thought to contribute to chronic pain states. Many of the channels involved in the afferent pain pathway are permeable to calcium ions, suggesting a role in cell signaling beyond the mere generation of electrical activity. In this article, we provide a broad overview of different calcium-permeable ion channels in the afferent pain pathway and their role in pain pathophysiology.

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

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

Acidic extracellular microenvironment and cancer

Acidic extracellular pH is a major feature of tumor tissue, extracellular acidification being primarily considered to be due to lactate secretion from anaerobic glycolysis. Clinicopathological evidence shows that transporters and pumps contribute to H⁺ secretion, such as the Na⁺/H⁺ exchanger, the H⁺-lactate co-transporter, monocarboxylate transporters, and the proton pump (H⁺-ATPase); these may also be associated with tumor metastasis. An acidic extracellular pH not only activates secreted lysosomal enzymes that have an optimal pH in the acidic range, but induces the expression of certain genes of pro-metastatic factors through an intracellular signaling cascade that is different from hypoxia. In addition to lactate, CO₂ from the pentose phosphate pathway is an alternative source of acidity, showing that hypoxia and extracellular acidity are, while being independent from each other, deeply associated with the cellular microenvironment. In this article, the importance of an acidic extracellular pH as a microenvironmental factor participating in tumor progression is reviewed.

Antinociceptive effects of amiloride and benzamil in neuropathic pain model rats

Amiloride and benzamil showed antinocicepitve effects in several pain models through the inhibition of acid sensing ion channels (ASICs). However, their role in neuropathic pain has not been investigated. In this study, we investigated the effect of the intrathecal amiloride and benzamil in neuropathic pain model, and also examined the role of ASICs on modulation of neuropathic pain. Neuropathic pain was induced by L4-5 spinal nerve ligation in male Sprague-Dawley rats weighing 100-120 g, and intrathecal catheterization was performed for drug administration. The effects of amiloride and benzamil were measured by the paw-withdrawal threshold to a mechanical stimulus using the up and down method. The expression of ASICs in the spinal cord dorsal horn was also analyzed by RT-PCR. Intrathecal amiloride and benzamil significantly increased the paw withdrawal threshold in spinal nerve-ligated rats (87%±12% and 76%±14%, P=0.007 and 0.012 vs vehicle, respectively). Spinal nerve ligation increased the expression of ASIC3 in the spinal cord dorsal horn (P=0.01), and this increase was inhibited by both amiloride and benzamil (P<0.001 in both). In conclusion, intrathecal amiloride and benzamil display antinociceptive effects in the rat spinal nerve ligation model suggesting they may present an alternative pharmacological tool in the management of neuropathic pain at the spinal level.

Mechanisms involved in the nociception triggered by the venom of the armed spider Phoneutria nigriventer

BACKGROUND

The frequency of accidental spider bites in Brazil is growing, and poisoning due to bites from the spider genus Phoneutria nigriventer is the second most frequent source of such accidents. Intense local pain is the major symptom reported after bites of P. nigriventer, although the mechanisms involved are still poorly understood. Therefore, the aim of this study was to identify the mechanisms involved in nociception triggered by the venom of Phoneutria nigriventer (PNV).

METHODOLOGY/PRINCIPAL FINDINGS

Twenty microliters of PNV or PBS was injected into the mouse paw (intraplantar, i.pl.). The time spent licking the injected paw was considered indicative of the level of nociception. I.pl. injection of PNV produced spontaneous nociception, which was reduced by arachnid antivenin (ArAv), local anaesthetics, opioids, acetaminophen and dipyrone, but not indomethacin. Boiling or dialysing the venom reduced the nociception induced by the venom. PNV-induced nociception is not dependent on glutamate or histamine receptors or on mast cell degranulation, but it is mediated by the stimulation of sensory fibres that contain serotonin 4 (5-HT4) and vanilloid receptors (TRPV1). We detected a kallikrein-like kinin-generating enzyme activity in tissue treated with PNV, which also contributes to nociception. Inhibition of enzymatic activity or administration of a receptor antagonist for kinin B2 was able to inhibit the nociception induced by PNV. PNV nociception was also reduced by the blockade of tetrodotoxin-sensitive Na(+) channels, acid-sensitive ion channels (ASIC) and TRPV1 receptors.

CONCLUSION/SIGNIFICANCE

Results suggest that both low- and high-molecular-weight toxins of PNV produce spontaneous nociception through direct or indirect action of kinin B2, TRPV1, 5-HT4 or ASIC receptors and voltage-dependent sodium channels present in sensory neurons but not in mast cells. Understanding the mechanisms involved in nociception caused by PNV are of interest not only for better treating poisoning by P. nigriventer but also appreciating the diversity of targets triggered by PNV toxins.

Reversal of acid-induced and inflammatory pain by the selective ASIC3 inhibitor, APETx2

BACKGROUND AND PURPOSE

Inflammatory pain is triggered by activation of pathways leading to the release of mediators such as bradykinin, prostaglandins, interleukins, ATP, growth factors and protons that sensitize peripheral nociceptors. The activation of acid-sensitive ion channels (ASICs) may have particular relevance in the development and maintenance of inflammatory pain. ASIC3 is of particular interest due to its restricted tissue distribution in the nociceptive primary afferent fibres and its high sensitivity to protons.

EXPERIMENTAL APPROACH

To examine the contribution of ASIC3 to the development and maintenance of muscle pain and inflammatory pain, we studied the in vivo efficacy of a selective ASIC3 inhibitor, APETx2, in rats.

KEY RESULTS

Administration of APETx2 into the gastrocnemius muscle prior to the administration of low pH saline prevented the development of mechanical hypersensitivity, whereas APETx2 administration following low-pH saline was ineffective in reversing hypersensitivity. The prevention of mechanical hypersensitivity produced by acid administration was observed whether APETx2 was applied via i.m. or i.t. routes. In the complete Freund's adjuvant (CFA) inflammatory pain model, local administration of APETx2 resulted in a potent and complete reversal of established mechanical hypersensitivity, whereas i.t. application of APETx2 was ineffective.

CONCLUSIONS AND IMPLICATIONS

ASIC3 contributed to the development of mechanical hypersensitivity in the acid-induced muscle pain model, whereas ASIC3 contributed to the maintenance of mechanical hypersensitivity in the CFA inflammatory pain model. The contribution of ASIC3 to established hypersensitivity associated with inflammation suggests that this channel may be an effective analgesic target for inflammatory pain states.

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.

Acid-Sensing Ion Channels and Pain

Pathophysiological conditions such as inflammation, ischemia, infection and tissue injury can all evoke pain, and each is accompanied by local acidosis. Acid sensing ion channels (ASICs) are proton-gated cation channels expressed in both central and peripheral nervous systems. Increasing evidence suggests that ASICs represent essential sensors for tissue acidosis-related pain. This review provides an update on the role of ASICs in pain sensation and discusses their therapeutic potential for pain management.

Acid-sensing ion channels 3: a potential therapeutic target for pain treatment in arthritis

Acid-sensing ion channels 3 (ASIC3) is the most sensitive to such a pH change, predominantly distributed in the sensory peripheral nervous system, and strongly correlated with pain. Recently, there is increasing evidence that ASIC3 may contribute to the pathogenesis of chronic inflammatory pain diseases due to it is predominantly expressed in dorsal root ganglia (DRG) neurons making it a good candidate for a pain sensor. Elevated expression of ASIC3 was found in DRG of rodents with inflamed hind paws. In addition, it has been shown that ASIC3 gene knock-out mice (ASIC3-/-) exhibited no enhanced hyperalgesia in inflamed joint. All theses findings suggest that ASIC3 have important biological effects in inflammation that might be a promising therapeutic target for arthritis pain. In this review, we will briefly discuss the biological features of ASIC3 and summarize recent advances on the role of ASIC3 in the pathogenesis and treatment of arthritis pain.